Composite with a Glassy Nonporous Coordination Polymer Enhances Gas Adsorption Selectivity.

A glass-crystal composite (g-NCP/PCP), comprising a glassy nonporous coordination polymer (g-NCP) and a crystalline porous coordination polymer (PCP)/metal-organic framework, was synthesized by using a melt-quenched method. Compared to that of the PCP itself, g-NCP/PCP has an enhanced gas adsorption selectivity. The results should stimulate further studies of the chemistry of g-NCP/PCP glass-crystal composites.

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe-N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands.

Fe/N/C single-atom catalysts containing Fe-Nx sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe-N4 sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe-N4 sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe-N4 sites catalyzing ORR with a TOF value of 0.21 e site-1 s-1 at 0.8 V vs. RHE. This indicates that the formation of Fe-N4 sites is controlled by precise tuning of the chemical structure of the iron complex precursor.

Deprotonation of a dinuclear copper complex of 3,5-diamino-1,2,4-triazole for high oxygen reduction activity.

A dinuclear copper(II) complex of 3,5-diamino-1,2,4-triazole is one of the highly active copper-based catalysts for the oxygen reduction reaction (ORR) in basic solutions. Our in situ X-ray absorption near edge structure measurements revealed that deprotonation of the triazole ligand might cause coordination geometrical changes, resulting in the enhancement of the ORR activity.

Ultrafast dynamics of photogenerated electrons in CdS nanocluster multilayers assembled on solid substrates: effects of assembly and electrode potential.

The ultrafast dynamics of photogenerated electrons in multilayer assemblies of CdS nanoparticles prepared on quartz and indium-tin oxide (ITO) substrates were followed by femtosecond (fs) visible-pump/mid-IR probe spectroscopy. Based on the observation of the photoinduced transient absorption spectra in the broad mid-IR range at the multilayer assembly of CdS nanoparticles, the occupation and fast relaxation of higher electronic states (1P(e)) were clarified. As compared with the electron dynamics of isolated (dispersed in solution) nanoparticles, the decay of photoexcited electrons in the multilayer assembly was clearly accelerated probably due to both electron hopping and scattering during interparticle electron tunneling. By using an ITO electrode as a substrate, the effect of the electric field on the photoelectron dynamics in the multilayer assembly was also investigated in situ. Both the amplitude and lifetime of photoexcited electrons gradually reduced as the potential became more positive. This result was explained by considering the reduction of the interparticle tunneling probability and the increase in the electron-transfer rate from the CdS nanoparticle assembly to the ITO electrode.

In situ fluorescence yield soft X-ray absorption spectroscopy of electrochemical nickel deposition processes with and without ethylene glycol.

The electrochemical Ni deposition at a platinum electrode was investigated in a plating nickel bath in the presence and absence of ethylene glycol (EG) using fluorescence yield soft X-ray absorption spectroscopy (FY-XAS) in the Ni L2,3-edge and O K-edge regions under potential control. At ≤+0.35 V vs. the reversible hydrogen electrode (RHE), the electrochemical Ni deposition was detected by the Ni L2,3-edge FY-XAS in the presence of EG whereas almost no such event was observed in the absence of EG. A drastic decrease of FY-XAS intensities in the O K-edge region was also observed in the presence of EG at >+0.35 V vs. RHE, suggesting that the nano-/micro-structured Ni deposition initiated by the removal of water molecules occurs on the Pt electrode. The complex formation of Ni2+ with EG and the adsorption of EG on the Ni surface could play an important role in the Ni deposition. This study demonstrates that the in situ FY-XAS is a powerful and surface-sensitive technique to understand (electro)chemical reactions including polyol synthesis and electrocatalysis at solid-liquid interfaces.

Effects of Interfacial Interactions on Electrocatalytic Activity of Cytochrome c Oxidase in Biomimetic Lipid Membranes on Gold Electrodes.

Effects of interfacial interactions on the electrocatalytic activity of protein-tethered bilayer lipid membranes (ptBLMs) containing cytochrome c oxidase (CcO) for the oxygen reduction reaction are studied by using protein film electrochemistry and surface-enhanced infrared absorption (SEIRA) spectroscopy. Mammalian CcO was immobilized on a gold electrode via self-assembled monolayers (SAMs) of mixed alkanethiols. The protein orientation on the electrode is controlled by SAM-CcO interactions and is critical to the cytochrome c (cyt c) binding. The CcO-phospholipid and CcO-cyt c interactions modulate the electrocatalytic activity of CcO, and more densely packed ptBLMs show higher electrocatalytic activity. Our study indicates that spectroscopic and electrochemical studies of ptBLMs can provide insights into the effects of relatively weak protein-protein and protein-lipid interactions on the enzymatic activity of transmembrane enzymes.

Electrochemical modification of surface morphology of Au/Ti bilayer films deposited on a Si prism for in situ surface-enhanced infrared absorption (SEIRA) spectroscopy.

