| Summary: | Hydrogen as an environmentally friendly fuel can be produced by photocatalytic procedures from aqueous systems, utilizing H<sub>2</sub>S, an industrial side-product, by conversion and storage of renewable solar energy. Although composites of CdS and ZnS prepared by co-precipitation are very efficient in heterogeneous photocatalytic H<sub>2</sub> generation, the optimal conditions for their synthesis and the effects of the various influencing factors are still not fully clarified. In this work, we investigated how the efficiency of Cd<sub>0</sub>.<sub>25</sub>Zn<sub>0</sub>.<sub>75</sub>S composites modified with Ni(II) was affected by the doping method, Ni-content, hydrothermal treatment, and presence of a complexing agent (ammonia) used in the preparation. The composition, optical, and structural properties of the photocatalysts prepared were determined by ICP, DRS, XRD, TEM, and STEM-EDS. Although hydrothermal treatment proved preferable for Ni-free composites, Ni-modification was more efficient for untreated composites precipitated from ammonia-containing media. The best efficiency (14.9% quantum yield at 380 nm irradiation, 109.8 mmol/g/h hydrogen evolution rate) achieved by surface modification with 0.1–0.3% Ni(II) was 15% and 20% better than those for hydrothermally treated catalyst and similarly prepared Pt-modified one, respectively. Structural characterization of the composites clearly confirmed that the Ni<sup>2+</sup> ions were not embedded into the CdS-ZnS crystal lattice but were enriched on the surface of particles of the original catalyst in the form of NiO or Ni(OH)<sub>2</sub>. This co-catalyst increased the efficiency by electron-trapping, but its too high amount caused an opposite effect by diminishing the excitable surface of the CdS-ZnS particles.
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