The development of visible-light-responsive photocatalysts to realize water splitting is a challenging research topic but essential because of the potential to generate clean hydrogen fuel from renewable energy resources [1]. So far, a variety of nanomaterials, such as sulfides, nitrides, carbides, and silicides, have been developed as potential candidates for visible light-driven photocatalysts [2-5]. Among photocatalyst candidates, CdS has been a focus of great interest owing to its suitable band gap (E_g = 2.4 eV) for visible light absorption and a more negative conduction band level (-0.9 eV vs. NHE) than the reduction potential of H⁺/H₂ [6]. However, short-lived charge carriers, photocorrosion under visible light, environmental risk of toxic cadmium as well as the requirement for noble metals as co-catalysts prohibits the wide application of CdS [6-10]. The alloying of semiconductors of different band-gap potentials with CdS to construct solid solution photocatalysts with tunable electronic structure, such as Cd_1-xZn_xS, Cd_1-xMn_xS, and Cd_1-xNi_xS, is considered a feasible strategy to address these problems [11-13]. Among them, the ternary Zn-Cd-S alloy system has gained significant interest owing to the easy formation of uniform solid solutions, composition-tunable optoelectronic properties as well as the various potential applications, especially in photocatalysis [14-23]. To date, several techniques have been reported for the growth of Zn_xCd_1-xS solid solutions with various sizes and morphologies to achieve superior photocatalytic performance. Unfortunately, these conventional approaches usually involve the use of high temperatures above 160 ℃ and the presence of toxic chemicals including organic solvents, surfactants, and reducing agent to control the activity of the ions [14-17]. In particular, the use of expensive and highly poisonous sulfur agents including Na₂S, thioactamide, thiourea, and dimethyl sulfoxide leads to the liberation of a large amount of environmentally toxic H₂S, and thus, is not suitable for large-scale production [18-23].

Keeping the above points in mind, our aim was to develop a facile, green, template-free hydrothermal route under mild conditions for the synthesis of Zn_xCd_1-xS solid solutions by employing the nontoxic biomolecule L-cystine as the sulfur source. As known, L-cystine contains several functional groups, such as -SH, -NH₂ and -COOH, which exhibit a strong affinity towards metal cations to form a metal-ligand complex [24-27]. The complex would release S^2- by thermal decomposition in a sustained manner at a relatively slow rate, which is beneficial for the nucleation and growth of well-dispersed Zn_xCd_1-xS nanoparticles. Hence, in this case, L-cystine acts as both a sulfur source and the coordinating agent. Unlike the aforementioned methods, no additional templates, highly-toxic sulfur precursors, and poisonous organic solvents are involved in this process. More interestingly, the as-obtained Zn_0.9Cd_0.1S with less cadmium exhibits an efficient and stable activity towards H₂ production under visible-light irradiation without a co-catalyst, with a rate of 4.4 mmol h^-1 g^-1, which far exceeds that of CdS. The origin of such a super-high photocatalytic performance of Zn_0.9Cd_0.1S under visible light was studied by BET specific surface area measurements, the transient photocurrent (TPC) responses, and electrochemical impedance spectroscopy (EIS) techniques.

Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), L-cystine (C₆H₁₂N₂O₄S₂), cadmium acetate dihydrate (Cd(CH₃COO)₂· 2H₂O), and NaOH were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). All chemicals used in the experiments were of analytical grade.

In a typical synthesis, 1.5 mmol of L-cystine was dissolved in 16 mL of deionized water. The pH of the solution was adjusted to 10-11 by the addition of an aqueous NaOH solution. Meanwhile, different molar ratios of Zn(OAc)₂·2H₂O and Cd(OAc)₂·2H₂O with a total sum of 6 mmol were dissolved in another 16 mL of deionized water. Then, the above two solutions were mixed together with vigorous stirring to form a milky suspension. Finally, the mixed slurry was transferred into a 45 mL stainless steel autoclave, which was heated to 140 ℃ and maintained at this temperature for 10 h. After they were cooled naturally to room temperature, the samples were collected, alternately rinsed with deionized water and ethanol, and then dried at 60 ℃ prior to use.

The structure and phase of the products were identified by X-ray diffraction (XRD) analysis on a Bruker D8 Advance diffractometer with Cu-K_α irradiation operating at 40 kV and 40 mA. UV-visible spectra were recorded on a Perkin-Elmer Lambda 3 spectrophotometer by adopting BaSO₄ as a 100% reference standard and were converted from reflectance to absorbance by the Kubelka-Munk function. The morphologies and particle sizes of the samples were revealed by transmission electron microscopy (TEM, FEI Tecnai Spirit) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30). The samples were grounded, ultrasonically dispersed in ethanol, and then stuck to a Cu grid. The surface electronic state was analyzed with an ESCALAB250 X-ray photoelectron spectroscopy (XPS) with Al-K_α (1486.6 eV) radiation under ultrahigh vacuum (< 2 × 10^-9 Torr). All the binding energies were calibrated to the C 1s peak at 284.6 eV. The Brunauer-Emmett-Teller (BET) specific surface areas were evaluated by N₂ adsorption-desorption isotherms at -196 ℃ (Beckman Coulter SA3100) after the samples were outgassed at 100 ℃ for 2 h under vacuum.

