Au(111)-Supported Pt Monolayer as the Most Active Electrocatalyst toward Hydrogen Oxidation and Evolution Reactions in Sulfuric Acid

As a subnanometer thick platinum (Pt) film can have catalytic properties different from those of the Pt bulk, the research on the preparation and characterization of a Pt monolayer is fundamentally intriguing and may lead to cost-effective fuel cells. We devise an electroless deposition method to fabricate a Pt monolayer and use scanning tunneling microscopy (STM) to characterize its atomic structures. This method involves the use of carbon monoxide (CO) molecules as the reducing agent for PtCl₆^2– complexes, yielding a CO-capped Pt film on an Au(111) substrate. The deposition of the Pt film stops at one atom thick. To expose the Pt film, the CO adlayer is stripped off by pulsing the potential to 0.96 V (vs hydrogen reversible electrode) for 3 s in H₂-saturated 0.1 M H₂SO₄. Atomic resolution STM imaging shows that the Pt adatoms arrange in two hexagonal arrays with different atomic corrugation patterns and a notable difference (5.5%) in the Pt–Pt distance at 0.1 V. The Pt film with a larger interatomic spacing of 0.287 nm is 2× more active than that of Pt(111), and may be the most active catalyst toward hydrogen evolution and oxidation reactions (HER and HOR) reported thus far

Effects of Anions on the Electrodeposition of Cobalt on Pt(111) Electrode

Voltammetry and in-situ scanning tunneling microscopy (STM) were used to examine electrodeposition of cobalt (Co) on a stationary Pt(111) electrode in 0.1 M K₂SO₄ + 1 mM H₂SO₄ + 10 mM CoSO₄ (or the sulfate solution) without and with 10 mM chloride (the chloride solution). Under- and overpotential deposition (UPD and OPD) of Co resulted in reduction peaks at −0.52 and −0.74 V (vs Ag/AgCl), respectively. Hydrogen evolution was the major obstruction to Co²⁺ reduction, which limited the efficiency of Co deposition at ∼63% in both solutions. UPD of Co resulted in a highly ordered honeycomb structure in the sulfate solution, whereas that formed in the chloride solution was clearly disordered. Multilayer Co deposit formed by OPD at −0.74 V in the sulfate medium was crystalline, forming moiré structures for the first eight layers, followed by pyramids made of stacked triangles. These results suggested face-centered cubic stacking of the Co deposit. Co film produced in the chloride solution was also layered, except perimeters of Co layers were mostly rugged. Distinct screw dislocations and spiral defects were seen in the Co thin films produced in both solutions

STM Characterization of Self-Assembled Monolayers of Cysteine Betaine on Au(111) Electrode in Perchloric and Sulfuric Acids

A bioderived zwitterionic molecule, cysteine betaine (Cys-b), can be used as a biomaterial coating to evade fouling and damage by light radiation. In situ scanning tunneling microscopy (STM) has been used to study the structures of the cysteine betaine (Cys-b) molecule adsorbed on an Au(111) electrode in 0.1 M HClO₄ and H₂SO₄. A number of Cys-b structures have been identified in 0.1 M HClO₄ before adsorbed Cys-b is irreversibly oxidized, including (4 × 8), (6 × 6), and (√19 × 3√3). By contrast, very different Cys-b structures, including (√7 × 4), which is an incommensurate structure, and disordered structures, are seen in the same potential region in H₂SO₄. These results are reconciled by a coadsorption scheme involving the Cys-b cation and ClO₄^– (or HSO₄^–). The coverages of Cys-b are 1.31 × 10¹⁴ and 2.32 × 10¹⁴ molecules/cm² at the same potential in HClO₄ and H₂SO₄. Although Cys-b molecules are tethered to the Au(111) substrate via their S-ends, their spatial structures are influenced greatly by the interactions with the coadsorbed anions. As ClO₄^– and HSO₄^– anions are hydrated in the aqueous electrolyte, their hydrated shells can affect their interactions with the Cys-b cation, leading to different ordered structures as seen by STM

Scanning Tunneling Microscopy of Superfilling in FormulaContaining Chloride, Polyethylene Glycol andBis-3-Sodiumsulfopropyl-Disulfide

