Fabrication of Ag3PO4/GO/NiFe2O4 composites with highly efficient and stable visible-light-driven photocatalytic degradation of rhodamine B

Effective visible-light-driven Ag₃PO₄/GO/NiFe₂O₄Z-scheme magnetic composites were successfully fabricated by a simple ion-exchange deposition method. The Ag₃PO₄/GO/NiFe₂O₄ (8%) composite exhibited excellent photocatalytic activity (degradation efficiency was ∼96% within 15 min and kinetic constant reached 0.1956 min⁻¹) and stability when compared to Ag₃PO₄, NiFe₂O₄, and Ag₃PO₄/NiFe₂O₄ for rhodamine B (RhB) degradation. Furthermore, by electrochemical and fluorescence measurements, the Ag₃PO₄/GO/NiFe₂O₄ (8%) material also showed larger transient photocurrent, lower impedance, and longer fluorescence lifetime (7.82 ns). Comparing the activity result dependence with characterization results, it was indicated that photocatalytic activity depended on fast charge transfer from Ag₃PO₄ to NiFe₂O₄ through GO sheet. The h⁺ and ·O₂⁻ species played important roles in RhB degradation under visible-light. A possible Z-scheme mechanism is proposed over the Ag₃PO₄/GO/NiFe₂O₄ (8%) composite. This study might provide a promising visible light responsive photocatalyst for the photocatalytic degradation of organic dyes in wastewater.

Effective visible-light-driven Ag₃PO₄/GO/NiFe₂O₄Z-scheme magnetic composites were successfully fabricated by a simple ion-exchange deposition method. The composites exhibited excellent photocatalytic activity and stability for RhB degradation.     

Enhanced photocatalytic activity of a flower-like In2O3/ZnGa2O4:Cr heterojunction composite with long persisting luminescence

The development of new photocatalysts with high photocatalytic efficiency and catalytic stability, and long persisting luminescence is critical for ensuring environmental protection and clean energy production. In this study, we develop a flower-like In₂O₃/ZnGa₂O₄:Cr heterojunction composite with enhanced ultraviolet (UV) photocatalytic activity using a facile two-step hydrothermal method. The spectral response range of the heterojunction composite is widened to the visible-light range owing to the presence of the ZnGa₂O₄:Cr persistent luminescence nanoparticles with sizes of less than 10 nm. The heterojunction composite is dispersed on the flower petals of In₂O₃. The In₂O₃/ZnGa₂O₄:Cr/1:1 composite exhibits photo-degradation performance for rhodamine B degradation that is superior to those of pure In₂O₃, ZnGa₂O₄:Cr, In₂O₃/ZnGa₂O₄:Cr/1:0.5 and In₂O₃/ZnGa₂O₄:Cr/1:2, achieving complete degradation after 80 min under UV light irradiation. Moreover, it exhibits long afterglow luminescence that lasts for more than 72 h. Thus, the In₂O₃/ZnGa₂O₄:Cr/1:1 composite shows great potential for use in round-the-clock photocatalytic applications.

Flower-like In₂O₃/ZnGa₂O₄:Cr heterojunction composites not only have high photocatalytic efficiency for rhodamine B degradation, but also have a long persisting luminescence performance.     

Fabrication of a magnetic ternary ZnFe2O4/TiO2/RGO Z-scheme system with efficient photocatalytic activity and easy recyclability

A magnetic composite based on TiO₂ nanosheets, ZnFe₂O₄ and reduced graphene oxide (RGO) was synthesized by a one-step hydrothermal synthesis method, which possessed the band structure of a Z-scheme photocatalytic system. The properties and structures of the samples were characterized by XRD, UV-Vis DRS, Raman spectroscopy, SEM, EDS, XPS and PL spectroscopy. Compared with TiO₂ nanosheets and the TiO₂/RGO composite, the obtained ternary composite with 3 wt% RGO exhibited a significant enhancement in photocatalytic activities, attributed to the efficient charge separation induced by the fabricated Z-scheme system. About 99.7% of p-nitrophenol (p-NP) degraded within 60 min under simulated solar irradiation. Trapping experiments showed that superoxide anions (˙O₂⁻) and hydroxyl radicals (˙OH) were the main active species in the p-NP photocatalytic degradation. Finally, a possible photocatalytic mechanism of Z-scheme ZnFe₂O₄/TiO₂/RGO was proposed based on the results of trapping experiments and the energy bands of TiO₂ and ZnFe₂O₄.

A magnetic separable Z-scheme composite based on ZnFe₂O₄, TiO₂ nanosheets and RGO exhibits efficient photocatalytic degradation of p-NP.     

Enhanced extraction of organophosphorus pesticides from fruit juices using magnetic effervescent tablets composed of the NiFe2O4@SiO2@PANI-IL nanocomposites

The reported ionic liquid (IL)-based magnetic effervescent tablets are a result of direct addition of ILs and magnetic nanoparticles (MNPs). In effervescent reaction-enhanced microextraction procedures, the dissociation between ILs and MNPs easily leads to loss of ILs due to aqueous solubility, thereby decreasing the extraction efficiency. Herein, we attached a hydrophilic IL ([BMIM]Br) onto the surface of NiFe₂O₄@SiO₂@polyaniline (NiFe₂O₄@SiO₂@PANI-IL) to prepare novel core–shell-like multi-layer nanocomposites. Magnetic effervescent tablets were composed of Na₂CO₃ as an alkaline source, tartaric acid as an acidic source and as-synthesized nanocomposites as an extractant. The nanocomposites were used in an effervescent reaction-enhanced magnetic solid-phase extraction (ERMSE) for the extraction of four organophosphorus pesticides (OPPs) in fruit juices prior to HPLC-DAD detection. Under optimized conditions, this method provided low limits of detection (0.06–0.17 μg L⁻¹), high recoveries (80.6–97.3%) and excellent precision (1.1–5.2%) for OPP quantification in five fruit juices. Notably, the three-layer core–shell nanocomposites were efficiently recycled for at least eight extraction cycles with a recovery loss of <10%. The novelty of this study lies in: (1) for the first time, the ILs-based hybrid magnetic nanocomposites were prepared with appropriate pore size/volume and more active sites for OPPs; (2) the combination of the nanocomposites with effervescent tablets realizes rapid dispersion of CO₂ bubbles, and convenient magnetic separation/collection into one synchronous step; and (3) due to there being no requirement of electrical power, it is feasible for use in field conditions. Thus, the ERMSE method has excellent potential for conventional monitoring of trace-level OPPs in complex fruit juice matrices.

The reported ionic liquid (IL)-based magnetic effervescent tablets are a result of direct addition of ILs and magnetic nanoparticles (MNPs).     

