Magnetite-free Sn-doped hematite nanoflake layers for enhanced photoelectrochemical water splitting

Magne i e-F ee Sn-Doped Hema i e Nano lake Laye s o Enhanced Pho oelec ochemical Wa e Spli ing Hyo-Jin Ahn,[a, b, c] S epan Kmen ,*[a, d] Jeong Eun Yoo,[b] Nha T uong Nguyen,[b] Albe o Naldoni,[a] Radek Zbo il,[a, d] and Pa ik Schmuki*[a, b] In he p esen wo k, we epo a p epa a ion s a egy o hema i e phase-pu e pho oanodes consis ing o Sn-doped hema i e nano lakes/hema i e hin ilm bilaye nanos uc u e (Sn-HB). This app oach is based on a wo-s ep annealing p ocess o pu e i on ilms deposi ed on luo ine doped in oxide (FTO) subs a es by ad anced magne on spu e ing. While he high densi y hema i e ul a hin nano lakes (HNs) wi h de imen- al i on oxide laye s (Fe3O4and/o FeO) a e gene a ed du ing he i s annealing s ep a 400°C o wo hou s, he second he mal ea men a 800°C o 15 minu es oxidises all he undesi ed i on oxide phases o a pho oac i e hema i e laye as well as is p o iding e icien Sn doping o a d op-cas ed SnCl4in o de o inc ease he conduc i i y. The op imized Sn-HB shows an a ound 11 imes highe pho ocu en densi y (0.71 mAcm2 a 1.23 VRHE) compa ed wi h a e e ence hema i e pho oanode p oduced om i on oil unde he same condi ions. In oduc ion Due o en i onmen al conce ns ega ding he use o ossil uels, o e he pas decades enewable ene gy esou ces ha e ecei ed g ea a en ion.[1] Among hem, pho oelec ochemical (PEC) wa e spli ing is a p omising g een app oach as i leads o oxygen and hyd ogen gases solely by using an aqueous elec oly e and sola ene gy.[2,3] A a ie y o oxide semiconduc- o s, such as TiO2,[4] α-Fe2O3,[5] ZnO,[6] Ta3N5,[7] WO3, [8] and BiVO4,[9] ha e been used as pho oanodes. In pa icula , α-Fe2O3(i.e., hema i e) is one o he mos p omising pho oanode ma e ials because o a band gap (2.1 eV), which allows abso p ion o a signi ican po ion o isible sola ligh and hus he high heo e ical sola - o-hyd ogen (STH) e iciency o 15 ~16%. The ma e ial has a high (pho o)chemical s abili y, can be ab ica ed by simple p ocessing, and bea s low cos .[10–12] Ne e heless, he eal PEC pe o mance o hema i e is s ill limi ed owing o a high ecombina ion a e o pho ocha ges esul ing om a sho pho ogene a ed hole di usion leng h (Lh�2–4 nm), slow oxy- gen e olu ion eac ion (OER) kine ics, and o en a limi ed cha ge ans e a in e nal in e aces o pho oanodes.[13–15] While add essing hese d awbacks, a la ge numbe o app oaches including he ab ica ion o nanos uc u ed hema i e,[16] he e oa om-doping,[17] and su ace ca aly ic modi ica ions[18] ha e been epo ed o enhance he e iciency o pho oelec odes.[19] The bene icial e ec s o each s a egy is oughly summa ized in he ollowing pa . Fi s , one dimen- sional (1D) nanoscale hema i e s uc u es (e.g., nano ods, nano ubes, nano lakes)[20–23] inc ease he su ace a ea o wa e oxida ion and educe he ecombina ion o pho op oduced cha ges wi hin 1D s uc u e because o an o hogonal cha ge sepa a ion wi h a s ongly dec eased di usion pa h o pho o- exci ed holes. Second, sui able elemen al doping (e.g., Ti4+,[24] Sn4+,[20] Si4+,[25] and P 4+[12]) inc eases he dono densi y o hema i e, enhances he elec ical conduc i i y, and can he e- o e imp o e he PEC pe o mance o a pho oanode. Thi d, su ace modi ica ions using co-ca alys s (e.g., CoPi,[26] I O,[11] FeOOH[21] and laye ed double hyd oxides[14]) imp o e he OER kine ics o hema i e, leading o a ca hodic shi o he OER onse po en ial. The combina ion o hese h ee key ea u es signi i- can ly enhances he PEC pe o mance o a hema i e pho oanode.[10,14,27] Among α-Fe2O3nanos uc u es, hema i e nano lakes (HNs) show an ul a- hin and high-aspec - a io mo phology ha has been conside ed an ideal nanos uc u e o PEC wa e spli ing because o an aniso opic cha ge ans e o elec on-hole-pai s (EHPs) in he hema i e la ice.[28] Howe e , HNs a e syn hesized om a me al subs a e (such as a shee o a oil) by he mal oxida ion. This he mal oxida ion s ep leads no only o hema i e bu also o magne i e (Fe3O4). This sub-oxide laye is inhe en o high empe a u e oxida ion o i on and o ms unde nea h he hema i e nanos uc u e a he in e ace o he [a] D . H.