In situ scanning tunneling microscopy (STM) was used to study copper deposition at vacancy defects on a copper thin film underpotentiostatic conditions at −0.20 V (vs. Ag/AgCl) in a formula containing sulfuric acid, chloride, polyethylene glycol (PEG), andbis-3-sodiumsulfopropyl-disulfide (SPS) – the widely used mixture to facilitate Cu superfilling at recessed features in semiconductorprocessing. The vacancy island measuring ∼70 nm wide and 12 nm deep sat in the middle of a facetted surface structure at thebeginning. Cu deposit nucleated mainly at the rim of the vacancy and grew into stacked Cu(111) facets. These local pyramidal Custacks could restructure into wider Cu(111) terraces by transferring Cu atoms rapidly from higher to lower planes. Voltammetricresults showed that Cu deposition was suppressed in a plating bath containing 1 mM KCl + 88 μM PEG8000 + 10−7 M SPS.Steps with sharp edges bunched in the course of Cu deposition. The vacancy island was filled with Cu deposit assuming smoothterraces with sharp step edges aligned mainly in the 121 directions of the Pt(111) electrode, suggesting crystalline packing in theCu deposit. Atomic-resolution STM imaging revealed a hexagonal array presumed to be the (√3 × √3)R30◦ – Cl− adlattice

Electrodeposition of Copper on a Pt(111) Electrode in Sulfuric Acid Containing Poly(ethylene glycol) and Chloride Ions as Probed by in Situ STM

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H2SO4 + 1 mM CuSO4+ 1 mM KCl + 88 μM PEG. At the end of Cu underpotential deposition (∼0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × √7)-Cu + HSO4 − structure was observed in 1 M H2SO4 + 1 mM CuSO4. This adlattice restructured upon the introduction of poly(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (∼0 V), a Pt(111)-(√3 × √3)R30°-Cu + Cl− structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl− were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H2SO4] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu

Epitaxial Electrodeposition of Nickel on Pt(111) Electrode

Artificial nickel thin films, potentially useful as magnetic materials and electrocatalysts, have been prepared by electrodeposition on noble transition metal electrodes. This study employed scanning tunneling microscopy (STM) and cyclic voltammetry to study electrodeposition of Ni on Pt(111) from 0.1 M KClO₄ + 1 mM HCl + 0.06 M NiCl₂. Deposition of Ni was noted at potentials more positive than its Nernst potential, as proton discharge and hydrogen evolution occurred concomitantly. Bulk deposition of Ni commenced at potentials more negative than −0.6 V (vs Ag/AgCl), where reduction of water to hydrogen was imminent. The reduction reaction of Ni²⁺ ion to Ni metal was a slow process under the present experimental conditions, and not all Ni deposit was removed from the Pt electrode, as indicated by irreversible changes in the voltammetric profiles. In-situ STM provided direct views of the growth process and the atomic structures of the Ni thin film. The first Ni adlayer deposited at <i>E</i> > −0.525 V or the underpotential deposited (UPD) layer was disordered but was transformed into an ordered structure supporting the subsequently deposited Ni adlayers. From the second all the way up to the tenth Ni adlayers, STM imaging revealed prominent moiré patterns exhibiting long-ranged intensity modulations undulating along the ⟨110⟩ direction of the Pt(111) substrate. These moiré patterns are proposed to arise from a stack of Ni(111)-like planes on the Pt(111) electrode. The periodicities of the moiré patterns decreased from 3.0 to 2.5 nm as the Ni deposit thickened from the second to the fourth layer, suggesting that the spacing between Ni adatoms decreased from 0.254 to 0.25 nm

Electrodeposition of Copper on a Pt(111) Electrode in Sulfuric Acid Containing Poly(ethylene glycol) and Chloride Ions as Probed by in Situ STM

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H₂SO₄ + 1 mM CuSO₄+ 1 mM KCl + 88 μM PEG. At the end of Cu underpotential deposition (∼0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × √7)-Cu + HSO₄^– structure was observed in 1 M H₂SO₄ + 1 mM CuSO₄. This adlattice restructured upon the introduction of poly­(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (∼0 V), a Pt(111)-(√3 × √3)­R30°-Cu + Cl^– structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl^– were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H₂SO₄] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu