Solar-Driven Water Splitting over a BaTaO2N Photoanode Enhanced

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Solar-driven Water Splitting over a BaTaON Photoanode Enhanced by Annealing in Argon Jeongsuk Seo, Mamiko Nakabayashi, Takashi Hisatomi, Naoya Shibata, Tsutomu Minegishi, and Kazunari Domen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00908 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 4, 2019

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ACS Applied Energy Materials

Solar-driven Water Splitting over a BaTaO2N Photoanode Enhanced by Annealing in Argon

Jeongsuk Seo1, Mamiko Nakabayashi2, Takashi Hisatomi1, Naoya Shibata2, Tsutomu Minegishi3, and Kazunari Domen1,3,*

1

Research Initiative for Supra-Materials (RISM), Shinshu University, 4-17-1 Wakasato, Nagano 3808553, Japan;

2

Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan;

3

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-31 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

* Corresponding author: [email protected] Keywords: water oxidation, oxygen evolution reaction, perovskite oxynitride, surface defects, surface crystallinity.

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Abstract BaTaO2N prepared by the nitridation of Ba5Ta4O15 drives photoelectrochemical water oxidation in response to photoexcitation up to 660 nm. However, a high concentration of defects and an amorphous surface promote the recombination of photogenerated holes and electrons in this material, thus reducing its performance. In this work, annealing in an Ar flow is used to activate the BaTaO2N surface and thus improve its water oxidation activity. The results show that annealing at 1073 K both crystallizes the amorphous BaTaO2N surface and increases the bulk crystallinity. Following surface modification to enhance charge separation during the photoreaction, a photoanode made of the annealed BaTaO2N generates an unprecedented photocurrent of 6.5 mA cm-2 at 1.23 VRHE during sunlight-driven water oxidation, and retains 79% of the initial photocurrent over 24 h. The half-cell solar-to-hydrogen energy conversion efficiency reaches 1.4% at 0.88 VRHE, representing the highest value yet reported for any perovskite-type oxynitride with intense visible light absorption. This remarkable improvement demonstrates that a surface treatment based on annealing in Ar effectively enhances the photoreaction over oxynitrides that otherwise tend to have amorphous surfaces.


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1. Introduction There is currently a gradually increasing global demand for clean, renewable fuels such as hydrogen, and sunlight-driven water splitting into hydrogen and oxygen using semiconductor photoelectrodes is a potential means of producing hydrogen. A solar-to-hydrogen (STH) energy conversion efficiency greater than 10% is necessary to achieve the commercial viability of photoelectrochemical (PEC) water splitting cells composed of a photoanode and a photocathode.1,

2

However, meeting this target requires the

development of semiconductors having certain characteristics. Thermodynamically, the semiconductor must have a band-gap energy (Eg) less than 2.3 eV (equivalent to a wavelength, , > 530 nm), while the valence band maximum potential for an n-type semiconductor must also be positioned at a potential more positive than 1.23 V. In addition, semiconductors for use in PEC systems must be chemically stable during functioning of the cell. The perovskite-type oxynitride semiconductor BaTaO2N has the smallest Eg (1.9 eV, =660 nm) among Ta-based oxynitrides such as TaON, Ta3N5 and ATa(O,N)3 (A=Ca, Sr, Ba and La), and satisfies the thermodynamic requirements. This material has the potential to generate a maximum photocurrent density of 17.6 mA cm-2 during water oxidation under sunlight, assuming an incident photon-to-current efficiency (IPCE) of unity, leading to an STH energy conversion efficiency of 21.6% if no applied bias is needed. Based on its favourable optical properties, BaTaO2N has been studied as a photoanode for 3 ACS Paragon Plus Environment


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water oxidation under visible light.3-8 The anodic photocurrent onset potential of BaTaO2N during water oxidation is approximately 0.6 to 0.7 VRHE, which is comparable to that of Ta3N5 and sufficient for a PEC water splitting cell.2, 9 However, the anodic photocurrent exhibited by this material at 1.23 VRHE has been found to be approximately 4 mA cm-2 at most, which is lower than the theoretical maximum value that BaTaO2N can produce.5 B-site cations (e.g., Ta5+) in perovskite-type AB(O,N)3 (A=Ca, Sr, Ba and La; B=Ti, Ta and Nb) compounds are readily reduced to lower oxidation states (e.g., Ta4+) during high-temperature nitridation, particularly when they have high electronegativities.10-12 The reduction of cations in this manner generates defects associated with anion vacancies and oxygen impurities to offset charge imbalance. These defects, especially if they are located on the surface of the oxynitride (where photoreactions take place), can act as recombination sites for photogenerated holes and electrons. Our previous study using X-ray photoelectron spectroscopy (XPS) demonstrated that the water oxidation photocurrent over a BaNbO2N photoanode was greatly decreased with increasing number of reduced Nb species on the oxynitride surface.13 Thus, a novel protocol composed of a mild nitridation and subsequent annealing in a flow of Ar was developed for the synthesis of low-defect BaNbO2N.14 The Ar annealing step effectively suppresses the formation of reduced defects and also enhances the crystallinity of the amorphous surface, thus allowing the BaNbO2N photoanode to produce a high photocurrent of 5.2 mA cm-2 at 1.23 VRHE 4 ACS Paragon Plus Environment

