Nan Zhu

Processing of graphene for electrochemical application: noncovalently functionalize graphene sheets with water-soluble electroactive methylene green.

To explore graphene applications in various fields, the processability of graphene becomes one of the important key issues, particularly with the increasing availability of synthetic graphene approaches, because the direct dispersion of hydrophobic graphene in water is prone to forming agglomerates irreversibly. Here, a facile method is proposed to increase the dispersity of graphene through noncovalent functionalization graphene with a water-soluble aromatic electroactive dye, methylene green (MG), during chemical reduction of graphene oxide (GO) with hydrazine. Atomic force microscopic and UV-vis spectrophotometric results demonstrate that chemically reduced graphene (CRG) functionalized with MG (CRG-MG) is well-dispersed into water through the coulomb repulsion between MG-adsorbed CRG sheets. The electrochemical properties of the formed CRG-MG are investigated, and the results demonstrate that CRG-MG confined onto a glassy carbon (GC) electrode has lower charge-transfer resistance and better electrocatalytic activity toward the oxidation of NADH, in relation to pristine CRG (i.e., without MG functionalization). This method not only offers a facile approach to dispersing graphene in water but also is envisaged to be useful for investigations on graphene-based electrochemistry.

Transfer Printing of Metal Nanoparticles with Controllable Dimensions, Placement, and Reproducible Surface-Enhanced Raman Scattering Effects

This paper describes the fabrication of single, multiple strand, and three-dimensional patterning of metal nanoparticles by nanotransfer edge printing (nTEP), a method comprising nanoparticle self-assembly, nanotransfer printing (nTP), and edge lithography. In the process proposed here, 20 nm Au nanoparticles (AuNPs) are deterministically arranged in precise placement by manipulating a topographically patterned poly(dimethylsiloxane) (PDMS) stamp, and Ag nanoparticles are conjugated with AuNP patterns to construct surface-enhanced Raman scattering (SERS)-active substrate to detect trace amounts (10(-13) mol/L) of biological molecules such as thrombin with enhancement up to 10(10). The simple, convenient, and inexpensive procedure has extended nTP and nTEP from using evaporated thin metal film to using self-assembled nanoparticles, and may stretch to other organic and inorganic species to find broad applications in many areas.

Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the Point Dipole Approximation

Understanding of Förster resonance energy transfer (FRET) in thin films composed of quantum dots (QDs) is of fundamental and technological significance in optimal design of QD based optoelectronic devices. The separation between QDs in the densely packed films is usually smaller than the size of QDs, so that the simple point-dipole approximation, widely used in the conventional approach, can no longer offer quantitative description of the FRET dynamics in such systems. Here, we report the investigations of the FRET dynamics in densely packed films composed of multisized CdSe QDs using ultrafast transient absorption spectroscopy and theoretical modeling. Pairwise interdot transfer time was determined in the range of 1.5 to 2 ns by spectral analyses which enable separation of the FRET contribution from intrinsic exciton decay. A rational model is suggested by taking into account the distribution of the electronic transition densities in the dots and using the film morphology revealed by AFM images. The FRET dynamics predicted by the model are in good quantitative agreement with experimental observations without adjustable parameters. Finally, we use our theoretical model to calculate dynamics of directed energy transfer in ordered multilayer QD films, which we also observe experimentally. The Monte Carlo simulations reveal that three ideal QD monolayers can provide exciton funneling efficiency above 80% from the most distant layer. Thereby, utilization of directed energy transfer can significantly improve light harvesting efficiency of QD devices.

Direct measurement and modulation of single-molecule coordinative bonding forces in a transition metal complex

Coordination chemistry has been a consistently active branch of chemistry since Werners seminal theory of coordination compounds inaugurated in 1893, with the central focus on transition metal complexes. However, control and measurement of metal-ligand interactions at the single-molecule level remain a daunting challenge. Here we demonstrate an interdisciplinary and systematic approach that enables measurement and modulation of the coordinative bonding forces in a transition metal complex. Terpyridine is derived with a thiol linker, facilitating covalent attachment of this ligand on both gold substrate surfaces and gold-coated atomic force microscopy tips. The coordination and bond breaking between terpyridine and osmium are followed in situ by electrochemically controlled atomic force microscopy at the single-molecule level. The redox state of the central metal atom is found to have a significant impact on the metal-ligand interactions. The present approach represents a major advancement in unravelling the nature of metal-ligand interactions and could have broad implications in coordination chemistry.

