Role of microporous Janus silica nanosheets in the assembly of ultra-small Ag nanoparticles with high catalytic activity

Abstract

As an emerging nano-silica material, two-dimensional (2D) silica nanosheets (SiNSs) have been derived from natural layered kaolinite and applied as a substrate for the highly efficient and dispersed assembly of functional materials, such as noble metal nanoparticles (NPs). In this work, the nature of SiNSs and its particular role in the assembly of ultra-small AgNPs via the reduction-growth method using a Sn(II) reductant were further researched. By adjusting the Sn(II) content x (1.2–6.0 wt%), it was found that the surface areas of the Sn(II)-activated SiNSs (xSn-SiNSs) had almost no change, and their reducibility did not fully increase with the increased x values, due to the saturated adsorption of the Sn(II) reductant by the surface hydroxyls of the SiNSs, which subsequently caused the decrease of the adsorbed Ag(I) precursor by the hydroxyls on the xSn-SiNSs (x ≥ 4.8 wt%). Accordingly, the sizes and loading amounts of the resultant AgNPs mainly showed a similar trend of increase before decrease. Furthermore, the regulated AgNPs with diverse mean sizes ranging from 1.71 to 2.16 nm were all ultra-small (more than half were nanoclusters < 2 nm) and highly dispersed, owing to the high electrostatic attraction of the negatively charged hydroxyls and the anchoring effect of the micropores on the hydroxylated surface of the Janus SiNSs. Therefore, the Ag/xSn-SiNSs nanocomposites displayed better catalytic properties for 4-nitrophenol reduction than most Ag-based supported catalysts, and the optimal Ag/2.4Sn-SiNSs catalyst exhibited quick reaction within 80 s and turnover frequency (TOF) of 3.34 min⁻¹. It reveals the key role of negatively charged surface hydroxyls and micropores of Janus SiNSs in the highly efficient and dispersed assembly of functional materials.