-J. Ahn, D . S. Kmen , D . A. Naldoni, P o . R. Zbo il, P o . P. Schmuki Regional Cen e o Ad anced Technologies and Ma e ials and Czech Ad anced Technology and Resea ch Ins i u e Palacký Uni e si y Olomouc Šlech i elů 27, Olomouc, 783 71, Czech Republic E-mail: [email p o ec ed] [b] D . H.-J. Ahn, D . J. E. Yoo, P o . N. T uong Nguyen, P o . P. Schmuki Depa men o Ma e ials Science and Enginee ing Uni e si y o E langen-Nu embe g Ma enss asse 7, D-91058 E langen, Ge many E-mail: [email p o ec ed] [c] D . H.-J. Ahn xEne gy and Ca alys LSTME Busan B anch 31, Gwahaksandan 1- o 60beon-gil, Gangseo-gu, 46742 Busan, Republic o Ko ea [d] D . S. Kmen , P o . R. Zbo il Nano echnology Cen e, Cen e o Ene gy and En i onmen al Technologies VŠB – Technical Uni e si y o Os a a 708 00 Os a a-Po uba, Czech Republic Suppo ing in o ma ion o his a icle is a ailable on he WWW unde h ps://doi.o g/10.1002/celc.202200066 © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. This is an open access a icle unde he e ms o he C ea i e Commons A ibu ion License, which pe mi s use, dis ibu ion and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed. ChemElec oChem www.chemelec ochem.o g Resea ch A icle doi.o g/10.1002/celc.202200066 ChemElec oChem 2022,9, e202200066 (1 o 7) © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. Wiley VCH Mon ag, 30.05.2022 2211 / 247129 [S. 116/122] 1 21960216, 2022, 11, Downloaded om h ps://chemis y-eu ope.onlinelib a y.wiley.com/doi/10.1002/celc.202200066 by Technical Uni e si y Os a a, Wiley Online Lib a y on [23/11/2022]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License me al.[29] This una oidable magne i e laye s ongly hampe s he cha ge ans e om he hema i e laye o he cha ge collec o , hus ep esen ing one o he mos impo an p ob- lems o he HNs pho oanodes.[30,31] To o e come he d awbacks o such sub-oxide (magne i e) o ma ion, se e al s a egies ha e been epo ed, including Sn-ion doping o magne i e o a emp s o add a gold laye be ween he magne i e laye and he i on oil.[28,32,33] Howe e , hese e o s canno ully o e come he de imen al e ec s o he magne i e laye . He ein, we in oduce a no el wo-s ep app oach o he ab ica ion o magne i e- ee hema i e nano lakes consis ing o a bilaye s uc u e o Sn-doped hema i e nano lakes/ hin ilm (Sn-HB). The idea is o achie e nano lake g ow h om an op imized ini e laye o Fe, i.e., o p e en magne i e o ma ion by achie ing o al oxida ion o a hin Fe ilm, hus a oiding magne i e o ma ion a he o he wise mo ing oxida ion boun- da y. Fo his, we used hin Fe ilms (200–800 nm) deposi ed on luo ine doped in oxide (FTO) glass by magne on spu e ing. Then he Fe hin ilm/FTO samples we e placed in a u nace a 400°C o syn hesizing a nano lake s uc u e. In his in e medi- a e s a e, we deco a ed he s uc u es wi h Sn. A second he mal s ep a 800°C was hen applied o he d i e-in o he doping and ull oxida ion o he nano lakes. Du ing his high- empe a u e annealing p ocess, no only can Sn4+ions be inco po a ed in hema i e la ice bu also he emnan s o sub- oxides a e ully con e ed o α-Fe2O3because o he accele a ed O2di usion a an ele a ed empe a u e and he absence o a me allic subs a e ha could o m Fe3O4.[34] The op imized Sn- HB pho oanode exhibi ed a pho ocu en densi y o 0.71 mAcm2a 1.23 VRHE ( e e sible hyd ogen elec ode, RHE); his ep esen s a oughly 245% enhanced PEC ac i i y o e a con en ional Sn-doped hema i e s uc u e on i on oil due o he absence o he de imen al Fe3O4laye s. Resul s and Discussion To in es iga e he e ec o he bo om laye s (Fe3O4and FeO) on he α-Fe2O3pho oanode, me allic Fe laye s wi h he hick- ness o 200, 400, 600, and 800 nm we e deposi ed on he FTO glass by magne on spu e ing. The c oss-sec ion SEM images o he spu e ed i on ilms a e shown in Figu e S1. The mo phology o he deposi ed i on ilms was he same o all he samples since all he magne on spu e ing condi ions we e kep cons an and only he ime o he deposi ion a ied. In a se o p elimina y expe imen s and in acco d wi h li e a u e,[36] he op imal he mal g ow h condi ion o hema i e nano lakes is a 400°C in ai . Du ing he he mal oxida ion o he Fe hin ilm on he FTO glass in he u nace, he hema i e nano lakes (HNF) and he Fe3O4laye g ow as a esul o he Fe and oxygen di usion along he Fe g ain bounda ies.