Synthesis and nanostructural investigation of TiO 2 nanorods doped by SiO 2

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1 PRAMANA c Indian Academy of Sciences Vol. 78, No. 2 journal of February 2012 physics pp Synthesis and nanostructural investigation of TiO 2 nanorods doped by SiO 2 MRIAZIAN 1,2, and A BAHARI 2 1 Department of Chemistry, Tonekabon Branch, Islamic Azad University, Tonekabon, Iran 2 Department of Physics, University of Mazandaran, Babolsar, Iran Corresponding author. m.riazian@umz.ac.ir MS received 16 March 2011; revised 5 July 2011; accepted 15 July 2011 Abstract. TiO 2 Nano rods can be used as dye-sensitized solar cells, various sensors and photocatalysts. These nanorods are synthesized by a hydrothermal corrosion process in NaOH solution at 200 CusingTiO 2 powder as the source material. In the present work, the synthesis of TiO 2 nanorods in anatase, rutile and Ti 7 O 13 phases and synthesis of TiO 2 nanorods by incorporating SiO 2 dopant, using the sol gel method and alkaline corrosion are reported. The morphologies and crystal structures of the TiO 2 nanorods are characterized using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) study. The obtained results show not only an aggregation structure at high calcination temperatures with spherical particles but also Ti O Si bonds having four-fold coordination with oxygen in SiO 4. Keywords. Nanostructure; nanorods; TiO 2 ;SiO 2 dopant; sol gel method. PACS Nos Qa; 66; Introduction TiO 2 is an important compound that is used in many industrial applications related to photo-splitting of water [1], photocatalysis, [2] photovoltaic devices, [3] etc. It is known to have three natural polymorphs, i.e. rutile, anatase and brookite. Only anatase is generally accepted to have significant photocatalytic activity. The photocatalytic performance of this compound depends on the characteristics of the TiO 2 crystallites, such as the size and the surface area. Therefore, modification of its physical and chemical properties is of interest to researchers. One possible way to modify the property of TiO 2 crystallites is by adding a second semiconductor into the TiO 2 matrix. Recently, many researchers [1 5] demonstrate that mixed silicon metal oxides enhance the photocatalytic performance due to improved surface adsorption and increase in surface hydroxyl groups. SiO 2 has been incorporated into the TiO 2 matrix to enhance the photocatalytic process because of its high thermal stability and excellent mechanical strength. It can also help to create new catalytic DOI: /s y; epublication: 23 December

2 M Riazian and A Bahari sites due to the interaction between TiO 2 and SiO 2 [6] and to obtain a large surface area as well as a suitable porous structure [7]. Many studies dealing with TiO 2 doping with different metals and synthesis and characterization of one-dimensional (1D) nanostructures (nanowires, nanotubes, nanorods) have received considerable attention due to their unique properties and novel applications [8 11]. However, more investigations are still needed to study the synthesis processes and dopant effects on the photocatalytic semiconductor surface for determining the cooperation of the micro- and nanoscale hierarchical surface structures, the orientation of the crystal planes and the surface photosensitivity. Many methods have been successfully developed for fabricating 1D nanostructures, including vapour solid, vapour liquid solid and solution liquid solid, template-based synthetic approaches and laser ablation [12 14]. However, almost all these methods used either catalysts or physical templates, which unavoidably brought some contamination to the products. Therefore, one has to explore a new approach to synthesize 1D nanomaterials without using preformed templates or catalysts. The sol gel process is a versatile method to prepare ceramic materials. Currently, the sol gel process is employed quite often for the synthesis of nanosized catalytic materials. The incorporation of an active metal in the sol during the gelation stage allows the metal to have a direct interaction with support, thereby the material possesses special catalytic properties. In the present work, TiO 2 nanorod is synthesized using hydrolysis procedure in simple wet chemical approach of tetraisopropyl orthotitanate at 300, 600 and 900 C. Shortly after that, it is doped into the SiO 2 matrix. The morphologies and crystal structures of the TiO 2 nanorods are characterized using FE-SEM, Hitachi S-4160, AFM, Easy Scan 2 Flex (Switzerland) and XRD, GBC-MMA 007 (2000) with Cu K α1 radiation, λ = Å. The obtained results indicated that the nanorod properties depend on the preparation procedures and calcination temperature. 2. Experimental procedures and details The composition of the starting solution and the experimental conditions used for TiO 2 nanorods are listed in table 1. Figure 1 illustrates the preparation procedures. The precursors: tetraethoxysilane (TEOS, Merk, 99%), tetraisopropyl orthotitanate (TIOT, Merk, Table 1. Composition of starting solutions and experimental conditions for TiO 2 nanorod preparation. Sol gel method Sol gel method Stirring time used step Precursor Molar ratio (MR) (h) ph Alkoxide route 1 TIOT TIOT/EtOH/H 2 O = :0.022: TEOS TIOT/TEOS= Precipitation Dropwise with ammonia 320 Pramana J. Phys., Vol. 78, No. 2, February 2012

