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Fe effect on the optical properties of TiO2:Fe2O3 nanostructured composites supported on SiO2 microsphere assemblies

The effect of Fe ion concentration on the morphological, structural, and optical properties of TiO2 films supported on silica (SiO2) opals has been studied. TiO2:Fe2O3 films were prepared by the sol-gel method in combination with a vertical dip coating procedure; precursor solutions of Ti and Fe were deposited on a monolayer of SiO2 opals previously deposited on a glass substrate by the same procedure. After the dip coating process has been carried out, the samples were thermally treated to obtain the TiO2:Fe2O3/SiO2 composites at the Fe ion concentrations of 1, 3, and 5 wt%. Scanning electron microscopy (SEM) micrographs show the formation of colloidal silica microspheres of about 50 nm diameter autoensembled in a hexagonal close-packed fashion. Although the X-ray diffractograms show no significant effect of Fe ion concentration on the crystal structure of TiO2, the μ-Raman and reflectance spectra do show that the intensity of a phonon vibration mode and the energy bandgap of TiO2 decrease as the Fe+3 ion concentration increases.


Background
SiO 2 microsphere assemblies coated with metal oxide films have been considered as promising candidate materials for applications in photocatalysis due to their large surface area, excellent accessibility to the inner surface, and suitable morphology in comparison with powder materials. For example, mesoporous TiO 2 films perform quite well in the decomposing of pollutants and in the generation of hydrogen by water splitting [1]. Also, the TiO 2 is the most promising catalyst due to its high efficiency, stability, and low cost. It has been used widely in the photocatalytic degradation of phenol. However, one disadvantage of TiO 2 for industrial applications is the need of filtration after the photodegradation process. Recently, the research has been focused on modifying the surface or the bulk of a semiconductor catalyst by adding transition metal impurities, which give rise to the mixed oxide semiconductor formation [2].
However, few research studies have been concerned to surface wetting application. One interest point in our current research is to observe the effect of light adsorption on surface materials with respect to their wetting properties particularly of the Fe 2 O 3 :TiO 2 /SiO 2 composites; this topic has attracted significant scientific attention particularly for biological systems. A method for the fabrication of thin films using sol-gel is the dip coating procedure, and it is an appropriate technique to obtain TiO 2 [3], Fe 2 O 3 :TiO 2 [4], and TiO 2 /SiO 2 [5], among others. This method has some advantages, for example, low investment cost for production facilities, large variety of coating materials, and high uniformity on large coating areas. Furthermore, an additional advantage of this technique is the fact that both sides of the substrate can be coated simultaneously. The versatility of this technique allows us to control the vertical dipping velocity, which is very important in the wetness of the forming layer and in the gelation of the layer by the solvent evaporation in order to get uniformity in the film thickness. The α-Fe 2 O 3 has the important feature of absorbing a large part of the visible solar light due to its energy bandgap of 2.1 eV [6].
Its chemical stability, no toxicity, abundance in nature, and low cost-effective features make it a good material candidate for applications in many fields.
The goal of this work is to synthesize a set of TiO 2 , Fe 2 O 3 , and TiO 2 -Fe 2 O 3 thin films deposited on SiO 2 microsphere assemblies by the sol-gel method in combination with a controlled vertical dip coating procedure. Mainly, we have analyzed the effect of Fe ion concentration on the structural and optical properties of these composites.
The synthesis of SiO 2 microparticles was carried out by the Stöber method. Following this method, it is possible to control the diameter of the spheres from the TEOS concentration. First, we made a solution by mixing ammonium hydroxide, ethanol, and water with volumes of 20, 38.4, and 41.6 ml, respectively. Then, we made a second solution with 6.6 ml of TEOS and 93.4 ml of ethanol, and later on, we mixed both solutions and stirred it for 1 h at room temperature. The TiO 2 precursor solution was made by mixing and stirring the following chemical compounds for 2 h: 19.2 ml of ethanol, 3.8 ml of hydrochloric acid, 7.7 ml of water, and 19.2 ml of titanium(IV) buthoxide at room temperature. The Fe 2 O 3 precursor solution was obtained as follows: 15.15 g of iron nitrate was dissolved in 60 ml of 2methoxyethanol and 6.8 ml of monoethanolamine, and this solution was stirred for 2 h at room temperature. Finally, to have solutions to the 1, 3, and 5 wt% of Fe 3+ with respect to the TiO 2 precursor solution, 0.0331, 0.0995, and 0.1658 g of iron nitrate, respectively, were required.
