Elaboration,physical and photo-electrochemical properties of NiCr2O4. Application to hydrogen production under visible irradiation

The physical and photoelectrochemical characterization of NiCr₂O₄, prepared by sol gel route, were investigated to be applied for the H₂ production. The thermal gravimetry (TG) indicates that the single phase is formed above 530 °C as confirmed by X-ray diffraction (XRD). The Nano powder crystallizes in a tetragonal structure with lattice constants: a = 8.3276 Å and c = 8.5542 Å and a particle size of 63 nm, smaller than that obtained by Transmission Electronic Microscopy (TEM) analysis; the latter gives sizes between 80 and 150 nm, indicating crystallites agglomeration. The variation of the dielectric constant (ε) with temperature gives a relative value of 26 at 10 kHz. A direct optical transition at 1.79 eV is determined from the diffuse reflectance spectroscopy assigned to Cr³⁺ octahedrally coordinated. The thermal variation of the conductivity shows that 3d-electrons are localized and the data are modelled by a lattice-polaron hopping with an activation energy of 0.17 eV. The dependence of the interfacial capacitance on the potential (C⁻² - E) indicates p-type behavior with a flat band potential (E_fb) of −0.23 VSCE and holes density (N_A) of 5.88 × 10¹⁶ cm⁻³. The potential of the conduction band (−1.85 V_SCE) is below the H₂O/H₂ level (∼-1.2 V_SCE), allowing a spontaneous H₂-release under visible light. The O₂ evolution occurs at high over-voltage as shown from the intensity-potential (J-E) characteristic in Na₂SO₄ solution (0.1 M) and a hole scavenger was used to preclude the photo corrosion. The NiCr₂O₄ mass, pH and the hole scavenger (S₂O₃²⁻ and NO₂⁻) were optimized. The H₂ volume reached 65 μmol with an evolution rate of 8.6 μmol g⁻¹ min⁻¹, liberated under optimized conditions {1.2 g catalyst L⁻¹, pH ∼9 with thiosulfate S₂O₃²⁻ [10⁻³ M]}.     

Enhanced photoelectrocatalytic hydrogen production performance of porous MoS2/PPy/ZnO film under visible light irradiation

Photoelectrochemical (PEC) water splitting provides a “green” approach for hydrogen production. However, the design and fabrication of high-efficient catalysts are the bottleneck for PEC water splitting owing to the involved thermodynamic and kinetic challenges. Herein, we report a new strategy for constructing a porous MoS₂/PPy/ZnO thin film photocatalyst with large specific surface area and excellent conductivity to achieve photoelectrochemical water splitting under visible light irradiation. Porous PPy/ZnO was synthesized via template-assisted electrodeposition, and MoS₂ was further electrodeposited to construct porous MoS₂/PPy/ZnO thin film photocatalyst. The hydrogen evolution rate of MoS₂/PPy/ZnO exhibits about 3.5-fold increase to 40.22 μmol cm⁻² h⁻¹ under visible light irradiation. The enhancement for photoelectrochemical hydrogen production is not only ascribed to enlarged specific surface area of the porous structure, but also attributed to the synergistic effects of MoS₂ and porous PPy/ZnO, which could dramatically improve its visible light absorption capacity and enhance the separation and transfer of photogenerated charges. Thus, more abundant photogenerated electrons and holes participate in photoelectrochemical process, which significantly enhances its photoelectrochemical hydrogen production performance.     

H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor

SrTiO₃:Cr/Ta powders were prepared by spray pyrolysis from polymeric precursors. Effects of the amount of co-dopant and additives on the photocatalytic activity for hydrogen evolution from aqueous methanol solution under visible light irradiation (λ > 415 nm) were investigated. For the photocatalyst prepared by spray pyrolysis from polymeric precursor, the hydrogen evolution rate was increased by a factor of ∼100 and induction period was decreased by a factor of 8 compared with a photocatalyst prepared by solid state reaction. These enhancements result from increased roughness of surface, and the compositional uniformity which are intrinsic characteristics of spray pyrolysis. In addition, photocatalyst prepared by spray pyrolysis from polymeric precursor have large BET surface area and small amount of Cr⁶⁺ ion which is responsible for long induction period. It should be noted that the reduction of Cr⁶⁺ ion was achieved without hydrogen reduction process.     

