Spatially Localized Synthesis of Metal Nanoclusters on Clay Nanotubes and Their Catalytic Performance

Halloysite nanotubes (HNTs) are highly stable aluminosilicate with a hollow tubular structure. It features a unique architectural effect from both interior and exterior surfaces. These special characteristics make HNT an interesting nano-object in many areas such as a vehicle for drug delivery or as a template for chemical reactions. In this work, HNT was used as a versatile template for the fabrication of a novel composite material. Specifically, thiolate ligand protected ultrasmall gold nanoclusters (AuNCs; diameter less than 3 nm), which exhibits molecular like properties such as highest occupied molecular orbital-lowest unoccupied molecular orbital transition and photoluminescence, were synthesized selectively within the interior and on the exterior surface of HNT (referred as AuNCs@HNT-In and AuNCs@HNT-Out, respectively). Such spatially selective localization of NCs was achieved by loading the intermediate Au(I)-thiolate complexes (formed during the synthesis of AuNCs) through electrostatic interaction, followed by their in situ mild reduction at moderate temperature. All these composite materials are highly stable at ambient conditions and luminescent under UV light. Finally, the catalytic efficiency of the composite materials was evaluated by the reduction reaction of 4-nitrophenol. At a similar experimental condition, AuNCs@HNT-Out showed the highest catalytic activity when compared with individual AuNCs and AuNCs@HNT-In, mainly because of the higher exposure of the active sites of NCs while being supported on the outermost surface of HNT. This clay-NC synergy may make these composites suited not only for catalysis but also for sensing, drug delivery, and environmental applications.