Photocatalytic overall water splitting is one of the most important approaches to artificial photosynthesis for solar H₂ production [1-3]. The use of particulate photocatalysts is considered vital for future practical applications of this technology, based on the primary advantages such as their simplicity and cost-effectiveness [4-6]. There are two routes to photocatalytic overall water splitting by particulate photocatalysts based on one- and two-step photoexcitation processes. As a thermodynamic prerequisite, the semiconductor for a one-step photoexcitation in overall water splitting process must have conduction band minimum (CBM) and valence band maximum (VBM) energy levels that straddle the potentials of the hydrogen evolution and oxygen evolution reactions. As a result, few photocatalysts have been reported capable of visible light-responsive overall water splitting [7]. In contrast, the two-step excitation system (also called the Z-scheme system) only requires that the semiconductor photocatalysts satisfy the band position condition for either the hydrogen evolution or water oxidation reaction. Therefore, this approach reduces the thermodynamic requirements and expands the choice of semiconductors, and in particular, allows the consideration of narrow bandgap materials [8]. To date, various types of visible-light-responsive photocatalysts have been applied in Z-scheme systems, including oxides, (oxy)nitrides, (oxy)sulfides, oxychlorides, and selenides [9-14]. However, considering that the driving force for the redox reactions decreases with decreasing bandgap, the development of narrow bandgap semiconductors for efficient Z-scheme overall water splitting is still significantly challenging [15].

Metal selenides are typical narrow bandgap semiconductors and have absorption edge wavelengths longer than the corresponding metal (oxy)sulfides. This is because the VBMs of the former are typically associated with Se 4p orbitals and thus, are shallower than those associated with O 2p and S 3p orbitals. In particular, solid solutions of zinc selenide and copper gallium selenide (denoted as ZnSe:CGSe) can have tunable band structures and semiconductor characteristics, depending on the Zn/(Zn+Cu) and Ga/Cu molar ratios [13]. Recently, our group has used these metal selenides as hydrogen evolution photocatalysts (HEPs), in conjunction with CoO_x/BiVO₄ and Au as the oxygen evolution photocatalyst (OEP) and electron mediator, respectively, for the Z-scheme overall pure water splitting. However, this system requires the vacuum deposition of an Au layer. It would be preferable to avoid using expensive noble metals and to simplify the preparation technology, so as to permit the future large-scale applications of solar water splitting.

Graphene-based materials consisting of earth-abundant elements have been extensively studied because of their useful properties such as high charge carrier mobilities and exceptional conductivities [16]. It has been reported that reduced graphene oxide (RGO) can act as an electron mediator in the Z-scheme overall water splitting systems, and that such systems can be assembled simply by mixing it with an OEP under light irradiation [17, 18]. In the present work, ZnSe:CGSe solid solutions with various Zn/(Zn+Cu) and Ga/Cu molar ratios were developed, and RGO-based powder suspension Z-scheme overall water splitting systems were constructed using ZnSe:CGSe photocatalysts as HEPs and CoO_x/BiVO₄ as the OEP. Both sacrificial H₂ evolution and Z-scheme overall water splitting reactions were examined to improve our understanding of the most important factors involved in constructing photocatalytic Z-scheme processes involving selenide-based materials.

X-ray diffraction (XRD) patterns for (ZnSe)_x(CuGa₅Se₈)_1−x solid solutions having Zn/(Zn+Cu) ratios in the range 0–1 are presented in Fig. 1a and 1b. Here, the samples having Zn/(Zn+Cu) ratios of 0 and 1 correspond to CuGa₅Se₈ and ZnSe, respectively. CuGa₅Se₈ and ZnSe have chalcopyrite and zinc blende structures, respectively [19, 20]. With increasing Zn/(Zn+Cu) ratio, the diffraction peak at approximately 28° gradually shifted to lower angles, implying the formation of a solid solution of CuGa₅Se₈ and ZnSe (Fig. 1b). The scanning electron microscopy (SEM) images indicated that the selenide particles had undefined morphologies, with a wide range of particle sizes starting from hundreds of nanometers to several micrometers (Fig. S1). The UV-vis diffuse reflectance spectroscopy (DRS) data showed that the absorption edge shifted to shorter wavelengths, from 675 to 480 nm, with increasing Zn/(Zn+Cu) ratio.

