Synthesis of TiO 2 -SiO 2 powder and thin film photocatalysts by sol-gel method

Transcription

1 Indian Journal of Chemistry Vol. 48A, July 2009, pp Synthesis of TiO 2 -SiO 2 powder and thin film photocatalysts by sol-gel method Radhiyah Abd Aziz & Iis Sopyan* Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University Malaysia (IIUM), PO Box 10, Kuala Lumpur, Malaysia sopyan@iiu.edu.my Received 4 February 2009; revised and accepted 19 June 2009 Binary metal oxide TiO 2 -SiO 2 powder and thin film photocatalysts have been synthesized via sol-gel method using tetraethoxysilane and a titanium chelate compound as the silicon and titanium precursors respectively. The physico-chemical properties of the TiO 2 -SiO 2 powder and thin film are characterized using TG-DTA, FTIR, XRD, FESEM, and UV-vis data. XRD analysis shows that the TiO 2 -SiO 2 powder and thin film obtained are in fully anatase structure after calcination at 500 C, which is also confirmed by TG-DTA analysis. The FESEM picture shows that the TiO 2 -SiO 2 particles in both powder and film are nano-sized spheres. The addition of low composition SiO 2 to the TiO 2 matrix suppresses the crystal growth of TiO 2 and enhances the photocatalytic performance. The photocatalytic properties have been evaluated by studies on the degradation of gaseous acetaldehyde. Keywords: Catalysts, Photocatalysts, Sol-gel processes, Oxides, Binary oxides, Titania, Silica, Thin films IPC Code: Int. Cl. 8 B01J21/00; B01J37/00 Titanium dioxide (TiO 2 ) is well known as a photocatalyst and widely applied for air and waste water purification. The use of conventional powder TiO 2 photocatalyst especially in water and air purification pollutes the environment. Since nanosized photocatalyst particles can cause respiratory problems, it is not suitable for air purification 1. In water treatment application, the use of conventional powder photocatalysts has the disadvantage of requiring stirring during reaction and separation after the reaction 2. Immobilizing the TiO 2 as a thin film on the substrate overcomes these disadvantages and thus can be extended to industrial applications. Unlike the powder form, the thin film form can prevent the scattering of light and enhance the transmittance of light, therebye resulting in high reaction efficiency 3. However, film type photocatalysts normally have lower surface areas than powdered ones, and the intrinsic photocatalytic activity of films is usually less than that of powders 4. Gan et al. 5 have reported that immobilization TiO 2 in a thin film form would lead to surface area constraints. The photocatalytic performance of these compounds depends on the characteristics of the TiO 2 crystallites, such as the size and surface area 6. Therefore, modification of its physical and chemical property is of interest to researchers. One of the possible ways to modify the property of TiO 2 crystallites is by adding a second semiconductor into the TiO 2 matrix. Silicon dioxide (SiO 2 ) has been incorporated into the TiO 2 matrix to enhance the photocatalytic process 2,7-9. SiO 2 has high thermal stability, excellent mechanical strength and helps to create new catalytic active sites due to interaction between TiO 2 and SiO Recently, Zhou et al. 9 demonstrated that mixed metal oxides (TiO 2 -SiO 2 ) enhance the photocatalytic performance due to improved surface adsorption and increasing surface hydroxyl group in the thin film. Also, at the same time SiO 2 acts as the carrier of TiO 2 and helps to obtain a large surface area as well as a suitable porous structure 11. Sol-gel method has been widely proven to be effective to produce binary oxide glass, either in the form of powder or film. In the present work, the TEOS is partially hydrolyzed in methanol, water and hydrochloric acid under controlled conditions that allow the solution, i. e., sol, to yield a formable, loosely cross-linked matrix, i.e., gel. Then, titanium chelate compound (PTP) was added to form polytitanosiloxane solution (TiSi) via polycondensation reaction (Scheme 1) 12. TiSi precursor solution consists of siloxane (Si-O-Si) and Si-O-Ti linkages as a main chain. Spin coating technique was used to deposit the precursor solution on the glass substrate. The TiO 2 -SiO 2 powder has also been obtained from the same precursor solution via dissolution process of the precursor solution in acetone and hexane. The samples of powder and thin film have been calcined and characterized using X-ray diffraction (XRD),

