A review on ipce and pec measurements and materials p.basnet

Journal Club Presentation
By:
Pradip Basnet, Ph.D. Candidate
The University of Georgia
Department of Physics & Astronomy
pbasnet@physast.uga.edu
PEC characterization: towards IPCE measurement
and development of new materials
- A review
P. Basnet
2/17/2015

P. Basnet
Photoelectrochemical (PEC) Reactions
ANALOGY
Light absorption
Biased potential
Electrolyte and so on…
2/17/2015
chemwiki.ucdavis.edu
Electron transfer in plants (vs. semiconductor/electrolyte solution)
-- most of the energy storage in photosynthesis
is in water splitting, not CO2 fixation!

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What we can make to happen in PEC Cell?
http://chemistry.harvard.edu/people/daniel-g-nocera

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PEC Cell
metal
e-
e-
A
D
A
D
e
-
h+
Light is Converted to
Electrical + Chemical Energy
LiquidSolid
?

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Outline:
1. PEC: Overview
- Measurements: Apparatus; Setup; Techniques etc.
- What Do We Measure/Compare in PEC Characterization?
2. Development of the New Materials:
- Theoretical, Computational & Experimental Viewpoints
3. Literature Summary: New Materials & Their Efficiencies

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PEC: Overview
Via light absorption & creating electron –
hole pairs in photoelectrodes.
Then electron–hole pairs separation in
electrodes (photo-cathode/anode).
Electrode processes & related charge
transfer within PECs.
Light into Electrical and/or Chemical Energy; how?

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PEC: Measurement; Apparatus; Setup
TWO- COMPONENTS
Lamp
Monochromator
(I): P(λ)
Lock-in amplifier
Shutter
Using Si detector
(II): I(λ)
S1227
Pre-amplifier
connected to S1227
(of known responsivity)
Source of monochromatic light
1. Record incident P(λ)
Potentiostat
CE
Ref.
A
V A = current
V = bias voltage
2. Record V, I(t) (=A)

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Chen et. al., J. Mat. Res., 2010, 25, 3-16.
What do/should we measure for PEC
characterization ?

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J. A. Turner, Science 285, 687 (1999)
PEC cell for hydrogen (H2) generation

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Assessment of materials
for photoelectrodes ?
( , )absorption
CB VBSemiconductor h Semiconductor e h  
 
Requirements ?
> 1.5 V (  < ca. 830 nm)
-- NOT ENOUGH
2hVB
+ + H2O -> ½ O2 (g) + 2H+
2eCV
- + 2H+ -> H2 (g)
The overall reaction :
2h + H2O -> H2(g) + ½ O2 (g)
PEC cell for hydrogen (H2) generation

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Generation of PEC voltage required for water decomposition
Ideal Condition for Water Splitting
H2O(l)  H2(g) + ½ O2
G0 = 237 kJ/mol
E0 = G0/nF = 1.23 V
hν ≥ 400 nm
A. Kudo and Y. Miseki, Chemical Society Reviews, 2009

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Why is it difficult to achieve?
• Oxides
• Stable but efficiency is
low (large gap)
• III-Vs
• Efficiency is good but
surfaces corrode
• Approaches
• Dye sensitization (lifetime
issues)
• Surface catalysis
• No practical PEC H2
production demonstrated
with single material yet.
• Efficiency and lifetime
Adapted from M. Grätzel, Nature 414, 388 (2001)

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What are the fundamental issues?
Band structure engineering
To match water redox potentials and achieve
high solar efficiency
Fundamental understanding of the electrode/
electrolyte interface
To accelerate water splitting reaction and reduce
corrosion

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Standardized Measurement: PEC
performance
A. J. Bard and L. R. Faulkner, Electrochemical Methods- Fundamentals and Applications, 2nd edition,
JOHN WILEY & SONS, INC., New York, 2001.
- can measure current in a cell when a potential is applied (or
voltage when current is applied)
What is Potentiostat?

