himmelspach - us china 2009

8/14/2019 Himmelspach - US China 2009

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Chemical and physical synthesisof TiO2-based nanocomposites for

solar energy production and otherenvironmental applications

US China Workshop

17 October 2009

Kimberly A. Gray

Institute for Catalysis in Energy Processes,Civil & Environmental Engineering, Chemical & BiologicalEngineering,

Northwestern UniversityEvanston, IL

CB

VB

e-

h+

et

ht

O2

O2-

OH-

OH

E hv


2/71

The Photocatalysis Challenge: SolarFuels

Basic Research Needs for Solar Energy Utilization", Report of the 2005 BasicEnergy Sciences Workshop on Solar Energy Utilization, US Department of Energy.


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Shift photoresponse into visible light region

Increase photoefficiency Retard

recombination

Target chemical reaction - CO2 reduction to CO,CH4 or

CH3OH (catalytic active sites or hot spots)

CO2 Redox Potential: - 1.9 V NHE (single electron

transfer)

- 0.103 V NHE (presence of H+-donor)

Challenge for Solar Energy Capture,Conversion & Storage

CO2 + 2H+ + 2e- CO + H2O

CO2 + CH3CH=CH2 CO + CH3CH-CH2

O


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CO2 adsorption/ CO2 speciation

Role of water

- Hole scavenger

- Proton donor

- Hydroxylate catalyst surface

- Solvate CO2 (H2CO3, HCO3-, CO3

2- )

Hydrogen management (H+

, H

, H-

) Reaction mechanism & retard back-reactions

Multiple electron transfer reaction/product

control

Photoefficiency how high can we push it. . .

But, there are some other issues, as

well:


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H2O H2 + O2 CO2 CH4 + CH3OHFujishima and Honda, Nature1972, 238, 37

Inoue et al., Nature1979, 277, 637Gratzel, Nature2001, 414, 338

TiO2 Photocatalysis for Solar Fuel Generation

Anatase

Ebg = 3.2 eV; m = 385 nm

Rutile

Ebg = 3.0 eV; m = 415 nm

ruby.colorado.edu/~smyth/min/tio2.htmlhttp://www.che.kyutech.ac.jp/chem23

Brookite


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-0.85 CO2/HCOOH

-0.76 CO2/CO

-0.72 CO2/HCHO

-0.66 H+/H2

-0.62 CO2/CH3OH

-0.48 CO2/CH4

pH = 7Anatase

(bulk)

Potential

/V

vsSCE

+2.50

3.23 eV-0.50

-1.00 -0.85 CO2/HCOOH

-0.76 CO2/CO

-0.72 CO2/HCHO

-0.66 H+/H2

-0.62 CO2/CH3OH

-0.48 CO2/CH4

pH = 7Anatase

(bulk)

Potential

/V

vsSCE

+2.50

3.23 eV-0.50

-1.00

Inoue et al, Nature1979, 277, 637; Yoneyama, Catal. Today1997, 39, 169

Relative energies for CO2 reduction


7/71

Conceptual Picture

e-

e-

h+

CB

VB

hv

ht

CB

VB

et

ht

et

Rutile Anatase

e-h+

1 2

3

Solid-Solid Interface -

Key to Highly Efficient & Reactive Photocatalysts

Location of adlineation or defect sites

4


8/71

EPR studies of P25 photochemistry

Electron trapping sites are rutile and anatase lattice sites

2.10 2.05 2.00 1.95 1.90 1.85

g

anatase, UV(black) and visible (red)

rutile, visible illumination

Degussa P25 Visible

UV

anatase: g= 1.990

rutile: g= 1.975

anatase: g =1. 957

rutile:g =1.940

Hurum, Gray, Rajh, Thurnauer. J. Phys. Chem. B2003, 107, 4545


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~0.45 m

2.05 2.00 1.95 1.90

g

~1.5 m

P25 Slurry

~0.2 m

Hurum, Gray, Rajh, Thurnauer, J. Elect. Spect. 2006, 150, 155 ; J. Phys. Chem. B2003, 107,

4545

Better electron transfer

Higher photoactivityEPR spectra of size-fractionated P25

Photochemistry of P25 (1): NanostructuredAssembly

Anatase

1.990Rutile1.975

Visible light

(> 400 nm)


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Direct evidence of surface dominated recombination

Intensity =517.6 au

Intensity =1197.5 au

Intensity=922.3 au

Intensity=1354.3 au

2.10 2.05 2.00 1.95 1.90 1.85

g

Interfacialg=1.979

Anatase

surfaceg=1.930

Recombination is dominated by surfacereactions.

Experimental evidence of an interfacial electrontrapping site.

