Elsevier

Surface Science

Volume 601, Issue 18, 15 September 2007, Pages 4515-4520
Surface Science

On the hydrophilicity of nitrogen-doped TiO2 thin films

https://doi.org/10.1016/j.susc.2007.04.156Get rights and content

Abstract

Nitrogen doping was proposed recently as a low-cost means to shift the absorption edge of titania towards 450–500 nm. Results are reported here on the preparation of nitrogen-doped 250 nm thick TiO2 films on glass substrates by RF magnetron sputtering. The nitrogen content in the films was modified by changing the N2 partial pressure in the discharge from 0.00 to 0.27 Pa. An additional amount of oxygen was added in the deposition chamber (pO2 = 0.11 Pa) to compensate for oxygen depletion in the films. Surface composition and structure were derived from XPS and XRD data, respectively. XRD data revealed a mixture of amorphous phase and rutile polymorph in the as-deposited films, while an anatase-rich phase occurs in the samples deposited under N2 pressure of 0.05 and 0.16 Pa. Above 0.16 Pa, a rutile-rich phase is present.

Surface wettability was investigated from contact angle data of de-ionized water with solid surface. The films deposited under mid-range N2 pressure in the discharge are super-hydrophilic. The photo-induced super-hydrophilicity decays after 36 h. Film wettability is discussed in terms of the synergic effects of surface roughness (measured by AFM) and rutile–anatase TiO2 phase transition, induced by doping. The TiO2 surface super-hydrophilicity (and catalytic efficiency) are related to beneficial properties of anatase phase (Eg = 3.2 eV), associated with a low recombination rate of charge carriers in the surface.

Introduction

TiO2 has been intensively studied, due to its strong oxidation power and hydrophilicity. Numerous applications of this material occur in photonics, electronics, environment protection, and medicine [1], [2]. Owing to large optical band gap and chemical stability, self-cleaning titanium dioxide films feature excellent photocatalytic and hydrophilic properties [3]. The main-stream of the effort is focused at present on enhancing TiO2 photocatalytic and hydrophilic characteristics, including the idea of shifting the upper limit of photoactivation using blue–green light. This would also allow for surface photoactivation under weak indoor illumination (1 μW/cm2).

We report here on the super-hydrophilic properties of the nitrogen-doped TiO2 films. The results are discussed taking into account the combined effects of surface morphology, structure and composition.

Section snippets

Experimental

Pure and nitrogen-doped TiO2 films, 250 nm thick, were grown by rf sputtering (Hüttinger PFG-300RF, 13.56 MHz, 150 W) in a conventional planar magnetron facility. A 2-in. sintered ceramic TiO2 disk (99.99% purity) was used as a sputtering target. Briefly, the setup consisted of a stainless steel deposition chamber, which could be evacuated to a base pressure of 1.3 × 10−5 Pa by a 250 l/s turbo-molecular pump. Ar, O2 and N2 gases were introduced into the deposition chamber via mass flow controllers.

Results and discussion

The AFM images of the sample surfaces reveal the presence of nanometer sized grains (Fig. 1), with roughness values, Rrms below 5 nm (Table 1). A well-defined minimum of surface roughness occurs for the samples prepared in the mid-range nitrogen partial pressure. The XRD patterns of all samples show that an important amount of amorphous phase is present in the as-deposited samples. A large rutile (1 0 1) peak, along with weak anatase (1 0 1) and (0 0 4) peaks, occur in the XRD pattern of the reference

Conclusion

Two hundred and fifty nanometer thick nitrogen-doped TiO2 thin films have been deposited onto glass, (1 0 0)Si and fused quartz substrates by RF sputtering. Film hydrophilicity was evaluated from contact angle measurements. By UV–Vis irradiation, the doped films become super-hydrophilic, if the N/Ti atomic ratio in the surface is less than 4.1 × 10−2. Film hydrophilicity is degraded in time, as a function of dopant concentration in the surface. The super-hydrophilic properties of films in the

Acknowledgements

This work was supported by the CEEX-Matnantech-23/2005 contract, granted by the Romanian Ministry of Education and Research. The authors are grateful to Radu-Paul Apetrei for experimental support during sample preparation.

References (11)

  • C.M. Teodorescu et al.

    Nucl. Instrum. Meth. Phys. Res. A

    (1994)
  • D. Mardare et al.

    Appl. Surf. Sci.

    (2000)
  • S. Schiller et al.

    Thin Solid Films

    (1981)
  • C.-C. Ting et al.

    J. Appl. Phys.

    (2000)
  • Y. Yamada et al.

    J. Vac. Sci. Technol. A

    (2001)
There are more references available in the full text version of this article.

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