Designing Z-scheme CdS/WS2 heterojunctions with enhanced photocatalytic degradation of organic dyes and photoreduction of Cr (VI): Experiments, DFT calculations and mechanism

In this work, Z-scheme CdS/WS2 heterojunctions with high-efficiency photocatalytic degradation for organic dyes and photoreduction for Cr(VI) were designed using a straightforward and green two-step hydrothermal method. Tuning the molar ratio of CdS and WS2 can effectively adjust the energy band structure in CdS/WS2 composites with the enhancement of separation efficiency for photogenerated electron-hole and the decrease in the recombination for photogenerated carrier, which leads to the improved photocatalytic activities. A highest photocatalytic activity is obtained for the CdS/WS2 composite with 30% WS2, where the apparent rate constant of RhB and the reduction rate of Cr(VI) are 0.06153 and 0.0846 per min, respectively. According to Density functional theory (DFT), the work function of CdS crystal surface is smaller than that of WS2. The trapping experiment indicates that, during the photocatalytic reaction, O2– and h+ are the two predominant active species. Notably, both experimental and theoretical analyses confirm the formation of Z-scheme heterojunctions with built-in electric fields in CdS/WS2 composite. In addition, the CdS/WS2 photocatalysts maintain good stability after four cycles of photocatalytic degradation of RhB and reduction of Cr(IV). Combing with the DFT calculations, a possible photocatalytic mechanism is proposed. Further, this study brings out a unique strategy for the construction of CdS based Z-scheme heterojunctions and the applications in remediation of polluted environment.