Semiconductor-based photocatalytic hydrogen evolution can convert and store solar energy in the form of clean hydrogen and is believed to be one of the most promising strategies to tackle the global energy shortage and environmental pollution [1, 2]. Ever since the pioneer work by Fujishima and Honda [3] on a TiO₂-based photoelectrochemical cell for hydrogen evolution, great efforts have been devoted to the development of semiconductors for photocatalytic hydrogen evolution. A variety of semiconductor-based photocatalysts have been developed for hydrogen evolution under visible light during the past two decades [4-9]. Among them, metal sulfides are promising photocatalysts due to their lower band gaps, which fall in the visible light region [2, 10-12]. As a ternary chalcogenide, hexagonal ZnIn₂S₄ has attracted extensive interest in photocatalysis owing to its suitable band gap of ca. 2.4 eV and considerable stability [13-17]. However, the photocatalytic activity for hydrogen evolution over bare ZnIn₂S₄ is low because of the poor separation efficiency and low migration ability of the photoexcited charge carriers.

For semiconductor-based photocatalytic hydrogen evolution systems, cocatalysts are usually required because cocatalysts can provide catalytic active sites by lowering the overpotential for hydrogen evolution over the semiconductor photocatalysts [11, 18]. Although noble metals are usually used as cocatalysts for hydrogen evolution, their high price and scarcity restrict their practical applications [19]. Recently, using of inexpensive layered transition metal chalcogenides, like MoS₂ [20], WS₂ [21] and CoS [22], as noble-metal-free cocatalysts to promote the semiconductor-based photocatalytic hydrogen evolution has been extensively studied. As a p-type semiconductor, NiS is well known to be a good electrocatalyst for cathodic hydrogen evolution in water electrolysis [23]. The application of NiS as a cocatalyst to enhance the photocatalytic hydrogen evolution over CdS has been reported [24]. Our previous studies also revealed that NiS can be an appropriate cocatalyst for ZnIn₂S₄, and the hydrothermally prepared NiS/ZnIn₂S₄ shows superior photocatalytic activity for hydrogen evolution under visible light [25].

A fast electron transfer from the photo-excited semiconductor to the cocatalyst is important for the efficient photocatalytic hydrogen evolution, which depends not only on the relative band positions of the semiconductor and the cocatalyst, but also on their interface as well as the electronic conductivity of the nanocomposite [26, 27]. It is therefore anticipated that the introduction of a component with high electronic conductivity to promote the charge transfer in the semiconductor-cocatalyst system should enhance its photocatalytic activity for hydrogen evolution. Carbon quantum dots (CQDs), a novel class of nanocarbons, has shown many potentials in a variety of fields, ascribing to their fascinating properties [28-32]. Due to their good electronic conductivity, CQDs have been used in photocatalysis to fabricate binary semiconductor/CQDs nanocomposites to promote the charge separation in semiconductor-based photocatalysts [33-35]. It is therefore anticipated that the introduction of CQDs as a bridge into the binary NiS/ZnIn₂S₄ system would promote the charge transfer from ZnIn₂S₄ to NiS, which could result in a further improvement of its performance for photocatalytic hydrogen evolution.

Herein, we reported the rational design and fabrication of the ternary NiS/CQDs/ZnIn₂S₄ nanocomposite as a superior noble-metal-free photocatalyst for hydrogen evolution under visible light. The self-assembly of ZnIn₂S₄ microspheres in the presence of preformed NiS/CQDs resulted in the efficient ternary NiS/CQDs/ZnIn₂S₄ nanocomposite for hydrogen evolution (Scheme 1), in which CQDs act as a bridge to promote the electron transfer from photo-excited ZnIn₂S₄ to NiS. The photocatalytic performance over ternary NiS/CQDs/ZnIn₂S₄ is much higher than ternary CQDs/NiS/ZnIn₂S₄, which was obtained via deposition of NiS on the preformed CQDs/ZnIn₂S₄. This study provides a strategy to promote the photocatalytic hydrogen evolution over semiconductor-based photocatalysts by incorporating CQDs.

