International Journal of Pure and Applied Physics ISSN 0973-1776 Volume 7, Number 1 (2011), pp. 105–115 © Research India Publications http://www.ripublication.com/ijpap.htm

Synthesis of Nano-TiO2 by Sol-Gel Route: Effect of Solvent and Temperature on the Optical Properties

R. Vijayalakshmi* and K.V. Rajendran

Department of Physics, Presidency College, Chennai, TamilNadu, India

*Corresponding author E-mail: viji2302@gmail.com

Abstract

Nano-sized TiO2 particles were synthesized by sol-gel process using Titanium Tetraiso propoxide mediated in different solvent and calcination temperature. The decomposition of organics in the sample was systematically investigated using Thermo gravimetric Analysis (TGA). The crystalline structures and morphologies of as synthesized TiO2 were characterized by X-Ray Diffraction (XRD), Scaning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The synthesized TiO2 nanoparticles possessed a grain size of 10nm with specific surface area 143.88m2/g that composed of anatase and exhibited significant blue shift in its Ultra Violet-Vis spectrum. The photocatalytic degradation of Methyl Orange in TiO2 suspension was investigated. The influences of solvent and calcination temperature on the size and its optical properties of TiO2 nanopowders were discussed.

Introduction In the past few years, nanoscale one-dimensional structures have attracted many people to research because of the unique electronic, optical and mechanical properties. Among the semiconductor structures, titanium dioxide (TiO2) is an important metal-oxide semiconductor and has a broad range of industrial applications in the areas including photo catalytic reaction, solar cells, gas sensors, electrochromic devices, pigment material, ceramics, deodorization, environmental purification and so on. TiO2 exists in three polymorphs viz., anatase, rutile and brookite [1]. Rutile is a thermodynamically stable phase possessing smaller band gap energy (3.0eV) than the anatase phase (3.2eV) [2]. Recent research on TiO2 has aimed at understanding its strong photo catalytic activity, which is useful in various environmental clean-up applications, such as water purification and wastewater treatment [3-5]. The catalytic performance was suggested to be dependent on a quite number of parameters such as phase, particle size, surface

106 R. Vijayalakshmi and K.V. Rajendran

area and method of preparation. For most photo catalytic reaction systems it was generally accepted that anatase demonstrates a higher activity than rutile and this enhancement in photo activity has been ascribed to the Fermi level of anatase being higher than that of rutile by about 0.2eV [6]. There are many methods of producing TiO2 nanopowders such as, solvothermal [7], hydrothermal [8], hydrolysis [9], the sol-gel technique [10], micro emulsion [11] and chemical vapor deposition (CVD) [12]. Among the methods sol-gel technique offers unique advantages in synthesizing nano-scale TiO2 powders such as better control over stoichiometric composition, easy route of synthesis, better homogeneity and production of high purity powder. In this article, nanosized TiO2 particles were prepared by sol-gel process using TTIP mediated in different solvents (CH3OH, C2H5OH and i-C3H7OH). Calcined powders were characterized for their phases, morphologies and optical properties. The influence of different solvents and calcination temperatures were discussed. Solvent and temperature found to play a very significant role in this process. The photocatalytic degradation of Methyl Orange in TiO2 suspension was also investigated.

Experimental Procedure Nano-sized TiO2 powders were synthesized in the molar ratio 1:4:4:0.07 of Titanium tetraisopropoxide, methanol, water and catalyst. The precursor TTIP was first mixed with the methanol to form a clear solution at 30oC. Next, the solution was rapidly poured into distilled water containing few drops of nitric acid, which acts as a catalyst for hydrolysis. The resultant solution was stirred vigorously for 60min and peptized overnight. The hydrolysis leading to the formation of TiO2 was represented by the following reaction:

C12H28O4Ti+ 2H2O→ TiO2 + 4C3H7OH After a period, the gel was collected and dried at 120oC for several hours, typical

yellow block crystals appeared. Procedure for the preparation of TiO2 was schematically shown in Fig.1. And thus, obtained crystals were ground into fine powder and further calcined at 450, 550 and 650OC for definite time to form TiO2

powder. The heat treatment at high temperature was necessary for removal of organic components and to acquire pure and perfect nanocrystalline TiO2. The same procedure was followed for other solvents ethanol and isopropanol.

