Underpotential Deposition-Mediated Layer-by-Layer Growth of Thin Films
Increase in catalytic activity for the oxygen reduction reaction (ORR) after coating palladium-on-carbon catalysts with four monolayers of platinum by underpotential deposition.
Platinum is a very good, albeit expensive, electrocatalyst. In order to increase the catalytic activity of an electrocatalyst per mass of noble metal (the mass activity), nanoparticles of less expensive materials are coated with atomically thin layers of platinum. First a monolayer of a sacrificial base metal such as copper or lead is deposited onto a core particle via underpotential deposition. Then the base metal is replaced by galvanic displacement with the catalytically active more noble metal. The coated composite can be used as an electrocatalyst in place of pure platinum, for example, in fuel cells.Description
This invention utilizes copper underpotential deposition (UPD) potential cycles to enhance the interlayer diffusion rate during a diffusion-controlled overpotential deposition of platinum on palladium nanoparticles, for example. The deposition starts at the potential near Cu bulk deposition where a full monolayer Cu covers the Pd surface. As the Cu monolayer is partially stripped in the positive potential sweep, Pt is deposited under diffusion control. When the potential sweep turns in negative direction, Cu UPD covers the deposited Pt preventing multiplayer growth before the completion of the first Pt layer. The role of Cu deposition/stripping is (1) to slow down Pt deposition rate and to enhance the Pt interlayer diffusion rate so that added Pt atoms in each potential cycle can settle down at the most energetically favored sites, which leads to layer by layer growth; (2) the current response to the potential cycles serves as an in situ monitor for the deposition processes.Benefits
This method allows deposition of smooth, atomically thin layers of noble metal catalysts, reducing the amount of precious metal used and so the cost of the catalysts.
Applications and Industries
Fuel cell catalysts; oxygen-reduction; heterogeneous catalysisMore Information
Wang, et al., “Overpotential deposition of Ag monolayer and bilayer on Au(1 1 1) mediated by Pb adlayer underpotential deposition/stripping cycles,” Surf. Sci., 540, 230-236 (2003). DOI: 10.1016/S0039-6028(03)00792-1Patents and Patent Applications
|Title and Abstract||
Underpotential deposition-mediated layer-by-layer growth of thin films
A method of depositing contiguous, conformal submonolayer-to-multilayer thin films with atomic-level control is described. The process involves the use of underpotential deposition of a first element to mediate the growth of a second material by overpotential deposition. Deposition occurs between a potential positive to the bulk deposition potential for the mediating element where a full monolayer of mediating element forms, and a potential which is less than, or only slightly greater than, the bulk deposition potential of the material to be deposited. By cycling the applied voltage between the bulk deposition potential for the mediating element and the material to be deposited, repeated desorption/adsorption of the mediating element during each potential cycle can be used to precisely control film growth on a layer-by-layer basis. This process is especially suitable for the formation of a catalytically active layer on core-shell particles for use in energy conversion devices such as fuel cells.
|Brookhaven National Laboratory||05/19/2015
|Technology ID||Development Stage||Availability||Published||Last Updated|