Fabrication of an Efficient BiVO4âTiO2 Heterojunction Photoanode for

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Fabrication of Efficient BiVO-TiO Heterojunction Photoanode for Photoelectrochemical Water Oxidation Bo-Yan Cheng, Jih-Sheng Yang, Hsun-Wei Cho, and Jih-Jen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05489 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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ACS Applied Materials & Interfaces

Fabrication of Efficient BiVO4-TiO2 Heterojunction Photoanode for Photoelectrochemical Water Oxidation Bo-Yan Cheng, Jih-Sheng Yang, Hsun-Wei Cho and Jih-Jen Wu* Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

KEYWORDS: Photoelectrochemical Water Oxidation, Bismuth vanadate, Titanium dioxide, Heterojunction, Charge separation and injection efficiencies.

ACS Paragon Plus Environment

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ABSTRACT

In this work, a simple planar BiVO4/TiO2 heterojunction photoanode has been prepared on fluorine-doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. The measurements of surface photovoltage, photocurrent transient behavior and hole-scavengerassisted PEC performance indicate that charge separation efficiency is improved compared to that of the BiVO4/FTO photoanode. This improvement is caused by the formation of the staggered

BiVO4/TiO2

heterojunction.

However,

the

photocurrent

densities

of

the

BiVO4/TiO2/FTO photoanode are higher than those of the BiVO4/FTO one only at the potentials > 1.2 V vs. reversible hydrogen electrode (RHE) although the two BiVO4 layers with comparable light harvesting efficiencies were prepared by the same method. The hole-scavenger-assisted PEC measurements reveal that the hole injection efficiency of the BiVO4/TiO2/FTO photoanode is inferior to that of the bare BiVO4/FTO one for oxygen evolution. It shows that the surface property of the BiVO4 layers is altered as they are deposited on different substrates. Based on these characterizations, the co-catalyst cobalt phosphate (Co-Pi) is further deposited on the surface of BiVO4/TiO2/FTO photoanode to improve the hole injection efficiency. Subsequently, the photocurrent density and stability of the Co-Pi/BiVO4/TiO2/FTO photoanode are significantly improved compared to those of the bare BiVO4/FTO one.


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Introduction The issues of global warming and increasing demand for energy have led to growing worldwide interest in renewable energy sources.1,2 Harvesting energy directly from sunlight is the most abundant renewable energy source. It has been recognized as a good solution to the energy challenge with minimal environmental impact.1-3 The production of solar fuels that store solar energy in the form of chemical bonds is a promising technique for practical utilization of solar energy.1-4 Solar fuels from thermodynamically uphill reactions of water splitting and CO2 reduction can be driven by photocatalysis and photoelectrochemistry.4,5 Solar energy conversion to fuels based on semiconductor photocatalysis has stimulated research efforts intensively over the past few decades.6 Water oxidation is a crucial step for both photocatalytic water splitting and CO2 reduction to provide the necessary electrons for the proton and CO2 reduction.6,7 Photocatalytic water oxidation comprises a sequence of steps, i.e., light harvesting, charge generation, charge separation, charge transport and charge injection (surface reaction). Charge recombinations occur in the bulk phase and at surface states of the photoanode. These are competing steps with charge separation and charge injection that degrade the water oxidation efficiency of the photoanode. Constructing staggered (type II) heterojunction semiconductor photoelectrode is one of the most effective approaches to improve charge separation.8 However, the morphology and surface properties of the semiconductor photocatalyst in the heterojunction photoelectrode may be altered compared to that of the plain semiconductor photoelectrode prepared by the same method. This alteration is due to the substrate effect on the deposition of the semiconductor photocatalyst. Therefore, careful evaluation of the factors influencing the photoelectrochemical (PEC) performance of the heterojunction photoelectrode is required for further rational design