All the reagents were commercially available and used without further purification. The CQDs was prepared by heating citric acid according to the literature [36]. For the preparation of NiS/CQDs nanocomposite, a mixture of CQDs (29 mg) and Ni(OAc)₂ (56 mg, 0.32 mmol) was dispersed in a degassed N, N-dimethylformamide (DMF) (16 mL). Then 1.6 mL Na₂S (0.2 mol/L) aqueous solution was added. Afterwards, the resultant solution was stirred at 90 ℃ in an oil bath for 4 h. To synthesize NiS/CQDs/ZnIn₂S₄ nanocomposites, different amounts of the as-obtained NiS/CQDs were dispersed in water (60 mL) and ultrasonicated for 10 min. Then ZnCl₂·6H₂O (0.136 g, 1.0 mmol) and InCl₃·4H₂O (0.586 g, 2.0 mmol) were added to the above suspension containing NiS/CQDs nanocomposite under vigorous stirring. Thioacetamide (TAA, 0.3 g, 4.0 mmol) was added and the resultant suspension was sealed in a Teflon-lined double-walled digestion vessel and reacted at 120 ℃ for 60 min using a microwave system (Ethos A, Milestone, Italy). After reaction, the resultant product was collected by centrifugation, washed with deionized water and absolute ethanol, and dried at 60 ℃. The NiS/CQDs/ZnIn₂S₄ nanocomposites with different amounts of NiS/CQDs were denoted as x wt% NiS/CQDs/ZnIn₂S₄, in which x = 0.5, 1.0, 1.5, 2.0 based on the ratio in the feed materials.

Bare ZnIn₂S₄ was synthesized from ZnCl₂·6H₂O, InCl₃·4H₂O and TAA under microwave heating at 120 ℃ for 60 min. NiS/ZnIn₂S₄ and CQDs/ZnIn₂S₄ were prepared from ZnCl₂·6H₂O, InCl₃·4H₂O and TAA in the presence of preformed NiS and CQDs, respectively, under microwave heating at 120 ℃. The ternary CQDs/NiS/ZnIn₂S₄ nanocomposite was prepared by deposition of NiS in the presence of preformed CQDs/ZnIn₂S₄ under microwave treatment.

X-ray diffraction (XRD) of the as-obtained products was carried out on a D8 Advance X-ray diffractometer (Bruker, Germany) using Cu K_α (λ = 1.5406 ) radiation at a voltage of 40 kV and 40 mA. XRD patterns were scanned over the angular range of 10°–70° (2θ) with a step size of 0.02°. IR analyses were carried out on a Nicolet IS50 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system (PHI, USA) with a monochromatic Al K_α source and a charge neutralizer. All the binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. The morphology of the samples was obtained by a field emission scanning electron microscopy (SEM, JSM-6700F). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained in a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. The sample suspension in ethanol was sonicated and a drop was dripped on the support film. UV-visible diffuse reflectance spectra (UV-DRS) of the powders were obtained with BaSO₄ used as a reflectance standard in the UV-visible diffuse reflectance experiment. Electrochemical impedance spectroscopy (EIS) was measured on an electrochemical analyzer (Zahner, Germany) in a standard three-electrode system using the prepared samples as the working electrodes with an active area of ca. 0.25 cm², a Pt wire as the counter electrode, and Ag/AgCl (saturated KCl) as a reference electrode. The electrolyte was a mixed aqueous solution containing K₃[Fe(CN)₆] (1 mmol/L), K₄[Fe(CN)₆] (1 mmol/L) and KCl (0.1 mol/L). Impedance data were fitted with ZSimpWin software (Princeton Applied Research). Inductively coupled optical emission spectrometry (ICP-OES) was performed on Optima 8000 (PerkinElmer). Before the ICP-OES experiment, the solid sample was digested in a mixture of HNO₃ and Milli-Q water.

Photocatalytic experiments for hydrogen evolution were carried out in a closed gas circulation and evacuation system fitted with a top Pyrex window. 50 mg of photocatalyst was dispersed in 100 mL of aqueous solution containing 10 mL triethanolamine (TEOA) as sacrificial reagent. The suspension was irradiated with a 300 W Xe lamp equipped with a 420 nm cut-off filter to provide the visible light irradiation. The reactant solution was maintained at room temperature by a flow of cooling water during the photocatalytic reaction. The amount of hydrogen evolved was determined with an on-line gas chromatograph equipped with a TCD detector.

