Weiyong Yuan

Direct Modulation of Localized Surface Plasmon Coupling of Au Nanoparticles on Solid Substrates via Weak Polyelectrolyte-Mediated Layer-by-Layer Self Assembly

For the first time, a pH-controllable weak polyelectrolyte/metal nanoparticle composite film was successfully constructed on a solid substrate through layer-by-layer (LbL) assembly, and its localized surface plasmon coupling (LSPC) was investigated. The degree of LSPC can be modulated by controlling pH of the weak polyelectrolyte used. The LSPC was tunable and stable, demonstrated by a large shift of the longitudinal band peak position over a range of 625-741.5 nm as a function of pH, while shifting insignificantly at a fixed pH for a month. The modulation of LSPC of the LbL nanocomposite film can be ascribed to changes in the assembled weak polyelectrolyte, where the charge density and conformation can be easily controlled by pH to tailor the interparticle spacing in the nanoparticle clusters. This work provides a rational approach for preparation of stable nanocomposites with easily tunable LSPC and scientific insight into the effect of film morphology on the optical properties of assembled nanoparticles. The spectral response to the environment has great potential in applications such as plasmonics, biosensing, and medical therapy.

Controllably layer-by-layer self-assembled polyelectrolytes/nanoparticle blend hollow capsules and their unique properties

Fabrication of a polymer/nanoparticle composite capsule with controllable nanoparticle properties remains a great challenge in material science. In this work, a three-component layer-by-layer (LbL) self-assembly process using a weak polyelectrolyte-nanoparticle blend and another weak polyelectrolyte was systematically studied to fabricate the composite capsules. The pH of the assembled polyelectrolyte and blend ratio of polyelectrolyte to nanoparticle were used to controllably construct blend multilayers on colloidal templates without aggregation. The capsules were then produced by removal of the templates and possess well-dispersed assembled nanoparticles with desired concentration, size and interparticle spacing for large tunability of localized surface plasmon resonance (LSPR) and ultrahigh permeability, which have not been accomplished by the reported polyelectrolyte or polyelectrolyte/nanoparticle capsules. This work also investigated the fundamental insights of the blend LbL self-assembly process while providing a universal approach to fabricate well-dispersed microcapsules with desired nanoparticle properties for various applications such as intelligent drug delivery, biosensing and bioimaging applications.

Polymer-Mediated Self-Assembly of TiO2@Cu2O Core-Shell Nanowire Array for Highly Efficient Photoelectrochemical Water Oxidation.

Phototoelectrochemical (PEC) water splitting represents a highly promising strategy to convert solar energy to chemical energy in the form of hydrogen, but its performance is severely limited by the water oxidation reaction. We conformally grew an ultrathin and continuous coating of Cu2O on TiO2 nanowire array (NWA) to form a truly core-shell TiO2@Cu2O NWA via a new facile, economical, and scalable polymer-mediated self-assembly approach, in which the polymer serves as a stabilizer, reductant, and linker simultaneously. This heteronanostructure was subsequently directly used as a photoanode for PEC water splitting, showing a photocurrent density of 4.66 mA cm(-2) at 1.23 V vs RHE in 0.5 M Na2SO4 solution and a maximum photoconversion efficiency of 0.71%, both of which are the highest reported for TiO2-based photoanodes measured under the same conditions (neutral conditions and without any sacrificial agent). The superior PEC performance of the TiO2@Cu2O NWA toward water oxidation is primarily due to much enhanced visible light collection and charge separation for high charge carrier density as well as greatly facilitated charge transfer and transport. This work not only offers a novel TiO2@Cu2O core-shell NWA photoanode for highly efficient PEC water oxidation and investigate its enhancement mechanism but also provides scientific insights into the mechanism of the polymer-mediated self-assembly, which can be further extended to fabricate various other core-shell nanoarchitectures for broad applications.

