Fast formation of thick and transparent titania nanotubular films from sputtered Ti

doi:10.1016/j.elecom.2009.05.001

Electrochemistry Communications

Volume 11, Issue 6, June 2009, Pages 1308-1311

https://doi.org/10.1016/j.elecom.2009.05.001 Get rights and content

Abstract

Ti films sputtered on transparent fluorine-doped tin oxide glass substrates were anodized in fluoride-containing organic electrolyte in the presence of H₂O. In this work, anodic TiO₂ nanotubes (ATNs) as long as 9.2 ± 0.3 μm were obtained with high growth rate of 0.64 ± 0.3 μm min⁻¹. We demonstrated the optimum anodization conditions for ATN growth on foreign substrates, were within the range of 0.3–0.5% (wt) NH₄F, with 3–5% (vol) H₂O at 60 V. XPS and ICP-MS were utilized to elucidate the increase of thickness and volume expansion obtained from the sputtered Ti film to their ATN forms. The ATN films exhibited excellent uniformity and adhesion to the substrates.

Introduction

Since the discovery of anodic nanoporous alumina [1], various nanostructures have been obtained by anodizing Ti [2], W [3], Ta [4], Zr [5]. In particular, the TiO₂ nanotubular structure obtained from Ti anodization has been explored for a wide range of applications, such as sensors [6], catalytic system [7], biomaterials [8], tunable surface wettability [9], electrochromic devices [10] and photovoltaic devices [11].

To date; anodic TiO₂ nanotubes (ATNs) have been grown with lengths up to 1 mm [12]; tube properties such as wall thickness and diameter have been precisely tailored to suit targeted applications [13]; and tubes have also been doped with impurities to further enhance their performances [14]. However, most of these achievements have been obtained on Ti foil, which limits potential ATNs applications. Although ATNs based dye sensitized solar cells (DSCs) were reported to have enhanced light scattering ability and electron collection efficiency when compared with conventional nanoparticulate TiO₂ based DSCs [11], [15], the optimised ATN-based DSCs on Ti foil have a limited efficiency of ∼6.1% [16]. This is mainly due to the inefficient illumination from the back-side of the cell, where light loses a significant amount of intensity after travelling through the counter-electrode, Pt coating and the redox mediator system [16]. Although successful transfer of ATN layers via a lift-off process onto foreign substrates has been reported [17], such a method requires delicate handling of the ATN films, and suffer from poor adhesion between the films and the substrates. Grimes and co-workers [18], [19] pioneered the fabrication of the transparent ATN films with thickness up to 3.6 μm. Later these films were implemented in DSCs, and achieved up to 4.7% efficiency by front-side illumination [16], [19]. Increasing the thickness of the transparent ATN films will have a profound impact on increasing the efficiency of DSCs.

We previously reported ATNs films with relatively short tube lengths produced from RF sputtered Ti film on indium doped tin oxide glass [20] and silicon [21]. In this work, we investigate the anodization of RF sputtered Ti films utilising ammonium fluoride (NH₄F) in ethylene glycol (EG) with different water contents at relatively high applied voltages, which result in the fast formation of ATN layers of up to 9.2 ± 0.3 μm thick.

Section snippets

Experimental

The Ti films of 0.9, 1.8 and 3.2 μm were deposited by a radio frequency (RF) sputtering system fitted with a Ti target of 99.995% purity. Ti was sputtered on commercial fluorine-doped tin oxide (FTO) glass (13 Ω square⁻¹, NSG Co. Ltd.). Sputtering was conducted in argon at 20 mTorr with 1.78 W cm⁻² applied RF power density and substrate temperature of 300 °C.

Anodization of the sputtered Ti films was carried out by a conventional anode (target sample) – cathode (platinum plate) system at room

Results and discussions

Since the effect on ATN due to different anodization parameters, such as applied potential, fluorine concentration and H₂O content were well studied, they are not detailed here. In this work, extremely high growth rates in the range of 1–1.76 μm min⁻¹ that are associated with either high applied potential (80 V) or high NH₄F concentration (1–1.5%) caused the ATN films partially delaminated from substrates. ATN films produced from prolonged anodization due to low growth rates were partially etched

Conclusion

We reported on the fabrication of thick and uniform ATN films from the RF sputtered Ti films on FTO substrates in fluoride-containing organic electrolyte with regulated addition of H₂O. It was found that a uniform and fast transformation from sputtered Ti films to ATN films occurs when the steady-state anodic current was within 10–20 mA m⁻² at 60 V. Our study found that the volumes of formed ATN films expanded within the range of compact titanium oxide volume expansion, while the formed ATN films

Acknowledgements

Appreciation to Mr. Paul Morrison and Ms. Emma Geothals from the School of Applied Sciences, Applied Chemistry of RMIT University for the use of ICP-MS.

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Fast formation of thick and transparent titania nanotubular films from sputtered Ti

Abstract

Introduction

Section snippets

Experimental

Results and discussions

Conclusion

Acknowledgements

Electrochimica Acta

Electrochemistry Communications

Biomaterials

Electrochemistry Communications

Solar Energy Materials and Solar Cells

Electrochemistry Communications

Science

Journal of the Electrochemical Society

Small

Sensor Letters

Small

Small