Elsevier

Chemical Physics Letters

Volume 428, Issues 4–6, 20 September 2006, Pages 421-425
Chemical Physics Letters

On wafer TiO2 nanotube-layer formation by anodization of Ti-films on Si

https://doi.org/10.1016/j.cplett.2006.07.062Get rights and content

Abstract

In this work, we present the self-organized formation of anodic TiO2 nanotubular layers on flat and thin sputter-deposited layers of titanium on silicon. The TiO2 nanotubes were grown by anodization of these substrates in HF/H2SO4 electrolytes at different potentials between 5 V and 20 V resulting in tubes with diameter ranging from 30 to 100 nm. The key to achieve an ordered layer from the thin film samples is to operate at low temperatures. This results in a conversion efficiency of Ti to TiO2 nanotubes close to 100% and prevents loss of large amounts of the tubes due to chemical dissolution.

Graphical abstract

Using a new technique, highly organized TiO2 nanotubular surfaces can be grown by the anodization of titanium on silicon substrates. The tubes are open at the top and their dimensions are given by the applied potential. The key to grow them is using temperatures close to 0 °C during anodization.

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Introduction

Since Zwilling and coworkers have shown the possibility to grow self-organized and ordered TiO2 nanotubes by anodic oxidation of titanium and its alloys [1], there have been many attempts to improve the structure and to exploit the functionality of these highly organized structures on titanium [2], [3], [4], [5], [6], [7]. In view of exploiting specific TiO2 properties, such as its use for solar energy conversion [8], splitting of water [9] or photocatalysis [10], the nanotubular layers provide a TiO2 morphology with a high surface area and controlled geometry. In this context, we have already shown that nanotubular structures show a considerable light-to-electric current conversion efficiency [11]. To improve their visible response, they can be dye-sensitized [12], or doped with nitrogen by means of N-ion implantation [13], [14] and annealing in NH3[15].

Within the growth process of self-organized nanotube layers, a steady-state situation is established between anodic TiO2 formation and its chemical dissolution in F containing electrolytes. Typically the chemical dissolution rate in acidic HF or neutral fluoride-containing solutions, (forming soluble [TiF6]2− complexes) is considerably high. As a certain initiation period for self-organization is required [3], [16], this leads to the situation that considerable time has to pass and significant material dissolution has to take place (i.e. considerable amount of substrate thickness is ‘eaten up’) before a highly regular TiO2 nanotubular structure is established. The starting substrate material for anodization is typically metal (or an alloy) in form of sheet or foil. However, it is of high technological interest to form such nanotube layers also from thin films of Ti, deposited on surfaces, such as silicon wafers or glass.

Very recently, Yu and coworkers have explored anodization of Ti on Si (prepared by DC magnetron sputtering) [17] in HF electrolytes at low temperatures. Although in certain cases a regular porous TiO2 structures could be formed, the conditions were limited to potentials only up to 3 V as at higher potentials they reported to have formed only a compact oxide layer with some rough features.

In the present work, we explore the use of a H2SO4/HF mixed electrolyte operated at low temperatures to achieve nanotubular layers over a wide potential range on thin film Ti substrates.

Section snippets

Experimental

The specimens – 500 nm thin Ti films were deposited on a Si wafer by ion-beam sputtering of a Ti target (purity; 99.5%) using 1.0 keV Ar+ ions. Prior to deposition, the substrates were sputter-cleaned to eliminate the contamination layer on the surface. The process chamber then was evacuated to the pressure of 1.0 × 10−7 torr before Ti deposition using a cryopump and maintained at 1.0 × 10−4 torr during deposition. Prior to electrochemical experiments,the Ti-coated Si samples were degreased by

Results and discussion

To grow self-organized nanotube layers on the thin film Ti layers, preliminary experiments were performed. These experiments indicated that anodization at 20 V in a 1 M H2SO4/0.15 wt.% HF solution can be successfully used.

Fig. 1a shows dependence of the current density on the anodization time while holding the samples at 20 V. The curves are recorded for different electrolyte temperatures – 20, 15 and 2 °C. Previous work established that at 20 °C and in potential range of 2–20 V [16], [18],

Conclusions

The results of the present work show that self-organized TiO2 nanotubes can be fabricated by anodization of thin Ti films sputtered on Si wafers. This can be achieved by using mixtures of sulfuric and hydrofluoric acid at low temperatures, where the chemical dissolution rate of TiO2 can be drastically diminished. On originally 500 nm thick Ti layers, nanotubes with length up to 400 nm with diameter in the range of 30 up to 100 nm could be formed. The lower practical limit of the anodization

Acknowledgements

The authors would like to acknowledge Hans Rollig and Martin Kolacyak for valuable technical help.

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