Moderate band-gap-broadening induced high separation of electron-hole pairs in Br substituted BiOI: a combined experimental and theoretical investigation.

We, for the first time, demonstrate band-gap-broadening as a new approach to remarkably enhance the photocatalytic activity of Br(-) substituted BiOI photocatalysts, which were fabricated via a facile chemical precipitation route. The successful incorporation of Br(-) ions into the crystal lattice of BiOI was confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The photocatalytic experiments demonstrated that all of the Br-BiOI samples exhibited highly improved photocatalytic performances pertaining to rhodamine B (RhB) and phenol degradation under visible light (λ > 420 nm). The active species trapping and electron spin resonance (ESR) experiments also suggested that more superoxide radicals (˙O2(-)), serving as the main active species, were generated over Br-BiOI than pristine BiOI in the photooxidation process. Based on the results from experiments and theoretical calculations, the enhancement of photocatalytic activity should be attributed to the lowered valence band (VB) potential and enlarged band gap induced by the Br(-) replacement, which greatly facilitated the high separation of photoinduced electron-hole pairs, as verified by the photoluminescence (PL) experiments, electrochemical impedance spectra (EIS) and Bode-phase spectra. This work sheds light on a new method to improve the photocatalytic performance of photocatalysts.