Organic-inorganic composite of g-C3N4–SrTiO3:Rh photocatalyst for improved H2 evolution under visible light irradiation

The organic-inorganic composite g-C₃N₄–SrTiO₃:Rh was prepared for the first time as a photocatalyst for hydrogen production and the resulting hydrogen evolution rate under visible light irradiation from aqueous methanol solution was measured. A high hydrogen evolution rate of 223.3 μmol h⁻¹ was achieved by using 0.1 g of as-prepared photocatalyst powder comprised of 20 wt.% g-C₃N₄ 80 wt.% SrTiO₃:Rh (0.3 mol%). The hydrogen evolution rate was greater than that obtained by SrTiO₃:Rh (0.3 mol%) by a factor of 3.24. The quantum efficiency of as-prepared composite photocatalyst was 5.5% at 410 nm for hydrogen evolution. The high activity of the composite photocatalyst for hydrogen evolution stemmed from its electron–hole separation and transportation capabilities due to the hetero-junctions of the organic-inorganic composite materials. The proposed mechanism for the electron–hole separation and hydrogen evolution of the g-C₃N₄–SrTiO₃:Rh composite under visible light irradiation featured the reduced recombination of the photo-generated charge carriers. The doping of Rh ions into the SrTiO₃ has contributed to the high photocatalytic activity by forming a donor level from the valance band to the conduction band.     

One-pot calcination preparation of graphene/g–C3N4–Co photocatalysts with enhanced visible light photocatalytic activity

Photocatalytic technology for hydrogen evolution from water splitting and pollutant degradation is one of the most sustainable methods. Here, the graphene/g–C₃N₄–Co composite materials have been prepared by one-pot calcination method. The results show that g-C₃N₄ grow on the surface of graphene and form a sandwich structure, meanwhile, the introduction of Co increases the active sites, which promotes the photocatalytic performance. The influences of graphene and Co content on photocatalytic activity were also studied by UV–visible spectrophotometry (DRS), photoluminescence spectroscopy (PL), photocurrent, degradation MB, and hydrogen production. The apparent reaction rate constant k of graphene/g–C₃N₄–Co (3%) is 0.946 h⁻¹, which is 4.90 and 2.18 times faster than g-C₃N₄ and graphene/g-C₃N₄, respectively. And the hydrogen production rate of graphene/g–C₃N₄–Co (3%) (892.3 μmol h⁻¹ g⁻¹) is 3.53 and 1.61 times higher than g-C₃N₄ and graphene/g-C₃N₄, respectively.     

Synthesis and characterization of g/Ni–SiO2 composite for enhanced hydrogen storage applications

Hydrogen is considered as one of the most important clean energy carriers for the future. Many experimental and theoretical investigations have focused on the adsorption and activation of H₂ on the metal surfaces. Metal oxides and semiconductors are suitable materials for this purpose. Gelatin assisted Ni loaded SiO₂ (g/Ni–SiO₂) was prepared and its structural properties, morphology, composition and surface properties were analyzed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM), Elemental mapping and energy dispersive spectrum (EDS), High resolution transmission electron microscopy (HR-TEM) and Brunauer-Emmett-Teller (BET) surface area measurements. The prepared material was effectively utilized for H₂ storage applications at room temperature. The H₂ storage capacity of g/Ni–SiO₂ was twice that of pristine SiO₂. This may be due to large change in pore volume and pore diameter of g/Ni–SiO₂, which may enhances the H₂ storage capacity of the sample. The H₂ storage capacity of other materials such as ZnO, anatase TiO₂, g/TiO₂, g/ZnO, g/TiO₂–SO₄^2-, Sb doped TiO₂, Ag₂S/TiO₂, Sb₂O₃, CdS and SiO₂/CdS also studied and compared with g-Ni/SiO₂.     