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• Do you need to know how a Potentiostat works?
• No. But if you know it, is a plus.
• Do you need to be able to recognize when something is
wrong?
• Yes! You must!
• Why would something go wrong?
• Because the performance of the Potentiostat is affected by
the electrical characteristics of the sample…or something in
the cell is causing a problem…or the Potentiostat is busted!
What one “MUST” know about the
following:

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DATA EVALUATION!
 Electrochemical data is “ALWAYS” a collection of
individual data points…one followed smoothly by another.
 Noisy data is “BAD”.
 Flat-lined data is “BAD”.
 Overloads are “BAD”.
 It is VERY rare to collect bad data that looks good.

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Potential or Voltage (E, sometimes V):
• Unit: Volt
• The Potential is the driving force for the redox reaction.
• The potential is related to the thermodynamics of the
system:
ΔG = -n F ΔE (negative ΔG is spontaneous)
• Potential is always measured versus a Reference
Electrode.
• A positive voltage is oxidative and a negative voltage is
reductive.
• 0 Volts is not nothing!
What is Potential?
A. J. Bard and L. R. Faulkner; 2001.

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What is Current?
Current (i):
• Unit: Ampere
• Electron flow is the result of a redox reaction.
• Current measures the rate of the reaction (electrons per
second).
• Zero current is nothing, i.e., if the current is zero, no
redox reactions are occurring (that’s not quite true in
corrosion!).
• Anodic (oxidation) and cathodic (reduction) currents
have different polarity (signs).
• Current may be expressed as current or current density.

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Ideal Electrodes

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Real Electrodes

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Electrochemical cell setup
2 electrode setup 3 electrode setup
Measure the current
Working
Electrode (WE) Ref/Counter electrode
Control voltage
RE

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3 Electrode Measurements
Control the potential of
the working electronic
vs. known potential of
the ref. electrode
This voltage is set so
the counter electrode
can pass the same
current as the working
electrode

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Table. Reference electrode potential conversion (if needed).
Choice of Reference electrode
E (vs RHE) = E (vs Ag/AgCl) + EAg/AgCl (reference) + 0.0591 V × pH
(EAg/AgCl (reference) = 0.1976 V vs. NHE at 25 ºC)
Zhebo Chen, Huyen N. Dinh and E. Miller, Springer, 2013.

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Pay Special Attention to the RE
• A Potentiostat needs a low impedance Reference Electrode!
• Use large junction reference electrodes
• Replace isolation frits regularly
• Avoid narrow Luggin Capillaries
• Potentiostats are less forgiving of high-impedance Reference
Electrodes than pH Meters!
• If there’s an problem with the cell, it’s almost always the Reference
Electrode!

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Role of the Counter Electrode

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Hodes, G., J., Phys. Chem. Lettr., 2012, 3(9), pp 1208-1213
Ideal vs. Real Counter Electrodes

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Overpotential

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Overpotential
 Electron transfer across charge double layer
 Depletion of concentration at electrode surface
 Chemical reactions that must occur before electron
transfer
 And more...

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ftp.kdis.edu.cn/211-xkkr-12/doc/10.1007_978-1-4614-8298-7.pdf
Cell Setup
Connections for 3- & 2-Electrode configurations
(a) Sketch of a horizontal PEC test setup and (b) picture of test cell
(a)

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How to know if the Data is Bad?
1. Calibrate the Potentiostat.
2. If calibration is successful, check the Potentiostat by
running a dummy cell (a network of resistors/capacitors
that give a known result).
3. If the instrument is OK, then check the cell. Check the
Reference Electrode first!
4. If the cell is OK, then it’s something in your sample
chemistry.
5. At some point, you should contact your Potentiostat
supplier for technical support.

