Role of the Nitrogen Source in Determining Structure and Morphology of N-Doped Nanocrystalline TiO2

The photocatalytic activity of N-doped nanostructured TiO2 (TiO2:N) in the visible region strongly depends on the close, yet not fully understood, interplay among crystal structure distortions, nature, and concentration of lattice defects and bulk electronic states. In this work, we study correlations among the chemical identity of the nitrogen source and the microscopic features of biphasic (anatase: brookite) TiO2:N nanoparticles through a broad starting doping range. Triethylamine, urea, and ammonia were considered as independent nitrogen supplies. Synchrotron X-ray photoelectron spectroscopy measurements confirmed the presence of nitrogen within the nanoparticles, while X-ray powder diffraction experiments performed at both synchrotron light sources and conventional laboratory-based instruments found that the dopant monotonically lengthens the cell edge module |c| along the unique C4-axis, until a plateau is reached for starting N/Ti ratios greater than 0.2. The chemical nature of the precursor determines (i) the morphology of the powder at the mesoscale, (ii) the actual magnitude of the maximum lengthening of the c-vector module, and (iii) the anatase phase enrichment. Overall, we found useful hints on possible routes to control and tailor one or more of the specific features of the material (polymorph enrichment, dopant levels, surface area).