Rapid Adsorption Mechanism of Methylene Blue onto a Porous Mixed Ti-Nb Oxide

This study explores the incorporation of aqueous methylene blue (MB+) into a specially prepared metal oxide host.  Derived from the chemical exfoliation of KTiNbO5 into nanosheet colloids, the host material was synthesized in water using acid to restack the colloids into aggregates of nanosheets.  The metal oxide host has a large open pore, disordered, and turbostratic layered structure.  When exposed to aqueous solutions of MB+, within minutes the novel host rapidly intercalated MB+ to saturation, to produce an organic-inorganic composite with an MB+ loading of 226 mg/g.  Well-rinsed composites exhibited a deep purple color, indicative of the high internal content of MB+.  The MB+ loading was quantified using EDX and UV-Vis spectrophotometry.  Small-angle x-ray scattering (SAXS) measurements were carried out and analyzed using a unified exponential/power-law (UEP) model to describe the composite’s nanostructure.  SAXS analyses indicated that intercalated material is composed of two phases, each with different layer spacings for the restacked sheets.  Compared to transmission spectra of aqueous MB+, diffuse reflectance UV-Vis absorption spectra of composite revealed changes in the absorbance maxima of the intercalated MB+, indicating that the MB+ molecules were interacting strongly with each other and with the oxide host.  Raman and IR spectra also revealed significant host-guest interactions.  As determined by x-ray diffraction, the measured layer spacing between restacked nanosheets in the composite is consistent with a molecular orientation of MB+ standing on the end but tilted 40.4° away from the plane of the sheets.  Electron microscopy analysis showed that there were no significant morphological changes occurred in the porous host aggregates during the intercalation of MB+.  From an electrostatics evaluation, the new organic-inorganic composite materials were found to contain 40 % of the theoretical maximum of MB+, which resulted in an empirical formula of (MB)0.4H0.6TiNbO5.