Microstructure of Precipitated Au Nanoclusters in TiO 2
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Microstructure of Precipitated Au Nanoclusters in TiO2 C. M. Wang, V. Shutthanandan, Y. Zhang, D. R. Baer, L. E. Thomas, and S. Thevuthasan Environmental Molecular Science Laboratory Pacific Northwest National Laboratory, 3335 Q Avenue, Richland, WA 99352, USA ABSTRACT Gold nanoclusters dispersed in single crystal TiO2 (110) have been formed by 2MeV Au2+ implantation to an ion fluence of 1×1017 cm-2 at 300 K and 975 K followed by annealing at 1275 K for 10 hours. The morphological features, size and crystallographic orientation of the Au nanoclusters with respect to the TiO2 matrix have been investigated using conventional transmission electron microscopy and electron diffractions. In particular, the interface structure between the Au nanoclusters and TiO2 was investigated. Atomic structural model of interface between Au and TiO2 was established based on high-resolution transmission electron microscopy (HRTEM) imaging and HRTEM image simulations. Two types of orientation relationship can be identified, Au//TiO2[001] and Au{111}//TiO2(200), and Au//TiO2[001] and Au{111}//TiO2(110). Au clusters are faceted along Au{112}and Au{111} or Au{220} and Au{002} planes. The precipitated Au clusters show extensive (111) twins. These orientation relationships are the same for Au clusters grown on TiO2(110) surface, indicating that lowering interfacial energy to be a governing factor for setting the orientation relationship. INTRODUCTION It is well known that metal particles of several nanometers in dimension attached on an oxide surface possess unique physical and chemical properties and have been explored for numerous applications ranging from microelectronics to catalysts and gas sensors.1-6 One of the examples is Au/TiO2 system, which shows high activity for low temperature oxidation of CO to CO21,4 and high sensitivity as a low temperature CO gas sensor1. Valden et al.4 have prepared gold clusters ranging in diameter from 1 to 6 nanometers on single crystalline surfaces of TiO2 in ultrahigh vacuum and they have investigated the unusual size dependence of the lowtemperature catalytic oxidation of carbon monoxide. They have found that the sensitivity of this reaction is related to a quantum size effect. One of the major goals in preparation of the dispersed or surface attached Au clusters is to keep the size within a range of several nanometers. Cosandey et al.1-3 have systematically investigated the growth and morphological features of Au clusters attached on the TiO2(110) surface by vacuum deposition and subsequent high temperature annealing. They have found that in the early stage of growth, Au atoms cluster as a 2D-like island followed by 3D growth of clusters with the size ranging in several tens of nanometers. Ion implantation and subsequent high temperature annealing has been proved to be a reliable method for preparation of matrix embedded structures.7,8 In this paper, ion implantation with 2 MeV Au ions and subsequent annealing at 1275 K are employed to prepare Au nanoclusters embedded in single crystal TiO2.
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