A non-noble metal MoS₂–Cd_0.5Zn_0.5S photocatalyst with efficient activity for high H₂ evolution under visible light irradiation

Abstract

Here we describe the synthesis and characteristics of MoS₂–Cd_0.5Zn_0.5S, an efficient non-noble metal photocatalyst of H₂ evolution from water under visible light irradiation. This new quaternary composite was successfully synthesized by using a two-step hydrothermal method. The formation of a uniform Cd_0.5Zn_0.5S solid solution through hydrothermal coprecipitation and the subsequent loading of a suitable amount of MoS₂ were essential to enhance the catalytic activity of the MoS₂–Cd_0.5Zn_0.5S photocatalyst, which exhibited a synergistic effect for H₂ evolution derived from splitting water. This photocatalyst exhibited good photostability and high photocatalytic activity, which was maintained even after being re-used three times. An H₂ evolution rate of 12.30 mmol h⁻¹ g⁻¹ was achieved under visible light irradiation from an aqueous solution of lactic acid, indicative of superior water-splitting activity for this noble metal-free photocatalyst.

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1. Introduction

The gradual depletion of fossil fuels and serious environmental pollution have in recent times become issues of considerable concern. Photocatalytic technology offers an effective way to transform solar radiant energy into chemical energy,¹ and has been considered as an ideal candidate for the replacement of fossil fuels. Since the first report on photocatalytic water splitting was published in 1972,² hydrogen evolution via photocatalysis has received much attention due to its recycling potential and environmental friendliness.³ Because visible light makes up 40% of the radiant energy in solar light, much effort has been devoted to developing visible-light-responsive photocatalysts (such as CdSe,⁴ CdS,⁵ C₃N₄,⁶ In₂S₃ ⁷ and other composites like CdSe–MoS₂,⁸ (AgIn)_xZn_2(1−x)S₂ solid solution,⁹ MoS₂–CdS,¹⁰ CdS/ZnS/In₂S₃ ¹¹ and nickel-based cocatalysts).¹² CdS, with a relatively narrow band gap (2.4 eV), has in particular shown high performance in photocatalytic H₂ production under visible light irradiation.¹³ However, most of the reported photocatalysts have shortcomings such as photocorrosion¹⁴ and a reliance on noble metals as co-catalysts,¹⁵ which limit their wide application. Therefore, the development of inexpensive photocorrosion-resistant photocatalysts is still needed for H₂ evolution.

Some CdS catalysts loaded with non-noble metal compounds, such as MoS₂,^10c Ni(OH)₂ ¹⁶ and NiS,¹⁷ have been reported to exhibit enhanced H₂ evolution rates of 1.47 mmol h⁻¹ g⁻¹, 5.09 mmol h⁻¹ g⁻¹ and 2.18 mmol h⁻¹ g⁻¹, respectively. But CdS loaded with these metal compounds is still not superior to CdS loaded with noble metals, such as the Pt–PbS/CdS photocatalyst, with a high H₂ evolution rate of 8.77 mmol h⁻¹ g⁻¹,^15b (Zn_0.95Cu_0.05)_0.5Cd_0.5S loaded with Pt, exhibiting an H₂ evolution rate of 3.63 mmol h⁻¹ g⁻¹,¹⁸ and Pt-loaded ZnIn₂S₄, showing an H₂ evolution rate of 8.42 mmol h⁻¹ g⁻¹.¹⁹ Therefore, there is a great urgency to develop an ideal visible-light-responsive photocatalyst, i.e., one that is inexpensive as well as highly stable and active. Such ideal properties may be provided by the newly discovered photocatalyst consisting of a solid solution of Zn_xCd_1−xS. When formed as a solid solution, Zn_xCd_1−xS is more stable and active than are CdS and ZnS. In an investigation by Li et al.,²⁰ the Cd_0.5Zn_0.5S solid solution exhibited a high H₂ evolution rate of 7.42 mmol h⁻¹ g⁻¹ under visible light irradiation. Many methods have been used to fabricate Zn_xCd_1−xS solid solutions, such as coprecipitation,²¹ thermal sulfuration,²² thermolysis,^20,23 chemical bath deposition²⁴ and so on. The Zn_0.62Cd_0.16S obtained through coprecipitation method showed an H₂ evolution rate of 2.01 mmol h⁻¹ g⁻¹ under visible light irradiation.²¹ When prepared by the thermal sulfuration method, the Cd_0.2Zn_0.8S photocatalyst exhibited an H₂ evolution rate of about 0.83 mmol h⁻¹ g⁻¹,²² and the titania nanotube–Cd_0.65Zn_0.35S nanocomposite also showed a high evolution rate.²⁵ Many doping methods with different metal ions have been considered for improving the photocatalytic activity of the solid solution.²⁶ A Zn_xCd_1−xS photocatalyst modified with a co-catalyst such as NiOH exhibited an H₂ evolution rate of 7.16 mmol h⁻¹ g⁻¹, higher than that other than Zn_xCd_1−xS photocatalysts.²⁷ In addition, controlling the molar ratio of ZnS to CdS has proven to be an effective way to enhance the photocatalytic activity of Zn_xCd_1−xS solid solution.^20,28 Most of the Zn_xCd_1−xS photocatalysts synthesized through the above methods show irregular shapes with large and nonuniform sizes. Small and uniform-sized Zn_xCd_1−xS photocatalysts would be expected to improve the catalytic activities of the solid solutions and of their composited photocatalysts.

