Synthesis and Analysis of Palladium Modified Bi₂WO₆/BiOCl Composites for Enhanced Water Decontamination

Abstract

Over the past several decades, rapid industrial and population growth has put a strain on the environment and clean water supplies by exposing them to harmful toxins such as heavy metals and organic and inorganic pollutants. As such, water decontamination remains a vital issue that needs to be addressed in the 21st century. Over the years, various water treatment strategies have been deployed, including ion exchange, membrane filtration, adsorption, and precipitation. However, many of these techniques are hindered by incomplete treatment and high operation and energy costs. In more recent times, advanced oxidation processes (AOPs), including UV disinfection, ozone disinfection, and heterogeneous photocatalysis, have sought to provide environmentally effective and friendly solutions to water treatment. Among these, semiconductor photocatalysis has attracted growing interest due to its ability to degrade a plethora of organic and inorganic pollutants into harmless compounds such as H₂O and CO₂. Photocatalysis uses light energy to activate a photoactive material, usually in the form of a semiconductor, to allow for the degradation of harmful pollutants in water. When photocatalysts are exposed to light with energy equal to or greater than their band gap, their electrons (e⁻) are excited from the valence band (VB) to the conduction band (CB), leaving positively charged holes (h⁺) in their place. These charge carriers can subsequently participate in reduction and oxidation reactions to break down undesirable pollutants. Historically, TiO₂ has been regarded as an ideal photocatalyst for its high stability, commercial availability, non-toxicity, and low cost. However, due to its wide band gap (>3.2 eV), it is only active under UV light. This hinders the application of TiO₂ in water decontamination processes that rely on visible-light-driven photocatalysis. Hence, modern work in this field focuses on the development of photocatalysts that access a larger portion of the solar spectrum. This work concerns the synthesis and analysis of a novel Pd-Bi₂WO₆/BiOCl heterojunction photocatalyst. Bi₂WO₆/BiOCl was prepared using a hydrothermal technique, whose conditions, including composite ratio, calcination temperature, and calcination time, were optimized based on their performance in the degradation of Rhodamine B (RhB) under visible light. Pd-Bi₂WO₆/BiOCl samples with varying amounts of palladium were fabricated by the photoreduction method. The novel catalyst was analyzed with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), and N2-sorption testing. It was demonstrated that the palladium-modified composites exhibited improved RhB photodegradation, with 0.5 wt% Pd-Bi₂WO₆/BiOCl showing maximum photoactivity, representing approximately 25% improvement over Bi₂WO₆/BiOCl.The improved results were attributed to the formation of a Schottky junction and the surface plasmon resonance (SPR) effect, which suppressed the recombination of e⁻ and h⁺ and boosted the visible light responsiveness. Quenching experiments revealed that holes and hydroxyl radicals were the main oxidizing species responsible for the breakdown of RhB molecules. The stability and reusability of the catalyst were demonstrated with a four stage recycle test. Experiments were also done to study the effects of key operational parameters, including the initial RhB concentration, catalyst dosage, reaction temperature, and water pH. From the experimental findings and characterization analyses, it was proposed that Pd-Bi₂WO₆/BiOCl forms a type I heterojunction that uses h⁺ and •OH to degrade RhB via the N-de-eethylation process. Overall, this study endorsed the use of plasmonic heterojunction photocatalysts for the effective treatment of organic pollutants in water.