Enhancing the solar photocatalytic hydrogen generation of ZnS films by UV radiation treatment

Abstract

This work reports the synthesis of transparent ZnS films for the hydrogen production using a low-cost chemical bath deposition method (CBD). Some ZnS films were subjected to UV treatment and became darker. According to the X-ray diffraction patterns, both, the transparent and dark films presented the same crystalline wurtzite phase. However, the morphological analysis by scanning electron microscopy (SEM) revealed that the dark ZnS films were formed by smaller ZnS grains (10–46 nm) in comparison with the transparent films (17–70 nm). The dark coloration of the films was caused by the presence of Zn vacancies, S vacancies and sulfate groups as confirmed by the XPS spectra and absorbance measurements. In fact, the presence of these defects and sulfates enhanced the absorbance and photocurrent in the dark ZnS films with respect to the transparent ZnS films. Consequently, the maximum hydrogen generation rate was 250% higher in the dark ZnS films (655 μmol∙h−1 g−1) in comparison to the transparent ZnS films (187 μmol∙h−1 g−1). In presence of a sacrificial agent (Na2SO3), the hydrogen generation rates increased up to 2903.6 and 5202.4 μmol∙h−1 g−1 for the transparent ZnS and dark ZnS films, respectively. The ZnS films studied here represent advance with respect to the previous powder-type binary/ternary photocatalyst nanocomposites, since they can be easily removed from water after the photocatalytic reaction (an advantage over the powders), they do not contain expensive/rare metals and their synthesis procedure is not as complex as these reported for binary/ternary nanocomposites.