Fabrication of TiO2/Fe2O3/CdS systems: effects of Fe2O3 and CdS content on superior photocatalytic activity

A heterostructured material of CdS and Fe₂O₃ nanoparticle-modified TiO₂ nanotube array (NTA) photoelectrode (TiO₂/Fe₂O₃/CdS) is reported in this work. TiO₂/Fe₂O₃ was prepared by annealing TiO₂ NTAs pre-loaded with Fe(OH)₃, which was uniformly clung to TiO₂ NTAs using sequential chemical bath deposition (S-CBD). Subsequently, CdS nanoparticles were deposited on TiO₂/Fe₂O₃ using the successive ion layer adsorption and reaction (SILAR) technique. Three-dimensional (3D) TiO₂/Fe₂O₃/CdS samples generated a photocurrent of approximately 4.92 mA cm⁻², with a photoconversion efficiency of 4.36%, which is more than 20 times higher than that of bare TiO₂ NTAs (0.22%) and 6 times that of TiO₂/Fe₂O₃ (0.71%). The photocatalytic activity was evaluated by the degradation of p-nitrophenol (PNP) under visible light (λ > 420 nm). The TiO₂/Fe₂O₃/CdS exhibited the best photocatalytic activity among all samples. Almost all PNP was degraded by TiO₂/Fe₂O₃/CdS within 120 min. The enhancement of photocatalytic activity could be attributed to the promoted photo-induced electron and hole separation and migration on the basis of photoluminescence spectra, photocurrent measurements, and open-circuit photovoltage responses. In addition, the newly synthesized TiO₂/Fe₂O₃/CdS can maintain high photocatalytic efficiency for five reuse cycles. Our findings provide a new idea for the low cost synthesis of high performance photocatalysts for the photodegradation of organic pollutants in aqueous solution.

New idea for the low cost synthesis of high performance photocatalysts for the photodegradation of organic pollutants in aqueous solution.     

Fullerene-modified magnetic silver phosphate (Ag3PO4/Fe3O4/C60) nanocomposites: hydrothermal synthesis,characterization and study of photocatalytic,catalytic and antibacterial activities

In this work, fullerene-modified magnetic silver phosphate (Ag₃PO₄/Fe₃O₄/C₆₀) nanocomposites with efficient visible light photocatalytic and catalytic activity were fabricated by a simple hydrothermal approach. The composition and structure of the obtained new magnetically recyclable ternary nanocomposites were completely characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, Brunauer–Emmett–Teller (BET) specific surface area analysis, vibrating sample magnetometery (VSM), diffuse reflectance spectroscopy (DRS), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy and transmission electron microscopy (TEM). This novel magnetically recyclable heterogeneous fullerene-modified catalyst was tested for the H₂O₂-assisted photocatalytic degradation of MB dye under visible light. The results show that about 95% of the MB (25 mg L⁻¹, 50 ml) was degraded by the Ag₃PO₄/Fe₃O₄/C₆₀ nanocomposite within 5 h under visible light irradiation. The catalytic performance of the Ag₃PO₄/Fe₃O₄/C₆₀ nanocomposite was then examined for 4-nitrophenol (4-NP) reduction using NaBH₄. This new nanocomposite showed that 4-NP was reduced to 4-aminophenol (4-AP) in 98% yield with an aqueous solution of NaBH₄. In both photocatalytic and catalytic reactions, the Ag₃PO₄/Fe₃O₄/C₆₀ nanocomposite exhibited higher catalytic activity than pure Ag₃PO₄. Moreover, the Ag₃PO₄/Fe₃O₄/C₆₀ nanocomposite could be magnetically separated from the reaction mixture and reused without any change in structure. The antibacterial activity of the nanocomposites was also investigated and they showed good antibacterial activity against a few human pathogenic bacteria.

Fullerene-modified magnetic silver phosphate (Ag₃PO₄/Fe₃O₄/C₆₀) nanocomposites prepared by a hydrothermal route were used as photocatalysts/catalysts for the efficient degradation and reduction of MB dye and 4-nitrophenol, respectively.     

BiVO4/Fe2O3/ZnFe2O4; triple heterojunction for an enhanced PEC performance for hydrogen generation

n/n/n triple heterojunction photoanodes made up of Zr:W-BiVO₄, Fe₂O₃, and ZnFe₂O₄ metal oxides are fabricated through a simplistic spray pyrolysis method. Use of Zr and W as dopants in BiVO₄ plays an important role as Zr increases the carrier density and W reduces the charge recombination. Further, Fe₂O₃ and ZnFe₂O₄ serve as a protective layer for Zr:W-BiVO₄, which augmented the photoelectrochemical performance and achieved a 1.90% conversion efficiency in the triple heterojunction. XRD measurements display the crystalline nature and reduction in particle size due to strain in the sample, UV-vis absorbance shows an extended absorption towards the visible region and the FE-SEM imaging confirms the successful deposition of ZnFe₂O₄ over BiVO₄/Fe₂O₃. By analyzing the band edge position, it was observed that on formation, the triple heterojunction not only suppresses the charge carrier recombination but also utilizes the band edge offset for the water splitting reaction using solar energy.

n/n/n triple heterojunction photoanodes made up of Zr:W-BiVO₄, Fe₂O₃, and ZnFe₂O₄ metal oxides.     

Dextran mediated MnFe2O4/ZnS magnetic fluorescence nanocomposites for controlled self-heating properties

Dextran mediated MnFe₂O₄/ZnS opto-magnetic nanocomposites with different concentrations of ZnS were competently synthesized adopting the co-precipitation method. The structural, morphological, magnetic, and optical properties of the nanocomposites were exhaustively characterized by XRD, HRTEM, FTIR, VSM techniques, and PL spectroscopy. XRD spectra demonstrate the existence of the cubic spinel phase of MnFe₂O₄ and the cubic zinc blend phase of ZnS in the nanocomposites. HRTEM images show the average crystallite size ranges of 15–21 nm for MnFe₂O₄ and 14–45 nm for ZnS. Investigation of the FTIR spectra reveals the incorporation of ZnS nanoparticles on the surface of MnFe₂O₄ nanoparticles by dint of biocompatible surfactant dextran. The nanocomposites exhibit both magnetic and photoluminescence properties. Photoluminescence analysis confirmed the redshift of the emission peaks owing to the trap states in the ZnS nanocrystals. The room temperature VSM analysis shows that the saturation magnetization and coercivity of MnFe₂O₄ nanoparticles initially increase then decrease with the increasing concentration of ZnS in the nanocomposite. The induction heating analysis shows that the presence of dextran enhances the self heating properties of the MnFe₂O₄/ZnS nanocomposites which can also be controlled by tailoring the concentration of the ZnS nanoparticles. These suggest that MnFe₂O₄/Dex/ZnS is a decent candidate for hyperthermia applications.