In situ fabrication of CDs/g-C3N4 hybrids with enhanced interface connection via calcination of the precursors for photocatalytic H2 evolution

Novel carbon dots (CDs)/graphitic carbon nitride (g-C₃N₄) hybrids were fabricated via an in situ thermal polymerization of the precursors, urea and glucose. This heterojunction catalyst exhibited enhanced photocatalytic H₂ evolution activity under visible-light (λ > 420). A sample of CDs/g-C₃N₄ hybrids, CN/G0.5, which was prepared from 0.5 mg of glucose in 6.0 g of urea (8.3 × 10^?3 wt% glucose), exhibited the best photocatalytic performance for hydrogen production from water under visible light irradiation, which is about 4.55 times of that of the bulk g-C₃N₄ (BCN). The improvement of photocatalytic activity was mainly attributed to the construction of built-in electric field at the interface of CDs and g-C₃N₄, which could improve the separation of photogenerated electron-hole pair. Moreover, the tight connection of CDs with g-C₃N₄ would serve as a well electron transport channel, which could promote the photocatalytic H₂ evolution ability.     

Fabrication and band structure of Ag3PO4–TiO2 heterojunction with enhanced photocatalytic hydrogen evolution

Ag₃PO₄ was deposited on TiO₂ by in-situ precipitation to fabricate Ag₃PO₄–TiO₂ heterojunction with different ratios of Ag₃PO₄. The electronic band structures of TiO₂ and Ag₃PO₄ were determined by means of ultraviolet photoelectron spectroscopy and UV–visible spectrometry. The Fermi levels of TiO₂ and Ag₃PO₄ were calculated to be −5.09 eV and −5.95 eV, respectively, and accordingly the energy band diagrams were constructed. The hydrogen production rates of bare TiO₂ and Ag₃PO₄–TiO₂ heterojunctions were measured under 300 W Xe lamp with a solar filter (AM 1.5). The heterojunction formed with 12 wt% Ag₃PO₄ showed the highest hydrogen evolution rate (44.5 μmol g⁻¹h⁻¹) which is 5.7 times higher than that of TiO₂. When Au nanoparticles were deposited on this heterojunction, it resulted in 10.2 times (453.0 μmol g⁻¹h⁻¹) further improvement of hydrogen generation. The hydrogen evolution performance is consistent with the result of photoluminescence analysis.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

Effects of MoS2 addition on the hydrogen storage properties of 2LiBH4–MgH2 systems

A 2LiBH₄–MgH₂–MoS₂ composite was prepared by solid-state ball milling, and the effects of MoS₂ as an additive on the hydrogen storage properties of 2LiBH₄–MgH₂ system together with the corresponding mechanism were investigated. As shown in the TG–DSC and MS results, with the addition of 20 wt.% of MoS₂, the onset dehydrogenation temperature is reduced to 206 °C, which is 113 °C lower than that of the pristine 2LiBH₄–MgH₂ system. Meanwhile, the total dehydrogenation amount can be increased from 9.26 wt.% to 10.47 wt.%, and no gas impurities such as B₂H₆ and H₂S are released. Furthermore, MoS₂ improves the dehydrogenation kinetics, and lowers the activation energy (E_a) 34.49 kJ mol⁻¹ of the dehydrogenation reaction between Mg and LiBH₄ to a value lower than that of the pristine 2LiBH₄–MgH₂ sample. According to the XRD test, Li₂S and MoB₂ are formed by the reaction between LiBH₄ and MoS₂, which act as catalysts and are responsible for the improved hydrogen storage properties of the 2LiBH₄–MgH₂ system.     

Enhanced hydrogen storage properties of TiN–LiAlH4 composite

The hydrogen storage properties of LiAlH₄ doped efficient TiN catalyst were systematically investigated. We observe that TiN catalyst enhances the dehydrogenation kinetics and decreases the dehydrogenation temperature of LiAlH₄. The dehydrogenation behaviors of 2%TiN–LiAlH₄ are investigated using temperature programmed desorption (TPD), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). Interestingly, the onset hydrogen desorption temperature of 2%TiN–LiAlH₄ sample gets lowered from 151.0 °C to 90.0 °C with a faster kinetics, and the dehydrogenation rate reached a maximum value at 137.2 °C. By adding a small amount of as-prepared TiN, approximately 7.1 wt% of hydrogen can be released from the LiAlH₄ at 130 °C. Interestingly, the result of the FTIR indicates that the 2%TiN–LiAlH₄ maybe restore hydrogen under 5.5 MPa hydrogen. Moreover, 2%TiN–LiAlH₄ displayed a substantially reduced activation energy for LiAlH₄ dehydrogenation.     

Promoting hydrogen production by loading PdO and Pt on N–TiO2 under visible light

Nitrogen/titanium dioxide (N/TiO₂) visible light photocatalysts were prepared using the sol–gel method. The catalysts were characterized using transmission electron microscopy, reflective UV–visible spectroscopy, specific surface area measurements, and X-ray diffraction. The prepared catalysts were used to generate hydrogen gas through the water-splitting reaction under visible light (wavelengths greater than 400 nm). Various N/Ti addition ratios were tested, and the hydrogen generation rates were compared to determine the optimal ratio. The maximal hydrogen production rate (approximately 55 μmol h⁻¹ g⁻¹) was attained when the N/Ti ratio of N–TiO₂ was 10. When PdO and Pt were loaded onto the N–TiO₂ catalyst, the hydrogen generation rates increased to 544 and 772 μmol h⁻¹ g⁻¹, respectively. The highest hydrogen production rate (2460 μmol h⁻¹ g⁻¹) was obtained when bimetallic 0.05 wt% PdO-0.10 wt% Pt/N–TiO₂ was used. After three times use the hydrogen yield of the catalyst was maintained as 83%. A possible mechanism of water splitting catalyzed by this visible light photocatalyst is proposed.     

Rehydrogenation and cycle studies of LiBH4–CaH2 composite

Rehydrogenation behavior of 6LiBH₄ + CaH₂ composite with NbF₅ has been studied between 350 and 500 °C after dehydrogenation at 450 °C. The composite exhibits the best rehydrogenation feature at 450 °C in terms of the overall rehydrogenation rate and the amount of absorbed hydrogen. It is found that about 9 wt% hydrogen is absorbed at 450 °C for 12 h. Up to 10 dehydrogenation–hydrogenation cycles have been carried out for the composite. It is demonstrated that 6LiBH₄ + CaH₂ with 15 wt% NbF₅ maintains a reversible hydrogen storage capacity of about 6 wt% at 450 °C after a slight degradation between the 1st and 5th cycles. The addition of NbF₅ seems to improve the cycle properties by retarding microstructural coarsening during cycles.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Al–Li3AlH6: A novel composite with high activity for hydrogen generation

A novel material for hydrogen generation with high capacity of H₂ generation has been successfully prepared by ball milling the mixture of Al and home-made fresh Li₃AlH₆ powder. Its theoretical capacity of hydrogen released is higher than that of pure Al. Results obtained have shown conversion efficiency of Al–Li₃AlH₆ composite can be close to 100% by increasing the content of Li₃AlH₆. When the content of Li₃AlH₆ is 20 wt%, the maximum hydrogen generation rate and hydrogen yield are 2737.6 mL g⁻¹ min⁻¹ and 1513.1 mL g⁻¹, respectively, at room temperature. By XRD, SEM analyses and reaction heat measurements, it demonstrates that the additive Li₃AlH₆ can provide an additional source of H₂ and an alkaline environment (LiOH) as well as additional heat to promote the Al/H₂O reaction. Therefore, the Al–Li₃AlH₆ composite has a very high activity and high capacity of hydrogen released.     

Synthesis of reduced graphene oxide–TiO2 nanoparticle composite systems and its application in hydrogen production

The utilization of solar energy for the conversion of water to hydrogen and oxygen has been considered to be an efficient strategy to solve crisis of energy and environment. Here, we report the synthesis of reduced graphene oxide–TiO₂ nanoparticle composite system through the photocatalytic reduction of graphite oxide using TiO₂ nanoparticles. Photoelectrochemical characterizations and hydrogen evolution measurements of these nanocomposites reveal that the presence of graphene enhances the photocurrent density and hydrogen generation rate. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm⁻² and 127.5 μmole cm⁻²h⁻¹ in 0.5 M Na₂SO₄ electrolyte solution under 1.5AM solar irradiance of white light with illumination intensity of 100 mW cm⁻². In graphene–TiO₂ nanocomposite, photogenerated electrons in TiO₂ are scavenged by graphene sheets and percolate to counter electrode to reduce H⁺ to molecular hydrogen thus increasing the performance of water-splitting reaction.     

