A review on TiO 2 -based Z-scheme photocatalysts

TiO2-based photocatalysts from type-II to S-scheme heterojunction and their applications

Photocatalysis is a promising sustainable technology to remove organic pollution and convert solar energy into chemical energy. Titanium dioxide has drawn extensive attention in this field owing to its high activity under UV light, good chemical stability, large availability, low price and low toxicity. However, the poor quantum efficiency derived from fast electron/hole recombination, the limited utilization of sunlight, and a weak reducing ability still hinder its practical application. Among the modification strategies of TiO2 to enhance its performance, the construction of heterojunctions with other semiconductors is a powerful and versatile way to maximise the separation of photogenerated charge carriers and steer their transport toward enhanced efficiency and selectivity. Here, the research progress and current status of TiO2 modification are reviewed, focusing on heterojunctions. A rapid evolution of the understanding of the different charge transfer mechanisms is witnessed from traditional type II to the recently conceptualised S-scheme. Particular attention is paid to different synthetic approaches and interface engineering methods designed to improve and control the interfacial charge transfer, and several cases of TiO2 heterostructures with metal oxides, metal sulfides and carbon nitride are discussed. The application hotspots of TiO2-based photocatalysts are summarized, including hydrogen generation by water splitting, solar fuel production by CO2 conversion, and the degradation of organic water pollutants. Hints about less studied and emerging processes are also provided. Finally, the main issues and challenges related to the sustainability and scalability of photocatalytic technologies in view of their commercialization are highlighted, outlining future directions of development.

Interface engineering in a nitrogen-rich COF/BiOBr S-scheme heterojunction triggering efficient photocatalytic degradation of tetracycline antibiotics

Tetracycline (TC) antibiotics, extensively utilized in livestock farming and aquaculture, pose significant environmental challenges. Photocatalysis, leveraging renewable sunlight and reusable photocatalysts, offers a promising avenue for mitigating TC pollution. However, identifying robust photocatalysts remains a formidable challenge. This study introduces a novel hollow-flower-ball-like nanoheterojunction composed of a nitrogen-rich covalent organic framework (N-COF) coupled with BiOBr (BOB), a semiconductor with a higher Fermi level. The synthesized N-COF/BOB S-scheme nanoheterojunction features an expanded contact interface, strengthened chemical bonding, and unique band topologies. The N-COF/BOB composites showcased exceptional TC degradation performance, achieving an 81.2% removal of 60 mg/L TC within 2 h, markedly surpassing the individual efficiencies of N-COF and BOB by factors of 3.80 and 5.96, respectively. Furthermore, the total organic carbon (TOC) removal efficiency highlights a superior mineralization capacity in the N-COF/BOB composite compared to the individual components, N-COF and BOB. The toxicity assessment revealed that the degradation intermediates possess diminished environmental toxicity. This enhanced performance is ascribed to the robust S-scheme nanoheterojunction structure, which promotes efficient photoinduced electron transfer from BOB to N-COF. This process also augments the separation of photogenerated charge carriers, resulting in an increased yield of superoxide radicals (∙O2-) and hydroxyl radicals (∙OH). These reactive species significantly contribute to the degradation and mineralization of TC. Consequently, this study introduces a sustainable approach for addressing emerging antibiotic contaminants, employing COF-based photocatalysts.

Enhanced Photocatalytic Degradation of Rhodamine B Dye by Iron-Doped Europium Oxide Nanoparticles

As a wide-bandgap rare-earth oxide, Eu2O3 was often utilized as an auxiliary material of other photocatalysts because its photocatalytic performance was limited by the luminescence characteristics of Eu3+ and low light utilization. In this study, we improved the photocatalytic degradation performance of the Eu2O3 nanoparticles by doping with Fe cations. The Eu2O3 nanoparticles with different Fe-doping concentrations (1, 3, and 5%, noted as EF1.0, EF3.0, and EF5.0, respectively) were synthesized via chemical precipitation and calcination methods. It was found that doping could reduce Eu2O3's bandgap, which probably originated from the introduction of oxygen vacancies with lower energy levels than the conduction band of Eu2O3. Compared with the undoped Eu2O3 nanoparticles with a removal efficiency of 22% for degrading rhodamine B dye within 60 min, the photocatalytic degradation efficiencies of EF1.0, EF3.0, and EF5.0 were demonstrated to be improved to 42, 48, and 33%, respectively, and EF3.0's performance was the best. The enhanced photocatalytic performance of the doped samples was related to the oxygen vacancies acting as capture centers for electrons, such that the photogenerated electron-hole pairs were efficiently separated and the redox reactions on the surface of the nanoparticles were enhanced accordingly. Additionally, the enhanced light absorption and broadened spectral band further improved EF3.0's degradation efficiency.

Photocatalytic degradation of fluoranthene in soil suspension by TiO2/α-FeOOH with enhanced charge transfer capacity

Polycyclic aromatic hydrocarbons (PAHs) in soil are potentially harmful to human health. However, the use of photocatalysis technology to treat soil contaminated with PAHs remains challenging. Therefore, TiO2/α-FeOOH composite photocatalyst has been synthesized by hydrothermal method and sol-gel method and applied to photocatalytic degradation of fluoranthene in soil. The morphology, elements, crystal structure, optical properties, electrochemical characteristics, and photocatalytic activity of TiO2/α-FeOOH have been characterized. Results showed that TiO2 is tightly fixed on the surface of α-FeOOH, and TiO2/α-FeOOH had higher photocatalytic activity on photocatalytic degradation of fluoranthene in soil under simulated sunlight. The degradation efficiency of TiO2/α-FeOOH is 3.0 and 4.8 times higher than that of TiO2 and α-FeOOH, respectively. This is attributed to enhanced photocatalytic ability by enhancing the transfer capacity of electrons and holes and broadening the spectrum absorption range. The highest degradation efficiency was achieved when the pH of the soil is neutral, the ratio of water/soil is 10:1, and the dosage of catalyst is 50 mg/g. In addition, it was proved that •O2-, h+, and 1O2 are the main active substances in the photocatalysis of TiO2/α-FeOOH. The possible mechanism of a Z-type electron transfer structure was also proposed. The degradation products of fluoranthene were detected, and the degradation pathway was deduced.

