Photocatalytic N-doped TiO2 for self-cleaning of limestones

Nanocrystalline nitrogen-doped TiO2 photocatalysts were applied as surface coatings to three typical Israeli limestones, Maccabim, Halila and Hebron, widely used for historic buildings. Two different N-doped TiO2 sols were synthesised by sol–gel method starting from titanium oxysulfate (TiOSO4) as Ti precursor: a new N-doped TiO2 coating (N-TiF), obtained in neutral conditions, was developed and compared to a N-doped TiO2 (N-TiA), obtained in acidic medium. XRD and Raman spectroscopy confirm the nanocrystalline nature of N-doped TiO2, mainly in anatase phase, with size ~ 4 nm as revealed by TEM analysis. Diffuse Reflectance Spectroscopy indicates a red-shift in the absorption spectra for both nanopowders, even if the X-ray Photoelectron Spectroscopy results are not conclusive on the state and location of the N doping ions. The self-cleaning efficiency of the two TiO2 coatings was assessed on the three Israeli carbonate stones and compared to commercial TiO2 (Evonik P25) by the photoinduced degradation of two organic dyes (Methyl orange and Rhodamine B) and by following hydrophilicity changes, through static contact angle measurements. The harmlessness of these coatings was confirmed by colorimetric and water capillary absorption measurements.