Raman study of the size effect and the non-stoichiometry effect on the structure of TiO2

Abstract. TiO2 nanocrystal powders have been prepared by sol-gel routes. By controlling pH, TiO2 nanoparticles of sizes ranging from 7 to 14 nm can be synthesized. X-ray diffraction (XRD) analysis indicated that both the anatase and the rutile phase appears in the powder when pH < 2. At this pH, a single anatase phase is obtained. These results are clarified by SEM and Raman spectroscopy. Moreover, the Raman spectroscopy was used to discuss the size effect and non-stoichiometry effect based on the blue shift and broadening of the lowest-frequency Eg 144 cm−1 Raman mode.

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JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-0029 Mathematical and Physical Sci., 2015, Vol. 60, No. 7, pp. 35-40 This paper is available online at RAMAN STUDY OF THE SIZE EFFECT AND THE NON-STOICHIOMETRY EFFECT ON THE STRUCTURE OF TiO2 Nguyen Cao Khang and Nguyen Van Minh Faculty of Physics, Hanoi National University of Education Abstract. TiO2 nanocrystal powders have been prepared by sol-gel routes. By controlling pH, TiO2 nanoparticles of sizes ranging from 7 to 14 nm can be synthesized. X-ray diffraction (XRD) analysis indicated that both the anatase and the rutile phase appears in the powder when pH < 2. At this pH, a single anatase phase is obtained. These results are clarified by SEM and Raman spectroscopy. Moreover, the Raman spectroscopy was used to discuss the size effect and non-stoichiometry effect based on the blue shift and broadening of the lowest-frequency Eg 144 cm−1 Raman mode. Keywords: Raman spectroscopy, size efect, non-stoichiometry. 1. Introduction Titanium oxide (TiO2) is a functional material for which there are several technological applications strongly related to its crystalline structure and nanocrystal size and morphology [1-5]. It has one stable phase, the rutile (tetragonal), and two metastable polymorph phases, the brookite (orthorhombic) and the anatase (tetragonal). Both metastable phases become the rutile (stable) when the material is submitted to temperatures above 700 ◦C (in a pure state with no additives). TiO2 is a versatile semiconductor oxide with potential applications as photocatalyst [6, 7], solar cell [8, 9] and gas sensor [10]. Furthermore, TiO2 nanocrystals are non-toxic compounds and can be a candidate for biological applications [11]. The applications of nanosized anatase TiO2 are primarily determined by its physicochemical properties such as crystalline structure, particle size, surface area, porosity and thermal stability. Proper control of these properties, especially size effect and non-stoichiometry depending on the preparation conditions of nanosized TiO2, represents some of the key issues in this area. Among the various techniques to characterize TiO2, Raman spectroscopy has certain unique advantages because it is very sensitive to nanocrystallinity of the anatase TiO2. The anatase phase is evident from the characteristic Raman modes at 144 (Eg), 403(B1g), 515(A1g, B1g) and 638 (Eg) cm−1. The changes in the Raman spectrum of nanocrystalline anatase, the phase most commonly synthesized at ambient conditions, have been interpreted as originating from the size effect [12], non-stoichiometry [13] or internal stress/surface tension effects [14]. Although the majority of published studies point out size effect as the main factor responsible for the changes Received September 10, 2015. Accepted October 26, 2015. Contact Nguyen Cao Khang, e-mail address: khangnc@hnue.edu.vn 35 Nguyen Cao Khang and Nguyen Van Minh observed in the Raman spectrum of nanocrystalline anatase, some researchers have interpreted their results favoring other factors, considering structural characteristics of nanopowders. The main purpose of this work is to clarify the role of size effect and non-stoichiometry on Raman spectra by simultaneously analysing the data of XRD, TEM and Raman spectroscopy measurements of TiO2 anatase prepared by controlled synthesis process. 2. Content 2.1. Experiment The synthesis of TiO2 nanocrystals is accomplished with a drop wise addition of 5 mL aliquot of Ti[OCH(CH3)2]4 dissolved in isopropyl alcohol (5 : 95) to 900 mL of doubly distilled water. By controlling the pH of the solution, TiO2 nanocrystals with different size can be synthesized. This solution was then evaporated for drying in a steam bath. The dry residue was then transferred to a horizontal muffle furnace and heated at 400 ◦C for 2 h to obtain the final sample. To study the effect of nonstoichiometry oxygen in TiO2, the gel obtained was also heated in O2, N2, and air, respectively. The structure of TiO2 samples were determined using X-ray diffractometer D5005 (Siemen) with CuKα radiation. The low resolution TEM images were taken on a JEOL 1200EX transmission electron microscope operated at 80 kV. High resolution transmission electron microscopy (HRTEM) images were obtained on a Tecnai F30 HRTEM machine operated under 300 kV. Measurements of Raman spectra were recorded using a T64000 Raman spectrometer (Jobin-Yvon). 