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during sunlight-driven water oxidation. There is no crystalline oxide precursor for BaTaO2N having a Ba/Ta ratio of unity. Consequently, the nitridation of Ba-rich Ba5Ta4O15 produces BaTaO2N, which has an amorphous surface with a high level of defects, similar to the BaNbO2N system.15 For this reason, annealing is required to effectively heal the inactive surface of BaTaO2N and enhance its oxygen evolution activity. In the present study, greatly enhanced water oxidation performance was obtained using a particulate BaTaO2N photoanode, the surface properties of which were dramatically transformed by annealing in an Ar atmosphere. As-prepared BaTaO2N is thermally stable in an Ar flow, so that the high-temperature annealing step does not deteriorate the crystallinity of this material, and annealing at 1073 K was found to improve both the bulk and surface properties of inactive as-prepared BaTaO2N. As a result, the anodic photocurrent produced by annealed BaTaO2N was increased by an order of magnitude relative to that obtained over the as-prepared oxynitride, providing a half-cell STH energy conversion efficiency of 1.4%. This is a new record for a photoanode based on perovskite-type AB(O,N)3, demonstrating that the annealing treatment is highly effective at improving the photoactivity of BaTaO2N.

2. Results and discussion



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In this study, BaTaO2N was prepared via the thermal nitridation of highly crystalline Ba5Ta4O15 under an NH3 flow at 1223 K for 30 h. Figure 1A presents X-ray diffraction (XRD) patterns generated by the powder products obtained from the nitridation process. These patterns show that the initial layered perovskite-type Ba5Ta4O15 was gradually transformed to perovskite-type BaTaO2N without the appearance of intermediate phases or by-products. The nitridation to give single-phase BaTaO2N was complete after 30 h. Removal of residual Ba species after the nitridation by washing with distilled water increased the intensity of the diffraction peaks attributable to the BaTaO2N phase. Subsequently, the as-prepared BaTaO2N was annealed in an Ar flow at various temperatures in the range of 873-1173 K for 1 h. The XRD patterns produced by the annealed oxynitrides are shown in Figure 1B. Based on the full width at half maximum (FWHM) values for the (110) peaks, the bulk crystallinity was unchanged during annealing up to 973 K, but was appreciably enhanced after the treatments at 1073 K or above. These results differ from those obtained when annealing BaNbO2N, which does not undergo changes in its bulk crystallinity.13 The patterns also demonstrate that impurity phases assignable to Ba2TaO3N and Ta2O5 were generated. An elemental composition analysis showed that the high-temperature annealing released nitrogen from the oxynitride (Table S1, Supporting Information). The as-prepared oxynitride was partly decomposed into Ba2TaO3N and Ta2O5 as a result of the loss of nitrogen during the annealing. Interestingly, the degree of crystallinity was not reduced, even when the


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annealing treatment was extended to 10 h (Figure S1). This result indicates that the decomposition of BaTaO2N into Ba2TaO3N and Ta2O5 occurred only during the early stage of the annealing, and that the remaining BaTaO2N was sintered. The as-prepared BaTaO2N was capable of harvesting visible light up to ca. 660 nm, in agreement with previous reports (Figure S2).5, 12 The background absorption above 660 nm was slightly increased after the high-temperature annealing, although the onset light absorption wavelength was unchanged. Figure 2 shows thermogravimetric/differential thermal analysis (TG/DTA) results for the asprepared BaTaO2N as a function of temperature in air and Ar flows. A mass loss originating from the desorption of H2O/OH- adsorbed on the oxynitride surface was observed up to approximately 700 K, irrespective of the atmosphere. In air, the oxynitride underwent a typical oxidation process (with an associated mass increase) and subsequently released nitrogen (leading to a mass decrease).16,

17

Interestingly, the thermal oxidation of the BaTaO2N started at a relatively high temperature (ca. 980 K) compared to those reported for other oxynitrides (673–873 K).14, 18, 19 In trials using an Ar flow, the mass was almost unchanged between 700 and 1200 K due to the absence of oxygen, demonstrating that the asprepared BaTaO2N was thermally stable up to 1200 K. This finding is consistent with observations that the bulk crystallinity of BaTaO2N was not deteriorated during annealing at 1073 K or above in Ar.



