Cu-modified Zn₆In₂S₉ photocatalyst for hydrogen production under visible-light irradiation

As energy and environmental problems caused by economic development become more serious, hydrogen is attracting attention as the next generation of clean energy. Photocatalytic hydrogen generation is a method with low environmental impact because it uses natural energy sources such as water and sunlight. Indium zinc sulfide is a photocatalyst with visible light response and chemical stability. The aim of this study was to improve the photocatalytic activity of indium zinc sulfide while reducing the use of expensive indium.

Copper-modified zinc indium sulfide was synthesized by heating and stirring zinc chloride, indium chloride tetrahydrate, copper(I) chloride, and thioacetamide in an autoclave at 180 °C for 18 h. Six types of catalysts were prepared by varying the amount of copper added. To evaluate the photocatalytic activity, 40 mg of catalyst was added in 40 mL of an aqueous solution containing 20 mL of 0.25 M Na₂SO₃/0.35M Na₂S (sacrificial agent) and 1.2 mL of hexachloroplatinic acid solution (co-catalyst). The solution was purged with nitrogen for 30 min, with stirring, followed by irradiation with visible light (λ ≥ 420 nm) for 6 h. Hydrogen production was measured using gas chromatography every 3 h. Catalyst characterization was also performed.

Compared to the hydrogen production rate of Zn₆In₂S₉, the hydrogen production rate of Zn_5.7Cu_0.3In₂S₉ was about five times higher. Therefore, the catalysts were characterized, and the characterization showed that the addition of copper suppressed the recombination of electron–hole pairs, increased the optical absorption in the visible light region and narrowed the band gap. The results indicate that the addition of copper to indium zinc sulfide improves the photocatalytic activity. The quantum yields for Zn_5.7Cu_0.3In₂S₉ were found to be consistent with the DRS trend, indicating that hydrogen production occurs from the photocatalyst.