Photocatalytic H₂-production experiments were conducted in a closed gas circulation using a 300 W Xe lamp with a cut-off filter (λ ≥ 420 nm). 50 mg of the Zn_xCd_1-xS powder was dispersed into 200 mL of an aqueous solution of 0.43 mol L^-1 Na₂S-0.5 mol L^-1 Na₂SO₃. Prior to the reaction, the system was thoroughly evacuated and then filled with approximately 30 Torr of Ar gas. A shutter window filled with water was positioned between the lamp and the reaction pool to eliminate infrared irradiation. The temperature of the working solution was maintained at about 10 ℃ by circulating water. The evolved H₂ was analyzed online by gas chromatography (TCD, molecular sieve 5-Å column, and Ar carrier).

The TPC and EIS analysis were measured in a standard three-electrode quartz cell with a CHI604B electrochemical workstation (Shanghai Chenhua Instrument Corp., China). A Pt foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, and the as-obtained samples coated on F-doped SnO₂ (FTO) conductive glass was employed as the working electrode. The working electrode was fabricated using the screen-printing technique described in previous work [28]. All electrodes had a similar thickness (10-11 μm). A 0.43 mol L^-1 Na₂S-0.5 mol L^-1 Na₂SO₃ aqueous solution was used as the electrolyte and a 300-W Xe lamp (λ ≥ 420 nm) was used as the visible light source.

Powder XRD was conducted to analyze the crystalline phase and purity of the products. The XRD patterns of the Zn_xCd_1-xS (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) solid solutions as well as the standard diffraction peaks of CdS and ZnS reported by the JCPDS are shown in Fig. 1(a). All the samples exhibited similar diffraction characteristics that could be assigned to a zinc-blende-type structure. The absence of any other phases or impurities indicated the high purity of the as-obtained products. With an increasing amount of Zn²⁺ content, the diffraction peaks systematically shifted to the higher-angle sides from CdS (JCPDS 65-2887) to ZnS (JCPDS 80-0020), indicating the formation of homogeneous cubic Zn_xCd_1-xS solid solutions instead of merely a physical mixture of CdS and ZnS. The calculated lattice constants for the Zn_xCd_1-xS solid solutions obtained by fitting the XRD patterns according to the MDI Jade5.0 software are listed in Table 1. Fig. 1(b) displays the dependence of the lattice parameters (1) on the Zn²⁺ content x. The lattice constant decreased almost linearly as the Zn²⁺ content increased. This phenomenon obeyed the well-known Vegard's law [29, 30], which rules out the separate nucleation of CdS or ZnS nanocrystals. The subsequent fringe lattice constriction was attributed to the larger ionic radius of Cd²⁺ (0.97 Å) relative to that of Zn²⁺ (0.74 Å) [31]. Additionally, the relatively small discrepancy in the electronegativity between Cd (1.69) and Zn (1.65) was favorable for the formation of Zn-Cd-S solid solutions [32]. Meanwhile, the structural similarity between ZnS and CdS confirmed by XRD data also facilitated the formation of a Zn-Cd-S alloyed structure [32].

Fig. 2 displays the UV-vis diffuse-reflectance absorption spectra of Zn_xCd_1-xS, CdS, and ZnS. All the Zn_xCd_1-xS samples showed an intense absorption band with a steep edge, which indicated that the absorption was caused by an intrinsic band-gap transition from the valence band to the conduction band rather than the transition from impurity levels [33]. The absorption edges of the Zn_xCd_1-xS solid solutions shifted monotonically to longer wavelengths with increasing Cd²⁺/Zn²⁺ molar ratio, which was consistent with the shift of the XRD peaks to higher angles (Fig. 1), further confirming the formation of Zn_xCd_1-xS solid solutions. The band gaps of the solid solutions were estimated to be 2.11-3.19 eV (x = 0-1) from the plots of (αhν)² versus hν (in which α and hν are the absorption coefficient and the incident photon energy, respectively) by linearly extrapolating to zero across the x axis (a straight line to the x-axis).

The TEM and HRTEM images of the Zn_0.9Cd_0.1S sample with the maximum H₂ generation rate are displayed in Fig. 3. Clearly, the sample was composed of nanoparticles with a size range from 30 to 100 nm. Further observation indicated that each nanoparticle was composed of smaller primary nanocrystals with a size of 3-8 nm. The well-defined lattice fringes of the Zn_0.9Cd_0.1S nanocrystal demonstrate its highly crystalline nature. The spacing of the lattice fringes was approximately 0.326 nm, corresponding to the interplanar distance of the (111) plane of Zn_0.9Cd_0.1S with a cubic zinc blende phase [34]. The surface compositions and electronic structures of the Zn_0.9Cd_0.1S sample were further analyzed by XPS, as presented in Fig. 4. The spectrum of the survey scan clearly demonstrated the existence of Zn, Cd, and S as well as C and O. The appearance of very small amounts of C and O signals resulted from the carbon tape and the adsorbed oxygen, respectively. The doublet peaks for Zn 2p (1021.7 and 1044.5 eV), Cd 3d (411.5 and 404.7 eV), and S 2p (161.3 and 162. 1 eV) were assigned to the spin-orbit split components of the Cd²⁺, Zn²⁺, and S^2- ions. The binding energies were in good agreement with the data reported in the literature [35, 36].

