RESUMO
CeO2 nanorods were synthesized by a hydrothermal method and used as the support for preparing a series of Ni/CeO2 nanorod catalysts. The surface area of the catalysts decreased when the Ni percent over the CeO2 nanorods was increased. SEM results showed that the CeO2 is formed by nanorods approximately 1 µm in length. TEM and HREM revealed that the width of the nanorods is about 8 nm and it grew along the [1 1 1¯] axis. The catalytic activity of the catalysts was improved as the Ni was loaded onto CeO2 nanorods. The exposed planes of the CeO2 nanorod structure along the zone axis [0 1 1] for Ni impregnation were (1¯ 1¯ 1), (1 1 1¯), (1 1¯ 1), (1¯ 1 1¯), (2 0 0) and (2¯ 0 0) and they were more reactive for methanol conversion than (2¯ 2¯ 0), (2¯ 0 2¯), (0 2 2¯), (0 2¯ 2), (2 0 2) and (2 2 0) planes from the [1 1 1¯] axis (growth direction of the nanorod). This finding is mainly ascribed to the synergistic effect of the CeO2 nanorods and the Ni.
RESUMO
Silver nanowires were used as templates to synthesize silver-doped CeO(2) (Ag-CeO(2)) nanotubes by the precipitation method. The precipitated solid was dried at 100 °C for 24 h and calcined at 500 °C for 5 h. A TEM, HRTEM, LV-SEM and XRD study was carried out to determine the micro and nanostructural characteristics of the samples. LV-SEM analysis allowed us to observe microtubular empty structures constituted by Ce, O and Ag as indicated by EDS. These tubular structures, with an external diameter from 120 to 280 nm and an internal diameter from 40 to 80 nm, were mainly composed of 11 nm ceria nanoparticles. This kind of structures was obtained when CeO(2) nanoparticles covered the Ag nanowires during the synthesis. Due to the presence of ammonium hydroxide used during the synthesis, a fraction of the silver nanowire reacts and Ag atoms begin to migrate outside the ceria microtube. When the sintering process is applied, the Kirkendall effect can occur. So, out-diffusion of the remnant Ag through the interface is faster than the in-diffusion of the shell material (CeO(2)), which eventually results in a coaxial nanotube on completion of the non-equilibrium interdiffusion, leaving the central core completely empty, driving the formation of hollow tubular Ag-CeO(2) structures as a result.
RESUMO
Different coalescence processes on 1D silver nanostructures synthesized by a PVP assisted reaction in ethylene glycol at 160 °C were studied experimentally and theoretically. Analysis by TEM and HRTEM shows different defects found on the body of these materials, suggesting that they were induced by previous coalescence processes in the synthesis stage. TEM observations showed that irradiation with the electron beam eliminates the boundaries formed near the edges of the structures, suggesting that this process can be carried out by the application of other means of energy (i.e. thermal). These results were also confirmed by theoretical calculations by Monte Carlo simulations using a Sutton-Chen potential. A theoretical study by molecular dynamics simulation of the different coalescence processes on 1D silver nanostructures is presented, showing a surface energy driven sequence followed to form the final coalesced structure. Calculations were made at 1000-1300 K, which is near the melting temperature of silver (1234 K). Based on these results, it is proposed that 1D nanostructures can grow through a secondary mechanism based on coalescence, without losing their dimensionality.