Surface-enhanced infrared absorption (SEIRA)-active Au/Ti bilayer films sputter deposited on Si substrates have been prepared by an electrochemical annealing (ECA) treatment for the first time. The application of Au/Ti bilayer films on Si substrates to the spectroscopic technique is a promising alternative to the conventional technique using directly deposited Au films on Si substrates, offering excellent adhesive durability of the deposited metal films. However, Au/Ti bilayer films have never been selected for the spectroscopy technique because the films in the as-prepared state exhibit relatively smooth surface morphology: the excitation of the localized surface plasmon is vital to achieving SEIRA enhancements but could hardly be observed on the smooth morphology. It is shown by ex situ scanning tunneling microscopy measurements that the unfavorable smooth morphology of the as-prepared Au/Ti bilayer films can be modified by the ECA treatment to a reasonably rough, island-structure morphology similar to that of the conventional SEIRA-active Au films. In situ infrared absorption spectroscopy of adsorbed sulfate anions has been conducted on the Au/Ti bilayer film both before and after ECA treatment. The spectroscopy measurements demonstrate that the SEIRA activity of the film after being subjected to the treatment is significantly improved so that the technique could detect adsorbates on the film electrodes even with the submonolayer coverage. As an additional benefit, the ECA treatment has brought about a substantial increase in the fraction of Au(111) domains on the polycrystalline Au film surfaces. Accordingly, this approach enables us to prepare SEIRA-active Au films having sufficient adhesion to the Si substrates as well as the highly preferred (111) orientation.

Terahertz Raman Spectroscopy of Ligand-Protected Au8 Clusters.

For ligand-protected gold clusters, geometrical differences of gold cores and/or the presence of secondary gold core-ligand interactions influence their unique optical and electronic properties and can, in principle, be detected by spectral changes of gold core vibrations (phonon modes) in ultralow-frequency Raman spectroscopy. We report experimental and theoretical Raman spectra of Au8 clusters protected by phosphine ligands particularly in the "gold cluster fingerprint" region from 50 to 150 cm-1 Raman shift (1.5 to 4.5 terahertz, THz). A characteristic core breathing mode observed at ca. 123 cm-1 was sensitive to differences of core geometries. A new band was found at ca. 150 cm-1, originating from a local strain on a polyhedral gold core caused by weak Au···π interactions. THz Raman spectroscopy can be utilized for metal nanoclusters to visualize core structural changes and Au···π interactions, which cannot be captured by single crystal X-ray analysis.

Enhancement of Electrocatalytic Oxygen Reduction Activity and Durability of Pt-Ni Rhombic Dodecahedral Nanoframes by Anchoring to Nitrogen-Doped Carbon Support.

Pt-based nanostructured electrocatalysts supported on carbon black have been widely studied for the oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells. Because sluggish ORR kinetics are known to govern the cell performance, there is a need to develop highly active and durable electrocatalysts. The ORR activity of Pt-based electrocatalysts can be improved by controlling their morphology and alloying Pt with transition metals such as Ni. Improving the catalyst durability remains challenging and there is a lack of catalyst design concepts and synthetic strategies. We report the enhancement of the ORR activity and durability of a nanostructured Pt-Ni electrocatalyst by strong metal/support interactions with a nitrogen-doped carbon (NC) support. Pt-Ni rhombic dodecahedral nanoframes (NFs) were immobilized on the NC support and showed higher ORR electrocatalytic activity and durability in acidic media than that supported on a nondoped carbon black. Durability tests demonstrated that NF/NC showed almost no activity loss even after 50 000 potential cycles under catalytic conditions, and the Ni dissolution from the NFs was suppressed at the NC support, as confirmed by energy dispersive X-ray spectroscopy analysis. Physicochemical measurements including surface-enhanced infrared absorption spectroscopy of surface-adsorbed CO revealed that the strong metal/support interactions of the NF with the NC support caused the downshift of the d-band center position of the surface Pt. Our findings demonstrate that tuning the electronic structure of nanostructured Pt alloy electrocatalysts via the strong metal/support interactions with heteroatom-doped carbon supports will allow the development of highly active and robust electrocatalysts.

Surface-Enhanced Infrared Absorption Spectroscopy of Bacterial Nitric Oxide Reductase under Electrochemical Control Using a Vibrational Probe of Carbon Monoxide.

Nitric oxide reductases (NORs) reduce nitric oxide to nitrous oxide in the denitrification pathway of the global nitrogen cycle. NORs contain four iron cofactors and the NO reduction occurs at the heme b3/nonheme FeB binuclear active site. The determination of reduction potentials of the iron cofactors will help us elucidate the enzymatic reaction mechanism. However, previous reports on these potentials remain controversial. Herein, we performed electrochemical and surface-enhanced infrared absorption (SEIRA) spectroscopic measurements of Pseudomonas aeruginosa NOR immobilized on gold electrodes. Cyclic voltammograms exhibited two reduction peaks at -0.11 and -0.44 V vs SHE, and a SEIRA spectrum using a vibrational probe of CO showed a characteristic band at 1972 cm-1 at -0.4 V vs SHE, which was assigned to νCO of heme b3-CO. Our results suggest that the reduction of heme b3 initiates the enzymatic NO reduction.

Development of a spectro-electrochemical cell for soft X-ray photon-in photon-out spectroscopy.