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and fabrication of efficient photoanode for water oxidation besides monitoring the PEC current or oxygen production. In this work, monoclinic bismuth vanadate (BiVO4) and TiO2 were selected as the model semiconductors to form the simple planar heterojunction photoanode for investigating the aforementioned evaluation issue on the heterojunction photoanode for water oxidation. TiO2 is one of the most attractive photoanode for water oxidation.9-11 However, only the UV part of solar irradiation (4% of the incoming solar energy on the Earth surface) can be absorbed by TiO2 due to its large band gap. With an appropriate band gap of ~ 2.4 eV for solar light harvesting and suitable valence band edge position for O2 evolution, BiVO4 is one of the most promising visible-light-driven photocatalysts for water oxidation.12,13 However, intrinsic BiVO4 shows an inferior photocatalytic activity due to the severe photocharge recombination in the bulk phase and to the slow hole kinetics of the O2 evolution reaction on the surface.13 Improvements of water oxidation efficiency of BiVO4 photoanodes have been reported by using various strategies, such as impurity doping for increasing electrical conductivity,14,15 heterojunction formation for improving charge separation16-18 and surface modification with O2 evolution co-catalyst.19-21 In the present work, BiVO4 layers were prepared on bare fluorine-doped tin oxide (FTO) and TiO2coated FTO substrates using the same metal organic decomposition (MOD) method.22 The influences of BiVO4/TiO2 heterojunction formation on the charge separation and injection efficiencies of the BiVO4 photoanode have been carefully studied for further design and fabrication of efficient photoanode for water oxidation.


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Experimental Section TiO2 layers were deposited on FTO substrates using a wet chemical route combining hydrothermal and chemical bath deposition methods.23 The hydrothermal process was carried out by immersing the FTO substrate in a solution of 40 mL of 8 M HCl in deionized water and 0.5 mL of titanium(IV) tert-n-butoxide at 150 oC for 3.5 h. Then chemical bath deposition was conducted in a solution of 40 mL of 0.25 M HCl and 1 mL of titanium(IV) tert-isopropoxide at 95 oC for 4 h. Another hydrothermal process was performed further for 1 h and it was followed by annealing at 450 oC for 1 h. BiVO4 layers were prepared on FTO and TiO2/FTO substrates using metal organic decomposition method.22 0.07M bismuth nitrate hexahydrate and 0.07 M vanadylacetylacetonate were added into 5 mL solution of 1:8.25 acetic acid and acetylacetone. The dark green solution was then sonicated for 15 min. The BiVO4 layer was prepared by repeating the procedures of spin coating the obtained solution onto the FTO or TiO2/FTO substrate and subsequent annealing at 500 oC for 10 min. The volume of the BiVO4 layer was controlled by the repeated cycles of the aforementioned procedure. Finally, the BiVO4 layer was annealed at 500 oC for 2 h. Cobalt phosphate (Co-Pi) was electrodeposited on the surface of the BiVO4 layer in a three-electrode cell with the electrolyte of 0.5 mM cobalt nitrate in 0.1 M potassium phosphate solution buffered at pH 7.24 The BiVO4 photoanodes, Pt foil and an SCE electrode were employed as the working, counter and reference electrodes, respectively. The deposition was conducted at 1 V vs. SCE for 2 min. The morphologies of the photoanodes were examined with scanning electron microscope (SEM, Hitachi SU8010). The structures of the BiVO4 photoanodes were characterized with Xray diffractometer (XRD, Rigaku D/MAX-2000) and Raman scattering spectrometer (integrated by Protrustech Corporation Limited) at excitation wavelength of 532 nm. Optical absorptions of


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the photoanodes were measured by a UV-vis-IR spectrophotometer (JASCO V-670). The surface potential images of the photoanodes were taken by scanning Kelvin probe force microscope (KPFM, Veeco diInnova) both in the dark and under white-light LED illumination. The details for the KPFM measurements have been reported in our previous work.25 PEC performances of the BiVO4 photoanodes were examined with a three-electrode system under AM 1.5 simulated sunlight at 100 mWcm-2 (100 W, Model 94011A, Oriel). The supporting electrolyte was prepared from 0.5 M Na2SO4 with KPi buffered at pH=7. The BiVO4 photoanodes with a well-defined area of 0.44 cm2, Pt foil and an Ag/AgCl electrode were employed as the working, counter and reference electrodes, respectively. PEC performances of the BiVO4 photoanodes were also monitored by using 0.5 M hydrogen peroxide as a hole scavenger. Incident-photon-to-current efficiency (IPCE) spectra were obtained by using 500 W xenon light source (Oriel) and a monochromater (Oriel Cornerstone) equipped with Si detector (Model 71640, Oriel).