Controllable synthesis of graphene supported MnO2 nanowires via self-assembly for enhanced water oxidation in both alkaline and neutral solutions

The sluggish water oxidation reaction represents a significant challenge in water splitting for energy storage using hydrogen. We herein report the synthesis of MnO2 nanowires with the ultrasmall diameter and aspect ratio as high as 125 on graphene using a novel in situ polymer-mediated self-assembly approach in aqueous solution and under ambient conditions. The self-assembly process is simple and controllable by the concentration and pH of the polymer solution, in which the polymer serves as a soft template to direct the growth of MnO2 nanowires and also stabilize the structure, forming a unique graphene supported MnO2 nanowire, G@MnO2 NW. This nanostructure shows the most significant improvement of the catalytic activity compared to the graphene supported MnO2 nanoparticle and commercial Pt/C toward water oxidation under both alkaline and neutral conditions, and demonstrates for the first time a remarkable effect of the shape of MnO2 nanostructures on water oxidation catalysis. For example, at 0.7 V, it produces a current density of 5.9 mA cm−2, 14.8 times that of the graphene supported MnO2 nanoparticle (4.0 mA cm−2) and 8.4 times that of Pt/C (0.7 mA cm−2) in alkaline solution. Furthermore, it displays the highest turnover frequency reported among all the Mn oxides used for water oxidation catalysis. The G@MnO2 NW shows great potential as a water oxidation catalyst for energy storage applications.

Significance of wall number on the carbon nanotube support-promoted electrocatalytic activity of Pt NPs towards methanol/formic acid oxidation reactions in direct alcohol fuel cells

For the first time it has been demonstrated that number of walls in pristine carbon nanotube (CNT) supports has significant effects on the electrocatalytic activity of Pt nanoparticles (NPs) towards methanol and formic acid oxidation reactions in alkaline solution, showing a distinctive volcano-type dependence of the catalytic performance on the number of walls of CNTs. In this study, Pt NPs of a similar size were uniformly self-assembled on CNTs with different numbers of walls and outer diameters to be used as model catalysts. The electrocatalytic activity and stability of Pt NPs showed distinctive volcano-type curves as a function of the number of walls with the best results being obtained using CNTs with an average of 7 walls. Such an unusual promotion effect can be ascribed predominantly to the high affinity of oxygen-containing species such as OHads, which facilitate the removal of adsorbed CO species and promote fast electron transfer between the outer walls and the inner tubes of CNTs, most likely through electron tunneling under the electrochemical polarization driving force. This study demonstrates the new possibility of tailoring the electrocatalytic properties of precious metal catalysts via altering the electronic and quantum transport properties of their CNT supports.

Charged drug delivery by ultrafast exponentially grown weak polyelectrolyte multilayers: amphoteric properties, ultrahigh loading capacity and pH-responsiveness

Exponentially growing layer-by-layer hierarchical nanoporous films have been used as a promising system for controlled drug loading/release applications. However, its growth mechanism and factors affecting the drug loading/release are still unclear. In this study, high molecular weight branched poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAA) were utilized as model weak polyelectrolytes to investigate the growth mechanism and the drug loading/release of the multilayers. The pH-dependent growth behavior, interdiffusion of PEI and morphological changes of the film indicate that a pH-dependent polyelectrolyte interdiffusion mechanism is involved in the ultrafast exponential growth process. It is discovered, for the first time, that the fabricated films possess a pH-triggered switchable polarity and tunable charge density associated to the outermost layer, which can enable the loading of anionic or cationic drugs while offering a broad range of pH-controlled release rates and ultralong release times. The multi-layered film has also achieved the highest pH-controlled drug loading/release capacity. This study not only provides a superior platform for the controlled delivery of charged drugs, but also proposes an exponential growth mechanism for weak polyelectrolyte multilayered films.

CoP Nanoparticles in Situ Grown in Three-Dimensional Hierarchical Nanoporous Carbons as Superior Electrocatalysts for Hydrogen Evolution

The development of efficient and low-cost hydrogen evolution reaction (HER) catalysts is critical for storing energy in hydrogen via water splitting but still presents great challenges. Herein, we report synthesis of three-dimensional (3-D) hierarchical nanoporous carbon (HNC) supported transition metal phosphides (TMPs) for the first time by in situ growth of CoP nanoparticles (NPs) in CaCO3 NP-templated Cinnamomum platyphyllum leaf extract-derived carbon. They were subsequently employed as a HER catalyst, showing an onset potential of 7 mV and an overpotential of 95.8 mV to achieve 10 mA cm(-2), a Tafel plot of 33 mV dec(-1), and an exchange current density of 0.1182 mA cm(-2), of which the onset overpotential and the Tafel plot are the lowest reported for non-noble-metal HER catalysts, and the overpotential to achieve 10 mA cm(-2) and the exchange current density also compare favorably to most reported HER catalysts. In addition, this catalyst exhibits excellent durability with negligible loss in current density after 2000 CV cycles ranging from +0.01 to -0.17 V vs RHE at a scan rate of 100 mV s(-1) or 22 h of chronoamperometric measurement at an overpotential of 96 mV and a high Faraday efficiency of close to 100%. This work not only creates a novel high-performance non-noble-metal HER electrocatalyst and demonstrates the great advantages of the in situ grown 3-D HNC supported TMP NPs for the electrocatalysis of HER but also offers scientific insight into the mechanism for the in situ growth of TMP and their precursor NPs, in which an ultralow reactant concentration and rich functional groups on the 3-D HNC support play critical roles.