3 TiO 2 nanorods doped by SiO 2 Figure 1. Schematic flowchart illustrating the steps in the synthesis of TiO 2 nanorods. 98%), 0.1 N nitric acid (Merk, 65%), ethanol (Merk, 97%) and distilled water are used without further purification. The starting point for the synthesis of a targeted system is a solution prepared by mixing the precursors according to the molar ratio given in table 1. Precursors are chosen (TIOT, deionized water, ethanol). TEOS of ph = 4 is dissolved in 20 cm 3 deionized water and stirred at room temperature (RT) for 48 h to get a solution of ph = 5. Then, for precipitation, ammonia solution is added dropwise so that ph = 8, then centrifuged at 1500 rpm to gather the precipitate. The precipitate is heated at 50 C for 24 h. The formed powder can be made into nanorods by immersing the powder in 10 N NaOH solution in teflon balloon. Once more, the powders are gathered with centrifuge and are purified with 0.1 N nitric acid and distilled water to eliminate the Na ions. At last, the powders are dried at 50 C for 48 h in air and calcined at three different temperatures (300, 600 and 900 C). The effects of different calcination temperatures are studied with as-prepared, 300, 600 and 900 C samples. 3. Characterization of the TiO 2 nanorods 3.1 X-ray powder diffraction study XRD patterns are measured on an GBC-MMA 007 (2000) X-ray diffractometer. The diffractograms are recorded with (K α (Cu), Å, 0.02 step size where the speed is 10 /min) radiation over a 2θ range of Pramana J. Phys., Vol. 78, No. 2, February

4 M Riazian and A Bahari 3.2 Field-emission electron microscopy (FE-SEM) Field-emission electron microscopy (FE-SEM) (S-4160 Hitachi) is routinely used to investigate the morphology of the nanoparticles. 3.3 Atomic force microscopy AFM, Easy Scan 2 Flex (Switzerland), silicon tip is used. The measurements are made at 20 C and in a relative humidity of 45%. 3.4 FT-IR study FT-IR measurements are performed using a 1730 infrared Fourier transform spectrometer (Perkin-Elmer) using potassium bromide as the background. Figure 2. XRD patterns of pure TiO 2 nanorod obtained from (a) without hydrothermal treatment (as-prepared), and when (b) calcined at 300,(c) calcined at 600,(d) calcined at 900 C. 322 Pramana J. Phys., Vol. 78, No. 2, February 2012

5 TiO 2 nanorods doped by SiO 2 Figure 3. XRD patterns of SiO 2 -doped TiO 2 nanorod obtained (a) without hydrothermal treatment (as-prepared), and when (b) calcined at 300 C, (c) calcined at 600 C and (d) calcined at 900 C. 4. Results and discussion Crystallographic phases of the TiO 2 nanorods are investigated using XRD technique and the results are shown in figures 2 and 3. The characteristics of the XRD peaks are summarized in tables 2 and 3. As shown in these figures, different crystalline phases are formed with different hydrothermal treatments. The powders are obtained from gels after 2 h drying. The powders are calcined at 300, 600 and 900 C with 10 C/min gradient and annealed in the same conditions. Figures 2 and 3 also show amorphous structures for the as-prepared and 300 C samples due to the short-range ordering of the network (see also refs [11, 15 17]). Samples obtained from 600 C and 900 C annealing have a high degree of crystallinity. The grain size values are calculated from Scherrer equation: r = 0.9λ 2B cos θ, (1) where λ = nm and θ is the reflection angle. Pramana J. Phys., Vol. 78, No. 2, February

