First-principles study of initial stage of Ni thin-film growth on a TiO 2 (110) surface
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First-principles study of initial stage of Ni thin-film growth on a TiO2 (110) surface P.L. Cao Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China, and Department of Physics & Astronomy and Materials Research Center, Northwestern University, Evanston, Illinois 60208
D.E. Ellis Department of Physics & Astronomy and Materials Research Center, Northwestern University, Evanston, Illinois 60208
V.P. Dravid Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 7 December 1998, accepted 18 June 1999)
The bonding structure and binding character for the initial stage of thin-film growth of Ni on a rutile (110) surface were studied using first-principles density functional theory. Our results show that, in the first monolayer, Ni atoms are preferentially adsorbed on top of bridging oxygen atoms and upon secondary surface oxygen. The bond strength between Ni adatom and substrate is much stronger than that between Ni adatoms. About 0.3 electrons are transferred from Ni atoms to substrate in low coverage; the adsorption of additional Ni atoms on neighboring sites decreases this transfer. In addition to the ionic bonding component, some covalent character is found in the Ni adatom–substrate bond. I. INTRODUCTION
Metal–ceramic interfaces form a topic of significant technological interest with considerable scientific appeal. On the fundamental front, it is important to know how bonding changes occur across interfaces: Is the transition from metallic bonding to predominantly mixed ionic/ covalent character abrupt or gradual? What are the atomic configurations at the interface? The nature and character of electronic structure at and in the vicinity of such interfaces dictate a myriad of phenomena which are at the heart of technological applications of metal– ceramic composites, where the interfacial phenomena have a direct bearing on macroscopic mechanical and electronic properties. 1,2 TiO 2 and metal-promoted TiO2 are of use in an extremely wide variety of applications, among them heterogeneous catalysis, gas sensors, microelectronics and photolysis of water on TiO2 electrodes.3,4,5,6,7,8 Ni metal dispersed on a metal oxide support has high catalytic activity. For example, rutile supported Ni exhibits a higher activity than either Ni/SiO2 or Ni/Al2O3 for CO hydrogenation.9 The Ni−TiO2 metal–ceramic system has characteristics of model interfaces which are amenable to high resolution spectroscopic measurements of interfacial electronic structure. TiO2 is a convenient material to work with for experimentalists, with high thermal stability and well defined morphology and surface structure. Modern scanning transmission electron microscopes (STEMs) and 3684
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J. Mater. Res., Vol. 14, No. 9, Sep 1999 Downloaded: 14 Mar 2015
transmission electron microscopes (TEMs) now have excellent spatial resolution ( 0 or Eint < 0 means that there is attraction or repulsion between
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