Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Corresponding author: K. Suzuki, k-suzuki@esi.nagoya-u.ac.jp

Preparation and Photocatalytic Properties of TiO2-SiO2 Mixed Oxides with Different TiO2/SiO2 Ratio

and Brownmillerite Type Calcium Ferrite

Jiro Ikawa 1, Shin’ichi Komai 2, Daisuke Hirabayashi 1, Feng Ya’ning 3, Miao Lei 4 and Kenzi Suzuki 1

1 EcoTopia Science Institute, Nagoya University

2 Technical Center of Nagoya University, Nagoya University 3 Department of Environmental Technology & Urban Planning, Nagoya Institute of Technology

4 Materials R&D Laboratory, Japan Fine Ceramics Centre

Abstract: TiO2-SiO2 mixed oxides with different TiO2/SiO2 molar ratios in the range, 0.01 ~ 0.2, were prepared by a sol-gel method. Their physicochemical properties were investigated by XRD, UV visible spectroscopy, TEM and N2 adsorption-desorption measurements. The photocatalytic activity of these samples was also measured by the adsorption of methylene blue (MB). It was found that the photocata-lytic activity improved with increasing TiO2/SiO2 molar ratio, the highest at 0.1. It was found that the par-ticle size and the number of TiO2 particles strongly influenced the photocatalytic activity. The possibility of Brownmillerite type calcium ferrite, Ca2Fe2O5, included in SiO2 as a visible light responding photo-catalyst was also examined. The formation of this phase was found to respond to the visible light. Keywords: Photocatalysis, TiO2-SiO2 mixed oxide, CaO-Fe2O3-SiO2 mixed oxide, Calcium Ferrite, UV light, Visible Light, Methylene Blue

1. INTRODUCTION It is known that TiO2 has many functions of deodoriza-

tion, sterilization, VOCs decomposition, NOx decomposi-tion, antifouling, self cleaning, antifog, antibacterial, etc. Using these functions of TiO2 a variety of products, air cleaner, refrigerator, road related products, glass coated, mirror coated, medical device, etc. are produced. Our living environment is contaminated with various hazard-ous chemicals and many health hazards such as respira-tory tract disease, allergic disease etc. are arising. Under such situation, the technology for environmental cleanup and preservation is strongly required for the construction of a sustainable society. Photocatalysis is one of the real-izing methods to achieve this. The photocatalyst used in the environmental cleanup is mostly TiO2. Since the con-centration of hazardous chemical existed in the environ-ment is very low, the technology of adsorption and con-centration of the hazardous chemicals is strongly required. Therefore, the high dispersion technology of TiO2 to the porous material with high specific surface area is neces-sary. In this study, the synthesis of TiO2-SiO2 mixed ox-ide by a sol-gel method was investigated.

Though near ultraviolet radiation, less than 380 nm, is necessary for the excitation of TiO2, the visible light re-sponding photocatalyst is required in order to effectively use the solar energy. SiC (413 nm), CdS (496 nm), Fe2O3 (539 nm), GaP (551 nm), CdSe (730 nm) etc. are already known as a visible light responding photocatalyst by a conventional study [1]. Moreover, RbPb2Nb3O10 (475 nm), BiVO4 (540 nm) and Cu2+ doped ZnS (520 nm) etc. are also known.

We have studied the catalytic properties of Brownmil-lerite type calcium ferrite, Ca2Fe2O5, until now. Our re-search clearly indicated that, calcium ferrite has high

oxidative catalytic activity upon heating [2-4]. Calcium ferrite is an n-type semiconductor, so that, the second content of this study is to confirm the possibility as a visible light responding photocatalyst of Brownmillerite type calcium ferrite, Ca2Fe2O5 [5, 6]. In this study, the photocatalytic function of Ca2Fe2O5 was evaluated under the coexistence of SiO2 with high surface area, since the specific surface area of Ca2Fe2O5 powder is very small, under 2 m2/g.

