Physical Properties Study of TiO2 Nanoparticle ...

Advanced Materials Research Vol. 772 (2013) pp 365-370 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.772.365

Online: 2013-09-04

Physical Properties Study of TiO2 Nanoparticle Synthesis Via Hydrothermal Method using TiO2 Microparticles as Precursor Mohd Hasmizam Razali1, a, Ahmad-Fauzi M. N1,b, Abdul Rahman Mohamed2,c and Srimala Sreekantan1,d 1

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 USM, Nibong Tebal, Pulau Pinang, Malaysia

2

School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 USM, Nibong Tebal, Pulau Pinang, Malaysia [email protected], [email protected], [email protected], [email protected]

Keywords: Nanomaterials, Photocatalyst, Hydrothermal

Abstract. Titanium dioxide (TiO2) nanoparticles were successfully synthesised by hydrothermal method using TiO2 microparticle powder (Merck) as precursor. TiO2 microparticles powder (~160 nm) was mixed with 10 M NaOH and treated hydrothermally at 150 °C and 2 MPa pressure in autoclave for 24 hours. After hydrothermal reaction was completed, the sample was washed, dried and heated at 500 °C for 2 hours to produce TiO2 nanoparticles. The synthesised nanoparticles were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and raman spectroscopy. UV-Vis DRS was used to determine the band gap energy. Field emissions and transmissions electron microscopy images revealed that nanoparticles obtained was about 14 nm. X-ray diffraction patterns showed that TiO2 nanoparticles were anatase phase (tetragonal). The band gap energy of TiO2 nanoparticles was determined to be 3.32 eV. Introduction The performance of TiO2 based photocatalyst is largely influenced by the size of the nanometric TiO2 building units because of the increased surface-to-volume ratio that facilitates reaction/interaction between them and the interacting media, which mainly occurs on the surface or at the interface [1]. In particular it has been found that TiO2 nanoparticles will enhanced the total photoreactivity of TiO2 due to their larger active surface area and capability to reduce the recombination rate of electron-hole pairs. Moreover, the size of the TiO2 particles is known to alter the width of the band gap and the band bending at the interfaces, and thus influence their photochemical properties.To beneﬁt from the nanoparticles speciﬁc properties described above, various technique had been applied to produce TiO2 nanoparticle such as sol–gel method [2], chemical vapor deposition [3], solvothermal process [4], reverse micelle method [5], and liquid phase deposition [6]. These techniques were classified as liquid process, whereas liquid titanium especially titanium alkoxide and titanium tetrachloride had been used as precursor. Production of TiO2 nanoparticle using these techniques is difficult because of the preparation condition such as precursor’s concentration, water and liquid titanium mixture molar ratio and pH of prepared solution should be optimized [7]. Furthermore, the use of organic precursors can lead to a contamination of the product and needs to be avoided as far as possible for environmental reasons. On top of that, the issues of high cost of liquid titanium salts precursor had also limited the application of these techniques. Thus, in this study we reported the application of hydrothermal method to synthesized TiO 2 nanoparticles without using liquid titanium salts (alkoxides or chlorine) as precursors. Commercial TiO2 microparticle powder (Merck) had been used as a precursor.

