Vertically oriented TiO<inf>2</inf> nanotube arrays prepared by anodic oxidation

Abstract

Vertically oriented TiO2 nanotube arrays transformed from Ti thin film on silicon substrates are very attractive due to their potential for nanostructure integration on silicon [1]. We fabricated the vertically oriented TiO2 nanotube arrays using Ti thin film anodization under various processing conditions. To obtain the optimum condition for the vertically oriented TiO2 nanotube arrays, the various anodization voltages and temperatures were investigated in aqueous solutions mixed with highly corrosive Na2SO4/NaF/citric acid and NH4F in glycerol (1,2,3-propanetriol). In case of a good quality Ti thin film deposited at 500degC, vertically oriented TiO2 nanotube arrays were formed in NH4F in glycerol (1,2,3-propanetriol) at 20degC. FE-SEM images of vertically oriented and highly ordered TiO2 nanotube arrays were shown in Fig. 1. The pore diameter and center to center spacing of nanotube arrays were -50 nm and -100 nm, respectively. However, in case of Ti thin film deposited at room temperature, very thin TiO2 layer of-70 nm with a worm-like structure was grown on the porous layer [2]. Fig. 2 shows a typical current density transient curve recorded during anodization under a constant voltage for self-organized pore formation. The pore formation behavior of anodic titanium oxide was similar to that of porous anodic alumina (PAA) [3]. And the dependences of pore morphology and pore formation rate on process parameters were evaluated in Fig 3. The porous anodic oxide layer grown on titanium by electrochemical anodization was studied and compared to the mechanism governing the formation of porous alumina. X-ray diffraction patterns revealed that as-anodized titania nanotube arrays with amorphous phase were crystallized to anatase phase by annealing at 500degC. The fabrication of nanoporous anatase titania with well-defined cylindrical pores and adjustable pore spacing has been attracting considerable fundamental and technological interests because of the important applications in sensing and detection, separation, electronics, optoelectronics and as host materials for various nanostructures [4, 5]. The vertically oriented and highly ordered TiO2 nanotube arrays have applications to dye-sensitized solar cell and bio-sensing materials.