Enhanced photo-electrochemical water oxidation on MnOx in buffered organic/inorganic electrolytes

Article abstract of DOI:10.1039/c5ta03850b

Manganese oxide (MnO_x) materials have been widely studied as electro-catalysts for the water oxidation reaction. Although the electronic structure of MnO₂ (band gap ～2 eV) indicates it could be a promising photo-anode material for solar water splitting, so far only quite small photocurrents have been obtained from manganese oxides. Here, we show that the photo-electrochemical water oxidation performance of MnO_x films can be significantly improved by using buffered aqueous electrolytes containing amine ionic liquids. The buffered conditions to maintain a constant proton activity and the amine salt are both crucial for achieving high photocurrents. Photocurrents as high as 4.5 mA cm^-2 were obtained at 1.0 V vs. SCE (η = 540 mV) in Bi buffered n-butylammonium nitrate (BAN) electrolyte at pH 9. The incident photon-to-current efficiency (IPCE) was found to be greater than 3% at 400 nm and 4% at 370 nm. Both H₂O₂ and O₂ are produced simultaneously in this system, with the potential subsequent decomposition of the H₂O₂ to form oxygen. An acceleration of the decomposition of H₂O₂ under illumination is proposed to explain the photocurrent improvement.

Full text of DOI:10.1039/c5ta03850b

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DOI: 10.1039/C5TA03850B
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ARTICLE(
Enhanced(Photo4Electrochemical(Water(Oxidation(on(MnOx(in(
Buffered(Organic/Inorganic(Electrolytes
Fengling(Zhou,a,b(Ciaran(McDonnellBWorth,a,b(Haitao(Li,(a,b(Jiaye(Li,b(Leone(Spiccia*a,b(and(Douglas(
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Manganese( oxide( (MnOx)( materials( have( been( widely( studied( as( electroBcatalysts( for( the( water( oxidation( reaction.(
Although(the(electronic(structure(of(MnO2((band(gap(~2(eV)(indicates(it(could(be(a(promising(photoBanode(material(for(
solar(water(splitting,(so(far(only(quite(small(photocurrents(have(been(obtained(from(manganese(oxides.(Here,(we(show(
that( the( photoBelectrochemical( water( oxidation( performance( of( MnOx( films( can( be( significantly( improved( by( using(
buffered( aqueous( electrolytes( containing( amine( ionic( liquids.( The( buffered( conditions( to( maintain( a( constant( proton(
activity(and(the(amine(salt(are(both(crucial(for(achieving(high(photocurrents.(Photocurrents(as(high(as(4.5(mA(cmB2(were(
obtained(at(1.0(V(vs(SCE((η(=(540(mV)(in(Bi(buffered(nBbutylammonium(nitrate((BAN)(electrolyte(at(pH9.(The(incident(
photonBtoBcurrent(efficiency((IPCE)(was(found(to(be(greater(than(3(%(at(400(nm(and(4%(at(370(nm.(Both(H2O2(and(O2(are(
produced(simultaneously(in(this(system,(with(the(potential(subsequent(decomposition(of(the(H2O2(to(form(oxygen.(An(
acceleration(of(the(decomposition(of(H2O2(under(illumination(is(proposed(to(explain(the(photocurrent(improvement.(
potential,( indicating( that( the( actual( catalyst( is( a( manganese(
oxide.[10](Different(manganese(oxide(phases(such(as(birnessite(
1. Introduction
PhotoBelectrochemical((PEC)(water(splitting(is(considered(to(be(
a( very( promising( method( to( store( solar( energy,[1]( and( has(
attracted( extensive( investigation.[2]( One( of( the( key( factors(
restricting( the( application( of( water( splitting( as( an( energy(
technology( is( the( lack( of( highly( efficient,( low( cost( and(
sustainable( catalysts.( In( recent( years,( oxygen( evolution(
catalysts( (OECs)( based( on( earthBabundant( elements( such( as(
manganese( oxides( (MnOx),[3,( 4]( cobalt( oxides( (CoOx),[5]( iron(
oxides( (FeOx)[6]( and( nickel( oxides( (NiOx)[7]( have( been( well(
studied( due( to( their( high( catalytic( activity.( Manganese(
catalysts,( which( have( been( identified( as( the( key( element( in(
Photosystem(II,(are(extremely(interesting.[8](We(have(reported(
that(manganese(molecular(clusters(doped(into(Nafion(exhibit(
good(water(oxidation(catalytic(activity.[9](Our(subsequent(study(
revealed( that( the( manganese( cluster( evolved( into( a( material(
with( a( birnessite( structure( (δBMnO2)( after( applying( a( certain(
[15]
(δBMnO2),[11,(12](pyrolusite((βBMnO2),[13](Mn2O3[14]and(Mn3O4
(
have( been( studied( as( catalysts( for( the( OER.( Among( these(
manganese(oxides,(MnO2(and(Mn2O3(have(proven(to(offer(the(
best(catalytic(performance.[14,(16](The(electrodeposition(process(
used(for(the(preparation(of(the(material(is(also(important(for(
achieving( high( catalytic( activity.( For( example,( the( Dau( group(
reported( that( the( electrodeposited( MnO2( by( voltageBcycling(
method(exhibits(good(catalytic(activity(while(electrodeposition(
at(constant(anodic(potential(produced(an(“inactive”(material(in(
their(study.[3](By(applying(an(ionic(liquid(electrolyte,(MnOx(was(
electrodeposited( at( high( temperature( and( found( to( exhibit(
higher(catalytic(activity(than(aqueous(electrodeposits.[16]
(
In(addition(to(the(nature(of(the(catalyst,(the(electrolyte(is(
another(important(factor(that(can(impact(the(water(oxidation(
process(and(catalyst(performance.[17,(18](The(effects(of(pH(have(
been(studied,(and(the(MnO2(catalyst(shown(to(exhibit(higher(
catalytic( activity( at( higher( pH.[11]( Recently,( we( reported( a(
relatively(low(overpotential((~150(mV)(water(oxidation(process(
on( electrodeposited( MnOx( that( was( achievable( only( in( a(
certain(family(of(organic(salt((e.g.,(nBbutylammonium(sulphate)(
electrolytes.[18,(19](In(this(case,(water(can(be(oxidised(at(lower(
overpotential(primarily(to(hydrogen(peroxide((H2O2),(which(is(
stabilised( by( hydrogen( bonding( with( the( ammonium( ion.( In(
this( respect,( the( process( can( be( regarded( as( an( nB
butylammonium( assisted( formation( of( H2O2( electrocatalytic(
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reaction,(and(this(process(provides(an(alternative(pathway(for( under( strong( illumination.( Here,( we( study( the( effect( of(
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water(splitting(at(lower(overpotentials.[18]
(
DOI: 10.1039/C5TA03850B
concentrated( solar( illumination( on( MnOx( water( oxidation(
Although( much( effort( has( been( devoted( to( investigations( electrodes.(For(MnOx(to(become(an(efficient(photoBelectrode,(
of( MnOx,( most( studies( have( focused( on( the( dark( electroB further(work(is(needed(to(improve(the(photoBelectrochemical(
catalytic(activity.[3,(4](In(fact,(MnOx(could(also(be(a(promising( performance,(by(optimising(both(the(electrolyte(and(also(the(
material( for( photoBelectrochemical( water( oxidation( as( crystalline(structure(and(morphology[6](to(enhance(the(photonB
indicated(by(a(band(structure(of(MnOx(that(has(a(more(positive( absorption( efficiency( and( the( photoBgenerated( carrier(
valence( band( energy( than( the( H2O/O2( reaction( potential(( transport(rate.(We(focus(on(the(role(of(the(electrolyte(in(the(
(figure(1),[20](and(a(band(gap((Eg(≈(2(eV,(which(corresponds(to( photoBwater( oxidation( process( and( demonstrate( that( the(
an(absorption(edge(at(~600(nm)(that(enables(light(absorption( photoBelectrochemical(oxidation(of(water(is(enhanced(by(the(
in(the(visible(part(of(the(spectrum.[21](However,(thus(far,(only(a( introduction( of( a( buffered( alkylammonium( ionic( liquid(
few( studies( have( probed( the( photoBelectrochemical( water( electrolyte.(The(process(is(shown(to(produce(both(oxygen(and(
oxidation( process( on( MnOx,( and( these( show( that( the( MnOx( hydrogen( peroxide( at( low( overpotentials.( The( effects( of(
films(exhibit(quite(poor(photoBactivity(for(water(oxidation.[22,( substrate,(film(thickness(and(illumination(direction(are(studied(
23]( WellBdefined,( dispersed( nanoparticles( MnOx( formed( by( and(optimised(and,(thereby,(photocurrents(as(high(as(~4.5(mA(
incorporating(Nafion(showed(significant(photoBactivities,(with( cm2((η(=(540(mV)(have(been(achieved(on(the(MnOx(film.((((
photocurrents( similar( to( the( dark( currents( measured( at(
overpotentials(of(150B550(mV.[24]((However,(the(low(loading(of(
MnOx( in( these( photoBanodes( meant( that( the( photocurrents(
were( relatively( small( (<0.1( mA( cmB2).( In( general,( the( photoB
activity( on( MnOx( is( still( quite( low( compared( to( other( typical(
photoBelectrochemical( water( oxidation( catalysts,( such( as(
Fe2O3[6]( and( BiVO4,[25]( and( the( investigation( of( MnOx( as(
photoanode(is(still(at(a(very(early(stage.((
(
2. Experimental Section