2 952 INDIAN J CHEM, SEC A, JULY 2009 FTIR, UV-vis, TG-DTA, and FESEM data. The photocatalytic activity on acetaldehyde gas has been measured by gas chromatography. Experimental For preparation of TiSi as precursor solution, titanium (di-isopropoxide) bis (acetylacetonate) (PTP) (Strem Chemicals, 75% in isopropanol), tetraethoxysilane (TEOS) (Alfa Aesar, 98%), methanol and hydrochloric acid (SYSTERM, 36%) were used. Acetone and hexane were used to produce the powder via precipitation method. The polytitanosiloxane precursor solution (TiSi) was produced by mixing TEOS and 30 ml of methanol in a round-bottomed flask with stirring. Into this solution, a mixture of HCl (6N), distilled water and 30 ml of methanol was added dropwise with a disposable pipette with stirring for 10 min to hydrolyze the TEOS partially. The molar ratio of HCl/TEOS, H 2 O/TEOS and PTP/TEOS are 0.1:1, 2:1 and 9:1, respectively. The partial hydrolysis of TEOS was followed by adding PTP dropwise with a disposable pipette. On complete addition of the PTP solution into the flask, it was heated until the reflux temperature reached about 74 C. The heating was stopped after 10 min to finally obtain the TiSi solution. This TiSi solution was further used to produce different forms of TiO 2 -SiO 2 powder and thin film. For the synthesis of TiO 2 -SiO 2 powder samples (P1 and P2), the TiSi solution (12.5 ml) was dried in air until the solution become almost solid. Then, 25 ml of acetone was added and stirred until a yellow translucent solution formed. About 6-8 ml of the yellow translucent solution was added dropwise into 150 ml hexane with vigorous stirring. A light yellow precipitate was obtained, which was filtered to get the gel powder. The gel powder was dried in an oven at 200 C for 1 hr and calcined at 500 C for 7 hr. For the synthesis of TiO 2 -SiO 2 thin film sample (Tf1 and Tf2), the soda lime glass substrate was carefully cleaned with acetone in ultrasonic bath and dried in an oven. A spin coating machine (model WS- 400B-6NPP/LITE, Laurell Technologies Corporation) was used to coat the TiSi solution on the substrate. The spinning condition was fixed such that uniform distribution of TiSi solution was obtained and the thickness of the thin film on the substrate could be controlled. The first stage of spinning was fixed at 1000 rpm for 10 s to ensure that the TiSi solution was uniformly dispersed on the surface of the substrate. The second stage of spinning was fixed at 3000 rpm for 30 s to get the final thickness of TiSi solution on the substrate. The wet gel thin film of TiO 2 -SiO 2 obtained was dried at 200 C for 1 hr in an oven. The coating process was repeated three times to increase the thickness of the thin film. After finishing the coating and drying process, the thin film was calcined at 500 C for 7 hr in air to obtain anatase structure TiO 2. Physical properties and crystal structure of sol-gel TiO 2 -SiO 2 thin film and powder were determined using Shimadzu diffractometer (model XRD 6000) with Cu Kα radiation. Both samples were scanned between 20 and 70 at a scan rate of 2 /min. Physical properties such as size and shape of particle and

3 NOTES 953 surface morphology were characterized by using Jeol FESEM instrument (model JSM 6700F). Estimation of thickness of the thin film was done using the same equipment. Differential and thermogravimetric analyses were carried out on the amorphous TiO 2 -SiO 2 powder in ambient air using Perkin Elmer instrument (model PYRIS Diamond) at a 10 C/min heating rate to determine the thermal properties of the sample. FTIR spectra were recorded on a Perkin Elmer instrument (model Spectrum 100), while the UV-vis absorption edge of thin film sample was recorded using Perkin Elmer spectrophotometer (model Lambda 35) in the range of nm. Perkin Elmer gas chromatograph (model Clarus 500) was used to measure the decrease in concentration of acetaldehyde gas. In order to evaluate the photocatalytic performance, an airtight container photocatalytic system with volume of 1 L was used. The TiO 2 -SiO 2 thin film with apparent surface area of cm -2 was placed at the center of the container. The distance of the photocatalyst from the UV light source was about 14 cm. At this point, the incident UV light intensity at the photocatalyst surface was 1.4 mw cm -2. Saturated acetaldehyde gas (7 ml, Merck, Germany) was injected into the airtight reactor using a syringe, so that the concentration was 7000 ppmv. The UV irradiation at room temperature was started after equilibrium between gaseous and adsorbed phases was reached (about 60 min), followed by the measurement of the decrease in concentration of the acetaldehyde gas. The measurements were made