Electrospun, flexible and reusable nanofiber mat of graphitic carbon nitride: Photocatalytic reduction of hexavalent chromium

Porous organic semiconductor/PET composite fibre for the synergistic removal of hexavalent chromium and organic pollutants under sunlight

In this study, the porous graphite phase carbon nitride photocatalyst (P-g-C3N4) is prepared by the CaCO3 template method, and then P-g-C3N4/T-polyethylene terephthalate (T-PET) catalytic fibre is prepared by the padding method. P-g-C3N4 can provide more active sites than g-C3N4 as proved by the Brunauer-Emmett-Teller and the UV-Visible diffuse reflectance test. P-g-C3N4 powder catalyst successfully supports PET fibre as proved by scanning electron microscope, Fourier infrared spectroscopy and X-ray diffraction spectroscopy. The photocatalytic performance of P-g-C3N4/T-PET catalytic fibre is tested by constructing a single hexavalent chromium or hexavalent chromium/organic pollutant binary pollution system. The potential application value of P-g-C3N4/T-PET catalytic fibre is further explored by simulating the complex actual water environment. After five recycles, P-g-C3N4/T-PET catalytic fibre shows good catalytic performance. The mechanism of P-g-C3N4/PET photocatalytic degradation of organic pollutants is proposed through the capture agent experiment and electron paramagnetic resonance spectroscopy. Among them, •O2- is the most important active species of P-g-C3N4 catalytic fibre, which is used for the oxidation of organic pollutants. At the same time, photoelectrons generated by the catalytic fibre are used to reduce hexavalent chromium. The efficiency of P-g-C3N4 to remove pollutants is improved by using PET fibre as a carrier, which not only solves the problem of difficult recovery of powder catalysts but also provides more active sites.

A novel tungsten diselenide nanoparticles for enhanced photocatalytic performance of Cr (VI) reduction and ciprofloxacin (CIP)

Nanoparticles (NPs) fabrication is a significant approach to enhance the visible light response of photocatalysts, to realize inexpensive and more harmful compound removal, at larger scale. The poor electrons and holes separation capability and low light activity of bulk materials can be notably enhanced through developing NPs. From photocatalytic investigation, better performance was received in the tungsten diselenide (WSe2) NPs than that in bare WSe2, exhibiting the action of restrained recombination of charge carriers in the NPs. The photocatalytic Cr(VI) reduction efficiency of WSe2 NPs is 2.7 folds greater than that by bare WSe2. On the other hand, the photocatalytic efficiency follows the order of nano WSe2-3 > nano WSe2-2 > nano WSe2-1 > bare WSe2, nano WSe2-3 is nearly 2.7 folds greater than that of bare WSe2. The results imply the fabrication of WSe2 NPs and it possesses improved visible light utilization. The proposed WSe2 NPs have merged with the three aspects of photocatalytic capability including the visible light activity, the valid separation of photo-response charge carriers and enough surface active sites owing to the nanoscale formed. This research endows conduct on the potential style of NPs for photo-response water environmental remediation.

Highly exfoliated graphitic carbon nitride for efficient removal of wastewater pollutants: Insights from DFT and statistical modelling

The present work entails the synthesis of thermally modified graphitic carbon nitride (GCN) using a two-step thermal treatment procedure and its subsequent use in the photocatalytic reduction of toxic pollutants such as rhodamine B dye (RhB) and chromium (VI) (Cr(VI)) from aquatic environments. The as-synthesised exfoliated GCN (GCNX) is characterised by X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), Brunauer-Emmett-Teller analysis (BET), diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). These characterisations helped to elucidate the phase formation, chemical structure, composition, surface area, optical properties, and morphology of the sample. With assistance from a visible light source, GCNX can degrade RhB dye within 30 min in the presence of hydrogen peroxide (H₂O₂) and reduce Cr(VI) to Cr(III) in under 2 h in the presence of formic acid (FA/HCOOH). Variations in different catalytic parameters, including catalyst amount, pH of the solution, initial RhB or Cr(VI) concentration, and variation in H₂O₂ or FA concentration, are performed to inspect their effects on the photodegradation activity of GCNX. Moreover, the GCNX catalyst exhibits impressive stability and reusability. A thorough statistical evaluation follows the response surface methodology to understand the complex interaction between the factors contributing to the catalytic activity. The band alignment of differently functionalised GCN blocks in their pristine form and their H₂O₂/FA-adsorbed states is investigated using first-principles calculations to provide a further understanding of the RhB and Cr(VI) reduction mechanisms. The modified GCN can thus be effectively employed as a low-cost material for removing contamination from aquatic environments.

