Zr-doped BaTaO₂N photocatalyst modified with Na–Pt cocatalyst for efficient hydrogen evolution and Z-scheme water splitting

Abstract

BaTaO₂N exhibits hydrogen evolution activity under visible light with wavelengths up to 650 nm and is applicable to Z-scheme overall water splitting (ZOWS). However, the insufficient activity and selectivity of BaTaO₂N in the presence of redox mediators limit the efficiency of this process. Herein, we report the use of modified BaTaO₂N as a hydrogen evolution photocatalyst in combination with BiVO₄ and [Fe(CN)₆]^3−/4− as the oxygen evolution photocatalyst and redox mediator, respectively. Zr-doped BaTaO₂N (BaTaO₂N:Zr) synthesized by flux-assisted thermal nitridation and decorated with Na and Pt (Na–Pt) as cocatalysts was found to provide higher hydrogen evolution activity than undoped BaTaO₂N. Zr doping extended the lifetime of electrons in the BaTaO₂N and promoted electron injection into the Na–Pt cocatalysts. Consequently, Na–Pt/BaTaO₂N:Zr modified with Cr₂O₃ to suppress reverse reactions evolved hydrogen from an aqueous K₄[Fe(CN)]₆ solution. Optimizing the BaTaO₂N photocatalyst and the reaction conditions provided a ZOWS system capable of operating under visible light at wavelengths up to 520 nm. This work indicates that ZOWS systems operable under visible light can be constructed based on detailed investigations of photocatalysts, cocatalysts and redox mediators.

---

Broader context

Direct decomposition of water into hydrogen and oxygen using particulate photocatalysts is a simple and cost-effective approach towards renewable solar hydrogen production on a large scale. Z-scheme overall water splitting (ZOWS) comprising two-step excitation of a hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) has attracted significant interest, because narrow bandgap photocatalysts can be applied to this process if these materials are active during either the hydrogen or oxygen evolution reactions. However, the past ZOWS systems mostly used wide bandgap semiconductors as either HEPs or OEPs, restricting the use of long-wavelength visible light. This work extends the wavelength range of visible light that can be used for ZOWS by improving the performance of a narrow bandgap oxynitride (BaTaO₂N) as the HEP and by selecting an appropriate redox couple for electron transfer. This represents an essential advance towards achieving the solar energy conversion efficiency required for practical photocatalytic hydrogen production. The practice of comprehensive refinement of narrow bandgap oxynitride photocatalysts, cocatalysts, and redox mediators effective for ZOWS will guide the research and development of photocatalytic solar hydrogen production.

Introduction

Z-scheme overall water splitting (ZOWS) comprising two-step excitation of a H₂-evolving photocatalyst (HEP) and O₂-evolving photocatalyst (OEP) has attracted significant interest as a means of harvesting solar energy. Narrow-bandgap materials can be applied to this process if these compounds are active during either the hydrogen or oxygen evolution reactions, and photocatalyst sheets have exhibited efficient scalable ZOWS.¹ The use of p-type semiconductors as HEPs is likely to be essential in the sheet system because the ZOWS mechanism is similar to that for photoelectrochemical cells. Many Cu-based chalcogenides are narrow-bandgap p-type semiconductors, but improving the durability and efficiency of such materials in ZOWS remains challenging.^2,3 In contrast, ZOWS systems based on a suspended HEP and OEP together with ionic electron mediators can operate even when both the HEP and OEP are n-type semiconductors. This is possible because charge transfer between the HEP and OEP can occur via photocatalytic reactions of mediator ions instead of at junctions between bulk solid materials.^4–8 Many ZOWS systems have been reported to date. As an example, Qi et al. demonstrated a ZrO₂/TaON–[Fe(CN)₆]^3−/4−–BiVO₄ system.⁹ The ZrO₂/TaON and BiVO₄ photocatalysts can utilize visible light up to 500 and 520 nm, respectively. However, achieving the target solar-to-hydrogen energy conversion efficiency (STH) of 5% or greater required for practical photocatalytic hydrogen production will necessitate the utilization of materials capable of using solar radiation at wavelengths above 530 nm for ZOWS.

BaTaO₂N has an absorption edge wavelength of 650 nm and so is a promising candidate for photocatalytic water splitting under visible light.^10–15 Single-crystalline particulate BaTaO₂N can be obtained by RbCl flux-assisted nitridation.¹⁶ When loaded with a Pt cocatalyst decorated with Na, this compound exhibits excellent hydrogen evolution activity from aqueous methanol solutions, while unmodified BaTaO₂N is inactive.¹⁷ The addition of Na ensures wide dispersion of the Pt nanoparticles during the hydrogen reduction treatment and these highly-dispersed nanoparticles effectively catalyze the hydrogen evolution reaction (HER) on the BaTaO₂N. Domen et al. reported ZOWS using BaTaO₂N as the HEP, WO₃ as the OEP and IO₃⁻/I⁻ as the ionic shuttle.^16,17 Despite the ability of BaTaO₂N to utilize visible light, the use of WO₃, which has a wide bandgap, restricts the use of wavelengths above 450 nm by this system. Therefore, it would be an important breakthrough to combine BaTaO₂N with OEPs having narrower bandgaps, such as BiVO₄.¹⁸ Unfortunately, the low activity of BaTaO₂N in the presence of reversible redox mediators hampers the realization of such ZOWS systems.

Doping is a simple but effective means of tuning the properties of photocatalysts so as to improve their activity. In the case of BaTaO₂N, doping with Zr tends to enhance performance by suppressing the formation of Ta³⁺ defects during the thermal nitridation process. Abe et al. demonstrated a negative shift in the photoanodic current onset potential when using Zr-doped BaTaO₂N.¹⁹ Our own group also synthesized BaZrO₃–BaTaO₂N solid solutions that exhibited improved photocatalytic hydrogen evolution from water under visible light.²⁰ Accordingly, it is expected that the hydrogen evolution activity of BaTaO₂N synthesized using nitridation in conjunction with a RbCl flux could be improved by doping with Zr. Another important consideration related to ZOWS systems is the ionic electron mediator, which can induce backward reactions. It is therefore important to improve not only the photoreduction activity but the selectivity for the HER associated with the reduction of the ionic electron mediator. This can be accomplished by surface modification of the BaTaO₂N photocatalyst to control the reaction selectivity.

