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Article Cite This: ACS Omega 2019, 4, 7815−7821
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Preparation of Nanostructured Ta3N5 Electrodes by Alkaline Hydrothermal Treatment Followed by NH3 Annealing and Their Improved Water Oxidation Performance Ashraf Abdel Haleem,*,†,‡ Nagaraju Perumandla,† and Yoshinori Naruta*,† †
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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, 63514, Egypt S Supporting Information *
ABSTRACT: Solar water splitting is a clean and sustainable process for green hydrogen production. It can reduce the fossil fuel consumption. Tantalum nitride (Ta3N5) is one of the limited candidates of semiconductors, which absorb a broad range of visible light and are thermodynamically able to split water without external bias potential. In the present work, we introduce a facile method to prepare a nanostructured Ta3N5 photoanode in a two-step process: hydrothermal deposition of perovskite-type NaTaO3 in a hydrofluoric acid-free NaOH aqueous solution followed by heat treatment in NH3 atmosphere. The resulted bare Ta3N5 electrode was subsequently modified with a Ni-doped CoFeOx (Ni:CoFeOx) as a water oxidation catalyst. After the cocatalyst loading, the electrode shows a photocurrent of about 5.3 mA cm−2 at 1.23 V vs reversible hydrogen electrode. The electrode maintained about 82% of its initial photocurrent after 7 h irradiation. In addition, a continuous oxygen evolution occurred for 3 h at Faraday efficiency of 96%. This performance is superior to that of the single-layer-modified Ta3N5 photoanodes reported so far. This remarkable improvement on the photochemical performance could be due to the uniform nanostructured surface morphology of the present Ta3N5 photoanode. Other alkaline salt treatments, such as LiOH and KOH, do not give such nanostructured morphology and accordingly exhibit lower performance than the one treated in NaOH.
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INTRODUCTION Solar water splitting is admittedly a promising alternative process of hydrogen production due to its cleanness and sustainability.1−4 Further studies, however, are required to improve the overall efficiency, the durability, and the cost of the photoconversion materials, as well as the optimization of the water splitting systems for their practical use. Tantalum nitride (Ta3N5) is a promising choice among the limited candidates of photoanodes, which absorb majority of visible light and are thermodynamically able to split water without any external bias potential.5,6 Therefore, Ta3N5 has recently attracted attentions to its solar water splitting application.7−14 The photochemical properties of the semiconductor photoelectrodes are governed by several factors including crystallinity, surface area, electrical conductivity and charge transport, carrier density and lifetime, band gap, and band edges. Particularly, the increased surface area of the nanostructured photoelectrodes increases the amount of cocatalyst loaded on the electrode surface. Subsequently, improved water splitting performance can be achieved. Nevertheless, there are a limited number of attempts to prepare nanostructured Ta 3 N 5 photoelectrodes for solar water splitting applications. Ta3N5 is basically prepared by nitrization of the corresponding precursor oxides.15 Thus, the preparation of TaOx-based precursors with improved properties is indispensible to the © 2019 American Chemical Society
production of Ta3N5 photoelectodes exhibiting an improved photoelectrochemical (PEC) water splitting performance. There have been several attempts to produce TaOx-based precursors in different surface morphologies. For instance, nanotube arrays of the tantalum oxide precursor electrodes were grown by electrochemical anodization technique in a hydrofluoric acid (HF)-containing highly acidic solution.16−19 Furthermore, nanorod arrays were deposited by the following three methods: the through-mask anodization in an acidic solution at a direct current bias voltage as high as 650 V, hydrothermal technique in a HF-containing solution at an elevated temperature (240 °C) for 24 h, or vapor-phase hydrothermal induced self-assembly technique in HF-containing bath at 200 °C for 6 h or 180 °C for 24 h.20−24 In our recent publication, we utilized the anodic spark deposition (ASD), which is a facile technique, to fabricate a highly active and a tightly adhesive Ta3N5 layer on the Ta substrate.25 However, the control on the surface morphology of the resultant electrodes is still challenging in the use of the ASD. Received: February 10, 2019 Accepted: April 17, 2019 Published: April 30, 2019 7815
