La2O3/BaO Composites With Enhanced Interfacial Charge Transport for High-Performance Photocatalytic Dye Degradation

Background: Reactive dyes are persistent water contaminants that can be difficult to remove by conventional treatment. Herein, we report La2O3/BaO composites for the ultraviolet (UV)-assisted degradation of a model dye—Reactive Blue 194 (RB 194)—and examine the influence of the La:Ba precursor ratio on their physicochemical properties. Methods: La2O3/BaO composites were synthesized by coprecipitation using La:Ba precursor ratios of 1:1, 1:2, and 2:1 (mol/mol) and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis–differential thermal analysis, UV–visible spectroscopy, and photoluminescence (PL) spectroscopy. Based on the results of this characterization, the composite prepared at a La:Ba precursor ratio of 1:2 (La1/B2) was selected for RB 194 degradation. We assessed the effects of catalyst dosage, initial dye concentration, pH, and temperature on photocatalytic performance, as well as degradation kinetics, reactive-species scavenging, electrochemical behavior, transformation products, and reusability. Results: The composites featured coexisting La- and Ba-containing crystalline phases and composition-dependent optical and PL responses. La1/B2 exhibited an apparent indirect band gap of ~4.25 eV and a point of zero charge of 8.92, achieving ~99% apparent decolorization within 120 min under the selected conditions (20 mg L−1 RB 194, 70 mg catalyst per 100 mL, pH 6, 20 °C, and UV irradiation at 450 W). Given that the decolorization extent was calculated from absorbance loss at 600 nm and Total Organic Carbon (TOC)/Chemical Oxygen Demand (COD) analyses were not performed, the value of 99% does not represent complete mineralization. Among the evaluated kinetic models, the pseudo-first-order model provided the best fit (apparent rate constant = 0.04103 min−1 at 20 mg L−1). The charge-transfer resistance decreased from 32.6 Ω for pristine La2O3 to 23.2 Ω for La1/B2, consistent with improved interfacial electrochemical transport. Scavenging experiments indicated an important contribution from hole-associated surface reactions. Conclusions: La1/B2 outperformed pristine La2O3 in terms of RB 194 decolorization efficiency and electrochemical charge transport, thus holding promise for dye removal. However, direct band-position measurements, mineralization analysis, and long-term stability assessment are required to elucidate the charge transfer mechanism and practical applicability of this composite.