[35] Figu e 1 and Figu e 2 shows he c oss-sec ional scanning elec on mic oscopy (SEM) images o he HNF samples and Sn- HB samples ab ica ed om di e en hickness o he Fe hin ilms, espec i ely. The desc ip ion o he sample designa ion codes is p o ided in he expe imen al pa . The HNF-200 sample consis s o a e y low densi y o hema i e nano lakes (HNs) and 363 nm o a dis inc laye comp ising o α-Fe2O3/Fe3O4mixed- phase laye (Figu e 1 and Figu e S2), which has been iden ically epo ed in a numbe o s udies.[28,29,33] The hickness o he α- Fe2O3/Fe3O4unde nea h he laye o HNF-400, HNF-600, HNF- 800 samples (Figu es 1b–1d) and o he HNF-i on oil (Fig- u e S3) inc eased o 714, 843, 852, and 900 nm, espec i ely. These samples also showed much highe densi y o he HNs on hei su aces. On he o he hand, he leng h and he mo phology o he hema i e nano lakes we e simila among he HNF samples ega dless o he inc ease in he α-Fe2O3/Fe3O4 bo om laye , as i can be seen in Figu es 1b–1d and Figu es S2–S3. Al hough a clea bounda y be ween he α-Fe2O3/ Fe3O4laye unde he HNs can be obse ed, when he hickness o he Fe ilm is highe han 600 nm ( ed lines, Figu e 1c–1d), he e is no clea bounda y be ween he α-Fe2O3/Fe3O4laye s in he c oss-sec ional SEM images o HNF-200 (Figu e 1a). How- e e , as shown la e in Figu e 3, s ong X- ay di ac ion (XRD) peaks co esponding o magne i e phase can be obse ed in all HNF samples. The e o e, i is a ional o de ine his bo om laye Figu e 1. C oss sec ional SEM images o (a) HNF-200, (b) HNF-400, (c) HNF- 600, and (d) HNF-800, which a e he mally oxidized 200, 400, 600, and 800 nm Fe ilms on he FTO glass a 400°C o 2 h, espec i ely. Figu e 2. C oss sec ional SEM images o (a) Sn-HB 200, (b) Sn-HB 400, (c) Sn- HB 600, and (d) Sn-HB 800, espec i ely. The inse s in (a–d) a e op iew SEM images. ChemElec oChem Resea ch A icle doi.o g/10.1002/celc.202200066 ChemElec oChem 2022,9, e202200066 (2 o 7) © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. Wiley VCH Mon ag, 30.05.2022 2211 / 247129 [S. 117/122] 1 21960216, 2022, 11, Downloaded om h ps://chemis y-eu ope.onlinelib a y.wiley.com/doi/10.1002/celc.202200066 by Technical Uni e si y Os a a, Wiley Online Lib a y on [23/11/2022]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License as a mix u e o α-Fe2O3and Fe3O4. I is no ewo hy ha he hickness o he α-Fe2O3/Fe3O4laye does no change signi i- can ly when he hickness o he Fe ilm is highe han 600 nm. This is a consequence o he limi ed annealing ime a 400 °C and he di usion-limi ed oxide g ow h (see Figu es 1c–d). Fu he mo e, he olume expansion o he Fe-oxide laye s, which a e hicke han he s a ing Fe seed laye , is also e iden om he SEM images o HNF-600 and HNF-800, espec i ely. Nex , he Sn-doped hema i e bilaye s uc u es, which consis ed o hema i e nano lakes and Sn-HB hin laye s, we e p epa ed by 20 μl o SnCl4solu ion ea men on he HNF samples ollowed by d ying a oom empe a u e o 10 min and a sin e ing p ocess a 800°C o 15 min. Finally, he Sn-HB samples we e dipped in 1 M KOH solu ion o 12 h in o de o emo e he excess SnO2laye s, as epo ed elsewhe e.[36] Figu e 2 displays SEM images o he Sn-HB samples. The o iginal leng h o he nano lakes o ca. 2 μm on he HNF samples dec eased o 1.56, 1.12, and 0.924 μm, espec i ely, a e he annealing p ocess. Mo eo e , he shape o he nano lakes on he Sn-HB samples is also con ac ed compa ed o HNFs, which esul s om a s uc u al de o ma ion o hema i e.[37,38] When HNF is con e ed o Sn-HB, he bo om laye s (α-Fe2O3/Fe3O4and/o FeO/Fe) in he HNF samples a e ully oxidized o α-Fe2O3in consequence o he high empe - a u e annealing, which is e iden om he XRD analysis (see Figu e 3b). In addi ion, he hickness o he bo om laye s inc eases o 420 nm, 785 nm, 1.52 μm, and 1.93 μm, espec- i ely, as also e iden in Figu e 2. Al hough he e we e se e e olume expansions, he de achmen o he ilms was no obse ed (See Figu e 2 and Figu e S4). The mo phology changes o Sn-HB 600 sample om Fe ilm o Fe2O3bilaye s uc u e was summa ized in Figu e S5. Fu he mo e, we con i med ha he e is educed e lec ion o hema i e bilaye s uc u e (Sn-HB 600) compa ed wi h single ilm s uc u e (Sn- HB 200) due o hyb id s uc u es, consis ing o hema i e nano lakes and ilm (See Figu e S6). In o de o s udy he c ys alline s uc u e o he samples, XRD measu emen s we e pe o med. Figu e 3a shows XRD pa e ns o he HNF samples wi h he ypical e lec ions a 24.13°, 35.61°, 40.83°, 49.42°, 57.51°, 62.40°, and 63.98° co esponding o (012), (110), (113), (024), (122), (214), and (300) plane o hema i e, espec i ely. The XRD peaks a 30.07°and 43.05°we e assigned o (220) and (400) plane e lec ions o magne i e (Fe3O4) in he HNF samples.