Robust Z-scheme g-C3N4/WO3 heterojunction photocatalysts with morphology control of WO3 for efficient degradation of phenolic pollutants

Efficient photodegradation of paraquat herbicide over TiO2-WO3 heterojunction embedded in diatomite matrix and process optimization

Photodegradation of paraquat herbicide was assessed over several TiO2-WO3 heterojunctions embedded in the diatomite matrix. The characterization results indicated that WO3 embedding in the TiO2 decorated-diatomite matrix could not only enhance the adsorption capacity, visible-light response, and distribution of semiconductor species but also lessen the recombination rate and band gap energy. These characteristics were more noticeable as 5 wt.% of WO3 was embedded. Despite better optical properties of immobilized TiO2-WO3 nanocomposites, overloading WO3 generally alleviates the synergetic effect of tungsten due to surface coverage of diatomite matrix and, subsequently, the significant attenuation of textural properties, more formation of agglomerations and defects as trapping centers in the oxidation sites of heterostructures, and also, less likely of forming TiO2-WO3 heterojunction. In accordance with characterization results, the highest UV-photodegradation of paraquat was attained over heterostructured nanocomposite containing 5 wt.% WO3 (T25-W5/Di). The effects of significant operating parameters were also investigated, modeled, and optimized using response surface methodology (RSM)-central composite design (CCD). Under optimized operation conditions, the experimental removal efficiency of paraquat reached 97.1 and 80% using UV and simulated solar light, respectively. Moreover, the reusability results confirm the sustained activity of the T25-W5/Di nanocomposite.

Heterojunction photocatalysts for the removal of nitrophenol: A systematic review

Nitrophenols are the most widely used raw materials in the chemical, pesticide, and pharmaceutical industries. Due to improper waste management and excessive usage, nitrophenol is listed as a priority pollutant and garnered global research attention. This review highlights the recent progress on heterojunction photocatalysts toward eliminating nitrophenols. The detailed mechanisms of the electron-hole pair separation using different heterojunctions such as traditional, p-n, Z-scheme, S-scheme, and Schottky heterojunctions are elaborated. The performance of the photocatalysts is evaluated using quantum efficiency. Among the heterojunctions, Z-scheme exhibited maximum removal efficiency of 100% and found superior over other heterojunctions. Even though heterojunctions exhibit good efficiency, the reusability of the heterojunction photocatalyst is not reported beyond 5 cycles. Further research is indeed to develop a highly reusable photocatalyst for environmental remediation.

Highly Efficient Solar Light Driven g-C3N4@Cs0.33WO3 Heterojunction for the Photodegradation of Colorless Antibiotics

This study facilitates the synthesis of a graphitic carbon nitride/cesium tungsten oxide (g-C₃N₄@Cs_0.33WO₃) heterojunction using a solvothermal method. The photocatalytic activities of the prepared samples were examined for the photodegradation of colorless antibiotics, namely tetracycline, enrofloxacin, and ciprofloxacin, as well as cationic and anionic dyes, such as methyl orange, rhodamine B, neutral red, and methylene blue, under full-spectrum solar light. We have purposely selected different kinds of wastewater pollutants of colorless antibiotics and cationic and anionic organic dyes to investigate the potential application of this heterojunction toward different groups of water pollutants. The results revealed that the g-C₃N₄@Cs_0.33WO₃ heterojunction showed an outstanding photocatalytic activity toward all the pollutants with concentrations of 20 ppm each at pH 3 by photocatalytically removing 97% of tetracycline within 3 h, 98% of enrofloxacin within 2 h, 97% of ciprofloxacin within 2.25 h, 98% of methylene blue in 1 h, 99% of rhodamine B within 2 h, 99% of neutral red in 1.25 h, and 95% of methyl orange in 2 h. These findings indicate that the developed photocatalyst possesses excellent photocatalytic properties toward seven different water pollutants that make it a universal photocatalyst. The developed g-C₃N₄@Cs_0.33WO₃ oxide heterojunction also presented a photocatalytic performance better than those of reported solar light active photocatalysts for photodegradation of rhodamine B and tetracycline. The efficient photocatalytic performance of the heterojunction can be ascribed to its extended light-absorbing ability, effective charge separation and fast charge transfer properties, and a high surface area. Moreover, an active species detection experiment also confirmed that superoxide radicals, hydroxyl radicals, and holes played significant roles in the photocatalysis of the organic dyes and tetracycline.

