Transcript Slide 1

By
Dr. Sarika Phadke-Kelkar
National Chemical Laboratory
24-March-2011
Outline
•
•
•
•
Energy Crisis
Alternative Fuels
Hydrogen as fuel
Hydrogen production from water using solar
energy
– Photo-chemical decomposition of water
– Photo-electro-chemical water splitting
• Materials: Selection criteria, important
candidates
• Current Status & Future Trend
Historical and Projected Variations in Earth’s
Surface Temperature
IPCC Reports
Years
Energy Demand in present and near future
* Present : 12.8 TW
2050년 : 28-35 TW
* Needs at least 16 TW
Bio
: 2 TW
Wind : 2 TW
Atomic : 8 TW (8000 power plant)
Fossil : 2 TW
* Solar: 160,000 TW
2010
2020
Hydrogen
• Hydrogen, a gas, will play an important
role in developing sustainable transportation
in the United States, because in the future it may be produced in virtually
unlimited quantities using renewable resources.
• Hydrogen and oxygen from air fed into a proton exchange membrane fuel
cell produce enough electricity to power an electric automobile, without
producing harmful emissions. The only byproduct of a hydrogen fuel cell is
water.
• Currently there are no original equipment manufacturer vehicles available
for sale to the general public. Experts estimate that in approximately 1020 years hydrogen vehicles, and the infrastructure to support them, will
start to make an impact.
Applications of Hydrogen Fuel
What is a Fuel Cell?
• A Fuel Cell is an electrochemical device that
combines hydrogen and oxygen to produce
electricity, with water and heat as its byproduct.
How can Fuel Cell technology be used?
• Transportation
– All major automakers are working
to commercialize a fuel cell car
– Automakers and experts speculate
that a fuel cell vehicle will be
commercialized by 2010
– 50 fuel cell buses are currently in
use in North and South America,
Europe, Asia and Australia
– Trains, planes, boats, scooters,
forklifts and even bicycles are
utilizing fuel cell technology as well
How can Fuel Cell technology be used?
• Stationary Power Stations
– Over 2,500 fuel cell systems have been installed
all over the world in hospitals, nursing homes,
hotels, office buildings, schools and utility power
plants
– Most of these systems are either connected to
the electric grid to provide supplemental power
and backup assurance or as a grid-independent
generator for locations that are inaccessible by
power lines
How can Fuel Cell technology be used?
• Telecommunications
– Due to computers, the Internet and sophisticated
communication networks there is a need for an
incredibly reliable power source
– Fuel Cells have been proven to be 99.999%
reliable
How can Fuel Cell technology be used?
• Micro Power
– Consumer electronics could
gain drastically longer
battery power with Fuel Cell
technology
– Cell phones can be powered
for 30 days without
recharging
– Laptops can be powered for
20 hours without recharging
Hydrogen Production
• The biggest challenge regarding hydrogen
production is the cost
• Reducing the cost of hydrogen production so
as to compete in the transportation sector
with conventional fuels on a per-mile basis is a
significant hurdle to Fuel Cell’s success in the
commercial marketplace
Hydrogen Production
• There are three general categories of
Hydrogen production
– Thermal Processes
– Electrolyte Processes
– PhotocatalyticProcesses
Hydrogen Production
• PhotocatalyticProcesses
– Uses light energy to split water into hydrogen and
oxygen
– These processes are in the very early stages of
research but offer the possibility of hydrogen
production which is cost effective and has a low
environmental impact
– Two types: a) Photochemical
b) Photo-electro-chemical
Photo-catalytic water splitting
1. Direct Water Splitting:
2. Water Splitting using photoelectrochemical cell (PEC):
H2
H+/H2
p
n
eeh2
TCO with
ohmic
contact
O2
h+
h1
h+
h
H2O/O2
Experimental setup
Direct Water Splitting:
PEC water splitting:
Potentiostat
Phto-electrochemistry of water
decomposition
• Basic principle
N-type semiconductor
N-type semiconductor
Metal
Metal
P-type semiconductor
P-type semiconductor
Reaction Mechanism
2hν→ 2e′ + 2h+
(1)
2h+ + H2O(liquid) → 1/2O2(gas) + 2H+ (2)
2H+ + 2e′ → H2(gas)
(3)
Overall Reaction
2hν + H2O(liquid) → 1/2O2(gas) + H2(gas)
= 1.23 eV
Electrochemical decomposition of water is possible when EMF of
cell ≥ 1.23 V
Band model representation
A
B
C
D
Materials Aspects of PEC
• Two main functions of photoelectrodes
– Optical function: maximum absorption of solar energy
– Catalytic function: water decomposition
• Desired properties of photoelectrodes
–
–
–
–
–
–
–
Bandgap
Flatband potential
Schottky barrier
Electrical resistance
Helmholtz potential
Corrosion resistance
Microstructure
Band structure of photoelectrode
material
WHY SEMICONDUCTOR ?
