Carbon nitride (CNx) was prepared by pyrolysis of melamine at 550 °C, and this procedure was modified for preparation of the floating CNx composite: HGMs (10 g) were thoroughly mixed with melamine at a specified mass ratio (1:1, 1:2, 1:3, 1:4; HGM:melamine), then heated at 1 °C min -1 to a final temperature of 550 °C for 3 h in an alumina crucible with a lid. After cooling, the resulting yellow solid (HGM/CNx) was collected and crushed to a fine powder using a mortar and pestle, then transferred to a 2 L separatory funnel. The HGM/CNx was washed repeatedly by gravimetric separation with deionised water to recover the floating fraction of the composite (by draining away any non-floating solid with the aqueous phase) until a transparent, colourless aqueous phase was achieved with a floating layer of catalyst. This floating fraction was then drained from the funnel and dried at 80 °C overnight.