Cathodic shift of a photo-potential on a Ta₃N₅ photoanode by post-heating a TiO₂ passivation layer

Abstract

Ta₃N₅ is a promising photoanode material for solar water splitting due to its suitable band gap and high theoretical solar energy conversion. A high onset potential of Ta₃N₅ limits its photoelectrochemical performance due to serious surface charge recombination. In a previous study, a TiO₂ passivation layer was usually coated on the surface of Ta₃N₅ to reduce the surface recombination and improve the performance of a sample. However, to date, there are no studies on the effect of conductivity of the TiO₂ passivation layer on the photoelectrochemical properties of a Ta₃N₅ photoanode. In this work, for the first time, the conductivity of TiO₂ is increased by post-heating of a TiO₂ passivation layer, leading to a 90 mV cathodic shift of the photo-potential of Ta₃N₅. After further loading with a Ni(OH)_x/FeOOH bi-layer electrocatalyst, the Ta₃N₅ photoanode achieves a current density of 6.4 mA cm⁻² at 1.23 V_RHE and a HC-STH (half-cell solar to hydrogen efficiency) of 0.72% under a sunlight simulator (100 mW cm⁻²), which are the highest values among the Ta₃N₅ photoanodes prepared by thermal oxidation and nitridation of Ta foil.

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1. Introduction

Since Fujishima and Honda reported the use of a TiO₂ photoanode to split water into H₂ and O₂ under illumination,¹ solar water splitting has become a promising method to convert solar energy into clean and high energy density H₂.^2,3 In the past forty years, three kinds of photoelectrochemical (PEC) cells, including PV (photovoltaic) + electrolysis, PV + PEC and a p–n tandem cell, have been explored.^4–9 In a p–n tandem cell, a n-type semiconductor as a photoanode is ohmically contacted with a p-type semiconductor as a photocathode. A two-photon system is used to split water in a p–n tandem cell and has higher efficiency than that of a conventional single-photon cell. Moreover, no expensive PV cells are used. Therefore, a p–n tandem cell is the most promising one due to its high theoretical efficiency and low cost.^10,11

Recently, some n-type semiconductors, BiVO₄, Fe₂O₃ and Ta₃N₅, have been intensively studied as photoanodes.^12–15 The band gap of BiVO₄ is too wide (2.4 eV), which leads to a low theoretical efficiency (9.1%), even lower than the minimum efficiency (10%) for practical applications.¹² Though Fe₂O₃ has a high theoretical efficiency (16%), most of the samples achieve photocurrent lower than 4 mA cm⁻², which leads to an experimental efficiency lower than 5%.^16,17 The low efficiency is due to an intrinsic short hole diffusion length caused by a local d-band composition in valence band of Fe₂O₃.¹⁸ In 2002, Ta₃N₅ was firstly reported as a visible light photocatalyst for water splitting by Domen's group.¹⁹ Though Ta₃N₅ has a similar band gap (2.1 eV) and theoretical photocurrent with Fe₂O₃, a valence band of Ta₃N₅ is composed of N2p and has a longer hole diffusion length than that of Fe₂O₃. Moreover, the bottom of conduction band of Ta₃N₅ is at −0.3 V_RHE, 0.6 V higher than that of Fe₂O₃, which means a more negative onset potential and a lower bias for photoelectrochemical water oxidation.^20,21 Therefore, it is easier for a Ta₃N₅ photoanode to obtain high efficiency than Fe₂O₃. In recent years, photocurrent of Ta₃N₅ has been boosted to a theoretical maximum value.^14,22–24 However, an onset potential of a Ta₃N₅ photoanode (0.6–0.8 V_RHE) is still much positive than the theoretical value (−0.3 V_RHE) even after coating efficient electrocatalysts on the surface of Ta₃N₅ to accelerate interface charge transfer.^22,25–27 The positive onset potential of a Ta₃N₅ photoanode mainly originates from poor bulk transport of majority carriers and severe surface carrier recombination.^14,28,29 In order to further lower an onset potential, a TiO₂ surface passivation layer was coated on the surface of Ta₃N₅ because TiO₂ could reduce surface recombination of photo-generated carriers.^30–33 However, to date, there are few studies on effect of conductivity of the TiO₂ passivation layer on photoelectrochemical property of a Ta₃N₅ photoanode.

