TiO2 Nanomaterials Studied by 44Ti(EC)44Sc Time Differential Perturbed Angular Correlations: Volume and Surface Properties

Abstract

TiO2 nanoparticles (anatase) with diameters between 2 and 4 nm were synthesized by controlled hydrolysis of a solution of titanium(IV)isopropoxide to which 44Ti in 4M HCl was added. Inactive nanoparticles were analyzed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), the active ones were analyzed by measuring the nuclear quadrupole interaction (NQI) of the I = 1 state in 44Sc using time differential perturbed angular correlation (TDPAC). Rather broad distributions were obtained. We also synthesized nanowires with typical diameters of 2 nm and 100 nm length using shape controllers. They were analyzed by HRTEM and XRD. The material turned out to be TiO2(B). The 44Ti was added by impregnation and diffusion at 180°C for two hours. Two well-defined NQI signals were observed which we tentatively assigned to the volume fraction and the “surface” fraction, i.e. Ti probes with OH-termination. In addition, we studied AMT-100 (anatase, uncoated, 6 nm) from Tayca, Eusolex T-2000 (rutile, Al2O3-coated, 20×20×100 nm3, simethicone additive) and P25 (mainly anatase, uncoated, 20 nm diameter) using the impregnation and diffusion method. P25 and the isolated rutile fraction from P25 yield spectra which correspond to anatase and rutile volume signals plus their surface signals, respectively. TDPAC thus proved very useful in characterizing the nanomaterials, especially their disorder, by measuring the NQI. In addition, information on surface properties is obtained. The relatively narrow surface signals indicate a lower degree of disorder and are possibly also a result of partial motional averaging of Ti-signals with OH-bonds due to mobile H-atoms.