Jai-Pil Choi

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.

Measurements of the core and ligand monolayer compositions of small gold nanoparticles (NPs) using electrospray ionization (ESI) mass spectrometry were performed by incorporating ionization tags, methoxy penta(ethylene glycol) thiolate ligands (-S-PEG), into the ligand monolayers via ligand exchange. During ESI, alkali metal ions (M+) coordinate to the -S-PEG ligands and give the NPs positive charge. Atomically precise, high-resolution measurements show unequivocally that the NP composition is Au25(ligand)18. The predominant ions, M4Au25(ligand)183+ and M5Au25(ligand)184+, have 1− charge on the core. Because ligand exchange is a statistical process, there is a distribution of mixed-monolayer exchange products, which is reflected in the mass spectra.

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.

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 (ΔGH) 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.

We report reactivity of the gold nanoparticle [TOA+] [Au25(SC2Ph)18]1− (TOA+ = tetraoctylammonium; SC2Ph = phenylethanethiolate = L; [Au25(SC2Ph)18]1− = Au25L181−) with Ag+, Cu2+, and Pb2+ ions. Titration of solutions of Au25L181− in CH2Cl2 with one and two equivalents of Ag+ produces changes in absorbance spectra with isosbestic points, and a titration curve break at 1:1 mol ratio, indicating a stoichiometric interaction. Similar effects are seen with Cu2+ and Pb2+ additions, but the break occurs at 0.5:1 mol ratio metal/nanoparticle. Changes in Au25L181− absorbance and fluorescence spectra are qualitatively similar to those accompanying oxidation of the Au25L181− nanoparticle anion, but the spectra of the stoichiometric products differ slightly according to the metal ion. Addition of higher excess of Ag+ or Cu2+ causes loss of characteristic [Au25(SC2Ph)18]1− UV−vis spectral fine structure and apparent irreversible refining into larger nanoparticles. Voltammetric currents for nanoparticle 0/-1 and +1/0 redox waves are depressed by Ag+ addition. Electrospray ionization mass spectra of products of addition of up to two equivalents Ag+ show prominent peaks for [Au25(SC2Ph)18]1− but also peaks corresponding to bimetal nanoparticles [Au24Ag(SC2Ph)18]2+, [Au23Ag2(SC2Ph)18]2+, and [Au22Ag3(SC2Ph)18]2+. We propose a redox model of reaction of the Au25L181− nanoparticle with metal ions, in which the Au25L181− nanoparticle acts as a reductant toward the metal ion, forming Au25M(SC2Ph)18 adducts that become oxidatively dissociated in the mass spectral cationization environment to yield the bimetals observed.