Revisiting structural and photocatalytic properties of g-C3N4/TiO2: Is surface modification of TiO2 by calcination with urea an effective route to “solar” photocatalyst?

A Review of Synthesis Methods, Modifications, and Mechanisms of ZnO/TiO2-Based Photocatalysts for Photodegradation of Contaminants

The environment being surrounded by accumulated durable waste organic compounds has become a critical crisis for human societies. Generally, organic effluents of industrial plants released into the water source and air are removed by some physical and chemical processes. Utilizing photocatalysts as cost-effective, accessible, thermally/mechanically stable, nontoxic, reusable, and powerful UV-absorber compounds creates a new gateway toward the removal of dissolved, suspended, and gaseous pollutants even in trace amounts. TiO2 and ZnO are two prevalent photocatalysts in the field of removing contaminants from wastewater and air. Structural modification of the photocatalysts with metals, nonmetals, metal ions, and other semiconductors reduces the band gap energy and agglomeration and increases the affinity toward organic compounds in the composite structures to expand their usability on an industrial scale. This increases the extent of light absorbance and improves the photocatalytic efficiency. Selecting a suitable synthesis method is necessary to prepare a target photocatalyst with distinct properties such as high specific surface area, numerous surface functional groups, and an appropriate crystalline phase. In this Review, significant parameters for the synthesis and modification of TiO2- and ZnO-based photocatalysts are discussed in detail. Several proposed mechanistic routes according to photocatalytic composite structures are provided. Some electrochemical analyses using charge carrier trapping agents and delayed recombination help to plot mechanistic routes according to the direction of photoexcited species (electron-hole pairs) and design more effective photocatalytic processes in terms of cost-effective photocatalysts, saving time and increasing productivity.

Selective efficient photocatalytic degradation of antibiotics and direct Z-type migration pathway for hierarchical core-shell TiO2/g-C3N4 composites

Constructing superior Z-type photocatalytic heterojunction is beneficial to effectively enlarge interface contact, improve the photo-generated carrier separation rate, and retain the high redox ability. In this work, we designed a hierarchical core-shell g-C3N4/TiO2 structure to build Z-type heterojunction via combining simple template method and pyrolysis process. A close-knit Z-type heterojunction was constructed using TiO2 as a thick core and g-C3N4 as an ultra-thin shell. The effects of lamp source, wavelength, tetracycline (TC) concentration, and photocatalyst dose on the degradation performance on TC of g-C3N4/TiO2 were inspected. 0.1TiO2/g-C3N4 photocatalyst had the best degradation rate and highest removal rate within 30 min, and its degradation rate was about 49, 23, and 5 times than pure g-C3N4, TiO2, and commercial TiO2/g-C3N4 in respect. Moreover, compared with degradation ability under Xenon lamp, LED irradiation for g-C3N4/TiO2 composites showed a remarkable selective degradation. The fast and efficient Z-type transfer pathway of 0.1 g-C3N4/TiO2 was realized by forming an optimized interface and abundant surface active sites ascribed to the combined action of thick TiO2 core and ultra-thin g-C3N4 shell. In addition, the degradation intermediates were analyzed by LC-MS and suggested pathways of degradation. The work could provide novel design concept to obtain reliable Z-type photocatalysts with hierarchical core-shell structure applied in degradation of antibiotic wastewater.

One-Step Fabrication of the ZnO/g-C3N4 Composite for Visible Light-Responsive Photocatalytic Degradation of Bisphenol E in Aqueous Solution

The ZnO/g-C₃N₄ composite was successfully synthesized by a simple one-step calcination of a urea and zinc acetate mixture. The photocatalytic activity of the synthesized composite was evaluated in the degradation of bisphenol E (BPE). The morphology, crystallinity, optical properties, and composition of the synthesized composite were characterized by using various analytical techniques such as scanning electron microscopy (SEM), transmitted electron microscopy (TEM), field emission-electron probe microanalysis (FE-EPMA), nitrogen adsorption and desorption isotherm measurement, Fourier-transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The degradation rate of BPE with the ZnO/g-C₃N₄ composite was 8 times larger than that obtained with pure g-C₃N₄ at the optimal conditions. The excellent photocatalytic activity was attributed to the synergistic effect between the g-C₃N₄ and ZnO, which enhanced the efficiency of charge separations, reduced the e^-/h⁺ pairs recombination, and increased the visible light absorption ability. The radical scavenger studies indicated that the ^•O₂ ^- and h⁺ species were mainly responsible for the degradation of BPE. The stability test suggested the chemical and photostability of the synthesized composite. Two possible photocatalytical mechanisms have been suggested.