Metals
CB
No band gap
Only reduction or oxidation
Depends on the band
position
CB
VB
Insulators
High band gap
CB
H+/H2
E
H2O/O2
VB
SC
VB
Metals
Insulators
High energy requirement
26
Concepts –Why semiconductors are chosen as
photo-catalysts?
For conventional redox reactions, one is interested in either
reduction or oxidation of a substrate.
For example consider that one were interested in the
oxidation of Fe2+ ions to Fe 3+ ions then the oxidizing agent
that can carry out this oxidation is chosen from the relative
potentials of the oxidizing agent with respect to the redox
potential of Fe2+/Fe3+ redox couple.
The oxidizing agent chosen should have more positive potential
with respect to Fe3+/Fe2+ couple so as to affect the
oxidation, while the oxidizing agent undergoes reduction
spontaneously. This situation throws open a number of possible
oxidizing agents from which one of them can be easily chosen.
27
Bandgap
Flatband potential
Other important parameters
• Electrical Conductivity
• Helmholtz Potential Barrier
• Corrosion Resistance:
– Electrochemical corrosion resistance
– Photocorrosion resistance
– Dissolution
Criterion for PE corrosion stability
Photo anode
Free enthalpy of oxidation reaction
Photo cathode
Free enthalpy of reduction reaction
What modifications?
• various conceptual principles have been incorporated into
typical TiO2 system so as to make this system responsive
to longer wavelength radiations.
These efforts can be
classified as follows:
• Dye sensitization
• Surface modification of the semiconductor to improve the
stability
• Multi layer systems (coupled semiconductors)
• Doping of wide band gap semiconductors like TiO2 by
nitrogen, carbon and Sulphur
• New semiconductors with metal 3d valence band instead of
Oxide 2p contribution
• Sensitization by doping.
• All these attempts can be understood in terms of some
kind sensitization and hence the route of charge transfer
has been extended and hence the efficiency could not be
increased considerably.
In spite of these options being
elucidated, success appears to beeluding the researchers.
33
Conditions to be satisfied?
•
The band edges of the electrode must overlap with the
acceptor and donor states of water decomposition
reaction, thus necessitating that the electrodes should
at least have a band gap of 1.23 V, the reversible
thermodynamic decomposition potential of water. This
situation
necessarily
means
that
appropriate
semiconductors alone are acceptable as electrode
materials for water
•
The charge transfer from the surface of the
semiconductor must be fast enough to prevent photo
corrosion and shift of the band edges resulting in loss
of photon energy.
