Morphology Of Pt Nanoparticles Research Articles

One-dimensional assemblies of platinum nanoparticles on a graphite surface using nonionic/ionized mixed hemicylindrical micelle templates

One-dimensional (1-D) self-assemblies of Pt nanoparticles on a graphite surface have been synthesized via a template-directed sintering process of individual nanoparticles, using nonionic/cationic mixed hemicylindrical micelle templates of dodecyldimethylamine oxide surfactant at graphite/solution interfaces. The dimension and morphology of Pt nanoparticles can be widely controlled by the concentration of Pt ions equivalent to the mixing ratio of nonionic and cationic species in the surfactant micelle. This approach could be extended to fabricate a wide range of self-assembling metallic nanostructures on surfaces using various nonionic/cationic mixed micelle-like self-assemblies carrying metal ions at interfaces, while providing a fundamental insight into a 1-D self-assembly from individual nanoparticles.

Annular Dark Field Tomography in TEM

S. Bals,* C. Kisielowski,** M. Croitoru,* and G. Van Tendeloo* * EMAT-University of Antwerp, Groenenborgerlaan 171, Antwerp B-2020, Belgium ** National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley CA 94720, USA The use of bright field TEM for tomography in physical sciences is often hampered by Bragg contributions. In Figure 1 it is illustrated that the morphology of Pt nanoparticles (5-7 nm) is well reconstructed, but artificial cavities are observed inside the 3D reconstructions of the particles. The different HREM projections in a tilt series of a Pt nanoparticle are simulated using multislice calculations. Using these images as an input for weighted back projection, artificial cavities are again found in the 3D reconstruction. This is not the case when we use the simulated phase of the exit wave as an input for 3D reconstruction. This allows us to conclude that strong scattering and channeling effects are possible reasons for the formation of these artefacts. An experimental method to avoid Bragg contributions is to combine electron tomography with annular dark field TEM (ADF-TEM) [1-3]. In this technique, the central beam and all electrons scattered up to a certain semi-angle are excluded from imaging by an annular objective aperture. In this manner, the traditional high-resolution TEM image contrast (linear interference between the central beam and diffracted beams) is suppressed and a mass-thickness contrast is generated that depends exponentially on sample thickness. This means that ADF-TEM fulfills the projection requirement and that the technique can be used for tomography. Using this approach, we have successfully obtained a 3D reconstruction of CdTe tetrapods as shown in Figure 2. It is found that ADF-TEM is a chemically sensitive technique and a heavier material will appear with higher intensity compared to a lighter material. This explains the indentation observed in one of the branches of the 3D reconstruction of the tetrapod. This area corresponds to the CdSe seed used during growth of the CdTe tetrapods [4]. The indentation does not correspond to a thinner part in one of the branches since the bright field TEM tomogram presented in Figure 3 shows that all branches have equal thickness. An additional advantage of using ADF-TEM for tomography is that the images require exposure times of only 3-5 seconds, which means that the acquisition time of a tomographic tilt series is remarkably reduced compared to e.g. HAADF-STEM or EFTEM where longer scanning times or multiple images for every tilt angle are required. Also scanning noise is avoided using the approach proposed in this work [5].

Preparation of Pt nanoparticles on carbon nanotubes and graphite nanofibers via self-regulated reduction of surfactants and their application as electrochemical catalyst

An in situ synthetic method is reported for preparing and decorating metal nanoparticles at sidewalls of sodium dodecyl sulfate micelle functionalized single wall/multiwall carbon nanotubes and graphite nanofibers. The amount and morphology of Pt nanoparticles depend on the types of carbon nanomaterials (C NM). The results demonstrate for the attachment of metal nanoparticles to the carbon nanotubes’ (CN) surface via non-destructed surfactant modification. These nanocomposites are successfully used as catalysts for oxidation of methanol, the results of which reveal an increase of oxidizing power of Pt/C NM upon increasing the C NM diameter.

Polyol Synthesis of Platinum Nanoparticles: Control of Morphology with Sodium Nitrate

Morphological control over platinum nanoparticles was realized by varying the amount of NaNO3 added to a polyol process, where H2PtCl6 was reduced by ethylene glycol to form PtCl42- and Pt0 at 160 °C. As the molar ratio between NaNO3 and H2PtCl6 was increased from 0 to 11, the morphology of Pt nanoparticles evolved from irregular spheroids with rounded profiles to tetrahedra and octahedra with well-defined facets. Absorption spectroscopy studies suggest that nitrate was reduced to nitrite by PtCl42- in the early stage of the synthesis, and the nitrite could then form stable complexes with both Pt(II) and Pt(IV) species. As a result, the reduction of Pt precursors by ethylene glycol was greatly slowed. This change in reaction kinetics altered the growth rates associated with different crystallographic directions of the Pt nanocrystals and ultimately led to the formation of different morphologies.

Effect of CO on the morphology of Pt nanoparticles supported on TiO 2(1 1 0)-(1 × n)

The effect of CO on the morphology of Pt nanoparticles of different sizes (1–4 nm) supported on TiO 2(1 1 0)-(1 × n) surface was investigated by scanning tunnelling microscopy (STM). The CO pressure was varied in the range of 10 −3–10 1 mbar. It was found that CO adsorption induces a significant increase in the initial size of the Pt nanoparticles of 1–2 nm even at room temperature. The agglomeration, however, is characteristically preceded by disruption of the smaller Pt nanoclusters at lower CO pressures. This finding was explained by CO-assisted Ostwald ripening, in which the mass transport proceeds via surface carbonyl intermediates.

2 Dimensional Dendrites and 3 Dimensional Growth of Electrodeposited Platinum Nanoparticles

Platinum (Pt) nanoparticles are of particular interest as catalysts or electrocatalysts in the electrochemical reduction of oxygen. The morphology of Pt nanoparticles is correlated to their catalytic activity. A mechanism for heteroepitaxial Pt particle dendrite formation (2D growth) and coalescence (3D growth) on highly ordered pyrolytic graphite (HOPG) was observed. The mechanism of 2D dendrite growth, which appeared to be the dominant mechanism at low ionic concentration, was similar to colloidal aggregation, while 3D growth occurred at a high ionic concentration. With increasing number of Pt atoms in a cluster, the simulated interaction energy between the Pt cluster and HOPG substrate decreases. The DLVO theory gives some insight into two different growth mechanisms. The energy barrier between coalescing particles decreases with increasing concentration of electrolytes. Various morphologies of Pt electrodeposition on HOPG were suggested based on the competition between 3D growth and 2D particle growth.