Enhanced Performance of Pristine Ta3N5 Photoanodes for Solar

Article pubs.acs.org/JPCC

Enhanced Performance of Pristine Ta3N5 Photoanodes for Solar Water Splitting by Modification with Fe−Ni−Co Mixed-Metal Oxide Cocatalysts Ashraf Abdel Haleem,*,†,‡ Samit Majumder,† Nagaraju Perumandla,† Zaki N. Zahran,†,§ and Yoshinori Naruta*,†,∥ †

Center for Chemical Energy Conversion Research and Institute of Science and Technology Research, Chubu University, Kasugai, Aichi 487-8501, Japan ‡ Department of Engineering Mathematics and Physics, Faculty of Engineering, Fayoum University, Fayoum, Egypt § Faculty of Science, Tanta University, Tanta, Egypt ∥ JST ACT-C, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Solar water splitting is an alternative way of clean and sustainable hydrogen production. Tantalum nitride (Ta3N5) is one of the promising candidates that recently attracted a great amount of attention as photoelectrodes for solar water splitting. Nevertheless, it suffers severely from photocorrosion in an aqueous solution. Therefore, the precise selection of a cocatalyst, in terms of the material, the amount, and the way of its deposition, is indispensable to highly improve its water splitting performance. In the present work, we introduce a Fe−Ni−Co mixed-metal oxide as a water oxidation cocatalyst that remarkably improved the photocurrent and photostability of the pristine Ta3N5 photoanode. The cocatalyst-modified electrode showed a photocurrent of about 4.0 mA cm−2 at 1.23 V vs RHE in 1 M NaOH. The electrode maintained 100% and 96% of the initial photocurrent after irradiation times of 1 and 2 h, respectively. In addition, a continuous evolution of hydrogen and oxygen occurred for 2 h at quantitative Faraday efficiencies (>96%). This photostability is superior compared to that of the other singlelayer modified Ta3N5 photoanodes reported so far. It is noteworthy that the anodic spark deposition is used to fabricate precursor electrodes (NaTaO3), which then were converted to Ta3N5 by nitridation in an ammonia atmosphere.

1. INTRODUCTION

ysis of tantalum-oxide-based layers. The precursor oxide layers have been prepared in different shapes including nanorods,24 nanotubes,25,26 microcubes,14 and compact films.13 The precursor Ta3O5 or NaTaO3 thin films would be converted to Ta3N5 through thermal annealing in NH3 atmosphere at 850−1000 °C. The anodic spark deposition (ASD) technique is a well developed, an exceedingly understood, and a widely used method in many applications.27 It is an electrochemical oxidation of valve metals (i.e., Ta, Ti, Fe, W, V, etc.) in alkaline or acidic electrolytes. In this process, once the applied voltage exceeds the breakdown limit of the formed highly resistive metal oxide thin layer, sparking starts, and the oxide layer grows further. In the present study, the precursor NaTaO3 thin films have been prepared by the ASD. Subsequently, NaTaO3 thin films were converted to Ta3N5 electrodes through

Nowadays, clean and sustainable energy resources are continuously and urgently required to suppress the emission of greenhouse gases and to satisfy the growing energy demand. Hydrogen fuel is one of the promising solutions. In addition, hydrogen production through photoelectrochemical (PEC) water splitting is an environmentally benign process.1−4 Ta3N5 (Eg = 2.1 eV) absorbs a major part of visible light, and its band edges straddle the redox potentials of water photolysis;5−9 thus, it is a choice among the limited number of photocatalysts. Theoretically, Ta3N5 is able to produce a photocurrent up to 12.5 mA cm−2 and thus split water without any external voltage bias at a solar-to-hydrogen efficiency of about 15%.1,10,11 Consequently, Ta3N5 has recently attracted intensive interest, and several synthetic methods have been developed to configure (oxy)nitride photoelectrodes, including high-temperature nitridation of oxide layers,12−17 reactive magnetron sputtering,18−20 electrophoretic deposition,21 flux coating method,22 and particle transfer.23 So far, the most efficient Ta3N5 photoelectrodes have been prepared by the ammonol© 2017 American Chemical Society

