metal oxide–carbon nanocomposites for energy storage and

METAL OXIDE–CARBON NANOCOMPOSITES FOR ENERGY STORAGE

AND CONVERSION

by

Wijayantha Asanga Perera

APPROVED BY SUPERVISORY COMMITTEE:

___________________________________________

Dr. Kenneth J. Balkus, Jr., Chair

___________________________________________

Dr. John P. Ferraris

___________________________________________

Dr. Yves J. Chabal

___________________________________________

Dr John W. Siber

Copyright 2017

Wijayantha Asanga Perera

All Rights Reserved

Dedicated to my loving parents, Neel Perera, Jayanthi Herath and my wife Anuradha for their

endless love, wisdom, support and encouragement


AND CONVERSION

by

WIJAYANTHA ASANGA PERERA, BS, MS

DISSERTATION

Presented to the Faculty of

The University of Texas at Dallas

in Partial Fulfillment

of the Requirements

for the Degree of

DOCTOR OF PHILOSOPHY IN

CHEMISTRY

THE UNIVERSITY OF TEXAS AT DALLAS

May 2017

v

ACKNOWLEDGMENTS

First of all I would like to express my deepest gratitude to my research supervisor, Dr. Kenneth J.

Balkus, Jr., for his constant support and encouragement with guidance throughout my PhD

program. His kind guidance encouraged me to identify my weaknesses and improve myself. I have

been very fortunate and proud to be one of his students. I would like to extend my special thanks

to him for guiding me to the end and giving me moral and emotional support to be successful. I

am also grateful to Dr. John P. Ferraris, my research collaborator who always helped me when

needed and gave me an opportunity to access cutting edge facilities. And also I would like to

express my special thanks to Dr. Yves Chabal for helping me in many ways. His kind, encouraging

words helped me to reach my goals. I want to express my gratitude to supervising committee

member Dr. John W. Sibert, for his support and guidance. Their valuable ideas and suggestions

helped me to understand and successfully overcome research challenges.

I would like to extend my gratitude to past and present members in the Balkus lab. They have been

a family for me throughout the years. I would also like to acknowledge all past and present

chemistry department staff members, especially Dr. George D. McDonald, Dr. Winston Layne,

Betty Maldonado, Linda L. Heard and Lydia Selvidge who helped in many ways to succeed. I

would like to extend my thanks to all my friends back in Sri Lanka and in USA who helped me in

many ways. I would like to thank all my Sri Lankan friends at UTD for giving me support during

my rough times. We spent a lot of enjoyable time together. Finally, I would like to express my

deepest gratitude to my mother Jayanthi Herath, my father Neel Perera, my brother Sanjaya Perera,

my sister Sandaruwani Perera and my loving wife Anuradha Liyanage, for their endless love and

vi

encouragement offered during difficult time periods. Without their support and love I would not

be able to reach this far.

March 2017

vii


AND CONVERSION

Wijayantha Asanga Perera, PhD

The University of Texas at Dallas, 2017

ABSTRACT

Supervising Professor: Dr. Kenneth J. Balkus, Jr.

Increased energy demand with the exponential growth in population has become one of the major

challenges that mankind has to face. To overcome the growing energy demand, there is a

significant need to find either a sustainable and renewable energy source or efficient ways to store

energy. Therefore, development of novel energy storage devices have attracted a great attention.

Among different energy storage devices, batteries are the most convenient and accessible devices

that are commercially available for a wide range of consumer devices. However, low power density

of batteries greatly limits its use in applications requiring quick burst of energy. Thus, one of the

overall goals of this study is to develop novel electrode nanostructures and compositions for the

next generation of electrochemical devices that are capable of delivering high energy density, high

power density and high capacitance.

In the first part of the dissertation, the preparation of metal oxide-carbon nanocomposites

using different methodologies and the evaluation of their performance will be discussed. Currently,

RuO2 is considered as the best metal oxide that possesses the highest pseudocapacitive properties.

viii

In the second chapter, the use of RuO2 as the pseudocapacitive metal oxide and carbon nanotubes

as the electrical double layer capacitive (EDLC) material will be discussed. Moreover, a novel

method was introduced to prepare RuO2 nanoribbons. Since RuO2 is an expensive material,

incorporating it with a cheaper alternative, i.e. vanadium oxide (V2O5) will be discussed in the

third chapter. To achieve this, RuO2 nanodots were deposited on V2O5 nanorods. The use of V2O5

significantly decreased the material cost. Since this novel method used RuO2 quantum dots in low

compositions, harvesting the great electrochemical performance of RuO2 without increasing the

material cost was successfully achieved. In the fourth chapter, graphene oxide mediated sodium

niobate nanotubes were prepared and used as the supercapacitor electrode material. Recently, the

use of high surface area carbon nanomaterials for electrochemical energy storage devices has

gained more attention. In the fifth chapter, the incorporation of high surface area wrinkled

mesoporous carbon to supercapacitor electrodes will be presented. Different amounts of RuO2

nanoparticles were deposited on to the wrinkled mesoporous carbon and the electrochemical

performance of supercapacitors were evaluated.

Another major challenge associated with the increasing population is rapid industrialization.

With the development of new industries, more waste are generated and released to the

environment. Most of these industrial waste contain organic pollutants and eventually they are

collected in free water bodies such as oceans and water streams ultimately resulting in the

accumulation of toxic organic components in the biomass.

The second part of the dissertation will be focused on the development of novel TiO2

nanotube/ RuO2 nanoribbons/graphene oxide composites for photocatalytic degradation of organic

pollutants. This novel photocatalyst significantly increased the photocatalytic remediation of

ix

organic dye due to reduced rate of electron-hole recombination. These results suggest that TiO2

nanotube/ RuO2 nanoribbons/graphene oxide composite is capable of efficiently degrading toxic

organic components present in industrial waste.

x

TABLE OF CONTENTS

ACKNOWLEDGMENTS ………………………….……………………….…….……….......... v

ABSTRACT …..…………………………………………………………….…………..............vii

LIST OF FIGURES …………………………………………………………………….……. ...xiv

LIST OF TABLES …………………………………………………….…………………...….. xvii

CHAPTER 1 INTRODUCTION ....................................................................................................1

1.1 Introduction ..............................................................................................................1

1.2 References ................................................................................................................2

CHAPTER 2 RUTHENIUM OXIDE NANORIBBON – CARBON NANOTUBE COMPOSITE

ELECTRODES FOR HIGH PERFORMANCE SUPERCAPACITORS .......................................4

2.1 Introduction ..............................................................................................................4

2.2 Materials and methods .............................................................................................6

2.3 Synthesis of ruthenium oxide nanoribbons (RuO2 NR) ...........................................7

2.4 Preparation of ruthenium oxide nanoribbon/ carbon nanotube composite

electrodes .................................................................................................................7

2.5 Supercapacitor assembly ..........................................................................................9

2.6 Characterization .......................................................................................................9

2.7 Results and discussion ...........................................................................................10

2.8 Characterization of RuO2 nanoribbons ..................................................................10

2.9 Fabrication and characterization of hybrid RuO2 nanoribbon carbon nanotube

composite papers ....................................................................................................13

2.10 Electrochemical characterization of MCNT- RuO2 NR composite papers ...........16

2.11 Conclusion .............................................................................................................23

2.12 Supporting information ..........................................................................................24

2.13 References ..............................................................................................................27

CHAPTER 3 RUTHENIUM OXIDE NANODOT DECORATED VANADIUM OXIDE

NANOROD– CARBON NANOTUBE COMPOSITES FOR SUPERCAPACITORS ................37

3.1 Introduction ............................................................................................................37

xi

3.2 Materials and methods ...........................................................................................39

3.3 Synthesis of vanadium oxide nanorods (VNRs) ....................................................39

3.4 Synthesis of ruthenium oxide nanodot (RuO2 ND) decorated vanadium oxide

nanorods (VNRs) ...................................................................................................40

3.5 Preparation of ruthenium oxide nanodot (RuO2 ND) chemically bound vanadium

oxide nanorods/ carbon nanotube composite electrodes ........................................40

3.6 Supercapacitor assembly ........................................................................................41

3.7 Characterization .....................................................................................................41


3.9 Characterization of ruthenium oxide nanodots (RuO2 NDs) decorated vanadium

oxide nanorods (VNRs) .........................................................................................42

3.10 Electrochemical characterization of RuO2 NDs /VNRs composite papers ...........47

3.11 Conclusion .............................................................................................................53

3.12 References ..............................................................................................................53

CHAPTER 4 BINDER FREE GRAPHENE–SODIUM NIOBATE NANOTUBE/ NANO-ROD

COMPOSITE ELECTRODES FOR SUPERCAPACITORS .......................................................58

4.1 Introduction ............................................................................................................58

4.2 Experimental ..........................................................................................................61

4.3 Material and methods .............................................................................................61

4.4 Synthesis of graphene oxide (GO) .........................................................................61

4.5 Synthesis of hydrothermally reduced graphene oxide (hGO) ................................62

4.6 Synthesis of sodium niobate nanotubes on hydrothermally reduced graphene

oxide (hGO) ...........................................................................................................62

4.7 Synthesis of NaNbO3 nanorods/hGO composite ...................................................63

4.8 Fabrication of coin cell type supercapacitors .........................................................63



4.11 Characterization of hGO and NaNbO3 nanorods ...................................................64

4.12 Characterization of NaNbO3 nanotube/hGO composites ......................................68

4.13 Proposed mechanism for NaNbO3 nanorod formation. .........................................69

4.14 Proposed mechanism for NaNbO3 nanotube formation.........................................72

4.15 Electrode preparation .............................................................................................73

xii

4.16 Conclusion .............................................................................................................80

4.17 References ..............................................................................................................82

CHAPTER 5 RUO2 NANODOTS SUPPORTED WRINKLED MESOPOROUS CARBON

FORSUPERCAPACITORS………………...………………………………………………….91

5.1 Introduction ............................................................................................................91

5.2 Materials and methods ...........................................................................................92

5.3 Synthesis of wrinkled mesoporous silica (WMS) ..................................................92

5.4 Synthesis of wrinkled mesoporous carbon (WMC) ...............................................93

5.5 Synthesis of ruthenium oxide nanodots on wrinkled mesoporous carbon

(WMCR) ................................................................................................................93

5.6 Synthesis of ruthenium oxide nanodots on wrinkled mesoporous carbon using a

reducing method.....................................................................................................94

5.7 Preparation of ruthenium oxide on wrinkled mesoporous carbon (WMCR)

composite electrodes ..............................................................................................94

5.8 Supercapacitor assembly. .......................................................................................95



5.11 Characterization of ruthenium oxide nanodots grown on wrinkled mesoporous

carbon (WMCR) ....................................................................................................96

5.12 Conclusion ...........................................................................................................102

5.13 References ............................................................................................................102

CHAPTER 6 HYDROTHERMAL SYNTHESIS OF TIO2 NANOTUBE (TNT)/ RUO2

NANORIBBON (NR)/ GRAPHENE OXIDE COMPOSITES WITH ENHANCED

PHOTOCATALYTIC ACTIVITY ..............................................................................................104

6.1 Introduction ..........................................................................................................104

6.2 Experimental section ............................................................................................107

6.3 Materials and methods .........................................................................................107

6.4 Characterization. ..................................................................................................107

6.5 Synthesis of graphene Oxide (GO). .....................................................................108

6.6 Synthesis of TNT/RuO2 NR/hGO (TRG) composites. ........................................108

6.7 Photocatalytic measurements. ..............................................................................109

6.8 Results and discussion .........................................................................................110

xiii

6.9 Characterization of TNT/ RuO2 NR/ hGO composites ........................................110

6.10 Morphology of TNT/RuO2 NR/ hGO composite.................................................115

6.11 Photocatalytic performance of TNT/RuO2 NR/ hGO composite. .......................118

6.12 Conclusion ...........................................................................................................123

6.13 Supporting information ........................................................................................123

6.14 References ............................................................................................................126

BIOGRAPHICAL SKETCH …………………………………………………………….……. 132

CURRICULUM VITAE ………………………………………………………………………133

xiv

LIST OF FIGURES

Figure 2-1 SEM images of RuO2 nanoribbons at a) low and b) high magnification. A histogram

for the ribbon width (inset). ...............................................................................................10

Figure 2-2 a) XRD pattern of annealed RuO2 nanoribbons with the simulated pattern (JCPDS-00-

040-1290) b) Crystal structure of RuO2 viewed in (110) direction, calculated using crystal

maker and the crystallographic data in reference.87 ...........................................................12

Figure 2-3 a) Top view TEM images of RuO2 NRs a) and b) with different magnification and

inset of high resolution image showing (110) plane with the d-spacing of 0.318 nm .......12

Figure 2-4 TEM image of a) MCNT- RuO2 nanoribbon composite electrode (R-3), b) high

resolution image showing planes of RuO2 nanoribbons lattice fringes with the d spacing

of 0.318 nm (110) and MCNT. ..........................................................................................14

Figure 2-5 XPS spectrum of RuO2 NR with the peak assignments. ..............................................15

Figure 2-6 Cyclic voltammograms of composite paper having different RuO2 nanoribbon

compositions b) galvanostatic charging and discharging curves measured at constant

current density 1.0 A g-1 c) charge–discharge profile of R-3 supercapacitor at 1 A g−1 for

the 1st and 1000th cycle d) capacitance retention of R-3 over 1000 charge/discharge cycles

evaluated from the galvanostatic discharge curves. ...........................................................16

Figure 2-7 Three electrode cyclic voltammogram of RuO2 NRs with reference to Ag/Ag+ in 1M

EMIM TFSI in acetonitrile at 50 mV s-1 ............................................................................18

Figure 2-8 a), b) Electrochemical impedance spectroscopy (EIS) of different RuO2 NRs

composite electrodes. .........................................................................................................19

Figure 2-9 Ragone plot for hybrid composite paper electrodes MCNT, R-1, R-2 and R-3 paper

electrode. ............................................................................................................................22

Figure 2-S. 1 Structure of 1-ethyl-3-methyl imidazolium (EMIM+)…………………………… 25

Figure 2-S. 2 shows the nitrogen adsorption–desorption isotherms of the RuO2 NRs sample….26

Figure 2-S. 3 XPS spectrum of MCNTs a) before b) after wash with 30% HNO3 with the peak

assignments………………………………………………………………………..……..27

Figure 2-S. 4 digital photograph of MCNT- RuO2 nanoribbon composite paper electrode (VR-3)

………………………………………………………………………………………...27

Figure 2-S. 5 FT-IR spectra of (a) pure Triton X-100 and (b) VR-3 composite electrode………28

xv

Figure 3-1 a) SEM b) TEM image of as synthesized VNRs (inset) size distribution histogram ...42

Figure 3-2 Scheme for the preparation of VNRs decorated with RuO2 NDs ................................43

Figure 3-3 a),b) TEM images of RuO2 ND bound VNRs c) particle size distribution of RuO2

NDs on VNRs ....................................................................................................................44

Figure 3-4 XRD pattern of a) VNRs b) RuO2 NDs decorated VNRs. ...........................................44

Figure 3-5 XPS spectrum of RuO2 NDs bound VNRs with peak assignments for a) V2O5 b)

RuO2 ...................................................................................................................................46

Figure 3-6 Cyclic voltammograms b) discharge curves of composite electrodes at 25 mV s-1 c)

charge discharge curve of 1st and 1000th cycle and d) capacitance retention for VR-5 .....47

Figure 3-7 Three electrode cyclic voltammogram (CV) of RuO2 NDs decorated VNR (VR-5)

with reference to Ag/Ag+ in 0.1 M LiTFSI in EMITFSI ...................................................51

Figure 3-8 a) Electrochemical impedance spectroscopy (EIS) b) Ragone plot of different

composite electrodes ..........................................................................................................52

Figure 4-1 a), b) SEM c), d) TEM image of NaNbO3 nanorods e) Raman spectra for GO, hGO,

graphite and NaNbO3 Nt-hGO (Nb-1) ...............................................................................66

Figure 4-2 Crystal structure of NaNbO3 in a) (111) b) (100) direction, calculated using crystal

maker and the crystallographic data in ref 36 c) digital image of flexible paper electrode

(Nb-2) XRD pattern for d) NaNbO3 nanorods e) as synthesized hGO-NaNbO3 nanotube

composite (Nb-2) f) simulated XRD pattern JCPDS 33-1270...........................................67

Figure 4-3 SEM images at a) low and b) high magnification, TEM image of c) low and d) high

magnification of Nb-2 composite electrode .......................................................................68

Figure 4-4 Schematic diagram for the formation mechanism of NaNbO3 nanorods .....................71

Figure 4-5 Schematic diagram of formation mechanism of NaNbO3 nanotube with the presence

of hGO ...............................................................................................................................73

Figure 4-6 Cyclic voltammograms of a) NaNbO3 nanotube/hGO composites b) NaNbO3

nanorods/hGO composite c) all the composite electrodes at 25 mV s-1 scanned rate. ......75

Figure 4-7 a) Charge discharge profile for coin cell series at 1 A g-1 b) Cell voltage vs. discharge

time of sample Nb-2 in 1 M LiTFSI at different discharge current densities c) Charge–

discharge profile of Nb-2 supercapacitor in 1 M LiTFSI at 1.5 A g−1 for the 1st and 7000th

cycle d) capacitance retention of Nb-2 over 1000 charge/discharge cycles evaluated from

the galvanostatic discharge curves .....................................................................................76

xvi

Figure 4-8 Three electrode cyclic voltammogram of NaNbO3/hGO (Nb-2) composite electrode

and hGO with reference to Ag/Ag+ in 1M LITFSI in acetonitrile at 50 mV s-1 ................78

Figure 5-1 a), wrinkled mesoporous silica (WMS) b) SEM image of wrinkled mesoporous

carbon (WMC) ...................................................................................................................96

Figure 5-3 TEM images of a), b) 10 wt% c), d) 20 wt% e), f) 40 wt% and g), h) 80 wt%

WMCR ...............................................................................................................................97

Figure 6-1 Graphical illustration of the synthesis of TNT/RuO2 NR/hGO composites. .............106

Figure 6-2 XRD patterns of (a) TG-1, (b) RG-2, (c) TRG-3, (d) TRG-4 and (e) TRG-5. ..........110

Figure 6-3 Raman spectra of (a) RuO2 NR and (b) RuO2 /hGO (RG-2). Inset Raman spectra

showing the blue shift of the Eg band of the composite. ..................................................112

Figure 6-4 Raman spectra of TNT (a), TG-1 (b), TRG-3 (c), TRG-4 (d) and TRG-5 (e). Inset

Raman spectra showing the blue shift of the Eg band of the composites. .......................112

Figure 6-5 Characteristic D and G bands of a) GO b) RG-2 c) TRG-3 d) TRG-4 and e) TRG-5

composites........................................................................................................................113

Figure 6-6 Deconvoluted peak of high resolution XPS core level of a) Ru 3d/ C 1s b) Ti 2p of

TRG-5 composite. ............................................................................................................114

Figure 6-7 SEM image of a) RuO2 NRs on hGO b) TNTs on RuO2 NRs, high resolution TEM

image of c) RuO2 NRs and TNTs on hGO sheet (TRG-5) d) RuO2 NRs and TNTs in

composite TRG-5. ............................................................................................................117

Figure 6-8 Plot of C/Co (%) versus time for the photocatalytic degradation of malachite green in

a quartz reactor. ................................................................................................................120

Figure 6-9 Graphical illustration of photodegradation of MGO in the presence of RuO2/

TiO2/hGO composite. .......................................................................................................121

Figure 6-S. 1 TEM-EDAX spectrum accrued at 10 nm magnification. ..................................12124

Figure 6-S. 2 a), b) High resolution TEM images of bulk TNTs.. ...........................................12124

Figure 6-S. 3 UV-vis diffuse reflectance spectra of composites .............................................12125

Figure 6-S. 4 a) Direct bandgap model of TRG-5 and RuO2 NRs b) indirect bandgap model of

TNTs. ...........................................................................................................................12125

xvii

LIST OF TABLES

Table 2-1 Specific capacitance, energy and power densities for different compositions calculated

from galvanostatic charging/discharging curves. ..........................................................…21

Table 2-S. 1 Composition of composite electrodes……………………………………...………26

Table 2-S. 2 d-spacing comparison calculated by XRD and TEM image………………...……..26

Table 3-1 Specific capacitance and IR drop of composite electrodes. ..........................................48

Table 3-2 Energy densities of composite electrodes ......................................................................50

Table 4-1Summarized parameters for electrode preparation .........................................................74

Table 6-1 Rate constant comparison for the oxidation of organic dyes ......................................122

Table 6-S. 1 Amount of TiO2, RuO2 and hGO in composites .....................................................122

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Energy consumption continually increases as the world population increases, resulting in greater

demands for energy to support human existence. Therefore, research towards renewable energy

and energy storage devices have been gaining more attention.1 Among the energy storage devices,

supercapacitors may have potential applications in the near future.2 Therefore, research has been

done to improve the performance of high surface area carbons and different redox active metal

oxide nanostructures.

