Graphene, a single layer of graphite, possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area, and can be produced on a large scale at low cost. Thus, it has been regarded as an important component for making various functional composite materials. Especially, graphene-based semiconductor photocatalysts have attracted extensive attention because of their usefulness in environmental and energy applications. This critical review summarizes the recent progress in the design and fabrication of graphene-based semiconductor photocatalysts via various strategies including in situ growth, solution mixing, hydrothermal and/or solvothermal methods. Furthermore, the photocatalytic properties of the resulting graphene-based composite systems are also discussed in relation to the environmental and energy applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation and photocatalytic disinfection. This critical review ends with a summary and some perspectives on the challenges and new directions in this emerging area of research (158 references).

To solve the problem of the global energy shortage and the pollution of the environment, in recent years, semiconductor photocatalytic technology that converts solar energy into chemical fuel has been widely studied. Regarding semiconductor-based photocatalysts, CdS has attracted extensive attention due to its relatively narrow bandgap for visible-light response and sufficiently negative potential of the conduction band edge for the reduction of protons. Studies have shown that CdS-based photocatalysts possess excellent photocatalytic performance in terms of solar-fuel generation and environmental purification. This critical review presents the recent advances and progress in the design and synthesis of various CdS and CdS-based photocatalysts. The basic physical and chemical properties of CdS and the related growth mechanism have been briefly summarized. Moreover, the applications of CdS-based photocatalysts have been discussed such as in photocatalytic hydrogen production, reduction of CO₂ to hydrocarbon fuels and degradation of pollutants. Finally, a brief perspective on the challenges and future directions for the development of CdS and CdS-based photocatalysts are also presented.

Graphene-modified TiO₂ nanosheets with exposed (001) facets (graphene/TiO₂ ) were prepared by microwave-hydrothermal treatment of graphene oxide (GO) and hydrothermally synthesized TiO₂ nanosheets with exposed (001) facets in an ethanol–water solvent. These nanocomposite samples showed high photocatalytic H₂-production activity in aqueous solutions containing methanol, as sacrificial reagent, even without Pt co-catalyst. The optimal graphene content was found to be ∼1.0 wt%, giving a H₂-production rate of 736 μmol h⁻¹ g⁻¹ with a quantum efficiency (QE) of 3.1%, which exceeded the rate observed on pure TiO₂ nanosheets by more than 41 times. This high photocatalytic H₂-production activity is due to the deposition of TiO₂ nanosheets on graphene sheets, which act as an electron acceptor to efficiently separate the photogenerated charge carriers. The observed enhancement in the photocatalytic activity is due to the lower absolute potential of graphene/graphene˙⁻ (−0.08 V vs. SHE, pH = 0) in comparison to the conduction band (−0.24 V) of anatase TiO₂ , meanwhile the aforementioned absolute value is higher than the reduction potential of H⁺ (0 V), which favors the electron transfer from the conduction band (CB) of TiO₂ to graphene sheets and the reduction of H⁺ , thus enhancing photocatalytic H₂-production activity. The proposed mechanism for the observed photocatalytic performance of TiO₂ nanosheets, modified with a small amount of graphene, was further confirmed by photoluminescence spectroscopy and transient photocurrent response. This work not only shows a possibility for the utilization of low cost graphene sheets as a substitute for noble metals (such as Pt) in the photocatalytic H₂-production but also for the first time shows a significant enhancement in the H₂-production activity by using metal-free carbon material as an effective co-catalyst.