Studies of electrochemical surface alloying and dealloying by in situ high-speed STM.

The electrochemical formation and dissolution of a lead/copper surface alloy on Cu(100) in chloride-containing electrolyte solutions were studied on the atomic scale by in situ scanning tunneling microscopy with high temporal and spatial resolution. Alloy formation, induced by a negative potential sweep, starts predominantly at the Cu steps, followed by the formation of a novel transient (4 × 3) alloy phase with 0.25 ML Pb coverage, which continuously is transformed into the 0.375 ML coverage c(4 × 4) phase, observed under UHV conditions. Both of these phases consist of rows of Pb atoms embedded into the Cu surface and exhibit highly dynamic structural fluctuations on sub 100 ms time scales. Upon increasing the potential again, a second c(4 × 4) phase with a different appearance in the STM images forms, which is attributed to partial dealloying, involving desorption of Pb from energetically less favorable sites. Further dealloying results in the formation of ribbon-like structures, already reported in previous studies. These ribbons are shown to consist of Pb atoms decorating domain boundaries in the c(2 × 2) chloride adlayer, left behind on the Cu surface by the dissolving surface alloy phase. Furthermore, dynamic observations of the subsequent coarsening of the ribbon network and the attachment/detachment of isolated Pd adsorbates to the ribbons are presented. Both isolated Pb adsorbates and Pb atoms in the ribbons are proposed to be stabilized by coadsorbed Cl.

In-situ high-speed atomic force microscopy observation of dynamic nanobubbles during water electrolysis.

The nanobubbles (NB) formed on a solid surface has been reported, although the stability in a solution is still discussed. The atomic force microscopy (AFM) has shown the unique shape of the flatted NB, however the dynamic behavior has not investigated yet. Recently, the high scanning speed AFM (HS-AFM) has been developed and applied to the several interfaces. Here, we present in-situ HS-AFM observation during water electrolysis. The hydrogen and oxygen NB evolution on highly oriented pyrolytic graphite (HOPG) electrode are studied. Our video data is the first time to demonstrate the NB nucleation and the growth. The processes are different between both gases. The hydrogen NB grows with active coalescence, while the oxygen one is smaller and irregularly moves on HOPG surface. Using this technique, we will be able to study the NB stability influence by some factors, such as the surface potential and electric capillarity.

Efficient Hydrogen Isotope Separation by Tunneling Effect Using Graphene-Based Heterogeneous Electrocatalysts in Electrochemical Hydrogen Isotope Pumping.

The fabrication of a hydrogen isotope enrichment system is essential for the development of industrial, medical, life science, and nuclear fusion fields, and therefore, efficient enrichment techniques with a high separation factor and economic feasibility are still being explored. Herein, we report a hydrogen/deuterium (H/D) separation ability with polymer electrolyte membrane electrochemical hydrogen pumping (PEM-ECHP) using a heterogeneous electrode consisting of palladium and graphene layers (PdGr). By mass spectroscopic analysis, we demonstrate significant bias voltage dependence of the H/D separation factor with a maximum of ∼25 at 0.15 V and room temperature, which is superior to those of conventional separation methods. Theoretical analysis demonstrated that the observed high H/D factor stems from tunneling of hydrogen isotopes through atomically thick graphene during the electrochemical reaction and that the bias dependence of H/D results from a transition from the quantum tunneling regime to the classical overbarrier regime for hydrogen isotopes transfer through the graphene. These findings will help us understand the origin of the isotope separation ability of graphene discussed so far and contribute to developing an economical hydrogen isotope enrichment system using two-dimensional materials.

In situ video STM studies of the hydrogen-induced reconstruction of Cu(100): potential and pH dependence.

The surface structure of Cu(100) electrodes in perchloric acid solutions of pH 1 to 3 was studied in the potential range of hydrogen evolution by video-rate scanning tunneling microscopy, focusing on the recently reported hydrogen-induced surface reconstruction [H. Matsushima et al., J. Am. Chem. Soc. 2009, 131, 10362]. Potential-dependent measurements reveal a two step formation process: at potentials close to the onset of hydrogen evolution a p(1×8) phase emerges, where Cu surface atoms in stripe-like structures are laterally and vertically displaced; at ≈30 mV more negative potentials a transition to a c(p×8) structure with an expanded Cu surface lattice occurs. Correlation of these observations with electrochemical data and studies on hydrogen interactions with Cu(100) surfaces under vacuum conditions support that these phases are induced by hydrogen in subsurface sites, pointing towards a high hydrogen coverage on this electrode surface under reaction conditions.

Reconstruction of Cu(100) electrode surfaces during hydrogen evolution.

Electrochemical hydrogen evolution on (100)-oriented copper electrodes is shown to induce a novel surface reconstruction, which substantially influences the rates of this electrochemical reaction. As revealed by in situ video-STM the formation of this phase starts with lateral displacements of Cu surface atoms from lattice positions, resulting in stripe-like structures, followed by expansion of the surface lattice along the stripe direction.

A Superhydrophilic Aluminum Surface with Fast Water Evaporation Based on Anodic Alumina Bundle Structures via Anodizing in Pyrophosphoric Acid.

A superhydrophilic aluminum surface with fast water evaporation based on nanostructured aluminum oxide was fabricated via anodizing in pyrophosphoric acid. Anodizing aluminum in pyrophosphoric acid caused the successive formation of a barrier oxide film, a porous oxide film, pyramidal bundle structures with alumina nanofibers, and completely bent nanofibers. During the water contact angle measurements at 1 s after the water droplet was placed on the anodized surface, the contact angle rapidly decreased to less than 10°, and superhydrophilic behavior with the lowest contact angle measuring 2.0° was exhibited on the surface covered with the pyramidal bundle structures. As the measurement time of the contact angle decreased to 200-33 ms after the water placement, although the contact angle slightly increased in the initial stage due to the formation of porous alumina, at 33 ms after the water placement, the contact angle was 9.8°, indicating that superhydrophilicity with fast water evaporation was successfully obtained on the surface covered with the pyramidal bundle structures. We found that the shape of the pyramidal bundle structures was maintained in water without separation by in situ high-speed atomic force microscopy measurements.

Novel PEFC Application for Deuterium Isotope Separation.

The use of a polymer electrolyte fuel cell (PEFC) with a Nafion membrane for isotopic separation of deuterium (D) was investigated. Mass analysis at the cathode side indicated that D diffused through the membrane and participated in an isotope exchange reaction. The exchange of D with protium (H) in H2O was facilitated by a Pt catalyst. The anodic data showed that the separation efficiency was dependent on the D concentration in the source gas, whereby the water produced during the operation of the PEFC was more enriched in D as the D concentration of the source gas was increased.