Jou nal o Physics D: Applied
Physics
PAPER • OPEN ACCESS
Nes ing BiVO4 nanoislands in ZnO nanodend i es
by wo-s ep elec odeposi ion o e icien sola
wa e spli ing
To ci e his a icle: Ali Can Güle
e al
2024
J. Phys. D: Appl. Phys.
57 355110
View he a icle online o upda es and enhancemen s.
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Jou nal o Physics D: Applied Physics
J. Phys. D: Appl. Phys. 57 (2024) 355110 (15pp) h ps://doi.o g/10.1088/1361-6463/ad5212
Nes ing BiVO4nanoislands in ZnO
nanodend i es by wo-s ep
elec odeposi ion o e icien sola
wa e spli ing
Ali Can Güle 1, Jan An oˇ
s1, Milan Masaˇ
1, Michal U bánek1, Michal Macho sk´
y1,
Rajesh Dagupa i2, Michal
ˇ
Zi ˇ
nan2, José J Velázquez2, Duˇ
san Galusek2,3
and I o Kuˇ
i ka1,4,∗
1Cen e o Polyme Sys ems, Tomas Ba a Uni e si y in Zlin, T . T. Ba i 5678, 760 01 Zlin,
Czech Republic
2Cen e o Func ional and Su ace Func ionalized Glass, Alexande Dubcˇek Uni e si y o T encˇín,
ˇ
S uden ská 2, SK-911 50 T encˇín, Slo akia
3Join Glass Cen e o he IIC SAS, TnU AD, and FChPT STU, T encˇín, Slo akia
4Depa men o Chemis y, Facul y o Technology, Tomas Ba a Uni e si y in Zlín, Va ecˇko a 5669,
760 01 Zlín, Czech Republic
E-mail: ku i ka@u b.cz,gule @u b.cz,an os@u b.cz,masa @u b.cz,mu banek@u b.cz,macho sky@u b.cz,[email p o ec ed],
[email p o ec ed],[email p o ec ed] and dusan.g[email p o ec ed]
Recei ed 15 Ma ch 2024, e ised 29 Ap il 2024
Accep ed o publica ion 30 May 2024
Published 12 June 2024
Abs ac
Pho oanodes wi h a la ge elec ochemically ac i e su ace a ea, apid cha ge ans e , and
b oadband ligh ha es ing capaci y a e equi ed o maximize he pho oelec ochemical (PEC)
wa e spli ing pe o mance. To add ess hese ea u es, we demons a e ha 3D hie a chal ZnO
nanodend i es (NDs) can be sensi ized wi h BiVO4nanoislands by chemical and he mal
ea men s o elec odeposi ed Bi me al ilms. The la band measu emen s and op ical
cha ac e iza ion sugges ed ha he esul ing he e ojunc ion had ype-II band alignmen wi h a
iable cha ge ans e om BiVO4 o ZnO NDs. In pa allel, PL analysis e ealed inhibi ion o
he cha ge ecombina ion a e by he elec on ans e be ween BiVO4and ZnO NDs. Upon AM
1.5 G illumina ion, BiVO4/ZnO NDs he e ojunc ion yielded he highes pho ocu en e iciency
(0.15 mA·cm−2a 1.2 V s. NHE), which was a ibu ed o i s enhanced su ace a ea (due o he
p esence o small dend i e b anches), ex ended b oadband ligh abso p ion ex ending om UV
o isible ligh egions, and he mos e icien in e acial cha ge ans e as p o en by
elec ochemical impedance spec oscopy (EIS) s udies. Besides, he inciden pho on- o-cu en
con e sion e iciency and applied bias pho on- o-cu en e iciency es s con i med an imp o ed
spec al pho o esponse o he he e ojunc ion based pho oanode, pa icula ly owa ds he isible
ligh spec um. The esul s ou line a p omising syn hesis ou e o building he e ojunc ions
be ween isible ligh ac i e and wide band gap semiconduc o s o he use as a highly e icien
pho oanodes in a PEC cell.
∗Au ho o whom any co espondence should be add essed.
O iginal con en om his wo k may be used unde he e ms
o he C ea i e Commons A ibu ion 4.0 licence. Any u -
he dis ibu ion o his wo k mus main ain a ibu ion o he au ho (s) and he
i le o he wo k, jou nal ci a ion and DOI.
1 © 2024 The Au ho (s). Published by IOP Publishing L d
J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
Supplemen a y ma e ial o his a icle is a ailable online
Keywo ds: ZnO nanodend i es, BiVO4, elec odeposi ion, he e ojunc ion pho oanode,
pho oelec ochemical
1. In oduc ion
Sola ene gy con e sion o chemical ene gy (i.e. hyd o-
gen) using semiconduc ing ma e ials ia pho oelec ochem-
ical (PEC) wa e spli ing has been ega ded as e icien sus-
ainable ene gy echnology since he pionee ing demons a-
ion by Honda and Fujishima [1,2]. The o e po en ial o
wa e oxida ion (oxygen e olu ion eac ion (OER)) and wa e
educ ion (hyd ogen e olu ion eac ion (HER)) in his p ocess
is pa ially compensa ed by he pho oinduced ol age o he
ligh abso be [3,4]. The semiconduc o mus hence possess
an app op ia e band alignmen wi h he wa e edox po en ials.
