Graphite Oxide with Different Oxygen Contents as Photocatalysts for Hydrogen and Oxygen Evolution from Water

Abstract

Graphite oxide (GO) photocatalysts were derived from graphite oxidation. 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 oxidation of water. The photocatalytic activity of GO specimens with various oxygen contents was measured in methanol and AgNO 3 solutions for H 2 and O 2 evolution, respectively. The H 2 evolution was strong and stable over time whereas the O 2 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 negligibly change with the oxygen content. When NaIO 3 was used as a sacrificial reagent to suppress the mutual reduction mechanism under illumination, strong O 2 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. IntroductionHydrogen generation from water decomposition under light irradiation has received much attention.1-7 Photolysis of water using semiconducting photocatalyst powders is considered a prospective candidate because of the low cost in instrumentation and high interfacial contact area for reaction. [8][9][10][11][12][13][14][15][16][17] The essential requirements for an effective semiconducting photocatalyst are more negative conduction band edge and more positive valence band edge, to facilitate respectively reduction and oxidation of water. Additionally, the contact area between the catalyst and water should be large enough to execute photochemical reactions at sufficiently high gas-evolution rates. Most photocatalysts are metal-containing inorganic solids for H 2 and/or O 2 evolution from water. [18][19][20][21][22][23][24][25][26][27][28] High temperature calcination of these metal-containing photocatalysts is generally conducted for high crystallinity, 29 but it shrinks the contacting surface area with water. Molecule-like materials with accurate electronic band structure, high surface area, and stability are an alternative to the crystalline solid. [19][20][21]30,31 Graphite oxide (GO) is a polymer-like material made of carbon, oxygen, and hydrogen, having a large exposable surface area after being dispersed in water to molecular scale. 32 We demonstrated that GO can serve as a photocatalyst for H 2 evolution from water. 33 However, the conduction and valence band levels relative to those for water reduction and oxidation are yet to be explored.GO is derived from extensive oxidation of graphite and contains hydrophilic oxygen functional groups on constituting graphene sheets. [34][35][36][37][38][39][40] Therefore, GO easily disperses in aqueous solutions and exfoliates in the manner of wrinkled paper. The electronic properties of GO are related to the composition of oxygen bonding on grap...

Nanocomposite of TiO2 @ Ni- or Co-doped graphene oxide for efficient photocatalytic water splitting

“…This indicated that the bandgap energies for TiO 2 @ CoO-doped GO and TiO 2 @NiO-doped GO are lower than that of anatase TiO 2 . This finding is agreement with previously stated [14]. It was presence of transition the p-type metal oxide on GO sheet forms a heterojunction with n-type TiO 2 which generates the synergistic effect via promoting the extension of electron/hole lifetime due to difference Fermi levels.…”

TiO2-reduced graphene oxide nanocomposites: Microsecond charge carrier kinetics

“…Several research articles have reported higher photocatalytic activity and faster reaction rates when GO/rGO-modified TiO 2 photocatalysts were used for organic pollutant photodegradation, dye photoremoval, bacterial inactivation and H 2 production [30][31][32][33][34]. Depending on the reduction degree of GO the valence band and conduction band position are shifted, which means that when oxygen content in GO is high enough GO could even act as a photocatalyst or a photosensitizer [35,36]. Therefore, it is crucial to know the role played by rGO in TiO 2 -graphene nanocomposites since depending on the bands position and the excitation wavelength, the photocatalytic process can take place by different mechanisms, for example, electrons could be transferred from TiO 2 to rGO or vice versa [28].…”