Current status on designing of dual Z-scheme photocatalysts for energy and environmental applications

Emerging bismuth stannate semiconductor and its photocatalytic applications in pollutant degradation via Z/S-scheme heterostructures

Bismuth stannate (Bi2Sn2O7) has emerged as a promising compound for heterostructure applications due to its outstanding photocatalytic, structural, and optical properties. As a pyrochlore-type semiconducting material, Bi2Sn2O7 demonstrates a suitable bandgap, strong visible-light absorption, and high chemical stability, making it attractive for environmental remediation. Heterostructures based on Bi2Sn2O7 have gained significant attention because of their enhanced charge carrier separation efficiency, improved charged carrier mobility, and synergistic effects that boost photocatalytic performance. Different strategies have been utilized to construct Bi2Sn2O7-based heterostructures, including doping, vacancies generation, coupling with other semiconductors to form Z-scheme and S-scheme heterojunctions. These engineered interfaces effectively reduce charge recombination, thereby enhancing photocatalytic efficiency for pollutant degradation. Furthermore, various synthesis techniques have been reviewed viz. hydrothermal, solvothermal solid-state reaction method, in-situ, and co-precipitation, etc for Bi2Sn2O7 photocatalyst in which the hydrothermal method was most preferable due to yield efficiency, crystallinity, morphology, cost-effectiveness, eco-friendly, and energy conserving. This review highlights the structural, and optical properties, synthesis, modification strategies, and application of Bi2Sn2O7-based heterostructures in environmental technologies. The challenges as well as future prospects of these materials are also analyzed, emphasizing their potential for next-generation photocatalysts. Further research is required to optimize material stability, enhance charge transport, and develop scalable synthesis methods for commercial applications.

Effect of TiO2 Morphology on the Properties and Photocatalytic Activity of g-C3N4/TiO2 Nanocomposites Under Visible-Light Illumination

This study focused on the preparation and investigation of g-C3N4/TiO2 photocatalysts using different TiO2 morphologies (anatase nanoparticles (TPs), poorly crystalline nanotubes (aTTs), and well-crystalline anatase nanorods (TRs)) and self-synthesized g-C3N4 (CN). The synthesis of the g-C3N4/TiO2 composites was carried out using a mortar mixing technique and a g-C3N4 to TiO2 weight ratio of 1:1. In addition, the g-C3N4/TiO2 composites were annealed in a muffle furnace at 350 °C for 2 h in air. The successful formation of a g-C3N4/TiO2 composite with a mesoporous structure was confirmed using the results of XRD, N2 physisorption, and FTIR analyses, while the results of microscopic analysis techniques confirmed the preservation of TiO2 morphology in all g-C3N4/TiO2 composites investigated. UV-Vis DR measurements showed that the investigated g-C3N4/TiO2 composites exhibited visible-light absorption due to the presence of CN. The results of solid-state photoluminescence and electrochemical impedance spectroscopy showed that the composites exhibited a lower charge recombination compared to pure TiO2 and CN. For example, the charge transfer resistance (RCT) of the CNTR/2 composite of TR and CN calcined in air for 2 h was significantly reduced to 0.4 MΩ, compared to 0.9 MΩ for pure TR and 1.0 MΩ for pure CN. The CNTR/2 composite showed the highest photocatalytic performance of the materials tested, achieving 30.3% degradation and 25.4% mineralization of bisphenol A (BPA) dissolved in water under visible-light illumination. In comparison, the pure TiO2 and CN components achieved significantly lower BPA degradation rates (between 2.4 and 11.4%) and mineralization levels (between 0.6 and 7.8%). This was due to (i) the presence of Ti3+ and O-vacancies in the TR, (ii) enhanced heterojunction formation, and (iii) charge transfer dynamics enabled by a dual mixed type-II/Z scheme mechanism.