The OER and HER could be d i en by he PEC wa e spli -
ing a po en ials below 1.23 V and abo e 0 V wi h e e ence
o he e e sible hyd ogen elec ode (RHE), espec i ely [5].
The impo an a gumen a ec ing he sola - o-hyd ogen con-
e sion e iciency (STH) a e ligh abso p ion, cha ge sepa a-
ion/ anspo and su ace ca aly ic eac ion (cha ge ans e )
[6,7].
Many p ope ies o semiconduc o nano ods (NRs) can be
signi ican ly al e ed when hei adial dimension (diame e )
is equal o below he cha ac e is ic leng h o he in e es ing
solid s a e phenomena such as he exci on Boh adius, exci on
di usion leng h, wa eleng h o i adia ion and phonon mean
ee pa h [8,9]. Fo ins ance, Song e al ab ica ed a com-
posi e i ania hin ilm composed o quasi-aligned u ile NRs
embedded in ana ase agg ega es and i s PEC beha io was
signi ican ly enhanced [10]. Recen ly, mo e e o s ha e been
de o ed on cons uc ing h ee-dimensional (3D) hie a chical
nanos uc u es om low-dimensional subuni s. Compa ed o
1D mo phologies, 3D complex zinc oxide (ZnO) mo pholo-
gies p esen a la ge ac i e su ace a ea, which esul s in a mo e
apid cha ge ans e p ocess and wa e oxida ion kine ics [11,
12]. This hie a chical s uc u al design can inc ease he num-
be o ligh anspo pa hs and he eby imp o e ligh ha -
es ing capaci y [13,14]. ZnO nanodend i es (NDs) included
in his class o nanos uc u es a e e y p omising candida e
o sola ene gy con e sion applica ions due o hei low-cos
and a acile syn hesis p ocess. In con as o he con en ional
me hods in which 3D ZnO NDs a e g own o e sequen ially
eseeded ZnO NRs su ace in aqueous phase [15–17], Wu e al
pu posed an al e na i e ab ica ion me hod u ilizing a supe -
sa u a ed solu ion wi hou any sequen ial seed laye o o ganic
s uc u e-di ec ing agen [18]. Ne e heless, besides all o he
asse s, a wide band gap (∼3.2 eV) o ZnO is conside ed as he
undamen al obs acle o achie ing mode a e STH e iciency
ia i s u iliza ion in he PEC wa e spli ing. A no el ma e ial
design is he e o e needed o in luence he ac o s men ioned
abo e.
Pho osensi iza ion wi h a na ow band gap semiconduc o
has eme ged as an e ec i e means o ex ension o he op ical
ac i i y o ZnO NDs in o he isible egion. Among he di -
e en pho osensi ize s ha show pho oac i i y unde isible
ligh , monoclinic bismu h anada e (BiVO4) was chosen as
he ligh abso be due o i s he sui able band gap (∼2.4 eV),
app eciable band edge posi ions o wa e oxida ion and he
ma ched band s uc u e wi h ha o ZnO [19,20]. The mono-
clinic BiVO4alone demons a ed pho oca aly ic ac i i y o
he O2e olu ion om an aqueous sil e ni a e solu ion unde
isible ligh i adia ion [21,22]. I was also epo ed ha he
o ma ion o he e ojunc ion pho oanode be ween he wu zi e
ZnO and monoclinic BiVO4p omo es he cha ge sepa a ion
and anspo o elec ons and holes assis ed by he buil -in
elec ic ield a he in e ace and ligh abso p ion a a wide
wa eleng h ange (UV and is), which hen boos s he PEC
pe o mance [23,24].
In his sense, Yan e al epo ed he ab ica ion o
ZnO NRs/BiVO4he e ojunc ion h ough chemical ba h
deposi ion ollowed by successi e ionic laye deposi ion
[25]. They obse ed ha pho ocu en o ZnO NRs/BiVO4
(1.72 mA·cm−2a 1.2 V s. Ag/AgCl) was highe han ha o
ZnO NRs pho oanode a he same po en ial. The imp o emen
in pho ocon e sion was associa ed wi h he ex ended spec al
esponse owa ds he isible spec um and lowe ecombin-
a ion a e o pho ogene a ed cha ge ca ie . Moniz e al suc-
cess ully syn hesized 1D ZnO coupled wi h nanopa icula e
BiVO4and cobal phospha e (Co–Pi) as a hole accep o [26].
