Zinc Tantalum Oxynitride (ZnTaO2N) Photoanode Modified with Cobalt Phosphate Layers for the Photoelectrochemical Oxidation of Alkali Water

Photoanodes fabricated by the electrophoretic deposition of a thermally prepared zinc tantalum oxynitride (ZnTaO2N) catalyst onto indium tin oxide (ITO) substrates show photoactivation for the oxygen evolution reaction (OER) in alkaline solutions. The photoactivity of the OER is further boosted by the photodeposition of cobalt phosphate (CoPi) layers onto the surface of the ZnTaO2N photoanodes. Structural, morphological, and photoelectrochemical (PEC) properties of the modified ZnTaO2N photoanodes are studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet visible (UV−Vis) diffuse reflectance spectroscopy, and electrochemical techniques. The presence of the CoPi layer significantly improved the PEC performance of water oxidation in an alkaline sulphate solution. The photocurrent-voltage behavior of the CoPi-modified ZnTaO2N anodes was improved, with the influence being more prominent at lower oxidation potentials. A stable photocurrent density of about 2.3 mA·cm−2 at 1.23 V vs. RHE was attained upon visible light illumination. Relative to the ZnTaO2N photoanodes, an almost three-fold photocurrent increase was achieved at the CoPi/ZnTaO2N photoelectrode. Perovskite-based oxynitrides are modified using an oxygen-evolution co-catalyst of CoPi, and provide a new dimension for enhancing the photoactivity of oxygen evolution in solar-assisted water-splitting reactions.


Introduction
The development of active and proficient semiconductor photocatalysts for the direct conversion of solar energy to chemical energy has gained attention in the desire to fulfil future energy and fuel demand [1,2]. The use of solar photocatalytic water splitting to produce H 2 and O 2 over a heterogeneous photocatalyst has been studied extensively for several years, and has been found to be a favorable process for clean and renewable hydrogen generation. The heterogeneous photocatalytic process function at a solar-energy conversion efficiency of 10% is theoretically able to generate hydrogen at a rate of $1.63/kg H 2 [3]. However, the development of a photocatalyst for the oxidation of water to yield oxygen gas remains the main hurdle that needs to be overcome in order to establish a technology that is based on water-splitting. The water oxidation reaction is the most difficult half reaction process as it includes four-electron transfer to generate an oxygen molecule, O 2 ; therefore there are significant thermodynamic and kinetic limitations to this process [3]. Several polycrystalline photocatalyst materials have been established for solar-assisted water splitting [1,4]; on the other hand, the materials separation and collection of holes generated at the surface of the photoanodes, subsequently improving photocatalytic performance.
Herein, we describe about the cocatalytic influence as well as the PEC performance of the Co-Pi incorporated ZnTaO 2 N photoanodes. The PEC properties of the ZnTaO 2 N electrodes are enhanced predominantly when modified with the Co-Pi catalyst, and its effect is greatest at a lower potential, while the photocurrent is enriched by a factor of 2.5 times relative to that of a bare electrode. The enhancement of the performance can be credited to an improvement in hole collection and charge-separation efficiency at the surface of the oxynitride photoanodes. The present work also demonstrates a viable way for an improvement in the energy-conversion efficiency by coupling OECs with oxynitride photoanodes.

XRD and DRS Analysis
The ZnTaO 2 N catalyst was prepared using the conventional ammonolysis method, and for comparison, TaON was prepared using a similar procedure. The as-synthesized ZnTaO 2 N powders show an orange-yellow color, while the TaON powder had a yellow color. The electrophoretic deposition (EPD) method was employed to prepare photoanodes of these materials. The average thickness of the ZnTaO 2 N-deposited film on the indium tin oxide (ITO) substrate was measured using a profilometer, and was about 2.5 µm. Figure 1 demonstrates the XRD patterns for the TaON, ZnTaO 2 N, and CoPi/ZnTaO 2 N photoanodes that were fabricated by EPD followed by loading CoPi using photodeposition methods on ITO substrates, and annealing in a nitrogen atmosphere at 450 • C. The XRD pattern of as-synthesized ZnTaO 2 N reveals that all the major peaks can be assigned to a perovskite-type phase that is structurally similar to that of ZnTaO 2 N [28]. The TaON XRD pattern obtained was consistent with the formation of the baddeleyite (monoclinic ZrO 2 ) structure. The XRD pattern of TaON photoanodes (Figure 1a) can be well indexed with the phase of TaON (JCPDS # 70-1193). In ZnTaO 2 N films, a small level of peak broadening was observed, which implies a reduction in particle size compared to the TaON photoanodes. The XRD pattern of the electrodeposited ZnTaO 2 N photoanodes ( Figure 1b) displays the mixed phase of ZnTa 2 O6 (JCPDS # 39-1484) and TaON. Nanomaterials 2018, 1, 48 3 of 14 reaction co-catalysts. The loading of the CoPi OER co-catalyst could amplify the charge separation and collection of holes generated at the surface of the photoanodes, subsequently improving photocatalytic performance.
Herein, we describe about the cocatalytic influence as well as the PEC performance of the Co-Pi incorporated ZnTaO2N photoanodes. The PEC properties of the ZnTaO2N electrodes are enhanced predominantly when modified with the Co-Pi catalyst, and its effect is greatest at a lower potential, while the photocurrent is enriched by a factor of 2.5 times relative to that of a bare electrode. The enhancement of the performance can be credited to an improvement in hole collection and charge-separation efficiency at the surface of the oxynitride photoanodes. The present work also demonstrates a viable way for an improvement in the energy-conversion efficiency by coupling OECs with oxynitride photoanodes.