Materials(
All( reagents( were( purchased( from( commercial( supplies( and(
used(as(received(unless(stated(otherwise.(Deionised(water(was(
used(for(all(experiments.(Ethylammonium(nitrate((EAN)(or(nB
butylammonium( nitrate( (BAN)( were( prepared( by( neutralising(
the(ethylamine((EA)(or(butylamine((BA)(with(dilute(nitric(acid(
(HNO3),( followed( by( removal( of( the( water( under( reduced(
pressure(by(rotary(evaporation(at(50(oC(for(2(hours.(
Film(deposition(
(
(
The( manganese( oxide( (MnOx)( films( were( prepared( by(
electrodeposition(in(a(threeBelectrode(electrochemical(system(
from(an(ionic(liquid(electrolyte,(as(described(in(greater(detail(
elsewhere.[16,( 18]( The( deposition( electrolyte( (2( ml( in( volume)(
contained(10(mM(Mn(CH3COO)2(in(a(1:9(water(to(EAN(mixture.(
It(was(acidified(slightly(by(adding(10(μl(of(2.0(M(HNO3(to(give(a(
10( mM( excess( of( acid.( The( MnOx( film( prepared( using( this(
electrolyte( was( denoted( as( sample( A1( in( our( previous(
report.[16](FluorineBdoped(tin(oxide((FTO)(conducting(glass(or(a(
gold((Au)(sheet(with(an(active(electrode(area(of(0.5(cm(x(0.5(
cm(was(used(as(substrate(for(film(deposition.(The(MnOx(film(
was(electrodeposited(at(120(oC(by(applying(a(constant(current(
density( of( 200( µA( cmB2.( Following( exploratory( studies( to(
determine(the(optimum(deposition(time,(a(time(of(30(min(was(
generally(used(to(prepare(the(film,(unless(otherwise(specified.(
After( deposition,( the( films( were( rinsed( thoroughly( with(
distilled(water.(
(
Figure( 1.( Schematic( band( structures( of( manganese( oxides(
(Birnessite)(with(reference(to(the(H2O2/O2(reaction.(
(
The(light(intensity(has(been(proven(to(affect(the(photocurrent(
generated(on(photoelectrodes(in(previous(studies,[26B28,(29](and(
an(illumination(of(AM1.5(100(mW(cmB2(is(usually(applied(in(the(
research.[6,(25,(30B32](On(the(other(hand,(in(practical(applications,(
the( solar( light( intensity( can( be( easily( magnified( using( solar(
concentrators.(This(approach(has(been(used(in(concentrating(
photovoltaic( (PV)( systems( to( focus( sunlight( onto( a( small( PV(
chip,( producing( highly( efficiency.[33]( This( could( potentially(
reduce( the( overall( cost( of( solar( systems( in( general( since( the(
solar( collector( is( typically( less( expensive( than( the( equivalent(
area(of(solar(cells.(Similarly,(in(solar(water(splitting,(the(use(of(
a( solar( concentrator( has( the( exciting( potential( to( generate(
higher( efficiencies( and( result( in( lower( costs.( Thus,( it( is(
important( for( photoelectrodes( to( maintain( high( efficiency(
Photo4electrochemical(measurements(
The( photoBelectrochemical( performance( was( measured( in( a(
threeBelectrode( system( on( the( VMP2( potentiostat( system.((
Generally,(the(MnOx(films(and(Ti(foil(were(used(as(the(working(
and(counter(electrodes,(respectively(with(Ag/AgCl(or(saturated(
calomel(electrode((SCE)(reference(electrode.(The(potential(of(
the(Ag/AgCl(reference(electrode(was(B60(mV(vs(SCE.(
2(|(J.!Name.,(2012,(00,(1B3(
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(A(1.0(M(nBbutylammonium(nitrate((BAN)(solution(of(pH9( were(varied(from(200(kHz(to(0.1(Hz,(using(an(amVpieliwtuArdtiecl(eoOfn(l1in0e(
DOI: 10.1039/C5TA03850B
or(pH10(was(prepared(by(adding(1.2(mol(%(and(11.2(mol(%(of( mV.( ( The( spectra( were( fitted( to( an( equivalent( circuit( model(
extra( nBbutylamine( (BA)( into( the( BAN( solution,( respectively.( using(the(ZView(software.(
The( 0.20( M( of( borate( buffer( (Bi)( at( pH9( was( prepared( by(
adding(boric(acid(solution((H3BO3)(into(a(0.05(M(sodium(borate(
solution( (Na2B4O7r10H2O),( whereas( the( pH10( buffer( was(
prepared(by(adding(sodium(hydroxide(solution(into(the(sodium(
borate(solution.(The(mixed(electrolytes(at(pH9(or(pH10(were(
obtained( by( mixing( the( (pH9/pH10)( BAN( solutions( with( Bi(
buffer((pH9/(pH10),(and(then(slightly(adjusting(the(pH(of(the(
mixture(to(9(or(10(by(adding(either(H3BO3(or(BA.(Typically,(10(
ml( of( electrolyte( was( used( for( the( photoBelectrochemical(
measurements.(
Film(characterization(
Film( thickness( was( measured( by( using( a( Dektak( 150( surface(
profiler.((The(absorption(spectrum(of(the(film(is(measured(on(a(
Jasco(VB670(UVBVis(spectrophotometer.(The(morphology(of(the(
film( was( analysed( using( the( scanning( electron( microscopy(
(SEM)(secondary(image,(which(was(conducted(at(30(kV(with(a(
working(distance(of(6(mm(on(a(JEOL(7001F(FEG(SEM.(Raman(
spectra( were( recorded( with( a( Renishaw( inVia( Raman(
microscope(using(a(514(nm(incident(laser.((
NMR(Spectroscopy!
Linear( scan( voltammetry( (LSV)( and( controlled( potential(
electrolysis( (CPE)( were( used( to( characterize( the( catalytic(
performance(with(respect(to(water(oxidation.(The(LSVs(were(
usually(carried(out(by(sweeping(from(0.3(V(to(1.2(V(vs(SCE(at(a(
scan( rate( of( 5( mV( sB1.( The( CPE( was(conducted( by( applying( a(
constant(potential(of(0.4(B(1.2(V(vs(SCE(at(room(temperature.(
In(the(LSV(test,(the(dark(and(light(LSV(were(measured(in(the(
same( electrolyte,( with( the( dark( LSV( tested( first.( In( all( other(
measurement,(fresh(electrolytes(were(used.((
Except( for( the( IPCE( measurements,( all( photoB
electrochemical(measurements(were(made(using(a(300(W(Xe(
lamp( as( the( light( source.( The( intensity( of( the( light( was(
controlled( by( varying( the( distance( between( the( light( source(
and(sample.(In(typical(measurements,(the(distance(was(kept(at(
2(cm,(where(the(light(intensity(equals(about(9(suns((calibrated(
with( Thorlabs( optical( power( and( energy( meter( PM110D(
equipped(with(S310C(thermal(sensor),(and(most(of(the(photoB
electrochemical(experiments(were(done(at(this(light(intensity(
using(backBside(illumination(unless(otherwise(specified.((((
The(electrolyte(for(NMR(spectral(measurement(was(prepared(
by(using(D2O(as(the(solvent,(and(the(electrolyte(contained(0.10(
M(Bi(and(1.0(M(BAN,(at(pH9.(Proton(NMR(measurements(were(
conducted( on( a( Bruker( AvanceIII( 400( spectrometer.( NMR(
signals( were( collected( on( the( electrolytes( before( and( after(
photoBelectrochemical(testing(under(~9(suns(illumination.(
Oxygen(and(hydrogen(peroxide(measurements(
Oxygen( measurements( were( conducted( in( a( homeBbuilt( gasB
sealed(threeBelectrode(cell.(A(6(ml(solution(of(0.10(M(Bi(buffer(
containing(1.0(M(BAN(at(pH9(was(added(into(the(cell,(leaving(a(
headBspace(volume(of(26(ml.((The(oxygen(in(the(head(space(
was( measured( using( a( fluorescence( probe,( as( detailed(
elsewhere.[16](The(fluorescence(probe(is(very(sensitive(to(light,(
and( the( gas( expansion( under( illumination( causes( a( pressure(
change.( Thus,( the( oxygen( measurement( under( illumination(
condition(was(conducted(by(measuring(the(oxygen(level(in(the(
head(space(of(the(cell(before(and(after(PEC(experiment.((
The( amount( of( hydrogen( peroxide( was( measured( by(
titration( in( a( UVBVis( cell,( using( potassium( permanganate( as(
oxidant,( as( detailed( in( the( literature.[18]( Typically,( 0.25( ml( of(
the( electrolyte( was( added( into( 0.25( ml( of( 1M( H2SO4,( and( a(
solution(of(0.00755(M(potassium(permanganate(was(used(for(
the(titration.(UVBVis(absorption(spectra(were(recorded(at(room(
temperature(on(a(Cary(300(Bio(UVBVisible(spectrophotometer(
and( the( permanganate( concentration( was( determined( from(
the( absorbance( at( 565( nm.[18]( A( control( experiment( was(
conducted(and(the(results((figure(S1)(show(that(in(this(titration(
no(amine(was(oxidized(by(the(permanganate.(
(
Incident(Photon(to(Current(Efficiency((IPCE)(measurements(
were(made(using(a(150(W(Oriel(solar(simulator,(equipped(with(
an(AM1.5G(filter,(as(the(light(source.((A(series(of(filters(were(
used(to(cut(the(light(to(monochromatic(light(from(370(nm(to(
570(nm(having(bandwidths(of(20(nm.(The(total(light(intensity(
for(the(IPCE(measurement(was(about(3(suns,(hence(for(each(20(
nm( band( the( intensity( is( about( 1~6%( sun( (1~6( mW( cmB2)(
depending( on( the( transparency( of( filters.( An( electrolyte(
containing(0.20(M(Bi(buffer(and(1.0(M(BAN(at(pH9(was(used(to(
measure(the(IPCE,(and(a(constant(potential(of(1.0(V(vs(SCE(was(
applied(during(the(test.((The(IPCE(was(calculated(as(follows:(
Hydrogen(production(is(observed(on(the(counter(electrode(
with( formation( of( bubbles( during( the( PEC( process.( The( GC(
measurement(of(the(collected(gas(confirms(the(production(of(
hydrogen.(
!"!#$%&'(/!"#
!ℎ!"!!"/!"#
! !" !×1240
IPCE!% =
=
!×100(
! !" ×!(!")
