Nonequilibrium Deposition in Epitaxial BiVO4 Thin Film Photoanodes

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Non-Equilibrium Deposition in Epitaxial BiVO Thin Film Photoanodes for Improving Solar Water Oxidation Performance Jaesun Song, Kyoung Soon Choi, Min Ji Seo, Yong-Ryun Jo, Jongmin Lee, Taemin Ludvic Kim, Sang Yun Jeong, Hyunji An, Ho Won Jang, Bong-Joong Kim, Cheolho Jeon, and Sanghan Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02131 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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Chemistry of Materials

Non-Equilibrium Deposition in Epitaxial BiVO4 Thin Film Photoanodes for Improving Solar Water Oxidation Performance Jaesun Song,† Kyoung Soon Choi,‡ Min Ji Seo,† Yong-Ryun Jo,† Jongmin Lee,† Taemin Ludvic Kim,§ Sang Yun Jeong,† Hyunji An,† Ho Won Jang,§ Bong-Joong Kim,† Cheolho Jeon,‡ and Sanghan Lee*,† † School of Materials Science and Engineering, Gwangju Institute of Science antd Technology, Gwangju, 61005, Republic of Korea. ‡ The Advanced Nano Surface Research Group, Korea Basic Science Institute, Daejeon 34133, Republic of Korea. § Department of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea. ABSTRACT: To improve the photoelectrochemical (PEC) performance of photoelectrodes, various modifications such as the doping of electron donors, morphology control, and adoption of catalysts have been widely implemented, among which the formation of type-II heterojunctions has been recognized as an effective method to significantly improve the charge transport efficiency of photoelectrodes. In this regard, we report on an in situ high-quality epitaxial BiVO4/Bi4V2O11 type-II heterojunction thin-film photoanode fabricated by using pulsed laser deposition (PLD) on the basis of only one BiVO4 ceramic target using the transition between BiVO4 and Bi4V2O11 crystalline phases. Herein, for the first time, we report on the structural and chemical transition between monoclinic BiVO4 (010) and orthorhombic Bi4V2O11 (001) crystalline phases by simply controlling the oxygen partial pressure. Subsequently, the growth of epitaxial BiVO4/Bi4V2O11 heterojunction thin film is achieved by controlling only the oxygen partial pressure based on band alignment. At 1.23 VRHE, the photocurrent density of heterojunction BiVO4/Bi4V2O11 structure is also significantly higher than that of the epitaxial BiVO4 thin film owing to the effective charge transfer of the Bi4V2O11 thin film. This study strongly suggests that the non-equilibrium deposition of epitaxial BiVO4 thin films can propose a new paradigm in the structural design of photoanodes.

INTRODUCTION Photoelectrochemical (PEC) water splitting has been drawing a great deal of attention recently owing to its ability to harvest and easily store solar energy as an environmentally friendly hydrogen fuel.1–4 Among the components for PEC water splitting, the semiconductor photoelectrodes are the most important factors because they are responsible for supplying photogenerated carriers for hydrogen production. Thus, most of the research studies have focused on how to enhance the efficiency of photoelectrodes, because it is most directly related to the solar-to-hydrogen (STH) conversion efficiency.2 However, until now, the STH conversion has not been commercialized because of the poor charge transport efficiency in photoelectrodes. In particular, photoanodes have much poorer charge transport efficiency than that of photocathodes.5 Therefore, designing a high-performance photoanode is the most important factor for the commercialization of STH conversion. For this reason, various strategies such as the doping of electron donors, morphology control, and adoption of catalysts have been implemented,2,6–9 among which the formation of type-II heterojunctions has been widely recognized as an effective method to greatly improve the charge transport efficiency of photoanodes.10–15 Of type-II heterojunction structures, monoclinic bismuth vanadate (BiVO4)-based composite photoanodes are the most widely researched photoanodes because BiVO4 has a relatively narrow direct band gap of ~2.4 eV and a desirable band edge position for water oxidation,3,16,17 resulting in a fairly high theoretical maximum photocurrent of 7.5 mA cm-2 for BiVO4.3 BiVO4 is most frequently formed by coupling it with tungsten oxide

