Metal/Semiconductor Nanohetrostructures: Interfacial Charge Transfer Dynamics and Their

Photoconversion Applications

Wei-Ta ChenAdvisor : Dr. Yung-Jung Hsu

10.19.2013

• Au-CdS Core−Shell Nanocrystals with Controllable Shell Thickness and Photoinduced Charge Separation Property and Interfacial Charge Carrier Dynamics Interfacial Charge Carrier Dynamics

• Au/ZnS Core/Shell Nanocrysals As an Eficient Anode Photocatalyst in Direct Methanol Fuel Cells

• L-Cysteine-Assisted Growth of Core-Satellite ZnS-Au Nanoassembles with Remarkable Photocatalytic Efficiency

• Know Thy Nano Neighbor. Plasmonic versus Electron Charging Effects of Metal Nanoparticles in Dye-Sensitized Solar Cells

• Realizing Visible Photoactivity of Metal Nanoparticles. Excited State Behavior and Electron Transfer Properties of Silver (Ag8) Clusters

Outline

Chem. Mater. 2008, 20, 7204–7206

J. Phys. Chem. C 2010, 114, 11414–11420

• Why metal/semiconductor hetrostructures?

Introduction

Ultrafast charge transfer!

Long-live charge separation time!

CdSe-Au

CdSe-Pt

[Wu et al., J. Am. Chem. Soc. 2012, 134, 10337][Costi et al., Nano lett. 2008, 8, 637]

[V. Iliev, D. Tomova, L. Bilyarska, G. Tyuliev, J. Mol. Catal. A: Chem. 2007, 263, 32.][P. V. Kamat et. al., J. Phys. Chem. C 2007, 111, 2834.]

[J. Qi et. al., ACS nano 2011, 5, 7108.]

Motivation

1. Prevention of Chemical poisoning

2. Visible-Light-Driven Catalytic Activity

Sunlight Driven!

• Tri-functional reagent, L-Cysteine (Cys):

Synthesis of Au-CdS core-shell nanocrystals

- SH : complexing with Cd2+ (Cys/Cd)

- NH2 : coupling Cys/Cd with Au - COOH : promoting the dispersion of Au

N1= Au-NN2= CNN3=NH2

C1= C-CC2= C-NC3=COO-

Photophysical properties

A B

C D

Volume

thickness

λest λexp

1 mL 9.0 nm 555 552

2 mL 11.9 nm

558 558

4 mL 13.9 nm

560 562

8 mL 18.6 nm

562 578

Theoretical calculation of SPR position for Au-CdS :

red shift

[T. Hirakawa et. al., J. Am. Chem. Soc. 2005, 127, 3928.] [G. Oldfield et. al., Adv. Mater. 2000, 12, 1519.]

[A. C. Templeton,et. al., J. Phys. Chem. B 2000, 104, 564.]

TEM images UV spectra

Electron transfer!!

e-

Au

CdS

hvh+ CdS emission

Excited state interaction studies

PL spectra

Photocurrent measurement

Au-CdS excited state interaction studies

Electron transfer rate constant , ket

A

B

Time-resolved PL spectra

Au–CdS + hν Au(e–)–CdS(h+) (1)

Au(e–)–CdS(h+) + H2O Au(e–)–CdS + H+ + ·OH (2)

RhB + ·OH oxidation products (3)

Au(e–)–CdS + O2 Au-CdS + ·O2– (4)

Photocatalytic applications

A

B

Reduce precious metal usage by light irradiation!!

Hole participates methanol oxidation reaction

Efficient hole exaction process by coupling metal with semiconductor!!

Photo-assisted direct methanol fuel cell

Department of Materials Science and Engineering, National Chiao Tung

University, Hsinchu, Taiwan 30010, Republic of China.

Chem. Commun., 2013, 49, 8486-8488

100 nm

A

5 nm

ZnS(002)

Au(111)

BZnS (002)Au (111)ZnS (103)ZnS (203)

0.31 nm

0.24 nm

A

B

TEM images of Au-ZnS core-shell nanocrystals

EDAX analysis

Core component

Shell component

Electrophotocatalysis oxidation of methanol

Effective degradation containment catalyst!

Methylene blue Thionine + ne- Convert to harmless form Langmuir 2010, 26, 5918–5925

ZnS- Au + hν Au(e–)–ZnS(h+) (1)

Au(e–)–ZnS(h+) + TH ZnS(h+) + Au + TH. (colorless form) (2)

ZnS(h+) + EtOH ZnS + EtOH.(3)

Photocatalytic applicationsA

B

• The results show that the Au/CdS and ZnS core-shell structure provides excellent oxidation reaction efficiency because the electron-hole pathway results in oxidation(reduction) reaction, rather than self-recombination.

