Nucleation of titanium nanoparticles in an oxygen-starved environment. II: theory

Abstract

The nucleation and growth of pure titanium nanoparticles in a low-pressure sputter plasma has been believed to be essentially impossible. The addition of impurities, such as oxygen or water, facilitates this and allows the growth of nanoparticles. However, it seems that this route requires so high oxygen densities that metallic nanoparticles in the hexagonal Tiphase cannot be synthesized. Here we present a model which explains results for the nucleation and growth of titanium nanoparticles in the absent of reactive impurities. In these experiments, a high partial pressure of helium gas was added which increased the cooling rate of the process gas in the region where nucleation occurred. This is important for two reasons. First, a reduced gas temperature enhances Ti2 dimer formation mainly because a lower gas temperature gives a higher gas density, which reduces the dilution of the Ti vapor through diffusion. The same effect can be achieved by increasing the gas pressure. Second, a reduced gas temperature has a "more than exponential" effect in lowering the rate of atom evaporation from the nanoparticles during their growth from a dimer to size where they are thermodynamically stable, *. We show that this early stage evaporation is not possible to model as a thermodynamical equilibrium. Instead, the single-event nature of the evaporation process has to be considered. This leads, counter intuitively, to an evaporation probability from nanoparticles that is exactly zero below a critical nanoparticle temperature that is size-dependent. Together, the mechanisms described above explain two experimentally found limits for nucleation in an oxygen-free environment. First, there is a lower limit to the pressure for dimer formation. Second, there is an upper limit to the gas temperature above which evaporation makes the further growth to stable nuclei impossible.3 the gas temperature is the same as the vacuum chamber wall temperature. For details on the experimental arrangements, see Gunnarsson et al [4][5].

Magnetically Collected Platinum/Nickel Alloy Nanoparticles as Catalysts for Hydrogen Evolution

“…Ar (99.9997%) is used as the process gas and is flown through the two cathodes at a total rate of 120 sccm. A small amount of O 2 is added, from outside the mesh cage, to support nanoparticle nucleation. , To efficiently add and control the O 2 content, it is diluted in Ar (95%), resulting in an effective flow of 0.025 sccm.…”

“…A small amount of O 2 is added, from outside the mesh cage, to support nanoparticle nucleation. 30,31 To efficiently add and control the O 2 content, it is diluted in Ar (95%), resulting in an effective flow of 0.025 sccm.…”

Plasma Synthesis and Self-Assembly of Magnetic Nanoparticles

“…been suggested that a two-body collision between a target atom and a water molecule could be enough for a dimer to form, as the target atom binds to the oxygen while a hydrogen molecule is ejected to release the excess energy. [23] Fig. 3 | Schematic image of three-body collision as two target atoms and a sputter gas atom collides.…”

“…Above this size the cluster will, on average, only grow bigger, and it is defined as being nucleated. [23] The probability of an atom to interact with a cluster depends on the size of the cluster as well as density and motion of the atoms. A larger cluster will have a larger "target surface" and is more likely to be hit by an atom.…”

Toward size-dependent thermodynamics of nanoparticles from quantum chemical calculations of small atomic clusters: a case study of (B2O3)n

“…At the same time, the thermodynamic (thermochemical) size-dependent properties of clusters, such as the enthalpy, entropy, Gibbs free energy, specific heat capacity, are of particular importance, 1,14,18,29–37 since knowledge of these attributes in a wide range of temperatures is necessary for predictive modeling chemical transformations involving clusters and nanoparticles in many areas of basic and applied research, namely, chemical vapor deposition, 4,31,38 plasma etching and other plasma-based technologies, 4,8,34,39 formation of interstellar/interplanetary dust and dusty plasma, 37,40,41 and, at last, formation of condensed phase during combustion. 42–47…”