Frank Lu

Adlattice structure and hydrophobicity of Pt (111) in aqueous potassium iodide solutions: Influence of pH and electrode potential

Abstract Measurements by means of Auger spectroscopy and LEED of the composition and structure of Pt (111) surfaces after immersion into aqueous iodide solutions are reported as a function of pH and electrode potential. Voltammetric current for reductive desorption of halogen was observed near −0.3 V (vs. Ag/AgCl reference). Comparison of the coulometric charge for halogen reductive desorption with the original packing density of iodine determined by Auger spectroscopy revealed that a one-electron reduction of the halogen had occurred: I(adsorbed) + e− → I−. The packing density of I atoms was a function of electrode potential, declining at the negative extreme of potential due to the reductive elimination process and also declining at the positive extreme of potential due to oxidation of the adsorbed halogen atoms and of the Pt surface. On the basis of LEED data, the halogen layer was found to be ordered in registry with the Pt (111) surface. The halogen adlattice structure was potential dependent: a (3 × 3) adlattice (ΘI = 4/9) at relatively positive potentials; a adlattice (Θ1 = 3/7) at potentials in mid-range; a adlattice (ΘI = 1/3) at relatively negative potentials; and a virtually halogen-free surface at potentials approaching the negative limit. The pH dependence of iodine adsorption was relatively slight, evidently because strong adsorption of halogen suppressed adsorption of OH and related species. The Pt (111)(3 × 3)-I and Pt (111) surfaces were remarkably hydrophobic, while the Pt surface was distinctly hydrophilic. Ionic species were not retained to a measurable degree at the hydrophobic surface, although normal retention of solution films and ions occurred at the surface in its hydrophilic states.

Structure and composition of the Ag(111) surface as a function of electrode potential in aqueous halide solutions

Studies by means of Auger spectroscopy, LEED and voltammetry are reported of the surface layers which were formed when a well-defined Ag (111) surface was immersed into aqueous halide solutions (KF, KCl, KBr or KI) at controlled pH and electrode potential. Electrode potentials spanning the range from water reduction to silver oxidation were studied at pH 4 and 10. Surface composition (F, Cl, Br or I, O, K and Ag) was monitored as a function of electrode potential by means of Auger spectroscopy following immersion. Strong adsorption of Cl, Br and I but not F occurred throughout most of the potential range, including simple immersion at open-circuit. Strength of adsorption of Cl and Br diminished significantly at extremely negative potentials near the solvent-reduction limit at pH 10; however, iodide was adsorbed strongly at all potentials studied. Reductive desorption of Cl and Br from Ag (111) involved transfer of one electron per halogen atom in a very broad voltammetric peak spanning much of the accessible potential range, Cl and Br formed Ag (111) ()R30° structures, while iodide yielded some of the same complex structures as reported for Ag deposition at iodine-pretreated Pt (111). That is, the silver surface is reconstructed in I− solutions.

Surface chemistry of mercaptopyridines at Ag(111) electrodes studied by EELS, LEED, Auger spectroscopy and electrochemistry