Photocatalytic H2 evolution under visible light from aqueous methanol solution on NaBixTa1−xO3 prepared by spray pyrolysis

Solid–solution NaBi_xTa_1−xO₃ photocatalyst powders were prepared by spray pyrolysis from aqueous and polymeric precursor solutions. The effects of Bi ions and additives in the precursors on the photocatalytic activities of the resulting catalysts were investigated by measuring hydrogen evolution rate from aqueous methanol solution under visible light irradiation (λ > 415 nm). The effects of NiO co-catalyst on hydrogen evolution rate are also tested. Hydrogen evolution rate was enhanced almost 20 times (to 1355 μmol g⁻¹ h⁻¹), with an induction period of 1 h, compared with a photocatalyst prepared by hydrothermal method because of the compositional uniformity and unique surface morphology with large BET surface area of the photocatalyst. Bi ions in the photocatalyst were confirmed by XPS to be mainly present as Bi⁵⁺. Polymeric additives to the precursors and NiO co-catalyst resulted in maximized hydrogen evolution rate when used at 300 mol% and 0.2 wt.%, respectively. The composition of the photocatalyst was optimized at NaBi_0.07Ta_0.93O₃.     

H2 evolution under visible light irradiation on La and Cr co-doped NaTaO3 prepared by spray pyrolysis from polymeric precursor

Photocatalysts of Na_1−xLa_xTa_1−xCr_xO₃ and NaTa_1−xCr_xO₃ were prepared by spray pyrolysis from aqueous and polymeric precursor solution. Apart from the contribution of La³⁺ ions co-doped into NaTa_1−xCr_xO₃ on the BET surface area and the surface morphology by preventing crystal growth, this co-doping contributed to the increased Cr³⁺ concentration by partially tuning the electron configuration from A⁺B⁵⁺O₃ to (A⁺A′³⁺)²⁺(B⁵⁺B′³⁺)⁴⁺O₃ in the lattice of the photocatalyst. Na_1−xLa_xTa_1−xCr_xO₃ prepared from polymeric precursor solution reduced the induction period to 33% and enhanced the hydrogen evolution rate 5.6-fold to 1467.5 μmol g⁻¹ h⁻¹ compared with the equivalent values of NaTa_1−xCr_xO₃ prepared from aqueous precursor. The optimum amounts of dopant and additives comprising the polymeric precursor to maximize the hydrogen evolution rate were x = 0.003 and 300 mol%, respectively.     

Effects of charge balance with Na on SrTiO3:Mo prepared by spray pyrolysis for H2 evolution under visible light irradiation

Photocatalyst powders of SrTi_1−xMo_xO₃ and Sr_1−2xNa_2xTi_1−xMo_xO₃ were prepared by spray pyrolysis for hydrogen evolution for the first time from aqueous methanol solution under visible light irradiation. The co-doping of Mo⁶⁺/Na⁺ ions resulted in increase of BET surface area and pore volume, and formation of unique morphology with wrinkled, furrowed and porous surface, without significant distortion of lattice structure of host material. The hydrogen evolution rate of Sr_1−2xNa_2xTi_1−xMo_xO₃ photocatalyst was enhanced up to 1115.8 μmol g⁻¹ h⁻¹ with an induction period of 1 h under visible light irradiation, which was 1.5 times higher than that of SrTi_1−xMo_xO₃. The co-dopant Na⁺ ion contributed to the charge balance in the host material by compromising the excess positive charge of Mo⁶⁺, which was effective for enhancing the hydrogen evolution rate. The optimum composition of photocatalyst corresponding to the maximum hydrogen evolution rate was Sr_1−2xNa_2xTi_1−xMo_xO₃ (x = 0.004).     