The crystalline phases of TiO2 were determined using X-ray diffractometer Schimadzu model: XRD 6000 with CuKα radiation in the range 20-80o (λ=0.154nm). A thermogravimetric analysis (TGA) using Shimadzu 50-TG-DTA was used to investigate the pyrolysis temperature of organics in the sample at a heating rate of 20oC/min. UV-Vis absorption spectra were recorded on a Varian Cary 5E spectrophotometer at room temperature in the wavelength region between 200 to 1000nm. The grain size and microstructure of as-prepared TiO2 were determined and observed on a Scanning Electron Microscope Hitachi S-4500. Transmission Electron Microscope (TEM) were obtained using a model JEOL-2010 microscope. A Photoluminescence (PL) spectrum was recorded on a Perkin-Elmer Lambda 900 spectrophotometer at room temperature using a Xe lamp.

Synthesis of Nano-TiO2 by Sol-Gel Route: Effect of Solvent and Temperature 107

Figure 1: Flow chart for the sol-gel preparation of nano-TiO2.

Measurement Of Photocatalytic Activity: A photocatalytic experiment was carried out to investigate the photo degradation of methyl orange as a model organic compound. The experiment was carried out in a 600 ml cylindrical vessel with an 8W mercury vapor lamp (365 nm) placed at the axis of the vessel. The lamp was installed in a quartz glass tube to protect it from direct contact with the aqueous solution. Reaction suspensions were prepared by adding the required amount of TiO2 into 500 ml of methyl orange (MeOr) solution with an initial concentration of 3x10-5 mol/l. Prior to the photodegradation, the suspension was magnetically stirred in a dark condition for 30 min to establish adsorption-desorption equilibrium conditions. The aqueous suspensions containing MeOr and photocatalyst were irradiated under UV light with constant stirring. The analytic samples from the suspension were collected at equal intervals of time, centrifuged and filtered. The concentration of methyl orange was analysed by UV-Vis Spectrometry at the wavelength 463 nm. The photocatalytic efficiency is calculated from the expression η= (1- Cr/Co), where Co is the concentration of methyl orange before illumination and Cr is the concentration of methyl orange after a certain irradiation time.

Results and Discussion Thermo gravimetric Analysis (TGA) of the synthesized amorphous powder dried at 120oC was shown in Fig.2. The significant weight loss occurred before 400oC due to desorption of water and decomposition of alcohol, after 400oC showed a nearly flat characteristic. Thus, 400oC was selected as the optimum calcination temperature high

108 R. Vijayalakshmi and K.V. Rajendran

enough to achieve crystallization, and optimum to reduce the thermal growth of the particles to maintain nano-scale features in the calcined powder [13].

Figure 2: TGA Curve of prepared TiO2 nanoparticles.

XRD Analysis Fig.3a, b&c shows the X-Ray Diffraction (XRD) patterns mediated in CH3OH, C2H5OH and i-C3H7OH TiO2 sintered at different temperatures 450,550 and 650oC. All the peaks in the XRD pattern can be indexed as anatase and/or rutile phases of TiO2 depending on the calcination temperatures, and the diffraction data were in good agreement with JCPDS files # 21-1272 and # 21-1276. The average crystallite size calculated from the diffraction peaks was 10,13&15 nm at 450oC using Debye Scherer’s equation. When the sample is annealed at 550oC, additional peaks were observed in the XRD pattern, which is due to rutile phase of TiO2 [14], and the amount of phase transformation was calculated using the Spurr formula

X = (1+ 0.8 IA/IR)-1 Where X is the weight fraction of rutile in the sample, IA and IR are the intensities

of the anatase (101) and rutile (110) peaks, respectively. It was calculated that at 550oC around 44.9,48.3 & 51.6% of crystalline anatase phase was transformed into rutile. The XRD pattern of the sample calcined at 650oC shows that the pure rutile characteristic peaks of TiO2 i.e., the sample has completely transformed from anatase to rutile phase when the calcination temperature was changed from 550oC to 650oC. Qiu.S et.al. [15] have also reported a similar phase transformation. It is well known that both anatase and rutile TiO2 can grow from TiO6 octahedra and that the phase transition proceeds by the rearrangement of the octahedra. Arrangement of octahedra through face sharing initiates the anatase phase while the edge sharing leads to the rutile phase. In aqueous medium, protonated surfaces of TiO6 octahedra easily combine with –OH groups of other TiO6 octahedra to form Ti- O-Ti oxygen bridge bonds by elimination of water molecule. The protonation followed by the possible face-sharing TiO6 octahedra will result in formation of anatase phase while edge sharing leads to rutile phase. The particle size as a function of calcination temperature