The mechanisms of recombination is randomflight.

Anatase

TiO O

O O

O

O

TiO O

O OO

OTi

OO

O


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Conceptual Picture

e-

e-

h+

CB

VB

hv

ht

CB

VB

et

ht

et

Rutile Anatase

e-h+

1 2

3

4

Extend photoresponse into visible. Spatially separate and stabilize charge. Create interfacial sites having unique chemistry and

reactivity - adlineation (defect) sites.

The high activity of mixed phase TiO2 due to

rutile-anatase interactions that:

c


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Current Work

Hypothesis:

The solid-solid interface in nanocomposite materials iskey to overcoming the three grand challenges of TiO2

photocatalysis

(high activity, tailored chemistry, visiblephotoresponse)*

Methods to synthesize materials with high densitiesof

solid-solid interface & controlled defects

- Solvothermal method for TiO2nanocomposites; Low T, tune phase composition by HCl:H2O**

- Reactive DC magnetron sputtering - target power,substrate bias, oxygen partial pressure, and

deposition angle***

* G. Li, K.A. Gray (2007). The Solid-Solid Interface: Explaining the High and Unique Reactivity of TiO2-based NanocompositeMaterials. Chemical Physics,

339:173-187.

**G. Li, K.A. Gray (2007). Preparation of Mixed-phase Titanium Dioxide Nanocomposites via Solvothermal Processing. Chemistry of Materials, 19:1143-

1146.

***Chen, Graham, Li and Gray, Fabricating Highly Active Mixed Phase TiO2 Photocatalysts by Reactive DC Magnetron Sputter Deposition,Thin Solid Films

2006, 515, 1176-1181.

0

20

40

60

80

100

0 10 20 30 40 50

A%R%B%

WeightPerc

HCl/Ti molar ratio


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Thermal treatmentAnatase (A)

RA

A

R

Rutile (R)

Tetrahedral Ti Sites During PhaseTransformation

Li, Dimitrijevic, Chen, Nichols, Graham, Rajh and Gray, JACS.2008, 130:5402-5403.


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CO2Sampling

Port

Cooling Water

Stir Bar

CO2Sampling

Port

Cooling Water

Stir Bar

Mixed-Phase TiO2: Highly Active

PhotocatalystsUV, isopropanol

A+R 773 K

A+R 373 K

3100 3200 3300 3400 3500 3600

A R

Field (Gauss)

UV

Li, Ciston, Saponjic, Chen, Dimitrijevic, Rajh and Gray, J. Catal.2007, 253:105-110.


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Preparation of mixed phase TiO2 thin film by

magnetron sputtering

Cryo Pump

Load Lock

Rotating Substrate Holder

Valve

FlowMeter

FlowMeter

Valve

O2

BaratronFlowController

Ar

Valve

MassSpectrometer

Slave FlowController

Slave FlowController

MasterController

FlowMeter

CryoPump

Target Target

Pulsed dcPower

rf/Pulseddc Power

Pulsed dcPower

Schematic of dual-cathode unbalanced

magnetron system

Cryo-pumped chamberand load lock

Closed-field unbalancedmagnetron targetarrangement (13cm x 38cm targets)

Arc suppression (pulsedpower) for targets

RF power for substrate Mass spectrometer control

of reactive gas partialpressure

Rotating substrate table

Target: pure titanium; Substrates: glassslides


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Process Effects for TiO2Conditions XRD results

Low angle deposition,

Power: 3kW, Bias: 300W,

TP: 6mTorr, Anneal: 400C (1 hr)

Pure anatase

Normal deposition,

Power: 5.8kW, Bias: 150W,

TP: 3.5mTorr, No anneal

Almost pure rutile

Low angle deposition,



Mixed phase.More anatase

Normal deposition,



Mixed phase,more rutile

xrd f or pure anatase made by pvd metho

0

1000

2000

3000

4000

5000

6000

20 30 40 50 60

2 thet a

intensity(counts)

xrd for ruti l e PVD sampl

0

100

200

300

400

500

600

700

800

900

1000

20 30 40 50 60

2 theta

intensity(counts)

xr d f or mi xed phase PVD (

0

500

1000

1500

2000

2500

3000

20 30 40 50 60

2 thet

intensity(counts)

xr d f or mi xed phase PVD(

0

500

1000

1500

2000

20 30 40 50 60

2 thet

intensity

(counts)


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The morphology and structure of the films

TEM figures

TEM results

A(110)

A(112)

R(101)

TEM plan-view image of themixed phase film preparedby magnetron sputtering

(JEOL JEM-2100F FAST TEM )

TEM plan-view selected areadiffraction pattern of the sputteredmixed phase film

(Hitachi H-8100 )

Anatase crystals and rutile crystals are completely mixedtogether, indicating a high density of rutile-anataseinterfaces were created.


