Solid mediator Z-scheme heterojunction photocatalysis for pollutant oxidation in water: Principles and synthesis perspectives

Amorphous Titanium Dioxide-Based Heterojunction with Locally Enhanced Electron Transport Multipathways for High-Efficient Photodegradation of Tetracycline

A novel amorphous titanium dioxide (AT)-based heterojunction, composed of AT, silver bromide, and silver nanoparticles (Ag NPs), was synthesized and designated as AgBr/Ag/AT-4%. The band structure and active species of AgBr/Ag/AT-4% composites were investigated, and the existence of multiple electron transport pathways in the composites was determined. The Channel I is an all-solid-state Z-scheme heterostructure formed between AT and AgBr by Ag acting as an electron transport bridge, and Channel II is excited by the localized surface plasmon resonance effect induced by the Ag NPs on the photocatalyst surface. The Channel III is an electronically bridged medium with Ti3+/Ti4+ redox coupling pairs and oxygen vacancy-mediated trap states constructed from defective structures. The activity of the sample was assessed by the photocatalytic degradation of tetracycline. It is hoped that this work will provide a new idea for the preparation of an amorphous titanium dioxide-based heterojunction with locally enhanced electron transport multiple pathways.

A review on development and modification strategies of MOFs Z-scheme heterojunction for photocatalytic wastewater treatment, water splitting, and DFT calculations

Increasing water pollution and decreasing energy reserves have emerged as growing concerns for the environment. These pollution are due to the dangerous effects of numerous pollutants on humans and aquatic organisms, such as hydrocarbons, biphenyls, pesticides, dyes, pharmaceuticals, and metal ions. On the other hand, the need for a clean environment, finding alternatives to fossil and renewable fuels is very important. Hydrogen (H2) is regarded as a viable and promising substitute for fossil fuels, and a range of methodologies have been devised to generate this particular source of energy. Metal-organic frameworks (MOFs) are a new generation of nanoporous coordination polymers whose crystal structure is composed of the juxtaposition of organic and inorganic constituent units. Due to their flexible nature, regular structure, and high surface area, these materials have attracted much attention for removing various pollutants from water and wastewater, and water splitting. MOFs Z-scheme heterojunctions have been identified as an economical and eco-friendly method for eliminating pollutants from wastewater systems, and producing H2. Their low-cost synthesis and unique properties increase their application in various energy and environment fields. The heterojunctions possess diverse properties, such as exceptional surface area, making them ideal for degradation and separation. The development and formulation of Z-scheme heterojunctions photocatalytic systems using MOFs, which possess stable and potent redox capability, have emerged as a successful approach for addressing environmental pollution and energy shortages in recent times. Through the utilization of the benefits offered by MOFs Z-scheme heterojunctions photocatalysts, such as efficient separation and migration of charge carriers, extensive spectrum of light absorption, among other advantages, notable enhancements can be attained. This review encompasses the synthesis techniques, structure, and properties of MOFs Z-scheme heterojunctions, and their extensive use in treating various wastewaters, including dyes, pharmaceuticals, and heavy metals, and water splitting. Also, it provides an overview of the mechanisms, pathways, and various theoretical and practical aspects for MOFs Z-scheme heterojunctions. Finally, it thoroughly assesses existing challenges and suggests further research on the promising applications of MOFs Z-scheme in industrial-scale wastewater treatment.

Design of double Z-scheme Ag-Ag3O4/CuO-CuFe2O4 magnetic nanophotocatalyst via starch-templated microwave-combustion hybrid precipitation method and modified with corona-plasma: Remediation of dye contaminants

Herein, the novel double Z-scheme Ag-Ag3O4/CuO-CuFe2O4 magnetic nanophotocatalyst with nanosphere-on-nanosheet-like morphology was synthesized via the corona-plasma-assisted starch-templated microwave-combustion-precipitation method to remove the dye pollutants. The CuO-CuFe2O4 meso/macroporous nanophotocatalyst was synthesized using a one-pot-stage combustion-microwave process with/without starch as a hard-template. Subsequently, surface modification was carried out by DC corona-plasma discharge technology at various voltages, namely 500, 1000 and 1500 V. Then, the Ag3O4 photocatalyst was deposited on the CuO-CuFe2O4 fabricated with starch-hard-template and treated with 1000 V corona-plasma (denoted as: Ag-Ag3O4/CuO-CuFe2O4 (Starch) 1000 P). The properties of the synthesized nanophotocatalysts were analyzed using various techniques, including X-ray diffraction (XRD), Diffuse reflectance spectroscopy (DRS), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller and Barrett-Joyner-Halenda (BET-BJH), Vibrating Sample Manetometer (VSM), and Photoluminescence (PL). The XRD analysis corroborated the presence of CuO, CuFe2O4 and Ag3O4 in the structure of all samples. The BET-BJH analysis indicates that the specific surface area of the Ag-Ag3O4/CuO-CuFe2O4 (Starch) 1000 P nanophotocatalyst as the best sample is 2 m2/g, higher than other samples. Additionally, the DRS analysis revealed that the band gap of the Ag-Ag3O4/CuO-CuFe2O4 (Starch) 1000 P nanophotocatalyst is about 1.68 eV with the surface plasmon resonance. The performance of the ternary heterostructured Ag-Ag3O4/CuO-CuFe2O4 (Starch) 1000 P nanophotocatalyst was 96.2% and 89.1% in the degradation of the crystal violet (10 mg/L) and acid orange 7 (10 mg/L), respectively, proving its outstanding degradation capacity.