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In summary, NaOH and KOH alkaline salts react actively with the Ta sheet in the hydrothermal process and result in the formation of the TaOx-based material. The optimal deposition conditions of the precursor TaOx-based sample in NaOH solution are (NaOH 23 wt %; 250 °C; 5 h) and that for KOH is (KOH 16 wt %; 150 °C; 1 h). For simplicity, Ta3N5 samples, whose precursor films were prepared through the optimal deposition condition in NaOH and KOH solutions, are named S−NaOH and S−KOH, respectively. Characterizations of Alkaline-Treated Precursor Electrodes. Structural Measurements. The X-ray diffraction spectra measurement of the precursor electrodes shows that the precursor film grown in NaOH solution at 250 °C for 5 h is a single-phase NaTaO3 (PDF card no. 25-0863) with perovskite structure as shown in Figure S8 and that grown in the KOH solution at 150 °C for 3 h is a single-phase pyrochlorate-type K2Ta2O6 (PDF card no. 35-1464) as shown in Figure S9. Therefore, the precursor NaTaO3 and K2Ta2O6 electrodes are most likely formed in a two-step hydrothermal process: initially, a Ta2O5 thin film was formed on top of the Ta substrate and Ta2O5 was finally converted to NaTaO3 or K2Ta2O6 in NaOH or KOH solution, respectively.28,29 In addition, Figure 1 shows the X-ray diffraction spectra of the
As another way, hydrothermal treatment of Ta metal in alkaline solutions has been reported to prepare (Ta, Na)Ox25−27 and (Ta, K)Ox.28 Generally, Ta oxide formation from metal Ta requires a higher concentration of bases and elevated temperature. In the present study, we optimized the preparation conditions of MTaOn precursors (M = Na, K) to enhance the PEC water oxidation performance (photocurrent and photostability) of the resultant Ta3N5 electrodes. In summary, the NaTaO3 precursor were prepared in a pure NaOH aqueous solution (23 wt %) at 250 °C for 5 h, at higher temperature and longer reaction time than those of the reported ones.25−27 These conditions gave the best PEC water oxidation performance among the others, after the conversion to the corresponding Ta3N5 and modification with the cocatalyst. To the best of our knowledge, the present study is the first report of the preparation of nanostructured Ta3N5 photoanodes through the hydrothermal method in a F−-free alkaline solution.
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RESULTS AND DISCUSSION Optimization of Hydrothermal Conditions. For the preparation of MTaOx, we optimized the key factors of the hydrothermal process, i.e., alkaline salt concentration, temperature, and reaction time. First, in the use of NaOH solution, samples deposited in baths containing 23 wt % NaOH solution under the following two conditions: at 250 °C, 5 h and at 200 °C, 8 h. Their nanostructures surface morphologies are shown in Figure S2. The figure reveals that the nanostructures are more distinguishable on samples deposited at the higher temperature (250 °C). This could explain the higher photoactivity of samples deposited at 250 °C (Figure S3). However, even at temperatures as high as 250 °C, the samples deposited in solutions with lower NaOH concentration (e.g., 16 wt %) and those deposited at shorter deposition time (e.g., 3 h) did not show clear nanostructures at their surfaces, as shown in Figures S4 and S5, respectively. Additionally, uniform TaOx precursor samples could not be successfully deposited at lower deposition temperatures (e.g., 150 °C) or at lower NaOH concentrations (e.g., 13 wt %). When KOH solution is used, the oxidation reaction of the Ta foil in the solution seems much faster than that in NaOH, since uniform TaOx-based precursor samples were deposited even at 150 °C for a short deposition time of 1 h. The resultant cocatalyst-loaded Ta3N5 sample showed remarkable photoactivity (∼4.0 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE)), as shown in Figure S6. In addition, the figure reveals that the photoactivity was relatively deteriorated when the deposition time was extended to 3 h. This could be due to the formation of microcracks at the film−substrate interface, as shown in Figure S7. Furthermore, the figure reveals that lower temperature (e.g., 120 °C) is not suitable to deposit samples with acceptable performance. It is worthy to mention that films grown at higher temperature (200 °C) were fragile and partially washed out during postdeposition cleaning. This could be due to the increased growth rate of TaOx-based precursor in KOH at such high temperature. When LiOH solution is used, TaOx-based precursor thin film growth is very slow even at 250 °C for 24 h. After that treatment, the resultant Ta metal surface showed metallic luster. Thus, the LiOH treatment is excluded from our further study.