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Experiments Preparation. 2 g of TiO2 precursor powder (Merck) was dispersed in 10 M NaOH (100 ml) and was subjected to hydrothermal treatment at 150°C for 24 hours in autoclave. When the reaction had completed, white solid was obtained and collected. The white solid was washed with 0.1 M HCl (200 ml) followed by distilled water until pH 7 of washing solution was obtained. Then, the white solid was separated and collected from solution and subsequently dried at 80°C for 24 h. After drying, the obtained powder (named as-synthesised sample) was then heated at 500oC in static air for 2 hours to produce TiO2 nanoparticles. Characterization Field emission scanning electron microscope (FESEM) and Transmission Electron Microscope (TEM) was used to investigate the morphology of the sample. FESEM micrographs and TEM micrographs were captured using a ZEISS SUPRATM 35VP FESEM and Philips CM12 TEM respectively. Raman spectra were analysed using RENISHAW Invia Raman microscope and recorded in the range of 100-1000 cm-1 wavenumber. X-Ray powder diffraction (XRD) analysis was performed using a Bruker D8 Diffractometer with Cu-Kα (λ = 1.54021 Å) and scans were performed in step of 0.2o/second over the range of 2θ from 20 up to 80 o. UV-Vis was carried out using Perkin Elmer Lambda UV-Vis spectrometer for band gap measurements. Results and discussion Fig. 1a shows the FESEM micrograph of TiO2 precursor, revealing agglomerated irregular shape particles. On the other hand spherical particles of the TiO2 precursors can be seen clearly in the TEM micrograph with the particle size being about 160 nm (microparticle) (Fig. 1b). While, figure 2 shows the FESEM and TEM micrographs of product obtained in this study. TEM micrographs of the samples revealed that the particles were about 14 nm (Fig. 2b) indicating the nanoparticle materials were produced. (a) (a)

(b) (b)

100 nm Fig. 1: (a) FESEM and (b) TEM micrographs of TiO2 precursor

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20 nm Fig. 2: (a) FESEM and (b) TEM micrographs of TiO2 nanoparticle The mechanism of the nanoparticles formation could be proposed as followed. During hydrothermal treatment, the spherical titanium dioxides (TiO2) precursor reacts with NaOH forming highly disordered phase of Na2Ti3O7 which presence in the layered structure form [8]. TiO2 is an amphoteric oxide, it can react as an acid or base depending on the pH of the solution. Since, the reaction was carried out in 10 M NaOH (high pH~14 with high basicity), TiO2 acted as acid and react with NaOH (alkaline) to produce layered titanate of Na2Ti3O7 and H2O (Eq. 1) [9]. 3TiO2 + 2NaOH Na2Ti3O7 + H2O (Eq. 1) + Washing the Na2Ti3O7 with dilute HCl and distilled water promoted ion exchanged of Na by H+. Based on the density functional theory, sodium ions can be replaced by hydrogen ion although the Na2Ti3O7 structure is very stable. This is possible since the sodium ions are only weakly bonded to the negatively charged Ti3O72- layers. While the Na–O bond length in Na2Ti3O7 is above 2Å, the bond length of H–O in H2Ti3O7 is about 1Å. Therefore, the hydrogen ion exchange is irreversible. Furthermore elution strength of H+ is larger than Na+, therefore the ion exchange between H+ and Na+ is possible to occur. Ion exchanged process important in controlling the amount of Na+ ions remaining in the sample solution, thus influencing the bending of the layered titanate. Zhang et al. [9] stated that due to the imbalance of H+ and Na+ ion concentration on two different sides of the layered-like structure, giving rise to excess surface energy and thus resulting in bending of layered like structure to form nanotubes of H2Ti3O7 according to eq. 2. Na2Ti3O7 + 2HCl H2Ti3O7 + 2NaCl (Eq. 2) The FESEM micrographs of H2Ti3O7 (Fig. 3a) showed the presence of fiber-like structured with ±10 nm in diameter. Further observation with TEM exposed the existence of hollow inside the fiber-like structure, thus indicating the formation of nanotubes (Fig. 3b). The inner and outer diameters of nanotubes are about 4 nm and 10 nm respectively. (a)

(b)

20 nm Fig. 3: (a) FESEM and (b) TEM micrographs of H2Ti3O7 (as-synthesised sample) After heat treatment at 500°C for 2 hours, the tubular structures were broken into small particles (Fig. 3b) due to dehydration of inter-layered OH groups that destroyed the nanotubes structure [10]. It could be interpreted as decomposition of H2Ti3O7 nanotubes to produce TiO2 nanoparticle after heated at 500 oC for 2 hours (Eq. 3) [11].