In this study, we found that the large amount of OH⁻ group exist in an as-deposited TiO₂ passivation layer to form TiO_x(OH)_y, which impeded interface hole transfer and limited a conversion efficiency of Ta₃N₅. A strategy of post-heating of TiO₂ passivation layer was proposed to cathodically shift photo-potential of Ta₃N₅ by about 90 mV. After post-heating at a low temperature, TiO_x(OH)_y in the as-deposited passivation layer was dehydrated into TiO_x(OH)_y−δ, which enhanced the interface hole transfer. Therefore, an onset potential of a Ta₃N₅ photoanode was cathodically shifted. To the best of our knowledge, it is the first time to shift the onset potential of Ta₃N₅ by heating a TiO₂ passivation layer at a low temperature. This strategy not only deepens understanding the role of a TiO₂ passivation layer in a photoelectrochemical cell but also offers reference to improve the performance of other photoelectrodes.

2. Experimental section

2.1 Preparation of Ta₃N₅ photoanodes

Ta₃N₅ photoanodes were prepared by modified oxidation and nitridation of Ta foils at a high temperature.¹⁴ Ta foils (7 × 16 mm) were firstly rinsed by isopropanol, acetone and ethanol before use, respectively. In order to remove the oxygen-rich surface recombination layer, Ta foils were etched in 5 mL hydrofluoric acid solution (40%) for 225 min at room temperature (25 °C), and then rinsed with distilled water and ethanol. The etched Ta foils were calcined in air at 554 °C for 30 min to obtain Ta₂O₅ film, followed by a further nitridation treatment under a flow of ammonia gas (800 mL min⁻¹) at 900 °C for 8 h to become Ta₃N₅. A Ta foil without etching was also calcined in the same conditions as a reference to investigate the etching effect.

2.2 Deposition of a TiO₂ passivation layer

A TiO₂ passivation layer was deposited by CBD method.²⁴ 200 μL of titanium trichloride solution (20%) was added into 20 mL of deionized water in ice water bath. A Ta₃N₅ sample was vertically immersed into the obtained solution, and a TiO₂ passivation layer was deposited on the surface of a Ta₃N₅ sample at 70 °C for 6 min. After washed with distilled water, the TiO₂ coated Ta₃N₅ electrode was calcined in nitrogen atmosphere at different temperatures (100 °C, 150 °C, 200 °C, and 250 °C) for 30 min in a tube furnace. A TiO₂ coated Ta₃N₅ electrode without calcination was also prepared as a reference.

2.3 Coating of a Ni(OH)_x/FeOOH bi-layer electrocatalyst

A FeOOH layer and a Ni(OH)_x layer were deposited on a TiO₂ coated Ta₃N₅ photoelectrode by CBD method and electro-deposition method,²⁴ respectively. A FeOOH layer was deposited in a mixture of 50 mM Fe(NO)₃ and 500 mM NaNO₃ aqueous solution at 100 °C for 20 min. After washed with distilled water and dried in air, a Ni(OH)_x layer was electro-deposited with a potential of −0.85 V vs. saturated calomel electrode (SCE) in 0.1 M NiSO₄ aqueous solution at room temperature. The deposition charge was about 12 mC cm⁻².

2.4 Characterization of samples

The crystal structures of samples were measured by X-ray diffraction patterns (XRD, Bruker D8 ADVANCE diffractometer). The morphologies of samples were observed by field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI). XPS spectra of samples were obtained by using a Thermo ESCALAB 250 machine and the binding energy was calibrated by C1s (284.8 eV).

