Transformation of a Polyoxometalate Precursor into Carbon Doped Bismuth Molybdate Nanosheets for the Visible Light Photocatalytic Degradation of Aqueous Pollutants

Heterogeneous photocatalysis is a convenient and effective tool for eliminating toxic organic effluents from the textile and pharmaceutical industries. The analogous photocatalytic properties of polyoxometalates (POMs) with metal oxide-based semiconductors make them applicable for photocatalytic wastewater treatment applications. However, the high aqueous solubility and low coverage of the visible solar spectrum limit the wider applicability of traditional POMs as heterogeneous photocatalysts. Herein, we report, for the first time, the hydrothermal transformation of a UV-active polyoxometalate precursor, (C16H36N)4[Mo8O26] abbreviated as TBA-Mo8, into a visible light active photocatalytic carbon-doped Bi2MoO6 nanosheets (C-BMO) in the presence of a Bi3+ salt and studied its catalytic activities toward the degradation of a dye pollutant, malachite green (MG), and a pharmaceutical pollutant, ciprofloxacin (CF). For comparison purposes, an undoped Bi2MoO6 (U-BMO) nanosheet was also prepared under identical conditions starting from an in situ generated POM precursor H4[Mo8O26]. The successful transformation of TBA-Mo8 and H4[Mo8O26] into Bi2MoO6 photocatalysts under hydrothermal conditions was confirmed through a series of analytical and spectroscopic techniques, including Fourier transform infrared, Raman, X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron spectroscopy, high-resolution transmission electron spectroscopy, and other analyses. The results of the photocatalytic experiments revealed superior catalytic activities of C-BMO compared to U-BMO and the parent cluster TBA-Mo8 under visible light irradiation. C-BMO achieved 70 and 87% degradation of MG and CF under visible light irradiation against the 29 and 35% degradation, respectively, shown by U-BMO. A plausible reason for the higher photocatalytic activity of C-BMO is its better charge separation due to the band gap modification resulting from carbon doping. The detailed pathways of MG and CF degradation have also been proposed based on scavenger studies and ESI-MS analyses of the photocatalytic degradation products. This work, therefore, summarizes how simple polyoxometalate can serve as precursors for developing better and more efficient visible light active photocatalysts under milder experimental procedures.