34
ENGINEERING THE SEMICONDUCTOR
ELECTRONIC STRUCTURES
 without deterioration of the stability
 should increase charge transfer processes at the interface
 should improvements in the efficiency
35
Positions of bands of semiconductors relative to the standard potentials of several
redox couples
36
THE AVAILABLE OPPORTUNITIES
 Identifying and designing new semiconductor materials
with considerable conversion efficiency and stability
 Constructing multilayer systems or using sensitizing
dyes - increase absorption of solar radiation
 Formulating multi-junction systems or coupled
systems - optimize and utilize the possible regions of
solar radiation
 Developing nanosize systems - efficiently dissociate
water
37
ADVANTAGES OF SEMICONDUCTOR NANOPARTICLES
 high surface area
 morphology
 presence of surface states
eV
 wide band gap
 position of the VB & CB edge
CdS – appropriate choice for
the hydrogen production
38
The opportunities
• The opportunities that are obviously available
as such now include the following:
– Identifying and designing new semiconductor
materials with considerable conversion efficiency and
stability
– Constructing multilayer systems or using sensitizing
dyes so as to increase absorption of solar radiation.
– Formulating multi-junction systems or coupled
systems so as to optimize and utilize the possible
regions of solar radiation.
– Developing catalytic systems which can efficiently
dissociate water.
39
Opportunities evolved
• Deposition techniques have been considerably perfected
and hence can be exploited in various other applications
like in thin film technology especially for various devices
and sensory applications.
• The knowledge of the defect chemistry has been
considerably improved and developed.
• Optical collectors, mirrors and all optical analysis
capability have increased which can be exploited in
many other future optical devices.
• The understanding of the electronic structure of
materials has been advanced and this has helped to our
background in materials chemistry.
• Many electrodes have been developed, which can be a
useful for all other kinds of electrochemical devices.
40
Limited success – Why?
The main reasons for this limited success in all these directions
are due to:
• The electronic structure of the semiconductor controls the
reaction and engineering these electronic structures without
deterioration of the stability of the resulting system appears
to be a difficult proposition.
• The most obvious thermodynamic barriers to the reaction and
the thermodynamic balances that can be achieved in these
processes give little scope for remarkable improvements in
the efficiency of the systems as they have been conceived
and operated. Totally new formulations which can still satisfy
the existing thermodynamic barriers have to be devised.
• The charge transfer processes at the interface, even though
a well studied subject in electrochemistry has to be
understood more explicitly, in terms of interfacial energetics
as well as kinetics. Till such an explicit knowledge is available,
designing systems will have to be based on trial and error
rather than based on sound logical scientific reasoning.
41
• Nanocrystalline (mainly oxides like TiO2, ZnO, SnO and Nb2O5
or chalcogenides like CdSe) mesoscopic semiconductor materials
with high internal surface area
If a dye were to be
adsorbed as a monolayer, enough can be retained on a given
area of the electrode so as to absorb the entire incident light.
• Since the particle sizes involved are small, there is no
significant local electric field and hence the photo-response is
mainly contributed by the charge transfer with the redox
couple.
• Two factors essentially contribute to the photo-voltage
observed, namely, the contact between the nano crystalline
oxide and the back contact of these materials as well as the
Fermi level shift of the semiconductor as a result of electron
injection from the semiconductor.
42
Another
aspect of thee nano crystalline state is the alteration of
the band gap to larger values as compared to the bulk material which
may facilitate both the oxidation/reduction reactions that cannot
normally proceed on bulk semiconductors.
The
response of a single crystal anatase can be compared with that
of the meso-porous TiO2 film sensitized by ruthenium complex (cis
RuL2 (SCN)2, where L is 2-2’bipyridyl-4-4’dicarboxlate).
The
incident photon to current conversion efficiency (IPCE) is only
0.13% at 530 nm ( the absorption maximum for the sensitizer) for
the single crystal electrode while in the nano crystalline state the
value is 88% showing nearly 600-700 times higher value.
43
This
increase is due to better light harvesting capacity of the dye
sensitized nano crystalline material but also due to mesoscpic film
texture favouring photo-generation and collection of charge
carriers .
It
is clear therefore that the nano crystalline state in
combination with suitable sensitization is one another alternative
which is worth investigating.