(WO3) because the valence and conduction band edge position of WO3 is more positive than that of BiVO4.18,19 Formation of this band alignment facilitates the migration of the photogenerated electrons of BiVO4 toward the bottom electrode, and WO3 serves as a hole-blocking layer to prevent the movement of the photogenerated holes to the bottom electrode. In addition to WO3, many research groups have investigated other metal-oxide semiconductor materials or methods for further photocatalytic enhancement of BiVO4based composite photoanodes. First, ferroelectric materials such as BiFeO3 were coupled with BiVO4.20–23 and the photocatalytic properties of BiVO4 coupled with ferroelectric materials were enhanced owing to the reduction of carrier recombination in the photoanode. It is well known that the unique spontaneous polarization properties of ferroelectric materials can induce an internal electric field in a photoanode, improving the charge separation efficiency. However, in terms of the energy band structure of the photoanode, inadequate energy band alignment between ferroelectric BiFeO3 and BiVO4 makes it difficult to improve the photocatalytic properties and lessens the possibility of maximizing the potential of the ferroelectric materials for solar water splitting.22 Additionally, recently, Bi-based semiconductors such as Bi2O3,24 Bi2WO6,25 Bi2MoO6,26 and Bi20TiO3227 have been extensively investigated as potential candidates for improving the photocatalytic properties owing to the peculiar electronic configuration of bismuth ions (Bi3+).28 Bi4V2O11 has been reported to have an Aurivillius layered-perovskite structure with alternating layers of (Bi2O2)2+ and (VO3.5)2-, as shown in Figure 1a, and it exhibits strong polar responses.28,29 For this reason, when BiVO4 and Bi4V2O11 are coupled as a heterojunction, a high internal electric field owing to the spontaneous polarization of Bi4V2O11

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can significantly improve the charge separation efficiency.23,30,31 However, the most important property of Bi4V2O11 is that type-II heterojunctions are formed by coupling with BiVO4 owing to the appropriate energy-band structure between BiVO4 and Bi4V2O11,30,32 which facilitates the migration of photogenerated carriers. Hence, the PEC performance of BiVO4/Bi4V2O11 heterojunctions was investigated.30,32 A relative photocatalytic improvement of BiVO4/Bi4V2O11 heterojunctions compared to that of BiVO4 was identified, but the photoanodic properties of these heterojunctions revealed an extremely low photocurrent of only 0.05 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode (RHE, VRHE) owing to lowquality heterojunction films, and it has not yet been systematically analyzed. In other words, a systematic analysis is still required to extract the potential for further enhancement of the PEC performance of BiVO4/Bi4V2O11 heterojunctions, and it is essential to grow epitaxial single-crystalline thin films to systematically investigate the fundamental photocatalytic properties without any other suppressing factors such as impurities, grain boundaries, or structural defects. In this study, we proposed in situ high-quality epitaxial BiVO4/Bi4V2O11 type-II heterojunction films fabricated by using pulsed laser deposition (PLD) technique on the basis of only one BiVO4 ceramic target by employing the interesting the transition between BiVO4 and Bi4V2O11 crystalline phases. Herein, we for the first time observed the unique properties of BiVO4, which induce the structural and chemical transition between monoclinic BiVO4 (a = 5.1956 Å, b = 5.0935 Å, c = 11.6972 Å, and β = 90.387º)33 and orthorhombic Bi4V2O11 (a = 5.5488 Å, b = 5.5528 Å, and c = 15.4673 Å)30 crystalline phases by simply controlling only the oxygen partial pressure (Figure 1). The growth of b-axis oriented epitaxial BiVO4 thin films was deposited on a SrTiO3 (001) (STO, a = 3.905 Å) substrate covered by a SrRuO3 (SRO, a = 5.573 Å, b = 5.538 Å, and c = 7.856 Å) bottom electrode under high oxygen partial pressure. Additionally, it has been recently reported that it has considerably improved charge transport properties in the b-axis oriented BiVO4.34 Meanwhile, under low oxygen partial pressure, Bi4V2O11 thin films were grown epitaxially in the caxis orientation owing to the effect of the propagation dynamics of a laser ablation plume and lattice mismatch with the substrate. In addition, to our knowledge, the epitaxial growth of Bi4V2O11 has not yet been reported. Additionally, the epitaxial competitive crystalline phases of BiVO4 could provide a new route for improving its photoactivity through exploring its original photocatalytic properties without any other suppressing factors such as structural defects. The effects of biaxial strain and the manipulation of the energy band structure by controlling the chemical composition in epitaxial BiVO4 thin films with different oxygen partial pressures could also be investigated.35 In this regard, we have fabricated heterojunction BiVO4/Bi4V2O11 structures epitaxially by simply controlling only the oxygen partial pressure based on the energy-band alignment between BiVO4 and Bi4V2O11. As a result, at 1.23 VRHE, the photocurrent density of a heterojunction BiVO4/Bi4V2O11 structure (3.13 mA cm-2) is significantly higher than that of epitaxial BiVO4 thin film (2.20 mA cm-2) owing to the effective charge transfer of the Bi4V2O11 thin film. These results strongly suggest that the non-equilibrium deposition could be a new strategy to improve the charge transport efficiency of BiVO4, and can propose a new paradigm in the structural design of photoanodes for solar water splitting.