High-efficiency removal of microcystis aeruginosa using Z-scheme AgBr/NH2-MIL-125(Ti) photocatalyst with superior visible-light absorption: Performance insights and mechanisms

Algal blooms have become a widespread concern for drinking water production, threatening ecosystems and human health. Photocatalysis, a promising advanced oxidation process (AOP) technology for wastewater treatment, is considered a potential measure for in situ remediation of algal blooms. However, conventional photocatalysts often suffer from limited visible-light response and rapid recombination of photogenerated electron-hole pairs. In this study, we prepared a Z-scheme AgBr/NH2-MIL-125(Ti) composite with excellent visible light absorption performance using co-precipitation to efficiently inactivate Microcystis aeruginosa. The degradation efficiency of AgBr/NH2-MIL-125(Ti) for chlorophyll a was 98.7 % after 180 min of visible light irradiation, significantly surpassing the degradation rate efficiency of AgBr and NH2-MIL-125(Ti) by factors of 3.20 and 36.75, respectively. Moreover, the removal rate was maintained at 91.1 % even after five times of repeated use. The experimental results indicated that superoxide radicals (•O2-) were the dominant reactive oxygen species involved. The photocatalytic reaction altered the morphology and surface charge of algal cells, inhibited their metabolism, and disrupted their photosynthetic and antioxidant systems. In conclusion, this study presents a promising material for the application of photocatalytic technology in algal bloom remediation.

Synergistic effect of Zn-AgIn5S8/CdS Z-scheme heterojunction and S-doped rGO for efficient removal of chromium from contaminated water

This study aimed to synthesize a Zn-AgIn5S8/CdS/SrGO nanocomposite for Cr(vi) removal from contaminated water under solar irradiation. To prevent photo corrosion of CdS, a Z-scheme heterojunction was formed between CdS and Zn-AgIn5S8. The introduction of Ag2+ plasmonic materials extended the light absorption range and stabilized the photocatalyst. Further, to improve the catalytic surface area, electrical conductivity, and minimize the rate of electron and hole pair recombination, the Zn-AgIn5S8/CdS Z-scheme heterojunction was loaded onto S-doped rGO. The morphological and structural analysis of the synthesized nanomaterials (NMs) was done using various techniques, including XRD, FT-IR, UV-vis DRS, FESEM, TEM, EDAX, photoluminescence, and Raman spectroscopy. Results revealed that the Zn-AgIn5S8/CdS/SrGO nanocomposite removed 85% of Cr(vi) at an initial concentration of 50 mg L-1 in 180 min when exposed to solar irradiation. The simulated first-order kinetic model fitted to the experimental data for Cr(vi) reduction by the nanocomposite exhibits a high correlation coefficient (R 2 ≥ 0.97) and the K app value for Zn-AgIn5S8/CdS/SrGO (K app = 0.0114 min-1) is around 1.6 times larger than that of bare ZnAgIn5S8. Moreover, Zn-AgIn5S8/CdS/SrGO heterojunctions show excellent reusability up to 4 cycles. Further, the possible photocatalytic mechanism of Cr(vi) reduction has been proposed. Therefore, the Zn-AgIn5S8/CdS/SrGO nanocomposite could serve as an alternative photocatalyst system driven by solar light for Cr(vi) reduction.

Novel defect-transit dual Z-scheme heterojunction: Sulfur-doped carbon nitride nanotubes loaded with bismuth oxide and bismuth sulfide for efficient photocatalytic amine oxidation

The rational design of Z-scheme heterojunction hybrid photocatalysts is considered a promising way to achieve high photocatalytic activity. In this study, a dual Z-scheme heterojunction with bismuth sulfide (Bi2S3) nanorods and bismuth oxide (Bi2O3) nanoparticles anchored Sulfur-doped carbon nitride (S-CN) nanotubes (Bi2S3/S-CN/Bi2O3) is designed and fabricated through the ordinal metal ion adsorption, pyrolysis, and sulfidation processes using supramolecular rods as precursor. Compared with pristine Bi2S3, Bi2O3, and CN, the dual Z-scheme tube-shaped Bi2S3/S-CN/Bi2O3 catalyst exhibited a significantly improved photocatalytic activity in amine oxidation. The optimized Bi2S3/S-CN/Bi2O3 nanostructure exhibits a 97.6 % benzylamine conversion and 99.4 % imine selectivity within 4 h under simulated solar light irradiation. The excellent activity of Bi2S3/S-CN/Bi2O3 nanotubes can be attributed to the characteristic hollow defect band structure and efficient charge separation and transfer achieved by the dual Z-scheme charge transfer mechanism, which was systematically studied using electron spin resonance spectroscopy, Kelvin probe force microscope, and other techniques. The optimized dual Z-scheme heterojunction hybrid photocatalyst maintains the high oxidizing ability of Bi2S3 and Bi2O3 and the excellent reducing ability of CN, thereby significantly enhancing the photocatalytic activity. This research provides a facile and feasible synthesis strategy for designing dual Z-scheme heterojunctions with defect band structure to improve the photocatalytic activity.

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.

