Influence of the Zn/Al molar ratio over the photocatalytic hydrogen production by ZnS/ZnAl-LDH composites

Abstract

In this work, ZnS/ZnAl-LDH composites were prepared by the sulfuration of pristine Zn–Al LDHs with Zn:Al molar ratios between 2 and 4. The sulfured samples (ZA2-S, ZA3-S, ZA4-S), were evaluated in the photocatalytic hydrogen production to verify the effect of the Zn:Al molar ratio. The X-ray powder diffraction revealed the presence of a well-crystallized hydrotalcite-like structure in all samples, but ZA4 also exhibited hydrozincite and Zn(OH)2 reflections. It was observed that as the Zn:Al ratio increased in the sulfured samples, more intense reflections of ZnS were detected. The sample with the highest amount of ZnS (ZA4-S) displayed the lowest bandgap value (3.54 eV) and the highest photocatalytic H2 production (4409.0 μmol g−1 h−1); although, it presented the lowest specific surface area (16 m2 g−1). Moreover, it was confirmed that after 5 cycles uses its activity not only was maintained but it increased slightly. As ZnS itself displayed a lower hydrogen production capacity compared to the ZA4-S composite, the photocatalytic activity of the prepared samples was explained in terms of the formation a type II heterojunction between ZnS and the LDH.