Figure 1. X-ray diffraction (XRD) patterns of Ta3N5, whose precursor electrodes are grown (a) in 23 wt % NaOH solution at 250 °C for 5 h and (b) in 16 wt % KOH solution at 150 °C for 1 h after annealing in NH3 at 1000 °C for 2 h.
two films after thermal annealing in ammonia atmosphere. In both cases, all peaks are of polycrystalline Ta3N5 (PDF card no. 79-1533), whereas other peaks denoted the presence of the tantalum subnitride phases, i.e., TaN (PDF card no. 39-1485) and Ta2N (PDF card no. 26-0985) in addition to another peak that represents the Ta substrate (PDF card no. 04-0788). The subnitride phases are most likely grown at the interface between Ta3N5 and Ta substrate as previously reported.22 It is widely accepted that the electrical conductivity of the subnitride phases is higher than that of Ta3N5 phase.22 The presence of such a conductive thin layer at the interface could facilitate the transfer of the photogenerated electrons from Ta3N5 to the Ta substrate and thus enhance the photoactivity of the photoanode. Morphology Measurements. Figure 2a−d shows the surface morphologies and the cross-sectional images of the produced Ta3N5 electrodes. The measurements denoted that the S−NaOH electrode is composed of homogeneously distributed nanoparticles of size 50−100 nm on the film surface as shown in Figure 2a. The low-magnification surface image (Figure S10) revealed that the film is generally smooth without voids or cracks. The cross-sectional image (Figure 2b) showed that the film thickness is about 7.0 μm, and there is no 7816
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surface morphology with an increased surface area due to the nanostructure formation and a stronger mechanical adhesion to the substrate. Cocatalyst Loading. Pristine S−NaOH and S−KOH samples were modified with Fe−Ni−Co mixed metal oxides, as a water oxidation catalyst, for photoelectrochemical water oxidation measurements. The X-ray photoelectron spectra (XPS) of the Fe 2p, Co 2p, and Ni 2p core levels are shown in Figure S12. The atomic percentages of Fe, Co, and Ni metals, calculated based on the 2p3/2 spectra of iron, cobalt, and nickel, are about 39.0, 59.0, and 2.0, respectively. This result agrees well with the quantitative study of the deposited cocatalyst by the inductively coupled plasma (ICP) analysis, which revealed that the atomic percentages of Fe, Co, and Ni are about 36.2, 56.5, and 7.3%, respectively. Therefore, the cocatalyst is mainly Ni-doped Co−Fe mixed metal oxides, abbreviated as Ni:CoFeOx. In the present study, it is worthy to mention that the loading of this cocatalyst is imperative to achieve a high solar water oxidation performance. Figure S13 reveals that the activity of the bare Ta3N5 is drastically improved after the cocatalyst loading. Concerning the stability, the photocurrent of the bare Ta3N5 electrode, unsurprisingly, almost vanished after few minutes of light illumination, data not shown. Photoelectrochemical (PEC) Measurements. The PEC water oxidation properties of the cocatalyst-modified Ta3N5 (Ni:CoFeOx/Ta3N5) of S−NaOH and S−KOH photoanodes are shown in Figure 3, which shows the current−potential curves of the two electrodes. Under dark conditions, the samples show negligibly low current in the selected voltage range 0.5−1.23 V vs RHE (hereafter, all voltages referred to RHE are shown as VRHE). Under light irradiation, the two samples showed remarkable PEC activity with onset potentials of about 0.8 and 0.76 V for Ni:CoFeOx/S−KOH and Ni:CoFeOx/S−NaOH, respectively. It is noteworthy that the current Ni:CoFeOx/S−NaOH sample has a sharp increase with the applied voltage; however, that of Ni:CoFeOx/S− KOH tends to saturate as the voltage increases. The increased surface area of S−NaOH could be due to its nanostructured surface morphology that facilitates charge separation at the electrode−electrolyte interface. The amount of the loaded cocatalyst on the S−NaOH and S−KOH samples based on the ICP measurements are ∼16.36 and ∼11.22 μg cm−2, respectively. These results confirm that S−NaOH has a larger surface area than S−KOH. In addition, the Mott−Schottky plots (Figure S14), which are derived from the electrochemical
Figure 2. Scanning electron microscopy (SEM) images of Ta3N5 whose precursor electrodes grown in 23 wt % NaOH solution at 250 °C for 5 h ((a) top and (b) cross-sectional views) and in KOH solution at 150 °C for 1 h ((c) top and (d) cross-sectional views).