XRD and DRS Analysis
The ZnTaO2N catalyst was prepared using the conventional ammonolysis method, and for comparison, TaON was prepared using a similar procedure. The as-synthesized ZnTaO2N powders show an orange-yellow color, while the TaON powder had a yellow color. The electrophoretic deposition (EPD) method was employed to prepare photoanodes of these materials. The average thickness of the ZnTaO2N-deposited film on the indium tin oxide (ITO) substrate was measured using a profilometer, and was about 2.5 μm. Figure 1 demonstrates the XRD patterns for the TaON, ZnTaO2N, and CoPi/ZnTaO2N photoanodes that were fabricated by EPD followed by loading CoPi using photodeposition methods on ITO substrates, and annealing in a nitrogen atmosphere at 450 °C. The XRD pattern of as-synthesized ZnTaO2N reveals that all the major peaks can be assigned to a perovskite-type phase that is structurally similar to that of ZnTaO2N [28]. The TaON XRD pattern obtained was consistent with the formation of the baddeleyite (monoclinic ZrO2) structure. The XRD pattern of TaON photoanodes (Figure 1a) can be well indexed with the phase of TaON (JCPDS # 70-1193). In ZnTaO2N films, a small level of peak broadening was observed, which implies a reduction in particle size compared to the TaON photoanodes. The XRD pattern of the electrodeposited ZnTaO2N photoanodes (Figure 1b    The XRD patterns of the CoPi-loaded ZnTaO 2 N photoanodes show that the CoPi deposition did not alter the crystalline phase of ZnTaO 2 N photoanodes, and no XRD peaks corresponding to metal impurities or simple metal oxides were observed. This may be credited to the good dispersion of nanocrystalline CoPi particles over the ZnTaO 2 N. A small shift towards the lower 2θ values was observed in the presence of CoPi-loaded photoanodes, compared with the parent photoanodes, and this was due to a small difference in the sample height in the powder diffractometer. Figure 2 displays diffuse reflectance spectroscopy (DRS) spectra for the various TaON-based photoanodes. All fabricated TaON-based photoanodes have visible light, and the onset of light-absorption features of the band-gap excitation of TaON, ZnTaO 2 N, and CoPi/ZnTaO 2 N was observed at 420, 447, and 452 nm, respectively.  However, CoPi/ZnTaO2N photoanodes display dissimilar absorption spectra from that of ZnTaO2N, and the absorption edge point is mainly lifted to the higher wavelengths compared with TaON. The BG of the fabricated TaON-based photoanodes was estimated via Kublenka-Munk (K-M) functions, and the data are presented in Table 1. The assessed optical band gaps decreased in the order TaON > ZnTaO2N > CoPi/ZnTaO2N > LaTa0.3Nb0.7O2N > LaNbO2N (Table 1), which is consistent with a similar effect for perovskite tantalum oxides [42].  However, CoPi/ZnTaO 2 N photoanodes display dissimilar absorption spectra from that of ZnTaO 2 N, and the absorption edge point is mainly lifted to the higher wavelengths compared with TaON. The BG of the fabricated TaON-based photoanodes was estimated via Kublenka-Munk (K-M) functions, and the data are presented in Table 1. The assessed optical band gaps decreased in the order TaON > ZnTaO 2 N > CoPi/ZnTaO 2 N > LaTa 0.3 Nb 0.7 O 2 N > LaNbO 2 N (Table 1), which is consistent with a similar effect for perovskite tantalum oxides [42].  faces, indicating a superior crystallinity of these anodes. This is likely to favor the process of photohole generation, which can reach the active reaction sites at the boundary between the electrode/electrolyte [30]. However, the presence of large quantities of grain boundaries and loose inter-particle connections may lead to the incoherence of electron transport between the particles. The SEM image in Figure 3c for ZnTaO 2 N covered with CoPi clearly reveals that the loaded CoPi layer does not alter the morphology of ZnTaO 2 N particles. In order to prove the deposition of the CoPi layer at the ZnTaO 2 N surface, we examined the structure using energy-dispersive X-Ray analysis (EDX), and the results are shown in Figure 3d. The EDX characterization confirms the existence of Ta, Co, and P elements in the photoanodes at 46.25, 1.23, and 0.58 wt %, respectively. generation, which can reach the active reaction sites at the boundary between the electrode/electrolyte [30]. However, the presence of large quantities of grain boundaries and loose inter-particle connections may lead to the incoherence of electron transport between the particles. The SEM image in Figure 3c for ZnTaO2N covered with CoPi clearly reveals that the loaded CoPi layer does not alter the morphology of ZnTaO2N particles. In Order to prove the deposition of the CoPi Layer at the ZnTaO2N surface, we examined the structure using energy-dispersive X-Ray analysis (EDX), and the results are shown in Figure 3d. The EDX characterization confirms the existence of Ta, Co, and P elements in the photoanodes at 46.25, 1.23, and 0.58 wt %, respectively.  Figure 4 shows the existence of Co, P, Zn, Ta, O, and N in the wide-scan X-ray photoelectron spectroscopy (XPS) spectrum of CoPi/ZnTaO2N photo-anodes. The molar ratio of Co and P in CoPi/ZnTaO2N was estimated to be 1:2.2. The major peaks of binding energies at 26.8 and 28.5 eV related to the spin-orbit separation of the Ta 4f5/2 and Ta 4f7/2 ingredients, respectively (Figure 4b), demonstrating the development of the Ta 5+ [43]. The two dissimilar binding energies of the O element can be allocated to the characteristics of the Ta-O bond (530.1 eV) and oxygen in carbonate species or hydroxyl groups (531.8 eV) (Figure 4c) [44]. The major peaks for Ta 4P2/3 and N 1s in Figure 4d indicate that the N 1s region centered at 396.4 eV is associated with the binding energy of about 403.5 eV for Ta 4P2/3, further confirming the creation of various Ta-N bonds [44].  Figure 4 shows the existence of Co, P, Zn, Ta, O, and N in the wide-scan X-ray photoelectron spectroscopy (XPS) spectrum of CoPi/ZnTaO 2 N photo-anodes. The molar ratio of Co and P in CoPi/ZnTaO 2 N was estimated to be 1:2.2. The major peaks of binding energies at 26.8 and 28.5 eV related to the spin-orbit separation of the Ta 4f 5/2 and Ta 4f 7/2 ingredients, respectively (Figure 4b), demonstrating the development of the Ta 5+ [43]. The two dissimilar binding energies of the O element can be allocated to the characteristics of the Ta-O bond (530.1 eV) and oxygen in carbonate species or hydroxyl groups (531.8 eV) (Figure 4c) [44]. The major peaks for Ta 4P 2/3 and N 1s in Figure 4d indicate that the N 1s region centered at 396.4 eV is associated with the binding energy of about 403.5 eV for Ta 4P 2/3 , further confirming the creation of various Ta-N bonds [44].

