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TiO2 is a versatile wide bandgap transparent semiconducting oxide with three well known polymorphs: Anatase, Rutile and Brookite. These crystal structures are desired for many industrial applications mainly for their photocatalytic activity, oftentimes in thin-film form. Thin-film crystalline polymorphs of TiO2 are often made by physical vapor deposition on a heated substrate, mostly yielding anatase and rutile, but rarely brookite.
However, in the current literature, the correlations of precursors with the end products are still unclear. Our research aims to understand the formation behavior of these metastable polymorphs of TiO2 as related to precursor properties such as defects and density. In this study, TiO2 thin films are deposited at room temperature on fused silica and silicon substrates by pulsed laser deposition and RF magnetron sputtering under different oxygen pressures. All the films are annealed with similar annealing process.
We have found a means to control the polymorph that crystallizes from the amorphous precursor and can produce large-area films of phase-pure brookite, anatase and rutile. The pulsed laser and RF-sputtered amorphous thin-film precursors are produced at up to 150 nm thickness at varying degrees of oxygen substoichiometry, which is the most important factor determining the final polymorph, annealed at moderate temperature, near 400 ˚C for a few minutes and allowed to cool. Additionally, we have later found that the deposition rate, annealing atmosphere, and thicknesses of the films are also important parameters needed to be adjusted properly in order to stabilize each polymorph.
We found that the precursors deposited at the lower oxygen pressure form rutile, and at higher pressures form anatase only. There is a small window of pressure in which brookite is formed, sometimes exclusively. We have also found that lower deposition rate favors anatase and higher deposition rate favors rutile. Anatase is also the dominant phase below 20 nm and above 120 nm thicknesses. On the other hand, thicknesses from 30 to 70 nm favors brookite and 80 to 120 nm favors rutile.
We have investigated the crystallization behaviors of each polymorph from amorphous precursors. We have found that anatase and rutile follow circular growth patterns, but brookite grows anisotropically and follows rhomboid or elliptical structures depending on the film. Rutile has the highest crystallization site count per area followed by brookite and then anatase. Rutile also has the highest Avrami exponent Avrami exponent average (4.8), followed by Brookite (3.6). Anatase having the lower average (3) showing that nucleation rate is higher for Rutile and lower for Anatase with the assumption of 2-dimensional growth. Rutile’s very high Avrami exponents and nucleation rate suggest a continuous heterogenous nucleation and a rapid generation of new nuclei by each newly formed rutile crystallite. We have found that Avrami exponents are not drastically affected by the thicknesses. However, total crystallization time of the anatase and brookite films decrease by increasing thickness that suggests a volume induced crystallization mechanism. We have also found that deposition rate is a critical factor on crystallization time and speed. Activation energies are in between 130-231 kJ/mol (1.4-2.4 eV) which are in agreement with the literature. We concluded this study having more in-depth description and insight of growth behaviors of the three major polymorphs of TiO2.
During this study, microstructural and physical properties of TiO2 thin films were investigated by XRD/XRR, Raman spectroscopy, optical microscopy, Hall Effect measurement, SEM, TEM, RBS, optical transmission, and reflection spectroscopy.