Sandwiched confinement of quantum dots in graphene matrix for efficient electron transfer and photocurrent production

Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3 × 109 s−1. Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications.

Surface self-assembled hybrid nanocomposites with electroactive nanoparticles and enzymes confined in a polymer matrix for controlled electrocatalysis

A three-dimensional network of highly branched poly(ethyleneimine) (PEI) is designed and synthesized on gold electrode surfaces. A self-assembled monolayer (SAM) of dithiobis(succinimidyl propionate) (DTSP) on a gold electrode was first prepared, which is confirmed by the reductive desorption of Au-S units. The PEI polymer was then covalently immobilized onto the DTSP layer, leaving free primary amine groups acting as a 3D skeleton for high loading of electroactive enzyme-size Prussian blue nanoparticles (PBNPs, 6 nm) via electrostatic trapping. Atomic force microscopy was used to disclose the microscopic structures of the different layers during the surface architecture formation. The resulting surface-bound nanostructured composite shows high electrochemical activity arising from confined PBNPs, and acts as an efficient electrocatalyst towards H2O2 reduction. Facile electron communication is achieved as reflected by a large electron transfer (ET) rate constant (ks) of 200 s-1, and the possible electron propagation mechanisms in the polymer network are discussed. This surface/interfacial nanocomposite can be further used in the accommodation of enzymes for electrochemical bio-catalysis. Glucose oxidase (GOD) was used towards this end, in a proof-of-concept study. This enzyme can be co-trapped in the PEI matrix and is interconnected with PBNPs, leading to highly efficient electrocatalyic oxidation and detection of glucose.

Long-range interfacial electron transfer and electrocatalysis of molecular scale Prussian Blue nanoparticles linked to Au(111)-electrode surfaces by different chemical contacting groups

We have explored interfacial electrochemical electron transfer (ET) and electrocatalysis of 5–6 nm Prussian Blue nanoparticles (PBNPs) immobilized on Au(111)-electrode surfaces via molecular wiring with variable-length, and differently functionalized thiol-based self-assembled molecular monolayers (SAMs). The SAMs contain positively (−NH3+) or negatively charged (–COO–) terminal group, as well an electrostatically neutral hydrophobic terminal group (–CH3). The surface microscopic structures of the immobilized PBNPs were characterized by high-resolution atomic force microscopy (AFM) directly in aqueous electrolyte solution under the same conditions as for electrochemical measurements. The PBNPs displayed fast and reversible interfacial ET on all the surfaces, notably in multi-ET steps as reflected in narrow voltammetric peaks. The ET kinetics can be controlled by adjusting the length of the SAM forming linker molecules. The interfacial ET rate constants were found to depend exponentially on the ET distance for distances longer than a few methylene groups in the chain, with decay factors (β) of 0.9, 1.1, and 1.3 per CH2, for SAMs terminated by −NH3+,–COO–, and–CH3, respectively. This feature suggests, first that the interfacial ET processes follow a tunneling mechanism, resembling that of metalloproteins in a similar assembly. Secondly, the electronic contact of the SAM terminal groups that anchor non-covalently the PBNP are crucial as reported for other types of molecular junctions. Highly efficient PBNP electrocatalysis of H2O2 reduction was also observed for the three linker groups, and the electrocatalytic mechanisms analyzed.

Drastic difference between hole and electron injection through the gradient shell of CdxSeyZn1−xS1−y quantum dots

Ultrafast fluorescence spectroscopy was used to investigate the hole injection in CdxSeyZn1-xS1-y gradient core-shell quantum dot (CSQD) sensitized p-type NiO photocathodes. A series of CSQDs with a wide range of shell thicknesses was studied. Complementary photoelectrochemical cell measurements were carried out to confirm that the hole injection from the active core through the gradient shell to NiO takes place. The hole injection from the valence band of the QDs to NiO depends much less on the shell thickness when compared to the corresponding electron injection to n-type semiconductor (ZnO). We simulate the charge carrier tunneling through the potential barrier due to the gradient shell by numerically solving the Schrödinger equation. The details of the band alignment determining the potential barrier are obtained from X-ray spectroscopy measurements. The observed drastic differences between the hole and electron injection are consistent with a model where the hole effective mass decreases, while the gradient shell thickness increases.