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.     

Hydrogen storage properties of Mg1−xCaxNi2−y defect solid solutions

Ternary Mg_1−xCa_xNi_2−y solid solutions were synthesized by powder sintering. The phase structures and hydrogen storage properties of the sintered samples were investigated. In a certain range of x and y values, the samples are a single C15 Laves phase with various types of defects. The reduction of Ni content leads to the formation of omission solid solution with vacancies on the sites of Ni. These vacancies increase the hydrogen storage capacity, but decrease the reversibility of hydrogen absorption and desorption.     

Photocatalytic hydrogen production under visible light over Cd0.1SnxZn0.9−2xS solid solution photocatalysts

A series of Cd_0.1Sn_xZn_0.9−2xS solid solution was successfully synthesized by hydrothermal method and employed as photocatalyst for photocatalytic hydrogen evolution under visible light irradiation. The structures, optical properties and morphologies of the solid solutions were studied by X-ray diffraction, diffuse reflectance UV-visible spectroscopy and field emission scanning electron microscopy. From the characterizations, it was confirmed that Sn can form solid solution with Cd_0.1Zn_0.9S and the high crystallinity can be maintained as well. Among all samples, the highest photocatalytic activity was observed on Cd_0.1Sn_0.01Zn_0.88S photocatalyst, with average rate of hydrogen production 3.52 mmol/h, which was ca. 1.5 times higher than the Cd_0.1Zn_0.9S photocatalyst. In addition to the high activity, the Cd_0.1Sn_0.01Zn_0.88S also showed high stability at long irradiation time. The role of Sn in preventing electron-hole recombination and photocorrosion was proposed.     

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.     

Hierarchical microarchitectures of AgGa1−xInxS2: Long chain alcohol assisted synthesis,band gap tailoring and photocatalytic activities of hydrogen generation

An enhanced visible-light-driven photocatalyst, AgGa_1−xIn_xS₂, was prepared by a facile long chain alcohol assisted hydrothermal route. Phase structures, morphologies, optical properties, energy level and specific surface areas of the products were characterized by X-ray diffraction, scanning electron microscope, UV–vis spectra and BET analysis. The obtained products show unique hierarchical microarchitectures which were fabricated by micro-sphere and nano-sheets. The introducing of long chain alcohol (pentanol and heptanol) was investigated to be crucial for the forming of hierarchical microarchitectures. On the other hand, the band gap energy of AgGa_1−xIn_xS₂ (x = 0–1) estimated from the onset of absorption edge was found to be reduced from 2.71 eV (x = 0) to 1.93 eV (x = 1) by indium substitution. The photocatalytic activity for hydrogen evolution was investigated under visible light irradiation (λ > 400 nm). The rate of hydrogen evolution reached up to 813 μmol/h when the amount of In (x) was 0.1 and the roles of long chain alcohol and Ga/In were also carefully investigated.     

Alkaline earth metal as a novel dopant for chalcogenide solid solution: Improvement of photocatalytic efficiency of Cd1−xZnxS by barium surface doping

Novel Ba(x)-Cd_0.8Zn_0.2S photocatalysts, which has not been reported yet, was synthesized for the first time by thermal sulfuration method. These samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, N₂ absorption-desorption isotherm, ultraviolet visible diffuse reflectance spectroscopy, density functional theory calculation and photoluminescence spectroscopy. Results showed that Ba mainly existed on the surface of Cd_1−xZn_xS particles, because of the largest atom radius compared to Cd and Zn atoms, in the chemical form of Ba-S-Cd/Zn bonding. The successful surface doping finally induces the element gradient from bulk phase to surface, which efficiently promotes the photogenerated charges’ transition and separation. On the basis of various characterization results, the mechanism of this promotion effect is proposed. The element gradient efficiently enhances the transition of electrons, while the noticeable doping influence on valence band significantly promotes the holes’ migration. Thus, the bulk recombination of photoelectrons and holes was efficiently suppressed, and the hydrogen production was improved efficiently.     