[28] The peak in ensi y o magne i e displayed in he inse o Figu e 3a is well consis en wi h he hickness o he α-Fe2O3/Fe3O4laye , as desc ibed abo e (Figu e 1). A e he second annealing s ep, he XRD peaks o magne i e in he Sn-HB samples (Figu e 3b) disap- pea ed, e i ying ha he magne i e phase had been ully ans o med o hema i e a e he high empe a u e annealing p ocess o he HNF samples. Since he oxygen di usion a e inc eases wi h an ele a ed empe a u e, he bo om laye s (α- Fe2O3/Fe3O4and/o FeO/Fe) o he HNF a e easily con e ed o hema i e laye s, as e iden om Figu e 2. Wi h he aim o examine he su ace composi ion o he HNF and he Sn-HB samples, X- ay pho oelec on spec oscopy (XPS) was ca ied ou , and he co esponding high- esolu ion Sn 3d and Fe 2p spec a a e shown in Figu e 4. The XPS analysis o he Sn-HB samples shows wo majo peaks a 487.3 eV (Sn 3d5/2) and 495.7 eV (Sn 3d3/2), which a e well consis en wi h he epo ed XPS da a o Sn-doped hema i e, demons a ing subs i u ional doping o Sn4+ions in he hema i e la ice (see Figu e 4(a)).[39] Figu e 4b shows XPS peaks a 711 eV (Fe 2p3/2) and 724.5 eV (Fe 2p1/2), which we e de ec ed in Sn-HB 600 and HNF-600, espec i ely. Since Sn4+ions can educe he Fe3+ o Fe2+, he XPS peak in ensi y o Fe2+(716 eV) o Sn-HB 600 was highe han ha o HNF-600, sugges ing a highe concen a ion o Fe2+ions on he su ace o Sn-HB 600 han HNF-600.,[40][41] The mola a io o Sn and Fe ob ained om he XPS analysis demons a es an a omic pe cen age (Sn/(Sn+Fe)) o 0.03% and 27.54% o he HNF-600 and Sn-HB 600 (Table S1). Figu e 5a displays pho ocu en densi y-po en ial (J-V) cu es eco ded in 1 M KOH elec oly e and by using AM1.5 chopped ligh o samples HNF-200, HNF-400, HNF-600, HNF- 800, as well as o he HNF-i on oil. The pho ocu en densi y o he HNF-200 sample is 0.021 mAcm2a 1.23 VRHE, which is simila o p e iously epo ed hema i e hin ilm pho oanodes. Conside ing he sho ligh abso p ion leng h and sho di usion leng h o pho oholes (Lh�2–4 nm), he pho ocu en densi y o HNF-200 wi h he hickness o 363 nm o Fe2O3/Fe3O4 hin ilm (Figu e 2a) is acco dingly low. A e he g ow h o he HNs on he Fe2O3/Fe3O4 hin ilm, he pho ocu en densi y o HNF-400 eaches 0.1 mAcm2a 1.23 VRHE. Howe e , o hicke laye s, he pho ocu en densi y again dec eases o 0.0675 mAcm2(HNF-600), 0.0513 mAcm2(HNF-800) and Figu e 3. XRD pa e ns o (a) HNF-200, HNF-400, HNF-600, and HNF-800, espec i ely, and (b) Sn-HB 200, Sn-HB 400, Sn-HB 600, and Sn-HB 800, espec i ely. The inse in (a) is a magni ied XRD peak o he magne i e (220) plane. Figu e 4. XPS spec a o (a) Sn 3d and (b) Fe 2p o HNF-600 and Sn-HB 600, espec i ely. ChemElec oChem Resea ch A icle doi.o g/10.1002/celc.202200066 ChemElec oChem 2022,9, e202200066 (3 o 7) © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. Wiley VCH Mon ag, 30.05.2022 2211 / 247129 [S. 118/122] 1 21960216, 2022, 11, Downloaded om h ps://chemis y-eu ope.onlinelib a y.wiley.com/doi/10.1002/celc.202200066 by Technical Uni e si y Os a a, Wiley Online Lib a y on [23/11/2022]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License 0.045 mAcm2(HNF- i on oil) a 1.23 VRHE. This end is due o an inc ease in he hickness o he bo om laye s (Fe2O3/Fe3O4 and FeO/Fe), as shown in Figu e 2a and Figu e 5a. Figu e 5b u he shows he J-V cu es o he samples a e he second he mal- and Sn- ea men s ep. An inc ease in he pho o- cu en densi y o HNF-600 o 0.71 mAcm2a 1.23 VRHE was obse ed, a e he high empe a u e annealing o he Sn- ea ed samples was ca ied ou (Sn-HB 600). This d as ic inc ease is due o he absence o he sub-oxide bo om laye s and