The NiS/CQDs nanocomposite was obtained by a hydrothermal treatment in a mixture of CQDs, Ni(OAc)₂ and Na₂S. The XRD of the as-obtained product shows characteristic peaks at 30.1°, 34.7°, 46.0° and 53.5° assignable to (100), (101), (102) and (110) crystallographic planes of hexagonal NiS (JCPDS No. 75-0613), indicating the formation of NiS nanoparticles during the hydrothermal treatment (Fig. 1(a)) [37]. The FT-IR spectrum shows two peaks at 1393 and 1583 cm⁻¹, corresponding to symmetric and asymmetric stretches of the carboxylate groups of CQDs. As compared with those observed over bare CQDs (1388 and 1576 cm⁻¹), these peaks shift slightly to longer wavelength, suggesting the existence of interaction between NiS and CQDs (Fig. 2(a) and (b)) [38]. It is believed that the rich hydroxyl and carboxylate groups on the CQDs surface can act as the adsorption sites for Ni²⁺. The reaction of these surface adsorbed Ni²⁺ with S^2– to form NiS nuclei, which further grow up to form the resultant NiS nanoparticles on the surface of CQDs. The mass ratio of NiS to CQDs in the as-obtained NiS/CQDs was determined to be 1.19:1 by ICP-OES.

The as-obtained NiS/CQDs nanocomposite was further used to fabricate ternary NiS/CQDs/ZnIn₂S₄ nanocomposites with different amounts of NiS/CQDs. The rich functional groups on the surface of CQDs act as adsorption sites for cationic Zn²⁺ and In³⁺ ions, which react with S²⁻ anions provided by the decomposition of TAA under microwave conditions to form ZnIn₂S₄ nuclei deposited on CQDs surface. The ZnIn₂S₄ nuclei grow up to form ternary the NiS/CQDs/ZnIn₂S₄ nanocomposite [39]. The mass ratio of NiS:CQDs:ZnIn₂S₄ in the as-obtained 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite was determined to be ca. 1.19:1:200 based on ICP-OES measurements. The XRD pattern of the as-obtained ternary NiS/CQDs/ZnIn₂S₄ nanocomposite (take 1.0 wt% NiS/CQDs/ZnIn₂S₄ for example) shows characteristic peaks of hexagonal ZnIn₂S₄ (JCPDS No. 65-2023) at 2q values of 21.5°, 27.6°, 30.4°, 39.8°, 47.2°, 52.4° and 55.6°, indicating that hexagonal ZnIn₂S₄ has been successfully obtained (Fig. 1(b) and 1(c)) [40, 41]. Although no characteristic peaks assignable to either NiS or CQDs are observed in the XRD patterns of the resultant product due to their low amount in the nanocomposite, the presence of CQDs is evidenced from the FT-IR spectrum of 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite, by showing peaks at 1393 and 1586 cm⁻¹, corresponding to symmetric and asymmetric stretches of the carboxylate groups (Fig. 2(c)) [38]. In addition, the XPS spectrum of the 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite in the C 1s region shows three peaks at 284.6, 286.5 and 288.3 eV, ascribable to the sp² hybridized carbon (C–C), epoxy/hydroxyl carbon (C–O) and carboxy carbon (O–C=O), respectively, further confirming the presence of CQDs in the NiS/CQDs/ZnIn₂S₄ nanocomposite (Fig. 3(a)) [42]. The XPS spectrum in the Ni 2p region shows two sets of peaks. The two peaks at 855.2 and 872.5 eV can be assigned to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2 in NiS, while the other two small peaks at 856.8 and 874.2 eV can be assigned to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2 coordinated with O, suggesting the interaction between NiS and CQDs (Fig. 3(b)) [43]. The XPS spectrum of NiS/CQDs/ZnIn₂S₄ in the S 2p region shows a broad peak, which can be deconvoluted to two sets of S^2–, with the peaks at 162.3 and 163.5 eV assigned to S^2– in NiS, while those at 161.8 and 163.0 eV to S^2– in ZnIn₂S₄, by comparison with bare ZnIn₂S₄ (Fig. 3(c)) [44, 45]. The XPS in the O 1s region can also be deconvoluted to three peaks, with the peak at 530.7 eV ascribed to O^2– in Ni–O bonds, in consistent with the XPS spectrum in the Ni 2p region, while the peaks at 531.2 and 532.1 eV assigned to the C–O and O–H bonds, respectively, further confirming the presence of the NiS/CQDs [42]. Peaks in Zn²⁺ 2p (1022.4 eV and 1045.8 eV) and In³⁺ 3d (445.1 and 452.5 eV) regions are also observed. However, as compared with bare ZnIn₂S₄, all the peaks shift to higher binding energy, indicating the existence of strong interaction between hexagonal ZnIn₂S₄ and NiS/CQDs (Fig. 3(c), (e) and (f)) [46].