at intervals of 15 minutes after irradiation of UV light. Results and discussion Thermal properties of the mixed TiO 2 -SiO 2 powder were studied by TG-DTA analysis, which was used to determine the crystallization temperature of the powder and to investigate the effect of addition of a second semiconductor into the TiO 2 matrix on its thermal property. The amorphous powder was heated from room temperature to 1100 C at a rate of 10 C /min. The TGA curve shows a sharp drop in weight of the powder below 200 C due to the decomposition of water. A continuous weight loss was observed in temperature range of 200 C-350 C, which is due to the combustion of the solvents (methanol) and other residual organic compounds (acetylacetonate). Figure 1(a & b) shows the exothermic peaks at 375 C and 432 C for P1 and P2 powder, respectively. Besides, anatase-rutile transformation temperature for P1 and P2 was observed at 548 C and 643 C, respectively. From the result, it is seen that the temperature of amorphous-to-anatase and anatase-to-rutile transitions shifts towards higher temperature as the SiO 2 composition increases. This may be due to the bulkier crystal structure of SiO 2 blocking the TiO 2 densification and crystal growth. From the TGA curve, it is observed that ca. 61% weight of P1 powder diminished after being heated to 1100 C. Calculations show that 68% of weight loss occurs after crystal transformation. Therefore, it is assumed that the possible structure of the polytitanosiloxane is ladder type as shown in Fig. 2. Compared to the P2 powder, only 35% of weight loss occurs until the end of the thermal analysis. This may be attributed to the thermal stability of P2 as compared to that of P1, and may be due to the existence of Si-O-Si bond which is more in P2 than in the P1 powder since the composition of Si in P2 is high. Even though the Si-O-Si bond is very bendable and flexible, addition of bulky group into the structure can increase the rigidness of the structure. This bulky Fig. 1 TG-DTA curves of TiO 2 -SiO 2. [(a) P1; (b) P2].

4 954 INDIAN J CHEM, SEC A, JULY 2009 Fig. 2 Possible structure of polytitanosiloxane. group also hinders the flexibility and raises the transition temperature. More Si-O-Ti bonds are formed in P2 powder as shown in the FTIR spectrum, leading to decrease in flexibility of structure and a small weight loss during heating. Compared to P1, fewer Si-O-Ti bonds do not hinder the flexibility of the structure, and hence a higher weight loss occurs. Figure 3 shows the XRD pattern of TiO 2 -SiO 2 powder and thin film for each composition. All samples have been calcined at 500 C for 7 hr. Fig. 3(a) shows that the most intense reflection at 2θ=25.3 and 25.4 may be assigned to the characteristic peak of anatase (d 101 ) crystal structure for P1 and P2, respectively. A small shift of the anatase peak to slightly higher 2θ and change of particle size as the composition of SiO 2 increased was also observed (Table 1). The shift in the anatase peak shows the Si atom substitution into the TiO 2 lattice. Since the atomic radii of Si atom is smaller than Ti, the TiO 2 particle experiences a contraction and its crystal growth is retarded due to the Si atom. This agrees with the small value of average grain size which has been calculated using Scherer s equation (Table 1). However, high composition of SiO 2 component leads to the formation of larger second particles of TiO 2. This is due to the SiO 2 which behaves as a neck and connects the TiO 2 particles 9 and thereby, forming bigger particles of P2. From the XRD pattern, it can be seen that higher crystalline powder was obtained in P1 as compared to that of P2. It can be inferred that excessive doping of SiO 2 in the TiO 2 matrix leads to amorphous mixed oxides. The XRD patterns show that the thin film has lower crystallinity than the powder. This may be because the thin film sample is composed of dense particles as compared to the dispersed particles in the powder form. This dense structure retards the crystal growth of the particles on the thin film resulting in a low crystallinity phase. This is further corroborated by unchanged 2θ angle of anatase peak in the XRD pattern even though the composition of SiO 2 is increased (refer Table 1). This low crystallinity is also Fig. 3 XRD patterns of TiO 2 -SiO 2 calcined at 500 C for 7 hours. [(a) powder; (b) thin film]. Table 1 The 2θ angle and particle size of different sample for each composition. Samples Angle (2θ) Particle size (nm) P P Tf Tf attributed to the absence of TiO 2 rearrangement due to the coexistence of SiO 2 when its composition is high 9. The suppressive effect of SiO 2 was not observed in the thin film sample as indicated by a narrow peak of anatase at high composition of SiO 2. Therefore, high composition of SiO 2 leads to the formation of larger TiO 2 particles. However, the particle size of TiO 2 on the thin film is still smaller as compared to that of the TiO 2 particles in the dispersed powder form. This result is consistent with the theory that dense structure

