Yongxing Hu

Formation of Hollow Silica Colloids through a Spontaneous Dissolution–Regrowth Process

tence of a large number of silicate species and their rich chemical interactions makes the dissolution and growth of silica challenging to study. However, this complexity also provides enormous opportunities for the development of materials with new structures and functionalities. For example, systematic investigation of the dissolution and formation of silica nanoparticles has made it possible to control the nucleation and growth, and subsequently the crystal size and shape, of zeolite materials. [4–6] Herein, we report that amorphous silica colloids, when dispersed in an aqueous solution of NaBH4, undergo a spontaneous morphology change from solid to hollow spheres. Concurrent but separate coredissolution and shell-growth processes appear to be responsible to the formation of the hollow structures. Besides the interesting fundamental aspects of this spontaneous process, this work also provides an effective self-templated route for the preparation of hollow silica nanostructures, which may find immediate applications in fields such as catalysis and drug delivery. [7–12] Since silica can coat many nanostructures through simple sol–gel processes, our discovery also allows convenient transformation of core–shell particles into yolk– shell structures, which are promising for use as nanoscale reactors and controlled-release vehicles. Compared to widely adopted methods using polymer beads and micelle and

Magnetically Recoverable Core–Shell Nanocomposites with Enhanced Photocatalytic Activity

Core-shell structured Fe(3)O(4)/SiO(2)/TiO(2) nanocomposites with enhanced photocatalytic activity that are capable of fast magnetic separation have been successfully synthesized by combining two steps of a sol-gel process with calcination. The as-obtained core-shell structure is composed of a central magnetite core with a strong response to external fields, an interlayer of SiO(2), and an outer layer of TiO(2) nanocrystals with a tunable average size. The convenient control over the size and crystallinity of the TiO(2) nanocatalysts makes it possible to achieve higher photocatalytic efficiency than that of commercial photocatalyst Degussa P25. The photocatalytic activity increases as the thickness of the TiO(2) nanocrystal shell decreases. The presence of SiO(2) interlayer helps to enhance the photocatalytic efficiency of the TiO(2) nanocrystal shell as well as the chemical and thermal stability of Fe(3)O(4) core. In addition, the TiO(2) nanocrystals strongly adhere to the magnetic supports through covalent bonds. We demonstrate that this photocatalyst can be easily recycled by applying an external magnetic field while maintaining their photocatalytic activity during at least eighteen cycles of use.

Hierarchical Magnetite/Silica Nanoassemblies as Magnetically Recoverable Catalyst-Supports

We report the synthesis of magnetically responsive hierarchical assemblies of silica colloids that can be used as recoverable supports for nanocatalysts. Each assembly is composed of a central magnetite/silica composite core and many small satellite silica spheres. The two regions are held together as a stable unit by a polymer network of poly(N-isopropylacrylamide). The central magnetite particles are superparamagnetic at room temperature with strong magnetic response to external fields, thus providing a convenient means for separating the entire assembly from the solution. The satellite silica particles provide large surface areas for loading nanocatalysts through the well-developed silane chemistry. As an example, we demonstrate the use of such magnetically responsive hierarchical assemblies as recoverable supports for Au nanocatalysts for the reduction of 4-nitrophenol with NaBH4.

Seeded growth of uniform Ag nanoplates with high aspect ratio and widely tunable surface plasmon bands.

Silver nanoplates with an extremely high aspect ratio (up to over 400) and a widely tunable surface plasmon resonance (SPR) band have been successfully synthesized by combining the concepts of selective ligand adhesion and seeded growth. Citrate ligands are used as the sole surfactant to effectively block overgrowth on the basal {111} facets and only allow growth in the lateral direction. By slowing down the reaction rate using Ag-citrate complex as precursor, the thin nature of Ag nanoplates is maintained with the edge length grown up to 4 μm, which ensures the high aspect ratio and the widely tunable SPR band. We also observe a size distribution focusing effect that helps to produce uniform nanoplates as well as narrow SPR bands over a wide range, which is important in many practical applications.

Tailored Synthesis of Superparamagnetic Gold Nanoshells with Tunable Optical Properties

Multifunctional Au nanoshells with tunable optical properties and fast magnetic response have been fabricated through a sequence of sol-gel, surface-protected etching, and seed-mediated growth processes. The use of a porous silica layer enhances the uniformity of nanoshell growth, the reproducibility of the synthesis, and the structural and optical stability of the products.

Fully alloyed Ag/Au nanospheres: combining the plasmonic property of Ag with the stability of Au.

