Noble metal nanoparticles represent a large class of functional nanomaterials with unique physical and chemical properties that are significantly different from those of their atomic or bulk forms. Traditionally, nanoparticle surface functionalization has been achieved by using mercapto derivatives as the ligands of choice by forming metalthiolate covalent linkages, but such interfacial linkages lack interesting chemistry. In recent years, metalcarbon covalent bonds have been explored for surface functionalization of noble metal nanoparticles with the formation of, for instance, metalcarbon (sp2 hybridized) single bond (MCsp2), metalcarbene (MC), metalacetylide (MC≡C)/metalvinylidene (MCC) bonds.

Because of the conjugated metalcarbon interfacial bonds, the resistance at the metalligand interface is significantly reduced, and intraparticle charge delocalization between the particle-bound functional moieties occurs, leading to the emergence of new optical and electronic properties that are analogous to those of their dimeric counterparts. More importantly, such electronic communication may be further manipulated by modifying the electronic structures of the nanoparticle cores and enabling specific interactions between the organic ligand shells and selective molecules/ions in local chemical environment. The fundamental insights may be further developed for sensitive chemical/biological sensing as well as electrocatalysts in fuel cell electrochemistry with enhanced performance.

Within this thesis, detailed discussion will be focused on metal nanoparticles functionalized with alkyne and alkene derivatives with respect to their interfacial structure, manipulation of intraparticle charge delocalization, and futher exploration as electrocatalysts for oxygen reduction reaction (ORR).

By comparing the deuterium labelled platinum nanoparticles (PtDC12, stabilized with dodec-1-deuteroyne) with the unlabeled counterparts (PtHC12, capped by 1-dodecyne), the interfacial bonds at the metalligand interface of alkyne-functionalized metal nanoparticles have been identified as the Ptvinylidene (PtCC) bonds dominated rather than Ptacetylide (PtCC) + platinum-hydride (PtH) bonds .

The manipulation of intraparticle charge delocalization was achieved for acetylene derivatives functionalized platinum nanoparticles. 4-Enthynylpheynyl boronic acid pinacol ester functionalized platinum nanoparticles (PtEPBAPE) were found to exhibit apparent photoluminescence due to the particle-bound acetylene moieties that behaved analogously to their dimeric derivatives. More importantly, the nanoparticle photoluminescence can be selectively modified with fluoride ions by forming BF bonds, indicating the effective manipulation of the intraparticle charge delocalization within the PtEPBAPE nanoparticles by the local chemical environment. The manipulation of intraparticle electronic communication has also been accomplished by deliberately tuning the metal core size. This was demonstrated with spectroscopic and electrochemical studies of two types of ethynylferrocene functionalized platinum nanoparticles with different sizes of the metal cores (Pt314eFc and Pt10eFc, with metal core consisting of about 314 and 10 platinum atoms, respectively). As the particle size increased, infrared measurements showed a more pronounced red-shift of the C≡C/CC and ferrocenyl CH vibration bands, and electrochemical measurements demonstrated enhanced intervalence charge transfer by displaying two pairs of voltammetric peaks with a larger potential spacing. This strongly suggests the enhanced (weakened) intraparticle charge delocalization with increasing (decreasing) particle size, which can be accounted for by the different conductivity of the metal cores. Furthermore, Pt10eFc exhibited improved intervalence charge transfer under UV photoirradiation because of enhanced Pt10 core (semiconductor-like) conductivity, while no such effect was observed for the Pt314eFc (metal-like) sample.

Alkene derivatives were explored as new ligands of choice for platinum nanoparticle functionalization by self-assembly. It was first demonstrated with 1-octadecene functionalized Pt nanoparticles (PtODE). The Ptacetylide (PtC≡C)/Ptvinylidene (PtCC) bonds were identified as one form of the interfacial interactions as a result of dehydrogenation and transformation of the olefin moieties catalyzed by platinum. This is suggested by the spectroscopic results showing apparent photoluminescence characteristics and a red-shift of the C≡C/CC vibration bands, similar with those of alkyne-functionalized platinum nanoparticles. This was then further confirmed with the selective reactivity of the nanoparticles with imine derivatives that was specific to Ptacetylide (PtC≡C)/Ptvinylidene (PtCC) bonds, as manifested by NMR and electrochemical measurements. Further X-ray absorption near-edge spectroscopy (XANES) suggested the Ptligand interfacial bonds were in intermediate between those of PtC≡C/PtCC and PtCsp2. The para-substituents effects were then examined by using para-substituted styrene derivatives (4-tert-butylstyrene, 4-methoxystyrene, and 4-(trifluoromethyl)styrene) functionalized platinum nanoparticles (PtTBS, PtMOS, and PtTFMS, respectively). The nanoparticle core size increases in the order of PtTFMS < PtMOS < PtTBS due to the reduced dehydrogenation rate with the decreasing Taft (polar) constants of the para-substituents. Apparent photoluminescence was also observed for all three nanoparticles, ascribed to the effective intraparticle delocalization between particle-bound functional moieties. More importantly, the emission peaks were found red-shifted with decreasing Hammett constant of the para-substituents. Additionally, electrochemical measurements suggested the PtTBS nanoparticles exhibited the best performance as ORR electrocatalysts, which may be accounted for by the optimal combination of the nanoparticle core size and ligand effects on the oxygen affinity of Pt metal cores.

A series of 1-dodecyne functionalized AgAu alloy nanoparticles with varied Ag/Au ratios were also synthesized and evaluated as electrocatalysts for ORR. The alloy nanoparticles exhibited a core size in the range of 35 nm with gold concentrations varied from 25 at% to 55 at%. The successful surface functionalization of 1-dodecyne was confirmed with IR measurements. Electrochemical studies displayed that all samples showed apparent catalytic activity for ORR in alkaline media, which was much enhanced compared with pure Ag catalysts, and the best performance was observed from the one containing 35.5 at% gold, which was even comparable to that of commercial Pt/C catalysts. The improved electrocatalytic activity was accounted for by the enhanced oxygen affinity of the AgAu nanoparticles due to the alloy effect and an optimal affinity led to the best catalytic performance in the series.