observation of voids or cracks at the film/substrate interface that may denote the tight adhesion of the film to the substrate. The perfect mechanical connection between the film and the substrate was confirmed by the high stability of the film under 2 min of sonication in pure water and acetone sequentially. However, the top-view image of the S−KOH sample (Figure 2c) does not show distinguished nanoparticles on the film surface. Moreover, the low-magnification surface image of this sample, shown in Figure S11, revealed that the film is generally rough with obvious superficial cracks. Regarding the film adhesion to the base metal, the film of S−KOH sample seems not strong: most of the materials left the surface during the 2 min ultrasonic treatment. This observation becomes clear when the hydrothermal deposition time extended to 3 h: obvious voids and cracks are generated at the film−substrate interface as shown in Figure S7. It is worthy to conclude that the S−NaOH compared with the S−KOH sample has a better
Figure 3. (a) Current−voltage curves and (b) current−time curves at 1.23 VRHE of Ni:CoFeOx-loaded Ta3N5, whose precursor electrodes grown in the 23 wt % NaOH solution at 250 °C for 5 h (black) and in the 16 wt % KOH solution at 150 °C for 1 h (red) under dark (broken) and light (solid) conditions (100 mW cm−2) in 1 M NaOH (pH 13.6) electrolyte. 7817
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carriers. Accordingly, the photoactivity and the photoelectrochemical stability of the electrode were remarkably improved. Additionally, the capacitance C is 998.2 and 11.7 μF and the series resistance RS is 10.63 and 8.93 Ω for the Ni:CoFeOx/S−NaOH and Ni:CoFeOx/S−KOH photoanodes, respectively. The presence of the low-resistive subnitrides (namely, TaN, Ta2N) at the Ta−Ta3N5 interface may explain these relatively low series resistances. Water Oxidation Performance. Since the nanostructured Ni:CoFeOx-modified S−NaOH samples possess remarkably improved photocurrent and stability, the wavelength dependence of the incident photon-to-current conversion efficiency (IPCE) and the gas detection measurements have been additionally performed for them to examine their water oxidation performance. Figure 5a shows the IPCE and the integrated photocurrent that is theoretically estimated by integrating the IPCE over the standard solar spectrum of the ASTM G173-03 for the Ni:CoFeOx/S−NaOH sample at 1.23 V. The resultant IPCE is about 55% in a wavelength range of 400−520 nm and the photoresponse wavelength is up to ≈620 nm, which is close to the absorption edge of Ta3N5 as shown in Figure S15. The theoretically calculated and the actually measured photocurrents are 5.5 and 5.3 mA cm−2, respectively. This reveals the reliability of our IPCE measurement. Additionally, the calculated value of the applied bias photonto-current efficiency (ABPE) is about 0.83% at 1.0 V vs RHE. This efficiency is slightly lower than the recently reported value (1.0%) for the CoPi/Ta3N5 photoanode, in which Ta3N5 was prepared by sputtering−nitridation process.12 However, we owe the advantage of the present work to the simplicity, the cost-effectiveness, and the ability of large-scale preparation of the hydrothermal method. Figure 5b shows the detected amount of oxygen gas evolved at the Ni:CoFeOx/S−NaOH electrode during 3 h of illumination (100 mW cm−2) at 1.23 VRHE in 1 M NaOH. The measurement showed a continuous evolution of oxygen, with the Faraday efficiency of 96%. Such a high efficiency confirmed that the majority of photogenerated holes of Ta3N5 were effectively consumed to oxidize water. Simultaneously, vigorous evolution of oxygen and hydrogen gas bubbles was observed during the experiment on the Ni:CoFeOx/S−NaOH electrode and platinum electrode, respectively.
