Stanko R. Brankovic

Spontaneous deposition of Pt on the Ru(0001) surface

Spontaneous deposition of Pt on a Ru(0001) single crystal surface has been demonstrated by in situ scanning tunneling microscopy and linear sweep voltammetry techniques. The immersion of a ultra-high vacuum (UHV) prepared Ru single crystal in a platinum-ion-containing solution results in the formation of monolayer-to-multilayer Pt deposits without application of an external potential. The coverage and morphology of the Pt deposit depend on the concentration of platinum ions and the time of immersion. This simple method can be applied to Pt deposition on Ru nanoparticles, which can considerably reduce Pt loading in Pt/Ru electrocatalysts. The electrochemical behavior of such Pt/Ru(0001) bimetallic surfaces depends on the coverage of the Ru electrode and amount and morphology of the Pt deposit, which indicates a pronounced electronic modification of the Pt adlayer due to a strong Pt/Ru interaction.

Kinetic Characterization of PtRu Fuel Cell Anode Catalysts Made by Spontaneous Pt Deposition on Ru Nanoparticles

Hydrogen oxidation kinetics without and with trace amounts of CO in H2 were investigated for carbon-supported catalysts consisting of Pt submonolayers on Ru nanoparticles prepared by spontaneous deposition and commercial Pt, Ru, and PtRu alloy catalysts. Thin catalyst layers were deposited onto a glassy carbon rotating disk electrode without using Nafion film to stabilize them. Nonlinear fittings of the entire polarization curves at several rotation rates were used to determine the exchange current, the Tafel slope, and the Levich slope. To ensure full utilization of the catalyst, the mass-specific activity was determined by finding the minimum Pt loading needed to have all three kinetic parameters close to those found for a polycrystalline Pt electrode. For the PtRu20 , PtRu10 , and PtRu5 samples prepared by spontaneous deposition of 1/9 to 4/9 monolayer Pt on Ru, the minimum loading is 5 nmol/cm 2 (1mgPt /cm 2 ). This is only one-third of that for Pt or PtRu ~E-TEK! catalysts and only double the atomic density of aP t~111! surface, indicating that the high activity of Pt metal for hydrogen oxidation is retained when the atomic assemblies are reduced to submonolayer level on Ru. The enhanced CO tolerance was studied at low potentials by correlating the loss of the

Stoichiometry of Pt Submonolayer Deposition via Surface-Limited Redox Replacement Reaction

The work exploring the stoichiometry of Pt deposition via surface-limited redox replacement of the underpotentially deposited (UPD) Cu monolayer on Au(111) is presented. The Cu UPD monolayer is formed from 10 -3 M Cu 2+ + 0.1 M HClO 4 solution, whereas the Pt deposition via surface-limited redox replacement reaction is carried out in 10- 3 M {PtCl 6 } 2- + 0.1 M HClO 4 solution at open-circuit potential. Our results indicate that the Pt submonolayers have two-dimensional morphology and linear dependence of their coverage on the amount (coverage) of the replaced Cu UPD monolayers. Our analysis shows that the oxidation state of Cu during redox replacement reaction is 1+, suggesting that four Cu UPD adatoms are replaced by each deposited Pt adatom. This work stresses the general importance of the anions, determining the stoichiometry of metal deposition reaction via surface-limited redox replacement of the UPD monolayers.

Electrosorption and catalytic properties of bare and Pt modified single crystal and nanostructured Ru surfaces

The electrosorption and catalytic properties of bare and Pt modified Ru(0001) and Ru(10 − 10) single crystal surfaces and carbon supported Ru nanoparticles have been studied by electrochemical, surface X-ray scattering, scanning tunneling microscopy, Fourier transform infrared spectroscopy and high resolution transmission electron microscopy techniques. The electrochemical surface oxidation of Ru(0001) in H2SO4 is an one-electron process resulting in 1 monolayer oxygen uptake and the increased spacing between the top two Ru layers from 2.13 A at 0.1 V to 2.20 A at 1.0 V. About 1/3 monolayer of bisulfate anions are coadsorbed with hydronium cations at low potentials. In HClO4 solution, the adsorption process at 0.1 V is due to the surface oxidation apparently to RuOH rather than to hydrogen adsorption. The oxidation of Ru(10 − 10) is quite facile and a progressive growth of the oxide layer is observed in repeated potential cycles. Spontaneous deposition of a submonolayer-tomultilayer of Pt on metallic Ru surfaces is a new phenomenon involving a noble metal deposition on a noble metal substrate through a local cell mechanism. The electrocatalysts prepared by spontaneous deposition of Pt on Ru nanoparticles have high activity and high CO tolerance exceeding those of the state-of-the-art commercial catalysts containing several times higher Pt loadings. Electronic effects appear to play a role in providing enhanced CO tolerance of Pt submonolayers on Ru nanoparticles.

Carbon monoxide oxidation on bare and Pt-modified and Ru(0001) single crystal electrodes

Abstract Carbon monoxide oxidation on bare and Pt-modified ruthenium surfaces with the (10 1 0) and (0001) orientations was investigated with cyclic voltammetry, scanning-tunneling microscopy and in situ Fourier transform infrared spectroscopy. Facile oxidation kinetics of CO on Ru (10 1 0) are observed, in contrast with a slow reaction on Ru(0001). Scanning tunneling microscopy (STM) measurements confirmed that spontaneous deposition of Pt produces island-like structures on both single crystal Ru surfaces. CO oxidation on a bimetallic Pt / Ru (10 1 0) surface with a Pt coverage of approximately one monolayer occurs at potentials that are 140 mV more negative than those for bare Pt. This potential is, however, more positive than the potential of the onset of the oxidation on Ru (10 1 0) . IR spectroscopy shows one peak for linearly adsorbed CO on bare and Pt-modified Ru (10 1 0) surfaces, while two peaks are visible for the Pt-modified Ru(0001) structure. A single broad peak for the bimetallic Pt / Ru (10 1 0) surface may result from addition of the red-shifted peak for Pt and the peak for the Ru (10 1 0) substrate. A large red shift of CO on the Pt / Ru (10 1 0) surface requires further work to be explained. A negative shift of CO oxidation on Pt / Ru (10 1 0) indicates a decrease of the PtCO bond strength on that surface compared with the bond with bulk Pt.

