James Goebl

A Systematic Study of the Synthesis of Silver Nanoplates: Is Citrate a “Magic” Reagent?

In this work we have carried out systematic studies and identified the critical role of hydrogen peroxide instead of the generally believed citrate in the well-known chemical reduction route to silver nanoplates. This improved understanding allows us to develop consistently reproducible processes for the synthesis of nanoplates with high efficiency and yields. By harnessing the oxidative power of H(2)O(2), various silver sources including silver salts and metallic silver can be directly converted to nanoplates with the assistance of an appropriate capping ligand, thus significantly enhancing the reproducibility of the synthesis. Contrary to the previous conclusion that citrate is the key component, we have determined that the group of ligands with selective adhesion to Ag (111) facets can be expanded to many di- and tricarboxylate compounds whose two nearest carboxylate groups are separated by two or three carbon atoms. We have also found that the widely used secondary ligand polyvinylpyrrolidone can be replaced by many hydroxyl group-containing compounds or even removed entirely while still producing nanoplates of excellent uniformity and stability. In addition to the general understanding of NaBH(4) as a reducing agent, it has also been found to act as a capping agent to stabilize the silver nanoparticles, prolong the initiation time required for nanoplate nucleation, and contribute to the control of the thickness as well as the aspect ratio of silver nanoplates. The improved insight into the specific roles of the reaction components and significantly enhanced reproducibility are expected to help elucidate the formation mechanism of this interesting nanostructure.

Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity

Mesoporous hollow TiO2 shells with controllable crystallinity have been successfully synthesized by using a novel partial etching and re-calcination process. This method involves several sequential preparation steps as follows: 1) Synthesis of SiO2@TiO2@SiO2 colloidal composites through sol–gel processes and crystallization by calcination, 2) partial etching to preferentially remove portions of the SiO2 layers contacting the TiO2 surface, and 3) re-calcination to crystallize the TiO2 and finally etching of the inner and outer SiO2 to produce mesoporous anatase TiO2 shells. The partial etching step produces a small gap between SiO2 and TiO2 layers which allows space for the TiO2 to further grow into large crystal grains. The re-calcination process leads to well developed crystalline TiO2 which maintains the mesoporous shell structure due to the protection of the partially etched outer silica layer. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as-prepared mesoporous TiO2 shells show significantly enhanced catalytic activity. In particular, TiO2 shells synthesized with optimal crystallinity by using this approach show higher performance than commercial P25 TiO2.

Magnetochromatic Microspheres: Rotating Photonic Crystals

Magnetochromatic microspheres have been fabricated through instant assembly of superparamagnetic (SPM) colloidal particles inside emulsion droplets of UV curable resin followed by an immediate UV curing process to polymerize the droplets and fix the ordered structures. When dispersed in the liquid droplets, superparamagnetic Fe(3)O(4)@SiO(2) core/shell particles self-organize under the balanced interaction of repulsive and attractive forces to form one-dimensional chains, each of which contains periodically arranged particles diffracting visible light and displaying field-tunable colors. UV initiated polymerization of the oligomers of the resin fixes the periodic structures inside the droplet microspheres and retains the diffraction property. Because the superparamagnetic chains tend to align themselves along the field direction, it is very convenient to control the orientation of such photonic microspheres and, accordingly, their diffractive colors, by changing the orientation of the crystal lattice relative to the incident light using magnetic fields. The excellent stability together with the capability of fast on/off switching of the diffraction by magnetic fields makes the system suitable for applications such as color display, rewritable signage, and sensors. As a simple demonstration, we have fabricated a display unit that has on/off bistable states by embedding the magnetochromatic microspheres in a matrix that can thermally switch between solid and liquid phases.

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.

Role of salt in the spontaneous assembly of charged gold nanoparticles in ethanol.

This paper investigates the role of salt in the spontaneous linear assembly of charged gold nanoparticles in ethanol and attempts to clear up a misunderstanding on the role of ethanol in this process. Many prior reports have noted that the addition of ethanol to an aqueous solution of gold nanoparticles causes their aggregation into linear assemblies. It was therefore believed that ethanol plays the determining role during the assembly process. In this work, we carried out systematic studies which indicate that residual salt in conjunction with ethanol, instead of ethanol itself, induces the assembly of gold nanoparticles in ethanol. In the absence of salt, gold nanoparticles can be well dispersed in an ethanol solution. Furthermore, we find that the chainlike assemblies can disassemble upon dilution of the salt or the evaporation of ethanol if the gold nanoparticles are protected with a sufficiently strong ligand.

Monitoring the Shape Evolution of Silver Nanoplates: A Marker Study

Out of the frame: A marker study using gold frames was designed to reveal that silver nanoplates undergo a shape transition during their seeded growth from triangular to circular to hexagonal plates before ultimately returning to triangular structures with an orientation 180° relative to that of the original triangular seeds (see picture, the original gold triangular frame is visible at the center of the silver nanoplate).

