Probing the Electronic Surface Properties and Bandgap Narrowing of in situ N, W, and (W,N) Doped Magnetron-Sputtered TiO2 Films Intended for Electro-Photocatalytic Applications

Abstract

We report on the optical and electronic properties of undoped, nitrogen (N) doped, tungsten (W) doped, and (W,N) doped TiO2 films deposited by RF magnetron sputtering process. The optical absorption edge of the N doped TiO2 films is shown to red-shift significantly as compared to the undoped TiO2 films. This tendency was found to continue with the codoped films, as their absorption edge substantially red-shifted to ∼420 nm. This red-shifting is due to the effective narrowing of the band gap of the doped films as determined via Tauc plot analysis, which confirmed a reduction of the bandgap of the (W,N) codoped TiO2 films to a value as low as 2.2 eV (in comparison with the 3.2 eV value of the undoped TiO2 films). From X-ray photoelectron spectroscopy (XPS) analysis, it is shown that the N and W atoms were incorporated into the O and Ti lattice sites of TiO2 respectively. On the other hand, valence band spectra of these films suggest that it primarily consists of O 2p derived states and the maximum upshift o...