2.2. Results Figure 1. SEM, TEM and HR-TEM images of TiO2 nanoparticles prepared in a solution with pH = 8, 7, 4, 3 and 2 36 Raman study of the size effect and the non-stoichiometry effect on the structure of TiO2 The crystalline morphology, particle size, and lattice spacing of the products were first investigated using SEM and HR-TEM. Figure 1 shows the SEM and TEM images of TiO2 nanoparticles prepared usi ng the sol-gel method. At a pH of 2 to 8, particle size was 7.6 nm to 20 nm. The results from Figure 1 imply that the pH value of the precursor solution is a decisive factor in controlling final particle size and shape, crystal phase and agglomeration. Figure 1 also shows a high-resolution TEM image of the as-synthesized TiO2 nanocrystallites calcined with pH = 2. It shows clear lattice fringes, indicating the established crystallinity of TiO2 crystallites. Since a crystallite can be defined by studying the orientation of the lattice fringes, one can see that the average crystallite size in the synthesized TiO2 powder is about 7 nm in diameter. Powder XRDwas performed to study the crystallographic structure of the samples. Figure 2 shows the XRD spectra of the prepared nanoparticle. The width of the peak becomes broader indicating a particle size decrease. This is in agreement with SEM and TEM images. At a pH of 2 to 8, single phase anatase samples are obtained with particle size of 7.6 nm to 18.2 nm, calculated using Scherrer’s equation (Figure 2b). Besides the anatase phase, when pH = 1.7, the rutile phase also appears in our samples when particle size was approximately 7.5 nm (Figure 2a). Figure 2. The XRD spectra of TiO2 particle prepared in a solution with pH = 1.7, 2, 3, 4, 7 and 8. The caption figure are the XRD of the sample with a range from 24 to 28◦ (a) and paticle size of the samples dependent on pH (b) Figure 3 shows the Raman spectra of the obtained anatase TiO2 powder in various sizes. Six Raman peaks at 144, 200, 398.48, 513, 519.54 and 643.74 cm−1. Those peaks were assigned to Eg, Eg, B1g, A1g, B1g, and Eg, respectively, as suggested in previous reports [15]. The positions and intensity of the six Raman active modes are also in good agreement with the reference values determined previously for anatase structure TiO2. At a pH of 2 to 8, the full widths at half maximum (FWHM) of the first Eg 144 cm−1 bandrange from 15 to 12 cm−1. The position of this mode for different TiO2 samples ranges between 145 and 147 cm−1, as can be seen in Figure 3b. 37 Nguyen Cao Khang and Nguyen Van Minh Figure 3. Raman spectra of TiO2 particle prepared in a solution with pH = 1.7, 2, 3, 4, 7 and 8 The caption figure are the full widths at half maximum (FWHM) of the first Eg 144 cm−1 (a) and the possion of the first Eg 144 cm−1 (b) samples dependent on paticle size Figure 4. Raman spectra of TiO2 particles prepared by heating Ti(OH)4 in N2, O2 and in air Figure 4. shows the Raman spectra of TiO2 nanoparticle prepared by heating Ti(OH)4 in N2, O2 and in air. For stoichiometry anatase crystals of 7 nm diameter, annealed in air or in oxygen at 400 ◦C, a FWHM of 17 cm−1 is estimated, whereas the width of the band in a larger crystal is about 8 cm−1. The Raman spectrum of a non-stoichiometry sample (annealed in nitrogen at 400 ◦C) is dominated by broad anatase features: the full widths at half maximum (FWHM) of the 144 cm−1 band is 19 cm−1, whereas the width of band in the sample annealed in air or in oxygen at 400 ◦C are about 8 cm−1. The band broadening is significant. The first Eg peaks of all 38 Raman study of the size effect and the non-stoichiometry effect on the structure of TiO2 samples heated in N2, O2 and in air are around 152 cm−1. It is clear that the main cause of the Raman band shifts to a higher wave number as particle size decreases. From the Raman spectra of nanophase TiO2 in Figure 4, it is clear that the oxygen non-stoichiometry can be assessed by Raman scattering. This technique is useful because both phases, anatase and rutile, are measurable and sensitive to the non-stoichiometry. Although there is not yet a theoretically justified functional dependence for the spectral features as a function of O/Ti ratio, the curves are smoothly monotonic and can yield a qualitative assessment of the non-stoichiometry of material. Comparing the Raman spectra in Figure 3, it is clear that the Raman bands shift towards a higher wave number and their intensities decrease as particle size decreases. The result in Figure 4 show that the broadening of the spectral peaks are due to the oxygen non-stoichiometry of the material. Thus, the observed shift is due to the effect of decreasing particle size on other properties of the nanoparticles. When particle size decreases to the nanometer scale, two effects on the vibrational properties of these materials might occur. First, a volume contraction occurs within the nanoparticles that is due to size-induced radial pressure, which leads to increases in the force constants as a result of the decreases in the interatomic distances. In vibrational transitions, the wave number varies approximately in proportion to k1/2, where k is the force constant. Consequently, the Raman bands shift towards a higher wave number due to the increasing force constants. Second, the contraction effect induces decreases in the vibrational amplitudes of the nearest neighbor bonds, which can be interpreted as a measure of the static disorder and thermal vibrational disorder of a material. This decrease in vibrational amplitude with decreasing particle size affects the intensity of the Raman bands. We conclude that the observed shift in the Raman spectra of TiO2 nanoparticles is due to the effect of smaller particle size on the force constants and vibrational amplitudes of the nearest neighbor bonds. 3. Conclusion We have presented the process for synthesis TiO2 nanomaterials. At a pH of 2 to 8, single phase anatase samples are obtained with particle size of 7.6 nm to 18.2 nm. The size effect was found to be the main cause of the blue shift of the main Raman peak in anatase nanocrystals and the main cause of the broadening due to the non-stoichiometry effect. 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