Effect of SWNTs on the reversible hydrogen storage properties of LiBH4–MgH2 composite

Single-walled carbon nanotubes (SWNTs) were mechanically milled with LiBH₄/MgH₂ mixture, and examined with respect to its effect on the reversible dehydrogenation properties of the Li–Mg–B–H system. Experimental results show that the addition of SWNTs results in an enhanced dehydriding rate and improved cyclic stability of the LiBH₄/MgH₂ composite. For example, the LiBH₄/MgH₂ composite with 10 wt% purified SWNTs additive can release nearly 10 wt% hydrogen within 20 min at 450 °C, with an average dehydriding rate over 2 times faster than that of the neat LiBH₄/MgH₂ sample. Based on the results of phase analysis and a series of designed experiments, the mechanism underlying the observed property improvement was discussed.     

Mg2FeH6–LiBH4 and Mg2FeH6–LiNH2 composite materials for hydrogen storage

Two composite hydrogen storage materials based on Mg₂FeH₆ were investigated for the first time. The Mg₂FeH₆–LiBH₄ composite of molar ratio 1:5 showed a hydrogen desorption capacity of 5.6 wt.% at 370 °C, and could be rehydrogenated to 3.6 wt.% with the formation of MgH₂, as the material was heated to 445 °C and held at this temperature. The Mg₂FeH₆–LiNH₂ composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of 4.3 wt.% and released hydrogen at 100 °C lower then the Mg₂FeH₆–LiBH₄ composite, but this mixture could not be rehydrogenated. Compared to neat Mg₂FeH₆, both composites show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at these low temperatures. In particular, Mg₂FeH₆–LiNH₂ exhibits a much lower desorption temperature than neat Mg₂FeH₆, but only Mg₂FeH₆–LiBH₄ re-absorbs hydrogen.     

Facile synthesis of C–Ta4+ co-doped NaTaO3 and rGO nanocomposites with enhanced visible light photocatalytic performance

Nanocomposites of C–Ta⁴⁺ co-doped NaTaO₃ and reduced graphene oxide (C-NTO/rGO) with highly efficient photocatalytic activity were fabricated by a one-step solvothermal reaction. The optimum composites C-NTO/3rGO (with 3 wt% rGO) exhibited superior photocatalytic activity for the degradation of RhB over a mono-component counterpart under visible light irradiation. The photocatalytic efficiency was found to be as high as 97% for 90 min. Both XRD and XPS characterizations indicated that the doping of carbon into the NaTaO₃ lattice leads to the reduction of Ta⁵⁺ to Ta⁴⁺. Furthermore, the co-doping significantly narrows the band gap (about 2.8 eV). Photo-luminescence (PL), time resolved transient PL decay spectra and photo-current results confirmed that photo-induced hole-electron pairs can be effectively separated resulting from the introduction of rGO. The synergistic effect of a decreased band gap and highly effective separation of photo-generated electrons and holes may be responsible for the enhanced photocatalytic activity of the fabricated nanocomposites. Both h⁺ and ?OH active radicals were found to be the key factors for the photo-decomposition process of RhB over the nanocomposites, while ?O₂^? was also involved to a smaller extent. A possible photo-degradation mechanism for C-NTO/rGO was proposed on the basis of the experiments results.     

Preparation and electrochemical properties of Co–Si3N4 nanocomposites

Cobalt nanoparticles on an amorphous Si₃N₄ matrix were synthesized by direct ball-milling of Co and Si₃N₄ powders for an improvement of their electrochemical performance. The microstructure, morphology and chemical state of the ball-milled Co–Si₃N₄ composites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of Co–Si₃N₄ composites was investigated by galvanostatic charge–discharge process and cyclic voltammetry (CV) technique. It is found that metallic Co nanoparticles of 10–20 nm in size are highly dispersed on the amorphous inactive Si₃N₄ matrix after the ball-milling. The composite with a Co/Si molar ratio of 2/1 shows the optimized electrochemical performance, including discharge capacity and cycle stability. The formation of Co nanoparticles with a good reaction activity is responsible for the discharge capacity of the composites. The reversible faradic reaction between Co and β-Co(OH)₂ is dominant for ball-milled Co–Si₃N₄ composite. The surface modification of the hydrogen storage PrMg₁₂–Ni composites using Co–Si₃N₄ composites can enhance the initial discharge capacity based on the hydrogen electrochemical oxidation and Co redox reaction.     