They ound ha Co–Pi/BiVO4/ZnO exhibi ed 12- old inc ease
in pho ocu en (∼3 mA·cm−2) compa ed o he BiVO4. The
enhancemen esul s om highe ligh abso p ion, elec on
low om BiVO4 o ZnO, and hole ans e o Co-Pi o
a o able OER. Recen ly, Yang and Wu cons uc ed no el
Co–Pi/BiVO4/ZnO NDs pho oanode based on me al o ganic
deposi ion o BiVO4on he su ace o hyd o he mally g own
ZnO NRs [27]. The esul an elec ode yielded an op imized
pho ocu en densi y o 3.5 mA·cm−2a 1.23 V s. RHE a e
being ully deple ed a a low bias po en ial (0.8 V s. RHE).
Along wi h he supe io ligh ha es ing capabili y and cha ge
injec ion e iciency by co-ca alys Co–Pi, he au ho s claimed
ha ully deple ed junc ion o igina ing om hin BiVO4shells
led o be e cha ge anspo , which in u n enhanced he PEC
ac i i y. Howe e , syn hesis echniques a ailable o p oduce
nanopo ous mo phology elec odes a e qui e limi ed. Kim
and Choi p esen ed elec ochemically deposi ed nanopo ous
BiVO4 om bismu h oxyiodide (BiOI) on luo ine-doped in
oxide subs a es [28]. They sugges ed ha he oids be ween
2D c ys al s uc u e o BiOI allowed he deposi ion o ul a-
hin pla es (∼20 nm) by inhibi ing he g ain o ma ion o
2
J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
BiVO4du ing he con e sion p ocess. Kang e al demon-
s a ed ha he elec odeposi ed Bi dend i ic elec odes ol-
lowed by he in oduc ion o a V p ecu so solu ion du ing
he oxida ion p ocess can lead o BiVO4NPs. The esul an
BiVO4 hin ilm had a high su ace a ea and a good elec ical
con inui y among he pa icles [29]. Using he simila elec-
odeposi ion p ocedu e, Bai e al ab ica ed Cu2O/BiVO4
p–n he e ojunc ion pho oanode and ob ained he maximum
pho ocu en densi y o 1.72 mA·cm−2(1.23 V s RHE),
which is 4.5 imes highe han ha o p is ine BiVO4 hin ilm
(∼0.38 mA·cm−2) a he same applied po en ial [30].
In iew o he ad an ages men ioned abo e, he ein, we
in oduce a acile s a egy o he cons uc ion o BiVO4/ZnO
NDs he e ojunc ion pho oanode wi h a no el nanos uc u e
o PEC wa e spli ing. The b anches wi h he leng hs o
300–400 nm we e di ec ly o med on hyd o he mally g own
p ima y ZnO NRs wi h a diame e o 84 nm in he absence o
any seed laye o o ganic s uc u e di ec ing agen o p oduce
ZnO NDs. BiVO4nanoislands nes ed in ZnO NDs we e p e-
pa ed by elec odeposi ion o Bi ilm ollowed by he in oduc-
ion o V solu ion and he mal ea men . A wo-s ep elec ode-
posi ion p ocedu e gene a ed an in ima e con ac a he in e -
ace be ween he cons i uen semiconduc o s. BiVO4/ZnO
NDs he e ojunc ion was iden i ied o ha e s agge ed band
a angemen ( ype II), leading o an e icien pho ogene a ed
elec on–hole sepa a ion and apid in e acial cha ge ans e .
Unde AM 1.5 G illumina ion, he he e ojunc ion pho oan-
ode achie ed a highe PEC pe o mance han he BiVO4, ZnO
NDs, and ZnO NRs pho oelec odes p epa ed in his s udy.
This new app oach may pa e he way o a ional design o
he e ojunc ions be ween na ow band gap and wide band gap
semiconduc o s wi h op imal s uc u e and composi ions.
2. Expe imen al sec ion
2.1. P epa a ion o ZnO NDs pho oanode
The ZnO NR a ays we e syn hesized by hyd o-
he mal me hod. Ini ially, 0.8 M zinc ace a e dihyd a e
(Zn(CH3CO2)2.2H2O, Pen a) and 0.8 M die hanolamine
(CH2CH2OH)2NH, CDH Fine Chemicals) we e dissol ed in
isop opanol ((CH3)2CHOH, Mic ochem), and s i ed a 50 ◦C
o 1 h. A e an o e nigh aging, he sol was spin-coa ed on
cleaned indium in oxide coa ed (ITO, 5–15 Ωsq−1, Sigma
Ald ich) glass subs a es a 3000 pm o 30 s. The coa ed sub-
s a e was calcined in an ambien a mosphe e a 400 ◦C o 1 h
o ob ain he ZnO seed laye . The ypical g ow h solu ion con-
sis ing o 0.025 M zinc ni a e hexahyd a e (Zn(NO3)2·6H2O,
Sigma Ald ich), 0.025 M hexame hylene e amine ((CH2)6N4,
Lachne ), and 0.5 ml polye hyleneimine (PEI, b anched, a e -
age Mw∼800 by LS, Sigma Ald ich) was p ehea ed o 2 h a
95 ◦C. The aged g ow h solu ion u ned o yellow s aw colo .
