Deposition Of Cocatalysts Research Articles

<i>In Situ</i> Synthesis of CNTs/Mg Composite Powders by CVD at Low Temperatures

The emphasis of this study was in-situ synthesis of carbon nanotubes (CNTs) on Mg matrix at 480°C. The process involves homogeneous deposition of Co catalyst onto Mg by deposition-precipitation route with low Co content (1.0 wt.%) and in situ synthesis of CNTs by chemical vapor deposition. The morphologies and microstructure of the as-obtained CNTs/Mg composite powders was characterized by SEM, TEM, Raman spectrum, and XRD. The results indicated that CNTs were well graphitized and uniformly distributed on the surface of Mg powders, which would be beneficial to the mechanical property of CNTs/Mg composites.

Photocatalytic Water Splitting with Suspended Calcium Niobium Oxides: Why Nanoscale is Better than Bulk – A Kinetic Analysis

The layered Dion–Jacobson phase KCa2Nb3O10 is known to catalyze photochemical water reduction and oxidation under UV light in the presence of sacrificial agents. The same reactions are catalyzed by tetrabutylammonium hydroxide-supported HCa2Nb3O10 nanosheets obtained by chemical exfoliation of the parent phase. Here we describe a factorial study into the effects of nanoscaling, sacrificial charge donors, cocatalysts, and cocatalyst deposition conditions on the activity of these catalysts. In water, nanoscaling leads to a 16-fold increase in H2 evolution and an 8-fold increase in O2 evolution over the bulk phase under the same conditions. The sacrificial electron donor methanol improves H2 production by 2–3 orders of magnitude to 20–30 mmol of H2/h/g, while the electron acceptor AgNO3 increases O2 production to 400 μmol of O2/h/g. Rates for H2 and O2 evolution further depend on the presence of cocatalysts (Pt or IrOx) and, in the case of H2, inversely on their particle size. To rationalize these findings and the increased activity of the nanoscale particles, we propose a kinetic model for photocatalysis with semiconductor particles. The model calculates the electronic rate of the catalysts as a product of terms for charge generation, charge and mass transport, chemical conversion, and charge recombination. The analysis shows that the activity of the catalysts is limited mainly by the kinetics of the redox reactions and by the rate of charge transport to the water–catalyst interface. Mass transport in the solution phase does not play a major role, and neither does surface charge recombination.

Atomic layer deposition of anatase TiO2 coating on silica particles: growth, characterization and evaluation as photocatalysts for methyl orange degradation and hydrogen production

Atomic layer deposition is used to deposit crystalline anatase TiO2 on 500 nm silica support particles over a range of shell thicknesses. The core–shell morphology combines nanometer thickness titania with silica that is readily separable from liquid media. The composite particles are evaluated as photocatalysts for destruction of the model pollutant methyl orange and, after deposition of platinum co-catalyst, for the sacrificial generation of hydrogen from water. The catalytic activities normalized to surface area are higher than those of P25 mixed anatase/rutile titania nanoparticles.

Mesoporous α-Fe2O3 thin films synthesized via the sol–gel process for light-driven water oxidation

This work reports a facile and cost-effective method for synthesizing photoactive α-Fe(2)O(3) films as well as their performances when used as photoanodes for water oxidation. Transparent α-Fe(2)O(3) mesoporous films were fabricated by template-directed sol-gel chemistry coupled with the dip-coating approach, followed by annealing at various temperatures from 350 °C to 750 °C in air. α-Fe(2)O(3) films were characterized by X-ray diffraction, XPS, FE-SEM and electrochemical measurements. The photoelectrochemical performance of α-Fe(2)O(3) photoanodes was characterized and optimized through the deposition of Co-based co-catalysts via different methods (impregnation, electro-deposition and photo-electro-deposition). Interestingly, the resulting hematite films heat-treated at relatively low temperature (500 °C), and therefore devoid of any extrinsic dopant, achieve light-driven water oxidation under near-to-neutral (pH = 8) aqueous conditions after decoration with a Co catalyst. The onset potential is 0.75 V vs. the reversible hydrogen electrode (RHE), thus corresponding to 450 mV light-induced underpotential, although modest photocurrent density values (40 μA cm(-2)) are obtained below 1.23 V vs. RHE. These new materials with a very large interfacial area in contact with the electrolyte and allowing for a high loading of water oxidation catalysts open new avenues for the optimization of photo-electrochemical water splitting.

Low Temperature Synthesis of Carbon Nanotube on Si substrate Using Alcohol Gas Source in High Vacuum

ABSTRACTConventional alcohol catalytic chemical vapor deposition (ACCVD) growth of carbon nanotubes (CNTs) have been carried out under ambient gases from 103 to105 Pa. These ambient gas pressures have prevented in situ observations using an electron beam during CNT growth, such as scanning electron microscopy (SEM), scanning tunneling microscopy (STM). Therefore, in order to realize the in situ observations and to clarify the growth mechanism of nanotube, CNT growth in a high vacuum is essential. In addition, the effects of residual gases also may be avoided in the growth under high vacuum. In this study, we carried out CNT growth under high vacuum using an alcohol gas source in an ultrahigh vacuum (UHV) chamber and we achieved CNT growth below 400°C without any excitation processes of carbon source. After deposition of Co catalyst of 1 nm in thickness on SiO2/Si substrate, ethanol gas was supplied to the substrate surface through a stainless steel nozzle in the UHV chamber. The growth temperature was monitored by a pyrometer during the growth, and set between 350 and 900°C. The supply of ethanol gas was controlled by monitoring an ambient pressure, which was varied from 1 ∼10-1 to 1 ∼10-4 Pa. The grown CNTs were characterized by SEM and Raman spectroscopy. The G/Si intensity ratio reached its maximum at 700°C, when the pressure was 1 ∼10-1 Pa. The maximum point of the G/Si peak intensity shifted to a lower temperature as the growth pressure decreased. When the pressure was 1 ∼10-4 Pa, the G/Si intensity ratio reached its maximum at 400°C, at which clear RBM peaks were observed in the Raman spectrum. From the RBM peaks, the CNT diameters were estimated to be between 0.9 to 1.7 nm, and CNTs of 1.2-1.4 nm in diameter were dominant at 1 ∼10-1 Pa, whereas thinner CNTs (diameter is below 1.0 nm) were increased with the reduction of the pressure. Our largest G/D ratio was about 40 for the sample grown at 1 ∼10-1 Pa, which is considerably larger than the reported value for the CNTs grown under low pressure. From these results, we conclude that the reduction of the growth pressure lowers the growth temperature. This technique can be applied to in situ observation, and may also be useful for low temperature growth of CNTs, which opens new possibilities for the fabrication of CNT based nanodevices.