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Comparison of Acetaldehyde Decay (UVirradiation)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50

time(min)

C/C0

mixed phase magnetron sputtering (71%1.12um2/um2

anatase magnetron sputtering1.10um2/u

P25 dip coating 2.18um2/um2

mixed phase solgel (70%A) 1.09um2/um

L. Chen, M.E. G raha m, G. Li, K.A. Gra y (2006) Fabrica ting Highly Ac tive Mixed Phase T iO2

Photocatalysts b y React ive D C M agnetron Sputter Deposition,Thin Solid Films

, 515(3):1176-1181.


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0. 00E+00

1. 00E+04

2. 00E+04

3. 00E+04

4. 00E+04

5. 00E+04

6. 00E+04

7. 00E+04

420 440 460 480 500 520 540 560 580 600

energ loss/ eV

counts

TEM results

100 nm

TEM Cross section, 70% A

(Hitachi HF-2000 )

Electron Energy

Lossspectroscopy

results

T

N

T

N

T: in the column

N: at the interfaces of the columns

Ti-L edge

(mixture of Ti3+ and Ti4+ )

O-K edge

More oxygen at point T (Ti:O=44.2: 55.8)

than point N (Ti:O=70:30);

Ti-L edge is shifted to low energy state

for point N lower Ti-valence.

More oxygenvacancies at the

interfaces of the

columns

50nm


20/71

CO2 reduction results

UV condition (mercury vapor lamp

100W)+ water added as holescavenger

Film (1): O2: 0.07 pa,with minimumnitrogen

Film (2): O2: 0.07 pa,with no nitrogenFilm (3): O2: 0.08 paFilm (4): O2: 0.035 paFilm (5): O2: 0.12 pa

No detectionof methanefor P25coated film

Film(1)

Film(3)


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Photoefficiency Comparison

Kaneco, S., et al., Photocatalytic reduction of high pressure carbon dioxide using TiO2 powderswith a positive hole scavenger. Journal of Photochemistry and Photobiology a-Chemistry, 1998.115(3): p. 223-226.

0.00%

0.01%

0.02%

0.03%

0.04%

0.05%

0.06%

1 2 3 4 5

Photoefficiency

1. Mixed phase low angle film with oxygen vacancies under UVlight

2. High angle mixed phase film under UV light

3. Titania nanotubes (TiNT) under UV light4. Mixed phase film with oxygen vacancies under visible light5. P25 suspended in isopropyl alcohol illuminated with light

>345nm with about 28atm of CO2 (Kaneco, et. al)

Continued work with TiNT - > 0.1% conversion efficiency, butcatalyst deactivation


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X-ray diffraction patterns for TiO2 and

Ti1-x NbxO2 thin films. Films are prepared

under idential sputtering conditions withvarying concentrations of Nb.

UV-visible absorption spectra for TiO2

andTi1-x NbxO2 thin films

Nb additions promote the growth of

rutileAS PREPARED films are amorphousabove 32%NbCrystalline films with 40%Nb have beenprepared upon post deposition annealingat 550C

0% Nb, 81% anatase

15% Nb, rutile

9.6% Nb, 17% anatase

45%Nb, amorphous

Nb-doped films have red-shifted

photoresponsePhotocurrent measurements confirmbandgap excitation with visible light

Film growth and phaseformation

Optical response

Nb-doping to red-shift photoresponse


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Conclusions Solid-solid interface

- facilitates charge transfer and separation; hindersrecombination, increases photoefficiency

- location hot spots (tetrahedral Ti4+ , oxygen deficiencyTi3+ )

By varying sputtering power, substrate bias, totalpressure, depostion angle of reactive DC magnetronsputtering

- prepared a variety of TiO2 films having high interfacialdensities & with different phases, structures,

function.

Mixed phase sputtered film proved to be superior tothe other films as measured by oxidation andreduction reactions.

- The surface of the mixed phase sputtered TiO2 film has

columnar structure composed of well mixed and


24/71

Techniques to synthesize TiO2-based composites -

Second Generation TiO2-x photocatalysts

- Oxygen vacancies help to extend the catalysts light responseto the visible light range while maintaining the high activity of thecatalysts. They are located mainly at the interfaces of the columns.

- There is an optimum pN2 stabilizes oxygen control and films

The ability to reduce CO2

to fuel under visible light provides thelarge potential for both environmental and energy areas.