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Equation/s we need to know
•Ohm’s Law
•E = iR
•If I apply 100 mV to a 1000 ohm resistor, I should
measure a current of…
•100 µA

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1) Cyclic voltammetry (CV): Redox potentials and stability
2) Linear sweep voltammetry (LSV): Onset Potential
3) Photocurrent (or density), Iph: Dynamic photoresponse
(light on/off cycles at a certain frequency)
4) IPCE%: Efficiency (quantitative comparison)
PEC Measurements/Characterizations
100
)(
1240
)w(
)amp(
% 
nmp
I
IPCE ph

… Iph and P(λ) need to be measured

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Other imp. features in PEC characterizations:
Band Bending
http://solar.iphy.ac.cn/index.php/?page_id=1775&lang=en
Photoelectrochemical water splitting systems using n-type
semiconductor photoanode (a), p-type semiconductor
photocathode (b), and tandem system (c)

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Outline:
1. PEC: Overview
► Development of the New Materials:
3. Literature Summary: New Materials & Their Efficiencies

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Development of Materials
1. Theoretical calculation
2. Computational
3. Experimental
Making Novel Semiconductor-Based Materials for Solar
Energy Conversion
Theoretical: Physical parameters that
might affect the photocatalytic activities
using quantum mechanics. For example,
density of states, absorbance, bandgaps
and so on.
Computational: the structural, optical,
electronic, and PEC properties. First
principle calculation is one of the well-
known computational methods.
Experimental: Honda’s work is an
example. Also based on results from
above models.

Periodic Table of the Elements
Contd.: Development of Materials
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Candidate Materials ?
General Requirements?
Contd.: Development of Materials
 Uv-Vis light absorption
 Chemically stable
 No-toxic etc.
 Efficient
For ex. spontaneous water splitting ?

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Approximation: Jmax- STH vs. Eg
Theoretical predicted values for
Jmax- STH as a function of
material Bandgap (Eg). The
calculation was made assuming
a complete collection and
conversion to electronic-current
of photogenerated electron-hole
pairs by using the following
eqn.:
d: sample’s thickness; λ :
wavelength; q: electronic
charge; Φ(λ): photon flux of the
AM 1.5 solar spectrum
   ddqJ   ))(exp(1)(max

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S. Chen and L.-W. Wang, Chem Mater, 2012, 24, 3659-3666.
Stability of a photoelectrode: Thermodynamic
viewpoint

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Contd.: Stability of a photoelectrode
S. Chen and L.-W. Wang, Chem Mater, 2012, 24, 3659-3666.

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PH dependent band edges: redox couple

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Source of apparent dark reduction reaction
Thermodynamic conditions: Ered<ECBMEVBM<Eox and
VB
Oxidation
CB
Reduction
First principles scheme to evaluate band edge
positions: Towards Stability of photoelectrode
L. I. Bendavid and E. A. Carter, J. of Phy.
Chem. B, 2013, 117, 15750-15760.

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Outline:
1. PEC: Overview
2. Development of the New Materials:
► Literature Summary: New Materials & Their Efficiencies

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Literature Summary: Current Reported Efficiencies
A. Fujishima and K. Honda, Nature, 1972, 238, 37-38.
 In 1972, Fujishima and Honda first showed direct water splitting
using n-TiO2 (photoanode), using a PEC Cell (at 840 mV bias).
 Since then, a large number of semiconductor materials have
been investigated for PEC electrodes for H2 production.
 Metal oxides, with wide bandgap
(2.5–3.5 eV) are polpular
 TiO2, ZnO, Fe2O3, SrTiO3, WO3 and
Cu2O are typically more stable in
aqueous media.
 Best multijunction/PV bias is: 8.5%
(S. Kahn et. al., Science, 297, (2002)

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Cu2O for overall water splitting under
visible light irradiation
Hara et. al. Chem. Comm., 1998
water into H2 and O2 on Cu2O
under visible light irradiation is
investigated; the photocatalytic
water splitting on Cu2O powder
proceeds without any noticeable
decrease in the activity for more
than 1900 h.
Cu2O has been regarded as an unstable material for water decomposition
under light irradiation from the results of PEC; this study has revealed
Cu2O to be a photocatalyst able to decompose water into H2 and O2 under
visible light irradiation. NO reaction mechanism was available.
The particle size and surface area of
P-type Cu2O were estimated to be 0.3–
0.5 mm and 6 m2 g21, respectively.
H2 and O2 evolution

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Cu2O: a catalyst for the PEC decomposition of
water?
P. E. de Jongh et. al., Chem. Commun. 1999, 1069.
Using 3 –electrode:
Current density at - 0.4 V
measured on a 0.5 μm
thick Cu2O electrode
illuminated with 350 nm
in 0.5 M Na2SO4 solution:
(a) bubbled with air, (b)
bubbled with Ar and (c)
bubbled with Ar and with
40 mM MV2+ added to
the solution.O2 + 2 H2O + 2e-  H2O2 + 2 OH- = +0.03 V
The hydrogen peroxide is further reduced to water.

