Applied Mechanics and Materials Vol. 624 (2014) pp 129-133 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.624.129
Effect of Annealing Treatment on Growth of Rutile TiO2 Nanorods Prepared by Chemical Bath Deposition Method on Silicon Substrate Abbas M. Selman1,2,a*, Z. Hassan1,b 1
Nano-Optoelectronics Research and Technology Laboratory (N.O.R.), School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia.
2
Department of Pharmacology and Toxicology, College of Pharmacy, University of Kufa, Najaf, Iraq. Email:
[email protected], b
[email protected]
Keywords: Rutile TiO2, Nanorods, CBD, Annealing temperature.
Abstract. Effects of annealing treatment on growth of rutile TiO2 nanorods on structural, morphological and optical properties of TiO2 nanorods were investigated. The nanorods were fabricated on p-type (111)-oriented silicon substrates and, all substrates were seeded with a TiO2 seed layer synthesized by radio-frequency reactive magnetron sputtering system. Chemical bath deposition (CBD) was carried out to grow rutile TiO2 nanorods on Si substrate at different annealing temperatures (350, 550, 750, and 950˚C). Raman spectroscopy, X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analyses showed the tetragonal rutile structure of the synthesized TiO2 nanorods. Optical properties were examined by photoluminescence spectroscopy. The spectra exhibit one strong UV emission peak which can be seen at around 390 nm for all of the samples. In the visible region, TiO2 demonstrated two dominant PL emissions centered at around 519 and 705 nm. The experimental results showed that the TiO2 nanorods annealed at 550˚C exhibited the optimal structural properties. Moreover, the CBD method enabled the formation of photosensitive, high-quality rutile TiO2 nanorods with few defects for future optoelectronic nanodevice applications. Introduction Titanium dioxide (TiO2) is a wide band gap (3.02 eV for rutile and 3.20 eV for anatase) semiconducting material; one-dimensional structures nanosized TiO2 including nanotubes, nanowires, nanonrods, and nanobelts has been extensively studied due to its large effective surface area. TiO2 exists in three main crystalline structures (i.e., anatase, rutile, and brookite) and each crystalline form exhibits different physicochemical properties [1], [2]. Among these structures, the rutile exhibits unique optoelectronic properties such as a high dielectric constant, high refractive index, chemical and physical stability, high hardness, excellent mechanical strength, transparency in visible region, and high ultraviolet (UV) ray absorption rate [3], [4]. Therefore, rutile TiO2 nanorods has been one of the most attractive materials for investigation during the last few decades because of its variety of applications [3], such as dye-sensitized solar cell [5] photovoltaic [6], gas sensing [7], photocatalysis [8], and UV detectors [9]. At present, although some techniques have been widely used to prepare nanostructures, such as sol–gel method [10], thermal evaporation [11], electrochemical etching [12], chemical vapor deposition [13], hydrothermal [14], template synthesis [15], the chemical bath deposition (CBD) is not only simple but also cost-saving among these methods[16]. Recently, Zhao et al. reported the synthesis method for single-crystalline rutile TiO2 nanorods on the fluorine doped tin oxide (FTO) substrate using the hydrothermal method, and they studied the influence of growth time and annealing on the structures were they employed to fabricate dye-sensitized solar cell (DSSC) [17]. Vishwas at el. studied the Influence of annealing temperature on Raman and photoluminescence spectra of TiO2 thin films that were deposited on fused quartz substrates by electron beam evaporated [18]. Xia at el. fabricated TiO2 nanostructures with high quality by direct annealing of the Ti foil with Pd catalyst [19]. In the present work, the effects of annealing treatment on the growth of rutile TiO2 nanorods deposited onto p-Si (111) substrate were examined. All substrates were seeded with a TiO2 seed layer synthesized by radio-frequency (RF)
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reactive magnetron sputtering. CBD was carried out to grow rutile TiO2 nanorods on the Si substrate. This process was simple and used low-cost instruments compared with other wet chemical methods. Therefore, more studies in this field particularly in nanostructure systems must be conducted. The surface morphology, crystal structure and other characteristics of the synthesized TiO2 nanorods were examined by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), photoluminescence (PL), and Raman spectroscopy. Experimental Details The rutile TiO2 nanorod synthesis has been described in our previous work [9]. In brief, CBD was used to synthesize the TiO2 NRs by heating an acidic solution of titanium (III) chloride containing the immersed seed layer (TiO2)-coated Si substrate. Titanium (III) chloride liquid was mixed with distilled water in appropriate quantities. Exactly 4 ml of TiCl3 (15 wt.% in HCl; Merck Sdn Bhd, Malaysia) was added to 50 ml distilled water. Urea (NH2CONH2; 0.1 M) was added to adjust the pH to approximately 0.7. After stirring at room temperature (27 °C) for 1 h, a homogeneous violet solution was obtained. The substrate was then vertically immersed in the aforementioned bath that was then heated at 55 °C. The substrate coated with TiO2 NRs were removed after 2 h, rinsed with distilled water, and dried with nitrogen gas. To improve crystallinity, the film was heat treated at different temperatures (350, 550, 750 and 950 °C) for 2 h in air. Results and Discussion The XRD patterns of the prepared TiO2 nanorods onto TiO2 seed layer-coated p-type (111)-oriented silicon substrates at different annealing temperature are depicted in Fig. 1. The scanning Bragg angle of the three samples ranged from 2θ = 20° to 80°. Distinct peaks were noted in the XRD patterns at 27.4°in all samples except for the sample prepared without annealing treatment. The temperature of annealing affected the degree of crystallinity. Compared with the other samples, a high crystallinity was found in the sample prepared at annealing temperature of 550°C, which had five diffraction peaks that correspond to the presence of the (110), (101),(200), (210) and (211) planes of TiO2 material. This result is in agreement with JCPDS card No.01-078-1508. The TiO2 NRs was polycrystalline and can be indexed as tetragonal rutile phase based on the XRD patterns.
