Fluorocarbon-Free Hierarchical Slippery Copper Surfaces via Solid-State Electrochemical Etching

Micro- and nanoscale copper patterns are essential for applications in flexible electronics, thermal management devices, and advanced surface-functional materials. However, the existing copper patterning techniques rely on complex, chemically intensive, and multistep processes, while solid-state electrochemical etching using polymer electrolyte membranes (PEM) has been limited to submicrometer-scale etching depths. In this study, a copper-ion-loaded PEM (CuSO₄ PEM) stamp is introduced to enable efficient and scalable solid-state electrochemical etching of copper. The CuSO₄ PEM exhibits markedly enhanced ionic conductivity and electrochemical stability compared with water-equilibrated PEM, enabling uniform through-etching of 10 μm thick copper foils and deep etching up to approximately 30 μm, achieved at 500 mV for 1 h under controlled high-humidity conditions (90%). Well-defined submillimeter-scale patterns, including line-and-space, honeycomb, and square-grid geometries, were transferred with high fidelity to produce transparent and conductive through-patterned copper foils. Using a hierarchical PEM stamp, 3D copper micro/nanohierarchical structures were fabricated, forming nanopillar arrays uniformly across the top surfaces and sidewalls of the microscale patterns. These hierarchical structures exhibit enhanced wetting control and enable the fabrication of slippery liquid-infused porous surfaces with pronounced direction-dependent droplet mobility without fluorinated surface treatment. Overall, this solid-state electrochemical etching approach reduces the environmental impact and processing cost, provides a simple route for high-resolution copper patterning, and shows high potential for applications in thermal management, microfluidic systems, transparent metal electrodes, and surface-functional interfaces.