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

[email protected]

PEC characterization: towards IPCE measurement

and development of new materials

- A review

P. Basnet2/17/2015

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!

http://chemwiki.ucdavis.edu/Biological_Chemistry/Photosynthesis/Photosynthesis_overview/The_Light_Reactions

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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

Shu

tte

r

Using Si detector

(II): I(λ)S1227

Pre-amplifierconnected to S1227(of known responsivity)

Source of monochromatic light

1. Record incident P(λ)

Potentiostat

CERef.

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 persecond).

• 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 A. J. Bard and L. R. Faulkner; 2001.

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

IIPCE

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 ConversionTheoretical: 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

hEg

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)

E vs S

HE

(V

olts)


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.

PO

TE

NT

IAL

VS

SH

E (

VO

LT

S)

-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)

http://www.nature.com/nnano/journal/v9/n1/full/nnano.2013.272.html

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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 gof catalyst; 0.1 M pH 7.0 potassium phosphate buffer solution containing 0.8 g of NaIO3 solution (200mL); reaction time, 3 h; light source, Xe lamp (300 W) with a cutoff filter (λ ≥ 420 nm). (b) Photocurrentdensities of CoPi/ BiVO4 electrodes with different deposited times measured at 0.3 V against the SCEreference 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/BiVO4electrodes 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 ofMOx measured (scan rate, 10 mV/s) with chopped light. Electrolyte: 0.1 M pH 7.0 potassium phosphatebuffer solution.

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(a) XRD of Bi0.5Y0.5VO4 synthesizedby solid state reaction. (b) UV−visdiffuse reflectance spectra ofBi0.5Y0.5VO4 synthesized by solidstate reaction.

Photocatalytic water splitting onBi0.5Y0.5VO4 loaded with CoPi and Pt frompure water. Reaction conditions: 0.2 g ofcatalyst; 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 photocatalystswith different Cu/Cd molar ratios and (b) long-time photocatalytic test ofCu2S/CdS-0.05 sample for hydrogen production. Reaction conditions: 0.2 g ofCdS 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 Xelamp 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

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(a) Low-magnification side-view and top-view and high-magnification top-view SEMimages of vertically orientatedGaAsP p−n homojunctionnanowires grown onto a p+ -Si(111) substrate. (b) Dark fieldTEM images and diffractionpatterns taken in differentpositions of a GaAsP p−nhomojunction nanowire. (c)Line element mapping imageof a GaAsP p−n homojunctionnanowire across the radial axis.The element composition isnormalized to the nanowirethickness

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(a) PEC water splitting device structure for GaAsP nanowires grown on Si substrates.

(b) Current density potential characteristics of GaAsP homojunction nanowires

photocathode.


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(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.


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.


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.


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)

2/17/2015P. Basnet

(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

2/17/2015P. Basnet


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

2/17/2015P. Basnet

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

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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

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

Science

pec characterization

pec performancea

water splitting

hn h2o h2g o2 g pec

development of new materials

adadeh light

new york

basnetassessment of