A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt.

Isolable, water-soluble gold clusters protected by monolayers of tiopronin (tiopronin−MPCs) or coenzyme A (CoA−MPCs) were synthesized by a procedure of comparable simplicity to the Brust synthesis for alkanethiolate monolayer-protected gold clusters. High-resolution transmission electron microscopy shows that, like their alkanethiolate−MPC counterparts, the average core diameters of tiopronin−MPCs can be systematically controlled by varying the tiopronin:Au mole ratio employed in the reaction. The UV−vis spectra of tiopronin−MPCs exhibit pH and core size dependency of the surface plasmon band position and intensity, respectively. Thermogravimetric analysis of the tiopronin−MPCs gave average numbers of tiopronin ligands per cluster; for example, tiopronin−MPCs with an average core size of 1.8 nm (∼Au201) are protected with an average of 85 tiopronin ligands. 1H NMR reveals a size-dependent evolution of spectral features interpreted as reflecting differences in attachment sites (terrace, defects) and/or restriction in ligand mobility. Infrared spectroscopy reveals strong hydrogen bonding in the monolayer and provides evidence for intercluster association, and acid/base titrations produce pK A values similar to the free ligand in the presence of a charge-screening supporting electrolyte, but higher in its absence. The same analytical methods were also applied to CoA-MPCs.

Carbon nanoparticles were prepared by refluxing the combustion soot of natural gas in nitric acid. Transmission Electron Microscopy measurements showed that the resulting particles exhibited an average diameter of 4.8 ± 0.6 nm, and the crystalline lattices were consistent with graphitic carbons. 13C NMR and FTIR spectroscopic measurements further confirmed the presence of sp2 carbons in the form of aryl and carboxylic/carbonyl moieties. The resulting carbon nanoparticles were found to emit photoluminescence with a quantum yield of approximately 0.43%. Additionally, the emission band energy of the carbon nanoparticle was very similar to that of much smaller carbon nanoparticles obtained from candle soot, suggesting that the photoluminescence might arise from particle surface states, analogous to the behaviors of semiconductor quantum dots with an indirect bandgap. In electrochemical measurements, two pairs of well-defined voltammetric waves were observed, which might be ascribed to the peripheral functional moieties that were analogous to phenanthrenequinone derivatives. Interestingly, the carbon nanoparticles might also be exploited as nanoscale structural scaffolds for the deposition of nanostructures of varied transition metals, leading to the formation of metal−carbon functional nanocomposites.

Bigger isn't better: Gold nanoclusters exhibit enhanced electrocatalytic activity in oxygen reduction when the core size decreased from Au(140) to Au(11), as reflected by the onset potential E(onset) and peak current density J(K) (see picture). The increasing fraction of surface atoms with low coordination numbers and the shift of the d-band center to the Fermi level facilitate oxygen adsorption.

Electrocatalysts with single metal atoms as active sites have received increasing attention owing to their high atomic utilization efficiency and exotic catalytic activity and selectivity. This review aims to provide a comprehensive summary on the recent development of such single-atom electrocatalysts (SAECs) for various energy-conversion reactions. The discussion starts with an introduction of the different types of SAECs, followed by an overview of the synthetic methodologies to control the atomic dispersion of metal sites and atomically resolved characterization using state-of-the-art microscopic and spectroscopic techniques. In recognition of the extensive applications of SAECs, the electrocatalytic studies are dissected in terms of various important electrochemical reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Examples of SAECs are deliberated in each case in terms of their catalytic performance, structure−property relationships, and catalytic enhancement mechanisms. A perspective is provided at the end of each section about remaining challenges and opportunities for the development of SAECs for the targeted reaction.

Thermally removable nanoparticle templates were used for the fabrication of self-supported N-doped mesoporous carbons with a trace amount of Fe (Fe-N/C). Experimentally Fe-N/C was prepared by pyrolysis of poly(2-fluoroaniline) (P2FANI) containing a number of FeO(OH) nanorods that were prepared by a one-pot hydrothermal synthesis and homogeneously distributed within the polymer matrix. The FeO(OH) nanocrystals acted as rigid templates to prevent the collapse of P2FANI during the carbonization process, where a mesoporous skeleton was formed with a medium surface area of about 400 m(2)/g. Subsequent thermal treatments at elevated temperatures led to the decomposition and evaporation of the FeO(OH) nanocrystals and the formation of mesoporous carbons with the surface area markedly enhanced to 934.8 m(2)/g. Electrochemical measurements revealed that the resulting mesoporous carbons exhibited apparent electrocatalytic activity for oxygen reduction reactions (ORR), and the one prepared at 800 °C (Fe-N/C-800) was the best among the series, with a more positive onset potential (+0.98 V vs RHE), higher diffusion-limited current, higher selectivity (number of electron transfer n > 3.95 at +0.75 V vs RHE), much higher stability, and stronger tolerance against methanol crossover than commercial Pt/C catalysts in a 0.1 M KOH solution. The remarkable ORR performance was attributed to the high surface area and sufficient exposure of electrocatalytically active sites that arose primarily from N-doped carbons with minor contributions from Fe-containing species.

Hydrogen evolution reaction is an important process in electrochemical energy technologies. Herein, ruthenium and nitrogen codoped carbon nanowires are prepared as effective hydrogen evolution catalysts. The catalytic performance is markedly better than that of commercial platinum catalyst, with an overpotential of only −12 mV to reach the current density of 10 mV cm-2 in 1 M KOH and −47 mV in 0.1 M KOH. Comparisons with control experiments suggest that the remarkable activity is mainly ascribed to individual ruthenium atoms embedded within the carbon matrix, with minimal contributions from ruthenium nanoparticles. Consistent results are obtained in first-principles calculations, where RuCxNy moieties are found to show a much lower hydrogen binding energy than ruthenium nanoparticles, and a lower kinetic barrier for water dissociation than platinum. Among these, RuC2N2 stands out as the most active catalytic center, where both ruthenium and adjacent carbon atoms are the possible active sites.

Subnanometer-sized copper nanoclusters were prepared by a one-pot procedure based on wet chemical reduction. The structural characteristics of the 2-mercapto-5-n-propylpyrimidine-protected nanoclusters, Cu(n) (n ≤ 8), were determined by mass spectrometry. The Cu nanoclusters displayed apparent luminescence, with dual emissions at 425 and 593 nm, with quantum yields of 3.5 and 0.9%, respectively, and high electrocatalytic activity in the electoreduction of oxygen.