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Figure 1. Schematics for the transition between (a) orthorhombic Bi4V2O11 structure and (b) monoclinic scheelite BiVO4 structure.

RESULTS AND DISCUSSION Structural Properties of Epitaxial Bi-based Semiconductor Thin Films. To address the varying transition between BiVO4 and Bi4V2O11 crystalline phases, we first fabricated Bibased semiconductor thin films under various oxygen partial pressures ranging from 50 to 400 mTorr on the SRO-buffered STO (001) substrate using PLD, and only one BiVO4 ceramic target was used to deposit the Bi-based semiconductor thin films (Figure S1). First, to identify the epitaxial crystalline structure and determine the crystallographic orientation of the fabricated Bi-based semiconductor thin films, the structural characterization of Bi-based semiconductor thin films was investigated by using X-ray diffraction (XRD). Figure 2a shows the XRD θ–2θ scans for 100-nm-thick Bi-based semiconductor thin films deposited under oxygen partial pressures of 50 and 400 mTorr. In case of the Bi-based semiconductor thin films deposited under an oxygen partial pressure of 400 mTorr, the XRD patterns revealed that monoclinic BiVO4 thin film was grown epitaxially in the b-axis orientation, while it could identify only orthorhombic Bi4V2O11 (00l) diffraction peaks in the Bi-based semiconductor thin films deposited under an oxygen partial pressure of 50 mTorr. These thin films could not identify any other second phases or orientations, which indicated that b-axis oriented epitaxial BiVO4 and caxis oriented epitaxial Bi4V2O11 thin films were grown in the out-of-plane directions by inducing the competitive crystalline phases depending on oxygen partial pressures. As shown in Figure 2b,c, the crystalline quality of epitaxial Bi-based semiconductor thin films were verified through the reflection rocking curves and the full width at half maximum (FWHM) values. The FWHM value of the Bi4V2O11 (006) diffraction peak (0.168°) is narrower than that of the BiVO4 (020) diffraction peak (0.382°), which is comparable to the FWHM values of approximately 0.3° reported for epitaxial BiVO4 thin films.36 Additionally, to clearly discern the additional in-plane epitaxial and exact crystalline structures of Bi-based semiconductor thin films, high-resolution XRD reciprocal space mappings (RSM) were employed (Figure 2d,e). First, the RSM pattern of Bi4V2O11 around the (102) diffraction peak of the STO (001) substrate directly reveals as an epitaxial single (109) diffraction peak located at (1, 0, 2.26 5), which indicates the epitaxial growth of Bi4V2O11 thin film in the in-plane direction. In addition, considering the structural alignment between Bi4V2O11 and the STO (001) substrate, epitaxial Bi4V2O11 thin film is


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Figure 2. (a) Out-of-plane θ-2θ XRD patterns of 100-nm-thick epitaxial BiVO4 (010) and Bi4V2O11 (001) thin films deposited by using only one BiVO4 ceramic target. Reflection rocking curves and the full-width at half maximum (FWHM) value for (b) Bi4V2O11 (006) and (c) BiVO4 (020) diffraction peak. RSM patterns for 100-nm-thick epitaxial (d) Bi4V2O11 (001) and (e) BiVO4 (010) thin film around 102 diffraction peak of STO (001) substrate. (f) Schematic of in-plane structure of epitaxial BiVO4 (010) and Bi4V2O11 (001) thin films.