where! I(mA)( indicates( the( photocurrent( (mA( cmB2),( P(mW)(
indicates(the(input(light(intensity((mW(cmB2)(and((λ(represents(
the(wavelength(of(monochromatic(light.(The(number(of(1240(
is( the( value( of( hc/(e! x( 1059),( with( h,! c! and! e! are( Planck’s(
constant,(speed(of(light(and(electron(charge,(respectively.(
3. Results and Discussion
3.1(Photo4electrochemical(Performance((
The(MnOx(film(deposited(from(acidic(ionic(liquid((A1)(for(10(mins(at(
120(oC(on(FTO(substrate(served(at(the(photoBanode(for(the(photoB
electrochemical(measurements.(The(obtained(film(has(a(nanoBsized(
fibrous(structure((figure(S2a)(with(a(thickness(around(900(nm.(XRD(
has( been( conducted( in( our( previous( study,[16]( and( no( peak(
Electrochemical(
impedance(
spectroscopy(
(EIS)(
measurements( were( carried( out( from( 0.4( to( 1.0( V( vs( SCE( in(
dark(and(under(illumination.(The(EIS(measurement(frequencies(
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attributable( to( MnOx( was( observed.( Extra( characterization( of( the( time,( the( thicker( the( films.[22]( Figure( S3( shows( the( thickness( of(
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DOI: 10.1039/C5TA03850B
material(carried(out(in(that(work(by(EXAFS(and(EDS(indicates(that( MnOx( films( obtained( from( different( deposition( time,( exhibiting( a(
this(film(corresponds(to(the(typical(birnessite(phase,(as(detailed(in( linear(relationship.(LSVs(of(MnOx(films(with(different(thickness(were(
our(previous(report.[16]
(
measured(in(the(dark(and(under(light(as(shown(in(figures(S4(and(3.(
The(linear(scan(voltammetry((LSV)(was(measured(in(several( It(can(be(seen(that(the(film(deposited(for(5(minute(has(the(highest(
electrolytes(in(the(dark(and(under(illumination(to(evaluate(the( activity(in(dark,(whereas(the(thicker(film(shows(a(slightly(decreased(
photoBelectrochemical(performance(of(the(MnOx(film.(Figure(2( dark( current( (figure( S4).( This( could( be( due( to( lower( carrier(
shows(the(LSVs(of(the(MnOx(in(three(electrolytes(at(pH9:((i)(1.0( collection(efficiency(in(the(thicker(film.[22](On(the(other(hand,(due(to(
M( nBbutylammonium( nitrate( (BAN);( (ii)( 0.20( M( borate( (Bi)( more(material(in(the(thicker(film,(more(light(can(be(absorbed,(and(
buffer;(and((iii)(a(mixture(of(0.10(M(Bi(buffer(with(0.50(M(BAN.( this(results(in(higher(photocurrents,(as(shown(in(figure(3.(Whereas,(
The(LSV(measured(in(the(Bi(buffer((black(dashed(line)(exhibits( for( the( very( thick( films,( both( the( resistance( within( film( increases(
a( clear( onset( of( dark( current( at( about( 0.9( V( vs( SCE,( while( it( and( also( the( pathway( for( charge( carrier( migration( to( the(
appears( at( around( 0.6( V( vs( SCE( in( the( BAN( solution( (blue( semiconductor(surface(increases(which(increases(the(recombination(
dashed( line).( For( the( BAN/buffer( mixture( (red( dashed( line),( rate( and( limits( the( photocurrent.[36]( In( this( study,( the( film( with(
which(was(obtained(by(mixing(the(BAN(solution(at(pH9(and(Bi( thickness(of(4(μm(from(a(30(min(deposition(exhibited(the(highest(
buffer(at(pH9(and(slightly(adjusting(the(pH(to(9,(the(onset(of( photocurrent( while( the( photocurrent( decreased( at( a( longer(
dark(current(occurs(at(0.6(V(vs(SCE,(which(is(same(as(that(in( deposition(time((45(min).((Therefore,(unless(otherwise(stated,(the(
the(BAN(solution((figure(2,(blue(dashed(line).((
MnOx(films(prepared(using(a(30(min(deposition(time(were(used(in(
Under( illumination( (solid( lines),( the( film( exhibits( photoB the(measurements(and(characterization(studies(that(follow.((
activity( in( all( the( three( electrolytes,( shown( as( solid( lines( in(
(
figure( 2.( There( is( a( decrease( in( the( onset( potential( and( a(
current(density(increase(under(light.(Under(illumination,(the(Bi(
buffer( (black( lines)( exhibits( either( little( or( no( photocurrent(
4.0
5 min deposition
10 min deposition
20 min deposition
30 min deposition
3.5
below(0.8(V.(The(highest(photocurrent(in(the(measured(region(
3.0
in(the(Bi(buffer(is(~0.5(mA(cmB2(at(1.2(V.(The(most(important(
45 min deposition
2.5
observation(is(that(the(photocurrent(in(the(mixture((red(line)(is(
much(higher(than(that(of(either(of(the(two(other(electrolytes(
2.0
1.5
(black(lines(for(Bi(and(blue(for(1.0(M(BAN).(The(photocurrent(
increases(with(the(applied(potential(below(1.0(V,(and(reaches(a(
1.0
plateau( at( higher( potentials,( as( is( normally( observed( with(
0.5
photoBelectrodes.[6,(34]((
0.0
(
0.6
0.7
0.8
0.9
1.0
1.1
1.2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
E vs SCE / V
1.0 M BAN-dark
1.0 M BAN- light
0.20 M Bi-dark
0.20 M Bi-light
0.10 M Bi+0.50 M BAN- dark
0.10 M Bi+0.50 M BAN- light
(
Figure(3.(Net(photoBcurrent(density(of(MnOx(films(with(different(
deposition(times,(measured(in(0.10(M(Bi+0.50(M(BAN(electrolyte(at(
pH9.(
(
The( influence( of( the( substrates( was( also( considered( and( it( was(
found( that( the( film( deposited( on( Au( exhibits( a( slightly( higher(
activity(than(that(on(FTO(glass(under(illumination((figure(S5).(This(
might(be(due(to(the(better(conductivity(of(the(Au(substrate,(which(
decreases(the(system(resistance(and(allows(the(electrode(to(carry(
higher( currents;( the( reflectance( of( the( Au( electrode( may( also(
increase(the(optical(path(length(in(the(semiconductor(and(results(in(
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
E vs SCE / V
(
better(utilization(of(the(light.[37,(38]
(
Figure(2.(LSVs(of(MnOx(film(measured(in(different(electrolyte(at(pH9(
in(dark(and(under(~9(suns(illumination(from(back(side(of(FTO.((
The( effect( of( the( side( of( illumination( on( the( photoB
electrochemical(performance(on(MnOx(was(investigated(on(the(
FTO(glass(electrodes.(Figure(S6(shows(the(CV(of(MnOx(in(dark(
and( under( different( illumination( conditions.( The( CV( plot(
exhibits( preBpeaks( (0.59( V( in( dark( and( 0.55( V( under(
illumination)( before( water( oxidation( starts.( These( peaks( has(
(
The( thickness( of( the( films( has( been( proven( to( be( an( important(
factor(in(the(photoBactivity(of(photoBelectro(active(materials.[35](In(
the( preliminary( experiment( shown( in( figure( 2( the( MnOx( film( was(
obtained( by( a( 10( minute( deposition.( The( films( thickness( can( be(
tuned(by(varying(this(deposition(time,(i.e.,(the(longer(the(deposition(
been(related(to(the(changes(of(the(surface(oxidation(states.[39]
(
4(|(J.!Name.,(2012,(00,(1B3(
This(journal(is(©(The(Royal(Society(of(Chemistry(20xx(
Please(do(not(adjust(margins(

Page 5 of 11
Journal of Materials Chemistry A
Please(do(not(adjust(margins(
Journal(Name(
(ARTICLE(
It(can(be(seen(that(the(photocurrents(measured(for(backBside( Further(investigation(of(the(effect(of(film(thickness(on(the(Tafel(
View Article Online
DOI: 10.1039/C5TA03850B
(substrate( side)( and( frontBside( (electrolyte( side)( illumination( slope( is( shown( in( figure( S7.( Compared( with( the( 30( mins(
were( very( similar.( The( illumination( direction( can( have( a( deposited( films,( a( 5( mins( deposited( film( exhibits( lower( Tafel(
significant( influence( on( the( performance( of( nanostructured( slopes.(The(larger(Tafel(slope(for(the(thicker(film(is(expected(
semiconductor( films.( Normally,( such( films( exhibit( lower( due( to( the( decreasing( electroBactivity( in( the( thicker( films( as(
photocurrents( under( frontBside( than( under( backBside( shown(in(the(dark(current(plot((figure(S4).(The(change(of(slope,(
illumination,(as(has(been(observed(on(Fe2O3[40](and(BiVO4.[31]
( for( example( in( the( mixture( from( 93( mV/dec( to( 59( mV/dec(
This(is(because(for(frontBside(illumination(the(photoBgenerated( (figure( S7b),( indicates( that( the( catalysis( mechanism( is(
charge( carriers( need( to( penetrate( the( film( before( they( are( different.(A(similar(effect(that(thinner(films(exhibit(lower(Tafel(
collected(by(the(conducting(substrate,(whereas(for(backBside( slopes(was(also(observed(on(a(Pt–Pd–rGO(electrode(used(for(
illumination(they(can(be(more(directly(and(efficiently(collected( the(hydrogen(evolution(reaction.[41](This(might(be(due(to(the(
by( the( substrate,( leading( to( higher( photocurrents.[31,( 40]( In( surface( states( varying( in( the( thicker( films,( and( lowering( the(
contrast,( Sasaki( and( coBworkers[23]( reported( higher( IPCEs( for( electroBcatalytic( activity.( Increasing( resistance( in( the( thicker(
frontBside(illumination(than(that(for(backBside(illumination(on( film(could(also(contribute(to(a(larger(Tafel(slope.(
MnO2,( and( proposed( that( only( the( topmost( layer( could(
generate(photocurrent(in(their(films.((
The(influence(of(the(BAN(and(Bi(concentration(in(the(mixed(
electrolyte( on( the( dark( current( and( photocurrent( was( also(
