Diffusion kinetics of gold in TiO₂ nanotube arrays for formation of Au@TiO₂ nanotube arrays

A simple approach is presented to form Au@TiO₂ nanotube arrays. This approach consists of the formation of pure TiO₂ nanotube arrays by a two-step anodization process, the coating of a Au nanofilm on the top of the pure TiO₂ nanotube arrays, and the heat treatment of the TiO₂ nanotube arrays coated with the Au nanofilm. The heat treatment leads to the diffusion of Au atoms into the TiO₂ nanotube arrays, resulting in the formation of Au nanocrystals on the outer surface of the TiO₂ nanotubes. X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy are used to characterize the microstructural evolution of the Au@TiO₂ nanotube arrays, which reveals the phase change of TiO₂ from amorphous phase to anatase phase. The kinetics of the migration of Au atoms into the TiO₂ nanotube arrays is studied. The diffusivity for the diffusion of Au atoms on the outer surface of the TiO₂ nanotubes is in the range of 4.14-19.4 × 10^-18 m² s^-1 for temperature in the range of 400-500 °C. The activation energy for the migration/diffusion of Au on the outer surface of the TiO₂ nanotubes in the temperature range of 400 to 500 °C is 67.2 kJ mol^-1. The growth of Au nanocrystals on the surface of the TiO₂ nanotubes can be described as a first order reaction.