TiO2 Heterostructure: Enhanced

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Band Edge Engineering in BiVO4/TiO2 Heterostructure: Enhanced Photoelectrochemical Performance through Improved Charge Transfer Aadesh Pratap Singh, Bodh Raj Mehta, Alexander Held, Leonhard Mayrhofer, and Michael Moseler ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00956 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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Band Edge Engineering in BiVO4/TiO2 Heterostructure: Enhanced Photoelectrochemical Performance through Improved Charge Transfer Aadesh P. Singh1*, Bodh R. Mehta1*, Alexander Held2,3, Leonhard Mayrhofer2,3, Michael Moseler2,3 1

Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas,

New Delhi-110016, India 2

Fraunhofer IWM, Wöhlerstr. 11, 79108 Freiburg, Germany

3

Freiburg Materials Research Center FMF, University of Freiburg, Stefan-Meier-Str. 21,

79104 Freiburg, Germany

ABSTRACT: The efficient separation of photogenerated electron-hole pairs and stability against corrosion are critical preconditions for a photoelectrode to achieve a high photoelectrochemical performance. In this work it is shown how both criteria can be met by employing a heterostructure of bismuth vanadate (BiVO4) and titanium dioxide (TiO2) as the photocatalyst. Using electronic structure calculations, an alteration of the band alignment is predicted at the heterojunction from type I to type II by hydrogen treatment of the top TiO2 layer. Guided by this idea, heterostructures of BiVO4 and TiO2 are fabricated and the effect of hydrogen treatment is studied. The achieved band engineering results in a significant improvement in photocurrent density, up to 4.44 mA cm-2 at 1.23 V vs RHE, and a low onset potential,

-0.14

V

vs

RHE,

under

visible

light

illumination.

The

enhanced

photoelectrochemical performance originates in facilitated hole transport to the electrode surface and enhanced photoabsorption in the TiO2 layer. This work is an example how hydrogenation can be used to tailor the properties of BiVO4/H:TiO2 heterostructures and provides valuable insights for the further development of similar material combinations. Keywords: water splitting, photoelectrochemical, band edge engineering, ab initio, heterojunction 1 ACS Paragon Plus Environment

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1. INTRODUCTION The efficient production of clean fuels from sustainable energy sources is one of the pressing problems of mankind. Splitting water with the help of sunlight is a promising route to succeed in this endeavor.1 For photoelectrochemical (PEC) water splitting, semiconductor materials are driving the fuel production reaction by photogenerated electron-hole pairs. The optoelectronic properties of the semiconductor are thus of utmost importance for device performance. Among the various material nano-architectures, heterojunction photoelectrodes of two or more materials play a crucial role in improving the system efficiency as such heterojunctions create regions with electric potential gradients inside the photoelectrode which

enhance

electron−hole

separation

and

the

carrier

transfer

rate

at

the

electrode−electrolyte interface.2,3 In order to achieve fast electron-hole separation, a type II band alignment, where the jump in the valence and conduction band edge goes in the same direction is advantageous, see Scheme 1.4 Possible combinations of different metal oxides in a heterojunction system are therefore limited by the fact that most of the metal oxides are not in favor of a type II band alignment. On the other hand, some heterostructures made of nonoxide semiconductors possess a type II band alignment and demonstrate the advantages of this configuration5 but have stability issues. Very recently, efforts have been made to control the flow of charge carriers at oxide heterojunctions by localized defect states.6 Among the wide variety of heterojunction systems, tungsten trioxide/bismuth vanadate (WO3/BiVO4) has been one of the most studied metal oxide heterostructures for solar water splitting. The combined properties of WO3 and BiVO4 allow this heterojunction system to have a wider range of photon absorption wavelengths due to the relatively narrow band gap of BiVO4 as well as to provide better charge transfer owing to the type II alignment at the BiVO4/WO3 interface.7,8 McDowell et al9 also reported that the use of dual-layer modification by a thin layer of TiO2 and further Ni coating on BiVO4 can improve the stability as well as

2 ACS Paragon Plus Environment

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enhance the photoelectrochemical response under simulated light. However, excessive electron–hole recombination, poor charge transport properties, and poor water oxidation kinetics have been found to be key limiting factors for water oxidation with these heterostructures. Therefore, the construction of a heterojunction between BiVO4 and another highly photoactive and corrosion resistant metal oxide semiconductor with a suitable band alignment is required to solve these problems. In this study, the particular interest to use TiO2 as a top layer in a heterostructure with BiVO4 is due to its well-known high water splitting ability and corrosion stability. However, heterostructures of BiVO4 with TiO2 reveal a type I alignment of the semiconductor band edges.10 This means that the valence band edge (VBE) of TiO2 is below the VBE of BiVO4 such that hole transport from BiVO4 to TiO2 is energetically impeded, see Scheme 1(a). Thus, holes that are photoexcited within BiVO4 are not able to reach the TiO2 surface to oxidize water. Hence, the mismatched band edge alignment prevents efficient charge carrier separation and appears to be the limiting factor for photoelectrochemical applications. However, our ab initio density functional theory simulations predict a transition from type I to type II band alignment at the BiVO4/TiO2 interface with increasing distortion of the oxygen sub-lattice in TiO2. Since hydrogen treatment typically induces lattice disorder11–14, our calculations suggest a favorable realignment of the band edges of the hydrogen treated H:TiO2 and BiVO4, see Scheme 1(b). These theoretical predictions are in accordance with the experimentally observed strongly enhanced photocurrent densities after the hydrogen treatment which are among the highest photocurrent densities obtained for BiVO4 heterostructures.7,8,15,16 2. EXPERIMENTAL SECTION 2.1 Preparation of BiVO4 Thin Films