Unraveling the impact of microwave-assisted techniques in the fabrication of yttrium-doped TiO2 photocatalyst

In this study, we investigate the role of microwave technology in the fabrication of yttrium-doped TiO2 through a comparative analysis of hydrothermal techniques. Microwave-assisted hydrothermal synthesis offers advantages, but a comprehensive comparison between microwave-assisted and conventional methods is lacking. Therefore, in our investigation, we systematically evaluate and compare the morphological, structural, and optical properties of yttrium-doped TiO2 samples synthesized using both techniques. The X-ray diffraction (XRD) patterns confirm the anatase tetragonal structure of the synthesized TiO2-Y systems, while the larger ion radius of yttrium (Y3+) compared to titanium (Ti4+) presents challenges for yttrium to incorporate into the TiO2 lattice. The X-ray Photoelectron Spectroscopy (XPS) revealed a significant difference in the atomic content of yttrium between the TiO2-Y systems synthesized using microwave-assisted and conventional methods. This finding suggests that the rapid microwave method is more effective in successfully doping TiO2 with rare earth metals such as yttrium. The photo-oxidation of carbamazepine (CBZ) using TiO2-Y systems demonstrated high efficiency under UV-LED light. Microwave-synthesized TiO2-Y demonstrates improved photo-oxidation efficiency of CBZ, attributed to enhanced absorption, charge transfer, surface area, and crystallite size. Overall, the microwave-synthesized TiO2-Y systems showed promising performance for the photo-oxidation of CBZ, with improved efficiency compared to conventional synthesis methods.

Ferrite bismuth-based nanomaterials: From ferroelectric and piezoelectric properties to nanomedicine applications

Bismuth ferrite (BiFeO3), a perovskite-type oxide, possesses unique morphology and multiferroicity, rendering it highly versatile for various applications. Recent investigations have demonstrated that BiFeO3 exhibits enhanced Fenton-like and photocatalytic behaviors, coupled with its piezoelectric/ferroelectric properties. BiFeO3 can catalytically generate highly oxidative reactive oxygen species (ROS) when exposed to hydrogen peroxide or light irradiation. Consequently, bismuth ferrite-based nanomaterials have emerged as promising candidates for various biomedical applications. However, the precise fabrication of BiFeO3-based materials with controllable features and applications in diverse biomedical scenarios remains a formidable challenge. In this review, we initially summarize the Fenton reaction property, ferroelectric, and piezoelectric properties of BiFeO3. We further survey the current methodologies for synthesizing BiFeO3 nanomaterials with diverse morphologies. Subsequently, we explore the effects of element doping and heterojunction formation on enhancing the photocatalytic activity of BiFeO3, focusing on microstructural, electronic band structure, and modification approaches. Additionally, we provide an overview of the recent advancements of BiFeO3-based nanomaterials in biomedicine. Finally, we discuss the prevailing obstacles and prospects of BiFeO3 for biomedical applications, offering valuable insights and recommendations for forthcoming research endeavors.

Degradation of Organic Dyes by the UCNP/h-BN/TiO2 Ternary Photocatalyst

In this study, upconversion nanoparticles (UCNPs) with a flower-like morphology were prepared using a urea coprecipitation method. A ternary photocatalyst was first prepared using a solvothermal method involving the use of titanium oxide (TiO2), hexagonal boron nitride (h-BN), and UCNPs (Y2O3, Yb3+, and Tm3+) as raw materials. The surface morphology, crystal structure, and functional groups of these materials were then characterized and analyzed through scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ultraviolet-visible spectrophotometry, and other techniques. Photocatalytic experiments were also conducted to investigate the effects of different catalyst types, raw material doping ratios, pH values, and catalyst quantities on the photocatalytic degradation of rhodamine B (RhB). The results indicated that doping with h-BN and UCNPs reduced the band gap width of RhB, increased its light absorption rate, and decreased the recombination rate of its photogenerated electrons and holes so that the photocatalytic degradation effect reached 100% within 2 h. After five experimental cycles, the 30% UC-BN-Ti photocatalyst remained highly durable and stable. To investigate the effects of different trapping agents on the degradation of RhB, benzoquinone, isopropanol, and ethylenediaminetetraacetic acid disodium salt were used as free-radical-capturing agents. The results indicated that •O2- was the primary active species in the degradation process. Finally, the pathway and mechanism of the degradation of RhB through ternary composite photocatalysis were identified.

Improving photocatalytic hydrogen production by switching charge kinetics from type-I to Z-scheme via defective engineering

By providing the spatial separation of the active sites and retaining high oxidative and reducing capacity, the direct Z-scheme heterostructure is considered the most potential structure for yielding photo-electric response. However, challenges still exist in the directional transfer of charge carriers between two semiconductors in direct Z-scheme structures. In this regard, by constructing the Vzn defect and p-n junction, a direct Z-scheme ZnxCd1-xS@ZnS-NiS heterostructure was obtained for the regulated electronic structure, which ensured high-yield hydrogen properties. The Zn vacancy in the partially-coated ZnS shell led to the Vzn energy level, and the addition of NiS led to the p-n structure, which caused a drastic downshift of the band edge potentials in comparison to that of pristine CdS. This variation gave rise to a staggered band edge alignment between ZnxCd1-xS and NiS, resulting in the variation of charge transfer kinetics from type-I to direct Z-scheme. Through careful characterization, it was found that the optimal photocatalytic hydrogen precipitation activity reached 16 683.6 μmol g-1 h-1, which was 70 times that of CdS, and this improvement was considered to form a spatial barrier, providing a clear direction and path for carrier transmission.