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The surface morphologies of the starting Ba5Ta4O15 and the as-prepared BaTaO2N were observed using scanning electron microscopy (SEM), as shown in Figures 3(a) and 3(b). These images demonstrate that the Ba5Ta4O15 particles were ~5 μm in size. The BaTaO2N surface became porous due to the exchange of three O2- ions for every two N3- anions during the nitridation, although the particle size remained similar to that of the oxide. The morphology of the BaTaO2N was unchanged following annealing in Ar (Figure S3), whereas the surface crystallinity of each annealed oxynitride was different, depending on the annealing temperature. Figures 3(c-e) present high-resolution transmission electron microscopy (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns for BaTaO2N particles, both as-prepared and after annealing in Ar at 873 or 1073 K for 1 h. The surface of an as-prepared BaTaO2N particle was composed of polycrystalline and amorphous domains, as demonstrated by the multiple spots and hollow rings in the ED pattern. During the annealing at 873 K, the amorphous layer became thinner so that lattice fringes indicating a degree of crystallinity were observed at the surface of the particle. At 1073 K, the surface was fully crystallized, producing clear lattice fringes with d(110) spacing (PDF card No. 01-078-1455), and the ED pattern corresponds to a single BaTaO2N phase along the [211] zone axis. Therefore, the high-temperature annealing at 1073 K enhanced the surface crystallinity of the as-prepared BaTaO2N as well as its bulk crystallinity, as


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demonstrated by the XRD patterns. This result also suggests that the impurity phases were segregated from the BaTaO2N particles. Changes in the cation stoichiometry on the BaTaO2N surface during the surface crystallization resulting from annealing were investigated by XPS. Figure 4 provides narrow-scan Ta 4f XPS spectra of the BaTaO2N photoanodes, both as-prepared and after annealing at different temperatures. The peak intensities were enhanced by the annealing and the spectra gradually shifted to higher binding energies with increasing annealing temperature. The spectra were deconvoluted into three doublets ascribed to Ta5+, Ta4+ and Ta3+, associated with 4f7/2 binding energies of 26, 24.7 and 23.5 eV. 20-22 The proportions of these surface Ta species are summarized in Table S2. The amorphous surface of the as-prepared BaTaO2N was primarily covered with Ta4+ and Ta3+, although annealing at 873 K led to a decrease in the concentration of reduced species and an increase in the number of Ta5+ species on the oxynitride surface. The as-prepared BaTaO2N included an excess of oxygen (Table S1), which would serve to oxidize the surface of the material even under an Ar flow. Nevertheless, the BaTaO2N surface was still amorphous after annealing at 873 K, while the BaNbO2N surface was crystallized into BaNbO3.13 The reduced species Ta3+ was eliminated at 973 K while the proportion of Ta5+ was simultaneously increased. At 1073 K or above, the proportions of the various surface Ta species were almost unchanged.



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Interestingly, the high-temperature annealing contributed to the release of nitrogen although it did not induce the reduction of Ta species to compensate for the charge imbalance. The inactive, amorphous surface was instead separated into crystalline impurities such as Ba2TaO3N and Ta2O5. This surface decomposition was also accompanied by enhanced bulk crystallinity. These phenomena are different from those observed during the annealing of BaNbO2N13 and are primarily attributed to the high thermal stability of BaTaO2N. Therefore, the annealing of BaTaO2N in Ar was effective at crystallizing the amorphous surface of the oxynitride that initially had a high defect density. Photoanodes made of the annealed BaTaO2N were fabricated by a particle transfer (PT) method.23, 24 In this process, a monolayer of BaTaO

2N particles was deposited on Ta ohmic conductor and Ti current

collector layers (Figure S4). Subsequently, the oxygen evolution reaction (OER) electrocatalysts Co(OH)x and FeOy were loaded on the photoanode (Figure S5). Analyses by energy dispersive X-ray spectroscopy (EDS) confirmed that Co and Fe layers were deposited to a thickness of approximately 50 nm. The data also showed that loading the electrocatalysts did not affect the surface crystallinity of the BaTaO2N. PEC water oxidation was examined using bare BaTaO2N photoanodes, both as-prepared and after annealing at various temperatures (Figure S6). The water oxidation photocurrent was increased after the annealing treatment. However, the photocurrent onset was relatively positive. The bare BaTaO2N 10 ACS Paragon Plus Environment

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photoanodes suffered from self-oxidation (2N3- + 6h+ → N2) during the photoreaction, analogous to other (oxy)nitrides.3,