Once the SiO 2 spheres have been obtained, a solution with 20 wt% of SiO 2 microspheres was prepared and impregnated on a glass substrate by the dip coating procedure at 3 mm/min rate. The glass substrate with the SiO 2 spheres was immersed into the corresponding TiO 2 precursor solution using the same vertical dip coating procedure at 1.5 mm/min rate. All immersion procedures were done at room temperature and normal pressure. Then, the sample containing the TiO 2 coating was annealed at 500°C with air flux 1 ml/s for 6 h.
The Fe 2 O 3 coating of the SiO 2 opals was obtained by following the same dip coating procedure with the Fe precursor solution under the same conditions. Finally, the TiO 2 :Fe 2 O 3 (1, 3, 5 wt%) films supported on the SiO 2 opals were obtained by adding the appropriate amounts of ferric nitrate nonahydrate into the TiO 2 precursor solution, and the same dipping procedure was carried out.
The samples were characterized by the technique: X-ray diffraction (XRD; D8 Discover CuK α (λ = 1.5406 Å), Bruker AXS, Inc., Madison, WI, USA), μ-Raman spectroscopy (RS; Horiba Jobin Yvon Lab Ram HR, HORIBA, Ltd., Kyoto, Japan), and diffuse reflectance spectroscopy (Varian Cary 100, Agilent Technologies, Inc., Santa Clara, CA, USA). The surface morphology of the samples was studied with a scanning electron microscope (JEOL JSM-6610LV, JEOL Ltd., Akishima-shi, Japan). Figure 1a shows a scanning electron microscopy (SEM) micrograph at ×10,000 of the surface morphology of the SiO 2 microspheres deposited on a glass substrate. It is seen in this figure that in some regions, the SiO 2 microspheres form a close-packed arrangement, and in others, some fissures are present. Figure 1b shows a lateral view at ×35,000 of the SiO 2 microparticles. It is seen that the SiO 2 spheres are smooth   surfaces and their average size is about 365 nm. It is seen also that the SiO 2 sphere assembly was obtained in a single-layer form; each sphere is surrounded by six nearest spheres in such a way that this arrangement is the most dense of identical circles packed in a plane. The formation of this assembly is mainly due to interparticle repulsion and particle-water interaction. Along the surface, there are fissures suggesting that the packing is not completely homogeneous; this ruptures on the surface are probably produced by the deposition silica method, that is, the glass substrate is immersed into an aqueous solution where the silica microparticles are dispersed and thus the hexagonal closed packed is not very well controlled.

Results and discussion
Guo et al. [7] described a controllable method to fabricate hexagonal close-packed Langmuir-Blodgett silica particulate monolayers modifying the silica surface by using a binary surfactant and solvent systems, reducing in this way fissures along the surface layer. The difficulty of this method to be applied in our work is that the TiO 2 adsorption on the silica-modified surface is not possible due to the hydrophilic character of TiO 2 and the hydrophobic character of the silica. Figure 2 shows SEM images of the SiO 2 microspheres coated with TiO 2 :Fe 2 O 3 . In Figure 2a at ×2,500, it is observed the presence of 'micro-shavings' dispersed on the glass substrate. This is due to the incorporation of TiO 2 :Fe 2 O 3 coating, which destabilizes the SiO 2 closepacked arrangement as shown in Figure 1. In Figure 2b, a lateral view at ×25,000 shows with more detail the curving of the SiO 2 single layer. In Figure 2c, a micrograph at ×65,000 is shown. The SiO 2 microspheres are peeled off; thus, the image shows an inverse opal like showing that the TiO 2 :Fe 2 O 3 composite has covered completely the SiO 2 surface and the interstitial spaces among spheres as well. Furthermore, Figure 3a,b at 50,000 and 80,000 magnifications, respectively, shows a granular surface morphology of the TiO 2 :Fe 2 O 3 composite coating, with grain diameter of about 50 nm. In Figure 3c, the mapping distribution of elements of the composite shows the presence of oxygen, titanium, and iron elements. The standardless EDS quantification is 56.78 wt% oxygen, 38.27 wt% silicon, 4.74 wt% titanium, and 0.20 wt% iron. Figure 4 shows X-ray diffractograms of all prepared samples. The Fe 2 O 3 X-ray diffractogram presents peaks corresponding to the alpha phase of this compound according to the JCPDS data card. It is observed that the peak intensity is enhanced as the Fe concentration is increased. The most intense peak appears at 28°and corresponds to the (101) plane of the anatase TiO 2 structure. It is also observed that there is no significant effect of Fe on the TiO 2 crystalline structure. However, there is a considerable effect of iron concentration on the TiO 2 phonon modes of vibration as is shown in the μ-Raman spectra (see Figure 5).