2. EXPERIMENTAL 2.1 Preparation of sample 2.1.1 TiO2-SiO2 mixed oxide

Titanium tetraisopropoxide {Ti[OCH(CH3)2]4, TTIP} and tetramethoxy silane {Si(OCH3)4, TMOS} were used as starting materials, and TiO2-SiO2 mixed oxides with TiO2/SiO2 molar ratios of 0.01, 0.05, 0.10, 0.15 and 0.20 were prepared by a sol-gel method. Ethanol solutions of TTIP and TMOS were first prepared and they were mixed at a desiring TiO2/SiO2 ratio with stirring. Addition of an aqueous ammonia solution to the TTIP and TMOS solu-tion generated TiO2-SiO2 mixed sol. The obtained sol was kept at room temperature for 12 h in order to progress the polymerization reaction, and then the wet gel was ob-tained. The xerogel of TiO2-SiO2 was obtained by drying the wet gel at room temperature for 12 h. TiO2-SiO2 mixed oxide was prepared by heating the xerogel at 400 ˚C for 2 h in air. The list of sample prepared is shown in Table 1. Pure TiO2 was also prepared by the similar method. In addition, a standard TiO2 sample was also ob-tained from Catalysis Society of Japan (JRC-TIO-4, crystal structure: anatase, specific surface area: 50±15 m2/g) was used without any pretreatment.

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

2.1.2 CaO-Fe2O3-SiO2 mixed oxide Ethanol solution of TMOS, and aqueous solutions of

Ca(NO3)2 and Fe(NO3)3 were mixed at a desiring CaO:Fe2O3:SiO2 molar ratio under stirring. The resulting mixture was aged at room temperature for 12 h for crys-tallization. The wet gel turned into a xerogel of CaO-Fe2O3-SiO2 upon drying at room temperature for 12 h. A mixed oxide of CaO-Fe2O3-SiO2 was obtained by heating the xerogel at 800 ˚C for 5 h in air. The Ca : Fe : Si mole ratios in CaO-Fe2O3-SiO2 mixed oxide were 0.10 : 0.10 : 1.0, 0.20 : 0.20 : 1.0 and 0.30 : 0.30 : 1.0. Binary, Fe2O3-SiO2 mixed oxide without CaO was also prepared in the similar manner with the Fe/Si mole ratio of 0.10. Table 1 shows the list of sample prepared.

2.1.3 Calcium ferrite, Ca2Fe2O5

Powder sample of calcium ferrite, Ca2Fe2O5, was pre-pared by solid state reaction. The starting materials were Ca(OH)2 and FeO(OH) powders. These hydroxides were calcined individually at 1000 ˚C for 7 h to obtain CaO and Fe2O3, respectively. These oxides were mixed physi-cally, ground in a pestle and mortar and then, calcined at 800 ˚C for 3 h in air.

2.2 Measurement 2.2.1 Characterization

The crystal structure of samples was measured by X-ray powder diffractmeter by using CuKα radiation (50 kV, 100 mA). The porosity such as specific surface area and pore volume was measured by using N2 adsorp-tion-desorption measurements at 77 K. The particle size and morphology of photocatalyst particles were observed with a transmission electron microscope (TEM).

2.2.2 UV/VIS absorption spectrum

The UV/VIS absorption spectrum was measured by diffuse reflectance spectroscopy. MgO was used as a standard sample for calibrating the reflectance spectrum. The absorption spectrum of the sample was obtained by converting the diffuse reflectance spectrum using the Kubelka-Munk function (Equation 1).

F(R∞) = k/s = (1-R∞)2/2R∞ = (1-r∞)2/2r∞ (1)

Where, F(R∞): Kubelka-Munk function, k: absorption

coefficient, s: scattering coefficient, R∞: reflectivity of the sufficiently thick sample, r∞: relative reflectivity.

The optical band gap of sample was obtained from equation 2.

[F(R∞)hν]2 = A(hν-Eg) (2)

Where, hν: energy of the light, A: constant, Eg: energy

gap. The optical band gap was obtained by following method, after the graph of [F(R∞)hν]2 vs. hν was de-scribed. The tangent was described on the graph, and hν at [F(R∞)hν]2=0 was made to be the optical band gap.