The chapter 2 of the dissertation reports a novel method to prepare RuO2 nanoribbons for

supercapacitor applications. RuO2 is one of the best pseudocapacitive materials reported.3 It is

important to obtain a high surface area, high capacitive material which can deliver high energy

and high power density. RuO2 nanoribbons were combined with carbon nanotubes to prepare

composite electrodes and evaluate its performance. The nanocomposite paper having highest RuO2

nanoribbon loading displayed ideal capacitive behavior with 1510 W kg-1 of power density and

160.8 Wh kg-1 energy density and 276.66 F g-1 specific capacitance.

There are many energy storage devices available ranging from fuel cells to batteries.4 Batteries are

a convenient method to store energy but with low power density. In the third chapter, a novel

strategy to prepare V2O5 nanorods modified with RuO2 nanodots is reported. RuO2 is expensive,

while V2O5 is a cheaper, layered pseudocapacitive metal oxide. In order to reduce the cost we have

prepared V2O5 nanorod decorated with RuO2 nanodots. The RuO2 provides extra capacitance with

2

high conductivity to enhance the performance of the supercapacitors. Composite electrodes were

prepared using different amounts of RuO2 nanodots and V2O5 nanorods with a constant amount of

carbon nanotubes. The composite electrode with 1:2 (wt%) shows the best performance where

capacitance of 158 F g-1, energy density of 157.08 Wh kg-1 and highest power density of 10.1 kW

kg-1 was observed.

In chapter 4, the preparation of sodium niobate nanorods and nanotubes is reported. It was found

that graphene oxide facilitates the formation of sodium niobate nanotubes. A possible mechanism

for the preparation of graphene oxide mediated sodium niobate nanotubes is proposed.

A high surface area wrinkled mesoporous carbon was prepared and RuO2 nanoparticles deposited

in the mesopores. The WMC/ RuO2 nanodots were combined with carbon nanotubes and the high

performance electrodes was evaluated in chapter 5.

Managing waste and waste treatment is a challenging task. In the second part of the dissertation,

the development of a new heterojunction photocatalytic material is discussed.5 Using high surface

area graphene oxide, ruthenium oxide nanoribbons and titanium oxide nanotubes a sequential

synthesis was developed and the photocatalytic activity of the composite was evaluated. The

highest loading of RuO2 NR/TNTs on hGO showed the highest photodegradation efficiency with

a 0.9625 min-1 rate constant. We have shown that a RuO2 NR/TNT/ hGO heterojunction helps to

improve the photodegradation efficiency of organic dyes by decreasing the electron hole

recombination.

1.2 References

1. H. Jiang, P. S. Lee and C. Li, Energy & Environmental Science, 2013, 6, 41-53.

3

2. E. Frackowiak and F. Béguin, Carbon, 2001, 39, 937-950.

3. B. J. Lee, S. R. Sivakkumar, J. M. Ko, J. H. Kim, S. M. Jo and D. Y. Kim, Journal of

Power Sources, 2007, 168, 546-552.

4. J. Liang, F. Li and H.-M. Cheng, Energy Storage Materials, 2016, 2, A1-A2.

5. M. T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, M. M. Müller, H.-J. Kleebe, K.

Rachut, J. Ziegler, A. Klein and W. Jaegermann, The Journal of Physical Chemistry C,

2013, 117, 22098-22110.

4

CHAPTER 2

RUTHENIUM OXIDE NANORIBBON – CARBON NANOTUBE COMPOSITE

ELECTRODES FOR HIGH PERFORMANCE SUPERCAPACITORS

2.1 Introduction

Supercapacitors have gained considerable attention in recent years due to their ultra-fast charge

and discharge rate, excellent stability, long cycle life, and very high power density.1-3 There are

two types of supercapacitors depending on their charge storage mechanism- electric double layer

capacitors (EDLC) and pseudocapacitors.4-13 In EDLC, charges are stored at the electrode surface

14-21 while pseudocapacitors generate a large number of charges due to Faradic reactions.18, 22-27

Hydrous RuO2 has been extensively studied as a pseudocapacitive electrode material because of

its high electrical conductivity, capacitance and energy densities in aqueous electrolytes.28-34 RuO2

is considered one of the best pseudocapacitive materials in terms of charge storage and fast,

reversible reaction kinetics compared with other metal oxides.35-43 Additionally, the availability

of several oxidation states for RuO2 helps store large quantities of charge in RuO2 electrodes.44-

48,89-91 Zheng et. al reported that RuO2 exhibits a specific capacitance as large as 720 F g−1 in

aqueous electrolytes.49-50 High aspect ratio nanostructures such as nanoribbons, nanorods,

nanotubes and nanowires can exhibit high surface areas which helps to increase performance.

However, there are relatively few examples of different RuO2 nanostructures reported including

thin films and nanoparticles.1, 51-54 Methods that have been used to prepare RuO2 nanostructures

include electrostatic spray deposition (ESD) 55, synthesis of ruthenium oxide aerogels 56, radio

frequency rf magnetron sputtering 57 and electrodeposition. 58-59 These methods are expensive and/

or time consuming with little or no control of morphology. Chang et. al reported the synthesis of

5

hydrous RuO2 tubular arrays by anodic deposition with tubes about 200 nm in diameter for

supercapacitor applications.60 Dubala and coworkers reported a chemical bath deposition method

to synthesize RuO2 nanograins of size less than 20 nm to form thin films for supercapacitors and

obtained 167 F g−1 in a polyvinyl alcohol gel electrolyte.61 Bhowmik et al. reported the growth of

one-dimensional RuO2 nanowires (~10 nm in width) on g-carbon nitride, an active and stable

bifunctional electrocatalyst for hydrogen and oxygen evolution.62 Wang et al. synthesized hydrous

ruthenium oxide nanoparticles (~ 5 nm ) anchored to graphene and carbon nanotube hybrid foams

and have achieved a maximum energy density of 13.09 Wh kg-1, a power density of 42.67 kW kg-

1 and a specific capacitance of 502.78 F g−1 in 2 M Li2SO4 aqueous electrolyte.1 Park et. al reported

graphene as a template for growing one dimensional RuO2 nanorods (~ 50 nm width) using a CVD

(chemical vapor deposition) technique.63 Barronco and coworkers have also grown RuO2

nanoparticles (3-15 nm) on amorphous carbon nanofibers for supercapacitor electrodes.64 Kim et

al. reported a template-free synthesis of ruthenium oxide nanotubes (diameter 5-7 nm) for

electrochemical capacitors using a microwave-hydrothermal process and achieved 511 F g-1 in

aqueous electrolyte. In this study, the synthesis of RuO2 nanoribbons is reported for the first time

using a surfactant assisted process.

Pseudocapacitance of a hybrid supercapacitor arises, when the application of a potential induces

faradaic current from reactions such as electrosorption or from the oxidation/ reduction of

electroactive materials.65 When using aqueous electrolyte, ruthenium dioxide exhibits

pseudocapacitance via a coupled proton–electron transfer according to the Eq. (1).66

𝑅𝑢𝑂2 + 𝑛𝐻+ + 𝑛𝑒− ⇌ 𝑅𝑢𝑂2−𝑛(𝑂𝐻) (1)

6

In the past, the presence of protons in equation 1 has restricted the study of ruthenium oxide to

aqueous electrolytes. Unfortunately, the use of aqueous electrolytes is limited to an operating

voltage of about 1.23 V above which water decomposes. In contrast, organic electrolytes can

provide a higher operating voltage up to 4 V.67-72 In the present study, the electrochemical behavior

of RuO2 NR/ Multiwall carbon nanotube composite electrodes in a non-aqueous ionic liquid

electrolyte is reported. 1-Ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide (EMIM

TFSI) has interesting properties and has been shown to facilitate redox chemistry in RuO2

pseudocapacitors.38, 73-77. Some imidazolium salts have recently attracted attention as ionic liquid

electrolytes operating at ambient temperature.78 Electrolytic salts with 1,3-substituted imidazolium

cation form ionic liquids which have low viscosity and good fluidity.79-80 Molecular structure of

1-ethyl-3-methyl imidazolium (EMIM+) has shown in Figure S.1-1. Surprisingly, a significant

amount of hydrogen bonding is predicted for nonprotic ionic liquids such as EMIM TFSI.81-82

Egashira et al. studied pseudocapacitive reactions based on imidazolium cation with RuO2.78 The

imidazolium cation has been proposed to form hydrogen bonds and adsorb on the ruthenium oxide

surface.38 Another study involving EMIM/ RuO2 shows that the EMIM cation can intercalate in

the RuO2.74 In this study, EMIM TFSI was used with RuO2 NRs and MNCTs to form coin cells,

where the RuO2 nanoribbons form binderless free standing papers with the MCNTs. Additionally,

the RuO2 provides conducting pathways in the composites.

2.2 Materials and methods

All reagents were used as received. Ruthenium chloride (RuCl3.xH2O) was purchased from

Pressure chemical co. SPAN-80 surfactant was purchased from Sigma Aldrich. High purity

7

multiwall carbon nanotubes (CNTs) (∼ 50 µm in length) were purchased from Sun Innovations

Inc. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) was obtained

from Sigma Aldrich. Typical coin cell packaging (CR-2032) was used to assemble the coin cell

type supercapacitors 67. A Teflon film (Gore Company) was used as the separator between the two

electrodes.

2.3 Synthesis of ruthenium oxide nanoribbons (RuO2 NR)

Hydrous ruthenium oxide nanoribbons were synthesized using a hydrothermal synthesis method

in the presence of a surfactant. First, 0.2 g of ruthenium chloride (RuCl3.xH2O) were dissolved in

10 mL of deionized water. Then, 1.5 mL of Span-80 surfactant was added and stirred at room

temperature until it dissolved. Then 10 mL of butanol were added to the mixture and stirred for

two hours at room temperature. The mixture was transferred to a Teflon lined autoclave with 0.6

g of sodium hydroxide and heated at 180 oC for 15 hours. The resulting product was isolated by

centrifugation and washed with deionized water and ethanol five times. The final product was

vacuum dried at room temperature overnight. The resulting black solid was annealed at 350 oC for

two hours in air.

2.4 Preparation of ruthenium oxide nanoribbon/ carbon nanotube composite electrodes

Prior to the use, MCNTs were washed with 30% (v/v) HNO3 to remove any impurities, followed

by filtration using Millipore (0.22 μm) hydrophilic polycarbonate membrane, with the aid of a

vacuum pump, and washed thoroughly with DI water. XPS spectra were taken before and after

8

washing with 30% (v/v) HNO3 and are reported in supplementary section (Figure 2-S.3). Lee et.

al reported the electrochemical properties of multi-walled carbon nanotubes treated with nitric acid

for a supercapacitor electrode. In this study capacitance increased seven times by treating MCNTs

with HNO3 acid for three hours. Oxidized defect sites provide ionic interaction sites for the

electrolyte and plays a key role in the formation of the electric double layer.83 The MCNTs were

dried at ambient temperature for 15 h using a vacuum oven. The carbon nanotubes (0.2 g) were

dispersed in DI water (200 mL) containing Triton-X 100 (2 g), then bath sonicated for 1 h and

probe sonicated for 15 min at (13 W) in order to disperse the MCNTs in the solution. The MCNT-

RuO2 NR composites were made by mixing w/w ratios of MCNT: RuO2 NR, 50:5 mg (R-1), 50:10

mg (R-2), 50:30 mg (R-3) and 50 mg of MCNTs as a control as shown in table 2-S.1. In order to

establish a well dispersed network of carbon nanotubes and RuO2 nanoribbons in a composite

paper, a high frequency resonant acoustic mixing technique was applied for 10 min prior to

filtration. Both high frequency (acoustic mixing) and low frequency (probe sonication) are very

important to break agglomerates and blend the two components. The composite dispersions were

suction filtered using a Nylon filter paper (Varian Chromatography System-Nylon 66, 0.45 μ m

pore size and 47 mm in diameter). Excess surfactant (Triton-X 100) was removed by washing with

DI water (250 mL) under suction filtration. Complete removal of Triton-X surfactant was

confirmed by FTIR (Figure 2-S.5). The composite papers were allowed to dry at room temperature

for 24 hours and then peeled off the filter paper as a flexible freestanding paper.

9

2.5 Supercapacitor assembly

The MCNT/RuO2 NR and the MCNT electrodes were immersed in the electrolyte, EMIM TFSI

for 1 h at room temperature. The composite paper anode and the MCNT cathode were separated

by a Teflon film. The coin cell packaging (CR2032) was used to assemble the supercapacitors as

previously described. Carbon coated aluminum sheets were used as the current collectors.

Additional EMIM TFSI electrolyte (~ 0.2 ml) was introduced to each electrode and sealed in the

coin cell using a coin cell crimper (Shenzhen Yongxingye precision machinery mold) by pressing

at 1100 psig.

2.6 Characterization

Transmission electron microscope (TEM) and scanning electron microscope (SEM) images were

acquired using a JEOL JEM-2100 TEM at 200 kV (JEOL Co. Ltd.) and a Leo 1530 VP field

emission electron microscope. X-ray powder diffraction (XRD) patterns were obtained using a

Rigaku Ultima IV diffractometer (CuKα radiation). Cyclic voltammograms (CV) and

galvanostatic charge/discharge curves were obtained using Arbin battery testing system (BT2000)

in the range of –2.0 to 2.0 V (voltage window of 4V). X-ray Photoelectron Spectroscopy (XPS)

measurements were performed using a Perkin Elmer PHI 5600 System. The photoelectrons were

excited using monochromatic Al Kα radiation (h ν = 1486.6 eV) and the spectra were acquired with

a 45o emission angle, using 0.125 eV step size and a pass energy of 29.35 eV in the hemispherical

analyzer. The porosity and pore volume were measured from the nitrogen adsorption isotherms at

10

-196.15 oC K (Quantachrome Instruments Autosorb-1). The specific surface area of the composites

was determined by the Brunauer–Emmett–Teller (BET) method

2.7 Results and discussion

2.8 Characterization of RuO2 nanoribbons

Figure 2-1 SEM images of RuO2 nanoribbons at a) low and b) high magnification. A histogram

for the ribbon width (inset).

11

Figure 2-1. shows the typical scanning electron microscope images of the as synthesized RuO2

nanoribbons at different magnifications. Figure 2-1.a shows a low magnification SEM image that

reveals the densely packed network of stacked nanoribbons. The width of the RuO2 NRs ~ 10-12

nm (insert 1.a). A higher magnification SEM image of the RuO2 in Figure 2-1.b clearly shows the

ribbon nature. The thickness of the RuO2 NRs is ~ 2 nm and up to 600 nm long. The nanoribbons

tend to aggregate in to bundles.

Figure 2-S.2 shows the nitrogen adsorption–desorption isotherms of the RuO2 NRs. The surface

area of the RuO2 NRs was 148 m2 g-1 which is higher than what has been reported for other RuO2

nanostructures. For example, Jeon et al reported RuO2 nanoparticles (size ~12.2 nm) with a surface

area of 78.2 m2 g-1 and Sivakami et. al reported RuO2 nanoparticles (~20 nm) with a surface area

of 118–133 m2 g-1.84-85 Hyoung et. al reported ruthenium and ruthenium oxide nanoparticles

supported carbon nanofibers having a surface area of 54.7 m2 g-1.86 The as synthesized RuO2

nanoribbons were also characterized using X-ray diffraction (XRD) as shown in Figure 2-2 where

the characteristic peaks for RuO2 (JCPDS-00-040-1290) are assigned to the (110), (101), (200),

(111), (211), (220) and (022).87

Figure 2-3. a) and b) shows the top views of TEM image of RuO2 NRs. The higher resolution TEM

image (inset) shows the RuO2 NRs (~ 9 nm) are oriented in the (110) direction (Figure 2-2.b) along

the ribbon axis consistent with the d-spacing of 0.318 nm (table 2-S.2). The RuO2 (110) orientation

is generally more stable and readily reducible orientation than other orientations.88

12

Figure 2-2 a) XRD pattern of annealed RuO2 nanoribbons with the simulated pattern (JCPDS-00-

040-1290) b) Crystal structure of RuO2 viewed in (110) direction, calculated using crystal maker

and the crystallographic data in reference.87

Figure 2-3 a) Top view TEM images of RuO2 NRs a) and b) with different magnification and

inset of high resolution image showing (110) plane with the d-spacing of 0.318 nm

13

2.9 Fabrication and characterization of hybrid RuO2 nanoribbon carbon nanotube

composite papers

Recently, multiwall carbon nanotubes (MCNTs) have attracted interest as electrode materials for

supercapacitors due to their unique structure and morphology, low mass density, outstanding

chemical stability, electronic conductivity and mechanical performance.89-91 Most importantly,

MCNTs can be used to prepare binder free flexible electrodes.67, 92-94 MCNT composites with

various metal oxides, such as RuO2, MnO2 and V2O5 have been used as electrode materials for

pseudocapacitors due to their large capacitance and fast redox kinetics.30, 54, 67, 95-97 The low

electrical conductivity of most metal oxides decreases the power density in asymmetric

capacitors.60 In an effort to increase electrical conductivity, metal oxide nanostructures have been

combined with conductive carbons such as graphene, MCNTs, activated carbon, etc.67, 98-100 RuO2

is a well-known conducting metal oxide and the electrical conductivity of the RuO2 NRs was

measured to be 1390 S m-1. The conductivity of the RuO2 NR/MCNT composites were measured

using a four probe measurement and found to be 1210 S m-1 for R-1, 1260 S m-1 for R-2, 1264 S

m-1 for R-3 and only 2.054 S m-1 for just the MCNTs. Therefore, the RuO2 provides

pseudocapacitance and increases the conductivity of the electrodes. The RuO2 NRs are only ~ 100-

600 nm long and do not form free standing papers themselves. In contrast, the MCNTs are ~ 50

µm long and readily form free standing papers. The MCNTs combined with the RuO2 NRs also

form free standing flexible composite paper electrodes. A digital image of the MCNT- RuO2 NR

(R-3) composite paper is shown in Figure 2-S.4. The binderless free standing composite papers

are flexible and can be cut in to electrodes with constant area in order to fabricate the coin cells.

14

The R-3 MCNT- RuO2 nanoribbon composite electrode (R-3) was further characterized using HR-

TEM as shown in Figure 2-4.a. The RuO2 nanoribbons and MCNTs are well dispersed and

connected, which is important for migration of electrons and ions through the electrode. The

thickness of the all the composite electrodes (R-1, R-2, R-3 and MCNT) was measured to be 0.05

± 0.01 mm.

Figure 2-4 TEM image of a) MCNT- RuO2 nanoribbon composite electrode (R-3), b) high

resolution image showing planes of RuO2 nanoribbons lattice fringes with the d spacing of 0.318

nm (110) and MCNT.

Figure 2-4.b shows a high resolution TEM image of a selected area, where the d-spacing was

measured to be 0.318 nm again consistent with the (110) plane orientation of RuO2. The d-spacing

was calculated using the XRD pattern and Bragg’s law in order to compare with the TEM images

and are reported in the supplementary section table 2-S.2. The calculated d-spacing match well

with the d-spacing measured from the TEM images.

15

XPS studies provide further insight into the local environment and oxidation state of the ruthenium

oxide nanoribbons. Figure 2-5 shows the deconvoluted XPS spectrum of RuO2 NRs, where the

two bands at 286.2 eV and 281.5 eV, readily assigned to Ru 3d3/2 and Ru 3d5/2.39 There are four

peaks appeared after deconvolute the two spin orbital-doublet. These high binding energy satellite

peaks can be identify as a second spin doublet due to the strong coulombic interaction between

the electrons in the d-orbital and the hole generated by the photoionization.101 Foelske and

coworkers have done extensive study on X-ray photoelectron studies of RuO2 at different

annealing temperatures and found out the second doublet evolve with the increasing annealing

temperature.102 Morgan et. al reported similar study and showed that the second doublet is an

indication of dehydration level of the crystalline structure and available RuO2 oxidation state is

only +4.103 Peak intensity increase or decrease with the hydration level and crystallinity of the

structure.