Effect of tri-doping on H2 evolution under visible light irradiation on SrTiO3:Ni/Ta/La prepared by spray pyrolysis from polymeric precursor

Tri-doped photocatalyst, SrTiO₃:Ni/Ta/La, was prepared by spray pyrolysis from aqueous and polymeric precursor solutions. The third dopant, La³⁺, contributed to the BET surface area and porous morphology by preventing crystal growth, and increased the Ni²⁺/Ni³⁺ ratio by affecting the electron configuration in the lattice structure, which is closely related to the hydrogen evolution rate. The hydrogen evolution rate of the tri-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%)/La(0.3 mol%), was increased by about 60%–895.2 μmol g⁻¹ h⁻¹ from the value of 561.2 μmol g⁻¹ h⁻¹ for the co-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%), and was further enhanced to 2305.7 μmol g⁻¹ h⁻¹ when a polymeric precursor was used instead of an aqueous precursor in spray pyrolysis. The optimum additive content for polymeric precursor solution was 300 mol%.     

Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation

The photocatalytic water splitting is a promising process for producing H₂ from two abundant renewable sources of water and solar light, with the aid of a suitable photocatalyst. In this work, a combination of sensitizer addition and noble metal loading was employed to modify perovskite photocatalysts in order to achieve the enhancement of photocatalytic H₂ production under visible light irradiation. The dependence of the H₂ production on type of mesoporous-assembled perovskite titanate nanocrystal photocatalysts (MgTiO₃, CaTiO₃, and SrTiO₃), calcination temperature of photocatalyst, Pt loading, type and concentration of electron donor (diethanolamine, DEA; and triethanolamine, TEA), concentration of sensitizer (Eosin Y, E.Y.), photocatalyst dosage, and initial solution pH, was systematically studied. The experimental results showed that the 0.5 wt.% Pt-loaded mesoporous-assembled SrTiO₃ nanocrystal synthesized by a single-step sol-gel method and calcined at 650 °C exhibited the highest photocatalytic H₂ production activity from a 15 vol% DEA aqueous solution with dissolved 0.5 mM E.Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic H₂ production activity were found to be 6 g/l and 11.6, respectively.     

High photocatalytic performance for hydrogen production under visible light on the hetero-junction Pani-ZnO nanoparticles

The present work is devoted to the preparation of the hetero-junction of Polyaniline-Zinc oxide nanoparticles (Pani-ZnO_Nps) and its photo-electrochemistry to assess its photocatalytic properties for the water reduction into hydrogen. The semiconducting characterization of the Pani-ZnO_Nps synthetized by in situ chemical oxidative polymerization was studied for the hydrogen evolution reaction (HER) upon visible light illumination. The forbidden bands E_g (= 1.64 eV, Pani) and (3.20 eV, ZnO_NPS) were extracted from the UV–Visible diffuse reflectance data. The Electrochemical Impedance Spectroscopy (EIS) showed the predominance of the intrinsic material with a bulk impedance of 71 kΩ cm². The semi conductivity was demonstrated by the capacitance measurements with flat band potentials (E_fb = - 0.7 and - 0.3 V_SCE) and carriers concentrations (N_A = 1.77 × 10¹⁹ and N_D = 4.80 × 10²⁰ cm^?3) respectively for Pani and ZnO_NPS. The energetic diagram of the hetero-junction Pani-ZnO_Nps predicts electrons injection from Pani to ZnO_NPS in KOH electrolyte. An improvement of 78% for the evolved hydrogen was obtained, compared to Pani alone; a liberation rate of 61.16 μmol g^?1 min^?1 and a quantum yield of 1.15% were obtained. More interestingly, the photoactivity was fully restored after three consecutive cycles with a zero-deactivation effect, indicating clearly the reusability of the catalyst over several cycles.     