Preparation of MnO2-Carbon Materials and Their Applications in Photocatalytic Water Treatment

Water pollution is one of the most important problems in the field of environmental protection in the whole world, and organic pollution is a critical one for wastewater pollution problems. How to solve the problem effectively has triggered a common concern in the area of environmental protection nowadays. Around this problem, scientists have carried out a lot of research; due to the advantages of high efficiency, a lack of secondary pollution, and low cost, photocatalytic technology has attracted more and more attention. In the past, MnO₂ was seldom used in the field of water pollution treatment due to its easy agglomeration and low catalytic activity at low temperatures. With the development of carbon materials, it was found that the composite of carbon materials and MnO₂ could overcome the above defects, and the composite had good photocatalytic performance, and the research on the photocatalytic performance of MnO₂-carbon materials has gradually become a research hotspot in recent years. This review covers recent progress on MnO₂-carbon materials for photocatalytic water treatment. We focus on the preparation methods of MnO₂ and different kinds of carbon material composites and the application of composite materials in the removal of phenolic compounds, antibiotics, organic dyes, and heavy metal ions in water. Finally, we present our perspective on the challenges and future research directions of MnO₂-carbon materials in the field of environmental applications.

Keggin Cluster Modulated Photocatalytic Activity of Aryl Sulfonium Polyoxometalate Hybrids toward Dichromate Reduction

Dichromate (Cr₂O₇^2-) ion having chromium in its +6 oxidation state is a carcinogen and a potential threat to humans and aquatic life. The photocatalytic reduction of toxic Cr(VI) species into less toxic Cr(III) is an important target in heterogeneous catalysis. In this work, the catalytic activities of a series of Keggin cluster-based aryl sulfonium polyoxometalate hybrids, (FPDS)₃[PMo₁₂O₄₀] (1), (FPDS)₃[PW₁₂O₄₀] (2), (FPDS)₄[SiMo₁₂O₄₀] (3), and (FPDS)₄[SiW₁₂O₄₀] (4), toward the photocatalytic reduction of Cr(VI) have been analyzed and compared. Here, we used the aryl sulfonium counterions to modulate the POM cluster's solubility in water and stabilize the photogenerated e^--h⁺ pair on the cluster. All of the hybrids 1-4 catalyzed the reduction of Cr(VI) to Cr(III) under ultraviolet (UV) irradiation, and their photocatalytic efficiencies followed the order hybrid 1 > hybrid 3 > hybrid 2 > hybrid 4, with the rate-constant values of 0.048, 0.0056, 0.0035, and 0.0028 min^-1, respectively. Hybrid 1 with [PMo₁₂O₄₀]^3- Keggin cluster exhibited the best photocatalytic activity in the series yielding a 99% reduction in 120 min. The reasons behind the best photocatalytic activity of hybrid 1 are identified as its low band gap, less charge recombination, and fast photoresponse. The electron-trapping analyses performed using AgNO₃ revealed electrons as the main reactive species responsible for the photocatalytic reduction of Cr(VI). A plausible photocatalytic mechanism has also been proposed based on electron-trapping experiments. The present study shows that aryl sulfonium Keggin hybrids can function as efficient photocatalysts for Cr(VI) reduction, and their catalytic efficiency varies with the nature of the Keggin cluster.

Photocatalytic Reduction of Hexavalent Chromium Using Cu3.21Bi4.79S9/g-C3N4 Nanocomposite

The photocatalytic reduction of hexavalent chromium, Cr(VI), to the trivalent species, Cr(III), has continued to inspire the synthesis of novel photocatalysts that are capable of achieving the task of converting Cr(VI) to the less toxic and more useful species. In this study, a novel functionalized graphitic carbon nitride (Cu3.21Bi4.79S9/gC3N4) was synthesized and characterized by using X-ray diffraction (XRD), thermogravimetry analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), and scanning electron microscope (SEM). The composite was used for the photocatalytic reduction of hexavalent chromium, Cr(VI), under visible light irradiation. A 92.77% efficiency of the reduction was achieved at pH 2, using about 10 mg of the photocatalyst and 10 mg/L of the Cr(VI) solution. A pseudo-first-order kinetic study indicated 0.0076 min−1, 0.0286 min−1, and 0.0393 min−1 rate constants for the nanoparticles, pristine gC3N4, and the nanocomposite, respectively. This indicated an enhancement in the rate of reduction by the functionalized gC3N4 by 1.37- and 5.17-fold compared to the pristine gC3N4 and Cu3.21Bi4.79S9, respectively. A study of how the presence of other contaminants including dye (bisphenol A) and heavy-metal ions (Ag(I) and Pb(II)) in the system affects the photocatalytic process showed a reduction in the rate from 0.0393 min−1 to 0.0019 min−1 and 0.0039 min−1, respectively. Finally, the radical scavenging experiments showed that the main active species for the photocatalytic reduction of Cr(VI) are electrons (e−), hydroxyl radicals (·OH−), and superoxide (·O2−). This study shows the potential of functionalized gC3N4 as sustainable materials in the removal of hexavalent Cr from an aqueous solution.