Boosting photocatalytic performances of lamellar BiVO4by constructing S-scheme heterojunctions with AgBr for efficient charge transfer

Successful construction of heterojunction can improve the utilization efficiency of solar light by broadening the absorption range, facilitating charge-carrier separation, promoting carrier transportation and influencing surface-interface reaction. Herein, visible-light-driven AgBr was deposited on the surface of lamellar BiVO₄which was prepared by a facile hydrothermal process to improve charge carrier separation, and subsequent photocatalytic effectiveness. The catalyst with an optimal AgBr/BiVO₄ratio exhibited a superbly enhanced photocatalytic decolorization ability (about 6.85 times higher than that of pure BiVO₄) and high stability after four cycles. The unique photocatalytic mechanism of S-scheme carrier migration was investigated on the bases of radical trapping tests and photo/electrochemical characterizations. Results showed that the enhanced migration strategy and intimately interfacial collaboration guaranteed the effective charge carriers separation/transfer, leading to magnificent photocatalytic performance as well as excellent stability.

Electrically conductive TiO2/CB/PVDF membranes for synchronous cross-flow filtration and solar photoelectrocatalysis

Combining photocatalysis (PC) and membrane filtration (MF) has emerged as an attractive technology for water purification, however, the water purification efficiency and membrane fouling are still challenging. Herein, we report a novel photoelectrocatalytic (PEC) membrane mediated by a ternary polyvinylidene fluoride (PVDF)-carbon black (CB)-TiO₂ composite conductive membrane synthesized by a phase inversion method assisted by the mixed surfactants of polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS). The resultant electrically conductive TiO₂/CB/PVDF membrane features a homogeneous surface with obvious pore size of 20-150 nm, a thickness ∼116 μm, and an average resistivity as low as ∼3.165 Ω∙m. The cooperation of PVP and SDS surfactants dramatically improves the organic-inorganic interactions and thus eventually enhances the porosity, stability of porous structure, mechanical stability, and conductivity and electrochemical properties of the hybrid membrane. Upon the solvent evaperation of the wellblended casting solution and the phase inversion, TiO₂/CB preferentially exist on the surface of PVDF membrane, enabling the efficient PEC degradation of organic pollutants. The synergistic coupling of TiO₂ and CB in PVDF membrane results in efficient PEC properties with bi-functional membrane antifouling and enhanced water purification in azo dyes decolorization under the stationary mode and in our lab-made continuous cross-flow PEC system, superior to those by photocatalysis and electrocatalysis. The developed synchronous MF and PEC system mediated by the conductive TiO₂/CB/PVDF membrane proves to a feasible route to improving the self-cleaning properties of the polymer membrane while simultaneously increasing the water decontaminating efficiency.

Characterization Methodology and Activity Evaluation of Solar-Driven Catalysts for Environmental Remediation

Solar-driven photocatalysis mediated by semiconductors has been rapidly developed as a green and sustainable technology for environmental remediation. Continuous efforts have been devoted to novel semiconducting photocatalysts to boost the efficiency of the photocatalytic system. However, controversy has widely existed in materials characterization and photocatalytic activity evaluation. This review overviews the recent advances in characterization methodology and photocatalytic activity evaluation of solar-driven catalysts (SDCs) for environmental remediation. After a general and brief introduction of different SDCs, the compositional, structural, and optical characterizations of SDCs are summarized. Moreover, the characterization methods and challenges in the doped and coupled SDCs are discussed. Finally, the challenges in the evaluation of current evaluation methods for the photocatalytic activity of SDCs are highlighted.

Heterojunction formation between AgNbO3 and Co3O4 for full solar light utilization with improved charge-carrier separation

In the present study, the charge-carrier recombination of visible light active perovskite silver niobate (AgNbO₃) was reduced by forming heterojunction with Co₃O₄ through simple impregnation and calcination route. The loading percentage of Co₃O₄ was varied as 2, 5, and 10 wt.%. The XRD study revealed reduced interlayer spacing in the composite due to the replacement of the bigger Ag⁺ ions by the smaller Co²⁺ and Co³⁺ ions of Co₃O₄. It was observed that the light harvesting efficiency of the materials was increased with increased loading of Co₃O₄. The TEM and XPS analysis confirmed the presence of Ag nanoparticles over the perovskite in the composite. The electrochemical analysis revealed enhanced charge-carrier number density and increased charge-carrier lifetime in the composite as a result of the presence of both silver and cobalt ions in the lattice. Further this enhanced charge-carrier separation of the composites was established through photocatalysis of Bisphenol-A under both solar and LED light. Charge-trapping study indicated *O₂^- and *OH as the major radicals involved and Z-scheme as the predominant charge transfer pathway for generation of these reactive oxygen species.

