Introduction Hydrogen energy, as an environmentally friendly and clean energy source, is expected to optimize the energy structure and alleviate energy consumption due to its high energy density, good combustion characteristics and abundant reserves. Hydrogen production from photolyzed water is considered as the most ideal way to obtain hydrogen source in the future. However, the disadvantages of weak visible light response, low catalytic activity, and poor chemical stability are one of the main factors limiting the application of traditional photocatalytic materials in photolytic water hydrogenation to real life. Zinc indium sulfide (ZIS) is proved to be an ideal semiconductor photocatalyst due to its good chemical stability, moderate forbidden bandwidth (2.2-2.5 eV), suitable conduction band position (0.79 eV), and good visible light absorption (absorption edge about 540 nm). However, ZIS suffers from a high photogenerated carrier complex rate and a weak photostability. Scholars optimized the ZIS energy band structure, expanded the spectral absorption range, improved the photoelectron dynamics behavior, and enhanced the photostability of ZIS by different approaches like morphology regulation, elemental doping, co-catalysis, and heterojunction. S-type heterojunctions have attracted recent attention due to their excellent photogenerated carrier dynamics and highly efficient photodissociation of hydrogen from water. In this paper, CWO/ZIS composite nanomaterials were synthesized by a two-step hydrothermal/water-bath method, and the photocatalytic mechanism of CWO/ZIS heterojunctions was discussed via constructing S-type heterojunctions, using the built-in electric field to enhance the photogenerated electron/hole separation efficiency and the migration rate.Methods Cobalt tungstate (CWO) nanoparticles were firstly prepared by a hydrothermal method, and then S-type CWO/ZIS heterojunctions were prepared by a water bath method. The composite samples named as CWO/xZIS were prepared at different molar ratios of Co2+ to Zn2+ (i.e., x = 0, 1%, 2%, 3%, 4%, 5%, 6%, and 10%). The properties of the samples (i.e., physical phase components, morphology, crystal structure, elemental distribution, chemical state, photocatalytic hydrolysis performance, photogenerated carrier separation efficiency, photogenerated charge transport rate, and photogenerated electron/hole pair recombination rate) were characterized.The photocatalytic hydrogen production performance of the samples was tested by a 30 mL customized photoreactor and a Timex GC7900 gas chromatograph. A 300 W Xenon lamp was equipped with a cut 420 nm filter (wavelength range: 420-780 nm) and an all-reflector was used as a visible light source. 5 mg of the sample was added to a mixed solution containing 9 mL of deionized water and 1 mL of triethanolamine (TEOA) and transferred to a 30 mL photoreactor sealed well. The air in the reactor was removed via charging the reactor with argon gas for 10 min. Afterwards, the reactor was mounted on the reaction platform, the circulating cooling water was turned on, the temperature of the reaction system was controlled at 10 ℃, and a stirrer was used for continuous stirring. A Xenon lamp 10 cm away from the reactor was turned on for illumination, and 100 μL of gas was extracted from the reactor for each 30 min and injected into the gas chromatograph for gas analysis for 3 h. The cyclic experiments were repeated under the same conditions for several times.Results and discussion According to the experimental results by X-ray diffraction (XRD), pure ZIS and pure CWO samples can be prepared. The characteristic peaks of ZIS and CWO both appear in the XRD patterns of the CWO/xZIS sample, indicating that CWO and ZIS are compounded. The surface morphology and microstructure of pure ZIS, pure CWO nanomaterials and their composite samples are determined by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). Based on the FESEM images, pure ZIS appears a typical two-dimensional nanosheet structure, and the size of pure CWO nanoparticles is approximately 50 nm. The FESEM images of CWO/3%ZIS samples show that CWO nanoparticles are attached to ZIS nanosheets. The HRTEM images show that the coexistence of lamellar and granular forms on the CWO/3%ZIS sample occur, confirming that the product contains both CWO and ZIS. Two sets of different lattice stripes corresponding to different crystalline surfaces of pure ZIS and CWO appear in the product, which further confirms that CWO and ZIS are composited. The X-ray photoelectron spectra indicate that CWO/3%ZIS samples contain ZIS and there is astrong intense interaction between CWO and ZIS. From the results of hydrogen production tests on CWO/xZIS samples, the samples of CWO/3%ZIS show the maximum hydrogen production activity with a photocatalytic hydrogen production rate, which is twice greater than that of pure ZIS. The reproducibility test proves that the samples of CWO/3%ZIS have a good photochemical stability. From the results of electrochemical performance tests on different samples, CWO/3%ZIS sample has the maximum photocurrent density and the minimum electrochemical impedance, thus indicating that it has the maximum photogenerated carrier separation efficiency and transport rate, which is also consistent with the photocatalytic hydrolysis to production hydrogen performance. Conclusions Pure CWO nanoparticles were synthesized by a hydrothermal method and S-type CWO/ZIS heterojunctions were prepared via loading CWO nanoparticles onto ZIS nanosheets by a water bath method. The results of XRD, FESEM, HRTEM, and XPS confirmed the formation of a close contact between CWO and ZIS and the construction of a good heterogeneous interface. The results of visible-light water splitting hydrogen production tests showed that the hydrogen production performance of the CWO/xZIS composite samples was improved, compared with that of pure ZIS, in which the hydrogen production rate of the CWO/3%ZIS samples reached 4 296.8 μmol·g1·h1, which is twice greater than that of pure ZIS (i.e., 2 178.2 μmol·g1·h1). The electrochemical performance test results showed that the S-type heterojunction photogenerated carrier transport channel was constructed due to the well-matched energy band structures of CWO and ZIS, resulting in the composite samples with a higher separation efficiency of the photogenerated electron/hole pairs, a faster photogenerated charge transport rate, and a smaller electron/hole recombination rate, thus leading to the enhancement of the photocatalytic hydrolysis into hydrogen generation efficiency of the CWO/xZIS heterojunction.