Growth and structure of MBE grown TiO 2 anatase films with rutile nano-crystallites Rui Shao, Chongmin Wang, David E. McCready, Timothy C. Droubay, Scott A. Chambers * Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, WA, USA Received 11 October 2006; accepted for publication 18 January 2007 Available online 1 February 2007 Abstract We have explored the systematics of TiO 2 polymorph nucleation during film growth by molecular beam epitaxy on perovskite sub- strates. The accidental lattice match between anatase (0 0 1) and LaAlO 3 (0 0 1) or SrTiO 3 (0 0 1) typically results in anatase nucleation at the interface. However, the growth conditions dictate whether or not rutile also nucleates, and the associated morphological and struc- tural properties of the composite film. Four symmetry equivalent epitaxial orientations of rutile on anatase are observed when rutile nucleates as discrete particles on LaAlO 3 (0 0 1). Such films constitute model systems for studying the anatase/rutile interface, which is of considerable current interest in photochemistry. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Anatase; Rutile; Molecular beam epitaxy 1. Introduction Titanium dioxide (TiO 2 ) is important in applications including heterogeneous catalysis, gas sensors, pigments and electronic devices [1–3]. TiO 2 comes in three polymor- phs: rutile, anatase and brookite. Rutile and anatase are being actively investigated for their utility in photocatalytic water splitting [4–6] as well as oxidative destruction of organic pollutants [7,8]. The underlying mechanism in heter- ogeneous photochemistry is thought to be the separation and transport of photo-generated charge carriers on the TiO 2 surface, resulting in long carrier lifetimes and enhanced redox reactions under UV irradiation [9]. It has long been realized that anatase nanoparticles display supe- rior photocatalytic properties compared to rutile. TiO 2 powders consisting of mixed rutile and anatase phases, such as Degussa P25, have higher photocatalytic activities than either pure phase. The nanoparticle structure in such composites was originally suspected to consist of an ana- tase core and a rutile shell [10]. However, later experiments by Datye et al. [11] and Ohno et al. [12] suggested that rutile and anatase nanoparticles are separate, but in intimate contact in these composites. It has thus been proposed that the enhanced photocatalytic activity is due to longer elec- tron–hole pair lifetimes associated with the spatial separa- tion of carriers across anatase/rutile interfaces as a result of a staggered band-alignment. This hypothesis has not been adequately tested for want of a model TiO 2 system with well-defined anatase/rutile interfaces. To address this issue, we report the epitaxial growth of anatase thin films containing nanocrystalline rutile inclu- sions. The films were grown on the (0 0 1) face of LaAlO 3 (LAO) and SrTiO 3 (STO) substrates, using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). Epitaxial anatase films have been grown elsewhere by metal-organic chemical vapor deposition [13], MBE [14], pulsed laser deposition [15], and sputtering [16]. Although rutile is the most thermodynamically stable phase of TiO 2 , anatase nucleation occurs because of similar in-plane lattice dimen- sions for the (0 0 1) face of anatase (a = 3.78 A ˚ ) and those of LAO (a = 3.79 A ˚ ) and STO (a = 3.90 A ˚ ). However, rutile may form as a minor phase in these films depending 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.01.039 * Corresponding author. Tel.: +1 509 376 1766; fax: +1 509 376 1044. E-mail address: sa.chambers@pnl.gov (S.A. Chambers). www.elsevier.com/locate/susc Surface Science 601 (2007) 1582–1589