44
• The second option is to promote water splitting in the visible
range using Tandem ells. In this a thin film of a
nanocrystalline WO3 or Fe2O3 may serve as top electrode
absorbing blue part of the solar spectrum. The positive
holes generated oxidize water to oxygen
• 4h+ + 2H2O --- O2 + 4 H+
• The electrons in the conduction band are fed to the second
photo system consisting of the dye sensitized nano crystalline
TiO2 and since this is placed below the top layer it absorbs
the green or red part of the solar spectrum that is
transmitted through the top electrode. The photo voltage
generated in the second photo system favours hydrogen
generation by the reaction
• 4H+ + 4e- --- 2H2
• The overall reaction is the splitting of water utilizing visible
light.
The situation is similar to what is obtained in
photosynthesis
45
• Dye sensitized solid hetero-junctions and extremely
thin absorber solar cells have also been designed
with light absorber and charge transport material
being selected independently so as to optimize solar
energy harvesting and high photovoltaic output.
However, the conversion efficiencies of these
configurations have not been remarkably high.
• Soft junctions, especially organic solar cells, based
on
interpenetrating
polymer
networks,
polymer/fullerene blends, halogen doped organic
crystals and a variety of conducting polymers have
been examined.
Though the conversion efficiency
of incident photons is high, the performance of the
cell declined rapidly. Long term stability will be a
stumbling block for large scale application of
polymer solar cells.
46
New Opportunities
1.
New semi-conducting materials with conversion
efficiencies and stability have been identified.
These are not only simple oxides, sulphides but
also multi-component oxides based on perovskites
and spinels.
2.
Multilayer configurations have been proposed for
absorption of different wavelength regions. In
these systems the control of the thickness of
each layer has been mainly focused on.
47
New Opportunities
3.
Sensitization by dyes and other anchored
molecular species has been suggested as an
alternative to extend the wavelength region of
absorption.
4.
The coupled systems, thus giving rise to multijunctions is another approach which is being
pursued in recent times with some success
5.
Activation of semiconductors by suitable
catalysts for water decomposition has always
fascinated scientists and this has resulted in
various metal or metal oxide (catalysts) loaded
semi conductors being used as photo-anodes
48
New opportunities (Contd)
• Recently a combinatorial electrochemical synthesis and
characterization route has been considered for
developing tungsten based mixed metal oxides and this
has thrown open yet another opportunity to quickly
screen and evaluate the performances of a variety of
systems and to evolve suitable composition-function
relationships which can be used to predict appropriate
compositions for the desired manifestations of the
functions.
• It has been shown that each of these concepts,
though has its own merits and innovations, has not
yielded the desired levels of efficiency. The main
reason for this failure appears to be that it is still
not yet possible to modulate the electronic structure
of the semiconductor in the required directions as well
as control the electron transfer process in the desired
direction.
49
PREPARATION OF CdS NANOPARTICLES
1 g of Zeolite (HY, H, HZSM-5)
1 M Cd(NO3)2 , stirred for
24 h, washed with water
Cd / Zeolite
1 M Na2S solution, stirred for
12 h, washed with water
CdS / Zeolite
48 % HF, washed with water
CdS Nanoparticles
50
XRD PATTERN OF CdS
Intensity (a.u.)
CdS- 
CdS-Z
CdS-Y
CdS (bulk)
20
30
40
50
60
70
80
2 theta
M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)
51
dSPACING AND CRYSTALLITE SIZE
Debye Scherrer Equation
0.89
T
 cos 
d-spacing (Å)
Catalyst
 = diffraction angle T = Crystallite size
 = wave length
 = FWHM
(0 0 2)
(1 0 1)
(1 1 2)
Crystallite
Size(nm)
CdS (bulk)
1.52
1.79
2.97
21.7
CdS (bulk)
(HF treated)
1.52
1.79
2.93
21.7
CdS-Y
1.53
1.79
2.96
8.8
CdS-
1.52
1.78
2.93
8.6
CdS-Z
1.52
1.79
2.97
7.2
52
UV –VISIBLE SPECTRA OF CdS SAMPLES
Absorbance (a.u.)
