Effects of reducing agents on the continuous-flow supercritical hydrothermal synthesis of silver nanoparticles

In this work, we investigated the effects of reducing agents on the continuous-flow supercritical hydrothermal synthesis of Ag nanoparticles, with a focus on Ag-species conversion, particle-size evolution, and morphology development. In addition to comparing formic acid, sodium citrate, and acetic acid, we performed a no-reducing-agent control experiment to distinguish the contribution of added reducing agents from the thermal conversion of Ag-containing species in supercritical water. The experimental results suggest that Ag nanoparticle formation can be interpreted by a “rapid reduction/decomposition–burst nucleation–growth–stabilization” pathway, in which the identity and concentration of the reducing agent influence the particle size and size distribution. The no-reducing-agent control indicated that Ag nanoparticles can form through thermal conversion or decomposition under supercritical hydrothermal conditions. However, the addition of reducing agents accelerated Ag-species conversion and reduced the final particle size. Among the three reducing-agent systems, the formic acid system showed the fastest apparent Ag-species conversion and yielded the smallest particles, whereas the acetic acid system showed slower apparent conversion and produced larger particles; sodium citrate showed intermediate behavior. Increasing the reducing-agent concentration decreased the final particle size in all three systems. This trend is attributed to the combined effects of earlier supersaturation formation, enhanced nucleation, and inhibited subsequent growth in the reaction environment created by stronger reducing agents. Comparative kinetic analysis using an apparent pseudo-first-order model showed that the fitted apparent rate constants followed the order formic acid > sodium citrate > acetic acid. In addition, representative non-spherical morphologies were observed, and their formation was discussed in relation to seed structure, growth kinetics, and PVP interfacial adsorption. Overall, this work provides a systematic experimental basis for understanding the effects of reducing agents on the controllable synthesis of Ag nanoparticles in continuous-flow supercritical hydrothermal systems.