The seeded ilm wi h i s conduc i e side acing downwa ds
was imme sed in he p ehea ed solu ion (in yellow s aw colo )
and kep a 95 ◦C o 6 h.
The b anches o ZnO NDs we e o med di ec ly on he ZnO
NRs wi hou any assis ance om ano he seeds and o ganic
s uc u e-di ec ing agen , as desc ibed elsewhe e [31]. The
ZnO NRs g own on ITO subs a e we e i s imme sed in o
an aqueous solu ion o 0.057 M zinc ace a e dihyd a e and
0.5 M sodium hyd oxide (NaOH, Lachne ) o 20 min a
oom empe a u e. Du ing his p ocess, he e ch pi s o med
ha se ed as g ow h cen e s o he o ma ion o nanocac us
(NCs), which hen u ned in o b anches a e 1 h g owing a
100 ◦C o in he same solu ion. The ob ained ZnO NDs we e
ca e ully insed wi h deionized wa e and d ied a 60 ◦C.
I should be men ioned ha b anch de elopmen only s a -
ed om a supe sa u a ed solu ion in a me as able s a e a a
high concen a ion o NaOH. The supe sa u a ion o zinc ace -
a e and NaOH solu ion can be eached be o e he solu ion u ns
opaque a e ∼5 min a oom empe a u e. The e o e, i is e y
impo an o imme se ZnO NRs in a clea solu ion be o e p e-
cipi a ion. A mo e de ailed in o ma ion can be ound whe e i
was p ima ily obse ed [18].
2.2. P epa a ion o BiVO4and BiVO4/ZnO NDs pho oanodes
Scheme 1p esen s he syn hesis p ocedu e o p epa e ZnO
NDs coa ed wi h BiVO4NPs, which could be con e ed om
elec odeposi ed Bi me al using a modi ied e sion o he p e-
iously desc ibed chemical and he mal ea men s [29]. In
he i s s ep, he pla ing solu ion was p epa ed by dissol ing
20 mM o bismu h(III) ni a e pen ahyd a e (Bi(NO)3.5H2O,
Sigma Ald ich) in 100 ml e hylene glycol (HOCH2CH2OH,
Sigma Ald ich) solu ion. The deposi ion was achie ed by
passing ∼0.033 C cm−2a −1 V agains he Ag/AgCl elec-
ode. The o he de ails o Bi me al elec odeposi ion a e gi en
in he supplemen a y in o ma ion ( igu e 1S). Following ha ,
100 µl o dime hyl sul oxide (DMSO, Sigma Ald ich) con-
aining 150 mM ammonium mono anada e (NH4VO3, Sigma
Ald ich) was d op-cas ed on o he en i e Bi ilm (a ea =2 cm2)
as he second s ep. The V p ecu so -inco po a ed ilm was
calcined a 500 ◦C o 2 h in ai . By he mal ea men , Bi
and VO2+oxidized o Bi2O3and V2O5, which eac ed o
o m BiVO4. Any esidual anadium pen aoxide (V2O5) on
he elec ode was emo ed by soaking i in 0.125 M NaOH
solu ion o 30 s. The esul an pu e BiVO4/ZnO NDs pho-
oanode was ho oughly washed by deionized wa e , and d ied
a 60 ◦C. Fo syn hesis o BiVO4, all he deposi ion condi ions
we e he same, excep o ITO subs a es used as sca olds.
2.3. PEC measu emen s
The PEC pe o mance es s o BiVO4sensi ized ZnO NDs
we e pe o med in h ee-elec ode con igu a ion wi hin a
plas ic cu e e unde on -side illumina ion a 87.5 mW·cm−2
ligh in ensi y om a PicoTM sola simula o (G2V Op ics)
wi h a s anda d AM 1.5 G il e . The inciden ligh in UV
and isible egions (350–800 nm) is a enua ed by 10% (da a
no shown) while passing h oughou he PEC cell cu e e
illed wi h elec oly e solu ion, co esponding ha he ligh
in ensi y a he sample su ace was 78.75 mW·cm−2. 0.5 M
Na2SO4(pH 7) was used as an elec oly e solu ion ha was
3
J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
Scheme 1. Schema ic ep esen a ion o he ab ica ion o BiVO4/ZnO NDs he e ojunc ion pho oanode.
degassed by ni ogen o 10 min o emo e any dissol ed oxy-
gen be o e he PEC measu emen s. The ab ica ed elec odes
wi h a ixed su ace a ea o 0.32 cm2, P oil and Ag/AgCl
(sa u a ed wi h KCl) we e used as he wo king, coun e , and
e e ence elec odes, espec i ely. The po en ials measu ed s.
