Stephen D. Rosasco

Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); Studies by leed, auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I2 vapor to form the Pt(111) (7 × 7)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.

Electrodeposition on a well-defined surface: Silver on Pt(111)(√7×√7)R19.1°−I

Abstract An exploration is reported of the structures formed by electrodeposition of silver on well-defined Pt(111) surfaces in various amounts up to a few monolayers. Prior to electrodeposition, the Pt(111) surface was treated with I2 vapor to form a Pt(111)(√7×√7)R19.1°−I superlattice which effectively protected the Pt and Ag surfaces from attack by the aqueous HClO4 electrolyte and residual gases. Silver electrodeposited in three widely separated underpotential deposition stages, forming distinct lattice structures having (3×3) or (√3×√3)R30° LEED patterns at all coverages studied. Formation of ordered Ag layers has therefore been demonstrated. Measurements of Auger electron spectroscopic current for Pt, Ag and I revealed that the silver was located underneath the iodine atomic layer, which remained attached during multiple cycles of electrodeposition and dissolution of silver from iodine-free solutions.

Preparation of well-defined surfaces at atmospheric pressure: Studies of structural transformations of I, Ag-adlattices on Pt(111) by LEED and electrochemistry

Pt(111) surfaces disordered by ion-bombardment or electrochemical oxidation were converted to well-defined, ordered states by annealing in iodine vapor at atmospheric pressure. A structure not obtainable in vacuum was formed, Pt(111)(33 × 93)R30°-I, containing 0.62 I atoms per surfa ce Pt atom in a slightly distorted hexagonal array. The I-I interatomic distances in this structure, 0.33 and 0.36 nm, were less than the Van der Waals distance, 0.43 nm. Gentle heating of this structure under pure Ar yielded I2 molecules, I atoms and a series of structures: Pt(111)(33 × 9 3)R30°-I(3 × 3)R30°-IPt(111) (clean surface). The Pt(111)(7 × 7 )R19.1°-I adlattice proved to be identifiable from its distinctive electrochemical behavior in electrodeposition of Ag from aqueous solutions of AgClO4, which consists of three prominent structural transitions. Kinematic calculations of the directions and qualitative intensities of the LEED beams at selected kinetic energies has led to proposed structures consisting of Ag atoms close-packed in registry with the three-fold sites of Pt but with I atoms substituted for Ag atoms at the (3 × 3)R30° positions. Phase boundaries caused by reversals of the two packing sites of the 3 unit mesh at intervals 17 Pt unit vectors divide the surface into hexagonal antiphase domains.

Electrochemical processes at well-defined surfaces

Abstract The structures of layers of atoms and ions formed on well-characterized single-crystal electrode surfaces in vapor and in electrolytic solutions at atmospheric pressure have been investigated by means of LEED, Auger spectroscopy, cyclic voltammetry and related techniques. Electrodeposited layers of metals were generally found to be highly ordered when deposited onto well-defined substrates. Layer structure proved to be a sensitive function of the structure of pre-adsorbed adlattices. In related studies, organic compounds of various types were found to form a layer of oriented adsorbed molecules on atomically smooth substrates. Reactivity of these oriented adsorbed intermediates was sharply dependent upon orientation. Findings of this latter type involved accurate packing density measurements using thin-layer electrodes. Recent work will be reviewed and additional findings presented.

Cation competition in the electrical double-layer at a well-defined electrode surface: Li+,Na+,K+,Cs+,H+,Mg2+,Ca2+,Ba2+,La3+, tetramethylammonium, choline and acetylcholine cations at Pt (111) surfaces containing an ordered layer of cyanide

