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Research Projects in the Chen GroupRESEARCH THEME: ELECTRON TRANSFER ON THE NANOSCALE
Nanoparticle-Mediated Intervalence Charge Transfer. Traditonally, intervalence transfer is observed with organometallic complexes at mixed valence. One good example is ferrocene oligomers which exhibit two instead of one pair of voltammetric waves along with a modestly intense absorption peak in the near-infrared region. However, intervalence transfer involving nanoparticle materials has remained largely unexplored. This research project is mainly focused on electronic communication of redox-active moieties that are bound onto the nanoparticle surface by a conjugated covalent bond. Ruthenium nanoparticles stabilized by carbene derivatives are used as the
illustrating example, which may undergo olefin metathesis reactions with vinyl-terminated derivatives (e.g., vinylferrocene) for further chemical functionalization. Electrochemical measurements of the resulting ferrocene-functionalized particles exhibit two instead of one pair of voltammetric peaks. In addition, NIR measurements depicts a strong absorption peak around 1900 nm. Both features are characteristics of Class II behaviors of intervalence transfer,
most probably as a result of the mediation of the nanoparticle core electrons. The results are further verified by density functional theory calculations. Further work is underway to further elucidate the fundamental mechanism which is important in exploting the unique optoelectronic properties of the resulting particles for a variety of applications.
Solid-State Electronic Conductivity of Nanoparticle Ensembles. In this project, we examine the electron transfer chemistry of nanoparticles assemblies in
solid state. These include transition-metal as well as semiconductor nanoparticles. We employ self-assembling and Langmuir-Blodgett techniques to fabricate nanoparticle organized
assemblies and investigate the effects of interparticle interactions, photoexcitation, as well as chemical environments on the conductivity properties of the ensemble structures.
For metal nanoparticles, we found that by deliberate control of the interparticle separation, lateral single electron transfer could be achieved; whereas for semiconductor particles,
the electronic conductivity could be drastically enhanced by photoexcitation with the photon energies greater than the ensemble effective bandgap. Comparative studies are also carried
out with scanning probe microscopic measurements (e.g., AFM, STM, and LFM). Such activities may be of fundamental importance to molecular electronics as well as photovoltaic applications.
Solid-state single electron transfer across a nanoparticle monolayer as an analog to the STM-based COulomb staircase, which is exemplified by Ag nanoparticles of varied sizes
Nanoparticle Catalysts and Fuel Cell Electrochemistry. The performance of a fuel cell is largely determined by the electrocatalysts for both the anode and cathode reactions, which are essentially nanoparticle materials.
Thus design and engineering of the nanoparticle structure and properties play a critical role in fuel cell research. In this project, our interest is two-fold: (i) to prepare novel nanoparticle materials by exploiting the unique metal-ligand interactions (e.g., metal-carbon covalent linkages);
and (ii) to examine the electrocatalytic activities within the context of particle structures, arrangements, loading, as well as external fields (e.g., magnetic fields). Specifically, the anodic oxidation of fuel molecules as well as oxygen reduction at the cathod will both be carefully investigated. For instance, in the studies of the electrocatalytic activities of FePt alloy nanoparticles in the oxidation of formic acid and in the reduction of oxygen,
we found that the optimal composition corresponded to an atomic ratio of Fe:Pt around 1:1, most likely as a consequence of the bifunctional mechanism and electronic effects, and there appeared to be a minimal loading of particles onto the electrode surface for optimal performance.
Functional Nanoparticles by Interfacial Engineering. In this project our interest is to develop effective experimental protocols for the preparation of
functional nanoparticle materials based on interfacial engineering, such as nanoparticles protected by metal-carbon covalent bonds, particles exhibiting amphiphilic surface characters (the so-called Janus nanoparticles, see right), etc. Of these, Janus nanoparticles constitute a unique class of functional nanomaterials, where the segregated distribution of the surface ligands may be exploited for anisotropic functionalization of the nanoparticle surface. A wide array of experimental techniques will then be used to unravel the nanoscale molecular structures, such as contact angle, NMR, FTIR, DLS, UV-Vis, AFM, and STM.
The ultimate goal is to achieve organized assemblies of the nanoparticles and to further manipulate their opto-electronic properties for possible device applications. For instance, electron-donors and acceptors may be situated on the two hemispheres of the nanoparticles. When the particles are self-assembled into a superparticular structures, these donor-acceptor pairs may serve as an effective mechanism for charge separation upon photoirradiation, an important process in photovoltaics. Some of the work has been included in the Wikipedea entry of "Janus particles".
Antimicrobial Activity of Functional Nanomaterials. As antibiotic resistant strains of pathogenic bacteria become increasingly prevalent and cause widespread infection in communities around the world, the development of novel, highly-effective antibacterial materials has become vital to world health. Metal nanoparticles as well graphene derivatives have both proven to be leading candidates for this application as they exhibit broad-spectrum antimicrobial effects against a variety of microorganisms. Within this context, our goal is to exploit the fundamental insights that we gain in the studies of nanoparticle surface functionalization and structural engineering to optimize the antimicrobial activity with the ultimate goal of estaiblishing an unambiguous structure-activity correlation.
Growth curves of E. coli cultures grown in luria broth containing (left) silver nanoparticles and (right) silver nanoprisms irradiated for 24 hours (B) at various concentrations.