Surface electrochemical studies of 2-pyridinethiol (2PyT, 2-mercaptopyridine) and 4-pyridinethiol (4PyT, 4-mercaptopyridine) at Ag(111) single-crystal electrode surfaces in aqueous fluoride solutions are reported. LEED patterns observed for 2PyT adsorbed at Ag(111) from a 1 mM aqueous solution containing 2 mM HF (pH 3) at potentials between −0.54 V and −0.64 V (vs. Ag/AgCl reference) reveal that an ordered layer is formed having a rectangular unit mesh, Ag(111) (2×0.53)R30°-2PyT, containing one molecule of 2PyT. Adsorption at potentials more positive than −0.54 V results in LEED patterns which are relatively diffuse and have not yet been identified. A weak but observable Ag(111) (30.53× 30.53)R30° LEED pattern is obtained for adsorption of 4PyT at −0.40 V from a 1 mM aqueous solution (pH 3). Molecular packing densities (nmolcm2) measured by means of Auger spectroscopy indicate that 2PyT and 4PyT molecules are adsorbed with the pyridine ring perpendicular to the Ag(111) surface. The packing density of these adsorbates is virtually independent of adsorbate concentration from 0.01 mM to near saturation (about 0.2 M) at −0.60 V. Packing density of 2PyT increases slightly but abruptly when the electrode potential is increased above about −0.4 V. The Ag(111) (2×0.53)R30°-2PyT structure contains about 0.57 nmolcm2, slightly less than the saturation packing density, 0.67 nmolcm2, observed at more positive electrode potentials. EELS spectra of adsorbed 2PyT indicate that attachment to the Ag(111) surface at potentials less than −0.2 V occurs primarily through the sulfur atom, with dissociation of the sulfhydryl hydrogen atom. At potentials more positive than 0.0 V, a coupling reaction between an adsorbed 2PyT and a dissolved 2PyT occurs at the Ag(111) surface. The occurrence of this coupling reaction is more apparent for 4PyT at the Ag(111) surface. Amplitudes of the EELS bands due to CH bending and CS stretching depend upon the electrode potential at which adsorption is carried out. EELS spectra of 2PyT adsorbed at Ag(111) at negative potentials (E < −0.4 V) closely resemble the IR spectrum of the unadsorbed compound; the same is true for 4PyT. Apart from potential-dependent adsorption/desorption and coupling reactions, adsorbed 4PyT and 2PyT are relatively inert towards electrochemical oxidation and reduction. The 2PyT and 4PyT adsorbed layers are stable in UHV.

Studies of adsorbed unsaturated alcohols at well-defined Pt (111) electrode surfaces by cyclic voltammetry assisted by vibrational spectroscopy (EELS)

Abstract Surface vibrational states (electron energy-loss spectroscopy, EELS), molecular packing density (Auger spectroscopy) and adsorbed layer electrochemical reactivity (cyclic voltammetry) were investigated for the following unsaturated alcohols adsorbed from neat liquids and aqueous solutions at Pt (111): benzyl alcohol (BZA); 4-pyridylcarbinol (4PDC); 3-pyridylcarbinol (3PDC); allyl alcohol (AAL); propargyl alcohol (PGA); cis-2-butene-1,4-diol (CBED); and 2-butyne-1,4-diol (BYD). Chemisorption of the unsaturated centers of these alcohols is the principal mode of surface attachment (phenyl, pyridyl, CC or CC). Electrochemical oxidation of adsorbed BZA, AAL. PGA, CBED and BYD in aqueous fluoride electrolyte proceeds efficiently to CO2, while adsorbed 4PDC and 3PDC are relatively inert. The adsorbed layer of each of these alcohols is stable in vacuum, such that surface spectroscopic methods in vacuum are directly applicable to the liquid/solid interfacial chemistry and electrochemistry of these compounds. 4PDC and 3PDC are adsorbed in a tilted vertical orientation with an angle near 70° between the pyridine ring and the Pt surface. The C-C bond-order for PGA and BYD is reduced dramatically to near that of a double bond as a result of the adsorption process. The aromatic or alkene moieties of BZA, 4PDC, 3PDC, AAL and CBED are retained and only small shifts in their vibrational frequencies are found. Ordered layers of these adsorbed alcohols were not found by LEED, although uniform molecular orientation is suggested by the packing density data and voltammetric behavior.

Surface chemistry of five-membered heteroaromatics at Pt(III) electrodes studied by EELS, LEED, Auger spectroscopy and electrochemistry: furan, pyrrole and thiophene

Abstract Reported here are studies of the chemisorption, surface vibrational spectroscopy, elemental and molecular packing densities, and electrochemical reactivity of three related five-membered heterocycles at Pt(III): pyrrole (PRR), furan (FRN), and thiophene (TPE). Packing density and stoichiometry were investigated by the use of Auger spectroscopy. None of the adsorbed layers exhibited long-range order detectable by LEED. Vibrational spectra of the chemisorbed species were obtained by means of electron energy-loss spectroscopy (EELS), and were compared with infrared spectra of the unadsorbed compounds. Electrochemical oxidation of the adsorbed molecules was explored by the use of cyclic voltammetry (CV). The results reveal that PRR adsorbs at Pt(III) mainly in a horizontal orientation with the ring intact; electrochemical oxidation of adsorbed PRR proceeds completely to NO2 and CO2. However, FRN is converted by hydrolytic, ring-opening isomerization to adsorbed pi-bonded butenoic acid (BTA); electrochemical oxidation of adsorbed BTA proceeds to CO2. TPE forms a mixed layer consisting of S atoms and adsorbed TPE molecules S-bonded and near-vertically oriented; adsorbed TPE is oxidized to CO2 and SO42−.