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Overview of
the redox pot.
of the relevant
reactions w.r.t.
the estimated
position of the
Cu2O band
edges.
It depends…

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Hydrogen-Evolving Photoelectrochemical Cells Based on p-Cu2O Films
h
Eg
VB
H+/H2
O2/H2O
CB
1.229 V
e-
h+
VB
H+/H2
D/D+
CBe-
h+
VB
A/A-
O2/H2O
CBe-
h+
a) b) c)
It depends…

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2H+ + 2e H2
-1.0
1.5
1.0
0.5
0.0
-1.5
p - Cu2O
VB
CB
2CuO + H2O + 2e Cu2O + 2OH-
Cu2O + H2O + 2e 2Cu + 2OH-
O2 + 2H2O + 4e 4OH-
Electrolyte (pH = 7)
EvsSHE(Volts)
Visible light
h  h+ + e-
2.1 eV
Why the TiO2 stable then?

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TiO2:
Stable n-type semiconductor.
Band gap energy is ~3.2 eV.
Wavelength region that suits this energy is < 400
nm (UV region).
The efficiency of solar energy conversion is low.
POTENTIALVSSHE(VOLTS)
-1
3
2
1
0
n -TiO2
3.2 eV
p -Cu2O
2.1 eV
Contd.:

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Efficient nanocrystalline CoO
L. Liao, et. al., Nat Nano., 2014, 9, 69-73.
TEM of CoO nanoparticles.Conversion of black Co3O4 to brown CoO powders
through thermal decomposition.
Before this, Solar to H2 conversion eff. ~ 0.1%; Here we show that
cobalt(II) oxide (CoO) nanoparticles can carry out overall water
splitting with a solar-to-hydrogen efficiency of around 5%
solar water-splitting using CoO- prepared by two distinct
methods (femtosecond laser ablation and mechanical ball
milling)

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Bandgaps and band-
edge positions of
CoO nanoparticles
and micropowders.

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Characterizations of hydrogen and oxygen evolutions with gas chromatography and
mass spectrometry. (a) A typical GC trace of evolved hydrogen and oxygen.
(b) Production of H2 and O2 from CoO nanoparticles (~12 mg) as a function of incident
laser power. The laser wavelength is 532 nm.

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(a) SEM images of the as-synthesized In2O3 nanocubes on FTO substrates.
(b,c) HRTEM image and HRTEM image of a single In2O3 nanocube. (d)
Enlarged image of the white square marked in (b). (e,f) Enlarged image of the
selected part.
Oxygen vacancies promoting PEC
performance of In2O3 nanocubes
Gan et. al., Sci. Reports 3, 1021 (2013)

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Fig. Comparison of XRD patterns of as-prepared In(OH)3 without heat
treatment and In2O3 nanocubes obtained by calcination in air at 250°C,
350°C and 450°C, respectively.
In2O3 nanocubes

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Fig. Linear sweeps voltammogram collected from In2O3nanocubes obtained by
calcination in air at 250°C, 350°C, and 450°C with (a) sunlight on and off and (b)
visible light (400<λ<700 nm) on and off.
In2O3 nanocubes

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Plot of the best IPCE efficiency at the
visible wavelength over 400 nm of
In2O3nanocubes obtained by calcination
in air at 250°C, 350°C, 450°C, as a
function of the relative oxygen vacancy
amount.
IPCE spectra of In2O3 nanocubes
obtained by calcination in air at
250°C, 350°C, 450°C, collected at the
incident wavelength range from 300
to 650 nm at a potential of −0.6 V vs
Ag/AgCl.
In2O3 nanocubes