Received: May 9, 2017 Revised: August 25, 2017 Published: August 29, 2017 20093

DOI: 10.1021/acs.jpcc.7b04403 J. Phys. Chem. C 2017, 121, 20093−20100

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The Journal of Physical Chemistry C

deposition, films were ultrasonically cleaned in water then in acetone. Afterward, the prepared NaTaO3 films were converted to Ta3N5 electrodes through thermal annealing in ammonia atmosphere at 1000 °C for 2 h. Deposition of Fe−Ni−Co MMO and Related Catalysts. The (Fe, Co, Ni)-based cocatalysts were deposited according to the reported electrodeposition method40 after some modifications. An aqueous solution containing nickel(II) acetate tetrahydrate (16 mM), anhydrous cobalt(II) chloride (16 mM), and iron(III) sulfate (5 mM) was prepared and sonicated to reach a complete dissolution of the chemicals, and the final pH value of the resultant electrolyte was about 5.3. The cocatalysts were deposited by a light-assisted electrochemical deposition in a three-electrode cell: platinum wire as a cathode, Ag/AgCl (in 3 M NaCl) electrode as a reference, and Ta3N5 electrode as an anode. A voltage sweep voltammetry (in a range of 0.0 to 0.7 V vs Ag/AgCl) was applied for 5−50 cycles under light illumination with intensity of 100 mW cm−2. For comparison, different combinations of mono- and bimetal oxide cocatalysts based on Fe, Ni, and/or Co, namely, FeOx, NiOx, CoOx, FeNiOx, FeCoOx, and NiCoOx, have been deposited onto Ta3N5 electrodes from acetate buffer solution (pH 5.3) using the same linear sweep voltammetry potential and light illumination as that used to deposit the Fe−Ni−Co MMO cocatalyst at the same number of deposition cycles. Deposition of Co-Pi Catalyst. Potassium phosphate buffer solution (0.1 M, pH 7) containing cobalt(II) nitrate (0.5 mM) is used as an electrolyte. Potentiostatic electrodeposition (V = 1 V vs Ag/AgCl) is used to deposit the material onto a Ta3N5 working electrode for 8 min of deposition time. The electrodes were washed with DI water and were left to dry in air.41 Characterization. Field emission scanning electron microscopy (HR-SEM; Hitachi model S-4300) operated at 15 kV accelerating voltage was used to characterize the surface morphology and the cross-section of the thin films. X-ray diffraction (Rigaku RINT-2100/PC) spectra were recorded using Cu Kα radiation (λ = 0.15405 nm, operated at 40 kV and 40 mA). X-ray photoelectron spectroscopy (XPS; Ulvac Phi VersaProbe CU) was executed using Mg Kα radiation at 10 mA and 8 kV. A gas chromatograph Shimadzu GC-2014, equipped with a capillary column (0.53 mm ID × 15 m) bearing a molecular-sieve 5A layer at 40 °C with Ar, was used to quantify the evolved hydrogen and oxygen gases. Electrochemical Measurements. The electrochemical measurements were carried out on an Autolab Potentiostat/ Galvanostat model PGSTAT128N with an impedance measurement unit. A three-electrode cell used for cocatalyst deposition, mentioned herein above, was used to study PEC properties of the bare as well as the cocatalyst-loaded Ta3N5 photoanodes in an alkaline electrolyte (1 M NaOH, pH 13.6). The linear sweep voltammetry (LSV) with a voltage range of 0.5−1.6 V vs RHE at an increasing rate of 20 mV s−1 and the bulk photoelectrolysis at V = 1.23 V vs RHE under dark condition and light illumination (100 mW s−1 from a calibrated 300 W xenon lamp without an AM1.5 filter) conditions were measured. The incident photon-to-current efficiency (IPCE) was measured for Ta3N5 samples modified with Fe−Ni−Co MMO cocatalyst at V = 1.23 V vs RHE under monochromatic light illumination, from a monochromator light source (M10-T, Bunkoukeiki Co., Ltd.), at wavelength range of 400 to 640 nm at intervals of 20 nm. The IPCE was calculated based on the following equation

