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.     

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.     

Zn2x(CuIn)1−xS2 photocatalysts synthesis by a hydrothermal process using H4EDTA as complexing agent

The study of the Zn_2x(CuIn)_1−xS₂ (0 ≤ x ≤ 1) solid solutions formation by hydrothermal synthesis using ethylenediaminetetraacetic acid (H₄EDTA) as a complexant and surfactant agent is reported for the first time. Different synthesis parameters were varied: the H₄EDTA concentration, the initial pH value, the Tu concentration, the duration and temperature of autoclaving process. The as obtained powders were characterized by X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and UV/vis/NIR diffuse reflection spectroscopy (DRS). The XRD and EDX results show that, with a careful adjustment of the reaction conditions, especially of the H₄EDTA concentration and initial pH value of the precursor solution, a mixture of solid solutions with sphalerite type structure can be obtained, which transforms into a single phase solid solution after heat treatment. The particles, as revealed by SEM investigations, have nanoporous hexagonal microplates morphology, about 1 μm thick and several microns in diameter. The Cu_0.159In_0.111Zn_1.778S₂ photocatalyst obtained by this method presents photocatalytic activity for hydrogen evolution from aqueous solutions containing S²⁻ ions as sacrificial agent, even without cocatalyst.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

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.     

3D hierarchical ZnIn2S4: The preparation and photocatalytic properties on water splitting

Hexagonal ZnIn₂S₄ photocatalysts with 3D-hierarchical persimmon-like shape have been successfully synthesized via an oleylamine (OA)-assisted solvothermal method. Hydrogen evolution experiments revealed that the obtained hierarchical ZnIn₂S₄ possessed good photocatalytic activity, e.g. hydrogen production rate reached to 220.45 μmol h⁻¹ and the quantum yield was up to 13.16% when 3% Pt was loaded. Further delicate tuning the percentage of exposed facet of the obtained ZnIn₂S₄ crystals, it was found that the increase of {006} facets, terminated by metal ions, would improve their photocatalytic activity, and the relationship between the crystal structure and photocatalytic properties had been studied.     

Low-temperature preparation of AgIn5S8/TiO2 heterojunction nanocomposite with efficient visible-light-driven hydrogen production

AgIn₅S₈ and AgIn₅S₈/TiO₂ heterojunction nanocomposite with efficient photoactivity for H₂ production were prepared by a low-temperature water bath deposition process. The resultant AgIn₅S₈ shows an absorption edge at ∼720 nm, corresponding to a bandgap of ∼1.72 eV, and its visible-light-driven photoactivity (100.1 μmol h⁻¹) for H₂ evolution is 9 times higher than that (11 μmol h⁻¹) of the product derived from a hydrothermal process, while the obtained AgIn₅S₈/TiO₂ heterojunction nanocomposites prepared by using commercially available TiO₂ nanoparticles (P25) as TiO₂ source exhibit remarkably improved photoactivity as compared to the pristine AgIn₅S₈, and the AgIn₅S₈/TiO₂ nanocomposite with molar ratio of 1:10 shows a maximum photocatalytic H₂ evolution rate (371.1 μmol h⁻¹), which is 4.3 times higher than that (85 μmol h⁻¹) of the corresponding AgIn₅S₈/TiO₂ nanocomposite derived from a hydrothermal method. This significant enhancement in the photocatativity of the present AgIn₅S₈/TiO₂ nanocomposite can be ascribed to the better dispersion of the AgIn₅S₈ formed on TiO₂ nanoparticle surfaces and the more intimate AgIn₅S₈/TiO₂ heterojunction structure during the water bath deposition process under continuously stirring as compared to the corresponding nanocomposite derived from a hydrothermal method. This configuration of nanocomposite results in fast diffusion of the photogenerated carriers in AgIn₅S₈ towards TiO₂, which is beneficial for separating spatially the photogenerated carriers and improving the photoactivity. The present findings shed light on the tuning strategy of spectral responsive region and photoactivity of photocatalysts for efficient light-to-energy conversion.     

A red La(OH)3/TiO2:B,N composite photocatalyst for broad-band visible-light-driven hydrogen evolution

A red La(OH)₃/TiO₂:B,N composite photocatalyst with broad-band visible-light-response was successfully synthesized through a three-step synthetic route. Structural, morphologic, optical and surface characteristics of the as-prepared photocatalysts were characterized by XRD, SEM, TEM, UV–vis, BET and XPS measurements. The 1.5% La added sample exhibited a high photocatalytic performance (48 μmol h⁻¹) in the presence of K₂CO₃ as the sacrificial agent under visible-light irradiation (λ > 400 nm), which is almost a 24-fold increase compared with the sample without La added.     

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.     

A simplified method for synthesis of band-structure-controlled (CuIn)xZn2(1-x)S2 solid solution photocatalysts with high activity of photocatalytic H2 evolution under visible-light irradiation

(CuIn)_xZn_2(1-x)S₂ (x = 0.01–0.5) microspheres were prepared by a simple hydrothermal method. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectra (UV–Vis), Raman scattering spectra and Brunauer-Emmett-Teller (BET) surface area measurement. It was found that the (CuIn)_xZn_2(1-x)S₂ samples formed solid solution only in the presence of surfactant cetyltrimethylammonium bromide (CTAB). CTAB enabled the increase of the surface area for the (CuIn)_xZn_2(1-x)S₂ solid solutions. Diffuse reflection spectra of the solid solutions shifted monotonically to long wavelength side as the value of x increased. The photocatalytic H₂ evolution activity under visible-light irradiation of the solid solutions was evaluated. The result showed that the activity depended on their corresponding compositions closely. Ru (1.5 wt%)-loaded (CuIn)_0.2Zn_1.6S₂ showed the highest photocatalytic activity of 198.09 μmol h⁻¹ under visible-light irradiation, and the apparent quantum yield amounted to 15.45% at 420 nm. Furthermore, the density functional theory (DFT) calculations showed that the solid solution with the ration of ZnS and CuInS₂, 6:1, was a direct band gap semiconductor. The valence band consisted of the hybrid orbital of S 3p and Cu 3d and the conduction band consisted of In 5s5p orbital mixed with Zn 4s4p. The energy band structure resulted in the visible-light response of the solid solution, and affected its photocatalytic performance.     