The(negligible(difference(between(frontBside(and(backBside( investigated(and(the(data(are(shown(in(figures(S8(and(5a.(In(
illumination( in( this( study( indicates( that( the( mobility( of( the( the(dark,(the(current(could(be(improved(by(increasing(the(Bi(
photoBgenerated( electrons( in( the( MnOx(film(is( not( a( strongly( concentration(while(variation(of(the(BAN(concentration(had(no(
limiting( factor( in( photocurrent( generation.( This( is( consistent( significant( effect( (see( figure( S8).( Under( illumination,( an(
with( the( film( thickness( study( above( that( showed( that( a( increase(in(the(BAN(concentration(in(the(mixture(improves(the(
reasonably(thick(film(generates(the(highest(photocurrent.((
photocurrent( (figure( 5a).( Variation( of( the( Bi( buffer(
To( further( understand( the( intrinsic( activity( of( the( film( in( concentration( in( the( mixture( also( affects( the( photocurrent(
different( electrolytes,( a( TafelBanalysis( was( performed( at( pH9( (figure(5a);(the(0.1(M(Bi(with(1.0(M(BAN(electrolyte(results(in(a(
under(dark(and(illumination(conditions,(as(shown(in(figure(4.(( smaller(photocurrent(density((3(mA(cmB2(at(1.0(V)(than(0.20(M(
The(30(min(deposited(film(in(the(Bi(buffer(exhibits(a(large(Tafel( Bi( with( 1.0( M( BAN( (4.5( mA( cmB2( at( 1.0( V,! η( =( 540( mV).( The(
slope( (cyan( line,( 153( mV/dec),( while( notably( in( the( BAN/Bi( highest(photocurrent(of(~4.5(mA(cmB2,(measured(in(0.20(M(Bi(
buffer(mixture((black(line),(the(Tafel(slope(is(much(smaller((~( with(1.0(M(BAN,(is(quite(remarkable(when(it(is(compared(to(a(
90( mV/dec).( Furthermore,( the( Tafel( slopes( for( the( 30( mins( value(of((<0.1(mA(cmB2(reported(previously(for(MnO2(studied(
deposited(film,(measured(in(the(mixture(electrolyte(in(the(dark( under(more(intense(illumination(conditions((~10(suns)[22](than(
(figure(4,(black(line,(96(mV/dec)(and(under(illumination((figure( used(herein(((~9(suns).(((
4,( red( line,( 91( mV/dec)( are( the( same;( however,( the( Tafel(
region(shifts(to(lower(overpotentials(under(illumination(due(to(
the( photoBenhanced( activity.( For( the( BAN( electrolytes,( the(
Tafel(slope(is(92(mV/dec((figure(4,(blue(line),(which(is(similar(to(
the(value(in(the(mixture(electrolyte.(The(significant(difference(
of( the( Tafel( slopes( measured( in( the( different( electrolytes(
indicates(that(the(water(oxidation(mechanism(may(be(different(
in(the(presence(and(absence(of(BAN.((
(
(a)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.20 M Bi pH9
0.20 M Bi + 0.50 M BAN pH9
0.20 M Bi + 1.0 M BAN pH9
0.10 M Bi + 1.0 M BAN pH9
0.10 M Bi + 0.50 M BAN pH9
1.0 M BAN pH9
(
k
r
a
0.9
0.8
0.7
0.6
0.5
0.4
0.3
d
-
i
B
,
c
e
d
/
V
m
4
±
3
5
1
0.6
0.7
0.8
0.9
1.0
1.1
1.2
(b)
E vs SCE/ V
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Borate buffer (0.10 M) with 1.0 M BAN
Bicarbonate buffer (0.10 M) with 1.0 M BAN
k
r
a
d
-
N
,
A
B
h
t
i
w
i
k
r
B
a
,
d
c
-
e
N
d
/
A
V
B
m
c
3
e
d
/
±
6
9
V
m
3
±
2
9
t
h
g
i
l
-
N
A
B
h
t
i
w
i
B
,
c
e
d
/
V
m
1
±
1
9
-2.5
-2.0
-1.5
-1.0
-0.5
Log ( J / mA cm-2)
(
Figure(4.(Tafel(plots(of(MnOx(film(deposited(for(30(mins,(measured(
in(different(electrolytes(at(pH9(in(dark(and(under(light.(
0.6
0.7
0.8
0.9
1.0
1.1
1.2
E vs SCE /V
(
(
This(journal(is(©(The(Royal(Society(of(Chemistry(20xx!
J.!Name.,(2013,(00,(1B3(|(5(
Please(do(not(adjust(margins(

Please(do(not(adjust(margins(
Journal of Materials Chemistry A
Page 6 of 11
ARTICLE(
Journal(Name(
Figure( 5.( (a)( Photocurrent( density( of( MnOx( films( measured( in(
different(electrolytes(at(pH9.((b)(MnOx(photocurrents(measured(in(
Bi(buffered(BAN(electrolyte((black),(and(bicarbonate(buffered(BAN(
electrolyte( (red)( at( pH9.( The( net( photocurrent( is( obtained( from(
LSVs(plots(by(subtracting(the(dark(current((figure(S8)(from(the(total(
current.(
(
View Article Online
DOI: 10.1039/C5TA03850B
8
7
6
5
4
3
2
1
0
(
In(order(to(investigate(whether(the(increase(in(photocurrent(
for(the(electrolyte(mixture(is(specifically(due(to(the(presence(
of(the(borate(buffer,(a(bicarbonate(buffer(mixed(with(the(BAN(
solution(was(also(studied.(Replacement(of(the(Bi(buffer(with(
the( bicarbonate( buffer( results( in( a( similar( photocurrent,( as(
shown(in(figure(5b.(However,(if(the(Bi(buffer(is(replaced(with(
NaNO3,(the(LSV(behaviour(is(similar(to(that(measured(with(the(
BAN( electrolyte( solution( (see( figure( S9);( i.e.,( only( small( dark(
and( photocurrents( were( observed.( This( indicates( that( the(
improved(photocurrent(is(probably(due(to(the(use(of(buffered(
electrolytes,(rather(than(any(particular(buffer(anions/cations.((
((((((Ethylammonium( nitrate( (EAN)( is( another( common( amine(
based( ionic( liquid( with( a( similar( structure( to( BAN.( Replacing(
BAN( in( the( electrolyte( mixture( with( EAN( reveals( that( both(
electrolytes(result(in(high(photocurrent(densities((figure(S10).(
Thus,(the(high(photocurrents(are(not(limited(to(the(BABbased(
electrolyte,(indicating(that(other(amine(cations(can(indeed(be(
used.( The( slightly( lower( photocurrent( for( the( EAN/Bi(
electrolyte(mixture(may(be(due(to(the(slightly(smaller(size(of(
the( cation( used( or( the( pKa( difference( (BA:( 10:59;[42]( EA:(
10.70[43]);(this(is(under(further(investigation.(((
The( borate( in( the( mixture( could( just( play( the( role( of( a(
buffer( for( the( amine( ions( based( electrolytes.( As( the( pKa( of(
butylamine( is( 10.59,[42]( a( BAN( electrolyte( with( a( higher( pH(
(~10)(could(be(considered(to(be(a(selfBbuffered(electrolyte((nBA(
:(nBAN=(0.11:1(at(pH=10).(In(order(to(find(whether(the(borate(
based( inorganic( electrolyte( is( essential( for( obtaining( high(
photocurrent,( further( testing( was( conducted( in( BAN(
electrolytes(with(and(without(Bi,(and(the(results(are(shown(in(
figure( 6.( In( order( to( better( compare( the( activity( at( different(
pHs,( the( applied( potentials( have( been( converted( to(
overpotentials( (η( =( E( B( E0).( The( dark( current( density( in( the(
BAN/Bi( mixture( at( pH10( (2.7( mA( cmB2( at( η( =( 0.5( V)( is( much(
higher(than(that(in(the(electrolytes(with(the(same(composition(
at(pH(9((1.1(mA(cmB2(at(η(=(0.5(V).(This(is(probably(due(to(the(
expected(increase(in(activity(of(MnOx(at(higher(pH.[11](Under(
illumination,( the( photocurrent( measured( for( the( BAN(
electrolyte(at(pH10((2.9(mA(cmB2(η(=(0.5(V),(is(more(than(three(
times(than(that(at(pH9((0.5(mA(cmB2(at(η(=(0.5(V)).(Similarly,(
the(photocurrent(in(the(pH10(BAN/Bi(electrolyte(mixture(is(3.5(
mA(cmB2,(which(is(50%(higher(than(that(at(pH9((2.5(mA(cmB2).(
Thus,(increasing(the(pH(clearly(results(in(a(effective(increase(
photocurrent( in( the( BAN,( and( it( indicates( that( the( buffer(
conditions( (by( selfBbuffering( or( adding( inorganic( buffer(
electrolyte)(in(the(presence(of(the(BAN(is(very(important(for(
and(affords(substantial(increases(in(high(photoBactivity.(((
0.2
0.3
0.4
0.5
0.6
0.7
η
/
V
(
Figure(6.(Dark(current((filled(bar(with(grid)(and(photocurrents((filled(
bar( without( grid)( on( MnOx( measured( at( different( potentials(
obtained(from(LSV(plots,(in(1.0(M(BAN(+(0.10(M(Bi(at(pH10((blue),(
1.0(M(BAN(at(pH10((pink),(1.0(M(BAN(+(0.10(M(Bi(at(pH9((orange),(
1.0(M(BAN(at(pH9((green).(
(
CPE(measurements(were(conducted(at(different(potentials(
on(the(MnOx(deposited(on(FTO(with(back(side(illlumination,(as(
shown( in( figures( 7( and( S11.( The( steadyBstate( photocurrents((
determined( by( this( means( are( independent( of( the( scan( rate(
and( measurement( history.[34]( ( Figure( 7( clearly( shows( that(
illumination( generates( a( reproducible( photocurrent.( The(
steadyBstate(photocurrent(when(a(pH9(buffer/BAN(electrolyte(
is( used( is( ~3( mA( cmB2( is( much( higher( than( for( the( Bi( buffer(
(~0.2(mA(cmB2)(and(BAN(solution((~(0.3(mA(cmB2),(as(expected(
from(the(LSV(results(in(figure(2.((
The(decay(of(the(photocurrent(back(to(zero(from(the(time(
the( light( is( switched( off( takes( as( long( as( ~150( s( for( the(
BAN/buffer((mixture((see(last(cycle(of(the(blue(line(in(figure(7).(
This(behaviour(is(not(normal(for(semicondutors,(but(has(also(
been( observed( for( some( other( photoelectrodes.[44]( This( slow(
photocurrent(decay(may(originate(from:(1)(the(heating(effect(
of(the(intense(light;(and/or(2)(slow(charge(carrier(diffusion(in(
the( MnOx( film( or( electrolyte.( The( temperature( of( the(
electrolytes(was(measured(before(and(after(each(test((overall(
illumination(for(120(s(as(shown(in(figure 7)(and(the(observed(
variation(is(only(~2(oC,(indicating(that(heating(effects(may(not(
be(an(important(factor.(This(conclusion(is(strongly(supported(
by(fact(that(a(similar(slow(current(decay(is(observed(when(the(
BAN( electrolyte( without( the( Bi( buffer( is( used( (red( trace( in(
figure( 7),( and( that( such( currentBdecay( behaviour( is( not(
observed( for( the( measurements( applying( only( the( Bi( buffer(
