Self-assembly and chemical reactivity of alkenes on platinum nanoparticles

Impacts of interfacial charge transfer on nanoparticle electrocatalytic activity towards oxygen reduction

Interfacial electron transfer within platinum and non-platinum-based nanocatalysts plays a significant role in the manipulation of the electronic interactions between oxygen species and the catalyst surfaces, which may be exploited as an effective mechanism to enhance and optimize the activity towards oxygen reduction.

Surface Functionalization of Metal Nanoparticles by Conjugated Metal-Ligand Interfacial Bonds: Impacts on Intraparticle Charge Transfer

Noble metal nanoparticles represent a unique class of functional nanomaterials with physical and chemical properties that deviate markedly from those of their atomic and bulk forms. In order to stabilize the nanoparticles and further manipulate the materials properties, surface functionalization with organic molecules has been utilized as a powerful tool. Among those, mercapto derivatives have been used extensively as the ligands of choice for nanoparticle surface functionalization by taking advantage of the strong affinity of thiol moieties to transition metal surfaces forming (polar) metal-thiolate linkages. Yet, the nanoparticle material properties are generally discussed within the context of the two structural components, the metal cores and the organic capping layers, whereas the impacts of the metal-sulfur interfacial bonds are largely ignored because of the lack of interesting chemistry. In recent years, it has been found that metal nanoparticles may also be functionalized by stable metal-carbon (or even -nitrogen) covalent bonds. Because of the formation of dπ-pπ interactions between the transition-metal nanoparticles and terminal carbon moieties, the interfacial resistance at the metal-ligand interface is markedly reduced, leading to the emergence of unprecedented optical and electronic properties. In this Account, we summarize recent progress in the studies of metal nanoparticles functionalized by conjugated metal-ligand interfacial bonds that include metal-carbene (M═C) and metal-acetylide (M-C≡)/metal-vinylidene (M═C═C) bonds. Such interfacial bonds are readily formed by ligand self-assembly onto nanoparticle metal cores. The resulting nanoparticles exhibit apparent intraparticle charge delocalization between the particle-bound functional moieties, leading to the emergence of optical and electronic properties that are analogous to those of their dimeric counterparts, as manifested in spectroscopic and electrochemical measurements. This is first highlighted by ferrocene-functionalized nanoparticles that exhibit nanoparticle-mediated intervalence charge transfer (IVCT) among the particle-bound ferrocenyl moieties, as manifested in electrochemical and spectroscopic measurements. Such intraparticle charge delocalization has also been observed with other functional moieties such as pyrene and anthracene, where the photoluminescence emissions are consistent with those of their dimeric derivatives. Importantly, as such electronic communication occurs via a through-bond pathway, it may be readily manipulated by the valence states of the nanoparticle cores as well as specific binding of selective molecules/ions to the organic capping shells. These fundamental insights may be exploited for diverse applications, ranging from chemical sensing to (nano)electronics and fuel cell electrochemistry. Several examples are included, such as sensitive detection of nitroaromatic derivatives, metal cations, and fluoride anions by fluorophore-functionalized metal nanoparticles, fabrication of nanoparticle-bridged molecular dyads by, for instance, using nanoparticles cofunctionalized with 4-ethynyl-N,N-diphenyl-aniline (electron donor) and 9-vinylanthracene (electron acceptor), and enhanced electrocatalytic activity of acetylene derivatives-functionalized metal/alloy nanoparticles for oxygen reduction reaction by manipulation of the metal core electron density and hence interactions with reaction intermediates. We conclude this Account with a perspective where inspiration from conventional organometallic chemistry may be exploited for more complicated nanoparticle surface functionalization through the formation of diverse metal-nonmetal bonds. This is a unique platform for ready manipulation of nanoparticle properties and applications.

Effects of para-substituents of styrene derivatives on their chemical reactivity on platinum nanoparticle surfaces

Stable platinum nanoparticles were successfully prepared by the self-assembly of para-substituted styrene derivatives onto the platinum surfaces as a result of platinum-catalyzed dehydrogenation and transformation of the vinyl groups to the acetylene ones, forming platinum-vinylidene/-acetylide interfacial bonds. Transmission electron microscopic measurements showed that the nanoparticles were well dispersed without apparent aggregation, suggesting sufficient protection of the nanoparticles by the organic capping ligands, and the average core diameter was estimated to be 2.0 ± 0.3 nm, 1.3 ± 0.2 nm, and 1.1 ± 0.2 nm for the nanoparticles capped with 4-tert-butylstyrene, 4-methoxystyrene, and 4-(trifluoromethyl)styrene, respectively, as a result of the decreasing rate of dehydrogenation with the increasing Taft (polar) constant of the para-substituents. Importantly, the resulting nanoparticles exhibited unique photoluminescence, where an increase of the Hammett constant of the para-substituents corresponded to a blue-shift of the photoluminescence emission, suggesting an enlargement of the HOMO-LUMO band gap of the nanoparticle-bound acetylene moieties. Furthermore, the resulting nanoparticles exhibited apparent electrocatalytic activity towards oxygen reduction in acidic media, with the best performance among the series of samples observed with the 4-tert-butylstyrene-capped nanoparticles due to an optimal combination of the nanoparticle core size and ligand effects on the bonding interactions between platinum and oxygen species.

Highly Active Hydrogen Evolution Electrodes via Co-Deposition of Platinum and Polyoxometalates

A highly active hydrogen evolution reaction (HER) electrode with low Pt loading on glassy carbon (GC) has been prepared by anodic platinum dissolution and co-deposition of polyoxometalates. TEM, EDS, XPS, CV, and ICP-MS analyses gave a Pt loading of 50-100 ng/cm2, corresponding to a Pt coverage of only 0.08-0.16 monolayer. With an overpotential of 65 mV at 20 mA/cm2, the modified GC has a HER activity comparable to that of the commercial Pt working electrode.