The present work devised an efficient ZOWS system based on Zr-doped BaTaO₂N (denoted herein as BaTaO₂N:Zr) as the HEP combined with BiVO₄ modified with Au and CoO_x (denoted as CoO_x/Au/BiVO₄) as the OEP and [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as the redox mediator.⁹ The BaTaO₂N:Zr photocatalyst was additionally loaded with a Pt cocatalyst together with Na, as well as with Cr₂O₃.²¹ The resulting ZOWS system (denoted as Cr₂O₃/Na–Pt/BaTaO₂N:Zr–[Fe(CN)₆]^3−/4−–CoO_x/Au/BiVO₄) exhibited stable H₂ and O₂ evolution at the expected stoichiometric molar ratio during a 30 h trial under simulated sunlight with a STH of 0.022%. The effects of the Zr doping, Na–Pt cocatalyst loading and Cr₂O₃ coating on the ZOWS activity of this material were evaluated.

Results and discussion

Cation-doped BaTaO₂N was prepared via the thermal nitridation of a mixture of the corresponding starting materials with an RbCl flux. The effect of doping was investigated by assessing the hydrogen evolution activity of doped BaTaO₂N in an aqueous methanol solution under visible light after loading the Na–Pt cocatalysts. As shown in Fig. 1a, Zr doping enhanced the hydrogen evolution activity of the BaTaO₂N in aqueous methanol while the other dopants investigated in this work actually lowered the activity. Zr⁴⁺ has the closest valency to Ta⁵⁺ and the disturbance of the anion compositions by cationic doping would be minimum, which may account for the lower hydrogen evolution reaction activity. On this basis, the optimal Zr doping amount was also investigated. The incorporation of 1% Zr in the BaTaO₂N enhanced the photocatalytic H₂ evolution activity to the greatest extent while higher doping levels reduced the activity (Fig. 1b).

Fig. 1 Photocatalytic H₂ evolution rates obtained for (a) Na–Pt/BaTaO₂N doped with various cations (M) with a M/Ta ratio of 0.01 and (b) Na–Pt/BaTaO₂N:Zr with different Zr/Ta molar ratios in an aqueous solution containing methanol (15 vol%). Reaction conditions: 0.23 wt% Na 0.3 wt% Pt/BaTaO₂N photocatalyst, 100 mg; 15 vol% aqueous methanol solution, 150 mL; light source, 300 W Xenon lamp equipped with a cut-off filter (420 nm < λ < 800 nm). Reaction system, Pyrex top-illuminated vessel connected to a closed gas-circulation system with a 10 kPa Ar background pressure.

Fig. 2a presents powder XRD patterns obtained from crystals of BaTaO₂N:Zr with Zr/Ta = 0, 0.01 and 0.1. The major product in all cases was evidently a perovskite-type material based on a comparison with the standard JCSD pattern for bulk BaTaO₂N.¹⁷ However, some small peaks ascribed to Ta₃N₅ were observed even in the pattern for the undoped BaTaO₂N. The formation of Ta₃N₅ in the undoped BaTaO₂N was attributed to a loss of barium through volatilization during the high-temperature nitridation process.²² Upon adding Zr, a larger amount of Ta₃N₅ was generated as a by-product because Zr⁴⁺ ions were substituted into the Ta⁵⁺ sites and a corresponding amount of Ta₂O₅ was segregated and nitrided into Ta₃N₅. The deficiency of barium species with respect to the B-site elements (tantalum and zirconium) was further confirmed by chemical composition analysis (Table S1, ESI†). The Ba/(Ta + Zr) atomic ratio was found to decrease from 1.01 in BaTaO₂N to 0.95 and 0.88 in BaTaO₂N:Zr0.01 and BaTaO₂N:Zr0.1, respectively. The specific surface areas of undoped BaTaO₂N and BaTaO₂N:Zr0.01 were 2.3 and 2.9 m² g⁻¹, respectively, suggesting a decrease in the average particle size of BaTaO₂N by Zr doping. This may partly account for the enhancement in the photocatalytic H₂ evolution activity at the low doping amount. Notably, the Zr/Ta ratio in the nitrided products was virtually the same as that in the corresponding starting materials. This result indicates that the added Zr species were not lost during the flux-assisted nitridation and subsequent rinsing processes. Despite the substitution of the lower valence element Zr for Ta, the O/(N + O) molar ratio was unchanged, possibly as a consequence of the formation of Ta₃N₅ (as was indicated by the XRD data). A broad absorption peak in the range of 700–750 nm, characteristic of Ta₃N₅, was not observed in the DRS spectra (Fig. 2b). However, the addition of Zr shifted the light absorption onset to shorter wavelengths with increasing concentration. Doping with lower valence cations results in exchange of nitride ions with oxide ions, decreasing the nitride ion content of the material to maintain the charge neutrality. As a result, the band gap was widened by doping Zr.

Fig. 2 (a) XRD patterns, (b) DRS spectra, and TAS decays probed at (c) 5000 cm⁻¹ (2000 nm; corresponding to electron dynamics) and (d) 15 400 cm⁻¹ (649 nm; corresponding to hole dynamics) with excitation at 470 nm for BaTaO₂N, BaTaO₂N:Zr0.01 and BaTaO₂N:Zr0.1.

The effect of zirconium doping on the dynamics of photogenerated electrons and holes was studied by TAS. Fig. 2c and d provide transient absorption intensity profiles acquired at 5000 cm⁻¹ (2000 nm) and 15 400 cm⁻¹ (649 nm), respectively. These data reflect the dynamics of the intraband transitions of free and/or shallowly trapped electrons and photoexcited holes, respectively.^23,24 The addition of 1 mol% Zr evidently increased the lifetime of shallowly trapped electrons, in agreement with earlier work by Hojamberdiev et al.,²⁵ but had no appreciable effect on the dynamics of trapped holes. Doping with lower valence cations may suppress the formation of reduced Ta species acting as recombination centers. However, according to the Ta 4f XPS analysis, the chemical states of Ta species were not appreciably changed by Zr-doping, probably due to the low dopant concentration (Fig. S1, ESI†). In-depth analysis of semiconducting properties and defects of particulate oxynitride materials will be required to clarify the physicochemical origin for the prolonged electron lifetime. In contrast, doping with 10 mol% Zr accelerated the decay of both electrons and holes. It is likely that the formation of Ta₃N₅ impurities, which typically have short carrier lifetimes, promoted charge recombination in the specimens.²⁶ This effect would explain why excessive Zr doping lowered the H₂ evolution activity.

