Oxide content optimized ZnS–ZnO heterostructures via facile thermal treatment process for enhanced photocatalytic hydrogen production

• ZnS–ZnO composite synthesized by thermal treatments from preformed ZnS particles.
• Improved charge separation in the ZnS–ZnO composite photocatalyst.
• Temperature and O2 partial pressure used to control oxide content in ZnS–ZnO composite.
• Optimized oxide content in ZnS–ZnO composite for H2 production of 494.8 μmol g−1 h−1.

The ZnS–ZnO heterostructured photocatalysts are synthesized by thermal treatments of the ZnS materials at various thermal processing temperatures (150 °C – 550 °C) with controlling O2 partial pressures (7.8 kPa – 33.8 kPa). The ZnS–ZnO composite structure shows much higher photocatalytic hydrogen production than those from the ZnS and ZnO pure substances. This phenomenon is mainly caused by effective charge separation between the photoexcited electrons and holes. The thermal oxidation of ZnS materials proceeds at temperatures higher than 500 °C. In addition to the thermal processing temperature, O2 partial pressure is also chosen for an experimental variable in order to control the atomic composition minutely. The ZnS–ZnO photocatalyst composite fabricated at 500 °C under 16.9 kPa of O2 partial pressure shows the highest hydrogen production rate of 494.8 μmol g−1 h−1 under 1 sun irradiation condition, and it is 37 times higher than that (13.5 μmol g−1 h−1) from the ZnS pure substance. At this optimized production rate, the Zn/S/O atomic compositions are measured as 45.9/46.9/7.2 (XPS) and 53.3/42.1/4.6 (ICP-AES), respectively.