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Abstract: Two types of mesoporous Co–B nanocatalysts were prepared by the reduction of cobalt chloride with Sodium Borohydride (SBH) in the presence of cationic and non-ionic surfactant templates, namely n-cetyl-trimethyl-ammonium bromide (CTAB) and Pluronic (P123) respectively. Nitrogen adsorption–desorption isotherms revealed the presence of slit-like pores on the catalyst surface which provide high effective surface area. These surface enhanced catalysts were tested for hydrogen production by hydrolysis of sodium borohydride. The mesoporous Co–B catalysts showed much higher activity (4 times) in comparison to the non-porous Co–B, which can be attributed to the higher surface area of the mesoporous structures. Co–B/P123 catalyst showed the highest hydrogen generation rate owing to the presence of wide uniform pores which facilitated easier interaction of the reactants to release hydrogen. The lack of stability in the pore structure is observed at elevated temperatures for both the mesoporous Co–B catalyst. [Copyright &y& Elsevier]

Highlights: [•] Co-B catalyst particles were supported over three type of mesoporous silica of different pore sizes. [•] Co-B supported over SBA-15 showed highest H2 generation rate for hydrolysis of AB. [•] Confinement of Co-B within the pores creates smaller NPs (6nm) with uniform size distribution. [•] Thicker pore walls of SBA-15 avoid agglomeration of Co-B NPs at elevated temperature (873K). [Copyright &y& Elsevier]

Abstract: Catalyst in the form of Co-B nanoparticles supported on mesoporous silica particles (MSP) was synthesized by chemical impregnation–reduction method. This catalyst powder was used for hydrogen production by hydrolysis of Ammonia Borane (AB) and was compared with Co-B catalyst supported on non-porous silica particles (NSP) and unsupported Co-B powder. It was found that the MSPs synthesized in the present studies have average pore size of about 3.1nm which were efficient to anchor the Co-B particles with average size of ~8nm on the surface to provide high active surface area and improved degree of dispersion. MSP-supported Co-B catalyst was able to produce expected amount of H2 gas from hydrolysis of AB with significantly superior generation rate, about 3 times higher than that produced by unsupported and NSP-supported Co-B catalyst. Availability of a large number of under-coordinated Co active atoms owing to the size of nanoparticles, better dispersion, high surface area and good stability against agglomeration during the reaction are the main features acquired by the Co-B nanoparticles supported on MSP that exhibit high catalytic efficiency. [Copyright &y& Elsevier]

Abstract: In order to sensitize TiO2 in visible light and to reduce photo-induced charge recombination, the multilayer films of Indium–Tin Oxide (ITO)/V-doped TiO2 were synthesized by radio-frequency magnetron sputtering. V-doped TiO2 thin films showed red shift in TiO2 absorption edge with increasing dopant concentration and, most importantly, the dopant energy levels are formed in the TiO2 band gap due to V5+/V4+ ions as confirmed by UV–Visible and XPS spectra. Multilayer films with different numbers of ITO/V-doped TiO2 (6 at.%) bilayers (namely, 2-, 3-, 4-, 5-, 6- and 7-bilayers) were deposited, in order to reduce the charge recombination rate, by keeping the total thickness of TiO2 constant in each multilayer film. In multilayer films, when exposed to visible light the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/V-doped TiO2. The measured enhanced photocurrent is attributed to: 1) ability of V-doped TiO2 to absorb visible light, 2) number of space-charge layers in form of ITO/TiO2 interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space-charge layer by decreasing the V-doped TiO2 layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by the photocurrent kinetic curves. The superior photocatalytic efficiency of the 6-bilayers film is also reflected in hydrogen production rate through water-splitting: we obtained indeed 31.2 μmol/h of H2 production rate. [Copyright &y& Elsevier]

