Clean synthesis of ZnSe–rGO Nanocomposites under mild conditions: effect on structure, crystallinity, band gap, catalytic and photocatalytic activity

Semiconductor doped rGO composites offer a dual advantage: (i) increase in conductivity of the semiconductor due to increased electron–hole separation; (ii) increase in the low band gap of the rGO upon doping. However, there are two bottlenecks in the synthesis of doped rGO materials: (i) use of toxic chemicals and (ii) requirement of extremely high temperatures. To overcome these bottlenecks and considering the importance of semiconductor-doped rGO composites, two relatively clean and mild methods of synthesis were explored viz. the γ-irradiation based method and the hydrothermal method for synthesis of ZnSe–rGO nanocomposites. Interestingly, the band gap (E_g) of the semiconductor ZnSe drastically increases upon doping into rGO sheets (from 2.90 eV in pristine ZnSe to 3.71 eV in the ZnSe–rGO composites synthesized by the γ-irradiation method and 3.22 eV in the composites synthesized by the hydrothermal method). The increased Eg value will surely result in decreased electron–hole recombination in the ZnSe–rGO nanocomposites compared to pristine ZnSe. The two different methods result in different crystalline forms of ZnSe in the composites—while the hydrothermal method results in pure fcc Zincblende, the γ-irradiation method produces a mixed phase comprising fcc ZnSe and a hexagonal primitive phase. The ZnSe nanocomposites were used as catalysts for Mannich type conversion of indole to 3-substituted indole (3-PPMI). Both composite catalysts show superior catalytic efficiency irrespective of the crystalline form i.e. 82–84% yield of 3-PPMI was obtained within 1 h of reaction as compared to almost no yield after 4 h using rGO only and 65% yield in 4 h using pristine ZnSe only. The prepared ZnSe–rGO composites also functioned as efficient photocatalysts in the degradation of a pollutant 4-nitrophenol (4-NP). In photocatalysis, the composites synthesized by the γ-irradiation method perform better than those synthesized by the hydrothermal method probably due to better electron–hole separation. However, both types of composites show much enhanced photocatalytic efficiency than pristine ZnSe as assessed from degradation rates (k), k being 6.8 × 10⁻⁴ min⁻¹ for pure ZnSe and 4.75 × 10⁻³ min⁻¹ and 5.78 × 10⁻³ min⁻¹ for the composite photocatalysts prepared by hydrothermal and γ-irradiation method respectively, with approximately 60% degradation in 150 min. Thus this work presents a useful guideline into selection of cleaner and milder methods for synthesis of ZnSe–rGO nanocomposites as per the requirement.