Syngas reactivity over (LaAg)(CoFe)O3 and Ag-added (LaSr)(CoFe)O3 anodes of solid oxide fuel cells

The syngas, H₂ + CO gas mixture with various H₂/CO ratios, is used as the anode fuel of solid oxide fuel cell with La_0.7Ag_0.3Co_0.2Fe_0.8O₃ (LACF) and 2 wt% Ag-added La_0.58Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) as the anode, respectively, both being in composite with 50 wt% Ce_0.9Gd_0.1O_1.95 (GDC). Both the current-voltage and the fixed-voltage measurements are performed at 800 °C. The reactivity with H₂ as the fuel is larger than that with CO. The syngas reactivity increases with increasing H₂ content. The results of the current-voltage and the fixed-voltage measurements are in agreement with each other. Ag-added LSCF-GDC has better reactivity with H₂, CO and syngas and better stability in the H₂ atmosphere than LACF-GDC as the anode material.     

Crystal structure solution of KMg(ND)(ND2): An ordered mixed amide/imide compound

An ordered mixed deuterated amide/imide potassium–magnesium compound was synthesized with the intent of solving its structure using neutron diffraction technique with help of “ab-initio” methods. Obtained powder diffraction patterns were compatible with the orthorhombic P2₁2₁2₁ space group, and lattice parameters a = 9.8896(3) Å; b = 9.3496(3) Å; c = 3.6630(1) Å, respectively.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

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.     

Hydrogen solid solution thermodynamics of V1−xAlx (x: 0, 0.18, 0.37, 0.52) alloys

Thermodynamic parameters such as enthalpy and entropy of the vanadium–hydrogen solid solution are investigated as a function of aluminum content using hydrogen solubility data and the Sievert's constant. The enthalpy decreases with increase in the aluminum content. Entropy shows anomalous behavior as it first increases with the aluminum content for V_1−xAl_x (x: 0, 0.18, 0.37) but then substantially decreases for V_0.48Al_0.52. The lattice parameters and the electrical resistivity of the alloys are calculated to explain the mechanical and electronic effects on the thermodynamic parameters. It is found that the electrical resistivity of vanadium systematically decreases and the lattice constant increases with increase in aluminum content. The hardness of the alloys increases with aluminum which indicates that aluminum hardens the vanadium by simple solid solution effect. The variation of enthalpy and entropy is explained on the basis of change in Fermi energy level of the host matrix vanadium, the strong bonding nature of V–Al in the alloy and increased activity of hydrogen due to aluminum in the alloy.     

Order-disorder transformation and enhanced oxide-ionic conductivity of (Sm1−xDyx)2Zr2O7 ceramics

(Sm_1−xDy_x)₂Zr₂O₇ (0 ≤ x ≤ 1) ceramics are prepared by a solid state reaction process at 1973 K for 10 h in air. (Sm_1−xDy_x)₂Zr₂O₇ (0 ≤ x ≤ 0.3) ceramics exhibit a single phase of pyrochlore-type structure, while (Sm_1−xDy_x)₂Zr₂O₇ (0.5 ≤ x ≤ 1.0) possess a defective fluorite-type structure. The full width at half-maxima in the Raman spectra increases with increasing Dy content, which indicates that the degree of structural disorder increases as the Dy content increases. The ionic conductivity of (Sm_1−xDy_x)₂Zr₂O₇ ceramics is investigated by impedance spectroscopy over a frequency range of 0.2 Hz to 8 MHz in the temperature range of 873-1173 K in air and hydrogen atmospheres, respectively. The ionic conductivity has a maximum near the phase boundary between the pyrochlore- and the defective fluorite-type phases under identical temperature levels. The ionic conductivity is determined by the degree of structural disorder or unit cell free volume, which is depending on the Dy content. As the ionic conductivity in the hydrogen atmosphere is almost the same as that obtained in air, the conduction of (Sm_1−xDy_x)₂Zr₂O₇ is purely ionic with negligible electronic conduction.     