Oriented adsorption at well-defined electrode surfaces studied by Auger, leed, and eels spectroscopy

Abstract Quantitation by use of Auger spectroscopy and cyclic voltammetry of molecular layers adsorbed at Pt(111) and Pt(100) surfaces from aqueous electrolytes is examined in this work for the following compounds: hydroquinone (HQ); phenol (PL); catechol (CT); 3,4-dihydroxyphenylacetic acid (DOPAC); L-3,4-dihydroxyphenylalanine (DOPA); l -tyrosine (TYR); l -phenylalanine (PHE); nicotinic acid (NA); 2,5-dihydroxy-4-methyl-benzyl mercaptan (DMBM); thiophenol (TP); benzylmercaptan (BM); 3-thiophene carboxylic acid; and 2,5,2′,5′-tetrahydroxybiphenyl (THBP). Two independent methods of measurement of packing density based upon Auger spectroscopy, and one based upon cyclic voltammetry are employed and the results compared. Voltammetric oxidation/reduction of adsorbed layers formed from these compounds at Pt surfaces in aqueous electrolyte is found to be essentially the same whether carried out before or after lengthy evacuation. Therefore, the results of surface spectroscopy in UHV are directly applicable to the liquid—solid chemistry and electrochemistry of these adsorbed compounds. Packing densities measured by means of two Auger spectroscopic methods were in good agreement with each other and with the voltammetric measurements.

Adsorption of ferricycanide at Pt (111) as a function of electrode potential studied by Auger spectroscopy

Measurements by means of Auger spectroscopy of the composition of Pt (111) surfaces after immersion into aqueous potassium ferricyanide solution (pH = 4) as a function of electrode potential are reported. The ferrocyanide complex was adsorbed with retention of its usual stoichiometry and ionic charge while ferricyanide was partially displaced by adsorption of an iron-free species such as cyanide. Hydrogen predominated at negative extremes of potential, while iron oxides and chlorides were present at positive extremes. Cations such as H+, Cs+ and K+ were retained relatively strongly at Pt (111) in ferricyanide/ferrocyanide solutions. The LEED patterns contained only beams due to the Pt (111) surface; no fractional-index beams were observed. Evidently, the layer of iron complexes did not possess long-range order.

Electrochemical oxidation of adsorbed terminal alkenes as a function of chain length at Pt (111) electrodes: Studies by cyclic voltammetry, EELS, and auger spectroscopy

Abstract Studies are reported of the surface packing density, vibrational spectroscopy and electrochemical oxidation of a series of straight-chain terminal alkenes adsorbed from the vapor at a Pt (111) electrode surface. Compounds studied were: ethene (ETE), propene (PPE), 1-butene (BTE), 1-pentene (PTE), 1-hexene (HXE), 1-octene (OCE) and 1-decene (DCE). Vibrational spectra of the adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities (nmol/cm2) in the adsorbed layer were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer in aqueous inert electrolyte (KF+ HF) was investigated by means of linear potential scan voltammetry. Attachment to the surface is through the CC bond. Based upon molecular packing densities, the aliphatic chains are pendant; regardless of its chain length, each chemisorbed alkene molecule occupies an area similar to that of chemisorbed PPE. The packing densities of ETE and PPE indicate an average orientation in which the CC moiety is parallel to the Pt (111) surface. EELS spectra indicate that the CC double bond is preserved in the adsorbed state of each 1-alkene studied. Measurement of the average number of electrons, nox, required to desorb an adsorbed 1-alkene molecule electro-oxidatively reveals that the catalytic electrochemical oxidation process involves primarily the CC double bond and one adjacent saturated carbon atom.

