Temperature dependence of oxide decomposition on titanium surfaces in ultrahigh vacuum

Abstract

It is well known that the surface of titanium forms stable oxides of Ti2O, TiO, Ti2O3, Ti3O5, TinO2n−1 (4<n<38) and TiO2 and, as a result, the surface exhibits a high degree of immunity against attack by acids and chlorides. However, titanium is also a very active getter. To characterize the gettering process, we have investigated the temperature dependence of stable oxides on titanium in ultrahigh vacuum (UHV) using Auger electron spectroscopy, quadrupole mass spectrometry (QMS), and x-ray photoelectron spectroscopy (XPS) in the temperature range from room temperature (RT) to 600 °C. The surfaces of titanium specimens were prepared by electropolishing (EP), and a thin titanium oxide layer, principally TiO2, was formed on the surface at RT. On increasing the temperature to above 300 °C, the OKLL Auger intensity began to decrease in a drastic way, while in contrast, the TiLMM Auger intensity was increased. Above 450 °C, no oxygen was detected on the titanium surface in UHV. However, the outgassing rates of H2O, O2, CO, and CO2 from the surface did not show any changes in QMS measurements below 450 °C. This behavior is explained as gettering, which is an enhancement of diffusion of oxygen from the surface into the bulk. To investigate the temperature dependence of the chemical states of titanium oxides on an EP surface, XPS analysis was also performed in the same way in the range of 200–450 °C. The results showed drastic changes of the titanium oxide core XPS levels such that the TiO2 decomposed into suboxides at 180 °C, and even the remaining TiO2 was still decreasing on heating up to 420 °C. The surface oxide composition on titanium is very sensitive to the temperature.