Potential Dependence of the Buckling Structure of the Interfacial Water Bilayer on a Graphene Electrode

The interfacial structure between aqueous electrolytes and the epitaxial graphene on a SiC(0001) electrode has been determined using X-ray diffraction. The electrolyte and electrode potential dependences are investigated, and it is found that the water bilayer is stabilized on the graphene surface in a similar fashion to icelike structure. There are no specific adsorbed ions and no layer formation of electrolyte ions at the Helmholtz plane, which differs from the double-layer structure found on metal electrodes remarkably. The layer spacing of the water bilayer depends on the electrode potential, indicating that water reorientation occurs. The applied electrode potential is strongly related to the potential drop across the interface induced by the electric dipole field of the bilayer water. A small double-layer current results from non-faradaic charge by the reorientation of the bilayer water

Vibrational Spectroscopic Observation of Atomic-Scale Local Surface Sites Using Site-Selective Signal Enhancement

Molecule–substrate interactions are sensitively affected by atomic-scale surface structures. Unique activity in heterogeneous catalysts or electrocatalysts is often related with local surface sites with specific structures. We demonstrate that adsorption geometry of a model molecule with an isocyanide anchor is drastically varied among one-fold atop, two-fold bridge, and three-fold hollow configurations with increasing the size of atomic-scale local surface sites of Pd islands on an Au(111) model surface. The vibrational spectroscopic observation of such local information is realized by site-selective and self-assembled formation of hotspots, where Raman scattering intensity is significantly enhanced via excitation of localized surface plasmons

Ethanol Oxidation on Well-Ordered PtSn Surface Alloy on Pt(111) Electrode

Surface and subsurface structures of PtSn surface alloy on Pt(111) were determined using in situ scanning tunneling microscopy (STM) and X-ray diffraction. Different ordered structures of the PtSn alloy layer were observed by STM in HClO₄ at coverage of θ_Sn ≤ 0.23. Superstructure of (√3 × √7)­R19.1° with small domain size was formed at θ_Sn = 0.23. This structure promoted the catalytic activity for the ethanol oxidation reaction with high durability. X-ray structural analysis showed that the ratio of Sn in the subsurface was below 3(2)%, The PtSn alloy layer was mainly formed at the surface of the Pt(111) electrode. The Sn atoms protruded by 0.02 nm from the Pt layer, which was similar to the surface structure of Pt₃Sn­(111). One Pt atom in the (√3 × √7)­R19.1° structure contacts to one or two surrounding Sn atoms, which lead to the highest activity for the EOR

Structural Dynamics of the Electrical Double Layer during Capacitive Charging/Discharging Processes

Transitional structures of Cs⁺ at the outer Helmholtz plane (OHP) have been determined using time-resolved X-ray diffraction during the double-layer charging/discharging on the Ag(100) electrode in CsBr solution. At the double-layer potential region at which c(2 × 2)-Br is formed on Ag(100), the transient current comprises two exponential terms with different time scales: a rapid and a slow one are due to the dielectric polarization of water molecules and the transfer of Cs⁺, respectively. The slow term is composed of different dynamic processes of Cs⁺ during charging and discharging. When the potential is stepped in the positive direction, the coverage of Cs⁺ at the OHP decreases. In this step, the transient X-ray intensity at the (0 0 1) reflection, which is sensitive to the OHP structure, shows that Cs⁺ is released from the OHP according to exponential function of time. The decay of transient intensity of X-ray has a time scale similar to that of the current transient measurement. On the other hand, the accumulation process of Cs⁺ from the diffuse double layer to the OHP comprises two different kinetic processes after a potential step in the negative direction: a rapid one is the accumulation of Cs⁺ near the outer layer, and a slow one is the structural stabilization of the Cs⁺ layer

Structural Effects on the Incident Photon-to-Current Conversion Efficiency of Zn Porphyrin Dyes on the Low-Index Planes of TiO₂

The structural effects of substrates on the incident photon-to-current conversion efficiency (IPCE) of Zn porphyrin (ZnP) dyes (ZnP-ref, YD2, and ZnPBAT) have been studied on well-defined single-crystal surfaces of rutile TiO₂ (TiO₂(111), TiO₂(100), and TiO₂(110)). IPCE of ZnP-ref depends on the structure of the substrates remarkably: TiO₂(100) < TiO₂(110) < TiO₂(111). IPCE of ZnP-ref/TiO₂(111) is 13 times as high as that of ZnP-ref/TiO₂(100) at 570 nm. YD2 and ZnPBAT also give the highest IPCE on TiO₂(111). The relative coverages of the porphyrin dyes give the following order: TiO₂(111) < TiO₂(110) < TiO₂(100). This order is opposite to that of IPCEs. The orientation of the dyes is predicted using density functional theory calculations on simplified models of TiO₂ surfaces. The highest IPCE on TiO₂(111) is attributed to the high rate of electron transfer through the space due to the fluctuation of the tilt angle of the adsorbed dyes