Synthesis of Nano-TiO2 by Sol-Gel Route: Effect of Solvent and Temperature 109

was plotted and it was observed that the particle size increases with increase of temperature (Fig.3d). The surface area of the samples prepared from different solvents and temperatures were calculated by elaborting the crystallite sizes obtained from X-ray diffraction spectra by means of the following formula and the results were tabulated in Table 1.

S= 6 x 103 /ρL Where S is the specific surface area (m2 g-1), L is the average crystallite size, ρ is

the density of the titania (4.17 g m-3) As seen in table.1 the smallest crystallites were obtained at 450oC as the

temperature increases the particle size also increases drastically, which is attributed to the thermally promoted crystallite growth. The specific surface area at 450oC was higher than that of temperatures 5500C and 6500C. Apparently, the surface areas and crystalline sizes are inversely propotional to each other. As the temperature raised to 650oC, the surface area decreases to 55.34m2/g. The materials show a high degree of crystalline and existence of fully anatase phase at 450oC. We chose 450oC as calcination temperature as the temperature was found to have highest activity among samples calcined at different temperature.

The TEM image and the select area electron diffraction (SAED) pattern of methanol mediated sample calcined at 450oC are shown in Figs.4a and 4b, respectively. The calcined TiO2 particles were well dispersed and spherical in shape with an average size of about 10 nm, which is in good agreement with the XRD value. In Fig 4b, the first four rings are assigned to the (101), (004), (200), (005) reflection plane of the anatase phase.

Ultra Violet (UV) -Vis absorption spectra of TiO2 particles mediated by different solvents and calcined at 450oC are shown in Fig.5 (a-c). The absorption band edges were estimated around 332,346 and 354 nm by extrapolating the steep slopes in the curve to the longer wavelength side. The calculated band gap energies (Eg) of as-prepared TiO2 nanoparticles (3.75, 3.54 and 3.34eV) were greater than that of the bulk (3.2eV). It can be noted from the Fig.5a that the methanol-mediated sample exhibited a prominent blue shift to compare to that of ethanol and iso-propanol solvents (Fig.5b and 5c) [16].

Fig.6 shows the Scanning Electron Microscope (SEM) images of TiO2 powder prepared from different solvents at 450oC. Fig.6b and 6c show the ethanol and isopropanol mediated TiO2 nanoparticles have a size of 15-17nm with non-uniform distribution. A well-defined finer and closer particles of 10 nm were observed in the case of methanol mediated TiO2 as in Fig.6a. Further, the powders prepared with CH3OH have a better dispersivity, than those prepared with C2H5OH and i-C3H7OH existed in small or big conglomeration. The size and activity of the solvents molecule can explain the above process. Since methanol has a small size, and a more active –OH and OCH3 it can react with titanium tetraisopropoxide more easily to form a polymer precursor with a high polymerization than –OC2H5 and -i-OC3H7OH which cannot react quickly due to its bigger size and lower activity of –OH. As to the ethanol, the situation is neutral compared with CH3OH and i-C3H7OH. Thus, the size and activity of solvent have obvious influence on the reacting progress.

110 R. Vijayalakshmi and K.V. Rajendran

PL spectrum of TiO2 calcined at 450oC exhibits a strong PL signal with the excited wavelength of 300nm as shown in Fig 7. The emission spectrum presents two bands at 430 and 460 nm, possibly the former mainly resulting from band edge free excitons and the latter due to the binding excitons. The peaks at 430 nm might originate from the luminescebce centers formed by such Titanium interstitials or dangling in the present TiO2 nanoparticles, but that is not yet clear. We attribute the emission at 460 nm to electron transistion mediated by defects levels in the band gap such as oxygen vacancies. In the present TiO2 nano crystals, the intrinsic defects such as oxygen vacancies, which act as luminescent centers, can form defect levels located highly in the gap, trapping electron from the valance band to make a contributaion to the luminescence [17].

