Visible-light-driven ZnIn2S4/CdIn2S4 composite photocatalyst with enhanced performance for photocatalytic H2 evolution

ZnIn₂S₄/CdIn₂S₄ composite photocatalysts (x = 0–1) were successfully synthesized via a hydrothermal route. Compositions of ZnIn₂S₄/CdIn₂S₄ composite photocatalysts were optimized according to the photocatalytic H₂ evolution rate. XRD patterns indicate the as-prepared samples are mixtures of hexagonal and cubic structures. FESEM and TEM images show that the as-prepared samples are composed of flower-like microspheres with wide distribution of diameter. There is obviously distinguishing distribution of Zn, Cd elements among the composite architectures. UV–vis absorption spectra of different compositions exhibit that absorption edges of ZnIn₂S₄/CdIn₂S₄ composites slightly move towards longer wavelengths with the increment of CdIn₂S₄ component. A typical time course of photocatalytic H₂ evolution from an aqueous Na₂SO₃ and Na₂S solution over unloaded and PdS-loaded ZnIn₂S₄/CdIn₂S₄ composite photocatalyst is carried out. The initial activity for H₂ evolution over 0.75 wt% PdS-loaded sample is up to 780 μmol h⁻¹. And the activity of unloaded sample also reaches 490 μmol h⁻¹ with consistent stability.     

Photocatalytic hydrogen generation in the presence of glucose over ZnS-coated ZnIn2S4 under visible light irradiation

ZnS coated ZnIn₂S₄ (ZnS–ZnIn₂S₄) photocatalysts were prepared in methanol by a facile solvothermal process. The photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), UV–Vis diffusive reflectance spectroscopy (DRS), BET, and electrochemistry measurements. ZnS–ZnIn₂S₄ photocatalysts have hexagonal crystal phase and complex morphology such as micro-spheres, micro-tubes and micro-ribbons. Using glucose as an electron donor, photocatalytic hydrogen generation over Pt/ZnS–ZnIn₂S₄ was investigated. The results show that photoactivity of hydrogen generation over Pt/ZnS–ZnIn₂S₄ was improved notably with simultaneous degradation of glucose. The factors which affect photocatalytic hydrogen generation, such as composition of ZnS-ZnIn₂S₄, initial concentration of glucose and concentration of NaOH, were investigated. The prepared ZnS–ZnIn₂S₄ photocatalysts exhibit better activity for hydrogen generation than pure ZnIn₂S₄, which may be attributed to enhancement of the adsorption of glucose by ZnS on the ZnIn₂S₄ surface. The effect of glucose concentration on the hydrogen generation rate is consistent with a Langmuir model. The basic condition is favorable for the photocatalytic hydrogen generation. A large number of ·OH radicals generated in ZnS–ZnIn₂S₄ system, have been tested by a TA-FL (terephthalic acid-fluorescence) method. A possible mechanism was discussed.     

Fabrication of graphene/CaIn2O4 composites with enhanced photocatalytic activity from water under visible light irradiation

A series of graphene/CaIn₂O₄ composites were synthesized using a facile solvothermal method to improve the photocatalytic performance of CaIn₂O₄. The reduction of graphene oxide to graphene and the deposition of CaIn₂O₄ nanoparticles on the graphene sheets can be achieved simultaneously during the solvothermal process. The photocatalytic activities of as-prepared graphene/CaIn₂O₄ composites for hydrogen evolution from CH₃OH/H₂O solution were investigated under visible light irradiation. It was found that graphene exhibited an obvious influence on the photocatalytic activity of CaIn₂O₄. The graphene/CaIn₂O₄ composite reached a high H₂ evolution rate of 62.5 μmol h⁻¹ from CH₃OH/H₂O solution when the content of graphene was 1 wt%. Furthermore, the 1 wt% graphene/CaIn₂O₄ composite did not show deactivation for H₂ evolution for longer than 32 h. This work could provide a new insight into the fabrication of visible light driven photocatalysts with efficient and stable performance.     