Photoelectrochemical (PEC) Properties of the ZnTaO2N Photoanodes
The PEC properties of ZnTaO2N photoanodes were examined by performing cyclic voltammetry (CV) and chronoamperometric (CA) measurements in an H-shaped cell. Figure 5a shows the CV measurements logged at 50 mV·s −1 in 1.0-M Na2SO4 for ZnTaO2N/ITO photoanodes under AM 1.5 G simulated sunlight at various thicknesses of the film prepared by varying the EPD deposition time duration. The results show that the current density of the photoanode changes dramatically as the deposition time increases. As shown in Figure 5b, the maximum current density (measured at 2.0 V vs. RHE) was observed around 0.5 ± 0.05 mg of loaded ZnTaO2N on ITO substrate (equivalent to deposition for 4.0 min and a thickness of 2.0 μm), and at higher loading (>0.5 mg), the photoanode current density significantly decreased. This can be clarified by the catalytic water oxidation reaction at the CoPi/electrolyte interface, and when CoPi loading at higher loading (>0.5 mg), the photoholes need to be transferred in amid numerous CoPi molecules and CoPi/electrolyte interface, which affects the sluggish kinetics of the hole transfer, and subsequently, a smaller photocurrent is detected [32]. Moreover, as shown in Figure 5c (taken from Figure 5a), at a ZnTaO2N loading of around 0.5 ± 0.05 mg, the onset potential of water oxidation markedly shifted to a less positive value, indicating a more favorable process. To understand the effect of pH on the PEC behavior of the ZnTaO2N photoanode, Figure 5d shows the linear sweep voltammetry (LSV) plot for ZnTaO2N photoanode at different pH in 1.0-M Na2SO4 solution. It is evidenced that the current density of water oxidation was significantly enhanced at alkaline pH of 13, which is almost eight times superior than that at pH of 12. Additionally, the onset potential of water oxidation was shifted 120 mV more cathodic compared with the value obtained at pH of 12.