Fig. 3 shows a TEM image of a crystal of the cocatalyst-loaded BaTaO₂N:Zr0.01, which exhibited the best H₂ evolution activity. A nearly cubic particle with 100–300 nm in size with clear facets can be seen. Note that rod-like particles typically formed by Ta₃N₅ crystals were not observable due to the low amount. The well-defined SAED pattern with no additional diffraction spots (inset to Fig. 3) and the clear lattice fringes observed by HRTEM (inset to Fig. 3) indicate that these BaTaO₂N:Zr0.01 particles were single crystals. Elemental mapping of the cross section of a BaTaO₂N:Zr0.01 sample with STEM-EDS (Fig. 4) confirmed that the Zr dopant was not segregated from the BaTaO₂N particles. Fig. 4 also shows that the other four elements (Ba, Ta, O and N) were uniformly distributed throughout the BaTaO₂N:Zr crystals. These results indicate the homogeneous composition and single-crystalline nature of the BaTaO₂N:Zr0.01 crystals.

Fig. 3 TEM, HRTEM (inset) and SAED (inset) images of the cross-section of a Cr₂O₃/Na–Pt loaded BaTaO₂N:Zr0.01 sample.

Fig. 4 ADF-STEM image and corresponding STEM-EDS elemental maps of the cross-section of a Cr₂O₃/Na–Pt loaded BaTaO₂N:Zr0.01 sample.

TAS was also used to examine the effect of zirconium doping on the interaction of the photocatalyst and the Na–Pt cocatalyst (Fig. 5a and b). The addition of the Na–Pt cocatalyst to the BaTaO₂N was found to accelerate the decay of electrons while inhibiting that of holes, similar to the findings of our previous work.¹⁷ This effect occurred because the Na–Pt cocatalyst captured electrons in the BaTaO₂N. Interestingly, the acceleration of the electron decay following cocatalyst loading was stronger in the case of the BaTaO₂N:Zr0.01, suggesting that electron injection into the Na–Pt cocatalyst was promoted by Zr doping. However, the decay of holes was not retarded appreciably. Zr doping may induce deep-trapping and quenching of photoexcited holes by modifying the anion composition that, in turn, affects the valence band structure of the BaTaO₂N. Additional in-depth spectroscopic investigations will be required to better explain the TAS observations in the present work.

Fig. 5 TAS decays probed at (a) 5000 cm⁻¹ (2000 nm; corresponding to electron dynamics) and (b) 15400 cm⁻¹ (649 nm; corresponding to hole dynamics) with excitation at 470 nm for BaTaO₂N:Zr0.01 and BaTaO₂N before and after the addition of Na–Pt cocatalysts.

The hydrogen evolution activity of a HEP in an aqueous solution containing an ionic redox mediator depends on the characteristics of the mediator, such as the redox potential, number of electrons involved in the redox reaction, and adsorption properties.⁹ Thus, an appropriate redox mediator for the present ZOWS system was determined by experimental screening. The hydrogen evolution activities of BaTaO₂N:Zr0.01 samples loaded with the Na–Pt cocatalyst followed by the photodeposition of Cr₂O₃ from aqueous K₄[Fe(CN)₆], NaI or FeCl₂ solutions were evaluated. Here, Cr₂O₃ was employed to suppress backward reactions (i.e., the reduction of oxidized forms of the redox shuttle).²⁷ As seen in Fig. 6a, the Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 was almost inactive in aqueous solutions containing I⁻ or Fe²⁺ shuttle ions, while the same photocatalyst exhibited decent activity when [Fe(CN)₆]⁴⁻ was employed as the electron donor. The [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox couple has a more negative redox potential than the IO₃⁻/I⁻ or Fe³⁺/Fe²⁺ redox mediators.⁹ As a result, photogenerated holes could be transferred from the valence band of the BaTaO₂N:Zr0.01 to the [Fe(CN)₆]⁴⁻ ions more efficiently. In addition, the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox couple works in less extreme pH environments (pH 6–7) than the Fe³⁺/Fe²⁺ mediator (pH 2–3, at which Cr₂O₃ is unstable). The former couple is also cycled by accepting/donating one electron instead of multiple electrons such as the six electrons required by the IO₃⁻/I⁻ shuttle. These features allowed the Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 to evolve hydrogen efficiently and steadily, and so the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox shuttle shows promise as a component of ZOWS systems.

Fig. 6 (a) Photocatalytic H₂ evolution activities of Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 (100 mg) in aqueous solutions of NaI, FeCl₂ and K₄[Fe(CN)₆] (150 mL, 2 mM each) with pH values of 6–7, 2.3 and 6, respectively. (b) Time courses of H₂ evolution over Cr₂O₃/0.23 wt% Na 0.3 wt% Pt/BaTaO₂N:Zr0.01 (100 mg) with varying amounts of Cr in an aqueous 50 mM sodium phosphate buffer solution at pH 6 (150 mL) containing 6 mM K₄[Fe(CN)₆]. Light source: 300 W xenon lamp (420 nm < λ < 800 nm).

Pt nanoparticles were well used as a cocatalyst for hydrogen evolution.^28,29 Note that it was important to apply a Cr₂O₃ layer to the Na–Pt nanoparticles to suppress backward reactions during the HER. As shown in Fig. 6b, without Cr₂O₃ deposition, hydrogen evolution ceased after a few hours due to the progression of backward reactions. With the optimal amount of Cr₂O₃, the H₂ evolution performance of the Na–Pt/BaTaO₂N:Zr0.01 was promoted because the access of [Fe(CN)₆]³⁻ to the Na–Pt cocatalyst was suppressed.³⁰ The optimum amount of Cr based on photodeposition was found to be 0.9 wt% with respect to the photocatalyst mass. Greater amounts of Cr actually lowered the hydrogen evolution activity because of the excessive photodeposition of Cr₂O₃ in the presence of methanol. The action spectrum of a Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 photocatalyst for the hydrogen evolution reaction from aqueous K₄[Fe(CN)₆] solution is provided in Fig. S2 (ESI†). The photocatalyst exhibited the hydrogen evolution activity up to 640 nm, which was consistent with the absorption edge of the material. The AQY at 420 nm was measured to be 0.4%.

