Niobium Segregation in Niobium-Doped Titanium Dioxide (Rutile)

Abstract

This work determined the effect of niobium segregation on the surface and near-surface composition of Nb-doped TiO 2 , containing 0.18 atom % Nb and 0.018 atom % Nb, after annealing in the gas phase of controlled oxygen activity. The segregation-induced concentration profiles were determined using a range of analytical techniques of different depth resolution, including secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). The XPS analysis of the 0.18 atom % Nb specimen annealed at 1273 K in oxidizing conditions and showed segregation-induced niobium surface enrichment of 2.83 atom % and 2.35 atom % in p(O 2 ) = 75 kPa and p(O 2 ) = 10 Pa, respectively. However, annealing at 1273 K in strong reducing conditions, p(O 2 ) = 10 −10 Pa, leads to depletion of the surface layer in niobium to the level of 0.05 atom % (desegregation). The results of SIMS, RBS, and XPS are consistent. The derived theoretical model, explaining the effect of oxygen activity on niobium segregation, considers contribution from three driving forces of segregation, including (i) strain relaxation, (ii) the formation of a lowdimensional surface structure, and the (iii) electric field associated with the surface charge. The established effect of oxygen activity on niobium segregation/desegregation may be used as a technology for imposition of (i) controlled surface composition that is required to achieve enhanced performance of TiO 2 in solar-to-chemical energy conversion and (ii) chemically induced electric field required for charge separation.

Segregation in Titanium Dioxide Co‐Doped with Indium and Niobium

“…The effect of segregation of aliovalent extrinsic ions on the chemical composition of the surface layer of rutile is profoundly influenced by oxygen activity in the gas phase . The effect of segregation on surface properties of TiO 2 has been studied for a range of ions, including indium and niobium . These studies revealed that segregation‐induced enrichment of the surface layer in these ions depends on oxygen activity.…”

“…These studies revealed that segregation‐induced enrichment of the surface layer in these ions depends on oxygen activity. The obtained composition data may be considered in terms of theoretical models based on defect chemistry …”

“…These ions have been selected for the following reasons: The valence of these cations differs from the host cation of TiO 2 , which is a tetravalent titanium ion. The incorporation of pentavalent niobium and tetravalent indium results in the formation of donors and acceptors, respectively. These two ions have recently been the subject of intensive studies on the effect of oxygen activity on segregation . This knowledge seems essential in better understanding the mechanism of co‐segregation. …”

“…Comprehensive studies on the effect of equilibrium segregation on surface versus bulk composition were recently performed for TiO 2 doped with indium and niobium . These studies revealed that segregation‐induced enrichment for both In‐doped TiO 2 and Nb‐doped TiO 2 increase with the increase of oxygen activity.…”

“…These studies revealed that segregation‐induced enrichment for both In‐doped TiO 2 and Nb‐doped TiO 2 increase with the increase of oxygen activity. It has been documented that the segregation‐induced enrichment of the surface layer results in the formation of a low‐dimensional surface layer that is entirely different from the bulk phase in terms of composition and the structure …”