Ag/AgCl elec ode (EAg/AgCl) we e con e ed o no mal hyd o-
gen elec ode (NHE) po en ials (ENHE) by using he equa ion:
ENHE =EAg/AgCl + 0.2 V.(1)
In linea sweep ol ammog ams, LSV (J-V), he scan a e
was 20 mV s−1and he scan ange was 0 V–1.5 V ( s.
Ag/AgCl) bo h unde da k and illumina ed condi ions. The
inciden pho on- o-cu en con e sion e iciency (IPCE) es s
we e also ca ied ou in he h ee-elec ode se up using 12
monoch oma ic channels o he sola simula o (PicoTM wi h
AM 1.5 G il e ) as he ligh sou ce. The IPCE was calcula ed
using he ollowing equa ion:
IPCE =Jph (λ)×1240
Pmono (λ)×λ(2)
whe e, Jph (in mA·cm−2) is he pho ocu en densi y eco ded
unde monoch oma ic illumina ion a wa eleng h λ(in nm),
Pmono (in mW·cm−2) is he ligh in ensi y o he monoch o-
ma ic sou ce a each wa eleng h, he cons an 1240 (hc/ein
V·nm) equals o he p oduc o Planck’s cons an and he speed
o ligh di ided by he cha ge o an elec on. The applied bias
pho on- o-cu en e iciency (ABPE), analogue o he STH
e iciency wi h no bias, was calcula ed by he o mula:
ABPE =[J(mA.cm−2)×(1.229 − |Vapp|)(V)×ηF
P o al (mW.cm−2)](3)
in which, 1.23 V is he s anda d s a e e e sible po en ial o
wa e spli ing, Vapp is he applied bias (measu ed s. P ), ηFis
he Fa adaic e iciency o hyd ogen e olu ion (ηF=1 in his
case), and P o al is he in ensi y o he ligh sou ce.
Elec ochemical impedance spec oscopy (EIS) measu e-
men s we e conduc ed in he same elec oly e and se -up as
employed in he pho ocu en measu emen s. The EIS da a
unde illumina ed condi ion was ob ained a 10 mV ampli ude
o AC signal o e a equency ange o 100 kHz o 0.1 Hz.
The Mo –Scho ky (MS) analysis was ca ied ou in da k a
1.5 kHz wi h 30 po en ial s eps.
2.4. Cha ac e iza ion
The c ys al s uc u e o he samples was de e mined by x- ay
powde di ac ome y (XRD, Panaly ical Empy ean DY1098)
wi h Cu Kɑ adia ion (λ=0.15405 nm) unde 40 kV and
45 mA in s eps o 0.02◦. Scanning elec on mic oscopy (SEM,
No a NanoSEM 450) was employed o s udy he mo phology
o he samples. The elemen al composi ion o he p oduced
he e os uc u e was de e mined h ough ene gy-dispe si e x-
ay spec oscopy (EDX). The mic os uc u e o he ilms was
moni o ed by ansmission elec on mic oscopy (TEM, JEM-
2100Plus, Jeol). The op ical cha ac e is ics o he syn hesized
pho oanodes we e acqui ed by a 150 mm InGaAs in eg a -
ing sphe es module o a UV-Vis-NIR abso p ion spec ome e
(Lambda 1050 Pe kin Elme ). The pho oluminescence spec-
a a oom empe a u e we e ob ained by luo escence spec-
opho ome e (PL, Fluo olog FL3-21, Ho iba) equipped wi h
del a diode lase DD-375 l 376 nm as an exci a ion sou ce.
Raman spec a we e collec ed by a mic o-Raman spec opho-
ome e (Renishaw in Via Re lex) a he exci a ion wa eleng h
4

J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
o 532 nm. X- ay pho oelec on spec oscopy (XPS, Axis
Ul a DLD spec ome e , K a os Analy ical L d) wi h a mono-
ch oma ized Al Kα adia ion (hν=1,486.7 eV), ope a ed
a 150 W (10 mA, 15 kV), was used o explo e he ele-
men al composi ion and oxida ion s a es o he elemen s on
he su ace o he BiVO4/ZnO NDs. The XPS spec a we e
ob ained using an analysis a ea o ∼300 µm×700 µm and
analyzed by Casa XPS so wa e. Elec ochemical wo ks a ion
(SP-200 Po en ios a , BioLogic) wi h EIS was u ilized o es
he PEC/elec ochemical pe o mance as well as in e acial
cha ge ans e p ope ies o he p oduced elec odes.
3. Resul s and discussion
BiVO4nanoislands nes ed in sequen ial hyd o he mally g own
ZnO NDs was p epa ed by elec odeposi ion o Bi ilm ol-
lowed by he in oduc ion o V solu ion and he mal ea men .
