The growth of Pt nanofilms on well-defined Au(111) electrode surfaces, using electrochemical atomic layer epitaxy (EC-ALE), is described here. EC-ALE is a deposition method based on surface-limited reactions. This report describes the first use of surface-limited redox replacement reactions (SLR(3)) in an EC-ALE cycle to form atomically ordered metal nanofilms. The SLR(3) consisted of the underpotential deposition (UPD) of a copper atomic layer, subsequently replaced by Pt at open circuit, in a Pt cation solution. This SLR(3) was then used a cycle, repeated to grow thicker Pt films. Deposits were studied using a combination of electrochemistry (EC), in-situ scanning tunneling microscopy (STM) using an electrochemical flow cell, and ultrahigh vacuum (UHV) surface studies combined with electrochemistry (UHV-EC). A single redox replacement of upd Cu from a PtCl(4)(2-) solution yielded an incomplete monolayer, though no preferential deposition was observed at step edges. Use of an iodine adlayer, as a surfactant, facilitated the growth of uniformed films. In-situ STM images revealed ordered Au(111)-(square root 3 x square root 3)R30 degrees-iodine structure, with areas partially distorted by Pt nanoislands. After the second application, an ordered Moiré pattern was observed with a spacing consistent with the lattice mismatch between a Pt monolayer and the Au(111) substrate. After application of three or more cycles, a new adlattice, a (3 x 3)-iodine structure, was observed, previously observed for I atoms adsorbed on Pt(111). In addition, five atom adsorbed Pt-I complexes randomly decorated the surface and showed some mobility. These pinwheels, planar PtI(4) complexes, and the ordered (3 x 3)-iodine layer all appeared stable during rinsing with blank solution, free of I(-) and the Pt complex (PtCl(4)(2-)).
This paper describes platinum nanofilm formation via the electrochemical form of atomic layer deposition (E-ALD), where the E-ALD cycles are based on surface limited redox replacement (SLRR) reaction. SLRR is where an atomic layer (AL) of a reactive (sacrificial) metal is exchanged for a more noble metal. In the present study both Cu and Pb AL were investigated as sacrificial atomic layers for replacement with both Pt(II) and Pt(IV) precursors. 25 E-ALD cycles were used to form Pt nanofilm deposits. Initial deposits contained an order of magnitude more Pt than expected, evidenced by a factor of 7 increase in surface roughness. Overly positive potentials achieved during the exchange promoted excess deposition and surface roughening. It is proposed that anionic Pt precursors adsorbed more strongly at high potentials, making them difficult to rinse from the cell. Those remaining adsorbed Pt anions are then reduced to Pt o when the potential was shifted negative for deposition of the sacrificial element. The result was Pt o formation at a large overpotential, which, contributed to excessive Pt deposits and roughening. Increased rinsing of the anionic Pt precursors from the cell eliminated the excess Pt deposition and roughening, resulting in the expected layer by layer growth of an ALD process.
The effect of elastic strain on catalytic activity of platinum (Pt) towards oxygen reduction reaction (ORR) is investigated through de-alloyed Pt-Cu thin films; stress evolution in the dealloyed layer and the mass of the Cu removed are measured in real-time during electrochemical de-alloying of (111)-textured thin-film PtCu (1:1, atom %) electrodes. In situ stress measurements are made using the cantilever-deflection method and nano-gravimetric measurements are made using an electrochemical quartz crystal nanobalance. Upon de-alloying via successive voltammetric sweeps between -0.05 and 1.15 V vs. standard hydrogen electrode, compressive stress develops in the de-alloyed Pt layer at the surface of thin-film PtCu electrodes. The de-alloyed films also exhibit enhanced catalytic activity towards ORR compared to polycrystalline Pt. In situ nanogravimetric measurements reveal that the mass of de-alloyed Cu is approximately 210 ± 46 ng/cm 2 , which corresponds to a de-alloyed layer thickness of 1.2 ± 0.3 monolayers or 0.16 ± 0.04 nm. The average biaxial stress in the de-alloyed layer is estimated to be 4.95 ± 1.3 GPa, which corresponds to an elastic strain of 1.47 ± 0.4%. In addition, density functional theory calculations have been carried out on biaxially strained Pt (111) surface to characterize the effect of strain on its ORR activity; the predicted shift in the limiting potentials due to elastic strain is found to be in good agreement with the experimental shift in the cyclic voltammograms for the dealloyed PtCu thin film electrodes. IntroductionEnhanced catalytic activity of de-alloyed PtCu electrodes towards oxygen reduction (ORR) reaction has been demonstrated in a number of recent investigations in thin-film and coreshell geometries [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The enhancement in catalytic activity is typically attributed to the electronic interaction between the core and shell constituents as well as to the epitaxial mismatch strain in the metallic shell. For example, Strasser et al. [4] demonstrated enhanced catalytic activity of de-alloyed PtCu core-shell nanoparticles towards ORR, which was attributed to the pseudomorphic compressive strain in the Pt-enriched shell. The magnitude of the strain in the Pt-enriched shell was estimated using lattice-constant measurements (via X-ray diffraction). In this work, real-time stress and nano-gravimetric measurements during electrochemical dealloying of thin-film PtCu (1:1, atom %) electrodes are reported. In situ stress measurements are made using the cantilever-deflection method, and nano-gravimetric measurements are made using an electrochemical quartz crystal nanobalance (EQCN). Upon de-alloying via successive voltammetric sweeps between -0.05 and 1.15 V vs. standard hydrogen electrode (SHE), compressive stress develops in the de-alloyed layer region near the surface of thin-film PtCu electrodes. In situ nanogravimetric measurements reveal that the mass of de-alloyed Cu is approximately 210 ± 46 ng/cm 2 , which co...
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