Facile synthesis of ZnBi<Subscript>2</Subscript>O<Subscript>4</Subscript>-graphite composites as highly active visible-light photocatalyst for the mineralization of rhodamine B

Novel highly active visible-light photocatalysts in the form of zinc bismuth oxide (ZnBi₂O₄) and graphite hybrid composites were prepared by coupling via a co-precipitation method followed by calcination at 450 °C. The as-prepared ZnBi₂O₄-graphite hybrid composites were tested for the degradation of rhodamine B (RhB) solutions under visible-light irradiation. The existence of strong electronic coupling between the two components within the ZnBi₂O₄-graphite heterostructure suppressed the photogenerated recombination of electrons and holes to a remarkable extent. The prepared composite exhibited excellent photocatalytic activity, leading to more than 93% of RhB degradation at an initial concentration of 50mg·L^-1 with 1.0 g catalyst per liter in 150min. The excellent visible-light photocatalytic mineralization of ZnBi₂O₄-1.0graphite in comparison with pristine ZnBi₂O₄ could be attributed to synergetic effects, charge transfer between ZnBi₂O₄ and graphite, and the separation efficiency of the photogenerated electrons and holes. The photo-induced h+ and the superoxide anion were the major active species responsible for the photodegradation process. The results demonstrate the feasibility of ZnBi₂O₄-1.0graphite as a potential heterogeneous photocatalyst for environmental remediation.     

Investigation of lithium ion kinetics through LiMn2O4 electrode in aqueous Li2SO4 electrolyte

Spinel LiMn₂O₄ was prepared by sol–gel method and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscope. Cyclic voltammogram, galvanostatic charge/discharge testing, and electrochemical impedance spectroscopy (EIS) techniques were employed to evaluate the electrochemical behaviors of LiMn₂O₄ in 1 M Li₂SO₄ aqueous solution. Two redox couples at E _SCE = 0.78/0.73 and 0.91/0.85 V were observed, corresponding to those found at E _Li/Li ⁺= 4.05/3.95 and 4.06/4.18 V in organic electrolyte. The discharge capacity of pristine LiMn₂O₄ in aqueous electrolyte was 57.57 mAh g⁻¹, and the capacity retention of the electrode is 53.7 % after 60 cycles. Only one semicircle emerged in EIS at different potentials in aqueous electrolyte, while three semicircles were observed in organic electrolytes. There was no solid electrolyte interface film on the surface of spinel LiMn₂O₄ electrode in aqueous electrolyte. The change of kinetic parameters of lithium ion insertion in spinel LiMn₂O₄ with potential in aqueous electrolyte for initial charge process was discussed in detail, and a suitable model was proposed to explain the impedance response of the insertion materials of lithium ion batteries in different electrolytes.     

CuGaO2/TiO2 heterostructure nanosheets: Synthesis,enhanced photocatalytic performance,and underlying mechanism

Effective separation and fast transport of photogenerated carriers are vital links determining the photocatalytic performance. Heterostructure constructed by two complementary semiconductors is a feasible strategy to achieve this goal. By one-pot hydrothermal method, 0D-TiO₂ nanoparticles are loaded onto 2D-CuGaO₂ nanosheets, forming a mixed dimension, closely combined heterostructure. The photocurrent density of CuGaO₂/TiO₂ heterostructure is ∼16.6 μA/cm², which is 1.24 times higher than that of pristine CuGaO₂ nanosheets (∼13.4 μA/cm²) and 15 times higher than that of TiO₂ (∼1.1 μA/cm²). In the tetracycline hydrochloride degradation experiment, the degradation efficiency of tetracycline hydrochloride by CuGaO₂/TiO₂ heterostructure reached 99% within 90 min, which was 1.2 times the degradation efficiency of CuGaO₂ nanoparticles (82%) and 20.2 times the degradation rate of TiO₂ (4.9%). A series of experimental characterizations combined with density functional theory calculations revealed that it is the built-in electric field in the CuGaO₂/TiO₂ interface region that drives the photogenerated electron–hole pairs to travel in the opposite direction, thus inhibiting their recombination. Furthermore, the energy band offset of the CuGaO₂/TiO₂ interface makes it easier for the photogenerated holes and electrons to gather onto the valence band of the CuGaO₂ nanosheets and the conduction band of the TiO₂ nanoparticles, respectively. Therefore, appropriate interface lattice matching, suitable configuration of band gap and band edge positions, and strong opposite drive of interface electric field enable CuGaO₂/TiO₂ heterostructure to achieve wide spectral response and effective separation of photogenerated electron–hole pairs at the same time.     

