A graphite oxide (GO) semiconductor photocatalyst with an apparent bandgap of 2.4–4.3 eV is synthesized by a modified Hummers' procedure. The as‐synthesized GO photocatalyst has an interlayer spacing of 0.42 nm because of its moderate oxidation level. Under irradiation with UV or visible light, this GO photocatalyst steadily catalyzes H2 generation from a 20 vol % aqueous methanol solution and pure water. As the GO sheets extensively disperse in water, a cocatalyst is not required for H2 generation over the GO photocatalyst. During photocatalytic reaction, the GO loses some oxygen functional groups, leading to bandgap reduction and increased conductivity. This structural variation does not affect the stable H2 generation over the GO. The encouraging results presented in this study demonstrate the potential of graphitic materials as a medium for water splitting under solar illumination.

Nitrogen-doped graphene oxide quantum dots exhibit both p- and n-type conductivities and catalyze overall water-splitting under visible-light irradiation. The quantum dots contain p-n type photochemical diodes, in which the carbon sp(2) clusters serve as the interfacial junction. The active sites for H2 and O2 evolution are the p- and n-domains, respectively, and the reaction mimics biological photosynthesis.

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Graphite oxide (GO) photocatalysts derived from graphite oxidation can have varied electronic properties by varying the oxidation level. Absorption spectroscopy shows the increasing band gap of GO with the oxygen content. Electrochemical analysis along with the Mott–Schottky equation show that the conduction and valence band edge levels of GO from appropriate oxidation are suitable for both the reduction and the oxidation of water. The conduction band edge shows little variation with the oxidation level, and the valence band edge governs the bandgap width of GO. The photocatalytic activity of GO specimens with various oxygenated levels was measured in methanol and AgNO3 solutions for evolution of H2 and O2, respectively. The H2 evolution was strong and stable over time, whereas the O2 evolution was negligibly small due to mutual photocatalytic reduction of the GO with upward shift of the valence band edge under illumination. The conduction band edge of GO showed a negligible change with the illumination. When NaIO3 was used as a sacrificial reagent to suppress the mutual reduction mechanism under illumination, strong O2 evolution was observed over the GO specimens. The present study demonstrates that chemical modification can easily modify the electronic properties of GO for specific photosynthetic applications.

Graphite oxide (GO) synthesized from the oxidation of graphite powders exhibits p-type conductivity and is active in photocatalytic H 2 evolution from water decomposition. The p-type conductivity hinders hole transfer for water oxidation and suppresses O 2 evolution. Treating GO with NH 3 gas at room temperature tunes the electronic structure by introducing amino and amide groups to its surface. The ammonia-modified GO (NGO) exhibits n-type conductivity in photoelectrochemical analysis and has a narrower optical band gap than GO. Electrochemical analysis attributes the band gap reduction to a negative shift of the valence band. An NGO-film electrode exhibits a substantially higher incident photo-to-current efficiency in the visible light region than a GO electrode. Photoluminescence analyses demonstrate the above-edge emission characteristic of GO and NGO. NH 3 treatment enhances the emission by removing nonirradiative epoxy and carboxyl sites on the GO. In half-reaction tests of water decomposition, NGO effectively catalyzes O 2 evolution in an aqueous AgNO 3 solution under mercury-lamp irradiation, whereas GO is inactive. NGO also effectively catalyzes H 2 evolution in an aqueous methanol solution but shows less activity than GO. Under illumination with visible light (λ > 420 nm), NGO simultaneously catalyzes H 2 and O 2 evolutions, but with a H 2 /O 2 molar ratio below 2. The ntype conductivity of NGO may hinder electron transfer and form peroxide species instead of H 2 molecules. This study demonstrates that the functionality engineering of GO is a promising technique to synthesize an industrially scalable photocatalyst for overall water splitting.