A new route for the synthesis of highly-active N-doped TiO2 nanoparticles for visible light photocatalysis using urea as nitrogen precursor

Abstract

Nitrogen-doped TiO2 nanoparticles with high specific surface area and photocatalytic activity under visible light were successfully produced using a modified sol-gel method with urea introduced as the nitrogen source. Different synthesis approaches and parameters such as doping temperature and urea to TiO2 molar ratio were tested to examine the best outcome regarding photocatalytic activity for both UV-A and visible light irradiation. UV–vis diffuse reflectance characterization revealed a decrease in band gap from 3.24 to a minimum of 2.79 eV with the N-doping process. The photocatalytic Methylene Blue dye degradation assays suggests that the introduction of nitrogen in the TiO2 lattice cell can provide a higher efficiency under both UV-A and visible irradiation. The maximum photocatalytic activity was achieved for the nitrogen-doped powders prepared with the lowest urea:TiO2 molar ratio (1.5) as it was the formulation that promoted an enhancement in N-doping and particle specific surface area to 182 m2 g−1, despite the fact that the highest specific surface area was registered for undoped TiO2 nanoparticles (228 m2 g−1). A synthesis step variation was performed to enhance the specific surface area of nanoparticles and consequently the photocatalytic activity. This modification promoted an increase in specific surface area by a factor of ∼5. XPS spectra confirmed a successful introduction of nitrogen in the TiO2 lattice up to 1.5 at.% for optimized powders, which is strongly dependant of the type of synthesis and the amount of dopant species added during the doping process.