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Splitting water with rust: α-Fe2O3
T. W. Hamann, Dalton Transactions, 2012, 41, 7830-7834
Hamann Studied Efficiency vs. thickness: “Nano-cauliflower”
hematite developed by Gratzel and co-workers. J. Am. Chem. Soc.
2006, 128, 15714-15721

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Steady state J-V curves of α-
Fe2O3 electrode with (purple)
and without (green) a Co-Pi
catalyst in contact with an H2O
electrolyte.
Steady state J-V curves of Fe2O3
electrode in contact with a H2O
(green curve) and [Fe(CN)6]3-/4-
(orange curve) electrolyte.
How do the electrolyte and co-catalysts affect ?

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water oxidation on BiVO4
Figure 2. SEM images of (a) BiVO4, (b)
BiVO4 at high resolution, (c) CoPi/BiVO4,
and (d) CoPi/BiVO4 at high resolution.
BiVO4 with CoPi cocatalyst & EDS data.

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(a) Photocatalytic O2 evolution on CoPi/BiVO4 with different loadings of CoPi. Reaction conditions: 0.2 g
of catalyst; 0.1 M pH 7.0 potassium phosphate buffer solution containing 0.8 g of NaIO3 solution (200
mL); reaction time, 3 h; light source, Xe lamp (300 W) with a cutoff filter (λ ≥ 420 nm). (b) Photocurrent
densities of CoPi/ BiVO4 electrodes with different deposited times measured at 0.3 V against the SCE
reference electrode. Electrolyte: 0.1 M pH 7.0 potassium phosphate buffer solution

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Photocatalytic O2 evolution on 0.44 wt % MOx/BiVO4 prepared by the impregnation method. Reaction
conditions: 0.2 g of catalyst; 0.1 M pH 7.0 potassium phosphate buffer solution containing 0.8 g of
NaIO3 (200 mL); reaction time, 3 h; light source, Xe lamp (300 W) with a cutoff filter (λ ≥ 420 nm). (b)
Photocurrent densities of MOx/BiVO4 electrodes with the same loading of MOx measured at 0.5 V
against the SCE reference electrode. Electrolyte: 0.1 M pH 7.0 potassium phosphate buffer solution.

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Photocurrent measurement. (a) Photocurrent−potential characteristics of BiVO4 and CoPi/BiVO4
electrodes with different deposited times of CoPi measured (scan rate, 10 mV/s) with chopped light. (b)
Photocurrent−potential characteristics of BiVO4 and MOx/BiVO4 electrodes with the same loading of
MOx measured (scan rate, 10 mV/s) with chopped light. Electrolyte: 0.1 M pH 7.0 potassium phosphate
buffer solution.

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(a) XRD of Bi0.5Y0.5VO4 synthesized
by solid state reaction. (b) UV−vis
diffuse reflectance spectra of
Bi0.5Y0.5VO4 synthesized by solid
state reaction.
Photocatalytic water splitting on
Bi0.5Y0.5VO4 loaded with CoPi and Pt from
pure water. Reaction conditions: 0.2 g of
catalyst; pure water (200 mL); light source,
Xe lamp (300 W), reaction time, 9 h.

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Back up Slides

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Noble-metal-free Cu2S-modified
Chen et. al., RSC Adv., 2015, 5, 18159
XRD patterns of CdS, CuxS, and
CuxS/CdS samples with various
Cu/Cd molar ratios.
(a) Cu 2p XPS spectra and (b) the Auger Cu LMM
spectra of CuxS and CuxS/CdS-0.05 samples.
- enhanced photocatalytic H2 production by forming nanoscale p–n
junction structure

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FESEM images of (a and b) CdS;
(c and d) Cu2S/CdS-0.05; and
(e and f) Cu2S/CdS-0.1.
(a) FESEM image of Cu2S/CdS-0.05 and (b–d) the
corresponding EDX-SEM mapping images of Cd, Cu
and S in Cu2S/CdS- 0.05. The scale bar is 2 mm in
each image
(a) TEM image and (b) HRTEM image of
Cu2S/CdS-0.05.

