Influence of Deposition Temperature on Titania Films Deposited by Ultrasonic Spray Pyrolysis

A. Nakaruk, D. S. Perera and C. C. Sorrell

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AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract
Introduction
Methodology
Results And Discussion
      Mineralogy
      Film Thickness
      Interfacial Reflections
      Degree of Crystallinity
      Grain Size
Summary And Conclusions
Acknowledgements
References
Contact Details

Abstract

Titania (TiO₂) films were coated on microscope glass slides using ultrasonic spray pyrolysis for 1 h in air at the deposition temperatures of 300°, 325°, 350°, 375°, and 400°C. The films were characterised using glancing-angle X-ray diffraction (mineralogy), focussed ion beam milling (thickness), and UV-VIS spectrophotometry (transmittance and optical band gap). At 300°C, the films were amorphous; higher deposition temperatures yielded films of the anatase polymorph of titanium dioxide. The thickness of the films increased, the optical transmission decreased, and the indirect optical band gap decreased as a function of increasing deposition temperature, with the latter decreasing from 3.47 eV to 3.23 eV between 300° and 400°C, respectively. The trend of decreasing band gap as a function of increasing deposition temperature was considered in terms of the (a) mineralogy, (b) film thickness, (c) interfacial reflections, (d) degree of crystallinity, and (e) grain size. In explanation of this trend: (a) mineralogy may be responsible for the trend but there are no data to support this conclusion, (b) thickness is irrelevant, (c) interfacial reflections support the trend, (d) the degree of crystallinity offers opposing perspectives in terms of the optical density and the refractive index, and (e) a grain size effect is contrary to the present data.

Introduction

Titanium dioxide (titania, TiO₂) is one of the most widely used materials in thin films because it has numerous applications, such as solar cells, photocatalysts, gas sensors [1], optical coatings [2], and self-cleaning materials [3]. In general, titania is very inert, surpassing glass in its resistance to attack by common solvents and acids. It acts as a catalyst for various organic reactions and, as a thin film, it is used as a dielectric for thin film capacitors and as an antireflection coating on silicon. In recent years, it has been investigated as an electrode for photoelectrochemical cells [4-6], and for detectors of H₂O [7], oxygen [8], and hydrogen [9].

There are many techniques to prepare titania films, including sputtering [10], pulsed laser deposition (PLD) [11], sol-gel [12], gel oxidation [13], anodic oxidation [14], electrophoretic deposition [15], and spray pyrolysis [16]. Spray pyrolysis is an attractive, versatile, and practical method to prepare titania thin films due to its advantages, such as low cost, simple operation, simple experimental setup, no need for vacuum, capacity for mass production, ease of doping, reproducibility, and rapid growth rates.

Briefly, spray pyrolysis is a simple technique, requiring only a liquid source, atomiser, and heated substrate. The droplet size depends on the method of atomisation, e.g., aerosol and ultrasonic spraying produce larger and smaller initial droplets, respectively. Greater control over the transit of the initial droplets to the substrate (heated typically with a hot plate) can be achieved through air entrainment. These entrained droplets subsequently form thin or thick films upon immediate approach to or impingement on the heated substrate.

From the preceding, it can be seen that the deposition temperature is one of the most important parameters in the preparation of titania films. Earlier reviews recognised the effects of deposition temperature and droplet size on film qualities, including mineralogy, thickness, morphology, transmission, and optical band gap [17-20]. However, these literature reviews were generic and so did not concentrate on the preparation and characteristics of titania films; this has been done recently by the authors [21]. Other researchers have examined the effects of the deposition temperature on titania thin films produced by spray pyrolysis but these studies were done at conventionally higher temperatures [22,23] while the present work was done at lower temperatures.

Methodology

The precursor consisted of titanium butoxide ([Ti(OCH₂CH₂CH₂CH₃)₄], Reagent Grade, 97 wt%, Sigma-Aldrich) dissolved in methanol (Reagent Plus =99 wt%, Sigma-Aldrich) at a titanium concentration of 0.5 M. The aerosol was produced using a commercial ultrasonic generator of 1.7 MHz frequency, placed directly in the solution. Further details of the experimental conditions have been described elsewhere [24]. The aerosol was carried to the microscope glass slides (25 mm x 25 mm x 1 mm) by using a conventional air entrainment arrangement. The deposition temperature was varied at 300°, 325°, 350°, 375°, and 400°C, with a 1 h deposition time and consistent solution feed rate.

The films were characterised by the following techniques: (1) The mineralogy of the films was examined using glancing angle X-ray diffraction (GAXRD, angle of incidence 1°, Phillips X’pert Materials Research Diffraction). (2) The film thickness was determined using single-beam focussed ion beam (FIB) milling (FEI XP200) following application of a ~20 nm thickness chromium (Cr) coating applied by sputtering. In this method, gallium ions (Ga3+) are used to erode a square hole in the film and an image of the cross-section of the layers is viewed at an angle of 45°. (3) The transmission spectrum in the visible region (300-800nm) was obtained using a dual beam UV-VIS spectrometer (Perkin Elmer Lambda 35).

Results And Discussion

Figure 1 shows the GAXRD patterns of titania films. It can be seen that the as-deposited films at 300°C were amorphous. A small (101) peak of anatase appeared for the as-deposited films processed at 325°C and the intensity of this peak increased with increasing deposition temperatures. For the films as-deposited at 350°, 375°, and 400°C, the GAXRD patterns showed that the films were polycrystalline anatase, with the increase in peak intensities indicating that increasing deposition temperature increased the amount of crystalline phase.

Figure 1. Representative glancing angle X-ray diffraction (GAXRD) patterns (measured at room temperature) for TiO₂ processed at different temperatures.

