Bing-Wei Mao

The Electrode/Ionic Liquid Interface: Electric Double Layer and Metal Electrodeposition

The last decade has witnessed remarkable advances in interfacial electrochemistry in room-temperature ionic liquids. Although the wide electrochemical window of ionic liquids is of primary concern in this new type of solvent for electrochemistry, the unusual bulk and interfacial properties brought about by the intrinsic strong interactions in the ionic liquid system also substantially influence the structure and processes at electrode/ionic liquid interfaces. Theoretical modeling and experimental characterizations have been indispensable in reaching a microscopic understanding of electrode/ionic liquid interfaces and in elucidating the physics behind new phenomena in ionic liquids. This Minireview describes the status of some aspects of interfacial electrochemistry in ionic liquids. Emphasis is placed on high-resolution and molecular-level characterization by scanning tunneling microscopy and vibrational spectroscopies of interfacial structures, and the initial stage of metal electrodeposition with application in surface nanostructuring.

Do Molecular Conductances Correlate with Electrochemical Rate Constants? Experimental Insights

We measured single-molecule conductances for three different redox systems self-assembled onto gold by the STMBJ method and compared them with electrochemical heterogeneous rate constants determined by ultrafast voltammetry. It was observed that fast systems indeed give higher conductance. Monotonous dependency of conductance on potential reveals that large molecular fluctuations prevent the molecular redox levels to lie in between the Fermi levels of the electrodes in the nanogap configuration. Electronic coupling factors for both experimental approaches were therefore evaluated based on the superexchange mechanism theory. The results suggest that coupling is surprisingly on the same order of magnitude or even larger in conductance measurements whereas electron transfer occurs on larger distances than in transient electrochemistry.

Can surface Raman spectroscopy be a general technique for surface science and electrochemistry

Surface-enhanced Raman spectroscopy has almost been restricted to the study of only three noble metals of Au, Ag and Cu for two decades. Recently, a new confocal Raman microscope and special surface pretreatments have allowed the acquisition of high-quality surface Raman spectra of organic and inorganic molecules adsorbed on bare Pt, Ni, Co, Fe, Pd, Rh, Ru and Si electrodes over a wide applied potential range for the first time. The present results demonstrate several advantages of in situ surface Raman spectroscopy that could probably make it a general technique widely used in surface science and electrochemistry.

Surface enhanced Raman scattering from transition metal nano-wire array and the theoretical consideration

Co, Ni, Pt and Pd nano-wire arrays with diameter of about 50 nm were fabricated by means of template synthesis. By alternating current (AC) electrodeposition these metals were filled into channels of anodic aluminum oxide (AAO) film respectively. Nano-electrode arrays having good electric contact with the substrate was also fabricated by employing combined electroless deposition and the AC electrodeposition. Strong surface enhanced Raman scattering (SERS) was observed from both metal nano-wire arrays and nano-electrode arrays after partial removal of the AAO film. The SERS intensity of probe molecules adsorbed at the arrays depends critically on the length of the nano-wire explored at the surface. The experimental results agree well with the corresponding theoretic calculations based on electromagnetic enhancement. The lightning rod effect may play an important role for the enhancement of the Ni nano-rod under the favorable length. It has been shown that metal nano-wire arrays can be developed to a new generation of substrate exhibiting very high SERS activity, especially for transition metals. These well-ordered surface nano-structures can also be served as a proper model for the SERS mechanism study.

SURFACE RAMAN SPECTROSCOPIC STUDIES OF PYRAZINE ADSORBED ONTO NICKEL ELECTRODES

Abstract Surface enhanced Raman scattering from pyrazine adsorbed on roughened Ni electrodes was observed for the first time. A special surface roughening procedure was carried out by a HNO3-etching method to obtain high-quality surface spectra. The significant differences derived from the comparison of the spectra at different electrodes suggest stronger interactions of pyrazine with Ni than with the typical SERS active metals of Ag, Au and Cu. Some forbidden vibrational bands of pyrazine were detected due to the lowering of the symmetry from D2h to C2v by the strong adsorption on the surface.

Extending the capability of STM break junction for conductance measurement of atomic-size nanowires: An electrochemical strategy

The STM break junction (STM-BJ) and mechanically controllable break junction (MCBJ) are the two most widely applied techniques to fabricate atomic-size nanowires for conductance measurement. However, the drawbacks of the mechanical crashing between the two electrodes of the same material in these techniques hamper its capability of application in view of the variety of metals as well as the environment to perform the measurements. In this paper, we present an electrochemical strategy for STM-BJ by establishing a chemically well-defined metallic contact through a jump-to-contact mechanism between the tip and substrate of dissimilar metals, wherein the tip is in situ and electrochemically deposited with a thin film of a foreign metal of interest. The feasibility of the approach is demonstrated by taking Cu as a model system, followed by generalizing to Pd and Fe for which the conductance has been found otherwise difficult to measure at room temperature. The preferential point-contact conductance at 1, 0.9, and 0.86 G0 was measured for Cu, Pd, and Fe, respectively. The strategy present in this work not only extends the capability of STM-BJ to create a variety of metal nanowires including magnetic nanowires for further investigations but also provides opportunities to construct metal-molecule-metal junctions with a variety of choices of metals in the junctions.

DNA-modified electrodes - Part 3: spectroscopic characterization of DNA-modified gold electrodes

DNA-modified gold electrodes were characterized by scanning tunneling microscopy (STM), Raman spectroscopy, in situ UV/Vis reflection spectroscopy, X-ray photoelectron spectroscopy (XPS) and alternating current (AC) impedance. It has been found that dsDNA adsorbed firmly on gold surfaces lies strand-on in an ordered saturated monolayer, and ssDNA strands exist in a honeycomb-like form on the surfaces. The bases and phosphate groups of DNA backbone interacting with gold electrode surfaces play an important role in DNA immobilization onto gold electrode surfaces.

Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals

The spontaneous α-to-δ phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI3)1-x(MAPbBr3)x to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect α-phase perovskite without segregation is x = 0.1-0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 μs, which is over 20 times of that of FAPbI3 single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA+ is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder. Moreover, we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of δ-phase. These findings benefit the understanding of α-phase stabilization behavior, and have led to fabrication of perovskite solar cells with highest efficiency of 19.9% via solvent management.

Studies on silicon etching using the confined etchant layer technique

Silicon surface etching in HBr solutions using the confined etchant layer technique (CELT) as well as scanning electrochemical microscopy (SECM) has been carried out and comparison between the two methods has been made in terms of the etching resolution. It has been shown that the lateral diffusion of the etchant in SECM configuration can be suppressed in CELT by a homogeneous scavenging reaction and thus the etching resolution of surface, especially for those with slow etching rate such as Si can be improved. H3AsO3 was added to the solution containing HBr which reacts rapidly and homogeneously with the electrogenerated bromine, resulting in a very thin bromine diffusion layer surrounding the tip. The size of the etching spot at the Si wafer surface obtained using the CELT matches that of the tip very well.

Gating of Redox Currents at Gold Nanoelectrodes via DNA Hybridization

DNA immobilized at the surface of cone-shaped gold nanoelectrodes provides a favorable platform for artificial ion channels that can gate the redox reaction of ferricyanide. This effect arises from the nanometer-scale curvature and the enhanced mass transfer at nanoelectrodes. The label-free feature based on this ion-channel effect provides a means to develop label-free electrochemical DNA sensors.