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UV-vis absorption spectra of
Cu2S/CdS hybrid photocatalysts
with different Cu/Cd molar ratios.
PL spectra of CdS and Cu2S/CdS-0.05.
The excitation wavelength was 420 nm.

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(a) Photocatalytic hydrogen production over Cu2S/CdS hybrid photocatalysts
with different Cu/Cd molar ratios and (b) long-time photocatalytic test of
Cu2S/CdS-0.05 sample for hydrogen production. Reaction conditions: 0.2 g of
CdS photocatalyst; appropriate amount of Cu2S deposited by the in situ method;
190 mL of aqueous solution containing 0.25 M Na2SO3/0.35 M Na2S; 300 W Xe
lamp equipped with a cutoff filter (420 nm)

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(a) The schematic diagram of forming p–n junction and charge transfer process
in Cu2S/CdS and (b) the band structure for Cu2S/CdS heterojunction and charge
separation process under illumination

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Ti-Fe2O3/Cu2O heterojunction photoelectrode
a) UV-visible absorption spectra for (A) pristine Cu2O, (B) hematite, (C) Ti-Fe2O3,
(D) Fe2O3/Cu2O, and (E) Ti-Fe2O3/Cu2O heterojunction thin films and Tauc plots
for (A) pristine Cu2O, (B) hematite, and (C) Ti-Fe2O3.
Sharma et. al., , Thin Solid Films, 2015, 574, 125-131.

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SEM images for
pristine Ti-Fe2O3,
Ti-Fe2O3/Cu2O
heterojunction
sample E, and the
energy-dispersive
X- ray image for
Ti-Fe2O3/Cu2O
heterojunction
thin film.
(a) AFM image for pristine Ti-Fe2O3.
(b) AFM image for Ti-Fe2O3/Cu2O
heterojunction sample E.

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Photocurrent density vs. applied potential curve for (A) pristine Cu2O, (B) hematite,
(C) Ti-Fe2O3, (D) Fe2O3/Cu2O, and (E) Ti-Fe2O3/Cu2O heterojunction thin films,
respectively, under visible light illumination in 0.1 M NaOH electrolytic solution.

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IPCE performance of for (A) pristine Cu2O, (B) hematite, (C) Ti-Fe2O3, (D)
Fe2O3/Cu2O, and (E) Ti-Fe2O3/Cu2O heterojunction thin films, respectively, under
visible light illumination in 0.1 M NaOH electrolyte.

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Vertically Aligned Ta3N5
-- fabricated by through-mask anodization and nitridation for water splitting.
The Ta3N5 nanorods, working as photoanodes of a photoelectrochemical cell,
yield a high photocurrent density of 3.8 mA cm−2 at 1.23 V vs RHE under AM
1.5G simulated sunlight and an incident photon-to-current conversion efficiency
of 41.3% at 440 nm, one of the highest activities reported for photoanodes so
far.
Nanorod Arrays for Solar Driven Photoelectrochemical Water Splitting
Li et. al., Adv. Mat. 2013, 25, 125-131.

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Vertically Aligned Ta3N5
Current–potential curves of the
IrO2/Ta3N5 nanorod arrays
nitrided at 850 ° C and 1000 °
C. The curves were measured
in a 0.5 M aqueous Na2SO4
solution (pH = 13) under
chopped AM 1.5G simulated
sunlight at 100 mW/cm2.
Current-potential curves for
IrO2 /Ta3N5 thin fi lm and
IrO2 /Ta3N5 nanorods
nitrided at 1000 ° C. The
curves were measured in a
0.5 M aqueous Na2SO4
solution (pH = 13) under
chopped AM 1.5G simulated
sunlight at 100 mW/cm 2 .
The inset shows the SEM
image of the cross sec-tion
of Ta 3 N 5 thin fi lm grown
on Ta foil. b) Wavelength
dependence of IPCE
measured at 1.23 V RHE for
IrO2 /Ta3N5 thin fi lm and
IrO2 /Ta3N5 nanorods.