2.5 Photoelectrochemical measurement

The photoelectrochemical properties were conducted in an electrochemical analyzer (CHI-660D, Shanghai Chenhua). A Ta₃N₅ electrode, SCE and Pt foil were used as the working electrode, reference electrode and counter electrode, respectively. 1 M NaOH (pH = 13.6) aqueous solution was used as electrolyte. A reversible hydrogen electrode (RHE) potential was obtained by the formula:

E(V vs. RHE) = E(V vs. SCE) + 0.242V + 0.059 × pH

A Xe lamp and AM 1.5G sunlight simulator (100 mW cm⁻², Newport oriel 92251A-1000) were used as light sources, respectively. The electrochemical impedance spectra (EIS) of samples were measured by an electrochemical analyzer (Solartron1260 + 1287) with a 10 mV amplitude perturbation and frequencies between 0.1 Hz and 10 MHz. The incident photon-to-current efficiency (IPCE) was measured under irradiation of different wavelengths of light generated by monochromatic filters. A photometer (Newport, 840-C, USA) was used to detect light intensity. The IPCE was calculated as follow:

IPCE = 1240 × I_ph/Pλ

where I_ph is photocurrent density (μA cm⁻²), P and λ are incident light intensity (μW cm⁻²) and wavelength (nm), respectively.

3. Results and discussion

3.1 Improved efficiency of Ta₃N₅ by etching a Ta foil precursor with HF solution

In our previous study, a smooth and thin layer of Ta₃N₅ with high oxygen impurity existed on the surface of an as-prepared sample, which played a role as surface recombination center and lowered the conversion efficiency of Ta₃N₅. By removing the Ta₃N₅ surface layer through thermal or mechanical exfoliation method, an efficient Ta₃N₅ photoanode was obtained.¹⁴ In this study, a new surface etching approach was carried out on a Ta foil precursor with HF aqueous solution before oxidation and nitridation. The oxidation temperature was 554 °C, lower than the critical temperature of 590 °C.¹⁴ Therefore, no surface thermal exfoliation was observed on the Ta₃N₅ samples in this study. Fig. 1a and b shows SEM images of Ta₃N₅ from Ta foil precursor without and with surface etching, respectively. Without surface etching, a smooth surface is observed on the sample. The surface becomes rougher after surface etching of Ta foil precursor and some particles, aggregates and grooves are observed. XRD (see Fig. S1a and Table S1†) and Raman (see Fig. S1b†) results suggest that there are no obvious differences in crystal structures and grain sizes between the two samples before and after etching. XPS indicates the ratio of O/Ta on the surface of Ta₃N₅ decreases from 0.35 to 0.24 after surface etching of Ta foil precursor (see Fig. 1c). The surface etching of Ta foil precursor with HF solution is a new method to remove an oxygen-rich surface layer. Fig. 1d indicates photoelectrochemical properties of Ta₃N₅ photoanodes from Ta foils with and without surface etching. After surface etching, the performance of a Ta₃N₅ photoanode increases remarkably, which comes from the surface oxygen-rich layer removal. The result is in good agreement with our previous study.

Fig. 1 SEM images of Ta₃N₅ from Ta foil precursor without (a) and with (b) surface etching; (c) O/Ta ratios on the surface of Ta₃N₅ by XPS from a Ta foil precursor with and without surface etching; (d) current–potential curves of Ta₃N₅ photoanodes from Ta foil precursor with and without surface etching in 1 M NaOH aqueous solution (pH = 13.6) under xenon lamp irradiation.