Constructing magnetically recoverable CdS/CaFe2O4 P–N heterojunctions for enhanced photoelectrochemical properties towards H2 evolution reaction

A novel CdS/CaFe₂O₄ (CS/CFO) heterogeneous p-n junction was created by thermal deposition of CaFe₂O₄ nanoparticles on CdS rods. The CS/CFO hetero-structured photocatalysts exhibited increasingly efficient visible light harvesting compared to the bare CdS. The CS/CFO composites also presented higher photocurrent and slower decay of photoluminescence, suggesting a better separation of the photo-generated electrons and holes. The photocatalytic H₂ evolution quantity on the optimized CS/CFO composite from water in the presence of ethanol was up to 2200 μmol after 3-h visible light illumination, which is more than twice that of the pristine CdS. The chemical interaction between CdS and CaFe₂O₄ was confirmed by the shifts in the XPS peaks, which made it possible for the charge carriers to transfer across the p-n junction interface. This research highlights the importance of forming an interfacial p-n heterojunction between two semiconductors for efficient charge separation and improved photocatalytic performance.     

Dy2BaCuO5/Ba4DyCu3O9.09 S-scheme heterojunction nanocomposite with enhanced photocatalytic and antibacterial activities

Semiconductor heterogeneous photocatalysis has been received much attention from the scientific and researchers in the last decade. The combination of two semiconductors with various energy diagram can dramatically enhance the lifetime and separation of the charge carriers, restrain photogenerated electron-hole recombination, and considerably enhance photocatalytic performance as compared with other single or binary components. In this regard, we introduced the Dy₂BaCuO₅/Ba₄DyCu₃O_9.09 nanocomposites as active photocatalysts below UV radiation. Dy₂BaCuO₅/Ba₄DyCu₃O_9.09 nanocomposites were prepared by a simple hydrothermal method and applied as a catalyst to treat water containing organic pollutions and microorganisms. Dy₂BaCuO₅/Ba₄DyCu₃O_9.09 nanocomposites degraded Methyl Orange (MO) about 87.0% after 120 min. In addition, these nanocomposites show antimicrobial activity against Gram-positive species, including a pathogenic strain of Enterococcus faecalis, and Staphylococcus aureus, and a Gram-negative species, including Klebsiella pneumonia and Escherichia coli.     

Oxygen-defects modified amorphous Ta2O5 nanoparticles for solar driven hydrogen evolution

An oxygen-deficient amorphous Ta₂O₅ nanoparticle photocatalyst was synthesized by a solvothermal method and thermal annealing in Ar atmosphere. The as-prepared Ta₂O₅ photocatalysts possess improved hydrogen evolution activities compared to pristine Ta₂O₅ and commercial Ta₂O₅ under the irradiation of simulated solar light. Therein, Ta₂O₅ nanoparticles annealing at 400 °C exhibit the most preferable catalytic performance of 10.67 μmol g^?1 h^?1. The enhanced performance can be attributed to the existence of oxygen defects in the samples, which can facilitate the separation and migration of photogenerated carriers thereby reducing the probability of recombination.     

Aminosilicate modified zinc oxide Nanorod-GO nanocomposite for DSSC photoanodes

Amine-functionalized ZnO nanorods@graphene oxide (ZnO-NR/NH₂/GO) nanocomposites prepared by a facile solution route have been investigated through X-ray diffraction, diffuse reflectance spectra, Raman spectra, scanning electron microscopy and transmission electron microscopy. The amine-functionalized ZnO-NR/NH₂/GO-2 nanocomposite exhibits very strong visible light absorption. Dye-sensitized solar cell (DSSC) made of ZnO-NR/NH₂/GO-2 nanocomposite (with optimum 2 wt % GO) photoanode delivers a power conversion efficiency (PCE) of 3.76% which is much higher than the efficiency of unmodified ZnO-NR/GO photoanodes based DSSC (2.27%). The enhancement of PCE is primarily caused by the increased current density, attributed to the incorporation of aminosilicate and GO on the surface of ZnO-NRs which facilitates rapid transfer of electron from conduction band of ZnO to conducting surface of FTO. This diminished recombination of photogenerated electrons and holes improve the electron transfer at the photoanode/electrolyte interfaces.     