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The BiVO4 thin films were prepared onto indium-doped tin oxide (SnO2:In, ITO) substrate by using RF magnetron sputtering of the bismuth vanadate target (BiVO4, 99.99% purity). Prior to deposition, the sputtering chamber was initially pumped down to approximately 2x106

Torr. The sputtering was carried out at optimized applied RF-power of 80W at room

temperature. To deposit the BiVO4 thin films, 20 sccm argon was used while maintaining the deposition pressure of 2.5x10-2 Torr. After the thin film deposition, the as-deposited BiVO4 samples were annealed at 400oC in air for 4 hr. 2.2 Preparation of TiO2 Thin Films Thin films of TiO2 were deposited onto indium-doped tin oxide (ITO) by RF-magnetron sputtering of titanium dioxide target (TiO2, 99.99% purity) at applied RF-power at 75W and substrate temperature at 300oC. 2.3 Preparation of BiVO4/TiO2 and BiVO4/H:TiO2 Heterostructures For making the heterostructures, 25 nm thin TiO2 films were deposited over the annealed BiVO4 films at 300oC. The band-edge engineering was carried out by hydrogen doping in the top TiO2 layer. Therefore, to achieve the type II band alignment, the prepared heterostructure was further annealed in 5% H2 in Ar atmosphere at 300°C at partial pressure of 2.5 × 10−2 Torr for 1 h. 2.4 Material Characterization The X-ray diffraction (XRD) analysis was conducted on a Philip’s X’Pert PRO-PW vertical system operating in reflection mode using Cu Kα (λ = 0.15406 nm) radiation. The 2θ scanning range was from 10o to 60o with a scanning speed of 0.02o s-1. Raman spectra of as synthesized samples were recorded on the powder sample by using an Invia Raman microscope under excitation by 514 nm Argon ion laser pulse, to determine the structural behavior of BiVO4 before and after TiO2 deposition and hydrogen treatment. All measurements were carried out at room temperature. XPS spectra were recorded on a load-


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locked Kratos XSAM 800 apparatus equipped with a dual anode X-ray source. The X-ray gun was operated at 12 kV and 10 mA at a pressure lower than 10-9 mbar, using a Mg Kα (1253.6 eV) excitation source. The high magnification analyzer mode was chosen to collect electrons from the smallest possible area on the specimen, approximately 4.0 mm2. All spectra were calibrated using the C1s photoelectron component peak of amorphous carbon (284.8 eV) present in the sample and also to the Au4f core lines; gold metal was in direct contact with the sample holder. Scanning electron microscopy (SEM) was performed with a FEI QUANTA 3D FEG Field-Emission Scanning Electron Microscope. The optical properties of as synthesized samples were analyzed by UV−vis spectroscopy recorded on a Perkin-Elmer Lambda 35 over a wavelength range of 200-800 nm and a resolution of 1 nm. 2.5 Photoelectrochemical Measurements For electrochemical measurements, thin films of BiVO4, BiVO4/TiO2 and BiVO4/H:TiO2 heterojunctions were converted into the photoelectrodes with an active surface area of about 0.45 cm2. For this purpose ohmic electrical contacts were made using silver paste and copper wire from the undeposited area of conducting glass substrate and later the undeposited area was covered with non-transparent and non-conducting epoxy resin. To investigate the photoelectrochemical performance

of

the

prepared

photoelectrodes,

linear

sweep

voltammetry scans under dark and visible light illumination were recorded. For the illumination, a 150W Xenon lamp fitted with a filter that cuts light with wavelengths ≤380 nm and having output illumination intensity of 100 mW/cm2 was used. Mott-Schottky measurements in dark condition were carried out in the potential range -1.0 to +1.0 V versus Ag/AgCl with a scan rate of 20 mV/s in 0.5M Na2SO4 electrolyte (pH = 7). The electrochemical cell was controlled using a CIMPS-2 (Controlled Intensity Modulated Photospectroscopy) system consisting of a Zennium Electrochemical Workstation (X-Pot Potentiostat). To study the intrinsic electronic properties, the capacitance (C) at the