H2O2‐Aided Seed‐Mediated Synthesis of Silver Nanoplates with Improved Yield and Efficiency

Two-dimensional silver nanoplates have drawn remarkable interest due to their strong shape-dependent optical properties. When the lateral dimension of silver nanoplates is much larger than their thickness, they possess an extreme degree of anisotropy, which favors high tunability of their localized surface plasmon resonance (LSPR) and therefore generates a maximum electromagnetic field enhancement. As a result, they are considered great candidates for applications in a wide variety of fields, such as surface-enhanced Raman scattering (SERS), near-field optical probes, optical lables, and catalysis. Since the first report by Mirkin et al. on the photo-induced synthesis of silver nanoplates, many additional synthetic strategies have been developed. Most conventional synthetic routes are based on solution-phase reactions, including photochemical processes, ligand-assisted chemical reductions, templating procedures, electrochemical syntheses, and sonochemical routes. Among these synthetic approaches, seed-mediated growth for the synthesis of silver nanoplates has been very popular, producing silver nanoplates with different aspect ratios resulting in their desired properties. In a typical seed-mediated method, two major steps are involved: 1) preparation of silver seeds; 2) growth of silver nanoplates based on the obtained silver seeds. In the first step, small silver seeds (usually <5 nm in size) are produced in the presence of citrate through rapid reduction of Ag by a strong reducing agent, such as NaBH4. In the second step, continued growth of seeds into well-defined nanoplates is promoted by reducing Ag ions with a milder reducing agent, such as ascorbic acid or hydrazine, in the presence of suitable surfactants, such as citrate and poly(vinyl)pyrrolidone (PVP). The milder reducing agent and surfactant impart a slower reaction rate which favours the anisotropic seeded growth. In between the two major steps, an important but often unrecognized, long aging period is typically required. After the initial reaction, the seed solution is usually aged in the dark for several hours prior to the growth step. We believe the primary purpose of this aging step, although not extensively discussed in literature, is to eliminate unreacted NaBH4, which, if present in the subsequent growth process, will produce spherical silver nanoparticles as the result of self-nucleation. According to our observations, this long aging period is the main cause of poor reproducibility of the seed-mediated process because any undesired disturbance, such as temperature variation, can greatly influence the decomposition of NaBH4. As a result, although numerous recipes have been proposed, seed-mediated growth methods are still not suitable for large-scale synthesis of silver nanoplates due to limitations in production efficiency, yield, and reproducibility. Recently, we have reported a systematic study on the synthesis of silver nanoplates using a direct chemical reduction approach, in which we identified that hydrogen peroxide (H2O2), instead of the generally attributed citrate, plays the determinant role. Although it remains an interesting assumption for future exploration, we proposed that H2O2 etching could remove the relatively unstable nanoparticles at the seeding stage, leaving only those with a higher degree of stability. Due to the protection of citrate ions, which preferentially bind to (111) facets of silver nanoparticles, plate-structured silver nuclei possess the highest relative stability since the majority of the surface is capped by ligands. Therefore, the net effect of H2O2 in this reaction, synergistically with citrate ions, is to promote the nucleation of plates by removing less stable silver nanoparticles possessing other structures. We have also found that the widely used secondary ligand, PVP, can be replaced by many hydroxyl-group-containing compounds or even removed entirely while still producing nanoplates of excellent uniformity and stability. This substantially improved insight into the specific roles of the reaction components and significantly enhanced reproducibility are expected to help elucidate the formation mechanism of this interesting nanostructure. Herein, we further extend our proposed mechanism to the widely used seed-mediated growth method in order to significantly improve the reproducibility, and production yield and efficiency. A typical recipe for seed-mediated growth of silver nanoplates has been adopted and modified from the literature, in which silver seeds were first obtained by mixing AgNO3, trisodium citrate (TSC) and NaBH4, followed by a seedmediated growth process in which ascorbic acid and TSC were utilized as reductant and surfactant, respectively. To prove our assumption that H2O2 can help obtain silver seeds with a twinned structure, H2O2 was used during the seeding process. The results show that H2O2 can not only improve the yield of silver nanoplates, but also shorten the reaction time by making the aging process unnecessary, which is a great advantage in comparison to traditional seed-mediated growth methods. Furthermore, we show that PVP is not only unnecessary for this synthetic system, but through its elimination, the stability of the obtained silver nanoplates can be significantly improved. [a] N. Li, Prof. Y. Gan School of Chemical Engineering & Technology Harbin Institute of Technology Harbin, Heilongjiang 150001 (P. R. China) E-mail : ygan@hit.edu.cn [b] N. Li, Q. Zhang, S. Quinlivan, J. Goebl, Prof. Y. Yin Department of Chemistry University of California, Riverside Riverside, CA 92521 (USA) Fax: (+1) 951-827-4713 E-mail : yadong.yin@ucr.edu

Seeded growth route to noble metal nanostructures

Seeded growth is a versatile solution-phase synthesis route to noble metal nanostructures. As the structure and concentration of the seeds as well as their growth kinetics can be manipulated independently, it enables convenient and precise control over the size and morphology of the target nanostructures. In this feature article, we summarize our recent progress in developing seeded growth techniques and provide a brief discussion of the principles for the rational design of noble metal nanostructures, emphasizing control over the reaction kinetics, the properties of the seed, the reduction potential of the involved reactive species, and the effect of spatial confinement imposed on the growth, for creating novel functional nanomaterials with well-defined sizes, structures, and properties.

Self-assembly and tunable plasmonic property of gold nanoparticles on mercapto-silica microspheres

We report the self-assembly behavior and tunable plasmonic property of gold nanoparticles on the surface of mercapto-silica colloidal spheres. The mercapto-silica spheres provide a quasi two-dimensional surface for nanoparticle immobilization through the strong gold–thiol interactions, making it convenient to control the interparticle separation by varying both the length of the capping ligands and the molar ratio of gold nanoparticles to the supporting mercapto-silica microspheres. With the ability to fine tune the interparticle separation and consequently the strength of the electromagnetic coupling, we demonstrate a high degree of control over the plasmon resonance property of the resulting composite microspheres. The colloidal form of the assembled nanostructures promises broad applications in which tailored plasmonic property and solution dispersibility are required.