Photocatalytic and photoelectrochemical properties of sol–gel TiO2 films of controlled thickness and porosity
Graphical abstract
Rate of decolorization of model DCIP ink for mesoporous and nonporous TiO2 films. Insert: rate of decolorization as a function of layer mass.
Introduction
Thin films of titanium dioxide deposited on various supports are very useful photocatalysts in a number of applications, primarily in environmental protection and in alternative energy generation. In the first field TiO2 coatings can oxidatively decompose organic deposits to inorganic products, such as CO2, H2O and mineral acids [1], [2]. Consequently, the dust particles no longer stick to the irradiated surface and can easily be washed off by rain water. Second application of TiO2 photocatalyst films is the photoelectrochemical water splitting [3]. Photogenerated electrons can be transferred via an external circuit to the auxiliary electrode. Holes formed may act as an oxidant, in this particular case to evolve molecular oxygen. On the auxiliary electrode, electrons are used to evolve hydrogen.
Great advantages of TiO2 are its low price, high stability and nontoxicity [4]. However, for practical applications, there are two important film parameters which should be addressed. The first is the amount of photocatalytically-active material (film thickness), which often determines the amount of light absorbed. The second is the film porosity. Porous films of TiO2 are more effective when the photodecomposition mechanism includes reactants or intermediates adsorbed on the surface. Furthermore, mesoporosity may ensure the fast transport of O2 and H2O, which are crucial for the photocatalytic degradation of organic deposits [5], [6]. On the other hand, particulate porous films can lack the electronic conductivity essential for the efficient photocurrent generation [7], [8].
The sol–gel technique enables TiO2 photocatalyst films to be prepared in a way that controls surface properties such as composition, thickness and morphology [9], [10]. This technique consists of several steps: (i) precursor synthesis, (ii) precursor deposition (usually by dip-coating), (iii) drying, and (iv) calcination at elevated temperatures. Layer thickness can be controlled by viscosity of the sol–gel precursor and by the withdrawal rate during dip-coating. Thicker (or multilayer) films can be obtained by repeating steps (ii), (iii) and (iv) [11]. A combination of the sol–gel technique with the addition of a suitable surfactant can be used to synthesize TiO2 films of well-defined porosity [12].
The aim of this work was to investigate the photocatalytically active sol–gel TiO2 coatings with particular focus on the effect of the layer thickness and porosity. Due to the potential applications (self-cleaning surfaces, photoelectrochemical water splitting, etc.) we focused on (i) photocatalytic degradation of thin solid films (degradation of the model ink 2,6-dichloroindophenol), and on the study of (ii) photocurrent and open circuit potential in aqueous media containing an inorganic salt.
Section snippets
Glass substrates
As conductive substrate two types of transparent conductive glass (TCO) were used. Indium tin oxide (ITO) conducting glass slides (5–15 Ω/square, size 50 × 12.2 × 1.1 mm) supplied by Delta-Technologies Ltd. and fluorine tin oxide (FTO) conducting glass slides (8 Ω/square, size 75 × 15 × 2 mm) supplied by Hartford Glass Company, Inc.
Preparation of nonporous TiO2 films
Nonporous films were prepared using titanium(IV) isopropoxide (97%, Sigma-Aldrich) as TiO2 precursor, absolute ethanol and ethyl acetylacetate (99% p.a. Fluka) as solvent and
Film characterization
XRD diffraction patterns of five-layer-thick nonporous (thickness 340 nm) and five-layer-thick mesoporous (thickness 1300 nm) TiO2 films deposited on FTO substrate are shown in Fig. 1. Both types of film have anatase crystalline structure but only 2 bands corresponding to anatase can be identified (due to overlapping of other bands with those corresponding to fluorine doped SnO2 layer), namely for position 2Θ = 25.2° (1 0 1) and 2Θ = 48.0° (2 0 0) and these bands were used for the calculation of crystal
Conclusion
TiO2 films of well-defined thicknesses and different values of porosity were synthesized by the sol–gel technique with dip-coating as the principal method of deposition. Two types of transparent conductive glasses namely indium tin oxide (ITO) and fluorine tin oxide (FTO) were used. Thickness and mass of TiO2 films depend linearly on the number of deposition cycles (dip-coating, drying and calcinations). Due to the different viscosities of the precursors the thickness of a single layer of
Acknowledgement
Authors acknowledge the financial support by the Grant Agency of the Czech Republic (project number P108/12/2104).
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