Illustration of the variation of SAR and ILP values with different concentrations for ac magnetic fields of amplitude H = 161 G and H = 226 G.     

Facile fabrication of hierarchical p-Ag2O/n-Nb2O5 heterojunction microspheres with enhanced visible-light photocatalytic activity

Constructing p–n heterojunction is an efficient strategy to improve the photocatalytic efficiency. Here, we report a hierarchical Ag₂O/Nb₂O₅ heterojunction composite as a novel and efficient visible-light driven photocatalyst. Hierarchical Nb₂O₅ microspheres were prepared by a hydrothermal method, and then the in situ growth of Ag₂O nanoparticles on their surfaces was realized by a simple deposition method. Structural and textural features of the Ag₂O/Nb₂O₅ composites were investigated, revealing that Ag₂O nanoparticles were well distributed on the surface of Nb₂O₅ microspheres. Photocatalytic degradation of rhodamine B (RhB) was significantly enhanced by Ag₂O/Nb₂O₅ photocatalysts under visible light. The optimal Ag/Nb molar ratio was determined to be 0.15 : 1, which yielded a 21.8 times faster degradation rate constant than plain Nb₂O₅ microspheres and had excellent stability for at least 4 catalytic cycles. The superior photocatalytic performance of Ag₂O/Nb₂O₅ photocatalyst can be ascribed to the hierarchical superstructure as well as the heterojunction between Ag₂O and Nb₂O₅, which facilitated the separation of photogenerated charge carriers. This work has potential application in the future for solving environmental pollution.

Constructing p–n heterojunction is an efficient strategy to improve the photocatalytic efficiency.     

Synthesis of Zn3(VO4)2/BiVO4 heterojunction composite for the photocatalytic degradation of methylene blue organic dye and electrochemical detection of H2O2

In this study, a Zn₃(VO₄)₂/BiVO₄ heterojunction nanocomposite photocatalyst was prepared using a hydrothermal route with different molar concentration ratios. The as-synthesized nanophotocatalyst was characterized using XRD, SEM, EDS, XPS, FT-IR, Raman, BET, UV-vis DRS, EPR and PL. The effect of molar ratio on composition and morphology was studied. The as-prepared nanocomposite exhibited excellent photocatalytic response by completely degrading the model pollutant methylene blue (MB) dye in 60 min at molar concentration ratio of 2 : 1. In basic medium at pH 12, the Zn₃(VO₄)₂/BiVO₄ nanocomposite degrades MB completely within 45 min. The nanocomposite was also successfully used for the electrochemical detection of an important analyte hydrogen peroxide (H₂O₂). This study opens up a new horizon for the potential applications of Zn₃(VO₄)₂/BiVO₄ nanocomposite in environmental wastewater remediation as well as biosensing sciences.

In this study, a Zn₃(VO₄)₂/BiVO₄ heterojunction nanocomposite photocatalyst was prepared using a hydrothermal route with different molar concentration ratios.     

One-pot synthesis of Cu2O/C@H-TiO2 nanocomposites with enhanced visible-light photocatalytic activity

As an environment-friendly semiconductor, titanium dioxide (TiO₂), which can effectively convert solar energy to chemical energy, is a crucial material in solar energy conversion research. However, it has several technical limitations for environment protection and energy industries, such as low photocatalytic efficiency and a narrow spectrum response. In this study, a unique mesoporous Cu₂O/C@H-TiO₂ nanocomposite is proposed to solve these issues. Polystyrene beads ((C₈H₈)_n, PS) are utilized as templates to prepare TiO₂ hollow microspheres. Cu₂O nanocomposites and amorphous carbon are deposited by a one-pot method on the surface of TiO₂ hollow spheres. After the heterojunction is formed between the two semiconductor materials, the difference in energy levels can effectively separate the photogenerated e⁻–h⁺ pairs, thereby greatly improving the photocatalytic efficiency. Furthermore, due to the visible band absorption of Cu₂O, the absorption range of the prepared nanocomposites is expanded to the whole solar spectrum. Amorphous carbon, as a Cu₂O reduction reaction concomitant product, can further improve the electron conduction characteristics between Cu₂O and TiO₂. The structure and chemical composition of the obtained nanocomposites are characterized by a series of techniques (such as SEM, EDS, TEM, XRD, FTIR, XPS, DRS, PL, MS etc.). The experimental results of the degradation of methylene blue (MB) aqueous solution demonstrate that the degradation efficiency of Cu₂O/C@H-TiO₂ nanocomposites is about 3 times as fast as that of pure TiO₂ hollow microspheres, and a more absolute degradation can be achieved. Herein, a recyclable photocatalyst with high degradation efficiency and a whole solar spectrum response is proposed and fabricated, and would find useful applications in environment protection, and optoelectronic devices.

As an environment-friendly semiconductor, titanium dioxide (TiO₂), which can effectively convert solar energy to chemical energy, is a crucial material in solar energy conversion research.     

g-C3N4/ZnCdS heterojunction for efficient visible light-driven photocatalytic hydrogen production

To suppress the aggregation behavior caused by the high surface energy of quantum dots (QDs), ZnCdS QDs were grown in situ on a g-C₃N₄ support. During the growth process, the QDs tightly adhered to the support surface. The ZnCdS QDs were prepared by low-temperature sulfurization and cation exchange with a zeolitic imidazolate framework precursor under mild conditions. The heterojunction of g-C₃N₄/ZnCdS-2 (CN/ZCS-2, with a g-C₃N₄ to ZIF-8 ratio of 2.0) not only showed excellent optical absorption performance, abundant reactive sites, and a close contact interface but also effectively separated the photogenerated electrons and holes, which greatly improved its photocatalytic hydrogen production performance. Under visible light irradiation (wavelength > 420 nm) without a noble metal cocatalyst, the hydrogen evolution rate of the CN/ZCS-2 heterojunction reached 1467.23 μmol g⁻¹ h⁻¹, and the durability and chemical stability were extraordinarily high.

The zeolitic imidazolate framework-8 (ZIF-8) is used as a precursor to prepare ZnCdS/C₃N₄ heterojunctions to achieve visible light-driven water splitting hydrogen production effectively.