Photoelectrochemical water oxidation using hematite modified with metal-incorporated graphitic carbon nitride film as a surface passivation and hole transfer overlayer

Hematite (α-Fe₂O₃) is renowned as a promising photoanode for water oxidation, even though it displays poor photoconversion efficiency. In this study, ∼5 nm-thick graphitic carbon nitride (g-C₃N₄; CN) and metal-incorporated CN (M-CN; M = Ag, Fe, Co) films are uniformly deposited on hematite via a facile one-step evaporation method. Herein, the Co-CN layer leads to the highest photoelectrochemical activity with hematite-based photoanode. The subsequent loading of Co-CN layer with oxygen evolution catalysts (FeNiOOH and CoOOH) further enhances photocurrent density to ∼3.5 mA cm⁻² and oxygen evolution at > 95 % of Faradaic efficiency over 24 h at E = 1.23 V. Detailed analysis based on spectroscopic and electrochemical measurements demonstrate that the primary role of CN layer is improving the charge separation efficiency by passivating the hematite surface. Then the incorporated metals contribute to reducing charge transfer resistance and thereby mediating hole transfer to interfacial water.     

Graphene-Bi24O31Br10 composites with tunable architectures for enhanced photocatalytic activity and mechanism

Novel graphene-Bi₂₄O₃₁Br₁₀ (GR-BOB) composites were successfully synthesized via a simple solvothermal process for the first time. Bi₂₄O₃₁Br₁₀ nanosheets were observed to be tightly coupled with GR, and their hierarchical architectures could be freely adjusted from 3D to 2D by simply controlling the amount of graphene oxide. The photocatalytic activity of as-prepared samples was evaluated by the degradation of rhodamine B (RhB) under the visible light irradiation. All GR-BOB composites exhibited significantly enhanced photocatalytic activity, and 1.0%GR-BOB sample displayed the highest degradation efficiency, which was 2.3 times higher than that of pristine Bi₂₄O₃₁Br₁₀. The enhanced photocatalytic activity could be attributed to the enhanced light harvesting and effective separation and transfer of photogenerated carriers. GR-BOB composites also showed decent photochemical stability and recyclability, which is of especial importance for its practical applications. This work not only provided a novel functional material for environmental purification, but also a principle method to produce GR-contained composites with tight coupling in interfaces.     

Graphene-induced spatial charge separation for selective water splitting over TiO2 photocatalyst

Here we report that separately decorating TiO₂ (P25) nanoparticles and CoO_x cocatalyst on a single reduced graphene oxide (rGO) sheet results in an advanced photocatalyst, P25–rGO–Co, which exhibits enhanced photocatalytic performances for selective H₂ or O₂ evolution from water splitting. Characterization results show that rGO can not only efficiently accept and transport photogenerated electrons but also shuttle holes to CoO_x cocatalyst simultaneously, achieving an efficient spatial charge separation on P25–rGO–Co, thus improving the half-reaction efficiency.     

Gas Recombination in Sealed Ni–MHx Cells

This paper focuses on investigation of gas recombination in a positive-limited-sealed Ni–MH_x cell. The positive electrodes were prepared by electrochemical impregnation of fibrous nickel plaques. The metal hydride negative electrodes were made by pasting the mixture of rare-earth hydrogen storage alloy powders, conducting and binding agents on foamed nickel substrates. The measurement of the positive capacity at different charge times was used to estimate the partial current for oxygen evolution at the same time. The effects of charge rate, electrolyte saturation level and initial state of charge of the positive electrodes on the recombination were investigated in sealed Ni–MH_x cells. By determining the differential capacity of nickel hydroxide electrodes, an improved mathematical model was used to evaluate the gas recombination parameters during charge, overcharge, rest and discharge of the positive-limited-sealed Ni–MH_x cell. The gas recombination during rest, discharge and overdischarge was also examined. The oxygen recombination on the nickel hydroxide electrodes can be neglected due to the consumption of water when the nickel hydroxide electrodes were discharged. The longer overdischarge produced an increase in cell pressure for the sealed Ni–MH_x cell at an electrolyte unsaturated level and the evolving gas can be recombined by a following recharge operation. © 1997 SCI.     