TiO2 Phase Ratio's Contribution to the Photocatalytic Activity

Photocatalysis is one of the approaches for solving environmental issues derived from extremely harmful pollution caused by industrial dyes, medicine, and heavy metals. Titanium dioxide is among the most promising photocatalytic semiconductors; thus, in this work, TiO2 powders were prepared by a hydrothermal synthesis using titanium tetrachloride TiCl4 as a Ti source. The effect of the hydrochloric acid (HCl) concentration on TiO2 formation was analyzed, in which a thorough morpho-structural analysis was performed employing different analysis methods like XRD, Raman spectroscopy, SEM/TEM, and N2 physisorption. EPR spectroscopy was employed to characterize the paramagnetic defect centers and the photogeneration of reactive oxygen species. Photocatalytic properties were tested by photocatalytic degradation of the rhodamine B (RhB) dye under UV light irradiation and using a solar simulator. The pH value directly influenced the formation of the TiO2 phases; for less acidic conditions, the anatase phase of TiO2 crystallized, with a crystallite size of ≈9 nm. Promising results were observed for TiO2, which contained 76% rutile, showing a 96% degradation of RhB under the solar simulator and 91% under UV light after 90 min irradiation, and the best result showed that the sample with 67% of the anatase phase after 60 min irradiation under the solar simulator had a 99% degradation efficiency.

Unveiling the Latest Developments in Molecularly Imprinted Photocatalysts: A State-of-the-Art Review

Responding to the growing concerns about environmental pollutants, scientists are increasingly turning to innovative solutions rooted in the field of environmental science. One such promising avenue combines the robustness of traditional photocatalysis with the precision of molecular imprinting, leading to the proposition of molecularly imprinted photocatalysts (MIPCs). These MIPCs hold the potential to specifically target and eliminate environmental pollutants, marking them as a promising tool in modern environmental remediation. As researchers delve deeper into this field, the design and optimization of MIPCs have become hotbeds for scientific inquiry. This comprehensive overview delves into the multifaceted approaches to MIPC design, elucidating on aspects like the selection of appropriate photocatalytic bases, the pivotal role of templates, the choice of monomeric building blocks, and the integration of effective cross-linking agents. However, as with all burgeoning technologies, the development of MIPCs is not without its challenges. These potential impediments to the successful innovation and implementation of MIPCs are also explored.

Recent Developments of Inorganic Nanosensitizers for Sonodynamic Therapy

As a noninvasive treatment, sonodynamic therapy (SDT) has been widely used in the treatment of tumors because of its ability to penetrate deep tissue with few side effects. As the key factor of SDT, it is meaningful to design and synthesize efficient sonosensitizers. Compared with organic sonosensitizers, inorganic sonosensitizers can be easily excited by ultrasound. In addition, inorganic sonosensitizers with stable properties, good dispersion, and long blood circulation time, have great development potential in SDT. This review summarizes possible mechanisms of SDT (sonoexcitation and ultrasonic cavitation) in detail. Based on these mechanisms, the design and synthesis of inorganic nanosonosensitizers can be divided into three categories: traditional inorganic semiconductor sonosensitizers, enhanced inorganic semiconductor sonosensitizers, and cavitation-enhanced sonosensitizers. Subsequently, the current efficient construction methods of sonosensitizers are summarized including accelerated semiconductor charge separation and enhanced production of reactive oxygen species through ultrasonic cavitation. Furthermore, the advantages and disadvantages of different inorganic sonosensitizers and detailed strategies are systematically discussed on how to enhance SDT. Hopefully, this review could provide new insights into the design and synthesis of efficient inorganic nano-sonosensitizers for SDT.

Metal sulphides and their heterojunctions for photocatalytic degradation of organic dyes-A comprehensive review

Water pollution caused by organic dyes is one of the greatest threats to the ecosystem. The removal of dyes from water has remained a challenge for scientists. Recently, metal sulphides have emerged as a potential candidate for water remediation applications. The efficient charge transportation, greater surface-active sites, and low bandgap of metal sulphides make them an excellent choice of semiconductor photocatalysts for degradation of dyes. This review summarises the potential application of metal sulphides and their heterojunctions for the photocatalytic degradation of organic dyes from wastewater. A detailed study has been presented on the synthesis, basics of photodegradation and heterojunctions and photocatalytic activity. The effect of the use of templates, doping agents, synthesis route, and various other factors affecting the photocatalytic activity of metal sulphides have been summarised in this review. The synthesis techniques, characterisation techniques, mechanism of degradation of organic dyes by Z-scheme heterojunction photocatalyst, reusability and stability of metal sulphides, and the scope of future research are also discussed. This study indicates that Scopus-based core gathered data could be used to give an objective overview of the global dye degradation research from 2008 to 2023 (15 years). All data (articles, authors, keywords, and publications) is compiled in the Scopus database. For the bibliometric study, 1962 papers relevant to dye photodegradation by sulfide-based photocatalysts were found, and this number rises yearly. A bibliometric analysis provides a 15-year evaluation of the state-of-the-art research on the impact of metal sulfide-based photocatalysts on the photodegradation of dyes.

Enhanced nitrate reduction by metal deposited g-C3N4/rGO/TiO2 Z-schematic photocatalysts: Performance and mechanism comparison of Pd-Cu and Ag

Deposition of bimetals on Z-scheme photocatalysts has been reported to improve the nitrate nitrogen (NO3-) reduction properties. However, it is not clear whether bimetal deposition possesses advantage over single metal deposition and what is the different reaction mechanisms. In this work, the g-C3N4(Pd-Cu)/rGO/TiO2 and g-C3N4(Ag)/rGO/TiO2 composites with bimetallic Pd-Cu and single metal Ag deposited on the graphitic carbon nitride/reduced graphene oxide/titanium dioxide (g-C3N4/rGO/TiO2) Z-scheme photocatalyst were prepared, and their photocatalytic NO3- reduction properties and the mechanisms under visible light irradiation were studied. The results showed that the NO3- and total nitrogen (TN) removal efficiencies of g-C3N4(Pd-Cu)/rGO/TiO2 were 57.78% and 20.1%, respectively, 1.15 and 1.72 times higher than those of g-C3N4(Ag)/rGO/TiO2. This can be ascribed to that Pd-Cu enriched more electrons and absorbed more NO3- molecules due to the different charge densities, and the NO3- reduction process were enhanced by the staged NO3-→NO2- and NO2-→N2/NH4+ processes on Cu and Pd. The effects of reductive species were demonstrated to be photogenerated electrons > ·OH (·CO2-) > ·O2- in g-C3N4(Ag)/rGO/TiO2, while it was photogenerated electrons > ·O2- > ·OH (·CO2-) in g-C3N4(Pd-Cu)/rGO/TiO2, which may be caused by the better O2 reduction property of the latter. Finally, the cyclic experiment proved the good stability of both materials. This work provided some reference for design of metal deposited Z-scheme photocatalysts for various reduction reactions.

