Summary of Wei-Ta's PhD thesis, enjoy it!

1. Metal/Semiconductor
Nanohetrostructures: Interfacial
Charge Transfer Dynamics and Their
Photoconversion Applications
Wei-Ta Chen
Advisor : Dr. Yung-Jung Hsu
10.19.2013

2. Outline
• 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

3. Chem. Mater. 2008, 20, 7204–7206
J. Phys. Chem. C 2010, 114, 11414–11420

4. Introduction
• Why metal/semiconductor hetrostructures?
CdSe-Au
Ultrafast charge transfer!
Long-live charge separation time!
CdSe-Pt
[Wu et al., J. Am. Chem. Soc. 2012, 134, 10337]
[Costi et al., Nano lett. 2008, 8, 637]

5. Motivation
1. Prevention of Chemical poisoning
2. Visible-Light-Driven Catalytic Activity
Sunlight Driven!
[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.]

6. Synthesis of Au-CdS core-shell nanocrystals
• Tri-functional reagent, L-Cysteine (Cys):
- SH : complexing with Cd2+ (Cys/Cd)
- NH2 : coupling Cys/Cd with Au
- COOH : promoting the dispersion of Au
N1= Au-N
N2= CN
N3=NH2
C1= C-C
C2= C-N
C3=COO-

7. Photophysical properties
UV spectra
TEM images
A
B
red shift
C
D
Theoretical calculation of SPR position for AuCdS :
Volume thickness
λest
λexp
1 mL
555
552
2 mL
11.9 nm
558
558
4 mL
[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.]
9.0 nm
13.9 nm
560
562
8 mL
18.6 nm
562
578

8. Excited state interaction studies
Photocurrent measurement
PL spectra
hv
e- h+
CdS emission
Au
CdS
Electron transfer!!

9. Au-CdS excited state interaction studies
A
Time-resolved PL spectra
Electron transfer rate constant , ket
ket =
B
1
<t >
(Au - CdS) -
1
<t >
(CdS)

10. Photocatalytic applications
A
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)
B

11. Department of Materials Science and Engineering, National Chiao Tung
University, Hsinchu, Taiwan 30010, Republic of China.
Photo-assisted direct methanol fuel cell
Hole participates methanol oxidation reaction
Reduce precious metal usage by light irradiation!!
Efficient hole exaction process by coupling metal with semiconductor!!
Chem. Commun., 2013, 49, 8486-8488

12. TEM images of Au-ZnS core-shell nanocrystals
A
A
EDAX analysis
Core component
100 nm
B
B
ZnS (203)
ZnS (103)
Au (111)
ZnS (002)
0.31 nm
ZnS(002)
0.24 nm
Au(111)
5 nm
Shell component

13. Electrophotocatalysis oxidation of methanol

14. Effective degradation containment catalyst!
Methylene blue
+ neThionine
Convert to harmless form
Langmuir 2010, 26, 5918–5925

15. Photocatalytic applications
A
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)
B

16. Conclusions
• 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 photoassisted methanol oxidation applications, photocatalysis,
electron storage, nonvolatile memory device, etc.

17. ACS Nano, 2012, 6, 4418–4427

18. Synthesis of Au-TiO2 and Au-SiO2 core-shell
nanocrystals

19. Photophysical properties
TEM images
UV spectra
1.2
Au
Au@TiO2
Au@SiO2
Absorbance
0.9
c
Red shift
0.6
b
a
0.3
0.0
400
500
600
700
800
Wavelength (nm)
Increasing n value by coating shell layer
A
Absorbance
0.8
Au/TiO2
f
g
0.6
0.4
a. 0min
b. 1min
c. 3min
d. 6min
e. 10min
f. 15min
g. Air
0.6
a. 0min
b. 1min
c, 3min
d. 6min
e. 10min
f. 15min
Au/SiO2
a-f
0.4
0.3
a
0.2
0.2
300
B
0.5
Absorbance
1.0
400
500
600
Wavelength (nm)
700
800
400
500
600
Wavelength (nm)
Increasing Au core charge density
700
800

20. Dye-Sensitized Solar Cell by Incorporating with Au/TiO2 and Au/SiO2
I-V curve measurment
24
A
Current density (mA/cm 2)
A
B
20
c
16
a
b
12
TiO2 + N719
TiO2 + Au@TiO2 + N719
8
TiO2 + Au@SiO2 + N719
4
0
0.0
0.2
Table 1
0.4
0.6
0.8
Voltage (V)
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

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

22. Conclusions
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.

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

24. Synthesis of DHLA protected Ag8 cluster
Sonication
NaBH4
α-Lipoic acid
DHLA(light yellowish solution)
+ AgNO3
Ag-DHLA complex
Ag-DHLA complex
NaBH4
Ag
Ag8 cluster

25. Photophysical properties
UV spectra
b
a
QY=4.62%
Clusters size larger than theory of Ag8 ?
Why?
MV+
Ag8
TEM images
e
MV2+
Ag
h
h ’
Ag core/Ag8 Shell

26. Charge transfer between Ag8 and MV2+
Ag8 - MV2+ -light
MV2+ -dark
MV2+ -light
Ag8 - MV2+ -dark
Formation of MV.+
e- transfer occurred

27. Ag8-MV2+ Interfacial charge transfer dynamics
A
Ag8
B
Ag8 + MV2+
Formation of MV.+
C
D

28. 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.
e-
hν
ket = 2.74 x 1010 s-1
e- e-
h + h+
MV2+
h+
MV .+
τav= 28.7ps
Ag8

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