Figure 3a: Methanol mediated TiO2 nanopowder prepared at (a) 450oC (b) 550oC (c) 650oC.

Figure 3b: Ethanol mediated TiO2 nanopowder prepared at (a) 450oC (b) 550oC (c) 650oC.

Figure 3c: Iso-propanol mediated TiO2 nanopowder prepared at (a) 450oC (b) 550oC (c) 650oC.

Figure 3d: The Curve of particle size versus calcinations temperature.

Synthesis of Nano-TiO2 by Sol-Gel Route: Effect of Solvent and Temperature 111

Figure 4a: TEM image of TiO2 nanoparticles at 450oC.

Figure 4b: SAED pattern of TiO2 nanoparticles at 450oC.

Figure 5: UV-Vis absorption spectra of nano-TiO2 powders prepared by different mediated solvents at 450oC.

Figure 6a: SEM Micrograph of TiO2 nanoparticles mediated by methanol at 450oC.

Figure 6b: SEM Micrograph of TiO2 nanoparticles mediated by ethanol at 450oC.

112 R. Vijayalakshmi and K.V. Rajendran

Figure 6c: SEM Micrograph of TiO2 nanoparticles mediated by iso-propanol at 450oC. Figure 6: SEM Micrograph of TiO2 nanoparticles prepared by different mediated solvents at 450oC.

Figure 7: PL Spectra of TiO2 nanoparticles prepared by methanol solvent at 450oC.

Synthesis of Nano-TiO2 by Sol-Gel Route: Effect of Solvent and Temperature 113

Table 1: Crystallite size, specific surface area for different solvent and calcination temperatures of TiO2 samples. SOLVENT PARTICLE SIZE AT DIFFERENT

TEMPERATURE (OC) SPECIFIC AREA (m2g-1)

Methanol Ethanol Iso-propanol

450oC 550oC 650oC 450oC 550oC 650oC

10

13

15

17

20

22

26

29

31

143.88

110.68

95.92

84.36

71.94

65.4

55.34

49.62

46.4

Photocatalytic Activity of Anatase TiO2 Nanoparticle: Fig.8a showed the temporal UV-Vis absorption spectra of methyl orange on anatase TiO2 (calcined at 450oC) catalyst under visible light irradiation. In comparison with UV-Vis characteristic absorption peak of the initial methyl orange solution, the apparent decrease of absorption intensity indicated the photocatalytic capability of the catalyst to degrade methyl orange Cheng et.al 2008. Fig. 8b shows the rate constant of degradation as a function of time. As observed in the graph, increasing the time result in decrease the degradation of Methyl orange and hence enhanced photocatalytic activity. The photocatalytic efficiency of our sample was found to be around 65% [18].

Figure 8a: UV-Vis absorption spectra of methyl orange on TiO2 catalyst under light irradiation.

Figure 8b: The Curve of Co/C versus time.

114 R. Vijayalakshmi and K.V. Rajendran

Conclusion Nanocrystalline anatase TiO2 has been successfully prepared by the sol-gel method. From XRD a pure anatase phase powders are obtained at 450oC with grain size of 10 nm and specific surface area 143.88m2/g at 450oC. As the calcination, temperature increases the particle size also increases and rutile phase was found at 650oC. The UV spectra show that the absorption edge was shifted to a higher energy (Eg=3.75, 3.54 and 3.34eV) due to the decrease in particle size. The PL emission spectra revealed a blue emission band centered at 435nm, the emission might be related with oxygen vacancies. Seen from the SEM and TEM, the spherical and uniform distribution of nano-TiO2 particles with average size of 10 nm were obtained as calcined at 450oC. Photodegradation experiments indicated that the MeOr solution can be markedly degraded in the aqueous TiO2 suspension and the photocatalytic efficiency of our sample was found to be around 65%. The decomposition of organics in the sample was investigated using TGA. In the formation of polymerization, CH3OH has a small size and more active –OH, it can react with precursor more easily than the other solvents. So finally, we conclude that the methanol proved to be a better solvent compared to that of other solvents.

