Nano-sized blue spectral shift in sol–gel derived mesoporous titania films
- 960 Downloads
Following the spectral energy shift of the energy gap (blue shift) of the TiO2 sol–gel derived films we have evaluated diameters of the nanocrystallites. The TiO2 films were deposited by dip-coating technique. Two types of mesoporous films were studied: films with porosity ~16% and refractive index (2.15 at wavelength 633 nm) and films with porosity ~46% and refractive index (1.61 at wavelength 633 nm). High porosity and consequently low refractive index was achieved by adding the non-ionic surfactant Triton X-100 to the starting solution as template. The principal goal of the work is to establish the influence of the Triton X-100 on the morphology as well as to establish a possible correlation between the morphology and optical features of the titania films. The surface morphology was explored using AFM method. And the energy gap was determined from the transmission spectra. Analysis of the blue energy spectral shift is performed following the excitonic model.
KeywordsSol–gel Titanium dioxide Nano confined effects Anatase Band energy gap
Titanium dioxide TiO2 is a promising optoelectronic wide energy band gap semiconductor oxide film. It is of interest for use as gas sensor [1, 2], photoinduced optical operated materials , solar cell [4, 5, 6], photocatalyst [7, 8, 9], self-cleaning glasses , electrochromic films , antireflection coatings  and transparent conductors . Titanium dioxide is also a principal component used in planar waveguide technology [14, 15]. Several synthesis methods concerning titanium dioxide thin films are described in the literature, such as e-beam evaporation , magnetron sputtering [7, 16], ultrasonic spray pyrolysis , chemical vapor deposition [18, 19], metal organic chemical vapor deposition , pulsed laser deposition , and sol–gel method [1, 5, 6, 9, 10, 11, 12, 13]. The sol–gel method has advantages, due to low temperature processing, easy coating of large area, and being suitable for preparation of porous films and homogeneous multicomponent oxide films. The most important advantage of sol–gel over conventional coating methods is the ability to tailor the microstructure of deposited films , so using the sol–gel method one can produce titania films with desirable structure. Contrary to the other, the sol–gel method is very efficient and does not require an expensive technological equipment.
In this study, the surface morphology and optical properties near the band energy gap edge for sol–gel derived TiO2 films deposited on soda-lime glass substrates were studied. The films were prepared by the dip coating method. Two types of the films were studied: compact films with refractive index of ~2.15 and mesoporous films with refractive index of ~1.61 (at wavelength 633 nm). High porosity and consequently low refractive index was achieved by adding of the non-ionic surfactant Triton X-100 to the initial solution as template. The principal goal of the work is to establish the influence of the Triton X-100 on the morphology as well as to study a possible correlation between the morphology and optical features of the titania films.
2 Experimental procedures
2.1 Films fabrication
The principal processes of sol–gel technology are described in detail by Brinker and Scherer . The major problems in texturing titania is the high reactivity of Ti alkoxide precursors towards to rapid hydrolysis and condensation reactions. Rapid condensation processes lead to the formation of oxo-oligomers, clusters or small polymers prior to the formation of the textured template. However, the control of hydrolysis and condensation, through the addition of condensation inhibitors such as chelating agents or mineral acids, can overcome the problem [21, 22, 23]. As a consequence in the presented here research, the titania films were fabricated using the tetraethoxyorthotitanate Ti(OEt)4 (TET) as a precursor for titania, water H2O, ethyl alcohol C2H5OH (EtOH) as homogenizing agent and hydrochloric acid HCl as a condensation inhibitor. Two types of gel were synthesized, which were used for synthesis of the TiO2 films possessing different porosity and refractive indices.