Other environmental applications for Titania nanocomposites:

- Photocatalytic ceramic ultrafiltration membranes for watertreatment

- Photocatalysts for gas phase chemical oxidation for airtreatment


25/71

Northwestern University Le Chen, Gonghu Li, Shannon Ciston, Paul Desario Dr. Michael Graham (Materials Science and Engineering) Yuan Yao, Prof. Richard Lueptow (Mechanical Engineering) Drew Gentner (undergrad), Jamie Nichols (RET program) NUANCE, MRSEC, ASL

Argonne National Laboratory Dr. Nada Dimitrijevic Dr. Tijana Rajh Dr. Zoran Saponjic

Degussa for their generous donation of P25U.S. National ScienceFoundationU.S. Department of Energy

Honeywell Corporation

Acknowledgements


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TiO2 can only utilize ~5% of the solar spectrum

http://www.globalwarmingart.com/images/thumb/4/4c/Solar_Spectrum.png/400px-Solar_Spectrum.png


27/71

CB

VB

e-

h+

et

ht

O2

O2-

OH-

OH

Ehv

Artificial Photosynthesisvs.

Water splitting or PV

Why TiO2

- new focus

(solid-solid interface &reduction)

Understand relationshipbetween

synthesis-structure-function


28/71

Why do mixed phase TiO2

materials show such highphotoactivity?

- Degussa P25 used as model- Flame hydrolysis of TiCl4

- Composition approximately80% Anatase20% Rutile

- Anatase generally considered the

active phase (385 nm, 3.2 eV)- Rutile comparatively less activecatalytically (410 nm, 3.0 eV)

Why does mixing an inactivewith active phase yield higheractivity material?

Previous Work:

TiO O

O O

O

O

TiO O

O OO

1.980

(Ti-O)

1.949 (Ti-O)

TiO O

O O

O

O

TiO O

O OO

1.980 (Ti-O)

1.934 (Ti-O)

a,b =3.7842 c= 9.5146

Anatase

Rutile

a,b =4.5845 c= 2.9533

University of Colorado Mineral Structure Database

T ti t l d l f


29/71

Two competing conceptual models ofactivity

Rutile acts as an electron sink for anatase improving

activity by separating the hole and the electron. OR Rutile acts as an electron source for anatase

improving activity by both separating charges andextending the photoresponse of the catalyst.

h+

CB

VB

e-

hv

ht

CB

VB

et

ht

e-et

Rutile

Anatase

Anatase Anatase

Hurum, D. et.al. J. Phys. Chem. B,107, 4545 (2003)

Rutile

Rutile

Anatase

e -

+

Bickley, R. et al. J. Solid State Chem. 92, 178-190 (1991)

Mixed PhaseTiO2

Leytner, S., Hupp, J. Chem. Phys. Letter,330, 231 (2000)

Mi d Ph TiO Hi hl A ti


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Mixed-Phase TiO2: Highly Active

Photocatalysts

A+R 373 K

500 nm

Aldrich

Anatase

A+R 773 K

Hydrothermal

773 K

Li, Ciston, Saponjic, Chen, Dimitrijevic, Rajh and Gray, J. Catal.2007, 253:105-110.

Mixed Phase TiO : Magnetron


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Mixed-Phase TiO2: Magnetron

Sputtering

Chen, Graham, Li and Gray, Thin Solid Films2006, 515, 1176

Sampleholder

Titarget

20 25 30 35 40 45 50 55 60

2-Theta (degree)

A

R

200 nm 200 nm


32/71

Conceptual Picture

Solid-Solid Interface Non-stoichiometryLocation of Undercoordinated Ti & Oxygen-

deficiency

e-

e-

h+

CB

VB

hv

ht

CB

VB

et

ht

et

Rutile Anatase

e-h+

1 2

3

4

TiO2Ti

TiO2(phase

1)

MOx(phase

2)

TiO2-x

Ti

O

M

Chen et al. 2006. Thin Solid Films, 515(3):1176-1181; Chen et al. 2009. Thin Solid Films, in press; Chen et al 2009.

JVST-A, in press; Li & Gray (2007). Chem. Mater., 19:1143-1146; Li & Gray, 2007. Chemical Physics, 339:1-3:173-187; G. Li, N.M. Dimitrijevic, L. Chen, J.M. Nichols, T. Rajh, K.A. Gray (2008)JACS, 130:5402-5403.


