Sodium batteries become a novel energy storage device due to their higher safety factor and superior comprehensive performance. The existing electrode materials such as Na_xCoO₂, Na_xTiO₂, NaTi₂(PO₄)₃, etc. are investigated for sodium batteries. However, these electrodes have a disadvantage of poor ion/electronic conductivity, restricting their widespread application. Na₂Ti₃O₇ is an ideal alternative titanium-based negative electrode material for sodium batteries due to its Ti³⁺/Ti⁴⁺ variability and suitable redox pairs, as well as its low voltage plateau. Embedding carbon in Na₂Ti₃O₇ electrode for sodium ion batteries can improve their conductivity and electrochemical performance, especially for the large-scale energy storage applications with different energy density and volume requirements. In this paper, Na₂Ti₃O₇/C composite anode material for sodium ion battery was prepared by a special template method, and its electrochemical characteristics were investigated.

Na₂Ti₃O₇/C composite anode material for sodium ion battery was synthesized with natural spirulina as a biological template. Spirulina was added into Na₂Ti₃O₇ precursor solution, heated and stirred in a magnetic stirrer for 1 h, and then put them in a hydrothermal reactor (with a polytetrafluoroethylene inner tank capacity of 100 mL) at 180 ℃ for 18 h. In addition, Na₂Ti₃O₇ precursor solution was also placed into a tank to hydrothermal reaction under the same condition for comparing with the template method. The final products were calcined in an air furnace (with a heating rate of 1 ℃/min from room temperature to 850 ℃) at 850 ℃ for 5 h to form the perfect crystals and remove the spirulina templates. A CR2032-typed buckle battery was assembled in a glove box (GRS-1200, Wuhan Grace New Energy Co., Ltd., China) filled in argon gas and with the synthesized Na₂Ti₃O₇/C or Na₂Ti₃O₇ electrodes as anodes, high purity sodium tablets as counter electrodes.

The microstructure of the product was characterized by a model Quanta 200 environmental scanning electron microscopy (ESEM) (FEI Ltd., Netherlands), and the components were analyzed by an EDAX spectrometer in the ESEM microscope. The crystal structure was determined by a model X'Pert PRO X-ray diffracto meter (XRD, PANalytical B.V. Ltd., Netherlands). The 2θ range of XRD is 5° to 70°. Their electrochemical properties were analyzed by a model CT2001A programmatical-controllable Land battery testing system (Hubei Lanbo New Energy Equipment Co., Ltd., China). The test parameters involved the charge-discharge capacities at a constant current (i.e., 0.5 C=125 mA/g), cycle stability and coulomb efficiency, and discharge capacities after 20 cycles at different rates (i.e., 0.1, 0.2, 0.3, 0.4 C, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0 C, and 5.0 C). The alternating aurrent (AC) impedance was analyzed via a model CS350 electrochemical workstation (Wuhan Kesite Instrument Co., Ltd., China). The test parameters are the total impedance (R₁) of electrode and electrolyte in series, electrode contact resistance (R₂), sodium ion charge transfer resistance (R₃) and semi-infinite diffusion Warburg impedance of Na⁺ in electrolyte (Ws).

The ESEM results show that Na₂Ti₃O₇/C material prepared with the spirulina biological template has a good network strip structure, with a small amount of carbon residue (the mass fraction of carbon of 1.29%). The Na₂Ti₃O₇ materials by a hydrothermal method are nano-particles with the diameters of approximately 10-30 nm. Their XRD patterns of both materials are consistent with the standard spectra of Na₂Ti₃O₇ (JCPDS 31-1329), indicating a perfect crystal structure. The initial charge-discharge capacity of Na₂Ti₃O₇/C electrode material prepared with spirulina template is 109.3 mA·h/g and 117.6 mA·h/g, respectively. The initial charge-discharge capacity is 89.3 mA·h/g and 86.3 mA·h/g of Na₂Ti₃O₇ electrode material by the hydrothermal method, respectively. After 250 cycles, the average retention rate of the charge-discharge specific capacity of Na₂Ti₃O₇/C material is 70%, which is better than that of Na₂Ti₃O₇ material (i.e., 35%), showing a better cyclic stability and a higher Coulomb efficiency. Based on the results of the specific discharge capacities at different rates, the discharge attenuation of both the materials are faster at a lower current, while they are relatively stable at a higher current, which provides a good power support for high-power hybrid electric vehicles. At the same rate, the stability of Na₂Ti₃O₇/C electrode material is better than that of Na₂Ti₃O₇ electrode material. The electrochemical impedance spectra shows that R₁, R₂, R₃ and Ws resistances of Na₂Ti₃O₇/C material are smaller than those of Na₂Ti₃O₇ material.

The specific capacity decay rate of Na₂Ti₃O₇/C electrode obtained by the spirulina template method was slower than that of Na₂Ti₃O₇ electrode obtained by the hydrothermal method after charge-discharge cycles. This was due to the nano-network strip structure prepared by natural spirulina as a template. Na₂Ti₃O₇/C electrode reduced the detachment resistance of sodium ions on the electrode surface and the diffusion resistance in the electrolyte, increased the contact area between the electrolyte and the electrode surface, thereby proved a good channel for the detachment of sodium ions, thus effectively improving the electrochemical performance of electrode materials. Moreover, the residual carbon in calcination improved its conductivity of Na₂Ti₃O₇ electrode material, and enhanced its electrochemical performance, thus providing a basic support for the development and application of novel sodium batteries.

Mirabilite as a phase change energy storage material becomes a research hotspot because of its high latent heat, low material cost, and high thermal conductivity. However, the problems of supercooling, phase delamination, and easy leakage in the phase transition cycle seriously restrict its application. Some work indicate that stereotype-phase change materials can effectively solve these problems. In this paper, crosslinked carbon nanotubes (CL-CNTs) were prepared via the Friedel-Craft alkylation and high-temperature carbonization. The CL-CNTs shaped Na₂SO₄·10H₂O based phase change materials were prepared via melting, and their properties were investigated.

Pyrene (1×10^-3 mol), 1, 2-dichloroethane (20 mL), and FeCl₃ (3×10^-3 mol) were added into 100 mL beakers. After FeCl₃ was dissolved, methyl acetal (3×10^-3 mol) was added and stirred magnetically at room temperature for 5 h. At the end of the reaction, the solution was filtered, and the solid obtained was alternately rinsed with methanol, distilled water, methylene chloride, and acetone. The cleaned solid material was extracted with methanol Soxhlet for 48 h and then dried in vacuum for 7 h. The yellow powder was heated to 650 ℃ in a tube furnace with a nitrogen atmosphere for 4 h. Finally, it was cooled to room temperature and carbonized to obtain a black powder. 4.0% NaCl, 1.0% borax, and 0.1% sodium metaphosphate were added into a mixture of Na₂SO₄·10H₂O and Na₂CO₃·10H₂O (9:1) to prepare a phase change energy storage material of mirabilite. Afterwards, CL-CNTs were added into the phase change material with a mass fraction of 0, 1%, 2%, 3%, 4%, and 5%, respectively, and the phase change material was prepared by a melting intercalation method. These samples were ultrasonically treated for 24 h in the molten state and then stirred in a mixer for 2 h, which were labeled as PCMs-0, PCMs-1, PCMs-2, PCMs-3, PCMs-4, and PCMs-5, respectively.

