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 | JOURNAL OF SCIENCE OF HNUE DOI Mathematical and Physical Sci. 2015 Vol. 60 No. 7 pp. 35-40 This paper is available online at http 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 lt 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 .