Selective Brookite Polymorph Formation Related to the Amorphous Precursor State in TiO2 Thin Films

A wide variety of brookite TiO₂ synthesis methods have been published over the past several decades, but few studies discuss the underlying mechanism that stabilizes brookite over its stable counterparts, rutile and anatase. Here, we investigate of the effect of pulsed laser deposition parameters on the as-deposited amorphous precursor titania thin films, which subsequently crystallize into stable and metastable TiO₂ polymorphs upon annealing. We find that oxygen pressure in the deposition chamber strongly influences the non-equilibrium state of the amorphous precursor, which ultimately allows for selective polymorph formation. Rutile forms as the dominant phase at low pO₂ < 0.1 mTorr, while anatase is favored at high pO₂ > 5 mTorr. Brookite forms primarily at intermediate pO₂ (0.5–1.0 mTorr). Controlling the amorphous structure (i.e. Ti—O bonding and polyhedral arrangement) of the precursors via oxygen deficiency is therefore likely for the selective formation of crystalline TiO₂ polymorphs from sub-stoichiometric amorphous precursors. Directing phase selectivity by manipulating the structure and internal energy of the precursor amorphous state may have tremendous potential for synthesis of metastable crystalline phases that exhibit more desirable properties in comparison to their stable counterparts.