EXAFS Study of Dopant Ions with Different Charges in Nanocrystalline Anatase: Evidence for Space-Charge Segregation of Acceptor Ions

• Published in: Chemphyschem Vol. 10; no. 8; pp. 1238 - 1246
• Main Authors: Knauth, Philippe, Chadwick, Alan V., Lippens, Pierre E., Auer, Gerhard
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 02-06-2009; WILEY‐VCH Verlag; Wiley; Wiley-VCH Verlag
• Subjects: anatase; cations; Chemical Sciences; Condensed matter: structure, mechanical and thermal properties; Crystallization; Defects and impurities in crystals; microstructure; density functional calculations; doping; Doping and impurity implantation in other materials; Exact sciences and technology; EXAFS spectroscopy; Inorganic chemistry; Ions; Nanoparticles - chemistry; Physics; Spectrum Analysis; Structure of solids and liquids; crystallography; Thermodynamics; Titanium - chemistry; Vacancies; Anatase; Zirconium ions; Segregation; Doping; Space charge; Nanostructures; EXAFS; Yttrium ions; Niobium; Zinc; captions; Hot pressing; density functional calculations; Density functional method; EXAFS spectroscopy; Niobium ions; Zinc ions; Nanopowder; Nanocrystal
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200800806

Nanocrystalline TiO2 (anatase) is an essential oxide for environment and energy applications. A combination of EXAFS spectroscopy and DFT calculations on a series of dopants with quite similar ion radius, but increasing ion charge, show boundary space charge segregation of acceptor cations. The picture illustrates the Fourier‐transformed EXAFS spectrum for Sn4+‐doped TiO2.
A series of dopants, including acceptor ions (Zn2+, Y3+), isovalent ions (Zr4+, Sn4+) as well as a donor ion (Nb5+), were studied by EXAFS spectroscopy in nanocrystalline TiO2 anatase powders and nanoceramics. Similar results were found for nanocrystalline powders and nanocrystalline ceramics, made by hot‐pressing the powders. Boundary segregation was observed for the acceptor ions yttrium and zinc, whereas tin, zirconium and niobium ions were placed on substitutional bulk sites and did not segregate, whatever their concentration. These results can be interpreted based on defect thermodynamics, in the framework of a space charge segregation model with positive boundary core, due to excess oxide ion vacancies, and negative space charge regions, where ionized acceptors are segregated.
Nanocrystalline TiO2 (anatase) is an essential oxide for environment and energy applications. A combination of EXAFS spectroscopy and DFT calculations on a series of dopants with quite similar ion radius, but increasing ion charge, show boundary space charge segregation of acceptor cations. The picture illustrates the Fourier‐transformed EXAFS spectrum for Sn4+‐doped TiO2.