Search

A porous coating assembled with hierarchical 3D Co 3 O 4 urchin-like particles was synthesized by Pulsed Laser Deposition (PLD) and thermal oxidation. Laser ablation of Co B powder, used as the target material, in oxygen atmosphere formed core–shell particles on the coating surface with mainly a metallic Co core and a mixture of Co, B and O accommodating the shell. The thermal oxidation of these core–shell particles in air at 600 °C induces the morphological transformation to urchin-like particles consisting of nanowires (NWs) (diameter: 30–60 nm and length 1–3 μm) grown radially from the core surface. The extrusion marks on the surface of NWs indicate that the stress induced growth process is caused by difference in the thermal expansion coefficient. XRD, Raman, EXAFS and HRTEM analysis confirmed that the NWs are polycrystalline consisting of pure Co 3 O 4 phase. A wet-chemistry hydrothermal procedure was also employed to synthesize nanostructured urchin-like particles which are hollow and the structure is held together by the radially oriented nanorods (diameter: 40–150 nm). During photocatalysis, urchin-like particles synthesized by PLD displayed significantly higher (∼5 times) degradation rates when compared to chemical urchins for degradation of methylene blue dye via a photo-Fenton reaction in presence of H 2 O 2 and visible light. This is mainly attributed to poor stability of the nanorods in the chemical urchin structure. Features such as high surface area, enhanced stability against agglomeration, polycrystalline nature of the NWs, porous surface and superior adhesion, are responsible for the enhanced photocatalytic activity of Co 3 O 4 urchin-like particles assembled in a porous coating synthesized by PLD and thermal oxidation. Reusability tests also demonstrate the robust nature of the catalyst coating. [ABSTRACT FROM AUTHOR]

Ruthenium nanoparticles (NPs) supported over carbon thin films (Ru/C thin films) catalysts were synthesized by pulsed laser deposition and used as catalysts for hydrolysis of ammonia borane (AB). Highly irregular and porous carbon films with high surface area were deposited by varying Ar gas pressures during the deposition. By taking the advantage of phase explosion phenomena, occurring at high laser fluence, the surface of the carbon films were decorated with crystalline Ru NPs with size below ∼10 nm. Ru/C thin film catalyst produced H 2 with 6 times higher H 2 generation rate as compared to unsupported Ru NPs assembled film, and with a high turnover frequency value of 70.5 mol H 2 mol −1 Ru min −1 . A combination of morphological features and high content of sp 2 bonded C atoms provides good dispersion of Ru NPs over a large surface area. Both these features contribute in generating large number of active sites leading to the increase in catalytic efficiency. A possibility of using the present form of catalyst as an ON/OFF switch for H 2 production was also tested. Although the catalytic activity decreased with the number of hydrolysis cycles, Ru/C thin film catalyst was able to generate the expected amount of H 2 gas in each cycle when it was reused several times. The observed low activation energy (∼28 kJ mol −1 ) and high H 2 generation rate (15.5 L H 2 min −1 g −1 of Ru) by hydrolysis of AB suggest that Ru/C thin film catalyst is highly efficient. [ABSTRACT FROM AUTHOR]

Abstract: Co3O4 nanoparticles assembled coatings have been synthesized by physical (pulsed laser deposition (PLD) and electron beam deposition) and chemical (sol–gel and electroless) techniques. The morphological, structural, and optical properties of all the coatings were analyzed by SEM, XRD, Raman spectroscopy, XPS, and UV–vis spectroscopy. The catalytic activities of the several Co3O4 heterogeneous catalyst coatings have been compared by looking at their efficiency on degradation of methylene blue dye that occurs via photo Fenton reaction in presence of H2O2 or oxone oxidants. The coating synthesized by PLD exhibits the best photo-degradation rate of dye in presence of H2O2 (4h) or oxone (8min). The NPs produced by PLD process exhibit smaller average size (18nm), narrow size distribution (σ =3nm), perfect spherical shape, low degree of agglomeration, and mixed amorphous-nanocrystalline phase. These special features of NPs, along with better visible light absorption, are the main factors responsible for the enhanced photocatalytic activity of the PLD Co3O4 NPs assembled coating. It is also observed that although the concentration of oxone (0.5mM) was very small as compared to H2O2 (100mM) the photo-degradation of MB dye solution is much faster for oxone. [Copyright &y& Elsevier]

Abstract: Cobalt oxide (Co3O4) nanoparticles (NPs) assembled coating have been prepared by reactive pulsed laser deposition of Co, in O2 atmosphere, on Si or glass substrate ranging from room temperature to 250°C. The NPs, having narrow size distribution with average values of around 25–50nm, were characterized by Raman, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Co3O4 NPs synthesized at 150°C comprise a mixed amorphous–nanocrystalline phase (that is unique property for catalysis) while complete crystallization of Co3O4 occurs at 250°C with formation of spinel structure. Photocatalytic properties of Co3O4 NPs assembled coating for degradation of methylene blue solution under visible light irradiation are reported. The effect of H2O2 concentration and effect of pH variation on dye degradation rate has been reported. The reusability of the Co3O4 NPs assembled thin coating catalyst was further evaluated in several recycling runs. The activity of heterogeneous Co3O4 NPs assembled thin coating catalyst was compared with homogenous catalyst producing Co+2 ions in methylene blue solution. [Copyright &y& Elsevier]

