*3.3. Calcination*

By changing the treatment temperature and ambient gas condition, the post thermal treatment of metal sulfide photocatalysts at elevated temperature in air or oxygen usually results in effective electron–hole separation and enhanced photocatalytic activity, due to the formation of metal sulfide–metal oxide heterojunction. Hong et al. [97] reported that ZnS–ZnO composite prepared by thermal treatments from preformed ZnS particles showed improved charge separation and photocatalytic activity. Optimizing oxide content in ZnS–ZnO photocatalyst by controlling O2 partial pressure (16.9 kPa) and temperature (500 oC) can help to achieve a H2 production rate of 494.8 μmol g−<sup>1</sup> h<sup>−</sup>1.

The crystallinity and surface area of a photocatalyst can be modified by calcination. Based on previous studies, the H2 production rate increases as the calcination temperature increases. However, there is an optimum temperature for the maximum H2 evolution rate. Further increment of temperature will cause negative effects on the H2 production rate. This negative effect results from the decrease in the surface area after heating at high temperature. These findings have been evidenced in Ce-doped ZnO/ZnS [98], ZnS1−x−0.5yOx(OH)y(1:1) [99], and CdS/TiO2 [85]. Table 9 presents the influence of calcination treatment on the activity of photocatalysts.


**Table 9.** Effects of calcination treatment on photocatalytic performance.