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Proposal: A possible

pathway of CO2 reduction

33

Series of surfaces (progressing from model (110 rutile) to morerelevant (101, 001 anatase, nanocubes)

H+ H

H-

HCOOH

H- H-H+e-

H

O

H

O

H

O

TiO2

e-

e-

e-

H+

H+

H+

H+

e-

e-e- e-

-H2O H2COCO2

H

e- H+

H+ H+H

H

CH3OH CH4-H2O

H

e-


34/71

Standard Magnetron

Intensifies plasma at target

surface for higher sputtering

rate Minimizes heating of

substrate from ions and

electrons

Unbalanced Magnetron

Increases plasma density

close to substrate Increases numberof ions

hitting the substrate

Closed field further intensifies

plasma at substrate

(-) V BiasPump

VacuumChamber

Plasma

Ar O2

-V -VN S

substrate

Target(cathode)

Argon

atoms

MagnetAssembly

EDislodged Tiatoms

Argon ions and

electrons form

plasma

Preparation of mixed phase TiO2 thin film by

magnetron sputtering


35/71

Changing the band-gap structure of

TiO2

N. Serpone / J. Phys Chem B 2006, 110, 24287-24293

a) Bandgap of TiO2b) localized dopant levels near VB and CBc) band gap narrowing due to broadening of VB,d) localized dopant levels and electronic transitions to CBe) electronic transitions from localized levels near VB to corresponding excited states for Ti3+ and

F+ centers


36/71

SEM results

SEM (Leo)

Low angle, 70% A film

Low angle, 70% A film,

with oxygen vacancies

High angle, 70% A film

High angle, N-doped film

Columnar structure

Similar to Anpos

structures -

mixedphase column;

Anisotropy key to

optical and electronic

properties;

Increasing visible light

absorption withdecreasing O:Ti


37/71


38/71

Solar FuelGeneration

Methane production under UV light for films with different

oxygen partial pressure

0

20

40

60

80

100

120140

160

0 50 100 150 200

time (min)

m

ethaneproduction(umol)

O 4 E-8 minium N 1.3 E-9

O 3.7 E-8 minimum N: 2.3E-9

O 3.7 E-8 N 8E-9

O 4E-8, no N, not stable

O 4E-8, N 4E-9

O 4.2E-8, No N

Methane production after 4 hours under

visible light illumination

8.90

13.90

18.60

22.62

15.01

9.64 9.94

5.57

0.00

4.60

0

5

10

15

20

25

30

O:5E-

8

O:4

.6E-

8

O:4

.2E-

8

O:4E-

8,not

stable

O:4E-

8,N:1

.3E-

9

O:4E-

8,N:4E-

9

O:3

.5E-

8,N:3E-

9

O:3E-

8,N:6E-

9N>

OP2

5

O and N partial pressure gradient,65-80%A

mehaneproduction(umol)


39/71

GOAL Delineate details of mechanism

39

COgas CH4CH3OH

C2H4, C2H6HCOO-

CO2H+

(H)(e-) C3, > C3(H)

(:CH2)

n

H2C CH 2

:CH2

2e- 2H+

(H)H2C

-H2O

4H+ + 4e-(H)

slow

2H+ + 2e-

-H2O

e-H

H

CO22e-

H+

O

C

COads

(H)

Starting point:

Simplified Reaction Network at an ElectrodeSurface

?

Centi et al. 2007, Green Chemistry, 9, 671-678; Gattrell, Gupta & Co, 2006, J. Electroanal.Chem., 594, 1-19.

eac on resu s - e ane


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eac on resu s e aneProduction

CH4 producti on

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150

ti me (mi n)

CH4umol/surfacearea(

70%A sputtered(H)

5%A sputtered

P25

70%A sputtered(L)

pure A sputt ered (L)

With addition of I-PrOH

Normalized by surface

area

CH4 Production @180 min.

0

10

20

30

40

50

60

70

80

90

100

P25 A (L) 70% A (L) 70% A (H) 5% A (H)

CH4production(nmo/mm2)

Figure 3. CH4 production of sputtered films and P25

UVexcitation

Chen, Graham, Li, Gentner, Dimitrijevic and Gray, Thin Solid Films (2009). Photoreduction of CO2 by TiO2 Nanocomposites

Synthesized through Reactive DC magnetron Sputter Deposition, Thin Solid Films

Optimization of Methane Production

0

50

100

150

200

250

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (hr)

Methane(umol)

Exp 1

Exp 2

Exp 3

Exp 4


41/71

Objectives:1. Determine effect of Nb on film growth and phase formation2. Characterize films structurally and optically

XRD, SEM, AFM, XPS, DR, etc.1. Determine solubility limit for Nb in TiO

2

Goal:

To determine if Nb substitution in the TiO2 lattice is aneffective way to red shift the photo response of thematerial without deleteriously modifying itsphotochemical properties.