Photoelectrochemical (PEC) Properties of the ZnTaO 2 N Photoanodes
The PEC properties of ZnTaO 2 N photoanodes were examined by performing cyclic voltammetry (CV) and chronoamperometric (CA) measurements in an H-shaped cell. Figure 5a shows the CV measurements logged at 50 mV·s −1 in 1.0-M Na 2 SO 4 for ZnTaO 2 N/ITO photoanodes under AM 1.5 G simulated sunlight at various thicknesses of the film prepared by varying the EPD deposition time duration. The results show that the current density of the photoanode changes dramatically as the deposition time increases. As shown in Figure 5b, the maximum current density (measured at 2.0 V vs. RHE) was observed around 0.5 ± 0.05 mg of loaded ZnTaO 2 N on ITO substrate (equivalent to deposition for 4.0 min and a thickness of 2.0 µm), and at higher loading (>0.5 mg), the photoanode current density significantly decreased. This can be clarified by the catalytic water oxidation reaction at the CoPi/electrolyte interface, and when CoPi loading at higher loading (>0.5 mg), the photoholes need to be transferred in amid numerous CoPi molecules and CoPi/electrolyte interface, which affects the sluggish kinetics of the hole transfer, and subsequently, a smaller photocurrent is detected [32]. Moreover, as shown in Figure 5c (taken from Figure 5a), at a ZnTaO 2 N loading of around 0.5 ± 0.05 mg, the onset potential of water oxidation markedly shifted to a less positive value, indicating a more favorable process. To understand the effect of pH on the PEC behavior of the ZnTaO 2 N photoanode, Figure 5d shows the linear sweep voltammetry (LSV) plot for ZnTaO 2 N photoanode at different pH in 1.0-M Na 2 SO 4 solution. It is evidenced that the current density of water oxidation was significantly enhanced at alkaline pH of 13, which is almost eight times superior than that at pH of 12. Additionally, the onset potential of water oxidation was shifted 120 mV more cathodic compared with the value obtained at pH of 12. To further improve the PEC behavior of the ZnTaO2N photocatalyst for water oxidation reaction, the CoPi co-catalyst (OEC) was incorporated into the photoanodes using the photodeposition PD method [32,45]. Initially, the amount of CoPi loaded onto the ZnTaO2N film was optimized by varying the PD duration, followed by measurement of the photocurrent response using chronoamperometry at a constant applied potential of 1.7 V vs. RHE in 1.0-M Na2SO4 solution. Figure 6a shows the current time transients with a 10 s light pulse (1.5 AM) for a ZnTaO2N anode before and after the photodeposition of CoPi for a deposition time of 60 min. For clarity, the background current in the dark was subtracted for both photoanodes. Once the light pulse was applied, the current of both photoanodes increased significantly and the photocurrent density obtained at the ZnTaO2N anode was about 0.75 mA·cm −2 , while the CoPi/ZnTaO2N photoanode displayed a much superior photocurrent density of 2.9 mA·cm −2 under the similar conditions. Clearly, the photocurrent obtained at CoPi/ZnTaO2N is more than three times higher than in the case of the ZnTaO2N electrode, which indicates that the separation and collection processes of the photogenerated electron/hole pairs are more efficient at CoPi/ZnTaO2N than at a simple ZnTaO2N photoanode. To further improve the PEC behavior of the ZnTaO 2 N photocatalyst for water oxidation reaction, the CoPi co-catalyst (OEC) was incorporated into the photoanodes using the photodeposition PD method [32,45]. Initially, the amount of CoPi loaded onto the ZnTaO 2 N film was optimized by varying the PD duration, followed by measurement of the photocurrent response using chronoamperometry at 1.7 V vs. RHE in 1.0-M Na 2 SO 4 solution. Figure 6a shows the current time transients with a 10 s light pulse (1.5 AM) for a ZnTaO 2 N anode before and after the photodeposition of CoPi for a deposition time of 60 min. For clarity, the background current in the dark was subtracted for both photoanodes. Once the light pulse was applied, the current of both photoanodes increased significantly and the photocurrent density obtained at the ZnTaO 2 N anode was about 0.75 mA·cm −2 , while the CoPi/ZnTaO 2 N photoanode displayed a much superior photocurrent density of 2.9 mA·cm −2 under the similar conditions. Clearly, the photocurrent obtained at CoPi/ZnTaO 2 N is more than three times higher than in the case of the ZnTaO 2 N electrode, which indicates that the separation and collection processes of the photogenerated electron/hole pairs are more efficient at CoPi/ZnTaO 2 N than at a simple ZnTaO 2 N photoanode.  Figure 6b shows the relationship between the CoPi PD time and the photocurrent obtained for the CoPi/ZnTaO2N anode at a potential of 1.7 V vs. RHE. Clearly, using our present PD methodology, the optimum period for CoPi photo-deposition on ZnTaO2N photo-anodes was found to be around 60 min. The PEC characteristics were compared for both ZnTaO2N and CoPi/ZnTaO2N anodes to examine the effect of the photodeposited CoPi co-catalyst on the photocurrent response of the ZnTaO2N photoanode. Figure 6c shows the LSV at 50 mV·s −1 in the dark and under 1.5 AM light illumination for CoPi/ZnTaO2N and ZnTaO2N photoanodes in 1.0-M Na2SO4 solution (pH 13). In comparison, the LSV of the control anode made by the PD of the CoPi catalyst directly onto the ITO substrate is also shown in Figure 6c (i). The LSV for the control anode of CoPi/ITO (i) shows a small level of the oxygen evolution current under light illumination. The CoPi/ZnTaO2N photoanode clearly shows negative potential shifts of about 120 mV in the onset potential of oxygen evolution, and an increase in the photocurrent of more than 2.3 mA·cm -2 at 1.23 V vs. RHE relative to its parent ZnTaO2N photoanode. For the CoPi/ZnTaO2N anode, the photocurrent was enhanced by a factor of three compared with the parent ZnTaO2N electrode. It is clear that the magnitude of the CoPi    Figure 6b shows the relationship between the CoPi PD time and the photocurrent obtained for the CoPi/ZnTaO 2 N anode at 1.7 V vs. RHE. Clearly, using our present PD methodology, the optimum period for CoPi photo-deposition on ZnTaO 2 N photo-anodes was found to be around 60 min. The PEC characteristics were compared for both ZnTaO 2 N and CoPi/ZnTaO 2 N anodes to examine the effect of the photodeposited CoPi co-catalyst on the photocurrent response of the ZnTaO 2 N photoanode. Figure 6c shows the LSV at 50 mV·s −1 in the dark and under 1.5 AM light illumination for CoPi/ZnTaO 2 N and ZnTaO 2 N photoanodes in 1.0-M Na 2 SO 4 solution (pH 13). In comparison, the LSV of the control anode made by the PD of the CoPi catalyst directly onto the ITO substrate is also shown in Figure 6c (i). The LSV for the control anode of CoPi/ITO (i) shows a small level of the oxygen evolution current under light illumination. The CoPi/ZnTaO 2 N photoanode clearly shows negative potential shifts of about 120 mV in the onset potential of oxygen evolution, and an increase in the photocurrent of more than 2.3 mA·cm −2 at 1.23 V vs. RHE relative to its parent ZnTaO 2 N photoanode. For the CoPi/ZnTaO 2 N anode, the photocurrent was enhanced by a factor of three compared with Nanomaterials 2018, 8, 48 9 of 15 the parent ZnTaO 2 N electrode. It is clear that the magnitude of the CoPi loading is influenced by the photoanode morphology, time, and the employed deposition procedure. A larger amount of CoPi deposited on ZnTaO 2 N (>60 min) results in the suppression of the photocurrent, as revealed in Figure 6b. This phenomenon can be explained by considering that the water catalytic oxidation reaction takes place at the CoPi/electrolyte interface. For a thick layer of CoPi, the photoholes have to transfer through a thick CoPi layer to reach the interface between the CoPi/electrolyte; consequently, the hole migration becomes very slow, and a decreased photocurrent is observed. On the contrary, for a thinner CoPi layer, the cobalt ions link more directly to the ZnTaO 2 N surface, and rapidly acquire the photoholes in the water oxidation reaction [46,47].
Hydrogen peroxide (H 2 O 2 ) was introduced to the electrolyte (0.1 M) as an electron donor to evaluate the maximum photocurrent that might be acquired while injecting a photoexcited holes from the anode surface to the electrolyte solution was perfectly facilitated. It is believed that H 2 O 2 captures the photoexcited holes efficiently without substantial recombination [27]. To observe these behaviors on the prepared photoanodes, LSV plots of CoPi/ZnTaO 2 N were logged in the presence and absence of H 2 O 2 and the results are presented in Figure 6d. The LSV of ZnTaO 2 N and CoPi/ZnTaO 2 N photoanodes indicate that the photocurrent at applied potential values of more negative than 1.7 V vs. RHE was improved considerably in the presence of H 2 O 2 . This implies that the redox active Co species may be quickly oxidized at a lower applied potential (<1.4 V vs. RHE), while water could not. This results coincides with the results attained for CoPi/La(Ta,Nb)O 2 N photoelectrodes [32]. These PEC performances show that the CoPi/ZnTaO 2 N photoelectrodes, photocurrent at <1.4 V vs. RHE was comparable, irrespective of the introduction of H 2 O 2 , signifying that the CoPi had improved the surface redox reactions effectively.

