Electrochemical control of ion transport through a mesoporous carbon membrane.

We report a carbon-based, three-dimensional nanofluidic transport membrane that enables gated, or on/off, control of the transport of organic molecular species and metal ions using an applied electrical potential. In the absence of an applied potential, both cationic and anionic molecules freely diffuse across the membrane via a concentration gradient. However, when an electrochemical potential is applied, the transport of ions through the membrane is inhibited.

Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by MAu24 and M2Au36 Nanoclusters (M = Pt, Pd).

This paper describes the effects of doped metals on hydrogen evolution reaction (HER) electrocatalyzed by atomically controlled MAu24 and M2Au36 nanoclusters, where M = Pt and Pd. HER performances, such as onset potential ( Eonset), catalytic current density, and turnover frequency (TOF), are comparatively examined with respect to the doped metals. Doping Pt or Pd into gold nanoclusters not only changes the electrochemical redox potentials of nanoclusters but also considerably improves the HER activities. Eonset is found to be controlled by the nanocluster's reduction potential matching the reduction potential of H+. The higher catalytic current and TOF are observed with the doped nanoclusters in the order of PtAu24 > PdAu24 > Au25. The same trend is observed with the Au38 group (Pt2Au36 > Pd2Au36> Au38). Density functional theory calculations have revealed that the hydrogen adsorption free energy (Δ GH) is significantly lowered by metal-doping in the order of Au25 > PdAu24 > PtAu24 and Au38 > Pd2Au36 > Pt2Au36, indicating that hydrogen adsorption on the active site of nanocluster is thermodynamically favored by Pd-doping and further by Pt-doping. The doped metals, albeit buried in the core of the nanoclusters, have profound impact on their HER activities by altering their reduction potentials and hydrogen adsorption free energies.

NIR luminescence intensities increase linearly with proportion of polar thiolate ligands in protecting monolayers of Au38 and Au140 quantum dots.

The near-infrared photoluminescence of monolayer-protected Au38 and Au140 clusters (MPCs) is intensified with exchange of nonpolar ligands by more polar thiolate ligands. The effect is general and includes as more polar in-coming ligands: thiophenolates with a variety of p-substituents; alkanethiolates omega-terminated by alcohol, acid, or quaternary ammonium groups; and thio-amino acids. Remarkably, place exchanges of the initial phenylethanethiolates on Au38 MPCs by p-substituted thiophenolates and thio-amino acids and of hexanethiolates on Au140 MPCs by omega-quaternary ammonium terminated undecylthiolates result in increases in the near-infrared (NIR) luminescence intensities that are linear with the number of new polar ligands. The increased intensities are systematically larger for thiophenolate ligands having more electron-withdrawing substituents. Analogous effects on intensities are observed in the NIR emission of Au140 MPCs upon place exchange of alkanethiolates with thiolates having short connecting alkanethiolate chains to quaternary ammonium and to omega-carboxylic acid termini, and with oxidative charging of the Au cores. The observations are consistent with sensitivity of the luminescence mechanism to any factor that enhances the electronic polarization of the bonds between the Au core atoms and their thiolate ligands. The luminescence is discussed in terms of a surface electronic excitation, as opposed to a core volume excitation.

Electron self-exchange between Au140(+/0) nanoparticles is faster than that between Au38(+/0) in solid-state, mixed-valent films.

The well-defined one-electron steps in the voltammetry of solutions of the nanoparticles Au38(SC2Ph)24 and Au140(SC6)53 (SC2Ph = phenylethanethiolate; SC6 = hexanethiolate) enable preparation of solutions containing, for example, Au38(SC2Ph)24 and Au38(SC2Ph)24+(ClO4)- nanoparticles in known relative proportions. From these solutions can be cast dry, mixed-valent films demonstrably containing the same proportions. Electronic conduction in such mixed-valent films is shown to occur by a bimolecular electron self-exchange reaction at a rate proportional to the concentration product, [Au38][Au38+]. The observed Au38(+/0) rate constant, approximately 2 x 10(6) M(-1) s(-1), is much smaller than that previously observed for Au140(+/0) films (ca. 4 x 10(9) M(-1) s(-1); Wuelfing, W. P.; et al. J. Am. Chem. Soc. 2000, 122, 11465). To our knowledge, this is the first example of a significant size effect in metal nanoparticle electron-transfer dynamics. Thermal activation parameters for the electron-hopping conductivities of the two nanoparticles reveal that the rate difference is mainly caused by energy barriers (EA) for Au38(+/0) electron transfers that are larger by approximately 3-fold than those for Au140(+/0) electron transfers (ca. 20 vs 7 kJ/mol). Differences in pre-exponential terms in the activation equations for the two nanoparticles are a smaller contributor to the rate constant difference and can be partly ascribed to differences in tunneling distances.

Electrogenerated chemiluminescence 69: the tris(2,2'-bipyridine)ruthenium(II), (Ru(bpy)3(2+))/tri-n-propylamine (TPrA) system revisited-a new route involving TPrA*+ cation radicals.

The reaction occurring on electrooxidation of Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and tri-n-propylamine (TPrA) leads to the production of Ru(bpy)(3)(2+) and light emission. The accepted mechanism of this widely used reaction involves the reaction of Ru(bpy)(3)(3+) and a reduced species derived from the free radical of the TPrA. However, this mechanism does not account for many of the observed features of this reaction. A new route involving the intermediacy of TPrA cation radicals (TPrA(*+)) in the generation of Ru(bpy)(3)(2+) was established, based on results of scanning electrochemical microscopy (SECM)-electrogenerated chemiluminescence (ECL) experiments, as well as cyclic voltammetry simulations. A half-life of approximately 0.2 ms was estimated for TPrA(*+) in neutral aqueous solution. Direct evidence for TPrA(*+) in this medium was obtained via flow cell electron spin resonance (ESR) experiments at approximately 20 degrees C. The ESR spectra of the TPrA(*+) species consisted of a relatively intense and sharp septet with a splitting of approximately 20 G and a g value of 2.0038.