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H2Ti3O7

3TiO2 + H2O

(Eq. 3)

Fig. 4 illustrates the XRD patterns of TiO2 microparticles precursor, as-synthesised sample and after heat treatment at 500 °C. The TiO2 precursor shows a series of sharp and narrow peaks, with the highest intensity at 25.2° (101) which is characteristic of TiO2 anatase phase. Meanwhile, XRD pattern of as-synthesised sample is identical to hydrogen trititanate, (H2Ti3O7) [12] and after heat treatment at 500 °C for 2 hours, the peaks which is assigned to TiO2 anatase was appeared. From the XRD analysis, it may be inferred that the composition and structure of the as-synthesised sample is very similar to layered protonic titanate with a general formula of H2Ti3O7. After calcinations at 500 o C for 2 hours, H2Ti3O7 decompose to produce TiO2 [Eq. 3] with anatase phase.

Fig. 4: XRD patterns of (a) TiO2 precursor (b) as synthesised sample and (c) after heat treatments at 500°C for 2 hours.

Fig. 5: Raman spectra of (a) TiO2 precursor (b) as synthesised sample and (c) after heat treatments at 500°C for 2 hours.

Further characterization was employed using raman spectroscopy. Structurally, the anatase-phase TiO2 is tetragonal, space group I41/amd [19]. The lattice contains two formula groups per unit cell and 10 optical phonons of symmetry A1g + A2u + 2B1g + B2u + 3Eg + 2Eu, of which A1g, 2B1g, and 3Eg are Raman active; A2u and 2Eu are IR-active; and B2u is inactive in both Raman and IR spectra [13]. In this study, fives peaks at 143 cm-1(Eg), 197 cm-1(Eg), 396 cm-1(B1g), 516 cm-1 (A1g/B1g) and 639 cm-1(Eg) appeared for the TiO2 precursor (Fig. 5a) due the external vibration of the anatase TiO2 [14]. Therefore it could be concluded that TiO2 precursor is an anatase phase, which is in agreement with the XRD result. As-synthesised sample showed different spectrum than TiO2 anatase pattern, where only 3 peaks at about 274 cm-1, 451 cm-1 and 664 cm-1 were observed (Fig. 5b). The broad peak at 274 cm-1 is assigned to the characteristic phonon mode of titanate nanotubes structures [15] and the peak at 451 cm-1 belong to the Ti-O bending vibration involving six-coordinated titanium atoms and three coordinated oxygen atom [14]. On the other hand, the peak at 664 cm-1 is due to the Ti-O-H vibration [15]. After heat treatment at 500 oC for 2 hours (Fig. 5c), similar raman spectrum with TiO2 precursor was observed indicating that the product is TiO2 with anatase phase. The band gap energy for both samples was also evaluated. In this study, the band gap energy of titanium dioxide starting material was determined to be 3.20 eV (Fig. 6) based on the reflectance spectra in fig. 6b, being similar with the band gap value that was reported in the literature for pure

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TiO2 anatase [16]. The titanium dioxide nanoparticles on the other hand, show slightly larger in their band gap energy (~3.32 eV) (Fig. 7b), and shifted to the blue region wavelength in reflectance spectra (Fig. 7a). This is attributed to the size quantization in nanoparticle materials. In nanosize materials, the effect of size quantization was due to localization of electrons and positive holes in a confined volume of the materials [17]. This will result in a change of energy band structure, due to the separation of individual energy levels and an increase in effective optical band gap of the nanomaterials as compared with bulk materials.

(a) (b)

Fig. 6: (a) Reflection spectra and (b) energy band gap of titanium dioxide precursor 80

(a)

(b)

R(%)

60 40 20 0 350

400

450

500

550

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Wave lenght (nm) Fig. 7: (a) Reflection spectra and (b) energy band gap of titanium dioxide nanoparticles

Conclusion Titanium dioxide nanoparticles were synthesized by hydrothermal method using TiO2 microparticle powder (Merck) as precursor. It was obtained after heat treatment at 500 °C for 2 hours of as-synthesized sample, which is assigned as hydrogen titanate H2Ti3O7 by XRD and raman analysis. XRD and raman result showed that TiO2 nanoparticles were in anatase phase. The average particle size obtained was about 14 nm, which is in good agreement with SEM and TEM results. The band gap energy of titanium dioxide nanoparticles is 3.32 eV, slightly larger than TiO2 precursor (microparticles powder) due to the quantum size effect. Acknowledgements We are grateful to Universiti Sains Malaysia (USM) for providing the facilities to carry out this project. We would also like to thank Universiti Malaysia Terengganu (UMT) and Ministry of Higher Education of Malaysia (MOHE) for financial support.