Photocatalytic performance of ZnGa2O4 for degradation of methylene blue and its improvement by doping with Cd

ZnGa₂O₄ powder catalysts, synthesized by a sol–gel process, exhibit high photocatalytic activity for methylene blue (MB) degradation. The pH value at the point of zero charge (pH_pzc) was determined to be 5.9 by a titrations method and MB conversion increased with the increase of the pH value. The valence and conduction band positions were determined at 3.35 V and − 1.45 V (Normal Hydrogen Electrode, NHE), respectively, indicating high photocatalytic ability. Improved photocatalytic efficiency was achieved by substituting cadmium for zinc in the ZnGa₂O₄. The improvement can be attributed to high UV absorption efficiency and high separation of photogenerated electron-hole pairs.     

Synthesis of hollow TiO2@g-C3N4/Co3O4 core-shell microspheres for effective photooxidation degradation of tetracycline and MO

Traditional bulk photocatalysts often experience serious charge recombination and poor visible-light capturing, resulting in inefficient photocatalytic activity. However, proper nanostructure design usually helps to increase the activity of composite photocatalysts. Here, hollow TiO₂@g-C₃N₄/Co₃O₄ core-shell microspheres are first reported. The hollow structure of the heterostructured will directionally separate the photogenerated carriers, and the photogenerated holes transferred to the surface will be further captured by Co₃O₄ to achieve an exposed oxidized surface. The novel multi-stage hollow microspheres can simultaneously achieve effective transfer of photogenerated carriers and extended light absorption. Benefiting from these structural and compositional characteristics, the optimized TiO₂@g-C₃N₄/Co₃O₄ nanospheres have excellent photodegradation activity for tetracycline and MO. Under simulated sunlight, the degradation rates of TC (10 mg/L) and MO (25 mg/L) at 60 min are 91.6% and 97.8%, respectively. At the same time, high activity is maintained after multiple cycles of testing. Possible transfer paths for photogenerated carriers have also been proposed. This work will provide more inspiration for the design of multi-stage hollow photocatalytic systems.     

Type II Heterostructures: The Way Towards Improved Photoelectrochemical Activity of Graphitic Carbon Nitride

Photoelectrochemical (PEC) water splitting is promising approach of solar energy conversion. Graphitic carbon nitride can be given as an example of metal-free and cheap semiconducting material for photoelectrochemical reactions however its activity is very low, probably due to high rate of unfavorable charge carriers recombination. Type II heterostructure, graphitic carbon nitride–copper oxide (g-C₃N₄ and CuO), showed improved photoelectrochemical activity in comparison with neat g-C₃N₄. Visible-light irradiated composite generates cathodic photocurrents under middle bias and therefore it can be used as a photocathode for water splitting with hydrogen formation. The band bending existing in type II heterostructures drives the photogenerated electrons and holes to move in opposite directions, resulting in a spatial separation of the photogenerated charge carriers on different sides of heterojunction. Lower recombination rate and higher activity are the overall effects. Moreover, by using copper based underlayer (metallic copper) or overlayer (copper iodide) the PEC activity grows. In particular CuI@g-C₃N₄_CuO showed the highest photocurrent density and the “relax time” in dark conditions has an noticeable influence on the further increase in activity.     