Enhanced photocatalytic and antibacterial activities of S-scheme SnO2/Red phosphorus photocatalyst under visible light

The construction of wide bandgap semiconductors with heterojunctions is an effective strategy to improve the photocatalytic activity of narrow-bandgap semiconductors, such as red phosphorus (RP). The novel step-scheme (S-scheme) heterojunction can separate photocarriers effectively while retaining the high reduction-oxidation capacity of the catalyst. Herein, a SnO₂/hydrothermally treated RP (SnO₂/HRP) S-scheme heterojunction was constructed and was found to display superior performance in the photocatalytic degradation of pollutants and the disinfection of bacteria. The 5%SnO₂/HRP (mass ration of SnO₂ with 5 wt%) composite had the strongest photocatalytic activity. It could degrade 97.5% of Rhodamine B (RhB) after 12 min of light exposure. The photodegradation rate constant of this composite reached 2.96 × 10^-1 min^-1, which was 4.4 and 59.2 times higher than that of pure HRP and SnO₂, respectively. Furthermore, this S-scheme heterojunction composite exhibited a higher efficient photocatalytic antibacterial rate (99.4%) for Escherichia coli (E. coli) under visible-light irradiation, than pure HRP (66.4%) and SnO₂ (72.9%). Further mechanistic investigations illustrated that the intimate contact between HRP and SnO₂ in the S-scheme system heterojunction could effectively boost carrier transfer and improve the photocatalytic activity of the semiconductor. This investigation provided an efficient recyclable S-scheme heterojunction composite for the photocatalytic degradation of pollutants and bacteria.

Three-dimensional porous MoS2 nanobox embedded g-C3N4@TiO2 architecture for highly efficient photocatalytic degradation of organic pollutant

Herein, a simple, highly efficient and stable MoS₂ nanobox embedded graphitic-C₃N₄@TiO₂ (g-CN@TiO₂) nanoarchitecture was synthesized by a facile solvothermal approach. The nano-hybrid photocatalyst was constructed by TiO₂ nanoparticles anchored on the surface of g-CN nanosheets. Then highly crystalline three-dimensional porous MoS₂ nanobox was homogeneously distributed on the g-CN@TiO₂ surface. The g-CN@TiO₂/MoS₂ hybrid achieved a high photocatalytic degradation efficiency of 97.5% for methylene blue (MB) dye pollutant under visible-light irradiant in an hour which was much better than TiO₂@MoS_2, g-CN@TiO_2, MoS_2, TiO₂ and g-CN. Furthermore, the reaction rate (k) value of g-CN@TiO₂/MoS₂ for MB dye is as high as 3.18 X 10^-2 min^-1, which is ~ 2.65 times better than those of g-CN@TiO₂ and MoS₂. This work presents a rational structure design, interfacial construction and suitable band gap strategy to synthesize advanced nano-hybrid photocatalyst for degradation of organic pollutant with excellent performance and long-term stability.

N/Fe/Zn co-doped TiO2 loaded on basalt fiber with enhanced photocatalytic activity for organic pollutant degradation

To avoid the loss of catalytic material powder, a loaded catalytic material of TiO2 with basalt fiber as the carrier (TiO2@BF) was synthesized by an improved sol-gel method. The TiO2@BF was doped with different contents of N, Fe and Zn elements and was used to degrade rhodamine B (RhB) under ultraviolet light. The physical characterization analysis indicated that the co-doping of the N, Fe and Zn elements had the effects of reducing grain size, increasing sample surface area, and narrowing the electronic band gap. The electronic band gap of nitrogen-iron-zinc co-doped TiO2@BF (N/Fe/Zn_TiO2@BF) was 2.80 eV, which was narrower than that of TiO2@BF (3.11 eV). The degradation efficiency of RhB with N/Fe/Zn_TiO2@BF as a photocatalyst was 4.3 times that of TiO2@BF and its photocatalytic reaction was a first-order kinetic reaction. Quenching experiments suggested that the reactive species mainly include photoinduced holes (h+), superoxide radicals (˙O2 -) and hydroxyl radicals (˙OH). In brief, this study provides a prospective loaded catalytic material and routine for the degradation of organic contaminants in water by a photocatalytic process.

Cooperative TiO2 photocatalysis with TEMPO and N-hydroxysuccinimide for blue light-driven selective aerobic oxidation of amines

TiO₂ has been the focus of attention in semiconductor photocatalysis for several decades because it can potentially settle the grand energy and environmental issues with earth-abundant elements of Ti and O. However, because of its wide band gap, TiO₂ can only collect UV light, hindering its practical applications under the illumination of sunlight. In view of this, an interesting phenomenon of light-driven adsorption of amines onto TiO₂ to form a visible light-absorbing complex was adapted to assemble smart photocatalysis. The endurance of this complex was eminently refurbished by blue light-driven continuous adsorption of amines. This in turn promoted a vital selective chemical transformation, blue light-driven selective oxidation of amines into imines with atmospheric dioxygen (O₂). More importantly, the inclusion of TEMPO and N-hydroxysuccinimide (NHS) into the smart photocatalytic system could cooperatively expedite the blue light-driven selective aerobic oxidation of amines into imines through dual independent reaction channels, resembling that of enzymatic catalysis. This work underscores the importance of manoeuvring multiple reaction channels by cooperative photocatalysis during selective chemical transformations.