Results and Discussion A thin TiO2 layer was first grown on the FTO substrate by wet chemical method.23 The topview SEM image of the thin TiO2 layer reveals that the FTO surface is well-covered by the tiny TiO2 nanocrystals (Figures S1a and S1b, Supporting Information). The thickness of the TiO2 layer is 10 nm which has been determined by TEM characterization in our previous work.23 Formation of the rutile phase TiO2 layer was confirmed by Raman spectroscopy (Figure S1c, Supporting Information). The BiVO4/FTO and BiVO4/TiO2/FTO photoanodes were constructed by the deposition of BiVO4 nanostructures on the FTO and TiO2/FTO surfaces by MOD method.22 Figure 1 shows the top-view and cross-sectional SEM images of the BiVO4/FTO and


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BiVO4/TiO2/FTO photoanodes. They reveal that porous but continuous layers were formed both on the bare FTO and TiO2/FTO surfaces via the MOD method. However, the detailed morphologies of these two photoanodes are quite different.

Figure 1. Top-view and cross-sectional SEM images of (a,b) BiVO4/FTO and (c,d) BiVO4/TiO2/FTO photoanodes. The crystal structure of the photoanodes was characterized by X-ray diffraction. The XRD patterns of the BiVO4/TiO2/FTO and BiVO4/FTO photoanodes are displayed in patterns I and II of Figure 2a, respectively. In addition to the diffraction peaks corresponding to the FTO substrate (as shown in pattern III of Figure 2a), the diffraction peaks in patterns I and II can be indexed as those of monoclinic bismuth vanadate according to the JCPDS file no. 14-0688 (pattern IV in Figure 2a). It reveals that the polycrystalline BiVO4 were grown on both the TiO2/FTO and the bare FTO substrates. Examination of the crystal structure of these two BiVO4 photoanodes were also conducted by Raman spectroscopy, as shown in Figure 2b. The scattering peaks at 210, 324, 366, 710, and 826 cm-1 in the Raman spectra of the two photoanodes are pertaining to monoclinic bismuth vanadate,26 confirming the XRD results of the formation of crystalline bismuth vanadate.


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Figure 2. (a) XRD patterns of (I) BiVO4/TiO2/FTO, (II) BiVO4/FTO, (III) FTO, and (IV) JCPDS file no. 14-0688 (monoclinic bismuth vanadate). (b) Raman spectra of BiVO4/TiO2/FTO and BiVO4/FTO electrodes. In order to investigate the influences of BiVO4/TiO2 junction on the charge separation and injection efficiencies of the BiVO4 photoanode, identical light harvesting of both the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes is desired in this work. The light harvesting efficiencies of the BiVO4 photoanodes were determined by the absorption spectra that were obtained from the relation of (1-total transmittance (T)-total reflectance (R)). As shown in Figures S2a and S3a (Supporting Information), the absorption of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes can be varied through adjusting the volume of BiVO4 layers on the bare FTO and the TiO2/FTO substrates by the repeated cycles of spin coating and subsequent annealing.


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Accordingly, BiVO4/FTO and BiVO4/TiO2/FTO photoanodes with almost identical absorption spectra have been attained, as shown in Figure 3a. For comparison, the absorption spectra of TiO2/FTO and bare FTO substrates are also displayed in Figure 3a. The two BiVO4 photoanodes exhibit the absorption edge at the wavelength of ~510 nm and both the TiO2/FTO and the bare FTO substrates show negligible light harvesting at wavelengths > 350 nm. Notice that the characterizations shown in Figures 1 and 2 were conducted on the BiVO4/FTO and BiVO4/TiO2/FTO photoanodes which exhibit the same light harvesting efficiency (displayed in Figure 3a). The PEC performances of the BiVO4 photoanodes were examined by a three-electrode electrochemical configuration in 0.5 M Na2SO4 electrolyte. The photocurrent density-potential (J-V) curves of the photoanodes were monitored by linear sweep photovoltammetry measurements. The photocurrent densities of BiVO4/FTO and BiVO4/TiO2/FTO photoanodes were individually optimized in the aspect of the volume of the BiVO4 layers, as shown in Figures S2b and S3b (Supporting Information). Interestingly, the optimized BiVO4/FTO and BiVO4/TiO2/FTO photoanodes are the ones with almost identical light harvesting efficiency (absorption) illustrated in Figure 3a. Figure 3b shows the J-V curves of the TiO2/FTO anode and of the optimized BiVO4/FTO and BiVO4/TiO2/FTO photoanodes. It reveals that the contribution of solar-fuel conversion from the TiO2 layer is almost negligible. The photocurrent densities of BiVO4/TiO2/FTO photoanode are slightly lower than that of BiVO4/FTO photoanode at potentials < 1.2 V vs. reversible hydrogen electrode (RHE). By increasing further the potential, the PEC performance of BiVO4/TiO2/FTO photoanode is superior to that of the BiVO4/FTO photoanode. As shown in Figure 3b, the photocurrent densities of BiVO4/TiO2/FTO photoanode at 1.23 and 1.7 V vs. RHE are 0.80 and