Covalent Organic Framework-Semiconductor-Based Heterostructures for Photocatalytic Applications

Photocatalysis is a promising and sustainable technology in the fields of energy conversion/storage and environment purification; however, the utilization of individual component as photocatalyst is generally restricted due to the low catalytic activity deriving from the rapid recombination of photogenerated electrons/holes. Covalent organic framework (COF)-semiconductor-based composite photocatalysts with synergistic effects provide a feasible route to achieve high-performance photocatalytic reactions with more active sites, strong light utilization ability, and high stability. In recent years, significant progress has been made in the rational design and preparation of the COF-semiconductors-based heterostructures for photocatalytic water splitting, carbon dioxide (CO₂ ) reduction, and dye/pollutant degradation. In this Review, the synthetic strategies of COF-semiconductor-based heterostructures are first introduced, which includes the rational design of the morphology, connection modes, and type of heterojunctions. The performance of COF-semiconductor-based heterostructures in different photocatalytic reactions are comprehensively reviewed. The structure-activity relationship and the synergistic effects within the heterostructures are discussed, and the photocatalytic mechanism and the role of COFs during the photocatalytic process are also presented. Finally, an outlook and challenges of realizing COF-semiconductor-based heterostructures with simple synthesis methods, diverse functions, high performance, and well-defined reaction mechanisms are provided.

Photoreduction of Cr(VI) by TiO2 adsorbed gold nanoparticles and perylene as a novel organic-inorganic hybrid photocatalyst

The photoreduction of hexavalent chromium (Cr(VI)) using TiO₂ adsorbed gold nanoparticles and perylene (Au/Pe/TiO₂) as a novel organic-inorganic hybrid photocatalyst has been studied. The irradiation by a Xe lamp of a Cr (VI) aqueous solution (0.1 mM) with the Au/Pe/TiO₂ powder resulted in the reduction of the Cr(VI). The rate of Cr(VI) reduction reached 98.3% by the irradiation for 60 min. The reaction rate constant using Au/Pe/TiO₂ (0.0545 min^-1) was higher than that of TiO₂ (0.0218 min^-1), Pe/TiO₂ (0.0303 min^-1), or Au/TiO₂ (0.0393 min^-1). Gold nanoparticles and perylene synergistically accelerated the TiO₂ photocatalytic reaction. This result is due to the Z-scheme electron transfer between Pe and TiO₂ and the suppression of charge recombination by the gold nanoparticles. The irradiation of sunlight also led to the photocatalytic reduction of the Cr(VI) by Au/Pe/TiO₂. In addition, successive reduction of the Cr(VI) was achieved by using a column packed with the Au/Pe/TiO₂ powder immobilized by calcium alginate gel.

Metal-organic frameworks in photocatalytic Z-scheme heterojunctions: an emerging technology

There is an urgent need for cleaner production processes for chemicals. An efficient and promising alternative for such reactions is heterogeneous photocatalysis, which works on the principle of converting (visible) light, including solar energy, into chemical energy. To that end, properly designed semiconductor based photocatalysts are necessary to trigger the photocatalytic reactions. Many commonly used photocatalysts have too large bandgaps (3-3.4 eV) to use visible light and a too low surface area for efficient production. Metal-organic frameworks (MOFs) have emerged as an encouraging class of materials for photocatalytic applications due to their (i) large surface area and porosity that facilitate adsorption towards chemicals, (ii) tunable crystallinity and optical and electronic properties for efficient light absorption in the visible region, (iii) tunable composition and functionality that make them versatile photocatalysts for a wide range of reactions, and (iv) facile development of composites with other semiconductors to produce Z-scheme heterojunctions that can effectively suppress the recombination of photogenerated charges. Ongoing research has started focusing on the judicious construction of Z-scheme heterojunctions in MOFs, to mimic natural photosynthesis, such that the MOF photocatalysts have higher light harvesting capacity, spatially separated reductive and oxidative active sites, and well-preserved redox ability. This review provides a concise compilation of the recent progress in the development and applications of MOF-based Z-scheme photocatalysts, their advanced characterization, and future perspectives for further advancements.

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.

Oxygen Vacancy Mediated Band-Gap Engineering via B-Doping for Enhancing Z-Scheme A-TiO2/R-TiO2 Heterojunction Photocatalytic Performance

Fabrication of Z-scheme heterojunction photocatalysts is an ideal strategy for solving environmental problems by providing inexhaustible solar energy. A direct Z-scheme anatase TiO₂/rutile TiO₂ heterojunction photocatalyst was prepared using a facile B-doping strategy. The band structure and oxygen-vacancy content can be successfully tailored by controlling the amount of B-dopant. The photocatalytic performance was enhanced via the Z-scheme transfer path formed between the B doped anatase-TiO₂ and rutile-TiO₂, optimized band structure with markedly positively shifted band potentials, and the synergistically-mediated oxygen vacancy contents. Moreover, the optimization study indicated that 10% B-doping with the R-TiO₂ to A-TiO₂ weight ratio of 0.04 could achieve the highest photocatalytic performance. This work may provide an effective approach to synthesize nonmetal-doped semiconductor photocatalysts with tunable-energy structures and promote the efficiency of charge separation.