Prospecting carbon-based nanomaterials for the treatment and degradation of endocrine-disrupting pollutants

The presence of endocrine-disrupting chemicals (EDCs) in water resources has significant negative implications for the environment. Traditional technologies implemented for water treatment are not completely efficient for removing EDCs from water. Therefore, research on sustainable remediation has been mainly directed to novel decontamination approaches including nano-remediation. This emerging technology employs engineered nanomaterials to clean up the environment quickly, efficiently, and sustainably. Thus, nanomaterials have contributed to a wide variety of remediation techniques like adsorption, filtration, coagulation/flocculation, and so on. Among the vast diversity of decontamination technologies catalytic advanced oxidation processes (AOPs) outstand as simple, clean, and efficient alternatives. A vast diversity of catalysts has been developed demonstrating high efficiencies; however, the search for novel catalysts with enhanced performances continues. In this regard, nanomaterials used as nanocatalysts are exhibiting enhanced performances on AOPs due to their special nanostructures and larger specific surface areas. Therefore, in this review we summarize, compare, and discuss the recent advances on nanocatalysts, catalysts doped with metal-based nanomaterials, and catalysts doped with carbon-based nanomaterials on the degradation of EDCs. Finally, further research opportunities are identified and discussed to achieve the real application of nanomaterials to efficiently degrade EDCs from water resources.

A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)-Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing

g-C3N4 has drawn lots of attention due to its photocatalytic activity, low-cost and facile synthesis, and interesting layered structure. However, to improve some of the properties of g-C3N4, such as photochemical stability, electrical band structure, and to decrease charge recombination rate, and towards effective light-harvesting, g-C3N4-metal oxide-based heterojunctions have been introduced. In this review, we initially discussed the preparation, modification, and physical properties of the g-C3N4 and then, we discussed the combination of g-C3N4 with various metal oxides such as TiO2, ZnO, FeO, Fe2O3, Fe3O4, WO3, SnO, SnO2, etc. We summarized some of their characteristic properties of these heterojunctions, their optical features, photocatalytic performance, and electrical band edge positions. This review covers recent advances, including applications in water splitting, CO2 reduction, and photodegradation of organic pollutants, sensors, bacterial disinfection, and supercapacitors. We show that metal oxides can improve the efficiency of the bare g-C3N4 to make the composites suitable for a wide range of applications. Finally, this review provides some perspectives, limitations, and challenges in investigation of g-C3N4-metal-oxide-based heterojunctions.

In situ fabrication of a novel CdS/ZnIn2S4/g-C3N4 ternary heterojunction with enhanced visible-light photocatalytic performance

In this study, a novel g-C₃N₄-based ternary heterojunction was rationally designed and constructed by the in situ growth of ZnIn₂S₄ nanosheets and CdS nanoparticles onto the g-C₃N₄ nanosheets using a facile two-step oil-bath method. Through optimizing the proportion of ZnIn₂S₄ and CdS component, g-C₃N₄ nanosheets coupled with ZnIn₂S₄ nanosheets and CdS nanoparticles (denoted as CdS/ZnIn₂S₄/g-C₃N₄) exhibited obviously higher photocatalytic properties for RhB removal than the single-component and dual-component systems. Among the as-obtained ternary photocatalysts, it was found that the ternary CdS/ZnIn₂S₄/g-C₃N₄-0.2 photocatalyst displayed the optimum photocatalytic property (96%) within a short time (30 min), which was almost 27.42 and 1.17 times higher than that of pure g-C₃N₄ and binary ZnIn₂S₄/g-C₃N₄-0.7 composite. The excellent activity of the ternary CdS/ZnIn₂S₄/g-C₃N₄ heterostructure is assigned to the synergetic effects of CdS nanoparticles, ZnIn₂S₄ nanosheets and g-C₃N₄ nanosheets, which not only broaden the visible-light absorption range, but also improve the charge mobility and separation rate, thus boosting the visible-light-driven photocatalytic property of g-C₃N₄.

A novel ternary photocatalyst CdS/ZnIn₂S₄/g-C₃N₄ was designed and constructed by a calcination and two-step in situ deposition method with high-efficiency visible-light photocatalytic performance.

Hydrothermal construction of WO3.0.33H2O/g-C3N4 nanocomposites with enhanced adsorption and photocatalytic activity

Tungsten trioxide has attracted extensive attention for photocatalytic applications owing to its unique structure and intrinsic merits; however, small specific surface area and rapid electron-hole recombination still limit its adsorption...