The increase in film thickness as a function in increasing deposition temperature is shown in Figure 2. These data, which are summarised numerically in Figure 3, show that the thicknesses of the titania films increased over the range 370, 460, 610, 720, and 800 nm for the deposition temperatures 300°, 325°, 350°, 375°, and 400°C, respectively (the bars represent the instrumental variability). Since the solution feed rate was consistent for all of the solutions, then it is implicit that the film thickness increased with increasing deposition temperature owing to the corresponding increase in the reaction rate. The increase is not due to increasing crystallinity since the specific volume of a crystalline material is less than that of the analogue amorphous material [25].

Figure 2. Representative focussed ion beam (FIB) cross-section images of TiO₂ films processed at different temperatures.

Figure 3. Representative thin-film thicknesses as a function of deposition temperature.

Figure 4 shows the transmission spectra of the films as a function of the deposition temperature. It can be seen that all five of the spectra show interference fringes in the form of periodic humps. This indicates that the smoothness of these films is relatively high [26]. These data are supported by the FIB images in Figure 2, which show the consistency of the thicknesses and smoothnesses of the films. The general decrease in transmission intensity in the visible range with increasing deposition temperature, as exemplified over the range 450-525 nm, is likely to be due to interfacial reflections. That is, examination of Figure 2 suggests that the amounts of interfacial areas associated with grain boundaries and subgrain boundaries increase as the thicknesses (through-volumes) of the films increase [24].

Figure 4. Representative UV-VIS light transmission spectra for TiO₂ films processed at different temperatures.

The indirect optical band gap (Eg) can be evaluated by the optical transmittance method [24]. To achieve this, the absorption coefficient (a) is determined according to the formula:

Where  

As shown in Figure 5, when (a)1/2 is plotted on the ordinate against h? (photon energy) on the abscissa, then the intercept of the tangent to the absorption edge with the abscissa gives an estimate of the band gap energy, which is indirect in the case of anatase and rutile. The values obtained from these data and the other data obtained are given in Table 1.

Figure 5. Representative optical band gaps calculated from UV-VIS spectroscopy transmission data for films as a function of deposition temperature.

Table 1 shows that the band gap decreases with increasing deposition temperature. This trend is the reverse of previous work on films annealed at higher temperatures (600°-1000°C) to produce anatase/rutile mixtures by the present [24] and other authors [27,28]. If the present data are a correct reflection of the optical properties of these films, then the following explanatory comments are relevant:

• Mineralogy
• Film Thickness
• Interfacial Reflections
• Degree of Crystallinity
• Grain Size

Table 1.Summary of Analytical Data.

Mineralogy

Since, it is known that the optical band gaps of anatase and rutile are in the ranges 3.20-3.56 eV and 3.00-3.34 eV, respectively [24,27,28], and the anatase-to-rutile phase transformation is known to occur at temperatures as low as 390°C [29] and 465°C [30] in nanotitania, it is possible that incipient anatase-to-rutile phase transformation occurs but is at a level below the limit of detection of the GAXRD unit.

Film Thickness

The reverse trend cannot be attributed to increasing film thickness because this variable is included in the band gap calculation [24].

Interfacial Reflections

As discussed in relation to the transmission data, the amount of reflection was assumed to increase with increasing thickness (assuming a relatively constant grain size). For films in general, increasing reflection results in decreasing transmission and band gap. Table 1 confirms this trend. Hence, the reverse trend may result from the increasing reflection associated with increasing thickness.

Degree of Crystallinity

The few data considering the effect of degree of crystallinity are contradictory, with the present authors’ observing a direct relation between degree of crystallinity and band gap [24] and others’ observing a converse relation [31]. This contradication is likely to result from the two concurrent optical variables that are affected by the degree of crystallinity:

• Optical Density: As the crystallinity increases, the optical density increases, the transmission decreases, and the band gap decreases. This is in agreement with Aarik et al. [31].
• Refractive Index: As the crystallinity increases, the number of interfaces decreases, the amount of interfacial reflection decreases, the transmission increases, and the band gap increases. This is in agreement with Nakaruk et al. [24].

Grain Size

If the grain sizes suggested by the asperities in the surface topography in Figure 2 are not constant but increase with increasing deposition temperature, which is a general observation for materials, then the amount of reflection as a function of thickness would be expected to decrease. This is in agreement with the data at higher temperatures [24,27,28], where grain growth occurs more readily, but it is in disagreement with the present data at lower temperatures, where less grain growth appears to occur.

Summary And Conclusions

In the present work, titania thin films were prepared by ultrasonic spray pyrolysis using glass substrates at the deposition temperatures of 300°, 325°, 350, 375°, and 400°C. GAXRD patterns show that the films deposited at 300°C were amorphous while those processed at higher temperatures consisted of anatase, with the proportion of crystalline phase increasing with increasing temperature. FIB images indicated that increasing deposition temperature resulted in increasing film thickness, decreasing transmission, and decreasing band gap.

The decreasing band gap as a function of increasing deposition temperature was considered in terms of the mineralogy, film thickness, interfacial reflections, degree of crystallinity, and grain size. Mineralogy may be responsible for the trend but there are no data to support this conclusion, the thickness is irrelevant, interfacial reflection supports the trend, the degree of crystallinity offers opposing perspectives in terms of the optical density and the refractive index, and a grain size effect is contrary to the present data.

Acknowledgements

The authors are grateful for the financial support of Austral Brick Co. Pty. Ltd., the National Hydrogen Materials Alliance, and the Australian Research Council, which have allowed this and other developmental work to be undertaken.

References

Contact Details

C. C. Sorrell
School of Materials Science and Engineering
University of New South Wales, Sydney, NSW 2052, Australia

E-mail: [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 12[1] (2010) 1-8.