he enhanced conduc i i y in oduced by he Sn doping (Figu e 2b and Figu e 5b). This can be con i med by he Mo - Scho ky plo s which a e well consis en wi h he PEC pe o m- ances as shown in Figu e S7. In he Mo -Scho ky plo s, he dono densi y o he Sn-HB 600 is highe han hose o he o he samples (see able in Figu e S7) deno ing highe dono concen a ion ha migh be ano he e ec o he high densi y o he nano lakes and hus e icien doping by Sn. Fo compa ison, we a emp ed o achie e a simila high empe - a u e Sn ea men using he samples wi hou FTO. Tha is, he Sn-doped HNF on he i on oil sample was p epa ed by he co esponding 2-s ep annealing p ocess (400°C o 2 h and 800°C o 5 min wi h he Sn ea men ). The esul s a e summa ized in Figu e S8. Howe e , unde hese condi ions, he olume expansion o he i on oil was oo se e e, and, he e- o e, he g own HNs inally pa ially peeled-o om he oil (see pho og aph in Figu e S8d). When he hickness o he bo om hema i e laye was highe han ha o Sn-HB 600, he pho o- cu en densi y dec eased o 0.63 mAcm2(Sn-HB 800) and 0.29 mAcm2(Sn-HB on i on ilm) a 1.23 VRHE because o a high ecombina ion a e esul ing om a longe elec on pa hway o he FTO glass. This can be obse ed in he J-V cu e o Sn-HB 800 and Sn-HB on he i on oil (Figu e 5b). The inciden pho on- o-cu en e iciency (IPCE) spec a in Figu e 5c e ealed a highe quan um e iciency o Sn-HB 600 compa ed o ha o o he samples a he applied po en ial o 1.23 VRHE, which was well consis en wi h he JV measu emen esul s. The IPCE end o all he samples ollowed he op ical abso p ion o he HNs in he egion o wa eleng hs be ween 300–600 nm. The maximum IPCE alues o he Sn-HB samples can be ound a he wa eleng h o 330 nm and i inc eased om 2.83% (Sn-HB 200) o 16.10% (Sn-HB 600), which was in line wi h he ini ial i on ilm hickness. Nex , when he hickness o he ini ial i on ilm was highe han 600 nm, he maximum IPCE alue dec eased o 14.36% (Sn-HB 800) because o a high ecombina- ion a e. The inse o Figu e 5c shows he e alua ion o he band-gap (Eg) om indi ec elec on ansi ion ((Iphh )1/2 s. pho on ene gy (h )) o he Sn-HB samples leading o app ox- ima ely 1.91 eV, which is well in he ange o he epo ed Ego p is ine hema i e (1.9–2.1 eV). To s udy he cha ge ans e kine ics o he hema i e pho oanodes, elec ochemical impedance spec oscopy (EIS) was pe o med in 1 M KOH elec oly e a 1.23 VRHE applied po en ial, using a 369 nm ligh sou ce (Figu e 5d). The equi - alen ci cui model depic ed in Figu e S9 was used o i he da a o he Nyquis plo s. This equi alen ci cui ep esen s apping/de- apping esis ance o elec ons in hema i e, R1; he space cha ge capaci ance a he in e ace o he bulk hema i e, C1; he cha ge ans e esis ance be ween he elec oly e and he su ace o he hema i e, R2; he space cha ge capaci ance a he in e ace be ween he elec oly e and he su ace o he hema i e, C2; and he se ies esis ance o he elec ochemical cell, Rs.[17] The EIS i ing esul s o Sn-HB 200, Sn-HB 400, Sn-HB 600, and Sn-HB 800 a e summa ized in Table S1. The Rs alues o all samples a e ela i ely low, which indica es an e icien ans e o elec ons om he hema i e and he FTO conduc i e elec ode. The apping/de- apping esis ance o elec ons (R1) and cha ge ans e esis ance be ween he elec oly e/su ace o he hema i e (R2) o he Sn-HB 200 de ining he hin ilm s uc u e show he highes alues compa ed o he o he Sn-HB samples, which means ha he hema i e hin ilm laye shows he poo PEC pe o mance because o high ecombina ion and poo cha ge ex ac ion. I is in e es ing ha e en hough Sn-HB 600 has a hicke bo om hema i e laye compa ed wi h Sn-HB 200, he elec on ecombina ion a e in he bulk egion (R1) and he cha ge ex ac ion esis ance (R2) o Sn-HB 600 is lowe han ha o Sn-HB 200, as displayed in Figu e 5d. This may be a ibu ed o he dec eased ecombina ion a e due o cha ge compensa ion o pho o-gene a ed holes in he hema i e hin laye by addi ional elec ons om HNs. When he hickness o he bo om laye eached 1.93 μm (Sn-HB 800), he R1and R2o Sn-HB 800 inc eased again owing o he p olonged pa hway o elec on ans e , which was consis en wi h he J-V cu es o he Sn-HB samples. F om pho oelec ochemical p ope ies combined wi h im- pedance measu