Facile preparation of novel Sb2S3 nanoparticles/rod-like α-Ag2WO4 heterojunction photocatalysts: Continuous modulation of band structure towards the efficient removal of organic contaminants

Insights into the performance of binary heterojunction photocatalysts for degradation of refractory pollutants

The uncontrolled discharge of industry- and consumer-derived micropollutants and synthetic contaminants into freshwater bodies represents a severe threat to human health and aquatic ecosystem. Inexpensive and highly efficient wastewater treatment methods are, therefore, urgently required to eliminate such non-biodegradable, recalcitrant, and toxic organic pollutants. In this context, advanced oxidation processes, particularly heterogenous photocatalysis, have received enormous attention over the past few decades. Among the different classes of photocatalysts explored by the scientific community, heterojunction photocatalysts, in general, and binary heterojunction photocatalysts, in particular, have shown tremendous promise, attributed to their many distinct advantages. As such, the present review highlights the application of diverse array of binary heterojunction photocatalysts for eliminating water-borne contaminants. Specifically, a bibliometric analysis has been conducted to identify the ongoing research trend and future prospects of heterojunction photocatalysts. It appears that metal oxide/metal oxide-based heterojunctions have superior thermal and mechanical stability compared to other heterojunction photocatalysts. In contrast, metal oxide/non-metal semiconductor-based heterojunctions are extremely effective in pollutant degradation without significant leaching of metal ions. The review concludes by proposing novel strategic research guidelines in order to make further advances in this rapidly evolving cross-disciplinary field of topical interest.

Toxicity of α-Ag2WO4 microcrystals to freshwater microalga Raphidocelis subcapitata at cellular and population levels

Silver-based materials have microbicidal action, photocatalytic activity and electronic properties. The increase in manufacturing and consumption of these compounds, given their wide functionality and application, is a source of contamination to freshwater ecosystems and causes toxicity to aquatic biota. Therefore, for the first time, we evaluated the toxicity of the silver tungstate (α-Ag₂WO₄), in different morphologies (cube and rod), for the microalga Raphidocelis subcapitata. To investigate the toxicity, we evaluated the growth rate, cell complexity and size, reactive oxygen species (ROS) production and chlorophyll a (Chl a) fluorescence. The α-Ag₂WO₄ - R (rod) was 1.7 times more toxic than α-Ag₂WO₄-C (cube), with IC₁₀ and IC₅₀ values of, respectively, 8.68 ± 0.91 μg L^-1 and 13.72 ± 1.48 μg L^-1 for α-Ag₂WO₄ - R and 18.60 ± 1.61 μg L^-1 and 23.47 ± 1.16 μg L^-1 for α-Ag₂WO₄-C. The release of silver ions was quantified and indicated that the silver ions dissolution from the α-Ag₂WO₄ - R ranged from 34 to 71%, while the Ag ions from the α-Ag₂WO₄-C varied from 35 to 97%. The α-Ag₂WO₄-C induced, after 24 h exposure, the increase of ROS at the lowest concentrations (8.81 and 19.32 μg L^-1), whereas the α-Ag₂WO₄ - R significantly induced ROS production at 96 h at the highest concentration (31.76 μg L^-1). Both microcrystal shapes significantly altered the cellular complexity and decreased the Chl a fluorescence at all tested concentrations. We conclude that the different morphologies of α-Ag₂WO₄ negatively affect the microalga and are important sources of silver ions leading to harmful consequences to the aquatic ecosystem.

