XPS studies of the nitridation of MoO 3 thin films on alumina and silica supports

Abstract

X-ray photoelectron spectroscopy has been used to characterize thin films of MoO 3 formed on planar oxidized supports, namely AlO x (from aluminum strip) and SiO 2 (from silicon wafer). Comparisons are made with behaviour for MoO 3 on metallic Mo substrate. It is observed that on calcination at 450°C, the Mo 3d spectral features shift to higher binding energy for MoO 3/AlO x and to lower binding energy for MoO 3/SiO 2. On nitridation by heating in NH 3, it is found that the samples on the oxide supports show easier O–N replacement compared with the MoO 3/Mo system. In general, the nitridation behaviour for MoO 3/AlO x is similar to that of MoO 3/Mo, but Mo species in MoO 3/SiO 2 seem to be more easily reduced (Mo(0) is detected for the SiO 2 system but not for AlO x ). Comparisons of heating rates for the second nitridation step from 350 to 450°C were made for the MoO 3/Mo and MoO 3/AlO x samples. Differences between the high heating rate (100 K/h) and the low heating rate (40 K/h) are incremental but definite. The lower heating rate is favourable both for the O–N replacement and for the metal reduction. For example, more Mo(+3) is present after the nitridation to 450°C when the low-heating rate regime is used, compared with that formed when the heating is done at the higher rate.