The influence of pressure and gas flow on size and morphology of titanium oxide nanoparticles synthesized by hollow cathode sputtering

Abstract

Titanium oxide nanoparticles have been synthesized via sputtering of a hollow cathode in an argon atmosphere. The influence of pressure and gas flow has been studied. Changing the pressure effects the nanoparticle size, increasing approximately proportional to the pressure squared. The influence of gas flow is dependent on the pressure. In the low pressure regime (107 ≤ p ≤ 143 Pa) the nanoparticle size decreases with increasing gas flow, however at high pressure (p = 215 Pa) the trend is reversed. For low pressures and high gas flows it was necessary to add oxygen for the particles to nucleate. There is also a morphological transition of the nanoparticle shape that is dependent on the pressure. Shapes such as faceted, cubic and cauliflower can be obtained.

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

“…The gas flow velocity is given by g = Ar atm 10 −6 B g gz 2 Ar 60 (8) where gz is the radius of the growth zone that the gas travels within, and atm is the density of the argon gas at atmospheric pressures [10]. An increased gas mass flow Ar will increase the amount of gas atoms that has to be cooled down according to equation (5). From Eq.…”

“…Close to the exit of the hollow cathode the gas is approximated to be pure argon (with a low degree of ionization), and the diffusion coefficient is given by [14]: ( Ar + Ti ) 2 (11) and the ambipolar diffusion coefficient by: is the collision mean free path, th and i,th are the thermal velocities of neutrals and ions, Ar is the mass of an argon atom, Ti is the mass of a titanium atom, Ar is the collision diameter of an argon atom, and Ti is the collision diameter of a titanium atom which was estimated to be ~ 3 × 10 −10 [m]. For the ambipolar diffusion e is the electron temperature, i is the ion temperature, and Ti + is the elastic collision cross section between a titanium ion and an argon atom, for which the same value as in [5] was used.…”

“…The expression for this two-body collision rate is taken from [18] as (17) Here, H 2 O is the density of water vapor in the vacuum system, σ Ti + H 2 O is the cross section for collisions between a titanium ion and a water molecule, and rel is the relative collision velocity between a titanium atom and a water molecule given by rel = ( 8 B g µ ) 1/2 (18) where µ is the reduced mass of the two colliding species. For a numerical example we use the parameters in the experiments in the high vacuum system used in [5], at the oxygen limit that is marked (1) in figure 1 (c). We are interested in the fraction of the Ti + ions that form TiO dimers during the time they are in zone 2, after leaving the hollow cathode.…”

“…We therefore call this oxygen-assisted nucleation, and call the limit marked (1) in the ( , Ar ) survey the "oxygen limit" for nucleation. In the experiment in [5] the oxygen limit for nucleation had the form Ar >1.3 [Pa/sccm] (1) In the UHV system studied in the experimental companion paper [4], the nanoparticles is produced at such low impurity concentration that they obtain the metallic Ti-phase, far below the oxygen limit for nucleation (1) found in [5]. Here, two other types of limits for nucleation are identified in the ( , Ar ) survey.…”

“…The analysis here is based on results from two experiments, one in a high vacuum system [5] with nucleation in an oxygen-containing environment, and one in a ultra-high vacuum (UHV) system [4]. In the latter, helium replaces oxygen as being necessary for nucleation.…”