Fig. 7 provides ADF-STEM, TEM, and STEM-EDS elemental maps of BaTaO₂N:Zr0.01 loaded with 0.23 wt% Na 0.3 wt% Pt and Cr₂O₃ (0.9 wt% Cr). The loading of Pt nanoparticles along with Na was studied in our recent work.¹⁷ According to our previous work, the Pt nanoparticles in this sample had sizes in the range of 2–8 nm. The addition of Na improved the dispersion and structural stability of the Pt cocatalyst, although some cocatalyst particles are seen to have aggregated to form larger secondary particles on the edges of the BaTaO₂N crystal. Na species were not observed because of the low loading amount and the poor sensitivity of EDS for this element. The Cr species appeared to be present in the same positions as the Pt, suggesting the formation of a layer of Cr₂O₃ covering the Pt nanoparticles as in earlier reports,²¹ but were distributed over a somewhat wider area.

Fig. 7 (a) TEM and ADF-STEM (inset) images and (b) ADF-STEM image along with STEM-EDS elemental maps of the cross-section of a 0.9 wt% Cr₂O₃/0.23 wt% Na 0.3 wt% Pt/BaTaO₂N:Zr0.01 sample.

The BaTaO₂N:Zr0.01 photocatalyst loaded with Cr₂O₃/Na–Pt cocatalysts was found to function as a HEP in a ZOWS system in combination with CoO_x/Au/BiVO₄ as the OEP and [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as a redox mediator (Fig. 8), whereas the amounts of gaseous products were below the detection limits in the absence of the redox mediator. The same HEP and OEP showed H₂ and O₂ evolution activities from aqueous solutions containing the corresponding redox mediators, respectively (Fig. S3, ESI†). These observations indicate the occurrence of Z-scheme via the electron transfer mediated by the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox couple. The ZOWS performance of this system was examined under simulated sunlight with evacuation of the test apparatus followed by the reintroduction of Ar to 10 kPa at 10 h intervals. Both H₂ and O₂ were evolved at a molar ratio close to the 2 : 1 stoichiometric ratio, and the STH of this redox-mediated ZOWS system was determined to be 0.022% in the initial run. The AQY values at 420 nm and 520 nm under monochromatic light were 1.5% and 0.2% respectively (Fig. S4, ESI†). Compared with the previously reported BaTaO₂N-WO₃ ZOWS system, this system using BiVO₄ instead of WO₃ was able to harvest longer-wavelength visible light up to approximately 520 nm. This performance was enabled by doping with 1 mol% Zr, which promoted H₂ evolution on the BaTaO₂N, as well as by the modification of the Na–Pt cocatalyst with Cr₂O₃ to inhibit reverse reactions and the use of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as a suitable redox mediator.

Fig. 8 Time courses of gas evolution during the ZOWS reaction using 70 mg of Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 as the HEP under simulated sunlight. Conditions: 0.9 wt% Cr₂O₃/0.23 wt% Na 0.3 wt% Pt/BaTaO₂N:Zr0.01 as the photocatalyst; 0.5 wt% CoO_x/0.2 wt% Au/BiVO₄, 100 mg; 150 mL 25 mM sodium phosphate buffer solution (pH 6) containing K₄[Fe(CN)₆] (6 mM) under simulated sunlight (AM 1.5G, 92 mW cm⁻², 5.5 × 5.5 cm). Reaction system: Pyrex top-illuminated vessel connected to a closed gas-circulation system, 10 kPa Ar background atmosphere.

The H₂ evolution rate obtained for the Cr₂O₃/Na–Pt modified BaTaO₂N:Zr0.01 in an aqueous [Fe(CN)₆]⁴⁻ solution (37 μmol h⁻¹; Fig. 6) was slightly lower than the O₂ evolution rate for CoO_x/Au/BiVO₄ in a [Fe(CN)₆]³⁻ solution (51 μmol h⁻¹; Fig. S3, ESI†). These data indicate that the rate-determining component of this ZOWS system was the Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 photocatalyst. In fact, the present system using Cr₂O₃/Na–Pt/BaTaO₂N:Zr as the HEP showed inferior AQY and STH values compared with an earlier system using RhCrO_x/ZrO₂/TaON.⁹ Thus, it is still necessary to improve the preparation of the BaTaO₂N:Zr so that more photoexcited charge carriers become available at the surface and to enhance the injection of electrons into the cocatalysts. The present ZOWS system was also found to lose 27% of its original activity when reused, unlike a previously reported system using ZrO₂/TaON loaded with a Rh_2−yCr_yO₃ cocatalyst as the HEP.⁹ The evolution of N₂ was not detected and thus deterioration of the BaTaO₂N:Zr0.01 photocatalyst during the 30 h trial was likely negligible. The XRD pattern of the Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 photocatalyst was also not changed appreciably during the hydrogen evolution reaction (Fig. S5A, ESI†). Therefore, the activity loss was presumably a result of the degradation of the Cr₂O₃/Na–Pt cocatalyst. Degradation of the hydrogen evolution activity of Na–Pt/BaTaO₂N was also observed in our previous work.¹⁷ XPS analysis of the Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 photocatalyst before and after a ZOWS reaction suggests that the BaTaO₂N:Zr0.01 photocatalyst was apparently intact while the Pt cocatalyst was slightly oxidized during the reaction (Fig. S5, ESI†). This may be associated with the degradation of the HER and ZOWS activities. Our previous work showed that coloading of an appropriate water oxidation cocatalyst can improve the durability of the cocatalyst by reducing the probability of hole injection into the hydrogen evolution cocatalyst.^31,32 At present, it is difficult to coload BaTaO₂N with hydrogen evolution cocatalysts and oxygen evolution cocatalysts without sacrificing photocatalytic activity because of the insufficient reactivity and stability of the materials. The development of new technologies for the preparation and modification of narrow-bandgap particulate oxynitride photocatalysts will be vital to the future design of efficient solar-to-chemical energy conversion processes.

