Enhanced photocatalytic H2-production activity of graphene-modified titaniananosheets

Abstract

Graphene-modified TiO₂ nanosheets with exposed (001) facets (graphene/TiO₂) were prepared by microwave-hydrothermal treatment of graphene oxide (GO) and hydrothermally synthesized TiO₂ nanosheets with exposed (001) facets in an ethanol–water solvent. These nanocomposite samples showed high photocatalytic H₂-production activity in aqueous solutions containing methanol, as sacrificial reagent, even without Pt co-catalyst. The optimal graphene content was found to be ∼1.0 wt%, giving a H₂-production rate of 736 μmol h⁻¹ g⁻¹ with a quantum efficiency (QE) of 3.1%, which exceeded the rate observed on pure TiO₂ nanosheets by more than 41 times. This high photocatalytic H₂-production activity is due to the deposition of TiO₂ nanosheets on graphene sheets, which act as an electron acceptor to efficiently separate the photogenerated charge carriers. The observed enhancement in the photocatalytic activity is due to the lower absolute potential of graphene/graphene˙⁻ (−0.08 V vs. SHE, pH = 0) in comparison to the conduction band (−0.24 V) of anatase TiO₂, meanwhile the aforementioned absolute value is higher than the reduction potential of H⁺ (0 V), which favors the electron transfer from the conduction band (CB) of TiO₂ to graphene sheets and the reduction of H⁺, thus enhancing photocatalytic H₂-production activity. The proposed mechanism for the observed photocatalytic performance of TiO₂ nanosheets, modified with a small amount of graphene, was further confirmed by photoluminescence spectroscopy and transient photocurrent response. This work not only shows a possibility for the utilization of low cost graphene sheets as a substitute for noble metals (such as Pt) in the photocatalytic H₂-production but also for the first time shows a significant enhancement in the H₂-production activity by using metal-free carbon material as an effective co-catalyst.