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semiconductor/electrolyte junction was measured at 1kHz AC signal frequency under dark conditions in the three-electrode configuration and Mott-Schottky plots were generated to determine the flat band potential. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 100 kHz to 0.01 Hz with AC signal amplitude of 10 mV under open bias condition (i.e. 0.0 V versus Ag/AgCl reference electrode). For better representation of our results, we converted the Ag/AgCl reference potential to the normal hydrogen electrode (RHE) potential by using the following formula: RHE = V (Ag/AgCl) +0.61V for 0.5M Na2SO4 electrolyte solution (pH 7). This potential was used to calculate the photocurrent density, flat band potential and solar-to-hydrogen conversion efficiency. 2.6 Simulation Details Ab initio density functional theory (DFT) calculations have been carried out using the projector augmented-wave17,18 (PAW) method with a plane-wave basis set for the representation of the pseudo-wave functions in the VASP implementation. A plane-wave cutoff energy of 520 eV has been applied. The scalar-relativistic frozen-core approximation has been used for the [Xe]4f14 states of Bismuth, the [Ne] states of Vanadium, the [He] states of Oxygen and the [Ne] states of Titanium. The exchange-correlation functional has been approximated by the generalized gradient approximation of Perdew, Burke and Ernzerhof19,20 (PBE). Correlation effects of transition-metal d-states have been described by adding a Hubbard-like onsite Coulomb interaction21 with a U - J value of 2.7 eV for Vanadium22 and 3 eV for Titanium.23,24 Unless noted otherwise, all structures and cell parameters have been relaxed until all forces fell below 0.01 eV/Å. For this purpose, a 4x4x4 Monkhorst-Pack25 k-point sampling has been chosen for the primitive cells. For the calculation of the density of states and the band alignment, the Kohn-Sham potential and electron density have been fixed and the Kohn-


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Sham eigenvalues have been calculated using a Γ-centered 9x9x9 k-point mesh for the primitive cells. For this choice of the k-point sampling, the estimated numerical errors of the conduction band edges with respect to the charge neutrality levels in the band alignment procedure are below 5 meV (see Figure S13). Due to the application of a scissor operator, the errors for the valence band edges are the same. Primitive cells have been constructed using the ASE software.26 2.7 Alignment Procedure in the Interface Induced Gap States Model The only general assumption we make about the BiVO4/TiO2 interface is that it is non-polar. Additionally we neglect the intrinsic dipole contribution proportional to the difference in electronegativity of both materials27 as the electronegativities estimated from the geometric mean28 of the elemental Pauling values differ only by 150 meV. The band off-sets are then obtained by aligning the energy scales of both semiconductors at their charge neutrality level (CNL).27 As a starting point, we have taken experimental structures of monoclinic scheelite (clinobisvanite) BiVO429 and anatase TiO230. For the relaxed cell parameters we obtained a = 7.33 Å, b = 11.76 Å, c = 5.18 Å, β = 134.90° for BiVO4 and a = 3.85 Å, c = 9.71 Å for TiO2, which is in reasonable agreement with the experimental values a = 7.258 Å, b = 11.706 Å, c = 5.084 Å, β = 134.073° for BiVO4 at 4.5 K and a = 3.782 Å, c = 9.502 Å for TiO2 at 15 K. The CNL is approximated from the occupied and unoccupied Kohn-Sham eigenvalues as described by Schleife et al.31 The choice of the two parameters NVB and NCB controlling how many occupied and unoccupied bands are taken into account is ambiguous. In reference 31

, an uncertainty of up to 200 meV has been assigned to the CNLs calculated using this

method, depending on the choice of the parameters. As we study the effect of oxygen displacement in TiO2 within a primitive cell consisting of two formula units, we have chosen NVB = 8 twofold spin degenerate bands in order to cover all oxygen 2p states in the alignment


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procedure. We have considered the same number of bands for the conduction band. Test calculations using NVB = 9 and 10 did not reveal relevant changes compared to our choice of parameters. For the calculation of the band alignment with the oxygen displacement model, for TiO2 the same scissor operator with a fixed value has been applied to all configurations, such that the experimental band gap was reproduced for the d = 0 Å configuration without distortion. The validity of applying a scissor operator to correct the PBE+U band gap of TiO2 has been checked against additional HSE06 hybrid functional calculations for TiO2 as detailed below. 3. RESULTS 3.1 Simulation Results To provide guidance to the experimental studies, the electronic properties of BiVO4/TiO2 and BiVO4/H:TiO2 heterostructures were investigated by numerical simulations based on density functional theory (DFT). The VASP32-35 implementation was used at the GGA+U21 level of theory. As we have no information on the atomic details of the different possible nanocrystalline BiVO4/TiO2 interface structures, we resort to the empirical interface induced gap states (IFIGS) model in order to calculate the band alignment between BiVO4 and TiO2.27 The IFIGS model relies only on bulk properties of the semiconductors composing the interface. Therefore no assumptions on the atomic details of the interface have to be made. The band off-sets are obtained by aligning the energy scales of both semiconductors at their charge neutrality level (CNL).27 Approximate CNLs have been calculated from the KohnSham eigenvalues by averaging the midgap energy over the first Brillouin zone and the topmost NVB valence bands and bottommost NCB conduction bands as described by Schleife et al.31 for a choice of NVB = NCB = 8 (see experimental section). We have not included spinorbit coupling for BiVO4 as a good agreement to the experimental electronic structure was found using DFT calculations without spin-orbit coupling.36