A comprehensive appraisal on status and management of remediation of DBPs by TiO2 based-photocatalysts: Insights of technology, performance and energy efficiency

Disinfection has been acknowledged as an inevitable technique in water treatment. However, an inadvertent consequence of generation of carcinogenic and mutagenic disinfection byproducts (DBPs) is associated with the reaction of disinfectants and natural organic matter (NOM) present in water. More than 700 DBPs have been identified in drinking water. The conventional processes carried out in WTPs do not optimally ensure NOM elimination, which evokes the need for the incorporation of other processes. In this context, several physicochemical and advanced oxidation processes (AOP), such as adsorption, membrane techniques, photocatalysis, etc., have been studied for the removal of NOM from water. Photocatalysis using semiconductors has been one of the most proficient technologies, which utilizes light energy for the degradation of recalcitrant organics. The present study aims to provide a comprehensive appraisal on the performance of titanium dioxide (TiO₂) based photocatalysts in the remediation of DBPs concerning the efficacy and energy requirements of the system. Furthermore, the effect of process parameters, such as pH, catalyst dose, light intensity, etc. on the efficacy of the process was also studied. It was observed that conventional P25-TiO₂ powders were efficient in the degradation of dissolved organic carbon (DOC) (up to 90%). However, low photocatalytic activity under visible light activation is one of its significant downsides. Several modifications on the catalyst surface in many studies exhibited advantages, such as high humic acid (HA) degradation under visible light. Furthermore, doped TiO₂ catalysts have shown high total organic carbon (TOC) degradation. The photocatalytic systems have achieved a better decrease in trihalomethane formation potential (THMFP) when compared to haloacetic acid formation potential (HAAFP). The energy requirements of the photocatalytic systems are determined by electrical energy per order (EE/O), which has been observed to be lesser for doped TiO₂ and engineered TiO₂ catalysts when compared with P25-TiO₂ powders. Carbon, iron, silver, etc., based catalysts can be a promising alternative to TiO₂-based photocatalysts for the degradation of NOM, although further research is required in this direction. The present review provides critical highlights on the uses, opportunities, and challenges of TiO₂-based photocatalytic techniques for the management of DBPs and their precursors pertaining to an emerging area of water treatment.

Design of an S-scheme photo-catalyst utilizing a Cu-doped perovskite and MOF-5 for simultaneous degradation of organic pollutants under LED light irradiation: Application of EXRSM method for spectra separation and BBD-RSM modeling

Heterojunction photo-catalysts have attracted significant attention in solar energy conversion due to their ability to reduce suppressing electron-hole pairs and improve catalytic capability. Herein, we designed an S-scheme photo-catalyst by encapsulating a Cu-doped perovskite inside the pores of MOF-5 for the first time, exhibiting excellent efficiency in a pollutant degradation process. The pristine MOF cannot act in the visible light region because of its wide bandgap. However, the encapsulation modified its bandgap and but also increased its photo-catalytic activity. Simultaneous photo-degradation of two organic contaminants, methylene blue (MB) and paracetamol (PA), was investigated to evaluate the catalytic activity of this composite. As a challenge, the UV-vis spectra of PA strongly overlapped with MB in a binary mixture preventing direct measurement of its concentration without previous separation via conventional methodologies. Hence, we used a simple and fast technique called the extended ratio subtraction method (EXRSM) to separate their absorption spectra. The statistical investigations established that it could resolve the issue of signal overlapping. Also, a statistical approach, Box-Behnken (BBD-RSM), was used to model and optimize the degradation process providing a better way to explain the effect and interactions of main parameters on degradation efficiency. Now, an empirical model for each pollutant can make a relationship between them. The photo-degradation yield was obtained at 67.12% and 87.96% for PA and MB, respectively, under optimum conditions. Furthermore, the kinetics and mechanism of reaction were investigated, and the results revealed that it follows a pseudo-first-order model for each pollutant.

3D chrysanthemum-like g-C3N4/TiO2 as an efficient visible-light-driven Z-scheme hybrid photocatalyst for tetracycline degradation

Utilization of a solar-driven semiconductor as a photocatalyst to degrade antibiotic pollutants is a feasible and environmentally friendly technology. In this paper, 3D chrysanthemum-like g-C₃N₄/TiO₂ as a visible-light-driven hybrid photocatalyst with a Z-scheme heterostructure was firstly synthesized by the in situ hydrothermal synthesis method. Experiments proved that this 3D chrysanthemum-like g-C₃N₄/TiO₂ had better degradation performance toward tetracycline than TiO₂ and g-C₃N₄. In particular, when optimized g-C₃N₄/TiO₂-2 was applied for tetracycline removal (200 ml, 10 mg L^-1), the corresponding degradation efficiency could reach nearly 100% within 60 min. The improved photocatalytic activity was the result of better utilization of the heterostructure-induced visible light, more efficient charge transfer in the Z-scheme heterojunction as well as stronger redox capability. The Z-scheme degradation mechanism was supported by the trapping experiments of active species and ESR radical detection, and the whole photocatalytic process was controlled by the combined action of ˙O₂^-, h⁺ and ˙OH radicals. This study may be beneficial for the design of more efficient sunlight-driven hybrid photocatalysts and their applications in wastewater treatment.

Bi2O3 modified TiO2 nanotube arrays and their application towards unsymmetrical dimethylhydrazine degradation in wastewater by electroassisted photocatalysis

In the present research, the preparation process parameters of TiO₂ nanotube arrays (TNAs) prepared by anodic oxidation were systematically studied by the orthogonal experimental method for the first time. Herein, the parameters of nine factors were optimized; the optimal parameters were: the electrolyte was a 0.2 mol L^-1 NaF solution with 3% vol H₂O at pH 7, the anodic oxidation voltage was 40 V, the electrode spacing was 4 cm and the reaction was carried out for 60 minutes. The physicochemical properties of the materials were characterized by SEM, XRD, EDS, UV-vis, and PL spectroscopy. By electrodeposition of Bi₂O₃ modified TNAs, the degradation rate of unsymmetrical dimethylhydrazine (UDMH) wastewater on the TNAs-10 was 89.14% within 10 h, which was 2.69 times that on pure TNAs. A bias potential of +0.3 V (vs. open circuit potential) was applied to the modified TNAs-10. The degradation rate of UDMH was significantly enhanced on the TNAs-10 (bias) process as compared to the TNAs-10 process. The degradation rate of UDMH wastewater on TNAs-10 (bias) exhibited an exponential distribution. UDMH and its toxic by-products FDMH, NDMA were completely degraded after 8 h.