Figure 2-5 XPS spectrum of RuO2 NR with the peak assignments.

16

2.10 Electrochemical characterization of MCNT- RuO2 NR composite papers

Figure 2-6 Cyclic voltammograms of composite paper having different RuO2 nanoribbon

compositions b) galvanostatic charging and discharging curves measured at constant current

density 1.0 A g-1 c) charge–discharge profile of R-3 supercapacitor at 1 A g−1 for the 1st and

1000th cycle d) capacitance retention of R-3 over 1000 charge/discharge cycles evaluated from

the galvanostatic discharge curves.

Electric charge can be stored in the bulk ruthenium oxide in addition to the carbon

electrode/electrolyte interface. However, capacitance decreases rapidly as the scan rate increases

which can be described to electrolyte depletion and oversaturation during the charge discharge

process. In the case of pseudo-capacitors, reversible redox processes take place on the surface

when the valance electrons in the electroactive RuO2 are transferred across the electrode-

electrolyte interface depending on the applied potential window. The MCNTs contribute electric

double layer capacitance and also provide electrical pathways for electron transfer during charging

17

and discharging. The combined redox capacitance from the RuO2 nanoribbons and EDLC from

MCNTs in the composite paper is expected to give higher energy and power densities.30 The

electrochemical behavior of the composite was evaluated using two electrode CR2032 type coin

cell supercapacitors. The cyclic voltammograms (CVs) and galvanostatic charge/discharge curves

for the composite papers with different loadings of MCNTs are shown in Figure 2-6. All

electrochemical studies were performed in a voltage range of −2.0 to 2.0 V, which is more

beneficial in terms of the high energy densities compared to aqueous electrolytes.2

The cyclic voltammograms of different RuO2 NR/ MCNT electrodes in Figure 2-6.a show a quasi-

rectangular shape indicating ideal capacitive behavior over the selected potential range. It appears

the RuO2 NRs play an important role in electrochemical behavior of composite papers. The MCNT

electrode exhibits the lowest current output (Figure 2-6.a) as well as poor capacitive behavior. This

illustrates the necessity of optimizing the amount of RuO2 NRs in the composite to overcome the

low performance. The composite paper having higher amounts of RuO2 NRs (R-3) generated

higher current densities. It should be noted that increasing the amount of RuO2 NRs creates more

conducting paths, which increases the rate of electron flow from the electrode. Figure 2-6.b shows

the charge/discharge curves obtained at constant a current of 3 A g −1. Figure 2-6.c shows the

charge and discharge curves for the 1st and 1000th cycles which was performed in a 3.0 V potential

window. Over 1000 cycles, the charge and discharge time decreases about 15%. The retention of

the specific capacitance against the cycle number is shown in Figure 2-6.d. After the 500th cycle

~80% of the specific capacitance was retained while after 1000th cycles ~70% of the capacitance

was retained as shown in the Figure 2-6.d which shows good cycling ability. The capacitance

retention reflects in part the degradation of the coin cell since it was assembled in air. Compared

18

with the literature, Lee et. al reported a carbon nanofiber/hydrous RuO2 nanocomposite that

exhibits a 10% loss in capacitance over 300 cycles in 1M H2SO4 electrolyte.104 Kim et al also

reported a hydrous ruthenium oxide/ carbon nanocomposite which exhibits a ~10 % loss in

capacitance after 1000 cycles in 1M H2SO4 electrolyte.

With higher loadings of RuO2 NRs a redox couple can be observed in the CV (Figure 2-6.a) which

is due to the pseudocapacitive behavior of the RuO2 NRs. These redox couples cannot be identified

using a two electrode system (without reference). Therefore, a three electrode experiment was

conducted as shown in Figure 2-7.

Figure 2-7 Three electrode cyclic voltammogram of RuO2 NRs with reference to Ag/Ag+ in 1M

EMIM TFSI in acetonitrile at 50 mV s-1

The electrochemical behavior of the pure RuO2 NRs was analyzed by cyclic voltammetry using

1M EMIM TFSI in acetonitrile at 50 mV s-1 with a Pt counter electrode reference to Ag/Ag+ in the

potential range 2.0 to -1.0 V. Figure 2-7, shows pure RuO2 NR electrode cyclic voltammogram

that exhibits two redox couples situated at Eeq(I) = 0.12 V and Eeq(II) = 0.63 V in 1M EMIM TFSI

19

in acetonitrile. (Eeq = (Epa+ Epc)/2, where Epa is the potential value of the anodic peak and Epc the

potential value of the corresponding cathodic peak). Erwin et al. reported the redox couple related

to Ru+2/Ru+3 is at approximately 0.12 V which can be attributed to peak (I) and Ru+3/Ru+4 is

approximately at 0.6 V which can be assigned to peak (II).105 Therefore, the RuO2 NRs can

undergo oxidation and reduction in this system to achieve pseudocapacitance using EMIM-TFSI.

Figure 2-8 a), b) Electrochemical impedance spectroscopy (EIS) of different RuO2 NRs

composite electrodes.

Electrochemical impedance spectroscopy (EIS) measurements of the MCNT and the RuO2 NR

nanocomposite electrodes are shown in Figure 2-8.a and 2-8.b. The EIS plots consist of (1) a high-

frequency intercept on the real Z' axis, (2) a semicircle in the high-to-medium-frequency region,

and (3) a straight line at the very low-frequency region.106 The diameter of the semicircle decreases

with increasing RuO2 NR, contact showing the charge transfer resistance is decreasing with an

increasing amount of RuO2 NRs in the composite.107 The pure MCNT electrode has the highest

radius semicircle which shows the highest resistance. Increasing the RuO2 amount in R-1, R-2 and

20

R-3 decreases the diameter of the semicircle, corresponding to a decrease in internal resistance.

The R-3 composite does not have a visible semicircle in Figure 2-8.b showing the lowest internal

resistance. This is consistent with the electrical conductivity of RuO2 NRs and MCNT electrodes

(1390 S m-1 and 2.05 S m-1 respectively).

The energy and power densities for the composite papers were calculated according to the equation

E = (I ×Δ t×ΔV)/(2×m) and P = E/Δt, where I is the constant discharge current, Δt is the discharge

time, ΔV is the voltage difference after the voltage drop (due to internal device resistant) and m

is the total mass of both electrodes (carbon fiber electrode and the composite paper electrode).

Table 2-1 shows capacitance, power and energy densities at different loadings of RuO2 NRs. The

specific capacitance (Csp) was calculated according to the equation Csp = ( I×Δt )/( m×Δ V ) where

I is the discharge current and t is the time it takes to discharge to 0 V from the initial voltage (ΔV),

taking into account the IR drop at the beginning of discharge. The specific capacitance is found to

be around 277 F g−1 for the R-3 device (fabricated coin cell supercapacitor). It should be noted that

the Csp calculated is for a two electrode system based on the total weight of the positive and the

negative electrode. Only MCNTs were used in a control experiment and the calculated specific

capacitance was 20.5 F g-1, energy density was 13 Wh Kg−1 and power density was 1.5 kW Kg −1

at 1 Ag-1. R-1, R-2 and R-3 composite electrodes have increasing loading of RuO2 nanoribbons

and constant amount of MCNTs. R-1 has the minimum amount of RuO2 nanoribbons (10 wt%)

and a 43.75 F g-1 which is a 150% increase in specific capacitance compared to MCNTs. R-2 has

20 % (w/w) RuO2 nanoribbons and it shows specific capacitance around 65.62 F g-1 which is a

275% increase. R-3 has 60 % (w/w) RuO2 nanoribbons and it shows 276.66 F g-1 which is over a

1400 % increase in specific capacitance. R-1 shows the energy density 27 Wh Kg −1, a 107.6 %

21

increase over the pure MCNTs. R-2- shows 64 Wh Kg −1 which is 392.3 % increase in energy

density. R-3 shows 161 Wh Kg −1 which is over 1138.4 % increase in energy density.

Table 2-1Specific capacitance, energy and power densities for different compositions calculated

from galvanostatic charging/discharging curves.

Sample Specific

capacitance (Fg-1)

Energy density (Wh kg-1) Power density (W/kg)

1 A g-1 10 A g-1 1 A g-1 10 A g-1

MCNT 20.5 ± 2.0 12.9 ± 0.5 8.3 ± 0.5 1499 ± 10 15000 ± 25

R-1 43.7 ± 1.5 27.0 ± 0.5 12.5 ± 0.5 1500 ± 10 15100 ± 50

R-2 65.6 ± 1.0 63.7 ± 1.0 20.8 ± 1.0 1504 ± 8 15000 ± 25

R-3 276.6 ± 2.0 160.8 ± 1.0 110.4 ± 1.0 1510 ± 10 15100 ± 25

According to Table 2-1, the electrochemical contribution of RuO2 NRs can be seen by comparing

the capacitance, power and energy density of samples R-1, R-2 and R-3 which are in the order of

increasing amount of RuO2 NRs. The composite having the highest RuO2 NRs content (R-3)

exhibits a higher energy density. The higher energy density largely reflects an increase in

pseudocapacitance from the RuO2 NRs compared with the electric double layer capacitance where,

R-3 gives ~ 80 Wh kg-1 and MCNT gives ~13 Wh kg-1. Kim et. al reported hydrous ruthenium

oxide/ carbon nanotube base supercapacitors with capacitance 863 Fg-1 in 1M H2SO4 aqueous

electrolyte and reported a power density 4000 W kg-1 and 17.6 Wh kg-1 energy density.108 The

aqueous electrolyte gives a higher capacitance compared with ionic liquid because of its high

22

conductivity and ion diffusion. Shen et al synthesized carbon encapsulated RuO2 nanodots

anchored on graphene for asymmetric supercapacitors with 75 F g-1 and 84 Wh kg-1 at 1 A g-1 in

EMIM-BF4 ionic electrolyte.74

Figure 2-9 Ragone plot for hybrid composite paper electrodes MCNT, R-1, R-2 and R-3 paper

electrode.

Figure 2- 9 shows a Ragone plot derived from the Galvanostatic discharge curves measured at

different charge–discharge current densities (1 A g-1 – 10 A g-1) indicating that R-3 composite

delivers higher energy and power densities compared to that of MCNT electrode. When increasing

current densities, energy density drops. MCNT shows the minimum drop and R-3 shows the

highest. When increasing the current density, the coin cells charge and discharge very quickly.

When the charge and discharge process is fast there is less time to undergo redox reaction.

Therefore, when increasing current density redox process are limited. This will result to lower the

energy density with higher current densities. R-3 composite has higher RuO2 NRs loading of than

23

MCNT. Therefore, energy density drops faster in R-3 than the MCNT electrode. The significant

improvement in the performance of R-3 (RuO2 NR/MCNT composite) can be attributed to the

increasing conductivity of the electrode due to the RuO2 and the pseudocapacitive behavior.

Increasing the amount of RuO2 NRs in the composite leads to brittle electrodes. Therefore, there

is a limitation with regards to the amount of pseudocapacitive material because of mechanical

properties. In EDLCs the charges are stored at the surface of the electrode such that charge can be

accessed readily in a short time to deliver higher power densities. The pseudocapacitance from

RuO2 NRs is due to the generation of large number of charges from the redox reactions, which

results in higher energy densities.

RuO2 is one of the best pseudocapacitive materials, but supercapacitor applications have been

generally limited to aqueous electrolytes (1.2 V potential window). In this study ionic electrolytes

were used which are having a higher voltage window (4 V potential window). Additionally, the

RuO2 nanoribbons have increased the supercapacitor performance with pseudocapacitance.

2.11 Conclusion

The ribbon morphology enabled the preparation of freestanding flexible RuO2 NR-MCNT

nanocomposite paper without using organic binders. These composite electrodes were used as the

cathode and MCNTs used as an anode in a coin cell type supercapacitor. The nanocomposite paper

having highest RuO2 nanoribbon loading (R-3) displayed ideal capacitive behavior with 1510 W

kg-1 of power density and 160.8 Wh kg-1 energy density and 276.66 F g-1 specific capacitance based

on the total weight of the electrodes. Ability to use these hybrid nanocomposite papers in wide

range of applications was demonstrated with constant energy densities. Our results have proven

24

the ability to use ionic liquid electrolyte with RuO2 to obtain the pseudocapacitance. Novel

preparation method of RuO2 nanoribbons can also be applied to prepare other metal oxide

nanoribbons. These binder-free metal oxide nanoribbon composites are promising candidates for

application in high performance supercapacitors.

2.12 Supporting information

Figure 2-S. 1 Structure of 1-ethyl-3-methyl imidazolium (EMIM+)

Figure 2-S. 2 shows the nitrogen adsorption–desorption isotherms of the RuO2 NRs sample.

25

Table 2-S. 1 Composition of composite electrodes.

Sample MCNT (mg) RuO2 NR (mg)

CNT 50 -

R-1 50 5

R-2 50 10

R-3 50 30

Table 2-S. 2 d-spacing comparison calculated by XRD and TEM image

Plane d-spacing calculated by XRD (nm) d-spacing calculated by TEM (nm)

(110) 0.301 0.318

Figure 2-S. 3 XPS spectrum of MCNTs a) before b) after wash with 30% HNO3 with the peak

assignments

Figure 2-S. 3 shows the XPS spectra of MCNTs used to make composite electrodes. Prior to use,

MCNTs were washed with 30 % HNO3 to remove any metal catalysts and debris. During this

process conductivity was increased from 100 Sm-1 to 205 Sm-1 and also sp2 character was

increased. Casa XPS software were used to estimate the area under the curve. Peak at 284.0 eV

26

(C-C sp2) have increased 61 %, peak at 284.6 (C=O) have increased 18% and peak at 288.5 eV

(O-C=O) have increased 2%.

Figure 2-S. 4 digital photograph of MCNT- RuO2 nanoribbon composite paper electrode (VR-3).

Figure 2-S. 5 FT-IR spectra of (a) pure Triton X-100 and (b) VR-3 composite electrode

FTIR spectra of pure Triton X-100 and VR-3 composite was collected In order to confirm the

complete removal of Triton X-100 surfactant. Figure 2-S. 5 shows the FTIR spectra of pure Tritin

X-100 and VR-3 composite electrode. The bands at 2951 and 2871 cm−1 are owing to the

asymmetric and symmetric CH2 stretch corresponding to the Triton X-100 surfactant which is not

visible in VR-3 composite.

27

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37

CHAPTER 3

RUTHENIUM OXIDE NANODOT DECORATED VANADIUM OXIDE NANOROD–

CARBON NANOTUBE COMPOSITES FOR SUPERCAPACITORS

3.1 Introduction

Electrolytic capacitors produce high power densities but low energy densities.1 The ideal energy

storage device, should be able to achieve high energy density, high power density and cycling

ability. This device most likely will be a supercapacitor. The ability to deliver high power density

in short time intervals would be an added advantage for the application of portable electric power

sources.2 Supercapacitors are attractive for highly time-dependent power demands3 when charge

and discharge can occur in a short time of period. Supercapacitors can be classified into two

categories depending on their charge-storage mechanism. Electric double-layer capacitors

(EDLCs) store energy based on the adsorption and desorption of both anions and cations. In

contrast, pseudocapacitors, store energy through fast surface redox reactions.4-5 In EDLCs charges

are stored at the electrode surface and they are readily accessible and help to obtain high power

densities.6 Carbon-based materials such as activated carbons and carbon nanotubes (CNTs) are

being used as electrode materials due to their fast charge-discharge rates and longer cycling lives.1,

6-7 Composite electrodes that integrate CNTs are growing in interest.8-9 Carbon-based electrode

nanomaterials often display good cycling stability but their specific capacitance is not high.

Therefore, developing composite electrodes that combine both EDLC and pseudocapacitor type

materials has become more popular where redox active materials show higher specific capacitance

than conventional EDLCs.10-11 Different types of transition metal oxides such as RuO2, Fe3O4,

CuO, MnO2, CuO, WO3, V2O5 have been shown to be promising materials for

38

pseudocapacitance.8, 12-17 RuO2 has been studied widely as a pseudocapacitive electrode material

because of its high electrical conductivity, high capacitance and high energy densities.18-19 RuO2

is one of the best pseudocapacitive materials known and displays good electrical conductivity,20

however, it is expensive.13, 21 In contrast V2O5 costs ~2-3 times less than RuO2 and is a good

pseudocapacitive material.9 Unfortunately, V2O5 is not very conductive. Therefore, the addition of

a small amount of RuO2 could increase the conductivity of a V2O5 composite and improve

pseudocapacitance.

Pusawale et. al have reported RuO2- SnO2 film prepared by chemical bath deposition method and

obtained capacitance 150 F g-1 in aqueous electrolyte.22 Shen et. al synthesized carbon

encapsulated RuO2 nanodots (~ 1- 4 nm) anchored on to the graphene and prepared an asymmetric

supercapacitor in ionic liquid electrolyte (75 F g-1 capacitance23, 108 Wh Kg-1 energy density and

103 W h kg-1 power density).23 Leng et. al reported that pseudocapacitive behavior of RuO2

nanoparticles (~3-8 nm) dispersed on graphene sheets can deliver 542.5 F g-1 capacitance in

aqueous electrolyte.24 Wang and coworkers reported RuO2 nanoparticles anchored on a graphene

/ carbon nanotube hybrid foam exhibited a capacitance of 502.78 F g-1 in aqueous electrolyte.8

While these results are promising, replacing more of the RuO2 with V2O5 would result in an

economic saving.

In this study, RuO2 NDs were chemically attached with VNRs and used to make composite

electrodes with carbon nanotubes. Since VNRs are only partially covered by the RuO2 NDs, the

electrolyte can access and penetrate the VNRs. RuO2 NDs can also act as a pseudocapacitive

material while enhancing the conductivity of the composite.25-28 In this study 0.1 M LiTFSI in

EMIM TFSI ionic liquid electrolyte was used to provide a higher operating voltage window (4 V)

39

than aqueous electrolytes (1.23 V). To our knowledge this is the first example of V2O5 surface

modified with RuO2 used for supercapacitors.


All reagents were used without further purification. Ammonium metavanadate (NH4VO3) (99%)

was purchased from Sigma-Aldrich. Pluronic P123 surfactant was obtained from BASF

Corporation. Lithium bis(trifluoromethanesulfonamide) (LiTFSI) was obtained from TCI

America. A Teflon film (Gore Company) was used as the separator for supercapacitor assembly.

Ruthenium chloride (RuCl3.xH2O) was purchased from Pressure chemical CO. Thiolactic acid

(95%) was purchased from Sigma-Aldrich. High purity multiwall carbon nanotubes (CNTs) (∼ 50

µm in length) were obtain from Sun Innovations Inc. 1-Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide (EMIM TFSI) was obtained from Sigma Aldrich. All solvents

were used without further purification. Two electrode coin cell package (CR2032) was used to

assemble all coin cell type supercapacitors.15

3.3 Synthesis of vanadium oxide nanorods (VNRs)

VNRs were synthesized according to a previously reported procedure.15 Briefly, ammonium

metavanadate (0.3 g) and P123 (EO20PO70EO20) (0.5 g) were dissolved in 30 mL of deionized (DI)

water in the presence of 2M HCl (1.5 mL, ~ pH 1). The mixture was stirred at room temperature

for 7 h. Then the resulting solution was transferred to a 45 mL Teflon-lined autoclave and heated

at 120 oC for 24 h under static conditions. The product was then dispersed in 50 mL DI water and

40

probe sonicated (10 W) for 5 min to obtain a homogeneous dispersion. The resulting product was

isolated by centrifugation (4000 rpm) and washed with deionized water and ethanol five times.

The final green product was vacuum dried at room temperature overnight.

