Electrochemical Generation of Metal Nanostructures Using Self-Assembled Monolayers As Templates
Authors: Yao Zhen , She Zhe , Di Falco Andrea , Buehl Michael , Buck Manfred 
Published: February 2020

Controlling electrochemical processes by molecular self-assemblies offers interesting perspectives for nanotechnology as precision and flexibility intrinsic to electrochemistry combines favourably with the range of possibilities for chemical, electronic, and structural modification of electrodes by molecular systems.
We exploit this combination for metal electrodeposition using a coordination-controlled deposition scheme. Based on the reduction of a two-dimensional (2D) layer of metal ions coordinated to the tail groups of a self-assembled monolayer (SAM) (1-3), it is extended by using the nanoparticles formed initially from the 2D layer as seeds for deposition from the bulk (3D) electrolyte. A number of opportunities for the generation of nanoscale deposits arise from this particular 2D/3D scheme which, depending on conditions, range from isolated nanoparticles to continuous layers on top of the SAM. Furthermore, by choosing different species coordinated to the SAM and present in the bulk electrolyte, bimetallic nanoparticles can be synthesized.
Exploring this coordination-controlled deposition scheme towards the generation of ultra-small metal structures, we investigate the combination of Pd and Cu, which is of interest for the generation of both electrocatalytically active PdCu nanoparticles and membranes for hydrogen separation (4). Deposition is accomplished via complexation of Pd2+ ions to a pyridine-terminated thiol SAM on a Au (111) electrode, followed by an electrochemical reduction in an acidic Cu2+-electrolyte. Cyclic voltammetry (CV), chronoamperometry, and X-ray photoelectron spectroscopy (XPS) reveal three phases, Pd nanoparticle formation, seeding of Cu deposition and formation of a Pd/Cu alloy, followed by the deposition of bulk Cu. To shed light on the initial stage of Pd deposition, density functional theory (DFT) calculations were performed.
Contrasting other SAM based deposition schemes which rely on defects in the monolayer (5,6), the coordination controlled deposition harnesses a molecular property, thus providing a better control over the deposition process. The generation of nanostructures by templated electrodeposition was investigated using either SAMs patterned by electron beam lithography or patterns of PdCu nanoparticles produced by selective removal with tips of a scanning tunneling or atomic force microscope. Metal layers with an average thickness of less than 3 nm and structures with lateral dimensions ranging from the micrometer to the sub-10 nanometer range can be formed with this technique.
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