impedance spectroscopy (EIS) measurements of bare S− NaOH and S−KOH samples under dark condition, revealed that the donor concentration of the S−NaOH sample is clearly higher than that of the S−KOH one, 1022 and 1020 cm−3, respectively. Therefore, the increased surface area and the higher donor concentration of the S−NaOH sample compared to those of the S−KOH sample may explain its superior photoactivity. In the next, photocurrents of cocatalyst-modified Ta3N5 electrodes were observed in the 1 M NaOH solution. Figure 3b shows the bulk photoelectrolysis of the two samples measured at 1.23 V under light irradiation. The Ni:CoFeOx/ S−KOH electrode shows an initial photocurrent of about 4.3 mA cm−2, which decreased about 50% of the initial value after 3 h due to significant photocorrosion. However, the Ni:CoFeOx/S−NaOH sample has a significantly increased photocurrent (initially ∼5.3 mA cm−2 ) compared to Ni:CoFeOx/S−KOH. The stability of S−NaOH is remarkably better than that of Ni:CoFeOx/S−KOH: about 82% of the initial current remains after about 7 h of photoreaction. To the best of our knowledge, the water oxidation performance of the present Ni:CoFeOx/S−NaOH sample is superior to that of other nanostructured Ta3N5-based photoelectrodes reported so far, as shown in Table S1. Thus, the nanostructured surface morphology of the current Ni:CoFeOx/S−NaOH electrode leads to remarkable improvement on the PEC water oxidation efficiency of the photoanode compared to those of the previously reported (Ta3N5-ASD) by our group, although the two samples have been modified with the same electrocatalyst: the current increased by about 35% and the stability test extended to 7 h instead of only 2 h. This could be mainly due to the increased surface area of the nanostructure morphology of the current photoanode. Electrochemical Impedance Spectroscopy (EIS). The measurement aims at examining the interfacial properties between the electrode and the electrolyte; the Nyquist plot as well as the related equivalent circuit are shown in Figure 4.
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CONCLUSIONS Single-phase NaTaO3 and K2Ta2O6 thin films were successfully prepared onto tantalum substrates by the hydrothermal reaction in the NaOH and KOH solution, respectively. The as-grown electrodes were converted to Ta3N5 through heat treatment in NH3 atmosphere at 1000 °C for 2 h. The Ta3N5 sample, whose precursor, the TaOx-based electrode, was produced by optimized hydrothermal deposition in NaOH solution (S−NaOH), possesses nanostructured morphology; however, that whose precursor electrode was perpetrated in KOH solution (S−KOH) shows a rough morphology without distinguished nanostructures at its surface. To the best of our knowledge, this is the first study to report the preparation of nanostructured Ta3N5 photoanodes through the hydrothermal treatment in HF-free medium. The bare Ta3N5 electrodes were simply modified with the Ni:CoFeOx cocatalyst by the photoassisted electrochemical deposition. The cocatalystmodified S−NaOH photoanode showed a photocurrent ∼5.3 mA cm−2 at 1.23 VRHE, and about 82% of its initial value remained after about 7 h of photoirradiation. The modified
Figure 4. EIS of Ni:CoFeOx-modified Ta3N5, whose precursor electrodes grown in 23 wt % NaOH solution at 250 °C for 5 h (black) and in 16 wt % KOH solution at 150 °C for 1 h (red) at 1.23 VRHE and light (100 mW cm−2) in 1 M NaOH. Inset: The equivalent circuit. Rs represents the solution resistance; the capacitance C and resistance Rct characterize the charge-transfer behavior across the electrode−electrolyte interface.