coherently compressive strained with STO (001) substrate with a small lattice mismatch of approximately 0.5%, and the lattice parameter of epitaxial Bi4V2O11 thin film can be calculated as a = 5.52 Å and c = 15.53 Å on the basis of the Bi4V2O11 (109) diffraction peak. Similarly, the epitaxial single BiVO4 (303) diffraction peak located at (1, 0, 2.304), which suggests epitaxial growth in the in-plane direction, is shown around the (102) diffraction peak of the STO (001) substrate. Unlike the compressive strain of epitaxial Bi4V2O11 thin film with substrate, the epitaxial BiVO4 thin film is coherently tensile strained with STO (001) substrate with a small lattice mismatch of about 0.2%, and the lattice parameter of epitaxial BiVO4 thin film can be calculated as a = 11.72 Å and c = 5.08 Å on the basis of the BiVO4 (303) diffraction peak. The inplane structural schemes for epitaxial Bi4V2O11 and BiVO4 thin films are drawn based on the results of XRD (Figure 2f). The epitaxial Bi4V2O11 thin film grows parallel to the diagonal direction, exactly matching the lattice planes of STO (001) substrate as the conventional lattice matching epitaxy (LME) based on the lattice parameter value of bulk Bi4V2O11 (a = 5.5488 Å, b = 5.5528 Å, and c = 15.4673 Å).30 However, the epitaxial BiVO4 thin film grows by matching three lattice planes of the BiVO4 thin film with four lattice planes of STO (001) substrate as the domain matching epitaxy (DME) because the initial lattice mismatch between the a-axis of BiVO4 thin film and STO (001) substrate is considerably large

at 33%.37 Hence, even though the epitaxial BiVO4 thin film has a smaller overall lattice mismatch with STO (001) substrate than that between epitaxial Bi4V2O11 thin film and STO (001) substrate, the crystalline quality of the epitaxial BiVO4 thin film is worse. This is consistent with the tendency of the measured FWHM values of the reflection rocking curves for epitaxial BiVO4 and Bi4V2O11 thin films. In conclusion, the growth of an epitaxial BiVO4 thin film as DME not exactly matching the lattice planes of STO (001) substrate can be the first factor that causes the structural and chemical transition between epitaxial BiVO4 and Bi4V2O11 thin films. Chemical Properties of Epitaxial BiVO4 and Bi4V2O11 Thin Films. Herein, the possible origin of the non-equilibrium deposition for epitaxial BiVO4 thin films can be explained as the effect of the propagation dynamics of a laser ablation plasma plume,38 and it is well known that the oxygen partial pressure is closely related to the kinetic propagation properties of the expanding plasma plume. Hence, the oxygen partial pressure can directly affect the variations in the chemical composition of a plasma plume, which is dominant in the properties of thin films. In this respect, under low oxygen partial pressure, a plasma plume vaporized from BiVO4 ceramic target is composed of non-stoichiometric BiVO4, and Bi4V2O11 thin film on the STO (001) substrate grown epitaxially owing to the considerable small lattice mismatch of less than 1% with the substrate. In contrast, epitaxial BiVO4 thin film is grown



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Figure 3. (a) Wide-scan, (b) Bi 4f, (c) V 2p, and (d) O 1s XPS spectra of epitaxial BiVO4 and Bi4V2O11 thin films.