6 M Riazian and A Bahari Table 2. The 2θ angle, d-space, Miller indexes and grain sizes of pure TiO2 nanorod. Pure state Crystalline As-prepared Calcined at 300 C Calcined at 600 C Calcined at 900 C phase 2θ d-space (Å) Size (nm) 2θ d-space (Å) Size (nm) 2θ d-space (Å) Size (nm) 2θ d-space (Å) Size (nm) Anatase tetragonal a = Å c = Å Rutile tetragonal a = Å c = Å Ti2O3 hexagonal a = Å c = Å Ti8O15 hexagonal a = Å c = Å 324 Pramana J. Phys., Vol. 78, No. 2, February 2012

7 TiO 2 nanorods doped by SiO 2 Table 3. The 2θ angle, d-space, Miller indexes and grain size of SiO2-doped TiO2 nanorod. Pure state Crystalline As-prepared Calcined at 300 Calcined at 600 Calcined at 900 phase 2θ d-space (Å) Size (nm) 2θ d-space (Å) Size (nm) 2θ d-space (Å) Size (nm) 2θ d-space (Å) Size (nm) Anatase tetragonal a = Å c = Å Titanium silicide Orthorhombic a = Å b = Å c = Å SiO2 Tetragonal a = Å c = Å Pramana J. Phys., Vol. 78, No. 2, February

8 M Riazian and A Bahari Since the atomic radii of Si atom is smaller than Ti, the TiO 2 particle experiences a contraction and its crystalline growth is retarded due to the Si atom. 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 matrix which behaves as a neck and connects the TiO 2 particles [9,18]. Data in table 2 shows the influence of hydrothermal treatment (calcination) on the grain size of the anatase and tenorite phases. FE-SEM images of TiO 2 nanorods are shown in figures 4 and 5. As-prepared sample has spherical cover but contains many rods. It seems nanoparticles are formed in thin rope shape. In other images in figure 4 the rods are formed at high temperature, 600 C and 900 C. Figure 5 shows FE-SEM images of SiO 2 -doped TiO 2 nanorods. Spherical particles in the as-prepared state form nanorods with increasing calcination temperature. SiO 2 dopant affects the length and shape in such a way that it enlarges the length and separates the nanorods. The surface morphology of the SiO 2 -doped TiO 2 nanorods are shown in figures 6 and 7. As shown in these figures, the islands have quite compact shape with 2 to 3.5 μm length. It is clear that after that the islands will have as arm-like shapes. The growth seems to continue in a dendritic or irregular pattern while maintaining an arm width of about 6 μm. Two issues affect the rod structures: (1) Diffusion of adatoms into the matrix at higher concentration: a larger fraction of the deposited TiO 2 or SiO 2 impinges onto the existing Figure 4. FE-SEM images of pure TiO 2 nanorods (a) as-prepared and at different calcination temperatures (b) 300 C, (c) 600 C, (d) 900 C. 326 Pramana J. Phys., Vol. 78, No. 2, February 2012

9 TiO 2 nanorods doped by SiO 2 Figure 5. FE-SEM images of SiO 2 -doped TiO 2 nanorods (a) as-prepared and at different calcination temperatures (b) 300 C, (c) 600 C, (d) 900 C. islands. These adatoms can diffuse off of the first islands and condense at the step edge, thereby thickening the structure. (2) Formation of fractals, where nanoaggregates shapes have been formed: as the linear size of the islands become larger, a higher percentage of Figure 6. AFM images of pure TiO 2 nanorods in as-prepared condition (top) and congestion state (bottom) single nanoaggregate. Pramana J. Phys., Vol. 78, No. 2, February

10 M Riazian and A Bahari Figure 7. AFM images of SiO 2 -doped TiO 2 nanorods in as-prepared condition. TiO 2 or SiO 2 makes up fractal regions between the arms. TiO 2 or SiO 2 atoms in these regions tend to fill in and do not contribute to further radial growth. The radial growth, on the other hand, is slowed to the extent that the islands have not coalesced and can still be identified as individual entities. The dendritic shapes are due to kinetic limitation existing at room temperature, which can be concluded from their thermal instability [19] as shown in figures 8 and 9. The FTIR spectra of pure TiO 2 nanorods calcined at different temperature powder are recorded in the wave number range of cm 1 (figures 8 and 9). The Figure 8. FTIR spectra of pure TiO 2 nanorod calcined at different temperatures. 328 Pramana J. Phys., Vol. 78, No. 2, February 2012