With the continuous advancement of global industrialization and rapid development of the global economy, fossil energy is being consumed at an increasingly steep rate. This uncontrolled usage has caused many irreversible environmental problems, including global warming, ozone layer destruction, and acid rain.¹ To promote the green and sustainable development of the society, various clean renewable energy sources such as solar energy, tidal energy, wind energy, biomass energy, and hydrogen energy have been developed in succession. Solar energy is a universal, sustainable, abundant, and environmentally friendly renewable energy.² Hydrogen is another clean energy source (combustion product = water) with a high energy density (122 kJ g⁻¹). Therefore, hydrogen is considered as a great alternative to fossil fuels,³ and photocatalytic hydrogen production technology is among the most effective solutions to the above problems. Photocatalytic water splitting based on TiO₂ and Pt electrodes was first reported by Fujishima and Honda in 1972,⁴ sparking great interest in photocatalytic hydrogen production from semiconductor materials. Many remarkable achievements have been made in the past decades.Among various photocatalysts, Zn_1−xCd_xS solid solution is widely used in photocatalytic hydrogen production because it strongly responds to visible light and has an adjustable band gap. However, when used alone, this common semiconductor is prone to photo-corrosion and photogenerated carrier recombination.⁵ Moreover, the low specific surface area of Zn_1−xCd_xS synthesized by conventional methods impedes the photocatalytic process. A material''s photocatalytic activity notably depends on its size or morphology, and the rational design and preparation of ideal nanostructured transition metal sulfides are of practical significance.Quantum dots (QDs) are zero-dimensional semiconductor nanomaterials, which possess high visible light absorption coefficients, large specific surface area, a size quantization property, and high stability.⁶ The size quantization property of QDs means that the optical band gap is tunable with respect to size, shape, and composition.⁷ In kinetic studies, the electron transfer rate constant is higher in smaller QDs than in larger QDs, indicating that electrons can be transported more quickly in small QDs.⁷ Most of the current QDs preparation methods require complex reaction devices, harsh reaction conditions, or a long reaction time. They may also generate toxic by-products. Against this background, an environmentally friendly and convenient method for Zn_1−xCd_xS QDs fabrication is urgently sought.^8,9At present, QDs are prepared from metal–organic frameworks (MOFs) precursors.^10–12 MOFs are inorganic–organic hybrid materials bridged by metal clusters and organic ligands.¹³ Many studies have found that MOF templates or precursors largely retain the characteristics of the original MOFs; moreover, they can be converted into carbon/metal-based porous materials with higher stability and conductivity than pristine MOF materials.¹⁴ The MOF-derived porous materials achieve a larger specific surface area, higher porosity, and more uniform heteroatom doping than traditional catalysts.¹⁵ Therefore, many MOF-derived porous materials provide better photocatalytic performance than their original MOFs. Unlike most MOF materials, zeolite imidazolate framework (ZIF-8) can be simply synthesized at a high yield under mild conditions. With a metal center of Zn²⁺,¹⁶ ZIF-8 is used as the precursor in the preparation of Zn_1−xCd_xS QDs. However, the QDs inevitably aggregate owing to their high surface energy, which reduces the specific surface area and adversely affects the photocatalytic performance.¹⁷ To prevent QDs'' agglomeration, this work proposes the in situ growth of ZIF-8 on a graphitic-phase carbon nitride (g-C₃N₄) substrate.g-C₃N₄ is a polymer semiconductor with a band gap of approximately 2.7 eV, which absorbs visible light. As g-C₃N₄ contains only C and N elements, it can be prepared at a low cost; moreover, its aromatic C–N heterocyclic structure and strong interlayer van der Waals forces confer high thermal and chemical stability and the flexible structure to load various nanoparticles.¹⁸ Therefore, porous g-C₃N₄ with large specific surface area was selected as the compounding substrate. Like many single-component photocatalysts, ZnCdS undergoes rapid recombination of its photogenerated charge carriers, which greatly reduces the photocatalytic hydrogen production activity.¹⁹ Besides preventing QD agglomeration, the g-C₃N₄/Zn_1−xCd_xS heterojunction can effectively prolong the charge lifetime.To prevent QDs'' aggregation and effectively separate the photoexcited electron–hole pairs, ZIF-8 was grown in situ in the presence of a g-C₃N₄ carrier, and a g-C₃N₄/ZnCdS heterojunction nanocomposite was synthesized through low-temperature sulfurization and cation exchange strategies. The optimized sample achieved the highest photocatalytic hydrogen yield and superior chemical stability under visible light irradiation (wavelength > 420 nm) without any noble metal catalyst. Scheme 1 describes the preparation process of g-C₃N₄/ZnCdS. First, g-C₃N₄ was synthesized from urea by a facile template-free method with two pyrolysis steps. As is well known, g-C₃N₄ can be simply synthesized by pyrolysis polymerization of some nitrogen-rich precursors. The precursors and reaction parameters of the preparation process largely influence the crystallinity and energy band structure of g-C₃N₄. Recently, g-C₃N₄ from urea precursor has been shown to form a porous morphology, which provides a high specific surface area and abundant reactive sites.^20,21 Exploiting the affinity of g-C₃N₄ for Zn²⁺, the proposed method grew ZIF-8 nanocrystals in situ on porous g-C₃N₄. During this process, an appropriate amount of triethylamine was added to regulate the ZIF-8 crystal size. When the ZIF-8 size is constrained, the small MOF particles are more evenly distributed on the porous g-C₃N₄. To optimize the ratio of g-C₃N₄ and ZIF-8, the g-C₃N₄/ZIF-8 samples were prepared at different ratios. The g-C₃N₄/ZnS nanocomposite was then prepared by low-temperature vulcanization. When the ZIF-8 framework collapsed, the obtained ZnS QDs grew uniformly on the g-C₃N₄ surface. Because ZnS and CdS have similar lattice structures and coordination modes, a Cd source was added for cation exchange to form the g-C₃N₄/ZnCdS heterostructure. The band structure of ZnCdS can be precisely controlled by adjusting the molar ratio of Zn to Cd.Open in a separate windowScheme 1Schematic illustration of the construction of g-C₃N₄/ZnCdS.The crystal phase of the obtained g-C₃N₄ was monitored by powder X-ray diffraction (PXRD) (Fig. 1). The low-angle reflection peak at 12.89° originated from the crystal plane parallel to the c-axis; meanwhile, in bulk g-C₃N₄, the strongest peak at 27.53° reflected the characteristic interlayer stacking of the aromatic system.²² The SEM pictures demonstrated the porous structure of g-C₃N₄ (Fig. S1^†). When ZIF-8 was synthesized on the g-C₃N₄ surface, obvious MOF diffraction peaks were observed at low angles. When the ZIF-8 loading increased, the diffraction peaks of g-C₃N₄ became inconspicuous. The vulcanized ZIF-8 presented three obvious diffraction peaks at 27.84°, 47.73°, and 56.91° corresponding to the (111), (220), and (311) crystal planes of ZnS, respectively. After the cation exchange process, the corresponding diffraction peaks in the PXRD pattern intensified as the ZnCdS loading increased. The PXRD results implied that g-C₃N₄ and ZnCdS maintained their own phases in the g-C₃N₄/ZnCdS composites. The specific composition of samples were shown in Table S1.^†Open in a separate windowFig. 1(a) PXRD patterns of g-C₃N₄, g-C₃N₄/ZIF-8-2, g-C₃N₄/ZnS-2 and g-C₃N₄/ZnCdS; (b) PXRD patterns of g-C₃N₄/ZIF-8 and g-C₃N₄/ZnS.To characterize their microscopic morphology and lattice parameters, the samples were examined under a transmission electron microscope (TEM) (Fig. 2 and S2^†). As shown in Fig. S2(a),^† g-C₃N₄ formed a porous morphology and displayed a structure similar to thin nanosheets. After in situ loading on g-C₃N₄ nanoparticles, the ZIF-8 particles were approximately 20 nm wide, much smaller than ZIF-8 nanoparticles synthesized alone (without trimethylamine) (Fig. S2(b)^†). The TEM images also suggested a close connection between ZIF-8 and porous g-C₃N₄ and the uniform loading of ZIF-8 crystals on the porous g-C₃N₄ surfaces. From the TEM and high-resolution TEM (HRTEM) images of g-C₃N₄/ZnS, the lattice spacing of ZnS was determined as 0.311 nm, and the ZnS as uniformly distributed (Fig. S2(c) and (d)^†). Moreover, from increasing the amount of Zn source precursor obviously increased the ZnCdS content on g-C₃N₄ (Fig. 2(a)–(c)). When the load capacity was excessive, the porous g-C₃N₄ nanoparticles tended to bend. As shown in Fig. 2(d), the lattice fringes of ZnCdS was 0.320 nm which corresponding to the (111) facet of cubic ZnCdS. In the TEM and HRTEM images of ZnCdS QDs (Fig. S3^†), the nanoparticles were sized approximately 5 nm and seriously aggregated. When the ZnCdS QDs were directly vulcanized with ZIF-8 at low temperature, the ZnCdS QDs grown on the g-C₃N₄ surface were notably more separated. TEM mapping results proved that all elements distribute evenly in the samples (Fig. S4–S6^†).Open in a separate windowFig. 2TEM images of (a) g-C₃N₄/ZnCdS-1, (b) g-C₃N₄/ZnCdS-3, (c) g-C₃N₄/ZnCdS-2; (d) high-resolution TEM (HRTEM) image of g-C₃N₄/ZnCdS-2.The optical and physical properties of the samples were measured using solid ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy (Fig. 3(a)). The spectral absorptions of the three samples (g-C₃N₄, g-C₃N₄/ZIF-8-2, and g-C₃N₄/ZnS-2) at λ = 430 nm were very similar and corresponded to a band gap of 2.88 eV. However, the spectrum of Cd-doped g-C₃N₄/ZnS presented two absorption peaks in the visible region corresponding to the absorptions of g-C₃N₄ and ZnCdS. These absorption peaks were red-shifted with increasing loading of ZnCdS QDs on the porous g-C₃N₄ surface.Open in a separate windowFig. 3(a) UV-vis spectra and (b) Tauc plots of g-C₃N₄, g-C₃N₄/ZIF-8-2, g-C₃N₄/ZnS-2, g-C₃N₄/ZnCdS, and ZnCdS.Applying the Kubelka–Munk formula, the Tauc plots of the samples were calculated from the UV-vis data and are plotted in Fig. 3(b). As the loading capacity of ZnCdS increased, the band gap of g-C₃N₄/ZnCdS (CN/ZCS) was significantly narrowed from 2.45 eV to 2.35 eV.The surface chemical states of g-C₃N₄ and CN/ZCS were investigated with X-ray photoelectron (XPS) spectroscopy (Fig. 4). In the XPS full survey spectrum, the main elements in the composites were C, N, Zn, Cd, and S, as expected (Fig. 4(a)). In XPS measurements of the g-C₃N₄ and CN/ZCS-2 samples (g-C₃N₄ to ZIF-8 ratio = 2.0), the binding energies of C 1s, N 1s, Zn 2p, Cd 3d, and S 2p were slightly shifted, suggesting that charge transfer occurred between porous g-C₃N₄ and ZnCdS QDs. The C 1s signal (284.6 eV) was used as the reference peak for charge correction on the fine spectra of the other elements. In the C 1s spectrum (Fig. 4(b)) of g-C₃N₄ presented peaks at 284.6 and 288.1 eV, which were ascribed to sp² C C bonds and to sp²-bonded carbon in the N-containing aromatic rings (N–C N), respectively. In the C 1s spectrum of CN/ZCS, the peak ascribed to the sp² C C bonds decreased due to the electron-withdrawing effect of ZnCdS QDs on g-C₃N₄.