Scheelite-Type Wide-Bandgap ABO₄ Compounds (A = Ca, Sr, and Ba; B = Mo and W) as Potential Photocatalysts for Water Treatment

Abstract

In the present study, alkaline earth metal scheelite-type ABO4 compounds (A = Ca, Sr, and Ba; B = Mo and W) synthesized by a hydrothermal method were systematically studied. The as-obtained photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, UV–vis diffuse reflectance (DR/UV–vis) spectroscopy, photoluminescence, and thermoluminescence (TL) spectroscopy together with charge carrier lifetime measurements, electron paramagnetic resonance (EPR) spectroscopy, and electrochemical impedance spectroscopy (EIS). The photocatalytic activity was studied in the reaction of phenol degradation under simulated solar light. The obtained tungstates and molybdates revealed excellent photocatalytic activity despite the low surface area and wide bandgap typical for insulators. The mechanism of phenol degradation proceeded through hydroquinone and catechol formation in the presence of hydroxyl and superoxide radicals. The presence of electron traps allowed absorption of light with lower energy than resulting from the absorption edge. BaWO4 and SrWO4, with the most extended average carrier lifetime, were the most efficient photocatalysts from the obtained series. In general, molybdates exhibited lower photocatalytic activity toward phenol degradation due to deeper trap states and lower average charge carrier lifetimes than tungstates. Additionally, electrochemical studies demonstrated that molybdates exhibit more insulating behavior than tungstates. The overall results showed that wide-bandgap semiconductors, mainly tungstates, can be applied as earth-abundant photocatalytic materials for the degradation of persistent organic pollutants.

Synthesis, characterization, and photocatalytic CO2 reduction evaluation of the CaWxMo1−xO4 (x = 0–1) solid solution

“…There is a slight increasing trend in the crystallite size with low W concentration; however, higher concentrations significantly decreased this parameter. In this context, this reduction can be associated with introducing a slightly larger cation into the crystalline structure, as has been reported in some other studies 50 compared to those scheelite materials. The largest crystallite size was exhibited in the W 0.4 M 0.6 .…”

“…As can be observed, there is a ''proportional'' increase in the carrier lifetime values owing to the increase in W 6+ . In this context, according to Kowalkinska et al, 50 compared to molybdate compounds, the tungstate compounds could present more charge carrier traps, which is a strong reason to prolong the carrier lifetimes.…”

“…CaMoO 4 and CaWO 4 , which form structure-type scheelite and powellite, respectively, are materials with ABO 4 composition. 38 Owing to their excellent chemical stability, these compounds have been studied as Raman lasers, 39 fuel cells, 40 diodes, 41 and catalysts. 42 These materials present a tetragonal structure in which each B atom (B = Mo and W) is coordinated with four oxygen [BO 4 ] 2− , while divalent metal shares corners with eight oxygen from tetragonal B metal groups, forming a polyhedral AO 8 .…”

Epitaxial Growth of Lattice‐Matched BaWO4/CH3NH3PbX3(X = Cl, Br, I) Hetero‐Micro/Nanostructure with Suppressed Halide Ion Migration

“…BaWO 4 with a scheelite structure shows particular excitonic luminescence property. [ 16 ] Stemming from the large exciton binding energy between the hole on oxygen and the electron on tungsten upon UV‐light excitation, a restricted interfacial charge transfer is expected to prevent the luminescence quenching of MAPbBr 3 PQDs in heterostructure. [ 17 ] In this regard, we propose a solvent‐free thermally driven heteroepitaxial growth strategy to assemble MAPbX 3 PQDs on highly lattice‐matched BaWO 4 .…”

Surface and Trapping Energies as Predictors for the Photocatalytic Degradation of Aromatic Organic Pollutants

“…Following early work on photo-induced reactions over semiconductor particles, − photocatalytic processes have been widely developed and proposed as a possible way to induce redox reactions inside the chemical systems. − Primarily, environmental and energy-related applications are extensively studied, including photocatalytic water splitting, , H 2 generation, , CO 2 and heavy metal reduction, − as well as degradation of organic pollutants from water and air streams. − Each of these processes is initiated by photon absorption and creation of electron–hole pairs, which can further undergo charge transfer to the substrate present at the surface, inducing its chemical transformation. The efficiency of such a transfer and the number of transferred charge carriers directly determine the final efficiency of the process and strictly depend on the nature of the photocatalyst.…”