What's on the Surface? Silver Nanoparticle Fractal Assemblies and Their Impact on Catalysis

Hierarchical assembly of metal nanoparticles offers a powerful strategy to modulate catalytic performance, yet the interplay between nanoscale organization, surface accessibility, and facet-dependent reactivity remains poorly resolved. Here, we investigate how hierarchical organization of spherical silver nanoparticles governs catalytic performance by systematically correlating structure, facet distribution, and reaction kinetics. Bis(p-sulfonatophenyl) phenylphosphine (BSPP)-coated silver nanoparticles (AgNPs) were assembled into fractal (Ag fractals) and aggregate (Ag aggregate), and their catalytic activity was evaluated using sodium borohydride (NaBH₄)-mediated reduction of organic dyes. Brunauer-Emmett-Teller (BET) analysis shows a decrease in specific surface area from AgNPs (138.1 m² g^-1) to Ag fractals (32.5 m² g^-1) and Ag aggregate (14.0 m² g^-1). The corresponding rate constants for rhodamine B (RhB) reduction follow the same trend (0.422, 0.278, and 0.061 min^-1 for AgNPs, Ag fractals, and Ag aggregate, respectively). To probe the catalytic pathway, mechanistic studies were performed. Thiol-based surface passivation using cysteine and mPEG-SH (1 and 10 kDa) suppresses catalytic activity by >90%, while radical scavenging by p-benzoquinone (p-BQ) and metal ion chelation by ethylenediaminetetraacetic acid (EDTA) indicate a predominantly surface-mediated process involving electron transfer through radical intermediates, with minor contributions from dissolved Ag⁺ species. X-ray diffraction (XRD) reveals facet redistribution upon assembly, with Ag fractals enriched in stable {111} planes and Ag aggregates exhibiting broadened, low-intensity features indicative of structural heterogeneity, correlating with their lower catalytic activity relative to AgNPs. Together, these results show that catalytic performance in hierarchical silver assemblies is governed not solely by surface area but by the coupled effects of nanoscale organization and facet-dependent surface reactivity.