²³ The N 1s XPS spectrum of g-C₃N₄ contained three components at 398.5, 399.8, and 401.1 eV, corresponding to the bi-coordinated sp²-hybridized nitrogen C–N C groups of triazine rings, tertiary nitrogen groups (N₃C nitrogen atoms), and nitrogen bonded with hydrogen atoms in N–H_x functional groups, respectively (Fig. 4(c)).²⁴ In the fine spectrum of Zn 2p, the two peaks at binding energies of 1044.3 and 1021.3 eV were the characteristic peaks of the oxidation state of Zn²⁺ (Fig. 4(d)).²⁵ The normalized high-resolution Cd 3d spectra presented two characteristic peaks at 411.7 and 404.9 eV, which were attributed to Cd²⁺ 3d_3/2 and Cd²⁺ 3d_5/2, respectively (Fig. 4(e)).²⁶ The binding energies of S²⁻ 2p_1/2 and S²⁻ 2p_3/2 in ZnCdS were 162.6 and 161.4 eV, respectively.²⁷ These results are consistent with previous reports. Moreover, in the CN/ZCS nanocomposite, the binding energies of C 1s and N 1s were clearly shifted (by approximately 0.1 eV) towards high binding energies, while the binding energies of Zn 2p, Cd 3d, and S 2p were shifted downward (by approximately 0.2 eV) from those of pristine g-C₃N₄ and ZnCdS. These shifts can be attributed to changes in the surface electron density (partial electron transfer from ZnCdS to g-C₃N₄). Such results indicate a strong electronic interaction between ZnCdS QDs and g-C₃N₄, suggesting the successful formation of the heterojunction.²⁸Open in a separate windowFig. 4(a) XPS survey spectra of g-C₃N₄ and g-C₃N₄/ZnCdS; (b–f) high-resolution XPS of C 1s, N 1s, Zn 2p, Cd 3d, and S 2p, respectively.The photocatalytic H₂ production performances of the as-synthesized samples were determined in photoactive tests under visible light irradiation (λ > 420 nm) with sacrificial Na₂S/Na₂SO₃. As shown in Fig. 5(a), the H₂ production rates of g-C₃N₄ and ZnCdS were 72.46 and 110.51 μmol g⁻¹ h⁻¹, respectively, in the absence of any noble-metal co-catalyst. After the formation of the CN/ZCS nanocomposites, the H₂ evolution from the CN/ZCS heterojunction resembled a volcanic peak as the ZnCdS QDs loading increased. In the CN/ZCS-2 sample with the highest activity, the production rate reached 1467.23 μmol g⁻¹ h⁻¹. The activities of samples loaded with a cocatalyst (Pt nanoparticles introduced by photodeposition) were also evaluated. The cocatalyst significantly improved the performance of all samples, and the activity of CN/ZCS-2/Pt reached 3245.18 μmol g⁻¹ h⁻¹. Next, the cycle stability and durability of CN/ZCS-2 were examined in four consecutive photocatalytic hydrogen production cycle tests under visible light irradiation (total test time = 12 h). The results are shown in Fig. 5(b). After four cycles, the photocatalytic activity of the CN/ZCS-2 heterojunction had only negligibly decreased, indicating the high stability of the photocatalyst. Subsequently, the recycled photocatalyst was re-characterized by PXRD and HRTEM. The CN/ZCS-2 photocatalyst almost retained its original structure and morphology, and its crystallinity was intact (Fig. S7 and S8^†). The above experimental results prove the excellent durability and stability of CN/ZCS-2. The apparent quantum yield (AQY) of samples are shown in Table S2.^† The g-C₃N₄/ZCS-2 exhibit the highest AQY value, matching well with the results of photocatalytic hydrogen productions.Open in a separate windowFig. 5(a) Photocatalytic hydrogen production rates of g-C₃N₄, ZnCdS, and g-C₃N₄/ZnCdS with (red bars) and without (blue bars) Pt cocatalyst; (b) recycling performance of CN/ZCS-2 (each color represents one test cycle).To understand the photoactivity enhancement in CN/ZCS-2, the N₂ adsorption–desorption isotherm and its corresponding differential curve was acquired. g-C₃N₄ is usually obtained by the traditional one-step pyrolysis method, but the present g-C₃N₄ nanoparticles were synthesized by secondary pyrolysis of urea, which boosted their specific surface area to 86.17 m² g⁻¹ (Fig. S9^†).²⁹ However, the specific surface area of CN/ZCS-2 was approximately 77.94 cm² g⁻¹ (Brunauer–Emmett–Teller test), lower than that of pristine g-C₃N₄ nanoparticles. These data suggest that the change in the active surface area did not dominantly promote photocatalytic performance.To explore the separation and transfer efficiency of the photogenerated electrons and holes in CN/ZCS-2, the influence of the photogenerated carrier behavior at the resulting heterojunction structure was investigated by photoluminescence (PL) spectroscopy. The semiconductor generated carriers when excited by light. Electrons and holes in their conduction band (CB) and valence band (VB) reach the quasi-equilibrium state through a relaxation process. During recombination, the electrons and holes emit light of different wavelengths. A strong PL peak denotes a high recombination rate of electron–hole pairs, meaning a low separation efficiency.³⁰ As the loading content of ZnCdS increases, the PL intensity of the CN/ZCS heterojunction first decreases and then increased (Fig. 6(a)). The fluorescence intensity was relatively strong in pure g-C₃N₄, g-C₃N₄/ZIF-2, and g-C₃N₄/ZnS, but weak in CN/ZCS-2. This result shows that the CN/ZCS-2 sample promoted the separation of photoinduced charge carriers to effectively utilize the sunlight. To further clarify the merits of the CN/ZCS heterojunction, the fluorescence lifetimes of the samples were measured by time-resolved fluorescence spectroscopy. The fitting results are shown in Fig. 6(b) and Table S3.^† In general, a longer emission lifetime implies a longer carrier migration distance and slower annihilation of the charge carriers, thereby promoting the photocatalytic performance. The CN/ZCS-2 showed the slowest fluorescence emission decay among the samples, suggesting that the lifetime of its photogenerated electrons and holes was longer than that of in the other samples. The emission lifetimes of the samples were obtained by fitting their attenuation curves. The carrier lifetime was longest in the CN/ZCS-2 sample (τ_A = 7.91 ns), consistent with expectations. These data prove that the charge separation efficiency of the photocatalytic reaction process was higher in CN/ZCS-2 than in the other samples.Open in a separate windowFig. 6(a) Photoluminescence (PL) emission spectra, (b) time-resolved PL spectra, (c) photocurrent–time curves, and (d) electrochemical impedance spectroscopy Nyquist plots of g-C₃N₄, g-C₃N₄/ZIF-8-2, g-C₃N₄/ZnS-2, and g-C₃N₄/ZnCdS.The charge separation efficiencies of the samples under working conditions were explored in photoelectrochemical tests (photocurrent–time (I–T) test and electrochemical impedance spectroscopy (EIS)). The I–T test curve was recorded under irradiation by a 300 W Xe lamp (λ > 420 nm). When the light irradiated the working electrode, the photogenerated electrons migrated along the external circuit to the other electrode and generated a current. A large photocurrent implies a high separation efficiency. As presented in Fig. 6(c), the photocurrent intensity of CN/ZCS was significantly enhanced from that of C₃N₄, and the CN/ZCS-2 heterojunction exhibited the strongest photocurrent response, consistent with the PL results. That is, the photoexcited charge carriers of CN/ZCS-2 were effectively separated and transferred at the interface between the sample and the electrolyte. These conclusions were also verified in the EIS test. Fig. 6(d) shows the Nyquist plots and simulated equivalent circuit plots of the samples obtained under an external bias of 1.11 V vs. RHE. The reaction resistance R_t was determined from the radius of the semicircle in the Nyquist diagram, which is proportional to charge transfer resistance.³¹ Among the samples, CN/ZCS-2 exhibited the smallest semicircle on the Nyquist plot and hence the lowest charge transfer resistance (R_t = 926 Ω, see Table S4^†). The results mirrored the photocurrent response data, again supporting that the CN/ZCS-2 heterojunction achieved the fastest charge dynamics and excellent photocatalytic activity.Finally, the XPS VB and steady-state surface photovoltage (SPV) spectra of the ZnCdS and g-C₃N₄ samples were investigated (Fig. 7). After the image processing and a correlation calculation, the VB positions of ZnCdS and g-C₃N₄ were obtained as 0.85 and 2.31 eV, respectively. Based on this result and the band gap calculated from the Tauc plots, the CB positions of ZnCdS and g-C₃N₄ were determined as −1.42 and −0.54 eV, respectively. Scheme 2 describes the charge transfer and separation mechanism of the CN/ZCS-2 heterojunction during the photocatalytic reaction, deduced from the experimental results. When CN/ZCS-2 is irradiated by visible light, the photogenerated electrons in the VB of ZnCdS are excited to the CB, and then migrate to g-C₃N₄ for the water reduction reaction; simultaneously, the photogenerated holes in the VB of g-C₃N₄ migrate to ZnCdS, where they react with the hole sacrificing agents. In the SPV spectra (Fig. 7(b)), the SPV value of g-C₃N₄ was very weak and fluctuated owing to the poor conductivity of this material. The SPV value of CN/ZCS-2 was significantly improved over that of g-C₃N₄, indicating that more photogenerated electron–hole pairs were transferred to the surface of CN/ZCS-2 when forming the heterojunction.Open in a separate windowFig. 7(a) X-ray photoelectron spectroscopy valence band spectrum of ZnCdS and g-C₃N₄; (b) steady-state surface photovoltage spectra of g-C₃N₄ and CN/ZCS-2.Open in a separate windowScheme 2Schematic of charge transfer and separation in CN/ZCS-2.In summary, we successfully prepared a CN/ZCS heterojunction by a mild hydrothermal method. Through a series of physical and chemical characterizations, we confirmed that the porous g-C₃N₄ nanoparticles alleviated the aggregation of ZnCdS QDs and optimized the separation and migration efficiency of the photogenerated carriers. The porous g-C₃N₄ sample also enlarged the specific surface area for hydrogen production, thus enhancing the photocatalytic efficiency. The ZnCdS QDs were synthesized with ZIF-8 precursors, which retains the high specific surface area of the original MOF. The photocatalytic activity of CN/ZCS-2 reached 1467.23 μmol g⁻¹ h⁻¹, 20 and 13 times higher than that of pristine g-C₃N₄ and ZnCdS, respectively. The CN/ZCS-2 also exhibited the longest fluorescence lifetime (7.91 ns) among the samples. This mild and simple synthesis method provides a reference for the development and preparation of other energy or environmentally related metal sulfide QDs heterojunction photocatalysts with efficient and stable operation.     