Preparation of novel SrTiO3:Rh/Ta photocatalyst by spray pyrolysis and its activity for H2 evolution from aqueous methanol solution under visible light

SrTiO₃:Rh/Ta powder was prepared by spray pyrolysis from polymeric precursors containing citric acid and ethylene glycol. Co-doping Ta into SrTiO₃:Rh increased the presence of Rh³⁺ in the host material. The retention of balanced charges by the substitution of two Ti⁴⁺ ions in the host material by one ion each of Rh³⁺ and Ta⁵⁺ enhanced hydrogen evolution from aqueous methanol solution under visible light irradiation (λ > 415 nm) by 3.5 times (to 531 μmol h⁻¹) and reduced the induction period by 50% (to 1 h), when compared with SrTiO₃:Rh. Thorough mixing of the multi-component spray pyrolysis precursor solution resulted in highly dispersed Rh ions and porous photocatalyst particles, which showed enhanced hydrogen evolution rate.     

Dye-cosensitized graphene/Pt photocatalyst for high efficient visible light hydrogen evolution

In this work, a highly efficient and stable photocatalytic H₂ evolution catalyst was constructed on Pt deposited graphene sheets cosensitized by Eosin Y (EY) and Rose Bengal (RB). Under two-beam monochromic light irradiation (520 and 550 nm), a high quantum yield (QY) up to 37.3% has been achieved owing to maximum utilization of incident visible light. As a result of the excellent electron transport properties of graphene, it can greatly facilitate the forward electron transfer from photoexcited dye molecules to Pt catalyst and suppress back electron transfer, which significantly enhances photocatalytic efficiency for H₂ evolution. This efficient cosensitization strategy is also effective in enhancing H₂ evolution efficiencies of dye sensitized TiO₂ and multiwall carbon nanotubes (MWCNTs).     

Co-doping schemes to enhance H2 evolution under visible light irradiation over SrTiO3:Ni/M (M = La or Ta) prepared by spray pyrolysis

Two photocatalysts, SrTiO₃:Ni/La and SrTiO₃:Ni/Ta, were prepared by continuous spray pyrolysis. The effects of the co-dopants on hydrogen evolution over the uncalcined photocatalysts were evaluated under visible light irradiation. The co-doping of La³⁺ into SrTiO₃:Ni transformed the charge structure and increased the presence of Ni²⁺ at the expense of Ni³⁺ in the host lattice structure. The co-doping of Ta⁵⁺ into SrTiO₃:Ni also increased the Ni²⁺/Ni³⁺ ratio around the Ti⁴⁺ ions. Compared with SrTiO₃:Ni, SrTiO₃:Ni/La showed a 3 times greater rate of hydrogen evolution under visible light irradiation and SrTiO₃:Ni/Ta, a 4 times greater rate. The co-doping levels required for optimized hydrogen evolution over SrTiO₃:Ni/La and SrTiO₃:Ni/Ta prepared by spray pyrolysis were smaller than those prepared by other methods. Spray pyrolysis also produced particles with large surface areas and high roughnesses.     

Hydrogen production from organic fatty acids using carbon-doped TiO2 nanoparticles under visible light irradiation

The photocatalytic production of H₂ using carbon-doped TiO₂ (CTiO₂) nanoparticles has been investigated in single or mixed systems of organic fatty acids (OFAs) under visible light irradiation, including acetic acid, propionate acid, butyric acid and lactic acid. When OFAs were applied at the same electron density (10 e-eq L^?1), the H₂ evolution rates followed the order of propionic acid > butyric acid ≈ acetic acid > lactic acid, whereas at the same molar concentration (0.5 mol L^?1), that order changed to lactic acid > acetic acid > butyric acid ≈ propionic acid. This result implied that the electron transfer efficiency differed from four OFAs, probably due to their different affinity with CTiO₂. O₂^?? and CH₃? partially contributed to OFAs degradation and H₂ production. The quantum dynamics simulations of electron transfer revealed that the dominant mechanism of H₂ production was direct electron transfer from adsorbed OFAs to CTiO₂. This work aims to pursue the synergy of solar energy utilization and conversion of OFAs into H₂.     