5 NOTES 955 of particles on the thin film retards the crystal growth of TiO 2. However, this low crystallinity of the thin film increased when the number of thin film layer increased. The FTIR spectra of different composition of TiO 2 -SiO 2 powder and thin film sample along with the precursor solution spectra were recorded in the wave number range of cm -1. The polytitanosiloxane solution of each composition shows the fundamental vibrational modes of acetylacetonate group at 1580 cm -1 (C=O), 1520 cm -1 (C=C) and 1340 cm -1 (C-C). The absorption bands at cm -1 and 2960 cm -1 is due to the stretching vibration of hydroxyl group (O-H) and aliphatic group (C-H), respectively. All these vibration bands disappeared when the sample underwent calcination process. The spectra of the calcined samples, either powder or thin film, show the absorption peak of Ti-O bond at 400 cm -1. However, the absorption peak at 1090 cm -1 and 1010 cm -1 indicates the Si-O-Si linkage in the powder form. However, it was observed that this absorption peak is weak in the thin film form. The siloxane group can also create a strong bond between the photocatalyst and the substrate. In the TiO 2 -SiO 2 system, the absorption peak of Si-O-Ti linkage was observed at 925 cm -1. In dye-sensitized solar cells, functionalization of TiO 2 thin films with siloxane adsorbates has been shown to be useful as a surfacepassivation technique that hinders the recombination processes and improves the overall efficiency of lightto-electricity conversion 13. As the thin film sample undergoes calcination process, the absorption peak of Si-O-Ti linkage of the calcined TF1 totally disappears. These results provide strong evidence that TiO 2 nanoparticles on the thin film surface are independently dispersed without forming any distinct chemical bond with silica in the hybrids 7. Therefore, the amount of Si-O-Ti is too few to be detected 9. TiO 2 nanoparticles separated by SiO 2 matrix avoid the formation of aggregates. However, a weak absorption peak can still be observed in Tf2. Meanwhile, the absorption peak of Si-O-Ti linkage of the calcined powder still exists due the interaction between TiO 2 and SiO 2. Figure 4 shows FESEM picture of the TiO 2 -SiO 2 powder and thin film surface. Nano-size of Fig. 4 FESEM of the TiO 2 -SiO 2. [(a) P1; (b) P2; (c) Tf1; (d) Tf2].

6 956 INDIAN J CHEM, SEC A, JULY 2009 TiO 2 -SiO 2 particles with spherical shape is observed in both powder and thin film forms after calcination at 500 C for 7 hr. Figure 4(a) shows that the particle size of P1 ( 100 nm) is smaller than that of P2 (Fig. 4(b)). This is due to the suppressive effect of SiO 2 on the crystal growth of TiO 2. Some bigger particles exist surrounding the primary particles of P2 due to increased amount of SiO 2 that would behave as a neck to connect the TiO 2 particles and form larger secondary particles 9. SiO 2 behaves as the neck only when Ti-O-Si bonds have been formed. This is confirmed by the FTIR spectra of P2 powder which shows higher intensity of Si-O-Ti bond as compared to that of P1 powder. If the amount of Si-O-Ti is too few, most of TiO 2 and SiO 2 exist independently, and SiO 2 only limits the aggregation of TiO 2 particles. Therefore, smaller size of particles is observed in P1 as compared to that in P2 sample. Similar observations are also seen in thin film sample. Figure 4(c & d) shows that particles of TiO 2 -SiO 2 thin film appear to be highly dense and uniform, and no pores are formed. Low composition of SiO 2 leads to a small size of primary particles in the thin film (~ <10 nm). Hence, the suppressive effect of SiO 2 on the crystal growth of TiO 2 particles is favorable only when lower amount of SiO 2 is incorporated into the TiO 2 matrix rather than high composition of SiO 2. From Fig. 4, it can be seen that the size of particles in the thin film are smaller than that of powder form. This is in accordance with the results of XRD analysis, which shows that the restriction of crystal growth of TiO 2 particles on thin film is due to the dense and compact structure. This is also in agreement with the UV-vis data of Tf1 and