2.2.3 Adsorption isotherm of Methylene Blue (MB)

After 10 mg of sample powder and 40 ml of MB (C16H18ClN3S) solution (initial concentration; 1, 5, 10, 12, 15 and 20 mg/L) were mixed, the mixture was stored in the dark place for 3 h with stirring at room temperature and then the concentration of MB solution was measured. The MB equilibrium uptake on the sample was calculated from the concentration of MB solution.

.2.2.4 Photocatalytic activity

The photocatalytic activity of the sample was evaluated by decolorization reaction of MB solution at room tem-perature by the irradiation of a black light (27 W) or a fluorescent lamp (20 W) with UV cut-off filter under 400 nm. The equipment employed to measure the photocata-lytic activity is shown in Fig. 1. The MB concentration was determined by the absorbance of 668 nm by using a spectrophotometer. Before taking measurements, 10 mg of the sample powder was mixed with 40 ml of MB solu-tion (initial concentration; 20 mg/L) and, the mixture was stored in the dark place for 3 h under stirring. The change in the concentration of MB after 3 h was considered caused by MB adsorption on the sample. Afterwards, the mixture was irradiated in UV light or visible light, and the MB concentration was measured periodically

3. RESULTS AND DISCUSSION 3.1 XRD measurement

XRD patterns of SG1-SG5, TiO2 prepared by sol-gel method and JRC-TIO-4 are shown in Fig. 2. From TiO2 and JRC-TIO-4, it was possible to obtain the diffraction peaks attributed to the anatase TiO2. On the other hand, the clear diffraction peak was not obtained from SG1-SG5. However, in the diffraction patterns of SG4 and SG5 which contained relatively higher TiO2 com-

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

pared with SG1-SG3, the broad diffraction line corre-sponded to anatase TiO2 was identified around 25˚ 2θ. From the above result, it was considered that the content of TiO2 is too small, the crystallinity of TiO2 is very low, and/or the particle size of TiO2 is too small. The XRD results of SGF and SGCF series clearly indicate that, these samples are almost similar to those of the SG series. No diffraction peaks attributed to Fe2O3 or Ca2Fe2O5 have been clearly discerned.

3.2 N2 adsorption / desorption isotherm

Fig.3 displays the N2 adsorption / desorption isotherms of SG1-SG5. Based on the shape of the isotherms, the samples could be classified into two groups: Group-1 samples, which include SG1, SG2 and, SG4 and Group-2 samples that contain SG3, SG5. The isotherm of the for-

mer group, (Group-1) samples exhibit a large N2 uptake at the relative pressure (P/P0) of zero, and the N2 equilib-rium uptake increased with increasing relative pressure. In addition, a hysteresis loop also appear between adsorp-tion and desorption isotherms indicating that these sam-ples posses pores widely distributed from micropore to mesopore, and display high specific surface areas. On the other hand, for the Group-2 samples, although the N2 equilibrium uptake at the initial p/p0 is almost similar to those observed for Group-1 samples, the N2 equilibrium uptake is significantly less compared to Group-1 samples even upon increasing the relative pressure. It is also noted that these samples do not exhibit any hysterisis. These results suggest that the Group-2 samples do not posses mesopore although they exhibit micropores. The specific surface area (SBET), mean pore diameter and pore volume of both Group-1 and Group-2 samples are shown in Table 2. Except the sample, SG5, which exhibits a BET surface area of about 400 m2/g, all other samples show a high BET surface areas over 500 m2/g. Though it is not clear still on the above reason, the dispersion state of TiO2 par-ticles in the SiO2 seems to influence.

Since the molecular size of MB (1.6×0.8×0.5 nm) is

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

smaller than the pore diameter of the samples, it can ex-

pect that MB could be introduced in the pore of all sam-ples. In addition, because all the samples exhibit a high BET surface areas, a high adsorption capacity for MB could also be expected.

