Atomistic Study of the Clean and Hydroxylated TiO 2 Surface
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ATOMISTIC STUDY OF THE CLEAN AND HYDROXYLATED TiO2 SURFACE
R. PODLOUCKY*, S.G. STEINEMANN -- AND A.J. FREEMAN ... *Inst. f. Physical Chemistry, Univ. Vienna, Wihringerstrasse 42, A-1090 Vienna, Austria "**Inst. f. Experimental Physics, Univ. Lausanne, CH-1015 Lausanne-Dorigny, Switzerland ***Dept. of Physics, Northwestern Univ., Evanston, IL 60208-3112, USA ABSTRACT The coverage of an oxidized Ti-surface by amphoteric OH groups seems to be the driving mechanism for the attachement of bone material to Ti-implants. To contribute to the quantitative and fundamental understanding of this phenomenon, an ab-inito surface method was applied to calculate the electronic structure and energetics of the clean and hydroxylated (110) Ti02 rutile surface. Some of the most important relaxation and reconstruction effects of surface geometries were derived from total energy minimization.
INTRODUCTION Ti and Ti-alloys have been applied for fracture treatement for more then 20 years'. So far no case of incompatibilty in human body has been reported. Pure Ti is a very reactive material and forms spontaneously an oxide on its surface when exposed to air, water or any electrolyte. Because the Ti-oxide is one of the most resistant minerals, the surface oxide film in contact with tissue is almost insoluble and no ions are released which could react with organic molecules. In addition, metallic Ti and related alloys have good mechanical properties and they are many times stronger than cortical bone or dentin. Bone grows into the rough surface and bonds to the metal. Such a reaction is usually attributed to the so-called bioactive materials. This ankylotic anchoring or osseointegration forms the best possible basis for dental implants because it can withstand all possible mechanical loads, such as tensile, compressive and shear forces. The bond formation between bone and the Ti-based implant is not due to bioactivity in its original meaning because the protective oxide on the surface is inert in tissue. The mechanism for the attachement of bone material seems to be rather due to specific chemical processes on the surface. Therefore it is our aim is to provide quantitative and fundamental insight into the surface reactions of Ti-oxides by application of ab-initio calculations. Most metal oxides are hydroxylated, i.e. covered by OH groups when water or its vapour can react with the surface'. The OH groups are amphoteric in character because they may dissociate protons from ccordinatively bound water molecules or they may add protons to hydroxid groups according to
M-+(OH),,(OH2 ),,, [Un+(OH)n+i(OH M 2 )m.i]- + H+, M"+(OH).,(OH2 ), + H+ - [M'+(OH).,_i(OH2)m+,+]+. by keeping the coordination number constant. Amino acids as elementary building blocks of all biological molecules are also bipolar and they can react as an acid or as bases forming strong double bonds in form of so-called chelats with the hydroxylated Ti-oxide surface. This bonding mechanism of amino acids as well as bonding of soft tissue was investigated by X-ray photoelectron s
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