engleski

Raman spectroscopy of ball milled TiO2

The Raman spectroscopy is method to analyze different crystal structure of TiO2. We applied it in order to study structural changes during high-energy ball milling of TiO2 powder. TiO2 anatase (tetragonal structure, space group I4/amd = D4h19) was milled up to 10 hours using two different sets of vials and balls. The first set was made of wolfram carbide (WC) and the other of agate. In addition to structural changes, ball milling caused decrease of TiO2 particles to nanometric size. As a novel approach the low-frequency Raman spectroscopy was used for monitoring of diminution of the TiO2 particle and determination of nano sizes. The most intensive band of TiO2 anatase was shifted to higher frequency as an evidence of nanosized particles. The kinetics of milling and sequence of resulting structures depend on milling set. After five minutes of milling of anatase structure in WC set the new bands were detected in Raman spectrum. They were assigned as band of high-pressure TiO2(II) phase (a-PbO2 structure, space group Pbcn = D2h14). However, three hours of milling in WC set was needed for transformation from high-pressure phase to rutil (tetragonal structure, space group P42/mnm = D4h14). When milling was performed by agate set the anatase structure was transformed directly to rutil structure of TiO2. Raman spectra showed that transformation to rutil started after approximately 30 minutes of milling with agate set and it was completed after 10 hours of milling. Different behavior due to the different milling set was the consequence of different mass of balls, and therefore the different energy was transferred to TiO2 powder. Based on long time theoretical result of Lamb1, it was known that diameter of the particle is inverse proportional to the frequency of the lowest-energy spherical mode of a free particle corresponding to angular momentum l=0. In our work, the low-frequency Raman band was broad with maximum appeared in range from 10 to 30 cm-1 depending on milling time and milling set used. The broadness suggests the wide dispersion in nano-particle dimensions, while position of low-frequency band point at dimensions from 7 to 21 nano meters. Raman spectroscopy enabled us to monitor structural and dimensional changes caused by high-energy ball milling.

Raman; TiO2; ball-milling; phase transition; nano-particle

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