We developed a spectro-electrochemical cell for X-ray absorption and X-ray emission spectroscopy, which are element-specific methods to study local electronic structures in the soft X-ray region. In the usual electrochemical measurement setup, the electrode is placed in solution, and the surface/interface region of the electrode is not normally accessible by soft X-rays that have low penetration depth in liquids. To realize soft X-ray observation of electrochemical reactions, a 15-nm-thick Pt layer was deposited on a 150-nm-thick film window with an adhesive 3-nm-thick Ti layer for use as both the working electrode and the separator window between vacuum and a sample liquid under atmospheric pressure. The designed three-electrode electrochemical cell consists of a Pt film on a SiC window, a platinized Pt wire, and a commercial Ag|AgCl electrode as the working, counter, and reference electrodes, respectively. The functionality of the cell was tested by cyclic voltammetry and X-ray absorption and emission spectroscopy. As a demonstration, the electroplating of Pb on the Pt/SiC membrane window was measured by X-ray absorption and real-time monitoring of fluorescence intensity at the O 1s excitation.

Size-dependent carrier dynamics in CdS nanoparticles by femtosecond visible-pump/IR-probe measurements.

Ultrafast photoexcited carrier dynamics in CdS nanoparticles prepared by an AOT/n-heptane reversed micelle system were investigated by a femtosecond visible-pump/mid-IR probe technique. A mid-IR probe beam was found to mainly probe the ultrafast dynamics of photoexcited electrons in the conduction band. Dispersions of CdS nanoparticles with 8 different mean diameters from 2.9 to 4.1 nm were prepared by tuning the mole ratio between water and AOT (W = [H(2)O]/[AOT]) in the reversed micelle systems. The excited state lifetime strongly depended on the mean size of CdS nanoparticles with a maximum around a mean diameter of 3.5 nm. This result was explained by considering the balance between the carrier recombination rates via surface states and those via interior states. The relationship between the excited state lifetime and the size of CdS nanoparticles was drastically changed when the surface was terminated by thiol molecules.

Electrodeposition of flattened Cu nanoclusters on a p-GaAs(001) electrode monitored by in situ optical second harmonic generation.

In situ optical second harmonic generation (SHG) technique was employed to investigate the shape and density of Cu nanoclusters, which were electrochemically formed on p-GaAs(001) electrode surfaces. Since GaAs is not a centrosymmetric medium, a significant portion of SHG signal arises from the bulk dipole susceptibility, but it was possible to separate a surface-induced signal from a bulk-induced signal by choosing an appropriate experimental geometry and appropriate data processing. The rotational anisotropy (RA) pattern of the SHG signal from a p-GaAs(001) electrode changed in both shape and magnitude during potential cycling in an electrolyte solution containing Cu2+. The surface plasmon-induced SHG signal from Cu nanoclusters deposited on GaAs was attributed to the modulation source for the RA-SHG pattern. More detailed study was carried out with both in situ SHG and ex situ AFM measurements for Cu nanoclusters deposited by potential step. The results showed that the SHG signal at the present optical geometry was sensitive to the number of oblate or flattened Cu nanoclusters with lateral diameter larger than 30 nm and that the SHG enhancement occurred because of resonant coupling between the surface plasmon induced in the flattened Cu nanoclusters and the near-infrared fundamental light.

Preparation of a self-standing mesoporous carbon membrane with perpendicularly-ordered pore structures.

A self-standing mesoporous carbon membrane (sOMC) with perpendicularly-ordered pore structures was prepared through a simple synthetic method; the pores with a diameter of 8 nm were well ordered over a large area and perpendicularly-oriented to the surface without any external field; in the formation of this ordered structure, the drying process is key, and a porous alumina support is important to induce drying.

Photocontrolled magnetization of CdS-modified Prussian blue nanoparticles.

The first photocontrollable magnetic nanoparticles containing CdS and Prussian blue (PB) have been created using reverse micelles as nanoreactors. Photoinduced electron transfer from CdS to PB in the reverse micelle changed the magnetic properties of the composite nanoparticles from ferromagnetic to paramagnetic. The magnetization in the ferromagnetic region below 4 K was substantially decreased after UV light illumination and could be restored almost to its original level by thermal treatment at room temperature. This novel strategy of designing composite nanoparticles containing photoconductive semiconductors and magnetic materials to create photoswitchable magnetic materials may open many possibilities in the development of magneto-optical devices.

Optical recognition of surface chirality at Au(hkl) single crystalline surfaces by second harmonic generation rotational anisotropy.

Two-dimensional chirality at naturally chiral gold single crystalline surfaces was detected and characterized using optical second harmonic generation (SHG) measurements. SHG rotational anisotropy (SH-RA) patterns at Au(643)S and Au(643)R surfaces were mirror symmetric to each other. Systematic SH-RA measurements at chiral Au(hkl) surfaces with the same step and kink structures but different (111) terrace widths showed a linear correlation between surface step density and SH-RA fitting parameters arising from defects. These results indicate that SH-RA measurements provide information not only on surface chirality but also on density of surface defects.