on the STO (001) substrate by the effect of stoichiometry under high oxygen partial pressure, although BiVO4 thin films are not easy to grow on the STO (001) substrate. In other words, under low oxygen partial pressure, the effects of a lattice mismatch with the substrate may be viewed as the dominant factor in comparison with the effects of stoichiometry, and under high oxygen partial pressure and vice versa. Therefore, in terms of the stoichiometry, the chemical properties, including the oxidation states of the elements and the chemical composition of epitaxial BiVO4 and Bi4V2O11 thin films, were investigated through X-ray photoelectron spectroscopy (XPS). Figure 3a shows the wide scan spectra of both epitaxial BiVO4 and Bi4V2O11 thin films. The presence of all elements is equally observed in both epitaxial thin films without any extra peaks. First, the high-resolution XPS spectra for the Bi 4f core level of both epitaxial BiVO4 and Bi4V2O11 thin films clearly reveal two symmetric peaks for Bi 4f7/2 and Bi 4f5/2, respectively. This indicates that the Bi ions of both the epitaxial BiVO4 and Bi4V2O11 thin films are composed of only Bi3+ (Figure 3b).39,40 As shown in Figure 3c, similarly, the V 2p peak of both the epitaxial BiVO4 and Bi4V2O11 thin films reveals that the V ions are mainly V5+.41 However, there is a slight difference between epitaxial BiVO4 and Bi4V2O11 thin films for the Bi 4f and V 2p peak positions. The Bi 4f peaks of the epitaxial BiVO4 thin film are slightly shifted by about 0.1 eV toward higher-binding energies compared to those of epitaxial

Bi4V2O11 thin film, while for epitaxial BiVO4 thin film, the V 2p peaks are shifted by about 0.2 eV toward lower-binding energies in comparison with those of epitaxial Bi4V2O11 thin film. These shifts may be explained as the structural characteristics of Bi4V2O11, which has an Aurivillius layered-perovskite structure with alternating layers of (Bi2O2)2+ and (VO3.5)2-. Owing to this Aurivillius layered-perovskite structure, the overall oxidation states of Bi and V ions between the epitaxial BiVO4 and Bi4V2O11 thin films are the same, but the partial oxidation states for Bi and V ions between the epitaxial BiVO4 and Bi4V2O11 thin films may differ. This causes shifts in the binding energies, which is comparable to the tendency of binding energies for Bi and V between BiVO4 and Bi4V2O11, as reported previously.32 This also verifies that the epitaxial Bi4V2O11 thin films were grown with the crystalline structure of Bi4V2O11 reported in the literature.28,29 Similarly, the binding energies of the O 1s have a negligible difference between epitaxial BiVO4 and Bi4V2O11 thin films (Figure 3d), indicating that the oxygen vacancies within the epitaxial BiVO4 thin film are similar compared to those within the epitaxial Bi4V2O11 thin film. Additionally, through the elemental analysis, it was confirmed that Bi-based semiconductor thin-films were grown as BiVO4 crystalline phases under a relatively high oxygen partial pressure and as Bi4V2O11 crystalline phases under a relatively low oxygen partial pressure (Figure S2).


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Figure 4. (a) Estimated work function of epitaxial BiVO4 and Bi4V2O11 thin films. (b) Valence band (VB) spectra of epitaxial BiVO4 and Bi4V2O11 thin films. (c) Absorbance measurements for epitaxial BiVO4 and Bi4V2O11 thin films obtained at 400–750 nm wavelength. Inset shows optical band gaps for epitaxial BiVO4 and Bi4V2O11 thin films. (d) Energy-band alignment of epitaxial BiVO4/Bi4V2O11 type-II heterojunction structure.

Energy-Band Alignment between Epitaxial BiVO4 and Bi4V2O11 Thin Films. Prior to the fabrication of epitaxial BiVO4/Bi4V2O11 type-II heterojunction films using the transition between BiVO4 and Bi4V2O11 crystalline phases, the energyband alignment between epitaxial BiVO4 and Bi4V2O11 thin films was characterized by ultraviolet photoelectron spectroscopy (UPS) and absorbance measurements. The work function values of epitaxial BiVO4 and Bi4V2O11 thin films were evaluated by subtracting the secondary electron cut-off value from the incident photon energy of 21.2 eV, which gives work function values of 5.02 and 4.74 eV for the epitaxial BiVO4 and Bi4V2O11 thin films, respectively (Figure 4a). In addition, the differences between the Fermi level (Ef) and the valenceband maximum (Ev) were determined by the intercepts of the extrapolated lines for the spectra near the Fermi edge, which were obtained as 1.76 and 2.0 eV for the epitaxial BiVO4 and Bi4V2O11 thin films, respectively (Figure 4b). Finally, the optical band gaps for the epitaxial BiVO4 and Bi4V2O11 thin films were estimated as shown in Figure 4c. Based on the values measured above, the energy-band alignment of the epitaxial BiVO4/Bi4V2O11 type-II heterojunction structure was constructed as shown in Figure 4d. According to this energyband structure, the valence and conduction band edge positions of epitaxial Bi4V2O11 thin film are more positive than