Most CL-CNTs prepared show a tubular structure, and a few carbon spheres grow on the carbon nanotubes. Hydrophilic CL-CNTs were prepared with hydroxyl and benzene ring groups on the CL-CNTs. Na₂SO₄·10H₂O shaped phase change materials were prepared via melting intercalation at different mass fractions of carbon nanotubes (i.e., 1%, 2%, 3%, 4%, and 5%). The samples at >4% CL-CNTs show a superior stereotyped state. According to the analysis by SEM, a uniform mixture of PCMs-4 phase change material and the crystalline particles of the mirabilite phase change material exist around the carbon nanotubes, indicating that CL-CNTs shaped Na₂SO₄·10H₂O phase change material can be prepared. The thermal conductivity of PCMs-4 is greater than that of PCMs-0, and the thermal conductivity increases with the increase of temperature at 0-20 ℃ and at 30-50 ℃. At 20 ℃, the thermal conductivity has the maximum value. At 20-30 ℃, the thermal conductivity of PCMs-0 and PCMs-4 decreases due to the heat absorption during the solid-liquid phase change of hydrated salt. Based on the DSC data of PCMs-0 and PCMs-4 before and after 500 phase transition cycles, PCMs-4 has better cyclic stability than PCMs-0. The enthalpy loss rate of melting and crystallization before and after phase transition cycles of PCMs-0 is 18% and 25%, respectively, indicating that a phase stratification occurs as phase transition cycle increases. The latent heat release of phase change decreases, and the Na₂SO₄·10H₂O-based hydrate salt gradually fails. The melting enthalpy loss rate of sample PCMs-4 before and after the phase transformation cycle is 2%, and the crystallization enthalpy loss rate is 2.3%, indicating that, the potential heat of phase transformation is not significantly reduced as the number of phase transformation cycle is increased. It is indicated that adding CL-CNTs to PCMs-4 can effectively reduce the phase stratification and improve the cycle stability of mirabilite phase change materials.

The shaped Na₂SO₄·10H₂O based phase change materials were prepared, thus effectively solving the problems of supercooling and phase stratification of materials. The sample PCMs-4 had a good shape. The thermal conductivity reached a maximum value of 1.008 W·m^-1·K^-1 at 20 ℃. There was still a high melting latent heat of 230.1 J/g and solidification latent heat of 205.2 J/g after the phase transformation cycle, indicating that the latent heat of phase transformation was not reduced as the number of phase transformation cycle was increased.

It is necessary to explore efficient and environmentally friendly energy storage devices due to the rapid consumption of fossil fuels and increasing environmental pollution. Among various energy storage devices, supercapacitors have some advantages of high power density, short charging and discharging time, good cycle stability, safety and reliability. The related studies focused on transition metal oxides and sulfides. However, their conductivity and cycle stability are rather poor. Compared with transition metal oxides and sulfides, transition metal selenides as an advanced multifunctional material dominate the field of energy storage and conversion by virtue of their high theoretical capacity, good electrical conductivity, and good cycling stability. Also, the coexistence of different metal cations with multiple valence leaps enables multiple redox reactions and enriches structural defects, and the coupling between the bimetals favors the regulation of the electronic structure and improved electrical conductivity, resulting in better electrochemical activity than monometallic selenides. Transition metal selenide electrodes for electrochemical studies are usually prepared with polymer binders by conventional slurry coating methods, which increases the dead volume of the active transition metal selenide surface and decreases the contact with the electrolyte. Therefore, transition metal selenides can be designed and prepared on a conductive substrate with an ideal porous network to obtain better charge storage performance. In this paper, NiMo precursor was firstly synthesized on a nickel foam substrate by a hydrothermal method, and then NiSe/Mo₁₅Se₁₉ electrode materials were prepared via in-situ selenation reaction of this precursor with Se powder. In addition, the effect of NiMo ratio on their electrochemical properties was also investigated.

A 2 cm × 4 cm nickel foam (NF) substrate was firstly pretreated with 2 mol/L HCl, acetone, anhydrous ethanol and deionized water, and then 6 mmol of Ni(NO₃)₂-6H₂O and 3 mmol of Na₂MoO₄-2H₂O were dissolved in 60 mL of deionized water. The dissolved mixture and 1 block of pre-treated NF were added into a PTFE-lined reactor, and the hydrothermal reaction was carried out at 150 ℃ for 5 h. The effect of nickel and molybdenum molar ratio (i.e., 1∶1 and 3∶1) on the electrochemical performance was investigated. Finally, the precursor and 0.13 g of Se powder were placed at the two ends of a quartz boat into a tube furnace in Ar gas as a protective gas at 350 ℃ for 2 h. After natural cooling, NiSe/Mo₁₅Se₁₉ was obtained. The crystal structure of the samples was determined by a model D8AA25 X-ray diffractometer (XRD, Bruker). The microstructure of the samples was observed using a model S-4800 scanning electron microscope (SEM, Hitachi Co., Japan). The morphology, lattice, and lattice spacing of the samples were analyzed by a model F30G2 transmission electron microscope (TEM, Tecnai). The molecular structure, elemental valence states and chemical bonding were analyzed by a model Xi+ X-ray photoelectron spectroscope (XPS, ESCALAB). The electrochemical properties of the samples in the three-electrode system and the two-electrode system were tested by a model CHI660E electrochemical workstation (Chenhua Co., China) under an electrolyte of 6 mol/L KOH. Three-electrode measurements were carried out using Pt as a counter electrode and saturated calomel as a reference electrode. The ASC devices were made with NiSe/Mo₁₅Se₁₉ as a positive electrode and activated carbon (AC) as a negative electrode in the two-electrode system.

The SEM images show that the Ni∶Mo molar ratio has an effect on the morphology of the samples. When the Ni∶Mo molar ratio is 1∶1, the sample morphology is the coexistence of flower-like microspheres composed of nanorods and nanosheets, and the disorganized molecular pore arrangement is not conducive to the flow of electrolyte ions. The diameter of the microspheres composed of nanosheets is approximately 1 μm, the thicknesses of the nanosheets are 30-40 nm, the diameters of the nanorods are 150-200 nm, and the length of the nanorods is approximately 2 μm. When the Ni∶Mo molar ratio is 2∶1, the morphology of the sample is nanorods, and they are grown vertically oriented on the NF substrate, with the lengths of the nanorods of 1.0-1.5 μm, and the diameters of the nanorods of 50-120 nm. At the Ni∶ Mo molar ratio of 3∶1, the morphology of the sample is flower-like microspheres composed of nanosheets, the diameters of the microspheres are 1.5-2.0 μm, and the thicknesses of the nanosheets are 20-30 nm. The electrochemical test shows that the sample has the optimum electrochemical performance when the molar ratio of Ni∶Mo is 2∶1. The specific capacitance of the sample is 940.9 F/g at the current density of 1 A/g, and the specific capacitance of the sample is 940.9 F/g when the current density increases from 1 A/g to 10 A/g. The electrode with n(Ni)∶ n(Mo) of 2∶1 still maintains a specific capacitance of 75.5% when the current density increases from 1 A/g to 10 A/g. This is since the rod-like nanostructure possesses a large specific surface area, thus providing more active sites for the electrochemical reaction and being beneficial to the energy storage device. From the TEM images, NiSe/Mo₁₅Se₁₉ nanorods have a porous structure due to the release of gases during the calcination process. This unique porous structure can provide more transport channels for electrolyte ions and a large number of active sites in the Faraday reaction.

NiSe/Mo₁₅Se₁₉ composites had an optimum electrochemical performance at a Ni∶Mo molar ratio of 2∶1. The sample-specific capacitance was 940.9 F·g^-1 at a current density of 1 A·g^-1. In addition, this electrode material exhibited good multiplication properties and cycling stability. The NiSe/Mo₁₅Se₁₉//AC hybrid supercapacitor (ASC) made from NiSe/Mo₁₅Se₁₉ and activated carbon (AC) had a high energy (i.e., 46.5 Wh·kg^-1) and a power density (i.e., 800 W·kg^-1), indicating that NiSe/Mo₁₅Se₁₉ composites have a promising application potential in energy storage devices.

