Utilizing TiO₂ amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

Abstract

Selective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO 2 thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle-like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.

Near-Surface Composition, Structure, and Energetics of TiO2 Thin Films: Characterization of Stress-Induced Defect States in Oxides Prepared via Chemical Vapor Deposition versus Solution Deposition from Sol–Gel Precursors

“…Recent X-ray diffraction and electron microscopy studies of the thermally induced transition of amorphous pulsed laser deposition (PLD) TiO 2 films to either anatase or brookite crystal forms suggest that there is considerable tensile strain intrinsic to as-deposited amorphous oxide films arising from constrained Ti–O bonding environments, oxygen deficiencies, and the activation energies that must be surmounted to fully relax local microstructures. This strain provides an internal driving force for nucleation and crystallization of the oxide as annealing temperatures are raised, dependent upon initial deposition conditions, including O 2 partial pressure. , Similar X-ray diffraction studies have tracked the conversion from amorphous to crystalline TiO 2 thin films . Most interestingly, Klein and co-workers have recently demonstrated that SG TiO 2 layers deposited on certain FTO surfaces experience a kind of “templating” of the oxide to FTO grains, with thin oxide films resulting which are mixtures of anatase and rutile forms of TiO 2 …”

“…This strain provides an internal driving force for nucleation and crystallization of the oxide as annealing temperatures are raised, dependent upon initial deposition conditions, including O 2 partial pressure. 2,49 Similar X-ray diffraction studies have tracked the conversion from amorphous to crystalline TiO 2 thin films. 50 Most interestingly, Klein and co-workers have recently demonstrated that SG TiO 2 layers deposited on certain FTO surfaces experience a kind of "templating" of the oxide to FTO grains, with thin oxide films resulting which are mixtures of anatase and rutile forms of TiO 2 .…”

“…TiO 2 thin films are attractive options as electron-selective charge harvesting/injecting interlayers in thin film solar cells (PV) and light-emitting displays because of their energetic alignment with conduction band edges of several device relevant active layers, their high overall chemical stability, and because they can be readily formed by scalable deposition technologies such as chemical vapor deposition (CVD), sol–gel (SG) processing, sputtering, pulsed laser deposition (PLD), and atomic layer deposition (ALD). − Such ultrathin oxide thin films have also seen use as chemically stable “separation layers” between electrolytes and photoactive, possible unstable semiconductors (e.g., Si). The oxide layer is expected to protect the underlying semiconductor from corrosion and provide anchoring sites (and protection) for catalysts that can inject/extract charge from the photoexcited semiconductor through the thin oxide layer, while enhancing rates of fuel-forming electrochemical processes. − In all of these platforms there is a trade-off between chemical stability imparted by the thin oxide, adequate electrical conductivity typically provided by doping via “defects”, and minimized chemical reactivity and surface recombination, often occurring at these same dopant sites.…”

Stromataxic Stabilization of a Metastable Layered ScFeO3 Polymorph

“…So how does changing the deposited structure change which phases could be stabilized? There is well-established precedent for the influence of precursor structure on the morphology and crystallographic texture of the ground-state phase in biology, geology, and materials science. − For example, it has been shown in some transition-metal oxides that varying the deposition rate leads to the formation of different polymorphs, presumably due to changes in the local structure prior to crystallization. − Metastable phases have been stabilized by engineering the entropy or enthalpy of alloyed systems, but this route is not viable for less-complex chemistries. , Perhaps a metastable phase could be stabilized if the temperature is low enough or quenched quickly enough, so that the as-deposited structure could have remnant metastability, similar to what has been done in the shuttered layer-by-layer growth of Ruddlesden–Popper phases. − But when the temperature is high enough to induce diffusion in similar compounds as well as absorption-controlled growth, as in the present work (where the substrate temperature was kept above 750 °C), remnant capture cannot entirely account for the difference in phase formation pathways.…”