3.2 Effect of post-heating a TiO₂ passivation layer on a photo-potential of a Ta₃N₅ photoanode

Efficient Ni(OH)_x/FeOOH bilayer electrocatalysts for OER were coated on the surface of Ta₃N₅ photoanodes to accelerate the water oxidation process, following a previous report.²⁴ In this study, a Ni(OH)_x/FeOOH coated Ta₃N₅ sample is referred as Ta₃N₅/F/N. In order to further decrease the surface recombination of Ta₃N₅, post-heating a TiO₂ passivation layer in nitrogen atmosphere at different temperatures was carried out. Fig. 2a indicates photoelectrochemical properties of Ta₃N₅/TiO₂/F/N after post-heating TiO₂ at different temperatures, and Ta₃N₅/F/N references without a TiO₂ layer before and after calcining at 200 °C. After coating a TiO₂ passivation layer, an onset potential of Ta₃N₅/F/N shifts cathodically, which is in good agreement with previous study.³¹ However, in previous studies,^24,31 no post-treatment of a TiO₂ passivation layer was carried out to further improve the performance of a Ta₃N₅ photoanode. In this study, after post-heating a TiO₂ passivation layer in N₂ at different temperatures for 30 min, the onset potential of Ta₃N₅/F/N with a TiO₂ passivation layer is cathodically shifted. The optimized post-heating temperature is 200 °C. Compared to the sample post-heating at 200 °C, an onset potential of Ta₃N₅/TiO₂/F/N with post-heating at 250 °C is anodic shifted. An onset potential of Ta₃N₅/F/N without a TiO₂ passivation layer is also anodic shifted after post-heating Ta₃N₅ in N₂ at 200 °C for the same time. The results suggest that the cathodic shift of onset potentials of Ta₃N₅/F/N with a TiO₂ passivation layer after post-heating does not come from post-heating Ta₃N₅, but the TiO₂ passivation layer.

Fig. 2 (a) Current–potential curves of Ta₃N₅/F/N, Ta₃N₅/TiO₂ (25 °C)/F/N, Ta₃N₅/TiO₂ (100 °C)/F/N, Ta₃N₅/TiO₂ (150 °C)/F/N, Ta₃N₅/TiO₂ (200 °C)/F/N, Ta₃N₅/TiO₂ (250 °C)/F/N and Ta₃N₅ (200 °C)/F/N without TiO₂ in 1 M NaOH (pH = 13.6) under 300 W xenon lamp irradiation; (b) photo-potentials@1.5 mA cm⁻² of the samples vs. post-heating temperatures; (c) current density–time curves of Ta₃N₅/TiO₂ (25 °C)/F/N and Ta₃N₅/TiO₂ (200 °C)/F/N at 1.23 V_RHE under 300 W xenon lamp irradiation.

Fig. 2b shows the photo-potential (@1.5 mA cm⁻²) of the samples with post-heating TiO₂ at different temperatures. After coating a TiO₂ passivation layer at room temperature, a photo-potential of a Ta₃N₅/F/N is cathodically shifted about 40 mV. An photo-potential is further cathodically shifted about 10 mV, 60 mV and 90 mV after post-heating TiO₂ passivation layers at 100 °C, 150 °C and 200 °C, respectively. Two kinds of slopes of photo-potential decrease are observed at the post-heating temperature lower and higher than 100 °C, suggesting 100 °C is a critical temperature to activate the TiO₂ passivation layer. When the post-heating temperature is increased to 250 °C, a photo-potential is anodically shifted about 50 mV, which comes from decreased performance of a Ta₃N₅ photoanode after post-heated at such a high temperature. The result suggests that the improved performance of a TiO₂ coated Ta₃N₅ comes from the post-heating treatment of TiO₂ but not Ta₃N₅. Fig. 2c indicates the photo-stability of Ta₃N₅/TiO₂/F/N with and without post-heating TiO₂ at 200 °C. Photo-stability of the two samples is similar and the increased photocurrent can be kept during 1800s' illumination, suggesting that the post-heating treatment makes Ta₃N₅/TiO₂/F/N change irreversibly.