Oxygen reduction on bare and Pt monolayer-modified Ru(0001), Ru(100) and Ru nanostructured surfaces

Abstract Oxygen reduction kinetics on bare and Pt-modified Ru(10 1 0) and Ru(0001) single crystal surfaces, and on carbon-supported Ru nanoparticles have been investigated. Spontaneous deposition of Pt was used to form approximately 1.5 and 0.5 monolayers on Ru single crystals and nanoparticles, respectively. The reaction kinetics of O2 on single crystal surfaces has a small structural dependence. It is also affected by the oxidation state of the Ru surfaces. The reaction involves the exchange of approximately four electrons per O2 molecule, with the transfer of the first electron being the rate determining step. A deposit of 1.5 monolayers of Pt makes the surfaces considerably more active than bare Ru, but nevertheless still less active than bulk Pt. An electrocatalyst made by the deposition 0.5 monolayers of Pt on carbon-supported Ru nanoparticles is somewhat less active than commercial catalysts, but contains considerably lower Pt loadings.

Surfactant Mediated Electrochemical Deposition of Ag on Au(111)

Electrochemical surfactant mediated growth of Ag on Au(111) was investigated experimentally using in situ scanning tunneling microscopy. Almost ideal layer-by-layer growth of Ag was obtained using 0.8 of a monolayer of Pb as a surfactant. Atomically flat epitaxial Ag thin films, 200 monolayer in thickness were grown on Au(111) using this technique. Energy dispersive X-ray analysis and Rutherford backscattering spectrometry showed no traces of Pb in the Ag layers grown.

Pulse electrodeposition of 2.4 T Co/sub 37/Fe/sub 63/ alloys at nanoscale for magnetic recording application

Pulse deposition of 2.4 T Co/sub 37/Fe/sub 63/ alloy in photoresist features with 40 nm critical dimension and high aspect ratio is presented. The design of the pulse deposition parameters is described in terms of the transport limitations through the diffusion layer, electrochemical interface stability with respect to Fe(OH)/sub 3/ precipitation, and the optimum conditions for additive (Saccharin) adsorption. The alloy grain size and crystal structure in the nanoconfined electrode geometry is compared versus the thin film and relevant implications for magnetic recording are discussed. The 2.4 T Co/sub 38/Fe/sub 61/Pd/sub 1/ alloy is introduced as a possible way to improve the corrosion properties of 2.4 T Co/sub 37/Fe/sub 63/ alloy.

Influence of Additive Adsorption on Properties of Pulse Deposited CoFeNi Alloys

In this paper, we investigated the conditions at the electrochemical interface for additive adsorption during the pulse current deposition of a CoFeNi alloy. Depending on the magnitude of the pulse currents used, different potentials and the corresponding additive coverage of CoFeNi surface are established affecting the CoFeNi alloy composition, concentration of incorporated C, S, and O inclusions, crystal structure, magnetic properties, and the surface quality of the deposit. The maximum content of S, O, and C in the CoFeNi deposit is found for the pulse current where the electrode potential is in the range where the maximum additive coverage is observed, indicating a close correlation between additive adsorption and additive incorporation phenomena. The anomalous codeposition effect was moderate in the potential range where maximum surface coverage of additives occurs, causing the composition of the CoFeNi films and their crystal structure to have a relatively mild change for a broad range of pulse current densities. The surface quality and the coercivity of the CoFeNi alloy have a strong correlation to the additive coverage during the pulse stage, and practical aspects of these findings are discussed. Organic additives have been commonly used in the electrodeposition of magnetic alloys for many years. 1 Besides the commonly seen action of leveling and brightening of the deposit, the benefit of using additives in the plating bath for ferromagnetic alloys is usually attributed to improvement in the crystal structure of the deposit, 2,3 smaller grain size, and reduction of the residual stress in the deposit. 4 These improvements usually reflect positively on the overall magnetic properties of these alloys as compared to the ones produced without any additives in the bath. Additional benefits of additives in the plating solution are also seen through the suppression of hydrogen evolution and impediment of FesOHd3 4 precipitation at the electrochemical interface, and in some instances, a relative improvement of corrosion resistance of the deposit. 5 More recently, it was demonstrated that an appropriate choice of the additives and bath chemistry could substantially improve the magnetic properties of CoFeNi films. 2,3,6-9

Size Effects in Monolayer Catalysis—Model Study: Pt Submonolayers on Au(111)

The two-dimensional Pt submonolayers on Au(111) were used as model catalyst system to study kinetics of hydrogen oxidation reaction (HOR). The morphology of different Pt submonolayers was characterized by ex situ scanning tunneling microscopy combined with statistical image analysis. The HOR kinetics data were analyzed using Levich–Koutecky formalism and presented as a function of the mean size of Pt clusters for each Pt submonolayer. The Pt submonolayers with smaller Pt clusters were found less active for HOR. This trend is well correlated with the continuum elasticity analysis of the average active strain in Pt clusters indicating that smaller clusters have less tensile strain. The density functional theory calculations were found in agreement with our results demonstrating that the size-dependent strain in Pt clusters has significant effect on the energy of the d-band center, i.e., the Pt clusters’ activity.