the ammonolysis at 1000 °C for 2 h. To the best of our knowledge, this is the first study to report the fabrication of Ta3N5 photoanodes by utilizing the ASD technique. There is a single study reporting the fabrication of NaTaO3 by ASD; however, neither the deposition parameters have been optimized nor the work has been extended to fabricate (Ta, N)-based photoanodes.28 To produce porous Ta3N5 thin films, ASD has remarkable advantages over hydrothermal and thermal oxidation techniques that are frequently used, namely, the lowcost deposition due to the short deposition time (∼3 min) and the deposition at room temperature, the anodization in HF-free electrolytes, and the control to fabricate the film on a single side of the Ta sheet through the masking of the undesired areas by a protecting tape. Despite the promising photoactivity of Ta3N5 as a photoanode for water splitting, the photostability is still a challenge due to its photocorrosion. One of the keys to enhance its photostability is the quick transfer of remaining holes in the semiconductor to water oxidation catalysts after photoelectrochemical charge separation. It is worthy to mention that G. Liu et al. have recently reported the most photoactive Ta3N5-based photoanode that was modified with a sophisticated stack of 5 different layers (protective/hole storage/ inorganic catalyst/two successive molecular catalyst layers). Although the initial photocurrent reached about 94% of its theoretical limit, the photostability of that photoanode has to be improved.29 In addition, molecular catalysts are not the optimal choice as OER cocatalysts because they decompose or detach from the electrode surface over time under oxidizing conditions required to drive oxygen evolution.30,31 Consequently, there is a substantial need for heterogeneous (inorganic) OER catalysts based on earth-abundant metals that could be readily deposited with high catalytic activity and stability. Very recently, the Domen group has succeeded in stabilizing the Ta3N5 photoanode for 10 h by the modification of the CoPi/GaN bilayer, thanks to the GaN protection layer; however, CoPi could be replaced by a more active and a more robust cocatalyst.32 It was recently reported that the amorphous Ni−Fe−Co mixed-metal oxide (MMO) OER cocatalyst showed a higher catalytic activity and a long-term stability than those of other (Ni, Fe, Co)-based water oxidation cocatalysts including the state-of-the-art Fe−Ni mixed-metal oxide/hydroxide cocatalysts.33−35 However, to the best of our knowledge, there is no single study reporting the use of this cocatalyst with photoanodes for water splitting. Here, the Ta3N5 photoanode modified with the Fe−Ni−Co MMO cocatalyst showed a superior performance compared to that modified with Co(OH)x,13,36 Co3O4,14,36 Co−Pi,37 Ni(OH)x,38 and Ni−Fe LDH39 cocatalysts, which have been previously reported.

2. EXPERIMENTAL SECTION Preparation of Ta3N5 Films. First, pieces of Ta foil (10 mm × 25 mm × 0.2 mm, 99.95%, Nilaco) were sequentially cleaned by sonication for 10 min in acetone, isopropanol, and DI water. ASD was used to prepare a sodium tantalum mixed oxide (NaTaO3) film onto the Ta foil. In this process, the clean tantalum foil and Pt wire were used as an anode and a cathode, respectively, in a two-electrode setup, and the distance between the two electrodes was about 1 cm. Galvanostatic bias potential with current density of 1.0 A cm−2 was applied; NaOH (3, 6, or 9 M) was used as electrolytes; and the deposition time was 2.0−5.0 min with gentle stirring during the experiment. After 20094


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The Journal of Physical Chemistry C IPCE = [1240 × Iλ /(λ × Pinput)] × 100%

NH3 at 1000 °C for 2 h). The major peaks belong to polycrystalline Ta3N5 material (PDF card no. 79-1533) and Ta substrate (PDF card no. 04-0788). Additionally, there are some minor peaks that belong to tantalum subnitrides, namely, TaN (PDF card no. 39-1485) and Ta2N (PDF card no. 26-0985). These subnitride phases may grow at the interface between Ta3N5 and Ta-substrate as previously reported.37 It is wellknown that these subnitride phases possess a higher electrical conductivity than that of Ta3N5, which could facilitate the transfer of photogenerated electrons to the Ta-substrate and thus improve the photoresponse of the photoanode.37 It is worth mentioning that the XRD study of the cocatalyst-loaded Ta3N5, data not shown, did not show additional peaks, which confirms the amorphous nature of the Fe−Ni−Co MMO layer.35 Figure 2a shows the surface morphology of the Ta3N5 thin film. The film possesses a porous morphology with pores, 200−