3.3 A mechanism for improved performance of a Ta₃N₅ photoanode by post-heating a TiO₂ passivation layer

Fig. 3 indicates surface morphologies of Ta₃N₅, Ta₃N₅/TiO₂ (25 °C), Ta₃N₅/TiO₂ (200 °C) and Ta₃N₅/TiO₂ (200 °C)/F/N, respectively. Some pores are observed on the surface of Ta₃N₅, which comes from decreased quantity of anions after nitriding Ta₂O₅ into Ta₃N₅.³⁴ No obvious morphology change is observed after coating and post-heating a TiO₂ passivation layer at 200 °C, which suggests that the TiO₂ layer is very thin (Fig. 3b and c), several nanometer, according to previous study.²⁴ After loading a Ni(OH)_x/FeOOH bilayer electrocatalyst, some nanoparticles uniformly distribute on the surface of a Ta₃N₅ photoanode (see Fig. 3d). X-ray diffraction patterns (XRD) and Raman was also employed to characterize TiO₂ on the surface of Ta₃N₅ samples with post-heating TiO₂ at different temperatures (see Fig. S3 and S4†). All the samples show the same XRD and Raman patterns with a bare Ta₃N₅ sample and no TiO₂'s signal is observed possibly due to the thin and poor crystalline of a TiO₂ layer.

Fig. 3 Surface SEM images of (a) Ta₃N₅, (b) Ta₃N₅/TiO₂ (25 °C), (c) Ta₃N₅/TiO₂ (200 °C), (d) Ta₃N₅/TiO₂ (200 °C)/F/N.

Fig. 4a and b show XPS peaks of Ta4f and N1s in TiO₂ coated Ta₃N₅ samples at different post-heating temperatures, respectively. Binding energies of Ta4f (25.2 eV and 27.1 eV) in Fig. 4a are assigned to Ta⁵⁺ in a TiO₂ coated Ta₃N₅ without post-heating, which do not shift after post-heating at temperatures of 100 °C, 150 °C and 200 °C. However, the binding energies shift to lower values about 0.1 eV after post-heating at 250 °C. Fig. 4b indicates similar trend of N1s with that in Fig. 4a. The results suggest that the valence state of Ta⁵⁺ and N³⁻ in Ta₃N₅ did not change when post-heating at a temperature lower than 200 °C. However, Ta₃N₅ is oxidized during post-heating at 250 °C due to residual O₂ impurity in N₂ carrier gas.^35,36 The binding energies of Ti2p (465 eV and 459 eV) in Fig. 4c are assigned to Ti⁴⁺,^37,38 which shift to lower binding energy as post-heating temperatures increase. The peak shift possibly comes from dehydration of titanium hydroxides into TiO_x at high temperatures.³⁸

Fig. 4 XPS spectra of (a) Ta4f_5/2 and Ta4f_7/2, (b) Ta4p_3/2 and N1s, (c) Ti2p_1/2 and Ti2p_3/2 of Ta₃N₅/TiO₂ (25 °C), Ta₃N₅/TiO₂ (100 °C), Ta₃N₅/TiO₂ (150 °C), Ta₃N₅/TiO₂ (200 °C), Ta₃N₅/TiO₂ (250 °C).

Fig. 5a shows the XPS spectra of O1s in the Ta₃N₅/TiO₂ at different post-heating temperatures. Three kinds of peaks (530.4 eV, 531.5 eV and 533.5 eV) are used to fit XPS spectra of O1s, which are assigned to lattice O²⁻, lattice OH⁻ and adsorbed H₂O (see Fig. S5†), respectively.^39,40 The relative concentrations of these three kinds of oxygen species are calculated from XPS data and the results are shown in Fig. 5b. The ratio of OH⁻ to O²⁻ in the sample without post-heating is 0.83, which does not change obviously after post-heating at 100 °C, and decreases to 0.55 when the calcination temperature is 250 °C (see Fig. 5c). The OH⁻ content of the samples decreases with higher post-heating temperature, due to the dehydration reaction during the post-heating process:⁴¹ Ti-OH + HO-Ti → Ti–O–Ti + H₂O. Therefore, the post-heating of a TiO₂ coated Ta₃N₅ leads to a reduction of OH⁻ content in the TiO₂ passivation layer. However, when the post-heating temperature is lower than 100 °C, no OH⁻ decrease is observed. These results suggest that there is a barrier for the dehydration reaction.