Band Positions of the ZnTaO 2 N Photoanodes
As previously discussed, the band-edge position at the electrolyte and the carrier mobility in the semiconductor photocatalyst are both significant factors for the characterization of the PEC performance of the photoanodes. The Mott-Schottky (MS) method was performed and the Nyquist plot was obtained to determine the characteristics of ZnTaO 2 N and CoPi/ZnTaO 2 N electrodes in 1-M Na 2 SO 4 (pH 13) at a frequency of 100 Hz. The valence-band and conduction-band potentials of TaON, LaTaO 2 N, and ZnTaO 2 N were calculated using the Butler and Ginley method, and the values are précised in Table S1.
The conduction-band edge (E CB ) for ZnTaO 2 N was obtained at 0.32 V vs. SCE by employing the method proposed by Butler and Ginley [48] using the optical BG from the K-M spectra. As presented in Figure 7a, the intercept on the potential axis of the ZnTaO 2 N photoanode reveals a flat band potential (E FB ) at 0.14 V, while the intercept of the CoPi/ZnTaO 2 N photoanodes displays an E FB at 0.08 V vs. RHE at pH = 13. It is clear that CoPi/ZnTaO 2 N photoanodes have less positive E FB values than the parent photoanodes. The addition of the CoPi co-catalyst as a hole acceptor promotes the charge-separation reaction, which leads to the enhancement of the photocurrent during the water oxidation reaction. In addition, the CoPi co-catalyst can cause the oxygen evolution reaction to occur at lower oxidation potential by altering the reaction pathway [39]. The CoPi layer on the photoanode has to be thin enough to complement the fast transfer of the photogenerated hole from the ZnTaO 2 N photoanode to the water oxidation reaction, which relieves the charge accumulation at the electrode/electrolyte interface. Figure 7b shows Nyquist plots of the ZnTaO 2 N and CoPi/ZnTaO 2 N photo-anodes acquired in the light and in the dark. The equivalent circuit is shown in the inset of Figure 7b and the fitting results of the photoanodes are summarized in Table 2. The ZnTaO 2 N photo-anodes were perfect fitted to a RC circuit model which grasps a resistor and a RC circuit. RC circuit can be allocated to ZnTaO 2 N/electrolyte interface. For the CoPi/ZnTaO 2 N photo-anode, capacitances increased, whereas the charge transfer resistances decreased, signifying that CoPi-loading sustained charge separation at the bulk photo-anode and, as a consequence, enriched PEC water oxidation ability. CoPi loading on photoanodes advances the PEC behaviour by endorsing the charge separation and water oxidation phenomenon on the ZnTaO 2 N photo-anodes surface which is very consistent with the behavior of CoPi/LaTaNbO 2 N [32], CoPi/TiO 2 [39] and CoPi/BiVO 4 structures [47].

