Formation of palladium nanofilms using electrochemical atomic layer deposition (E-ALD)
Sheridan, Leah Bullard
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This dissertation discusses the layer-by-layer formation of Pd nanofilms on polycrystalline Au and Au(111) electrodes by electrochemical atomic layer deposition (E-ALD) using surface limited redox replacement (SLRR). E-ALD is a deposition technique that relies on surface limited reactions to form smooth, conformal films with controlled thickness. In SLRR, underpotential deposition (UPD) is used to form an atomic layer of one metal to act as a sacrificial layer, which is subsequently exchanged at open circuit for a more noble metal. In these studies, an atomic layer of Cu or Pb was deposited by UPD, followed by galvanic exchange in a Pd ion solution, resulting in a layer of Pd. That cycle was then repeated until achieving the desired film thickness. Films were characterized using coulometry, electron probe microanalysis (EPMA), cyclic voltammetry (CV), X-ray diffraction (XRD) and in-situ scanning tunneling microscopy (STM). The addition of Pd2+ complexing agents and their effect on the resulting deposits was studied. EPMA results showed improved film uniformity and suggested an SLRR mechanism dominated by indirect electron transfer from the CuUPD atom to the Pd ion through the electrode, rather than a direct electron transfer from atom to ion. The optimized Pd nanofilms on Au(111) were used as a platform for studying the hydrogen sorption (adsorption and absorption) and desorption properties of Pd as a function of film thickness. CV revealed that hydrogen adsorption is strongly influenced by the underlying Au, as well as film thickness for deposits below 5 cycles. No hydrogen absorption occurs for thinner films, while it increases with the number of E-ALD cycles for films formed with 3 cycles or more. Since Pd is often used as a material in hydrogen storage and sensing devices, the kinetics for hydrogen sorption/desorption from Pd are of critical importance. Varying amounts of Rh have been deposited on Pd nanofilms by direct electrodeposition and E-ALD. CV of these films indicates enhanced kinetics for hydrogen absorption and desorption in the presence of Rh, with the greatest enhancement observed for Rh deposited at 0 V for 60 s.