Based on the widely accepted equivalent circuit of the Ta3N5based photoanode, the semiconductor−electrolyte chargetransfer resistance, Rct, is the key parameter to characterize the semiconductor−electrolyte charge-transfer process.10,19 The fitted values of Rct are 445.2 and 1105 Ω for the Ni:CoFeOx/S−NaOH and Ni:CoFeOx/S−KOH photoanodes, respectively. This means that the formation of nanostructures reduced the charge-transfer resistance remarkably and thus decreased the recombination of photogenerated 7818
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Figure 5. (a) IPCE values, the integrated photocurrent estimated by integrating the IPCE values over the standard solar spectrum of the AM1.5G (ASTM G173-03), are also shown and (b) the theoretically estimated and the evolved oxygen gas of Ni:CoFeOx/Ta3N5 photoanode at 1.23 VRHE potential under light (100 mW cm−2) quantified by GC.
Ulvac Phi VersaProbe CU) using Mg Kα radiation at 10 mA and 8 kV was utilized. Inductively coupled plasma (ICP) spectrometer (Rigaku ICPE-9810) was used to quantify the amount of loaded cocatalyst on the photoelectrode surface. A gas chromatograph Shimadzu GC-2014, equipped with a capillary column (0.53 mm i.d. × 15 m) bearing molecularsieve 5A layer at 40 °C with Ar, was used to quantify the evolved oxygen gas. Electrochemical Measurements. The measurements were carried out using Autolab Potentiostat/Galvanostat (model PGSTAT128N) with an electrochemical impedance measurement setup. The three-electrode cell that was used to deposit the cocatalyst was utilized to examine the PEC properties of the samples in an alkaline electrolyte (1 M NaOH, pH 13.6). The photoactivity and the photostability of the samples were examined under the linear sweep voltammetry (with a voltage range of 0.5−1.23 V vs RHE at a sweeping rate of 20 mV s−1) and a constant voltage (V = 1.23 V vs RHE) under dark and light illumination (100 mW cm−2 from a calibrated 300 W Xenon lamp without an AM1.5 filter). Incident Photon-to-Current Efficiency (IPCE). IPCE was measured for the Ni:CoFeOx-modified Ta3N5 photoanode at V = 1.23 V vs RHE under monochromatic light, from a monochromator light source (M10-T, Bunkoukeiki Co., Ltd.), at a wavelength range of 400−640 nm at intervals of 20 nm. The IPCE was calculated according to the following formula
photoanode produced oxygen gas continuously for 3 h at the quantitative Faraday efficiency of 96%.
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EXPERIMENTAL SECTION Preparation of Ta3N5 Films. First, pieces of Ta foil (25 mm × 30 mm × 0.2 mm, 99.95%, Nilaco) were sequentially cleaned by sonication for 10 min successively in acetone, isopropanol, ethanol, and deionized (DI) water. The hydrothermal technique has been used to prepare sodium-tantalum [(Na, Ta)Ox] and potassium-tantalum mixed oxide [(K, Ta)Ox] films onto the Ta foil. In this process, a Teflon liner autoclave (140 mL) containing the cleaned Ta foil immersed in a concentrated aqueous solution of NaOH or KOH (100 mL) was heated up in a muffle furnace. The autoclave was left to cool down naturally in open air after a certain deposition time. The alkaline salt concentration (wt %), the heating temperature, and the deposition time were optimized as discussed hereinafter to achieve the optimal water oxidation performance. After the deposition, the film was rinsed with a diluted aqueous HCl solution and DI water, sequentially. Afterward, the prepared precursor electrodes were converted to Ta3N5 material through thermal annealing in ammonia atmosphere (NH3, flow rate: ∼200 mL min−1) at 1000 °C for 2 h in an alumina tube. A schematic diagram of the main steps of the sample preparation is shown in Figure S1. Deposition of Ni:CoFeOx Cocatalyst. Sixteen millimolar of nickel(II) acetate tetrahydrate, 16 mM of anhydrous cobalt(II) chloride, and 5 mM of iron(III) sulfate were dissolved in DI water. The pH value of the resulted aqueous solution was about 5.3.30 The cocatalyst was deposited through the light-assisted electrochemical deposition in a threeelectrode cell: platinum as a counter electrode, Ag/AgCl (in 3 M NaCl) as a reference electrode, and bare Ta3N5 electrode as a working electrode. A voltage sweep voltammetry (in a range of 0.1−0.5 V vs Ag/AgCl) was applied for five cycles under light with the intensity of 100 mW cm−2.25 Spectroscopic Characterizations of Electrodes. The high-resolution field emission scanning electron microscopy (HR-SEM; Hitachi Model S-4300) operated at 15 kV accelerating voltage was utilized to examine the surface morphology as well as the cross section of the samples. The X-ray diffraction (Rigaku RINT-2100/PC) spectra were detected using Cu Kα radiation (λ = 0.15405 nm, operated at 40 kV and 40 mA). X-ray photoelectron spectroscopy (XPS;