Quantification of Dioxygen Evolution during Photoactivation
The irradiation of ZnTaO2N and CoPi/ZnTaO2N photoanodes with visible-light photons eventually increases the evolution of dioxygen. Figure 8a shows the oxygen-evolution measurements obtained at 1.7 V vs. RHE using an oxygen sensor oxysense system. The corresponding photocurrent responses were obtained and the results are presented in Figure 8b. As anticipated, under similar conditions, the CoPi/ZnTaO2N photoanodes and their parent photoanodes with visible-light photons (λ > 420 nm) caused an increase in the dioxygen evolution. To obtain an estimate, the precise oxygen concentration was logged before and after PEC reactions. Figure 8a confirms that by switching on the photoelectrolysis, the concentration of oxygen starts to increase linearly with time, and no substantial quantity of oxygen was observed before (<15 min) or after (>45 min) the photoelectrolysis was turned off. The CoPi/ZnTaO2N photoanodes showed increased oxygen generation relative to parent ZnTaO2N, which is demonstrated by its oxygen-evolution rate. The oxygen-evolution rate is faster in the case of CoPi-coated ZnTaO2N compared with the parent photoanode ( Figure 8a). Conversely, it is evident that the incorporation of CoPi as a co-catalyst enhanced the photocurrent response and stability under continued illumination, as shown in Figure 8b. The durability of the CoPi/ZnTaO2N photoanodes was examined, and it displayed in Figure S1. The chronoamperometric results indicate that the photoanode remains stable until 180 min, after which small changes are observed. The stability of the CoPi/ZnTaO2N photoanodes under light irradiation was investigated by relating their absorption spectrum before and after visible light irradiation, signifying that the photoanodes are stable under visible-light illumination ( Figure S2). A more effective coupling of photoholes to reaction sites of the water-oxidation reaction can be anticipated, as discussed earlier, which validates the enhancement of oxygen-evolution performance as well as the stability of CoPi-loaded ZnTaO2N photoelectrodes.