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Figure 3.(a) Absorption spectra, (b) J-V curves, and (c) IPCE spectra of BiVO4/FTO and BiVO4/TiO2/FTO photoanodes. IPCE spectra were measured at 1.23 V vs. RHE. 2.14 mAcm-2, respectively. Figure 3c shows the IPCE spectra of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes measured at 1.23 V vs. RHE. The almost identical IPCE spectra


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confirm the similar photocurrent densities of these two photoanodes at 1.23 V vs. RHE. Moreover, the threshold of IPCE spectra of both BiVO4 photoanodes is at ~510 nm that is consistent with the absorption edge of their absorption spectra shown in Figure 3a.

Eisenberg et al. demonstrated that the PEC water oxidation performance of the discontinuous BiVO4 films on FTO substrate can be improved by the addition of an amorphous TiO2 over-layer.27 They concluded that the blocking of solution-mediated recombination on FTO by insulating exposed regions using the TiO2 over-layer contributes to the increase in photocurrent densities, especially at bias low enough. In this work, porous but continuous BiVO4 layers were formed on FTO and TiO2/FTO surfaces, as shown in Figure 1. The photocurrent densities of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes are almost the same at low potentials (shown in Figure 3b), although a continuous rutile TiO2 layer was formed between BiVO4 and FTO. We therefore suggest that solution-mediated recombination on FTO substrate is negligible in our BiVO4/FTO photoanode and the TiO2 interlayer does not play role in blocking the solution-mediated recombination on FTO. Surface potential (SP) of the BiVO4/TiO2/FTO and the BiVO4/FTO anodes are measured in the dark and under white LED light irradiation using KPFM to investigate charge distribution in the BiVO4 layers.23,25,28 The concept for surface potential measurements of n-type metal oxide semiconductors under ambient conditions was described in Supporting Information. The topographical images of the BiVO4/FTO and the BiVO4/TiO2/FTO anodes ((a) and (c)) as well as the corresponding SP images ((b) and (d)) measured in the dark and under illumination are illustrated in Figure 4. In the dark, the SP value of the BiVO4/TiO2/FTO is lower than that of the BiVO4/FTO as shown in the lower panels of Figures 4b and 4d, indicating the increase of band bending in the BiVO4 layer of the BiVO4/TiO2/FTO compared to that of BiVO4/FTO


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(Supporting Information). The increase of band bending in the BiVO4 layer is attributed to the formation of heterojunction with TiO2, which facilitates photocharge separation in the BiVO4/TiO2/FTO photoanode.

Figure 4. Topographical and the corresponding surface potential images of (a,b) BiVO4/FTO and (c,d) BiVO4/TiO2/FTO. SP images were taken in the dark (lower) and with white-light LED irradiation (upper). The SP images of the two photoelectrodes were further obtained under irradiation as shown in the upper panels of Figures 4b and 4d. The photovoltages (SPVs) of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes, that are the modulation of the SP values measured under illumination and in the dark, can be the index of the relative concentrations of steady-state excess carriers on the surfaces of the two BiVO4 layers.23,25 Compared to those measured in the dark, as shown in Figures 4b and d, increased SP values of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes are obtained under illumination. The average SPV values of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes are 20 mV and 70 mV, respectively. These values were


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calculated from the results shown in Figures 4b and 4d. It reveals that both BiVO4/FTO and BiVO4/TiO2/FTO photoanodes possess the positive SPV values and the latter has significantly larger SPV value. The positive SPV value results from the fact that certain amount of excess holes drift toward the surface to reduce band bending under illumination (Supporting Information).23 The larger SPV value measured in the BiVO4/TiO2/FTO photoanode indicates that more excess holes are able to reach the surface. Therefore, the KPM measurements determine that the band structure of the BiVO4/TiO2 heterojunction facilitates photoelectrons transferring from conduction band of BiVO4 to that of TiO2 and photogenerated holes drifting to the surface of BiVO4, demonstrating the formation of energy matched type II heterojunction of BiVO4/TiO2.