Charge transfer in TiO2-based photocatalysis: fundamental mechanisms to material strategies

Semiconductor-based photocatalysis has attracted significant interest due to its capacity to directly exploit solar energy and generate solar fuels, including water splitting, CO2 reduction, pollutant degradation, and bacterial inactivation. However, achieving the maximum efficiency in photocatalytic processes remains a challenge owing to the speedy recombination of electron-hole pairs and the limited use of light. Therefore, significant endeavours have been devoted to addressing these issues. Specifically, well-designed heterojunction photocatalysts have been demonstrated to exhibit enhanced photocatalytic activity through the physical distancing of electron-hole pairs generated during the photocatalytic process. In this review, we provide a systematic discussion ranging from fundamental mechanisms to material strategies, focusing on TiO2-based heterojunction photocatalysts. Current efforts are focused on developing heterojunction photocatalysts based on TiO2 for a variety of photocatalytic applications, and these projects are explained and assessed. Finally, we offer a concise summary of the main insights and challenges in the utilization of TiO2-based heterojunction photocatalysts for photocatalysis. We expect that this review will serve as a valuable resource to improve the efficiency of TiO2-based heterojunctions for energy generation and environmental remediation.

Nano-sized aggregate Ti3C2-TiO2 supported on the surface of Ag2NCN as a Z-scheme catalyst with enhanced visible light photocatalytic performance

Exposing the photocatalyst's highly active facets and hybridizing the photocatalyst with suitable cocatalysts in the proper spot have been recognized as strong methods for high-performance photocatalysts. Herein, Ag2NCN/TiO2-Ti3C2 composites were synthesized by applying simple calcination and physically weak interaction deposition processes to obtain an excellent photocatalyst for Rhodamine B (Rh B) degradation when exposed to visible light. The findings from the experiments reveal that the Ag2NCN/TiO2-Ti3C2400 composite exhibited an outstanding photocatalytic rate in 80 min, with the highest Rh B degradation rate (k = 0.03889 min-1), which was 16 times higher than that of pure Ag2NCN (k = 0.00235 min-1) and 2.2 times higher than that of TiO2-Ti3C2400 (k = 0.01761 min-1). The results from the following factors: (i) the powerful interfacial contact created by the in situ formation of TiO2, and the superior electrical conductivity of Ti3C2 that makes carrier separation possible; (ii) TiO2 with electron-rich (101) facets are deposited on the surface of Ag2NCN, significantly reducing charge carrier recombination by trapping photoelectrons; (iii) a Z-type heterojunction is constructed between nanosize aggregate Ti3C2-TiO2 and Ag2NCN with non-metal Ti3C2 as the solid medium, improving the transfer and separation of photogenerated charges and inhibiting the recombination of electrons and holes. Additionally, the redox ability of the composite photocatalyst is enhanced. Furthermore, the analyses of active species showed that photogenerated superoxide radicals and holes were the principal active agents inside the photodegradation of Rh B. Moreover, the composite exhibited outstanding photo-stability.

Synthesis of a novel solid mediator Z-scheme heterojunction photocatalysis Fe3O4/C/uio66-nh2: Used for oxidation of Rh6G in water

A novel mediator Z-scheme photocatalyst, Fe3O4/C/UiO-66-NH2, was designed, synthesized, and characterized using SEM, TEM, FTIR, XRD, EPR, and XPS. Formulas #1 to #7 were examined using dye Rh6G dropwise tests. Carbonization of glucose forms the mediator carbon, which connects two semiconductors, Fe3O4 and UiO-66-NH2, to construct the Z-scheme photocatalyst. Formula #1 generates a composite with photocatalyst activity. The band gap measurements of the constituent semiconductors support the mechanisms for the Rh6G degradation using this novel Z-scheme photocatalyst. The successful synthesis and characterization of the proposed novel Z-scheme confirm the feasibility of the tested design protocol for environmental purposes.

