Redox-driven green synthesis of silver nanoparticle/cellulose nanofiber composites using high-pressure wet-type jet-milling

Abstract

A sustainable method for efficiently producing and depositing silver nanoparticles (AgNPs) onto cellulose nanofibers (CNFs) was developed in this study by harnessing high-pressure wet-type jet-milling-induced redox reactions. An aqueous solution of silver nitrate was mixed with a CNF suspension and subsequently subjected to 0–4 jet-milling passes at a discharge pressure of 200 MPa. Additionally, the influence of the pipe fitting material on the formation of nanoparticles (NPs) was evaluated by using either 304 stainless steel (SUS304) or brass pipe fittings between the feed tank and the collision chamber. The X-ray diffraction pattern of the sample prepared using brass fittings and subjected to one jet-milling pass exhibited distinct peaks corresponding to metallic silver, confirming the presence of AgNPs, in addition to those from cellulose crystallites. In contrast, the sample prepared using SUS304 fittings exhibited only peaks corresponding to cellulose crystallites, with no detectable peaks attributable to metallic silver. Transmission electron microscopy revealed that the CNF surface of the sample processed with brass fittings were uniformly and densely covered with spherical AgNPs (diameters <5 nm), and that the particle size distribution was remarkably narrow. In contrast, the quantity of NPs prepared using SUS304 fittings was insufficient to achieve uniform coverage, with deposition confined to isolated regions on the CNF surface. Furthermore, the larger amount of AgNPs deposited with the brass fittings, as compared to the SUS304 fittings, proportionally increased with the number of jet-milling passes. This enhanced deposition is attributed to the presence of copper and zinc in the brass fittings, which likely promote the redox reaction-induced reduction of silver ions. By synergistically coupling the intense cavitation generated during high-pressure wet-type jet-milling with redox activity derived from brass fittings, the reduction of Ag(I) to Ag was markedly enhanced, enabling dense and uniform deposition of AgNPs on the CNF surface.