Direct Imaging of Epitaxial Layers by Auger Electrons

It has recently been demonstrated that the surface atomic structure of materials can be imaged by means of Auger electrons. Angular distribution Auger microscopy (ADAM) produces subatomic resolution images of atomic structure by measuring and displaying the complete angular distribution of Auger electrons emitted from atoms near the surface of a solid material or thin film. Auger angular distributions contain the “silhouettes” of surface atoms “backlit” by emission from atoms located deeper in the solid. The locations and shapes of these silhouettes directly reveal the relative positions of atoms near the surface. Consequently, ADAM is an exciting new technique for characterizing materials surfaces and epitaxially grown surfaces. Auger electrons have been employed for surface elemental analysis and depth- profiling for many years. The limited escape-depth of Auger electrons makes Auger spectroscopy inherently sensitive to the surface region. Depth profiling of materials, often performed in combination with ion etching, is widely used. ADAM offers the possibility of nondestructively obtaining profiles and structural information concerning buried interfaces and can also yield images of atomic structure.

Surface vibrational spectroscopy. A comparison of the EELS spectra of organic adsorbates at Pt(111) with IR and Raman spectra of the unadsorbed organics

In this study EELS spectra obtained for the adsorbed species formed from aqueous electrolytes at Pt(111) electrode surfaces are compared with the IR and Raman spectra of the unadsorbed compounds in order to reveal the changes in vibrational spectra resulting from chemisorption of various important functional groups, and to explore the differences in vibrational absorptivities between EELS spectra of adsorbed species and IR and Raman spectra of the corresponding unadsorbed compounds. Of particular interest are the variations in EELS vibrational frequency, bandwidth and absorptivity due to bonding with the surface, intermolecular interactions of adsorbed molecules and changes in adsorbate molecular orientation. The influence of surface bonding on the EELS spectrum of a functional group was explored through studies of phenol (PL), phenol-d6 (PLD6), benzyl alcohol (BZOH), catechol (CT), benzoic acid (BA), 2-picolinic acid (PA), 2,6-pyridine dicarboxylic acid (26PDCA), and propenoic acid (PPEA). The aromatic ring of adsorbed PL, PLD6, BZA, CT, BA, PA and 26PDCA is oriented parallel to the Pt(111) surface. The resulting strong interactions affect the frequencies and relative intensities of the EELS bands: weak CH stretching modes; a large CC stretching band (1600–1650 cm−1), and weak CH bending (700–800 cm−1). The carboxylic acid moieties of BA and PA interact strongly with the Pt surface, while those of 26PDCA do so only when adsorbed at relatively positive electrode potentials. OH stretching and bending are absent from the EELS spectra of adsorbed PL, BZOH and CT, perhaps due to dissociation of the hydroxyl hydrogen during adsorption of the molecule. Adsorption of alkenes at Pt(111) from solution preserves the characteristic CC stretching band near 1650 cm−1; examples are: PPEA; 1-hexene (HXE); propenol (PPEOH); 4-pentenol (PTEOH); and cis-2-butene-1,4-diol (CBED); adsorption of ethene, propene and butene from vacuum at room temperature has been reported to result in loss of double-bond character. Adsorption of propynol (PPYOH) lowers the frequency of the CC triple bond to approximately that of a double bond. EELS spectra of adsorbed pyridine (PY), 2-methylpyridine (2MPY) and 2,6-dimethylpyridine (26DMPY) were examined in order to contrast the vibrational behavior of adsorbates having the aromatic ring perpendicular to the Pt surface (PY and 2MPY), giving rise to strong perturbation of the vibrational spectra or parallel (26DMPY), leading to limited perturbation of the vibrational spectra. Compounds for which the surface interaction is limited primarily to bonding of a single sulfur atom were studied in order to observe the vibrational behavior of a comparatively unperturbed pendant ring or chain: thiophenol (TP); benzylmercaptan (BM); cysteine (CYS); and thiophene (TPE). The expected lack of perturbation was found, making TP, BM and CYS particularly suitable reference compounds for surface vibrational studies of aromatic rings and amino acids. In contrast, L-phenylalanine (PHE) interacts with the Pt(111) surface through the phenyl ring as well as the amino acid functionality. In general, chemisorption and intra-layer intermolecular interaction both lower the surface vibrational frequencies (EELS) and incidentally affect peak amplitudes. Orientation affects peak amplitudes and incidentally affects frequencies insofar as a change in orientation is accompanied by a change in mode of surface bonding. Surface roughness leads to a scrambling of adsorbed states which affects bandwidths, chemical/electrochemical reactivities, and in turn the frequencies and amplitudes of EELS peaks. Specific findings and conclusions are presented for each compound and correlated.