42/71

Solubility Preparation

Chargecompensation

Reference

>10at.% in anatase Sol-gel - Sacerdoti, Ruiz,Traversa

>40at.% in anatase DC co-sputtering

Low conc.: cationvacancy

High conc.: both Nb andTi reduced

Sheppard

>1at.% in rutile singlecrystals Diffusionaltechnique Low conc.: Ti reduced(Ti3+ )High conc.: Nb & Ti

reduced

Morris

40at.% in rutile MBE Nb substitutes as Nb4+ Gao

6.6at.% in rutile heattreatment

Nb substitutes as Nb5+ , Tireduced (Ti3+ )

Valigi

>10at.% in anatase,6% in rutile

Sol-gel Cation vacancies RuizNb doping: charge compensation1) One Ti4+ vacancy for every four Nb5+ ions2) One Ti4+ reduced to Ti3+ for every Nb5+ ion (or Nb5+ reduced to Nb4+ )3) One oxygen interstitial for two Nb5+ ions

L. Sheppard et al. / Thin Solid Films 510 (2006) 119-124


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Future Research Directions Does Nb segregate in rutile phase or at interphases?

LEAP (Local-Electrode Atom-Probe) tomography

Can we synthesize films at high concentrations of Nb withhigh anatase content? Post-deposition treatment

What is the mechanism of charge compensation in ourfilms?

Does red-shift in absorption lead to higher visible lightactivity? Photoreduction of CO

2and photooxidation of acetaldehyde


44/71

What is EPR?

Electron ParamagneticResonance (EPR) is aspectroscopic method whichcan be used to detect,identify, and quantifyparamagnetic species.

A single electron has a spinof +1/2 or -1/2, a doubletstate.

Any nonzero spin can beobserved. ie... triplet statesin phosphorescence, organicradicals, and transitionmetals.

O

Cl

Cl

Cl H

H

Ti

O

Ti
http://www.northwestern.edu/


45/71

Titanium Dioxide/Carbon Nanotube

Composites for Photo-reactive Filtration

Speaker: Yuan Yao

Advisor: Prof. Richard M. Lueptow

Co-advisor: Prof. Kimberly A. Gray

Colleagues: Dr. Gonghu Li

Shannon Ciston

Funded by NSF

Background
http://www.northwestern.edu/


46/71

Not all membrane filtration processes can remove organic compoundseffectively.

Organic compounds

Problem 1 BiofoulingAccumulation and growth of microbial communities (biofilms) on the

membrane surfaces.

Ref:

Biofouling in water treatment, in Biofouling and Biocorrosion in Industrial Water Systems, edited by H.C. Flemming (Springer-Verlag,Berlin, 1991), pp. 47-80.

www.micromemanalytical.com/ bacAA/bactAA.htm

Membrane filtration

EPS(extracellular polymeric substances)

Microorganisms

algaeTEM image of biofilm(Transmission Electron Microscropy)

widely used for water purification.

Disinfection by-products (DBP)

Biofouling

Problem 2 Organic compound removal

Background
http://www.micromemanalytical.com/bacAA/bactAA.htmhttp://www.micromemanalytical.com/bacAA/bactAA.htmhttp://www.micromemanalytical.com/bacAA/bactAA.htm


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TiO2

Electrons transfer reduced recombination

enhanced reactivity

Energ

y(eV)

Ref:D. C. Hurum, et al. J. Phys. Chem. B 107:4545-4549, 2003.

Example: Degussa P25 (composed of 70-80% anatase, 20-30% rutile)

High photocatalytic activity

Mixed-phase TiO2:

Mechanism:

TiO2 photocatalysis

CB

VB

e-

h+

Carbon nanotube

?

Background


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Synergistic effect enhanced photocatalysis

Oxidation of organic compounds & inactivation of microorganisms

Porous structure as filtration media

Reduction of biofouling

Objective:

TiO2coated on MW-CNTsMixture of TiO2& MW-CNTs

New

SW-CNTs attached on TiO2

To fabricate a TiO2/CNT composite with enhanced photocatalytic action

for membrane filtration.

10mg photocatalyst

Results


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Phenol degradation testPhenol degradation test

in a suspension:in a suspension:

10mg photocatalyst

100ml phenol (C0=400M)

Mercury UV lamp , (100W, with intense

lines at 366, 436 and 549nm)

0

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 90

Irradiation Time (minutes)

C/C0

P25 control

TiO 2 (100nm) control

20:1

SWCNTs control

TiO2 (100nm) /SW-CNTs

Results


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0

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 90

Irradiation Time (minutes)

C/C0

P25 control

10:1

TiO 2 (100nm) control

20:1100:1

SWCNTs control

Mixture 20:1 control

TiO2 (100nm) /SW-CNTs

Phenol degradation testPhenol degradation test

in a suspension:in a suspension:

10mg photocatalyst

100ml phenol (C0=400M)

Mercury UV lamp

Results


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TiO2(100nm)/MW-CNTs TiO2(100nm)/SW-CNTs

Lower degree of contact Greater degree of contact

MW-CNT diameter:

20-30nm

SW-CNT diameter:

~1.4nm (individual)

2-10nm (bundles)

Results


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TiO2(5nm)/SW-CNTs TiO2(5nm)/MW-CNTs

TiO2(5nm)

huge clumps bad reactivityTiO2 (5nm) and its composites

Results


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Laser activation

PL spectra collected

TiO2 has a broad

peak at ~ 540nm due

to recombination

Photoluminescence (PL) test:Photoluminescence (PL) test:

TiO2(100nm)/CNTs composites have reduced recombination.

