Combined experimental and theoretical studies on enlarged bandgap and improved photoelectrochemical properties of reduced graphene oxide film by hydrogen annealing

Abstract

• Relationship between thermal annealing, sp 2 domain, bandgap and properties in RGO. • Correlation between experimental data with calculations (GFN2-xTB, sTD-DFT). • The significant increase of 45.8% in the PEC hydrogen evolution for RGO1100 films. The effect of thermal annealing on the recovery of sp 2 carbon networks in the reduced graphene oxide (RGO) and its consequent bandgap enlargement and electrical conductivity improvement were thoroughly studied, experimentally and theoretically correlated. The annealed RGO film at 1100 °C (designated as RGO1100) exhibited a low sheet resistance (1250 Ω.cm −2 ) with a wide bandgap of 2.82 eV. The photoelectrochemical (PEC) experiments in 1 M H 2 SO 4 solution under solar simulation light (1.5 AM) showed photocurrent densities of 48 and 70 μA.cm −2 for non-annealed RGO (72.5% sp 2 fraction) and RGO1100 (87% sp 2 fraction) respectively, equivalent to an increase of 45.8% for RGO1100. The bandgap widening as well as electrical conductivity increase of RGO1100 were closely interpreted on the basis of sp 2 domain restoration, electronic and molecular orbital structures obtained from extensive experimental data as well as elaborated Geometry, Frequency, Noncovalent Extended Tight-binding (GFN2-xTB) and simplified Tamm-Dancoff density functional theory (sTD-DFT) calculations. The correlation between the electronic structure and the PEC performance of the RGO was also evaluated when the impact of the spatial distribution of oxidation and reduction regions on the recombination rate between photogenerated electrons and holes had been taken into account. The findings shed light on the structure-PEC relationship of RGO1100, which will definitely assist in designing and tailoring RGO-based, highly active materials for PEC applications.