The effects of the Ga/Cu molar ratio on the structural and optical properties of ZnSe:CGSe were also investigated. The selenides with a Zn/(Zn+Cu) ratio of 0.5 and Ga/Cu ratios ranging from 1.0 to 5.0 (denoted as (ZnSe)_0.5(CGSe)_0.5) are discussed. Fig. 2a and 2b show that all the diffraction peaks can be assigned to the chalcopyrite structure, suggesting the formation of solid solutions of ZnSe and CGSe. The shift in the main diffraction peak at ~28° to higher angles, with increasing Ga/Cu ratio, is primarily ascribed to a structural rearrangement (Fig. 2b) [19]. SEM images showed that these materials had similar particle sizes, with irregular morphologies and coarse surfaces (Fig. S2). UV-vis DRS data indicated that the absorption edge shifted to shorter wavelengths with increasing Ga/Cu ratio (Fig. 2c). A similar trend was also observed in our previous study [13].

The photocatalytic H₂ evolution activities of ZnSe:CGSe with different compositions in the presence of Na₂S and Na₂SO₃ as sacrificial reagents were evaluated under visible light irradiation. For each selenide photocatalyst, a Ru cocatalyst was loaded at a concentration of 1 wt% using the photodeposition method (see Supporting Information for details). As shown in Fig. 3a, a volcano-shaped trend was observed for the rate of H₂ evolution as a function of the Zn/(Zn+Cu) ratio, with the maximum rate at a molar ratio of 0.5. Based on this optimized condition, Ru/(ZnSe)_0.5(CGSe)_0.5 photocatalysts with different Ga/Cu ratios were evaluated. In the molar ratio range 1.0−5.0, the rate of H₂ evolution increased monotonically with increasing Ga/Cu ratio. The time courses of H₂ evolution on these Ru/ZnSe:CGSe photocatalysts (Fig. S3) also demonstrate that the catalytic activities were maintained for 5 h, irrespective of the Zn/(Zn+Cu) or Ga/Cu ratio. The optimized Ru/(ZnSe)_0.5(CuGa₅Se₈)_0.5 photocatalyst exhibited an apparent quantum yield (AQY) of 0.5% at 420 nm. Fig. 3c summarizes the relationship between the rate of H₂ evolution and the cutoff wavelength of the incident light. These data show that the onset of H₂ evolution is consistent with the onset of absorption in the UV-vis DRS data, confirming that the photocatalytic H₂ evolution over Ru/(ZnSe)_0.5(CuGa₅Se₈)_0.5 resulted from band gap excitation. Incident light up to a wavelength of 650 nm could be utilized for the proton reduction reaction in the presence of this photocatalyst.

A previous work by our group demonstrated that selenide semiconductors can be used as HEPs for Z-scheme pure water splitting when they are assembled with CoO_x-BiVO₄ and Au as the OEP and electron mediator, respectively [13]. In such a case, the photocatalyst system was constructed in the form of a sheet, and the Au layer was prepared by vacuum evaporation. In this work, a different Z-scheme pure water splitting system incorporating a selenide-based H₂-evolving photocatalyst and RGO was examined. Fig. 4a presents a schematic illustration of this new process. The primary differences in the new system are that the Au layer is replaced with RGO, and the assembled photocatalysts are in the form of a powder suspension. Thus, the assembly can be realized simply by mixing the photocatalysts together under light irradiation, without the requirement of a vacuum system. Prior to assembling the HEP and OEP in this system, a Pt reduction cocatalyst was deposited on the selenide semiconductor using an ion adsorption method [21], while RGO was deposited on the CoO_x/BiVO₄ surface (see the XRD pattern and UV-vis DRS data in Fig. S4) via photoreduction in an aqueous methanol solution (see Supporting Information for details).

