Role of graphene on the band structure and interfacial interaction of Bi2WO6/graphene composites with enhanced photocatalytic oxidation of NO

Abstract

The photocatalytic performance of Bi₂WO₆ was limited by slow electron transfer and fast charge recombination. In this report, Bi₂WO₆/graphene (2 wt%) composites were fabricated by a two-step approach using graphene as precursor, which can maintain the crystallinity, morphology and particle size of pristine hierarchical Bi₂WO₆ microspheres, providing unique opportunities to correlate interfacial interaction with photocatalytic activity. The interfacial electronic interaction between Bi₂WO₆ and graphene evidenced by X-ray photoelectron spectroscopy (XPS) resulted in positive shifting of the Fermi level and broadening of the valence band (VB) of Bi₂WO₆. These reveal a stronger oxidative power and faster mobility of photogenerated holes upon excitation, in combination with radical trapping and electron spin resonance (ESR) experiments providing clear evidence for this key property. Compared to pristine Bi₂WO₆, the composites exhibited not only higher photocatalytic activity toward the oxidation of NO, but also better selectivity for the formation of ionic species (NO₃⁻) as well as a ninefold enhancement of the photocurrent density. The significantly improved charge separation and migration in the Bi₂WO₆/graphene composite was demonstrated by electrochemical impedance spectroscopy (EIS). Moreover, the interfacial electron transfer rate determined for the composite was 7.97 × 10⁸ s⁻¹via time-resolved fluorescence decay spectra. It was therefore proposed that the enhanced photocatalytic activity of Bi₂WO₆/graphene could be directly ascribed to the deeper VB edge position as well as efficient charge transfer across the interface. The present study points out the key role of graphene in tuning electronic structure and interfacial charge transfer processes for the development of highly efficient photocatalysts.