The all-inorganic wide-band gap CsPbI₂Br perovskite (1.92 eV) trades off stability and light absorption performance, showing a great application potential in tandem solar cells and flexible solar cells. However, its power conversion efficiency (PCE) and stability still need to be further improved. Usually, the CsPbI₂Br perovskite films derived from solution process will inevitably generate various defects at interfaces or grain boundaries as non-radiative recombination centers for photogenerated carriers, and they are sensitive to water molecules to degrade, thus reducing the photovoltaic performance and stability of the devices. It was reported that the interface modification of functional cations (i.e., alkali metal ions, organic amine ions, etc.) and anions (i.e., halogen ions, acetic acid ions, etc.) had a positive effect on improving the efficiency and stability of perovskite solar cells (PSCs).

In this paper, a modifier tetrabutylammonium hexafluorophosphate (TBAPF₆) with both defect passivation and hydrophobic functions was used to modify the interface of CsPbI₂Br/Carbon for the improvement of the PCE and stability of hole-free carbon-based CsPbI₂Br PSCs with an architecture of indium tin oxide (ITO)/SnO₂/CsPbI₂Br/TBAPF₆/Carbon. TBAPF₆ as an ionic liquid with the Lewis base properties was used as a dopant or an interfacial modifier of perovskite films, which was conducive to passivating the defects of devices. The surface modification of CsPbI₂Br perovskite films was carried out at different concentrations of TBAPF₆, and its effect on the film quality of CsPbI₂Br perovskite and the photovoltaic performance and environmental the stability (including wet and thermal stability) of CsPbI₂Br PSCs devices was investigated.

Lead iodide (PbI₂, 99.999%, in mass fraction, the same below, Advanced Election Technology Co., Ltd.), lead bromide (PbBr₂, 99.999%, Advanced Election Technology Co., Ltd.), cesium iodide (CsI, 99.999%, Advanced Election Technology Co., Ltd.), tetrabutylammonium hexafluorophosphate (TBAPF₆, 98%, Aladdin Co.), dimethyl sulfoxide (DMSO, analytical reagent, Sigma-Aldrich Co.), tin (IV) oxide colloid (SnO₂, Alfa Co.), isopropanol (99.5%, Aladdin Co.), ITO-etched glass (square resistance 7-9 Ω/square, Advanced Election Technology Co., Ltd.), and conductive carbon paste (Huamin New Material Technology Co., Ltd.).

1.0 mol/L CsPbI₂Br perovskite precursor was prepared via dissolving CsI, PbBr₂, and PbI₂ at a molar ratio of 1.0∶0.5∶0.5 in DMSO. The ITO-etched glass substrates were cleaned with ethanol, acetone, isopropanol, and ethanol under ultrasonication for 15 min and dried in an oven, and then treated with UV ozone for 25 min. The SnO₂ colloid precursor was spin-coated onto ITO glass substrates at 3 000 r/min for 30 s and then put on a hot plate in ambient air at 150 ℃ for 30 min. After 10 min for UV ozone, 44 μL perovskite precursor solution was deposited on the SnO₂ layer through a spin-coating process at 1 000 r/min for 10 s and then 3 000 r/min for 30 s in anitrogen-filled glove box. After the spin-coating process, the obtained films were placed on a hot plate at 40 ℃ for 3 min and then annealed at 150 ℃ for 5 min. Afterwards, 45 μL TBAPF₆ solution in isopropanol (IPA) with different concentrations (i.e., 0, 0.5, 1.0 and 2.0 mg/mL) was spin-coated on the CsPbI₂Br films at 4 000 r/min for 30 s and then annealed at 120 ℃ for 10 min, respectively. Finally, the carbon electrode was deposited on top of the device by a doctor-blading method and then the prepared device was completed after annealing at 120 ℃ for 20 min.

The morphology of the absorber layers was observed by a model JSEM-5610LV field-emission scanning electron microscope (SEM, JEOL Co., Japan). The X-ray diffraction (XRD) pattern was recorded by a model AXS X-ray diffractometer (Bruker Co., Germany) with Cu K_α radiation (λ=1.54 Å). The current density-voltage (J-V) curves of devices were obtained by a model Keithley 2400 source meter under an illumination of AM 1.5G (100 mW/cm²) with cell area controlled at 0.09 cm² by a black metal mask. The ultraviolet photoelectron spectra (UPS) were determined by a model ESCALAB 250XI ultraviolet photoelectron spectroscope (Thermo Co., USA) and a model PHI 5 000 Versa Probe III (ULVAC-PHI. Inc., Japan), and the test light source energy was 21.22 eV. The UV-Vis absorption spectra were recorded by a model UV-3600plus spectrophotometer (Shimadzu Co., Japan) in a wavelength range from 300 to 800 nm at room temperature. The steady-state photoluminescence (PL) was obtained by a model FLS1000 UltraFast fluorescence spectrometer (Edinburgh Co., UK) with an excitation wavelength of 450 nm at room temperature. The time-resolved photolumine-scene (TRPL) spectra were obtained on a model Delta Flex fluorescence spectrometer (HORIBA Scientific Co., Japan) using a time-dependent single photon counting method. The electrochemical impedance spectroswpy (EIS) were carried out with a model PP211&CHI1030B electrochemical workstation under illumination with a bias at 0 V in a frequency range of 1 Hz-2 MHz.

Based on density functional theory, the Gauss View and Gaussian were used to conduct molecular modeling and electrostatic potential simulation analysis for TBAPF₆. The cation is a quaternary ammonium ion (TBA⁺) containing 4 butyl groups, and the anion is 6 fluorophosphate ion (PF₆^-). TBAPF₆ has the Lewis base properties, in which nitrogen has some uncoordinated lone pair electrons, which can effectively passivate the uncoordinated Pb²⁺ through electrostatic action. TBA⁺ can interact with negatively charged defects such as Cs⁺ vacancies through ionic and hydrogen bonds. Since anion PF₆^- is negatively charged, positively charged defects (such as halogen vacancies) are passivated by electrostatic action, and fluorine atoms on the anion also play a hydrophobic role to a certain extent.

The effect of TBAPF₆ interface modifier on the surface morphology of CsPbI₂Br perovskite films was analyzed by SEM. The surface morphology of the pristine sample is uneven and rugged. Clearly, there are many pinholes on the surface of the film and cannelures between the grains possibly due to the corrosion of water molecules on the grain boundaries. These characteristics are the main causes of leakage current. Compared with the pristine film, the surface morphology of TBAPF₆-modified film is more smooth, uniform and dense. There are few pinholes on the surface and no cannelures between grains. Therefore, TBAPF₆ modification can effectively improve the surface morphology of CsPbI₂Br perovskite films to prevent the erosion of water molecules in air, and the dense microstructure is conducive to improving the photovoltaic performance and stability of the devices. The phase composition of CsPbI₂Br perovskite films treated at different concentrations of TBAPF₆ was analyzed by XRD. Clearly, all the samples have characteristic diffraction peaks with 2θ of 14.5° and 29.6°, corresponding to the planes (100) and (200) of perovskite phase. The characteristic peak intensity increases significantly with the increase of TBAPF₆ concentration. When the concentration increases upto 1.0 mg/mL, the characteristic diffraction peak intensity reaches the maximum value. However, the characteristic peak intensity decreases when the concentration of TBAPF₆ further increases.