Modulation of Z-scheme photocatalysts for pharmaceuticals remediation and pathogen inactivation: Design devotion, concept examination, and developments

The recent outbreak of Covid-19 guarantees overconsumption of different drugs as a necessity to reduce the symptoms caused by this pandemic. This triggers the proliferation of pharmaceuticals into drinking water systems. Is there any hope for access to safe drinking water? Photocatalytic degradation using artificial Z-scheme photocatalysts that has been employed for over a decade conveys a prospect for sustainable clean water supply. It is compelling to comprehensively summarise the state-of-the-art effects of Z-scheme photocatalytic systems towards the removal of pharmaceuticals in water. The principle of Z-scheme and the techniques used to validate the Z-scheme interfacial charge transfer are explored in detail. The application of the Z-scheme photocatalysts towards the degradation of antibiotics, NSAIDs, and bacterial/viral inactivation is deliberated. Conclusions and stimulating standpoints on the challenges of this emergent research direction are presented. The insights and up-to-date information will prompt the up-scaling of Z- scheme photocatalytic systems for commercialization.

The Ultrahigh Adsorption Capacity and Excellent Photocatalytic Degradation Activity of Mesoporous CuO with Novel Architecture

In this paper, mesoporous CuO with a novel architecture was synthesized through a conventional hydrothermal approach followed by a facile sintering procedure. HR-TEM analysis found that mesoporous CuO with an interconnected pore structure has exposed high-energy crystal planes of (002) and (200). Theoretical calculations indicated that the high-energy crystal planes have superior adsorption capacity for H⁺ ions, which is critical for the excellent adsorption and remarkable photocatalytic activity of the anionic dye. The adsorption capacity of CuO to methyl orange (MO) at 0.4 g/L was approximately 30% under adsorption equilibrium conditions. We propose a state-changing mechanism to analyze the synergy and mutual restraint relation among the catalyst CuO, H⁺ ions, dye and H₂O₂. According to this mechanism, the degradation rate of MO can be elevated 3.5 times only by regulating the MO ratio in three states.

Comprehensive Review on g-C3N4-Based Photocatalysts for the Photocatalytic Hydrogen Production under Visible Light

Currently, the synthesis of active photocatalysts for the evolution of hydrogen, including photocatalysts based on graphite-like carbon nitride, is an acute issue. In this review, a comprehensive analysis of the state-of-the-art studies of graphic carbon nitride as a photocatalyst for hydrogen production under visible light is presented. In this review, various approaches to the synthesis of photocatalysts based on g-C₃N₄ reported in the literature were considered, including various methods for modifying and improving the structural and photocatalytic properties of this material. A thorough analysis of the literature has shown that the most commonly used methods for improving g-C₃N₄ properties are alterations of textural characteristics by introducing templates, pore formers or pre-treatment method, doping with heteroatoms, modification with metals, and the creation of composite photocatalysts. Next, the authors considered their own detailed study on the synthesis of graphitic carbon nitride with different pre-treatments and respective photocatalysts that demonstrate high efficiency and stability in photocatalytic production of hydrogen. Particular attention was paid to describing the effect of the state of the platinum cocatalyst on the activity of the resulting photocatalyst. The decisive factors leading to the creation of active materials were discussed.

A Brief Review of Photocatalytic Reactors Used for Persistent Pesticides Degradation

Pesticide pollution is a major issue, given their intensive use in the 20th century, which led to their accumulation in the environment. At the international level, strict regulations are imposed on the use of pesticides, simultaneously with the increasing interest of researchers from all over the world to find methods of neutralizing them. Photocatalytic degradation is an intensively studied method to be applied for the degradation of pesticides, especially through the use of solar energy. The mechanisms of photocatalysis are studied and implemented in pilot and semi-pilot installations on experimental platforms, in order to be able to make this method more efficient and to identify the equipment that can achieve the photodegradation of pesticides with the highest possible yields. This paper proposes a brief review of the impact of pesticides on the environment and some techniques for their degradation, with the main emphasis on different photoreactor configurations, using slurry or immobilized photocatalysts. This review highlights the efforts of researchers to harmonize the main elements of photocatalysis: choice of the photocatalyst, and the way of photocatalyst integration within photoreaction configuration, in order to make the transfer of momentum, mass, and energy as efficient as possible for optimal excitation of the photocatalyst.

TiO2 based Photocatalysis membranes: An efficient strategy for pharmaceutical mineralization

Among the various emerging contaminants, pharmaceuticals (PhACs) seem to have adverse effects on the quality of water. Even the smallest concentration of PhACs in ground water and drinking water is harmful to humans and aquatic species. Among all the deaths reported due to COVID-19, the mortality rate was higher for those patients who consumed antibiotics. Consequently, PhAC in water is a serious concern and their removal needs immediate attention. This study has focused on the PhACs' degradation by collaborating photocatalysis with membrane filtration. TiO₂-based photocatalytic membrane is an innovative strategy which demonstrates mineralization of PhACs as a safer option. To highlight the same, an emphasis on the preparation and reinforcing properties of TiO₂-based nanomembranes has been elaborated in this review. Further, mineralization of antibiotics or cytostatic compounds and their degradation mechanisms is also highlighted using TiO₂ assisted membrane photocatalysis. Experimental reactor configurations have been discussed for commercial implementation of photoreactors for PhAC degradation anchored photocatalytic nanomembranes. Challenges and future perspectives are emphasized in order to design a nanomembrane based prototype in future for wastewater management.