ij 1240 × I yz λz zz × 100 ICPE (%) = jjjj j λ × Pinput zz k {
(1)
where Iλ (mA cm−2) is the density of the steady-state photocurrent, λ (nm) is the wavelength of the monochromatic light, and Pinput (mW cm−2) is the incident photon density measured using a Si photodiode detector S1337-1010BQ (Hamamatsu Photonics K.K.). In addition, the electrochemical impedance spectroscopy (EIS) was measured for the pristine and the cocatalyst-modified Ta3N5 photoanodes at potential V = 1.23 V vs RHE in 1 M NaOH solution (pH 13.6) under light illumination. The amplitude of the applied sinusoidal signal was 10 mV with a frequency range of 0.1−105 Hz. Furthermore, the Mott−Schottky analysis for the optimized bare electrodes was performed based on the EIS measurements under dark condition in an alkaline electrolyte (1 M NaOH, pH 13.6) in a voltage range of 0.4−0.9 V vs AgCl. The Mott− 7819
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ACKNOWLEDGMENTS Y.N. acknowledges JST ACT-C (Grant No. JPMJCR12YV), JSPS KAKENHI (Grant No. 23245035), and a Chubu University Research Grant for financial support.
Schottky plots enabled us to calculate the donor concentration of the measured samples based on the following equation.32,33 ij d(C −2) yz 2 jj z jj dV zzz = qεε N o D k {
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where C is the capacitance per unit area (F cm−2), V is the applied potential (V), q is the electron charge constant (∼1.6 × 10−19 Coulomb), εo is the permittivity of free space (∼8.85 × 10−14 F cm−1), ε is the dielectric constant of Ta3N5, and ND is the donor concentration (cm−3). Faradic Efficiency. The three-electrode cell was perfectly sealed and purged with Ar gas for 30 min. The headspace gas was analyzed by utilizing the GC instrument before and after the measurement. We ensured that the sealed cell was completely depleted from H2, O2, and N2 gases. In this experiment, the Ni:CoFeOx/Ta3N5 photoanode was biased with 1.23 V vs RHE in 1 M NaOH under light illumination (100 mW cm−2) and the evolved O2 gas was quantified every 30 min during the experiment. Finally, the Faradic efficiency of the evolved oxygen was estimated. The three-electrode cell was used to measure the Faradic efficiency of evolved oxygen. The Ni:CoFeOx/Ta3N5 photoanode was biased at 1.23 V vs RHE in 1 M NaOH under light illumination (100 mW cm−2). The cell was sealed and purged by Ar for 30 min, and no H2, O2, or N2 was detected before the measurement. The headspace gas was analyzed by using the GC to quantify the evolved O2 gas. Applied Bias Photon-to-Current Efficiency (ABPE). It was calculated based on the following formula
where Jph is the photocurrent density obtained under an applied bias Vb V vs RHE and Ptotal is the total incident power density (100 mW cm−2).31
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00382. XRD of the precursor (Na, Ta)Ox and (K, Ta)Ox electrodes, low-magnification top-view SEM images of S−NaOH and S−KOH samples, cross-sectional SEM image of S−KOH whose precursor electrode was grown at 3 h of hydrothermal deposition, IPCE of Ni:CoFeOx/ S−NaOH versus absorbance of S−NaOH (PDF)
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ij J (mAcm−2) × (1.23 − |Vb|) V yz j ph zz zz ABPE = jjjj zz j Ptotal(mWcm−2) k {
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Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (A.A.H.). *E-mail:
[email protected] (Y.N.). ORCID
Ashraf Abdel Haleem: 0000-0003-1397-6996 Notes
The authors declare no competing financial interest. 7820
DOI: 10.1021/acsomega.9b00382 ACS Omega 2019, 4, 7815−7821
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Article
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