Herein, we aimed to achieve a Zn_xCd_1−xS solid solution with a uniform size and shape as well as high photocatalytic activity via hydrothermal coprecipitation. We also aimed to produce a new quaternary composite MoS₂–Cd_xZn_1−xS photocatalyst with superior photocatalytic activity through the subsequent loading of MoS₂ as a co-catalyst on the Zn_xCd_1−xS solid solution. In our work, the addition of potassium hydroxide and a two-step hydrothermal process were employed for the synthesis of uniform MoS₂–Cd_xZn_1−xS nanoparticles. The MoS₂–Cd_0.5Zn_0.5S photocatalyst without loaded noble metals was observed to be stable and to exhibit a high H₂ production activity of 12.30 mmol h⁻¹ g⁻¹ under visible light irradiation from an aqueous solution of lactic acid. Our new photocatalysis technique efficiently provided a non-noble metal photocatalyst for the H₂ evolution reaction.

2. Experimental

2.1. Synthesis of Zn_xCd_1−xS solid solution

Zn_xCd_1−xS solid solutions were prepared through a hydrothermal coprecipitation method with different molar ratios of Cd to Zn precursors. One millimole of cetyltrimethyl ammonium bromide (CTAB) was dissolved completely in 80 mL of distilled water at 50 °C, and was cooled to room temperature. Then x mmol cadmium acetate (Cd(CH₃COO)₂·2H₂O), 1 − x mmol zinc acetate (Zn(CH₃COO)₂·2H₂O), 2 mmol thiourea (CN₂H₄S) and 10 mmol KOH were added in that order into the CTAB solution prepared above with constant stirring (x = 0–1). The mixtures were stirred for 24 h, and then transferred into a Teflon vessel (100 mL), which was placed in a vacuum drying oven and heated at 200 °C for 24 h. The formed turbid liquid was filtered and washed with ethanol several times to remove CTAB and other ions, and the product was dried at 60 °C for 12 h. Finally, the Zn_xCd_1−xS powder was obtained. CdS and ZnS were also synthesized, using a similar method, for comparing the photocatalytic activities of different photocatalysts.