Exploring the visible light-assisted conversion of CO2 into methane and methanol, using direct Z-scheme TiO2@g-C3N4 nanosheets: synthesis and photocatalytic performance

The rapid growth of carbon dioxide (CO₂) emissions raises concern about the possible consequences of atmospheric CO₂ increase, such as global warming and greenhouse effect. Photocatalytic CO₂ conversion has attracted researchers' interests to find a sustainable route for its elimination. In the present study, a direct Z-scheme TiO₂/g-C₃N₄ composite (T-GCN) was fabricated via a facile hydrothermal route for the photocatalytic reduction of CO₂ into methane (CH₄) and methanol (CH₃OH), under visible light irradiation without an electron mediator. The microstructure of the as-obtained TiO₂/g-C₃N₄ nanocomposites was fully characterized for its physicochemical, structural, charge separation, electronic, and photo-excited carrier separation properties. The effect of CO₂ and H₂O partial pressure was studied to find the best operational conditions for obtaining maximum photocatalytic efficiency; the P_CO2 and P_H2O were 75.8 and 15.5 kPa, respectively, whereas, by increasing the light intensity from 20 to 80 mW/cm², a remarkable improvement in the reduction rate takes place (from 11.04 to 32.49 μmol.gcat^-1.h^-1 methane production, respectively). Finally, under the most favorable light, P_CO2 and P_H2O conditions, high methanol and methane rates were obtained from the CO₂ photocatalytic reduction through T-GCN (1.44 μmol.gcat.^-1.h^-1 and 32.49 μmol.gcat.^-1.h^-1, respectively) and an integrated proposition for the Z-scheme mechanism of photocatalytic reduction was proposed. This study offers a promising strategy to synthesize a Z-scheme T-GCN heterojunction with high photocatalytic performance for effective CO₂ conversion.

Enhancing the Photoinduced Interlayer Charge Transfer and Spatial Separation in Type-II Heterostructure of WS2 and Asymmetric Janus-MoSSe with Intrinsic Self-Build Electric Field

Two-dimensional heterostructure manipulation is promising to overcome the high recombination rates and limited redox abilities of photogenerated electron-hole pairs in a single photocatalyst. The built-in electric field (E_hetero) in the type-II heterojunction is normally unfavorable for the desired charge transfer, which is an important but easily neglected issue that needs to be solved. Here, on the basis of the density functional theory (DFT) and the nonadiabatic molecular dynamics (NAMD) calculations, we obtain a type-II band alignment in Janus-MoSSe/WS₂ heterostructure, which meets the band-edge position requirement for water splitting. Importantly, the intrinsic self-build electric field (E_self) of Janus-MoSSe can effectively weaken the hindrance effect of E_hetero for charge transfer by constructing a suitable Se-S stacking configuration, improving charge separation efficiency in the Janus-MoSSe/WS₂ heterostructure. Our work provides a materials-by-design paradigm and interlayer charge-transfer dynamics understanding of heterostructure engineering against asymmetric structures lacking reflection symmetry.

Recent Advances on Metal Oxide Based Nano-Photocatalysts as Potential Antibacterial and Antiviral Agents

Photocatalysis, a unique process that occurs in the presence of light radiation, can potentially be utilized to control environmental pollution, and improve the health of society. Photocatalytic removal, or disinfection, of chemical and biological species has been known for decades; however, its extension to indoor environments in public places has always been challenging. Many efforts have been made in this direction in the last two–three years since the COVID-19 pandemic started. Furthermore, the development of efficient photocatalytic nanomaterials through modifications to improve their photoactivity under ambient conditions for fighting with such a pandemic situation is a high research priority. In recent years, several metal oxides-based nano-photocatalysts have been designed to work efficiently in outdoor and indoor environments for the photocatalytic disinfection of biological species. The present review briefly discusses the advances made in the last two to three years for photocatalytic viral and bacterial disinfections. Moreover, emphasis has been given to the tailoring of such nano-photocatalysts in disinfecting surfaces, air, and water to stop viral/bacterial infection in the indoor environment. The role of such nano-photocatalysts in the photocatalytic disinfection of COVID-19 has also been highlighted with their future applicability in controlling such pandemics.

Constructing novel graphitic carbon nitride-based nanocomposites - From the perspective of material dimensions and interfacial characteristics

Two-dimensional (2D) graphitic carbon nitride (g-C₃N₄), a fascinating metal-free conjugated polymer, has garnered immense interest in the fields of solar power generation and environmental remediation. The construction of g-C₃N₄-based nanocomposites with materials of various dimensions can further improve their photocatalytic activities by surface area enlargement, bandgap tuning, heterojunction formation, etc. In this paper, we comprehensively reviewed the design, synthesis, and functionalities of g-C₃N₄-based nanocomposites based on their applications in hydrogen evolution, CO₂ reduction, and pollutants removal. We provided detailed analyses on the integration of 2D g-C₃N₄ with zero-, one-, two-, and three-dimensional materials with a focus on their interfacial characteristics and functional improvement. This review aims to stimulate fresh ideas on the interfacial engineering of g-C₃N₄-based nanocomposites to broaden their future applications.

Formation and crystallization of TiO2 nanostructures on various surfaces

A comparative study of the synthesis of TiO₂ nanorods on fluorine-doped tin oxide (FTO) glass, Si, SiO₂, Si/Ta, Si/TiN, Si/TiN/Ti and Si/HFO₂ substrates by hydrothermal reaction is presented. Detailed analysis on the growth of TiO₂ on pre-annealed Si/TiN/Ti and HfO₂ (HFO) surfaces is also given. For Si/TiN/Ti, a pre-annealing procedure led to the transformation of Ti to a TiO₂ layer which acts as a seed for aligned growth of TiO₂ nanorods. In contrast, Si/HFO does not provide a nucleation site for the formation of aligned nanorods. Various samples were prepared by varying the synthesis conditions, i.e. pre- and post-annealing temperatures and hydrothermal reaction time to figure out the optimum conditions which lead to unidirectional and highly aligned nanorods. X-ray diffraction, scanning electron microscopy, ultraviolet-visible spectroscopy and Raman spectroscopy were used to study structural, morphological and optical properties of synthesized samples. It is found that TiO₂ nanorods exhibit a rutile phase on the Si/Ti/TiN and Si/HFO substrates, but highly oriented vertical growth of nanorods has been observed only on pre-annealed Si/TiN/Ti substrates. On the other hand, TiO₂ nanorods form dandelion-like structures on Si/HFO substrates. Growth of vertically oriented TiO₂ nanorods on Si/TiN/Ti is attributed to the TiO₂ seed layer which forms after the process of pre-annealing of substrates at a suitable temperature. Variation in hydrothermal reaction time and post-annealing temperature brings further improvement in crystallinity and morphology of nanorods. This work shows that the pre-annealed Si/TiN/Ti substrate is the optimal choice to achieve vertically oriented, highly aligned TiO₂ nanorods.

