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

Abstract

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.

Catalytic Applications of Carbon Dots

“…Electronic structural analysis revealed that graphene oxide (GO) materials have conduction band minimum (CBM) and valence band maximum (VBM) levels suitable for generating H 2 and O 2 , respectively, under visible-light irradiation [80]. Size modulation and chemical modification readily tune the electronic properties of graphene.…”

Direct Observation of Quantum‐Confined Graphene‐Like States and Novel Hybrid States in Graphene Oxide by Transient Spectroscopy

“…This could be helpful for highly active photocatalytic processes. [ 43,56 ] It also renews the understanding of role of oxygen-containing functional groups in GO and its part-reduced derivatives. Moreover, a greater understanding of the fundamental physics of carbon nanomaterials will eventually be a benefi t for design and fabrication of novel carbon-based nano optical-electronic devices in the not-too-distant future.…”

“…This bandgap is close to the recent analysis results from incident photon to current conversion effi ciency and linear potential scans of GO. [ 43 ] Therefore, this kind of unusual bleaching signal mode directly indicates the bandgap of GO. This also means that ultrafast broadband transient absorption spectroscopy excited in visible light range can be used for detecting the bandgap of GO as well as rGO (see below).…”

Application of GO in Energy Conversion and Storage

“…GO exhibits p-type conductivity because of oxygen's high electronegativity compared to carbon. Likewise, n-type conductivity appears when graphene covalently bonds to electron donating nitrogen-containing functional groups [1]. As oxygen bonds to graphene, the valence band originates from the O 2p orbital rather than the π orbital of graphene, leading to a larger band gap for a high oxygen coverage rate of GO and thus an evolution from semimetal to insulator.…”

“…Meanwhile, the conduction band of GO is mainly contributed by the anti-bonding π* orbital, which has a higher energy level than that needed for H 2 generation [2]. Therefore, GO with an appropriately functionalization might be a promising photocatalyst [1][2][3][4][5][6][7].…”