Experimental

Synthesis of Zr-doped BaTaO₂N

BaTaO₂N:Zr samples were synthesized by RbCl (95.0%; FUJIFILM Wako Pure Chemical Corporation) flux-assisted nitridation of a mixture of ZrO(NO₃)₂·2H₂O (99.5%; Wako Pure Chemical Industries, Ltd.), BaCO₃ (99.9%; Kanto Chemical Co., Inc.) and Ta₂O₅ (99.9%; High Purity Chemicals). Note that all chemicals employed in this study were used directly as received. In a typical synthesis, a certain amount of the mixture (in which the Ba/Ta molar ratio was 1.1 regardless of the Zr/Ta molar ratio) and RbCl were blended thoroughly so that the solute concentration in RbCl flux was 10 mol% and the total mass of solute and RbCl flux was 2 g. The mixture was subsequently heated under an ammonia flow (200 mL min⁻¹) at 1223 K for 8 h, based on a procedure previously reported by our group.¹⁷ The resulting BaTaO₂N:Zr was washed with water and dried to give the final product. The BaTaO₂N:Zr specimens having varying Zr doping levels are referred to herein as BaTaO₂N:Zrx, where x represents the Zr/Ta molar ratio and ranges from 0 to 0.1. BaTaO₂N specimens doped with other cations at the cation/Ta molar ratio of 0.01 were also prepared for comparison purposes.

Synthesis of BiVO₄

BiVO₄ was prepared by a hydrothermal procedure.⁹ In this process, 5 mmol of NH₄VO₃ (99.0%; FUJIFILM Wako Pure Chemical Corporation) and 5 mmol of Bi(NO₃)₃·5H₂O (99.5%; FUJIFILM Wako Pure Chemical Corporation) were dissolved in 50 mL of a 2.0 M nitric acid solution (65%; FUJIFILM Wako Pure Chemical Corporation). The pH of this solution was then adjusted to 1.0 with an ammonia solution (28 wt%; FUJIFILM Wako Pure Chemical Corporation) while stirring the mixture, which generated a light-yellow precipitate. The solution was allowed to age for 2 h with stirring, after which the precipitate was removed and transferred to a Teflon-lined stainless-steel autoclave with a capacity of 100 mL and was hydrothermally treated at 453 K for 11 h. The single phase of diffraction structure of as-prepared BiVO₄ (PDF No. 14-0688) was confirmed as shown in Fig. S6 (ESI†).⁹

Deposition of cocatalysts and characterization

The cocatalysts were deposited on the BaTaO₂N:Zr by impregnation followed by hydrogen reduction.¹⁷ In a typical process, a 110 mg quantity of BaTaO₂N:Zr was immersed in an aqueous solution containing NaOH (0.1 mol L⁻¹; FUJIFILM Wako Pure Chemical Corporation) and H₂PtCl₆·6H₂O (98.5%; FUJIFILM Wako Pure Chemical Corporation) as the precursors. The amounts of Pt and Na in the solution were 0.3 and 0.23 wt%, respectively, with respect to the photocatalyst mass. The solution was subsequently heated on a boiling water bath, after which the product was completely dried and then heated at 523 K for 30 min in a flow of 10% H₂ in N₂. The resulting Na–Pt/BaTaO₂N:Zr was additionally modified with Cr₂O₃ by photodeposition to suppress backward reactions.²¹ This was accomplished by dispersing a quantity of the Na–Pt/BaTaO₂N:Zr powder in 100 mL of an aqueous solution of methanol (15 vol%), after which K₂CrO₄ (99.0%; FUJIFILM Wako Pure Chemical Corporation) was added as a Cr⁶⁺ precursor without pH adjustment. The amount of Cr₂O₃ was equivalent to a loading of 0.9 wt% Cr with respect to the photocatalyst mass. After complete degassing, the suspension was irradiated with visible light (420 nm < λ < 800 nm) for 1 h. The photocatalyst was then removed by filtering, washed with ultrapure water and dried at 313 K under vacuum.

The deposition of Au and CoO_x as reduction and oxidation cocatalysts, respectively, on the BiVO₄ was carried out using a stepwise photodeposition process previously reported in the literature.⁹ The prepared BiVO₄ (150 mg) was dispersed in deionized water to which a specific amount of HAuCl₄·4H₂O (99.0%; FUJIFILM Wako Pure Chemical Corporation) was added, equivalent to 0.2 wt% Au relative to the oxide mass. Thereafter, the suspension was irradiated under visible light (420 nm < λ < 800 nm) for 1 h, after which the powder was collected by filtration, washed and dried. The as-obtained powder was subsequently loaded with CoO_x (0.5 wt%) by photodeposition. This was accomplished by dispersing the powder in 100 mL of a 50 mM sodium potassium buffer solution (KH₂PO₄, 99.5%; FUJIFILM Wako Pure Chemical Corporation; 44 mM; Na₂HPO₄, 99.0%; FUJIFILM Wako Pure Chemical Corporation; 6 mM) at pH 6.0. A quantity of K₃[Fe(CN)₆] (99.0%; FUJIFILM Wako Pure Chemical Corporation) sufficient to give a concentration of 5 mM was added along with a specific amount of Co(NO₃)₂·6H₂O (98.0%; FUJIFILM Wako Pure Chemical Corporation), and the mixture was illuminated for 1 h. The resulting CoO_x/Au/BiVO₄ has been shown to work as an excellent OEP in ZOWS systems involving [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as the redox mediator.⁹

X-Ray diffraction (XRD) patterns were acquired using a Rigaku MiniFlex 300 powder diffractometer with a Cu Kα radiation source (λ = 1.5418 Å). The data were analyzed after mathematically removing the contribution from the Kα₂ line. (Scanning) transmission electron microscopy ((S)TEM) images, energy dispersive X-ray spectroscopy (EDS) mapping images, selected area electron diffraction (SAED) patterns, high-resolution transmission electron microscopy (HRTEM) and annular dark-field (ADF) STEM images were obtained with a JEOL JEM-2800 system equipped with an Oxford Instruments X-MAX 100TLE SDD detector. Diffuse reflectance spectroscopy (DRS) data were acquired with an ultraviolet-visible-near-infrared spectrometer (V-670, JASCO) and converted from reflectance to the Kubelka–Munk function. The Ba, Ta and Zr concentrations in the BaTaO₂N:Zr were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; ICPS-8100, Shimadzu). The oxygen and nitrogen contents of the BaTaO₂N were obtained using an oxygen–nitrogen combustion analyzer (Horiba, EMGA-620W). X-Ray photoelectron spectroscopy (XPS) was performed using a PHI Quantera II spectrometer with an Al Kα radiation source. All binding energies were referenced to the C 1s peak (284.8 eV) arising from adventitious carbon. The specific surface area of the samples was calculated from nitrogen adsorption isotherms measured at 77 K with a BELSORP Mini II apparatus (MicrotracBEL) using the Brunauer–Emmett–Teller method.