CdS (bulk)
CdS - 
CdS - Z
CdS - Y
500
600
Samples
Band Gap
(eV)
CdS – Z
2.38
CdS – Y
2.27
CdS - 
2.21
Bulk CdS
2.13
700
Wavelength (nm)
M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)
53
PHOTOCATALYTIC PRODUCTION OF HYDROGEN
35ml of 0.24 M Na2S and
0.35 M Na2SO3 in Quartz cell
N2 gas purged before the reaction and
constant stirring
0.1 g CdS
400 W Hg lamp
Hydrogen gas was collected over
water in the gas burette
54
Amount of Hydrogen (micro moles / 0.1g )
AMOUNT OF HYDROGEN EVOLVED BY CdS
PHOTOCATALYST
700
CdS - Y
CdS - Z
CdS - 
CdS - with HY
CdS (bulk)
600
500
400
300
200
100
0
0
1
2
3
4
5
6
Time (h)
55
TEM IMAGE OF CdS NANOPARTICLES
Particle Size
(nm)
Surface
area
(m2/g)
Rate of hydrogen
production
(  moles /h)
CdS - Y
8.8
36
102
CdS - Z
6
46
68
CdS - 
11
26
67
CdS - Bulk
23
14
45
Catalyst
CdS-Z
CdS- 
CdS-Z
100 nm
100 nm
56
SCANNING ELECTRON MICROGRAPHS
CdS-Z
CdS-Y
CdS-
CdS- bulk
57
PHOTOCATALYSIS ON Pt/TiO2 INTERFACE
Vacuum level
 Electrons are transferred
to metal surface
 Reduction of H+ ions
takes place at the metal
surface
 The holes move into
the other side of
semiconductor
 The oxidation takes
place at the
semiconductor surface
Aq. Sol
pH = 7
H+/H2
pH=0
Pt
TiO2
Aq. Sol
C.B
EF
V.B
T.Sakata, et al Chem. Phys.Lett. 88 (1982) 50
58
MECHANISM OF RECOMBINATION REDUCTION BY METAL DOPING
e-(M) <-- M+eEg
Conduction Band
e- e- e- e- e- e- e- e- e- e- e- eElectron/hole pair
recombination
Electron/hole pair
generation
Valence Band
h+ h+ h+ h+ h+ h+ h+ h+ h+ h+
Metallic promoter attracts electrons from TiO2 conduction
band and slows recombination reaction
52
Activity of the catalyst is directly
proportional to work function of the
metal and M-H bond strength.
(Amount of hydrogen (micro moles/ 0.1g))
PHOTOCATALYTIC HYDROGEN
EVOLUTION OVER METAL LOADED CdS
NANOPARTICLES
3500
H beta
Pt / CdS
Pd / CdS
Rh / CdS
CdS (Bulk)
Ru / CdS
3000
2500
2000
1500
1000
500
0
0
1
2
3
4
5
6
4
5
6
4000
H-ZSM-5
Pt / CdS
Pd / CdS
Rh / CdS
CdS (Bulk)
Ru / CdS
3500
3000
2500
2000
1500
1000
500
0
0
1
2
3
Time (h)
4
5
6
Amount of Hydrogen (micro moles / 0.1g )
Amount of Hydrogen (micro moles / 0.1g)
Time (h)
3000
HY
Pt / CdS
Pd / CdS
CdS (Bulk)
Rh / CdS
Ru / CdS
2500
2000
1500
1000
500
0
0
1
2
3
Time (h)
60
HYDROGEN PRODUCTION ACTIVITY OF METAL LOADED CdS PREPARED
FROM H-ZSM-5
Metal
Redox
potential
(E0)
Metal- hydrogen
bond energy
(K cal mol-1)
Work
function
(eV)
Hydrogen
evolution rate*
(µmol h-1 0.1g-1)
Pt
Pd
Rh
Ru
1.188
0.951
0.758
0.455
62.8
64.5
65.1
66.6
5.65
5.12
4.98
4.71
600
144
114
54
*1 wt% metal loaded on CdS-Z sample. The reaction data is presented
after 6 h under reaction condition.