Acknowledgment The authors are grateful to the University Grand Commission for extending financial assistance to carryout this work.

References [1] Yang.S, Liu.Y, Guo.Y, Zhao.J, Xu.H, Wang.Z.Preparation of rutile Titania

nanocrystals by liquid method at room temperature. Materials chemistry and Physics 77 (2002) 501-506.

[2] Vadivel Murugan.A, Samuel.V, Ravi.V. Syuthesis of nanocrystalline anatase TiO2 by microwave hydrothermal method. 60 (2006) 479-480.

[3] Kaifeng Yu, Jingzhe Zhao, Yupeng Guo, Xuefeng Ding, Hari-Bala, Yanhua Liu, Zichen Wang. Sol–gel synthesis and hydrothermal processing of anatase nanocrystals from titanium n-butoxide Materials Letters, 59, (2005), 2515-2518.

[4] M. Sasani Ghamsari, A.R. Bahramia High transparent sol–gel derived nanostructured TiO2 thinfilm Materials Letters, 62, (2008), 361-364.

[5] Lianfeng Zhang, Tatsuo Kanki, Noriaki Sano, Atsushi Toyoda Development of TiO2 photocatalyst reaction for water purification Separation and Purification Technology, 31, (2003),105-110.

[6] Akira Fujishima, Xintong Zhang, Donald A. Tryk TiO2 photocatalysis and related surface phenomena Surface Science Reports, 63, (2008), 515-582.

[7] Jae-Hun Yang, Yang-Su Han, Jin-Ho Choy TiO2 thin-films on polymer substrates and their photocatalytic activity. Thin Solid Films, 495, (2006), 266-271.

Synthesis of Nano-TiO2 by Sol-Gel Route: Effect of Solvent and Temperature 115

[8] Juan Yang, Sen Mei, José M. F. Ferreira Hydrothermal processing of nanocrystalline anatase films from tetraethylammonium hydroxide peptized titania solsJournal of the European Ceramic Society, 24, (2004), 335-339.10

[9] Kaifeng Yu, Jingzhe Zhao, Yupeng Guo, Xuefeng Ding, Hari-Bala, Yanhua Liu, Zichen Wang Sol–gel synthesis and hydrothermal processing of anatase nanocrystals from titanium n-butoxide Materials Letters, 59 2005,2515-2518.

[10] J.N. Hart, Y.B. Cheng, G.P. Simon, L. Spiccia Challenges of producing TiO2

films by microwave heating Surface and Coatings Technology, 198, 2005, 20-23.

[11] Anli Yang, Zuolin Cui ZnO layer and tubular structures synthesized by a simple chemical solution route. MaterialsLetters, 60,2006,2403-2405

[12] Liqiang.J, Xiaojun.S, Baifu.X, Baigi.W, Weimin.C, Honggang.F. The preparation and characterization of La doped TiO2 nanoparticles and their photocatalytic activity. Journal of solid-state chemistry 177 :2004:3375-3382.

[13] Sahni.S, Bhaskar Reddy.S, Murty.B.S. Influence of process parameters on the synthesis of nano-titania by sol-gel route. Materials science and Engineering A 452-453:2007:758-762.

[14] Qiu.S, Kalita.S. Synthesis, processing and characterization of nanocrystalline titanium dioxide. MAterials science and Engineering A. 435-436:2006:327-332.

[15] Zhao.Y, Li.C, Liu.X, Gu.F, Jiang.H, Shao.W, Zhang.L, He.Y. Syntheis and optical properties of TiO2 nanoparfticles. Materials Letters. 61:2007:79-83.

[16] Liqiang.J, Yichun.Q, Baigi.W, Shudan.L, Baojiang.J, Libin.Y, Wei.F,Honggang.F, Jiazhang.S. REview of photoluminescence performance of nanosized semiconductor materials and its relationships with photocatalytic activity.Solar energy materials & solar cells. 90:2006:1773-1787.

[17] Cheng.P, Deng.C, Gu.M, Dai.X. Effect of urea on the photoactivity of titani powder prepared by sol-gel method. Materials Chemistry nd Physics. 107:2008: 77-81.

116 R. Vijayalakshmi and K.V. Rajendran