Tuning the Electronic Structure of Graphite Oxide through Ammonia Treatment for Photocatalytic Generation of H₂ and O₂ from Water Splitting

Graphite oxide (GO) synthesized from the oxidation of graphite powders exhibits p-type conductivity and is active in photocatalytic H₂ evolution from water decomposition. The p-type conductivity hinders hole transfer for water oxidation and suppresses O₂ evolution. Treating GO with NH₃ 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. NH₃ treatment enhances the emission by removing nonirradiative epoxy and carboxyl sites on the GO. In half-reaction tests of water decomposition, NGO effectively catalyzes O₂ evolution in an aqueous AgNO₃ solution under mercury-lamp irradiation, whereas GO is inactive. NGO also effectively catalyzes H₂ evolution in an aqueous methanol solution but shows less activity than GO. Under illumination with visible light (λ > 420 nm), NGO simultaneously catalyzes H₂ and O₂ evolutions, but with a H₂/O₂ molar ratio below 2. The n-type conductivity of NGO may hinder electron transfer and form peroxide species instead of H₂ molecules. This study demonstrates that the functionality engineering of GO is a promising technique to synthesize an industrially scalable photocatalyst for overall water splitting

Graphite Oxide with Different Oxygenated Levels for Hydrogen and Oxygen Production from Water under Illumination: The Band Positions of Graphite Oxide

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

Extending the π‑Conjugation of g‑C₃N₄ by Incorporating Aromatic Carbon for Photocatalytic H₂ Evolution from Aqueous Solution

This study details the synthesis of high-activity g-C3N4 catalysts for H2 generation from a triethanolamine aqueous solution under visible light. We anneal a mixture of urea and NH4Cl to obtain g-C3N4 nanosheets, which are subsequently solvated with ethanol molecules and annealed to form aromatic carbon-doped g-C3N4. The results of analyses conducted using X-ray photoelectron, Fourier-transform infrared, and carbon-13 nuclear magnetic resonance spectroscopies demonstrated that annealing the ethanol molecules leads to the grafting of aromatic heterocycles on the g-C3N4 nanosheets and substitution of nitrogen with carbon. The grafted aromatic heterocycles and doped carbon atoms extend the π-conjugation system in g-C3N4 to reduce the band gap and facilitate the separation of photogenerated charges. The carbon-incorporating also preserve the crystallinity of g-C3N4 during high-temperature annealing, which facilitates the suppression of the recombination of photogenerated charges at defect sites. The developed aromatic carbon-doped g-C3N4 effectively catalyzes H2 generation from the aqueous solution, achieving apparent quantum yields of 14% and 2.2% under 420 and 550 nm monochromatic irradiation, respectively, whereas urea-derived g-C3N4 reached only 3.4% and 0.1%. The proposed strategy of extending the π-conjugation system is promising for promoting the activity of carbon-nitride photocatalysts

Elucidating Quantum Confinement in Graphene Oxide Dots Based On Excitation-Wavelength-Independent Photoluminescence

Investigating quantum confinement in graphene under ambient conditions remains a challenge. In this study, we present graphene oxide quantum dots (GOQDs) that show excitation-wavelength-independent photoluminescence. The luminescence color varies from orange-red to blue as the GOQD size is reduced from 8 to 1 nm. The photoluminescence of each GOQD specimen is associated with electron transitions from the antibonding π (π*) to oxygen nonbonding (n-state) orbitals. The observed quantum confinement is ascribed to a size change in the sp² domains, which leads to a change in the π*−π gap; the n-state levels remain unaffected by the size change. The electronic properties and mechanisms involved in quantum-confined photoluminescence can serve as the foundation for the application of oxygenated graphene in electronics, photonics, and biology