25, 26

Figure 5 presents current-potential curves for particulate Co(OH)x-

FeOy/BaTaO2N photoanodes, followed by the loading of electrocatalysts, under chopped simulated sunlight (AM 1.5G). The anodic photocurrent produced by the as-prepared BaTaO2N was nearly negligible at potentials lower than 0.8 VRHE and was less than 0.1 mA cm-2 even at a potential of 1.23 VRHE. This is a reasonable result, based on the thick, amorphous BaTaO2N surface layer and the high defect density in this material, which promoted the recombination of photogenerated holes and electrons. In contrast, the photocurrent density was remarkably enhanced after annealing in Ar, and the performance of the annealed material was also found to be stable during a 1 h photoreaction (Figure S7). Notably, annealing at 873 or 973 K enhanced the photoanodic current considerably despite the lack of any change in the bulk crystallinity or optical properties of the oxynitride. These data suggest that reducing the surface defect concentration and the thickness of the amorphous layer increased the anodic photocurrent density. The increased bulk crystallinity of the BaTaO2N that resulted from annealing at temperatures above 1073 K further improved the sunlight-driven water oxidation activity. A photocurrent was reached 6.5 mA cm-2 at 1.23 VRHE, regardless of the scan direction (Figure S8), representing the highest photocurrent density yet obtained using BaTaO2N. The anodic photocurrent was decreased by annealing at 1173 K, 11 ACS Paragon Plus Environment


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although the crystallinity of this material did not appear to have deteriorated. This decrease can possibly be attributed to an increase in anion defects associated with the oxygen concentration in the oxynitride, based on evident increases in light absorption beyond the absorption edge (Figure S2), and an increase in the O/Ta bulk ratio (Table S1). Figure 6 shows Nyquist plots of bare BaTaO2N series, both as-prepared and after annealing at various temperatures, and the corresponding fitting results using an equivalent circuit model. The charge transfer resistance at the interface between electrolyte and BaTaO2N particles, Rct, was dramatically decreased with increasing the annealing temperature in Ar. Rct was reached a minimum value in the BaTaO2N photoanode annealed at 1073 K. It is considered that the highly crystalline BaTaO2N surface results in the reduced resistance leading to more efficient electron transfer in the photoanode system. Thus, the enhanced photocurrent density for water oxidation, evident in Figure 5, can be largely ascribed to the surface property of the oxynitride improved by annealing. It also confirms that the surface characteristics are closely correlated with photoactivity. The anodic photocurrent over the BaTaO2N annealed at 1073 K was observed beginning at 0.6 VRHE. The potential at which the photocurrent appeared was also lower than those reported for other active perovskite-type oxynitrides, such as LaTiO2N (ca. 0.7 VRHE) and BaNbO2N (ca. 0.9 VRHE). 14, 27 This low onset potential is beneficial with regard to the use of this material in PEC water splitting cells. Even at a


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relatively low pH of 9.5, the annealed BaTaO2N produced a significant photocurrent density of 5.5 mA cm-2 at 1.23 VRHE, as shown in Figure 7A. The photocurrent onset exhibited a lag of approximately 0.3 VRHE compared to that for sulfite oxidation, and this difference suggests an overpotential associated with the slow OER.28 However, the photocurrent density at higher potentials was comparable to that for sulfite oxidation so that the efficiency of surface charge separation, ηsurface, was determined as presented in Figure 7B. In this study, the Co(OH)x-FeOy/BaTaO2N photoanode has nearly ηsurface of 92% at 1.23 VRHE, indicating highly efficient charge separation during the water oxidation process. The water oxidation performance of a photoanode made of BaTaO2N annealed at 1073 K was subsequently characterized, and Figure 8A shows the photocurrent produced by a Co(OH)xFeOy/BaTaO2N photoanode at 1.23 VRHE as a function of time over 24 h. The photocurrent decreased over the initial 10 h but then plateaued. After 24 h, the photoanode retained 79% of the initial photocurrent density. Figure 8B plots the amounts of H2 and O2 evolved on a Pt wire counter electrode and a BaTaO2N photoanode, respectively, during the initial 10 h of the water oxidation trial shown in Figure 8A, and demonstrates that H2 and O2 were evolved at the expected stoichiometric ratio of 2:1. The Faradaic efficiency of this PEC water splitting was nearly 100% and, on average, the PEC system incorporating the annealed BaTaO2N produced H2 at a rate of 93 μmol h-1 cm-2. The annealing treatment also greatly increased the half-cell STH (HC-STH) energy conversion efficiency, providing a value of



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1.4% at 0.88 VRHE (Figure 8C). This high STH value is attributed to an increase in the photocurrent density at potentials lower than 1.23 VRHE. Figure 8D plots the wavelength dependence of the IPCE values for the Co(OH)x-FeOy/BaTaO2N photoanode at applied potentials of 0.8, 0.9 and 1.23 VRHE. At all the applied potentials, the IPCE spectra became zero at 700 nm and the shapes were in good agreement with that of the light absorption spectrum for the BaTaO2N powder. The IPCE at 1.23 VRHE was found to increase as the wavelength became shorter, and reached a plateau value of approximately 43% at 540 nm. IPCE values of 28 and 20% at 540 nm were also observed at the lower potentials of 0.9 and 0.8 VRHE, respectively. These values are greater than those reported in previous works.3, 5 The HC-STH and IPCE results demonstrate remarkably high water oxidation activity at low potentials. These results, together with the long-term stability of this material, demonstrate that the annealed BaTaO2N has significant potential as a photoanode for efficient sunlight-driven water splitting.