(see( sharp( rise( on( switching( in( the( black( trace( in( figure( 7).(
Additional( CPE( measurements,( conducted( at( different(
potentials((see(figure(S11aBc)(provide(further(evidence(against(
heating(effects(being(a(factor.(At(low(potentials((0.4(V(in(figure(
S11a(and(0.6(V(in(figure(S11b),(the(photocurrent(increases(and(
decreases(instantly(on(switching(the(light(on(and(off.(It(is(at(
higher(potentials(in(the(presence(of(BAN,(where(the(currents(
6(|(J.!Name.,(2012,(00,(1B3(
This(journal(is(©(The(Royal(Society(of(Chemistry(20xx(
Please(do(not(adjust(margins(

Page 7 of 11
Journal of Materials Chemistry A
Please(do(not(adjust(margins(
Journal(Name(
(ARTICLE(
are( higher( (1.0( V( and( 1.2( V( in( figures( S11c( and( 7),( that( Rp( indicating( a( significant( decrease( in( the( water( oxidation(
View Article Online
DOI: 10.1039/C5TA03850B
switching( results( in( longer( photocurrent( transients.( This( charge( transfer( resistance,( as( expected( from( the( higher(
indicates( that( a( slow( build( up( and( decay( of( the( steady( state( photocurrents(shown(in(figures(2(and(3.(
population( of( photoBexcited( electrons( and( holes( may( be( the(
Figure( S12e( shows( the( impedance( spectroscopy( of( the(
origin(of(these(effects;(this(behaviour(will(be(further(discussed( MnOx(films(at(pH10(at(0.8(V.(Two(semicircles(can(be(observed(
later.(A(longer(term(experiment((more(than(1(hr(illumination(of( in( the( impedance( spectra( for( both( the( BAN( and( mixture(
the( MnOx( film)( does( not( show( an( obvious( decrease( of( the( electrolytes.( This( indicates( that( at( pH10,( where( the( BAN(
photocurrent( with( time( (figure( S11d)( and,( moreover,( the( solution( is( selfBbuffered,( there( is( a( similar( charge( transfer(
morphology( and( the( Raman( spectra( of( the( film( does( not( resistance( to( that( for( the( mixture.( This( is( again( concordant(
change( significantly( after( long( term( testing( (figure( S2).( Thus,( with(the(similar(photocurrents(shown(in(figure(6.((
the( MnOx( photoBcatalyst( is( stable( during( prolonged( photoB
electrochemical(water(oxidation.(
(
3.2(Optical(Properties(and(IPCE(Measurement(
(
UVBVis(spectrophotometry(was(used(to(investigate(the(optical(
properties(of(the(MnOx(films,(as(shown(in(figure(S13(and(Figure(
8((blue(line).((The(film(exhibits(absorption(up(to(650(nm.(The(
Tauc(plot(of(the(variation(of((αhν)1/2(and((αhν)2(with(photon(
energy,(inset(to(figure(8,(shows(an(energy(gap(for(the(indirect(
(2.25(eV)(and(direct(transitions((2.7(eV)(in(MnOx(films,(which(
are(close(to(literature(reports(for(birnessite(MnO2(films((2.1(eV(
and(2.7(eV,(respectively).[22](Calculations(of(vacancyBfree(MnO2(
shows(that(it(is(an(intrinsic(indirect(band(gap(material,(while(
the( existence( of( Ruetschi( defects( (Mn(IV))( can( introduce( a(
direct( excitation.[47]( Both( the( indirect( band( gap( energy( and(
direct(band(gap(energy(can(be(observed(in(MnOx,(as(shown(in(
this( research( and( in( the( literature.[22]( The( existence( of( the(
indirect( band( gap( is( considered( to( cause( a( decrease( in( the(
recombination( rate( of( the( photoBexcited( electronBhole( pairs,(
which( can( improve( the( photoBactivity( (for( example,( anatase(
TiO2(which(has(an(indirect(band(gap(usually(exhibits(a(higher(
photoBactivity( than( rutile( TiO2( which( has( only( a( direct( band(
gap).[48]((
4.0
light off
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
0.20 M Bi +1.0 M BAN, pH9
0.20 M Bi, pH9
1.0 M BAN, pH9
light on
0
50
100
150
200
250
300
350
400
450
Time /s
(
Figure( 7.(CPE(measured(on(MnOx(films(in(different(electrolytes(at(
pH9(under(chopped(illumination(from(the(back(side.(Arrows(show(
the(turning(of(the(light(on(and(off.(The(photocurrent(was(obtained(
by(subtracting(the(dark(current(from(the(overall(current.(Conditions(
of(test:(1.0(V((vs(SCE)(under(~9(suns(light(intensity.((
(
Electrochemical( impedance( spectroscopy( (EIS)( was( used( to(
study(the(charge(transfer(during(water(oxidation(as(shown(in(
figures(S12.(The(semicircle(in(the(high(frequency(range(for(a(
particular(electrolyte(shows(no(change(in(the(position(and(the(
radius(by(varying(the(applied(potential,(but(decreases(a(little(
on( illumination( (illuminated( and( dark( EIS( are( compared( in(
figure(S12(a,(b,(c).(The(fitted(parameters((table(S1)(show(that(
the(resistance(R1,(usually(related(to(the(resistance(of(the(film(
and(electrolyte(as(well(as(contacts(in(the(circuit,[38,(45](is(smaller(
in(the(mixture(and(BAN(solution(than(that(in(Bi(buffer.((This(is(
likely( due( to( the( better( conductivity( when( a( higher(
concentration(electrolyte(is(used.(At(low(frequencies,(another(
semicircle(appears(and(its(radius(under(illumination(is(smaller(
than( in( the( dark( at( higher( potentials.( This( semicircle( varies(
with(applied(potential((figure(S12a,(b,(c)(and(is(understood(to(
be( associated( with( the( water( oxidation( reaction.[38,( 46]( A(
comparison( of( the( Nyquist( plots( at( 1.0( V,( see( figure( S12d,(
shows(that(the(radius(of(the(semicircle(at(low(frequencies(is(
much(smaller((under(illumination)(in(the(Bi(buffer(and(BAN/Bi(
mixture(than(in(BAN.(The(fitted(parameters(indicate(a(higher(
charge(transfer(resistance((Rp)(for(the(water(oxidation(reaction(
in(the(BAN(electrolyte,(as(expected(from(the(LSV(plot(at(this(
potential((figure(2).(The(BAN/Bi(mixture(exhibits(the(smallest(
(
1.0x105
5.0
4.5
Absorption
8.0x104
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IPCE
2.0x104
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350
400
450
500
550
600
650
700
750
800
Wavelength / nm
(
Figure( 8.( UVBVis( spectra( and( IPCE( results.( (Blue( line)( Absorption(
coefficient( was( calculated( from( the( transmittance( spectra( of( the(
thinnest(MnOx(film((figure(S13).(Inset(shows(the(variation(of((αhν)1/2
(
and( (αhν)2( with( photon( energy( for( the( MnOx( film;( the( horizontal(
intercept( represents( the( energy( gap( of( the( indirect( and( direct(
transitions(respectively.((Red(dotted(line)(IPCE(results(for(the(MnOx(
film(on(FTO(in(0.20(M(Bi(buffer(+1.0(M(BAN(electrolyte((pH9).(The(
test(was(conducted(whilst(applying(a(constant(potential(of(1.0(V(vs(
SCE(under(monochromatic(light((1B6(mW(cmB2).((
(
This(journal(is(©(The(Royal(Society(of(Chemistry(20xx!
J.!Name.,(2013,(00,(1B3(|(7(
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Journal of Materials Chemistry A
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Journal(Name(
The(photocurrents(generated(under(monochromatic(light((20(
nm( bandwidth)( were( measured( at( a( relatively( high( potential(
(1.0(V(vs(SCE(in(0.20(M(Bi(with(1.0(M(BAN(electrolyte(at(pH9)(
to( ensure( the( accuracy( of( the( measurements.( The( incident(
photon(to(current(efficiency((IPCE)(is(calculated(as(detailed(in(
the(experimental(section,(and(is(plotted(as(a(function(of(the(
excitation( wavelength( in( figure( 8( (red( dotted( line).( The( IPCE(
follows(the(absorption(spectrum,(increasing(significantly(below(
450(nm.(The(highest(IPCE(of(~4%(was(achieved(at(the(lowest(
wavelength(measured((370(nm)(and(was(above(3%(at(400(nm.(
Thus,( the( MnOx( film( more( effectively( converts( incident(
photons(to(current(at(the(shorter(wavelengths.(Compared(to(
the( absorption( spectrum( (blue( line( in( figure( 8),( the( IPCE(
spectrum( shifts( slightly( to( shorter( wavelengths.( This(
phenomenon( has( been( observed( previously( in( MnO2[22]( and(
Fe2O3[40,( 49]( and( may( be( due( to( a( predominance( of( indirect(
bandBgap(transitions(under(illumination(at(longer(wavelengths,(
which(are(of(lower(energy(and(hence(lower(the(efficiency(of(
photocurrent(generation(at(these(wavelengths.[22,(49]((
(
View Article Online
DOI: 10.1039/C5TA03850B
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200
400
600
800
1000
Light intensity / mW cm-2
(
Figure(9.(The(influence(of(light(intensity(on(the(steady(state(photoB
current(density(of(MnOx(films(tested(in(0.20(M(Bi(buffer(+(1.0(M(
BAN(electrolyte((pH9)(at(1.0(V(vs(SCE.(Each(data(point(is(the(mean(
of(measurements(on(three(different(samples((error(bar(=(s.e.m.)(((
(
3.3(Oxygen(and(Hydrogen(Peroxide(Production(
The( IPCE( efficiencies( determined( in( this( study( are( much(
higher( than( previous( reported( values( for( MnOx,( which( well(
below( 1%.[22,( 23]( The( remarkable( improvement( in( IPCE(
presented( herein( could( be( partly( due( to( the( optimisation( of(
the(MnOx(film(with(respect(to(thickness,(and(more(importantly(
to( the( significant( enhancement( in( photoBelectrochemical(
activity(afforded(by(using(buffered(amineBbased(electrolytes.((
In(this(study,(the(photoBelectrochemical(performance(was(
mostly(recorded(under(a(high(light(intensity((~9(suns),(which(is(
much( higher( than( that( normally( used( in( similar( studies( (~1(