emen s o he Sn-HB samples, a cha ge ans e mechanism, as depic ed in Figu e 6, can be concluded. The gene ally high pho ocu en s o he hema i e nano lakes can be asc ibed o he high aniso opic conduc i i y o hema i e ha is up o ou o de s o magni ude highe elec on anspo along Figu e 5. (a) J-V cu e o HNF-200, HNF-400, HNF-600, HNF-800, and HNF on i on oil. (b) J-V cu e o Sn-HB 200, Sn-HB 400, Sn-HB 600, Sn-HB 800, and Sn doped HNF on i on oil. (c) IPCE o Sn-HB 200, Sn-HB 400, Sn-HB 600, and Sn-HB 800 measu ed a 1.23 VRHE. The inse shows he band gap calcula ion om a (Iphh )1/2 s. pho onene gy (h ) plo . (d) Nyquis plo s o Sn-HB 200, Sn-HB 400, Sn-HB 600, and Sn-HB 800 unde 369 nm ligh -emi ing diode (LED) sou ce a 1.23 VRHE. The Inse shows he enla ged high equency egion o Nyquis plo s. ChemElec oChem Resea ch A icle doi.o g/10.1002/celc.202200066 ChemElec oChem 2022,9, e202200066 (4 o 7) © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. Wiley VCH Mon ag, 30.05.2022 2211 / 247129 [S. 119/122] 1 21960216, 2022, 11, Downloaded om h ps://chemis y-eu ope.onlinelib a y.wiley.com/doi/10.1002/celc.202200066 by Technical Uni e si y Os a a, Wiley Online Lib a y on [23/11/2022]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License (110) hema i e c ys al planes han o hogonal o hem and o he high aspec a io s uc u e o HNs.[42] In o he wo ds, no only can he pho ogene a ed holes in he HNs easily each he elec oly e h ough he hin nano lake s uc u e, bu also he pho ogene a ed elec ons can low o he FTO glass along he p e e en ial (110) plane. We can hus sugges a possible cha ge ans e mechanisms conside ing also he hickness o he hema i e bo om laye as well as he densi y o HNs g ow, which can desc ibe he enhanced PEC ac i i y o he Sn-HB 600 sample. Fi s , Figu e 6a desc ibes he cha ge ans e mecha- nism o he Sn-HB 200 sample. As shown abo e, his sample con ains e y low densi y o he HNs on he op o a ela i ely hick hema i e laye . Because o gene ally e y low elec ical conduc i i y o hema i e ilms and sho di usion leng h o holes, he EHPs gene a ed in he bulk hema i e laye easily ecombine, which is simila o he ypical hema i e hin ilms. By con as , when he densi y o he HNs conside ably inc eases and he hema i e bo om laye has he op imal hickness, as in he case o sample Sn-HB 600, he pho ogene a ed holes in he bo om hema i e hin ilm can be compensa ed by he pho o- gene a ed elec ons om he uppe HNs. Since he pho o- exci ed holes om he HNs can be easily ex ac ed o elec o- ly e, he elec ons om he HNs can each he bo om laye and ac as a hole sca enge in he bulk hema i e hin ilm. These cha ge compensa ions be ween he elec ons om he HNs and he holes om he hin ilm laye can e ec i ely educe he p obabili y o a ecombina ion a e in he hin ilm laye and he HNs, as i is schema ically depic ed in Figu e 6b. Finally, i he hickness o he bo om laye exceeds a ce ain limi and simul aneously he leng h o he HNs is sho ened, he ecombina ion a e inc eases again. I is a consequence o p olonging he elec on pa hway o he FTO cha ge collec o and a poo compensa ion e ec by a dec eased numbe o elec ons esul ing om he educed ligh abso p ion o he sho ened HNs. Since he hole sca enge p esen in he elec oly e educes he cha ge ans e esis ance be ween hema i e su ace and elec oly e, ou a o emen ioned mecha- nism can be con i med by compa ing he EIS da a wi h and wi hou he hole sca enge .[43] Figu e S10a and S10b show he EIS da a o he samples measu ed in 1 M KOH solu ion a 1.0 VRHE and unde 1 sun illumina ion wi hou and wi h 0.5 M Na2SO3as he hole-sca enge , espec i ely. In bo h cases he sample Sn-HB 600 show he lowes esis ance implying educed ecombina ion o cha ges also in he bulk hema i e laye and hus he bes cha ge ans e abili y. In addi ion, he cha ge sepa a ion (ηsep) and cha ge injec ion (ηinj) e iciency o Sn- HB600 shows he highes alue compa ed o ha o o he samples, indica ing educed cha ge ecombina ion in pho o- anode (Figu e S12). Simila mechanism can also be conside ed o he sample Sn-HB 800 as depic ed in Figu e 6c. This sample acco ding o he SEM analysis (see Figu e 2d) shows he hickness o he bo om hema i e laye o 1.5 μm and he