Transient absorption (TA) spectroscopic measurements were carried out using a Nd:YAG laser system (Continuum, Surelite I; pulse duration: 6 nm) equipped with custom-built spectrometers.³³ The probe beams coming from the halogen lamp (producing visible light) and MoSi₂ coil (producing IR light) were used in this measurement for monitoring the absorption signals at 649 and 2000 nm, respectively. In the case of 2000 nm probing, the beam was focused on the sample and was transmitted from the sample and then detected by an mercury cadmium telluride detector. For the 649 nm probing, we used reflection mode, of which the incident probe beam was focused on the sample and then the reflected light from the sample was detected by a Si photodetector. The output electric signal was amplified with an AC-coupled amplifier (Stanford Research Systems, SR560, 1 MHz). The time resolution of the spectrometer was limited to 1 μs by the response of the photodetectors. One thousand responses were accumulated to obtain the transient profiles (that is, the decay curves) at 649 and 2000 nm. The various BaTaO₂N:Zr photocatalysts loaded with Na and/or Pt were photoexcited using 470 nm pump pulses with a fluence of 3 mJ pulse⁻¹. Each powder sample was fixed on a circular CaF₂ substrate by drop-casting at a density of 1.24 mg cm⁻². These TAS analyses were carried out in a vacuum at room temperature.

Photocatalytic H₂ evolution reaction and OWS reaction

The HER over each BaTaO₂N photocatalyst was performed at room temperature in a Pyrex top-irradiation reaction vessel connected to a glass closed gas circulation system. In each trial, a quantity of Na–Pt/BaTaO₂N:Zr (100 mg) was dispersed in an aqueous methanol solution (15 vol%, 150 mL) using a magnetic stirrer. The vessel was subsequently evacuated several times to ensure complete air removal. The activity of a photocatalyst can depend on the pressure in the reaction system.¹⁶ Thus, to minimize the effects of changes in pressure resulting from gas evolution and heating of the reaction solution during irradiation, Ar was introduced into the reaction system prior to the reaction to a pressure of approximately 10 kPa. The reaction was initiated by irradiation with a 300 W Xe lamp fitted with a cutoff filter and a dichroic mirror, emitting at 420 nm < λ < 800 nm. A flow of cooling water was used to keep the suspension at approximately 283 K throughout the trial. The evolved gases were analyzed by gas chromatography (GC; Shimadzu, GC-8A with a thermal conductivity detector, MS-5 A columns and Ar as the carrier gas). The H₂ evolution rates obtained from aqueous solutions of NaI, FeCl₂ and K₄[Fe(CN)₆] and the O₂ evolution rates from an aqueous K₃[Fe(CN)₆] solution were also examined. In each case, an aqueous solution (150 mL) containing the photocatalyst sample (100 mg) was prepared and the pH was adjusted to a specific value for NaI (pH 6–7, 2 mM), FeCl₂ (pH 2.3, 2 mM), K₄[Fe(CN)₆] (pH 6, 2 mM) and K₃[Fe(CN)₆] (pH 6, 5 mM). An aqueous HNO₃ solution was used to adjust the pH of the FeCl₂ solution while the pH values of the K₄[Fe(CN)₆] and K₃[Fe(CN)₆] solutions were maintained at 6 using a 50 mM sodium potassium buffer solution.

ZOWS reactions were carried out using the same reactor and system described above. In each experiment, Cr₂O₃/Na–Pt/BaTaO₂N:Zr (70 mg) and CoO_x/Au/BiVO₄ (100 mg), acting as the HEP and OEP, respectively, were suspended in 150 mL of an aqueous sodium potassium buffer solution (pH 6.0, 25 mM) containing K₄[Fe(CN)₆]·3H₂O (99.5%; FUJIFILM Wako Pure Chemical Corporation; 6 mM). After complete degassing, Ar gas was introduced to the reaction system to an initial pressure of 10 kPa. Following this, the suspension was irradiated with either a Xe lamp or solar simulator (AM 1.5G, 92 mW cm⁻², 5.5 × 5.5 cm), and a cooling water system was employed to maintain the solution at 283 K, as described above. The evolved gases were also analyzed by the same GC instrumentation.

Apparent quantum yield (AQY) and STH

The AQY values for the ZOWS reactions were determined using a procedure similar to that described in our previous paper.³⁴ The same light source was employed but it was equipped with various band-pass filters. The number of incident photons illuminating the reaction cell was measured using a grating spectroradiometer (LS-100, EKO Instruments). The AQY was then calculated as

AQY (%) = [A × R]/I × 100, (1)

where R and I represent the amounts of evolved H₂ and O₂ and the number of incident photons per unit time, respectively, and A is the number of electrons and holes consumed to generate one molecule of H₂ and one molecule of O₂. The values of A are 4 and 8 for Z-scheme water splitting based on two-step photoexcitation, respectively. The average of the AQY values calculated for H₂ and O₂ values was adopted.

The STH values for ZOWS reactions under simulated solar radiation were calculated as

STH (%) = (R(H₂) × ΔG_r)/(P × S) × 100, (2)

where R(H₂), ΔG_r, P, and S denote the rate of hydrogen evolution determined from the rate of water splitting during the ZOWS reaction, the Gibbs energy change for the reaction H₂O (l) → H₂ (g) + 1/2 O₂ (g), the energy intensity of the AM 1.5G solar irradiation (92 mW cm⁻²) and the irradiated sample area (5.5 × 5.5 cm), respectively. The rate of water splitting was assumed to equal the average of the H₂ evolution rate and twice the O₂ evolution rate based on the reaction stoichiometry. The value of ΔG_r used for the calculations was 229 kJ mol⁻¹ at 283 K.³⁴

Conclusions

This work demonstrated the effective modification of a BaTaO₂N photocatalyst by Zr doping and by incorporating a Na–Pt cocatalyst together with a Cr₂O₃ coating to produce a HEP for ZOWS. Doping with 1 mol% Zr extended the lifetime of electrons and promoted electron injection into the Na–Pt cocatalyst, leading to improved H₂ evolution activity for an aqueous methanol solution under visible light. The Cr₂O₃ coating also allowed the Na–Pt/BaTaO₂N:Zr0.01 photocatalyst to evolve H₂ from an aqueous [Fe(CN)₆]^3−/4− solution.