3.4 Synthesis of ruthenium oxide nanodot (RuO2 ND) decorated vanadium oxide nanorods

(VNRs)

VNRs (0.3 g) were re-dispersed in 30 mL of DI water and 25 µL of Thiolactic acid (TLA) (95%)

were added to the solution and then stirred for 24 h. RuCl3.xH2O (0.1 g) was then added to the

solution and stirred for another 24 h (~ pH 5). Solid was isolated by centrifuging and washed with

DI water several times to remove any excess RuCl3 and dried at room temperature. Then, the VNRs

were treated with 20 mL of a 1M NaOH solution for 24 h at room temperature. The dark green

solid was isolated by filtration and washed several times with DI water to remove any unbound

RuO2. The resulting dark green solid was then annealed at 250 oC for 3 h in air.

3.5 Preparation of ruthenium oxide nanodot (RuO2 ND) chemically bound vanadium oxide

nanorods/ carbon nanotube composite electrodes

To prepare the electrodes, different concentrations of RuO2 NDs modified on VNRs were mixed

with a constant amount of CNTs as follows. Samples VR-1, VR-2, VR-3, VR-4, VR-5 and VR-6

were prepared by mixing 0.025 g, 0.05 g, 0.075 g, 0.1 g, 0.2 g and 0.3 g of RuO2 NDs bound VNRs

with 0.1 g of CNTs and 4% PTFE as a binding agent to bind everything together. As a control 0.1

41

g of CNT was used to prepare CNT electrode. Additionally, 0.1 g of CNTs and 0.1 g of VNRs

were mixed to prepare a VNRs /CNT composite electrode.

3.6 Supercapacitor assembly

The VR composite electrodes and the CNT electrodes were immersed in the electrolyte 0.1 M

LiTFSI in EMIM TFSI (~ 1 ml) for 1 h at room temperature. The composite paper anode and the

CNT cathode were separated by a Teflon film. The coin cell packaging (CR2032) was used to

assemble the supercapacitors as previously described.29 Carbon coated aluminum sheets were used

as the current collectors. Additional electrolyte (~ 0.2 mL) was introduced to each electrode and

sealed in coin cell using a crimper (Shenzhen Yongxingye precision machinery mold) by pressing

at 1500 psig.


High resolution transmission electron microscope (HR-TEM) and scanning electron microscope

(SEM) images were acquired using a JEOL JEM-2100 TEM at 200 kV (JEOL Co. Ltd.) and a Leo

1530 VP field emission electron microscope. X-ray powder diffraction (XRD) patterns were

obtained using a Rigaku Ultima IV diffractometer (Cu Kα radiation). Cyclic voltammograms (CV)

and galvanostatic charge/discharge curves were obtained using Arbin battery testing system

(BT2000) in the range of – 2.0 to 2.0 V (voltage window of 4V). X-ray Photoelectron Spectroscopy

(XPS) measurements were obtained using a Perkin Elmer PHI 5600 System.

42


3.9 Characterization of ruthenium oxide nanodots (RuO2 NDs) decorated vanadium oxide

nanorods (VNRs)

Figure 3-1 a) SEM b) TEM image of as synthesized VNRs (inset) size distribution histogram

Vanadium nanorods (VNRs) were synthesized using a hydrothermal synthesis method.9 The

morphology of VNRs was characterized using scanning electron microscopy (SEM) as shown in

Figure 3-1.a. The Figure 3-1.a inset shows the size distribution histogram for the VNRs where the

average diameter and length are about 1-3 μm long and ~ 60 nm in diameter. Figure 3-1.b shows

the top view of transmission electron microscope of VNR. Calculated d-spacing of 0.96 nm

corresponds to the plane (001).9

43

Figure 3-2 Scheme for the preparation of VNRs decorated with RuO2 NDs

The VNRs were modified with RuO2 NDs following the procedure developed for growing PbS

quantum dots on TiO2 nanotubes as shown in Figure 3-2.30 First, the surface of the VNRs were

functionalized with thiolactic acid where the carboxylate group of the thiolactic acid binds to the

surface of the VNRs. Then the thiol group binds to the Ru+3 ions. Growth of RuO2 NDs occurs

after reaction with NaOH (Figure 3-2). This experimental design results in RuO2 NDs attached to

the surface of the VNRs and prevents the formation of bulk RuO2 nanoparticles since there is no

free Ru+3 ions in solution. Additionally, by changing the TLA concentration, the amount of Ru+3

is limited and in theory the size of the RuO2 NDs can be controlled. Transmission electron

microscopy (TEM) images and the corresponding size distribution of the RuO2 NDs on the VNRs

are shown in Figure 3-3. The size of the RuO2 NDs are 1-4 nm in size, where ~ 50 % of the RuO2

NDs were 2 nm in size. The RuO2 NDs are well dispersed on the VNRs as shown by TEM (Figure

3-3b). Leng et. al reported the growth of RuO2 nanoparticle (~ 3 – 8 nm) on graphene sheets.24 A

study by Shen and coworkers also obtained ~ 3 nm RuO2 nanoparticles on graphene. Wang et. al

reported ~ 5 nm hydrous RuO2 nanoparticles were anchored on graphene/ carbon nanotube

44

composites.8 Uddin et. al prepared RuO2 nanoparticle (~ 2-5 nm) deposited TiO2 nanoparticles (~

15- 18 nm) for photocatalysis.

Figure 3-3 a),b) TEM images of RuO2 ND bound VNRs c) particle size distribution of RuO2

NDs on VNRs

Figure 3-4 XRD pattern of a) VNRs b) RuO2 NDs decorated VNRs.

Figure 3-4 shows the XRD pattern of the VNRs (Figure 3-4.a) and RuO2 ND modified VNRs

(Figure 3-4.b) after annealing process. The characteristic peaks for dehydrated V2O5 assigned to

(200), (001), (101), (110), (400), (011), (111), (310), (002), (102), (411), (600), (021), (020), (610),

45

(601), (021), (020), (601), (021), (320) and (710) planes are labeled in Figure 4. For RuO2 the

reflections are assigned to the (110), (101), (111) and (211) planes as shown in Figure 3-4.b.31-34

The X-ray photoelectron spectra (XPS) spectra of RuO2 NDs combined with VNRs are shown in

Figure 3-5.a. The peak at 517.4 eV corresponds to the binding energy of the V2p3/2 electrons for

vanadium in the + 5 oxidation state. The peak at 515.9 eV is due to the partial reduction of the + 5

ions to + 4 during the hydrothermal synthesis of the VNRs.9, 33 Figure 3-5.a shows the

deconvoluted XPS spectrum of the RuO2 NDs, where the two bands at 285.2 eV and 281.2 eV, are

readily assigned to Ru 3d3/2 and Ru 3d5/2. Besides the two major spin orbit-doublet, there is a

second doublet that appears after peak deconvolution. The second doublet at B.E = 282.1 eV and

286.2 eV occurs due to the RuO2 crystalline structure. Foelske et. al reported on X-ray

photoelectron spectroscopy study of RuO2 at different annealing temperatures and showed that

with increasing temperature (above 200 oC) a second spin-orbit doublet evolves.35 The peak

intensity of the second doublet increases with the annealing temperature. The second doublet was

generated due to the final state screen effects coursed by the strong coulombic interaction between

d-orbital electrons and photoionized holes. Similar studies by Morgan and coworkers showed that

the second doublet is on indication of dehydration and crystallinity of the structure.36 Kim et. al

reported core level X-ray photoelectron spectra of RuO2 and shown that the second doublet is

strictly due to the final-state screening effects of the Ru+4 oxidation state.37

46

Also they have shown that the only possible oxidation state present in RuO2 is Ru+4.

Figure 3-5 XPS spectrum of RuO2 NDs bound VNRs with peak assignments for a) V2O5 b) RuO2

47

3.10 Electrochemical characterization of RuO2 NDs /VNRs composite papers

Figure 3-6 Cyclic voltammograms b) discharge curves of composite electrodes at 25 mV s-1 c)

charge discharge curve of 1st and 1000th cycle and d) capacitance retention for VR-5

The electrochemical behavior of the composite electrodes was investigated using an asymmetric

coin cell set up using composite electrode as an anode and the CNT as the cathode. Various

electrochemical measurements were performed to contrast the electrochemical performance of the

different composite electrodes and carbon nanotubes (CNTs). Cyclic voltammetry (CV) curves of

the composites were collected at a scan rate of 25 mV s-1, in the potential range of -2.0 to 2.0 V

(Figure 3-6.a). In this study EMIM TFSI (4 V) was used as the electrolyte since organic electrolyte

have larger potential windows than aqueous electrolyte (1.23 V).38 EMIM TFSI have been use

with RuO2 in order to achieve higher capacitance, energy densities and power densities.23, 39 The

48

CV curves from the composites were compared to the CV curves of CNTs and VNRs/CNTs. The

cyclic voltammograms of the composites in Figure 3-6.a show a quasi-rectangular shape that

indicate ideal capacitive behavior. When RuO2 NDs were bound on to the VNRs, the capacitance

increased ~ 12 times. With increasing amounts of RuO2 NDs on the VNRs, the capacitance rapidly

increased (VR-5, 158.3 F g-1). Discharge curves for the CNTs and VNRs /CNT electrode at 1 Ag-

1 are shown in Figure 3-6.b. The coin cells were charged up to 3.5 V and time calculated to reach

0 V. The initial IR drop in the discharge curve is related to the device resistance. The IR drop

varies with the conductivity of the electrodes. With increasing amounts of the RuO2 ND on VNRs

the IR drop decreases. Table 3-1 shows the IR drop for each electrode related to each discharge

curve. The highest IR drop was observed for the CNT and VNR/ CNT electrodes. Upon

incorporation of RuO2 NDs on to the VNRs, the IR drop decreases significantly as a result of

increased conductivity. Furthermore, the conductivity of the VNRs and RuO2 ND bound VNRs

was also measured using a four probe conductivity meter. V2O5 is a poor electrical conductor 40

and the conductivity of VNRs found to be 0.2 S m-1 which is similar to previous reports for V2O5.41

When the RuO2 NDs were added to the VNRs, the conductivity increased to 40 S m-1.

Table 3-1 Specific capacitance and IR drop of composite electrodes.

Sample name Specific capacitance (F g-1) IR drop (Ω)

CNT 17.5 0.41

VNR /CNT 20.1 0.46

VR-1 24.2 0.21

VR-2 40.1 0.15

49

VR-3 61.3 0.13

VR-4 81.2 0.10

VR-5 158.3 0.10

VR-6 110.1 0.31

The energy and power densities for the composite electrodes were calculated using the following

equation. E = (I ×Δ t×ΔV)/(2×m) and P = E/Δt, where I is the constant discharge current, Δt is

the discharge time, ΔV is the voltage difference after the voltage drop and m is the total mass of

the electrodes. Table 3-1 shows capacitance at different amounts of the VNRs with RuO2 nanodots.

The specific capacitance (Csp) was calculated according to the equation Csp = ( I×Δt )/( m×Δ V ).

Where I is the discharge current and t is the time it takes to discharge to 0 V from the initial voltage

(ΔV). After taking into account the IR drop at the beginning of discharge curve, the highest specific

capacitance (Csp) calculated using a current density of 1 Ag-1, was 158 F g-1 for the VR-5

composite. The ratio of CNT to VNRs for the VR-5 composite is 1:2 by wt%. Perera et. al reported

vanadium oxide nanorods carbon nanotube composite with specific capacitance 48.5 F g-1 for a

ratio of 1:1 wt%.9 Saravanakumar et. al synthesized a V2O5 functionalized CNT hybrid

nanocomposite for supercapacitors with 64 F g-1 capacitance.42 Bonso et. al reported a VNR

composite with Exfoliated graphite nanoplatelets for supercapacitors with 70 F g-1 capacitance.43

The CNT and VNR/ CNT composite electrodes used as control and showed in the present study

specific capacitance of 17 F g-1 and 20 F g-1 respectively. When the VNRs were combined with

RuO2 NDs, the specific capacitance increased to 24 F g-1 (VR-1). Which is higher than both the

50

VNRs / CNT and CNTs. When the amount of RuO2 NDs/ VNRs, the specific capacitance increased

to 158 F g-1, which is nine times higher than that of the pure CNT electrode. The Energy density

of the CNT electrode and the VR-5 composite were 13 Wh kg-1 and 157.08 Wh kg-1 respectively

(table 3-2). Perera et. al reported a 46. 3 Wh kg-1 energy density for a VNR/ CNT nanocomposite.9

Bonso and coworkers prepared exfoliated graphite nanoplatelets – V2O5 nanotube composites with

an energy density 28 Wh kg-1 and the maximum power density was 10.1 kW kg-1 at 10 A g-1 current

density (> 6.36 kW kg-1).15

Table 3-2 Energy densities of composite electrodes

Sample name Energy density (Wh kg-1)

CNT 12.92

VNR/CNT 30.45

VR-1 78.37

VR-2 80.20

VR-3 90.42

VR-4 120.21

VR-5 157.08

VR-6 132.91

51

In order to evaluate the cycling stability sample VR-5 was charged up to 3.5 V and discharged to

0 V continuously up to 1000 cycles. Figure 3-6.c shows the charge discharge curves for 1st and

1000th cycle for VR-5. Capacitance retention was calculated using charge discharge curves. Figure

3-6.d shows the capacitance retention vs cycle number. After 500 cycles 95 % of the capacitance

was retained and 80 % retained after 1000 cycles for the VR-5 composite electrode (Figure 3-6.d).

Since the coin cell was assembled in air, some loss in capacitance was expected.

Figure 3-7 Three electrode cyclic voltammogram (CV) of RuO2 NDs decorated VNR (VR-5)

with reference to Ag/Ag+ in 0.1 M LiTFSI in EMITFSI

Figure 3-6.a shows quasi reversible redox peaks that arise from the metal oxides. Therefore, a three

electrode CV was collected in order to assign the redox peaks appear in the CVs (Figure 3-7). The

composite electrode (VR-5) was used as a working electrode, Pt wire as a counter electrode and

Ag/Ag+ as a reference electrode. The CV was obtained in 0.1 M LiTFSI in EMIM TFSI at 10 mV

s-1 in the potential range of - 2.0 V to 2.0 V. The CV plot of the composite shows two distinct

redox peaks at Eeq = – 0.57 V which can be attribute the Li ion intercalation and deintercalation

in to V2O5.43 Peaks appear at Eeq = 0.63 V can be attributed to the Ru+3/Ru+4 couple.43-44

52

Figure 3-8 a) Electrochemical impedance spectroscopy (EIS) b) Ragone plot of different

composite electrodes

Electrochemical impedance spectroscopy (EIS) measurements of the composite electrodes are

shown in Figure 3-8.a (VR-1, 6). The EIS plots consists of (1) a high-frequency intercept on the

real Z' axis and (2) a semicircle in the high-to-medium-frequency region.45 The diameter of the

semicircle decreases with the increasing amount of RuO2 NDs decorated VNRs, showing the

charge transfer resistance is decreasing as the amount of RuO2 NDs in the composite increases.14

The VR-1 has the highest radius semicircle which shows the highest resistance (~ 5 Ω). Increasing

the RuO2 NDs with VNRs amount in VR-1 (1:0.25), VR-2 (1:0.5), VR-3 (1:0.75), VR-4 (1:1) and

VR-5 (1:2) decreases the diameter of the semicircle, corresponding to a decrease in internal

resistance.

Figure 3-8.b shows a Ragone plot of energy density versus power density for the different

composite electrodes. VR-5 delivered higher energy and power density compared to the other

composite electrodes. In Figure 3-8.b the energy density decreased with an increase in current

53

density. This may be due to the low penetration of the ions into the RuO2 NDs and V2O5 nanorods

layers due to fast potential changes. According to the Ragone plot, the composite electrodes with

VNRs modified with RuO2 NDs show improved performance over the other composite electrodes.

3.11 Conclusion

In this study we have successfully synthesized RuO2 nanodots bound to V2O5 nanorods by using

thiolactic acid as a linker to bind Ru+3 ions followed by a reaction with NaOH. The size of the

RuO2 nanodots are on average 2-3 nm and homogeneously dispersed on the V2O5 nanorods. The

composite electrode with 1:2 (wt%) (VR-5) shows the best performance where capacitance of 158

F g-1, energy density of 157.08 Wh kg-1 and highest power density of 10.1 kW kg-1 in. This novel

preparation method can be implemented to prepare different metal oxide nanodots composite

materials to improve the performance of energy storage device such as supercapacitors and lithium

ion batteries.

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58

CHAPTER 4

BINDER FREE GRAPHENE–SODIUM NIOBATE NANOTUBE/ NANO-ROD

COMPOSITE ELECTRODES FOR SUPERCAPACITORS

4.1 Introduction

Energy consumption continually increases as the world population increases, resulting in greater

demands for energy to support human existence. Therefore, renewable energy and energy storage

devices such as supercapacitors have grown in interest. A supercapacitor can store one million

times more energy than a regular capacitor and exhibits higher energy density, power density and

good cycling ability.1-3 Electrochemical double-layer capacitors (EDLCs) are can be explain as

combination of two capacitors connected in series with a conducting electrolyte medium. The

capacitance of these devices is generated from storing of charges at the high surface area electrode

and electrolyte interface, creating a double layer. In pseudocapacitors, charges are generated by

undergoing redox reaction, which are then stored in the surface of the electrode material.

Pseudocapacitors show higher charge densities but slower charge transfer kinetics compared to

EDLCs due to the charges are stored at the interface and can readily accessible. Many EDLC

electrodes are based on high surface area carbon materials such as carbon nanotubes, activated

carbon and graphene sheets due to their long cycling stability, good processing ability, high surface

area and low electrical resistivity.4 The irreversible adsorption of solvated ions can dilute the ion

density on the carbon and limit the available active surface area. These ions help to catalyze the

degradation of the electrode under high electric field yielding low surface area and tend to

collapsed structures.5 These limitations can be overcome with a hybrid system. Stable redox

(pseudocapacitance) and double layer capacitance can be combined to increase the power and

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energy densities. Metal oxides such as Fe3O4, CuO, MnO2, and V2O5 have been used extensively

to replace expensive pseudocapacitive materials 6-10. Metal oxides such as MnO2 9, V2O5

11, SnO2

12 with reduced graphene oxide composites have been used for hybrid supercapacitors. Perera et

al. have studied vanadium oxide nanowire–carbon nanotube binder-free flexible electrodes for

supercapacitors and obtained 57.3 F g-1 capacitance, 65.9 Wh kg-1 and 8.32 Kw Kg-1.13-15 Niobium

and vanadium elements are in the same periodic group and show similar oxidation states. It is

possible to have pseudocapacitive properties for these compounds.13 Gum-Jae Park et al. reported

the preparation of a novel hybrid supercapacitor using a graphite cathode and a niobium (V) oxide

anode 16. The purpose of this study is to explore other possible pseudocapacitive electrode

materials such as sodium niobate. Alkaline metal niobates are excellent ferroelectric, piezoelectric,

electro-optic, nonlinear optical, photorefractive, photocatalytic and ion conductive materials 17. In

particular sodium niobate (NaNbO3) has generated interest for its use as lead free piezoelectric,

photocatalytic, dielectric and antiferroelectric materials. There are very few studies reported using

NaNbO3 as an electrode material for energy storage devices 18.

Morphology control has gained considerable attention in the synthesis of micro and nanostructures

when physical and chemical properties strongly depend on morphology 17. In this study NaNbO3

nanotubes were prepared for the first time. This synthesis involves the use of Pluronic P123 as a

soft template. At room temperature NaNbO3 forms orthorhombic crystals with antiferroelectric

properties 19. There is evidence that this structure is tolerant to ionic substitution, such as

replacement of Na+ by Li+, Mg+2, Mn+2 in solid solutions. Several synthesis routes have been

developed to prepare alkali-metal niobates including combustion, non-hydrolytic solution

reactions and hydrothermal processes 20. Herein, we report a hydrothermal synthesis route to

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prepare NaNbO3 nanorod - graphene oxide composites and NaNbO3 nanotube – graphene oxide

composites as well as the first example of NaNbO3 nanotubes mediated by graphene. Various

synthesis methods to obtain different morphologies of NaNbO3 have been reported. For example,

Liu et al. reported the synthesis of NaNbO3 nanowires using a temperature induced solid phase

oriented rearrangement route 21. Li et al. reported the effects of crystal structure and electronic

structure on photocatalytic H2 evolution and CO2 reduction over two phases of NaNbO3 22. For

supercapacitors high surface area is very important. 13, 23-24. High surface area redox active material

will enhance energy density and power density of the supercapacitors. High surface area carbon

materials have been extensively investigated and commercialized as an EDLC material due to their

long cycling stability, good processing ability, high surface area and high electrical conductivity

13, 25-26. However, there are limitations to have higher energy and power density of carbon based

electric double layer capacitors. This limitation can be overcome by combining the high surface

area carbon with a redox active material. During the fast faradic reduction, multi-electron transfer

can be achieved between the electrodes. In recent years, graphene has gain more attention as an

electrode material, in part because of its high theoretical surface area (2630 m2g-1), excellent

electron mobility (250,000 cm2 Vs-1) and good mechanical strength (1 TPa) 23. Vivekch et al.

reported the electrochemical performance of chemically reduced graphene as an electrode material

for supercapacitors. In this paper, a novel method for the preparation of NaNbO3 nanotubes on

hGO and nanorods mixed with hGO is reported for the first time.