2.2. Synthesis of the MoS₂–Cd_xZn_1−xS nanomaterial

The MoS₂–Cd_0.5Zn_0.5S sample was synthesized by using a two-step hydrothermal method. Ten micromoles of ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) and 2 mmol of thiourea were (completely) dissolved in 60 mL of distilled water, and then 1 mmol of Cd_0.5Zn_0.5S obtained from the above hydrothermal coprecipitation method was mixed in. After being ultrasonicated for 30 min, the mixture was transferred into a Teflon vessel (100 mL) and heated in a vacuum drying oven at 200 °C for 23 h. The obtained turbid liquid was filtered and washed with distilled water several times, and then dried under vacuum at 60 °C for 12 h. By varying the amount of the Mo precursor, different MoS₂–Cd_0.5Zn_0.5S samples were prepared. The obtained CdS and ZnS were also loaded with MoS₂ by the same hydrothermal method for comparison.

2.3. Characterization

X-ray diffraction (XRD), using a Bruker D8 Advance diffractometer (Germany Bruker AXS Ltd.), was employed for phase identification of the powders. The morphology and size of the obtained samples were visualized with a scanning electron microscope (SEM, S-4700F, Japan JEOL Ltd.). Transmission electron microscopy images were taken using a high-resolution transmission electron microscope (HR-TEM, J-3010, Japan JEOL Ltd.). Ultraviolet-visible (UV-Vis) diffuse reflectance spectra were recorded with a UV-Vis spectrophotometer (Tu-1901, Beijing Persee General Instrument Co. Ltd.). X-ray photoelectron spectroscopy (XPS) measurements were taken using an ultrahigh vacuum VG ESCALAB 250 electron spectrometer equipped with a multichannel detector. Element analysis was carried out on an inductively coupled plasma spectrometry-atomic emission spectrometer (ICP-AES, ULTIMA, JY Inc.). A Micromeritics ASAP 2020HD88 nitrogen adsorption apparatus was employed to determine the Brunauer–Emmett–Teller (BET) specific surface areas (SBET) of the samples.

2.4. Photocatalytic reaction and quantum efficiency (QE)

A photocatalytic H₂ evolution reaction was carried out in a 500 mL photochemical reactor equipped with a magnetic stirrer and cooling system in a dark box. A 500 W Xe lamp was used as a light source for the photocatalytic H₂ evolution. For a typical photocatalytic reaction, 60 mg of MoS₂–Zn_xCd_1−xS photocatalyst was dispersed in a 400 mL aqueous solution containing 40 mL of lactic acid as a sacrificial reagent under ultrasonic treatment for 10 min. Then, the mixture was transferred into the photochemical reactor. During the reaction under light irradiation, the reaction solutions were maintained at room temperature with cooling water. The amount of hydrogen evolved during the reaction was analyzed and determined by using gas chromatography (TDX-01 molecular sieve column, TCD, N₂ carrier).

For comparison, another experiment for H₂ production was carried out in a 50 mL Pyrex flask sealed with stopper-rubber. A 300 W Xe lamp with a UV-cutoff filter (λ ≥ 420 nm) was used as the visible light source. In contrast to the above experiment, 3 mg of MoS₂–Zn_xCd_1−xS photocatalyst was dispersed in a 20 mL aqueous solution containing 2 mL of lactic acid in this experiment. The quantum efficiency (QE) was measured under the same photocatalytic reaction conditions, except that a monotone filter (λ = 420 nm) was used instead of the UV-cutoff filter. The average intensity of irradiation and the irradiation area were determined to be 31.69 mW cm⁻² and 16.25 cm², respectively. The QE was calculated according to the following equation:

QE = (number of reacted electrons)/(number of incident photons) × 100%

= (number of evolved H₂ molecules × 2)/(number of incident photons) × 100%

3. Results and discussion

Using the hydrothermal coprecipitation method with the aid of potassium hydroxide, Zn_xCd_1−xS solid solutions were prepared and used for the subsequent hydrothermal syntheses of MoS₂–Zn_xCd_1−xS photocatalyst. MoS₂–Cd_0.5Zn_0.5S samples were obtained with the starting reactant molar ratios of 1 : 1 for Cd/Zn, 10 : 1 for KOH/(Cd + Zn), and 7.5 : 100 for Mo/(Cd + Zn) and were examined first. Their morphologies and crystal structures were characterized with electron microscopy and X-ray diffraction. Fig. 1a and b show SEM images of Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S, respectively. The obtained Cd_0.5Zn_0.5S particles were observed to be spherical with a uniform size of about 14 nm, without any evidence of adhesiveness and agglomerations. Thus, the present hydrothermal coprecipitation method provides a way to synthesize uniform Cd_0.5Zn_0.5S nanoparticles that are superior to the product obtained by other methods reported. Compared with the Cd_0.5Zn_0.5S nanoparticles, the obtained MoS₂–Cd_0.5Zn_0.5S particles were observed to be a little bit larger, with some of them showing a distorted petal shape. These different observed sizes and shapes indicate that MoS₂ may be generated on the surface of Cd_0.5Zn_0.5S particles.

Fig. 1 SEM images of Cd_0.5Zn_0.5S (a) and MoS₂–Cd_0.5Zn_0.5S (b); HRTEM image of MoS₂–Cd_0.5Zn_0.5S (c); X-ray diffraction patterns of Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S (d).

An HRTEM image of the MoS₂–Cd_0.5Zn_0.5S particles is shown in Fig. 1c. Petal-shaped MoS₂ lamellas were observed to be generated on Cd_0.5Zn_0.5S nanoparticles, which formed separate and distorted lamellar shells without overlap. A repeat distance of 2.40 Å was observed (Fig. 1c), which is close to the distance between the (102) crystallographic planes in the Cd_0.5Zn_0.5S solid solution (Fig. 1d). The distance between lamellas on the surface of Cd_0.5Zn_0.5S nanoparticles was observed to be about 2.72 Å, corresponding to the (100) plane of the MoS₂ nanosheets. The above SEM and HRTEM results indicate that MoS₂ formed and combined with the Cd_0.5Zn_0.5S solid solution, i.e., quaternary MoS₂–Cd_0.5Zn_0.5S nanoparticles formed. Energy dispersive X-ray spectrometry (EDS) mapping analysis of MoS₂–Cd_0.5Zn_0.5S confirmed a homogenous distribution of the Cd, Zn, Mo, and S elements. Similarly, the EDS mapping analysis of Cd_0.5Zn_0.5S also confirmed a homogenous distribution of the Cd, Zn, and S elements (ESI, Fig. S1†).

Fig. 1d shows the XRD patterns of Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S nanoparticles. The XRD patterns of the two samples are in good agreement with JCPDS file no. 89-2943, indicating a spinel phase Cd_0.5Zn_0.5S solid solution. Well-ordered diffraction peaks corresponding to the (100) (002) (101) (102) (110) (103) (112) (203) (211) (300) planes of Cd_0.5Zn_0.5S were observed, with no other peaks present. The positions of these peaks were for each case at slightly lower two-theta values than those in the JCPDS file, indicative of a slightly enlarged crystal lattice constant, which is in agreement with the difference between the HRTEM image of MoS₂–Cd_0.5Zn_0.5S and that of Cd_0.5Zn_0.5S. There is no great difference between the XRD pattern of MoS₂–Cd_0.5Zn_0.5S and that of Cd_0.5Zn_0.5S particles. The former (as well as of course the latter) only showed the diffraction peaks of Cd_0.5Zn_0.5S particles, with no MoS₂ diffraction peak observed. This observation can be attributed to the single-layer structure of MoS₂.²⁹

To gain deeper insight into the bulk compositions of the MoS₂–Cd_0.5Zn_0.5S photocatalyst, a quantitative elemental characterization on MoS₂–Cd_0.5Zn_0.5S was conducted by using ICP. The metal elements Cd, Zn, and Mo were detected in MoS₂–Cd_0.5Zn_0.5S using this method. And these metals made up, respectively, 45.67 mol%, 46.17 mol%, and 8.16 mol% of the total metal content, consistent with an approximately 1 : 1 molar ratio of Cd to Zn and metal molar ratios similar to those of the starting materials.