Nanocomposite of BiPO4 and reduced graphene oxide as an efficient photocatalyst for hydrogen evolution

Nanocomposites of BiPO₄ and reduced graphene oxide (BiPO₄/RGO) synthesized by hydrothermal method, hydrazine reduction, and UV-assisted photoreduction method were studied as photocatalysts for hydrogen evolution from ethanol aqueous solution under irradiation. The incorporation of RGO into BiPO₄ significantly enhanced the photocatalytic activity for H₂ evolution, and the photocatalytic activity increases in the order of BiPO₄/RGO-hydrothermal > BiPO₄/RGO-photoreduction > BiPO₄/RGO-hydrazine. The optimum proportion of GO is 2 wt% for all the samples prepared by different methods. The rate of H₂ production calculated for BiPO₄/RGO-hydrothermal (with 2 wt% GO) nanocomposite was about 306 μmol/h/g, which was almost 2 times as high as that for bare BiPO_4. The XRD, Raman and XPS characterization suggested that the original GO was successfully reduced to RGO. The more intimate contact between BiPO₄ and RGO, the higher photocurrent responses and the higher reduction degree of RGO was consistent with the higher photocatalytic performance.     

Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water

Highly dispersed CuO was introduced into TiO₂ nanotube (TNT) made by hydrothermal method via adsorption-calcination process or wet impregnation process, to fabricate CuO incorporated TNT photocatalysts (CuO-TNT) for hydrogen production. It was found that CuO-TNT possessed excellent hydrogen generation activity, which was constantly vigorous throughout 5 h reaction. Depending on the preparation method, hydrogen evolution rates over CuO-TNT were founded in the range of 64.2-71.6 mmol h⁻¹ g⁻¹_catalyst, which was much higher than the benchmark P25 based photocatalysts, and even superior to some Pt/Ni incorporated TNT. This high photocatalytic activity of CuO-TNT was mainly attributed to the unique 1-D tubular structure, large BET surface area and high dispersion of copper component. Compared to wet impregnation, adsorption-calcination process was superior to produce active photocatalyst, since it was prone to produce photocatalyst with more highly dispersed CuO.     

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.     

Facile hydrothermal synthesis of urchin-like NiCo2O4 spheres as efficient electrocatalysts for oxygen reduction reaction

In this work, we have successfully prepared urchin-like NiCo₂O₄ spheres via a facile hydrothermal method without any templates or surfactants. The as-prepared urchin-like NiCo₂O₄ spheres have uniform diameters of 6 μm with numerous small nanorods radially-grown from the center. Typical nanorods have diameters of 20–50 nm and lengths of 2–3 μm. Remarkably, urchin-like NiCo₂O₄ nanostructure as an alternative non-precious metal electrocatalyst for ORR exhibits catalytic performance and excellent cycling stability in alkaline environment.     

Visible-light sensitive hydrogen evolution photocatalyst ZnRh2O4

We investigated the ability of the oxide ZnRh₂O₄ to serve as a solar H₂-evolution photocatalyst due to the predicted potential of its conduction band bottom, which may allow thermodynamically favorable H₂ evolution in spite of its small band-gap of 1.2 eV. ZnRh₂O₄ produced H₂ in the presence of HCHO, but only scarcely in the presence of CH₃OH, indicating that the potential of the valence band top of ZnRh₂O₄ lies at ∼0.1 V (vs. SHE). Thus, the conduction band bottom potential (∼−1.1 V) lies much more negative than the potential of H⁺/H₂, allowing thermodynamically favorable H₂ evolution. In addition, the irradiated-light-wavelength dependence of the quantum efficiency (QE) for H₂ evolution was consistent with the solar spectrum, and the QE was quite high (∼27%), even at a wavelength of 770 ± 25 nm. Taken together, our findings indicate that ZnRh₂O₄ can utilize solar light effectively, not only the entire range of UV and visible light, but is also sensitive to infrared light.     

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.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Highly efficient TiO2 nanotube photocatalyst for simultaneous hydrogen production and copper removal from water

1-D mesoporous TiO₂ nanotube (TNT) with large BET surface area was successfully synthesized by a hydrothermal-calcination process, and employed for simultaneous photocatalytic H₂ production and Cu²⁺ removal from water. Cu²⁺, across a wide concentration range of 8-800 ppm, was removed rapidly from water under irradiation. The removed Cu²⁺ then combined with TNT to produce efficient Cu incorporated TNT (Cu-TNT) photocatalyst for H₂ production. Average H₂ generation rate recorded across a 4 h reaction was between 15.7 and 40.2 mmol h⁻¹ g⁻¹_catalyst, depending on initial Cu²⁺/Ti ratio in solution, which was optimized at 10 atom%. In addition, reduction process of Cu²⁺ was also a critical factor in governing H₂ evolution. In comparison with P25, its large surface area and 1-D tubular structure endowed TNT with higher photocatalytic activity in both Cu²⁺ removal and H₂ production.