Tuning the microstructural and magnetic properties of CoFe2O4/SiO2 nanocomposites by Cu2+ doping

Co–Cu ferrite is a promising functional material in many practical applications, and its physical properties can be tailored by changing its composition. In this work, Co_1−xCu_xFe₂O₄ (0 ≤ x ≤ 0.3) nanoparticles (NPs) embedded in a SiO₂ matrix were prepared by a sol–gel method. The effect of a small Cu²⁺ doping content on their microstructure and magnetic properties was studied using XRD, TEM, Mössbauer spectroscopy, and VSM. It was found that single cubic Co_1−xCu_xFe₂O₄ ferrite was formed in amorphous SiO₂ matrix. The average crystallite size of Co_1−xCu_xFe₂O₄ increased from 18 to 36 nm as Cu²⁺ doping content x increased from 0 to 0.3. Mössbauer spectroscopy indicated that the occupancy of Cu²⁺ ions at the octahedral B sites led to a slight deformation of octahedral symmetry, and Cu²⁺doping resulted in cation migration between octahedral A and tetrahedral B sites. With Cu²⁺ content increasing, the saturation magnetization (M_s) first increased, then tended to decrease, while the coercivity (H_c) decreased continuously, which was associated with the cation migration. The results suggest that the Cu²⁺ doping content in Co_1−xCu_xFe₂O₄ NPs plays an important role in its magnetic properties.