Preparation of ZnIn2S4/fluoropolymer fiber composites and its photocatalytic H2 evolution from splitting of water using Xe lamp irradiation

This paper focuses on the preparation of ZnIn₂S₄/fluoropolymer fiber composites and their performance for H₂ evolution from splitting of water using Xe lamp irradiation. Hexafluorobutyl acrylate-co-methacrylic acid (poly(HFBA-co-MAA)) is synthesized by a solution polymerization. Next, the fluoropolymer fibers, which have around 100 nm in average diameter, of poly(HFBA-co-MAA) and polyvinylidene fluoride (PVDF) mixtures are obtained by electrospinning. Then, zinc and indium ions are introduced onto the fiber surface by coordinating with carboxyls of MAA. After that, sulfide ions are incorporated to react with zinc and indium ions by a hydrothermal synthesis. Thus, ZnIn₂S₄ particles of around 800 nm in average size, are obtained and well loaded on the fiber surface. The absorption edge of ZnIn₂S₄/fluoropolymer fiber composites is at 510 nm within the visible-light region. Photocatalytic H₂ evolution from water was investigated using Xe lamp. It was found that the average rate of H₂ evolution of ZnIn₂S₄ powders gradually decreased, while the average rate of H₂ evolution of ZnIn₂S₄/fluoropolymer fiber composites increased from the first to the third run. The average rate of H₂ evolution using the ZnIn₂S₄/fluoropolymer fiber composites as the catalyst achieved 9.1 mL/h in the third run.     

Surfactant tunable hierarchical nanostructures of CdIn2S4 and their photohydrogen production under solar light

The hierarchical nanostructures of CdIn₂S₄ were selectively prepared through hydrothermal process in the presence of different surfactants. Structural study demonstrated existence of cubic spinel structure and micro structural study shown a pretty marigold flower like morphology without any surfactant. The effect of surfactants on the morphology and microstructure of CdIn₂S₄ has been studied by using Polyvinyl pyrrolidone (PVP) and Cetyltrimethyl ammonium bromide (CTAB) as a surfactants. The CdIn₂S₄ bipyramids with length of 0.7-1 μm have been obtained using PVP. Interestingly, the nanopetals (thin and transparent) of CdIn₂S₄ are self assembled into hollow spheres in the presence of CTAB. Considering the importance of these unique nanostructures, the growth mechanism has also been proposed. The optical properties demonstrated the band gap in the range of 2.12-2.29 eV which is well within the visible region. In this contest, photocatalytic activity for hydrogen production using the above nanostructures under visible light was also demonstrated. The prima-fascia observations shows that the bipyramidal CdIn₂S₄ exhibit excellent photocatalytic activity for hydrogen production (3238 μmolh⁻¹) than other nanostructures. Being a nanostructured semiconductor chalcogenide with a good stability will also have potential applications in solar cells and LED.     

Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light

The advantage of copper doping onto TiO₂ semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500 °C. It was found that 10 wt.% Cu/TiO₂ calcined at 300 °C for 30 min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.     

CuCr2O4/TiO2 heterojunction for photocatalytic H2 evolution under simulated sunlight irradiation

CuCr₂O₄/TiO₂ heterojunction has been successfully synthesized via a facile citric acid (CA)-assisted sol-gel method. Techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectrum (UV-vis DRS) have been employed to characterize the as-synthesized nanocomposites. Furthermore, photocatalytic activities of the as-obtained nanocomposites have been evaluated based on the H₂ evolution from oxalic acid solution under simulated sunlight irradiation. Factors such as CuCr₂O₄ to TiO₂ molar ratio in the composites, calcination temperature, photocatalyst mass concentration, and initial oxalic acid concentration affecting the photocatalytic hydrogen producing have been studied in detail. The results showed that the nanocomposite of CuCr₂O₄/TiO₂ is more efficient than their single part of CuCr₂O₄ or TiO₂ in producing hydrogen. The optimized composition of the nanocomposites has been found to be CuCr₂O₄·0.7TiO₂. And the optimized calcination temperature and photocatalyst mass concentration are 500 °C and 0.8 g l⁻¹, respectively. The influence of initial oxalic acid concentration is consistent with the Langmuir model.     

Pd-Gardenia-TiO2 as a photocatalyst for H2 evolution from pure water

The photocatalytic activity for H₂ evolution from pure water over Pd loaded TiO₂ prepared by gardenia extract (Pd-Gardenia-TiO₂) is systematically investigated. The as-prepared photocatalysts are characterized by X-ray diffraction, high resolution transmission electron microscopy, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. Gardenia extract functions as reducing and stabilizing agents simultaneously. The mean size of the as-prepared Pd nanoparticles is in the range of 2.3 ± 0.5 nm based on TEM images. The Pd-Gardenia-TiO₂ catalyst exhibits good photocatalytic activity for H₂ evolution (93 μmol · h⁻¹ · g⁻¹), which is much higher than that of Pd photodeposited on TiO₂. Possible factors for its photocatalytic activity from pure water are also investigated.     

Cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthesis of ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production

A series of ZnIn₂S₄ photocatalysts was synthesized via a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method. These ZnIn₂S₄ products were characterized by X-ray diffraction (XRD), UV–visible absorption spectra (UV–vis) and scanning electron microscopy (FESEM). The effects of hydrothermal time and CTAB on the crystal structures, morphologies and optical properties of ZnIn₂S₄ products were discussed in detail. The photocatalytic activities of the as-prepared samples were evaluated by photocatalytic hydrogen production from water under visible-light irradiation. It was found that the photocatalytic activities of these ZnIn₂S₄ products decreased with the hydrothermal time prolonging while increased with the amount of CTAB increasing. The highest quantum yield at 420 nm of ZnIn₂S₄ photocatalyst, which was prepared through the CTAB (9.6 mmol)-assisted hydrothermal procedure for 1 h, was determined to be 18.4%. The optimum amount of Pt loaded for the ZnIn₂S₄ photocatalyst was about 1.0 wt%, under the present photocatalytic system.     

Ag3PO4 photocatalyst: Hydrothermal preparation and enhanced O2 evolution under visible-light irradiation

Visible-light-driven semiconducting photocatalysts of Ag₃PO₄ were prepared by a hydrothermal method, and were optimized by adjusting reaction conditions, i.e., temperature, pH of reaction solution, concentration of feedstock, and time of hydrothermal process. The obtained photocatalysts were then systematically characterized by different instruments, such as XRD, UV–vis, FESEM, and BET, to reveal the physicochemical properties. Furthermore, activities of photocatalysts for visible-light-driven O₂ evolution were evaluated, demonstrating that the photocatalytic activity of Ag₃PO₄ prepared by hydrothermal reaction (initial rate of O₂ evolution, 1156 μmol g⁻¹ h⁻¹) was more than two times as that of sample prepared by room-temperature reaction (initial rate of O₂ evolution, 533 μmol g⁻¹ h⁻¹), which could be attributed to its better ability to utilize visible light and more regulated morphology.     

H2 evolution from a photoelectrochemical cell with n-Cu2O photoelectrode under visible light irradiation

H₂ evolution was observed for the first time from a photoelectrochemical cell using an n-type Cu₂O photoelectrode under visible light irradiation. Three-electrode configuration was used in the photoelectrochemical cell to observe H₂ evolution. AgCl/Ag calomel electrode and a platinum plate were used as the reference and counter electrodes, respectively. Fe²⁺/Fe³⁺ redox couple was used as the electrolyte. H₂ evolution was visible on the platinum electrode in the photoelectrochemical cell.     

Stability of an H2-producing photocatalyst (Ru/(CuAg)0.15In0.3Zn1.4S2) in aqueous solution under visible light irradiation

Efficient photocatalytic water-splitting systems require stable photocatalysts that have photocatalytic activity with repeated consecutive use. This study investigated H₂ production under visible light irradiation with an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst and KI as an electron donor. In addition, the stability and reusability of the catalyst were evaluated over multiple cycles of H₂ production and catalyst regeneration. The results show that sintering temperature influenced the crystallinity and photocatalytic activity, as indicated by the X-ray diffraction analyses and H₂ production rates. In particular, the catalyst sintered at 873 K yielded the highest quantum yield of 4.6% at 420 ± 5 nm of wavelength. After seven consecutive reaction cycles, the quantum yield decreased from 4.6% to 3.0% at the end of the seventh cycle. The decrease probably occurred because (1) particles of the catalyst underwent pronounced aggregation, which led to the increase in particle size; and (2) a release of significant metal ions was observed during H₂ production, which led to a loss of the catalyst mass and potential changes in the photocatalytic activity. This study will help facilitate a search of stable photocatalysts for water splitting.     

Bimetallic PtNi/g-C3N4 nanotubes with enhanced photocatalytic activity for H2 evolution under visible light irradiation

Bimetallic PtNi-decorated graphitic carbon nitride (g-C₃N₄) nanotubes were prepared through calcining the mixture of urea and thiourea in the presence of Pluronic F127, followed by deposition of bimetallic PtNi nanoparticles (NPs) via chemical reduction. It is found that the photocatalytic activity of PtNi/g-C₃N₄ nanotubes is strongly dependent on the molar ratio of Pt/Ni and the highest activity is observed for Pt₁Ni₁/g-C₃N₄. Under visible light (λ > 420 nm) irradiation, the H₂ generation rate over Pt₁Ni₁/g-C₃N₄ nanotubes is 104.7 μmol h^?1 from a triethanolamine (10 vol%) aqueous solution, which is higher than that of Pt/g-C₃N₄ nanotubes (98.6 μmol h^?1) and is about 47.6 times higher than that of pure g-C₃N₄ nanotubes. The cyclic photocatalytic reaction indicates that our Pt₁Ni₁/g-C₃N₄ nanotubes function as a stable photocatalyst for visible light-driven H₂ production. The effect of bimetallic PtNi NPs in the transfer and separation of photogenerated charge carriers occurring in the excited g-C₃N₄ nanotubes was investigated by performing photo-electrochemical and photoluminescence measurements. Our results reveal that bimetallic PtNi could replace Pt as a promising cocatalyst for photocatalytic H₂ evolution with better performance and lower cost.     