leng h o HNs o 1.12 μm. Howe e , his sample p o ides sligh ly lowe PEC ac i i y p esumably due o he de imen al e ec o low abso p ion coe icien and sho di usion leng h o he cha ges, which again s a o educe he PEC pe o m- ance. As men ioned be o e, ano he limi a ion o hema i e o an e icien applica ion o PEC wa e spli ing is poo oxygen e olu ion kine ics on i s su ace. The e o e, in o de o u he imp o e he PEC pe o mance, an OER ca alys , zinc-cobal laye ed double hyd oxide (ZnCo LDHs), was deposi ed on HNF-600 and Sn-HB 600 by imme sing in he dilu ed ZnCo LDH solu ion o 10 min as shown in Figu e 7. The ZnCo LDH co-ca alys was syn hesized using he same me hod as epo ed elsewhe e.[14] The SEM image (Figu e S13) and XPS analysis (Figu e S14) was used o con i m he p esence and he p ope chemical composi ion o he ZnCo LDHs deposi ed on he su ace o he Sn-HB 600 (ZnCo LDH/Sn-HB 600), espec i ely. The high- esolu ion XPS peaks o Zn 2p and Co 2p o ZnCo LDH in Figu e S14 show he cha ac e is ics o ZnCo LDH, which a e in line wi h he li e a u e.[14] The pho ocu en o ZnCo LDH/Sn-HB 600 in Figu e 7 shows 0.97 mAcm2a 1.23 VRHE, which ep esen s abou 15 imes enhanced PEC pe o mance compa ed wi h HNF-600 (0.0675 mAcm2a 1.23 VRHE), i.e., he ma e ial can indeed be u he imp o ed by using an OER ca alys . Fu he mo e, he onse po en ial o Zn- CO LDH/Sn-HB 600 shows a 119 mV ca hodic shi om 0.963 (Sn-HB 600) o 0.844 VRHE. O e all, his conside able imp o e- men is a ibu ed o he acile cha ge ans e by e ec i ely doped Sn ion, he absence o a poo ly conduc i e Fe3O4laye , and he enhanced OER p ope y. Figu e 6. Schema ic diag am o cha ge ans e mechanism o (a) Sn-HB 200, (b) Sn-HB 600 and (c) Sn-HB 800, espec i ely. Figu e 7. J-V cu e o HNF-600, Sn-HB 600, ZnCo LDH/Sn-HB 600, espec i ely. ChemElec oChem Resea ch A icle doi.o g/10.1002/celc.202200066 ChemElec oChem 2022,9, e202200066 (5 o 7) © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. Wiley VCH Mon ag, 30.05.2022 2211 / 247129 [S. 120/122] 1 21960216, 2022, 11, Downloaded om h ps://chemis y-eu ope.onlinelib a y.wiley.com/doi/10.1002/celc.202200066 by Technical Uni e si y Os a a, Wiley Online Lib a y on [23/11/2022]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Conclusion In conclusion, we ab ica ed an Fe3O4laye ee Sn-doped hema i e bilaye s uc u e o hema i e nano lakes/ hin ilm by using i s he magne on spu e ing o i on ilms wi h di e en hicknesses on o an FTO subs a e ollowed by simple wo-s ep annealing p ocess. Du ing he i s s ep, he as-deposi ed Fe ilms we e annealed a 400 °C o 2 h and hen he Sn ea men o he hema i e nanos uc u es was ca ied ou , ollowed by second annealing a 800°C o 15 min. No only does ou app oach con e he high esis i e bo om laye s o a pho o- esponsi e hema i e laye , bu i also makes a complemen a y ela ionship be ween he pho ogene a ed EHPs o hema i e nano lakes and ha o hema i e hin ilm. Taking ad an age o his e ec , he Sn-HB 600 shows he 0.71 mAcm2a 1.23 VRHE, which is 10.52 imes highe pho ocu en densi y compa ed wi h HNF-600 (0.0675 mA cm2a 1.23 VRHE). Ou s udy pa es he way o educing he ecombina ion o hin ilm pho oanode and also p o ides a s aigh o wa d way o sol e he ch onic issue o poo conduc i e sub-oxides bo om laye in he mal oxidized me als. Expe imen al Sec ion P epa a ion o HNF and Sn-HB Samples I on hin ilms o 200, 400, 600, and 800 nm in hickness we e p epa ed on FTO glass by using a magne on spu e ing. The i on ilms we e deposi ed by an ad anced plasma deposi ion me hod known as high-impulse magne on spu e ing (HiPIMS). The 4’’ i on a ge (pu i y 99.99%, Leske ) and A gon a mosphe e we e used as he sou ce o i on and wo king gas, espec i ely. The ilms we e deposi ed on ca e ully cleaned ( insed in isop opyl alcohol, e hanol, and deionized wa e espec i ely) FTO coa ed glass subs a es (Sola onix) a he oom empe a u e ( o he sake o cla i y he empe a u e o he FTO subs a e aised up o abou 80 °C due o he plasma bomba dmen du ing he deposi ion). The FTO samples we e placed in an ul a-high acuum chambe on a o a ing subs a e holde . The deposi ions o i on we e ca ied ou wi h he applied Di ec cu en (DC) powe o 650 W using a pulsed mode wi h he pulse equency o 100 Hz and du y cycle o 1%. The i on ilm/FTO samples we e placed in a u nace a 400°C o 2 h in o de o p epa e he HNF samples. Fo Sn-doping, 20 μl o SnCl4e hanol solu ion (e hanol:SnCl4=200:1) was d opped on he HNF samples and d ied in ai o 20 min, and hen annealed a 800°C in a u nace o 15 min o p epa ing Sn-HB samples.