Enhancing Pollutant Mineralization through Organic-Inorganic Defect-Transit Dual S-scheme with a Robust Internal Electric Field

Achieving superior photomineralization of pollutants relies on a rational design of a dual S-scheme with a robust internal electric field (IEF). In this study, to tackle the low mineralization rate in type-II In2 O3 /In2 S3 (IO/IS) systems, an organic-inorganic dual S-scheme In2 O3 /PDI/In2 S3 (IO/PDI/IS) nanostructured photocatalyst is synthesized via a method combining solvent-induced self-assembly and electrostatic forces. Due to the unique energy band position and strong IEF, the photoinduced defect-transit dual S-scheme IO/PDI/IS facilitates the degradation of lignin and antibiotics. Notably, a promising mineralization rate of 80.9% for sodium lignosulfonate (SL) is achieved. The charge transport pathway of IO/PDI/IS are further validated through the analysis of in situ X-ray photoelectron spectroscopy (in situ XPS), density functional theory calculations, and radical trapping experiments. In-depth, two possible pathways for the photocatalytic degradation of lignin are proposed based on the intermediates monitored by liquid chromatography-mass spectrometry (LC-MS). This study presents a new strategy for the design of organic-inorganic dual S-scheme photocatalysts with a robust IEF for pollutant degradation.

Dual Z-scheme TCN/ZnS/ ZnIn2S4 with efficient separation for photocatalytic nitrogen fixation

The development of an efficient catalyst that can use solar energy for NH3 production is of great significance in solving the environmental and energy crisis caused by the traditional ammonia synthesis process. In this work, a dual Z-scheme tubular carbon nitride/zinc sulfide/zinc indium sulfide ternary composited photocatalyst (TCN/ZnS/ZnIn2S4) with excellent nitrogen photofixation performance under visible light was prepared by self-assembly and hydrothermal methods. The crystal structure studies confirmed that tubular carbon nitride (TCN) had more active sites that could promote N2 adsorption. The photochemical studies proved that the double charge transfer channel provided by the dual Z-scheme heterojunction could improve the efficiency of electron-hole separation and achieve excellent photocatalytic nitrogen fixation. The ammonia production rate of the TCN/ZnS/ZnIn2S4 catalyst was up to 136.56 μmol/L, and it also has good stability and reusability. This work provides new insight into the development of Z-scheme heterojunction photocatalysts with green and efficient nitrogen fixation.

ZnSe-rGO nanocomposites as photocatalysts for purification of textile dye contaminated water: A green approach to use wastewater for maize cultivation

Disputes about the probable availability of safe water and the efficacy of processed wastewater are key issues that necessitate a suitable solution to enhance the quality of clean water. The current research emphasizes the synthesis of ZnSe-reduced graphene oxide nanocomposites (ZnSe:rGO) with different weight ratios of rGO (represented as X = 0.6, 1 and 1.6 g)via one-step hydrothermal method. The photocatalytic performance for the degradation of methyl violet (MV) dye was investigated under visible light irradiation by varying the reaction parameters. The crystal structure, elemental composition, surface functionality and morphology of the synthesized ZnSe-XrGO nanocomposites were estimated by powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopic (SEM) techniques. UV-visible spectroscopy was used to investigate the optical properties. The highest efficiency is obtained for ZnSe-XrGO in 1:1 and it showed pseudo 1st order behavior with rate constant of 0.0167min-1and 94 % photodegradation of MV in just 3 h. Furthermore, hazardous effects of MV were investigated on the germination and growth of Zea mays seeds by giving them aqueous solution of MV (0, 8, 12, 24 and 48 ppm) and the decontaminated water after photodegradation of MV with the synthesized photoactive composite. The results showed profound negative effect on both germination and seedling growth at higher concentration (>12 ppm) of the dye solution. No hazardous effects were observed on both these parameters when it was given the dye degraded water which reflects the practical use of the synthesized catalyst for water remediation. The current study fulfills the goal of designing an efficient visible-light active nano-photocatalyst and its direct applicability on life sciences for water purification.

Research progress of MOF-based membrane reactor coupled with AOP technology for organic wastewater treatment

MOF-based catalytic membrane reactor (MCMR), which can simultaneously achieve membrane separation and chemical catalytic degradation in an integrated system, is a cutting-edge technology for effective treatment of organic pollutants in water. The coupling of MCMR and advanced oxidation process (AOP) not only significantly improves the pollutant removal efficiency but also inhibits the membrane pollution through self-cleaning effect, thus improving the stability of MCMR. This paper reviews different MCMR systems combined with photocatalysis, Fenton oxidation, and persulfate activation, elucidates the reaction mechanism, discusses key issues to improve system effectiveness, and suggests future challenges and research directions.