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Self-Catalyzed 1.7. eV GaAsP Core-Shell Nanowire
Photocathode on Silicon Substrates
 Under AM 1.5G illumination, the GaAsP nanowire photocathode yielded a
photocurrent density of 4.5 mA/cm2 at 0 V versus RHE and a solar-to-
hydrogen conversion efficiency of 0.5%, which are much higher than the
values previously reported for wafer-scale III–V nanowire photocathodes.
 Plus, GaAsP reported to be more resistant to photocorrosion than InGaP.
These results open up a new approach to develop efficient tandem PEC
devices via fabricating GaAsP nanowires on a silicon platform.
Wu et. al., Nano Lett., 2014, 14 (4), pp 2013–2018

2/17/2015P. Basnet
(a) Low-magnification side-
view and top-view and high-
magnification top-view SEM
images of vertically orientated
GaAsP p−n homojunction
nanowires grown onto a p+ -Si
(111) substrate. (b) Dark field
TEM images and diffraction
patterns taken in different
positions of a GaAsP p−n
homojunction nanowire. (c)
Line element mapping image
of a GaAsP p−n homojunction
nanowire across the radial axis.
The element composition is
normalized to the nanowire
thickness

2/17/2015P. Basnet
(a) PEC water splitting device structure for GaAsP nanowires grown on Si substrates.
(b) Current density potential characteristics of GaAsP homojunction nanowires
photocathode.

2/17/2015P. Basnet
(c) Current density potential characteristics of GaAsP homojunction nanowires photocathode
with an InGaP passivation layer. (d) Comparison of the PEC performance between the GaAsP
nanowire photocathode without InGaP passivation layer (blue line) and the one with
passivation after stability test (red line). The insets in b and c are the steady-state current
density of the photocathodes measured at 0.1 V versus RHE under AM 1.5G illumination.
The current−potential curves were measured in 0.1 M KPi buffer solution (pH 7) at a scan
rate of 10 mV/s under chopped AM 1.5G illumination.

2/17/2015P. Basnet
(a) Current density potential characteristics of GaAsP homojunction nanowires photocathodes with an InGaP passivation layer
illuminated with normal incident light and off-normal incident light. The current−potential curves were measured in 0.1 M Na2SO4
solution (pH 10) at a scan rate of 10 mV/s under chopped AM 1.5G illumination. (b) Illustration of the illumination configuration
during PEC performance characterization. (c) Current density measured 0 V vs RHE and 0.2 V vs RHE as a function of off-normal
incident angle θ. (d) Current density in c normalized to the illumination area.

2/17/2015P. Basnet
WO3/BiVO4 Core/Shell Nanowire
Photoanode
practical water oxidation photocurrent (JH2O) is much lower due to the limited light
absorption, charge separation, and surface charge transfer efficiencies (ηabs, ηsep and ηtrans,
respectively) of the BiVO4 material, according to
JH2O = Jmax × ηabs × ηsep × ηtrans.
Fig. The WO3/W:BiVO4 core/shell nanowire (NW) photoanode. Structural schematic and
energy band diagram of the core/ shell NWs and type-II staggered heterojunction, in which
charges generated in both the W:BiVO4 shells and WO3 NW cores can contribute to the
water oxidation photocurrent. The band edges and water oxidation and reduction potentials
are plotted on the RHE scale.
Simultaneously Efficient Light Absorption
and Charge Separation for PEC water
Oxidation
Rao et. al., Nano Letters, 2014, 14, 1099-1105.

