Synthesis and growth mechanism of graded TiO2 nanotube arrays by two-step anodization

Two-step anodization is a novel method to fabricate graded nanotube arrays with particular morphologies. Through tailoring the electrochemical conditions, graded TiO2 nanotube arrays can be formed by two-step anodization. The growth mechanism of graded TiO2 nanotube arrays and influential factors in two-step anodization are investigated. We find that the nature of electrolyte used in the different anodization steps strongly influences the formation of graded nanotubular structure. In order to form graded TiO2 nanotube arrays, proper anodization sequence must be used: Step-1 anodization in the electrolyte which can produce higher electric filed intensity and faster chemical dissolution rate. Followed by Step-2 anodization in the electrolyte which can produce lower electric filed intensity and slower chemical dissolution rate.