Scrutinizing particle size related bond strengthening in anatase TiO2

A series of small, middle, and large anatase TiO2 particles were synthesized through the hydrolysis of titanium tetraisopropoxide (TTIP) to investigate the size-related chemical bond length and strength variation. Unit cell volume contraction with decreasing particle size is identified from Rietveld refinement of high-resolution synchrotron powder X-ray diffraction (PXRD) patterns. More titanium vacancies are also found for smaller anatase particles. Contrary to the variation in unit cell volume, a larger Debye temperature Theta(D)(TiO2) derived from the linear and nonlinear fitting of atomic displacement parameters (U-iso(TiO2)) as a function of temperature is revealed for smaller anatase particles. The length of the Ti-O bond is also shorter for smaller anatase particles. Furthermore, optical phonon frequencies blue-shifting with the decrease in anatase particle size are determined by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis rules out the presence of a large amount of Ti3+, while optical diffuse reflectance measurement eliminates the existence of a large number of oxygen vacancies in all particles. Combining the analysis results of PXRD, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR), more structural and surface hydroxyls (-OH) appear to exist in smaller anatase particles. It is the structural and surface -OH that are responsible for the size-related chemical bond length and strength variation in the as-synthesized anatase particles.