Structural and electronic modifications on TiO₂ anatase by Li, K or Nb doping below and above the solubility limit

Undoped and Nb-, K- or Li-doped TiO₂ nanopowders were synthesized using a bottom-up approach by sol–gel method. The changes of crystal structure in TiO₂ upon Nb, K or Li doping were followed with X-ray diffraction (XRD) and Raman spectroscopy techniques. XRD results have shown that anatase-to-rutile phase transition as a function of temperature was suppressed by potassium and niobium incorporation, whereas it was facilitated by lithium incorporation. X-ray detectable secondary phases were not observed in any of the cases. Additional Raman lines were observed at 362, 323 and 242 cm⁻¹ for K- or Nb-doped anatase, which were missing in both Li-doped and undoped anatase TiO₂. Those additional lines were attributed to brookite phase or brookite-like distortions introduced to Ti–O octahedra in anatase. Solubility limits of the aforementioned cations were determined with depth profiling using a surface etching technique. ICP-OES results of etched powders have shown that most of the K or Li was on the surface of the particles and only 0.2 cation% K and 0.7 cation% Li have been incorporated into bulk anatase. A wedge-shaped specimen was utilized for chemical analysis of Nb-doped anatase. EDXS measurements in a TEM were carried out above and below solubility limit. Strong surface accommodation took place in the sample doped above the solubility limit.