sun).[6,(30](In(order(to(study(the(influence(of(light(intensity,(the(
steadyBstate( photocurrents( at( different( intensities( were(
measured,( as( shown( in( figure( 9.( The( photocurrent( increases(
linearly( with( the( light( intensity( and,( over( the( intensity( range(
studied,(does(not(show(any(signs(of(reaching(a(saturation(limit(
of(the(sort(seen(in(other(materials(such(as(CdS.[26](The(linear(
relationship( reveals( that( charge( is( transferred( rapidly( at( the(
interfaces,[29]( otherwise( recombination( would( become( more(
We( previously( reported( that( the( nBbutylamine( (BA)( cation( is(
stable( without( any( decomposition( or( oxidation( observed(
during(water(oxidation(in(dark(conditions.[18](In(order(to(check(
whether( BA( decomposes( under( light,( NMR( spectra( of( the(
electrolyte( were( recorded( before( and( after( water( oxidation(
under(light.(The(spectra(show(no(evidence(of(decomposition(
products( being( formed( (figure( S14)( confirming( that( the(
increase(in(the(current(density(under(light(does(not(result(from(
decomposition(of(the(electrolyte.((
Our( previous( work( has( shown( that( H2O2( is( a( significant(
product( in( the( buffered( amine( electrolytes.[18,( 19]( The(
controlled(experiment(with(commercial(H2O2(shows(no(effect(
on( the( onset( of( the( LSV( plots( of( MnOx( in( the( examined(
electrolytes( (figure( S15).( To( determine( the( H2O2( production(
efficiency,(a(constant(potential(of(1.0(V(vs(Ag/AgCl(was(applied(
for(2(hours(on(the(MnOx,(both(in(the(BAN(electrolyte(and(the(
mixture( (and( also( in( Bi( as( a( control( experiment),( with( and(
without(illumination.(The(amount(of(H2O2(was(determined(by(
titration( in( a( UVBVis( cell,( as( detailed( in( the( experimental(
section( and( literature.[18]( The( H2O2( production( efficiency,(
calculated( by( using( Faraday’s( law( and( assuming( a( 2Belectron(
water(oxidation(process,(is(shown(in(table(1.((In(the(dark,(the(
1.0(M(BAN(produces(3.1(μmol(H2O2(in(2(hours,(equivalent(to(a(
70%(Faradaic(efficiency,(and(the(buffer/BAN(mixture(produces(
3.4(μmol(H2O2,(which(indicates(a(63%(Faradaic(efficiency.(The(
lower( than( 100%( H2O2( production( efficiency( is( due( to( the(
further( oxidation( or( decomposition( of( H2O2( to( O2,( or( the(
competing( process( of( water( oxidation( to( oxygen.[18]( In(
contrast,(in(the(Bi(buffer,(only(O2(is(produced(and(no(H2O2(was(
found( (table( 1),( as( reported( previously( under( the( similar(
conditions.[13]( The( plot( in( Figure( S16( shows( a( combined(
Faradaic(efficiency(of(94±7%(for(the(water(oxidation(process(
(H2O/H2O2( +( H2O/O2)( in( the( dark,( confirming( that,( within(
experimental(error,(all(the(current(is(used(for(water(oxidation.((
dominant(as(it(is(dependent(on(the(square(root(of(intensity.[29]
(
The( linear( dependence( observed( here( indicates( that( the(
photocurrents(generated(by(the(MnOx(films(can(potentially(be(
further(enhanced(by(working(at(even(higher(light(intensities.((
((The(photocurrent((Jph)(at(1(sun(can(also(be(estimated(by(
integrating(the(IPCE(spectra((Figure(8)(from(370B570(nm(using(
a(method(detailed(elsewhere;[50](and(yields(a(value(of(0.2(mA(
cmB2.(Under(1(sun(illumination,(the(measured(photocurrent(at(
1.0( V( was( 0.4( ±( 0.15( mA( cmB2( (Figure( 9).( The( slightly(
underestimation( in( Jph( from( the( IPCE( data( partly( arises( from(
the(fact(that(the(measurement(does(not(cover(the(whole(solar(
spectrum.(
(
(
(
(
8(|(J.!Name.,(2012,(00,(1B3(
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Page 9 of 11
Journal of Materials Chemistry A
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(ARTICLE(
(
Σ(–OH(+(OHB→(Σ(+H2O2(+eB(((((((((((((((((((((((((((((((((((((((equation(2)(
View Article Online
2H2O2(→(O2(+(2H2O((((((((((((((((((((((((((((((((((((((((((((((D(((O((I(:e1q0u.1a0t3io9/nC(35)T(A03850B
Table(1.(H2O2(production(and(Faradaic(efficiency((measured(at(1.0(V(
vs(Ag/AgCl)((
Once(formed,(the(peroxide((H2O2)(will(disproportionate(into(O2(
(equation(3)(or(be(further(oxidised(to(O2.[53](As(summarised(in(
the( schematic( diagram( in( figure( 10,( In( the( presence( of( the(
ammonium(cation/amine(mixture,(the(H2O2(is(solvated(and(its(
production( thereby( enhanced.[18]( ( However,( irradiation( is(
known( to( cause( a( significant( increase( in( the( rate( of( H2O2(
decomposition[52]( in( solution.( The( increased( photocurrent(
under( illumination( observed( in( the( present( work( could(
therefore( be( the( result( of( the( decomposition( of( H2O2( to(
dioxygen,(thus(decreasing(the(steady(state(H2O2(concentration(
and(accelerating(the(water(oxidation(reaction.(
(
H2O2(production(in(2(
H2O2(production(
hours((μmol)(a(
(
efficiency((%)(b(
1.0(M(BAN,(pH9(B(dark(
1.0(M(BAN,(pH9(B(light(
3.1±(0.1(
2.2±0.1(
70±3(
27±1(
0.10(M(Bi(buffer(+(1.0(
M(BAN,(pH9(B(dark(
3.4±0.2(
63±3(
24±1(
0.10(M(Bi(buffer(+(1.0(
M(BAN,(pH9(B(light(
3.3±0.1(
0.20(M(Bi,(pH9(–(dark(c( B0.06±0.15(
0.20(M(Bi,(pH9(–(light(c( B0.06±0.22(
B(
B(
(
a( amount( of( H2O2,( measured( from( by( titration( in( a( UVBVis( cell( as(
detailed( in( the( experimental( section( and( literature.[18]( b( H2O2(
production(efficiency(calculated(as(follows:(((amount(of(measured(
H2O2()(/((the(calculated(amount(of(H2O2(by(using(Faraday’s(law(and(
assuming( a( 2Belectron( water( oxidation( process)( x100.( c( the(
measured( amount( of( H2O2( in( Bi( in( dark( and( under( illumination( is(
just(errors,(indicating(no(H2O2(produced(in(Bi.((
(
(
Figure(10:(Schematic(diagram(showing(the(water(oxidation(reaction(
Under(illumination,(the(amount(of(H2O2(produced(is(similar(to(
and(H2O2(decomposition(on(MnOx.(
that(in(dark(in(both(electrolytes,(but(the(corresponding(H2O2(
production( efficiency( drops( significantly( (because( the( overall(
current(is(increased)(from(70%(to(27%(and(from(63%(to(24%,(
respectively.(Furthermore,(the(combined(Faraday(efficiency(for(
O2( (75±2%)( and( H2O2( (27±2%)( production,( determined( from(
the(same(experiment(under(illumination((Table(S2),(was(100%(
(102±4%).(This(implies(that(either((i)(the(photocurrent(increase(
(
The(CPE(measurement(with(the(slow(increase(at(light(on(and(
slow( decay( at( light( off( (figure( 6,( S11)( clearly( illustrates( the(
influence(of(illumination(on(the(photocurrent(in(the(presence(
of( BAN.( The( detailed( photocurrent( under( CPE( measurement(
shows( it( is( composed( of( two( parts:( an( initial( fast( response(
followed( by( a( slow( response( as( shown( in( figure( 11.( The( fast(
photocurrent(response(can(be(observed(instantly(with(light(on(
and( light( off,( followed( by( another,( slower( photocurrent(
response.( Normally,( the( photocurrent( resulting( from( the(
photoBgenerated(electronBhole(is(determined(by(the(mobility(
and( separation( efficiency( of( electronBhole( pairs,( and( the(
response(time(varies(but(does(not(exceeds((several(seconds(in(
normal(cases.[56](Thus,(the(fast(photocurrent(response(in(figure(
S11(is(attributed(to(these(photoBgenerated(electronBhole(pairs.(
However,(the(slow(photocurrent(response((30(s(during(rise(and(
more(than(150(s(during(decay)(is(not(likely(to(be(due(to(the(
slow( mobility/separation( of( electrons/holes.( Notably,( the(
photoBinduced(H2O2(decomposition(usually(takes(much(longer,(
as( shown( in( figure( S16( and( previous( studies.[52]( Hence,( the(
slow(photocurrent(response(with(illumination(is(probably(due(
to(the(decomposition(of(H2O2.(This(would(also(explain(why(the(
slow(response((figure(7)(is(only(obvious(in(the(presence(of(BAN(
but(not(in(Bi(buffer.(As(a(result,(the(effect(of(the(light(on(the(
photocurrent(is(improved(not(only(improving(the(MnOx(activity(
but(also(by(accelerating(the(H2O2(decomposition.(((
is(principally(the(result(of(a(direct(water(to(oxygen(process[51]
(
or((ii)(the(illumination(is(accelerating(the(H2O2(decomposition(
to( O2( on( or( near( the( electrode.[52]( The( accelerating( effect( of(
illumination(on(the(decomposition(of(H2O2(on(MnO2(particles(
is( confirmed( by( the( results( shown( in( Figure( S17.( This( is( in(
accordance( with( the( previous( results[52]( where( illumination(
was(found(to(result(in(rapid(decomposition(of(H2O2.(
(
3.4(Mechanistic(Considerations(
Much( fundamental( work( has( been( carried( out( on( the(
mechanism(of(water(oxidation(by(electrolysis.(Under(alkaline(
conditions,(the(first(step(in(the(water(oxidation(reaction(on(the(
nonBnoble( metal( oxides,( such( as( Ni,( Mn,( Co( and( SrFeO3( was(
proposed(to(involve(a(one(electron(process(to(form(a(surface(
(Σ)(bound(hydroxyl(species:[53,(54](((
Σ(+(H2O→(ΣBOH(+H+(+eB((((((((((((((((((((((((((((((((((((((((((((((equation(1)(((
Slow( oxidative( cleavage( of( the( ΣBOH( bond( then( forms( a(
peroxide(species:[53,(55]
(
(
This(journal(is(©(The(Royal(Society(of(Chemistry(20xx!
