Nano-quantum size effect in sol–gel derived mesoporous titania layers deposited on soda-lime glass substrate

Highlights

• We studied the sol–gel derived titania layers on soda-lime glass substrate.

• In titania layers, the quantum size effect is observed.

• Soda-lime glass substrate influence on energy band gap of the titania layers.

• The widths of energy gap and Urbach energy depend on titania layer thickness.

• The kind of substrate and layer thickness influence on titania layer morphology.

Abstract

The TiO₂ nanolayers were fabricated on soda-lime glass substrates with the application of sol–gel method and dip-coating technique. In the fabricated TiO₂ layers, the quantum size effect can be observed. For the sake of comparison, we investigated also the TiO₂ nanolayers fabricated on soda-lime glass substrates with a buffer silica layer. The fabricated layers were investigated with the application of optical measurement techniques and atomic force microscopy. The widths of energy gap and Urbach energy were determined. The diffusion of sodium ions Na⁺ from the glass substrate to the TiO₂ layer brings about the non-monotonic dependence of the energy band gap on the thickness of TiO₂ layer. In the TiO₂ layers fabricated on soda-lime glass substrates pre-coated with a SiO₂ layer, the influence of silicon ions on the direct energy band gap was found.

Introduction

Titanium dioxide TiO₂ is a wide energy band gap n-type semiconductor with wide application due to its optical and electrical properties. Titanium dioxide films are very versatile from the view point of their potential applications such as photocatalyst [1], [2], [3], self-cleaning glasses [4], antireflection [5] or high reflection coatings [6], electrochromic films [7], solar cell [8], [9], [10], transparent conductors [11], protective layers [12], and gas sensitive layers [13], [14], [15]. Titanium dioxide is a component used in planar optical waveguide technology [16], [17]. Many different technological procedures for the fabrication of thin films of titania have been reported, such as e-beam evaporation [14], magnetron sputtering [1], [18], ultrasonic spray pyrolysis [19], chemical vapor deposition [20], metal organic chemical vapor deposition [21], pulsed laser deposition [22], and sol–gel method [3], [4], [5], [6], [7], [10], [13], [15], [23]. The sol–gel method has advantages, such as low temperature processing, easy coating of large area, and being suitable for preparation of porous films and homogeneous multi-component oxide films. In contrast to the other, the sol–gel method is very efficient and does not require expensive technological equipment. The most important advantage of sol–gel over other coating methods is the ability to tailor the microstructure of deposited films [23], so using the sol–gel method titania films of controlled structure can be produced. The properties of sol–gel derived titania films depend on applied technological processes as well as applied substrates. Different procedures of fabrication technology of titania layer with the sol–gel method as well as the properties of the obtained layers have been widely reported in the literature [3], [4], [5], [6], [7], [10], [13], [15], [23], [24]. The influence of substrate type on photocatalytic properties of titania dioxide was discussed in the literature in the past [25], [26], [27]. However, to the best of our knowledge, the problem involving the influence of substrate type on the width of energy gap of the titania layer has not been discussed so far. While investigating thin titania films fabricated on soda-lime glass substrates, we found that there was a non-monotonic dependence of the indirect energy band gap on layer thickness. And that finding stimulated us to undertake the research presented here in order to clarify the origin of the effect. To the best of our knowledge such non-monotonous dependence of indirect energy band gap on layer thickness was not explored earlier. In this paper we present results of our studies of the influence of soda-lime glass substrate on the quantum size effect in sol–gel derived mesoporous titania layers which are demonstrated as blue-shift of optical energy band gap. Fabricated by us via sol–gel route titania layers on soda-lime glass (SLG) substrate exhibit both direct and indirect optical energy band gaps. For the indirect transitions we obtain non monotonic dependence of the energy band gap versus layer thickness while for the direct transition we obtain monotonic one. We present both results for titania layers on SLG substrates pre-coated with the silica films of different thicknesses. The objective of the work was to define the influence of the thickness of the buffer silica layer on the properties of titania layers. The data presented here show that the buffer silica layers are an effective barrier for Na⁺ ions. However, this layer is simultaneously a cluster of Si ions, which difunds to the TiO₂ layer. The concentration of Si⁴⁺ in the TiO₂ has an influence on the Urbach energy magnitude. The resulting titania layers were characterized by monochromatic ellipsometry, UV–vis spectrophotometry and atomic force microscopy (AFM). The work consists of two main sections. Section 2 of the work contains the description of technological procedures, the applied measurement methods and measurement apparatus, and the results of the carried out research and discussion are presented in Section 3.