Catalytic effect of MgFe2O4 on the hydrogen storage properties of Na3AlH6–LiBH4 composite system

The effect of MgFe₂O₄ on the hydrogen storage properties of the composite Na₃AlH₆4LiBH₄ was studied for the first time, where it was found that MgFe₂O₄ addition decreased the onset desorption temperature of Na₃AlH₆4LiBH₄. Hydrogen (～9.5 wt%) was released in three stages and the dehydrogenation temperatures were reduced to 80 °C, 350 °C, and 430 °C for the first, second, and third stage, respectively. The absorption kinetics of Na₃AlH₆4LiBH₄ was also significantly improved due to the catalytic effect of MgFe₂O₄. Using Kissinger analysis, the apparent activation energies of decomposition of the Li₃AlH₆ and NaBH₄ stages in Na₃AlH₆4LiBH₄-10 wt% MgFe₂O₄ were calculated to be 72 and 141 kJ/mol, respectively. These values were considerably lower than the corresponding values for the undoped composite. X-ray diffraction analysis revealed the formation of new products such as MgO and Fe during the heating process. Our results suggest that MgFe₂O₄ enhanced the hydrogen storage properties of Na₃AlH₆4LiBH₄ through the formation of active species, such as MgO and Fe.     

Noble metal-free NiCo nanoparticles supported on montmorillonite/MoS2 heterostructure as an efficient UV–visible light-driven photocatalyst for hydrogen evolution

In this study, a noble-metal-free photocatalyst, based on NiCo nanoparticles supported on montmorillonite/MoS₂ heterostructure (MMT/MoS₂/NiCo), was successfully synthesized and applied for photocatalytic water reduction to produce H₂. Under UV–visible light irradiation, the composite showed improved photocatalytic performance for H₂ evolution compared to MMT/MoS₂, MMT/MoS₂/Ni, MMT/NiCo, and MoS₂/NiCo. The as-synthesized MMT/0.79MoS₂/Ni_8.14Co_6.4 (0.79, 8.14 and 6.4 denote the weight ratios % of MoS₂, Ni and Co in the catalyst) photocatalyst exhibited a high H₂ production rate of 8.7 mmol g^?1 h^?1, 26.5 and 2.3 times higher than for MMT/0.79MoS₂ and MMT/Ni_8.14Co_6.4, respectively. The enhanced photocatalytic performance was attributed to the loaded MoS₂ and NiCo nanoparticles, introducing active sites, increasing the light-absorbing capacity and accelerating the charge transfer from the Eosin Y dye owing to their appropriate Fermi level energy alignment. This work presents a cost-effective method combining the 2D sheets of MMT and MoS₂, and NiCo nanoparticles to form a quaternary photocatalytic system showing highly efficient hydrogen evolution from water without using noble metals.     

Layered MoS2–graphene composites for supercapacitor applications with enhanced capacitive performance

Layered molybdenum disulfide (MoS₂)–graphene composite is synthesized by a modified l-cysteine-assisted solution-phase method. The structural characterization of the composites by energy dispersive X-ray analysis, X-ray powder diffraction, Fourier transform infrared spectroscopy, XPS, Raman, and transmission electron microscope indicates that layered MoS₂–graphene coalescing into three-dimensional sphere-like architecture. The electrochemical performances of the composites are evaluated by cyclic voltammogram, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Electrochemical measurements reveal that the maximum specific capacitance of the MoS₂–graphene electrodes reaches up to 243 F g⁻¹ at a discharge current density 1 A g⁻¹. The energy density is 73.5 Wh kg⁻¹ at a power density of 19.8 kW kg⁻¹. The MoS₂–graphene composites electrode shows good long-term cyclic stability (only 7.7% decrease in specific capacitance after 1000 cycles at a current density of 1 A g⁻¹). The enhancement in specific capacitance and cycling stability is believed to be due to the 3D MoS₂–graphene interconnected conductive network which promotes not only efficient charge transport and facilitates the electrolyte diffusion, but also prevents effectively the volume expansion/contraction and aggregation of electroactive materials during charge–discharge process. Taken together, this work indicates MoS₂–graphene composites are promising electrode material for high-performance supercapacitors.