High-fraction brookite films from amorphous precursors

James E S Haggerty,Kevin H Stone,Brian P Gorman,Janet Tate,Daniil A Kitchaev,Wenhao Sun,Lauren M Garten,John D Perkins,Gerbrand Ceder,Laura T Schelhas,Michael F Toney,John S Mangum,David S Ginley

doi:10.1038/s41598-017-15364-y

Abstract

Structure-specific synthesis processes are of key importance to the growth of polymorphic functional compounds such as TiO2, where material properties strongly depend on structure as well as chemistry. The robust growth of the brookite polymorph of TiO2, a promising photocatalyst, has been difficult in both powder and thin-film forms due to the disparity of reported synthesis techniques, their highly specific nature, and lack of mechanistic understanding. In this work, we report the growth of high-fraction (~95%) brookite thin films prepared by annealing amorphous titania precursor films deposited by pulsed laser deposition. We characterize the crystallization process, eliminating the previously suggested roles of substrate templating and Na helper ions in driving brookite formation. Instead, we link phase selection directly to film thickness, offering a novel, generalizable route to brookite growth that does not rely on the presence of extraneous elements or particular lattice-matched substrates. In addition to providing a new synthesis route to brookite thin films, our results take a step towards resolving the problem of phase selection in TiO2 growth, contributing to the further development of this promising functional material.

Highlights

TiO2 can form in many structures, the most prominent of which are the naturally-occurring rutile, anatase and brookite polymorphs depicted in Fig. 1, many other synthetically prepared structures have been reported[1,2]
We report a simple, substrate agnostic synthesis route that produces a high fraction of the brookite phase in a thin film by pulsed laser deposition (PLD) of amorphous TiO2 and subsequent annealing
To identify possible off-stoichiometric intermediates, we examine the formation of partially sodiated NaxTiO2+y compounds, with Na+ incorporated into the Ti-O matrices of each phase which are known to form in the Na-TiO2 space[31], so that the NaxTiO2+y phases may template the growth of a particular Ti-O framework

Summary

Introduction

TiO2 can form in many structures, the most prominent of which are the naturally-occurring rutile, anatase and brookite polymorphs depicted in Fig. 1, many other synthetically prepared structures have been reported[1,2]. Growth post-nucleation to a large crystal size, reconstructive process[3] Small crystallite sizes under high pressures[45] Large nanocrystals to macroscopic single crystals under high pressures[45] Controlled size in the tens of nanometers regime and the presence of surfactants[10,46,47].Twinning on anatase {112} facets[48]. Helper ions such as NaOH17, C2H2O4 + polymer[24], pH during hydrothermal reaction[8], or Cl- ions[25] during either solution or hydrothermal synthesis. Vapour deposition processes inspired by the proposed growth mechanisms do not consistently lead to the same results[26]

Methods

Results

Conclusion