Visible-light-driven photocatalytic H2 evolution from water splitting with band structure tunable solid solution (AgNbO3)1−x(SrTiO3)x

(AgNbO₃)_1−x(SrTiO₃)_x samples were successfully employed as photocatalysts for photocatalytic hydrogen evolution under visible light. The samples were characterized by a series of techniques, including X-ray diffractometry, scanning electron microscopy, UV–Vis spectrophotometry, and electrochemistry technology. The band gaps of (AgNbO₃)_1−x(SrTiO₃)_x solid solutions can be tuned continuously from 3.21 to 2.65 eV and the flat-band potentials (V_fb) can be shifted positively from −0.79 to −0.31 V vs. SHE when x decreased from 1 to 0. Band positions of (AgNbO₃)_1−x(SrTiO₃)_x samples were further testified by density functional theory, suggesting that the band gap narrowing of the solid solutions derived from the hybridization of (Ti 3d and Nb 4d) and (O 2p and Ag 4d) orbital. The photocatalytic activities of samples for H₂ evolution with Pt cocatalyst were evaluated in aqueous methanol solution under visible light irradiation. The highest photocatalytic activity was obtained at (AgNbO₃)_0.25(SrTiO₃)_0.75. Photocatalytic activity in hydrogen evolution of these solid solutions proved to be closely dependent on band structures.     

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.     

Sm0.2(Ce1−xTix)0.8O1.9 modified Ni-yttria-stabilized zirconia anode for direct methane fuel cell

Sm_0.2(Ce_1−xTi_x)_0.8O_1.9 (SCTx, x = 0-0.29) modified Ni-yttria-stabilized zirconia (YSZ) has been fabricated and evaluated as anode in solid oxide fuel cells for direct utilization of methane fuel. It has been found that both the amount of Ti-doping and the SCTx loading level in the anode have substantial effect on the electrochemical activity for methane oxidation. Optimal anode performance for methane oxidation has been obtained for Sm_0.2(Ce_0.83Ti_0.17)_0.8O_1.9 (SCT0.17) modified Ni-YSZ anode with SCT0.17 loading of about 241 mg cm⁻² resulted from four repeated impregnation cycles. When operating on humidified methane as fuel and ambient air as oxidant at 700 °C, single cells with the configuration of SCT0.17 modified Ni-YSZ anode, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃-Sm_0.2Ce_0.8O_1.9 (LSCF-SDC) composite cathode show the polarization cell resistance of 0.63 Ω cm² under open circuit conditions and produce a peak power density of 383 mW cm⁻². It has been revealed that the coated Ti-doped SDC on Ni-YSZ anode not only effectively prevents the methane fuel from directly impacting on the Ni particles, but also enhances the kinetics of methane oxidation due to an improved oxygen storage capacity (OSC) and redox equilibrium of the anode surface, resulting in significant enhancement of the SCTx modified Ni-YSZ anode for direct methane oxidation.     

Effect of O2 concentration on performance of solid oxide fuel cells with V2O5 or Cu added (LaSr)(CoFe)O3-(Ce,Gd)O2−x cathode with and without NO

A solid oxide fuel cell unit is constructed with Ni-(Ce,Gd)O_2−x (GDC) as the anode, yttria-stabilized zirconia as the electrolyte, and V₂O₅ or Cu added (LaSr)(CoFe)O₃-GDC as the cathode. The effect of the O₂ concentration on the open circuit voltage (OCV) is studied and a mass-transfer limited OCV is observed. The power density with Cu addition can be much higher than that with V₂O₅ addition but the effect of the O₂ concentration with Cu addition is larger than that with V₂O₅ addition. Without the presence of NO, both the power density and the OCV decrease with decreasing O₂ concentration. The OCV variation can be substantial with the variation of the flow rate, the O₂ concentration and the NO concentration. The presence of CO₂ can increase the OCV while that of NO can decrease the OCV; however, a synergistic effect can occur on the OCV when NO is present at a very low O₂ concentration which results in a sudden drop of the OCV.     