WO3 Fibers/g-C3N4 Z-Scheme Heterostructure Photocatalysts for Simultaneous Oxidation/Reduction of Phenol/Cr (VI) in Aquatic Media

A sequence of WO3/g-C3N4 composites was synthesized at various % weight ratios (1, 5, 6.5, 8, 10, and 15%) of WO3 into g-C3N4 via electrospinning and wet-mixing method. The prepared photocatalytic materials were characterized by X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, UV–vis diffuse reflection spectroscopy (DRS), scanning electron microscopy (SEM), N2 porosimetry and dynamic light scattering (DLS). Electrospun fibers of WO3 with diameter 250–300 nm was prepared using polyvinylpyrrolidone (PVP) polymer and used for the synthesis of composite WO3/g-C3N4 heterojunction structures. Results showed mesoporous materials with triclinic WO3 crystal phase, surface areas up to 67.7 m2g−1 and band gaps lower than 2.5 eV confirming the absorption to visible light region. The photocatalytic performance of the prepared photocatalysts were assessed towards the oxidation of phenol and reduction of Cr (VI), in single and binary systems using simulated solar light illumination, that followed first-order kinetics. The WO3/g-C3N4 composites were found to exhibit improved photocatalytic performances compared to the pure WO3 and g-C3N4 with 6.5 wt% WO3/g-C3N4 and 5 wt% WO3/g-C3N4 composites being the most efficient catalysts for the oxidation of phenolics and reduction of Cr (VI), respectively. The improved performance was explained by a Z-scheme photocatalytic mechanism which was proposed based on scavenging experiments and the determination of the corresponding energy levels of valence and conduction bands. The study demonstrated that such composites present interesting photocatalytic properties that can be further expanded to other environmental depollution applications as well as in energy applications.

The construction of NiFeSx/g-C3N4 composites with high photocatalytic activity towards the degradation of refractory pollutants

In this work, a novel NiFe layered double hydroxide-derived sulfide (NiFeSx)-modified g-C3N4 nanosheet photocatalyst (NiFeSx/g-C3N4) was synthesized, and its morphology, structure and visible light absorption capacity were simultaneously characterized by XRD, SEM, TEM, FT-IR, XPS, UV-Vis DRS, PL techniques and EIS Nyquist plots. Furthermore, it was discovered that at an optimum mass ratio of 3% (NiFeSx to g-C3N4), 3% NiFeSx/g-C3N4 composites exhibited the best degradation efficiency toward tetracycline hydrochloride refractory pollutants. The degradation rate of tetracycline hydrochloride by 3% NiFeSx/g-C3N4 composites was 92.54% under 70 min of visible light illumination, which was about 2.61 times higher than that of pure g-C3N4. The improved degradation activity may be attributed to the synergistic effect between the two constituents of as-synthesized composites, and the formed heterojunction reduced the efficiency of photogenerated carriers. More importantly, this work also gives some inspiration to synthesize some similar photocatalysts for a targeted environmental remediation.

Synthesis, characterization and photocatalytic performance of W6+ impregnated g-C3N4 for the removal of chlorophenol derivatives in natural sunlight exposure

The surface of the g-C₃N₄ was altered by impregnating W⁶⁺ ions that transformed to homogeneously coated oxide layer by a calcination process. An enhanced absorption and the suppressed de-excitation in the emission spectra, with the increasing W⁶⁺ loading, exposed the supporting role of the coated layer in extending the spectral response as well as the prolonged life span of excitons. The same was further supported by electrochemical impedance spectroscopy (EIS). The XRD and XPS analysis revealed the coated layer as highly crystalline pure phase monoclinic WO₃ with the majority of impregnated tungsten ions in 6+ oxidation state respectively, whereas the FESEM and HRTEM analysis substantiated the uniformity of the coated layer with the interlayer spacing of the 0.369 nm. Additionally, the probable formation of individual WO₃ nanoparticles or clusters was ruled out. The as-synthesized impregnated photocatalysts, in comparison to pure g-C₃N_4, were subjected to natural sunlight exposure for the photocatalytic removal of chlorophenol derivatives (2-CP, 3-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP and PCP) that revealed the 5 wt% coating as the optimum level for significant removal. The progress of the photocatalytic process was monitored by periodic HPLC analysis whereas ion chromatography (IC) was used for the estimation of released ions. The mineralization capability of the as-synthesized W⁶⁺ coated catalysts was measured by the time scale TOC measurements. As the formation of intermediates was indicated in HPLC analysis, selected samples were subjected to GC-MS analysis for the identification of the nature of intermediates. The variable degree of removal of chlorophenol derivatives signified the role of the position and orientation of Cl group. The kinetics of the removal process was evaluated with the calculation of rate constants. The results extracted from the analytical tools and the associated band edge potentials were correlated to speculate the probable mechanism as well as the identification of major reactive oxygen species (ROS) involved in the removal process.