In contrast to SG samples, the BET surface areas of

Ca2Fe2O5 and TiO2 are significantly less, 1.8 and 5.0 m2/g, respectively. It should be noted that the BET surface area of TiO2 is drastically decreased from 150 to 5.0 m2/g after heating at 400 ˚C. It is considered that the heat-resistance is very low for TiO2 xerogel. The BET surface area of SGCF10 and SGCF20, and that of CaO-Fe2O3-SiO2 mixed oxide are 385 and 403 m2/g, respectively. However, the BET surface area of SGCF30 sample that incorpo-rates both CaO and Fe2O3 is also less, 273 m2/g.

3.3 TEM images

TEM images of SG1, SG2, SG3 and SG4 are shown in Fig. 4. The black particles in the photograph are the TiO2 particles. The TiO2 particle sizes in SG2, SG3 and SG4 are almost ~5, 5~20 and 5~50 nm, respectively, and they increased with increasing TiO2 content. The TiO2 parti-cles could be formed in the following way: at the initial stage of the TiO2 sol generation, the crystalline nucleus of TiO2 is formed first. Then the growth of the crystalline nucleus is continuously generated in proportion to the TTIP concentration. On the other hand, the formation of TiO2 particle could not be observed in SG1, because the TiO2 content in this sample is very low with a TiO2/SiO2 molar ratio of 0.01.

3.4 UV/VIS absorption spectrum The UV/VIS absorption spectra of samples are shown in Fig. 5. The validity of this measurement method could be confirmed, because the absorption edge for JRC-TIO-4 which posses primarily anatase TiO2 phase, exhibit absorption at 371nm (3.34eV). Though the ab-sorption edges for SG3 - SG5 are also about 380nm, very close to anatase TiO2, the absorption edges for SG1 and SG2 are shifted to low-wavelength side by “blue shift” and this is caused by smaller TiO2 particle sizes, espe-cially below 10 nm [7]. It is clear from TEM results that the particle sizes of TiO2 in SG1 and SG2 are smaller than those in SG3 - SG5. In addition, the absorption spectra of SG3-SG5 are recognized even in longer-wavelength side over 400nm. From this fact, the possibility of the excitation by the visible light could be considered. The samples, SG3 - SG5 exhibited a faint brown coloration, while the samples SG1 and SG2

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

showed white color. The observed difference would have caused by the use of aqueous ammonia in the synthesis.

On the other hand, the absorption spectra of SGF10, SGCF10, Fe2O3 and Ca2Fe2O5 of which exhibited brown body color absorb in the visible light region. This fact shows the possibility as a visible light responding photo-catalyst in these samples. The bandgap obtained from equation 2 and wavelength of SG1 - SG5, JRC-TIO-4 and Ca2Fe2O5 are summarized in Table 3.

3.5 Adsorption isotherm of MB The MB adsorption isotherm for SG4 is shown in Fig. 6 and that is typical Langmuir type adsorption isotherm. The adsorption phenomenon of MB on the sample sur-face therefore corresponds to monolayer adsorption. The saturation adsorption amount of MB for SG4 could be calculated using Langmuir equation with 0.0705 g/g. This value is about 1/4 in comparison with the MB quantity (0.283 g/g) of monolayer adsorbed on the sur-face of SG4 (682 m2/g). 3.6 Photocatalytic activity

MB solution of 20 mg/L was stored with SG samples, JRC-TIO-4 and TiO2 for 3 h under stirring before UV ir-radiation. While the MB concentration lowered signifi-cantly after the adsorption equilibrium over SG samples, it remained almost unchanged over both JRC-TIO-4 and TiO2 samples (Table 4). A plot of the amount of MB ad-sorbed versus BET surface areas of samples shown in Fig.

7 indicates that the adsorption capacity increases and shows a maximum around 600 m2/g. However, we be-lieve that there are other parameters such as pore diame-ter, pore shape, pore volume and other surface properties of TiO2 particle that influences the adsorption properties rather than mere BET surface areas. Further experiments are in progress to clarify this.