The enhancement mechanism of ultra-active Ag3PO4 modified by tungsten and the effective degradation towards phenolic pollutants

A novel strategy of W modification was applied to overcome the disadvantages of Ag₃PO₄. Ultra-active Ag₃PO₄ with different W doping ratios were successfully synthesized by facile chemical precipitation method, among which 0.5%W-AP showed the best results. Meanwhile, the stability and yield were enhanced. XRD, Raman and ESR etc. were employed to investigate the morphology, structure and optical properties of samples. It was proved W⁶⁺ entered into the Ag₃PO₄ lattice, occupied the position of P⁵⁺ and doped in the form of WO₄^2-. The significant improvement of photocatalytic performance of W doped Ag₃PO₄ was attributed to the change of morphology, the decrease of particle size, the increase of crystallinity, the shrink of band gap energy and the reduction of photo-induced carriers recombination rate with W doping. The photocatalytic mechanism analysis showed h⁺ was the main oxidative species in the photocatalytic process, •O₂^- and •OH played minor roles. Under visible light irradiation, the impacts of the important operating parameters on the typical phenolic pollutants, phenol and bisphenol A, were evaluated with 0.5%W-AP. It was confirmed that 68% and 82% of phenol and bisphenol A were respectively degraded within 15 min and 40 min under optimized photocatalytic parameters: 0.4 g/L catalyst dosage, 20 mg/L pollutant concentration, pH 5.7 and 125 mW/cm² irradiation intensity, and the corresponding K' were 2.14 and 5.50 times of undoped samples. This work provides a new approach for effective degradation towards phenolic pollutants by Ag₃PO₄ with ultra-high photocatalytic activity, high applicability and enhanced stability and yield.

Visible light-conducting polymer nanocomposites as efficient photocatalysts for the treatment of organic pollutants in wastewater

This review compiles recent advances and challenges on photocatalytic treatment of wastewater using nanoparticles, nanocomposites, and polymer nanocomposites as photocatalyst. The review provides an overview of the fundamental principles of photocatalytic treatment along the recent advances on photocatalytic treatment, especially on the modification strategies and operational conditions to enhance treatment efficiency and removal of recalcitrant organic contaminants. The different types of photocatalysts along the key factors influencing their performance are also critically discussed and recommendations for future research are provided.

Construction of cerium oxide nanoparticles immobilized on the surface of zinc vanadate nanoflowers for accelerated photocatalytic degradation of tetracycline under visible light irradiation

Construction of Z-scheme heterojunction has been deemed to be an effective and promising approach to boost the photocatalytic activity on account of accelerating the separation efficiency of the photogenerated carriers and maintaining the strong redox ability. Herein, an attractive CeO₂/Zn₃V₂O₈ Z-scheme heterojunction photocatalyst was rationally constructed by zero-dimensional (0D) CeO₂ nanoparticles immobilized on the surface of three-dimensional (3D) Zn₃V₂O₈ nanoflowers using a simple mixing method, and applied to the photocatalytic degradation of tetracycline (TC) under visible light irradiation. As expected, it was observed that the prepared CeO₂/Zn₃V₂O₈ hybrid illustrated significantly boosted the photocatalytic activity for the elimination of TC compared to pure Zn₃V₂O₈. More importantly, the optimized CeO₂(40 wt%)/Zn₃V₂O₈ hybrid owned the largest elimination rate of TC with 1.13 × 10^-2 min^-1, which was around 8.1 and 3.8 times as high as single CeO₂ (0.14 × 10^-2 min^-1) and Zn₃V₂O₈ (0.30 × 10^-2 min^-1), respectively. The appreciable performance improvement was mainly ascribed to the formation of Z-scheme heterojunction between CeO₂ and Zn₃V₂O₈, facilitating the transfer rate of photogenerated carriers and remaining the high reducibility of photoexcited electrons in CeO₂ and strong oxidizability of photoinduced holes in Zn₃V₂O₈. Active species capture experiments and electron spin resonance spectra showed that superoxide radicals and holes were the main active species for TC degradation. Besides, the possible degradation pathways of TC were speculated by identifying degradation intermediates, and the reasonable degradation mechanism including migration and transport behaviors of charge carriers and generation processes of reactive species were revealed in depth. This investigation enriches Zn₃V₂O₈-based Z-scheme heterojunction photocatalytic system and offers a new inspiration for the construction and fabrication of high-efficiency Z-scheme heterojunction photocatalysts to remove the antibiotics from wastewater.