Integration of high visible-light-driven ternary dual Z-scheme AgVO3-InVO4/g-C3N4 heterojunction nanocomposite for enhanced uranium(VI) photoreduction separation

With deepening application of nuclear power technology, the problem of water ecological environment pollution caused by uranium (U(VI)) is becoming increasingly serious. Photoreduction separation of U(VI) on photocatalysts is considered as an effective strategy to solve uranium pollution. In this work, a novel ternary dual Z-scheme AgVO3-InVO4/g-C3N4 heterojunction (Z-AIGH) nanocomposite with high surface area (73.45 m2 g-1, Z-AIGH2) was designed. The batch adsorption experiment in dark environment showed that Z-AIGH2 nanocomposite had an excellent U(VI) adsorption performance. As for photocatalytic experiments, Z-AIGH2 exhibited a rapid photocatalytic response for separating U(VI) without any organic sacrifice agents. The U(VI) separation rate on Z-AIGH2 nanocomposite was over 98.7% after only 20.0 min visible light irradiation (T = 298 K, CU(Ⅵ) = 10.0 mg L-1, m/V = 0.1 g L-1 and pH = 7.0). Z-AIGH2 nanocomposite also showed good selectivity and cycle stability. The U(VI) removal rate of Z-AIGH2 nanocomposite after fifth cycles was about 96.1% (T = 298 K, CU(Ⅵ) = 10.0 mg L-1, m/V = 0.1 g L-1 and pH = 7.0). High photocatalytic activity of Z-AIGH2 for U(VI) was attributed to the construction of ternary dual Z-scheme heterojunction structure and ant nest-like hole structure. Based on above results, Z-AIGH2 nanocomposite had great potential for water environment renovation.

Potential of novel dual Z-scheme carbon quantum dots decorated MnIn2S4/CdS/Bi2S3 heterojunction for environmental applications

In this work, CQDs decorated MnIn₂S₄/CdS/Bi₂S₃ heterojunction was prepared successfully by hydrothermal technique for photocatalytic disinfection of Escherichia coli (E. coli) and mineralization of methyl orange (MO) dye. The charge transferal route and mineralization process in CQDs-MnIn₂S₄/CdS/Bi₂S₃ heterojunction were comprehensively investigated by advanced spectroscopic techniques. The improved visible-light activity and enhanced photo-generated charge transferal efficacy caused dual Z-scheme CQDs-MnIn₂S₄/CdS/Bi₂S₃ heterojunction to achieve boosted photodegradation ability. The catalytic degradation trend was followed as CQDs-MnIn₂S₄/CdS/Bi₂S₃ > MnIn₂S₄ > CdS > Bi₂S₃. The dye was mineralized within 180 min under visible light irradiation. The effect of reaction parameters, pH effect, catalyst dosage, and H₂O₂ addition on MO degradation was also investigated. The degradation rate was maximal at pH 4 with a pseudo-first-order rate constant, 0.0438 min^-1. The assessment of antibacterial properties revealed that CQDs-MnIn₂S₄/CdS/Bi₂S₃ composite effectively inactivated E. coli under visible light. Scavenging experiments, transient photocurrent response, and electron spin resonance spectroscopy suggested that ^•[Formula: see text] and holes were the dominant reactive species. The Z-scheme heterojunction is recyclable up to ten photocatalytic cycles according to recycling experiments. This research indicates the importance of dual Z-scheme CQDs decorated MnIn₂S₄/CdS/Bi₂S₃ heterojunction in wastewater remediation.

Photo-Fenton assisted AgCl and P-doped g-C3N4 Z-scheme photocatalyst coupled with Fe3O4/H2O2 system for 2, 4-dimethylphenol degradation