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4.2 Experimental

4.3 Material and methods

All reagents were used without any further purification. Nb metal powder was purchased from

Sigma-Aldrich. Pluronic P123 surfactant was purchased from BASF Corporation. Graphite was

obtained from Sigma-Aldrich. Lithium bis(trifluoromethanesulfonamide) (LiTFSI) was purchased

from TCI America. A Teflon film (Gore Company) was used as the separator between anode and

cathode electrodes. A typical coin cell package (CR2032) was used to assemble all coin cell type

supercapacitors.

4.4 Synthesis of graphene oxide (GO)

GO was synthesized using previously reported modified Hummer’s method 27. First 0.5 g of

graphite and 0.5 g of NaNO3 were dispersed in 23 mL of 1 M H2SO4 and stirred in an ice bath for

15 min. Then, 4.0 g of KMnO4 was slowly mixed in an ice bath to obtain a dark-green mixture.

The resulting suspension was transferred in to a 40 oC water bath and stirred for 90 min. The

resulting dark brown colored solid was diluted by the of 50 mL of deionized (DI) water and allowed

to stir for a further 10 min. Then, 6 mL of H2O2 (30%) were slowly added to the mixture to produce

a golden brown sol. The product was centrifuged and washed with warm DI water repeatedly to

adjust the pH to 6. Finally, the product was dried at 80 oC for 24 h under vacuum.

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4.5 Synthesis of hydrothermally reduced graphene oxide (hGO)

First, 50 mg of GO were sonicated in 50 mL of DI water for 1 h to have uniform dispersions of

GO 27. Then 10.5 g of NaOH were added and the mixture was transferred in to a Teflon-lined

autoclave and heated at 120 oC for 24 h. The resulting black colored solid was neutralized using

0.1 M HCl solution. The resulting product was washed with DI water five times and dispersed in

DI water. Finally, the hGO dispersion was suction filtered using nylon filter paper (Varian

Chromatography System Nylon 66, 0.45 mm pore size and 47 mm in diameter) to obtain flexible

paper electrodes.

4.6 Synthesis of sodium niobate nanotubes on hydrothermally reduced graphene oxide

(hGO)

First, 40 mL of hGO and 0.3 g of P123 surfactant were mixed in 10 mL of ethanol. The solution

was stirred for 15 min. Then, 0.1 g of niobium metal powder was added, followed by 8 g of sodium

hydroxide pellets to make a 10 M solution. The solution was stirred for 30 min and transferred to

a Teflon lined autoclave and heated 180 oC for 100 min. The above procedure was repeated using

0.25 g and 0.5 g of Nb metal powder. The resulting solutions were filtered using nylon filter paper

in order to make flexible electrodes and then these electrodes were washed several times with DI

water to remove any surfactant. These electrodes were used to prepare coin cell type

supercapacitors. To understand the mechanism of formation of graphene mediated NaNbO3

nanotubes, the reaction time was varied (Table 4-S.1) and samples were characterized using SEM

and TEM.

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4.7 Synthesis of NaNbO3 nanorods/hGO composite

First, 0.3 g of P123 surfactant were mixed in 10 mL of ethanol and stirred for 15 min at room

temperature. Then, three different 10 M solutions were made using, 0.1 g, 0.25 g, and 0.5 g of

niobium metal powder with 8 g of sodium hydroxide pellets. The solutions were stirred for 30 min

at room temperature and transferred to a Teflon lined autoclave and heated for 180 oC at 100 min.

The resulting solutions were centrifuged and air dried for one day to prepare the NaNbO3 nanorods.

Then 0.1 g, 0.25 g, and 0.5 g of NaNbO3 nanorods were dispersed separately in 40 ml of hGO

solution using probe sonication followed by the bath sonication for 30 min in order to make

homogeneous mixture. Well dispersed solution then filtered using a nylon filter paper in order to

make flexible homogenous electrodes for supercapacitors. The electrode were washed with DI

water several times to remove any residual surfactant.

4.8 Fabrication of coin cell type supercapacitors

Coin cells were fabricated according to the previously reported method using CR2032 packaging

11. Composite electrode and carbon nanotube electrode were used to make coin cells as the anode

and the cathode in order to make asymmetrical coin cells. The masses of the cathode (carbon

nanotube electrode) remained constant and the masses of the anode electrodes were changed and

reported in the supplementary material (table 4-S.2). 1 M LiTFSI (Lithium bis(tri-

fluoromethanesulfonimide)) in acetonitrile, was added to each electrode and sealed in coin cell

using a coin cell crimper by pressing at 1000 PSIG.

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Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku Ultima IV diffractometer

(Cu Kα radiation). Raman spectra were collected using a JY Horiba HR-800 spectrophotometer.

Transmission electron microscope (TEM) images were taken using a JEOL JEM-2100 TEM at 200

kV (JEOL Co Ltd), Scanning electron microscope (SEM) images were acquired using Leo 1530

VP field emission electron microscope. Cyclic voltammograms (CV) and galvanostatic

charge/discharge curves (CDC) were obtained using a Arbin battery testing system (BT2000) with

a 2.0 V (for LiTFSI) potential window. Three electrode cyclic voltammograms were obtained by

potentiostat/Galvanostat (model 273A). X-ray Photoelectron Spectroscopy (XPS) measurements

were performed ex situ, using a Perkin Elmer PHI System. Electrochemical impedance

spectroscopy (EIS) measurements were obtain on EG&G Princeton Applied Research

potentiostat/galvanostat (model 273A). The energy and power densities for composite electrodes

were calculated according to the equations in the supplementary section.


4.11 Characterization of hGO and NaNbO3 nanorods

The morphology of NaNbO3 nanorods were observed by SEM and TEM. Figure1 a) and b) shows

the SEM images of nanorods. NaNbO3 nanorods are 150 nm in length and diameter is about 50

nm. Figure 1 c) and d) shows the low and high magnification HR-TEM images. According to the

TEM images, NaNbO3 nanorods are about 50 nm in diameter, and the calculated d-spacing of 0.388

nm correspond to the plane (111). Graphene oxide was synthesized from graphite flakes using a

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modified Hummer’s method 27. Figure1e shows the Raman spectra of graphite, graphene oxide

and hydrothermally reduced graphene oxide which exhibit characteristic D and G bands at 1300

cm-1 and 1600 cm-1 respectively 11. The G band generated from in-plane vibrations of sp2 carbons

in graphitic domains. The D band is originate due to the edges, defects and structurally disordered

carbons found in the graphene sheets. The low intensity of the D band shows a low degree of

defects. Compared to graphite, the intensity of the D band in GO and hGO is remarkably increased

after the chemical treatment due to the intrction of different types of structural defects. Disorder

and defects in graphite lead to broad D and G bands, as well as an increased intensity of the D

band 28. The intensity ratio of D band to G band (ID/IG) indicates the degree of defects presence in

graphene materials. Table 4-S.3 in the supplementary section summarizes the Raman data for GO,

hGO, graphite and NaNbO3 Nt-hGO composite. Reduced GO shows an increased ID/IG ratio

relative to the reduced graphene oxide (hGO), suggesting a decrease the size of the average sp2

domains due to the introduction of oxygen functional groups.29-30 The ID/IG ratio for NaNbO3 Nt-

hGO composite is lower than that of GO and hGO, indicating the hydrothermally prepared

composite has more sp2 domains 31. The ID/IG ratio for hGO is 0.97 and for the composite it is

0.86. During the composite preparation, hGO was reduced and it will should increase the

conductivity of the composite. The domain size of the graphite microcrystals (La) were calculated

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according to the equation, La = 44/R, where R is ID/IG ratio 32. As reported in the Table S.3, graphite

has the highest domain size due to the uninterrupted graphite structure.

Figure 4-1 a), b) SEM c), d) TEM image of NaNbO3 nanorods e) Raman spectra for GO, hGO,

graphite and NaNbO3 Nt-hGO (Nb-1)

The oxidation of graphite introduces oxygen functional groups and disrupt the large continuous

Sp2 domains in to smaller domains (∼4 nm). The domain size for hGO, ∼4.5 nm, that is larger

than that of GO (∼4 nm) and smaller than the NaNbO3 Nt-hGO composite (~5.7 nm). This suggests

that, after the alkaline hydrothermal treatment and composite preparation, the size of the

undisrupted sp2 domains of GO increased dramatically 33The crystal structure of NaNbO3 (lushite)

is show in Figure 4-1 a) and Figure 4-1 b) viewed along the (111) and (100) directions. Like other

ABO3 perovskites, NaNbO3 is comprised of a three dimensional framework of corner sharing BO6

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groups occupying octahedral with sodium cations in its cavities. In Figure 4-1 a), the octahedra

represent NbO6 units and the spheres represent sodium ions.34-35

Figure 4-2 Crystal structure of NaNbO3 in a) (111) b) (100) direction, calculated using crystal

maker and the crystallographic data in ref 36 c) digital image of flexible paper electrode (Nb-2)

XRD pattern for d) NaNbO3 nanorods e) as synthesized hGO-NaNbO3 nanotube composite (Nb-

2) f) simulated XRD pattern JCPDS 33-1270

It has been shown that this structure is tolerant to ionic substitution in the literature. Na+ can be

replaced by other cations such as Li+, Mg2+ and Mn2+ for charge compensation 37. Therefore, it is

possible to exchange Na+ ions in the crystal structure, especially, when using a Li+ ion based

electrolyte, resulting in the exchange of Li+ ions and Na+ ions 38. As shown in Figure 4-2 c. all the

composite electrode materials were flexible. These composite electrodes are promising electrodes

for flexible supercapacitors applications. As synthesized NaNbO3 nanorods were further

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characterized using XRD. The X-ray diffraction pattern of the NaNbO3 nanorods is shown in the

Figure 4-2 d. The characteristic peaks corresponding to the NaNbO3 polymorph of lueshite,

matches well with the experimental pattern (JCPDS card 33-1270) 39.

4.12 Characterization of NaNbO3 nanotube/hGO composites

Perera et al. reported growing vanadium oxide nanowires on graphene sheets. Therefore, it was

reasoned that it may be possible to grow NaNbO3 nanorods on hGO sheets. The X-ray diffraction

(XRD) pattern of the hGO-NaNbO3 nanotube composite is shown in Figure 4-2 e. According to

the XRD pattern and the simulated pattern (JCPDS 33-1270), it is clear that the synthesized

NaNbO3 nanotubes have the same lushite structure as the nanorods phase.

Figure 4-3 SEM images at a) low and b) high magnification, TEM image of c) low and d) high

magnification of Nb-2 composite electrode

Figure 4-3a and Figure 4-3b show the SEM images of the hGO-NT composite electrode at different

magnifications and it can be seen NaNbO3 nanotubes were grown on graphene sheets and several

micrometers long. Transition electron microscopy (TEM) was used to investigate morphology and

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the d-spacing. Figure 4-3.c shows TEM images of (Nb-2 composite) nanotubes. The outer diameter

of the nanotube is around 30 nm and the inner diameter of the nanotube is around 10 nm and the

observed length is about 3 µm. Figure 4-3.d shows a TEM image of nanotube at higher

magnification revealing the crystal lattice planes. The calculated d-spacing is 0.390 nm which

corresponds to the 111 plane. The surface oxygens on hydrothermally reduced graphene oxide

serves as a nucleation site for the growth of the nanotubes on the hGO plates. When the same

procedure was followed without graphene, the NaNbO3 nanorods were made.

4.13 Proposed mechanism for NaNbO3 nanorod formation.

A possible mechanism for the formation of sodium niobate nanotubes and nanorods is based on

results obtained from time-variable experiments and the reported literature. Triblock copolymers

(P123) have been used extensively to prepare different rod like nanostructures such as PbS

nanorods,40 hydroxyapatite nanorods,41 lanthanide phosphates single-crystalline

nanowires/nanorods,42 Al2O3 nanorods, NdVO4 nanorods, GdVO4 nanorods43 and SBA-15

nanorods 44. In these experiments, all the variables (reaction concentrations, temperature and

volumes) were kept constant except time. Nga et. al have reported the formation mechanism of

surfactant-assisted size controlled hydroxyapatite nanorods for bone tissue engineering, which is

similar to the formation of sodium niobate nanotubes/ nanorods 41. Bu et al. have also reported the

surfactant assisted synthesis of lanthanide phosphates and the synthesis of single-crystal PbS

nanorods via a simple hydrothermal process using a PEO-PPO-PEO (P123) triblock copolymer as

a structure directing 40, 42. The poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide)

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(PEOm−PPOn−PEOm), surfactant normally form core−shell like micelles in aqueous medium

above the critical micelle concentration (CMC) or critical micelle temperature (CMT) 45-46. At

higher surfactant concentrations, block copolymers can also self-assemble into lyotropic liquid

crystals 45, 47. Self-aggregation of the PPO block helps to form a hydrophobic core with surrounded

by hydrophilic tail and hydrated PEO blocks 48-49. At low pluronic concentrations and above the

CMC, spherical micelles form or worm micelles as the temperature increases 48. The triblock

copolymer gel is bound together by reversible attachments between coronae of neighboring

micelles, due to the absence of covalent cross-linking between the micelles 50. The PEO and PPO

blocks are hydrophilic at low temperatures allowing them to form and disperse, yielding water

soluble transparent solutions 51. Here, the triblock copolymers remain as unimers surrounded by

water molecules.

Above CMC or CMT the molecular aggregation of hydrophobic PPO blocks in the solution leads

to the form micelles 52-54. These micelles exhibit a shape transition from spherical to rod-like or

worm-like micelles at elevated temperatures 55-57. The process of self-aggregation can therefore be

induced by increasing the concentration of triblock-polymer above the CMC 48, 58-65. Micelle

formation in triblock-polymers changes due to the following reasons. There is a broad temperature

range above the CMT that can coexist micelles in a solution with unimers 36, 51, 57, 66-71. A higher

temperature is needed to form micelles because the effective PEO-PEO, PPO-PPO, and PEO-PPO

interactions are highly temperature dependent. Above a certain temperature, the effective PPO-

PPO attraction will be higher than the PEO-PEO repulsion, and it will help to form micelles.

Micelle formation is a highly temperature-dependent entropy-driven process. The gradual growth

of the hydrophobic core with the increasing temperature is due to the increasing dehydration

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process of PEO blocks. In the corona introduced instability in the spherical micellar dispersion,

that will leads to the formation of rod-like structures. Another parameter that affects micelle

formation in water is the cloud point (CP).71-72 At temperatures well above the CMT, the polymer

solution becomes opaque because phase separation occurs between the polymer and water. The CP

phenomenon in the triblock-polymer is related to the core and corona model of the triblock-

polymer aggregates. The presence of ether-oxygen species in both the PPO and PEO units allows

considerable amount of water molecules to be in the core.

Figure 4-4 Schematic diagram for the formation mechanism of NaNbO3 nanorods

Even at optimum temperatures, an increase in the temperature dehydrates the triblock-polymer

micelles by expelling water molecules, which are weakly associated with the ether-oxygen species

through electrostatic interactions. 73-74. The cloud point for the P123 is 90 oC and this temperature

is well below the synthesis temperature of 180 oC. Therefore, there is a tendency to move hydrated

PPO blocks to the corona and dehydrated PEO into the core. A graphical representation of the

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formation mechanism of nanorods is shown in Figure 4-4. It is well studied that the tri block co-

polymer PEO20-PPO70-PEO20 (P123) surfactant molecules spontaneously organize into rod-shaped

micelles when their concentrations reach a critical values as shown in step 1 in Figure 4-4 40-41, 67,

75. These anisotropic structures can be used as soft templates to form 1D nanostructures. Since,

these are soft templates it can easily distort, there is a tendency to form non symmetrical

nanostructures. Na+ ions complexes with the hydrophilic functional groups in P123 as shown in

step 2 in Figure 4-4. When the Nb metal powder is added to the highly basic solution, it reacts with

NaOH to form Nb2O5 76. The Nb2O5 reacts further with OH- at high temperature and pressure to

form Nb6O19−8 at high temperature and pressure 75, 77. Then Nb6O19

−8 react with Na+ ions complexed

with P123 to form NaNbO3 77. Finally, when the surfactant molecules are removed, crystalline 1D

nanorods are obtained 75. Balanced chemical equations are provided in the supplementary section.

4.14 Proposed mechanism for NaNbO3 nanotube formation.

It is known that, at high pH values, hydrothermally reduced graphene oxide is hydrophilic 49.

Accordingly, the surface of the graphene remains hydrophilic after reduction of graphene oxide,

while the electrical conductivity of graphene is partially restored. Zeta potential of hydrothermally

reduced graphene oxide is around -60 mV, which is below the zeta potential at low pH 78. When

the pH was increased, the negative zeta potential increased. Higher pH values result in increased

edge charges and therefore increased hydrophilicity of the sheet. Consequently, hGO in high pH

media is hydrophilic. The XPS data of hGO show the presence of C-C, C-O and C=O, which is

consistent with the presence of surface COO- groups that contribute to the hydrophilic nature of

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hGO.79 When the surface environment is hydrophilic, it is possible to direct hydrophilic head group

in P123 towards the hGO sheets and the hydrophobic tail group towards inside so as to form

reverse micelles. Figure 4-5 shows a schematic diagram of nanotube formation. Similarly, Na+

ions complexes with the hydrophilic functional groups in P123. Under basic conditions the Nb

metal particles react with NaOH to form ultimately NaNbO3 on the outside of the micelle (vide

supra). Finally, when surfactant molecules are removed, single-crystalline 1D nanotubes are

obtained. The proposed mechanism is supported by the TEM images (Figure 4-S.1 in the

supplementary section) taken at different time intervals.

Figure 4-5 Schematic diagram of formation mechanism of NaNbO3 nanotube with the presence

of hGO

4.15 Electrode preparation

Electrodes were prepared with either, different amounts of pre-prepared nanorods mixed with fixed

amounts of hGO or different amounts of Nb metal powder were used to grow nanotubes on hGO

sheets. Electrodes were prepared according to the table 4-1. Both types of electrodes were

characterized using cyclic voltammetry with 1M LiTFSI in acetonitrile as an electrolyte. Different

amount of Nb metal powder were used to prepare different electrodes (Nb-2, Nb-4, Nb-6), as

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summarized in table 4-2. In this case the nanotubes were grown on hGO sheets. But NaNbO3

nanorods were mixed with hGO to prepare electrodes (Nb-1, Nb-3, Nb-5).

Table 4-1 Summarized parameters for electrode preparat

To investigate the electrochemical behavior of all the electrodes, cyclic voltammograms were

performed at 25 mV s-1 scan rates in the potential window of 3 V, using LiTFSI in acetonitrile as

the electrolyte. (Figure 4-6). When increasing the amount of NaNbO3 nanotubes or nanorods, the

electrode performance decreases. Composite electrodes generate higher current densities in a large

voltage window than aqueous electrolytes. Interestingly, the area under the curves decreases when

the amount of NaNbO3 nanotubes or nanorods is increased due to a decrease in conductivity. When

compared with the hGO-CNT electrode, it is clear that the NaNbO3 nanotubes and nanorods

Sample Amount of Nb or NaNbO3 nanorods

Nb-1 0.1 g Nanorods + hGO

Nb-2 0.1 g Nb to grow NaNbO3 Nt on hGO





hGO-CNT Control

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enhance the performance of the electrodes. Figure 4-6.c shows the cyclic voltammogram

comparison of all the composite electrode with hGO-CNT electrode.

Figure 4-6 Cyclic voltammograms of a) NaNbO3 nanotube/hGO composites b) NaNbO3

nanorods/hGO composite c) all the composite electrodes at 25 mV s-1 scanned rate.

