Zhenhua Sun

Understanding the photothermal conversion efficiency of gold nanocrystals.

Plasmon-based photothermal therapy is one of the most intriguing applications of noble metal nanostructures. The photothermal conversion efficiency is an essential parameter in practically realizing this application. The effects of the plasmon resonance wavelength, particle volume, shell coating, and assembly on the photothermal conversion efficiencies of Au nanocrystals are systematically studied by directly measuring the temperature of Au nanocrystal solutions with a thermocouple and analyzed on the basis of energy balance. The temperature of Au nanocrystal solutions reaches the maximum at ∼75 °C when the plasmon resonance wavelength of Au nanocrystals is equal to the illumination laser wavelength. For Au nanocrystals with similar shapes, the larger the nanocrystal, the smaller the photothermal conversion efficiency becomes. The photothermal conversion can also be controlled by shell coating and assembly through the change in the plasmon resonance energy of Au nanocrystals. Moreover, coating Au nanocrystals with semiconductor materials that have band gap energies smaller than the illumination laser energy can improve the photothermal conversion efficiency owing to the presence of an additional light absorption channel.

More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects

Lithium-sulfur (Li-S) batteries have attracted tremendous interest because of their high theoretical energy density and cost effectiveness. The target of Li-S battery research is to produce batteries with a high useful energy density that at least outperforms state-of-the-art lithium-ion batteries. However, due to an intrinsic gap between fundamental research and practical applications, the outstanding electrochemical results obtained in most Li-S battery studies indeed correspond to low useful energy densities and are not really suitable for practical requirements. The Li-S battery is a complex device and its useful energy density is determined by a number of design parameters, most of which are often ignored, leading to the failure to meet commercial requirements. The purpose of this review is to discuss how to pave the way for reliable Li-S batteries. First, the current research status of Li-S batteries is briefly reviewed based on statistical information obtained from literature. This includes an analysis of how the various parameters influence the useful energy density and a summary of existing problems in the current Li-S battery research. Possible solutions and some concerns regarding the construction of reliable Li-S batteries are comprehensively discussed. Finally, insights are offered on the future directions and prospects in Li-S battery field.

pH‐Controlled Reversible Assembly and Disassembly of Gold Nanorods

Gold nanorods exhibit rich surface-plasmon-resonance (SPR)derived properties, which have made discrete nanorods useful for many interesting applications such as optical data storage, submicrometer metallic barcodes, sensing, biological imaging, and controlled gene delivery. Future scientific and technological applications of Au nanorods require the capability to assemble into complex one-, two-, or even three-dimensional (3D) functional architectures. The assembly of Au nanorods also allows for the utilization of their collective properties that result from the coupling of the optical and electronic properties between neighboring individual nanorods. Several approaches have been developed for the assembly of Au nanorods in either end-to-end (EE) or side-by-side (SS) orientations. They include i) assembly through electrostatic interactions, hydrogen bonding, or covalent bonding, ii) antibody/antigen and streptavidin/biotin biorecognitions, iii) use of carbon nanotubes and silica nanofibers as templates, and iv) interactions between functionalized polymers in selective solvents. Au nanorods assembled by these approaches are generally difficult to disassemble. Even though significant progress has beenmade in the organizationof nanomaterials, reversible assembly and disassembly of Au nanorods in either EE or SS orientations has remained a big challenge. So far, reversible aggregation of spherical Au nanoparticles has been demonstrated by functionalizing them with thiol-modified DNA oligomers. Here, we report on a robust strategy for the reversible assembly and disassembly of Au nanorods in both EE and SS fashion. Thiol-containing bifunctional molecules are selectively bound to the end or side surface of individual Au nanorods. The bound molecules induce the assembly of Au nanorods if the pH of the nanorod solution is adjusted within an optimal range. Outside the optimal pH range, Au nanorods are disassembled. This pH-controlled assembly and disassembly is reversible and can be repeated many times. Moreover, the distances between assembled nanorods are estimated to vary from 0.080 to 1.8 nm for different assemblingmolecules and assembly orientations.

Conductive porous vanadium nitride/graphene composite as chemical anchor of polysulfides for lithium-sulfur batteries

Although the rechargeable lithium–sulfur battery is an advanced energy storage system, its practical implementation has been impeded by many issues, in particular the shuttle effect causing rapid capacity fade and low Coulombic efficiency. Herein, we report a conductive porous vanadium nitride nanoribbon/graphene composite accommodating the catholyte as the cathode of a lithium–sulfur battery. The vanadium nitride/graphene composite provides strong anchoring for polysulfides and fast polysulfide conversion. The anchoring effect of vanadium nitride is confirmed by experimental and theoretical results. Owing to the high conductivity of vanadium nitride, the composite cathode exhibits lower polarization and faster redox reaction kinetics than a reduced graphene oxide cathode, showing good rate and cycling performances. The initial capacity reaches 1,471 mAh g−1 and the capacity after 100 cycles is 1,252 mAh g−1 at 0.2 C, a loss of only 15%, offering a potential for use in high energy lithium–sulfur batteries.