In this study graphitic carbon nitride (g-C₃N₄ or GCN) and phosphorus doped graphitic carbon nitride (p-g-C₃N₄ or PCN) were prepared using facile thermal polycondensation method. Phosphorus doping was employed to preserve the non-metallic nature of GCN. The AgCl/PCN/Fe₃O₄ heterojunction was synthesized using a simple in-situ route. The photocatalytic performance of the GCN, PCN, Fe₃O₄ and AgCl/PCN/Fe₃O₄ was tested towards 2, 4-dimethylphenol (DMP) pollutant. The work explored improvement in physiochemical properties and reduction of band gap of GCN after P doping (through Tauc's plot method). Coupling with AgCl (silver halide) also enhanced photoinduced charge carriers' separation and migration ability due to apt band alignment among both AgCl and PCN photocatalysts which resulted in formation of direct Z-scheme charge transfer mechanism. Similarly, the incorporation of ferrimagnetic material i.e. Fe₃O₄ enhanced the generation of hydroxyl (^•OH) radicals via photo-Fenton process and facilitated photocatalysts easy separation from the aqueous medium. Through PL and EIS analysis the enhanced charge separation and migration ability in AgCl/PCN/Fe₃O₄ nanocomposite was validated. The attained DMP degradation efficiency of photo-Fenton assisted AgCl/PCN/Fe₃O₄/H₂O₂ Z-scheme nanocomposite was much higher i.e. 99% compared to other photocatalysts within 60 min of visible light irradiation following pseudo-first-order kinetics. Electron paramagnetic resonance (EPR) and scavenging tests confirmed the substantial role of ^•OH and ^•O₂^- radicals in the photo-Fenton reaction. Furthermore, liquid chromatography-mass spectrometry (LC-MS) analysis detected the generated oxidative products and mineralization pathways associated with DMP degradation. The proposed direct Z-scheme charge transfer route presented efficient charge separation and migration ability in AgCl/PCN/Fe₃O₄ nanocomposite. Recycle ability of the fabricated AgCl/PCN/Fe₃O₄ photocatalyst was tested up to 5 cycles with 90% removal efficacy, confirming the excellent reusability and stability of AgCl/PCN/Fe₃O₄ photocatalyst.

BiOCl Heterojunction photocatalyst: Construction, photocatalytic performance, and applications

BiOCl semiconductors have attracted extensive amounts of attention and have substantial potential in alleviating energy shortages, improving sterilization performance, and solving environmental issues. To improve the optical quantum efficiency of layered BiOCl, the lifetimes of photogenerated electron-hole pairs, and BiOCl reduction capacity. During the past decade, researchers have designed many effective methods to weaken the effects of these limitations, and heterojunction construction is regarded as one of the most promising strategies. In this paper, BiOCl heterojunction photocatalysts designed and synthesized by various research groups in recent years were reviewed, and their photocatalytic properties were tested. Among them, direct Z-scheme and S-scheme photocatalysts have high redox potentials and intense redox capabilities. Hence, they exhibit excellent photocatalytic activity. Furthermore, the applications of BiOCl heterojunctions for pollutant degradation, CO2 reduction, water splitting, N2 fixation, organic synthesis, and tumor ablation are also reviewed. Finally, we summarize research on the BiOCl heterojunctions and put forth new insights on overcoming their present limitations.

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.

Graphitic carbon nitride based immobilized and non-immobilized floating photocatalysts for environmental remediation

In solar photocatalysis, light utilization and recycling of powder from reaction solution are the main obstructions that hinder the photocatalytic efficacy of any photocatalyst. In this respect, a floatable system is effective for efficient solar photocatalysis by light utilization. Due to the maximum solar light absorption property, floating nanocomposite photocatalyst is an appealing substitute for effective wastewater treatment. Floating photocatalysts are a non-oxygenated and non-stirred solution that is a good light harvester, stable, non-toxic, biodegradable, naturally abundant in nature. They also have low density, a simple preparation process, no need to stir, and high porosity. Due to these characteristics, floating photocatalysts are widely favored and ideal candidates for practical environmental remediation. Several researchers have come up with new and innovative ways for immobilizing capable photocatalyst on a floatable substrate to produce floating nanocomposite photocatalytic material. In recent decades, g-C₃N₄-based floating photocatalysts have gained a lot of attention as g-C₃N₄ is a visible light active photocatalyst with unique and exceptional properties. It also has good photocatalytic activity in waste water treatment and environmental remediation. Many previous reports have studied the logical design and manufacturing method for heterojunction floating photocatalysts and immobilized floating photocatalysts. Based on those studies, we have focused on the g-C₃N₄ based immobilized and non-immobilized floating photocatalysts for pollutant degradation. We have also categorized immobilized floating photocatalyst based on several lightweight substrates such as expanded perlite and glass microbead. In addition, future challenges have been discussed to maximize solar light absorption and to improve the efficiency of broadband response floating photocatalysts. Floating photocatalysis is an advanced technique in energy conversion and environmental remediation thus requires special consideration.