Effect of pH on the synthesis and characteristics of RGO–CdS nanocomposites

•RGO–CdS nanocomposites were hydrothermally synthesized under different pH levels.•Photocatalytic activity of the composites was compared with pure CdS.•The best photocatalytic performance was shown by RGO–CdS synthesized at high pH.•Smaller size of CdS and higher number of π-electron improved the photocatalytic activity. The effect of pH on the synthesis of reduced graphene oxide (RGO)–CdS nanocomposites via a hydrothermal method was investigated. Higher pH is more favorable for the formation of good quality RGO–CdS nanocomposites. The photocatalytic performance of the composites was investigated by measuring the reduction of Cr(VI), and the composite synthesized at high pH (=11) exhibited a 76% removal rate of Cr(VI). At high pH, the development of CdS particles of smaller sizes and the maximum number of π-electrons available in RGO increased the effective surface area available for light absorption, fast electron transport, and enhanced light absorption intensity. The staircase band structure of CdS in combination with RGO helped to inhibit electron–hole recombination, which enhanced photocatalytic activity.