Impact of mesoporous pore distribution on adsorption of methylene blue onto titania nanotubes in aqueous solution

This study investigates the liquid-phase adsorption behavior of methylene blue (MB) dye onto titania nanotubes, prepared by hydrothermal method, at different temperatures ranging from 30 °C to 50 °C. The nanotubes are mainly mesoporous, and their pore sizes were found to increase with calcination temperature. The maximum of MB adsorption capacity was found to be ca. 290 mg/g at 30 °C. The isosteric heat (qst) of adsorption was obtained from the adsorption isotherm measurements. The heat of adsorption at low coverage is in the range ∼10.1–13.5 kJ/mol, while at the monolayer completion, the value of qst displays a minima within 4.6–6.2 kJ/mol. The magnitude of qst confirms that the adsorption mechanism is physisorption. The variation of isosteric heat with surface coverage reflects that the titania nanotubes have a surface heterogeneity for adsorption of MB in liquid phase. The heat of adsorption of MB on the nanotubes is the decreasing function of the tubular size of the nanotubes. The difference of qst value can be attributed to two concepts: (i) in narrow pores, the concave pore walls would provide overlapping potentials at low coverage; (ii) at high coverage, capillary process easily takes place in the smaller pore. This result supports the usefulness of open-ended titania nanotubes for dye adsorption in liquid phase.