One-pot approach for acoustic directed assembly of metallic and composite microstructures by metal ion reduction

Acoustic-directed assembly is a modular and flexible bottom-up technique with the potential to pattern a wide range of materials. Standing acoustic waves have been previously employed for patterning preformed metal particles, however, direct patterning of metallic structures from precursors remains unexplored. Here, we investigate utilization of standing waves to exert control over chemical reaction products, while also exploring their potential in the formation of multi-layered and composite micro-structures. Concentric micro-structures of gold and silver were obtained by introducing a metal precursor salt and a reducing agent into a cylindrical piezoelectric resonator that also served as a reservoir. In addition, we introduce an innovative approach to directly fabricate metallic multi-layer and composite structures by reducing different metal ions or adding nanoparticles during the reduction step. We showcased our method through the fabrication of layered structures, incorporating a combination of gold and silver, as well as structures composed of silver and hematite nanoparticles. The produced structures were characterized through both surface and cross-sectional analyses, revealing the influence of the acoustic field on the mesostructure as well. This innovative approach is promising for production of complex microstructures with enhanced functionality and controlled properties.