Controllably self-assembled graphene-supported Au@Pt bimetallic nanodendrites as superior electrocatalysts for methanol oxidation in direct methanol fuel cells

Controllable growth of highly dense and uniform Pt nanostructures on graphene could greatly increase the electrocatalytic activity and Pt utilization for methanol oxidation in direct methanol fuel cells (DMFCs), but still presents a great challenge. This study reports a novel strategy of combining self-assembly with in situ seeded growth to fabricate graphene supported Pt nanostructures. Using self-assembled gold nanoparticles as the seeds, highly dense, uniform and well-dispersed Au@Pt bimetallic nanodendrites supported on graphene were fabricated for the first time. The density, size and shape of the bimetallic nanostructure can be easily controlled by the fabrication conditions such as the number of deposition cycles, precursor concentration and reductant concentration. The graphene supported Au@Pt bimetallic nanodendrites display greatly enhanced electrocatalytic activity and durability toward methanol oxidation compared to graphene supported Pt nanostructures and commercial Pt/C (E-TEK). Their catalytic activity is also among the highest reported for other state-of-the-art commercial Pt/C and PtRu/C as well as for non-covalently polyelectrolyte-functionalized carbon-supported Pt and PtRu catalysts. Our strategy can be extended to other substrates and metal components to fabricate various supported core/shell bimetallic nanostructures for applications such as catalysis, sensing and electronics.

Synthesis of Cobalt Phosphide Nanoparticles Supported on Pristine Graphene by Dynamically Self-Assembled Graphene Quantum Dots for Hydrogen Evolution

A highly active, durable, and low-cost hydrogen evolution reaction (HER) catalyst is desirable for energy storage through water splitting but its fabrication presents great challenges. Herein, mediated by dynamically self-assembled graphene quantum dots (GQDs), small, uniform, high-density, and well-dispersed CoP nanoparticles were grown in situ on pristine graphene for the first time. This hybrid nanostructure was then employed as HER electrocatalyst, showing an onset potential of 7 mV, an overpotential of 91.3 mV to achieve 10 mA cm-2 , a Tafel slope of 42.6 mV dec-1 , and an exchange current density of 0.1225 mA cm-2 , all of which compare favorably to those of most reported non-noble-metal catalysts. The developed catalyst also exhibits excellent durability with negligible current loss after 2000 cyclic voltammetry cycles (+0.01 to -0.17 V vs. RHE) or 34 h of chronoamperometric measurement at an overpotential of 91.3 mV. This work not only develops a new strategy for the fabrication of high-performance and inexpensive electrocatalysts for HER but also provides scientific insight into the mechanism of the dynamically self-assembled GQDsmediated synthesis process.

Self-assembling microsized materials to fabricate multifunctional hierarchical nanostructures on macroscale substrates

It is very challenging to assemble microscale objects on a macroscale substrate due to the weak interaction and size/geometric mismatch. Herein a novel polyelectrolyte-mediated self-assembly approach with microsized ZnO nanoflowers as building blocks was successfully used to grow a hierarchical nanostructure on a substrate, which is mainly due to the loop and tail conformation of the weak polyelectrolyte used. Furthermore, a heating step was able to enhance the self-assembly process. The obtained ZnO flower hierarchical nanostructure possesses simultaneous non-light induced superhydrophilic, antifogging and antibacterial properties, thus providing great potential in applications such as biomedical devices, hospital building paints, and daily life uses. This demonstrated method could be extended to fabricate hierarchical nanostructures with other microscale nanostructured materials on various substrates for broad applications.