Figure 5. Photocurrent transient behaviors of BiVO4/FTO and BiVO4/TiO2/FTO photanodes in response to on-off irradiation. Figure 5 shows the photocurrent transient behaviors of the BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes. It illustrates that substantial photocurrent decay is recorded in the J-t curve of the BiVO4/FTO photoanode as irradiation swiftly switched on. The photocurrent decay that is observed typically in a PEC cell is mainly ascribed to holes trapped at surface states


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recombining with electrons from the conduction band, revealing that photogenerated electrons and holes are not effectively separated prior to recombination at surface states.10,23 As shown in Figure 5, the photocurrent decay is significantly reduced in the J-t curve of the BiVO4/TiO2/FTO photoanode, suggesting that the recombination can be suppressed in the BiVO4 layer due to the enhancement of charge separation by the formation of BiVO4/TiO2 heterojunction. Both SPV and J-t measurements show that charge separation in the BiVO4/TiO2/FTO photoanode is enhanced compared to BiVO4/FTO photoanode. However, the PEC performances of the BiVO4/TiO2/FTO and BiVO4/FTO photoanodes are comparable to each other at applied potentials < 1.2 V vs. RHE. The charge separation and injection efficiencies of the two BiVO4 photoanodes were further studied using hydrogen peroxide as a hole scavenger in the electrolyte.19,23,29 The J-V curves of the two BiVO4 photoanodes with and without the addition of hydrogen peroxide in the electrolyte are displayed in Figure S4 (Supporting Information). Based on the results in Figure S4 and the assumption of the 100% injection efficiency of the photoanode with H2O2 in the electrolyte, the charge separation efficiency of the two photoanodes can be estimated by the ratio of the photocurrent density obtained with H2O2 in the electrolyte to the photon absorption rate. The charge injection efficiency of the photoanode can be determined by the ratio between photocurrent densities measured without and with H2O2 in the electrolyte.19,23,29 The charge separation and injection efficiencies of the BiVO4/TiO2/FTO and the BiVO4/FTO photoanodes as functions of applied potential are shown in Figures 6a and 6b, respectively. Compared to the BiVO4/FTO photoanode, the charge separation efficiencies in the BiVO4/TiO2/FTO photoanode are improved and this is consistent with the SPV and J-t measurements. Moreover, the charge separation efficiency increases with the potential, as


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illustrated in Figure 6a. It is mainly attributed to the fact that the built-in potential established at the heterojunction of BiVO4/TiO2 combines with the external applied potential to substantially enlarge the band bending of the photoanode, which facilitates the charge separation in the BiVO4 layer as the potential is increasing.23

Figure 6. (a) Charge separation and (b) injection efficiencies of BiVO4/TiO2/FTO and BiVO4/FTO photoanodes as functions of applied potential. By contrast, Figure 6b shows that the charge injection efficiencies in the BiVO4/TiO2/FTO photoanode are inferior to those in BiVO4/FTO photoanode but the difference is getting smaller at high potentials. The charge injection efficiency estimated from the hole-scavenger-assisted PEC measurements inspects the fraction of those holes at the interface of photoanode and electrolyte injected into the electrolyte for water oxidation without recombining with electron at