Quantification of Dioxygen Evolution during Photoactivation
The irradiation of ZnTaO 2 N and CoPi/ZnTaO 2 N photoanodes with visible-light photons eventually increases the evolution of dioxygen. Figure 8a shows the oxygen-evolution measurements obtained at 1.7 V vs. RHE using an oxygen sensor oxysense system. The corresponding photocurrent responses were obtained and the results are presented in Figure 8b. As anticipated, under similar conditions, the CoPi/ZnTaO 2 N photoanodes and their parent photoanodes with visible-light photons (λ > 420 nm) caused an increase in the dioxygen evolution. To obtain an estimate, the precise oxygen concentration was logged before and after PEC reactions. Figure 8a confirms that by switching on the photoelectrolysis, the concentration of oxygen starts to increase linearly with time, and no substantial quantity of oxygen was observed before (<15 min) or after (>45 min) the photoelectrolysis was turned off. The CoPi/ZnTaO 2 N photoanodes showed increased oxygen generation relative to parent ZnTaO 2 N, which is demonstrated by its oxygen-evolution rate. The oxygen-evolution rate is faster in the case of CoPi-coated ZnTaO 2 N compared with the parent photoanode ( Figure 8a). Conversely, it is evident that the incorporation of CoPi as a co-catalyst enhanced the photocurrent response and stability under continued illumination, as shown in Figure 8b. The durability of the CoPi/ZnTaO 2 N photoanodes was examined, and it displayed in Figure S1. The chronoamperometric results indicate that the photoanode remains stable until 180 min, after which small changes are observed. The stability of the CoPi/ZnTaO 2 N photoanodes under light irradiation was investigated by relating their absorption spectrum before and after visible light irradiation, signifying that the photoanodes are stable under visible-light illumination ( Figure S2). A more effective coupling of photoholes to reaction sites of the water-oxidation reaction can be anticipated, as discussed earlier, which validates the enhancement of oxygen-evolution performance as well as the stability of CoPi-loaded ZnTaO 2 N photoelectrodes.

Preparation of the ZnTaO2N Catalyst
The ZnTaO2N powder was fabricated via conventional solid-state reaction described in a previous work [49]. In the synthesis described here, stoichiometric quantities of ZnCO3 and Ta2O5 (Aldrich, 99.9%, St. Louis, MO, USA) were well ground together in acetone in the presence of KCl (50% total weight), which is used as a mineralizer. The resulting mixture was heated at 850 °C under an ammonia flow (Air Products Electronic Grade, Riyadh, KSA) for 20 h at a flow rate of 7 dm 3 ·h −1 , and it was then permitted to cool to normal temperature under ammonia atmosphere. The mineralizer was leached from the products using excess de-ionized water, and the residual product was dried overnight at 80 °C. The TaON was prepared following similar procedure by heating pure Ta2O5 (Aldrich, 99.9%) at 850 °C under flowing ammonia for 18 h at a flow rate of 7 dm 3 ·h −1 .

Fabrication of the ZnTaO2N Photoanodes
ZnTaO2N film photoanodes were fabricated using the EPD process, as described in our previous work [32]. For instance, the ZnTaO2N (15 mg) and 3-mg iodine (Alfa-Aesar, Karlsruhe, Germany) powders were kept ultrasonically discrete in acetone (15 mL) to obtain a uniform suspension. The ITO substrates (174 nm, 1.0 × 1.0 cm, Asahi glass co., Ltd., Tokyo, Japan) were submerged and held parallel at about 1.0 cm apart from each other in the solution. A +10 V bias was then applied to the two electrodes for 2 min using a potentiostat (Bio-Logic SAS, VSP 0478, Seyssinet-Pariset, France). This deposition process was repeated twice, and the electrode was then dried and annealed at 450 °C under a flow of N2 gas at a rate of 500 mL/min for 1 h. This process resulted in the development of a ZnTaO2N layer (in total about 0.5 mg) with a comparatively uniform thickness of about 2 μm, as monitored using a surface profilometer. The cobalt phosphate (CoPi) co-catalyst layer was deposited on ZnTaO2N photoanodes using the PD method, as described in the literature [32]. CoPi/ZnTaO2N electrodes with various CoPi depositions were fabricated by altering the PD time (indicated as CoPi/ZnTaO2N). Characterization and XRD measurements were carried out using a MiniFlex 600 (Rigaku, CuKα, 40 kV, 15 mA, Tokyo, Japan). The photoanodes were further characterized using an ultraviolet visible (UV-Vis) spectrophotometer (Shimadzu UV-2600, Tokyo, Japan) and EDAX (JED-2200 series, Tokyo, Japan). Electrochemical impedance