X-ray photoelectron spectroscopy (XPS) can be used to help characterize the chemical states of nanomaterials. Fig. 2 shows the XPS spectra of MoS₂–Cd_0.5Zn_0.5S and Cd_0.5Zn_0.5S. The binding energy (BE) scales were referenced by setting the C 1s BE to 285.0 eV. The BEs of Cd 3d_5/2 and Cd 3d_3/2 for Cd_0.5Zn_0.5S were found to be 405.1 and 412.0 eV, respectively, slightly higher than those of bulk. For MoS₂–Cd_0.5Zn_0.5S, the BEs of Cd 3d shifted to values 0.1 eV higher than those of Cd_0.5Zn_0.5S. In contrast, the BEs of Zn 2p for MoS₂–Cd_0.5Zn_0.5S shifted to values 0.4 eV lower than those of Cd_0.5Zn_0.5S, for which the BEs of Zn 2p were observed to be 1022.6 and 1045.7 eV. These differences may be due to the differences in the chemical states of the Zn and Cd atoms in the MoS₂–Cd_0.5Zn_0.5S sample relative to those of the Cd_0.5Zn_0.5S sample. The BE peaks of S 2p for MoS₂–Cd_0.5Zn_0.5S were observed at 162.9 and 161.9 eV, lower than that of plain sulfur, and an S 2p_1/2 peak emerged. The altered S peaks probably resulted from the presence of Mo. The BEs of Mo 3d_5/2 and Mo 3d_3/2 centred at 228.5 and 232.4 eV, respectively. All of the XPS results indicated the involvement of positive-valence Cd, Zn and Mo metallic states and negative-valence sulfur, providing evidence for the formation of MoS₂–Cd_0.5Zn_0.5S and Cd_0.5Zn_0.5S. Moreover, the changed chemical states of Zn and Cd in the MoS₂–Cd_0.5Zn_0.5S atoms described above indicates an obvious interaction between MoS₂ and Cd_0.5Zn_0.5S.

Fig. 2 XPS spectra of MoS₂–Cd_0.5Zn_0.5S (A) and Cd_0.5Zn_0.5S (B) with Cd 3d (a), Zn 2p (b), S 2p (c) and Mo 3d (d) binding energies.

Diffuse reflectance spectroscopy is a useful tool to characterize the optical properties of photocatalysts. Fig. 3 shows the diffuse reflectance UV-Vis spectra of Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S nanoparticles. The inset was obtained by using the Kubelka–Munk function.³⁰ The band gap E_g of each sample was obtained by extrapolating the linear relation to [Ahν]^0.5 = 0. The optical absorption edge of the Cd_0.5Zn_0.5S nanoparticles was observed to be about 540 nm, corresponding to a band gap of 2.30 eV (Fig. 3). Furthermore, the MoS₂–Cd_0.5Zn_0.5S nanoparticles yielded a band gap of 2.25 eV with an optical absorption edge of about 551 nm. Taken together, our results reveal the uniform Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S nanoparticles to be responsive to visible light, and can be applied in visible light catalytic reactions.

Fig. 3 Diffuse-reflectance UV-Vis spectroscopy and the resulting band gap (inset) of MoS₂–Cd_0.5Zn_0.5S (A) and Cd_0.5Zn_0.5S (B).

Fig. 4 shows the nitrogen adsorption–desorption isotherms and pore-size distribution curves (inset) for the Cd_0.5Zn_0.5S solid solution and the MoS₂–Cd_0.5Zn_0.5S composite. Both Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S were observed here to have type IV isotherms. According to the Brunauer–Deming–Deming–Teller (BDDT) classification, the hysteresis loops of type H2 at a relative pressure range of 0.7–1 and 0.8–1 for the Cd_0.5Zn_0.5S solid solution and the MoS₂–Cd_0.5Zn_0.5S composite indicate the formation of mesopores of Cd_0.5Zn_0.5S and mesopores as well as macropores of MoS₂–Cd_0.5Zn_0.5S. The Cd_0.5Zn_0.5S solid solution exhibited a narrow distribution of pore sizes, and peaks in the range of 5–30 nm with a peak pore diameter of 14 nm (inset in Fig. 4). The MoS₂–Cd_0.5Zn_0.5S composite exhibited a broad distribution of pore sizes, and peaks in the range of 5–120 nm with a peak pore diameter of 50 nm. The specific surface areas of Cd_0.5Zn_0.5S solid solution and MoS₂–Cd_0.5Zn_0.5S composite were determined to be 26.7 m² g⁻¹ and 38.4 m² g⁻¹, respectively. Both the pore sizes and specific surface areas of Cd_0.5Zn_0.5S increased when it was combined with MoS₂.

