Junji Inukai

Structures of copper and halides on Pt(111), Pt(100) and Au(111) electrode surfaces studied by in-situ scanning tunneling microscopy

Abstract Structures of copper and halide (chloride, bromide or iodide) in a sulfuric acid solution on Pt(111), Pt(100) and Au(111) single-crystal electrode surfaces were systematically investigated by in-situ scanning tunneling microscopy. A (4 × 4) structure was imaged after the first cathodic current peak for Cu + Cl on Pt(111). Distorted (4 × 4) and (√3 × √3)R30° structures were found after the first and the second peaks, respectively, for Cu + Br on Pt(111). For Cu + I on Pt(111), a (√3 × √3)R30° structure was observed after the copper deposition peak. A c(2 × 2) structure was obtained for Cu + Cl, Cu + Br and Cu + I on Pt(100). On a Au(111) surface, a (5 × 5) structure was observed for Cu + Cl after both the first and the second cathodic current peaks. For Cu + Br on Au(111), (√7 × √7)R19.1° was found after the first peak, and (4 × 4) structures were imaged after both the second and third peaks. A (3 × 3) image was obtained for Cu + I after a single peak on Au(111). Structural models for the copper and the halide layers are discussed based on the adsorbate—substrate interaction at these surfaces. Coadsorption structures on platinum surfaces are understandable as those of halide adsorption on a pseudomorphic copper (1 × 1) layer on platinum, while on Au(111) the coadsorption layer is interpreted as an ultrathin crystal of CuX(X = Cl, Br, I).

Electrodeposition processes of palladium and rhodium monolayers on Pt(111) and Pt(100) electrodes studied by IR reflection absorption spectroscopy

Abstract The electrodeposition of palladium on Pt(111) or Pt(100) single-crystal electrodes, and of rhodium on a Pt(111) electrode was carried out in 0.5 M sulfuric acid aqueous solutions containing 1×10 −4 M Pd 2+ or Rh 3+ . Cyclic voltammograms showed the growth of the new sets of adsorption and desorption peaks at 0.22 V (vs. the standard hydrogen electrode) and 0.22 V for Pd/Pt(111), 0.10 and 0.12 V for Pd/Pt(100), and 0.14 and 0.19 V for Rh/Pt(111) respectively. The IR spectra of carbon monoxide on the surfaces during deposition gave evidence for the formation of two-dimensional islands of palladium and rhodium, respectively, by showing the bridge and the threefold absorption bands of carbon monoxide on those atoms. The epitaxial growth of palladium or rhodium monolayers in pseudomorphic (111) structures on Pt(111) and Pt(100) was suggested.

The structure of bisulfate and perchlorate on a Pt(111) electrode surface studied by infrared spectroscopy and ab-initio molecular orbital calculation

Abstract In situ infrared reflection absorption spectroscopy yielded the first spectroscopic evidence that perchlorate anion is adsorbed on a Pt(111) electrode surface in a 0.1 M perchlorate acid solution over the potential range, 0.6 to 0.9 V vs. NHE. Both perchlorate and bisulfate anions were chemisorbed reversibly in the form of unidentate or tridentate coordination on the surface. Bisulfate and perchlorate ions started to be adsorbed at 300 and 600 mV vs. NHE, respectively, and these potentials coincided with the onset of hydrogen desorption. The anomalously sharp spike peaks seen in the cyclic voltammetry of the Pt(111) in acid solutions were associated with the structure change of both anions. Ab-initio molecular orbital calculation supported the large frequency shift of bilsufate anions on a positively charged electrode surface.

Coverage and potential dependent CO adsorption on Pt(1111), (711) and (100) electrode surfaces studied by infrared reflection absorption spectroscopy

The structure of CO adsorbed on stepped electrode surfaces was studied by infrared reflection absorption method. Three kinds of (100) terrace surfaces which exhibited different step densities, Pt(1111)[6(100) × (111)], Pt(711)[4(100) × (111)] and Pt(100), were examined. At least four kinds of absorption bands of CO were found on each electrode surface: on-top CO on steps at 2045 cm−1, on-top CO on terraces at 2060 cm−1, symmetric bridge CO at 1880 cm−1, and asymmetric bridge CO at 1920 cm−1. The population at each CO adsorption site could be controlled by changing the electrode potential and CO coverage. At negative electrode potentials, CO preferred a bridge site on terrace atoms and an on-top site on step atoms, while at positive potentials, on-top CO on terraces predominated. The coverages at which on-top CO on the terrace surface started to appear were 0.40 on Pt(100), 0.35 on Pt(1111) and 0.25 on Pt(711).