2/17/2015P. Basnet
Fig. (b,c) Scanning electron
microscope (SEM, left) and
transmission electron
microscope (TEM, right)
images of the bare WO3
NW array (75 nm average
NW diameter) and
WO3/W:BiVO4 core/shell
NWs (60 nm average
W:BiVO4 shell thickness),
respectively. The W:BiVO4
shell consists of a single
layer of densely packed
nanoparticles.
Photoanode

2/17/2015P. Basnet
PEC response of the WO3/W:BiVO4 NW photoanode and control samples in
0.5 M potassium phosphate electrolyte buffered to pH 8. (a) Current−voltage
(J−V) curves (solid lines: simulated AM 1.5G illumination, dotted lines: dark)
and (b) Incident photon-to-current efficiency (IPCE) measured at 1.23 VRHE.
Photoanode

2/17/2015P. Basnet
Efficiencies of subprocesses that comprise the overall photoelectrochemical response of
the WO3/W:BiVO4 NW photoanode and control samples. (a) Light absorption efficiency
(ηabs) and (b) J−V curve under simulated AM 1.5G illumination with H2O2 added to the
potassium phosphate electrolyte as a hole scavenger, which demonstrates the photocurrent
achieved when the surface charge transfer efficiency is nearly 100%. (c) Charge separation
(ηsep) and (d) surface charge transfer efficiency (ηtrans) of the WO3/W:BiVO4 NWs and,
where appropriate, the bare WO3 NWs and same-mass W:BiVO4 film.
Photoanode

2/17/2015P. Basnet
- for Solar Fuel Production
One-D ZnO/TiO2 Hybrid Nanoelectrodes
FESEM top-view images of (a) TiO2
nanotubes after annealing for 4 h at
450 °C, and the ZnO/TiO2 nanotubes
heterojunctions formed via the
sputtering of ZnO for (b) 18, (c) 35, and
(d) 53 min, respectively, on the
annealed TiO2 nanotube films.
Shaheen et. al., J. Phys. Chem. C, 2013, 117 (36)

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(A) Photocurrent density vs potential in 0.5 M Na2SO4 solution under UV (320−400 nm,
100 mW/cm2) illumination for the pure titania nanotubes and the ZnO/TiO2 heterojunction
electrodes, (B) the corresponding photoconversion efficiency and (C) the IPCE of pure
TiO2 nanotubes and the 53-min ZnO/TiO2 electrodes under a constant bias of 0.6 V.
One-D ZnO/TiO2 Hybrid Nanoelectrodes

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PEC cell: 3-electrode each component
91
Reference electrode (RE) used to measure applied
voltage versus absolute reference.
Counter electrode (CE) used to complete
circuit, potential required to pass current at
CE usually not measured.
Semiconductor working electrode (WE)
control majority carrier Fermi level versus the
reference electrode and measure current.
Typically bubble O2 or H2
through solution to
maintain well-defined
Nernstian potential

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PEC efficiency measurements

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Photocatalysis: A source of energetic electrons
Christoph E. Nebel, Nature Materials 12, 780–781 (2013)
Fig. Valence-band maxima, bandgaps and conduction bands related to the vacuum
energy for a variety of semiconductors shown as function of energy (left) with relation
to EVac and as function of the electrode potential scale (right) relative to the standard
hydrogen electrode.

2/17/2015P. Basnet
Carbon Monoxide to Liquid Fuel on Oxide-Derived
Nanocrystalline Copper
Electroreduction of Carbon Monoxide to Liquid Fuel on Oxide-Derived Nanocrystalline Copper
Li et. al., Nature (2014) , 508

2/17/2015P. Basnet
CO2 Reduction at low overpotential on Cu electrodes resulting from the reduction of thick
Cu2O films. Li, C. W.; Kanan, M.W. J. Am. Chem. Soc. 2012, 134, 7231-7234.
CO2 Reduction at low overpotential on Cu electrodes

2/17/2015P. Basnet
Powdered Photocatalysts
Kudo et. al., Chem. Soc. Rev. 2009, 38, 253-278.
Advantage: No support, high surface area, easy to scale-up.
Disadvantage: 1. Single junction cell requires large Eg >2.5 eV to generate
required photopotential; fundamentally inefficient with solar spectrum. 2.
Separation of H2 and O2 flammable mixture difficult. How to prevent reverse
electrochemical reaction?

2/17/2015P. Basnet
NREL demonstrates 45.7% efficiency for
concentrator solar cell

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