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For the DFT calculations, primitive cells consisting of two formula units each have been constructed and relaxed to a local minimum. The relaxed structures are shown in Figure 1 (a) and (b) as reconstructed conventional cells. The band gap resulting from our GGA+U calculation is 2.3 eV for BiVO4, consistent with previous GGA+U calculations22 and in good agreement with the experimentally observed value of about 2.4 eV.37 As it is well known in the literature, the experimental band gap of TiO2 is underestimated at the GGA+U level of theory unless extremely high U-values are chosen.23 We have obtained a band gap of 2.4 eV for anatase TiO2, lower than the experimentally observed value of 3.2 eV.38 Consequently, we apply a scissor-operator shifting the conduction bands up to match the experimental band gap for both materials for the following band alignment procedure.39 The exact alignment procedure and simulation details are described in the experimental section. The calculated band alignment at the BiVO4/TiO2 interface is shown in Figure 1 (c). With pristine TiO2, a heterojunction with type I band alignment is formed, which hinders the transport of photogenerated holes from BiVO4 to the TiO2 surface exposed to the electrolyte, as indicated in Scheme 1. The effect of hydrogen treatment of the TiO2 layer on the band alignment was simulated in a next step. The exact mechanisms underlying the change in optoelectronic properties due to hydrogen treatment of TiO2 are still subject to active debate and depend on the details of the synthesis approaches.40 We follow the findings of Chen et al.11 in that we attribute the changes in the band structure to hydrogen induced disorder. For the motivational simulation part, we have neglected the possible formation of midgap states formed during hydrogenation which might act as additional transport channels for the charge carriers.41 It has been shown in a computational study that one of the most pronounced effects of hydrogen treatment is the elongation of Ti-O bonds.14 Based on this observation, we employ a simple disorder model that consists of distorting the oxygen sub-lattice of TiO2 by


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displacing the oxygen atoms away from the titanium atoms along the c-direction of the conventional unit cell, as indicated in Figure 1(b). This disorder model is very similar to the model of Liu et al.13 with the difference that we displace all oxygen atoms at once in order to account for strong lattice disorder typically found in the surface region of nanocrystalline TiO2.11,13,14 In order to implement the model, we have used the relaxed primitive cell of anatase TiO2. The cell and ionic positions were kept fixed, while the displacement d of the oxygen atoms has been varied from 0 to 0.35 Å in steps of 0.05 Å. The band alignment as a function of oxygen displacement is shown in Figure 1 (c). When the displacement exceeds about 0.15 Å, the VBE of TiO2 is shifted above the VBE of BiVO4 such that the band alignment of the BiVO4/TiO2 heterostructure changes from type I to type II. Oxygen displacements due to hydrogen treatment of up to 14% of the Ti-O bond length have been predicted in a simulation study.14 For our structure, this corresponds to d = 0.28 Å. Similar to the findings of Liu et al.13, the conduction band edge remains almost unchanged. These simulation results indicate that a band realignment facilitating hole transport from BiVO4 to TiO2 and hence favoring an efficient charge carrier separation is possible through hydrogen induced lattice disorder in TiO2. In order to test whether the application of a constant scissor operator for different oxygen displacements d is justified, we have performed additional HSE06 hybrid functional calculations for TiO2 using the experimental structure for the d = 0 Å starting configuration.42 Despite a slight overestimation of the experimental anatase band gap (3.6 eV instead of 3.2 eV) leading to a type I to II transition at a higher value for the oxygen displacement, no qualitative difference to our PBE+U calculations with the scissor operator could be found (see Figure S14). In order to elucidate the nature of the states forming above the original VBE when the oxygen sub-lattice is distorted, we have plotted the density of states for the oxygen pprojection and the titanium d-projection in Figure 2 (a) for a displacement of d = 0.2 Å. It can