Preparation of the ZnTaO 2 N Catalyst
The ZnTaO 2 N powder was fabricated via conventional solid-state reaction described in a previous work [49]. In the synthesis described here, stoichiometric quantities of ZnCO 3 and Ta 2 O 5 (Aldrich, 99.9%, St. Louis, MO, USA) were well ground together in acetone in the presence of KCl (50% total weight), which is used as a mineralizer. The resulting mixture was heated at 850 • C under an ammonia flow (Air Products Electronic Grade, Riyadh, KSA) for 20 h at a flow rate of 7 dm 3 ·h −1 , and it was then permitted to cool to normal temperature under ammonia atmosphere. The mineralizer was leached from the products using excess de-ionized water, and the residual product was dried overnight at 80 • C. The TaON was prepared following similar procedure by heating pure Ta 2 O 5 (Aldrich, 99.9%) at 850 • C under flowing ammonia for 18 h at a flow rate of 7 dm 3 ·h −1 .

Fabrication of the ZnTaO 2 N Photoanodes
ZnTaO 2 N film photoanodes were fabricated using the EPD process, as described in our previous work [32]. For instance, the ZnTaO 2 N (15 mg) and 3-mg iodine (Alfa-Aesar, Karlsruhe, Germany) powders were kept ultrasonically discrete in acetone (15 mL) to obtain a uniform suspension. The ITO substrates (174 nm, 1.0 × 1.0 cm, Asahi glass Co., Ltd., Tokyo, Japan) were submerged and held parallel at about 1.0 cm apart from each other in the solution. A +10 V bias was then applied to the two electrodes for 2 min using a potentiostat (Bio-Logic SAS, VSP 0478, Seyssinet-Pariset, France). This deposition process was repeated twice, and the electrode was then dried and annealed at 450 • C under a flow of N 2 gas at a rate of 500 mL/min for 1 h. This process resulted in the development of a ZnTaO 2 N layer (in total about 0.5 mg) with a comparatively uniform thickness of about 2 µm, as monitored using a surface profilometer. The cobalt phosphate (CoPi) co-catalyst layer was deposited on ZnTaO 2 N photoanodes using the PD method, as described in the literature [32].

Photoelectrochemical Characterization
The PEC properties of the ZnTaO 2 N and CoPi/ZnTaO 2 N photoanodes were obtained in H-shaped two-compartment glass cells with a 2-cm diameter quartz window. The fabricated ZnTaO 2 N-based photoanode film was taken as a working electrode (WE, 0.25 cm 2 ), the saturated calomel electrode as a reference electrode, and Pt foil was served as the counter electrode in the electrolyte solution containing 1-M Na 2 SO 4 , and the pH of the electrolyte solution was modified to 13 with the addition of KOH. Photoelectrochemical oxygen evolution was monitored by an oxygen analyzer (Oxysense Inc., Dallas, TX, USA, 300/5000 series). The compartment cell was sealed and argon gas was used for purging. Prior to the experiment, the electrolyte solution was bubbled with argon for 1 h, and the atmosphere above the electrolyte was maintained as argon throughout the measurements. Before starting the electrolysis, the cell setup was purged for 20 min to attain the equilibrium.

Conclusions
A photocatalyst of ZnTaO 2 N was initially synthesized via conventional solid state reaction and then photoanodes were fabricated by an electrophoretic deposition method into ITO. We studied the co-catalytic effect of a photo-assisted water oxidation reaction by photodeposition of CoPi oxygen electrocatalyst onto the ZnTaO 2 N photoanodes. An electrochemical investigation revealed that the PEC performance of ZnTaO 2 N was significantly enhanced after CoPi deposition and a 2.3 mA·cm −2 photocurrent density was reached at 1.23 V vs. RHE in a sulfate medium. Additionally, CoPi-loading assisted the PEC performance of ZnTaO 2 N film by reducing the charge recombination process and stabilizing the photo-anode performance for the oxygen evolution reaction under visible light illumination.