The first sol, which will be indicated as sol-A, was prepared with the following molar ratios of the components: TET:EtOH:H2O:HCl = 1:15:2.8:0.7. The second one, which will be indicated as sol-B, has been prepared similarly to the sol-A. To the sol-B it was additionally added acetylaceton (Acac) as a chelate and non-ionic surfactant Triton X-100 as a template. Tetraethoxytitanate was mixed with acetylaceton in the molar ratio TET:Acac = 1:1. A non-ionic surfactant Triton X-100 was added in the volume ratio TET:Triton X-100 = 1:0.5. to the starting solutions. After mixing of the components, the sols formation was performed during 3 h in a sealed glass crucible at temperature about 50 °C, using ultrasonic mixing. Ultra clean soda-lime glass substrates (microscope slides, Menzel-Glaser), was cleaned by a method described in the Ref. 15. The films were deposited on the cleaned substrates by dip coating method. The such prepared film were annealed at temperature of 500 °C during 1 h. For the substrate withdrawal speed from the sol-A of 1.7÷3.0 cm/min we have obtained films with thickness varying within 71–89 nm. For sol-B and the same withdrawal speed we have synthesized films with thickness varying within 113–130 nm. Sol-B containing Triton X-100 had higher viscosity with respect to sol-A. As a consequence, the thickness of the film fabricated from sol B is higher than those one fabricated from sol A at the same withdrawal rate of substrate.
At the beginning precursor was solvated in water-less ethanol and afterwards HCl acid was added successively. The chemical reactions in the Ti(OR)4/EtOH/H2O/HCl were studied in details by Soler-Illia and co-workers [22, 23, 24]. Upon dissolution, Ti(OR)4 precursors undergo fast exchange reactions with EtOH and HCl molecules, leading to formation of Ti(OR)n−x(OEt)x, TiCln−x(OEt)x nanoclusters, or a mixture of both. Subsequently, polycondensation reactions between metal alkoxides can occur, yielding metal-oxo condensates with general formula Ti(OR)x(OH)yO2−(x+y)/2 or TiClx(OH)yO2−(x+y)/2. Typically, x ≈ 0.3−0.7 and y ≈ 0−0.2; while the value of y increases with the water content, x decreases [22, 23, 24].
The role of Triton X-100 as a surfactant has been described by Avnir and co-workers in Ref. . Dag and co-workers have fabricated mesoporous TiO2 using Ti(OEt)4 as precursor and non-ionic surfactant CH3(CH2)n(OCH2CH2)mOH , which is similar to Triton X-100. Their studies have shown that appeared nanoclusteres during process of controlled Ti(OEt)4 effectively interact with surfactant CH3(CH2)n(OCH2CH2)mOH which favors formation of nanoclaster/non-ionic surfactant “titanotropic” mesophase that undergoes hydrolytic polycondensation to give mesostructured titania with a wormhole structure, an amorphous titania framework, and uniform pore size distribution. The surfactant pre-organizes the clusters, which are attached to its head group . We assume that for the case of use the Triton X-100 the similar reaction should be observed.
2.2 Sample characterization
The such prepared surface of the TiO2 films have been monitored by atomic force microscope (AFM), using Ntegra Prima (NT-MDT), monochromatic ellipsometer Sentech SE400 (Sentech, model 2003, Germany). Additionally spectrophotometer UV–VIS HR4000CG (OceanOptics) with spectral resolution 0.75 nm within 200–1,100 nm spectral wavelength range was used. The AFM experiments for TiO2 thin films were performed using semi-contact mode AFM. In AFM measurements the HA_NC (NT-MDT) silicon cantilever with nominal curvature radius of a tip equal to 10 nm and resonance frequency of 250 kHz was used. The AFM image analysis was carried out using commercial NOVA 220.127.116.114 (NT-MTD) software procedures to evaluate Root-Mean-Square (RMS) surface roughness. The ellipsometric measurements were done for wavelength of He–Ne cw laser λ = 633 nm and spectroscopic measurements were carried out in the spectra range within 200–1,100 nm.
3 Results and discussion
3.1 Surface morphology
The presented here results concerning the AFM demonstrate substantial influence of non-ionic surfactant Triton X-100 on the morphology if the such synthesized films. In the layers formed in sol B containing surfactant the grains do not form the aggregates, on the contrary to the layer formed from sol A (see Fig. 1). The surfactant decreases the capillary stresses and avoids a destroying of the layered structure during drying process . As a consequence the layer structure is more modified and its surface is more rough. For both types of the layers the maximal grain sizes are comparable, however, the maximal heights of grains are substantially different. The lower maximal values of grain heights (about 7 nm) for the layers formed from sol A without surfactant are caused by effect of collapse of structure for the layers formed from A during the drying and annealing processes.