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be seen that the new states arising above the original VBE of the undistorted sample are formed by oxygen p-states as it is the case for the valence band of pristine TiO2. In Figure 2 (b) and (c), we have plotted the partial electron density of the highest occupied states for an oxygen displacement of 0 and 0.2 Å. In both cases, the partial electron density mostly resides around the oxygen atoms with the only difference that the location of the oxygen atoms changes for the d = 0.2 Å case. In that sense, it is justified to assign the role of an upshifted VBE to the highest occupied state in the displacement model as it was done in the band alignment procedure to obtain Figure 1 (c). Within the applied approximation for the estimation of the CNL, the effect of the oxygen displacement on the alignment can now be understood as follows. The CNL is computed as the arithmetic mean of a Brillouin zone and band averaged valence and conduction band energy.31 The symmetry breaking induced by the oxygen displacement leads to a broadening of the oxygen p-states near the VBE. Yet, the Brillouin zone and band average of the valence band states shifts towards the CNL by only 160 meV when comparing the d = 0.2 Å to the d = 0 Å configuration, indicating an almost symmetric broadening hardly affecting the position of the CNL. Thus the VBE is shifted up significantly by the broadening while the Ti d-states and consequently the conduction band edge are unaffected by the symmetry breaking in the oxygen sub-lattice. 3.2 Experimental Results To test our hypothesis and to scrutinize the DFT simulation results experimentally, we have fabricated BiVO4/TiO2 heterojunction electrodes by sequential deposition of nanocrystalline BiVO4 and TiO2 using RF magnetron sputtering from the individual targets of BiVO4 and TiO2. Band edge engineering in TiO2 was carried out by hydrogen doping in the top TiO2 layer by post hydrogen treatment of TiO2 in 5% H2 balanced Ar atmosphere. Morphological characteristics were investigated by scanning electron microscopy (SEM) (Supplementary


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Figure S1). SEM images of pristine BiVO4 electrodes showed uniform deposition of a thin layer of thickness of about 300-400 nm with inter-particle pores as shown in Figure S1(a). Optimized thickness of TiO2 layer (thickness < 25 nm) over BiVO4 clearly showed a smooth coverage as shown in Figure S1(b). Figure 3(a) displays the optical absorption spectra of the TiO2, BiVO4, BiVO4/TiO2, and BiVO4/H:TiO2 samples in the wavelength range 300-800 nm. In all samples, a steep increase in absorption at wavelengths shorter than ≈ 470 nm can be attributed to the intrinsic band gap of crystalline BiVO4 and TiO2. Compared with TiO2, BiVO4 and the BiVO4/TiO2 heterostructure, the BiVO4/H:TiO2 sample possesses a significantly higher absorption in the visible light. In case of the BiVO4/H:TiO2 system, the optical absorption drastically increases at wavelengths longer than ≈525 nm, which is vastly superior to both BiVO4 and BiVO4/TiO2 and other reported heterostructures based on BiVO4.43,44 This high absorption can be attributed to the hydrogen treatment which is well known to generate black TiO2.11,12,40 Figure 3(b) describes the band diagram of TiO2 and H:TiO2 samples drawn on the basis of work function, band gaps, and valence band off-sets with respect to the Fermi level EF. The valence band off-sets were obtained from valence band X-ray photoelectron spectroscopy (XPS) (Supplementary Figure S7). The work function values were obtained by Kelvin probe force microscopy (KPFM) studies. The band gaps were determined from Tauc plots, see Figure S9. For pristine TiO2, the values for the valence band off-set, the work function and the band gap are 1.76 eV, 4.96 eV and 3.3 eV, and the corresponding values for H:TiO2 are 1.62 eV, 4.66 eV and 2.89 eV.14 As can be seen in Figure 3(b), the VBE of H:TiO2 is shifted upwards by 0.44 eV compared to pristine TiO2, while the conduction band remains practically unchanged when the vacuum level is taken as a reference. A detailed experimental study on hydrogen treated TiO2 thin films grown by rfsputtering has been reported previously.14 These experimental results confirm the basic effect of hydrogen-induced disorder on the band alignment with BiVO4 predicted by our


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simulations. This realignment of the band edge positions with respect to BiVO4 is a crucial factor for an efficient performance in a PEC cell. Also, the suitable alignment of the conduction and valence band edges with respect to the hydrogen evolution level (H2/H2O) and oxygen evolution level (H2O/O2) in H:TiO2 is in favor for overall water splitting. To examine the PEC performance of the investigated systems, the current-voltage characteristics under dark and illumination was measured in a three-electrode PEC cell. Figure 4 depicts the PEC performance in terms of the photocurrent density vs applied potential in dark and under illumination for pure BiVO4, BiVO4/TiO2 and BiVO4/H:TiO2 heterostructures. The PEC measurements were carried out in 0.5 M Na2SO4 aqueous solution. For illumination visible light (λ > 380 nm) having an output illumination intensity of 100 mW/cm2 was used. The photocurrent density of the pure BiVO4 photoanode is low, ~0.84 mA/cm2 at 1.23 V vs RHE, which is significantly high as compared to the previously reported values due to the difference in the film thickness45 and has an onset of photocurrent at 0.16 V vs RHE. A similar onset potential but an even lower photocurrent of ~0.28 mA/cm2 at 1.23 V vs RHE was observed with the BiVO4/TiO2 heterostructure. Finally, the hydrogen treatment of the top TiO2 layer in the BiVO4/H:TiO2 heterostructure substantially shifted the onset of photocurrent to -0.14 V vs RHE and the current density at 1.23 V vs RHE increased to 4.44 mA/cm2, which is to our best knowledge the highest reported photocurrent density for a metal oxide heterostructure made of BiVO4 and TiO2 on an ITO substrate, and hydrogen treated TiO2 thin films.14 The obtained value of photocurrent density for BiVO4/H:TiO3 heterostructured system is comparable to that of a recent FeOOH/NiOOH dual layer modified nanoporous BiVO4 photoanode.46 The saturated photocurrent of the BiVO4/H:TiO2 heterostructure photoanode is able to reach 4.8 mA cm-2 at 1.6 V vs RHE, which is noticeably higher than those of using a single material system as visible light photon absorber.45