The amoun o he deco a ed BiVO4was de e mined based
on measu ing he elec ode weigh be o e and a e coa ing.
Wi hou con ibu ion om ITO, he weigh o ZnO NDs was
de e mined as 0.89 mg while he weigh o BiVO4deposi s
on ZnO NDs was de e mined as 0.12 mg, co esponding o
o ma ion o BiVO4/ZnO NDs (∼13.4 w . %) he e os uc u e
pho oanode.
The XRD spec a o he p oduced elec odes a e shown in
igu e 1. The peak posi ions o ZnO NDs a e he same as hose
o ZnO NRs be o e g owing ZnO b anches, which co espond
o he cha ac e is ic di ac ion maxima o hexagonal wu zi e
s uc u e [32]. The dominan (002) peaks in he XRD pa e ns
o ZnO NRs as well as ZnO NDs sugges a p e e en ial o i-
en a ion along he c-axis no mal o he subs a e a he han
an aniso opic o ien a ion. In bo h samples, he (002) peak a
35◦wi h he ull-wid h hal maximum o less han 0.16◦con-
i ms a high c ys allini y. An in e es ing aspec o his p om-
inen peak is i s s onge in ensi y o ZnO NDs. This change
is a ibu ed o he occu ence o b anches since he c ys al-
line olume con ibu es o he di ac ion in ensi y [33]. The
XRD pa e ns o BiVO4we e ound o be consis en wi h he
cha ac e is ic di ac ion peaks o monoclinic scheeli e [34].
I is disce nible ha he elec odeposi ion o BiVO4did no
cause any conside able peak shi ing o c ys al phase change
o ZnO NDs. All XRD pa e ns o BiVO4/ZnO NDs a e he e-
o e assigned o he co-exis ence o BiVO4and ZnO phases
wi hou any undesi able impu i ies. Figu e 2S(a) shows he
XRD pa e ns o ITO glass and Bi me al elec odeposi ed
ITO glass be o e in oducing he V p ecu so (DMSO con-
aining ammonium mono anada e) o o m BiVO4. The peak
obse ed a 30.80◦is associa ed wi h he (222) plane o ITO
[35]. The peaks in he XRD pa e n o Bi-deposi ed ITO ilm
a e indexed as he e agonal phase o β-Bi2O3[36]. The mos
ele an di ac ion planes a e labeled, and no o he impu i y
phase was de ec ed. The XRD esul s indica e ha Bi me al
eac s wi h he a mosphe ic oxygen o o m Bi2O3.
The Raman spec a o he samples a e shown in igu e 2.
The cha ac e is ic weak peak o ZnO a 438 cm−1is obse ed
in he spec a o he ZnO NDs as well as BiVO4/ZnO NDs.
Figu e 1. XRD pa e ns o BiVO4, ZnO NRs, ZnO NDs and
BiVO4/ZnO NDs pho oanodes.
Figu e 2. Raman spec a o ZnO NDs, BiVO4, BiVO4/ZnO NDs
and BiVO4/ZnO NDs wi hou soaking in NaOH solu ion o emo e
V2O5.
The peaks a 1090 cm−1and 575 cm−1a e associa ed wi h
he de ec s in ZnO c ys als [37]. On he o he hand, he peaks
a ound 820, 706, 365, 210 and 145 cm−1in he Raman spec-
a o BiVO4and BiVO4/ZnO NDs e e o monoclinic BiVO4
[38], implying he o ma ion o he c ys alline BiVO4on ND
ZnO s uc u es. Raman spec al analysis was also applied o
he as-p epa ed BiVO4/ZnO NDs wi hou soaking in NaOH
solu ion o con i m success ul emo al o he esidual V2O5.
Addi ional ib a ional modes appea ed in he Raman spec a
o BiVO4/ZnO NDs wi hou soaking, when compa ed wi h he
Raman spec a o BiVO4/ZnO NDs. Among Raman peaks o
V2O5, he in e nal modes be ween 500–1000 cm−1in ol e
V–O s e ching ib a ions while he ex e nal modes be ween
200–500 cm−1in ol e V–O–V bending ib a ions [39]. The
5
J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
Figu e 3. High esolu ion XPS co e le el spec a o BiVO4/ZnO NDs (a) Zn 2p, (b) Bi 4 , (c) V 2p and (d) O 1 s.
esul indica es ha he excess o V2O5was i mly elimina ed,
and pu e BiVO4/ZnO NDs was o med.
The composi ion and chemical s a es in a BiVO4/ZnO NDs
he e ojunc ion sample was analyzed by XPS. The peak loca-
ions in all XPS spec a we e co ec ed using C 1 s a 284.5 eV.
The high esolu ion XPS spec um o Zn 2pin igu e 3(a)
e eals wo binding ene gies o 1021.0 (Zn 2p3/2) and 1044.0
(Zn 2p1/2) eV, signi ying ha Zn is p esen in he Zn2+s a e
[40]. The ypical binding ene gies o 158.0 (Bi 4 7/2) and 163.4
(Bi 4 5/2) eV in igu e 3(b) deno e Bi in he 3+s a e. As o he
V 2pXPS spec um in igu e 3(c), he majo peaks appea ed
a 515.7 (V 2p3/2) and 523.5 eV (V 2p1/2) indica e ha V in
BiVO4/ZnO NDs pho oanode is p esen in he oxida ion s a e
o V5+[41]. Figu e 3(d) demons a es he O 1 s co e le el
spec um wi h wo decon olu ed peaks a 529.7 and 531.7 eV.
The peak loca ed a 529.7 eV is mainly a ibu ed o he la ice
oxygen o ZnO c ys al while he peak loca ed a 531.7 eV is
ela ed o he hyd oxyl (–OH) g oups o med on he su ace o
specimen [42].
C oss-sec ional and op- iew SEM images and diame e
dis ibu ion o he p ima ily g own ZnO NR a e espec i ely
shown in suppo ing in o ma ion ( igu es 3S(a)–(c)). G ow h
o e ically aligned ZnO NRs on a 300 nm hick seed laye
a e 6 h du ing he hyd o he mal syn hesis esul ed in homo-
geneous hexagonal a ays wi h a leng h o ∼2µm and an
a e age diame e o 44 nm. Mo eo e , he spines g ew di ec ly
on he ZnO NRs o o m he NCs a oom empe a u e a e
20 min ( igu e 3S(d)). As shown in igu e 4(a), SEM image
demons a es ha he spines de eloped in o he b anches o
o m ZnO NDs wi h high su ace a ea a e subsequen hyd o-
he mal g ow h a 100 ◦C o 1 h. These b anches leng hs
anged om 300 o 400 nm. The diame e dis ibu ion o
he p ima y ZnO NRs wi hin NDs is shown in igu e 4(b),
demons a ing ha he a e age diame e inc eased om 44 nm
o 84 nm. I should be poin ed ou ha he elec odepos-
i ed Bi c ys als did no co e he en i e ITO su ace ( igu e
2S(b)). This island mo phology o Bi is a ibu ed o he poo
dissolu ion o Bi deposi s [28,29]. Howe e , as shown in
igu e 4(c), he ITO su ace was almos ully co e ed wi h
po ous, uni o m, and nanoc ys alline BiVO4. A e in odu-
cing V sou ce and a e hea ea men , he mo phology o
BiVO4al e ed signi ican ly compa ed o ha o he o iginal Bi
me al deposi s. The SEM image o BiVO4/ZnO NDs he e o-
junc ion shown in igu e 4(d) illus a es ha elec odeposi ion
p ocess success ully nes ed BiVO4nanoislands in and on op
o ZnO NDs. The size o he BiVO4nanos uc u es was abou
200–300 nm. I should be no ed ha he e is a di e ence in
appea ance be ween he g ow h o ien a ion o BiVO4NPs on
plana ITO and 3D complex ZnO s uc u es. In he o me ,
he nanopa icles accumula e la e ally, in he la e , he e is
6
J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
Figu e 4. SEM images o (a) ZnO NDs, (b) co esponding diame e dis ibu ion o ZnO NDs, (c) BiVO4, and (d) BiVO4/ZnO NDs.
a e ical g ow h o he po ous small nanopa icles s acked
on op o each o he appea like a single la ge clus e . The
quali y o semiconduc o -semiconduc o con ac is essen ial
o achie ing excellen PEC pe o mance o he he e ojunc-
ion de ice. BiVO4nanoislands in he uppe laye igh ly w ap
ZnO NDs and in e wine hem, leading o an in imida e con-
ac a he in e ace o he expec ed good cha ge sepa a ion.
Elec odeposi ion occu s on a conduc i e subs a e by acili -
a ing elec ons o he eac ion. I was epo ed ha ZnO NRs
displayed pola i y dependen high elec ical conduc i i y a y-
ing be ween 10.2 and 90.9 S cm−1[43]. This alue is much
smalle han he elec ical conduc i i y o ITO subs a e ha
is ∼104S cm−1[44]. Equally impo an is he elec ical con-
duc i i y o he elec odeposi ed compounds. Elec ons canno
easily each all he loca ions a he deposi s o he low con-
duc i i y phase while elec ons a e a aliable a all he su ace
si es o he high conduc i i y phase [45]. To emphasize again,
he ca hodic cu en du ing he elec odeposi ion o Bi me al
on ZnO eached as high as −1.1 mA, signi ying su icien elec-
ical conduc i i y o elec odeposi ed phase. Ou esul s sug-
ges ha ZnO in he o m o b anched NRs p o ides sui able
elec ical and anspo p ope ies o he uni o m o ma ion o
BiVO4NPs ia elec odeposi ed Bi me al. Besides, ZnO NDs
sligh ly co oded due o he imme sion in he acidic Bi me al
pla ing solu ion o 5 min.
