The role of oxygen pressure in nitrogen and carbon co-doped TiO2 thin films prepared by pulsed laser deposition method

TiO2 absorbs mostly the UV part of the solar spectrum, which results in low efficiency of absorption for visible light applications. Using pulsed laser deposition technique, we introduce defect states in the band gap of TiO2 by co-doping with N and C under different oxygen pressures (4, 50 and 100 mTorr) to tune the band gap towards visible region. This process yields anatase-rutile mixed phase films. As control samples, we deposit undoped TiO2 films with the above mentioned oxygen pressures. Several characterization techniques are used to examine thin film properties such as X-ray diffraction for crystal structure, Raman spectroscopy for phase analysis, X-ray photoelectron spectroscopy for electronic structure, scanning electron microscopy for planar and columnar morphology and ultraviolet-visible diffuse reflectance spectroscopy for photo absorption. While undoped TiO2 samples show only anatase phase, N and C co-doped TiO2 samples exhibit transition from rutile to anatase with increasing oxygen pressure. Oxygen pressure also affects the surface morphology and columnar structure of the films such that columnar structure is favored under oxygen-rich conditions. Finally, visible absorption around 550 nm was obtained.