Photo-Activated Direct Catalytic Oxidation of Gaseous Benzene with a Cu-Connected Serial Heterojunction Array of Co3 O4 /Cux O/Foam Cu Assembled via Layer upon Layer Oxidation

The foam copper (FCu) has been first used as a promising supporting material to prepare a photo-activated catalyst of Co₃ O₄ /Cu_x O/FCu, in which the fine Co₃ O₄ particles are inlayed on the Cu_x O nanowires to form a Z-type heterojunction array connected by substrate Cu. The prepared samples have been used as a photo-activated catalyst to directly decompose gaseous benzene and the optimized Co₃ O₄ /Cu_x O/FCu demonstrates a 99.5% removal efficiency and 100% mineralizing rate within 15 min in benzene concentration range from 350 to 4000 ppm under simulate solar light irradiation. To track the reactive mechanism, a series of MO_x /Cu_x O/FCu (M = Mn, Fe, Co, Ni, Cu, Zn) is prepared and a novel photo-activated direct catalytic oxidation route is proposed based on the comparative investigation of material properties. Moreover, the approach grew in situ via layer upon layer oxidation on FCu dedicates to the extra lasting reusability and the easy accessibility in the diverse situations. This work provides a novel strategy for the preparation of Cu connected series multidimensional heterojunction array and a promising application for the quick abatement of the high-leveled concentration gaseous benzene and its derivatives from the industrial discharged flow or the accident scene's leakage.

Synthesis of Mixed-Phase TiO2-ZrO2 Nanocomposite for Photocatalytic Wastewater Treatment

The use of TiO₂ nanoparticles for photocatalysis for the degradation of organic dyes under UV light for wastewater treatment has been widely studied. However, the photocatalytic characteristics of TiO₂ nanoparticles are inadequate due to their UV light response and higher band gap. In this work, three nanoparticles were synthesized: (i) TiO₂ nanoparticle was synthesized by a sol-gel process. (ii) ZrO₂ was prepared using a solution combustion process and (iii) mixed-phase TiO₂-ZrO₂ nanoparticles were synthesized by a sol-gel process to remove Eosin Yellow (EY) from aqueous solutions in the wastewater. XRD, FTIR, UV-VIS, TEM, and XPS analysis methods were used to examine the properties of the synthesized products. The XRD investigation supported the tetragonal and monoclinic crystal structures of the TiO₂ and ZrO₂ nanoparticles. TEM studies identified that mixed-phase TiO₂-ZrO₂ nanoparticles have the same tetragonal structure as pure mixed-phase. The degradation of Eosin Yellow (EY) was examined using TiO₂, ZrO₂, and mixed-phase TiO₂-ZrO₂ nanoparticles under visible light. The results confirmed that the mixed-phase TiO₂-ZrO₂nanoparticles show a higher level of photocatalytic activity, and the process is accomplished at a high degradation rate in lesser time and at a lower power intensity.

Understanding the Principles and Applications of Dual Z-Scheme Heterojunctions: How Far Can We Go?

In the past seven years, dual Z-scheme heterojunctions evolved as favorable approaches for enhanced charge carrier separation through direct or indirect charge transfer transportation mechanisms. The dynamics of the charge transfer is the major strategy for understanding their photoactivity and stability through the formation of distinctive redox centers. The understanding of currently recognized principles for successful fabrication and classification in different energy and pollution remediation strategies is discussed, and a universal charge transfer-type-based classification of dual Z-schemes that can be adopted for Z-scheme and S-scheme heterojunctions is proposed. Methods used for determining the charge transfer as proof of dual Z-scheme existence are outlined. Most importantly, a new macroscopic N-scheme and a triple Z-scheme that can also be adopted as triple S-scheme heterostructures composed of four semiconductors are proposed for generating both oxidatively and reductively empowered systems. The proposed systems are expected to possess properties that enable them to harvest solar light to drive important chemical reactions for different applications.

Construction of BiOCl/Ti-MOFs type-II heterojunction photocatalyst for enhanced photocatalytic performance for multiple antibiotics removal

The increased threats to environmental and human health caused by the widespread use of antibiotics have increased the need for efficient technologies for removing antibiotic remnants from wastewater after production. Photocatalysis, which is non-toxic, highly efficient, and low energy consumption, has played a vital role in wastewater treatment among the aforementioned technologies. Therefore, a MIL-125(Ti)/BiOCl type-II heterojunction photocatalyst was fabricated using solvothermal method. Investigations remarkably revealed that the enhanced photocatalytic performance of the photocatalyst for multiple antibiotics degradation (tetracycline and ofloxacin) was attributed to the construction of a heterojunction, which inhibits carrier recombination and enhances visible-light absorption. Furthermore, the radical trapping experiments and electron spin resonance determined superoxide radicals and holes to be the main species in the photocatalytic process. Finally, we presented a potential photocatalytic mechanism that could account for the observations. Overall, this study offered guidelines for developing more photocatalysts with visible-light responses and removing multiple antibiotics from water more efficiently.