• Reaction rate and electron transfer rate significantly enhances increasing CdS shell thickness.

• Our study provides an alternative design for such photo-assisted methanol oxidation applications, photocatalysis, electron storage, nonvolatile memory device, etc.

Conclusions

ACS Nano, 2012, 6, 4418–4427

Synthesis of Au-TiO2 and Au-SiO2 core-shellnanocrystals

400 500 600 700 8000.0

0.3

0.6

0.9

1.2

c

b

Abso

rban

ce

Wavelength (nm)

Au Au@TiO

2

Au@SiO2

a

Photophysical properties

Red shift

TEM images UV spectra

Increasing n value by coating shell layer

300 400 500 600 700 800

0.2

0.4

0.6

0.8

1.0

g

f

a. 0min b. 1min c. 3min d. 6min e. 10min f. 15min g. Air

Abso

rban

ce

Wavelength (nm)

a

A

400 500 600 700 800

0.2

0.3

0.4

0.5

0.6

Abso

rban

ce

Wavelength (nm)

a. 0min b. 1min c, 3min d. 6min e. 10min f. 15min

B

a-f

Au/TiO2 Au/SiO2

Increasing Au core charge density

0.0 0.2 0.4 0.6 0.80

4

8

12

16

20

24

Curr

ent den

sity

(m

A/c

m2 )

Voltage (V)

TiO2 + N719

TiO2 + Au@TiO

2 + N719

TiO2 + Au@SiO

2 + N719

A

ab

c

Table 1

Dye-Sensitized Solar Cell Performance

Support/Dye Jsc (mAcm-2) Voc (V) FF η (%)

TiO2/N719 18.28 0.729 0.697 9.29

TiO2+Au@TiO2/N719 18.281 0.771 0.694 9.78

TiO2+Au@SiO2/N719 20.31 0.727 0.691 10.21

Performances of DSSCs were measured with 0.18 cm2 working area under AM 1.5 illumination. Electrolyte: 0.6 M

DMPImI, 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile. Au@TiO2 and Au@ SiO2 loadings were kept

at 0.7% by weight. FF and correspond to fill factor and power conversion efficiency respectively.

Dye-Sensitized Solar Cell by Incorporating with Au/TiO2 and Au/SiO2

I-V curve measurment

A B

Distinguish the role of core/shell nanocrystals in solar cell devices

By incorporating these Au core@oxide shell nanoparticles in the DSSC, we have succeeded in identifying the influence of these effects.

The examples discussed in the presents study provides a convenient way to isolate the two effects. The surface plasmon resonance effects increases the photocurrent of DSSC while the charging effects leads to increase in photovoltage.

These observations opens up new opportunity to introduce both these paradigms and synergetically enhance the photocurrent and photovoltage of DSSC.

Conclusions

Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

Synthesis of DHLA protected Ag8 cluster

α-Lipoic acid DHLA(light yellowish solution)

+ AgNO3

Ag

Sonication

Ag-DHLA complex

Ag-DHLA complex

NaBH4

NaBH4

Ag8 cluster

QY=4.62%a

b

Clusters size larger than theory of Ag8 ?Why?

Ag

Ag8

hn

eMV2+

MV+

hn’

Ag core/Ag8 Shell

Photophysical properties

TEM images

UV spectra

Charge transfer between Ag8 and MV2+

Ag8 - MV2+ -light

Ag8 - MV2+ -darkMV2+ -darkMV2+ -light

Formation of MV.+

e- transfer occurred

Ag8-MV2+ Interfacial charge transfer dynamics

B

C D

A Ag8 Ag8 + MV2+

Formation of MV.+

Conclusions • Ag8 cluster excited state electron transfer event have

successfully demonstrated.• The photochemical activity established in the present study

offers another dimension to the fascinating properties of small metal nanostructures.

• Basic understanding of excited state processes in fluorescent metal clusters paves the way towards the development of biological using and catalysts in energy conversion devices.

MV2+

MV .+

e-

e- e-

h+ h+ h+

Ag8

hνket = 2.74 x 1010 s-1

τav= 28.7ps

Thank you!Those papers can be found in

Chemistry of Materials 2008, 20, 7204-7206 Journal of Physical Chemistry C 2009, 113, 17342-17346Chem. Comm. 2013, 2013, 49, 8486-8488 Langmuir 2010, 26, 5918-5925 Journal of Physical Chemistry C 2010, 114,11414-11420ACS Nano 2012, 6, 4418–4427 J. Phys. Chem. Lett. 2012, 3, 2493–2499

Acknowledgement