Facile fabrication of B-rGO/ZnFe2O4 p-n heterojunction-based S-scheme exciton engineering for photocatalytic Cr(vi) reduction: kinetics, influencing parameters and detailed mechanism.

The fabrication of p-n heterostructures was found to be an effective strategy to stimulate the interfacial exciton shipment and photocatalytic reactions. Herein, we report a p-n junction synthesized by combining p-type boron-doped reduced graphene oxide (B-rGO) with an n-type ZnFe2O4 semiconducting material for Cr(vi) reduction under LED light irradiation. The band structures of ZnFe2O4 and B-rGO were evaluated using UV-vis spectroscopy, Mott-Schottky (M-S) plots and photocurrent studies. The results indicated that ZnFe2O4 and B-rGO exhibit a conventional type-II charge transfer, and the Fermi-level (E F) of ZnFe2O4 was found to be much lower than that of the B-rGO material. Based on these investigations, an S-scheme charge-migration pathway was suggested and demonstrated by the photocatalytic activity and nitroblue tetrazolium (NBT) chloride experiments. The optimal 2 wt% B-rGO/ZnFe2O4 heterojunction exhibits the highest photocatalytic performance, i.e. 84% of Cr(vi) reduction in 90 min under 20 W LED light irradiation with a rate constant of 0.0207 min-1, which was 4.6- and 2.15-fold greater than that of ZnFe2O4 (ZnF) and B-rGO, respectively. The intimate interfacial contact, excellent photon-harvesting properties, effective exciton segregation and availability of active electrons are some factors responsible for enhanced photocatalytic Cr(vi) reduction. In order to fulfill the demand of applied waste-water management, the influences of various photocatalyst amounts, pH values and co-exiting ions on photocatalytic activities were evaluated. Finally, this work provides a way to fabricate S-scheme-based p-n-heterostructures for photocatalytic wastewater treatment.

Facilitated Visible-Light-Driven Peroxymonosulfate Activation by a Co-Fe Layered Double Hydroxide Derived p-n Heterostructure for Sulfadiazine Degradation: Affecting Parameters, Kinetics, and Mechanistic Insights.

The utilization of multivalence ionic metal species generated through a peroxymonosulfate (PMS)-assisted photocatalytic system is a promising platform for the selective degradation of water contaminants. However, achieving an effective electron transport and enhanced separation efficiency for these metal species is a daunting challenge. Thus, our current study addresses this challenge by using a Co-Fe-based layered-double-hydroxide template to synthesize a Co3O4/FeCo2O4 p-n heterojunction composite via a simple monosynthetic route. The resultant composite is thoroughly validated through advanced characterization techniques that efficiently activate PMS for sulfadiazine (SDZ) degradation under visible light, achieving a remarkable degradation efficiency of up to 90%. This accomplishment is attributed to factors including intimate interfacial contact, excellent light harvesting, mesoporosity, and oxygen vacancies within the composite. The formation of a distinct p-n heterojunction following the S-scheme charge dynamic significantly enhances photogenerated carrier separation and reduces charge recombination. The research delves into comprehensive investigations including degradation studies, active species trapping experiments, parameter exploration, and in-depth liquid chromatography-mass spectrometry for analysis of the degradation byproducts and pathway. Induced oxygen vacancies, strategically placed active surface sites, and mesoporosity in the Co3O4/FeCo2O4 composite synergistically boosted the sluggish PMS activation, leading to enhanced SDZ degradation. This study introduces a new perspective by demonstrating the potential of a single-material, mixed-metal oxide-based p-n heterojunction photocatalytic system following the S-scheme charge-transfer route for SDZ degradation. The findings contribute toward emphasizing the importance of tailored composite materials in tackling persistent contaminants.

ZnFe2O4@WO3-X /Polypyrrole: An Efficient Ternary Photocatalytic System for Energy and Environmental Application.