Fig. 4 Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) of Cd_0.5Zn_0.5S and MoS₂–Cd_0.5Zn_0.5S.

In contrast to many other methods for synthesizing Cd_0.5Zn_0.5S solid solution reported in the literature,^20-22 we introduced KOH into the synthesis reaction system. KOH was expected to promote the formation of uniform Zn_xCd_1−xS nanoparticles with high photocatalytic activity, and this expectation was based on the following considerations. First, adding KOH would increase the alkalinity of the reaction system and thus speed up the formation of the metal hydroxide and its colloidal solution.³¹ Secondly, sulphions would be easily generated from the hydrolysis reaction of thiourea in alkaline solution. The sulphions would react quickly with the hydroxides to form metal sulfides for the subsequent transformation into a solid solution with the assistance of a hydrothermal process. These two expected effects were observed in our experiments. Adding KOH was found to favor the formation of cadmium and zinc hydroxide colloidal solutions and accelerate the formation of their sulfides (ESI, Fig. S2†). That is, as indicated in Fig. 1, KOH actively effects the formation of a homogeneous Zn_xCd_1−xS solid solution and MoS₂–Zn_xCd_1−xS nanoparticles.

In the preparation of the Cd_0.5Zn_0.5S nanoparticles, several different MoS₂–Cd_0.5Zn_0.5S samples were synthesized with varying amounts of KOH in the preparation process based on the presence of Cd_0.5Zn_0.5S solid solution. The synthesized MoS₂–Cd_0.5Zn_0.5S quaternary composites made with these different amounts of KOH were used as photocatalysts in the water splitting reaction for H₂ evolution. The molar ratio of KOH to metal precursors (the total amount of Cd and Zn) varied between 2.1 and 20. Fig. 5a shows the rate of H₂ evolution from water over the MoS₂–Cd_0.5Zn_0.5S photocatalyst synthesized with the aid of KOH. The amount of KOH had a significant effect on the photocatalytic activity of MoS₂–Cd_0.5Zn_0.5S. An optimum amount of KOH was necessary for improving photocatalytic activity of the MoS₂–Cd_0.5Zn_0.5S photocatalyst. When the value of the molar ratio of KOH to Cd and Zn metal precursors was 10, the synthesized MoS₂–Cd_0.5Zn_0.5S exhibited superior photocatalytic activity with a high H₂ evolution rate of 12.30 mmol h⁻¹ g⁻¹ for a duration of five hours.

Fig. 5 The dependence of the MoS₂–Cd_0.5Zn_0.5S-catalyzed rate of H₂ evolution on the MoS₂–Cd_0.5Zn_0.5S synthesis conditions, including the molar ratios of KOH/(Cd + Zn) (a), Cd/(Cd + Zn) (b), and Mo/(Cd + Zn) (c).

