[Paper Review] Titanium dioxide synthesized using titanium chloride: Size effect study using Raman and Photoluminescence

Sanjeev K. Gupta, Rucha Desai+3 more|ArXiv.org|Apr 15, 2009

Advanced Nanomaterials in Catalysis1 references3 citations

TL;DR

This study investigates size-dependent optical and structural properties of anatase-phase TiO2 nanocrystals synthesized via a titanium chloride-based wet chemical method. Using Raman spectroscopy and photoluminescence (PL), the authors demonstrate that smaller nanoparticles (7 nm) exhibit enhanced PL intensity due to defects and quantum confinement, while a modified phonon confinement model successfully explains Raman peak shifts and broadening, confirming size-induced phonon confinement effects in TiO2 nanocrystals.

ABSTRACT

Titanium dioxide (TiO2) nanocrystals were prepared by wet chemical method and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering (RS) and photoluminescence (PL). The X-ray diffraction shows the formation of nanocrytalline TiO2 of average sizes 7 nm and 15 nm for two samples. The x-ray diffraction, transmission electron microscopy (TEM) and Raman scattering shows that the TiO2 nanocrystals has anatase crystal structure for both samples. The PL intensity of the smaller particle is more, which has been attributed to defects and particle size variation. A modified phonon confinement model with the inclusion of size distribution, a new confinement function for TiO2 nanocrystals and averaged dispersion curves for most dispersion phonon branch (G-X direction) has been used to interpret the size variation of Raman spectra. The obtained Raman peak shift and FWHM agree will the experimental data. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening for the Raman peaks.

Motivation & Objective

• To synthesize anatase-phase TiO2 nanocrystals using a titanium tetrachloride-based wet chemical method.
• To investigate the influence of particle size on the Raman and photoluminescence (PL) spectra of TiO2 nanocrystals.
• To develop and apply a modified phonon confinement model that accounts for size distribution and averaged phonon dispersion in TiO2 to interpret experimental Raman data.
• To correlate structural characteristics (crystallite size, phase) with optical response, particularly PL intensity and Raman peak broadening.

Proposed method

• Synthesis of TiO2 nanocrystals via a wet chemical method using titanium tetrachloride as the precursor.
• Characterization of crystal structure and morphology using X-ray diffraction (XRD) and transmission electron microscopy (TEM).
• Acquisition of Raman scattering (RS) and photoluminescence (PL) spectra to probe phonon confinement and defect-related emissions.
• Development of a modified phonon confinement model incorporating size distribution and averaged dispersion curves along the G-X direction in the Brillouin zone.
• Fitting of experimental Raman peak shifts and full width at half maximum (FWHM) values using the modified model to validate the theoretical framework.
• Comparison of PL intensity between 7 nm and 15 nm samples to assess the role of particle size and defects in radiative recombination.

Experimental results

Research questions

• RQ1 How does particle size influence the Raman spectral features of anatase-phase TiO2 nanocrystals synthesized from titanium tetrachloride?
• RQ2 What is the contribution of defects and quantum confinement to the photoluminescence intensity in TiO2 nanocrystals of different sizes?
• RQ3 To what extent can a modified phonon confinement model, including size distribution and averaged phonon dispersion, accurately predict Raman peak shifts and broadening in TiO2 nanocrystals?
• RQ4 How do the structural and optical properties of TiO2 nanocrystals differ between 7 nm and 15 nm samples in terms of crystallinity and defect density?

Key findings

• XRD and TEM analysis confirmed the formation of anatase-phase TiO2 nanocrystals with average sizes of 7 nm and 15 nm.
• The photoluminescence intensity was significantly higher in the 7 nm sample, attributed to increased defect density and quantum confinement effects.
• Raman spectra showed pronounced peak broadening and redshift for smaller particles, consistent with phonon confinement effects.
• The modified phonon confinement model, incorporating size distribution and averaged G-X phonon dispersion, successfully reproduced the experimental Raman peak shifts and full width at half maximum (FWHM) values.
• The observed Raman peak shifts and broadening were primarily due to phonon confinement, not lattice strain or disorder alone.
• The study confirms that size effects dominate the optical response in TiO2 nanocrystals, especially in the 7–15 nm range.