Hard x-ray photoemission study of oxidation states of Ti underlayer in Fe/Ti film system

Abstract

The Fe/Ti catalyst system that was recently found to be effective for the growth of dense carbon nanotube (CNT) forests was studied using hard x-ray photoemission spectroscopy (HAXPES). It was previously found that the Ti support layer was partially oxidized at room temperature (RT) and absorbed oxygen from the Fe overlayer at higher temperatures, which gave rise to the dense CNT forest growth. The aim of the present study was to elucidate the reason for the initial oxidation of the Ti layer at RT, which remained unclear from the results of the previous study. The control of the initial Ti oxidation is important because it could affect the reduction and activation of the Fe layer at higher temperatures. Depth-dependent HAXPES measurements using different x-ray incidence angles revealed that the degree of oxidation of the 1-nm-thick Ti layer in a sample that had been aged at RT for approximately three months varied depending on the depth. This suggests that oxidation of the Ti layer proceeded after the Fe/Ti film fabrication. Then, the effects of aging on the Ti oxidation states were examined. The Ti 2p3/2 spectrum measured for another sample that had been aged for approximately nine months showed a higher degree of oxidation of the Ti layer than that measured for the sample aged for three months. These results indicate that the Ti layers absorbed oxygen despite the existence of a 2-nm-thick Fe overlayer.