Niobium Segregation in Niobium-Doped Titanium Dioxide (Rutile)

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• 作者：Armand J. Atanacio ; Tadeusz Bak ; Janusz Nowotny
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：May 29, 2014
• 年：2014
• 卷：118
• 期：21
• 页码：11174-11185
• 全文大小：726K
• 年卷期：v.118,no.21(May 29, 2014)
• ISSN：1932-7455

文摘

This work determined the effect of niobium segregation on the surface and near-surface composition of Nb-doped TiO₂, 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(O₂) = 75 kPa and p(O₂) = 10 Pa, respectively. However, annealing at 1273 K in strong reducing conditions, p(O₂) = 10^鈥?0 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 low-dimensional 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 TiO₂ in solar-to-chemical energy conversion and (ii) chemically induced electric field required for charge separation.