Photoelectron spectroscopic study of Li oxides on Li over-deposited V 2 O 5 thin film surfaces Qi-Hui Wu * , A. Thissen, W. Jaegermann Surface Science Institute, Department of Materials Science, Darmstadt University of Technology, Peterssenstr. 23, 64287 Darmstadt, Germany Received 18 August 2004; accepted 13 December 2004 Available online 13 January 2005 Abstract The Li oxides species formed on Li over-deposited V 2 O 5 thin film surfaces have been studied by using X-ray and UV induced photoelectron spectroscopy (XPS and UPS). The photoelectron spectroscopic data show that the Li over-deposited V 2 O 5 system itself is not stable. Further chemical decomposition reactions are taken place even under UHV conditions and lead to form Li 2 O and Li 2 O 2 compounds on the surface. The formation of Li 2 O 2 causes to arise an emission line at about 11.3 eV in the valence band spectra. # 2004 Elsevier B.V. All rights reserved. Keywords: XPS; UPS; Li deposition; V 2 O 5 1. Introduction Adding alkali metals to metal oxide substrates often significantly alters their surface properties. The alkali metal doped transition metal oxides (i.e. NiO 2 , MoO 3 ,V 2 O 5 , MnO 2 ) are widely studied for applica- tions in the field of catalysis, electrochromics, and energy storage [1–4]. In general, these materials have suitable structural and electronic properties that allow reversible intercalation of small alkali metal (Na, Li) when the amount of alkali metal is small, as show below for V 2 O 5 example [5,6], xLi þ V 2 O 5 ! Li þ x V 2 O  5 ð0  x  2Þ (1) But when extra amount of alkali metal deposition on these oxide surfaces, chemical decomposition reaction between the alkali metal and substrate oxides will take place [7]. The exact mechanism of the formation of the alkali oxides on the transition metal oxides surfaces is not fully understood until now [8], but a direct Li induced decomposition reaction of the V 2 O 5 host could be considered [9]. Furthermore, very few inter- esting XPS data have been reported for the O 1s line of different alkali oxides. The aim of this work is to: (1) shed light on some of the unwanted surface processes on the Li over-deposited V 2 O 5 thin films; (2) assign www.elsevier.com/locate/apsusc Applied Surface Science 250 (2005) 57–62 * Corresponding author. Present address: Physical Chemistry I, Ruhr-University Bochum, Universita ¨tsstr. 150, 44780 Bochum, Germany. Tel.: +49 234 3224219; fax: +49 234 3214182. E-mail address: wu@pc.rub.de (Q.-H. Wu). 0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.023