Multicomponent Composite Membrane with Three-Phase Interface Heterostructure as Photocatalyst for Organic Dye Removal

A multicomponent composite membrane (P-S-T/C) with three-phase interface heterostructure is ingeniously designed. A polydopamine (PDA)-modified conductive carbon fiber cloth (CFC) is used as the substrate. Activated poly(vinylidene fluoride) (PVDF) with titanium dioxide (TiO2) and a silicon dioxide (SiO2) aerogel are electrospun as the top layer. The three-phase interface heterostructure was formed by TiO2, conductive CFC, and the SiO2 aerogel. Its photocatalytic performance is validated by photodegradation of organic dyes in a low-oxygen (O2) water environment. On combining with the capillary condensation of a bilayer structure, P-S-T/C exhibits excellent removal capability for anionic and cationic dyes. Moreover, P-S-T/C exhibits excellent stability and recyclability under simulated sunlight. The mechanism study indicates that the separated photogenerated carriers diffuse to the composite membrane surface rapidly on the three-phase interface of P-S-T/C. The abundant O2 adsorbed on the porous SiO2 aerogel surface acts as an electron (e-)-trapping agent, which can also decrease the work function of the composite materials. Superoxide radicals (•O2 -) play a dominant role in the reaction of photodegradation supported by a free radical-trapping experiment. This work paves a way to design a membrane with photocatalytic performance by constructing the interface heterostructure.

3D hollow MXene@ZnIn2S4 heterojunction with rich zinc vacancies for highly efficient visible-light photocatalytic reduction

Well-designed heterojunction semicounductor coupled with high-conductive cocatalyst can obtain boosted photocatalytic activity. Herein, a novel three-dimensional (3D) hollow heterojunction was prepared by coating the indium zinc sulfide (ZnIn₂S₄) nanosheets with rich-zinc vacancies (V_Zn) on 3D hollow titanium carbide (Ti₃C₂). The obtained 3D hollow heterojunction (Ti₃C₂@ZnIn₂S₄) achieved effective optical collection and promoted the separation and transmission of photogenerated carriers as well as the surface reaction of spatial separation. In addition, time-resolved photoluminescence and steady-state photoluminescence spectra indicated that the existence of V_Zn and the introduction of hollow Ti₃C₂ spherical shell effectively inhibited the recombination of photogenerated carriers and accelerated their separation and transmission, thus further enhancing the photocatalytic activity. In addition, the introduction of 3D hollow Ti₃C₂ benefited a larger specific surface area for heavy metal adsorption. Due to the unique structural and compositional characteristics, the heterojunction showed high efficiency of Cr(VI) reduction under visible light. In particular, the optimal Ti₃C₂@ZnIn₂S₄ heterojunction (1%-Ti₃C₂@ZnIn₂S₄) achieved 100% removal of Cr(VI) within 25 min, with a reaction rate constant of 0.225, which was 8.5 times higher than that of the pristine ZnIn₂S₄. The superior reusability and structural stability further indicated the MXene-based novel photocatalyst is promising for application in environmental remediation.

Photocatalytic self-cleaning properties of m-phenylene isophthalamide membranes enhanced by immobilization of GO-ZnO-Ag for dye wastewater disposal

Drawing self-cleaning technology into polymer membranes offers an extensive solution to overcome the membrane fouling obstacle. Herein, the GO-ZnO-Ag nano-hybrid particles with photocatalytic activity prepared via the microwave hydrothermal method were immobilized in poly(m-phenylene isophthalamide) (PMIA) membranes. Not only was the thorny issue in catalyst recovery avoided, but also the satisfactory photocatalytic self-cleaning performance of the polymer membrane was awarded. GO and noble metal silver nanoparticles (AgNPs) acted as a conductive electron transfer carrier and an electron host, respectively, which hindered the recombination rate of excited electrons and holes on ZnO under UV light. The UV-driven PMIA membrane photodegradation rates of MB and Cr(VI) were up to 97.2% and 94.3%, respectively, at 150 min on photodegradation kinetics. Also, MB was completely degraded at 180 min. For MB and Cr(VI) mix solutions, their degradation efficiencies were 78.1% and 71%, respectively. The BSA rejection rate of the modified polymer membrane was 93.8%, and the water flux was 408.8 (L·m−2·h−1). Permeability was maintained at a high level after repeated use. Therefore, the newly designed PMIA/GO-ZnO-Ag series membranes could expand the application of polymers in wastewater disposal industry.