Distinct role of hexagonal tungsten in tungsten/ceria heterojunction in efficient utilization of visible flux

Abstract

Mixed phase metal oxide photocatalysts at optimised phase ratio are known to perform better than their monophasic counterpart in visible spectrum. In tungsten oxide (WO3) too high visible activity is seen in multi-phasic systemhaving dominant hexagonal phase and is attributable to its superior photon harvesting characteristicsthan other phases. In a hetero-composite with ceria due to suitable band edge position further enhancement in the photoactivity is seen mainly with its monoclinic(m-WO3) phase. In this work, h-WO3/Ce3+/Ce4+based hetero-composites have been realized with different molar ratio of W/Ce.While h-WO3 dominates all the systems,m-WO3appears in the system at low concentration of Ce and disappears at higher concentrations. The resultant sharp increase (13.66X) in the visible photocatalytic performance of mixed phase-WO3/ceria composite compared to pristine h-WO3 shows a further jump (44X) in the monophasic-WO3/ceria composite with the highest concentration of ceria synthesized. The characterization by X-ray diffraction (XRD), UV-VIS spectroscopy (UV-VIS DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, transmission electronmicroscopy(TEM), and scanning electron microscopy(SEM) with energy dispersive x-ray analysis (EDX) reveal the complex role distinct phase junctions of homo-/hetero-composites. The mixed-phase h-WO3/m-WO3homojunction, h-WO3/Ce3+ composite heterojunction and h-WO3/Ce3+/Ce2+ with both hetero-composite (h-WO3/Ce3+ and h-WO3/Ce2+) and homo-composite(Ce3+/Ce4+) heterojunction in the samples has been identified, and their roles have been analyzed to correlate with their respective visible light photoactivity.