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ARTICLE(
Journal(Name(
relationship(at(low(light(intensities,(but(becomes(proportional(
DOI: 10.1039/C5TA03850B
to(the(square(root(of(light(intensity(for(TiO2, (or(reaches(a(
(
View Article Online
[27]
Slow response
Fast response
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saturation( in( the( case( of( CdS[26]( at( higher( illumination(
intensities.(Fe2O3(studies(have(shown(a(linear(relationship(at(
low( photocurrent( density( (<( 1( mA( cmB2).[28,( 29]( In( this( study,(
however,(the(MnOx(film(shows(no(limit(in(photocurrent,(even(
under(strong(illumination(and(reaches(a(high(photocurrent(at(
~9(suns((~(4.5(mA(cmB2(at(η(=(540(mV,(as(shown(in(figure(5a).((
Although( Fe2O3( and( BiVO4( are( highly( photoBactive,( their(
intrinsic( water( oxidation( activity( is( not( as( good( as( the( OECs,(
such( as( CoBPi,[2]( MnOx[3,(16]( and( NiOx.[7]( Thus,( a( second( phase(
serving(as(an(OEC(has(been(incorporated(with(the(Fe2O3[49,(58]or(
BiVO4[25,(31,(59]( to( enhance( their( overall( photoBelectrochemical(
water(oxidation(performance.(The(advantage(of(MnOx(as(the(
photoBanode(is(that(it(is(not(only(photoBactive(as(shown(in(this(
paper,( but( is( also( a( highly( active( and( earthBabundant( water(
oxidation( catalyst.[3,( 16]( ( Thus,( high( photoBactivity( and( water(
oxidation( performance( could,( in( principle,( be( achieved( on( a(
single(MnOx(material,(recalling(that(the(band(gap(in(MnOx(is(
~2.0( V.( Further( improvements,( in( particular( in( the( photoB
voltage( generated,( might( stem( from( further( studies( of( the(
photo( excitation( process( in( MnOx,( as( has( been( achieved( in(
light off
light on
300
350
400
450
70
75
80
85
90
95 100
Time /s
(
Figure(11.((Transient(photocurrent(of(a(MnOx(film(in(Bi+BAN(at(1.0(V(
with(light(on(and(light(off.(The(photocurrent(is(composed(of(a(fast(
reponse(and((slow(response,(marked(as(blue(and(pink,(respectively.(((
(
In(addition(to(the(BAN(contributing(to(the(high(photocurrent,(
the( buffered( conditions( have( a( strong( influence( on( the(
generation(of(a(high(photocurrent.(This(effect(of(buffering(on(
maintaining(a(high(water(oxidation(activity(is(also(reported(for(
a(cobalt(based(catalyst.[57](The(1.0(M(BAN(solution((10(ml)(at(
pH9(contains(1.2(mol%((120(μmol)(of(free(amine,(while(the(1.0(
M(BAN(at(pH10(contains(11.2%(free(amine((1120(μmol).(As(the(
pKa( of( butyl( amine( is( 10.59( and( the( range( of( pKa±1( can( be(
considered(as(the(effective(buffer(region,(the(BAN(at(pH10(can(
be(expected(to(act(as(a(buffer(while(the(BAN(at(pH9(has(very(
low(buffer(capacity.((In(the(unBbuffered(BAN(electrolyte,(the(
accumulation( of( H+( on( the( surface( generated( via( equation( 1(
(6.22!μmol!over!2hrs)!is(small(compared(with(the(excess(amine(
in(the(bulk(electrolytes,(but(it(could(significantly(influence(the(
local(proton(activity(on(the(surface((for(example(there(is(only(
1.5!μmol(excess(amine(in(a(0.5(cm(thick(diffusion(layer(at(the(
electrode).(Thus,(a(decrease(in(the(local(pH(could(occur(on(the(
catalyst(surface.(As(the(photoBelectrochemical(water(oxidation(
is( pH( dependent( (E0((V)( =1.23( B( 0.059( x( pH),( the( decrease( in(
local( pH( results( in( a( decrease( in( the( effective( overpotential.(
This( in( turn( decreases( both( the( dark( and( photocurrent( as(
shown( in( BAN( at( pH9.( On( the( other( hand,( in( the( pH10( BAN(
electrolyte,(which(has(sufficient(protonBaccepting(capacity,(the(
pH( on( the( surface( is( more( stable( than( in( the( unBbuffered(
electrolytes.( Thus,( the( buffered( BAN( electrolytes( (i.e.,( pH10(
BAN(and(pH9(BAN(with(Bi)(exhibit(much(higher(photocurrents(
than( the( unBbuffered( (pH9)( BAN.( Notably,( experiments(
examining( the( contributions( of( the( added( borate( buffer( vs(
bicarbonate( buffer( indicated( that( these( function( in( a( similar(
way(in(maintaining(high(photocurrents.((
recent(years(with(Fe2O3.[6,(28,(29,(60]
(
(
4. Summary and Conclusions
This( work( has( examined( the( photoBelectrochemical(
performance( of( MnOx( with( respect( to( the( water( oxidation(
reaction.( Investigations( of( the( effect( of( film( thickness,(
substrate,(illumination(direction(and(electrolyte(on(the(water(
oxidation(reaction(have(led(to(the(following(findings:(
1)(Water(oxidation(performance(on(MnOx(can(be(improved(
by( illumination,( and( the( buffered( amine( ionic( liquid(
electrolytes( significantly( enhance( the( photoBelectrochemical(
activity(by(producing(H2O2(and(O2(at(the(same(time.((
2)(The(photocurrent(generation(can(in(part(be(attributed(to(
accelerated(decomposition(of(H2O2(under(light.(
3)(The(highest(photocurrent(is(achieved(on(MnOx(films(with(
a(thickness(of(4(µm.(
4)( The( photocurrent( is( linearly( dependent( on( light(
intensity.((
This(study(provides(an(alternative(method(to(improve(the(
photoBactivity(in(water(oxidation(catalysis.(It(shows(that(MnOx(
is(not(only(a(very(good(electroBcatalyst,(but(is(also(a(promising(
photoBanode(for(water(oxidation.(Further(investigations(of(the(
relationship(between(photoBactivity(and(the(crystal(structure,(
morphology,( electronic( structure,( and( charge( transport(
behaviour(could(contribute(to(yet(further(improvements(in(the(
photoBelectrochemical(performance(of(MnOx.((
An(important(aspect(of(this(study(is(the(linear(relationship(
between(light(intensity(and(photocurrent(that(is(maintained(at(
intensities( as( high( as( 9( suns.( This( is( much( higher( than( the(
normal(illumination(conditions((~1(sun).[6,(25,(30B32](It(has(been(
reported( that( the( photocurrent( typically( shows( a( linear(
(
Acknowledgements
DRM( and( LS( gratefully( acknowledge( funding( from( the( Australian(
Research( Council( (ARC)( and( ARC( Centre( of( Excellence( for(
10(|(J.!Name.,(2012,(00,(1B3(
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Journal of Materials Chemistry A
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(ARTICLE(
30 (B.(Klahr,(S.(Gimenez,(F.(FabregatBSantiago,(T.(Hamann,(J.V(BieiswqAuretirctle,(JO.(nAlimne.(
Chem.(Soc.(2012,(134,(4294.(
31 (Y.(Liang,(J.(Messinger,(Phys.(Chem.(Chem.(Phys.(2014,(16,(12014.(
32 (M.(Fekete,(W.(Ludwig,(S.(Gledhill,(J.(Chen,(A.(Patti,(L.(Spiccia,(Eur.(J.(Inorg.(
Chem.(2014,(2014,(750.(
33 (Mathias,( Solar( Cell( Efficiency( World( Record( Set( By( Sharp( B(
44.4%,http://cleantechnica.com/2013/06/23/solarBcellBefficiencyBworldB
recordBsetBbyBsharpB44B4/,((July,(2013.(
34 (H.( Dotan,( K.( Sivula,( M.( Gratzel,( A.( Rothschild,( S.( C.( Warren,( Energy(
Environ.(Sci.(2011,(4,(958.(
Electromaterials(Science((ACES)(as(well(as(funding((DRM)(from(the(
Australian(Laureate(Fellowship(scheme.(FLZ(is(grateful(to(Australian(
Postgraduate( Award( (APA)( and( Endeavour( International(
Postgraduate(Research(Scholarship((IPRS).(The(authors(would(like(to(
acknowledge(Monash(University(Centre(for(Electron(Microscopy(for(
providing( electron( microscopic( facilities( and( F.( Huang( for( helpful(
discussion.((
DOI: 10.1039/C5TA03850B
35 (J.(Nissfolk,(K.(Fredin,(J.(Simiyu,(L.(Häggman,(A.(Hagfeldt,(G.(Boschloo,(J.(
Electroanal.(Chem.(2010,(646,(91.(
36 (B.(Yang,(P.(R.(F.(Barnes,(W.(Bertram,(V.(Luca,(J.(Mater.(Chem.(2007,(17,(
2722.(
37 (Y.(C.(Ling,(G.(M.(Wang,(D.(A.(Wheeler,(J.(Z.(Zhang,(L.(Yat,(Nano(Lett.(2011,(
11,(2119.(
38 (F.( L.( Zhou,( A.( Izgorodin,( R.( K.( Hocking,( V.( Armel,( L.( Spiccia,( D.( R.(
MacFarlane,(Chemsuschem(2013,(6,(643.(
39 (K.(L.(Pickrahn,(S.(W.(Park,(Y.(Gorlin,(H.BB.BR.(Lee,(T.(F.(Jaramillo,(S.(F.(Bent,(
Advanced(Energy(Materials(2012,(2,(1269.(
40 (T.( Lindgren,( H.( Wang,( N.( Beermann,( L.( Vayssieres,( A.( Hagfeldt,( S.BE.(
Lindquist,(Sol.(Energy(Mater.(Sol.(Cells(2002,(71,(231.(
41 (S.( Bai,( C.( Wang,( M.( Deng,( M.( Gong,( Y.( Bai,( J.( Jiang,( Y.( Xiong,( Angew.(
Chem.(Int.(Ed.(2014,(53,(12120.(
Notes(and(references(
1
2
3
(M.(Gratzel,(Nature(2001,(414,(338.(
(M.(W.(Kanan,(D.(G.(Nocera,(Science(2008,(321,(1072.(
(I.(Zaharieva,(P.(Chernev,(M.(Risch,(K.(Klingan,(M.(Kohlhoff,(A.(Fischerb,(H.(
Dau,(Energy(Environ.(Sci.(2012,(5,(7081.(
4
(M.( Huynh,( D.( K.( Bediako,( D.( G.( Nocera,( J.( Am.( Chem.( Soc.( 2014,( 136,(
6002−6010;(M.(Busch,(E.(Ahlberg,(I.(Panas,(J.(Phys.(Chem.(C(2013,(117,(
288;(P.(W.(Menezes,(A.(Indra,(P.(Littlewood,(M.(Schwarze,(C.(Goebel,(R.(
Schomaecker,(M.(Driess,(Chemsuschem(2014,(7,(2202;(X.(Yang,(C.(Du,(J.(
Xie,(D.(Wang.(J.(Catal.(2013,(304,(86;(N.(C.(Srandwiz,(D.(J.(Comstock,(R.(L.(
Grimm,( A.( C.( NicholsBNielander,( J.( Elam,( N.( S.( Lewis.( J.( Phys.( Chem.( C(
2013,(117,(4931.((
42 (H.(Hall,(J.(Am.(Chem.(Soc.(1957,(79,(5441.(
5
(M.( W.( Kanan,( J.( Yano,( Y.( Surendranath,( M.( Dinca,( Y.( Yachandra,( D.( G.(
Nocera,(J.(Am.(Chem.(Soc.(2010,(132,(13692.(
43 (J.(M.(Beale,(J.(H.(Block,(Eds.,!Wilson!and!Gisvold’s!Textbook!of!Organic!