The SEM image of 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite shows that it consists of flower-like microspheres with an average size of 3.0 μm, assembled by a large number of interwoven ultrathin nanosheets (Fig. 4(a)), similar to the morphology of bare ZnIn₂S₄ (Supporting Information Fig. S1). The mechanism for the formation of hexagonal ZnIn₂S₄ flower-like microspheres has been well elucidated previously and is believed to be driven by the anisotropy growth tendency of hexagonal ZnIn₂S₄, whose nuclei are prone to form nanosheets and a further self-assembly resulting in the flower-like microspheres [47]. The TEM image shows that nanoparticles of NiS/CQDs with a dimension of ca. 30 nm are deposited on ZnIn₂S₄ nanosheets (Fig. 4(b)). In addition to lattice fringes of 0.32 nm, which corresponds to (102) crystallographic planes of hexagonal ZnIn₂S₄, clear lattice fringes of 0.29 nm and 0.22 nm, which can be assigned to the (100) plane of NiS and (100) plane of graphite CQDs, respectively, can also be observed in the HRTEM image of the 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite (Fig. 4(c)) [48, 49]. The HRTEM image of ternary NiS/CQDs/ZnIn₂S₄ nanocomposite clearly shows CQDs contact with both ZnIn₂S₄ and NiS, indicating that CQDs act as a bridge to connect hexagonal ZnIn₂S₄ and NiS (Fig. 4(c)). On the contrary, even though CQDs and NiS are also deposited on the surface of ZnIn₂S₄ in the CQDs/NiS/ZnIn₂S₄ nanocomposite obtained via the deposition of NiS on the preformed CQDs/ZnIn₂S₄, no contact between CQDs and NiS is observed (Supporting Information Fig. S2). It is anticipated that the transfer of the photo-generated electrons from ZnIn₂S₄ to NiS is more facilitated over the NiS/CQDs/ZnIn₂S₄ nanocomposite as compared with that over CQDs/NiS/ZnIn₂S₄ nanocomposite due to the existence of a bridge of CQDs with excellent electronic conducting properties.

The UV-vis DRS of NiS/CQDs/ZnIn₂S₄ nanocomposite is shown in Fig. 5. As compared with bare ZnIn₂S₄, the incorporation of NiS/CQDs nanocomposite enhances the light absorption of ZnIn₂S₄ in 500–800 nm regions, in agreement with the color change from yellow for bare ZnIn₂S₄ to light green for NiS/CQDs/ZnIn₂S₄ nanocomposite [50].

The photocatalytic performance of the NiS/CQDs/ZnIn₂S₄ nanocomposite was investigated under visible light irradiation using TEOA as the sacrificial agent and was compared with that over NiS/CQDs, bare ZnIn₂S₄, CQDs/ZnIn₂S₄ and NiS/ZnIn₂S₄ (Fig. 6). Hydrogen evolved over irradiated NiS/CQDs was almost negligible, even though previous studies have revealed that CQDs can act as a photocatalyst for hydrogen evolution in the presence of EDTA as a sacrificial agent [38]. However, in our case, the photocatalytic activity of NiS/CQDs is much lower as compared with semiconductor-based photocatalysts like ZnIn₂S₄ (Fig. 6(a)). Bare ZnIn₂S₄ shows low activity for photocatalytic hydrogen evolution under visible light irradiation, with 26 μmol of hydrogen evolved in 5 h (Fig. 6(b)). The introduction of 1.0 wt% CQDs into ZnIn₂S₄ resulted in an improvement of the photocatalytic activity, with 61 µmol of hydrogen evolved in 5 h over 1.0 wt% CQDs/ZnIn₂S₄ (Fig. 6(c)). As previous studies indicated, NiS can be a suitable cocatalyst for ZnIn₂S₄, and improved photocatalytic activity for hydrogen evolution was observed over 1.0 wt% NiS/ZnIn₂S₄, with 88 μmol of hydrogen evolved in 5 h (Fig. 6(d)) [25]. A significantly enhanced photocatalytic activity was observed over the 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite, with 142 μmol of hydrogen evolved after irradiated for 5 h, 1.6 and 2.3 times that over 1.0 wt% NiS/ZnIn₂S₄ and 1.0 wt% CQDs/ZnIn₂S₄, respectively (Fig. 6(e)). The amount of hydrogen evolved over the ternary 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite increased linearly with the irradiation time, indicating its stability during the photocatalytic hydrogen evolution. In addition, no obvious loss of the activity was observed after three cycling tests, and the catalyst after three runs showed similar XRD patterns as that of fresh NiS/CQDs/ZnIn₂S₄ nanocomposite (Fig. 7).