The photocatalytic performance of the Zn_xCd_1-xS solid solutions for the decomposition of water to H₂ under visible light irradiation (λ > 420 nm) was evaluated using S^2-/SO₃^2- as hole scavengers, as presented in Fig. 5(a). The H₂ production rate was monitored during 3 h of continuous illumination. No H₂ was detected over ZnS because ZnS (3.21 eV) is only active under UV light [37]. Only a trace amount of hydrogen was produced over naked CdS (0.03 mmol h^-1 g^-1), owing to the rapid recombination of the photogenerated e^--h⁺ pairs [6]. In contrast, the Zn_xCd_1-xS solid solutions exhibited much superior performance for photocatalytic water reduction compared to CdS. A gradual improvement in the H₂ evolution activity was observed with the increase of the Zn²⁺ content. The highest activity was achieved over the Zn_0.9Cd_0.1S catalyst, with a H₂ evolution rate of 4.4 mmol h^-1 g^-1 even without the addition of a cocatalyst, which far exceeded that of CdS. A further increase of the Zn²⁺ content led to a deterioration of the photocatalytic performance. In previous reports, the optimum activity was generally realized in a solid solution with a higher Cd/Zn molar ratio in the hexagonal-wurtzite structure [14, 38]. However, in this work, the optimum composition consisted of a very low Cd²⁺ content in the cubic-blende structure. The difference in the optimum composition is considered to originate from the difference in the crystalline structure, resulting in different charge-transfer paths [34, 39, 40]. The stability of the Zn_0.9Cd_0.1S catalyst was investigated by performing consecutive recycle experiments under identical reaction conditions, as demonstrated in Fig. 5(b). There was a slight decrease in activity in the second run owing to the consumption of the sacrificial reagents with prolonged reaction time. The activity was recovered in the third cycle by adopting a fresh aqueous solution of S^2-/SO₃^2-, indicating that the photocatalyst was essentially stable during the photocatalytic water reduction to H₂ under visible light illumination.

Generally, photocatalysts with higher specific surface areas are favorable for the improvement of the H₂ production activity owing to the increase in the surface to volume ratio [41]. The BET surface areas of the as-synthesized samples were investigated by nitrogen adsorption-desorption measurement and are listed in Table 1. The high BET surface area of Zn_0.9Cd_0.1S is self-explanatory to validate higher activity. In addition, the charge separation and transfer efficiency of photoinduced e^- -h⁺ pairs was also considered to significantly influence the photocatalytic activity [42-44]. Thus, the surface and bulk charge-transfer behavior was examined by the TPC response and EIS by constructing a three-electrode system. A comparison of the photocurrent-time (I-t) curves for the as-fabricated CdS, Zn_0.9Cd_0.1S, and ZnS electrodes with five on-off cycles of intermittent visible light illumination is shown in Fig. 6(a). All electrodes showed an instant and reproducible photocurrent response upon irradiation. Further observations showed that the photocurrent of the Zn_0.5Cd_0.5S electrode was significantly enhanced as compared to those of the ZnS and CdS electrodes, indicating the improvement of the charge transfer and the suppression of the charge recombination by constructing an alloyed Zn_0.9Cd_0.1S solid solution [45, 46]. The electron-transfer behavior occurring in the Zn_0.9Cd_0.1S electrode was further corroborated by the EIS Nyquist plot (Fig. 6(b)). Generally, the semicircle in the Niquist plot reflects a lower charge-transfer resistance [47, 48]. The alloyed Zn_0.9Cd_0.1S electrode exhibited the smallest arc radius, which implied more efficient interfacial charge-carrier separation and transfer. Based on the TPC and EIS analysis, the separation and transport of photoinduced electron-hole pairs was facilitated owing to the formation of alloyed Zn_1-xCd_xS solid solutions, which, in turn, significantly boosted the photocatalytic performance for water reduction to hydrogen under visible light.

A series of efficient visible-light-responsive Zn_1-xCd_xS solid solutions with a cubic zinc blende structure were successfully synthesized through a low-temperature L-cystine-assisted hydrothermal approach. The whole process was facile, cost-effective, and ecofriendly, which is very promising for large-scale applications. The optimum activity was achieved over the Zn_0.9Cd_0.1S catalyst, which had a H₂ generation rate of 4.4 mmol h^-1 g^-1 even without the loading of a noble-metal co-catalyst. In addition, this photocatalyst contained a lower amount of toxic Cd, which is significantly important from an environmental point of view.

Acknowledgments

We sincerely thank Dr. Jingying Shi of State Key Laboratory of Catalysis, National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, for the helpful suggestions and discussions.