We report that fully alloyed Ag/Au nanospheres with high compositional homogeneity ensured by annealing at elevated temperatures show large extinction cross sections, extremely narrow bandwidths, and remarkable stability in harsh chemical environments. Nanostructures of Ag are known to have much stronger surface plasmon resonance than Au, but their applications in many areas have been very limited by their poor chemical stability against nonideal chemical environments. Here we address this issue by producing fully alloyed Ag/Au nanospheres through a surface-protected annealing process. A critical temperature has been found to be around 930 °C, below which the resulting alloy nanospheres, although significantly more stable than pure silver nanoparticles, can still gradually decay upon extended exposure to a harsh etchant. Nanospheres annealed above the critical temperature show a homogeneous distribution of Ag and Au, minimal crystallographic defects, and the absence of structural and compositional interfaces, which account for the extremely narrow bandwidths of the surface plasmon resonance and may enable many plasmonic applications with high performance and long lifetime, especially for those involving corrosive species.

Mesoporous TiO2 Nanocrystal Clusters for Selective Enrichment of Phosphopeptides

Protein phosphorylation plays a key role in most cellular processes. Studying phosphopeptides in complex biological samples has been a great challenge due to their low abundance as well as the coexistence of excessive amounts of salts or surfactants. In this work we demonstrate a general approach for selective separation of phosphopeptides using a class of novel mesoporous nanostructured materials. TiO(2) nanocrystals are first self-assembled into submicrometer clusters containing relatively uniform mesoscale pores and then stabilized by coating with a thin layer of silica. Calcination of the materials at high temperatures connects the neighboring nanocrystals together and enhances the mechanical stability of the clusters and at the same time removes the organic surfactants and makes the TiO(2) surface fully accessible to phosphopeptides. By coating the nanocrystal clusters with a layer of silica before calcination and removing it afterward through chemical etching, we have been able to make the cluster surface hydrophilic and negatively charged, thus enhancing the water dispersibility of the clusters and eventually their accessibility to phosphopeptides. The high selectivity and capacity of these mesoporous TiO(2) clusters have been demonstrated by effectively enriching phosphopeptides from digests of phosphoprotein (alpha- or beta-casein), protein mixtures of beta-casein and bovine serum albumin, milk, and human serum samples. We also demonstrate that the self-assembly process brings the flexibility of incorporation of multiple components, such as superparamagnetic nanocrystals, to further facilitate the peptide separation.

Self-Assembly and Field-Responsive Optical Diffractions of Superparamagnetic Colloids

Superparamagnetic Fe(3)O(4) colloids with highly charged surfaces have been assembled into ordered structures in water in response to external magnetic fields. The colloids form chainlike structures with regular interparticle spacings of a few hundred nanometers along the direction of the external field so that the system strongly diffracts visible light. The balance between attractive (in this case, magnetic) and repulsive (electrostatic) forces dictates interparticle spacing and therefore optical properties. By changing the relative strength of these two forces, one can tune the peak diffraction wavelength over the entire visible spectrum. We were able to optimize the diffraction intensity and the tuning range through studying their dependence on variables such as the size distribution and concentration of the Fe(3)O(4) colloids or ionic strength of the solutions. The fast, reversible response and the feasibility for miniaturization impart these photonic materials great potential in applications such as optoelectronic devices, sensors, and color displays.

One-pot synthesis and optical property of copper(I) sulfide nanodisks.

Copper(I) sulfide (Cu(2)S) nanodisks with controllable size and aspect ratio have been synthesized by using a one-pot colloidal process, in which no pre-prepared organometallic precursors are required. The reaction involves the injection of dodecanethiol into a hot solution containing copper salt, surfactants, and a high boiling-point organic solvent. Copper thiolate forms at the beginning of the reaction which effectively acts as a precursor whose decomposition leads to further nucleation and growth of Cu(2)S nanocrystals. The nanocrystals begin as small nanodots in the early stages of the reaction, gradually turning into nanodisks with aspect ratios (average disk diameter divided by thickness) up to 2.0, while the band gap of the nanocrystals decreases accordingly. As the growth of nanocrystals follows the monomer addition mechanism, the diameter, thickness, aspect ratio, and optical property of the Cu(2)S nanodisks can be tuned systematically by changing the reaction time, the amount of surfactants, and the concentration of the precursors. This synthesis provides a simple and highly reproducible method for the preparation of Cu(2)S nanocrystals that may find potential applications in the fabrication of photovoltaic devices.

Self-assembly of superparamagnetic magnetite particles into peapod-like structures and their application in optical modulation

Superparamagnetic Fe3O4/SiO2/TiO2 peapod-like nanostructures have been successfully synthesized by using Fe3O4/SiO2 core/shell particles as building blocks and TiO2 as the adhesive without the need of any hard or soft templates. The fabrication process involves chaining the Fe3O4/SiO2 cores during magnetic stirring and subsequent fixing of the chain structure during TiO2 coating. The number of Fe3O4/SiO2 cores arranged linearly in the chains could be effectively controlled by tuning the amount of titanium precursor or the magnetic stirring rate. The double layer coating of SiO2 and TiO2 enhances thermal and chemical stability of the nanopeapods, and the one-dimensional chain structure produces interesting properties that enable applications not possible with conventional magnetite materials. As a demonstration, we show here the use of these superparamagnetic peapod-like nanostructures for low-frequency optical modulation.