UC Santa Cruz

Nanoparticles represent a novel class of material consisting of hundreds to a few thousand atoms each, and their physical and chemical properties are significantly different than the bulk materials. The electronic structure, chemical and optical properties of nanoparticles can be tuned by the size, shape, surface modification and interaction with supporting materials, to fulfil the potential specific applications in catalysis, imaging and electronic devices. In the preparation of nanoparticles, protecting ligands play a crucial role in the dispersion, size control, and shape control of particles. Here in this thesis, ligand functionalization of metal nanoparticles and engineering of carbon nanomaterials were manipulated, and the influence of metal-organic interaction on the chemical, optical and electrochemical properties of nanoparticles and their applications in fuel cell electrocatalysis were studied.

Ruthenium nanoparticles protected by ferrocenecarboxylates (RuFCA) were synthesized. The carboxylate group were bound onto the nanoparticle surface via Ru–O bonds in a bidentate configuration which is highly polarized, leading to the diminishment of the electron density of the iron centers and the increase of formal potential of the ferrocenyl moieties by 120mV in electrochemical measurements. In addition, galvanic exchange reactions of the RuFCA nanoparticles with Pd(II) followed by hydrothermal treatment at 200 °C led to (partial) decarboxylation of the ligands such that the ferrocenyl moieties were now directly bonded to the metal surface, as manifested in voltammetric measurements that suggested intervalence charge transfer between the nanoparticle-bound ferrocene groups.

In a further study, decarboxylation was also found happened at the metal-ligand interface in the hydrothermal treatment of 2-naphthalenecarboxylate protected ruthenium nanoparticles at higher temperature 250°C, and the naphthalenyl moieties became directly bonded to the metal cores, which was confirmed by infrared and X-ray photoelectron spectroscopic measurements. In comparison with the as-produced RuCOONA nanoparticles, the decarboxylated nanoparticles (RuNA) exhibited markedly different optical and electronic properties due to electronic coupling between the particle-bound naphthalene groups. The intraparticle charge delocalization led to spilling of nanoparticle core electrons to naphthalene moieties, resulting in the negative shift of the formal potential.

In addition, the electron-transfer properties of the ruthenium nanoparticles protected by 1-dodecyne, laurate and 1-dodecanethiol were examined by scanning tunneling spectroscopic (STS) measurements. Ruthenium-vinylidene (Ru=C=CH–), –oxygen (Ru-O), and –thiolate (Ru-S) interfacial bonds were formed when protected by alkyne, carboxylate and thiol ligands, respectively, and the polarization of the interfacial bonds was found to increase in the order of Ru=C=CH– < Ru-S < Ru-O. The relatively large nanoparticles (dia. ∼ 3 nm) were found to show clearly-defined Coulomb staircase; and with diminishing particle core dimensions to below 1 nm, Coulomb blockade started to emerge. The nanoparticle molecular capacitance and effective nanoparticle dielectric constants were estimated and the dielectric constants increased inversely with the nanoparticle core dimensions; and at any given particle size, the dielectric constants varied with the specific metal-ligand interfacial bonds, increasing in the order of Ru-S < Ru=C=CH– < Ru-O.

Part of the dissertation research was devoted to carbon nanomaterials. In one of the study, nitrogen-doped graphene quantum dots (NGQDs) were prepared by a facile hydrothermal method and incorporated with ruthenium metal ions by exploiting the unique complexation of selected nitrogen dopants with ruthenium ions. Complexation of NGQDs with ruthenium ions likely occurred through the pyridinic nitrogen dopants, leading to the incorporation of multiple metal centers within the conjugated graphitic C sp2 scaffolds (Ru-NGQDs). Intervalence charge transfer between embedded Ru ions was studied electrochemically, the Ru-NGQD compounds exhibited two pairs of voltammetric waves, with a peak spacing of 150 mV, suggesting Class II delocalized system. Near-infrared spectroscopic measurements demonstrated an absorption band emerged at ca. 1450 nm at mixed-valence metal charge transfer, by using Ce(SO4)2 as the oxidizing reagent.

Covalently crosslinking of GQDs were accomplished by forming ensembles of a few hundred nanometers in size by McMurry deoxygenation coupling reactions of peripheral carbonyl functional moieties catalyzed by TiCl4 and Zn powders in refluxing THF. Photoluminescence measurements showed that after chemical coupling, the excitation and emission peaks blue-shifted somewhat and the emission intensity increased markedly, likely due to the removal of oxygenated species where quinone-like species were known to be effective electron-acceptors and emission quenchers.

Metal nanoparticles were also prepared and tuned for the applications as high-performance catalysts. In one of the research, gold core@silver semishell Janus nanoparticles were prepared by interfacial chemical etching of Au@Ag core–shell nanoparticles at the air/water interface. The resulting Janus nanoparticles exhibited an asymmetrical distribution of silver on the surface of the gold cores. Interestingly, the Au@Ag semishell Janus nanoparticles exhibited enhanced electrocatalytic activity in oxygen reduction reactions, as compared to their Au@Ag and Ag@Au core–shell counterparts, likely due to a synergistic effect between the gold cores and silver semishells that optimized oxygen binding to the nanoparticle surface. In another research, cysteine-stabilized Ag–Cu hollow nanoshells are prepared by the co-reduction of silver nitrate and cupric nitrate with sodium borohydride in the presence of sodium thiocyanate. When capped with 1-dodecanethiol, the hollow nanoshells become dispersible in apolar organic solvents and the cavity may be exploited for the effective phase-transfer of target molecules such as rhodamine 6G between water and organic media. The Ag–Cu nanoshells also show apparent catalytic activity toward the reduction of 4-nitroaniline by sodium borohydride, a performance that is markedly better than that of the solid counterparts and comparable to leading results in recent literature based on relevant metal catalysts.