Environmental protection and the necessity of green energy have become fundamental concerns for humankind. However, rapid recombination of photoexcitons in semiconductors often gets in the path of photocatalytic reactions and annoyingly suppresses the photocatalytic activity. In this study, a polypyrrole (PPY)-supported step-scheme (S-scheme) ZnFe2O4@WO3-X (PZFW15) ternary composite was fabricated by a multistep process: hydrothermal and calcination processes, followed by polymerization. During the formation of the heterojunction, the oxygen vacancy (OV) on WO3-X promotes effective separation and increases the redox power of the photogenerated excitons via the built-in internal electric field of S-scheme pathways between ZnF and WO3-X. The successful construction of the S-scheme heterojunction was substantiated through X-ray photoelectron spectroscopy, experimental calculations, radical trapping experiment, and liquid electron spin resonance (ESR) characterization, whereas the existence of OVs was well confirmed by EPR and Raman analyses. Meanwhile, the PPY served as a supporter, and the polaron and bipolaron species of PPY acted as electron and hole acceptors, respectively, which further enhances the charge-carrier transmission and separation in the ternary PZFW15 photocatalyst. The designed ternary nanohybrid (PZFW15) displays outstanding gemifloxacin detoxification (95%, 60 min) and hydrogen generation (657 µmol h-1), i.e., 1.5 and 2.2 times higher than the normal S-scheme ZFW15 heterostructure and pure ZnFe2O4 (ZnF), respectively, with an apparent conversion efficiency of 4.92%. The ESR and trapping experiments indicate that the generated •OH and •O2 - radicals from the PZFW15 photocatalyst are responsible for gemifloxacin degradation. This unique PPY-supported S-scheme heterojunction is also beneficial for the enhanced electron-transfer rate and provides abundant active sites for photocatalytic reactions.

Discriminatory {040}-Reduction Facet/Ag0 Schottky Barrier Coupled {040/110}-BiVO4@Ag@CoAl-LDH Z-Scheme Isotype Heterostructure.

Crystal facet engineering, a trending technique to acquire superior exciton pair anti-recombination and interfacial charge pair separation via an inherent functional exposed facet isotype junction, is the current research hotspot. Selectively controlling facet exposure factor with Schottky energy barrier architecture across discerned exposed functional facet attested to facilitate electron injection-separation via a shorter barrier height and closer surface distance. In this context, a {040/110}-BiVO4@Ag@CoAl-LDH Z-scheme isotype heterostructure with elevated {040} facet exposure factor tailored a {040/110} crystal facet isotype junction, and {040}-BiVO4 functional facet/metallic Ag0 nano-island semiconductor-metal selective Schottky contact was fabricated meticulously via a three-step reflux, photoreduction, followed by an in situ co-precipitation method. Inherent attribution of crystal facet isotype junction and minor semiconductor-metal Schottky barrier toward the nature of exciton pair separation and elevated photoredox activity was neatly demonstrated and well inferred, which is the novelty of the present research. The ternary isotype heterostructure corroborates impressive gemifloxacin detoxification (89.72%, 90 min) and O2 generation (768 µmol, 120 min), which are multiple folds that of respective pure and binary isotype heterostructures. The bottom-up photoredox activity was well ascribed to shorter Schottky barrier hot electron channelization provoked superior exciton pair separation and well attested via linear sweep voltammetry (315 µA), photoluminescence, electrochemical impedance spectroscopy, Bode, carrier density, and transient photocurrent analysis. The research illustrates a novel insight and scientific basis for the rational design of crystal facet isotype junction and selective Schottky contact vectorial electron shuttling promoted Z-scheme charge transfer dynamics isotype heterostructure systems toward photocatalytic energy-environmental remediation.

Bimetallic co-effect of Au-Pd alloyed nanoparticles on mesoporous silica modified g-C3N4 for single and simultaneous photocatalytic oxidation of phenol and reduction of hexavalent chromium.