The pristine device has a PCE of 9.93%, with an open circuit voltage (V_oc) of 1.16 V, a short circuit current density (J_sc) of 14.02 mA/cm², and a fill factor (FF) of 60%. As TBAPF₆ concentration increases from 0 to 1.0 mg/mL, the device with an architecture of ITO/SnO₂/CsPbI₂Br/TBAPF₆/carbon exhibits the maximum PCE of 12.04%, with V_oc of 1.24 V, J_sc of 14.60 mA/cm², and FF of 67%. To test the repeatability, 30 devices based on different concentrations of TBAPF₆ were set up. The overall PCE distribution of the modified device is significantly greater than that of the pristine device. The PCE distribution of the modified device at 0.5 mg/mL is mainly in a range of 11.0%-11.5%, and some can reach more than 11.5%. The PCE of 1.0 mg/mL modified device increases, mainly distributes 12%, which is the optimum overall distribution of PCE (i.e., 12.04%). The PCE of 2.0 mg/mL modified device is mainly distributed in a range of 11.0%-11.8%, which decreases slightly. Compared with the optimized device, the PCE of the pristine device is mainly distributed in a range of 7%-10%. Therefore, 1.0 mg/mL TBAPF₆ is an optimal concentration of the interface modifier.

The optical band gap (E_g) of the pristine and 1.0 mg/mL TBAPF₆-modified CsPbI₂Br perovskite films is 1.92 eV and 1.91 eV, respectively. The Fermi level (E_f) of the pristine and modified samples can be calculated as -5.23 eV and -5.25 eV, respectively, which drops from -5.23 eV to -5.25 eV before and after modification, and is closer to the valency band, indicating that CsPbI₂Br perovskite film changes from N-type to P-type semiconductor. The valence maximum (E_v) moves up by 0.05 eV, which reduces the energy level shift of the CsPbI₂Br/carbon electrode interface, thus effectively reducing V_oc loss and promoting the hole transfer between perovskite and carbon electrode.

At last, in the humidity environment of 35%-40%, 80% of the initial efficiency of the optimal 1.0 mg/mL TBAPF₆ modified device is maintained after 16 h, and in air at 85 ℃ after 72 h, 70.2% of the initial efficiency is maintained, indicating that the moisture and thermal stability are greatly improved, compared with the pristine device.

The results showed that TBAPF₆ interface modification agent could effectively passivate the surface defects of CsPbI₂Br perovskite films, reduce the non-radiative recombination of carriers, improve the energy level arrangement of CsPbI₂Br/carbon electrode interface, and promote the carrier transport. It played a hydrophobic role. Thus, the PCE and stability of all inorganic hole-free carbon-based CsPbI₂Br PSCs were improved. The microstructure of TBAPF₆ modified CsPbI₂Br films was more smooth and dense with a higher crystallinity. The E_g decreased slightly, and the light absorption capacity increased. The E_f was closer to the valence band, indicating the transition from N-type to P-type semiconductor. The upward movement of E_v was conducive to the hole transfer at the CsPbI₂Br/carbon electrode interface. The steady-state photoluminescence intensity was more intense, indicating a longer average carrier lifetime. The decrease of dark current and the increase of recombination resistance indicated that the defects of the modified device were effectively passivated, and the probability of carrier non-radiative recombination was reduced. The molecular modeling and electrostatic potential simulation showed that PF₆^- of TBAPF₆ passivated the positively charged halogen vacancies through electrostatic action, while TBA⁺ passivated the negatively charged Cs⁺ vacancies through ionic and hydrogen bonding. Some functional groups with long alkyl chains and fluorine atoms effectively prevented the intrusion of water molecules in air, thus improving the wet stability of the devices. The optimal 1.0 mg/mL TBAPF₆ modified device exhibited a champion PCE of 12.04%, with V_oc of 1.24 V, J_sc of 14.60 mA/cm², and FF of 67%, demonstrating the superior moisture and thermal stability, compared with the pristine device.

Graphite carbon nitride (g-C₃N₄) composite manganese cobalt based prussian blue (MnCoPBA/g-C₃N₄) catalyst was prepared by a self-assembly method, a coprecipitation method and a high-temperature annealing method. The catalyst prepared was used to analyze the catalytic degradation of rhodamine B (RhB) in the SR Photo Fenton like system. The MnCoPBA/g-C₃N₄ catalyst has a degradation efficiency of 96% for RhB within 10 minutes and exhibits a good cyclic stability. The catalyst also has a good catalytic degradation performance for RhB at pH values of 1-9, indicating a superior applicability in acidic, neutral, and weakly alkaline environments.

Catalytic performance test is an important indicator for evaluating MnCoPBA/g-C₃N₄ catalysts. A certain amount of MnCoPBA/g-C₃N₄ (0.05-0.20 g/L) catalyst was added into 100 mL of RhB (5-20 mg/L) solution. The suspension was stirred magnetically for 30 min to achieve adsorption-desorption equilibrium. Subsequently, 3 mL of suspension was taken and centrifuged to add the supernatant to a colorimetric dish. The data were recorded using a UV spectrophotometer. Subsequently, in a xenon lamp (350 W, >420 nm), a certain amount of PMS (5-60 mg/L) was added and 3 mL of suspension was treated by centrifugal force every 5 minutes to obtain the supernatant. The data were recorded in a UV spectrophotometer until the solution become colorless and the UV absorption spectrum data unchanged.

The cyclic stability of catalysts is an important indicator for evaluating their practical application. To reduce the possible experimental errors caused by catalyst loss in the cyclic experiment, the cyclic experiment was designed as a parallel experiment, where each cyclic experiment was conducted independently. Sampling was no longer conducted during the n-1st cycle before the n-th cycle experiment, and sampling was conducted again during the n-th cycle experiment. The experimental operation was the same as the catalytic experiment. The waste liquid was collected and centrifuged, and the product was washed and dried with deionized water and ethanol. The UV spectrophotometer was used to test the data and collect records.

Based on the analysis of the results, a possible catalytic degradation mechanism for the Photo-Fenton like system was proposed. The photocatalytic excitation of g-C₃N₄ semiconductor material in the Photo-Fenton like system generates photo- generated electrons and hole pairs, which are captured by metal ions in MnCoPBA and undergo a reduction reaction, achieving an effective separation of photo-generated carriers in the photocatalytic system and overcoming the defect of high photo-generated carrier recombination efficiency in g-C₃N₄ photocatalytic system. Simultaneously achieving a closed loop cycle of Fe(Ⅲ)/Fe(Ⅱ), Mn(Ⅲ)/Mn(Ⅱ) and Co(Ⅲ)/Co(Ⅱ) ions in the Fenton like system is obtained. An energy support for the Fenton like system is provided. The rate limiting step is broken through, and the defect of ion deficiency in the Fenton like system is overcome. Also, photo-generated electrons can directly activate PMS to generate SO₄^·-, triggering chain reactions to produce ·OH, and reacting with O² to form ·O₂^-. Various strong oxidizing radicals efficiently degrade RhB, thus improving the catalytic degradation performance.

MnCoPBA/g-C₃N₄ catalyst was prepared by a coprecipitation method combined with a high-temperature calcination method. The effect of catalytic condition on the catalytic degradation performance of RhB was investigated. The results indicated that when MnCoPBA/g-C₃N₄-2.5 had the optimal catalytic degradation efficiency for RhB under the optimum condition (i.e., the catalyst dosage of 0.1 g/L, RhB concentration of 10 mg/L, PMS dosage of 0.15 g/L, and pH values of 1-9). The catalytic degradation mechanism indicated that Mn and Co metal ions in MnCoPBA acted as mediators for photocatalytic and the Fenton like connections, capturing photo-generated electrons to slow down the rate of photo generated electron hole recombination. Also, the reduced metal ions could promote the Fenton like reaction to decompose PMS, producing the more intense oxidizing free radicals and accelerating the catalytic degradation of RhB. The catalytic degradation efficiency of RhB was 90% within 2 min, and 96% within 10 min. In addition, the catalyst also exhibited a good cycling stability.

