Elsevier

Thin Solid Films

Volume 357, Issue 2, 15 December 1999, Pages 173-178
Thin Solid Films

Preparation, fractal surface morphology and photocatalytic properties of TiO2 films

https://doi.org/10.1016/S0040-6090(99)00561-1Get rights and content

Abstract

Opaque nanostructured glass-supported TiO2 films were prepared from a commercial and a laboratory synthesised precursor, respectively. Qualitative film analysis was performed with atomic force microscopy (AFM), and surface parameters such as average grain diameter, roughness exponent and fractal dimension were determined. The TiO2 surfaces present important roughness due to the large aggregates formed by the interconnected TiO2 particles. Fractal dimension was found to range between 2.100 and 2.450, depending on the scanned range and the preparation method. The photocatalytic activity of the films towards the degradation of 2,4-dichlorophenol in acidic aqueous solutions has been investigated. The organic pollutant undergoes effective photodegradation upon photolysis with UV/near visible light. The film morphology, surface characteristics and efficiency remained intact after use. Comparison of the film activities showed that the films prepared from a commercial colloid possess a better photocatalytic performance than the sol-gel films and this can be understood in terms of their smaller particle size, higher roughness and specific surface area.

Introduction

Fabrication of titanium (IV) oxide thin films has attracted considerable attention in recent years, due to their unique electro-optic properties. These films exhibit a large potential in various fields such as electrochromic devices [1], photocatalytic detoxification of polluted water [2], air deodorisation [3], protective anti-reflection coatings [4], solar cells [5], [6], [7], [8], lithium insertion batteries [9], gas sensors [10] and practical use appears feasible in the near future. It is not surprising therefore, that much current research has focused on developing film preparative techniques where a plethora of different approaches has been undertaken. Selectively we mention, sol-gel [11], spray pyrolysis [12], electrophoresis [13], anodic oxidative hydrolysis of Ti3+[14] and chemical vapour deposition [15].

In the present work we focus on preparation methods for thin TiO2 films which meet the requirements for some environmentally friendly applications like solar to electrical energy conversion [5], [16], [17] and photocatalytic pollutant degradation [18].

Among the great variety of water pollutants, chlorophenols are of special interest because of their intensive use in a number of industrial processes. Chlorinated phenols are also chemical precursors of the highly toxic polychlorinated dibenzo-p-dioxins [19]. Various advanced oxidation processes have been used for their degradation, and of these, photocalytic mineralisation has proved very promising [20], [21], [22]. Thus, very recently, the photocatalytic decomposition of chlorinated organic pollutants has been investigated in homogeneous aqueous solutions using polyoxometallates (POM) and the reaction intermediates as well as the final degradation products were determined [23], [24]. However, the best results to date were obtained in heterogeneous systems upon photolysis with titanium oxide (TiO2) [21], [25], [26], [27]. The method involves illumination of the solution by UV and near-visible light in the presence of titanium oxide (in the form of a colloidal suspension or a thin film). Mineralisation of the pollutant occurs due to the formation of highly oxidising OH. radicals from the reaction of the photogenerated holes with water [28]. Immobilisation of the photocatalyst as a thin film presents the advantage of easier photocatalyst separation. Moreover, the possibility of electrochemical assistance for the TiO2 film deposited on a conductive support makes further work on the subject very encouraging. In fact, the recombination between the photogenerated electron-hole pairs can be suppressed by applying an external anodic bias, thus increasing the efficiency of the photocatalytic degradation [29], [30], [31].

In the above mentioned processes the texture and the spatial extension of the surface are very important factors that govern the efficiency of the process. Thus, the surface of the photocatalyst needs to be as large as possible in order to efficiently absorb the incident light energy. A net improvement of the real surface area of the photocatalyst may be achieved by increasing the height, roughness and frequency of the surface features. With these clear objectives in mind, two film-growing techniques are presented. The one starting from a liquid state precursor [Ti(i–OC3H7)4] while the other from a commercially available powder (P25 of Degussa AG). The films were compared in terms of their surface morphology and their photocatalytic efficiency against 2,4-dichlorophenol (DCP). An attempt has been made to rationalise the relation between the fractal surface characteristics and the photochemical behaviour.

Section snippets

Preparation of the TiO2 films

TiO2 thin nanostructured films were deposited on optically transparent microscopy glass substrates, ultrasonically cleaned prior to use. Two film preparative methods were employed. The specimen will be henceforth labelled as SOL (sol-gel) and TR (Triton), respectively. Method A (SOL films) is based on the sol-gel dip-coating process. For the preparation of the dipping solution titanium tetra-isopropoxide (1.1 g - Aldrich) was added dropwise (in order to avoid the formation of large

Results and discussion

Good uniformity was obtained in both cases of SOL and TR TiO2 films, while sol-gel processed films display a somewhat better scratch resistance and adherence. The drawback of the sol-gel method is that it is rather time and energy consuming since a significant number of repetition cycles is required in order to get a sufficiently thick TiO2 film.

In the diffraction pattern of the SOL films, peaks at 25.4, 37.7, 48.1 and 55.2° could be identified. They correspond to (101), (112), (200) and (211)

Conclusions

Titanium oxide films possessing different surface features were elaborated by sol-gel and sintering methods and characterised by means of AFM and XRD techniques. The photocatalytical properties of these films towards the 2,4-DCP degradation were investigated with the goal of elucidating the way the preparation procedure influences the morphological features of the surface. It has been demonstrated that the films photocatalytic properties are strongly dependent on their synthesis and processing

Acknowledgements

Thanks must be addressed to Dr. E. Papaconstantinou for helpful discussions and to M. Archimandriti for obtaining the AFM images.

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