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Physical Properties Study of TiO2 Nanoparticle Synthesis via Hydrothermal Method Using TiO2 Microparticles as Precursor 10.4028/www.scientific.net/AMR.772.365 DOI References [16] D.S. Bhatkhande, V.G. Pangarkar, A. A. Beenackers: Journal of Chemical Technology and Biotechnology Vol. 77 (2001), p.102. http://dx.doi.org/10.1002/jctb.532 [15] Z. Tang, L. Zhou, L. Yang, and F. Wang: Journal of Alloys and Compounds Vol. 481 (2009), p.704. http://dx.doi.org/10.1016/j.jallcom.2009.03.077 [14] H. Berger, H. Tang, F. Levy: J. Cryst. Growth Vol. 130 (1993), p.108. http://dx.doi.org/10.1016/0022-0248(93)90842-K [12] M. Qamar, C. R. Yoon, H.J. Oh, N. H. Lee, K. Park, D. H. Kim, K. S. Lee, W. J. Lee, S.J. Kim: Catalysis Today Vol. 131 (2008), p.3. http://dx.doi.org/10.1016/j.cattod.2007.10.015 [11] Q. Chen, W. Zhou, G. Du, L. M. Peng: Advanced Materials Vol. 14 (2002), p.1208. http://dx.doi.org/10.1002/1521-4095(20020903)14:173.0.CO;2-0 [10] L. Chung-Kung, W. Cheng-Cai, L. Meng-Du, J. Lain-Chuen, L. Shin-Shou, H. Shui-Hung: Journal of Colloid and Interface Science Vol. 316 (2007), p.562. http://dx.doi.org/10.1016/j.jcis.2007.08.008 [9] S. Zhang, L. M. Peng, Q. Chen, G. H. Du, G. Dawson, W. Z. Zhou: Phys. Rev. Lett. Vol. 91 (2003), p.256103. http://dx.doi.org/10.1103/PhysRevLett.91.256103 [7] T. Sugimoto, X. Zhou, A. Muramatsu: J. Colloidal Interface Sci. Vol. 259 (2003), p.43. http://dx.doi.org/10.1016/S0021-9797(03)00036-5 [6] J. G. Yu, H.G. Yu, B. Cheng, X.J. Zhao, J.C. Yu, W.K. Ho: J. Phys. Chem. B Vol. 107 (2003), p.13871. http://dx.doi.org/10.1021/jp036158y [5] X. Sui, Y. Chu, S. Xing, M. Yu, C. Liu: Colloid Surface A Vol. 251 (2004), p.103. http://dx.doi.org/10.1016/j.colsurfa.2004.08.015 [4] C.S. Kim, B.K. Moon, J.H. Park, S.T. Chung, S.M. Son: J. Cryst. Growth. Vol. 254 (2003), p.405. http://dx.doi.org/10.1016/S0022-0248(03)01185-0 [3] T. Maekawa, K. Kurosaki, T. Tanaka, S. Yamanaka: Surf. Coat. Technol. Vol. 202 (2008), p.3076. http://dx.doi.org/10.1016/j.surfcoat.2007.11.017 [2] T. Sugimoto, X. Zhou, A. Muramatsu: J. Colloid Interface Sci. Vol. 259 (2003), p.53. http://dx.doi.org/10.1016/S0021-9797(03)00035-3 [1] D. P. Macwan, S. Chaturvedi, P. N. Dave: Journal of Material Sciences Vol. 46 (2011), p.3669. http://dx.doi.org/10.1007/s10853-011-5378-y