[36] To emo e he excess SnO2, he samples we e imme sed in 1 M KOH solu ion o 12 h. Addi ional samples we e p epa ed on i on oil, wi hou FTO subs a e. The HNF on i on oil samples (i.e., hema i e nano lake on Fe wi hou FTO) we e p epa ed by he same p ocedu e as he men ioned one abo e. The Sn-HB on i on oil samples we e p epa ed by 5 min annealing ime a 800°C wi h SnCl4e hanol solu ion. In he ex , he samples a e coded acco ding o he hickness o he spu e ed i on ilms as ollows: i) hema i e nano lakes g own du ing he i s annealing s ep a 400°C o wo hou s a e named as HNF-#, #=200, 400, 600, and 800; ii) ully oxidized and Sn-doped hema i e nano lakes achie ed a e he second annealing s ep a 800°C o 15 min. a e named as Sn-HB #, #=200, 400, 600, and 800; and iii) o compa a i e easons, he hema i e nano lakes g own om commonly used i on oils a 400°C o wo hou s a e named as HNF-i on oil. P epa a ion o Zn-Co LDHs To syn hesize ZnCo LDHs, 10 ml deionized wa e (DI) solu ion, con aining 44 mg o zinc ni a e hexahyd a e (0.15 mmol), 87 mg o cobal ni a e hexahyd a e (0.3 mmol) and 144 mg o u ea (2.4 mmol) we e p epa ed and hen 40 ml e hylene glycol was added he e. This solu ion was placed in mic owa e eac o and ea ed mic owa e i adia ion o 10 min wi h 30 s on/o in e al and hen cooling na u ally. The ZnCo LDH was il e ed, washed wi h DI wa e , e hanol, and d ied a 60 °C o e nigh . P epa a ion o Zn-Co LDH/Sn-HB 600 The ZnCo LDH powde was dispe sed in DI wa e wi h a densi y o 0.1 mg ml1. A e ZnCo LDH solu ion was sonica ed o 10 min, Sn-HB 600 was imme sed in ZnCo LDH solu ion o 10 min. The ZnCo LDH/Sn-HB 600 was washed by DI wa e and d ied wi h N2 gas. Pho oelec ochemical measu emen s The pho oelec ochemical pe o mance o he hema i e pho o- anodes was measu ed in a h ee-elec ode PEC cell, whe e a P mesh and a Ag/AgCl (3 M KCl) elec ode ac ed as a coun e and e e ence elec ode, espec i ely, unde illumina ion o AM1.5G (100 mWcm2, 1 sun) in 1 M KOH elec oly e. Pho ocu en densi y s. applied po en ial (J-V) was measu ed by scanning he po en ial om 0.5 o 0.7 V a a scan a e o 2 mVs1. The po en ials s. Ag/ AgCl (3 M KCl) we e con e ed o he e e sible hyd ogen elec ode (RHE) ia he ollowing equa ion ERHE =EAg/AgCl +0.059pH +E0 Ag/AgCl, whe e EAg/AgCl is he expe imen ally measu ed po en ial, and E0 Ag/AgCl =0.209 V a 25°C o an Ag/AgCl elec ode in 3 M KCl. IPCE was acqui ed in he ange om 300 o 700 nm wi h 10 nm s eps a an applied po en ial o 1.23 VRHE in 1 M KOH. The EIS measu emen s we e ca ied ou in he equency ange om 100 kHz o 0.1 Hz a 1.23 VRHE wi h a pe u ba ion ampli ude o 10 mV in a h ee- elec ode se -up by using a 369 nm LED ligh sou ce a 0.335 mWm2. Acknowledgemen s We would like o acknowledge he ERC, he DFG, he E langen DFG clus e o excellence EAM, p ojec EXC 315 (B idge), he DFG unCOS and he Ope a ional P og amme Resea ch, De elopmen and Educa ion-Eu opean Regional De elopmen Fund, p ojec no. CZ.02.1.01/0.0/0.0/15_003/0000416. We acknowledge he unding om Czech Science Founda ion, p ojec GA CR – EXPRO, 19– 27454X. Open Access unding enabled and o ganized by P ojek DEAL. Con lic o In e es The au ho s decla e no con lic o in e es . Da a A ailabili y S a emen The da a ha suppo he indings o his s udy a e a ailable in he supplemen a y ma e ial o his a icle. ChemElec oChem Resea ch A icle doi.o g/10.1002/celc.202200066 ChemElec oChem 2022,9, e202200066 (6 o 7) © 2022 The Au ho s. ChemElec oChem published by Wiley-VCH GmbH. 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Wiley VCH Mon ag, 30.05.2022 2211 / 247129 [S. 122/122] 1 21960216, 2022, 11, Downloaded om h ps://chemis y-eu ope.onlinelib a y.wiley.com/doi/10.1002/celc.202200066 by Technical Uni e si y Os a a, Wiley Online Lib a y on [23/11/2022]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License