A ZOWS system based on Cr₂O₃/Na–Pt/BaTaO₂N:Zr0.01 as the HEP, CoO_x/Au/BiVO₄ as the OEP and [Fe(CN)₆]^3−/4− as redox ions evolved H₂ and O₂ under visible light up to 520 nm. The STH efficiency of this process was 0.022%. This newly developed ZOWS system was able to harvest visible light with longer wavelengths than earlier systems utilizing TaON as the HEP or WO₃ as the OEP. This achievement was made possible by improving the performance of the BaTaO₂N as the HEP and also by the selection of [Fe(CN)₆]^3−/4− as an appropriate redox couple. It is therefore important to optimize the photocatalyst, cocatalysts and reaction conditions to enhance the activity of ZOWS systems operating under long-wavelength visible light.

Author contributions

The manuscript was written through contributions of all authors. H. L. designed and performed the experiments, wrote and edited the manuscript. J. J. M. V. acquired the TAS spectra and analyzed these data together with A. Y. T. H. edited the manuscript and discussed the scientific results as a supervisor. M. N. characterized the morphology and composition of each specimen using STEM-EDS and HRTEM together with N. S. J. X., Z. P. W. L. and W. Z. discussed the scientific results. X. T. and S. C. discussed the synthesis of BiVO₄. T. T. provided resources as a supervisor. K. D. edited the manuscript and provided resources as a supervisor. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (grant no. 51402139), the China Scholarship Council (grant no. 201906185004 and 202008440289), the Artificial Photo-synthesis Project (ARPChem) of the New Energy and Industrial Technology Development Organization (NEDO), and by JST, PRESTO, Japan (grant no. JPMJPR20T9). Part of this study was supported by the University of Tokyo Advanced Characterization Nanotechnology Platform as part of the Nanotechnology Platform Project sponsored By the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (grant no. JPMXP09A-21-UT-0046). W. L. thanks 2021 MEXT Scholarship with Embassy Recommendation for financial support. The authors thank Ms Michiko Obata of Shinshu University for her assistance during the XPS analyses. The authors thank Dr Taro Yamada, Ms Keiko Kato and Ms Yasuko Kuromiya of the University of Tokyo for their assistance in the ICP-OES and oxygen–nitrogen combustion analyses.