(1)

where Iλ (mA cm−2) is the steady-state photocurrent density; λ (nm) is the wavelength of the monochromatic irradiation; and Pinput (mW cm−2) is the incident photon density measured using a Si photodiode detector (S1337-1010BQ, Hamamatsu Photonics K.K.). The electrochemical impedance spectroscopy (EIS) was measured for the bare as well as the cocatalyst-loaded Ta3N5 samples at potential V = 1.23 V vs RHE in 1 M NaOH solution (pH 13.6) under light irradiation. The frequency range was 0.1 Hz to 105 Hz and the amplitude of the sine-wave signal was 10 mV. Faradaic Efficiency. The three-electrode cell was used to measure the Faradaic efficiency. The FeNiCoOx/Ta3 N5 photoanode was biased at 1.23 VRHE in 1 M NaOH under light illumination (100 mW cm−2). The cell was sealed and was purged by Ar for 30 min, and no H2, O2, or N2 was detected before the Faradaic efficiency measurement. Evolved hydrogen and oxygen gases were detected by a gas chromatograph.

3. RESULTS AND DISCUSSION The XRD spectrum (Figure S1) of the as-grown thin film revealed that the precursor layer is mainly a polycrystalline NaTaO3 (PDF card no. 25-0863) containing secondary phases of (Ta, O) material (PDF card no. 19-1299). This result agrees with the previously published report.28 Interestingly, NaTaO3 is preferable to Ta3O5 precursor material to fabricate Ta3N5 electrode; the nucleation of NaTaO3 in the initial stages of nitridation resulted in an improved crystallinity of the resultant Ta3N5.42 The NaOH electrolyte concentrations as well as the deposition time of NaTaO3 were optimized to give the highest photoactivity of the finally fabricated cocatalyst-loaded Ta3N5 photoanode (Figures S2 and S3). SEM cross-sectional images of Ta3N5 fabricated from NaTaO3 that deposited at different NaOH concentrations (3, 6, and 9 M) and deposition times (2, 3, and 5 min) are shown in Figures S4−S6. The film thickness increases with the increase of NaOH molar concentration and with the extension of deposition time. It seems that when the film thickness exceeds a certain limit the PEC properties deteriorate, possibly due to the lower charge transport within a thicker film. The parameters of the optimized deposition are electrolyte 9 M NaOH, deposition time t = 3 min, and DC current density of 1.0 A cm−2. Structure and Morphology. Figure 1 shows the XRD spectrum of the postdeposition annealed bare electrode (in

Figure 2. SEM images of the Ta3N5 grown at galvanostatic condition (1.0 A cm−2) for 3 min in 9 M NaOH electrolyte and annealed in NH3 at 1000 °C for 2.0 h: (a) top and (b) cross-sectional views.

500 nm in diameter, distributed randomly over the film surface. The cross-section observation, shown in Figure 2b, confirms the roughness of the film surface, and thus it is quite difficult to give an exact value for the film thickness (it is in the order of 3−5 μm). In addition, the Ta3N5 thin film is tightly connected to the Ta-substrate since there is no observation of cracks or voids at the interface. The perfect mechanical adhesion of the grown layer was confirmed by its high stability under a long time of sonication in DI water and acetone. Cocatalyst Optimizations. The parameters of the deposition voltage (linear sweep voltammetry, LSV) used to load the cocatalyst onto the Ta3N5 electrodes were studied to give the optimal performance of the FeNiCoOx/Ta 3N 5 photoanode, namely, the starting voltage (V1), the ending voltage (V2), the step voltage (Vstep), and the number of deposition cycles (NC). First optimizing V2, XPS spectra of Fe 2p (Figure S7), Co 2p (Figure S8), and Ni 2p (Figure S9) as well as the relevant atomic percentage calculations (Table S1) revealed that V2 (0.3, 0.5, and 0.7 V) clearly affects the composition of the deposited cocatalysts. Films deposited at V2 = 0.3 V are highly Fe-rich (Fe/Co ∼ 2.5) and almost Ni-free, and those deposited at V2 = 0.7 V are highly Co-rich (Fe/Co ∼ 0.4) cocatalysts, whereas films deposited at V2 = 0.5 V, which resulted in a superior OER activity as shown in Figure S10, contain a moderate Fe/Co ratio (∼0.5) and Ni content at the doping level (roughly 0.38%). In addition, a deconvoluted Fe 2p spectrum (Figure S11) shows peaks centered at about 711.4 and 724.6 eV that can be ascribed Fe3+ species.43 The satellite signal centered at about 718.9 eV is a characteristic peak of Fe3+, and that located at about 714 eV may be attributed to Fe3+ bounded with the

