Effective activation of halloysite nanotubes by piranha solution for amine modification via silane coupling chemistry

The present work reports a novel modification methodology for halloysite nanotubes (HNTs) that includes two successive steps, i.e., activation by piranha solution and silanization reaction. A commercial silane coupling agent, 3-aminopropyltriethoxysilane (APS), was selected to modify the surface of HNTs. The presence of APS moieties on the HNT surface was characterized by the combination of Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen sorption. For the coupling reaction, the effect of reaction time, temperature, rehydration and APS concentration on the course of silanization degree was carefully investigated. The mechanism and the grafted product structure of the reaction between activated HNTs and APS were revealed through Si-29 solid-state NMR spectroscopy and XPS analysis. The result shows that piranha solution is an effective activation agent for silanization of HNTs. A higher reaction temperature (120 degrees C) contributed to a higher grafted amount compared with a lower temperature (70 degrees C). Moisture led to a higher degree of silanization. The grafted amount increased with APS concentration and leveled off at about 1%. Further increase in the APS concentration only led to a drastic decrease in grafting yield. The grafting reaction was confirmed by the presence of tridentate (T-3) and bidentate (T-2) bonded Si in Si-29 NMR. Free terminal amino groups and protonated amine groups were identified in modified HNTs by XPS.