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

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 of surface position of valence band maximum (0.8 eV) with respect to the Fermi level was observed for (W,N) doped films, which, in turn, is in accordance with the narrowing of the bandgap revealed by UV–vis spectra analysis. Finally, by quantitatively measuring the work function of undoped, monodoped, and codoped TiO2 films, by means of UV-photoelectron spectroscopy (UPS), we were able to achieve a complete reconstruction of their one-dimensional energy band structure. This fundamental information is highly valuable for the efficient use of doped TiO2 films in electro-photocatalytic and/or optoelectronic applications.