Figure 1. XRD pattern of Ta3N5 electrode grown under a galvanostatic condition (1.0 A cm−2) for 3 min in 9 M NaOH electrolyte and annealed in NH3 at 1000 °C for 2.0 h. 20095


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The Journal of Physical Chemistry C hydroxyl group.44 The measurement revealed that the oxidation state of iron did not change with V2 variation. Furthermore, curve fitting of the Co 2p spectrum (Figure S12) resulted in peaks centered at about 780.1 and 795.8 eV that can be assigned to Co3+, while that centered at about 781.9 and 797.5 eV can prove the existence of Co2+ species.45,46 Second, the effect of V1 values (0.0, 0.1, 0.2, and 0.3 V) on the resulted electrode was studied. Under the fixed values of V2 = 0.5 V and Vstep = 10 mV s−1, cocatalysts deposited at V1 equal to 0.0, 0.1, and 0.2 V have almost similar current−voltage curves; however, the OER activity decreased when V1 reached 0.3 V as shown in Figure S13. The XPS spectra (Figures S14− S16) and atomic percentage calculations (Table S2) pointed out that the film deposited at V1 = 0.3 V is a highly Co-rich cocatalyst (Fe/Co ∼ 0.4), and Fe and Co ions exist as Fe3+ and a mixture of Co2+ and Co3+, respectively, as shown in Figure S17. Consequently, based on the XPS analysis mentioned herein, the iron to cobalt ratio has a remarkable effect on the catalytic activity of the cocatalyst, Fe/Co about 0.5, and resulted in a superior performance. Third, the catalyst deposited at Vstep = 5 mV s−1 resulted in a better water splitting performance than that deposited at Vstep = 10 mV s−1 as shown in Figure S18. The XPS spectra of these two samples are shown in Figures S19−S21, where the peak shape and binding energy position did not change. In addition, the atomic percentages of Fe, Co, and Ni, shown in Table S3, are almost similar between the two samples. However, the peak intensities of Fe 2p, Co 2p, and Ni 2p of the sample deposited at Vstep = 5 mV s−1 are relatively higher than that of Vstep = 10 mV s−1, which may denote the increase of cocatalyst amount per a certain surface area. Accordingly, Vstep = 5 mV s−1 might result in a more compact cocatalyst layer and thus a better catalytic activity than that of Vstep = 10 mV s−1. Lastly, the number of deposition cycles decides the deposited amount of the cocatalyst. The optimal number of deposition cycles is NC = 15 as shown in Figure S22. The cocatalyst could partially block the incident light at a higher number of deposition cycles (e.g., NC = 30 or 20), while the lower number of deposition cycles (e.g., NC = 5 or 10) is likely not enough to produce a compact layer of the cocatalyst. The high-resolution SEM images shown in Figure S23 could support this explanation; the surface morphology details (the grain boundary lines) of the bare Ta3N5 film were partially disappeared by the loading of a very thin layer after 15 cycles and completely disappeared by loading a thicker cocatalyst layer at NC = 30 cycles. Furthermore, in order to investigate the significance of each metal in the Fe−Ni−Co MMO catalyst, Ta3N5 electrodes were modified with different combinations of (Fe, Ni, Co)-based metal or mixed-metal oxide OER cocatalysts at the same deposition cycles. To avoid the effect of the pH value on the deposited materials, reagents were dissolved in acetate buffer solution pH 5.3. The PEC properties of cocatalyst-loaded electrodes, which were studied in details elsewhere,47 have been measured. This study showed that CoOx is a superior OER cocatalyst among the other monometal oxides in terms of photoactivity, shown in Figure S24, and photostability, shown in Figure S25. Concerning the bimetal oxides, the NiCoOx cocatalyst resulted in a higher photoactivity, shown in Figure S26, and photostability, shown in Figure S27, than that of FeCoOx and FeNiOx. It is worthy to mention that the Co ion plays a crucial role to improve the PEC water splitting properties of Ta3N5 photoanodes since the presence of cobalt