During the charge cycle, partial reduction of Nb+5 to Nb+4 at -0.92 V occurs, as shown in the

equation 1. The potential window lies in between -1.0 V to 2.0 V. This shows that NaNbO3

nanotubes generate pseudocapacitance, while the hGO sheets effectively create conducting

pathways for the electrons. The hGO also helps to generate EDLC, which improves the overall

capacitance of the composite electrode.

NaNbO3 + xLi+ + xe− ↔ Lix+NaNb2−xNbx

+4O52− … … (1)

The charge-discharge behavior of hGO-NaNbO3 composite electrodes was characterized under

Galvano-static conditions (Figure 4-7.a, b). The galvanostatic charge-discharge curves were

collected at constant current density of 1 A g-1. Compared to the hGO-CNT electrodes, the

discharge time of the hGO-NaNbO3 nanotube (Nb-2) composite was significantly increased,

suggesting that the combination of NaNbO3 nanotubes and hGO offers a larger charge capacity,

which is consistent with the CV behavior (Figure 4-6).

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Figure 4-7 a) Charge discharge profile for coin cell series at 1 A g-1 b) Cell voltage vs. discharge

time of sample Nb-2 in 1 M LiTFSI at different discharge current densities c) Charge–discharge

profile of Nb-2 supercapacitor in 1 M LiTFSI at 1.5 A g−1 for the 1st and 7000th cycle d)

capacitance retention of Nb-2 over 1000 charge/discharge cycles evaluated from the

galvanostatic discharge curves

Energy- power densities and specific capacitance were based on the total weight of the anode and

the cathode (Table 4-S.1). The discharge profile of the asymmetric cell (Nb-2) at different

discharge current densities is shown in Figure 4-7.b. The retention of the specific capacitance

against the cycle number is shown in Figure 4-7.d. After 600th cycle ~80% of the specific

capacitance was retained while after 1000th cycle ~70% of the capacitance was retained. Cycle test

were carried out up to 7000th cycles and ~45 % of the capacitance was retained as shown in the

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Figure 4-S. 3. Should be noted that the coin cell were assembled in air and that absorbed water can

influence to the loss in capacitance with time. During the charge discharge process Li ions in the

medium can be diluted due to the Li ion intercalation. Unlike in Li ion batteries, where Li metal

foil act as a Li ion source. But in this study Li ion source is the LiTFSI.

In order to explain the cycling, the coin cells were dissembled after the electrochemical

characterizations and characterized using XRD and Raman spectroscopy. Figure 4-S 2 in the

supplementary materials shows the XRD pattern for the Nb-2 electrode before and after the

electrochemical characterization of Nb-2. The XRD pattern shifted towards higher 2-theta values.

This is evidence for exchange of the Na+ ions with Li+ ions causing contraction of the crystal

structure decreasing the d-spacing 4. Li+ ions are smaller than Na+ ion with ionic sizes 0.076 nm

and 0.102 nm, respectively 80. The Nb can have different oxidation states, with +5 and +4 being

the most common. Therefore, doping may also result in contraction of the lattice. In order to

confirm this phenomena, cyclic voltammogram (CV) for Nb-2 was performed in Na+ ion

containing electrolyte (1M Na2SO4). Different scan rates have different ion exchange rates.

Therefore, at each scan rate 100 cycles were cycled in five different scan rates (10, 25, 50, 75 and

100 mV s-1). All together 500 cycles were cycled at different scan rates. Then coin cell were

dissembled and XRD was taken. According to the Figure 4-.8, there is no peak shift in XRD

pattern. This implies there is no crystal contraction. Therefore, it can be confirm that when NaNbO3

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nanorods/hGO composite (Nb-2) in LiTFSI electrolyte exchange Li+ ions in electrolyte with the

Na+ ions in the crystal.

Figure 4-8 Three electrode cyclic voltammogram of NaNbO3/hGO (Nb-2) composite electrode

and hGO with reference to Ag/Ag+ in 1M LITFSI in acetonitrile at 50 mV s-1

The electrochemical behavior of NaNbO3/hGO composite electrode was analyzed by cyclic

voltammetry in 1M LITFSI in acetonitrile at 50 mV s-1 with Pt counter electrode reference to

Ag/Ag+ in the potential range 2.0 to -2.0 V. Carbon paste electrodes were prepared by using

NaNbO3/hGO composite carefully pressed in to the electrode. Figure 4-8 NaNbO3/hGO composite

electrode voltammograms exhibit two pairs of peaks I and II situated respectively at Eeq(I) = - 0.92

V, Eeq(II) = 0.57 V 1M LITFSI in acetonitrile (Eeq = (Epa+ Epc)/2, where Epa is the potential value of

the anodic peak and Epc the potential value of the corresponding cathodic peak). Kosho et al. have

reported the redox couple related to Nb+4/Nb+5 is approximately -0.92 V which can be assigned to

peak (I) 81. Yuyan Shao et al. reported the reduction of graphene oxide surface which can be

assigned to peak (II) 82-84. Even after chemical reduction, surface functional groups still exists.

These functional groups can undergo oxidation and reduction during electrochemical

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characterization. Elzbieta et al. reported the following possible reactions of electroactive functional

surface groups on hGO 85.

−C − OH ↔ C = O + H+ + 𝑒 – (2)

−COOH ↔ −COO + H+ + 𝑒− (3)

Thus the, two well defined couples I and II in these voltammograms can be attributed to the

Nb+4/Nb+5 and reduced graphene oxide (hGO) redox processes.81, 86 Electrochemical impedance

spectroscopy (EIS) measurements were performed to evaluate resistant components on both the

hGO and Nb-2 composite electrodes. The EIS measurements in Figure 4-S.4 indicate Nb-2

electrodes display ideal capacitive behavior with a semicircle at high-medium frequency and an

inclined line at low frequency, which corresponds to charge transfer and diffusion respectively and

explain in the supplementary section 87.

The Nb-1 (nanorods) in table 4-2 coin cell produced an energy density of 30 Wh kg-1 and power

density of 900 W kg-1 at 1 A g-1 discharge rates. The energy density and power density are higher

than the hGO-CNT electrodes. The Nb-2 coin cell showed an energy density of 44.86 Wh kg-1 and

931.76 W kg-1 at 1 A g-1 discharge rates. The specific capacitance of the supercapacitors (Csp) of

the electrode was calculated using the equation given in the supplementary section. The specific

capacitances of Nb-1 and Nb-2 are 232 F g-1 and 260 F g-1, respectively, which is consistent with

the CV and discharge profiles. Under the same conditions, the hGO-CNT electrode has a

capacitance of 120 F g-1, which is lower than Nb-1 and Nb-2 coin cells. Liu et. al have reported

the highest capacitance of 137 F g-1 with using lithium niobate nanoflakes in 1M H2SO4 aqueous

electrolyte 88. Ke et. al have used chemical vapor deposition to grow RuO2 on lithium niobate

substrate. Electrochemical performance have been conducted and capacitance were found to be

80

high as 569 F g-1 in 1M H2SO4 aqueous electrolyte 89. The morphology change from nanorods to

nanotubes enhances the performance of the coin cell. The energy density and power density of the

hGO-CNT electrodes were 17.6 Wh kg-1 and 900 W kg-1 respectively. With increasing

pseudocapacitance, the energy density was observed to increase, while the power density remained

somewhat constant, presumably because the electric double layer capacitance remains constant in

electrodes containing the NaNbO3 nanostructures. The surface areas of the hGO and NaNbO3-

hGO composite (Nb-2) were measured to be 46.7 m2 g-1 and of 128.8 m2 g-1 respectively. Isotherms

and pore size distribution diagrams are shown in the supplementary section Figure 4-S.5 and

correlate with the measured capacitance.

4.16 Conclusion

Graphene-mediated NaNbO3 nanotubes were prepared for the first time. The freestanding flexible

NaNbO3 nanotube/ nanorod-hGO nanocomposite papers were also prepared without using any

organic binders. Coin cell type supercapacitors were prepared using NaNbO3 nanotubes/nanorods

graphene composite electrodes as the anode and the free standing carbon nanotube (CNT)

electrode as the cathode to investigate the application of sodium niobate as a supercapacitor

material. XPS data show the partial reduction of Nb+5 to Nb+4, consistent with the pseudocapacitive

properties of the electrode and confirmed with the three electrode measurements. The Raman band

shifts also explain the enhancement of physical attraction between graphene and the

nanostructures. The nanocomposite electrode prepared from 0.25 g of Nb powder shows the

capacitance behavior with 260 F g-1, 931.76 W kg-1 of power density and 44.86 Wh kg-1 of energy

81

density. These values are higher than hGO-CNT electrode itself. In this study, we have

demonstrated the potential application of NaNbO3 as a supercapacitor electrode and new method

and concept to prepare NaNbO3 nanotubes. These flexible nanocomposite paper electrodes are

promising binder-free electrodes for application in the high-energy storage devices. Graphene-

mediated NaNbO3 nanotubes were prepared for the first time. The freestanding flexible NaNbO3

nanotube/ nanorod-hGO nanocomposite papers were also prepared without using any organic

binders. Coin cell type supercapacitors were prepared using NaNbO3 nanotubes/nanorods

graphene composite electrodes as the anode and the free standing carbon nanotube (CNT)

electrode as the cathode to investigate the application of sodium niobate as a supercapacitor

material. The nanocomposite electrode prepared from 0.25 g of Nb powder shows the capacitance

behavior with 260 F g-1, 931.76 W kg-1 of power density and 44.86 Wh kg-1 of energy density.

These values are higher than hGO-CNT electrode itself. In this study, we have demonstrated the

potential application of NaNbO3 as a supercapacitor electrode and new method and concept to

prepare NaNbO3 nanotubes. These flexible nanocomposite paper electrodes are promising binder-

free electrodes for application in the high-energy storage devices. Reported new strategy to prepare

NaNbO3 nanotubes and nanorods can be used for other application such as lead free piezoelectric

materials.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

Acknowledgements:

We thank the Robert A Welch Foundation Grant No AT-1153 for support of this research.

82

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91

CHAPTER 5

RUO2 NANODOTS SUPPORTED WRINKLED MESOPOROUS CARBON

FOR SUPERCAPACITORS

5.1 Introduction

There are two types of supercapacitors according to the charge storage mechanism which are

electric double layer and pseudocapacitance.1 Electric double layer capacitors (EDLC) store

charges at the interface by forming an electric double layer.2 High surface area carbon materials

are common electric double layer capacitor materials such as carbon nanotubes (215.5 m2 g-1)3,

graphene (theoretical 2630 m2 g-1 )4 and activated carbon (1367 m2 g-1)3 which have high surface

areas, long cycle lives and high electrical conductivity. Pseudocapacitors generate charges by

undergoing redox reactions. RuO25, V2O5

6, MnO27, CuO8 and Fe3O4

9 can undergo redox reactions

and generate a large number of charges. A combination of EDLC and pseudocapacitance in a

hybrid supercapacitor could result in higher performance using a high surface area carbon and a

high pseudocapacitive metal oxide. RuO2 is the best known pseudocapacitive metal oxide for

supercapacitors that gives over 700 F g-1 capacitance in aqueous electrolytes.10 In our previous

studies we have shown that RuO2 can be used in ionic liquid electrolytes. We have also synthesized

novel high surface area (1370 m2 g-1) carbons derived from wrinkled mesoporous silica.11 The

wrinkled spherical shaped carbon has conical mesopores that can be modified with metals such as

Pd12 and different metal oxides such as RuO213, V2O5

6, MnO27 and CuO8. In this study, RuO2, has

beendeposited in the pores of WMC for supercapacitors. The conical shape mesopores allow better

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diffusion of electrolyte. We have demonstrated two different methods that can be used to obtain

RuO2 nanoparticles inside the wrinkled mesoporous carbon framework.


All reagents were used as received. Ruthenium chloride (RuCl3.xH2O) was purchased from

Pressure chemical co. High purity multiwall carbon nanotubes (CNTs) (∼ 50 µm in length) were

purchased from Sun Innovations Inc. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)

imide (EMIM TFSI) was obtained from Sigma Aldrich. Typical coin cell packaging (CR-2032)

was used to assemble the coin cell type supercapacitors.6 A Teflon film (Gore Company) was used

as the separator between the two electrodes.

5.3 Synthesis of wrinkled mesoporous silica (WMS)

Wrinkled mesoporous silica (WMS) was synthesized using a previously reported procedure as

follows.11 1.8 g of urea were mixed with 15 mL of DI water and 1.2 mL of n-butanol with 1 g of

cetyltrimethylammonium bromide (CTAB) in the 40 ml round bottom flask and stirred at ambient

conditions to obtain a clear solution. Then, 15 mL of cyclohexane were added to the above mixture

and stirred for another 30 min. Next, 1.25 mL of tetraethylorthosilicate (TEOS) was added to the

mixture and stirred for 30 min at room temperature. Finally, the reaction mixture was heated at 70

oC for 24 h. The resulting product (WMS) was collected by centrifugation and washed with acetone

and water four times. The WMS was redispersed in 40 mL of ethanol and 4 mL of 12 M HCl and

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refluxed at 70 oC overnight to remove any residual template. The final WMC product was

centrifuged and dried at 60 oC for 24 hours.

5.4 Synthesis of wrinkled mesoporous carbon (WMC)

In order to prepare wrinkled mesoporous carbon (WMC), first 0.25 g of WMS were dispersed in

mixture of 0.31 g sucrose, 0.035 g of concentrated H2SO4 and 1.25 mL of DI water. The mixture

was dried 24 hours 100 0C and the temperature increased up to 160 oC for another 6 hours. Then

the resultant solid was immersed again in a solution mixture of 0.19 g of sucrose, 0.01 mL of

H2SO4 and 1.25 mL of DI water. Next the mixture was dried at 100 oC for 24 hours and 160 oC for

6 hours. Then the product was carbonized at 900 oC for 3 h under nitrogen. The silica template

was removed by dissolving in 1M NaOH solution with ethanol: water 1:1 mixture twice. Finally,

the WMC product was dried at 100 oC overnight after washing with ethanol and water.

5.5 Synthesis of ruthenium oxide nanodots on wrinkled mesoporous carbon (WMCR)

Different amounts of the RuO2 nanodots were bound to the WMC using the following procedure.

First 20 mg of WMC were dispersed in 20 mL of DI water and stirred for 6 h at room temperature.

Then 3.47 mg, 7.78 mg, 0.020 g and 0.24 g of RuCl3 were dissolved separately in order to prepare

WMCR-A, WMCR-B, WMCR-C and WMCR-D. Solid was isolated by centrifuging and washed

with DI water several times to remove any excess RuCl3 and dried at room temperature. Then, the

WMCR were treated with 20 mL of 1M NaOH solution for 24 h at room temperature. The mixture

was transferred in to a 45 mL Teflon lined autoclave and heated at 180 oC for 15 h. Then the

94

solution was filtered to remove any unbound RuO2 nanoparticles. The black solid was annealed at

250 oC for 3 h in air.

5.6 Synthesis of ruthenium oxide nanodots on wrinkled mesoporous carbon using a

reducing method.

First 20 mg of WMC were dispersed in a mixture of 10 mL DI water and 10 mL 90% ethanol.

Then 0.1 g of RuCl3 were added and stirred for 6 h. Then 0.1 g of NaOH were added and stirred

until dissolved. The resultant solution was transferred in to a 45 mL Teflon lined autoclave and

heated at 180 oC for 15 h. Then resultant solid was isolated by centrifugation and washed with DI

water. The solid product was dried at 60 oC overnight. Finally, the black product was annealed at

250 oC for 3 h.

5.7 Preparation of ruthenium oxide on wrinkled mesoporous carbon (WMCR) composite

electrodes

The composite electrodes (WMCR-A, B, C and D) were made by mixing 20 mg of WMCR with

20 mg of carbon nanotubes (MCNTs) and with 2% PTFE as a binder to make a free standing paper.

WMC/ CNT and CNT electrodes were prepared by using 20 mg of WMC and 20 mg of MCNT as

a control.

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5.8 Supercapacitor assembly.

Prior to use the electrodes were immersed in the 2 mL of EMIM TFSI electrolyte for 1 h at room

temperature. The composite paper anode and the MCNT cathode were separated by a Teflon film.

Coin cell packaging (CR2032) was used to assemble the supercapacitors as previously described.6

Carbon coated aluminum sheets were used as the current collectors. Additional EMIM TFSI

electrolyte (~ 0.2 mL) was introduced to each electrode and sealed in the coin cell using a coin cell

crimper (Shenzhen Yongxingye precision machinery mold) by pressing at 1100 psig.


The RuO2 deposited on WMC was characterized by X-ray powder diffraction (XRD) using Rigaku

Ultima IV diffractometer (Cu Kα radiation). Transmission electron microscope (TEM) images

were acquired using a JEOL JEM-2100 TEM at 200 kV (JEOL Co. Ltd.). Thermogravimetric

analysis (TGA) was performed using a TA Instruments Q600 Simultaneous TGA. Cyclic

voltammograms (CV) and galvanostatic charge/discharge curves were obtained using Arbin

battery testing system (BT2000) in the range of –2.0 to 2.0 V (voltage window of 4V).

96


5.11 Characterization of ruthenium oxide nanodots grown on wrinkled mesoporous carbon

(WMCR)

Figure 5-9 a), wrinkled mesoporous silica (WMS) b) SEM image of wrinkled mesoporous

carbon (WMC)

Figure 5-1.a shows a SEM image of a typical WMS synthesized from a microemulsion of urea,

cetylmethylammonium bromide, n-butanol, tetraethylorthosilicate and water. The WMS can

function as a template for making wrinkled mesoporous carbon (WMC) as shown in Figure 5-1.b.

These WMC were used as a framework to incorporate different amount of RuO2. The objective of

incorporating RuO2 was to increase the conductivity and pseudocapacitance. Figure 5-2. a-i, shows

the TEM images of different RuO2 loaded WMC composites labeled as WMCR-1 (10 wt% RuO2),

WMCR-2 (20 wt% RuO2), WMCR-3 (40 wt% RuO2) and WMCR-4 (80 wt% RuO2). Figure 5-2.

a, b, and c shows the TEM image with 10 wt% RuO2 loaded WMC. The particle size distribution

histograms (inset) shows the majority of the particles are about 2 nm. But is has ~ 1 – 9 nm range

size distribution. WMC has conical mesopores that act as a pockets to grow RuO2. These

97

mesopores are random in size and shape. Similarly, Figure 5-2.d, e and f shows the 20 wt% RuO2

loaded WMC with 2-3 nm average particle size. When increase the RuO2 amount in to 40 wt%

and 80 wt% particle size tend to increase.

Figure 5-10 TEM images of a), b) 10 wt% c), d) 20 wt% e), f) 40 wt% and g), h) 80 wt% WMCR

98

Figure 5-11 XRD patterns of a) WMC b) 10 wt% c) 20 wt% d) 40 wt% and e) 80 wt% WMCR

Synthesized wrinkled mesoporous carbon with different amount of RuO2 were characterized using

X-ray diffraction (XRD) as shown in Figure 5-3 where the characteristic peaks for RuO2 (JCPDS-

00-040-1290) are assigned to the (110), (101) and (220).

Furthermore, WMCRs were characterized using TGA analysis in order to quantify the amount of

RuO2 contain in each samples (Figure 5-4). WMC has decomposition temperature around 425 oC

in air while RuO2 is thermally stable up to 1300 0C. Therefore the weight loss around 424 oC

corresponds to the amount of WMC contained in each sample. According to the Figure 5-4, the

amount of RuO2 contain in each sample is approximately equal to the amount predict from

synthesis.

99

Figure 5-12 TGA of a) WMC b) 10wt% c) 20 wt% d) 40 wt% e) 80 wt% different RuO2 loading

and f) RuO2

Figure 5-5. shows the TEM images of RuO2 loaded WMC prepared by reduction method. During

this process first it forms Ru metal particles due to the reduction of RuCl3 with the presence of

ethanol in hydrothermal synthesis. Figure 5-5. b shows the well dispersed Ru metal particles on

100

WMC. During the annealing process these Ru metal particles oxidize in to RuO2 nanoparticles as

shown in Figure 5-5.d.

Figure 5-13 a), b) and c) TEM images of WMC with Ru metal nanoparticles and d) with RuO2

nanoparticles

101

Figure 5-14 Cyclic voltammograms of composite electrodes with different RuO2 loaded WMCs.