Compression structures of carbon monoxide on a Pt(111) electrode surface studied by in situ scanning tunneling microscopy

Abstract The structure of carbon monoxide adsorbed on a platinum (111) single crystal electrode surface was examined by in situ electrochemical scanning tunneling microscopy in a 0.5 M H 2 SO 4 solution containing CO. Four different structures were observed depending on the electrode potential and the CO coverage. At CO saturation, distorted and (3 × 1) structures were found on a Pt(111) electrode surface at the electrode potential of 0.55 V against the normal hydrogen electrode. At low coverages, the R30° structure known under ultrahigh vacuum was observed at 0.55 V, and a periodic domain structure based on R30° was also found at 0.30 V.

CO migration on Pt(100) and Pt(11 1 1) surfaces studied by time resolved infrared reflection-absorption spectroscopy

Abstract Carbon monoxide (CO) migration on Pt(100) and Pt(11 1 1) single crystal electrode surfaces in an acid solution was studied by electrode potential modulated infrared reflection-absorption spectroscopy and time resolved infrared reflection-absorption spectroscopy. The interconversion of CO adsorbed on both electrode surfaces between on-top, asymmetric and symmetric bridge-bonded sites was caused by the applied electrode potentials. Unstable CO intermediate species were created during the interconversion of their sites.

Potential-induced migration of top-layer atoms and molecules on Pt(110) electrode surface studied by infrared reflection absorption spectroscopy

Abstract The reconstruction process on a clean Pt(110) surface was monitored by infrared reflection absorption spectra of CO on the surface. The adsorbate-induced phase transition from the (1×2)-Pt(110) to the (1×1)-Pt(110) structure was completely suppressed under negative electrode potentials. The migration of copper ad-atoms on the Pt(110) surface prepared by underpotential deposition was also controlled by the electrode potential. At negative potentials, the mobility of the copper ad-atoms was depressed strongly, and the copper ad-atoms did not form a uniform film. Diffusion of the copper atoms started at a relatively positive potential. Thus the reconstruction of a Pt(110) surface and the migration of copper ad-atoms did not proceed under highly negative electrode potentials at room temperature.

Ex-situ IRAS and LEED studies of underpotentially deposited copper on a Pt(111) electrode in a sulfuric acid solution: layer exchanges of anions with copper

Abstract The electrodeposition of copper on a Pt(111) electrode in a 0.5M H 2 SO 4 solution was studied by means of voltammetry, low energy electron diffraction and ex-situ infrared reflection absorption spectroscopy. At any potential between 0.25 and 0.85 V versus the standard hydrogen electrode, the same diffraction pattern of (√3 × √3)-R30° was observed due to a top-layer structure of bisulfate or sulfate before and after the copper deposition. The voltammetric peak at 0.59 V was due to copper adsorption while that at 0.49 V originated from a bisulfate to sulfate conversion on copper during a cathodic sweep. At an electrode potential for underpotential deposition of copper, a bisulfate becomes weakly bound to platinum atoms allowing copper deposition; simultaneous adsorption of bisulfate on the copper layer prohibits further copper deposition.

Vibrational frequency shifts of dicyanoethylenedithiolate on a copper electrode surface induced by electrode potential

Abstract The vibrational frequencies of 1,1-dicyanoethylene-2,2-dithiolate ( i -marononitriledithiolate: i MNT) adsorbed on a copper electrode surface have been studied by surface enhanced Raman spectroscopy. i MNT on a copper electrode surface showed the CC stretching band at 1380 cm −1 at the negative potential of −850 mV (versus Ag/AgCl), while it shifted to 1423 cm −1 at −50 mV. This frequency shift is explained by the structural changes based on the degrees of charge delocalization due to the back-donation from the surface atoms to the π* orbitals of i MNT. This model was supported by the structural and vibrational studies of Na 2 i MNT and Cu 8 i MNT 6 complexes through X-ray crystal structure analysis and Raman spectroscopy. The result for 1,2-dicyanoethylene-1,2-dithiolate adsorbed on a copper electrode surface is also discussed.