resulted in a higher performance compared to other monometal oxides and bimetal oxide cocatalysts. This observation agrees well with the previously reported highly active Ta3N5 photoanodes since all of them were loaded with a cobalt-containing modifying layer as exhibited in Table S4. The minor oxidation peak that appeared in current−voltage curves of CoOx (Figure S23) and NiCoOx (Figure S25) at about 0.6 V vs RHE (below the onset potentials) can ascribe Co2+/Co3+ transition.48 The oxidation peak disappeared after adding Fe to the films; this may indicate that iron stabilizes higher oxidation state of cobalt after the initial oxidation process.34 In addition, adding Fe to the NiCoOx cocatalyst resulted in a cathodic shift of the onset potential by ∼150 mV and a sharp increase of the photocurrent beyond the onset potential. On the other hand, the current−voltage curve of FeNiOx cocatalyst did not reach a saturation state at voltages up to 1.6 V; this might point out the slow rate of charge transfer between the cocatalyst and Ta3N5. Interestingly, although the percentage of Ni-metal in the FeNiCoOx catalyst is very low based on the XPS study, the Ni ion plays a key role to improve the water oxidation activity of the cocatalyst; the photocurrent of FeNiCoOx/Ta3N5 is about 50% higher than that of FeCoOx/Ta3N5, as shown in Figure S27. It is interesting to note that the atomic ratio of iron to cobalt did not change significantly from FeCoOx to FeNiCoOx as shown in Table S5. Additionally, the inclusion of nickel did not alter the oxidation states of iron (Fe3+) and cobalt (Co2+ and Co3+) as revealed from peak-fitting Fe 2p and Co 2p core levels, shown in Figures S28 and S29, respectively. This study denoted the synergistic effect among the constituent metals of Fe−Ni−Co MMO cocatalyst. Optimized Fe−Ni−Co MMO Cocatalysts. The optimized conditions of Fe−Ni−Co MMO deposition are V1 = 0.1 V, V2 = 0.5 V, Vstep = 5 mV s−1, and NC = 15 cycles. This means that the proper potential window lying between 0.6 and 1.0 V vs RHE is less positive than the onset potential of the pristine Ta3N5 photoanode (Vonset ∼ 1.0 VRHE). Accordingly, the photogenerated holes are mostly used to oxidize the metal ions located at the electrode/electrolyte interface (not to oxidize water) and thus, a uniform deposition of Fe−Ni−Co MMO on the electrode surface could occur.49 The surface properties of the Fe−Ni−Co MMO cocatalyst deposited on Ta3N5 under the optimized conditions were explored by XPS measurements. Figure 3 shows the XPS spectra of the Fe 2p, Co 2p, and Ni 2p core levels. The iron ion takes a trivalent oxidation state (Fe3+). Curve fitting of the Co 2p3/2 peak demonstrates the coexistence of Co2+ and Co3+. The Ni 2p signal is too weak to perform a curve fitting and define the Ni oxidation state. The atomic percentages, calculated based on 2p3/2 spectra, of iron, cobalt, and nickel, are tentatively equal to 30.93, 68.67, and 0.4, respectively. Photoelectrochemical Studies. The PEC water-splitting properties of the pristine Ta3 N5 and FeNiCoOx/Ta3 N5 photoanodes are given in Figure 4. In addition, PEC properties of a Ta3N5 electrode modified with a well-documented OER cocatalyst (Co-Pi) are also shown for comparison. Since Co-Pi OER has been reported by the Nocera group,50 it has been widely used as a cocatalyst to improve the photoactivity and photostability of various photoanodes.24,51−56 Figure 4a shows the current−potential curves of the three electrodes. Under the dark condition, samples show negligible currents in the selected voltage range 0.4−1.6 V vs RHE (all voltage values hereafter are referred to RHE unless otherwise noted). Under the light 20096