Medicinal!Pharmaceutical!Chemistry5!12th!Edition,(Lippincott(Williams(&(
Wilkins,(Philadelphia,(PA,(USA(2009.(
44 (J.(Long,(H.(Chang,(Q.(Gu,(J.(Xu,(L.(Fan,(S.(Wang,(Y.(Zhou,(W.(Wei,(L.(Huang,(
X.(Wang,(P.(Liu,(W.(Huang,(Energy(Environ.(Sci.(2014,(7,(973;(S.(Dhara,(P.(
Giri,(Nanoscale(Res.(Lett.(2011,(6,(504.(
6
7
8
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(M.( M.( Najafpour,( S.( Heidari,( E.( Amini,( K.( M.( ,( R.( Carpentier,( S.( I.(
Allakhverdiev,(J.(Photochem.(Photobiol.,(B(2014,(133,(124.(
(R.(Brimblecombe,(A.(Koo,(G.(C.(Dismukes,(G.(F.(Swiegers,(L.(Spiccia,(J.(Am.(
Chem.(Soc.(2010,(132,(2892.(
9
45 (F.( FabregatBSantiago,( G.( GarciaBBelmonte,( J.( Bisquert,( A.( Zaban,( P.(
Salvador,(J.!Phys.!Chem.!B(2002,(106,(334.(
10 (R.(K.(Hocking,(R.(Brimblecombe,(L.(Y.(Chang,(A.(Singh,(M.(H.(Cheah,(C.(
Glover,(W.(H.(Casey,(L.(Spiccia,(Nat(Chem(2011,(3,(461.(
11 (T.(Takashima,(K.(Hashimoto,(R.(Nakamura,(J.(Am.(Chem.(Soc.(2012,(134,(
1519.(
12 (Y.(Meng,(W.(Song,(H.(Huang,(Z.(Ren,(S.BY.(Chen,(S.(L.(Suib,(J.(Am.(Chem.(
Soc.(2014,(136,(11452.(
46 [46](A.( K.( Manohar,( O.( Bretschger,( K.( H.( Nealson,( F.( Mansfeld,(
Electrochim.( Acta( 2008,( 53,( 3508;( A.( K.( Manohar,( O.( Bretschger,( K.( H.(
Nealson,(F.(Mansfeld,(Bioelectrochemistry(2008,(72,(149.(
47 (K.( D.( Kwon,( K.( Refson,( G.( Sposito,( Physical( Review( Letters( 2008,( 100,(
146601.(
48 (S.( Banerjee,( J.( Gopal,( P.( Muraleedharan,( A.( K.( Tyagi,( B.( Raj,( Curr.( Sci.(
2006,(90,(1378.(
13 (M.(Fekete,(R.(K.(Hocking,(S.(L.BY.(Chang,(C.(Italiano,(A.(F.(Patti,(F.(Arena,(L.(
Spiccia,(Energy(Environ.(Sci.(2013,(6,(2222.(
49 (J.(Y.(Kim,(G.(Magesh,(D.(H.(Youn,(J.BW.(Jang,(J.(Kubota,(K.(Domen,(J.(S.(Lee,(
Sci.(Rep.(2013,(3,(2681.(
14 (K.(L.(Pickrahn,(S.(W.(Park(,(Y.(Gorlin(,(H.BB.BR.(Lee(,(T.(F.(Jaramillo(,(S.(F.(
Bent,(Adv.(Energy(Mater.(2012,(2,(1269.(
50 (Y.(Chiba,(A.(Islam,(Y.(Watanabe,(R.(Komiya,(N.(Koide,(L.(Han,(Jpn.(J.(Appl.(
Phys.(2006,(40,(L638.(
15 (M.(R.(Gao,(Y.(F.(Xu,(J.(Jiang,(Y.(R.(Zheng,(S.(H.(Yu,(J.(Am.(Chem.Soc.(2012,(
134,(2930−2933.(
51 (I.( Katsounaros,( W.( B.( Schneider,( J.( C.( Meier,( U.( Benedikt,( P.( U.(
Biedermann,( A.( A.( Auer,( K.( J.( J.( Mayrhofer,( Phys.( Chem.( Chem.( Phys.(
2012,( 14,( 7384;( W.( Jia( ,( M.( Guo( ,( Z.( Zheng( ,( T.( Yu,( E.( G.( Rodriguez,( Y.(
Wang,(Y.(Lei,(J.(Electroanal.(Chem.(2009,(625,(27.(
52 (C.( A.( Páez( ,( D.( Y.( Liquet( ,( C.( Calberg,( S.( D.( Lambert,( I.( Willems,( A.(
Germeau,(J.BP.(Pirard,(B.(Heinrichs,(Catal.(Commun.(2011,(15,(132.(
53 (H.( Dau,( C.( Limberg,( T.( Reier,( M.( Risch,( S.( Roggan,( P.( Strasser,(
ChemCatChem(2010,(2,(724.(
54 (S.(P.(Mehandru,(A.(B.(Anderson,(J.(Electrochem.(Soc.(1989,(136,(158.(
55 (J.(O.(M.(Bockris,(T.(Otagawa,(J.(Phys.(Chem.(B(1983,(87,,(2960;(J.(O.(M.(
Bockris,(T.(Otagawa,(V.(Young,(J.(Electroanal.(Chem.(1983,(150,(633.(
56 (D.(Tafalla,(P.(Salvador,(R.(M.(Benito,(J.(Electrochem.(Soc.(1990,(137,(1810.(
57 (Y.(Surendranath,(M.(Dinca,(D.(G.(Nocera,(J.(Am.(Chem.(Soc.(2009,(131,(
2615.(
16 (F.(L.(Zhou,(A.(Izgorodin,(R.(K.(Hocking,(L.(Spiccia,(D.(R.(MacFarlane,(Adv.(
Energy(Mater.(2012,(2,(1013.(
17 (S.(Crawford,(E.(Thimsen,(P.(Biswas,(J.(Electrochem.(Soc.(2009,(156,(H346.(
18 (A.(Izgorodin(,(E.(Izgorodina,(D.(R.(MacFarlane,(Energy(Environ.(Sci.(2012,(
5,(9496.(
19 (C.(McDonnellBWorth,(D.(R.(MacFarlane,(RSC(Adv.(2014,(4,(30551.(
20 (D.(M.(Sherman,(Geochimica(Et(Cosmochimica(Acta(2005,(69,(3249.(
21 (C.(Sun,(Y.(Wang,(J.(Zou,(S.(C.(Smith,(Phys.(Chem.(Chem.(Phys.(2011,(13,(
11325;(S.(Pishdadian,(A.(M.(S.(Ghaleno,(Acta(Phys.(Pol.,(A(2013,(123,(741.(
22 (B.(A.(Pinaud,(Z.(B.(Chen,(D.(N.(Abram,(T.(F.(Jaramillo,(J.(Phys.(Chem.(C(
2011,(115,(11830.(
23 (N.(Sakai,(Y.(Ebina,(K.(Takada,(T.(Sasaki,(J.(Phys.(Chem.(B(2005,(109,(9651.(
24 (A.(Singh,(R.(K.(Hocking,(S.(L.BY.(Chang,(B.(M.(George,(M.(Fehr,(K.(Lips,(A.(
Schnegg,(L.(Spiccia,(Chem.(Mater.(2013,(25,(1098((
58 (D.( K.( Zhong,( M.( Cornuz.,( K.( Sivula.,( M.( Grätzel.,( D.( R.( Gamelin,( Energy(
Environ.(Sci.(2011,(4,(1759.(
25 (T.(W.(Kim(,(K.BS.(Choi,(Science(2014,(343,(990.(
26 (R.(H.(Bube,(J.(Appl.(Phys(1960,(31,(1301.(
59 (M.( Zhou,( J.( Bao,( W.( Bi,( Y.( Zeng,( R.( Zhu,( M.( Tao,( Y.( Xie,( Chemsuschem(
2012,(5,(1420.(
60 (J.(H.(Kennedy,(K.(W.(Frese,(J.(Electrochem.(Soc.(1978,(125,(709;(A.(Duret,(
M.(Gratzel,(J.(Phys.(Chem.(B(2005,(109,(17184.(
27 (T.BH.(Lim,(S.BD.(Kim,(Korean(J.(Chem.(Eng.(2002,(19,(1072.(
28 (S.(R.(Pendlebury,(A.(J.(Cowan,(M.(Barroso,(K.(Sivula,(J.(Ye,(M.(Gratzel,(D.(R.(
Klug,(J.(Tang,(J.(R.(Durrant,(Energy(Environ.(Sci.(2012,(5,(6304;(I.(Cesar,(K.(
A.,(J.(A.(G.(Martinez,(M.(Gratzel,(J.(Am.(Chem.(Soc.(2006,(128,(4582.(
29 (B.(D.(Chernomordik,(H.(B.(Russell,(U.(Cvelbar,(J.(B.(Jasinski,(V.(Kumar,(T.(
Deutsch,(M.(K.(Sunkara,(Nanotechnology((2012,(23,(194009.(
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This(journal(is(©(The(Royal(Society(of(Chemistry(20xx!
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