Tf2, which show absorption at nm and nm wavelength, respectively, i.e., the absorption edge of Tf1 is at shorter wavelength as compared to that of the Tf2. The shift is ascribed to the difference in crystallite size 2. Absorption edge at shorter wavelength range represents the existence of small particle size on the thin film. Thickness of 1 µm was obtained in one coating by using spin coating under fixed spinning conditions (not shown here). The thickness of the thin film should be in optimum for the photocatalyst activity to occur. Thicker film may lead to lower photocatalytic activity due to difficulty of transfer of charge carriers from the bulk to the surface of the thin film. It has been reported that the photocatalytic reaction is a surface reaction and only takes place on the surface of Fig. 5 Plots of the decrease in acetaldehyde gas concentration with irradiation time for TiO 2 -SiO 2 thin film. the thin film of photocatalyst 14,15. However, cracks were seen on the surface of the thin film in this study. The cracked surface may result in a high surface area to be exposed to the UV irradiation and generate more charge carriers. Photocatalytic performance of thin film sample has been investigated by measuring the change in concentration of acetaldehyde gas as a function of UV irradiation time. The decrease of concentration of acetaldehyde gas may be due to two different mechanisms; adsorption ability of the pollutant molecules of the photocatalyst and photocatalytic degradation under UV irradiation. Figure 5 shows that Tf1 has higher rate of photocatalytic activity as compared to that of Tf2. The difference in the photocatalytic activity between Tf1 and Tf2 may be because the thin film Tf2 contains a high amount of SiO 2 which leads to lesser TiO 2 on the surface. This is confirmed by the XRD result which showed low crystallinity of TiO 2 in Tf2 sample due to low mass of material on the substrate surface. Less TiO 2 content means less photocatalytic center. Moreover, SiO 2 and TiO 2 can exist independently if there is an excess of SiO 2 in the TiO 2 matrix. This excess composition of SiO 2 would hinder the TiO 2 particles from interacting with organic molecules and decelerate the photocatalytic activity. Sol-gel method has been used to synthesize the TiO 2 -SiO 2 powder and thin film precursor solution with different amounts of SiO 2. This precursor solution after filtration and coating process produces the photocatalyst in the form of powder or thin film. The anatase phase of TiO 2 with nano-sized particles was obtained for the sample calcined at 500 C for

7 NOTES hr. The suppressing effect of SiO 2 on crystal growth of TiO 2 is clearly observed at low concentration of SiO 2. High concentration of SiO 2 in the TiO 2 matrix leads to the formation of larger particles of TiO 2. The TiO 2 -SiO 2 thin film with low concentration of SiO 2 shows higher photocatalytic activity in degradation of acetaldehyde gas. This is due to the high crystallinity and small size of TiO 2 particles resulting in production of more charge carriers and high surface area for photocatalysis process. Acknowledgement The authors thank Research Management Center, International Islamic University Malaysia (IIUM) for financial support through a research project (No. EDW B ). References 1 Yu J G, Yu H G, Ao C H, Lee S C, Yu J C & Ho W K, Thin Solid Films, 496 (2006) Yu J, Yu J C & Zhao X, J Sol-Gel Sci Technol, 24 (2002) Otsuka E, Kurumada K, Suzuki A, Matsuzawa S & Takeuchi K, J Sol-Gel Sci Technol, 46 (2008) Sopyan I, Watanabe M, Murasawa S, Hashimoto K & Fujishima A, J Photochem Photobiol Part A: Chem, 98 (1996) Gan W Y, Lee M W, Amal R, Zhao H & Chiang K, J Appl Electrochem, 38 (2008) Matsuda A, Higashi Y, Tadanaga K, Tatsumisago M, J Mater Sci, 41 (2006) Meng X, Qian Z, Wang H, Gao X, Zhang S & Yang M, J Sol-Gel Sci Technol, 46 (2008) Machida M, Norimoto K, Watanabe T, Hashimoto K & Fujishima A, J Mater Sci, 34 (1999) Zhou L, Yan S, Tian B, Zhang J & Anpo M, Mater Lett, 60 (2006) Ennaoui A, Sankapal B R, Skryshevsky V & Lux-Steiner M C, Sol Energy Mater Sol Cells, 90 (2006) Cheng P, Zheng M, Jin Y, Huang Q & Gu M, Mater Lett, 57 (2003) Gunji T, Sopyan I & Abe Y, J Polym Sci Part A: Polym Chem, 32 (1994) Iguchi N, Cady C, Snoeberger III R C, Hunter B M, Sproviero E M, Schmuttenmaer C A, Crabtree R H, Brudvig G W & Batista V S, Physical Chemistry of Interfaces and Nanomaterials VII, Proc. Of SPIE 7034, (2008) 70340C1. 14 Sheng J, Shivalingappa L, Karasawa J & Fukami T, J Mater Sci, 34 (1999) Xianyu W X, Park M K & Lee W I, Korean J Chem Eng, 18 (2001) 903.