Nanoscale surficial films and a surface transition in V 2 O 5 –TiO 2 -based ternary oxide systems H. Qian, J. Luo * School of Materials Science and Engineering, Center of Optical Materials Science and Engineering Technologies (COMSET), Clemson University, Clemson, SC 29634, USA Received 3 April 2008; received in revised form 5 May 2008; accepted 16 May 2008 Available online 23 June 2008 Abstract Nanoscale, vanadia-based, quasi-liquid films of self-selecting (equilibrium) thickness were observed on TiO 2 surfaces in six ternary oxide systems (Ti–V–X–O; X = P, Na, K, Nb, Mo or W). It is demonstrated that the film appearance and thickness could be tailored via co-doping or changing the equilibration temperature. Furthermore, the observed discontinuous changes in film thickness, hysteresis and bimodal thickness distributions indicate a first-order monolayer-to-multilayer adsorption transition, which is interpreted as a cou- pled prewetting and premelting transition. The film thickness and stability are measured as functions of equilibration temperature, anneal time, thermal treatment history, co-doping, overall composition and TiO 2 phase and orientation. The characterization of more than 850 independent films represents the most systematic measurement of similar interfacial films to date, providing insights into the formation mechanisms and the thermodynamic stability of equilibrium-thickness surficial films and analogous intergranular films. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Wetting; Surface segregation; Phase transformation; Interfaces; Catalysis 1. Introduction Nanoscale, impurity-based films of similar character have been widely observed at grain boundaries (GB) in ceramics [1–6] and metals [3,7,8], at hetero-interfaces in ceramic–ceramic [3] and metal–oxide [9–12] systems, and on free surfaces in oxides [11,13–17]. These films are called intergranular (glassy) films (IGF) or surficial amorphous films (SAF), although some partial structural order gener- ally exists within them [3,13]. These surficial and intergran- ular films represent non-wetting conditions. Sufficient data are now available to interpret these IGF and SAF alterna- tively as equilibrium-thickness interfacial films [1,3,13,18,19] or multilayer adsorbates [2–6,13,16,19]. Nanometer-thick, impurity-based, quasi-liquid films can also be stabilized below the bulk solidus temperatures [3], where analogies to the simpler interfacial phenomena of premelting (in unary systems) [20] and prewetting (in bin- ary de-mixed liquids) [21] can be made. Further discussions of these related interfacial phenomena and terms/jargon can be found in recent reviews [3,13]. Recently, Tang et al. [22] proposed that subsolidus IGF can be understood as quasi-liquid interfacial films formed from coupled GB prewetting and premelting transitions in a diffuse-interface model, and these GB transitions can be first order. A similar theory has also been proposed for SAF [19]. Furthermore, Dillon et al. observed several disordered GB structures (complexions) in doped Al 2 O 3 , implying the existence of GB transitions between them [5,6]. Yet, first-order transitions from monolayer/sub- monolayer adsorption to nanoscale quasi-liquid films have not been directly observed at surfaces or GB in ceramics. In addition to their scientific significance, GB and surface transitions are of practical importance because they cause abrupt changes in transport kinetics [5,6] and material properties [3]. Consequently, this study sought a surface transition in V 2 O 5 –TiO 2 -based systems. 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.05.027 * Corresponding author. Tel.: +1 864 656 5961; fax: +1 864 656 1453. E-mail addresses: jianluo@clemson.edu, jluo@alum.edu (J. Luo). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 4702–4714