The Cu²⁺ doping content in Co_1−xCu_xFe₂O₄/SiO₂ plays an important role in tuning hyperfine interaction and magnetic properties.     

The synthesis of a Bi2MoO6/Bi4V2O11 heterojunction photocatalyst with enhanced visible-light-driven photocatalytic activity

A novel Bi₂MoO₆/Bi₄V₂O₁₁ heterostructured photocatalyst was successfully fabricated using a facile one-pot solvothermal method. This heterojunction consists of homogeneous dispersed Bi₄V₂O₁₁ nanocrystals anchored on the surface of Bi₂MoO₆ nanoflakes, endowing the heterojunction with nanosized interfacial contact. Based on the favorable interfacial contact, the band alignment at the heterojunction effectively facilitated photo-generated carrier transfer, which was verified by photoelectrochemical and photoluminescence measurements. Thereby, in contrast with pristine Bi₂MoO₆ and Bi₄V₂O₁₁, the as-synthesized heterojunction with nanoscale contact exhibited significantly enhanced photocatalytic activity towards the degradation of MB and the reduction of Cr(vi). In addition, the as-fabricated Bi₂MoO₆/Bi₄V₂O₁₁ heterojunction exhibited good cycling stability for MB degradation after 4 cycles. Finally, a plausible photocatalytic mechanism for MB degradation over the Bi₂MoO₆/Bi₄V₂O₁₁ heterojunction was discussed in detail. This work not only reports a highly efficient photocatalyst but also sheds light on the design and optimization of a heterojunction.

A Bi₂MoO₆/Bi₄V₂O₁₁ heterojunction exhibits remarkably enhanced photocatalytic activity for MB degradation and Cr(vi) reduction under visible-light illumination compared to its pristine samples.     

One-step hydrothermal synthesis of a ternary heterojunction g-C3N4/Bi2S3/In2S3 photocatalyst and its enhanced photocatalytic performance

In recent years, photoelectrocatalysis has been one of the hotspots of research. Graphite-like carbon nitride (g-C₃N₄) is one of the few non-metal semiconductors known and has good potential in the field of photocatalysis due to its simple preparation method and visible light effects. In this study, a method for compounding two semiconductor materials, In₂S₃ and Bi₂S₃, on the surface of g-C₃N₄via a one-step hydrothermal method is reported, and it was found that this resulting material showed remarkable properties. The advantages of this method are as follows: (1) the formation of a heterojunction, which accelerates the separation efficiency of photogenerated carriers; (2) a large number of holes and defects on the surface of g-C₃N₄ are conducive to the nucleation, crystallisation and growth of In₂S₃ and Bi₂S₃. Compared with its counterpart catalysts, the CN/In₂S₃/Bi₂S₃ composite catalyst has significantly improved performance. Due to its high degree of crystallinity, the adsorption capacity of the catalyst itself is also significantly improved. In addition, the stability of the composite material maintains 90.9% after four cycles of use, and the structure is not damaged. In summary, CN/Bi₂S₃/In₂S₃ composite materials are believed to have broad application potential in the treatment of dye wastewater.

The proposed photocatalytic mechanism for the degradation of RhB on the surface of the 0.05CN/Bi₂S₃/In₂S₃ composite.     

