Ultrafast Initial Growth Rate of Self-Assembled TiO[sub 2] Nanorod Arrays Fabricated by Ti Anodization

Abstract

The growth mechanism of TiO 2 nanorods fabricated by the anodization of Ti films using a mask of anodic aluminum oxide is investigated by examining the growth rate and morphology of the fabricated nanorods. It is found that the growth rate is 250 nm/s within the first 0.1 s at 117 V and decreases abruptly to 0.79 nm/s after 3 s. In addition, the diameter of the nanorods is larger near the top than near the root of the nanorods. A growth mechanism is proposed to explain these interesting phenomena.Zero-dimensional ͑0D͒ and one-dimensional ͑1D͒ metal oxide nanostructures have attracted considerable attention in recent years due to their unique physical properties and many potential applications in areas such as electronics, optoelectronics, and sensing devices. 1 A thin film of anodic aluminum oxide ͑AAO͒ is employed as a template to grow 0D and 1D metal oxide nanostructures. 2,3 With the aid of the AAO template, large-scale arrays of nanostructures can be fabricated on either semiconductor or glass substrates.In recent years, TiO 2 has drawn much attention due to its promising photoconductive properties that can be tailored for the development of photovoltaic devices. [4][5][6][7][8][9][10][11] The anodization process is adopted to form 0D and 1D nanostructures. 12-20 Because anodization is a cost-effective process, it has the potential to grow large-scale arrays of nanostructures for device applications. It is reported that arrays of TiO 2 nanotubes can be fabricated in bulk Ti and in Ti films. 13 Arrays of TiO 2 nanostructures such as nanodots and nanorods can be fabricated from the same materials using an AAO mask. 2,12 With an AAO mask, TiO 2 nanostructures can be fabricated on semiconductor or glass substrates for device applications. In addition, TiO 2 nanostructures grown on semiconductors, such as Si, are able to form heterojunctions and have interesting photoconductive properties. 21 The growth mechanism is well studied for the TiO 2 nanostructures grown from bulk Ti. However, the growth rate and growth mechanism are not clear for TiO 2 nanostructures grown in AAO pores by anodization.In this study, we investigated the growth mechanism of TiO 2 nanorods in AAO nanopores using anodic oxidation under various conditions. The growth rate and morphology are examined at different stages. It was found that the TiO 2 nanorods grow very quickly in the beginning and reach saturation in growth within 3 s. A possible growth mechanism is proposed to explain this interesting observation. ExperimentalTo fabricate anodic TiO 2 nanostructures, first, a Ti film with a thickness of 130 nm was deposited on a P-type ͑100͒ silicon substrate by an e-gun evaporator. Subsequently, a 700-nm Al film was deposited by a thermal evaporator. To control the morphology of the anodic TiO 2 nanostructures, a two-step anodization process was carried out on the Al/Ti films. 17,22 The first anodization was carried out in a 0.3 M oxalic acid ͑H 2 C 2 O 4 ͒ electrolyte by a 40-V bias at room temperature until the current dropped down to 1 mA. A...

Hierarchical decoration of anodic TiO2 nanorods for enhanced photocatalytic degradation properties

“…3(d). The length is observed to increase linearly at a rate of 0.62 GV m À1 , confirming and extending the data in of Chang et al's report on the length of nanorods grown at potentials between 40 and 120 V which had a growth rate of 0.65 GV m À1 [28]. Starting at 200 V and above, a local failure of the porous alumina layer via breakdown results in formation of oxygen bubbles at those sites.…”

TiO2 nanotube structures for the enhancement of photon utilization in sensitized solar cells

“…With such an AAO membrane as a template, anodic TiO 2 nanorod arrays with regular top morphologies are formed [243]. In TiO 2 NT films synthesized by anodization, although regular periodical structures in the tube direction by current pulse anodization have been achieved as discussed above, the NT mouth has a wide pore distribution in the film plane.…”

Fabrication of titanium dioxide with durable superhydrophilicity by anodization

“…1a, the sample AT-5 was preliminarily etched and obtained micro-scale pit-like structures with sizes ranging from several to tens of micrometers. 25 For a randomly selected zone, it is evident that the above micro-scale structures were actually composed of smaller adjacent pit-like structures with sizes of about 200-400 nm (Fig. 1b).…”