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Further, the solar-to-hydrogen (STH) conversion efficiency for the water splitting reaction in the PEC cell was calculated for all the samples under illumination through a 150 W Xenon lamp having output illumination intensity of 100 mW cm-2 using the expression47 % =

. − ×

where the photocurrent density, Jp is in mA cm-2, Io is the input intensity of incident light falling on the surface of the photoelectrode in mW cm-2, VApp = Vmea−Vaoc, where Vmea is the electrode potential (V vs RHE) of the working electrode at which the photocurrent was measured under illumination and Vaoc is the electrode potential (V vs RHE) of the same working electrode at open circuit condition under similar illumination conditions and in the same electrolyte. The potential, 1.23 eV is the redox potential (VRedox) at room temperature based on the Gibbs free energy change involved in water splitting of 237 kJ mol-1. The solar to hydrogen conversion efficiency was plotted vs applied potential (V vs Ag/AgCl) and is shown in Figure 5. Both BiVO4 and BiVO4/TiO2 photoanodes exhibited very low photoconversion efficiencies of ~0.27 and ~0.19 % at a bias of 0.6 V vs RHE. However, the BiVO4/H:TiO2 heterostructure resulted in a maximum photoconversion efficiency of 2.87% at a bias of 0.6 V vs RHE which is significantly higher as compared to pure BiVO4 and the BiVO4/TiO2 heterostructure. Additionally, Mott-Schottky measurements were conducted on all three samples of BiVO4, BiVO4/TiO2, and BiVO4/H:TiO2 heterojunctions at 1 kHz frequency under dark conditions to investigate the influence of hydrogen treatment on the intrinsic electrical properties. The Mott-Schottky curves (1/C2 versus Vapp, Figure 6) were generated to determine the flat band potential (VFB) by using the Mott-Schottky equation C-2 = (2/qεoεsND)[V-VFB-kT/q].48 The value of VFB was observed to shift towards the more cathodic side from 0.66V vs Ag/AgCl for BiVO4 to 0.43V vs RHE for the BiVO4/TiO2


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sample. However, the hydrogen treatment of the TiO2 layer significantly reduces the flat band potential values to 0.32 V vs RHE. Further, electrochemical impedance spectroscopic (EIS) measurements were performed under visible light illumination conditions and are shown in Figure 7. From the EIS Nyquist plots, it is evident that the radii of the semicircle on the EIS plots of the BiVO4/H:TiO2 heterostructure is much smaller than that of BiVO4/TiO2 which indicates that the hydrogen treatment in the TiO2 layer changes the charge distribution in BiVO4/H:TiO2 resulting in a lower magnitude of the equivalent series resistance in BiVO4/H:TiO2 heterojunction photoanodes indicating strongly improved charge transport properties compared to BiVO4/TiO2. 4. DISCUSSION For a typical n-type semiconductor photoanode, the magnitude of photocurrent corresponds to the number of photogenerated holes that reach the electrode surface and participate in the oxidation process before they can recombine with excited electrons. For an efficient charge separation, potential gradients driving holes in one and electrons in the other direction are highly beneficial. Semiconductor heterojunctions with a discontinuity of the band edges at the interface can therefore suppress electron-hole recombination and improve the transport properties for photoexcited charge carriers.7 However for photoanodes, a type I band alignment is detrimental since it hinders hole transport over the heterojunction to the electrode surface. Ab initio calculations of the band alignment at the BiVO4/TiO2 interface predicted that a distortion of the oxygen sub-lattice in TiO2 shifts up the VBE and can lead to a transition from a type I to a type II band alignment of the BiVO4/H:TiO2 heterostructures. Since it is well known that hydrogen treatment can cause such lattice disorder in TiO2, our simulations indicate that heterojunctions of BiVO4 and hydrogen treated TiO2 can lead to considerably enhanced hole transport from BiVO4 to TiO2 and hence to an efficient