Notes and references

1. Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I. Sharp, A. Kudo, T. Yamada and K. Domen, Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%, Nat. Mater., 2016, 15, 611–615.
2. K. Iwashina, A. Iwase, Y. Ng, R. Amal and A. Kudo, Z-Schematic Water Splitting into H₂ and O₂ Using Metal Sulfide as a Hydrogen-Evolving Photocatalyst and Reduced Graphene Oxide as a Solid-State Electron Mediator, J. Am. Chem. Soc., 2015, 137, 604–607.
3. S. Chen, J. Vequizo, Z. Pan, T. Hisatomi, M. Nakabayashi, L. Lin, Z. Wang, K. Kato, A. Yamakata, N. Shibata, T. Takata, T. Yamada and K. Domen, Surface Modifications of (ZnSe)_0.5(CuGa_2.5Se_4.25)_0.5 to Promote Photocatalytic Z-Scheme Overall Water Splitting, J. Am. Chem. Soc., 2021, 143, 10633–10641.
4. J. Nasir, A. Munir, N. Ahmad, T. Haq, Z. Khan and Z. Rehman, Photocatalytic Z-Scheme Overall Water Splitting: Recent Advances in Theory and Experiments, Adv. Mater., 2021, 33, 2105195.
5. B. Ng, L. Putri, X. Kong, Y. The, P. Pasbakhsh and S. Chai, Z-Scheme Photocatalytic Systems for Solar Water Splitting, Adv. Sci., 2020, 7, 1903171.
6. Y. Miseki and K. Sayama, Photocatalytic Water Splitting for Solar Hydrogen Production Using the Carbonate Effect and the Z-Scheme Reaction, Adv. Energy Mater., 2019, 9, 1801294.
7. K. Maeda, Z-Scheme Water Splitting Using Two Different Semiconductor Photocatalyst, ACS Catal., 2013, 3, 1486–1503.
8. R. Zhang, L. Zhang, Q. Zheng, P. Gao, J. Zhao and J. Yang, Direct Z-Scheme Water Splitting Photocatalyst Based on Two-Dimensional van der Waals Heterostructures, J. Phys. Chem. Lett., 2018, 9, 5419–5424.
9. Y. Qi, Y. Zhao, Y. Gao, D. Li, Z. Li, F. Zhang and C. Li, Redox-Based Visible-Light-Driven Z-Scheme Overall Water Splitting with Apparent Quantum Efficiency Exceeding 10%, Joule, 2018, 2, 2393–2402.
10. M. Hojamberdiev, K. Yubuta, J. Vequizo, A. Yamakata, S. Oishi, K. Domen and K. Teshima, NH₃-Assisted Flux Growth of Cube-like BaTaO₂N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity, Cryst. Growth Des., 2015, 15, 4663–4671.
11. S. Wei, S. Chang, F. Yang, Z. Fu, G. Liu and X. Xu, Stable and efficient solar-driven photoelectrochemical water splitting into H₂ and O₂ based on a BaTaO₂N photoanode decorated with CoO microflowers, Chem. Commun., 2021, 57, 4412–4415.
12. H. Zhang, S. Wei and X. Xu, Mg modified BaTaO₂N as an efficient visible-light-active photocatalyst for water oxidation, J. Catal., 2020, 383, 135–143.
13. S. Wei, G. Zhang and X. Xu, Activating BaTaO₂N by Ca modifications and cobalt oxide for visible light photocatalytic water oxidation reactions, Appl. Catal., B, 2018, 237, 373–381.
14. B. Dong, Y. Qi, J. Cui, B. Liu, F. Xiong, X. Jiang, Z. Li, Y. Xiao, F. Zhang and C. Li, Synthesis of BaTaO₂N oxynitride from Ba-rich oxide precursor for construction of visible-light-driven Z-scheme overall water splitting, Dalton Trans., 2017, 46, 10707–10713.
15. K. Hibino, M. Yashima, T. Oshima, K. Fujii and K. Maeda, Structures, electron density and characterization of novel photocatalysts, (BaTaO₂N)_1-x(SrWO₂N)_x solid solutions, Dalton Trans., 2017, 46, 14947–14956.
16. Z. Wang, Y. Luo, T. Hisatomi, J. Vequizo, S. Suzuki, S. Chem, M. Nakabayashi, L. Lin, Z. Pan, N. Kariya, A. Yamakata, N. Shibata, T. Takata, K. Teshima and K. Domen, Sequential cocatalyst decoration on BaTaO₂N towards highly-active Z-scheme water splitting, Nat. Commun., 2021, 12, 1005.
17. H. Li, D. Lu, S. Chem, T. Hisatomi, J. Vequizo, J. Xiao, Z. Wang, L. Lin, Q. Xiao, Y. Sun, Y. Miseki, K. Sayama, A. Yamakata, T. Takata and K. Domen, A Na-containing Pt cocatalyst for efficient visible-light-induced hydrogen evolution on BaTaO₂N, J. Mater. Chem. A, 2021, 9, 13851–13854.
18. A. Iwase, Y. Udagawa, S. Yoshino, Y. Ng, R. Amal and A. Kudo, Solar Water Splitting under Neutral Conditions Using Z-Scheme Systems with Mo-Doped BiVO₄ as an O₂-Evolving Photocatalyst, Energy Technol., 2019, 7, 1900358.
19. M. Higashi, Y. Yamanaka, O. Tomita and R. Abe, Fabrication of cation-doped BaTaO₂N photoanodes for efficient photoelectrochemical water splitting under visible light irradiation, APL Mater., 2015, 3, 104418.
20. K. Maeda, D. Lu and K. Domen, Solar-Driven Z-scheme Water Splitting Using Modified BaZrO₃-BaTaO₂N Solid Solutions as Photocatalysts, ACS Catal., 2013, 3, 1026–1033.
21. K. Maeda, K. Teramura, D. Lu, N. Saito, Y. Inoue and K. Domen, Noble-Metal/Cr₂O₃ Core/Shell Nanoparticles as a Cocatalyst for Photocatalytic Overall Water Splitting, Angew. Chem., 2006, 118, 7970–7973.
22. Y. Luo, Z. Wang, S. Suzuki, K. Yubuta, N. Kariya, T. Hisatomi, K. Domen and K. Teshima, Fabrication of Single-Crystalline BaTaO₂N from Chloride Fluxes for Photocatalytic H₂ Evolution under Visible Light, Cryst. Growth Des., 2020, 20, 255–261.
23. A. Yamakata and J. Vequizo, Curious Behaviors of Photogenerated Electrons and Holes at the Defects on Anatase, Rutile, and Brookite TiO₂ Powders: A Review, J. Photochem. Photobiol., C, 2019, 40, 234–243.
24. A. Yamakata, J. Vequizo, T. Ogawa, K. Kato, S. Tsuboi, N. Furutani, M. Ohtsuka, S. Muto, A. Kuwabara and Y. Sakata, Core-Shell Double Doping of Zn and Ca on β–Ga2O3 photocatalysts for Remarkable Water Splitting, ACS Catal., 2021, 11, 1911–1919.
25. M. Hojamberdiev, R. Vargas, Z. Kadirova, K. Kato, H. Sena, A. Krasnov, A. Yamakata, K. Teshima and M. Lerch, Unfolding the Role of B Site-Selective Doping of Aliovalent Cations on Enhancing Sacrificial Visible Light-Induced Photocatalytic H₂ and O₂ Evolution over BaTaO₂N, ACS Catal., 2022, 12, 1403–1414.
26. J. Vequizo, M. Hojamberdiev, K. Teshima and A. Yamakata, Role of CoO_x cocatalyst on Ta₃N₅ photocatalysts studied by transient visible to mid-infrared absorption spectroscopy, J. Photochem. Photobiol., A, 2018, 358, 315–319.
27. M. Yoshida, K. Takanabe, K. Maeda, A. Ishikawa, J. Kubota, Y. Sakata, Y. Ikezawa and K. Domen, Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall Water Splitting Studied by Model Electrodes, J. Phys. Chem. C, 2009, 113, 10151–10157.
28. L. Lin, Z. Lin, J. Zhang, X. Cai, W. Lin, Z. Yu and X. Wang, Molecular-level insights on the reactive facet of carbon nitride single crystals photocatalysing overall water splitting, Nat. Catal., 2020, 3, 649–655.
29. M. Liu, C. Wei, H. Zhuzhang, J. Zhou, Z. Pan, W. Lin, Z. Yu, G. Zhang and X. Wang, Fully Condensed Poly (Triazine Imide) Crystals: Extended π-Conjugation and Structural Defects for Overall Water Splitting, Angew. Chem., Int. Ed., 2022, 61, e202113389.
30. M. Qureshi, T. Shinagawa, N. Tsiapis and K. Takanabe, Exclusive Hydrogen Generation by Electrocatalysts Coated with an Amorphous Chromium-Based Layer Achieving Efficient Overall Water Splitting, ACS Sustainable Chem. Eng., 2017, 5, 8079–8088.
31. H. Lyu, T. Hisatomi, Y. Goto, M. Yoshida, T. Higashi, M. Katayama, T. Takata, T. Minegishi, H. Nishiyama, T. Yamada, Y. Sakata, K. Asakura and K. Domen, An Al-doped SrTiO₃ Photocatalyst maintaining sunlight-driven overall water splitting activity for over 1000 h of constant illumination, Chem. Sci., 2019, 10, 3196–3201.
32. T. Ohno, L. Bai, T. Hisatomi, K. Maeda and K. Domen, Photocatalytic Water Splitting Using Modified GaN:ZnO Solid Solution under Visible Light: Long-Time Operation and Regeneration of Activity, J. Am. Chem. Soc., 2012, 134, 8254–8259.
33. A. Yamakata, M. Kawaguchi, T. Nishimura, T. Minegishi, J. Kubota and K. Domen, Behavior and Energy States of Photogenerated Charge Carriers on Pt- or CoO_x-Loaded LaTiO₂N photocatalysts: Time-Resolved Visible to Mid-Infrared Absorption Study, J. Phys. Chem. C, 2014, 118, 23897–23906.
34. Z. Wang, T. Hisatomi, R. Li, K. Sayama, G. Liu, K. Domen, C. Li and L. Wang, Efficiency Accreditation and Testing Protocols for Particulate Photocatalysts toward Solar Fuel Production, Joule, 2021, 5, 344–359.

---

Footnote

† Electronic supplementary information (ESI) available: Characterization results including those obtained using ICP-OES and N–O analysis; photocatalytic HER data. See DOI: https://doi.org/10.1039/D2EY00031H

---