The EDX analysis o BiVO4/ZnO NDs ( igu e 4S) e eals
ha he he e os uc u e is only composed o zinc, bismu h,
anadium, and oxygen. The absence o any o he elemen con-
i ms he pu i y o he ab ica ed ilm.
Figu e 5. TEM image o BiVO4/ZnO NDs he e ojunc ion.
Figu e 5 ep esen s he TEM image o BiVO4/ZnO NDs
he e os uc u e ex ac ed om a la ge a ea o ND a ays.
ZnO NDs sligh ly co oded du ing he elec odeposi ion o
Bi me al ilm due o he acidi y o he pla ing solu ion as
7
J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güle e al
Figu e 6. (a) PL spec a (exci a ion wa eleng h 335 nm, he peak a 670 nm co esponds o i s second ha monics), (b) Abso p ion plus
sca e ing spec a (A+S=100 −R−T), (c) Nyquis plo s unde AM 1.5 G i adia ion and (d) Mo –Scho ky plo o BiVO4and ZnO NDs.
semiconduc o s.
was also obse ed by SEM. The su ace mo phology exam-
ina ion indica es ha a b anch g ew e ically om he s em
o p ima y ZnO NRs, which ha e a diame e o abou ∼65 nm.
Mo eo e , he size o BiVO4c ys als is abou ∼200 nm wi h
ela i ely da ke con as . I can also be deduced ha BiVO4
nanoislands a e i mly bound o ZnO NDs, leading o a good
elec ical con inui y be ween wo phases. All hese esul s a e
consis en wi h SEM obse a ions.
The semiconduc o he e ojunc ion pho oanode is
epo edly e y e ec i e o imp o emen o he in e acial
cha ge anspo a ion and sepa a ion e iciency [23,46]. To
e alua e he ecombina ion a e wi hin he ab ica ed pho o-
elec odes PL measu emen s we e pe o med unde he exci a-
ion wa eleng h o 335 nm as i s quali a i e indica ion. As seen
om igu e 6(a), wo dis inc i e emission peaks appea ed in
he PL spec a o ZnO NRs, ZnO NDs and BiVO4/ZnO NDs.
The peak a 376 nm esul s om he nea band edge (NBE)
ansi ions in ZnO [47]. Ano he wide and in ense emission
peak o he p is ine ZnO samples appea ed a 570 nm and was
associa ed wi h he p esence o he oxygen acancies [48]. As
BiVO4deposi s on ZnO NDs, a ed-shi o he de ec -induced
emission is obse ed a 620 nm. Annealing ZnO inc eases he
amoun o he oxygen in e s i ials and dec eases he amoun
o he oxygen acancies [49]. The e o e, his shi o is-
ible emission could be ela ed o he a ia ion o he local
su oundings o he de ec si es as a esul o annealing he
he e ojunc ion sample. In he case o BiVO4, wo b oad peaks
a 505 and 410 nm we e a ibu ed o he NBE ansi ion
and he deep le el de ec s, espec i ely [50]. In ac , high PL
in ensi y gene ally cha ac e izes a high ecombina ion a e o
he pho ogene a ed cha ge ca ie [51]. The ZnO NDs NBE
emission in ensi y shows a ema kable educ ion a e coa ing
i wi h BiVO4. Consequen ly, i can be s a ed ha he o ma-
ion o he e ojunc ion be ween BiVO4NPs and ZnO NDs can
signi ican ly inhibi he ecombina ion a e and dec ease he
numbe o c ys al de ec s. The e iciency o cha ge sepa a ion
plays a i al ole ega ding he PEC p ope ies.
UV- is spec opho ome y was also ca ied ou o s udy he
ligh abso p ion capaci y o he ZnO elec odes associa ed wi h
he s uc u al ans o ma ion and BiVO4pho osensi iza ion.
Abso p ion and sca e ing spec a (A+S=100 −R−T) o
he samples we e calcula ed by sub ac ing e lec ance ( igu e
5S(a)) and ansmi ance ( igu e 5S(b)) om 100% inciden
ligh and illus a ed in igu e 6(b). Among he elec odes, he
mos signi ican non-ze o baseline (λ > 400 nm) was obse ed
o ZnO NDs. The main eason is ha he ho izon ally aligned
dense b anches su ounding he p ima y NRs cause sca e ing.
Mos pho ons pass di ec ly h ough he ITO subs a e whe e
hey may be in e nally e lec ed o escape unmeasu ed (ligh
loss) [52]. The supe io ligh abso p ion eme ging om s uc-
u al e olu ion o he b anched ZnO nanos uc u es was epo -
ed p e iously and jus i ied in a simila way [12,33]. The spec-
a also indica e ha ZnO NRs and ZnO NDs ha e e y simila
UV abso p ion edges a 380 nm while he abso p ion edge o
8
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