Visible-light photocatalytic performance,recovery and degradation mechanism of ternary magnetic Fe3O4/BiOBr/BiOI composite

The ternary magnetic Fe₃O₄/BiOBr/BiOI (x : 3 : 1) photocatalysts were successfully synthesized by a facile solvothermal method. The samples were characterized by XRD, SEM, EDS, ICP-AES, XPS, UV-vis DRS, PL and VSM. Nitrogen-containing dye RhB was used as a degradation substrate to evaluate the photocatalytic degradation activities of the samples. The photocatalytic performance of Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) is superior to other Fe₃O₄/BiOBr/BiOI (x : 3 : 1). Compared with binary magnetic Fe₃O₄/BiOBr (0.5 : 1) prepared in our previous work, the Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) has obvious advantages in photocatalytic activity and adsorption capacity. And the specific surface area (48.30 m² g⁻¹) is much larger than that of the previous report (Fe₃O₄/BiOBr/BiOI (0.5 : 2 : 2)) synthesized by a co-precipitation method. Besides, after 25 s of magnetic field, Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) can be rapidly separated from water. After eight recycling cycles, the magnetic properties, photocatalytic activity, crystallization and morphology of the Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) catalyst remain good. The possible photocatalytic degradation mechanism of RhB under Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) photocatalyst was also proposed. The results indicate that the ternary magnetic Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) composite with high photocatalytic degradation efficiency, good magnetic separation performance and excellent recyclability and stability has potential application prospect in wastewater.

The ternary magnetic Fe₃O₄/BiOBr/BiOI (x : 3 : 1) photocatalysts were successfully synthesized by a facile solvothermal method.     

The fabrication and characterization of CeO2/Cu2O nanocomposites with enhanced visible-light photocatalytic activity

Spherical Cu₂O nanocrystals were synthesized and CeO₂/Cu₂O nanocomposites were successfully prepared from the spherical Cu₂O nanocrystals. Characterization analysis was performed via scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible diffusion reflectance spectroscopy (DRS) studies. In comparison with the Cu₂O nanocrystals, the CeO₂/Cu₂O nanocomposites exhibited high visible-light-induced photocatalytic activity for the degradation of methyl orange solution. Radical trapping experiments proved that photo-generated electrons played a very minor role, while photo-generated holes and superoxide radicals played a major role in the degradation process. The CeO₂/Cu₂O system could cause the internal energy band to bend, leading to the building of internal electric fields. The excited electrons and holes easily moved in opposite directions, promoting the effective separation of charges, which obviously enhanced the visible light photocatalytic activity of the catalyst.

Spherical Cu₂O nanocrystals were synthesized and CeO₂/Cu₂O nanocomposites were successfully prepared from the spherical Cu₂O nanocrystals.     

Synthesis of SrTiO3 submicron cubes with simultaneous and competitive photocatalytic activity for H2O splitting and CO2 reduction

Single crystalline strontium titanate (SrTiO₃) submicron cubes have been synthesized based on a molten salt method. The submicron cubes showed superior photocatalytic activity towards both water splitting and carbon dioxide reduction, in which methane (CH₄) and hydrogen (H₂) were simultaneously produced. The average production rate of methane up to 8 h is 4.39 μmol g⁻¹ h⁻¹ but drops to 0.46 μmol g⁻¹ h⁻¹. However, the average production rate of hydrogen is 14.52 before 8 h but then increases to 120.23 μmol g⁻¹ h⁻¹ after 8 h. The rate change of the two processes confirms the competition between the H₂O splitting and CO₂ reduction reactions. Band structure and surface characteristics of the SrTiO₃ submicron cubes were characterized by diffuse reflective UV-Vis spectroscopy, Mott–Schottky analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The results reveal that the simultaneous and competitive production of methane and hydrogen is due to a thermodynamics factor, as well as the competition between the adsorption of carbon dioxide and water molecules on the surface of the faceted SrTiO₃. This work demonstrates that SrTiO₃ photocatalysts are efficient in producing sustainable fuels via water splitting and carbon dioxide reduction reactions.

There is a clear competitive relationship between water splitting and photocatalytic reduction of carbon dioxide in the whole process of photocatalytic reduction of carbon dioxide with the prepared cubic SrTiO₃ as a photocatalyst.     

Photocatalytic activity of BiFeO3/ZnFe2O4 nanocomposites under visible light irradiation

Herein, BiFeO₃/ZnFe₂O₄ nanocomposites were synthesized via a glyoxylate precursor method using a two-pot approach. Phase evolution is investigated by X-ray diffraction and Raman spectroscopy, which confirm that no impurity phases are formed between BiFeO₃ and ZnFe₂O₄ following calcination at 600 °C. The specific surface area characterized by N₂ adsorption–desorption isotherms decreases from 30.56 to 13.13 m² g⁻¹ with the addition of zinc ferrite. In contrast, the magnetization increases from 0.28 to 1.8 emu g⁻¹ with an increase in the amount of ZnFe₂O₄. The composites show strong absorption in the visible region with the optical band gap calculated from the Tauc''s plot in the range from 2.17 to 2.22 eV, as measured by diffuse reflectance spectroscopy. Furthermore, the maximum efficiency for the photodegradation of methylene blue under visible light is displayed by the composite containing 25 wt% ZnFe₂O₄ due to the synergic effect between BiFeO₃ and ZnFe₂O₄, as confirmed by photoluminescence spectroscopy.

BiFeO₃-25 wt% ZnFe₂O₄ exhibits a low specific surface area, high magnetization, and maximum photocatalytic efficiency of 97%.     

The synthesis of CoxNi1−xFe2O4/multi-walled carbon nanotube nanocomposites and their photocatalytic performance

A series of Co_xNi_1−xFe₂O₄/multi-walled carbon nanotube (Co_xNi_1−xFe₂O₄/MWCNTs) nanocomposites as photocatalysts were successfully synthesized, where Co_xNi_1−xFe₂O₄ was synthesized via a one-step hydrothermal approach. Simultaneously, methylene blue (MB) was used as the research object to investigate the catalytic effect of the catalyst in the presence of hydrogen peroxide (H₂O₂). The results showed that all the photocatalysts exhibited enhanced catalytic activity compared to pure ferrite. In addition, compared with the other photocatalysts, the reaction time was greatly shortened a significantly higher removal rate was achieved using 3-CNF/MWCNTs. There was no significant decrease in photodegradation efficiency after three catalytic cycles, suggesting that Co_xNi_1−xFe₂O₄/MWCNTs are recyclable photocatalysts for wastewater treatment. Our results indicate that the Co_xNi_1−xFe₂O₄/MWCNT composite can be effectively applied for the removal of organic pollutants as a novel photocatalyst.

A series of Co_xNi_1−xFe₂O₄/multi-walled carbon nanotube (Co_xNi_1−xFe₂O₄/MWCNTs) nanocomposites as photocatalysts were successfully synthesized. The results implied that this composites can be effectively applied for the removal of organic pollutant as novel photocatalysts.