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

John Perkins, David Ginley, John Mangum, Okan Agirseven, James Haggerty, Laura Schelhas, Daniil Kitchaev, Michael Toney, Janet Tate, Brian Gorman, Aaron Holder

Research output: Contribution to journalArticlepeer-review

11 Scopus Citations

Abstract

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.

Original languageAmerican English
Pages (from-to)109-114
Number of pages6
JournalJournal of Non-Crystalline Solids
Volume505
DOIs
StatePublished - 1 Feb 2019

Bibliographical note

Publisher Copyright:
© 2018 Elsevier B.V.

NREL Publication Number

  • NREL/JA-5K00-72918

Keywords

  • Amorphous precursors
  • Oxygen deficiency
  • Polymorphs
  • Selective crystallization
  • Thin films
  • TiO

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