11 TiO 2 nanorods doped by SiO 2 Figure 9. FTIR spectra of SiO 2 -doped TiO 2 nanorods calcined at different temperatures. polytitanosiloxane-composite of each sample shows fundamental vibration modes. In the prepared gel, the 3200 cm 1 band has to be attributed to hydroxyl groups from water and ethanol, which are occluded in the titania pore. The OH bending band of water in the gel is observed at 1650 cm 1 and at the low-energy interval the Ti O bands are found at 1061 and below 1000 cm 1. The IR spectra show peaks characteristic of OH at the surface (3225 cm 1 ), molecular water (1621 cm 1 ), Ti O ( cm 1 ) and Ti O Ti ( cm 1 ). However the absorption peaks at 1090 cm 1 and 1010 cm 1 indicate the Si O Si linkage in the SiO 2 -doped TiO 2 nanorod. The absorption peak of Si O Ti linkage is 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 surface passivation technique that hinders the recombination processes and improves the overall efficiency of light-to-electricity conversion [8]. Pramana J. Phys., Vol. 78, No. 2, February

12 M Riazian and A Bahari When the composite is calcined at 900 C, the high-energy stretching band almost fades and the 1600 and 3225 cm 1 bending vibration band intensities decrease due to the vapourization of the liquid. In the low-energy interval, the band at 550 cm 1 is due to stretching vibrations of Ti O band and the broad bands between 800 and 1400 cm 1 are attributed to the lattice vibrations of titanium oxide. It should be noted that no traces of solvents such as ethanol and water have been observed in the calcined oxide composite. 5. Conclusion The homogeneous hydrolysis of metal alkoxide and corrosion with NaOH provided an excellent technique to prepare TiO 2 nanorods. Experimental results indicated that the homogeneous hydrolysis of tetraisopropys orthotitanate via sol gel route is a promising technique for preparing photosensitive materials with uniform nanoparticles. In this study, nanocrystalline TiO 2 nanorod particles have been successfully synthesised by chemical method and heat treatment process. Phase transformation of titanium dioxide depends on calcination temperatures. The addition of silica can effectively suppress the anatase rutile phase transformation which results in an increase of anatase rutile phase transformation temperatures. Average crystallite sizes increased with an increase in calcination temperatures in where an aggregate of spherical Ti nanoparticles and Ti O Si bonds with four-fold coordination are formed. Acknowledgement The authors thank Islamic Azad University, Tonekabon branch, for financial support through a research project. References [1] A Fujishima and K Honda, Nature 37, 238 (1972) [2] B O Regan and M GrGtzel, Nature 353, 737 (1991) [3] A L Linsebigler, G Lu and J T Yates, Chem. Rev. 95, 735 (1995) [4] Z Ding, X J Hu, G Q Lu, P L Yue and P F Greenfield, Langmuir 16, 6216 (2000) [5] Y Yin and A P Alivisatos, Nature 437, 664 (2005) [6] A Ennaoui, B R Sankapal, V Skryshevsky and M C Lux-Steiner, Sol. Energ. Mat. Sol Cells 90, 1533 (2006) [7] P Chang, M Zheng, Y Jin, Q Huang and M Gu, Mater. Lett. 60, 396 (2003) [8] N Iguchi, C Cady, R Snoeberger, B Hunter, E Sproviero, C Schmuttenmaer, R Crabtree, G Brudvig and V S Batista, Physical chemistry interfaces and nanomaterials VII, Proc. of SPIE 70340, 70340C (2008) [9] L Zhou, S Yan, B Tian, J Zhang and M Anpo, Mater. Lett. 60, 396 (2006) [10] T Sun, G Wang, L Feng, B Liu, Y Ma, L Jiang and D Zhu, Angew. Chem (2004); Angew.Chem.Int.Ed.43, 357 (2004) [11] A I Hochbaum, R Fan, R He and P Yang, Nano. Lett. 5, 457 (2005) [12] Y X Zhang, G H Li, Y X Jin, Y Zhang, J Zhang and L D Zhang, Chem. Phys. Lett. 365, 300 (2002) [13] R W Scott, M J Maclachlan and G A Ozin, Curr. Opin. (2003); Solid State Mater. Sci. 4, 113 (1999) 330 Pramana J. Phys., Vol. 78, No. 2, February 2012

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