A General Approach to the Synthesis of Gold–Metal Sulfide Core–Shell and Heterostructures†

Cores and effect: Water-dispersible core-shell structures and heterostructures incorporating gold nanocrystals of different shapes (polyhedra, cubes, and rods) and a variety of transition metal sulfide semiconductors (ZnS, CdS, NiS, Ag(2)S, and CuS) are synthesized using cetyltrimethylammonium bromide-encapsulated gold nanocrystals and metal thiobenzoates as starting materials.

Plasmon coupling in clusters composed of two-dimensionally ordered gold nanocubes.

Gold nanocubes are assembled into clusters of varying numbers and ordering on indium tin oxide substrates. The plasmon coupling in the clusters is studied with both dark-field imaging and finite-difference time-domain calculations. Generally, as a cluster becomes larger and more asymmetric, it exhibits more scattering peaks towards longer wavelengths. The coupling of the vertically oriented dipole in the nanocube with its image dipole in the substrate generates two scattering peaks. One is fixed in energy and the other red-shifts with increasing cluster size. The coupling of horizontally oriented dipoles among different nanocubes produces multiple scattering peaks at lower energies. Their positions and intensities are highly dependent on the number and ordering of nanocubes in the cluster. Au nanocubes in the clusters are further welded together by thermal treatment. The scattering peaks of the thermally treated clusters generally become sharper. The lower-energy scattering peaks arising from dipolar oscillations are red-shifted.

Effects of Dyes, Gold Nanocrystals, pH, and Metal Ions on Plasmonic and Molecular Resonance Coupling

The effects of various factors on the resonance coupling between elongated Au nanocrystals and organic dyes have been systematically investigated through the preparation of hybrid nanostructures between Au nanocrystals and the electrostatically adsorbed dye molecules. A nanocrystal sample is chosen for each dye to match the longitudinal plasmon resonance wavelength with the absorption peak wavelength of the dye as close as possible so that the resonance coupling strength can be maximized. The resonance coupling strength is found to approximately increase as the molecular volume-normalized absorptivity is increased. It is mainly determined by the plasmon resonance energy of the Au nanocrystals instead of their shapes and sizes. Moreover, the resonance coupling can be reversibly controlled if the dye in the hybrid nanostructures is pH-sensitive. The coupling can also be weakened in the presence of metal ions. These results will be highly useful for designing resonance coupling-based sensing devices and for plasmon-enhanced spectroscopy.

Curvature-Directed Assembly of Gold Nanocubes, Nanobranches, and Nanospheres

Gold nanocubes, nanobranches, and nanospheres were prepared in high yields using a seeded growth method in the presence of cationic surfactants. The resultant Au nanostructures are encapsulated with a surfactant bilayer and positively charged. The nanocubes are single-crystalline and enclosed with low-index facets. The nanobranches and nanospheres are multiply twinned. Each nanobranch possesses a varying number of sharp tips, which expose high-index facets. Glutathione was used to induce the assembly of the Au nanostructures, including both monocomponent (nanocubes and nanobranches) and bicomponent (nanocube-nanosphere and nanobranch-nanosphere) systems. The assembly was observed to occur predominantly at the vertices of the nanocubes and the sharp tips of the nanobranches. This curvature-directed assembly can be attributed to the preferential bonding of glutathione to the highly curved sites of the Au nanostructures. The fact that the curvature-directed assembly occurs for both the single-crystalline nanocubes and the multiply twinned nanobranches strongly suggests that the preferential bonding of glutathione to the curved sites is due to the less ordered surfactant bilayer at the curved sites than on the flat surfaces.

A Sulfur‐Rich Copolymer@CNT Hybrid Cathode with Dual‐Confinement of Polysulfides for High‐Performance Lithium–Sulfur Batteries

A sulfur-rich copolymer@carbon nanotubes hybrid cathode is introduced for lithium-sulfur batteries produced by combining the physical and chemical confinement of polysulfides. The binderfree and metal-current-collector-free cathode of dual confinement enables an efficient pathway for the fabrication of high-performance sulfur copolymer carbon matrix electrodes for lithium-sulfur batteries.

Fabrication of porous Sn–C composites with high initial coulomb efficiency and good cyclic performance for lithium ion batteries

In this paper, a cheap porous acrylic ion-exchange resin was introduced as a carbon source and metal ion supporter for the fabrication of Sn–C composites, which were used as anode materials for lithium ion batteries. The porous structure of the ion-exchange resin could be well preserved in the Sn–C composite, and Sn ions could be easily reduced to Sn nanoparticles. The as-prepared Sn–C composite showed high specific capacity and stable cycle performance. However, the large surface area and exposed metal nanoparticles of the porous Sn–C composites resulted in a low initial coulomb efficiency and led to potential safety problems. To remove these disadvantages, a surface carbon encapsulation process was applied to the porous Sn–C composite through a chemical vapour deposition process. The covered Sn–C composite showed an enclosed porous structure and all previously exposed Sn nanoparticles were perfectly encapsulated by amorphous carbon. The covered Sn–C composite also showed greatly improved chemical stability and a high initial coulomb efficiency over 80%.