Abstract: Co-B nanoparticles supported over carbon films were synthesized by using pulsed laser deposition (PLD) and used as catalysts in the hydrolysis of sodium borohydride (NaBH4) to produce molecular hydrogen. Amorphous Co-B-based catalyst powders, produced by chemical reduction of cobalt salts, were used as target material for nanoparticles-assembled Co-B film catalysts preparation through PLD. Various Ar pressures (10–50Pa) were used during deposition of carbon films to obtain extremely irregular and porous-carbon support with high surface area prior to Co-B film deposition. Surface morphology of the catalyst films was studied using scanning and transmission electron microscopy, while structural characterizations were carried out using X-ray diffraction. The hydrogen generation rate attained by carbon-supported Co-B catalyst film is significantly higher as compared to unsupported Co-B film and to conventional Co-B powder. Morphological analysis along with NaBH4 hydrolysis tests showed that the Co-B nanoparticles produced with PLD act as active catalytic centers for hydrolysis while the carbon support provides high initial surface area for the Co-B nanoparticles with better dispersion and tolerance against aggregation. The hydrogen generation rate obtained by the present catalyst film was also investigated as a function of Co-B loading, carbon morphology, and solution temperature. The high performance of our carbon-supported Co-B film is well supported by the obtained very low activation energy (∼31kJ (mol)−1) and exceptionally high H2 generation rate (8.1LH2 min−1 (gofcatalyst)−1) in the hydrolysis of NaBH4. [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: Hydrogen generation by catalytic hydrolysis of sodium borohydride (NaBH4) is studied by using Co–B-based thin film catalyst synthesized by pulsed laser deposition (PLD) technique. Co–B nanoparticles, produced in the catalyst film after the laser ablation process, act as active centers producing significantly higher H2 generation rate than Co–B bulk powder. Surface morphology was studied by using scanning electron microscopy and compositional analysis was established by using X-photoelectron and infrared spectroscopies. Films were deposited at different PLD set-up parameters in order to understand the possible role of size and density of the nanoparticles in the catalytic process. Cobalt has been found here to act as an efficient catalyst only when alloyed with boron which partially prevents cobalt oxidation. In particular we report that when used as suitable thin films, Co–B produces H2 with a maximum generation rate of about 3300ml/min per gram of catalyst. [Copyright &y& Elsevier]

Nanocatalysts, in the form of nanoparticles assembled coatings (NPACs) of Co 3 O 4 , were synthesized by pulsed laser deposition (PLD) with optimized parameters. Phase explosion phenomena occurring at high laser fluences produce nanoparticles (NPs) with an average size of ∼5 nm having narrow size distribution (3–10 nm) and low degree of agglomeration, which are randomly arranged in the form of coating on the substrate surface. In comparison with chemically synthesized Co 3 O 4 crystalline powder, the NPACs deposited by PLD showed significantly higher catalytic activity for H 2 generation by hydrolysis of NaBH 4 . Maximum H 2 generation rate obtained by NPACs (5010 ml min −1 g cat −1 ) is about 5 times higher than that produced by Co 3 O 4 powder (1000 ml min −1 g cat −1 ), which is mainly attributed to high surface area and large number of active sites provided by the Co 3 O 4 NPs in the coating owing to their size and shape. By varying the O 2 pressure during PLD, two different cobalt oxide phases, namely Co 3 O 4 and CoO, were formed in NPACs and found that Co 3 O 4 phase is more active for hydrolysis than CoO phase with lower oxidation number. The morphology and crystallinity of Co 3 O 4 NPACs were tuned by varying the laser fluence and substrate temperature respectively, and their effect on H 2 generation rate was studied. The results showed that the NPs with mixed amorphous-nanocrystalline phase on the surface act as active sites for favorable interaction and NaBH 4 conversion. [ABSTRACT FROM AUTHOR]

Abstract: The influence of substrate temperature, process gas, deposition pressure, and substrate type on the phase selection, orientation/epitaxy, and growth morphology of thin films in the SrNbO y (y≈3.0 or 3.5) family was investigated. Pulsed laser deposited films (from a Sr2Nb2O7 target) obtained in both oxygen and nitrogen atmospheres upon various substrates were characterized with X-ray diffraction, energy dispersive spectroscopy, atomic force microscopy, and transmission electron microscopy. In oxygen atmospheres, films adopted the (110)-layered perovskite structure of the target. Higher temperatures, lower pressures of oxygen, and use of (110)-oriented SrTiO3 substrates lead to highly crystalline, epitaxial films of Sr2Nb2O7. The use of nitrogen atmospheres resulted in cubic perovskite SrNbO3 formation: epitaxial, textured, or polycrystalline films were obtained depending on the substrate; no nitrogen incorporation could be observed on the anion sublattice. On SrTiO3, the cubic perovskite films followed a cube-on-cube epitaxy and planar defects were observed to occur on the (110) perovskite planes. [Copyright &y& Elsevier]