3. Conclusions In conclusion, annealing under Ar was found to improve the surface properties of previously inactive as-prepared BaTaO2N. The original BaTaO2N surface immediately after the thermal nitridation of Barich crystalline Ba5Ta4O15 was covered with an amorphous layer. However, annealing of the oxynitride decreased the relative proportion of surface reduced Ta species and also enhanced the crystallinity of


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near-surface regions, depending on the annealing temperature. The highest level of water oxidation performance was obtained following annealing at 1073 K, at which temperature both the surface and bulk of the BaTaO2N phase were most effectively crystallized, despite the segregation of by-products. A photoanode made using BaTaO2N annealed at this temperature produced a photocurrent of 6.5 mA cm-2 at 1.23 VRHE during solar water splitting, corresponding to a maximum HC-STH energy conversion efficiency of 1.4% at 0.88 VRHE. This photoanode also retained 79% of its initial photocurrent value over 24 h of water oxidation. This performance represents the state-of-the-art for a perovskite-type oxynitride photoanode during sunlight-driven PEC water oxidation. The results of this work also demonstrate that the annealing treatment enhanced the photoactivity of BaTaO2N, in the same manner as in previous work with the perovskite-type material BaNbO2N. The present study suggests that annealing in Ar represents a facile yet versatile method of effectively improving the surface characteristics of various oxynitrides that otherwise exhibit amorphous oxide layers formed by exposure to air and/or low bulk crystallinity owing to insufficient nitridation.

4. Experimental Section Synthesis of BaTaO2N: Ba5Ta4O15 powder was prepared by a solid-state reaction method. BaCO3 (99.9%, Kanto Chemical. Co., Inc.) and Ta2O5 (99.99%, Rare Metallic Co., Ltd.) were blended at a molar 15 ACS Paragon Plus Environment


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ratio of 5:2 and then calcined in air at 1573 K for 30 h in conjunction with two intermediate grinding processes. The resulting crystalline Ba5Ta4O15 was heated to 1223 K at a rate of 10 K min-1 and held at that temperature in a 250 mL min-1 NH3 flow for 30 h, with two gentle intermediate grinding processes. The nitrided powder was thoroughly washed with distilled water to remove residual Ba species and then allowed to dry upon standing. The BaTaO2N powder was subsequently heated to 873, 973, 1073 or 1173 K at a rate of 10 K min-1 and annealed at this temperature for 1 h under a 100 mL min-1 Ar (6N grade) flow. Photoelectrochemical water splitting: BaTaO2N photoanodes were fabricated by the PT method.23, 24

In this process, Ta ohmic conductor and Ti current collector layers were deposited sequentially on

BaTaO2N particles by radio frequency (RF) magnetron sputtering at 673 K, applying a power of 100 W for 5 min or 200 W for 6 h. The OER electrocatalysts, Co(OH)x and FeOy, were then loaded on the BaTaO2N/Ta/Ti photoanode using previously reported conditions, to accelerate the PEC water oxidation process.14 The electrocatalyst-loaded BaTaO2N electrodes, denoted as Co(OH)x-FeOy/BaTaO2N hereafter, were then heated in air at 623 K for 30 min. PEC water oxidation over the Co(OH)x-FeOy/BaTaO2N photoanodes was carried out using a conventional three-electrode system in conjunction with a potentiostat (Hokuto Denko, HZ-7000) under chopped AM 1.5G simulated sunlight (SAN-EI Electric, XES-301S). A Pt wire and an Ag/AgCl