M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)
61
EFFECT OF METALS ON HYDROGEN EVOLUTION RATE
Pt
Pt, Pd & Rh show higher
activity
 High reduction potential.
Pd
1000
Rh
Au
Cu
100
 Hydrogen over voltage
is less for Pt, Pd & Rh
Ag
Ni
10
Fe
Ru
3%
62
EFFECT OF SUPPORT ON THE CdS PHOTOCATLYTIC ACTIVITY
2, 5,10 and 20 wt % CdS on support - by dry impregnation method
80
CdS (ZSM-5)/MgO
Rate of hydrogen production
-1
-1
(µmol h 0.1g )
75
CdS (ZSM-5)/Al2O3
70
Alumina & Magnesia
supports enhance
photocatalytic activity
CdS (ZSM-5)
65
Bulk CdS/MgO
60
55
MgO support has higher
photocatalytic activity favourable band position
Bulk CdS/Al2O3
50
Bulk CdS
45
40
0
2
4
6
8
10 12
14
16
18
20
22
CdS (Wt %)
63
Pb2+/ ZnS
Absorption at 530nm (calcinations at 623-673K)
 Formation of extra energy levels between the band gap by Pb
6s orbital
 Low activity at 873K is due to PbS formation on the surface
(Zinc blende to wurtzite)
Eg
(a) 573 K, (b) 623 K, (c) 673 K, (d) 773 K, and (e) 873K
Band structure of ZnS doped with Pb.
I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1
64
PREPARATION OF MESOPOROUS CdS NANOPARTICLE
BY ULTRASONIC MEDIATED PRECIPITATION
250 ml of 1 mM
Cd(NO3)2
Rate of addition
20 ml / h
Ultrasonic waves
 = 20 kHz
250 ml of 5 mM
Na2S solution
The resulting precipitate was
washed with distilled water until
the filtrate was free from S2- ions
65
N2 ADSORPTION - DESORPTION ISOTHERM
 The specific surface area and pore volume are 94 m2/g and
0.157 cm3/g respectively
 The adsorption - desorption isotherm – Type IV (mesoporous nature)
140
 The maximum pore
volume is contributed by
45 Å size pores
100
Relative volume (%)
3
 Mesopores are in the
range of 30 to 80 Å size
Volume (cm /g)
120
8
6
4
2
80
0
0
60
20
40
60
80
100
Pore range (A)
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
66
X- RAY DIFFRACTION PATTERN
 XRD pattern of as-prepared CdS -U shows the presence of cubic
phase
 The observed “d” values are 1.75, 2.04 and 3.32 Å corresponding
to the (3 1 1) (2 2 0) and (1 1 1) planes respectively - cubic
 The peak broadening shows the
formation of nanoparticles
Intensity (a.u.)
The particle size is calculated
using Debye Scherrer Equation
(111)
(220)
(311)
 The average particle size of asprepared CdS is 3.5 nm
20
30
40
50
60
70
2 theta
M. Sathish and R. P. Viswanath Mater. Res. Bull(Communicated)
67
ELECTRON MICROGRAPHS
The growth of fine spongy particles of CdS-U is observed on the
surface of the CdS-U
 The CdS-bulk surface is found with large outgrowth of CdS particles
The fine mesoporous CdS particles are in the nanosize range
 The dispersed and agglomerated forms are clearly observed for
the as-prepared CdS-U
TEM
SEM
CdS-U
CdS-U
100 nm
CdS - Bulk
68
PHOTOCATALYTIC HYDROGEN PRODUCTION
Na2S and Na2SO3 mixture used
as sacrificial agent
Amount of hydrogen (µM/0.1 g)
Metal
CdS-U
CdS-Z
CdS
bulk
Amount of hydrogen/M 0.1g
-1
10000
Pt / CdS-U
Pd / CdS-U
Rh / CdS-U
CdS-U
8000
6000
4000
2000
0
0
-
73
68
45
Rh
320
114
102
Pd
726
144
109
Pt
1415
600
275
1
2
3
4
5
6
Time (h)
1 wt % Metal loaded CdS – U is 23 times more active than the
CdS-Z
69
LIMITED SUCCESS – WHY?