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surface traps.23 The factors contributing to the reduction of charge recombination at surface traps and the acceleration of the oxygen evolution kinetics will improve the charge injection efficiency. One of the reasons for the inferior charge injection efficiency in the BiVO4/TiO2/FTO photoanode is that the BiVO4 layer deposited on the TiO2/FTO substrate possesses a higher density of surface traps as compared to that on the FTO substrate. Nevertheless, the formation of the BiVO4/TiO2 heterojunction brings about the abundant enhancement of the charge separation efficiency in the BiVO4/TiO2/FTO photoanode as the applied potential increases, as shown in Figure 6a. Due to the enhanced difference in the charge separation efficiencies, the charge recombination at surface traps of the BiVO4/TiO2/FTO photoanode can be considerably reduced at higher potentials. Therefore, the variance of the charge injection efficiencies of the two BiVO4 photoanodes is getting reduced as the applied potential increases. Based on the results shown in Figure 6, the superior separation efficiency of BiVO4/TiO2/FTO photoanode (as compared to that of the BiVO4/FTO anode) can prevail over its initial inferior injection efficiency as the applied potential increases. It results in the PEC performance of the two BiVO4 photoanodes shown in Figure 3b. At low applied potentials, the much poorer charge injection in the BiVO4/TiO2/FTO photoanode results in slightly lower photocurrent density although more efficient charge separation occurs in the BiVO4/TiO2/FTO photoanode compared to that in the BiVO4/FTO one. At higher potentials, the enhancement in charge separation is more significant and the inferiority in charge injection is reduced in the BiVO4/TiO2/FTO photoanode. Consequently, the enhancement in the PEC performance is achieved at higher applied potentials by the formation of BiVO4/TiO2 junction.


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Figure 7. J-V curves of BiVO4/TiO2/FTO photoanodes with and without Co-Pi deposition Poor charge injection efficiency of the BiVO4/TiO2/FTO photoanode was characterized by the PEC measurements, suggesting that the slow kinetics for oxygen evolution on the BiVO4 surface of the BiVO4/TiO2 heterojunction is one of the limiting factors for water splitting. Co–Pi as an oxygen evolution catalyst19,20 was further electrodeposited on the surface of the BiVO4/TiO2/FTO photoanode in this work. The J-V curves of the BiVO4/TiO2/FTO photoanodes with and without the Co-Pi deposition are shown in Figure 7. With the Co-Pi deposition, the onset potential in the J-V curve of the BiVO4/TiO2/FTO photoanode shows a cathodic shift from 0.7 V to 0.4 V vs. RHE. Moreover, the photocurrent density of the BiVO4/TiO2/FTO photoanode is significantly improved. Photocurrent densities of 1.61 and 2.45 mAcm-2 are monitored in the Co-Pi deposited photoanode at 1.23 and 1.7 V vs. RHE, respectively, that are 100% and 14% enhancement compared to the photocurrent densities of the photoanode without Co-Pi deposition. As shown in Figure S5 (Supporting Information), with the Co-Pi on the surface, the BiVO4/FTO photoanode also shows a cathodic shift and increased photocurrent densities. However, photocurrent densities of 1.41 and 2.11 mAcm-2 are obtained in the Co-Pi/BiVO4/FTO


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photoanode at 1.23 and 1.7 V vs. RHE, respectively, that are inferior to those of the CoPi/BiVO4/TiO2/FTO photoanode.

Figure 8. Photoresponses of (a) BiVO4/FTO, (b) BiVO4/TiO2/FTO and (c) CoPi/BiVO4/TiO2/FTO photoanodes as functions of time under chopped illumination at an applied potential of 1.23 V vs. RHE.


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Figure 8 shows the photoresponses of the BiVO4/FTO, BiVO4/TiO2/FTO, and CoPi/BiVO4/TiO2/FTO photoanodes as functions of time under chopped illumination at an applied potential of 1.23 V vs. RHE. A significant decay of the photocurrent density is observed in the BiVO4/FTO photoanode over a period of 300 s, as revealed in Figure 8a, indicating severe photocorrosion occurring in the bare BiVO4/FTO photoanode.30,31 Due to the enhancement of the charge separation in the BiVO4 layer, the stability of the photoanode can be improved by the formation of the BiVO4/TiO2 heterojunction. The photocurrent densities of the bare BiVO4/FTO and the BiVO4/TiO2/FTO photoanodes over a period of 300 s is reduced by 34% and 16%, respectively, as shown in Figures 8a and b. In Figure 8c, the photocurrent density is decreased by 10% with the addition of Co-Pi on the BiVO4/TiO2/FTO photoanode. The accumulation of holes on the surface of the BiVO4 layer is diminished by the enhancement of the hole kinetics for water oxidation with the addition of Co-Pi. Therefore, the anodic photocorrosion of BiVO4 can be reduced to improve the stability of the photoanode. By using the simple planar BiVO4/TiO2 heterojunction photoanode as an example, we demonstrated that the formation of heterojunction photoanode may create the conflicting effects on charge separation and charge injection, resulting in its limited PEC performance. The measurements of SPV, photocurrent transient behavior and hole-scavenger assisted PEC performance indicate that charge separation efficiency in the BiVO4 is indeed improved by the formation of the BiVO4/TiO2 heterojunction photoanode. However, the enhancement of the photocurrent density is achieved only at the potentials > 1.2 V vs. RHE. Although the two BiVO4 layers with comparable light harvesting efficiencies were prepared by the same MOD method, we found that the surface properties of the BiVO4 layer on TiO2/FTO is inferior for oxygen evolution compared to that of the bare BiVO4/FTO photoanode. It brings about the