BaZrO3 hollow nanostructure with Fe (III) doping for photocatalytic hydrogen evolution under visible light

Visible-light-responded BaZrO₃ hollow nanostructure with Fe (III) doping was prepared through facile solvothermal reaction using ethanediamine (EDA) as solvent. Fe doping in BaZrO₃ as an efficient way of reducing its wide band gap (～4.96 eV) is realized in order to achieve visible light response. The corresponding trapping center introduced by Fe³⁺ doping contributes to the lower recombination of photo-induced holes and electrons. Meanwhile, the special hollow structure with larger specific area is also a key factor to increase optical absorption, charge carrier migration rate and redox reaction sites. Hence, the participation of Fe and hollow nanostructure are responsible for the visible-light-driven photocatalytic hydrogen evolution.     

SrS/CdS composite powder as a novel photocatalyst for hydrogen production under visible light irradiation

In this paper a novel SrS/CdS composite powders were prepared by coprecipitation method. The physicochemical properties of the photocatalysts were analyzed by XRD, UV–Vis, BET, PL and SEM. Photocatalytic hydrogen production results showed that these composite powders can work efficiently under visible light without loading noble metal, and it was found that the ratio of SrS/CdS equaling to 2/8 has the best performance among various SrS/CdS composite powders, and the hydrogen evolution rate amounted to 123 μmol/h under visible light irradiation. The apparent quantum yield for this photocalyst was calculated to be 2.85%, 4.59%, 9.63% at 420 nm, 440 nm and 480 nm respectively, and the apparent quantum yield under visible light was 5.83%. The reason for its high activity was analyzed.     

Enhanced photocatalytic hydrogen evolution under visible light over Cd1−xZnxS solid solution with cubic zinc blend phase

A series of Cd_1−xZn_xS solid solutions were synthesized at 80 °C with the assistance of sodium dodecylsulfate. The structures, optical properties and morphologies of the solid solutions have been studied by X-ray diffraction, UV–vis diffuse reflectance spectroscopy, and transmission electron microscopy. The photocatalytic H₂ evolution over the solid solutions under visible-light irradiation was investigated and the highest rate reached 2640 μmol h⁻¹ g⁻¹ even without any co-catalysts. The solid solution with optimum performance exhibited cubic structure rather than previously-reported hexagonal one and the possible reasons were discussed. Moreover, the effects of sacrificial reagents on the photocatalytic H₂ evolution were explored by using Na₂S solution with different concentration.     

Photocatalytic hydrogen production from aqueous methanol solutions under visible light over Na(BixTa1−x)O3 solid-solution

A series of equivalent substitution solid-solution Na(Bi_xTa_1−x)O₃ (x = 0–0.10) was prepared by a simple hydrothermal method using Ta₂O₅ and NaBiO₃ as precursors. The Na(Bi_0.08Ta_0.92)O₃ photocatalyst exhibited the highest performance of H₂ evolution (59.48 μmol h⁻¹ g⁻¹) under visible-light irradiation (λ > 400 nm) without co-catalyst, whereas no H₂ evolution is observed for NaTaO₃ under the same conditions. The UV-Vis spectra indicate that the Na(Bi_0.08Ta_0.92)O₃ powders can absorb not only ultraviolet light like pure NaTaO₃ powder but also the visible-light spectrum. The absorption edge corresponds to a band gap of 2.88 eV. The results of density functional theory calculation illuminate that the visible-light absorption bands in the Na(Bi_xTa_1−x)O₃ catalysts are attributed to the band transition from the O2p to the Bi2s + 2p + Ta5d hybrid orbital.     

