Available online at www.sciencedirect.com ScienceDirect Journal of the European Ceramic Society 35 (2015) 3103–3109 Enhanced structural and phase stability of titania inverse opals Robert M. Pasquarelli a,∗ , Hooi Sing Lee b , Roman Kubrin a,d , Robert Zierold c , Alexander Yu. Petrov b , Kornelius Nielsch c , Gerold A. Schneider a , Manfred Eich b , Rolf Janssen a a Institute of Advanced Ceramics, Hamburg University of Technology (TUHH), 21073 Hamburg, Germany b Institute of Optical and Electronic Materials, Hamburg University of Technology (TUHH), 21073 Hamburg, Germany c Institute of Nanostructure and Solid State Physics, Universität Hamburg, 20355 Hamburg, Germany d Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland Received 21 February 2015; received in revised form 20 April 2015; accepted 24 April 2015 Available online 15 May 2015 Abstract The applications and processing of nanostructured materials at high temperatures require stability of their morphology. However, in such envi- ronments (>1000 ◦ C), these structures are prone to significant undesired microstructural changes that result in a loss of functional properties. The thermal stability of titania inverse opal films, prepared from self-assembled templates of monodisperse polystyrene spheres by infiltration utilizing atomic layer deposition and subsequent calcination, was assessed. Resistance to grain growth and a shift in the anatase-to-rutile transformation to higher temperatures was observed, with dramatic stability under vacuum. Vacuum annealed samples retained the anatase phase and exhibited minimal grain growth even after 3 h at 1300 ◦ C. Photonic properties were retained until the transformation onset. The remarkable resistance was attributed to inhibition of surface diffusion and structure-substrate constraints. In addition to being technologically enabling, the results provide further insight into the titania system and its phase transformation mechanism. © 2015 Elsevier Ltd. All rights reserved. Keywords: Titania; Anatase-to-rutile; Phase transformation; Inverse opal; Photonic crystal 1. Introduction Titania is one of the most widely applied materials in cataly- sis, optics, and photonics [1–3]. Its high refractive index (anatase n = 2.5 and rutile n = 2.9) makes it particularly attractive for fab- rication of photonic crystals (PhCs) [4–6]. Synthetic methods to titania PhCs, typically in the form of three-dimensionally ordered macropores (3DOM, e.g. inverse opals), commonly yield the anatase polymorph. However, titania structures are rarely considered for high temperature applications due to a phase transformation from anatase to the more thermodynami- cally stable rutile polymorph, which starts at temperatures as low as ∼600 ◦ C for bulk and nanocrystalline material [1]. The transformation itself is a reconstructive nucleation and ∗ Corresponding author. Tel.: +49 40428783646. E-mail addresses: robert.pasquarelli@gmail.com (R.M. Pasquarelli), janssen@tuhh.de (R. Janssen). growth process. It occurs via surface/interface nucleation, which initiates rearrangement within the grain [7–9]. Additionally, this reconstructive phase transformation is accompanied by rapid coalescence and grain growth [10–13], leading to severe degra- dation of the morphology of the structural features and a loss of properties. While a recent study by Li et al. reported the transfor- mation of anatase inverse opals to rutile [6], no clear comparison between performance and morphology of the anatase and rutile structures has been made. Highly porous periodic structures, especially for high- temperature photonic applications in thermophotovoltaics (TPV) and thermal barrier coatings, require the thermal stabil- ity of these features at the desired service temperatures [14,15]. However, at temperatures on the order of 1000 ◦ C, even 3DOM structures made of refractory oxides, such as alumina or yttria- stabilized zirconia, exhibit significant undesired microstructural changes [16–19]. Thermal stabilization of macroporous struc- tures and photonic crystals remains a significant challenge. In order to achieve phase stabilization in the titania system, http://dx.doi.org/10.1016/j.jeurceramsoc.2015.04.041 0955-2219/© 2015 Elsevier Ltd. All rights reserved.