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0.77 V) and the photocurrent increased by five times (Jphoto = 2.5 mA cm−2) at a voltage 1.0 V compared to that of the Co-Pi/ Ta3N5 electrode. It is worthy to note that the current−potential curve of the FeNiCoOx/Ta3N5 electrode has a sharper increase beyond the onset potential (0.77−1.2 V) and a higher tendency for saturation in the high-potential region (1.2−1.6 V) than the other two electrodes. This might denote the higher chargetransfer and collection efficiency of the FeNiCoOx/Ta3N5 electrode compared to the other counterparts. In addition, Figure 4b shows the bulk photoelectrolysis of the photoanodes measured at 1.23 V under light irradiation. The pristine electrode shows a low current with a significant decay with time due to the severe photocorrosion, and the photocurrent almost disappeared after 1000 s. The Co-Pi/Ta3N5 photoanode shows a higher photocurrent (∼2.0 mA cm−2); however, it suffers from a remarkable photocorrosion, and only ∼66% of the initial photocurrent remained after 1 h of photoirradiation. The photocurrent, however, as well as the stability of the Ta3N5 was remarkably improved after Fe−Ni−Co MMO loading. The initial photocurrent of FeNiCoOx/Ta3N5 is 2 times higher than that of the Co-Pi/Ta3N5 photoanode. Interestingly, FeNiCoOx/Ta3N5 showed an improved photostability; exactly 100% and 96% of the initial photocurrent remained after 1 and 2 h of photoillumination, respectively, shown in Figure S30. Table S4 shows the comparison of PEC properties of the most interesting Ta3N5 photoanodes that have been recently reported. These Ta3N5 photoanodes could be categorized based on the modifying material into two sets; in set 1 the Ta3N5 photoanode was directly modified with the cocatalyst (the present work lies in this category), and in set-2 the Ta3N5 electrode was coated with a hole-storage and/or a protection layer before depositing the cocatalyst (shaded area in the table). The electrochemical impedance spectroscopy (EIS) measurement was carried out in order to explore the interfacial properties between the electrode and the electrolyte, and the Nyquist plot as well as the equivalent circuit are shown in Figure 5. The semiconductor−electrolyte charge-transfer resistance, Rct, is a key parameter to characterize the semiconductor−electrolyte charge transfer process. The fitted values of Rct yield 25.98 KΩ, 2,080 Ω, and 640 Ω for the Ta3N5, Co-Pi/Ta3N5, and FeNiCoOx/Ta3N5 photoanodes, respectively. This means that the Rct value of the FeNiCoOx/Ta3N5 electrode is about 40 and 3 times lower than that of bare Ta3N5

Figure 3. XPS spectra measured at the surface of FeNiCoOx-modified T3N5 electrode of (a) Co 2p, (b) Fe 2p, and (c) Ni 2p.

Figure 4. PEC properties of bare Ta3N5 (red-dotted), Co-Pi/Ta3N5 (blue-dashed), and FeNiCoOx/Ta3N5 (black-solid) electrodes under dark and light conditions (100 mW cm−2) from a 300 W xenon lamp. 1 M NaOH (pH 13.6) electrolyte is used for bare and FeNiCoOxmodifed electrodes and 0.5 M K2HPO4 (pH 13.6) in the case of CoPi/Ta3N5 (a) current−voltage curves and (b) current−time curves at 1.23 VRHE.

irradiation, the pristine and the Co-Pi/Ta3N5 photoanodes showed PEC performances with onset potentials 1.0 and 0.92 V, respectively. The Co-Pi/Ta3N5 photoanode showed a photocurrent of 0.58 mA cm−2 at a bias potential of 1.0 V (the onset potential of the pristine electrode). The PEC properties of the Ta3N5 were significantly improved after the Fe−Ni−Co MMO cocatalyst loading; the onset potential was shifted cathodically by 150 mV (Vonset =