The Raman lines located at 397, 522, and 642 cm −1 are, respectively, assigned to the B 1 , B 1g , and A 1g vibration modes of TiO 2 . According to the analysis of group theory, the TiO 2 anatase phase has six Raman active modes. The Raman spectrum of a TiO 2 anatase single crystal has been investigated by Ohsaka et al. [8]; they have concluded that the six characteristic allowed modes appear at 146, 197, 400, 516, 520, and 641 cm −1 .
The Raman lines appearing at 225, 245, 291, 410, 495, 611, and 1318 cm −1 are a characteristic of α-Fe 2 O 3 , i.e., the lines at 225 and 495 cm −1 are assigned to the A 1g vibration mode and the four peaks at about 245, 291, 410, and 611 cm −1 are attributed to the E g vibration mode [9]. It should be noted that there is a line at 663 cm −1 which is typical for Fe 3 O 4 [10]. In our study, the Raman spectra of the TiO 2 film supported on SiO 2 opals are slightly shifted with respect to the TiO 2 anatase phase obtained from TiO 2 bulk; this shift could be due to the presence of Fe ions in the composite. In Figure 5, it is observed that the intensity of the phonon vibration mode at 149 cm −1 is decreased as the Fe +3 concentration is increased. These results suggest that when the Fe ions are incorporated into TiO 2 crystal structure, species -Ti-O-Fe-O-Ti-O-type are formed. On the other side, in the Raman spectra of the TiO 2 :Fe 2 O 3 composites, phonon lines corresponding to α-Fe 2 O 3 are not observed.
Reflectance spectra of the samples TiO 2 , Fe 2 O 3 , and TiO 2 :Fe 2 O 3 composites at 1, 3, and 5 wt%, respectively, of Fe supported on the SiO 2 microspheres are shown in Figure 6. The diffuse reflectance spectra were fitted using the Kubelka-Munk theory where the intersection of the fitted straight line and the photon energy axis gives the energy bandgap value [11,12]. Table 1 Table 1. This effect has been observed in a previous work [12].
The decrement in energy bandgap of the TiO 2 -Fe 2 O 3 composites with respect to the one of TiO 2 bulk is probably due to the fact that Fe ions have been incorporated in the TiO 2 lattice as indicated in the Raman section.
In the case of the Fe 2 O 3 sample, we have observed at 2.44 eV a maximum which is considered to be the result of the pair excitation processes 6 A 1 + 6 A 1 → 4 T 1 ( 4 G) + 4 T 1 ( 4 G) [13]; in the case of 2.62 eV, other maximum in the reflection spectrum is observed due to the 6 A 1 → 4 E, 4 A 1 ( 4 G) transition, in accordance to the Tanabe-Sugano diagram [13]. Here, we have also observed a reflection edge between 3.00 and 3.25 eV (413 and 381 nm), which could be due to the transition from the oxygen 2p orbital to the iron 3d orbital [14] or by the transition from the valence band to the conduction band of iron oxide. The bandgap value found for this oxide was 2.33 eV, which is a little greater than the one of reference value of Fe 2 O 3 in bulk (2.22 eV). Additionally, there exists a maximum at 3.43 eV which is due to the contributions of the Fe 3+ ligand field transitions 6 A 1 → 4 E( 4 D) and 6 A 1 → 4 T 2 ( 4 D) [13]. Otherwise, when the TiO 2 :Fe concentration is 1 and 3 wt%, the same maxima have been observed between 2.44 and 2.62 eV, which are related to the formation of Fe 2 O 3 into the TiO 2 lattice supported by the SiO 2 microsphere assembly. However, the maximum at 3.43 eV disappears because more Fe ions (5 wt%) are introduced into the TiO 2 lattice diminishing the energy bandgap of TiO 2 which induces optical transitions in the visible region.

Conclusions
An important effect of the Fe ion concentration on the morphological and optical properties of the TiO 2 films supported on the single layer of SiO 2 microsphere assemblies has been found. We have prepared silica  microparticles of about 364 nm diameter by the Stöber method. We have been able to fabricate a single layer of SiO 2 microspheres assembled in hexagonal close-packed fashion on glass substrate with a controlled vertical dip coating procedure. We have observed that the TiO 2 film formation affects the stability of the SiO 2 layer cracking the layer and giving rise to the formation of 'micro-shavings' dispersed on the glass substrate, presenting a granular surface morphology with grain diameter of about 50 nm. The X-ray diffractograms do not show any significant effect of the Fe concentration into the TiO 2 lattice structure. However, a considerable effect of iron concentration on the intensity of the TiO 2 phonon vibration modes as shown in the μ-Raman spectra results has been observed. Furthermore, the diffuse reflectance spectra have shown that the energy bandgap of the TiO 2 :Fe 2 O 3 /SiO 2 composites are located in the range between the energy bandgap of TiO 2 and the one of Fe 2 O 3 bulk.