Different molar ratios of Zn to Cd precursors were also tested for their effects on the formation of the Zn_xCd_1−xS solid solution. The catalytic activities of the corresponding synthesized MoS₂–Zn_xCd_1−xS samples are given in Fig. 5b. High H₂ evolution over the MoS₂–Cd_0.5Zn_0.5S photocatalyst was observed here when the molar ratio of Cd to Zn was 1 : 1. In addition, the Mo precursor was reacted with thiourea in the presence of Cd_0.5Zn_0.5S solid solution during the second hydrothermal process. The molar amount of Mo precursor was varied from 2.5% to 10%. Fig. 5c shows the H₂ evolution results for the case of xMoS₂–Cd_0.5Zn_0.5S photocatalysts prepared from different amounts of Mo precursor, ranging from 2.5% to 10%. The optimum molar percentage of Mo was found to be 7.5%. Similarly, an appropriate amount of the used metal precursors was preferred. In summary, an efficient MoS₂–Cd_0.5Zn_0.5S photocatalyst can be synthesized through a two-step hydrothermal method when using optimum reaction conditions, i.e., with the molar ratios of Cd/Zn, KOH/(Cd + Zn) and Mo/(Cd + Zn) being set to 1 : 1, 10 : 1 and 7.5 : 100, respectively. The highest rate of H₂ evolution observed over a duration of five hours was 12.30 mmol h⁻¹ g⁻¹. For comparison with experimental conditions used in other published investigations, the catalytic activity of the MoS₂–Cd_0.5Zn_0.5S photocatalyst was also examined when irradiated under a 300 W Xe lamp. A high H₂ evolution rate of 11.49 mmol h⁻¹ g⁻¹ was determined from the lactic acid solution. The corresponding apparent quantum efficiency (AQE) at 420 nm reached 1.34% for the MoS₂–Cd_0.5Zn_0.5S photocatalyst.

In order to demonstrate the superior photocatalytic activity of MoS₂–Cd_0.5Zn_0.5S for H₂ production, pure CdS, ZnS, MoS₂, as well as the MoS₂–ZnS and MoS₂–CdS composites were synthesized using the same method for comparison. Photocatalytic reactions for H₂ evolution over the obtained different photocatalysts were carried out. ZnS, MoS₂ and MoS₂–ZnS hardly exhibited any photocatalytic activity for H₂ production, and CdS had weak photocatalytic activity for H₂ production, which was barely detected quantitatively. In contrast, as shown in Fig. 6a, MoS₂–CdS and Cd_0.5Zn_0.5S exhibited relatively high photocatalytic H₂ production activities of 4.15 mmol h⁻¹ g⁻¹ and 4.30 mmol h⁻¹ g⁻¹, respectively. Notably, MoS₂–Cd_0.5Zn_0.5S delivered the highest photocatalytic activity with an H₂ evolution rate of 12.30 mmol h⁻¹ g⁻¹, which was almost three times higher than those of MoS₂–CdS or Cd_0.5Zn_0.5S. The results revealed that the MoS₂–Cd_0.5Zn_0.5S quaternary photocatalyst exhibited a synergistic effect, which can improve H₂ evolution in water splitting reactions. In fact, it was the combination of MoS₂ and the Cd_0.5Zn_0.5S solid solution that provided the synergy rather than the combination of MoS₂, CdS and ZnS (see ESI Fig. S3†).

Fig. 6 Photocatalytic activities of different photocatalysts (a); and H₂ evolution for different recycling times of MoS₂–Cd_0.5Zn_0.5S photocatalysts (b).