The UV irradiation on the MB solution started after 3 h. The relationship between MB concentration and irradia-tion time is shown in Fig. 8. The MB concentration de-creased with the irradiation time in all samples. Espe-cially, the decreasing rate of the MB concentration is large, when JRC-TIO-4 and TiO2 are used as photocata-lysts. These samples, in fact, exhibit higher photocatalytic activities. In case of SG samples, on the other hand, the decreasing rate of MB concentration is small at SG1 and SG5, and it increases in the order SG4<, SG2<SG3. The graph of ln (C/C0) vs. irradiation is shown in Fig. 9. The Co is the initial concentration of MB in UV irradiation start, and C is the MB concentration after the irradiation time elapsed. The decomposition reaction of MB was first-order reaction, because the graph of ln (C/C0) vs. ir-radiation time fitted first order. Therefore, the rate con-stant of the reaction for each sample has been calculated from the slope of the graph, and the results are summa-rized in Table 5. In addition, the rate constant of reaction

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

for the MB decomposition in visible-light irradiation is also summarized in Table 5. It can be seen that both JRC-TIO-4 and TiO2 show high photocatalytic activity by the UV irradiation. The SG3 sample also exhibits a high photocatalytic activity by the UV excitation. It is inter-esting to note that although the JRC-TIO-4 sample ex-hibit a high photocatalytic activity of rate of 139×10-6 s-1 under UV irradiation, the sample exhibit a very low rate 4.44×10-6 s-1 under visible light irradiation. This is about 30 times loss in activity when the switching from UV to Visible light irradiation. On the other hand, the photo-catalytic activity decreases only by about 50 % in the case of SG3. The rate constant for SG3 (21.1×10-6 s-1) is about 5 times higher than that of JRC-TIO-4 sample (4.4×10-6 s-1) highlighting that the SG3 developed in this work is much more active under visible light than the commercial JRC-TIO-4 photocatalyst. The results also

suggest that the SG3 sample could be employed as a photocatalyst in both UV and visible range.

The catalytic activities of Fe2O3 and SGF10 which are considered to be promising materials as a visible light responding photocatalyst show very low photocatalytic activity compared to SG3 sample. The catalytic activity of SGCF10 in which CaO coexisted with Fe2O3 showed the high photocatalytic activity in either irradiation of UV and visible light. The rate constants were 20.8×10-6 and 25.2×10-6 s-1 in each irradiation. The absorption wave length of SGCF10 exists in the visible light region, indi-cating that SGCF10 also is a promising material as a visible light responding photocatalyst. 4. CONCLUSIONS

The photocatalyst with high dispersion of TiO2 parti-cles and high specific surface area could be obtained from sol-gel synthesized TiO2-SiO2 mixed oxides. The TiO2/SiO2 ratio was found to greatly influence the photo-catalytic activity. In this study, mixed oxides with TiO2/SiO2 = 0.10 in which TiO2 particles of about 10nm diameter highly dispersed in the silica showed the highest photocatalytic activity. On the other hand, the catalytic activity of Fe2O3-SiO2 mixed oxide under visible-light irradiation was found to be very low. Its catalytic activity could be improved when CaO is doped. Although the in-formation of Ca2Fe2O5 is unclear it is highly probable that this phase is present in these samples that are contributing to high catalytic activity. REFERENCES 1. The Chemical Society of Japan, “Jikken Kagaku

Koza”, Maruzen, p.558 (in Japanese). 2. D. Hirabayashi et al., Adv. Sci. Tech., 45, (2006) pp.2169-2175. 3. D. Hirabayashi et al., Catal. Lett., 110, (2006) pp.155- 160. 4. D. Hirabayashi et al., Hyperfine Interact., 167, (2006) pp.809-813. 5. Y. Yang et al., Mater. Chem. Phys., 96, (2006) pp.234-

239. 6. Y. Yang et al., Mater. Sci. Eng. B, 132, (2006) pp.311-

314. 7. M. Anpo et al., J. Phys. Chem., 91, (1987) pp.4305-

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