Structural and electrical properties of (Ba1−xSrx)(Zr0.9M0.1)O3, M = Y, La, solid solutions

The aim of the work was to study the structural and electrical properties of the (Ba_1−xSr_x)(Zr_0.9Y_0.1)O₃ and (Ba_1−xSr_x)(Zr_0.9Y_0.1)O₃ solid solutions. The powders of different strontium content (x = 0, 0.03, 0.05 and 0.1) were prepared by a thermal decomposition of organo-metallic precursors containing ethylenediaminetetraacetate acid. Some parameters describing stability and transport properties of the perovskite structure, such as tolerance factor, specific free volume and global instability index, were calculated. It was found that the introduction of strontium into both solid solutions caused the increase of specific free volume and global instability index—these structures became a little less stable but, on the other hand, better ionic conductor. All samples were cubic perovskite and the substitution of strontium for barium caused the decrease of respective lattice parameters. Electrical conductivity measurements were performed by the d.c. four-probe method in controlled gas atmospheres containing Ar, air, H₂ and/or H₂O at the temperature from 300 to 800 °C. It was found that the conductivity depended on a chemical composition of the samples and the atmosphere. In general, the electrical conductivity was higher in wet atmospheres which contained oxygen, being in accordance with the model of a proton transport in the perovskite structure which assumed the presence of the oxygen vacancy. The solid solution containing 5 mol.% of strontium showed the highest conductivity and the lowest activation energy of conductivity regardless of the atmospheres.     

Hydrogen storage properties of the pseudo binary laves phase (Sc1−xZrx)(Co1−yNiy)2 system

The (Sc_1−xZr_x)(Co_1−yNi_y)₂-H_z system has been studied using both experimental techniques and ab initio calculations. The material was synthesised through high temperature synthesis and characterised using powder XRD. Hydrogen absorption and desorption was studied in-situ using synchrotron radiation. Maximal storage capacity increased when Co replaced Ni and substitution of Sc for Zr increased the equilibrium pressure. Density functional based calculations reproduce the experimental trends in terms of cell parameters both for the non-hydrogenated systems as well as for the hydrogenated systems, and helped to quantitatively understand the observed hydrogen uptake properties.     

Novel photocatalysts based on Cd1−xZnxS/Zn(OH)2 for the hydrogen evolution from water solutions of ethanol

Multiphase photocatalysts Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂, Pt/Cd_1−xZn_xS/ZnO, and Pt/Cd_1−xZn_xS/Zn(OH)₂ were synthesized by a new two-step technique. The photocatalysts were characterized by a wide range of experimental techniques: X-ray diffraction, high-resolution transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, low-temperature N₂ adsorption/desorption, and UV/VIS spectroscopy. The photocatalytic activity was tested in a batch reactor in the reaction of H₂ evolution from aqueous solutions of ethanol under visible light irradiation (λ > 420 nm). The highest achieved photocatalytic activity was 2256 μmol H₂ per gram of photocatalyst per hour; the highest quantum efficiency was 10.4%. The activity of Pt/Cd_1−xZn_xS/Zn(OH)₂ was higher than that of Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂ and Pt/Cd_1−xZn_xS/ZnO. The explanation of enhanced activity of zinc–cadmium sulfide/ε-zinc hydroxide based on quantum calculations was suggested.     

Photocatalytic hydrogen evolution with simultaneous degradation of organics over (CuIn)0.2Zn1.6S2 solid solution

Using various organics as electron donor, (CuIn)_0.2Zn_1.6S₂ microsphere solid solution prepared via hydrothermal method as photocatalyst, hydrogen production by anaerobic photocatalytic reforming organics were researched. The photocatalytic hydrogen production activity was notably enhanced in the presence of the organic electron donors. Formic acid was found to be the most efficient sacrificial agent among methanol, glucose, triethanolamine and formic acid. The effects of initial formic acid concentration on hydrogen generation were investigated. When the initial formic acid concentration was 10 vol%, the photocatalytic activity reached the highest. The average activity in initial 10 h can amount to 144 μmol h⁻¹. The possible mechanism of photocatalytic reaction for hydrogen production with simultaneous formic acid degradation was discussed preliminary.