Abstract: Cr-doped-TiO2 thin films, with three different Cr concentrations (2, 5.5, and 9 at.%), have been synthesized by radio-frequency magnetron sputtering in order to sensitize TiO2 in visible light. UV–visible spectra showed that maximum narrowing (2.1 eV) of the TiO2 band gap is obtained for the highest Cr concentration. However, negligible photocurrent was measured with Indium Tin Oxide (ITO)/Cr-doped-TiO2 (9 at.%) single bilayer sample due to the increased recombination rate of the photo-generated charges on the defects associated to the Cr3+ ions. To lower the charge recombination rate in the Cr-doped-TiO2, multilayer films with different numbers of ITO/Cr-doped-TiO2 (9 at.%) bilayers (namely, 3-, 4-, 5-, 6- and 7-bilayers) were deposited by keeping the total thickness of TiO2 constant in each multilayer film. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Cr-doped-TiO2. The enhanced photocurrent is attributed to: 1) higher absorption of visible light by Cr-doped-TiO2, 2) number of space charge layers in form of ITO/TiO2 interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space charge layer by decreasing the Cr-doped-TiO2 layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by studying the photocurrent kinetic curve. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water-splitting: we obtained indeed 24.4 μmol/h of H2 production rate, a value about two times higher than that of pure TiO2 (12.5 μmol/h). [Copyright &y& Elsevier]

Abstract: Nanoparticle-assembled Co-B thin films were synthesized by Pulsed Laser Deposition (PLD) and used as catalysts for the hydrolysis of NH3BH3 (ammonia borane, AB) to produce molecular hydrogen. Amorphous Co-B-based catalyst powders, produced by chemical reduction of cobalt salts, were used as target material for Co-B thin film catalysts preparation through PLD. Surface morphology of Co-B powder and film catalyst was studied using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Compositional and structural characterizations were carried out using X-photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques, respectively. The efficiency of both powder and film catalysts was tested by comparative kinetic analysis of the AB hydrolysis for hydrogen production. It was observed that nanoparticles produced during the laser ablation process act as active catalytic centers to produce significantly higher rate (about 6 times) of H2 than the same amount of the corresponding Co-B powders. Almost complete conversion (95%) of AB was obtained, as confirmed by 11B NMR, by using Co-B films at room temperature. Active Co-B nanoparticles on the surface of the PLD-deposited films is able to decrease the activation energy, for hydrolysis of AB, to the very low value of 34kJmol−1. We also found that by adding small amount of NaBH4 to the NH3BH3 solution increases the efficiency of the Co-B catalyst films, thus generating H2 with higher rate. Maximum H2 generation rate of about ∼8.2LH2 min−1 (g of Co)−1 and ∼13LH2 min−1 (g of Co)−1 was measured by hydrolysis of AB and mixture of (AB+NaBH4) solutions, respectively. [Copyright &y& Elsevier]

Abstract: Cr- or Fe-ion-doped TiO2 thin films have been synthesized by radio-frequency magnetron sputtering and a sol–gel method to study hydrogen generation by photocatalytic water-splitting under visible light irradiation. The doping method, dopant concentration, charge transfer from metal dopants to TiO2, and type of dopants used for modification of TiO2 were investigated for their ability to enhance photocatalytic activity. UV–Visible spectra show that the metal-doped-TiO2 obtained by sputtering is much more efficient than that obtained by the sol–gel technique at inducing a red shift of the absorption edge in the visible light range. Low concentration metal ion doping must be done near the conducting indium tin oxide (ITO) – TiO2 interface to avoid the formation of recombination centers for photo-generated electron–hole pairs. H2 production rate (μmol/h) is higher for Fe-doped TiO2 (15.5μmol/h) than for Cr-doped TiO2 (5.3μmol/h) due to the ability of Fe ions to trap both electrons and holes, thus avoiding recombination, while Cr can only trap one type of charge carrier. A constant H2 generation rate is obtained for long periods of time by all the investigated TiO2 films because of the separate evolution of H2 and O2 gases, thus eliminating the back-reaction effect. [Copyright &y& Elsevier]

Abstract: TiO2 thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO2 film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H2 generation rate is obtained for long period of time by both the TiO2 films because of the separated evolution of H2 and O2 gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H2 generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO2 film by the energetic target ions during deposition in pure Ar gas pressure. [Copyright &y& Elsevier]