Catalytically active silver nanoparticles loaded in the lumen of halloysite nanotubes via electrostatic interactions

Halloysites are a kind of aluminosilicate clay with a morphology of a nanotube. The inner wall of halloysite is positively charged, and the external surface is negatively charged. In this work, we propose a simple and facile method to prepare Ag NPs loaded in the lumen of halloysite nanotubes (HNTs). Herein, N-acetyl-l-cysteine modified silver nanoparticles (Ag NPs) with negative charges spontaneously and stably resided in the lumen of HNTs via electrostatic interactions. The images of transmission electron microscopy and scanning transmission electron microscopy showed that Ag NPs with a size of ~2.6 nm were uniformly distributed in the lumen of HNTs. The catalytic activity of the obtained Ag NPs/HNTs composites was evaluated by the reduction reaction of 4-nitrophenol (4-NP) as a model reaction. When the molar ratio of Ag and 4-NP was set at 0.008, the rate constant of the reaction was found to be 0.91 min⁻¹, two times higher than that of Ag NPs adsorbed on the external surface of HNTs. Additionally, no Ag NPs were found in the supernatant after the Ag NPs/HNTs suspension was stirred for 30 min. Such structural stability implies good reusability as a catalyst.