Photocatalytic oxidation degradation of cyanide is an effective technology to treat cyanide wastewater. TiO₂/SiO₂ is a common catalyst. A preliminary research indicates that the pore structure of SiO₂ has a certain influence on the degradation of cyanide and the removal of zinc and copper. If TiO₂ particles are only loaded on the surface of mesoporous SiO₂, the contact between the surface of SiO₂ and the target substance in wastewater is reduced, thus affecting its adsorption performance. In this study, a macroporous SiO₂ was prepared, and TiO₂ particles were loaded on the surface and in three-dimensional interconnected channels of SiO₂, thus increasing the adsorption sites on the surface of SiO₂ and improving the adsorption performance of target pollutants, and enhancing its efficiency of treating cyanide in wastewater.

Gravity self-assembled polystyrene microspheres (PS) were used as hard templates, impregnated in silica sol for 8.0 h, dried in an oven at 60 ℃, and roasted in a Muffle furnace at 550 ℃ for 10 h to obtain macroporous SiO₂. SiO₂/TiO₂ precursors were prepared by a sol-gel method and roasted at 500 ℃ for 90 min. SiO₂/TiO₂ composites with a three-dimensional interconnected macroporous structure were prepared. The wastewater and materials were placed and mixed in a beaker in a certain proportion. After stirring magnetically for a period of time, a small air pump was used to pump air into the bottom of the beaker at room temperature, and a high-pressure mercury lamp was used as the light source for photocatalytic oxidation of the wastewater. The effects of TiO₂ particle loading mass, catalyst dosage, catalytic time, adsorption time on the treatment of cyanide wastewater were investigated. N₂ adsorption-desorption, scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR) and other characterization techniques were used to determine the structural characteristics of the material and its adsorption-catalytic degradation mechanism.

PS presents a monodisperse, independent and spherical structure with a particle size of 1.65 μm. PS templates are arranged in an orderly manner, independent of each other but closely connected, and they do not show a large continuous structure. SiO₂ is a three-dimensional ordered macroporous structure with a pore size of 1.2 μm, which is smaller than the diameter of microspheres, indicating that the skeleton structure shrinks during the roasting process. SiO₂ particle is broken mainly because of mechanical grinding. The long distance of the channels is orderly, the large holes are uniform and interconnected through small holes. The sizes of the small holes are 170-220 nm, the holes are formed due to the accumulation of microspheres. TiO₂ particles are fragmented and loaded on the concave surface of SiO₂. The particle size of TiO₂ is uneven, and smaller than the pore size of SiO₂. TiO₂ particles are evenly dispersed on the surface of SiO₂ or in the physical structure of the cavity, which eliminates the agglomeration of TiO₂. N₂ adsorption-desorption test shows that the SiO₂/TiO₂ material has a large pore structure, and the specific surface area is increased by 5 times, compared with TiO₂ nanoparticles. The thermogravimetric curve shows that the materials have less weight loss at 250-800 ℃. The X-ray diffraction pattern of the material shows that the diffraction peak intensity of SiO₂ loading with TiO₂ particles decreases. The experimental results show that the degradation efficiency of total cyanide and the removal efficiencies of copper, iron and zinc are 98.79%, 99.10%, 100.00% and 92.26%, respectively, under the optimum conditions (i.e., 10% of TiO₂ particle loading mass, 0.35 g of the material mass when treating 100 mL cyanide wastewater, 1.0 h of dark adsorption and 4.0 h of illumination time). The material was regenerated in a sulfuric acid solution by ultrasonic treatment. Under the same experimental conditions, the total cyanide degradation efficiency is reduced by 4.95% after three catalytic cycles, indicating that the material has strong stability and reusability. The XRD results before and after adsorption-catalysis show that ZnO, CuSCN, Cu₂Fe(CN)₆ and Fe₂O₃ are adsorbed on the surface of the material. The FTIR results show that a characteristic peak of C≡N appears on the surface of the material after 1-h adsorption and a characteristic peak of NO₃^- appears after 4-h catalysis. The EDS of the material after 1-h adsorption shows that cyanide is adsorbed on the surface of the material, and metal ions are uniformly distributed on the surface of the material. The EDS of the material after 4-h illumination shows that the metal content is higher than that after 1-h adsorption, and the distribution of Si and Ti is uniformly crossed. The results of free radical quenching experiment show that electron capture by oxygen is the main factor to improve the catalytic effect.

Macroporous SiO₂/TiO₂ materials were prepared by PS self-assembly template, impregnation, roasting and loading, which were used to treat cyanide wastewater. Under optimal experimental conditions, the degradation efficiency of total cyanide and the removal efficiencies of copper, iron and zinc were 98.79%, 99.10%, 100.00% and 92.26%, respectively. The three-dimensional ordered pore structure of macroporous SiO₂ made TiO₂ particles evenly dispersing on its surface and inside the cross-linked pore. The mechanism indicated that the cyanide was degraded into non-toxic nitrogen oxides and carbon oxides, and the metals were removed by precipitation.

Hydrogen peroxide (H₂O₂) is an important chemical that can be utilized as oxidant and fuel. Photocatalytic technology based on semiconductors can produce H₂O₂ from water and oxygen with sustainable solar energy as a sole energy input, which is a promising approach for industrial application. Nevertheless, the photoactivity of the common catalysts (i.e., graphitic carbon nitride, metal oxide, metal-organic frameworks, etc.) is low due to the poor utilization of solar light, easy recombination of electron-hole pairs, small quantity of reactive sites and weak redox ability. Developing efficient photocatalytic systems thus becomes an urgent target. Metal sulfides can be classified to single metal sulfide (i.e., CdS, In₂S₃, etc.), bimetallic sulfide (i.e., ZnIn₂S₄、CdIn₂S₄, etc.) and trimetallic sulfide (i.e., Cu₂ZnSnS₄). Sulfides exhibits the suitable band structures that can harness visible light and possess the proper redox capability. Meanwhile, the high structural symmetry enables them ultrafast charge carries transportation. The band structure of the semiconductor is a prerequisite both for optical absorption and redox ability. For photocatalytic H₂O₂ production, oxygen reduction reaction (ORR) and water oxidation reaction (WOR) channels can synergistically achieve the maximum activity. The ORR requires a potential that is more negative than -0.33 V vs. normal hydrogen electrode (NHE). The WOR needs a potential that is more positive than +1.78 V vs. NHE. The larger bandgap can induce the limited photon absorption. Thus, modulating the band structure of the metal sulfide through changing the metals ratio is an effective strategy to boost the photoactivity.

In_xSn_5-xS₈ samples were prepared through a hydrothermal approach. A certain amount of InCl₃·4H₂O and SnCl₄·5H₂O were dissolved into deionized water and then added L-Cysteine into the solution. Thereafter, the suspension was treated by a hydrothermal method at 180 ℃ for 24 h. In₂S₃ and SnS₂ were prepared by the same procedure without the addition of SnCl₄·5H₂O or InCl₃·4H₂O. The XRD patterns were characterized by X'Pert³MRD. The Raman spectra were obtained by LabRAM HR Evolution with 532 nm laser. The morphology structure was detected by a modelGemini SEM 500 scanning electron microscope and a modelJEM-2100 transmission electron microscope. The specific surface area and pore size were analyzed by a model ASAP2460BET instrument. The XPS spectra were obtained by an X-ray photoemission spectroscope with Al K_α excitation. The ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) was determined by an ultraviolet-visible absorbance spectroscope. The photoluminescence (PL) and time-resolved photoluminescence (TR-PL) spectra were collected on Horiba Fluorologat 375 nm. The transient photocurrent, electrochemical impedance spectra and Mott-Schottky plots were measured on a modelDH7000 electrochemical workstation.