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aforementioned limited PEC performance of the BiVO4/TiO2/FTO photoanode. Based on these characterizations and evaluations, the co-catalyst Co-Pi is further deposited on the surface of BiVO4/TiO2/FTO photoanode to improve the hole injection efficiency. Subsequently, the photocurrent density and stability of the Co-Pi/BiVO4/TiO2/FTO photoanode are significantly improved compared to those of the bare BiVO4/FTO one.

Conclusions In this work, a simple planar BiVO4/TiO2 heterojunction photoanode is constructed on FTO substrate. Charge separation efficiency in the BiVO4/TiO2/FTO heterojunction photoanode is improved compared to the BiVO4/FTO photoanode. This was confirmed by the SPV measurements, photocurrent transient behavior and hole-scavenger-assisted PEC performance. However, the photocurrent densities of the BiVO4/TiO2/FTO are superior to those of the BiVO4/FTO photoanodes only at potentials > 1.2 V vs. RHE although the two BiVO4 layers with comparable light harvesting efficiencies were created using the same method. The holescavenger-assisted PEC measurements reveal that the hole injection efficiency of the BiVO4/TiO2/FTO photoanode is inferior to that of the bare BiVO4/FTO one for oxygen evolution, that is pertaining to the alteration of the surface property of BiVO4 layers deposited on different substrates. Nevertheless, the superior separation efficiency of BiVO4/TiO2/FTO photoanode, as compared to BiVO4/FTO anode, can prevail over its initial inferior injection efficiency as the applied potential increases. Therefore, we conclude that the formation of heterojunction photoanode may create the conflicting influences on charge separation and charge injection, resulting in its limited PEC performance. To overcome the drawback of the slow kinetics for oxygen evolution, the hole injection efficiency of the BiVO4/TiO2/FTO photoanode


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is further improved by the formation of the co-catalyst Co-Pi on the surface. Consequently, the photocurrent density and stability of the Co-Pi/BiVO4/TiO2/FTO photoanode are improved significantly compared to those of the bare BiVO4/FTO one.

ASSOCIATED CONTENT Supporting Information. SEM image and Raman spectrum of TiO2/FTO electrodes, Absorption spectra and J-V curves of BiVO4/FTO and BiVO4/TiO2/FTO photoanodes with various volumes of BiVO4 layers, J-V curves of optimizedBiVO4/FTO and BiVO4/TiO2/FTO photoanodes with the addition of hydrogen peroxide in the electrolyte, J-V curve of BiVO4/FTO photoanode with Co-Pi deposition, Surface potential measurements of n-type metal oxide semiconductor under ambient conditions. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions Jih-Jen Wu conceived and designed the experiments; Bo-Yan Cheng, Jih-Sheng Yang and HsunWei Cho performed the experiments; Bo-Yan Cheng and Jih-Sheng Yang analyzed the data; JihJen Wu wrote the paper. All authors approved the final version of the manuscript. Funding Sources This research is supported by the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan and by the


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Ministry of Science and Technology in Taiwan under Contract No. MOST 102-2221-E-006-215MY3 and MOST 103-2221-E-006-245-MY3.

ACKNOWLEDGMENT Financial supports received from the Headquarters of University Advancement at the National Cheng Kung University, that is sponsored by the Ministry of Education, Taiwan and from the Ministry of Science and Technology in Taiwan under Contract No. MOST 102-2221-E-006-215MY3 and MOST 103-2221-E-006-245-MY3 are gratefully acknowledged.

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