A Multitechnique Study of the Interfacial Reaction between TiO2 Surfaces and Molybdenum

Since a few years, molybdenum oxides have attracted a lot of interest due to their numerous applications such as catalytic materials, lubricants, memory devices, gas sensors and solid state microbatteries. On the other hand, TiO2 surfaces supporting metals have generated a lot of works due to the strong relation between the morphology of the deposit and the electronic interactions between TiO2 and the deposited metal. For instance, deposition of molybdenum on (110) TiO2 surfaces leads to ultrathin films of MoOx. Therefore, oxygen deficient MoOx compound supported on TiO2 substrates attract a lot of interests, especially for their catalytic properties. Accordingly, numerous researches are developed in order to grow ordered MoOx nanostructures (nano-particles or ultrathin films) on TiO2. Nevertheless, no systematic studies of the role of the TiO2 crystallographic constitution and surface morphology on the interaction with ultrathin (< 5 nm) Mo films have been reported.
In this work, we aim to grow, by non-reactive DC sputtering of a metallic Mo target, MoOx films on home-made (reactive sputtering) and standard TiO2 supports. The TiO2 supports differ by their crystallographic constitution (amorphous, anatase and rutile). A particular attention is made on the influence of the TiO2 properties on the diffusion mechanism and oxidation process at the interface TiO2/MoOx using XPS and Tof-SIMS measurement. Periodic DFT calculations have been also performed to validate the observed behavior.
XPS measurements showed that the interfacial reaction between the TiO2 surface and the deposited Mo is affected by the crystalline properties of the TiO2 support. The strong oxidation of the first monolayer of the deposited Mo is evidenced.
Differences were found in this electronic exchange and, as a consequence, in the growth mode for the three TiO2 substrates. The emphasis is made with the Cabrera-Mott theory, ie , the dependence of the interface reaction on the bulk electronic structure of the three phases. Growth mechanisms were estimated using analysis of the peak and background shapes of the XPS signal using the 'QUASES' software. A layer by layer growth mode is observed for Mo deposited on the (110)-rutile substrate, a Volmer-Weber growth mode is observed on the (101)-anatase substrate and a Stranki-Krastanov growth on the amorphous one. Tof-SIMS depth profile measurements reveal the diffusion of oxygen at the interface with appearance of an oxygen depleted zone at the surface of the TiO2 support. Additional studies were performed with labelled 18O2 TiO2 substrate to assess the influence of heating of the device to the diffusion of oxygen across the interface.