3.2 Optical features
For the two studied types of films the energy gaps are higher with respect to the energy gap of the bulk anatase specimens E bulk = 3.20 eV. This is a consequences of quantum size effects [33, 34, 35, 36, 37]. It is crucial that following the values of spectral shift (blue energy shift) of the energy gap one can evaluate the diameters of the crystallites.
For the titania films fabricated from sol-A the blue energy shift is in the range 0.52–0.55 eV, but for the titania films fabricated from sol-B the blue energy shift is about 0.2 eV. Following the dependences presented in the Fig. 9 we have determined diameters of the anatase nanocrystallites, which were equal to 2R ≈ 2.5 nm for the layers produced from sol A and 2R ≈ 4 nm for the layers synthesized fir sol-B. As a consequence the grains presented in the figures (Sect. 3.1) have the polycrystalline structure. So one can see that presence of Triton X-100 leads not only to higher roughness of the synthesized layers, however, also to higher diameters of anatase nanocrystallites.
From these equation one can see that the exciton radius is less from the estimated radiuses R of nanocrystallites for the studied samples.
We have established the influence of the Triton X-100 on the morphology as well as on optical features of the titania films. The blue spectral shift of energy gap was determined for two types of TiO2 films fabricated using sol–gel technology. The synthesized layers possessed the layers with refractive indices ~2.15 i ~1.61, respectively. The low refractive indices are caused by high porosity of the layers, which is achieved due to adding of surfactant Tryton X-100. We have established a crucial influence of the microstructure and morphology of the layers on the value of energy gap. The micro porous layers possessing the refractive indices 2.15 possesses the direct energy gap, which depending on the film thickness is varied within E g ≈ 3.72÷3.75 eV. The mesoporous films with refractive index 1.61 possess the indirect energy gap equal to E g ≈ 3.40 eV. So following the degree of porosity one can control their energy gap. The principal fact consists in an evaluation of the nano-grain sizes. Following the determined blue energy shift we have evaluated diameters of the anatase nanocrystallites, which for the mesoporous layers are equal to ~4 nm, and for the low porous layers are equal to ~2.5 nm. The presence of Triton X-100 causes higher roughness and higher nanocrystallite sizes.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
- 8.Shibata H, Ohshika S, Ogura T, Watanabe S, Nishio K, Sakai H, Abe M, Hashimoto K, Matsumoto M (2011) Preparation and photocatalytic activity under visible light irradiation of mesostructured titania particles modified with phthalocyanine in the pores. J Photochem Photobiol A Chem 217:136–140CrossRefGoogle Scholar
- 14.Karasiński P (2011) Embossable grating couplers for planar evanescent wave sensors. Opto-Electron Rev 19:13–24Google Scholar
- 21.Brinker CJ, Scherer GW (1990) Sol-gel science: the physics and chemistry of sol-gel processing. Academic Press, San Diego, CAGoogle Scholar
- 22.Soler-Illia GJ de AA, Scolan E, Louis A, Albouy PA, Sanchez C (2001) Design of meso-structured titanium oxo based hybrid organic-inorganic networks. New J Chem 25: 156–165Google Scholar
- 23.Soler-Illia GJ de AA, Sanchez C (2000) Interactions between poly(ethylene oxid)-based surfactants and transition metal alkoxides: their role in the templated construction of mesostructured hybrid organic-inorganic composites. New J Chem 24:493–499Google Scholar
- 24.Soler-Illia GJ de AA, Louis A, Sanchez C (2002) Synthesis and characterization of mesostructured titania-based materials through evaporation-induced self-assembly. Chem Mater 14:750–759Google Scholar
- 31.Tauc J (1974) Amorphous and semiconductors. Plenum, LondonGoogle Scholar
- 40.Reddy KM, Monorama SV, Reddy AR (2002) Bandgap studies on anatase titanium dioxide nanoparticles. Mater Chem Phys 78:230–245Google Scholar