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separation of photoexcited electron-hole pairs. The theoretical considerations were substantiated by the experimental comparison of BiVO4, BiVO4/TiO2 and BiVO4/H:TiO2 photoanodes for the oxygen evolution reaction. The coating of BiVO4 by a pure TiO2 overlayer created a type I interface and lead to a poorer PEC performance. However, vast improvements were achieved upon additional hydrogen treatment. The photocurrent densities in BiVO4/H:TiO2 photoanodes could be significantly improved by a factor 15 (0.28 mA/cm2 to 4.44 mA/cm2 at a low bias voltage of 1.23 V vs RHE) and a cathodic shift of the onset potential by 0.3 V (0.16 V to -0.14 V) with respect to the BiVO4/TiO2 photoanode was observed. As EIS measurements showed, the BiVO4/H:TiO2 photoelectrode outperformed the untreated heterojunction and the BiVO4 sample by far with respect to the charge transport properties. This suggests that the respective band alignment hinders hole transport at the BiVO4/TiO2 heterojunction while it facilitated hole transport at the BiVO4/H:TiO2 interface as predicted by the simulations. In fact, experimentally an upshift of the VBE by 0.44 eV is observed in hydrogen treated TiO2. This value is close to the VBE upshift of 0.5 eV, which according to our simulations leads to a type I to II transition for the BiVO4/H:TiO2 heterojunction, see Figure 1. However, the experimental upshift of the VBE is considerably smaller than the upshift predicted for a displacement of the oxygen sub-lattice by d = 0.28 Å as theoretically expected in the work of Lu et al.14 We attribute the experimentally less pronounced upshift of the valence band to the fact that realistically not every O atom will be fully affected by the incorporation of hydrogen in TiO2. Indeed, calculations for a single displaced O atom within a supercell containing 16 TiO2 units showed that a displacement of d = 0.28 Å only leads to an upshift of about 0.1 eV (see Supplementary Figure S12). Hence the experimental value for the upshift lies between the two theoretical extremes. An aspect that is missing in the simulations is a disordered variation of the displacements among the different O atoms due to the hydrogen treatment. In such a case the VBE is smeared out and a tail of


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states is formed above the band edge.49 The existence of states above the band edge is confirmed by the optical absorption measurements which show a pronounced absorption at photon energies as low as 1.55 eV while the optical gap for H:TiO2 is 2.89 eV. Another experimental hint for additional states above the VBE is the increased intensity in the tail of the valence band XPS spectrum of H:TiO2 as compared to TiO2 (see supplementary Figure S7). Close to the band edge and at finite temperature these tail states might additionally contribute to hole transport49 and to the formation of an effective type II band alignment. In summary, both simulation and experiment indicate that hydrogen treatment considerably enhances the hole transport properties of BiVO4/TiO2 heterojunctions by creating channels for hole transport from the VBE of BiVO4 to newly formed occupied states in H:TiO2. Those states are absent in pristine TiO2, see Scheme 1. Moreover, the PEC performance might also profit from the hydrogen treatment because of increased visible light absorption generating additional holes near the electrode surface and a stronger band bending at the surface due to an enhanced charge carrier density as indicated by the downshift of the flat band potential. 5. CONCLUSION Guided by electronic structure calculations, we showed that the band edge engineering of TiO2 via hydrogen treatment is a practical route to considerably improve charge carrier separation and the photoelectrochemical performance of TiO2 coated low band gap oxide photoanodes. Especially, such a type of heterostructures allows the combination of visible light absorbing small band gap metal oxides with corrosion resistant and highly photocatalytically active TiO2 even if originally a type I band alignment hinders hole transport over the metal oxide interface and leads to poor charge transport properties and high recombination rates. Theoretically and experimentally the band edge realignment within a


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BiVO4/TiO2 heterostructure due to hydrogen treatment was traced back to an upshift of the VBE in TiO2 over the VBE of BiVO4 resulting in a favorable type II heterojunction. Both, our theoretical and our experimental approach can be applied to other TiO2 coated metal oxides and semiconductors as well and might become important tools for engineering efficient and stable photoelectrodes. ASSOCIATED CONTENT Supporting Information Crystal structure, surface morphology, XPS analysis and band gap determination of the pristine and modified hematite samples. Also, the photoanode stability test and energy band diagram of BiVO4/H:TiO2 heterostructure photoelectrodes and scheme of the charge transfer mechanism in BiVO4/H:TiO2 heterostructure photoanodes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (A.P.S.), [email protected] (B.R.M.) Tel: +91-11-26591333, Fax: +91-11-26581114 Author Contributions A.P.S. and B.R.M. conceived the idea and designed the experiments. A.P.S. have prepared and characterized the samples and analyzed the data for the experimental part of the manuscript. A.H., L.M. and M.M. planned and analyzed the ab initio simulations. A.H. realized the simulations. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. Notes The authors declare no competing financial interest. 18 ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS We gratefully acknowledge the financial support provided by the Department of Science and Technology, New Delhi India under the DST-UKERI programme and New Indigo project InSOL. B.R.M. acknowledges the support of Schlumberger Chair Professorship at IIT Delhi. A.P.S. is grateful to Department of Science & Technology, New Delhi, India for financial support in terms of INSPIRE Faculty award No. IFA12-PH-16. A.H., L.M. and M.M. kindly acknowledge financial support from the Insol Project (BMBF, Grant 01DQ14011) and the SOLAROGENIX Project (EC-FP7- Grant Agreement No. 310333). We greatly appreciate Ms. Nisha Kodan’s assistance for helping in sample preparation. We are also thankful to Dr Satheesh Krishnamurthy for his help in XPS measurements. The VESTA software50 has been used to create Figures 1 (a) and (b) and Figure 2 (b). Calculations were performed on the Joe cluster of Fraunhofer IWM and on the supercomputer JUROPA at Jülich Supercomputing Centre (JSC). The computing time granted by the John von Neumann Institute for Computing (NIC) and provided on JUROPA is gratefully acknowledged by the authors.