Therefore, MoS₂ was determined to play an important role in the high H₂ evolution for the MoS₂–Cd_0.5Zn_0.5S photocatalyst as well as in improving the photocatalytic activity of Cd_0.5Zn_0.5S. Both the average pore diameter and the specific surface area of MoS₂–Cd_0.5Zn_0.5S were determined by using BET to be larger than those of Cd_0.5Zn_0.5S, indicating the role of MoS₂ in changing the structure of the Cd_0.5Zn_0.5S solid solution. The improved pore size and surface area of the MoS₂–Cd_0.5Zn_0.5S composites yielded improved adsorption and consequently the enhancement of the photocatalytic activity. As Low and co-works reported,³² MoS₂ nanosheets could be used as a promising material for photocatalytic reactions. Their theoretical calculations and experimental results showed MoS₂ to be able to activate H₂, and showed the conduction band (CB) potential of nanoscale MoS₂ to be about −0.20 eV versus the normal hydrogen electrode potential (NHE) because of the quantum confinement effect.³³ An empirical equation (E_CB = χ − E^C − 0.5E_g) could be employed to determine the CB edge position of the prepared Cd_0.5Zn_0.5S solid solution. In the equation, E_CB is the CB edge potential and χ is the electronegativity of the semiconductor, while E^C is the energy of free electrons on the hydrogen scale (∼4.5 eV), and E_g is the direct band gap of the semiconductor. So the E_CB of the Cd_0.5Zn_0.5S solid solution was calculated to be −0.294 eV, which is more negative than that of MoS₂. Therefore, the photogenerated electrons may transfer from the CB of Cd_0.5Zn_0.5S to the CB of MoS₂. Fig. 7 shows the CB and VB edge potentials of the Cd_0.5Zn_0.5S solid solution and MoS₂. A possible mechanism of the MoS₂–Cd_0.5Zn_0.5S photocatalyst for the high H₂ evolution may be proposed. When the MoS₂–Cd_0.5Zn_0.5S photocatalyst was irradiated under visible light, forming electron–hole pairs, photogenerated electrons were transferred from the Cd_0.5Zn_0.5S solid solution to MoS₂ being used to evolve H₂, and the holes oxidized the sacrificial agents.^10a As a result, the co-catalyst MoS₂ accelerated the separation of electron–hole pairs, which is a significant factor for photocatalytic reactions, and restrained the recombination of electron–hole pairs, contributing a high H₂ evolution for the MoS₂–Cd_0.5Zn_0.5S photocatalyst. However, when excess MoS₂ was loaded, most of the surface of the Cd_0.5Zn_0.5S solid solution may have been covered by MoS₂ nanosheets, decreasing the number of the electron–hole pairs that were formed, or the excess MoS₂ nanosheets may have become recombination centres for photogenerated electrons–holes pairs, leading to a low photoactivity,³⁴ which was observed, as shown in Fig. 5c.

Fig. 7 Schematic illustration of the potential and band positions of the Cd_0.5Zn_0.5S solid solution and MoS₂.

Also, the MoS₂–Cd_0.5Zn_0.5S photocatalyst was reused in the catalytic reaction to evaluate its catalytic stability. Photocatalytic results of recycling the catalyst three times are shown in Fig. 6b. MoS₂–Cd_0.5Zn_0.5S displayed very stable photocatalytic behaviour with high H₂ evolution even when re-used three times.

Taken together, we achieved a visible-light-responsive non-noble metal MoS₂–Cd_0.5Zn_0.5S photocatalyst with good photochemical stability and a five-hour duration of photocatalytic activity for H₂ evolution of up to 12.30 mmol h⁻¹ g⁻¹.

4. Conclusions

In summary, Cd_0.5Zn_0.5S solid solution nanoparticles with uniform shape and small average sizes of 14 nm were synthesized by using the hydrothermal coprecipitation method. The obtained Cd_0.5Zn_0.5S nanoparticles showed a higher activity than those prepared by other methods reported. Based on the Cd_0.5Zn_0.5S solid solution, a new quaternary composite, MoS₂–Cd_0.5Zn_0.5S, was formed successfully via the growth of MoS₂ by using this hydrothermal method. The molar ratios of three metal precursors were optimized. The addition of a suitable amount of KOH and the formation of the Cd_0.5Zn_0.5S solid solution were found to be essential for achieving the small-sized and highly active MoS₂–Cd_0.5Zn_0.5S photocatalyst. Over the MoS₂–Cd_0.5Zn_0.5S photocatalyst, a high rate of hydrogen evolution from water of 12.30 mmol h⁻¹ g⁻¹ was achieved under visible light irradiation. The MoS₂–Cd_0.5Zn_0.5S photocatalyst exhibited superior catalytic activity and photochemical stability, and hence provides an efficient non-noble metal photocatalyst for many promising applications in the field of clean energy.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21106006) is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ta08365f

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