FabricatingZ-scheme C-doped TiO2/rGO nanocomposites for enhanced photocatalytic NO removal

It is attractive to explore practical approaches to optimize the photodegraded NO property of TiO₂. Herein, a typicalZ-shaped heterojunction C-TiO₂/rGO composed of carbon-doped TiO₂and reductive graphene oxide (rGO) was constructed to optimize the NO removal efficiency through anin situone-pot hydrothermal process with glucose as reductant and dopant. The C-TiO₂/rGO (0.11%) composite displays a remarkable NO removal performance of 40.6% under visible light illumination. It was found that the C-TiO₂nanoparticles were tightly attached to the rGO sheets and had strong interactions with rGO, which induced a positive impact on not only the light absorption and photo-generated charge separation but also the NO adsorption and reactive oxygen species formation, resulting in boosted photodegrade NO activity. As to the photodegrade NO process over the C-TiO₂/rGO, the HO^•and O₂^•-were the dominant radicals, of which the O₂^•-radical originated from the interactions between C-TiO₂and rGO. We proposed aZ-scheme mechanism to illuminate the advanced photocatalytic activity of C-TiO₂/rGO. This work affords an approach to developing effective photocatalysts in the NO purification field.

Photocatalytic Materials Obtained from E-Waste Recycling: Review, Techniques, Critique, and Update

Waste-derived materials obtained from the recovery and recycling of electronic waste (e-waste) such as batteries and printed circuit boards have attracted enormous attention from academia and industry in recent years, especially due to their eco-friendly nature and the massive increment in e-waste due to technological development. Several investigations in the literature have covered the advances achieved so far. Meanwhile, photocatalytic applications are especially of interest since they maintain mutual benefits and can be used for H2 production from solar water splitting based on semiconductor processing as a proper environmentally friendly technique for solar energy conversion. In addition, they can be utilized to degrade a variety of organic and non-organic contaminations. Nonetheless, to the best of the authors’ knowledge, there has not been any comprehensive review that has specifically been focused on e-waste-derived photocatalytic materials. In this regard, the present work is dedicated to thoroughly discussing the related mechanisms, strategies, and methods, as well as the various possible photocatalysts synthesized from e-wastes with some critiques in this field. This brief overview can introduce modern technologies and promising possibilities for e-waste valorization, photocatalytic processes, and new photocatalytic degradation methods of eco-friendly nature. This paper discusses various e-waste-obtained photocatalytic materials, synthesis procedures, and applications, as well as several types of e-waste, derived materials such as TiO2, ZnO, indium tin oxide, and a variety of sulfide- and ferrite-based photocatalytic materials.

Properties, optimized morphologies, and advanced strategies for photocatalytic applications of WO3 based photocatalysts

The development of WO₃ based photocatalysts has gained considerable attention across the world, especially in the realm of environmental remediation and energy production. WO₃ has a band gap of 2.5- 2.7 eV that falls under the visible region and is thus a potential candidate to utilize in various photocatalytic processes. As an earth-abundant metal oxide, WO₃ discovered in 1976 displayed excellent electronic and morphological properties, good stability, and enhanced photoactivity with diverse crystal phases. Also, it unveils non-toxicity, high stability in drastic conditions, biocompatibility, low cost, excellent hole mobility (10 cm² V^-1s^-1), and tunable band gap. This review provides a comprehensive overview of the different properties of WO₃ inclusive of crystallographic, electrical, optical, thermoelectrical, and ferroelectric properties. The different morphologies of WO₃ based on dimensions were obtained by adopting different fabrication methods including inspecting their effects on the efficiency of WO₃. Numerous strategies to construct an ideal photocatalyst such as engineering crystal facets, surface defects, doping, heterojunction formation explaining specifically type-II, Z-scheme, and S-scheme mechanisms with addition to carbonaceous based WO₃ nanocomposites are summed up to explore the photocatalytic performance. The typical application of WO₃ is deliberated in detail involving the role and efficiency of WO₃ in pollutant degradation, CO₂ photoreduction, and water splitting. Besides, other applications of WO₃ as gas-sensor, bio-sensor, decomposition of VOCs, heavy metals ions adsorption, and antimicrobial property are also included. Moreover, the numerous aspects responsible for the high efficiency of WO₃-based nanocomposites with their challenges, opportunities, and future aspects are summarized. Hopefully, this review may inspire researchers to explore new ideas to boost the production of clean energy for the next generation.

Structural and Optical Properties of Calcium Titanate Prepared from Gypsum

Ceramic materials have been used in various human health-related applications for considerable time. One of the important applications of ceramic materials is in electronics. Our work focuses on calcium titanate (CaTiO3). CaTiO3 is typically created via sintering. Gypsum particles is used to form calcium hydroxide, which is then combined with titanium dioxide to form rutile crystals. Thereafter, calcination is performed at 900°C, 1000°C, and 1100°C for 2 h. X-ray diffraction is employed to track the evolution of the CaTiO3 phase. Scanning electron microscopy is used to characterize the morphologies of the different preparation steps. As the calcination temperature increases from 900°C to 1000°C, the crystallite size of CaTiO3 increases from 35 nm to 45 nm. Furthermore, the energy gaps of the CaTiO3 powders obtained after calcination at 900°C and 1000°C are 5.32 eV and 5.43 eV, respectively, and their particle sizes are 150–200 nm and 200–300 nm, respectively.