Abstract: Surface oxidation of polycrystalline nickel foil in air and pure water at different temperatures and the thermal stability of the surface oxides have been investigated by means of XPS.In ad- dition to NiO,Ni_2O_3 is formed especially after long periods of air exposure.Nickel surfaces are much less reactive to pure water than to air.The thermal stability of the surface oxides is related to oxidative temperature.The surface species of oxides formed by air exposure at temperatures below 120℃ can be reduced into nickel metal after heating the sample in vacuum at 300℃ for only 10 minutes (in the case of room temperature) to 1 h (in the case of 120℃). This reduction is caused by reaction with surface carbon contaminants.However,the surface species of nickel oxides formed by air exposure with heating at temperatures above 200℃ can not be reduced into metal after heating the sample in vacuum at 300℃ for 1h.

Key words: XPS, nickel, surface oxide, thermal stability