The amount of NiS/CQDs in the ternary NiS/CQDs/ZnIn₂S₄ nanocomposite also influenced its photocatalytic activity. It was found that the photocatalytic activity was first improved with the increase amount of NiS/CQDs, with the optimum performance observed over the 1.5 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite, by showing 201 μmol of hydrogen evolved in 5 h (Fig. 8). However, a further increase of the amount of NiS/CQDs in the ternary NiS/CQDs/ZnIn₂S₄ nanocomposite led to a decrease of its performance.

The controlled experiment revealed that a mechanical mixture of 1.0 wt% NiS/CQDs and ZnIn₂S₄ showed an inferior performance for hydrogen evolution, with only 64 μmol of hydrogen generated in 5 h, as compared with 142 μmol of hydrogen evolved over ternary 1.0 wt% NiS/CQDs/ZnIn₂S₄ nanocomposite, indicating that the effectiveness of the electron transfer from ZnIn₂S₄ to NiS via CQDs is influenced by the interaction between the three components. In addition, to our expectation, ternary 1.0 wt% CQDs/NiS/ZnIn₂S₄ obtained via the deposition of NiS on the preformed CQDs/ZnIn₂S₄ also showed a lower photocatalytic activity for hydrogen evolution, with only 105 μmol of hydrogen evolved in 5 h (Fig. 9). This result confirms that the transfer of photo-generated electrons from ZnIn₂S₄ to NiS is more facilitated over the NiS/CQDs/ZnIn₂S₄ nanocomposite than over the CQDs/NiS/ZnIn₂S₄ nanocomposite. Unlike that in NiS/CQDs/ZnIn₂S₄, most of CQDs in CQDs/NiS/ZnIn₂S₄ do not directly contact with NiS, which makes it not as efficient as that in NiS/CQDs/ZnIn₂S₄ to promote the electron transfer from ZnIn₂S₄ to NiS. The superior charge transfer in the NiS/CQDs/ZnIn₂S₄ nanocomposite was evidenced from the electrochemical impedance spectra (EIS) analysis by showing a smaller semicircle as compared with that of bare ZnIn₂S₄, NiS/ZnIn₂S₄ and CQDs/NiS/ZnIn₂S₄ nanocomposite (Fig. 10) [51]. These results indicate that the rational design of the ternary CQDs-based system to enable CQDs to act as a bridge to promote the vectorial transfer of the photo-generated electrons from the semiconductor to the cocatalyst is important for realization of efficient photocatalytic hydrogen evolution. Therefore, the mechanism for the superior photocatalytic hydrogen evolution over the ternary NiS/CQDs/ZnIn₂S₄ nanocomposite under visible light irradiation is shown in Scheme 2. In the ternary NiS/CQDs/ZnIn₂S₄ nanocomposite, electrons and holes are generated when ZnIn₂S₄ is irradiated with visible light. A directional transfer of the photo-generated electrons from ZnIn₂S₄ to NiS, the cocatalyst for hydrogen evolution, can be promoted by CQDs acting as an electron mediator, ascribed to their excellent electronic conductivity [52, 53].

In summary, ternary NiS/CQDs/ZnIn₂S₄ nanocomposites which show superior photocatalytic hydrogen under visible light were obtained via self-assembly of ZnIn₂S₄ in the presence of preformed NiS/CQDs. The superior photocatalytic performance observed over ternary NiS/CQDs/ZnIn₂S₄ nanocomposite can be ascribed to the presence of CQDs acting as a bridge to promote the vectorial transfer of the photo-generated electrons from ZnIn₂S₄ to NiS. This study highlights the great potential of using CQDs for fabricating highly efficient ternary nanocomposites for photocatalytic hydrogen evolution.