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electrode were incorporated in the test system as counter and reference electrodes, respectively. Linear sweep voltammograms (LSVs) were recorded cathodically during the water oxidation process over the potential range of 1.3 to 0 VRHE at a scan rate of 10 mV s-1 at 298 K in an Ar-saturated aqueous 0.5 M potassium borate (KBi) electrolyte at pH 13. This electrolyte was prepared by adding KOH to a 0.5 M H3BO3 solution. The Ag/AgCl reference electrode was calibrated relative to a reversible hydrogen electrode (RHE) so that the potentials in the KBi solution could be expressed using the Nernst equation, ERHE = EAg/AgCl + 0.059 pH + 0.197. The electrochemical impedance spectroscopy (EIS; METEK Inc., VersaSTAT3) of BaTaO2N photoanodes with the area of 0.4 cm2 was measured at an applied potential of 1.23 VRHE in a frequency range from 104 to 10-1 Hz with an AC amplitude of 10 mV. The acquired data were fitted by a ZView program (Scribner Associates, Inc.) using an equivalent circuit model including resistances and constant phase element (CPE) values. The PEC sulfite oxidation in an aqueous 1 M Na2SO3 electrolyte was assumed to completely suppress a surface charge recombination (i.e., ηsurface = 100%) and to hardly affect a bulk charge separation. Thus, the efficiency of surface charge separation, ηsurface (%), can be determined as ηsurface = Jwater / Jsulfite × 100%, where Jwater and Jsulfite are the photocurrent densities for water oxidation and sulfite oxidation, respectively. The HC-STH energy conversion efficiency (η) of the BaTaO2N photoanode was calculated based on current-potential curves acquired under chopped sunlight, using the



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equation η = [(EOER - ERHE) × (Jlight - Jdark) / Plight], where EOER is the reversible water oxidation potential (1.23 VRHE), ERHE is the potential of the electrode vs. RHE, Jdark and Jlight are the photocurrent density (mA cm-2) in darkness and under AM 1.5G light, and Plight is the power density of the AM 1.5G irradiation (100 mW cm-2). The effect of wavelength on the IPCE of the anode was estimated using data obtained with monochromatic irradiation from a Xe lamp (Asahi Spectra, MAX-303) equipped with band pass filters covering the range of 420 to 700 nm. The IPCE at each wavelength was calculated using the equation IPCE = [(1240 (V · nm) × (Jlight - Jdark)]/[Plight × λ (nm)] × 100%, where λ is the irradiation wavelength. Finally, the amounts of O2 and H2 evolved from the BaTaO2N photoanode and the CrOxcoated Pt counter electrode were determined by micro-gas chromatography (Agilent Technologies, 490 Micro-GC). Structural characterization: The crystal structure of each BaTaO2N powder was determined by XRD (MiniFlex300, Rigaku) using Cu Kα radiation and operating at 30 kV and 10 mA. The optical properties of the materials were evaluated by UV-visible diffuse reflectance spectroscopy (UV-vis DRS; V-670, JASCO), employing an integrating sphere at room temperature and using Spectralon as a standard reflection reference. The thermal properties of the as-prepared BaTaO2N powders were investigated by TG/DTA (Rigaku, Thermoplus II) over the range of 298 to 1400 K under air or Ar at a flow rate of 100 mL min-1 and at a heating rate of 10 K min-1. The surface morphologies, crystallinities and elemental


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compositions of the BaTaO2N powders were characterized by SEM (SU8020, HITACHI) and HRTEM (JEM-2800, JEOL) instruments equipped with SAED and EDS capabilities. The proportions of Ba, Ta, O and N in the bulk BaTaO2N were estimated using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Shimadzu, ICPS-8100) and oxygen-nitrogen combustion analysis (Horiba, EMGA-620W). The chemical states of Ta on the BaTaO2N photoanodes were determined by XPS (JPS90SX, JEOL) using a non-monochromatic X-ray source producing Mg-Kα emission with a current of 10 mA and an acceleration voltage of 8 kV.

Acknowledgements

This work was supported by the Artificial Photosynthesis Project (ARPChem) of the New Energy and Industrial Technology Development Organization (NEDO) and Grant-in-Aids for Scientific Research (A) (No. 16H02417) and Young Scientists (No. 19K15676) from the Japan Society for the Promotion of Science (JSPS). A part of this work was conducted in association with the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by the Nanotechnology Platform of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors wish to sincerely thank Dr. Taro Yamada



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and Yasuko Kuromiya at the University of Tokyo for performing the ICP-AES and oxygennitrogen combustion analyses, respectively.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Physical and optical properties of prepared BaTaO2N powder, characterization of BaTaO2N photoanodes prepared by a PT method, and supplementary data for photoelectrochemical water oxidation activity over BaTaO2N photoanodes annealed at different temperatures.


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Figure 1. A) XRD patterns for (a) crystalline Ba5Ta4O15 and the resulting oxynitrides prepared by the nitridation of the oxide at 1223 K for (b) 5, (c) 10, (d) 20 and (e) 30 h and (f) after washing with distilled water. The washed oxynitride is referred to as the as-prepared BaTaO2N. B) XRD patterns for (a) the asprepared BaTaO2N and samples annealed in an Ar flow at (b) 873, (c) 973, (d) 1073 and (e) 1173 K for 1 h. The estimated full width at half maximum (FWHM) values for the (110) peak are provided for each specimen. The empty and filled triangles indicate the impurity phases Ta2O5 (ICSD #9112) and Ba2TaO3N (PDF card No. 00-047-1388), respectively.



