Elsevier

Thin Solid Films

Volumes 455–456, 1 May 2004, Pages 417-421
Thin Solid Films

Spectroellipsometric characterization of sol–gel TiO2–CuO thin coatings

https://doi.org/10.1016/j.tsf.2004.01.030Get rights and content

Abstract

TiO2 based nanustructured coatings have been prepared by sol–gel method on glass and SiO2/glass substrates. The films were thermally treated in oxidative and reductive conditions at 500 °C. The influence of the substrate and of the thermal treatment on the optical properties of the films was studied. The samples were characterized by spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS) and Raman spectroscopy. SE results supported by other optical and structural measurements have proven the transformation of CuO from as-prepared samples into Cu2O in samples heated in oxidative conditions and into metallic Cu in samples heated in reductive conditions as well as the transformation of the amorphous TiO2 in anatase.

Introduction

Titanium dioxide films have been shown to be an excellent photocatalyst for the degradation of several environmental contaminants [1]. It has antibacterial properties and is useful in the purification of water against Escherichia Coli bacteria [2].

Many deposition techniques have been used to prepare titanium oxide coatings, such as chemical vapor deposition [3], evaporation [4], reactive d.c. or r.f. diode, magnetron sputtering [5], [6], ion beam techniques [7], and sol–gel processes [8]. The sol–gel processes are particularly efficient in producing thin, transparent, doped, with controled porosity, multi-component oxide layers of many compositions on various substrates, including glass [9], [10], [11], [12], [13], [14].

In the present work, single TiO2–CuO films were deposited on glass and SiO2/glass substrate by sol–gel method. The transformation of the amorphous TiO2 into anatase and the transformation of the CuO into metallic Cu, when the samples are thermally treated (at 500 °C, 30 min in O2 and 2 h in H2), was followed using the spectroscopic ellipsometry (SE). Due to the complexity of the system approached, the ellipsometric data were corroborated by other optical and structural measurements (SEM, RBS and Raman spectroscopy).

Section snippets

Coating preparation

The TiO2–CuO coatings were deposited by the dip-coating technique on optically transparent microscope glass and on SiO2/glass substrates. The as-prepared samples were dried at room temperature and then were thermally treated (rate of 5 °C/min), at 500 °C for 30 min under a stream of oxygen in order to remove the organic components. Finally, the samples were additionally thermally treated under reductive atmosphere at 500 °C for 2 h (Table 1) in order to ensure Cu nanoparticles formation. A

Results and discussions

In order to model the ellipsometric spectra, a physically realistic model is needed. This requires a good knowledge of the number of layers and their components. In our case this was provided by several alternative methods of analysis: SEM, RBS and Raman spectroscopy.

A direct inspection of the film color also gives information about the valence state of copper.

The as-deposited coatings were amorphous, transparent and slightly colored in gray-lilac, indicating the presence of CuO. After the

Conclusions

A strong influence of the SiO2 buffer layer and of the atmosphere of the thermal treatment on the microstructure of the films was observed. The films deposited on SiO2/glass are much thicker and more porous than those deposited directly on glass. SE results supported by other optical and structural measurements (Raman, RBS, SEM) have proven the transformation of CuO from as-prepared samples into Cu2O in samples heated in oxidative conditions and into metallic Cu in samples heated in reductive

Acknowledgements

The authors wish to acknowledge the support of the EU Transactional Access Programme, under the RIMDAC scheme at the NMRC, University College Cork, Ireland.

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