Efficient hydrogen production by composite photocatalyst CdS–ZnS/Zirconium–titanium phosphate (ZTP) under visible light illumination

A novel composite CdS–ZnS/Zirconium–titanium phosphate (ZTP) photocatalyst working under visible light was successfully prepared by a two-step sulfidation procedure. The photocatalytic activity of the cadmium sulfide–zinc sulfide supported composite catalyst was evaluated toward hydrogen energy production in the presence of hole scavenger, sulfide (S²⁻) and compared with the activity of neat CdS, ZnS, ZTP, CdS–ZnS, CdS/ZTP and ZnS/ZTP without using any co-catalyst. The photocatalysts were characterized by X-ray diffraction (Small Angle X-ray diffraction Studies and Broad-Angle X-ray Diffraction studies), N₂ adsorption–desorption, diffuse reflectance UV–vis spectroscopy (DRUV-vis), photoluminescence (PL) studies, SEM/EDX, X-ray photoelectron spectroscopic (XPS) studies, transmission electron microscopy (TEM) etc. Amongst all the catalysts, 5CdS–ZnS/ZTP showed highest results toward hydrogen production (2142.7 μmol) with an apparent quantum efficiency of 9.6% under visible light illumination.     

Rh nanospheres anchored TaON@Ta2O5 nanophotocatalyst for efficient hydrogen evolution from photocatalytic water splitting under visible light irradiation

Rh nanospheres anchored TaON@Ta₂O₅ with shell-core structure was synthesized from in-situ Rh³⁺-doped Ta₂O₅ by one-step high-temperature ammonolysis, which exhibited efficient visible light photoactivity for H₂-evolution (39.41 μmol·g⁻¹·h⁻¹), much higher than unmodified Ta₃N₅@Ta₂O₅ (21.75 μmol·g⁻¹·h⁻¹). Rh-modifying inhibited the phase transformation from TaON to Ta₃N₅ during the nitridation process, and increased the specific surface area and active sites, and extended the optical absorption throughout visible light region due to localized surface plasmon resonance (LSPR) effect. Relative intensities of peaks at 827 cm⁻¹ for Ta–O bonds increased with increasing Rh-modifying amounts, which was beneficial to improving the stability. Notably, constructing novel Rh/TaON/Ta₂O₅ heterojunctions including a Rh/TaON Schottky junction and an n-n TaON/Ta₂O₅ mutant heterojunction facilitated photogenerated carriers directed transfer from inner to surface active sites, and decreased travel distance of charge carriers, and formed built-in electric fields to accelerating charge separation. Synergetic effects of the enhanced photocatalytic H₂-evolution activity were discussed in detail. This work provided a promising strategy to further design and develop efficient and stable Ta-based photocatalysts for solar water splitting.     

Hydrogen production from water–methanol solution over visible light active indium–titanium oxide photocatalysts modified with copper oxide

Titanium oxide coupled with different amount of indium oxides were studied for production of hydrogen under visible light irradiation from water–methanol solution. The photocatalysts were prepared by co-precipitation and characterized by surface area and pore analysis, X-ray diffraction, field emission scanning electron microscopy, UV–Vis diffuse reflectance spectra, and photoluminescence spectroscopy. With increases in indium oxide content, the surface area, visible light absorption and separation of photogenerated electron-holes were enhanced. For binary catalysts, the activity was highest for 16.7 at.% indium with hydrogen production of 1829 μmol/g/h. Incorporation of copper oxide further enhanced the activity with hydrogen production of 2149 μmol/g/h. The higher hydrogen production for ternary catalyst can be attributed to the synergistic effects of higher surface area, stronger absorption in visible light region and enhanced separation of photogenerated charge carriers. The hydrogen generation was attributed to partial oxidation of methanol to formaldehyde thereby producing pure hydrogen.