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Figure 2. TG/DTA data for the as-prepared BaTaO2N powder under Ar and air flows at a ramp rate of 10 K min-1.


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Figure 3. SEM images of (a) Ba5Ta4O15 and (b) BaTaO2N powder as-prepared by nitridation, and HRTEM images of (c) the as-prepared and annealed BaTaO2N particles at (d) 873 and (e) 1073 K for 1 h. The insets show the corresponding SAED patterns. The pattern in (e) corresponds to diffraction from crystalline BaTaO2N along the [211] zone axis. Scale bars, (a,b) 2 μm, (c-e) 5 nm.



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Figure 4. Narrow-scan Ta 4f XPS spectra obtained from BaTaO2N photoanodes (a) as-prepared by the nitridation, and annealed in an Ar atmosphere at (b) 873, (c) 973, (d) 1073 and (e) 1173 K for 1 h.


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Figure 5. LSV data for Co(OH)x-FeOy/BaTaO2N photoanodes (a) as-prepared by nitridation at 1223 K for 30 h and after annealing in an Ar flow at (b) 873, (c) 973, (d) 1073 and (e) 1173 K for 1 h during water oxidation under chopped AM 1.5G sunlight. The PEC data were acquired by sweeping the potential from 1.3 to 0.4 VRHE at a scan rate of 10 mV s-1 in a stirred Ar-saturated 0.5 M KBi aqueous electrolyte at pH 13.



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Rs [kΩ]

CPE-P; n

CPE-T; Q [Ω-1sn]

Rct [kΩ]

(a) as-prepared

0.030

0.92

1.3 × 10-5

302.74

(b) 873 K

0.020

0.90

4.4 × 10-5

34.51

(c) 973 K

0.022

0.92

4.9 × 10-5

14.19

(d) 1073 K

0.021

0.92

1.1 × 10-4

4.47

(e) 1173 K

0.021

0.92

1.0 × 10-4

6.55

Figure 6. Nyquist plots of bare BaTaO2N photoanodes (a) as-prepared by nitridation at 1223 K for 30 h and after annealing in an Ar flow at (b) 873, (c) 973, (d) 1073 and (e) 1173 K. The electrochemical impedance spectroscopy (EIS) measurements were performed in a stirred Ar-saturated 0.5 M KBi aqueous electrolyte at pH 13 under AM 1.5G simulated irradiation at the applied potential of 1.23 VRHE. The EIS data were acquired in the frequency range from 104 to 10-1 Hz at an AC amplitude of 10 mV. The table below summarizes fitting results, namely, resistances and constant phase element (CPE) values, of the Nyquist plots under an equivalent circuit model shown in an inset of the Figure.


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Figure 7. A) LSV data for Co(OH)x-FeOy/BaTaO2N photoanodes as-prepared by nitridation at 1223 K for 30 h and after annealing in Ar at 1073 K for 1 h during (a) water oxidation in a stirred Ar-saturated 0.5 M KBi aqueous electrolyte at pH 9.5 and (b) sulfite oxidation in a stirred Ar-saturated 1 M Na2SO3 aqueous electrolyte at pH 10.2 under chopped AM 1.5G sunlight. The PEC data were acquired by sweeping the potential from 1.3 to 0.1 VRHE at a scan rate of 10 mV s-1. B) Charge separation efficiency, ηsurface (%), on the surface of Co(OH)x-FeOy/BaTaO2N photoanode presented in (a).



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Figure 8. The solar water oxidation performance of a Co(OH)x-FeOy/BaTaO2N photoanode as-prepared by nitridation at 1223 K for 30 h and subsequent to annealing in an Ar flow at 1073 K for 1 h. A) A chronoamperometry curve acquired during long-term water splitting (24 h) at 1.23 VRHE. B) Time courses of O2 and H2 generation over the BaTaO2N photoanode and a Pt counter electrode during the initial 10 h of the water splitting trial shown in (A). The dashed lines indicate the amounts of O2 and H2 expected for a Faradaic efficiency of unity. C) The HC-STH energy conversion efficiency values (%) estimated from the corresponding LSV curve shown in Figure 5(d). D) The IPCE values at applied potentials of 0.8, 0.9 and 1.23 VRHE as functions of wavelength. The UV-vis DRS spectrum of the corresponding BaTaO2N powder is provided for comparison.