Recyclable 0D/2D ZnFe2O4/Bi5FeTi3O15 S-scheme heterojunction with bismuth decoration for enhanced visible-light-driven tetracycline photodegradation

Practical application of photocatalysis is often challenged by some intrinsic issues such as recombination of photogenerated charge carriers, stability and separation, etc. Herein, bismuth decorated 0D/2D ZnFe₂O₄/Bi₅FeTi₃O₁₅ (Bi/ZF/BFT) step-scheme (S-scheme) heterojunction was fabricated by an in-situ method. Due to the advantages of structure and composition, the Bi/ZF/BFT with the desired proportion (Bi/ZF/BFT-35) exhibits favorable photocatalytic performance towards tetracycline (TC) degradation. Compared with the pure ZF, the nanohybrid shows superior stability after 5 times cycle tests. Moreover, Bi/ZF/BFT-35 is convenient to be separated from the reaction system due to its magnetic nature. As identified by ESR measurement, ?O₂^? and ?OH radicals were involved in the photodegradation of TC, which supports that the S-scheme is successfully prepared. Also, the Bi/ZF/BFT-35 shows great ability of chemical oxygen demand (COD) removal in the practical wastewater as well. Importantly, antibacterial activity against E. coli test indicates that photodegraded TC has lower biotoxicity. The present work demonstrates that cocatalyst Bi modified ZF/BFT S-scheme can not only significantly improve its stability with good recyclability from the reaction system, but also inhibits the recombination of charge carriers, giving insight on the strategy of fabricating a promising photocatalyst for practical wastewater treatment.     

Partition of bromophenol blue in toluene/water/sodium bis(2-ethylhexyl)-sulfosuccinate water-in-oil microemulsions

The partition of bromophenol blue between oil-water interfaces and water droplets in water-in-oil microemulsions of toluene/water/sodium bis(2-ethylhexyl)sulfosuccinate was studied by a spectroscopic method at different water fractions and pH. The partition equilibrium constant, K _p , between the two domains decreased considerably with an increase in the water fraction and pH. The decrease in K _p with an increase in the water fraction suggests the retention of more dye molecules in water. The decrease in K _p with an increase in pH has been attributed to a lower tendency of the base form to associate with the anionic oil-water interface compared with that of the acid form and to an increase in the negative charge density at the oil-water interface at a higher pH.     

Synthesis and characterization of BiOCl-CoWO4 nanocomposites with improved photocatalytic activity

We have synthesized novel BiOCl-CoWO₄ heterostructured nanocomposites through chemical precipitation route with different amount of CoWO₄ using KCl as Cl source at a temperature of 100°C, 4 hours. X-ray diffraction, transmission electron microscopy, UV-visible NIR spectroscopy, photoluminescence spectroscopy, N₂ adsorption-desorption isotherms, and electrochemical impedance spectroscopy were performed to gain the crystal structure, morphology, optical properties, surface area, and charge separation of the prepared photocatalysts. BiOCl-CoWO₄ composites demonstrated the diffraction peaks of both monoclinic CoWO₄ nanoparticles and tetragonal BiOCl indicating the formation of the nanocomposite. TEM observations have shown that CoWO₄ nanoparticles were deposited on the BiOCl surface. Photoluminescence, fluorescence lifetime study, and Electrochemical impedance spectroscopy responses of materials indicated a good separation efficiency of charge carriers in BiOCl-CoWO₄-1. The photodegradation efficiency of the prepared materials was assessed by the decomposition of rhodamine B (RhB) dye solution under sunlight irradiation. Among the synthesized materials, the BiOCl-CoWO₄-1 composite photocatalyst exhibited maximum photocatalytic activity. Thus the resulting heterostructure favored the efficient charge and energy transfer between BiOCl and CoWO₄ nanoparticles across the interface. The investigations from the radical scavenger tests showed that photogenerated h⁺, O₂ ^{∙ −}, and ^•OH radicals were involved in the photodegradation of RhB.     

