Photocatalytic Oxidation of Hydrosulfide Ions by Molecular Oxygen Over Cadmium Sulfide Nanoparticles

Photocatalytic activity of CdS nanoparticles in hydrosulfide-ions air oxidation was revealed and thoroughly investigated. HS⁻ photooxidation in the presence of CdS nanoparticles results predominantly in the formation of SO₃ ²⁻ and SO₄ ²⁻ ions. Photocatalytic activity of ultrasmall CdS crystallites in HS^– photooxidation is much more prononced as compared to bulk CdS crystals due to high surface area of nanoparticles, their negligible light scattering, improved separation of photogenerated charge carriers etc. It was shown that hydrosulfide ions can be oxidized in two ways. The first is HS⁻ oxidation by the CdS valence band holes. This process rate depends on the rate of comparatively slow reaction between molecular oxygen and CdS conduction band electrons. The second reaction route is the chain-radical HS⁻ oxidation induced by photoexcited CdS nanoparticles and propagating in the bulk of a solution. In conditions favourable to chain-radical oxidation of HS⁻(i.e. at low light intensities and CdS concentration and high oxygen and Na₂S concentrations) quantum yields of the photoreaction reach 2.5.