Solar photoconversion using graphene/TiO ₂ composites: Nanographene shell on TiO ₂ core versus TiO ₂ nanoparticles on graphene sheet

Size controlled nanographene oxides (NGOs; <50 nm) were prepared by a two-step oxidation process and NGOs were self-assembled with TiO ₂ nanoparticles to form the core/shell structure. Nanosized GO-coated TiO ₂ nanoparticles (NGOTs) were then reduced by a photocatalytic process under UV irradiation to obtain graphene-coated TiO ₂. This is clearly different from the typical graphene/TiO ₂ composite with the particles-on-a-sheet geometry and is the first study on the core/shell structure of its kind. The physicochemical properties of NGOs and the reduced NGOTs (r-NGOTs) were characterized by various analytical and spectroscopic methods (AFM, FT-IR, XPS, TEM, EELS, etc.). The photocatalytic and photoelectrochemical activities of r-NGOT were compared with a composite of r-GO/TiO ₂ that has TiO ₂ nanoparticles loaded on a larger graphene sheet (r-LGOT). The photocatalytic production of hydrogen was measured in the aqueous suspension of the composite photocatalyst under UV irradiation (λ > 320 nm), and the photoelectrochemical behaviors were characterized using the electrode coated with the composite photocatalyst. The rates of H ₂ production and photocurrent generation were higher with r-NGOT than r-LGOT, which indicates that the presence of r-GO shell on the surface of TiO ₂ facilitates the interfacial electron transfer. The direct contact between r-NGO and TiO ₂ is maximized in r-NGOT by retarding the charge recombination and accelerating the electron transfer. The geometry of the core/shell structure should be effective in the design of a graphene/TiO ₂ composite for solar conversion applications.