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

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.
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Raman study of the size effect and the non stoichiometry effect on the structure of TiO2 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 http://stdb.hnue.edu.vn 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 technologicalapplications 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], solarcell [8, 9] and gas sensor [10]. Furthermore, TiO2 nanocrystals are non-toxic compounds andcan be a candidate for biological applications [11]. The applications of nanosized anatase TiO2are primarily determined by its physicochemical properties such as crystalline structure, particlesize, surface area, porosity and thermal stability. Proper control of these properties, especiallysize 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 uniqueadvantages because it is very sensitive to nanocrystallinity of the anatase TiO2 . The anatasephase 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 phasemost commonly synthesized at ambient conditions, have been interpreted as originating from thesize effect [12], non-stoichiometry [13] or internal stress/surface tension effects [14]. Although themajority of published studies point out size effect as the main factor responsible for the changesReceived 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 Minhobserved in the Raman spectrum of nanocrystalline anatase, some researchers have interpretedtheir 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-stoichiometryon Raman spectra by simultaneously analysing the data of XRD, TEM and Raman spectroscopymeasurements of TiO2 anatase prepared by controlled synthesis process.2. Content2.1. Experiment The synthesis of TiO2 nanocrystals is accomplished with a drop wise addition of 5 mLaliquot of Ti[OCH(CH3 )2 ]4 dissolved in isopropyl alcohol (5 : 95) to 900 mL of doubly distilledwater. By controlling the pH of the solution, TiO2 nanocrystals with different size can besynthesized. This solution was then evaporated for drying in a steam bath. The dry residue wasthen transferred to a horizontal muffle furnace and heated at 400 ◦ C for 2 h to obtain the finalsample. To study the effect of nonstoichiometry oxygen in TiO2 , the gel obtained was also heatedin 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 transmissionelectron 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 ...