Figure 5-6 shows the cyclic voltammograms (CV) of different composite electrodes. It clearly

indicates that when increasing the amount of RuO2, capacitance increases. Quasi rectangular CV

implies the ideal capacitance with pseudocapacitance. 80 wt% electrode shows the redox couple

that may be appeared due to the RuO2. Capacitance of the WMC is calculated to be 15 F g-1. When

incorporated 10 wt% RuO2 capacitance increases to 52 F g-1. 20 wt% and 40 wt% electrodes show

78 F g-1 and 118 F g-1 capacitance. The highest loading 80 wt% RuO2 containing electrode shows

148 F g-1 capacitance. This investigation shows the potential application of WMC incorporated

with RuO2 nanoparticles for supercapacitors.

102

5.12 Conclusion

In this study we have demonstrated the novel method of synthesizing wrinkled mesoporous carbon

derived from mesoporous silica with RuO2 for supercapacitor applications. We have reported two

different methods to obtain RuO2 nanoparticles in WMC. This strategy can be used to incorporate

different metal oxides for supercapacitors and catalytic applications.

5.13 References

1. M. Winter and R. J. Brodd, Chemical Reviews, 2004, 104, 4245-4270.

2. A. G. Pandolfo and A. F. Hollenkamp, J. Power Sources, 2006, 157, 11-27.

3. E. Frackowiak and F. Béguin, Carbon, 2001, 39, 937-950.

4. M. S. Chang, T. Kim, J. H. Kang, J. Park and C. R. Park, 2D Materials, 2015, 2, 014007.

5. M. Egashira, Y. Matsuno, N. Yoshimoto and M. Morita, Journal of Power Sources, 2010,

195, 3036-3040.

6. S. D. Perera, A. D. Liyanage, N. Nijem, J. P. Ferraris, Y. J. Chabal and K. J. Balkus Jr,

Journal of Power Sources, 2013, 230, 130-137.

7. S. D. Perera, M. Rudolph, R. G. Mariano, N. Nijem, J. P. Ferraris, Y. J. Chabal and K. J.

Balkus Jr, Nano Energy, 2013, 2, 966-975.

8. S. E. Moosavifard, M. F. El-Kady, M. S. Rahmanifar, R. B. Kaner and M. F. Mousavi,

ACS Applied Materials & Interfaces, 2015, 7, 4851-4860.

9. M. Egashira, Y. Tsubouchi, N. Yoshimoto and M. Morita, Electrochemistry, 2015, 83, 244-

248.

10. P. Wang, H. Liu, Q. Tan and J. Yang, RSC Advances, 2014, 4, 42839-42845.

11. Z. Wang and K. J. Balkus Jr, Materials Letters, 2017, 195, 139-142.

12. Z. Wang and K. J. Balkus, J. Porous Mater., 2017, DOI: 10.1007/s10934-017-0415-0, 1-

7.

103

13. B. J. Lee, S. R. Sivakkumar, J. M. Ko, J. H. Kim, S. M. Jo and D. Y. Kim, Journal of

Power Sources, 2007, 168, 546-552.

104

CHAPTER 6

HYDROTHERMAL SYNTHESIS OF TIO2 NANOTUBE (TNT)/ RUO2 NANORIBBON

(NR)/ GRAPHENE OXIDE COMPOSITES WITH ENHANCED

PHOTOCATALYTIC ACTIVITY

6.1 Introduction

TiO2 is the most widely studied semiconducting metal oxide for photocatalytic applications due to

its high photocatalytic ability.1-8 In the presence of UV light, valance band electrons in TiO2 can

be excited from the valence band (VB) to the conduction band (CB). This will create electron−hole

pairs (e- and h+) which are powerful reducing and oxidizing agents.9 These electron−hole pairs are

mainly involved in the photocatalytic activity.10-11 However, the photocatalytic activity of TiO2 is

reduced when the photogenerated electron-hole pairs recombine.12-13 Therefore, the goal is to

minimize electron hole pair recombination and improve the photocatalytic efficiency of TiO2.6, 14

Electron−hole pair recombination can be reduced by surface modification with noble metals and

metal oxides.6, 10-11, 15 Many current studies are aimed at develop new strategies that combine

different metal oxide particles and ions with TiO2.16 There is growing interest in graphene/metal

oxide nanocomposites as photocatalysts due to higher adsorptivity, conductivity, tunable optical

behavior and stability.17 Graphene shows excellent electrical (electron mobility - 250,000 cm2 V-1

s-1), mechanical (Young’s modules – 1 TPa) and thermal (thermal conductivity – 5000 W m-1 K-1)

properties due to its single layer two-dimensional graphitic structure.18-19 Graphene is an

inexpensive material with properties that are attractive for applications such as nanoelectronics,

photovoltaics, catalysis and energy storage devices.20-22 Various methods have been used to

prepare a single layer or a few layers of graphene sheets by exfoliation of naturally occurring

105

graphite flakes.23-26 Strong oxidizing agents can be used to exfoliate graphite in to individual

graphene oxide sheets. This process introduces extra oxygen functional groups on the surface and

edges of the graphite.27 These oxygen functional groups help to improve the solubility of GO sheets

in various solvents and allow additional surface modification.28-29 Surface oxygen functional

groups disrupt sp2 hybridization of exfoliated GO sheets and reduce the electrical conductivity of

GO. Hydrazine (N2H4), sodium borohydride (NaBH4) and alcohols can be used to prepare reduced

graphene oxide (rGO). These reducing agents can reestablish the sp2 hybridized system and

improve its electronic conductivity.30-32 The two dimensional reduced graphene sheets can act as

a substrate to grow nanostructured catalysts.33 Furthermore, the functional groups on reduced

graphene sheets can provide nucleation and growth sites for metal oxide nanoparticles.9, 34 The

graphene support prevents loss of metal oxide nanoparticles in to the solution.35

Reduced graphene oxide (rGO) has been used as a support for various metal oxides such as TiO2,36-

38 ZnO,35, 39-45 Cu2O,33 SnS2,46 ZnFe2O4,

47 CuFe2O448 and Bi2WO6.

49 These composites have shown

excellent photocatalytic activity for the decomposition of synthetic dyes. Previously, we reported

the preparation of graphene-TiO2 nanotube composites while showed enhanced photocatalytic

activity.9

There have been several studies that report improved photocatalytic activity of RuO2/TiO2

mesoporous heterostructures for photodecomposition of organic dyes.50-51-52-57 Ibhadon et al.

reported the photocatalytic activity of surface modified TiO2 nanoparticles with RuO2 and SiO2

nanoparticles for azo-dye degradation.50 TiO2 nanoparticles and RuO2 nanoparticles can aggregate

and lead to poor interfacial contact with the graphene surface. Therefore, improved interfacial

contact (chemically bound) between TiO2/RuO2 and RuO2 with the graphene (hGO) surface

106

without aggregation is important to enhance the photocatalytic activity of TiO2/RuO2/hGO

composites. Therefore, RuO2 nanoribbons and TiO2 nanotubes were employed to achieve better

contact between nanostructures. RuO2 has excellent electronic conductivity due to the partially

filled metal (d)- oxygen (p) π* band 58 and can act as an efficient electron hole transfer catalyst

when deposited on TiO2. Furthermore, RuO2 has high work function that is located above the

valance band of the TiO2.59 Uddin and coworkers have reported upward band bending for a TiO2/

RuO2 heterojunction.60 This will help electrons to flow from TiO2 to RuO2 and separate charges at

the RuO2/ TiO2 interface under illumination. The RuO2 will increase the conductivity of the

composite which facilitates the efficient charge transfer.61 Tamez and coworkers have prepared

RuO2 /TiO2 mesoporous heterostructures and reported enhanced photocatalytic properties by band

alignment.60 The band bending for this RuO2/ TiO2 heterostructure was 0.2 ± 0.05 eV,60 which

helped to minimize the electron-hole pair recombination. The preparation of TNT/RuO2 NR/hGO

composites is illustrated in Figure 6-1. First, hydrothermally reduced graphene oxide (hGO) was

prepared and then RuO2 NRs were grown on the hGO. Then the TNTs were grown on the RuO2

NRs. Good interfacial contact between RuO2 NRs/TNTs and graphene at the interface help to

separate charges enhancing the photocatalytic efficiency.

Figure 6-1 Graphical illustration of the synthesis of TNT/RuO2 NR/hGO composites.

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6.2 Experimental section


TiO2 nanoparticles (P25) were purchased from Evonik-Degussa. Ruthenium chloride

(RuCl3.xH2O) was purchased from Pressure chemical CO. SPAN-80 surfactant and graphite

powder was purchased from Sigma Aldrich. H2SO4 (EMD chemicals), HCl (Fischer Scientific),

NaNO3 (Sigma Aldrich), NaOH (Alfa Aesar), and KMnO4 (Baker analyzed) were used without

any purification. Malachite green oxalate was obtained from Alfa Aesar.

6.4 Characterization.

X-ray powder diffraction (XRD) patterns were collected using a Rigaku Ultima IV diffractometer

(Cu Kα radiation). Raman spectra were obtained using a JY Horiba HR-800 spectrophotometer.

Transmission electron microscope (TEM) images and Energy-dispersive X-ray spectra (EDX)

were collected using a JEOL JEM-2100 TEM at 200 kV (JEOL Co. Ltd.). Scanning electron

microscope (SEM) images were obtained using a LEO 1530 VP field emission electron

microscope. UV−vis spectra were collected using a Shimadzo UV-1601PC spectrometer with

integrating sphere. X-ray photoelectron spectroscopy (XPS) measurements were carried out exsitu,

using a Perkin-Elmer PHI System.

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6.5 Synthesis of graphene Oxide (GO).

GO was prepared by using a modified Hummer’s method.62 First, 0.5 g of graphite and 0.5 g of

NaNO3 were added to 23 mL of 12.1 M H2SO4 and stirred in an ice bath for 15 min. Next, 4.0 g

of KMnO4 was slowly mixed with the mixture in an ice bath to obtain a purple-green gel. Resultant

suspension was transferred to a 40 °C water bath and stirred for 90 min. Then 50 ml of deionized

water (DI) water was added to the dark brown colored paste and stirred for another 10 min. Then,

6 mL of H2O2 (30%) was added to the solution and obtained a brown sol. Then, 50 mL of DI water

was added, and the resulting product was isolated by centrifuge and solid was washed with warm

(60 oC) DI water five times. Finally the product was dried at 80 °C for 24 h.

6.6 Synthesis of TNT/RuO2 NR/hGO (TRG) composites.

The GO composites with different TNT/RuO2 NR content were synthesized using hydrothermal

method under alkaline conditions. To obtain homogeneous dispersions of GO, 10 mg of GO were

sonicated in 10 mL of DI water for 1 h. Ruthenium chloride (RuCl3.xH2O) was added to the GO

dispersions with stirring (0.1 g (TRG-3), 0.2 g (TRG-4) and 0.3 g (TRG-5)). Then, RuCl3 /GO

mixture was stirred for another 1 h at room temperature. 1.5 mL of Span-80 was added to the

RuCl3/ GO mixture and stirred at room temperature for 2 h. Then, 10 mL of butanol were added

to the mixture and stirred for two hours at room temperature. The mixture was transferred to a 40

mL Teflon lined autoclave with 0.6 g of sodium hydroxide and heated at 180 oC for 15 hours. Then

the product was isolated by centrifugation and washed with deionized water and ethanol five times.

The final product was vacuum dried at room temperature overnight. The resulting solid was

109

sonicated in 30 mL of DI-water for 15 min to obtain homogeneous dispersions of RuO2 NR/hGO.

TiO2 (P25) powder (0.1 g (TRG-3), 0.2 g (TRG-4) and 0.3 g (TRG-5)) were then added to the

RuO2 NR/hGO dispersions with stirring. The TiO2/RuO2 NR/hGO mixture was stirred for 1 h to

achieve complete mixing. Then 10.5 g of NaOH were added, and the mixture was transferred to a

45 mL Teflon lined autoclave and then heated at 120 °C for 24 h. The resulting precipitate was

washed with 0.1 M HCl solution and stirred overnight at room temperature. Then the product was

washed with DI water three times. Then centrifuged, dried at 80 °C and annealed at 250 °C for

120 min. Samples were prepared with constant amount of GO and different amount of RuCl3.xH2O

and TiO2 (P25) amounts. Sample labeled as TG-1 prepared using 10 mg of GO with 0.3 g of P25.

Sample labeled as RG-2 prepared by using 10 mg of GO and 0.3 g of RuCl3. Sample labeled as

TRG-3, TRG-4 and TRG-5 prepared using 10 mg of GO and 0.1 g, 0.2 g and 0.3 g of P25 and 0.1,

g, 0.2 g and 0.3 g of RuCl3 Table S.1 in the supplementary section shows the detail sample

preparation procedure.

6.7 Photocatalytic measurements.

The photocatalytic performance of the composites was investigated for the decomposition of

malachite green oxalate (MGO) (13.1 mg/L) in H2O. First 20 mg of the catalyst was stirred with

the 100 mL of the dye solution in a 250 mL quartz round-bottom flask and kept inside a dark box

with a water-cooled mercury lamp (450 W, quartz Hanovia). To obtain equilibrium, the mixture

was stirred in the dark for 1 h. Next, the mixture was irradiated with the 450 W mercury lamp.

Five milliliter aliquots were taken out after every 10 minutes and centrifuged to separate the

catalyst. Finally the absorbance measurements were taken using UV−vis spectroscopy.

110


6.9 Characterization of TNT/ RuO2 NR/ hGO composites

The TNT/RuO2 NR/hGO composites were dispersed in aqueous medium due to the hydrophilic

oxygen functional groups on the surface of the hGO sheets. Oxygen functional groups such as

carboxylates promote the formation of metal oxide on GO.9, 63 Similarly, these functional groups

help the RuO2 nanostructures to grow on GO.64 Combining RuCl3.xH2O and GO results in a dark

brown suspension. Hydrothermal treatment under basic conditions using a previously reported

procedure results in hGO supported RuO2 NRs. The color of the hGO- RuO2 NR dispersion

became black after the hydrothermal treatment. In order to grow TNTs on the supported RuO2 NR,

a second hydrothermal treatment was conducted under basic conditions as describe above. During

this step the hGO will be further reduced removing oxygen functional groups from the surface.

Figure 6-2 XRD patterns of (a) TG-1, (b) RG-2, (c) TRG-3, (d) TRG-4 and (e) TRG-5.

111

X-ray diffraction (XRD) patterns were collected for the TNT/RuO2 NR/hGO composites as shown

in Figure 6-2. TG-1 (TNT/ hGO) exhibits characteristic (101), (004), (200), (105) and (211)

reflections corresponding to the anatase phase (JCPDS PDF#: 00-021-1272) (Figure 6-2.a). The

XRD pattern in Figure 6-2.b (RG-2) shows the characteristic peaks for RuO2 at 2ɵ = 27.9o (d -

0.318 nm), 35.0o, 39.9o, 40.4o, which are assigned to the (110), (101), (200), (111) and (211).

(JCPDS PDF#: 00-040-1290). The corresponding peaks for the composites are broader than pure

TNT and RuO2 NR. The average crystal size of the TNT/hGO (TG-1), RuO2 NR/hGO (RG-2)

were calculated using the Scherrer equation based on the XRD peak related to TiO2 (101) and

RuO2 (110) (Supporting Information, Table 6-S.2). Calculated average crystal size of TNT was

about ~ 8 nm and RuO2 NR was about ~ 6 nm which is consistent with the TEM analysis (vide

infra).

The Raman spectra of the RuO2 NR and RuO2 NR/ hGO composite (RG-2) are shown in Figure

6-3. There are 15 optical modes of RuO2. But four modes are Raman-active (A1g, B1g, B2g and

Eg).65 Four major Raman features of rutile RuO2 are placed at 528, 646, 716 and 812 cm-1,

respectively. The broader peaks related to the Eg mode for RuO2 NR and RuO2 NR/ hGO

composites are notably blue-shifted from 528 cm-1 to 534 cm-1 (Figure 6-3, inset). The blue-shift

and peak broadening is ascribed to the surface pressure or phonon confinement effect which are

common for nanomaterials.66 Previously reported TNT/ hGO composites also showed a similar

blue-shift due to the strong interaction between TNT and hGO.9 These results suggests a strong

chemical interaction between RuO2 NR and hGO.

112

Figure 6-3 Raman spectra of (a) RuO2 NR and (b) RuO2 /hGO (RG-2). Inset Raman spectra

showing the blue shift of the Eg band of the composite.

Figure 6-4 Raman spectra of TNT (a), TG-1 (b), TRG-3 (c), TRG-4 (d) and TRG-5 (e). Inset

Raman spectra showing the blue shift of the Eg band of the composites.

Raman spectra of the TNTs and composites with different TNT/ RuO2 NR compositions are shown

in Figure 6-4. The Raman active 144 cm-1 (Eg), 399 cm-1 (B1g), 513 cm-1 (A1g) and 638 cm-1 (Eg)

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modes correspond to the anatase structure of TNTs.67 The peak related to the Eg mode for all

composites are broader and blue-shifted from 143 cm-1 to 158 cm-1 (Figure 6-4, inset). As

previously discussed, this could be due to the strong interaction between TNTs, RuO2 NR and

hGO. These interaction is important to reduce the electron hole recombination.

Figure 6-5 Characteristic D and G bands of a) GO b) RG-2 c) TRG-3 d) TRG-4 and e) TRG-5

composites.

Raman spectroscopy can be used to characterize the electronic structure of the carbon composites

figure 6-5 shows the characteristic D and G bands located at 1347 cm-1 and 1598 cm-1. The G band

provides information about sp2 carbons and the D band shows the sp3 defect sites.68-69 The intensity

ratio provides the level of defects in carbon material. The calculated Id/Ig ratio for GO is ~1.32

showing the high degree of defect sites, while TRG-5 was 0.953. In this case, the hydrothermal

114

process converts more sp3 hybridized carbons to sp2 hybridized carbons. The blue shift of the G

band can be ascribed to formation of graphene sheets from graphite.

Figure 6-6 Deconvoluted peak of high resolution XPS core level of a) Ru 3d/ C 1s b) Ti 2p of

TRG-5 composite.

XPS analysis helps to determine the chemical compositions of composites and oxidation states of

the ruthenium cations. Figure 6-6 (a, b) shows the XPS spectrum of the TRG-5 composite

(hGO/RuO2 NR/TNT). Figure 6.a shows two prominent bands at 280.1 eV and 285.2 eV, readily

assigned to 3d5/2 and 3d3/2 .70 The Ru 3d peak shifts ~ 1 eV to a lower binding energy compared to

pure RuO2 nanoribbons. Uddin et al. have reported the Ru 3d low binding energy shift when TiO2

deposited on RuO2 (~ 1 eV). X-ray photoelectron valance band spectra was used to determine the

core level binding energy shift and it was proposed that the shift reflect the band bending at the

interface.60 These peaks can also be deconvoluted in to four peaks with two higher binding energy

satellite peaks at 282.2 eV and 284.6 eV. This artifact is due to the strong interaction between the

d-orbital electrons and generated photoionized holes (final state screening effect).71 Foelske et. al

115

reported X-ray photoelectron spectroscopy of RuO2 and obtained an increase in satellite peak

intensity with increasing annealing temperature.72 Kim and coworkers have shown by X-ray

photoelectron spectroscopy that RuO2 is in the Ru+4 oxidation state.73 The peak at 285 eV

overlapped with carbon which can be attributed to hGO. Peaks at 285 eV, 287.1 eV, 289.0 eV,

289.6 eV can be assigned to sp2 C-C, C-O, C=O and O-C=O peaks respectively.17, 34, 60, 74 9. The

high resolution XPS spectra for Ti2p showed two prominent peaks in the Ti2p region (Figure 6.b).