those of epitaxial BiVO4 thin film. For this reason, the epitaxial Bi4V2O11 layer makes it easier to transfer the photogenerated holes to epitaxial BiVO4 thin film and collect the photogenerated electrons toward the bottom electrode, which cause a considerable suppression of carrier recombination. Consequently, epitaxial BiVO4/Bi4V2O11 type-II heterojunction films can expect a significant enhancement in photoactivity by the more effective charge separation efficiency of the electronhole pairs within the photoanode. Structural Properties of Epitaxial BiVO4/Bi4V2O11 TypeII Heterojunction Thin Films. Based on the above results, we have fabricated in situ epitaxial BiVO4/Bi4V2O11 type-II heterojunction films by controlling only the oxygen partial pressure. However, the peaks of BiVO4 was not confirmed in the out-of-plane θ-2θ XRD patterns for in situ epitaxial BiVO4/Bi4V2O11 type-II heterojunction films due to the similar peak position between Bi4V2O11 and BiVO4 and a decrease in the crystalline quality of BiVO4 thin film (Figure S3). Therefore, to identify the structural properties of epitaxial BiVO4/Bi4V2O11 type-II heterojunction films fabricated, highresolution transmission electron microscopy (HRTEM) was employed. Figure 5 shows HRTEM images of epitaxial BiVO4/Bi4V2O11 type-II heterojunction films taken along the [110]STO zone axis for Bi4V2O11 thin film and the [100]STO zone



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Figure 5. (a) High-resolution transmission electron microscopy (HRTEM) measurements for epitaxial BiVO4/Bi4V2O11 type-II heterojunction films. (b) The magnified images for near the interface between BiVO4 and Bi4V2O11. Insets show the fast Fourier transform (FFT) patterns for BiVO4 and Bi4V2O11. (c) Magnified selected-area electron diffraction (SAED) patterns of epitaxial BiVO4/Bi4V2O11 type-II heterojunction films.

axis for BiVO4 thin film, and it can confirm that the heterojunction films is precisely grown with well-aligned lattice fringes through the control of only oxygen partial pressure. Additionally, the out-of-plane lattice parameter of epitaxial Bi4V2O11 (d(002) = 7.69 Å) grown on the SRO-buffered STO (001) substrate is in good agreement with the c-axis of the bulk orthorhombic Bi4V2O11, and the out-of-plane lattice parameter of the subsequently deposited epitaxial BiVO4 (d(020) = 2.54 Å) is in good agreement with the b-axis of the bulk monoclinic BiVO4. The alignment from the substrate to the thin film is also identified in the out-of-plane direction through the magnified selected-area electron diffraction (SAED) patterns (Figure 5c). Although it has been fabricated as heterojunction films, these results show similar trends to the XRD results measured for the epitaxial BiVO4 and Bi4V2O11 thin films as shown in Figure 2. In other word, it is clearly confirmed that in situ BiVO4/Bi4V2O11 type-II heterojunction film fabricated by controlling only the oxygen partial pressure was grown epitaxially. Table 1. Comparison of charge transfer resistance values from the photoanode to electrolyte for epitaxial Bi4V2O11, BiVO4, and BiVO4/Bi4V2O11 heterojunction thin films. Photoanode

Rs (Ω·cm2)

R1 (Ω·cm2)

Bi4V2O11

1.75

930.43

BiVO4

1.55

337.16

BiVO4/Bi4V2O11

1.81

133.10

Photoelectrochemical Properties of Epitaxial BiVO4/Bi4V2O11 Type-II Heterojunction Thin Films. To confirm the effect of the energy-band alignment between epitaxial BiVO4 and Bi4V2O11 thin films with regard to their photoactivity, the photocatalytic properties of epitaxial 100-nm-thick BiVO4, 50-nm-thick Bi4V2O11, and 100-nm-thick BiVO4/50nm-thick Bi4V2O11 heterojunction thin-film photoanodes were measured under simulated AM 1.5G illumination (100 mW cm-2) in 0.5 M Na2SO4 with 0.5 M Na2SO3 aqueous solutions. Prior to the meas urement of the photocatalytic properties, it was confirmed that the surficial effects for epitaxial BiVO4, Bi4V2O11, and BiVO4/Bi4V2O11 heterojunction thin-film pho-

