Interfacial Stabilization of Metastable TiO₂ Films

Abstract

This work demonstrates a phenomenon that preserves the traditionally metastable anatase crystal structure of thin titania (TiO2) films along a two-dimensional oxide interface at temperatures well in excess of those that normally trigger a full polymorphic transformation to rutile in higher dimensionality crystalline powders. Whereas atomic surface mobility appears to dominate polymorph transformation processes within bulk TiO2 powders, a simple reduction in dimen-sionality to a two-dimensional TiO2 film (ca. 50−200 nm thick), supported upon a substrate, leads to a remarkable resistance to the calcination-induced anatase-to-rutile transformation. This stabilization does not appear to be specifically reliant on substrate character given its persistence for TiO2 films prepared on amorphous silica (SiO2) as well as crystalline TiO2 substrates. Instead, interface-mediated coordination of the TiO2 film with the substrate, and the inherent confinement of crystallites in two dimensions, is believed to resist polymorph transformation by mitigation of the atomic surface mobility. Only when temperatures (i.e., >800 °C) that are conducive to bulk atomic mobilization are reached does reconstructive grain growth convert the film into the thermodynamically stable rutile crystal structure.

Template-Induced Structuring and Tunable Polymorphism of Three-Dimensionally Ordered Mesoporous (3DOm) Metal Oxides

“…Indeed, the wetting of isolated crystallites by SiO2 thin films also appears to enhance the interfacial stabilization of anatase crystallites. Increased loading of TiO2 crystallites closely adhered to the SiO2 surface probably also leads to a degree of confinement-based stabilization as we have reported recently, 57 consistent with the apparent interfacemediated stabilization observed even for nonsiliceous templates (e.g., styrene 20 ) as discussed earlier.…”

“…This concept is derived from our recent report of anatase stabilization along the interface of thin TiO2 films deposited on oxide support layers (i.e., TiO2/SiO2). 57 Specifically, we have shown that the anatase polymorph can be stabilized by the TiO2-SiO2 interface at temperatures well in excess of the characteristic Huẗtig temperature identified for bulk TiO2. This results from the apparent interface-mediated mitigation of atomic surface mobility that typically favors the polymorphic transformation from anatase to rutile in bulk powders.…”

Improved Performance of Ternary Solar Cells by Using BODIPY Triads

“…PEDOT: PSS (PH 1000) was purchased from Ossila Ltd.(Sheffield, UK). TiO 2 sol-gel was prepared by following the procedure reported by Gregory et al [51]. Anhydrous sodium sulfate and potassium carbonate were purchased from Thermo Fisher Scientific (Thailand) Ltd. (Bangkok, Thailand).…”

Low-Temperature Processed TiOx/Zn1−xCdxS Nanocomposite for Efficient MAPbIxCl1−x Perovskite and PCDTBT:PC70BM Polymer Solar Cells

“…During the synthesis, 2.7 mL titanium (IV) butoxide (97%, Sigma Aldrich, MO, USA), 0.57 mL ethyl acetoacetate (99%, Sigma Aldrich), 57 μL 2,4-pentanedione (99%, Sigma Aldrich, MO, USA) and 57 μL 2,3-butanedione (97%, Sigma Aldrich, MO, USA) were mixed and stirred at room temperature overnight [37].…”