Mesoporous WO3/TiO2 spheres with tailored surface properties for concurrent solar photocatalysis and membrane filtration

The strategical integration of membrane water filtration with semiconductor photocatalysis presents a frontier in deep purification with a self-cleaning capability. However, the membrane fouling caused by the cake layer of the reclaimed TiO₂ nanoparticles is a key obstacle. Herein, mesoporous WO₃/TiO₂ spheres (∼450 nm in diameter) consisting of numerous self-assembled WO₃-decoated anatase TiO₂ nanocrystallites are successfully prepared via a facile wet-chemistry route. The decoration of monolayered WO₃ significantly affects the surface, photocatalytic, and optical properties of original mesoporous TiO₂ spheres. XRD and Raman analyses show the presence of monolayered WO₃ suppresses the crystal growth of TiO₂ during the calcination process, significantly improves the surface acidity, and causes an obvious red shift in absorption edge. These favorable textural properties, coupling the enhanced interfacial charge carrier separation, render mesoporous WO₃/TiO₂ spheres with a superior photocatalytic activity in degradation of methylene blue under UV, visible, and solar light irradiations. The optimal molar ratio of W/Ti is examined to 6%. The synthesized mesoporous WO₃/TiO₂ spheres also show much higher flux during membrane filtration in both dead-end and cross-flow modes, suggesting a promising photocatalyst for concurrent membrane filtration and solar photocatalysis.

A CuMn2O4 spinel oxide as a superior catalyst for the aerobic oxidation of 5-hydroxymethylfurfural toward 2,5-furandicarboxylic acid in aqueous solvent

A CuMn₂O₄ spinel oxide was prepared via a freezing-assisted sol–gel method and used in the aerobic oxidation of 5-hydroxylmethylfurfural (HMF) toward 2,5-furandicarboxylic acid (FDCA) in aqueous solvent.

Synthesis and fabrication of g-C3N4-based materials and their application in elimination of pollutants

With the fast development of industrial and human activity, large amounts of persistent organic pollutants, heavy metal ions and radionuclides are released into the natural environment, which results in environmental pollution. The efficient elimination of the natural environment is crucial for the protection of environment to against the pollutants' toxicity to human beings and living organisms. Graphitic carbon nitride (g-C₃N₄) has drawn multidisciplinary attention especially in environmental pollutants' cleanup due to its special physicochemical properties. In this review, we summarized the recent works about the synthesis of g-C₃N₄, element-doping, structure modification of g-C₃N₄ and g-C₃N₄-based materials, and their application in the sorption, photocatalytic degradation and reduction-solidification of persistent organic pollutants and heavy metal ions. The interaction mechanisms were discussed from advanced spectroscopic analysis and computational approaches at molecular level. The challenges and future perspectives of g-C₃N₄-based materials' application in environmental pollution management are presented in the end. This review highlights the real applications of g-C₃N₄-based materials as adsorbents or photocatalysts in the adsorption-reduction-solidification of metal ions or photocatalytic degradation of organic pollutants. The contents are helpful for the undergraduate students to understand the recent works in the elimination of organic/inorganic pollutants in their pollution management.

Molecularly imprinted carbon nanosheets supported TiO2: Strong selectivity and synergic adsorption-photocatalysis for antibiotics removal

In order to achieve strong specific recognition and remarkable synergy between adsorption and photocatalysis, carbon nanosheets supported TiO₂ (CT) was designed and embellished by molecular imprinting technology with ciprofloxacin (CIP) as template. The molecular imprinted CT (CT-MI) product exhibited remarkable synergy of adsorption-photocatalysis and high selectivity in both aspects, benefitted from specific recognition of imprinted layer, strong carbon adsorption and electroconductivity, and enhanced photocatalysis. Compared to the competitive pollutant, sulfamethoxazole (SMZ) in this study, selectivity coefficient was 7.2 for adsorption and 3.2 for photocatalysis, respectively. This is superior to most of the imprinted photocatalysts reported in the literature. In addition, effect of mass ratio between TiO₂ matrix to imprinted polymers, as well as water quality and composition, to the performance of final product was studied and favorable conditions were proposed. Electron transfer mode, selective recognition mode, and antibiotics degradation mechanism and pathways were also illustrated based on trapping experiments and HPLC-MS technology etc. This study confirmed that alliance between molecular imprinting, carbon nanosheets and well dispersed photocatalyst possessed broad prospect of applications in specific recognition and selective degradation of a highly toxic pollutant in a variety of mixed systems.