Graded ZnS/ZnSxO1−x heterostructures produced by oxidative photolysis of zinc sulfide: Structure, optical properties and photocatalytic evolution of molecular hydrogen

Abstract

Abstract The photolysis of microcrystalline and single crystal ZnS on the ambient air was found to result in the formation of metallic zinc clusters and radical sulfur species additionally to the elimination of sulfur in reactions with oxygen and generation of S vacancies. The latter process results in the formation of a graded alloyed ZnO x S 1−x layer with a continuously decreasing content of sulfur and increasing content of oxygen from the bulk to the surface of the crystals. These photolytic changes are observed within the surface layer with a thickness of 50–80 nm which is comparable to the depth of light penetration into the ZnS crystals. Coupling of ZnS to ZnO, both via pre-photolysis thermal oxidation of ZnS in the dark or by in situ formation of ZnO via the oxidative photocorrosion of ZnS accelerates the photolysis considerably, most probably, as a result of directed and opposite flow of the electrons and holes within the graded ZnO x S 1−x layer and suppression of the electron-hole recombination. The ZnS/ZnS x O 1−x heterostructures revealed a relatively high photocatalytic activity in hydrogen evolution from water/ethanol mixtures under illumination with the UV light with a quantum yield of up to 1.6% in the absence of any co-catalysts. The dependence between the rate of photocatalytic hydrogen formation and the duration of ZnS photolysis was found to be dome-shaped one as a result of the interplay of two factors—(i) an increase of the probability of the spatial separation of the electron and hole within the ZnO x S 1−x layer and (ii) a decrease of the probability of ethanol oxidation by the holes with an increase in the ZnS x O 1−x layer thickness (or photolysis duration as an equivalent). In the optimal conditions the rate of photocatalytic H 2 evolution was 30 times higher than that for original non-photolyzed ZnS owing to the unique graded structure of the alloyed zinc oxysulfide layer produced by the oxidative photolysis.