Perception of surface plasmonic resonance in heterogeneous photocatalysis not only has impact on basic science of sustainable energy development, but also generates green technologies for wastewater treatment, selective oxidation and reduction reactions. In the present study Au/Pd bimetallic alloyed nanoparticles were effectively decorated on mesoporous silica modified g-C3N4 (graphitic carbon nitride) nanosheets by a simple one-pot calcinations strategy. The formation of Au/Pd alloyed nanoparticles has been supported by XRD, UV-vis DRS, TEM and XPS studies. The photocatalytic performance of the photocatalysts were investigated by performing tandem reaction for simultaneous oxidation of phenol and reduction of Cr (VI). The photocatalytic performances were found to be significant for either single phenol species or single Cr (VI), but quite appreciable photocatalytic performance was observed for a solution containing Cr (VI)-phenol mixture. The synergetic effect of Au/Pd alloyed nanoparticle and enhanced photocurrent (1.4 mA/cm2) generated by the nano-composite further supports the activity. The results of tandem reaction not only reveals the feasibility of carrying out degradation of two important pollutants simultaneously from waste water, but also gives us an enlightenment to efficiently degrade mixture of pollutants without using any additional chemical as trapping agent in the photocatalytic process.

Construction of a Z-Scheme Dictated WO3-X /Ag/ZnCr LDH Synergistically Visible Light-Induced Photocatalyst towards Tetracycline Degradation and H2 Evolution.

Herein, we have designed nonstoichiometric WO3, coupled with ZnCr layered double hydroxide (LDH) nanosheeet through Ag nanoparticle as the solid-state electron mediator to form WO3-X /Ag/ZnCr LDH Z-scheme photocatalyst. The presence of oxygen defect levels in as-synthesized materials was confirmed by Raman, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) analyses. The photocatalytic performance of the catalysts was investigated by the tetracycline degradation and H2 energy production under visible light irradiation. The WO3-X /Ag/ZnCr LDH ternary heterostructure exhibits superior activity toward tetracycline degradation and hydrogen evolution. The excellent photocatalytic performance of the catalyst was attributed to the synergistic effects among three species (WO3-X , Ag, and ZnCr LDH) and the enhanced separation efficiency of photoinduced charge carriers through the Z-scheme WO3-X /Ag/ZnCr LDH system. In addition, the created oxygen deficiency on WO3-X could improve the photocatalytic behavior of ZnCr LDH in heterostructure by delaying the recombination efficiency of photoexcited electron-hole pairs. Furthermore, the higher affinity of tetracycline at the oxygen defect levels of the photocatalyst supports the high rate of tetracycline degradation. The enhanced photocatlytic activity of the catalysts was further supported by PL spectra and photoelectrochemical studies (electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) plot). The present research opens up a new strategy for designing highly efficient visible light-induced Z-scheme-based photocatalysts with high population of active sites for energy and environmental applications in a sustainable manner.

Fabrication of a Co(OH)2/ZnCr LDH "p-n" Heterojunction Photocatalyst with Enhanced Separation of Charge Carriers for Efficient Visible-Light-Driven H2 and O2 Evolution.

Photocatalytic generation of H2 and O2 by water splitting remains a great challenge for clean and sustainable energy. Taking into the consideration promising heterojunction photocatalysts, analogous energy issues have been mitigated to a meaningful extent. Herein, we have architectured a highly efficient bifunctional heterojunction material, i.e., p-type Co(OH)2 platelets with an n-type ZnCr layered double hydroxide (LDH) by an ultrasonication method. Primarily, the Mott-Schottky measurements confirmed the n- and p-type semiconductive properties of LDH and CH material, respectively, with the construction of a p-n heterojunction. The high resolution transmission electron microscopy results suggest that surface modification of ZnCr LDH by Co(OH)2 hexagonal platelets could form a fabulous p-n interfacial region that significantly decreases the energy barrier for O2 and H2 production by effectively separating and transporting photoinduced charge carriers, leading to enhanced photoreactivity. A deep investigation into the mechanism shows that a 30 wt % Co(OH)2-modified ZnCr LDH sample liberates maximum H2 and O2 production in 2 h, i.e., 1115 and 560 µmol, with apparent conversion efficiencies of H2 and O2 evolution of 13.12% and 6.25%, respectively. Remarkable photocatalytic activity with energetic charge pair transfer capability was illustrated by electrochemical impedance spectroscopy, linear sweep voltammetry, and photoluminescence spectra. The present study clearly suggests that low-cost Co(OH)2 platelets are the most crucial semiconductors to provide a new p-n heterojunction photocatalyst for photocatalytic H2 and O2 production on the platform of ZnCr LDH.