Tunable nanoporous metallic structures formed by hybrid laser deposition and chemical etching and their functionalization

Abstract

Nano-porous metals exhibit novel physical and chemical properties, showing a promising potential for scientific research and engineering applications such as catalyst, electrochemical catalysis, detecting and sensing etc. The nano-porous materials are normally made of noble metal films by chemical dealloying about several-hundred micrometers in thickness prepared by arc melting, magnetic spattering and electrodeposit, which limits their performance and potential applications. We report a novel approach to fabricate Cu-Mn nano-porous structures on any location of conventional metallic components by laser deposition hybrid with electrolyte dealloying. Cu-Mn alloy layers with specific composition were deposited by laser onto a mild steel substrate surface. The laser deposition parameters were optimized to achieve sound Cu-Mn layers free of cracks, pores or inclusion. The deposition dilution was specifically controlled to avoid the designed composition of the Cu-Mn alloy to be modified by the melted Fe from the substrate and thus to obtain a single solid-solution microstructure, as a first precondition for forming nano-porous structure by electrolyte dealloyibng. Laser deposition offers a flexible approach to deposit designed composition with designed microstructure and geometry onto any location. The microstructure of the as-deposited layer was further processed by laser remelting for grain-size refinement to meet the second precondition. The second-dendrite-arm-spacing (SDAS) of the as-deposited structure decreases with the increase of laser remelting speed. SDAS can be decreased to 1.6μm with a laser remelting speed up to 83mm/s. Electrolyte dealloying was applied on the as-deposited and remelted layer. The polarization curves of Cu-Mn alloy were analyzed in KNO3 and HCl electrolytes to figure out their electrochemical behavior. The Cu-Mn alloy shows selective dissolution characteristics in different electrolytes. Nanoporous Cu and nanoporous Mn structures can be formed with optimal electrochemically dealloying parameters in 0.1M HCl and 0.1M KNO3 electrolytes respectively. The nano porous structure is tunable by the laser parameters and the chemical etching conditions. Nanoporous Cu structure with pore size ranging from 30 to 50nm was achieved. The nano-Mn structure displays a ribbon-like morphology with ultrahigh roughness factor up to 900. Both Cu and Mn nanostructures exhibit good performance on Surface Enhanced Raman Scattering (SERS) and superhydrophobicity. We further used laser texturing to obtain a regular micro-structure on the laser deposited CuMn surface followed by selective chemical etching to obtain a random nanoporous structure on regular microstructure-a micro/nano hierarchical structure. The regular micro-scale roughness can be tuned from 5 μm to 30 μm Rz by laser texturing while the ligament width and spacing of the random nano porosity be 10-50 nm by selective chemical etching. This unique hierarchical structure demonstrates good photocatalytic performance.