-200

0

200

400

600

800

1000

475 500 525 550 575 600 625 650

Wavelength (nm)

Luminescen

ceIntensity(arb.

units)

TiO2(100nm)TiO2(100nm)/SWCNTs

SWCNTs

100:1 20:1TiO 2(100nm)/MWCNTs

Results


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Dip-coating results:Dip-coating results:

TiO2(100nm)/SW-CNTs 100:1

TiO2(100nm)


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Experiment

Exhaust GC FID He

He

CO2

Pump

Six-port Valve

Reactor

H2O

Exhaust GC FID He

He

CO2

Pump

Six-port Valve

Reactor

H2O

CO2 reduction reactor

Light condition:

a Black Ray UV lamp: The UV lamp provided light primarily at a

wavelength of 365nm and an energy density of ~1mW/cm2

A solar light lamp (SVLVANIA, 20W)A solar light lamp (SVLVANIA, 20W)

I-PrOH was added in some cases as a hole scavenger


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increasing magnetic field

h = gHE

In the absence of a magneticfield, the two states for theelectron are at the sameenergy.

In the presence of themagnetic field, the interactionof the magnetic moment withthe external field splits the

two states apart. The energyseparation of the states cannow be probed.

E=h =g H

Magnetic resonance

S TiO N it


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Summary: TiO2 Nanocomposites

TiO2 Nanocomposites as Highly Active Photocatalysts

How to fabricate highly active TiO2 nanocomposites?Solution phase method, magnetron sputtering

Importance of contact: simple mixing does not work

What is the origin of interfacial sites in mixed-phase TiO2?

Phase transformation by thermal treatment

Photoreduction of CO2 with H2O

Magnetron sputtering: oxygen deficiency

Role of Defect Sites in Photocatalysis

Corners, edges, steps, interfacial sites

Li and Gray, Chem. Phys.2007, 339, 173

H2

EvolutionRe

action

Jaramillo et al., Science2007, 317, 100


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0

2

4

6

8

10

12

14

0 2 4

ti me(hr)

methane(umol)

P25

sputtered f i l mwi th

O2 di f f i ci ency, 60%A

sputtered f i l m70%A

N doped f i l m(N/Of l owrate i s 3. 5)

CO2 reduction for sputtered

films under visible light

condition

Interfacial Sites in Mixed-Phase


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Interfacial Sites in Mixed PhaseTiO2

1. Baiker et al., Phys. Chem. Chem. Phys.2002, 4, 3514; 2. Penn and Banfield,Am. Miner.1999,

84, 871; 3. Zhang et al., J. Phys. Chem. B 2006, 110, 927

Mixed-phase TiO2: highly distorted, tetrahedrally coordinated interfacial sites

Tetrahedrally coordinated Ti species in flame synthesized materials(P25) 1

Distorted clusters with rutile-like character at anatase-anatasecontacts during phase transformation2

Phase transformation interfaces between anatase particles 3

Interfacial Sites in Thermally Treated


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Interfacial Sites in Thermally TreatedP25

EPR spectra in the presence of TCP

Li, Dimitrijevic, Chen, Nichols, Graham, Rajh and Gray, JACS,2008, 130:5402-5403.

20 25 30 35 40 45 50 55 60

2-Theta (degree)

P25

R

773 K

873 K

973 KA

3200 3250 3300 3350 3400 3450 3500 3550

Magnetic Field (Gauss)

R R

A AS

P25

773 K

873 K

973 K

Interfacial Sites in Synthesized Mixed-


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Interfacial Sites in Synthesized MixedPhase TiO2

Rutile-like character in A (773 K) Interfacial sites in A+R (873 K)

20 25 30 35 40 45 50 55 60

2-Theta (degree)

873 K

773 K

A

R

Li, Dimitrijevic, Chen, Nichols, Graham, Rajh and Gray, JACS,2008, 130:5402-5403.

