Silica Nanotubes with Widely Adjustable Inner Diameter and Ordered Silicas with Ultralarge Cylindrical Mesopores Templated by Swollen Micelles of Mixed Pluronic Triblock Copolymers

Abstract

Mixtures of commercially available Pluronic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers combined with appropriate swelling agents are proposed as templates for silica nanotubes and ordered silicas with very large cylindrical mesopores. In particular, swollen micelles of Pluronic F127 (EO106PO70EO106) with long poly(ethylene oxide) (PEO) chains and low poly(propylene oxide) (PPO) content (30 wt %) mixed with a Pluronic with a higher PPO content (60–70 wt %) and short PEO chains were found at 11 °C to template two-dimensional hexagonal SBA-15 silicas with ultralarge (100) interplanar spacings up to ∼30 nm. Moreover, when the ratio of the silica precursor to the triblock copolymer mixture was appropriately lowered, silica nanotubes were readily obtained. While the addition of the high-PPO-content Pluronic facilitated the uptake of the swelling agent (toluene), allowing one to achieve very large inner tube diameters (∼35 nm), smaller diameters down to ∼10 nm were systematically generated using lower relative quantities of the swelling agent. As-synthesized (surfactant-containing) nanotubes of very large diameter had a tendency to flatten into ribbons and twist or bend, resulting in materials that had rather featureless pore size distributions after the surfactant removal. However, the degradation of these nanotubes was suppressed by increasing the hydrothermal treatment temperature (from 100 to 110–130 °C). The nanotubes exhibited high surface areas (424–911 m2/g) and large total pore volumes (∼2 cm3/g), in addition to the well-defined and widely tunable inner diameter (10–35 nm), even if they tended to cluster upon removal of the surfactant. It is envisioned that swollen mixed PEO-based surfactants will provide new templating opportunities for nanotubes of different compositions and for other nanoporous materials.