An Experimental and Theoretical Approach to Understanding the Surface Properties of One-Dimensional TiO2 Nanomaterials

The present research focuses on the comparative investigation of the acid base surface properties (the isoelectric point, pH(iep) and point of zero charge, pH(pzc)) of one-dimensional TiO2 nanomaterials. Different one-dimensional TiO2 nanomaterials, nanotubes (NTs) and nanowires (NWs) were prepared by an alkaline hydrothermal synthesis procedure. The structural properties of the synthesized TiO2 nanomaterials were investigated with high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The NWs and NTs were characterized using Raman and Fourier transform infrared (FT-IR) spectroscopy as well as Brunauer Emmett Teller (BET) measurements. Surface properties, i.e. pH(iep) and pH(pzc) of NWs and NTs were determined from electrokinetic measurements, potentiometric mass and electrolyte titrations. The relative acidity for the NWs is found to be in the interval 3 < pH(iep) < 4 in comparison with the NTs, with 4 < pH(iep) < 6. The observed differences in the relative acidity are correlated with differences in crystal structure of the studied nanomaterials and their resulting morphology. In addition, our results reveal a strong electrolyte effect on the characteristic points, pH(iep) and pH(pzc), especially the higher cation affinity for both TiO2 nanomaterials surfaces that has a significant effect on the pH of the system. Application of the multisite complexation (MUSIC) model yields a satisfactory description of the electrokinetic data and can explain observed salt effect.