3200 3250 3300 3350 3400 3450 3500 3550

Magnetic Field (Gauss)

R R

A

A H2O

A+R H2O

A+R TCP


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Coating Micro-/nano-structure

Typically columnar

structure, highly textured,

tens to hundreds of

nanometers grain dia.

Nano-thicknessmultilayers possible.

Top surface can vary,

smooth or faceted, denseor open, and is controlled

by deposition conditions.

A

Film growth cross-section cartoon

Mixed phase TiO2

The growth of the films


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The growth of the films

Nucleation and film growth: creating solid-

solid interfaces among crystals or columns

Reactive sputtering: control reactive gas more control on defects formation.

http://www.alacritas-

consulting.com/thin_film_growth.html

Low mobilitycauses smallercolumns orgrains

The structurezone model of

Thornton

Our samples:dualcolumnar

structure!


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EPR Characterization of SputteredFil


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Films

Li, Chen, Graham and Gray, J. Mol. Catal. A2007, 275, 30

3200 3250 3300 3350 3400 3450 3500

Field (Gauss)

A

R

P25

MS Film

MS film: domain of crystallinity

200 nm

UV

1 m

3100 3200 3300 3400 3500 3600

Field (Gauss)

Ti3+

Ti2O3

MS Powder

MS powder: oxygen deficiency

(more like TiO2-x )

Dark

Photochemistry of P25 (1): Nanostructured


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~0.45 m

2.05 2.00 1.95 1.90

g

~1.5 m

P25 Slurry

~0.2 m

Hurum, Gray, Rajh, Thurnauer, J. Elect. Spect. 2006, 150, 155 ; J. Phys. Chem. B2003, 107,4545

Better electron transfer

Higher photoactivityEPR spectra of size-fractionated P25

Photochemistry of P25 (1): NanostructuredAssembly

Anatase1.990

Rutile1.975

Visible light

(> 400 nm)

Rutile

Rutile

Anatase

e-

+

h



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Intensity =517.6 au

Intensity =1197.5 au

Intensity=922.3 au

Intensity=1354.3 au

2.10 2.05 2.00 1.95 1.90 1.85

g

Interfacialg=1.979

Anatase

surfaceg=1.930

Recombination is dominated by surfacereactions.

Experimental evidence of an interfacial electrontrapping site.

The mechanisms of recombination is randomflight.

Hurum, Gray, Rajh and Thurnauer, J. Phys. Chem. B2005, 109, 977

Semiconductor assisted photocatalysis


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CB

VB

3.2 eV

TiO2Photoinduced charge separation

h

+

- -

+

Anodic part

Cathodic part

A/A-

D+/D

-

+

p y

+

-

Converting photons into chemicalenergy:

light-induced formation ofcharges

charge separation

charge-transfer reactions

Particulate TiO2 - behaves asminiature electrochemical cell

Background


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Background

Defects:

Fundamental issues in catalysis:What is the active site?

Schwab - 1930s; Adlineation of phases (metal clusters onsupport) creates active sites -Adlineation sites at solid-

solid interface.(Schwab, G. M.; Pietsch, E.Zeitschrift fuer PhysikalischeChemie, Abteilung B: Chemie der Elementarprozesse, Aufbau der Materie1928, 1,386-408.)

Somorjai (2006) catalytically active surfaces tend to bedisordered, while ordered surface are catalytically

inactive. (Somorjai, G. A.; Bratlie, K. M.; Montano, M. O.; Park, J. Y.Journal ofPhysical Chemistry B2006, 110, 20014-20022.)

Selloni & Diebold (2006)- Theoretical work models defectsites on specific crystal surfaces such as the anatase

TiO2 (001) surface. (Gong, X.-Q.; Selloni, A.; Batzill, M.; Diebold, U. NatureMaterials2006, 5, 665-670.)


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70

On rutile (110) single crystal - only observe CO2 adsorption &

reaction on oxygen deficient surface & in presence of H2O(Funk & Burghaus, 2006. Phys. Chem. Chem. Phys.,8, 4805-4813.)

- Ability to make anatase single crystals (101) & most reactivefacet (001)

Defects:

Yang et al. (2008) Nature453, 638-641;Selloni (2008)Nature Materials, 7, 613 615; P. Zapol, L. A. Curtiss,(2007)

J. Computational and Theor. Nanoscience. 4, 222.

Tetrahedrally Coordinated Titanium


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Anpo and Thomas, Chem. Comm.2006, 3273

Frei et al., J. Phys. Chem. B2004, 108, 18269

Tetrahedrally Coordinated TitaniumSpecies

TiO2 Nanocomposites as Highly Active Photocatalysts

What is the origin of interfacial sites in mixed-phase TiO2?

How to engineer the solid-solid interface for high density active sites?

Excitation: UV

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