toanodes is insignificant for the performance of the photoelectrochemical water splitting by using atomic force microscopy (AFM) (Figure S4). Figure 6a shows the linear sweep voltammetry (LSV) curves of epitaxial BiVO4, Bi4V2O11 and BiVO4/Bi4V2O11 heterojunction thin-film photoanodes. The photocurrent density of epitaxial BiVO4/Bi4V2O11 heterojunction thin films (3.13 mA cm-2) is significantly higher than those of the epitaxial BiVO4 (2.20 mA cm-2) and Bi4V2O11 (0.16 mA cm-2) thin films at 1.23 VRHE owing to the effective charge transfer of the epitaxial Bi4V2O11 thin film, which can be a tremendous improvement compared with the approximately 0.15 mA cm-2 reported for BiVO4/Bi4V2O11 heterojunction thin films.30 Additionally, the emergence of a peak near 0.5 VRHE in the LSV of epitaxial Bi4V2O11 thin-film photoanode indicates the formation of peroxo species on the surface of epitaxial Bi4V2O11 thin-film photoanode due to slow water oxidation kinetics at the Bi4V2O11 surface.5,42-45 When the photocurrent densities of epitaxial BiVO4/Bi4V2O11 heterojunction thin films with various thicknesses of epitaxial Bi4V2O11 thin films ranging from 30 to 80 nm were measured, it was confirmed that the 50-nm-thick Bi4V2O11 thin film is operated as the optimum charge transfer layer (Figure 6b). To investigate the charge transfer kinetics of an epitaxial BiVO4/Bi4V2O11 heterojunction thin-film photoanode, electrochemical impedance spectra (EIS) measurements were carried out as shown in Figure 6c. The measured EIS of the epitaxial BiVO4/Bi4V2O11 heterojunction thin-film photoanode exhibits the smallest impedance value, which is consistent with high photoactivity and the best kinetic charge transfer. In addition, the equivalent Randle circuit model is presented as shown in the inset of Figure 6c. It consists of the series resistance (Rs), constant phase element from the photoanode to electrolyte (CPE1), and the charge transfer resistance from photoanode to electrolyte (R1). In addition, the fitted values of Rs and R1 for the epitaxial BiVO4/Bi4V2O11 heterojunction thin-film photoanode are considerably lower than those of the epitaxial BiVO4 and Bi4V2O11 thin-film photoanodes, as presented in Table 1. Subsequently, it was identified that the incident-photon-to-current conversion efficiency (IPCE) value of the epitaxial BiVO4/Bi4V2O11 heterojunction thin-film photoanode is also higher than that of the epitaxial BiVO4 thin film (Figure 6d), and the increases in IPCE from a wavelength of about 500 nm correspond to optical band gaps for the epitaxial BiVO4 thin film. Additional stability of epitaxial BiVO4, Bi4V2O11, and


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Figure 6. (a) Linear-sweep voltammetry (LSV) curves of epitaxial 100-nm-thick BiVO4, 50-nm-thick Bi4V2O11, and 100-nm-thick BiVO4/50-nm-thick Bi4V2O11 heterojunction thin film photoanodes obtained in 0.5-M Na2SO4 with 0.5-M Na2SO3 aqueous solutions. (b) Linear sweep voltammetry curves of epitaxial 100-nm-thick BiVO4/Bi4V2O11 heterojunction thin-film photoanodes with various thicknesses of epitaxial Bi4V2O11 thin films ranging from 30 to 80 nm. (c) Electrochemical impedance spectra of 100-nm-thick epitaxial 100-nm-thick BiVO4, 50-nm-thick Bi4V2O11, and 100-nm-thick BiVO4/50-nm-thick Bi4V2O11 heterojunction thin-film photoanodes. Inset shows equivalent Randle circuit model for epitaxial thin films. (d) Incident-photon-to-current conversion efficiency (IPCE) spectra of epitaxial 100-nm-thick BiVO4, 50-nm-thick Bi4V2O11, and 100-nm-thick BiVO4/50-nm-thick Bi4V2O11 heterojunction thinfilm photoanodes.