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Table of Contents graphic

FIGURE CAPTIONS Scheme 1. Schematic for the energy band structure of (a) BiVO4/TiO2 and (b) BiVO4/H:TiO2. The hydrogen treatment results in a realignment of the band edges at the heterojunction leading to an enhanced separation of electron-hole pairs. Figure 1. Relaxed structures from the GGA+U calculation of (a) monoclinic scheelite BiVO4 and (b) anatase TiO2. The grey arrows in (b) indicate the oxygen displacement directions for the disorder model. The different elements are colored according to Bi: purple, V: light green, O: red and Ti: light blue. (c) Calculated band alignment between BiVO4 and TiO2 for varying oxygen displacement d. Disorder-free TiO2 is represented by d = 0. VBM is the valence band maximum. Figure 2. (a) Calculated projected densities of states for undistorted TiO2, (d = 0 Å, plotted upwards) and for the disorder model with an oxygen displacement of d = 0.2 Å (plotted downwards) for the p-projection onto all oxygen atoms (O p) and the d-projection onto all Ti atoms (Ti d). Energy zero: CNL from Figure 1 (c). (b) Isosurface plot of the partial electron


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density of the highest occupied Kohn-Sham orbital for d = 0 Å . Coloring by elements: O: red and Ti: light blue. (c) Same as (b) but for d = 0.2 Å. Figure 3. (a) Spectral absorbance of TiO2, BiVO4, BiVO4/TiO2, and BiVO4/H:TiO2 heterostructure (b) Band diagram of TiO2, and H:TiO2. All representations were built using experimental data from UV−vis spectroscopy, work function and valance band XPS analysis. Conduction band off-set was estimated using band gaps, and valance band off-sets. Figure 4. Measured photocurrent density versus applied potential curves for BiVO4, BiVO4/TiO2 and BiVO4/H:TiO2 heterostructure in 0.5M Na2SO4 solution. Figure

5. Calculated photoconversion efficiencies for BiVO4,

BiVO4/TiO2 and

BiVO4/H:TiO2 heterostructure in 0.5M Na2SO4 solution. Figure 6. Mott-Schottky plots for BiVO4, BiVO4/TiO2 and BiVO4/H:TiO2 heterostructure in 0.5M Na2SO4 solution. Figure 7. Measured EIS Nyquist plots for BiVO4, BiVO4/TiO2 and BiVO4/H:TiO2 heterostructures in 0.5M Na2SO4 solution.


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Scheme 1.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.

Photocurrent Density (mA/cm2)

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7 6

BiVO4 BiVO4/TiO2

5

BiVO4/H:TiO2

4 3 2 1 0 Dark scans

-0.5

0.0

0.5

1.0

Applied Potential (V vs. RHE)


1.5

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Figure 5.

Photoconversion Efficiency

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3.5 3.0

BiVO4 BiVO4/TiO2

2.5

BiVO4/H:TiO2

2.0 1.5 1.0 0.5 0.0 -0.2

0.0

0.2

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Figure 6.

0.5

BiVO4/H:TiO2 BiVO4/TiO2

0.4

2.0

0.3

0.32 V

2.5

0.2

0.43 V

1/C2x1010 (cm4F-2)

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1/C2x1010 (cm4F-2)

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0.1

1.5

0.0 0.0

0.2

0.4

1.0

BiVO4 BiVO4/TiO2

0.5

BiVO4/H:TiO2

0.0 -0.5

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V vs. RHE

0.0

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Figure 7.

3.0

BiVO4 BiVO4/TiO2

2.5

BiVO4/H:TiO2

2.0

1.0 0.8

1.5

-Z" (k Ω )

-Z" (kΩ)

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0.6

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0.4 BiVO4 BiVO4-TiO2 BiVO4/H:TiO2

0.2

0.5

0.0 0.0

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Z' (kΩ)

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6

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TiO2 Heterostructure: Enhanced

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