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Novel composite photocatalysts consisting of a cadmium and zinc sulfide solid solution (Cd 0.6 Zn 0.4 S) and zinc sulfide (ZnS) nanoparticles were successfully prepared by a simple hydrothermal treatment of suspended Cd 0.3 Zn 0.7 S at 120 °C. The as-obtained materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and UV-VIS diffuse reflection spectroscopy. The obtained photocatalysts were tested in the photocatalytic evolution of hydrogen from a Na 2 S/Na 2 SO 3 aqueous solution under visible light irradiation (λ = 450 nm). It is shown that the hydrothermal treatment of Cd 0.3 Zn 0.7 S at 120 °C increases the activity by a factor of 7.5 due to the phase transformations of the solid solution with the formation of the multiphase Cd 0.6 Zn 0.4 S/ZnS sample. The deposition of gold on the surface of Cd 0.6 Zn 0.4 S/ZnS leads to a further increase in activity: the achieved photocatalytic activity and quantum efficiency (450 nm) for 1%Au/Cd 0.6 Zn 0.4 S/ZnS are 17.4 mmol g−1 h−1 and 42.6%, respectively. This excellent performance is found to be attributable to the transformation of Cd 1-x Zn x S from the cubic to the hexagonal phase during the hydrothermal treatment. Additionally, photoelectrodes based on Cd 0.6 Zn 0.4 S/ZnS and FTO were synthesized and tested in a two-electrode cell. A high value of the photocurrent equal to 0.5 mA/cm2 is achieved for the Cd 0.6 Zn 0.4 S/ZnS/FTO electrode. An investigation by means of impedance spectroscopy reveals the longer lifetime of photogenerated charge carriers in the Cd 0.6 Zn 0.4 S/ZnS/FTO photoelectrode if to compare with Cd 0.3 Zn 0.7 S/FTO system. Image 1 • Au/Cd 0.6 Zn 0.4 S/ZnS photocatalysts were prepared via simple hydrothermal synthesis. • The highest value of the photocatalytic activity was 17.4 mmol H 2 per hour per gram. • The highest apparent quantum efficiency was 42.6% (450 nm). • Photocurrent 0.5 mA/cm2 was achieved for the Cd 0.6 Zn 0.4 S/ZnS/FTO electrode. [ABSTRACT FROM AUTHOR]

The promotion of the photocatalytic activity of the Cd 0.3 Zn 0.7 S nanoparticles via deposition of some copper species, such as Cu x S and Cu 0 , as a co-catalyst, in the hydrogen production under visible light irradiation from aqueous solutions of a sacrificial Na 2 S/Na 2 SO 3 system was studied. The state of the samples was characterized with a wide range of physical methods both in the initial state as well as after their photocatalytic operation. An evolution of both photocatalytic activity and the state of the photocatalyst particles was registered by a set of the physical methods. The highest apparent quantum efficiency and photocatalytic activity which were achieved at illumination by light with λ = 450 nm over the 0.1 wt% Cu x S/Cd 0.3 Zn 0.7 S sample for the photocatalytic hydrogen evolution from Na 2 S/Na 2 SO 3 aqueous solutions were 23.1% and 6.4 mmol g −1 h −1 , respectively. It was shown that the deposited Cu species undergo the transformation to Cu x S during the photocatalytic production of hydrogen, and these are the copper sulfide nanoparticles that increase the activity of the Cd 1-x Zn x S solid solutions. This statement agrees well with the principle of a constancy of the state of heterogeneous catalysts under their operation in the same reaction media established by G.K. Boreskov in the 1950s. [ABSTRACT FROM AUTHOR]