Fig. 5 (a) XPS spectra of O1s (b) relative concentrations of adsorbed H₂O (dark), lattice OH⁻ (red), lattice O²⁻ (blue) vs. post-heating temperatures and (c) ratio of OH⁻ to O²⁻ on the surface of Ta₃N₅/TiO₂ (25 °C), Ta₃N₅/TiO₂ (100 °C), Ta₃N₅/TiO₂ (150 °C), Ta₃N₅/TiO₂ (200 °C), Ta₃N₅/TiO₂ (250 °C).

In order to understand the effect of post-heating on photo-potential of a TiO₂ coated Ta₃N₅, electrochemical impedance spectroscopy (EIS) measurement of the samples was also carried out and the results are shown in Fig. 6a. An equivalent circuit is plotted in the inset of Fig. 6a. R_ct, R_s, and CPE represent a charge transfer resistance in semiconductor–electrolyte interface, an electrolyte resistance and a constant phase element of a double layer, respectively.³¹ The semicircle of a TiO₂ coated Ta₃N₅ with post-heating at 200 °C is the smallest, indicating a fastest interface charge transfer speed among the five samples. Fig. 6b shows the R_ct values of the TiO₂ coated Ta₃N₅ samples with post-heating at different temperatures. There is a positive correlation between the performance of a TiO₂ coated Ta₃N₅ and interface charge transfer. Therefore, reduced OH⁻ content in the TiO₂ passivation layer after the post-heating process, which leads to an increase of interface charge transfer and cathodic shift of the photo-potential.

Fig. 6 (a) EIS of Ta₃N₅/TiO₂ (25 °C), Ta₃N₅/TiO₂ (100 °C), Ta₃N₅/TiO₂ (150 °C), Ta₃N₅/TiO₂ (200 °C), Ta₃N₅/TiO₂ (250 °C); (b) interface charge transfer resistance R_ct vs. post-heating temperatures.

Photocurrent–potential curves of Ta₃N₅ before and after depositing TiO₂ layer in 1 M NaOH with H₂O₂ as sacrificial reagent were measured and the results are shown in Fig. S6a.† From Fig. S6a,† the photocurrent of Ta₃N₅ in the solution with sacrificial reagent are similar to the samples after depositing a TiO₂ layer and post-heating TiO₂ layer, which suggests that charge separation efficiency does not change after depositing the TiO₂ layer.¹⁵ Therefore, the improved performance of a TiO₂ coated Ta₃N₅ photoanode for water oxidation comes from enhanced interfacial charge transfer efficiency, which is in agreement with previous study.²⁴ Both loading oxygen evolution reaction (OER) catalysts and passivating surface defect states by noncatalysts can enhance interfacial charge transfer efficiency.⁴² In order to investigate the role of a TiO₂ layer, current–potential curves of FTO, FTO/TiO₂ and FTO/TiO₂ (200 °C) for oxygen evolution reaction are shown in Fig. S6b.† The OER current of FTO decreases after depositing a TiO₂ layer, whether post-heating or not. The results suggest that no catalysis effects are observed on the TiO₂ layer, which are in good agreement with previous studies.^24,32,33 Therefore, the TiO₂ layer plays a role as a passivation layer,^31,43 which reduces the surface carrier recombination and facilitate interfacial charge transfer.