Because the optimal Zn/(Zn+Cu) ratio for both sacrificial H₂ evolution and sheet-based overall pure water splitting was determined to be 0.5 [13], (ZnSe)_0.5(CGSe)_0.5 was employed to assess the effect of the Ga/Cu ratio. As shown in Fig. 4b, (ZnSe)_0.5(CGSe)_0.5 photocatalysts with Ga/Cu ratios ranging from 1.0 to 5.0 were assessed, and all the photocatalysts exhibited Z-scheme overall pure water splitting. The water splitting rate was highly dependent on the Ga/Cu ratio and resulted in a volcano-shaped curve, with a maximum at Ga/Cu ratio of 2.0, corresponding to an H₂ evolution rate of 3.3 µmol/h. The time courses of the overall water splitting on these photocatalysts indicated that the activities were maintained throughout the testing period of approximately 10 h (Fig. S5). It is worth noting that the optimized photocatalyst system generated H₂ and O₂ in a molar ratio close to the theoretical stoichiometric value of 2, indicating that the water splitting reaction occurred in a stable manner under visible light irradiation, with an effective suppression of the self-oxidation of (ZnSe)_0.5(CuGa₂Se_3.5)_0.5 (Fig. 4c). Similar to the Au layer in our previously reported photocatalyst sheet system, RGO played an important role in constructing the Z-scheme process and in stabilizing the selenide photocatalysts during the photocatalytic reaction. In this powder suspension system, photogenerated holes in the metal selenides were preferentially recombined with electrons from BiVO₄ via RGO, avoiding the self-oxidation of the selenides and leading to stable pure water splitting.

Interestingly, the sacrificial H₂ evolution rate was found to increase monotonically with increasing Ga/Cu ratio, while a volcano-shaped trend was observed in the Z-scheme water splitting rate as a function of the Ga/Cu ratio (Fig. 3b and 4b). That is to say, the activities of the sacrificial H₂ evolution and Z-scheme overall water splitting reactions were not linearly correlated. Our previous study showed that increasing the Ga/Cu ratio also increased the band gap energy of (ZnSe)_0.5(CGSe)_0.5. This effect was primarily due to a positive shift in the VBM potentials of these materials, which in turn allowed holes to be consumed more efficiently in response to the oxidation of the sacrificial reagents, owing to the greater driving force [13]. As a result, the photogenerated electrons were able to avoid recombination with holes and thus, contributed to the hydrogen evolution more efficiently, despite an approximate increase of 0.2 eV in the band gap energy. In contrast, the Z-scheme water splitting activity of the RGO-based powder suspension system examined in the present study varied with the Ga/Cu ratio in a manner similar to that for the Z-scheme-type photocatalyst sheet based on an Au conductor layer. The latter can be regarded as an assembly of numerous miniature p–n photoelectrochemical (PEC) cells [13]. Thus, this similarity suggests that the photocatalytic performance of the assembled unit consisting of an HEP, an OEP, and RGO is closely related to the PEC performance of such units. Therefore, the excellent PEC characteristics of the selenide-based materials are evidently responsible for the efficient Z-scheme activities exhibited by this powder suspension photocatalyst system [21, 22].

In summary, ZnSe:CGSe have been found to act as an HEP in a powder suspension Z-scheme overall pure water splitting system, when combined with CoO_x/BiVO₄ and RGO as the OEP and electron mediator, respectively. This is the first report of visible light-driven photocatalytic Z-scheme pure water splitting using a powder suspension that involves metal selenides with narrow bandgaps. A nonlinear relationship was observed between the photocatalytic activities during the Z-scheme overall water splitting and sacrificial H₂ evolution reactions. Based on the similar reaction characteristics of the present RGO-based Z-scheme system and a previously reported photocatalyst sheet system, it was revealed that selenide photocatalysts exhibiting high photocathodic currents at positive potentials when serving as photocathodes, should exhibit more efficient Z-scheme overall pure water splitting.