Solar hydrogen generation from organic substance using earth abundant CuS–NiO heterojunction semiconductor photocatalyst

This work explores the critical role of NiO co-catalyst assembled on the surface of a CuS primary photocatalyst which effectively improves interface properties and enhances solar-to-hydrogen production by prolonging lifetime of photo-excitons generated at the CuS surface. The nanoscale CuS/NiO heterojunction is formulated using hydrothermal and wet impregnation methods. The resultant CuS/NiO composite shows optical absorbance between 380 and 780 nm region. The type-II energetic structure formed at CuS/NiO heterojunction facilitates rapid charge separation and as a result, the CuS/NiO composite exhibits 13 folds higher photocatalytic water splitting performance than CuO and NiO. The champion CuO/NiO photocatalyst is first identified by screening the catalysts using a preliminary water splitting test reaction under natural Sunlight irradiation. After the optimization of the catalyst, it was further explored for enhanced photocatalytic hydrogen production using different organic substances dispersed in water (alcohols, amine and organic acids). The champion CuS/NiO catalyst (CPN-2) exhibited the photocatalytic hydrogen production rate of 52.3 mmol h^?1.g^?1_cat in the presence of lactic acid-based aqueous electrolyte and, it is superior than hydrogen production rate obtained in the presence of other organic substances (triethanolamine, glycerol, ethylene glycol, methanol) tested under identical experimental conditions. These results indicate that the energetic structure of CuS/NiO photocatalyst is favorable for photocatalytic oxidation or reforming of lactic acid. The oxidation of lactic acid contributes both protons and electrons for enhanced hydrogen generation as well as protects CuS from photocorrosion. The modification of surface property and energetic structure of CuS photocatalyst by the NiO co-catalyst improves photogenerated charge carrier separation and in turn enhances the solar-to-hydrogen generation efficiency. The recyclability tests showed the potential of CPN-2 photocatalyst for prolonged photocatalytic hydrogen production while continuous supply of lactic acid feedstock is available.     

LaCoO3 acts as a high-efficiency co-catalyst for enhancing visible-light-driven tetracycline degradation of BiOI

Exploring noble-metal-free co-catalysts highly flexible for separating the photogenerated charge carriers is of prime importance for the visible-light-driven photocatalysis. Herein, three-dimensional flower-like BiOI microspheres were fabricated and applied to support noble-metal-free LaCoO₃ co-catalysts to construct a unique LaCoO₃/BiOI hybrid photocatalyst with a higher ability in charge separation. As expected, the optimum tetracycline degradation rate of LaCoO₃ (4.0 wt%)/BiOI was up to 0.0161 min⁻¹, which was nearly 3.4 fold larger than that of pure BiOI (0.0048 min⁻¹). The enhanced photocatalytic performance was mainly ascribed to the vital role of LaCoO₃ co-catalyst, which acted as an excellent electron collector for capturing the electrons generated by BiOI, effectively promoting the separation of photogenerated charge carriers and prolonging the lifetime of photo-induced electron-hole pairs at the same time. Furthermore, the active species trapping experiments revealed that the excellent photocatalytic activity was primarily driven by photogenerated holes and superoxide radicals. This work is expected to provide a new inspiration for rationally designing and fabricating noble-metal-free co-catalyst system with high efficiency applied in environment purification.     

Aerosol-assisted chemical vapour deposition of α-Fe2O3 nanoflowers for photoelectrochemical water splitting

3-dimensional (3D) nanostructures have gained broad attention in the field of microelectronics and nanotechnology owing to their fascinating properties and potential for novel applications. To enable successful fabrication of the nanostructure, deep understanding on their growth mechanism is an absolute prerequisite. In this study, thin film of hematite (α-Fe₂O₃) nanoflakes is successfully converted to nanoflowers using aerosol-assisted chemical vapour deposition (AACVD) technique simply by supplying high amount of oxygen and regulating the deposition time. The crystal structure and morphological properties including thickness and roughness of the film are thoroughly investigated to provide a clear explanation on the growth mechanism of α-Fe₂O₃ by AACVD. Results indicate that (110) crystal plane is the predominant factor that influence the formation of nanoflowers with unique pyramidal nanostructure. This structure causes the film thickness to increase linearly while the surface roughness shows a logarithmic growth trend. The samples are further employed in photoelectrochemical (PEC) water splitting as photoanode where 40 min deposition period is the optimum condition for achieving PEC photocurrent density of up to 585 μA/cm² at 1.2 V vs. Ag/AgCl. The major contributor towards the performance enhancement is the large surface area and high light absorption of α-Fe₂O₃ nanoflowers as this parameter provides greater sites for photocatalytic activity, greater charge generation and enhanced charge carrier separation efficiency.