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

Abstract Plasma-assisted atomic layer deposition (PA-ALD) was adopted to deposit TiO 2- x N x ultrathin layers on Si wafers, calcined Ti foils and nanotubular TiO 2 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 20 s. 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 TiO 2 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 low photocurrents. These N subst are linked to the presence of Ti 3+ , which act as detrimental recombination center for photo-generated electron-hole pairs. On contrary, PA-ALD treated nanotubular TiO 2 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 TiO 2- x N x , regardless of the adopted recipe.