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TOC figure


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Figure 1. A) XRD patterns for (a) crystalline Ba5Ta4O15 and the resulting oxynitrides prepared by the nitridation of the oxide at 1223 K for (b) 5, (c) 10, (d) 20 and (e) 30 h and (f) after washing with distilled water. The washed oxynitride is referred to as the as-prepared BaTaO2N. B) XRD patterns for (a) the asprepared BaTaO2N and samples annealed in an Ar flow at (b) 873, (c) 973, (d) 1073 and (e) 1173 K for 1 h. The estimated full width at half maximum (FWHM) values for the (110) peak are provided for each specimen. The empty and filled triangles indicate the impurity phases Ta2O5 (ICSD #9112) and Ba2TaO3N (PDF card No. 00-047-1388), respectively. 277x146mm (300 x 300 DPI)

ACS Paragon Plus Environment


Figure 2. TG/DTA data for the as-prepared BaTaO2N powder under Ar and air flows at a ramp rate of 10 K min-1. 193x138mm (300 x 300 DPI)


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Figure 3. SEM images of (a) Ba5Ta4O15 and (b) BaTaO2N powder as-prepared by nitridation, and HRTEM images of (c) the as-prepared and annealed BaTaO2N particles at (d) 873 and (e) 1073 K for 1 h. The insets show the corresponding SAED patterns. The pattern in (e) corresponds to diffraction from crystalline BaTaO2N along the [211] zone axis. Scale bars, (a,b) 2 μm, (c-e) 5 nm. 243x163mm (300 x 300 DPI)



Figure 4. Narrow-scan Ta 4f XPS spectra obtained from BaTaO2N photoanodes (a) as-prepared by the nitridation, and annealed in an Ar atmosphere at (b) 873, (c) 973, (d) 1073 and (e) 1173 K for 1 h. 130x160mm (300 x 300 DPI)


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Figure 5. LSV data for Co(OH)x-FeOy/BaTaO2N photoanodes (a) as-prepared by nitridation at 1223 K for 30 h and after annealing in an Ar flow at (b) 873, (c) 973, (d) 1073 and (e) 1173 K for 1 h during water oxidation under chopped AM 1.5G sunlight. The PEC data were acquired by sweeping the potential from 1.3 to 0.4 VRHE at a scan rate of 10 mV s-1 in a stirred Ar-saturated 0.5 M KBi aqueous electrolyte at pH 13. 156x156mm (300 x 300 DPI)



Figure 6. Nyquist plots of bare BaTaO2N photoanodes (a) as-prepared by nitridation at 1223 K for 30 h and after annealing in an Ar flow at (b) 873, (c) 973, (d) 1073 and (e) 1173 K. The electrochemical impedance spectroscopy (EIS) measurements were performed in a stirred Ar-saturated 0.5 M KBi aqueous electrolyte at pH 13 under AM 1.5G simulated irradiation at the applied potential of 1.23 VRHE. The EIS data were acquired in the frequency range from 104 to 10-1 Hz at an AC amplitude of 10 mV. The table below summarizes fitting results, namely, resistances and constant phase element (CPE) values, of the Nyquist plots under an equivalent circuit model shown in an inset of the Figure. 162x135mm (300 x 300 DPI)


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Figure 7. A) LSV data for Co(OH)x-FeOy/BaTaO2N photoanodes as-prepared by nitridation at 1223 K for 30 h and after annealing in Ar at 1073 K for 1 h during (a) water oxidation in a stirred Ar-saturated 0.5 M KBi aqueous electrolyte at pH 9.5 and (b) sulfite oxidation in a stirred Ar-saturated 1 M Na2SO3 aqueous electrolyte at pH 10.2 under chopped AM 1.5G sunlight. The PEC data were acquired by sweeping the potential from 1.3 to 0.1 VRHE at a scan rate of 10 mV s-1. B) Charge separation efficiency, ηsurface (%), on the surface of Co(OH)x-FeOy/BaTaO2N photoanode presented in (a). 353x154mm (300 x 300 DPI)



Figure 8. The solar water oxidation performance of a Co(OH)x-FeOy/BaTaO2N photoanode as-prepared by nitridation at 1223 K for 30 h and subsequent to annealing in an Ar flow at 1073 K for 1 h. A) A chronoamperometry curve acquired during long-term water splitting (24 h) at 1.23 VRHE. B) Time courses of O2 and H2 generation over the BaTaO2N photoanode and a Pt counter electrode during the initial 10 h of the water splitting trial shown in (A). The dashed lines indicate the amounts of O2 and H2 expected for a Faradaic efficiency of unity. C) The HC-STH energy conversion efficiency values (%) estimated from the corresponding LSV curve shown in Figure 5(d). D) The IPCE values at applied potentials of 0.8, 0.9 and 1.23 VRHE as functions of wavelength. The UV-vis DRS spectrum of the corresponding BaTaO2N powder is provided for comparison. 361x295mm (300 x 300 DPI)


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TOC figure 274x250mm (300 x 300 DPI)


Solar-Driven Water Splitting over a BaTaO2N Photoanode Enhanced

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