How and to what extent do carbon materials catalyze solar hydrogen production from water?

The aim of this study is to find key physicochemical properties of carbon materials in catalyzing the photocatalytic H ₂ production from visible light-irradiated aqueous suspensions of carbon/CdSe composites. For this, we have employed five different carbon materials (activated carbons, carbon fibers, multi- and single-walled nanotubes, and graphites) without and with acid treatment, and two carbon materials obtained from graphite (graphite oxides and reduced graphene oxides). Detailed surface analyses for the bare carbon materials and carbon/CdSe composites were completed to characterize their physicochemical properties. Most virgin (non-treated) carbon materials are beneficial in catalyzing the photocatalytic H ₂ production of carbon/CdSe composites, and such catalytic effects are significantly enhanced by their acid treatment by a factor of 3-7. Systematic investigation indicates that the surface area, the degree of disordered carbon (i.e., I _D/I _G ratios), and the electrical conductivity (σ) of carbon materials display the correlations with H ₂ production of carbon/CdSe composites. However, the first two factors could not explain well the enhanced H ₂ production by acid treatment of carbon materials, whereas the last factor exhibits the linearity with the H ₂ amounts even for the acid-treated carbon materials. High electrical conductivity appears to facilitate the photogenerated electron transfer from CdSe to neighboring carbon materials, leading to inhibition of charge pair recombination as evidenced by photoluminescence study.