 Difficulties on controlling the semiconductor electronic
structure without deterioration of the stability
Little scope on the thermodynamic barriers and the
thermodynamic balances for remarkable improvements in the
efficiency
Incomplete understanding in the interfacial energetic as well
as in the kinetics
70
THE OTHER OPPORTUNITIES EVOLVED
 Deposition techniques -thin film technology, for various devices
and sensory applications.
Knowledge of the defect chemistry has been considerably
improved and developed.
Optical collectors, mirrors and all optical analysis capability
have increased
 Understanding of the electronic structure of materials
Many electrodes have been developed- useful for all other
kinds of electrochemical devices.
71
Thank you all for
your kind attention
Photo-electrochemical
H2 Generation
• Basic principle
N-type semiconductor
N-type semiconductor
Metal
Metal
P-type semiconductor
P-type semiconductor
Reaction Mechanism
•
•
•
•
•
2hν→ 2e′ + 2h+
(1)
2h+ + H2O(liquid) → 1/2O2(gas) + 2H+
(2)
2H+ + 2e′ → H2(gas)
(3)
Overall Reaction
2hν + H2O(liquid) → 1/2O2(gas) + H2(gas)
= 1.23 eV
Electrochemical decomposition of water is possible when EMF of
cell ≥ 1.23 V
Metal Oxide Requirements
• Two main functions of photoelectrodes
– Optical function: maximum absorption of solar energy
– Catalytic function: water decomposition
• Desired properties of photoelectrodes
–
–
–
–
–
–
–
Bandgap
Flatband potential
Schottky barrier
Electrical resistance
Helmholtz potential
Corrosion resistance
Microstructure
Bandgap
Flatband Potential
Other important parameters
• Electrical Conductivity
• Helmholtz Potential Barrier
• Corrosion Resistance:
– Electrochemical corrosion resistance
– Photocorrosion resistance
– Dissolution
Criterion for PE corrosion stability
Photo anode
Free enthalpy of oxidation reaction
Photo cathode
Free enthalpy of reduction reaction
Dye-Sensitized TiO2
J. AM. CHEM. SOC. 2009, 131, 926–927
Mesoporous Fe2O3
J. AM. CHEM. SOC. 9 VOL. 132, NO. 21, 2010
WO3 Nanowires
Double-Sided CdS and CdSe Quantum Dot Co-Sensitized
ZnONanowire Arrays for PEC Hydrogen Generation
NanoLett. 2010, 10, 1088–1092
Summary
• Metal oxide nanomaterials offer great
versatility in properties
• Optoelectronic properties can be tuned by
choosing/controlling the synthesis protocol
• Hybridization with organic/molecular
materials provide unique combinations of
properties
• Low temperature and solution based
processing is the key for future metal oxide
based energy devices
Thank You!
ww.mathworks.com
What is Nanoscale
www.physics.ucr.edu
Fullerenes C60
22 cm
12,756 Km
1.27 × 107 m
0.7 nm
0.22 m
10 millions times
smaller
0.7 × 10-9 m
1 billion times
smaller
How Small is Nano Really ?
InP nanoparticles
Quantum Phenomena
Large Surface to Volume Ratio
Gold nanoparticles of
different sizes
Passivated Carbon Nanodots
Sun et al. JACS 128, 7756 (2006)