Fig. 1 XRD patterns of rutile TiO2 nanorods grown on silicon (111) substrate at different annealing temperatures. Fig. 2 shows the FESEM images of the TiO2 nanostructures synthesized on p-type (111)-oriented silicon substrates with a TiO2 buffer layer using CBD at different annealing temperature. Fig. 2a (without annealing treatment) shows the uniform and high-density growth of nanorods growing almost vertically aligned to the substrate with a diameter of 20–22 nm and an average length of 80 nm. When the annealing temperature was increased to 350°C and 550°C a lot of flowerlike structures
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were observed in the samples. After magnification, we found that these flowerlike structures were composed of bundles of nanorods growing almost vertically aligned to the substrate with a diameter of 23-25, 26–28 nm, and an average length of 83 nm, 95 nm respectively (as shown in Fig.2b-c). A careful look at the image in Fig. 2(d), that shows the sample under 750°C annealing treatment, reveals that most of the rutile nanorods are randomly oriented growth surface and not perpendicular to the substrate surface, but lean toward the azimuthal direction with an angle of around 25°, also we found that the rod shape is destroyed with 950°C annealing process (as shown in Fig.2e). Comparing the images in Fig. 2, the TiO2 density in Fig. 2c was much higher.
Fig. 2 FESEM image of the rutile TiO2 nanorods grown on silicon (111) substrate at different annealing temperatures (a) without annealing, (b) 350°C, (c) 550 °C, (d) 750 °C and (6) 950 °C. Thus, the sample prepared at annealing temperature of 550°C had higher in length and diameter than the others; therefore, we conclude that different superstructures of rutile TiO2 nanorods can be obtained by controlling the annealing treatment. These results demonstrated that the TiO2 nanorods prepared at annealing temperature of 550°C had the optimal structural properties, i.e., larger and rougher surface areas, than the other samples. These larger and rougher surface areas of the TiO2 nanostructure might increase the efficiency of photo-scattering and improvement of the light-absorption [20] Moreover, The barrier effect of intercrystalline TiO2 is extremely reduced by usinglong nanorods instead of a porous TiO2 thin film composed of accumulated nanoparticles. This might contribute to the simpler electron transfer and reduce the ohmic loss through theTiO 2 layer in dye-sensitized solar cells [21]. Therefore, CBD method can be used to grow rutile TiO2 nanorods with few defects for future optoelectronic nanodevices applications. The Raman spectra of Si substrate and rutile TiO2 nanorods are shown in Fig. 3. Raman spectroscopy was performed to confirm further if the prepared TiO2 nanorods were in the rutile phase. Typical Raman bands of the rutile phase became clear at 143, 235, 447, and 612 cm-1, which can be attributed to B1g, two-phonon bands, Eg, and A1g modes, respectively [22]. All the position and the relative intensity of the observed Raman peaks are in agreement with the literature, where all peaks of crystalline rutile were observed for all prepared samples as shown in Table 1. The strong band at 520 cm−1 and a weak broad band near 970 cm-1 which resulted from Si substrate is in agreement with previous findings for Si substrate [23].
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Fig.3 Raman spectra of rutile TiO2 nanorods Fig.4 Photoluminescence spectra at room grown on silicon (111) substrate at different temperatureof rutile TiO2 nanorods grown on annealing temperatures. silicon (111) substrate at different annealing temperature. Table 1 Raman bands of prepared rutile TiO2 nanostructures. Annealing Raman bands of prepared rutile TiO2 nanostructures Temperature (°C) B1g (cm-1) two-phonon bands(*) (cm-1) Eg ( cm-1 ) A1g (cm-1) Without 151 296 431 615 350 No peak 232 436 609 550 143 233 445 610 750 145 302 439 610 950 132 305 No peak No peak PL emission spectroscopy is a good tool for characterizing the optical quality of semiconductor materials. This technique is used to study the transfer behavior of photogenerated charge carriers to understand their separation and recombination. In the present study, the optical properties of rutile TiO2 grown on p-type (111)-oriented silicon substrates were examined at room temperature by PL spectroscopy and the results are shown in Fig. 4. The spectra exhibit one strong UV emission peak which can be seen at around 390 nm for all of the samples except for the sample prepared without annealing, and can be attributed to the near-band-edge UV emission of the wide bandgap of TiO2, where the electrons in the valence band are transferred to the conduction band, and then the excited electrons by photoemission. In the visible region, TiO2 demonstrated two dominant PL emissions centered at around 519 and 705 nm (Fig.4). were stabilized These emissions were the radiative recombination of the self trapped excitons and the radiative transitions inside the substrate transitions inside the substrates initiated from the TiO2 surface [24].This result well agreed with those reported by Vishwas et al.[25] Conclusions High-quality rutile TiO2 NRs were grown on p-type Si (111) substrate via chemical bath deposition method. The effects of annealing treatment on rutile TiO2 nanorods grown on p-type (111)-oriented silicon substrates were investigated. The annealing temperature was found to affect the structural, morphological, and optical properties of TiO2 nanorods. The sample prepared at annealing temperature of 550°C had the optimal structural properties; therefore, we conclude that different superstructures of rutile TiO2 nanorods can be obtained by controlling the annealing treatment. Raman spectra depicted the rutile crystal phase of TiO2, and the highest PL UV intensity revealed the high optical quality of TiO2 nanorods with few defects. These films may have potential use in future optoelectronic nanodevices because of their interesting properties. Acknowledgements This work was supported by ERGS grant (203/PFIZIK/6730046), PRGS grant (1001/PFIZIK/846073) and Universiti Sains Malaysia.
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