Nitrogen-doping of bulk and nanotubular TiO2 photocatalysts by plasma-assisted atomic layer deposition

Plasma-assisted atomic layer deposition (PA-ALD) was adopted to deposit TiO2-xNx ultrathin layers on Si wafers, calcined Ti foils and nanotubular TiO2 arrays. A range of N content and chemical bond configurations were obtained by varying the background gas (O-2 or N-2) during the Ti precursor exposure, while the N-2/H-2-fed inductively coupled plasma exposure time was varied between 2 and 20s. On calcined Ti foils, a positive effect from N doping on photocurrent density was observed when O-2 was the background gas with a short plasma exposure time (5 and 10 s). This correlates with the presence of interstitial N states in the TiO2 with a binding energy of 400 eV (N-interst) as measured by X-ray photoelectron spectroscopy. A longer plasma time or the use of N-2 as background gas results in formation of N state with a binding energy of 396 eV (N-subst) and very ow photocurrents. These N-subst are linked to the presence of Ti3+, which act as detrimental recombination center for photo-generated electron-hole pairs. On contrary, PA-ALD treated nanotubular TiO2 arrays show no variation of photocurrent density (with respect to the pristine nanotubes) upon different plasma exposure times and when the O-2 recipe was adopted. This is attributed to constant N content in the PA-ALD TiO2-xNx, regardless of the adopted recipe. (C) 2015 Elsevier B.V. All rights reserved.