Figure 5. EIS of bare Ta3N5 (red), Co-Pi/Ta3N5 (blue), and FeNiCoOx/Ta3N5 (black) electrodes under 1.23 VRHE and light (100 W cm−2) in 1 M NaOH. Inset shows the equivalent circuit. Rs represents the solution resistance; the capacitance C and resistance Rct characterize the charge-transfer behavior across the electrode− electrolyte interface. 20097


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performance as a photoanode for water splitting. It is worthy to mention that the water splitting performance of the FeNiCoOx/Ta3N5 photoanode could be improved further if the surface area of Ta3N5 is increased (e.g., nanostructures).

and Co-Pi/Ta3N5 electrodes, respectively. Subsequently, Fe− Ni−Co MMO modification significantly reduced the charge transfer resistance and thus reduced the recombination of photogenerated carriers. This could illustrate, to a high extent, the superior performance of the FeNiCoOx/Ta3N5 photoanode compared to the other counterparts. Figure 6a shows the wavelength dependence of the incident photon-to-current conversion efficiency (IPCE) measurements

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04403. XRD and optimization of the deposition condition (NaOH electrolyte concentration and the deposition time) of the precursor NaTaO 3 electrode, the optimization of the FeNiCoOx deposition parameters (sweep voltage and the number of deposition cycles), and the PEC properties of the Ta3N5 loaded with different combinations of (Fe, Ni, Co)-based cocatalyst were demonstrated and XPS studies of cocatalysts deposited at different conditions (PDF)

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Figure 6. IPCE values (black). The integrated photocurrent estimated by integrating the IPCE values over the standard solar spectrum of the AM 1.5G (ASTM G173−03) is also shown (red dash-dotted) (a) and the gas (hydrogen and oxygen) evolution measurement (b) of FeNiCoOx/Ta3N5 photoanode at 1.23 VRHE potential under Xenon light (100 mW cm−2).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] and [email protected]. *E-mail: [email protected]. ORCID

Ashraf Abdel Haleem: 0000-0003-1397-6996 Zaki N. Zahran: 0000-0003-0371-1089

and the integrated photocurrent that is theoretically estimated by integrating the IPCE over the standard solar spectrum of the ASTM G173-03 for the FeNiCoOx/Ta3N5 photoanode at 1.23 V. The IPCE is about 45% in a wavelength range of 400−480 nm, and the photoresponse wavelength is up to ≈620 nm, which is close to the absorption edge of Ta3N5 as shown in Figure S31. The integrated and the actually measured photocurrents equal 4.3 and 4.0 mA cm−2, respectively. This could confirm the reliability of the present IPCE measurement. Figure 6b shows the detected amount of hydrogen and oxygen gases evolved at Pt and FeNiCoOx/Ta3N5 electrodes, respectively, during 2 h illumination (100 mW cm−2) at 1.23 VRHE in 1 M NaOH. The measurement revealed that a continuous production of hydrogen and oxygen, in the stoichiometric ratio 2:1, occurred with Faraday efficiencies of 98% and 96%, respectively. Such high efficiencies confirmed that the photogenerated holes of Ta3N5 were mainly consumed to oxidize water, thanks to the Fe−Ni−Co MMO cocatalyst.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS YN acknowledge JST ACT-C (Grant No. JPMJCR12YV), JSPS KAKENHI (Grant No. 23245035), and a Chubu University Research Grant for financial support.

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REFERENCES

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4. CONCLUSIONS Polycrystalline NaTaO3 thin films were prepared onto tantalum substrates by the anodic spark deposition. Subsequently, the deposited films were converted to polycrystalline Ta3N5 through thermal annealing in NH3 atmosphere. To the best of our knowledge, this is the first study to report the preparation of (Ta, N)-based photoelectrodes based on the anodic spark deposition. The pristine Ta3N5 electrodes were directly modified with the Fe−Ni−Co mixed-metal oxide (MMO) cocatalyst by the photoassisted electrochemical deposition. The modified photoanodes showed a photocurrent ∼4 mA cm−2 at 1.23 VRHE, and 100% and 96% of its initial value remained after 1 and 2 h of photoirradiation, respectively. The modified photoanode produced hydrogen and oxygen gas continuously for 2 h at quantitative Faraday efficiencies (>96%). This means that the present work presented new findings that could help to drive the Ta3N5 to a higher level of 20098


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