Improving photocatalytic activity for hydrogen evolution over ZnIn2S4 under visible-light: A case study of rare earth modification

A series of rare earth (RE) ions (La³⁺, Ce³⁺, Gd³⁺, Er³⁺ or Y³⁺) modified ZnIn₂S₄ photocatalysts (RE-ZnIn₂S₄) were prepared using the hydrothermal method and characterized by various analysis techniques, such as UV–Vis diffusive reflectance spectroscopy, X-ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller surface analyzer, photoluminescence spectroscopy and X-ray photoelectron spectroscopy. The results indicated that the RE element exists as the oxide RE₂O₃ and their modification can reduce ZnIn₂S₄ crystallite size, inhibit ZnIn₂S₄ grain growth, promote ZnIn₂S₄ crystallite self-organization into a micro-sphere flower-like morphology, increase ZnIn₂S₄ surface area and total pore volume, and bring rich defects to ZnIn₂S₄. The photocatalytic activities of RE-ZnIn₂S₄ were evaluated based on photocatalytic hydrogen production from water under visible-light irradiation and the hydrogen production efficiency increased by 46%, 53%, 61%, 69%, and 106% after adding 2.0 wt% of Y, Gd, Er, Ce and La, respectively. The relationship between the photocatalytic activity of RE-ZnIn₂S₄ and the RE properties was discussed.     

g-C3N4 coated SrTiO3 as an efficient photocatalyst for H2 production in aqueous solution under visible light irradiation

A highly active photocatalyst based on g-C₃N₄ coated SrTiO₃ has been synthesized simply by decomposing urea in the presence of SrTiO₃ at 400 °C. The catalyst demonstrates a high H₂ production rate ∼440 μmol h⁻¹/g catalyst in aqueous solution under visible light irradiation, which is much higher than conventional anion doped SrTiO₃ or physical mixtures of g-C₃N₄ and SrTiO₃. The improved photocatalytic activity can be ascribed to the close interfacial connections between g-C₃N₄ and SrTiO₃ where photo-generated electron and holes are effectively separated. The newly synthesized catalyst also exhibited a stable performance in the repeated experiments.     

Synthesis of g-C3N4/TiO2 with enhanced photocatalytic activity for H2 evolution by a simple method

A photocatalyst composed of graphite-like carbon nitride (g-C₃N₄) and TiO₂ was fabricated by a simple method to calcine the mixture of melamine and TiO₂ precursor. The photocatalyst has enhanced photoactivity for hydrogen evolution from water. Characterization by XRD, FTIR, SEM and elemental analysis showed that the crystal structure and morphologies of composites were affected by the amount of melamine in the composite. The UV–Vis characterization displayed that the optical absorption range of g-C₃N₄/TiO₂ hybrid was broadened with a synergistic effect. The photoactivity for H₂ evolution was shown that the best result obtained from the composite with 67 wt% melamine has about 5 times improvement compared with bare TiO₂ or pure g-C₃N₄. The enhanced photoactivity might be related with the favorable structure resulted from heat-treatment temperature, and the content of g-C₃N₄ participating in wide optical absorption, separation and transportation of electronic-holes, as well as morphology of composite.     

The pH effects on H2 evolution kinetics for visible light water splitting over the Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst

Much progress has been made in the development of novel visible light photocatalysts that split water into hydrogen (H₂) and oxygen (O₂). In this study, we examine the impact of initial solution pH on H₂ production using an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst under visible light irradiation. In addition, the reaction mechanism was analyzed by examining the oxidation products of the electron donor (I‾) at different solution pH values. The results show that the initial pH significantly influenced the rate of H₂ production and quantum yield (QY). In particular, the photocatalyst yielded the highest apparent QY (∼12.8%) at 420 ± 5 nm and highest H₂ production rate (∼525 μmol h⁻¹) at pH 2; with increasing pH, the H₂ production and QY decreased significantly. The oxidation product of I‾ at pH < 6 was mainly I₃‾, whereas at pH > 6 water splitting did not occur at all, so no IO₃‾ or I₂ were observed.