Size and phonon-confinement effects on low-frequency Raman mode of anatase TiO2 nanocrystal
Introduction
Recently anatase titanium dioxide (TiO2) nanocrystals have been attracted much attention owing to novel physical, chemical and electrical properties for various applications to electrochemical solar cells, rocking-chair lithium batteries, photocatalysis and so on [1], [2], [3], [4]. Among three natural phases including rutile and brookite, anatase phase is an important one. Anatase has tetragonal structure of space group with two formulas per unit cell and thus has six Raman active modes (). It was demonstrated that the blueshift and broadening of low-frequency mode (143 cm−1) observed in anatase TiO2 nanocrystals prepared with gas condensation technique were related to the oxygen stoichiometry instead of any internal stress or grain size effects [5]. Size-effects on the frequency and bandwidth of the mode in anatase nanocrystals derived from hydrolysis of tetrabutyl titanate and alkoxide were recorded to be ascribed to phonon confinement [6], [7], [8]. Besides conventional size-effects on the Raman frequency and bandwidth, size-effect on the phonon–phonon coupling is worthy to be examined. Temperature dependence of the Raman spectra show that phonon–phonon coupling was increased with decreasing the particle size in Si and CdSe nanocrystals [9], [10], but was not increased in CeO2 nanoparticles [11]. The size-effect on the phonon coupling in anatase TiO2 nanocrystals has not been discussed until now. In this Letter, we present Raman spectra of anatase TiO2 nanocrystals at temperature range 83–293 K and study in detail the size dependence and temperature dependence of the low-frequency mode. Not only we find an empirical formula for describing the relationship between the particle size and both frequency shift and line width due to phonon confinement, but also we obtain the size dependence of three-phonon anharmonic processes in anatase TiO2 nanocrystal derived from hydrolysis of tetrabutyl titanate.
Section snippets
Experiment details
Titanium dioxide (TiO2) nanocrystals of anatase phase were synthesized by hydrolysis method [8]. An ethanol solution of tetrabutyl titanate, Ti(OC4H9)4, was slowly added into deionized water which was vigorously stirred. The resulting titania gel was washed with deionized water, followed by evaporation in atmosphere at room temperature for a few days. Then as-prepared nanocrystal was annealed in air for 1 h at temperature range 373–923 K. The phase contents, the lattice parameters and the
Results and discussion
X-ray diffraction patterns of the TiO2 nanocrystals are shown in Fig. 1, where standard XRD pattern of anatase, brookite and rutile from JCPDS cards with relatively characteristic peaks are also plotted in order to identify diffraction lines. Annealed at the temperature below 873 K, anatase is main phase in our samples. Brookite content is about 30%wt in the as-prepared sample and decreases to near zero as annealing temperature is up to 773 K, at which rutile phase appears, and then reaches
Conclusion
Anatase TiO2 nanocrystal with particle size range 2.2–25.5 nm were prepared using a hydrolysis process of tetrabutyl titanate under heat treatment. The low-frequency mode shifts to higher frequency and asymmetrically broadens with a high-frequency shoulder as the particle size is decreased. The size dependence of the frequency, bandwidth and lineshape was calculated on the basis of the phonon confinement model with the theoretical phonon dispersion relationships. The results agree well with
Acknowledgments
This work was supported by the National Natural Science Foundation of China through Grants Nos. 10174034 and 10374047.
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