Recent Advances in the Development of Novel Iron–Copper Bimetallic Photo Fenton Catalysts

Advanced oxidation processes (AOPs) have been postulated as viable, innovative, and efficient technologies for the removal of pollutants from water bodies. Among AOPs, photo-Fenton processes have been shown to be effective for the degradation of various types of organic compounds in industrial wastewater. Monometallic iron catalysts are limited in practical applications due to their low catalytic activity, poor stability, and recyclability. On the other hand, the development of catalysts based on copper oxides has become a current research topic due to their advantages such as strong light absorption, high mobility of charge carriers, low environmental toxicity, long-term stability, and low production cost. For these reasons, great efforts have been made to improve the practical applications of heterogeneous catalysts, and the bimetallic iron–copper materials have become a focus of research. In this context, this review focuses on the compilation of the most relevant studies on the recent progress in the application of bimetallic iron–copper materials in heterogeneous photo–Fenton-like reactions for the degradation of pollutants in wastewater. Special attention is paid to the removal efficiencies obtained and the reaction mechanisms involved in the photo–Fenton treatments with the different catalysts.

Recent Progress in Photocatalytic Removal of Environmental Pollution Hazards in Water Using Nanostructured Materials

Water pollution has become a critical issue because of the Industrial Revolution, growing populations, extended droughts, and climate change. Therefore, advanced technologies for wastewater remediation are urgently needed. Water contaminants are generally classified as microorganisms and inorganic/organic pollutants. Inorganic pollutants are toxic and some of them are carcinogenic materials, such as cadmium, arsenic, chromium, cadmium, lead, and mercury. Organic pollutants are contained in various materials, including organic dyes, pesticides, personal care products, detergents, and industrial organic wastes. Nanostructured materials could be potential candidates for photocatalytic reduction and for photodegradation of organic pollutants in wastewater since they have unique physical, chemical, and optical properties. Enhanced photocatalytic performance of nanostructured semiconductors can be achieved using numerous techniques; nanostructured semiconductors can be doped with different species, transition metals, noble metals or nonmetals, or a luminescence agent. Furthermore, another technique to enhance the photocatalytic performance of nanostructured semiconductors is doping with materials that have a narrow band gap. Nanostructure modification, surface engineering, and heterojunction/homojunction production all take significant time and effort. In this review, I report on the synthesis and characterization of nanostructured materials, and we discuss the photocatalytic performance of these nanostructured materials in reducing environmental pollutants.

The state of the art review on photocatalytic Cr(VI) reduction over MOFs-based photocatalysts: From batch experiment to continuous operation

This state of the art review presented the photocatalytic reduction from highly toxic Cr(VI) to lowly toxic Cr(III) with metal-organic frameworks (MOFs) and their composites. The construction of composites facilitated the transportation of the photo-induced charges to enhance the Cr(VI) reduction, in which the corresponding mechanisms were clarified by both experimental tests and DFT calculations. The immobilized MOFs onto some substrates accomplished continuous operations toward Cr(VI) reduction even under real solar light. As well, the environmental applications of the Cr(VI) reduction were analyzed, in which the influence factors toward the Cr(VI) reduction were clarified. This review reported that a big breakthrough was achieved from the batch experiment to the continuous operation for Cr(VI) reduction, in which MOFs demonstrated a bright prospective in the field of photocatalytic Cr(VI) reduction.

Synthesis of a novel solid mediator Z-scheme heterojunction photocatalysis CuFe2O4/Cu/UiO-66-NH2 for oxidation of dye in water

Metal mediator Z-scheme photocatalyst comprises three elements: two semiconductors and a sandwiched metal mediator, so the catalyst can effectively degrade pollutants using visible lights. Proper design and synthesis of Z-scheme with targeted performance has not been systematically proposed. This work proposed the protocol to design and synthesize a Z-scheme photocatalyst with targeted performance. A novel metal mediator Z-scheme photocatalyst CuFe₂O₄/Cu/UiO-66-NH₂ was used to implement the design proposal. After determining synthesis protocol from the theory, the concentrations of three reagents - glucose, l-cysteine, and precursor of UiO-66-NH₂ for synthesizing Z-scheme photocatalyst were revised to achieve successful photocatalyst. Dropwise photocatalytic tests were performed to confirm the activities of the synthesized catalysts using 0.112 mmol/mL UiO-66-NH₂ precursor, 10 mmol/mL glucose, and 1 mmol/mL l-cysteine yielded effective photocatalyst to degrade rhodamine 6G. The dye degradation tests and EPR tests confirmed the successful synthesis of the designed Z-scheme photocatalyst.