Immersion of the Pt (111) surface into aqueous KCN solution produced an ordered ionic layer, Pt (111)(2√3×2√3)R30°−KCN. The adlattice consisted of isolated adsorbed CN− anions surrounded by adsorbed HCN molecules, and a layer of K+ counter-ions. When immersed at open circuit into aqueous solutions of simple chloride salts this surface underwent cation exchange without loss or rearrangement of the CN−/HCN layer, based upon Auger spectroscopy and LEED. Experiments in which cations were made to adsorb competitively from mixtures of chloride salts revealed that a remarkable degree of selectivity exists in the interaction of cations with this CN− layer. Highly charged cations predominated, followed by cations of relatively small size and those least strongly hydrated. Strength of cation retention varied in the order: La3+ ≫ Ba2+ > Ca2+ ≫ Mg2+ ≫ K+ > Na+ = Cs+ > NH4+ > H+ ≫ Li+ ≫ NH4+. Quaternary ammonium cations (tetramethylammonium, choline, acetylcholine), even when present in large excess, were unable to compete with other cations for retention by the double-layer.

Demonstration of the surface stability of the Van der Waals surface (0001) of MoSe2 by LEED and electrochemistry

Abstract The Van der Waals surface (0001) of a MoSe 2 crystal was studied by LEED and Auger spectroscopies. Almost no surface reactivity was measured with these techniques upon exposure of the crystal to I 2 vapor, liquid H 2 O, 1M HClO 4 and 10 −3 M NaI 3 . This behavior is in contrast to I 3 − adsorption which has been measured in situ on these surfaces in electrolytes. Electrochemical cycling in various electrolytes, subsequent and prior to the high vacuum measurements, also produced very little change in the composition or structure of the MoSe 2 surface.

Studies of electrodeposition of silver on an iodine-pretreated stepped surface: Pt(S)[6(111)×(111)]

Abstract Layers of Ag electrodeposited from aqueous solution onto a Pt(S)[6(111)×(111)](3× T )-I adlattice formed by pretreatment of the Pt(S)[6(111)×(111)] surface with I 2 vapor were studied by LEED, AES and thermal desorption. Stability of the I adlattice toward exposure to perchloric acid solution, and persistence of I on the surface during multiple cycles of electrodeposition and dissolution of Ag was demonstrated. The I adlattice served to protect the Pt and Ag electrodeposited surfaces from unwanted side reactions. Electrodeposition of Ag occurred in three well-resolved UPD regions. Subsequent UHV experiments, after each UPD peak and up to coverages of a few monolayers, revealed that stable and ordered Ag superlattices were formed, each UPD region leading to a change in LEED pattern and superlattice structure. These results are compared with previous results for smooth Pt(111), revealing clues as to the role of steps in electrodeposition.

Films formed on well-defined stainless steel single-crystal surfaces in oxygen and water: studies of the (111) plane by LEED, Auger and XPS

Abstract Reported here are studies by LEED, Auger and X-ray photo-electron spectroscopy of the (111) plane of a face-centered cubic Fe-Cr-Ni alloy single-crystal of composition (70 at% Fe, 18 at% Cr, 12 at% Ni) resembling type 304 stainless steel. Surface films resulting from treatment with O 2 , water vapor or liquid water at ambient temperature or with annealing were characterized. A disordered oxygen-containing layer was formed at ambient temperature. When formed from water, the layer contained oxide and undissociated water. Some of the water was co-ordinated and the remainder hydrogen bonded within the film. Annealing, or formation of the film at temperatures above 1000 K, produced an ordered oxide film consisting primarily of chromium oxides in the form of hexagonal meshes of Cr 2 O 3 (001), square meshes of CrO and an antiphase-domain adlattice of chemisorbed oxygen.

Iodine adsorption on a platinum stepped surface: Pt(s)[6(111) × (111)]

Abstract I 2 adsorption on Pt(s)[6(111) × (111)] surfaces under vacuum and atmospheric pressure conditions was studied by LEED, AES and thermal desorption. In contrast to smooth Pt(111), the surface structures were composed of multiple phase domains having (3 × 3) or ( 3 × 3 )R30° local geometry and structural coincidence of the adjacent terraces. No special stability or instability of iodine adsorption at steps was observed.