Photocatalytic H₂O₂ production was evaluated in a sealed three-neck round bottom flask with the mixture of deionized water and isopropanol under visible light illumination (λ≥420 nm). After purging with oxygen for 30 min, the reaction system was placed under Xenon lamp and then extracted the suspension at certain time interval. The concentration of H₂O₂ was estimated by iodometry methods. The reaction pathway was investigated by scavengers experiment and changing the gases.

The as-prepared In₄SnS₈ nanomaterials have a typical cubic structure with the flower-like morphology that is composed of ultrathin 2D nanosheets with the thicknesses of 5-10 nm. Apristine In₂S₃ presents a 3D solid sphere with the diameters of 4-7 μm, and SnS₂ has a plate-like structure with a lateral size of ~1 μm. The hierarchical structure of In₄SnS₈ endow it with the maximum specific surface area of 648.056 m²/g, benefiting for the surface photoreaction. Nevertheless, pure In₂S₃ and SnS₂ possess the small specific surface area of 70.631 m²/g and 30.411 m²/g. According the UV-Vis DRS, the bandgap of In₂S₃, In₉SnS₁₆, In₄SnS₈, In₃Sn₂S₈, In₂Sn₃S₈ and SnS₂ is 2.03, 1.98, 2.16, 1.87, 1.80 and 1.78 eV, respectively. According to Mott-Schottky (MS) plots, their conductive band (CB) potential is calculated to be -0.24、-0.26、-0.39、-0.24、-0.22 and -0.31 eV, respectively. Their corresponding valance band (VB) is thus 1.79、1.72、1.77、1.63、1.58 and 1.47 eV, respectively. CB and VB both for In₄SnS₈ can meet the requirement of ORR and WOR channels for H₂O₂ evolution. While other photocatalysts can only undergo the single pathway of direct two-electron reduction reaction.

Under visible light illumination, In₄SnS₈ nanomaterials have the maximum photoactivity with a H₂O₂ production rate of 1.936 μmol·L^-1·min^-1, which is 5.2- and 71.7- fold greater than that of pristine In₂S₃ and SnS₂. They also present a good stability after 4 cycles experiments. The photo-reactivity of In₉SnS₁₆, In₃Sn₂S₈ and In₂Sn₃S₈ dramatically decreases in N₂ atmosphere, indicating that ORR is a dominant pathway in the three systems above. For In₂S₃ and In₄SnS₈, the concentration of H₂O₂ is decreased by 30%-50% in N₂, implying that WOR pathway is also responsible for H₂O₂ evolution. The trapping agent experiment demonstrates that 2e^- WOR, direct one-step two-electron ORR and indirect sequential two-step single-electron ORR all exist in In₄SnS₈ system. Among all the photocatalysts, In₄SnS₈ exhibits the maximum photocurrent of 0.25 mA·cm^-2, the minimum interfacial electron transfer resistance, the lowest photoluminescence signal and the shortest charge carrier lifetime, indicating that the electron-hole pairs in In₄SnS₈ can efficiently separate and migrate to the surface, then boost the photo-reactivity.

A series of In_xSn_5-xS₈ materials were prepared through a hydrothermal process. In₄SnS₈ nanomaterials exhibited the superior photocatalytic performance and its visible-light-driven H₂O₂ production rate was 1.936 μmol·L^-1·min^-1, which was 5.2- and 71.7- fold greater than that of pristine In₂S₃ and SnS₂. The band structure analysis demonstratedthat the bandgap and band position of In_xSn_5-xS₈ could be controlled by In/Sn molar ratio. In₄SnS₈ had the proper bandgap of 2.16 eV and its CB and VB lied at -0.39 and 1.77 eV, thus satisfying the potential of two independent pathways for H₂O₂ generation, i.e., oxygen reduction reaction and water oxidation reaction. In addition, the hierarchical nanoflower-like structure of bimetallic In₄SnS₈ nanomaterials could also provide the more reactive sites, which were responsible for the improved photocatalytic performance.

Photocatalysis is one of the most potential techniques to solve energy crisis and environmental pollution problems, and how to obtain photocatalysts with a greater catalytic efficiency is always a research hotspot in the photocatalysis. The heterogeneous photocatalyst is proved as a promising efficient strategy to address this issue. The heterojunction photocatalysts can be utilized to expedite the separation of photo-induced electron-hole pairs. In particular, Z-scheme heterojunctions exhibit the dual advantages of suppression of recombination of electron-hole pairs, reserving a high redox ability for both semiconductors. The visible-light-driven photocatalysts Bi₂WO₆ and g-C₃N₄ exhibit a poor photocatalytic activity because of their fast recombination of photo-generated carriers. It is advisable to choose the g-C₃N₄ and Bi₂WO₆ to form an all-solid-state Z-scheme photocatalytic system to obtain the higher photocatalytic performance. It is reported that carbon materials often put up a low work function when they modify the electronic and optical properties of hybrids. In this paper, a novel amorphous carbon/g-C₃N₄/Bi₂WO₆ (C/C₃N₄/Bi₂WO₆) Z-scheme heterojunction with a visible light response was synthesized. The performance of C/C₃N₄/Bi₂WO₆ was measured by photo-degrading TC under the visible light (i.e., ≥420 nm). The as-synthesized composites played an enhanced visible-light photoactivity than the pure g-C₃N₄ and Bi₂WO₆. Furthermore, the possible mechanism of the enhanced performance of the C/C₃N₄/Bi₂WO₆ composite was discussed.

All reagents with analytical grade were used without further purification. First, the C/C₃N₄ sample was prepared by a milling/roasting method. When a mass ratio of glucose to urea was 0.1%, the as-fabricated C/C₃N₄ composite exhibited an optimum photocatalytic performance. The C/C₃N₄/Bi₂WO₆ nanocomposites were prepared through a facile hydrothermal method. The obtained products were denoted as C/C₃N₄/Bi₂WO₆-1, C/C₃N₄/Bi₂WO₆-2, C/C₃N₄/Bi₂WO₆-3 and C/C₃N₄/Bi₂WO₆-4, when the mass ratios of C/C₃N₄∶Bi₂WO₆ were 1.0∶0.1, 1.0∶0.4, 1.0∶0.7 and 1∶1, respectively. For the comparison, a pure Bi₂WO₆ sample was synthesized through the same procedur without introducing C/C₃N₄.

The crystal structure was detected by X-ray diffraction with Cu K_α radiation (λ=0.154 06 nm). The morphology and microstructures of the synthesized samples were investigated on high-resolution transmission electron microscopy (HRTEM). The ultraviolate-visible (UV-Vis) diffuse reflectance spectra (DRS) were taken on UV-Vis-near infrared (NIR) spectrophotometry with BaSO₄ as a reference. The chemical composition and chemical state of C/C₃N₄/Bi₂WO₆ were analyzed by X-ray photoelectron spectroscopy (XPS) with Al K_α excitation source. The photocurrent response under simulated solar light irradiation was recorded with an electrochemical workstation in a sandwich-type configuration, a Pt slice as a counter electrode, a saturated calomel electrode (SCE) as ae reference, and 0.1 mol/L Na₂SO₄ solution as electrolyte. A 300 W xenon arc lamp equipped with a simulated solar light filter calibrated to 100 mW/cm², which was measured with a radiometer, employed as a light source. Electrochemical impedance spectroscopy (EIS) Nyquist plots were obtained at 0.6 V with a small alternating current amplitude of 5 mV in a frequency range of 0.1 Hz-10⁵ Hz.

The photocatalytic activity of the C/C₃N₄/Bi₂WO₆ sample was determined by degrading the simulated pollutant tetracycline (TC). In the experimental setup, a 300W Xe lamp was employed as a light source and a 420 nm cut-off filter was used to provide only visible-light irradiation. The photocatalyst of 100 mg was added to 100 mL of TC solution (1×10^-4 mol/L). Before being irradiated, the suspensions were magnetically stirred in the dark for 3 h to reach the adsorption-desorption equilibrium between photocatalysts and TC. The solution was exposed to visible light irradiation under magnetic stirring. The aliquot of 3 mL was collected from the suspension and centrifuged immediately for every 10 min. The degradation of TC was monitored by checking the absorbance at 357 nm using an UV-Vis spectrometer.

The degradation ratios of TC are 25% and 50% after irradiation for 240 min by visible-light in the presence of g-C₃N₄ and Bi₂WO₆, respectively. Furthermore, it is clear that the C/C₃N₄ composite exhibits a higher photocatalytic activity in the degradation of TC than the g-C₃N₄ sample. All the C/C₃N₄/Bi₂WO₆ photocatalysts exhibit better photocatalytic performance than the C/C₃N₄ composite and Bi₂WO₆ under the same experimental condition. Specifically, the photocatalytic activities of C/C₃N₄/Bi₂WO₆ nanocomposites gradually improve as Bi₂WO₆ ratio increases, and the photocatalytic performance is optimal when the mass ratio of C/C₃N₄∶Bi₂WO₆ is 1.0∶0.7. The photocatalytic activities decrease when Bi₂WO₆ ratio continues to increase. Hence, the as-prepared C/C₃N₄/Bi₂WO₆-3 nanocomposite has the optimum photocatalytic performance, and the maximum degradation efficiency of 97.1% is attained after the irradiation for 240 min.

The improved photocatalytic performance of C/C₃N₄/Bi₂WO₆ composites is ascribed to the formed amorphous carbon-mediated Z-scheme heterojunction. When the heterojunction C/C₃N₄/Bi₂WO₆ is exposed to visible light (λ>420 nm), the electrons generated from CB of Bi₂WO₆ can transfer to the VB of g-C₃N₄ via the amorphous carbon mediator due to the good electrical conductivity of amorphous carbon, and then annihilate with the holes induces from the VB of g-C₃N₄, the photo-induced carriers can be effectively separated.

We constructed Z-scheme C/C₃N₄/Bi₂WO₆ heterojunction photocatalyst with enhanced photocatalytic activities in degradation of TC. When the mass ratio of C/C₃N₄∶Bi₂WO₆ was 1.0∶0.7, the as-fabricated C/C₃N₄/Bi₂WO₆ heterojunction exhibited an optimum photocatalytic performance. The mechanism illuminated that the efficient separation of charge carriers generated by the amorphous carbon-mediated Z-scheme mechanism mainly contributed to the improved photocatalytic performance of C/C₃N₄/Bi₂WO₆ heterojunction. We anticipated that the results of this work could favor the application of solar energy in water pollution.

Dyeing wastewater is one of environmental pollutions, having a serious threat to the environment and human-being health. Therefore, the purification of dyeing wastewater becomes a challenge. Among various methods for treating dyeing wastewater, photocatalysis is developed as an efficient environmental pollution control technology due to its mild reaction conditions, sustainable driving energy sources, and no secondary pollution after the reaction. Zinc tungstate (ZnWO₄) as a semiconductor photocatalyst material with stable physical and chemical properties has a promising application potential in photocatalytic removal of environmental pollutants. However, ZnWO₄ has a wide bandgap and a low migration efficiency of carriers, resulting in unsatisfactory photocatalytic intrinsic activity. Therefore, researchers attempted to optimize the photocatalytic performance of ZnWO₄ via regulating its morphology, size and crystallinity. Sonochemical method is a highly concerned method for synthesizing nanomaterials. Under ultrasonic cavitation conditions, the physical and chemical environment of the reaction system will undergo significant changes, i.e., the instantaneous generation of high temperature, high pressure, and strong shock waves, which are beneficial for strengthening the fracture of chemical bonds and the generation of new chemical bonds, thereby regulating the morphology and structure of material. In this work, ZnWO₄ with oxygen defects nanomaterials were prepared by a sonochemistry-assisted hydrothermal method, and the influence of sonochemical treatment time on the photocatalytic activity of ZnWO₄ was investigated as methylene blue was used as a target pollutant.

ZnWO₄ with oxygen defects nanomaterials were prepared by a sonochemistry-assisted hydrothermal method. In the preparation, 0.297 g Zn(NO₃)₃·6H₂O was dissolved in 8 mL distilled water under constant stirring, to obtain solution A. Also, 7 mL distilled water containing 0.392 g Na₂WO₄·2H₂O was added in dropwise to solution A and mixed by stirring for 30 min. Afterwards, LC-UP-400 ultrasonic signal connected Φ10 luffing rod (selecting ultrasonic power was 50%, ultrasonic treatment for 2 s, and stay for 2 s) was used to irradiate the white suspension for 1h, and then the suspensions were put into Teflon-lined autoclave, sealed and maintained at 140 ℃ for 20 h. After cooling to room temperature, the product was washed with distilled water and ethanol for several times. The final products were obtained after drying at 60 ℃ for 12 h. The samples of ZnWO₄ nanomaterials treated at different ultrasonic treatment time (X min) were named as ZnWO₄-X.

The photocatalytic activity of as-prepared samples was evaluated by MB photocatalytic removal in 10 mg/L of MB dyeing wastewater. Herein, MB dyeing wastewater under light illumination without the addition of photocatalysts as a blank test for comparison, having an unsatisfactory photoreduction effect. After 40 min of illumination, ZnWO₄ has a weak photocatalytic activity for MB photodegradation, i.e., 52% of MB is photocatalytically degradated by pure ZnWO₄. However, the induction of oxygen vacancy defects results in an enhanced photocatalytic performance for MB removal over ZnWO₄-5 min. The optimized ZnWO₄-5 min displays a superior photocatalytic activity for MB removal, which can remove 95.0% of MB after 40 min of illumination. The kinetic of MB photodegradation was further analyzed by plotting the -ln(c/c₀) as a function of irradiation time. The calculated rate constant k of ZnWO₄-5min is 0.067 min^-1, which is 4.5 times greater than that of ZnWO₄. The reusability of ZnWO₄-5min was investigated by performing 5 consecutive cycles for MB photodegradation. The results show that the photoreduction efficiency maintains at a high level (i.e., >90%) after five cycling experiments, indicating the superior stability and huge potential value of ZnWO₄-5 min for the treatment of dyeing wastewater. The oxygen vacancy defects induced by ultrasound cause the lattice contraction within ZnWO₄, which is beneficial for optimizing electronic structure, exposing more catalytic active sites, broadening the photocatalytic response range, and improving the migration efficiency of photogenerated carriers, thereby significantly enhancing the photocatalytic degradation performance of ZnWO₄ in dyeing wastewater.

ZnWO₄ with oxygen defects nanomaterials were prepared by a sonochemistry-assisted hydrothermal method, and the influence of sonochemical treatment time on the photocatalytic activity of ZnWO₄ was investigated when MB was used as a target pollutant. The results showed that sonochemical treatment could induce the formation of oxygen vacancy defects in ZnWO₄ nanoparticles, thereby optimizing electronic structure, exposing more active sites, broadening the light response range and improving the transfer efficiency of photogenerated carriers, thus enhancing the photocatalytic degradation performance of ZnWO₄ for dyeing wastewater. The optimized ZnWO₄-5 min displayed a superior photocatalytic activity for MB removal, which removed 95.0% of MB after 40 min of illumination. The calculated rate constant k of ZnWO₄-5 min was 0.067 min^-1, which was 4.5 times greater than that of ZnWO₄. Furthermore, ZnWO₄-5min displayed a superior photocatalytic stability, and the photoreduction efficiency maintained at a high level (>90%) after five cycling experiments. This indicated that sonochemically-induced oxygen vacancy defects be an effective method to improve the photocatalytic performance of ZnWO₄. This work can provide some ideas for the preparation of high-performance semiconductor photocatalytic materials for the treatment of dyeing wastewater.

Antimony mainly exists in the oxidation states of Sb(III) and Sb(V), where Sb(V) is the predominant species and exists as Sb(OH)₆^- in oxidic environments. The environmental contamination of Sb has received much attention due to the toxicity and adverse effect on the hematic, gastrointestinal, and respiratory systems of human-beings. Treatment of antimony-contaminated water is a prominent topic. Adsorption is considered as one of the most attractive and practical approaches for Sb remediation in aqueous media because of its operation simplicity, cost-effectiveness, high efficiency, and less sludge production. Magnetite (Fe₃O₄) as one of the most promising magnetic materials is extensively investigated. However, some previous studies showed that the removal capacity and reactivity for pollutants of the iron-based magnetic nanoparticles are size dependent. A high surface free energy results in a greater aggregation for nanoparticles. The formation of aggregates can decrease the surface area of the magnetic nanoparticles, thereby restricting the treatment performance for contaminants. Recent technologies are developed using some porous materials as mechanical supports to enhance the dispersibility of magnetic nanoparticles. Herein, acid-treatment sepiolite was prepared as support materials, and acid-sepiolite supported magnetite nanocomposite powder was obtained for Sb(V) adsorption. In addition, the effects of initial pH value, reaction time, initial mass concentration of Sb(V), adsorbent dosage, and coexisting ions (SO₄^2-, Ca²⁺, Mg²⁺, Fe³⁺, and humic acid) on the adsorption of Sb(V) were also investigated.

For acid-treatment of sepiolite, all chemical reagents used were of analytical grade. Acid-treatment sepiolite was firstly prepared, i.e., 1.0 g of sepiolite was added into 100 mL of 1.2 mol/L HCl solution and reacted at 60 ℃. After 12 h of stirring, the suspension was centrifuged, and washed with deionized water until the pH value was 7.0. Finally, the precipitate was collected and dried at 50 ℃.

For preparation of acid-sepiolite supported magnetite nanocomposite powder, the acid-sepiolite supported magnetite nanocomposite powder was prepared by a microwave-assisted reflux method. In the synthesis process, 1 mmol (0.27 g) of FeCl₃·6H₂O and 0.5 mmol (0.10 g) of FeCl₂·4H₂O were dissolved in 20 mL of ethylene glycol, and then 0.25 g of acid-treatment of sepiolite was dispersed in the solution under ultrasonication to form solution A. Afterwards, 0.16 g (4 mmol) of NaOH was dissolved in 3 mL of deionized water to form solution B. Solution B was introduced into solution A in a 100 mL round-bottomed flask. Subsequently, the round-bottomed flask with the reactants was equipped on the microwave reactor, and irradiated for 20 min at 80% of the full power (640 W). After cooling to room temperature, the precipitates were collected and washed with alcohol for several times, and finally dried in a vacuum oven at 60 ℃ for overnight.

A stock solution containing Sb(V) (600.0 mg/L) was prepared via dissolving K[Sb(OH)₆] with deionized water, and a series of solutions used were prepared via diluting the stock to the desired concentrations-actual concentrations measured by inductive coupled plasma-atomic emission spectroscopy (ICP-AES). In a typical adsorption run, 50 mg of adsorbent was added into 50 mL of solution containing 60.0 mg/L Sb(V) with constantly stirred at 180 r/min at 25 ℃. The initial pH of the solution was adjusted with HCl and/or NaOH solutions before the addition of adsorbent. The effect of pH value, contact time, adsorbent dosage, and initial concentration of Sb, coexisting anions (Ca²⁺, Fe³⁺, Mg²⁺, SO₄^2-, and humic acid) on the Sb adsorption were investigated in the same procedures. After adsorption, the residual level of Sb in solution was determined by ICP-AES.

After the microwave irradiation, the prepared Acid-Sep-Fe₃O₄ composite powder maintains a nanorod-like structure of sepiolite, and massive magnetite nanoparticles appear on the surface of the powder. The corresponding energy dispersive X-ray spectroscopy and X-ray diffraction patterns indicate the preparation of Acid-Sep-Fe₃O₄ composite powder. The BET surface area and total pore volume of the composite powder are 269.00 m²/g and 0.96 cm³/g, respectively. Moreover, the magnetic saturation of the powder is 10.4 emu/g. The obtained composite powder has high surface area and good magnetic nature, which are beneficial for Sb(V) adsorption and adsorbent recovery.

The Sb(V) adsorption by nanocomposite powder is dependent on the pH value with the maximum adsorption under acidic conditions and decreases with the increase of pH value. The adsorption percentage of Sb(V) increases with the increase of adsorbent dosage, and over 85% of Sb(V) is removed at the adsorbent dosage of 50 mg/50 mL. The prepared powder has a great adsorption rate to Sb(V) due to its high specific surface area. 81.4% of Sb(V) is adsorbed after 240 min of agitation. Moreover, the maximum removal capacity of the powder toward Sb(V) is 92.3 mg/g, which is greater than that of the nanoscale magnetite (79.2 mg/g), acid-treatment sepiolite (42.3 mg/g), and raw sepiolite (35.6 mg/g). The results indicate that the adsorption capacity of magnetite toward Sb(V) is enhanced after the support of acid-treatment sepiolite. The presence of SO₄^2-, Ca²⁺, Mg²⁺, and humic acid has a negligible effect on the adsorption of Sb(V), while the Sb(V) sorption process is enhanced with the addition of Fe³⁺, due to the formation of Fe precipitates. The adsorption behavior of nanocomposite powder toward Sb(V) fits the pseudo-second-order kinetic model and the Langmuir model. The XPS analysis reveal that the adsorption of composite powder toward Sb(V) mainly occurs via electrostatic interaction followed by chemical reactions.

Acid-treatment sepiolite supported Fe₃O₄ nanocomposite powder was prepared via acid-treatment of sepiolite and microwave-assisted reflux method with a natural nanorod-like sepiolite. The prepared composite powder exhibited a superior adsorption capacity toward Sb(V) with the maximum adsorption capacity of 92.3 mg/g, which was greater than that of the unsupported Fe₃O₄ nanoparticles (i.e., 79.2 mg/g), acid-treatment sepiolite (i.e., 42.3 mg/g), and raw sepiolite (i.e., 35.6 mg/g). The adsorption behavior of Sb(V) followed the pseudo-second-order kinetic model and Langmuir model. The Sb(V) adsorption by magnetic nanocomposites mainly occurred via electrostatic interaction followed by chemical reactions. The prepared acid-treatment sepiolite supported Fe₃O₄ nanocomposite powder could be used as an ideal adsorbent for Sb(V) due to the facile fabrication, efficient adsorption performance and easy magnetic separation.