BiVO4/Bi4V2O11 heterojunction thin-film photoanodes is described in Figure S5. In conclusion, the photocatalytic properties of the epitaxial BiVO4/Bi4V2O11 heterojunction thin-film photoanode are significantly enhanced compared to those of the epitaxial BiVO4 thin-film photoanode. This improvement in the epitaxial Bi VO4/Bi4V2O11 heterojunction thin-film photoanode is related to the formation of the energy-band alignment between BiVO4 and Bi4V2O11. This facilitates the migration of the photogenerated electrons of BiVO4 toward the bottom electrode, and Bi4V2O11 serves as a hole-blocking layer to prevent the transfer of the photogenerated holes to the bottom electrode.

CONCLUSIONS The structural and chemical transition between epitaxial monoclinic BiVO4 (010) and epitaxial orthorhombic Bi4V2O11 (001) crystalline phases was investigated for the first time by simply controlling the oxygen partial pressure. Subsequently, the growth of an epitaxial BiVO4/Bi4V2O11 heterojunction thin film by controlling only the oxygen partial pressure is achieved based on the energy-band alignment. As a result, the

photocurrent density of the heterojunction BiVO4/Bi4V2O11 structure is significantly enhanced compared with that of epitaxial BiVO4 thin film owing to the effective charge transfer of the Bi4V2O11 thin film. These results strongly suggest that the non-equilibrium deposition could be a new strategy to improve the charge transport efficiency of BiVO4, and can propose a new paradigm in the structural design of photoanodes for solar water splitting.

EXPERIMENTAL SECTION Epitaxial Thin-Film Growth and Characterization. Epitaxial BiVO4 and Bi4V2O11 thin films were fabricated by simply controlling the oxygen partial pressure employing only one BiVO4 ceramic target by the pulsed laser deposition (PLD) method using a KrF excimer laser (Coherent COMPexPro 205F). All thin films were grown on a SrRuO3 (SRO)buffered STO (001) substrate at a substrate temperature of 350 ºC and an oxygen partial pressure ranging from 50 to 400 mTorr. In the case of the growth of epitaxial BiVO4/Bi4V2O11 heterojunction thin film, Bi4V2O11 thin film was grown first under low oxygen partial pressure (50 mTorr). Then, BiVO4



thin film was deposited under high oxygen partial pressure (400 mTorr). The structural properties were characterized using X-ray diffraction (XRD, PANalytical X’Pert Pro diffractometer) using Cu Kα radiation (λ = 1.5418 Å). Reciprocal space mapping (RSM) was performed at the 3A beamline at the Pohang Light Source with a six-circle Paul Scherrer Institute (PSI) diffractometer. The chemical properties were measured through X-ray photoelectron spectroscopy (XPS, KRATOS AXIS Ultra DLD model). Optical properties such as the optical band gap were measured using UV-vis-NIR absorption spectroscopy (Varian Cary500scan). PEC Measurements. PEC performance (Ivium Technologies, Nstat) was measured using a standard three-electrode configuration with Ag/AgCl (1 M KCl) as the reference electrode and Pt wire as the counterelectrode. Epitaxial BiVO4, Bi4V2O11, and BiVO4/Bi4V2O11 heterojunction thin film photoanodes were measured as the working electrode. All measurements were performed in 0.5 M Na2SO4 with 0.5 M Na2SO3 aqueous solutions under simulated AM 1.5G illumination (100 mW cm-2). The electrochemical impedance spectra (EIS) measurements were carried out in a frequency range of 100 kHz to 0.01 Hz by applying -0.5 V vs. Ag/AgCl. The incident-photon-to-current conversion efficiency (IPCE) measurements were performed at 1.23 VRHE.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Out-of-plane θ-2θ XRD patterns, EDX, XPS and Topological AFM images

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Creative Materials Discovery Program (2017M3D1A1040828), and Basic Science Research Program (2016R1D1A1B03931748) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, and the Ministry of Education, and by GIST Research Institute(GRI) grant funded by GIST.

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Nonequilibrium Deposition in Epitaxial BiVO4 Thin Film Photoanodes

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