The peak at 459.2 eV and 465.1 eV corresponds to Ti (IV) 2p1/2 and Ti (IV) 2p3/2.75

6.10 Morphology of TNT/RuO2 NR/ hGO composite

The composite materials were characterized using SEM and high resolution TEM. Figure 6-7.a

shows a SEM image of RuO2 NRs grown on hGO sheets. Figure 6-7.b shows the SEM image of

the TRG-5 composite where the surface is fully covered with the interconnected TNTs and RuO2

NRs. The sheet like morphology of the hGO is retained even after hydrothermal synthesis and

annealing (Figure 6-7.b). Figure 6-S.1. shows TEM-EDAX spectrum accrued at 10 nm

magnification. Figure 6-S.2. a and b shows the High resolution TEM images of bulk TNTs. The

high resolution TEM images of TRG-5 in Figure 6-7.c and d show the RuO2 NRs and TNTs are

grown on hGO sheets making an interconnected network. The RuO2 NRs are in the range of ~ 6-

7 nm in width and TNTs are ~ 9 nm in diameter with ~ 6 nm pores. TNTs grown on the hGO

sheets have inner and outer diameters of ∼ 6 nm and ∼ 9 nm that are smaller than TNT prepared

without hGO (~ 10 nm) as shown in Figure 6-S.3. The TRG-5 composite contains the highest

loading of RuO2 NRs and TNTs but a similar morphology was observed for the other composites.

116

The composite materials were characterized using SEM and high resolution TEM. Figure 6-7.a

shows a SEM image of RuO2 NRs grown on hGO sheets. Figure 6-7.b shows the SEM image of

the TRG-5 composite where the surface is fully covered with the interconnected TNTs and RuO2

NRs. The sheet like morphology of the hGO is retained even after hydrothermal synthesis and

annealing (Figure 6-7.b). Figure 6-S.1. shows TEM-EDAX spectrum accrued at 10 nm

magnification. Figure 6-S.2. a and b shows the High resolution TEM images of bulk TNTs. The

high resolution TEM images of TRG-5 in Figure 6-7.c and d show the RuO2 NRs and TNTs are

grown on hGO sheets making an interconnected network. The RuO2 NRs are in the range of ~ 6-

7 nm in width and TNTs are ~ 9 nm in diameter with ~ 6 nm pores. TNTs grown on the hGO

sheets have inner and outer diameters of ∼ 6 nm and ∼ 9 nm that are smaller than TNT prepared

without hGO (~ 10 nm) as shown in Figure 6-S.3. The TRG-5 composite contains the highest

loading of RuO2 NRs and TNTs but a similar morphology was observed for the other composites.

117

Figure 6-7 SEM image of a) RuO2 NRs on hGO b) TNTs on RuO2 NRs, high resolution TEM

image of c) RuO2 NRs and TNTs on hGO sheet (TRG-5) d) RuO2 NRs and TNTs in composite

TRG-5.

Figure 6-S.3 shows the UV-vis spectra of the composites, RuO2 NRs and TiO2 NTs. The pure TiO2

showed a sharp absorption peak at 385 nm, which is characteristic for anatase phase.76 In contrast,

the heterostructure TRG-5 nanocomposites showed an absorption band in the visible range. The

intensity of the peaks became stronger as the RuO2 content was increased. This can be explained

by the excitation of discrete number of electron oscillations in the RuO2.60

The band gap energies

(Eg) of TRG-5, RuO2 NRs and TNTs were calculated using the following equation.

118

𝛼(ℎʋ) = 𝐴(ℎʋ − 𝐸𝑔)𝑛 (1)

where α - absorption coefficient, ʋ - light frequency, Eg - band gap energy, A and n - constant

parameter depending on the nature of the semiconductor.77 78 The indirect bandgap Eg of TiO2 NTs

and direct band gap Eg of TRG-5 composite and RuO2 NRs79 were calculated and compared

(Figure 6-S.4.a, b).

Therefore for the TiO2 NTs the indirect band gap was estimated using Kubelka-Munk theory.80

The direct band gap model was used to estimate the band gap energy of TRG-5 composite and

RuO2 NRs. The band gap energy of TiO2 NTs were estimated to be 3.2 eV which matches the

literature 60 using the indirect bandgap model. The TRG-5 composite band gap was 2.7 eV which

has band gap energy more towards visible spectrum. This shows these composites can absorb

visible light and act as a photocatalyst. Uddin et. al reported RuO2/TiO2 mesoporous

heterostructure with band gap energy 3.15 eV that was active in UV region.60

6.11 Photocatalytic performance of TNT/RuO2 NR/ hGO composite.

To evaluate the photocatalytic efficiency of the TNT/RuO2 NR/ hGO composites, the

photocatalytic decomposition of malachite green oxalate (MGO) was performed as a test

reaction.50 The maximum absorption peak of MGO at 616 nm decreases exponentially and decayed

completely under UV light irradiation after 80 min in the presence of the TNT/RuO2 NR/ hGO

composite. The color of the solution decreases with the time due to the decomposition of the MGO.

Furthermore, similar experiments were carried out using a RuO2 NR/hGO composite and a

119

TNT/hGO composite as controls. The degradation efficiency was calculated using C/C0, where C0

and C are the initial concentration and the concentration at time t, respectively.

In order to explain the effect of the RuO2 NRs/ TNT ratio on the photocatalytic activity, a set of

experiments were conducted using the TNT/RuO2 NR/ hGO composites containing different ratios

of RuO2 NRs/TNTs. The TNT/RuO2 NR/ hGO composites (TRG-5, TRG-4, TRG-3) had a higher

photocatalytic activity than pure RuO2 NR/hGO and TNT/hGO as shown in Figure 6-8. The TRG-

5 composite shows the best performance with a degradation efficiency of 95% after 80 min

exposure to UV light. Perera and coworkers have prepared TNT/hGO composite and it shows 80%

degradation efficiency after 80 min.9 For the TRG-4, TRG-3 samples, the degradation efficiency

was lower, i.e., 90% and 85%, respectively, but still higher than that of pure TNT/hGO (TG-1)

and RuO2 NR/hGO (RG-2), i.e., 40% after 80 min and 20% after 80 min respectively.

The photocatalytic degradation reaction follows a pseudo-first order kinetics −ln(C/C0) = kt, where

k is the apparent reaction rate constant (min−1), C- dye concentration after time t and Co- initial

dye concentration. The highest rate constant for photocatalytic activity (0.9625 min−1) was

obtained for TRG-5, which is greater than TNT/ hGO (0.674 min-1) and ~4.5 times greater than

that of TNTs (0.218 min-1).9 Uddin et. al reported a RuO2/ TiO2 nanoparticle heterostructure that

exhibited a rate constant of 0.239 min-1 for decomposition of methylene blue dye.60 Rate constants

for the RuO2 NR/ TNT composites were TRG-4 (0.6924 min−1), TRG-3 (0.4818 min−1), RG-2

(0.0741 min−1), TG-1 (0.1583 min−1) respectively. With the increase of RuO2 NRs/ TiO2 NTs

120

composition, rate constant of the reactions increased rapidly. This is mainly due to the minimizing

the electron hole recombination rate.

Figure 6-8 Plot of C/Co (%) versus time for the photocatalytic degradation of malachite green in

a quartz reactor.

The photocatalytic mechanism includes several steps. Start with the adsorption of the pollutant to

the catalyst, absorption of light (UV) by the photocatalyst and generate electrons and holes and

undergoing charge transfer reactions to create radicals to decompose the pollutants. Aromatic

industrial dyes are able to form π-π interactions with the graphene.81 This will help to increase the

adsorption process significantly. To obtain enhanced photocatalytic degradation rates, higher dye

concentration near the substrate is an important factor (Figure 6-9, step 1). Under UV light

irradiation, electrons are excited from the TiO2 valance band (VB) to the conduction band (CB)

forming holes in the VB (Figure 6-9, step 2). These electrons and holes (step 2) tend to react with

H2O to create radical oxygen species that can react with dyes to decompose them into small

121

molecules (Figure 6-9, step 3). Fast electron−hole pair recombination (10−9 s) can reduce the

photocatalytic activity. This is due to the reaction kinetic of the pollutants (10−8−10−3 s) on TiO2

is slower than the electron−hole recombination time. Generally, a small amount of electrons and

holes (< 1%) are contributing in photocatalytic reactions, while the majority (99%) of electrons

and holes recombine without engage in any chemical reaction. Therefore, it is essential to control

step 2 and step 3 to enhance the photocatalytic activity by reducing the electron-hole

recombination. In this study, RuO2 NRs has been coupled with TNTs and hGO to reduce the

recombination rate.

Figure 6-9 Graphical illustration of photodegradation of MGO in the presence of RuO2/

TiO2/hGO composite.

The photogenerated charge separation mechanism at the interface of the RuO2 /TiO2 heterojunction

has been discussed by several groups.11, 50, 60 The Fermi level of TiO2 located above the Fermi

level of the RuO2. Therefore, electrons are transferred from TiO2 to the metal like RuO2 when they

are in contact. This will bring Fermi level of TiO2 and RuO2 to the same level forming an electron

depletion region and an upward band bend in the TiO2. When irradiated, electrons from valence

band of TiO2 are excited to the conduction band creating holes in the valance band. Due to the

122

internal electric field at the interface, generated electrons and holes can be separated. Generated

electrons reduce dissolve oxygen to form superoxide radicals (O2’-) and hydroxyl species OH*

which are very strong oxidizing agents that can decompose organic pollutants. Table 6-1 compares

the reactivity of the TNT/RuO2 NR/ hGO composites with related catalysts. TRG-5 composite

shows the exceptional photocatalytic performance among the other composites reported.

Table 6-1 Rate constant comparison for the oxidation of organic dyes

Sample Rate constant (min-1) Reference

TRG-5 0.9625 This study

RG-2 0.0741 This study

TG-1 0.1583 This study

TNT 0.218 Ref 9

TNT/hGO 0.674 Ref 9

RuO2/TiO2 nanoparticles 0.239 Ref 60

RuO2/TiO2 (SiO2 5%) 0.429 Ref 50

RuO2/TiO2 (SiO2 10%) 0.530 Ref 50

TiO2 (SiO2 10%) 0.376 Ref 14

TiO2/ rGO 0.0889 Ref 37

123

6.12 Conclusion

In this study we have prepared TNT/RuO2 NR/hGO composites using a sequential hydrothermal

synthesis method. Their photocatalytic activity was evaluated and the highest loading of RuO2

NR/TNTs on hGO shows the highest photodegradation efficiency of MG (TRG-5). By comparing

TNT/hGO and RuO2 NR/hGO photodegradation efficiencies, it is clear that TNT/RuO2 NR

heterojunction helps to improve the photodegradation efficiency of organic dyes by decreasing the

electron hole recombination rate. This technique for synthesizing TNT/ RuO2 NR/ hGO

heterojunction composites can be used to prepare other metal oxide heterojunction composites for

different applications.

6.13 Supporting information

Table 6-S. 1 Amount of TiO2, RuO2 and hGO in composites

Sample TiO2 (g) RuO2 (g) hGO (mg)

TRG-5 0.3 0.3 10

TRG-4 0.2 0.2 10

TRG-3 0.1 0.1 10

RG-2 0.3 - 10

TG-1 - 0.3 10

124

Figure 6-S. 2 TEM-EDAX spectrum accrued at 10 nm magnification

Figure 6-S. 3 a), b) High resolution TEM images of bulk TNTs.

125

Figure 6-S. 4 UV-vis diffuse reflectance spectra of composites.

Figure 6-S. 5 a) Direct bandgap model of TRG-5 and RuO2 NRs b) indirect bandgap model of

TNTs

126

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BIOGRAPHICAL SKETCH

Wijayantha Asanga Perera was born in Kandy, Sri Lanka in 1984, son to Neel Perera and Jayanthi

Herath. He obtained his Bachelor’s degree in Chemistry (Hons) with Physics and Mathematics

from University of Peradeniya in 2009. Then he started his higher studies in Post Graduate Institute

of Science in University of Peradeniya and he earned his Master’s degree in Nanoscience and

Nanotechnology in 2012. During this time he worked as an assistant lecturer in Physics department

for undergraduate students. In fall 2012, he entered The University of Texas at Dallas to pursue

the degree of Doctor of Philosophy in Chemistry under the supervision of Prof. Kenneth J. Balkus,

Jr. His research was focused on “metal oxide-carbon nanocomposites for energy storage and

conversion.”

CURRICULUM VITAE

Wijayantha A. Perera

2200 Waterview Pkwy, 27210, Richardson, TX, 75080

[email protected]

https://www.linkedin.com/in/wijayanthaperera

(972) 876-6803

MATERIALS CHEMIST

Enthusiastic, goal-oriented scientist with excellent technical and nontechnical skills

Motivated chemist with interdisciplinary research experiences. Thrives in fast-paced

environments. Good interpersonal skills, and can work independently or as part of a team.

Reliable and energetic. Clear oral and written communication skills. Critical thinker with the

ability to develop innovative solutions. Quick learner with the desire to work in any field to gain

experience with scientific knowledge and skills in inorganic chemistry, analytical chemistry,

nanoscience and nanotechnology.

Certified User of Cleanroom Class 10,000 | HR-TEM | TEM-EDX | SEM | XRD | FTIR | Raman

| UV-Vis | Fluorescence | PLD | TGA | CV | HPLC | HPLC-MS | LC-MS | GC-MS |

PROFESSIONAL EXPERIENCE

University of Texas at Dallas, Texas

2012-2017 (May)

Graduate Assistant, Advisor: Prof. Kenneth J. Balkus, Jr.

Developed high efficient photocatalytic composite materials for water purification and

degradation of organic dyes by sequential synthesis of titanium oxide nanotubes, ruthenium

oxide nanoribbons and hydrothermally reduced graphene oxide.

Conducted sequential hydrothermal synthesis of composites to enhance photocatalytic

efficiency.

Designed, synthesized and conducted photochemical reaction to evaluate the

photocatalytic activity.

Developed a novel methodology to prepare sodium niobate nanotubes and nanorods. Designed,

synthesized and evaluated the sodium niobate nanotubes and nanorods composite materials with

graphene, graphene oxide and carbon nanotubes. Fabricated device prototypes using the

materials developed and evaluated the electrochemical properties.

https://www.linkedin.com/in/wijayanthaperera

Synthesized composite materials using hydrothermal synthesis.

Studied the structural and electrochemical behavior of nanostructures.

Designed and fabricated devices to analyze electrochemical performance using cyclic

voltammetry, charge/discharge characteristics and impedance spectroscopy.

Developed a new synthetic method to prepare different morphologies of ruthenium oxide

nanostructures to enhance the pseudocapacitive properties of supercapacitors (nanoribbons,

quantum dots).

Developed composite electrode with carbon nanotubes.

Developed a novel technology to decorate ruthenium oxide quantum dots on vanadium

oxide nanowires for high performance supercapacitor.

Studied the surface modification of vanadium oxide nanowires and carbon materials

for supercapacitor electrodes using ruthenium oxide quantum dots.

Developed novel methods to prepare pseudocapacitive carbon nanofibers by electrospinning

polymer solution polyacrylonitrile (PAN) with different metal oxide and nitride nanostructures.

In addition to the thesis projects mentored undergraduate students by teaching lab skills and

chemical concepts. Also conducted regular maintenance and repairing of lab equipment. Served

as the laboratory manager to get quotations and order laboratory supplies.

Post-Graduate Institute of Science, University of Peradeniya, Sri Lanka

2011-2012 Assistant Lecturer

Conducted nanoscience and nanotechnology laboratory courses (CHN 507 and CHN 508).

Post-Graduate Institute of Science, University of Peradeniya, Sri Lanka

2010-2012 Post-Graduate Researcher, Advisor: Prof. R.M.G. Rajapakse

Synthesized hydroxyapatite nanoparticles and investigated their mechanical properties for bio-

active prosthesis.

Department Of Physics, University Of Peradeniya, Sri Lanka

2010-2012 Assistant Lecturer

Instructed sophomore, junior and honors physics laboratory courses (PH103, PH204 and PH348)

through preparing and leading lab experiments, grading, and holding office hours.

SKILLS

Synthesis, nanofabrication, prototype fabrication and characterization of nanomaterials.

Characterization and laboratory techniques – Certified and trained to use cleanroom class

10,000 | Transmission electron microscopy | transmission electron microscopy-EDX | scanning

electron microscopy | X-ray diffraction spectroscopy | Raman spectroscopy | Fourier transform

infrared spectroscopy | Ultraviolet–visible spectroscopy | Pulsed laser deposition |

Thermogravimetric analysis |

Software – MS Office | OrginLab | Scifinder | ChemDraw | Crystal Maker | Material Studio |

Jade 9 | 3D Modeling with Autodesk Fusion 360 | Sketchup 2015

EDUCATION

PhD. Chemistry, Material Science.

2012-2016 University of Texas at Dallas, TX

M.S. Nanoscience and Nanotechnology

2010-2012 Post Graduate Institute of Science, University of Peradeniya

PROFESSIONAL AFFILIATIONS AND ACTIVITIES

Research collaborator at Laboratory for Surface and Nanostructure Modification, Department of

Material Science and Engineering, University of Texas at Dallas.

2012 – present Research collaborator at the Alan G. MacDiarmid Nanotech Institute, University of Texas at

Dallas.

2012 - present Member of the American Chemical Society, ACS Division of Inorganic Chemistry.

2012 - present Vice President of Sri Lankan Students’ Association, University of Texas at Dallas.

2014 - present

PUBLICATIONS

1) Wijayantha A. Perera, Yuzhi Gao, John Ferraris, Yves Chabal, Kenneth Balkus Jr.

“Hydrothermal synthesis of TiO2 nanotube (TNT)/ RuO2 nanoribbons (NRs)/ Graphene

composite for enhanced photocatalytic activity for water purification”. Manuscript

submitted.

2) Wijayantha A. Perera, Imalka Munaweera, Michel Trinh, Yuzhi Gao, John Ferraris, Yves

Chabal, Kenneth Balkus Jr. “Binder free Graphene–Sodium niobate nanotubes/ nano-rods

composite electrodes for supercapacitors”, ACS Appl. Mater. Interfaces, 2015, Manuscript

submitted.

3) Wijayantha A. Perera, Yuzhi Gao, John Ferraris, Yves Chabal, Kenneth Balkus Jr.

“Ruthenium Oxide nanoribbons for the supercapacitors” Manuscript submitted.

4) Wijayantha A. Perera, Yuzhi Gao, John Ferraris, Yves Chabal, Kenneth Balkus Jr. “RuO2

quantum dot decorated V2O5 nanowires for high performance supercapacitors”.

Manuscript submitted.

5) Zijie Wang, Wijayantha A. Perera, Sahila Perananthan, John P. Ferraris, and Kenneth J.

Balkus, Jr.*. Lanthanum Oxide Nanorod Templated Graphitic Hollow Carbon Nanorods

for Supercapacitors. ACS Energy Letters. In review.

6) Nimali C. Abeykoon, Velia Garcia, Rangana A. Jayawickramage, Wijayantha Perera,

Jeremy Cure, Yves J. Chabal,,Kenneth J. Balkus, Jr. and John P. Ferraris*, Novel binder-free

electrode materials for supercapacitors utilizing high surface area carbon nanofibers

derived from incompatible polymer blends of PBI/6FDA-DAM:DABA, RSC Advances,

2017, 7, 20947–20959.

Wijayantha A. Perera [email protected]

CONFERENCE PROCEEDING – RESEARCH PRESENTATIONS

W. A. Perera, K. J. Balkus Jr .; “ High surface area electrodes for supercapacitors using

polyacrylonitrile, tetracyanobenzene with the presence of zinc chloride” 46th ACS DFW Meeting

in Miniature, University of A&M (April 2013, Dallas TX)

W. A. Perera, K. J. Balkus Jr .; “ Niobium Oxide nanowire – Graphene binder free

nanocomposite flexible paper electrode for supercapacitors” ACS Southwest Regional Meeting

(November 2013, Waco, Dallas TX)

W. A. Perera, K. J. Balkus Jr .; “High surface area and redox active electrodes for

supercapacitors using lithium phthalocyanine and zinc phthalocyanine” 247th ACS National

Meeting and Exposition (March 2014, Dallas TX)

W. A. Perera, K. J. Balkus Jr .; “ Binder free graphene- sodium niobate naanotubes/ nanorods

composite electrodes for supercapacitors” ACS Southwest Regional Meeting (November 2014,

Fort Worth TX)

W. A. Perera, K. J. Balkus Jr .; “TiO2 nanotube/ RuO2 nanoribbons/ hGO composite for

photocatalytic activity” The International Chemical Congress of Pacific Basin Societies 2015

(December 2015, Honolulu, Hawaii)

metal oxide–carbon nanocomposites for energy storage and

Documents