Photoelectron spectroscopic study of Li oxides on Li over-deposited V2O5 thin film surfaces

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Abstract

The Li oxides species formed on Li over-deposited V2O5 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 V2O5 system itself is not stable. Further chemical decomposition reactions are taken place even under UHV conditions and lead to form Li2O and Li2O2 compounds on the surface. The formation of Li2O2 causes to arise an emission line at about 11.3 eV in the valence band spectra.

Introduction

Adding alkali metals to metal oxide substrates often significantly alters their surface properties. The alkali metal doped transition metal oxides (i.e. NiO2, MoO3, V2O5, MnO2) are widely studied for applications in the field of catalysis, electrochromics, and energy storage [1], [2], [3], [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 V2O5 example [5], [6],xLi + V2O5  Lix+V2O5 (0  x  2)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 V2O5 host could be considered [9]. Furthermore, very few interesting 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 V2O5 thin films; (2) assign the binding energies of O 1s and Li 1s for different Li oxides. The main techniques used in this study are XPS and UPS, which are proven to be the right tools to analyze the chemical composition and oxidation state of elements presenting on the solid surfaces [10], [11].

Section snippets

Experimental

V2O5 thin films were deposited on freshly cleaved HOPG substrates at room temperature (RT) from a homemade PVD effusion cell using a BN crucible attached by a NiCr/Ni thermocouple. During deposition the source temperature was approximately 670 °C. The deposition rate was determined to 2.3 Å/min by a quartz microbalance (QMB). 350 Å thick, nearly stoichiometric V2O5 films without any detectable contaminations were used in this study. The Li metal was evaporated from standard alkali metal dispensers

Result and discussion

In our previous study [7], we have reported that the maximum content for Li intercalation into V2O5 thin films was about 2.4Li/V2O5. The Li deposition time in our conditions is about 3 min. A further deposition of Li would lead to the formation of Li oxides on the surface. Two Li over-deposited V2O5 thin films samples were prepared in UHV chamber after 9 min Li deposition. One was kept inside the UHV chamber (the basic pressure is better than 10−10 mbar), the other was expored to the air. Fig. 1

Conclusions

Photoelectron spectroscopic data indicate that Li over-deposited V2O5 system itself is not stable in UHV chamber. Further chemical decomposition reactions will take place and form Li2O and Li2O2 on the surface probably due to the instability of the host crystal structure. When the sample was expored to the air, LiOH will form and completely cover on the sample surface. The binding energies of Li1s and O1s are 55.6 and 530.6 eV for Li2O, 56.4 and 533.1 eV for Li2O2, and 55.1 and 531.5 eV for LiOH,

Acknowledgement

The financial support from the Deutsche Forschungsgemeinschaft (DFG) is grateful.

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