According to XPS and EIS results, a mechanism of cathodic shift photo-potential after post-heating a TiO₂ layer is illustrated in Fig. 7. Before post-heating, holes are generated in the valence band of Ta₃N₅, which transfer to the TiO₂ passivation layer and then to electrocatalysts for water oxidation under illumination. From EIS in Fig. 6, large amount of OH⁻ in the as-deposited TiO₂ decreases conductivity of TiO₂ and limits hole transfer in the TiO₂ passivation layer. After post-heating, OH⁻ content in TiO₂ decrease and increase the conductivity of the passivation layer. Therefore, hole transfer in a TiO₂ passivation layer is increased after post-heating. However, if the post-heating temperature is too high (250 °C), Ta₃N₅ can be oxidized and the performance of the samples becomes lower. Therefore, an optimum post-heating temperature is observed.

Fig. 7 Illustration of energy band positions and interface charge transfer in a TiO₂ coated Ta₃N₅ before and after post-heating.

3.4 Performance of a Ta₃N₅ photoanode with post-heating a TiO₂ passivation layer under sunlight simulator illumination

Fig. 8a shows current–potential curves of the Ta₃N₅/TiO₂ (200 °C)/F/N under a standard measurement condition to compare the performance of our sample with previous studies. A photocurrent of 6.4 mA cm⁻² at 1.23 V_RHE is achieved in the Ta₃N₅/TiO₂ (200 °C)/F/N. The IPCEs at 1.23 V_RHE of the samples were measured and the results are shown in Fig. 8b. An integrated photocurrent is calculated as 6.2 mA cm⁻² from IPCEs at the standard solar spectrum, which is consistent with the measured value in Fig. 8a. Therefore, the solar photocurrent in Fig. 8a is reliable. A half-cell solar to hydrogen efficiency (HC-STH) is calculated as 0.72% at 1.02 V_RHE (Fig. 8c). The photocurrent at 1.23 V_RHE and HC-STH are the highest values among Ta₃N₅ photoanodes prepared by oxidation and nitridation of Ta foils method. The results suggest that post-heating is an effective strategy to obtain efficient TiO₂ coated Ta₃N₅ photoanodes for solar water splitting.

Fig. 8 (a) A current–potential curve of Ta₃N₅/TiO₂ (200 °C)/F/N photoanode and (b) the corresponding IPCEs at 1.23 V_RHE; (c) a half-cell solar to hydrogen efficiency electrolyte: 1 M NaOH (pH = 13.6), light source: AM 1.5G simulated sunlight (100 mW cm⁻²).

4. Conclusion

In summary, a new etching method was used to remove an oxygen-rich surface layer of Ta₃N₅ and enhanced the performance of a Ta₃N₅ photoanode. The conversion efficiency of Ta₃N₅ was improved by coating a TiO₂ passivation layer on the surface. We found that amount of OH⁻ decreased the conductivity of an as-deposited TiO₂ layer, which limited the performance of a TiO₂ coated Ta₃N₅. Therefore, a post-heating treatment was proposed to decrease OH⁻ species and increase the conductivity of TiO₂, which cathodically shifted the photo-potential of Ta₃N₅ about 90 mV. After the post-heating, a photocurrent of 6.4 mA cm⁻² at 1.23 V_RHE and a HC-STH efficiency of 0.72% were obtained in a TiO₂ coated Ta₃N₅ photoanode with Ni(OH)_x/FeOOH bilayer electrocatalyst, which are the highest values among Ta₃N₅ photoanodes prepared by thermal oxidation and nitridation of Ta foils.

Acknowledgements

This work is financially supported by the National Basic Research Program of China (973 Program, No. 2015CB932200, 2014CB239303 and 2013CB632404), the Natural Science Foundation of Jiangsu Province of China (No. 15KJB150010 and No. BK20150955), Synergetic Innovation Center for Organic Electronics and Information Displays, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Wenjun Luo and Wei Huang acknowledge a Newton Research Collaboration Programme Type 2 Award (No. NRCP/1415/264).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra04647b

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