Junzheng Wang

Two-Phase Synthesis of Monodisperse Silica Nanospheres with Amines or Ammonia Catalyst and Their Controlled Self-Assembly

A significant progress has recently been made in the synthesis of monodisperse silica nanoparticles less than 30 nm in diameter by using basic amino acids (e.g., lysine) as a base catalyst for hydrolysis of silicon alkoxide. Alternatively, a more versatile and economical amino acid-free method has been developed to synthesize uniform silica nanospheres (SNSs) with low polydispersity (<12%) in liquid-liquid biphasic systems containing tetraethoxysilane (TEOS), water, and primary amine (or ammonia) under precisely controlled pH conditions (pH 10.8-11.4). The diameter of the SNSs determined from scanning electron microscopy (SEM) can be tuned from ∼12 to ∼36 nm by simply changing the initial pH of the aqueous phase in the reaction mixtures. Furthermore, the as-synthesized sol was taken as the starting material for studying the influences of the type of base catalysts on the solvent evaporation-induced three-dimensional (3D) self-assembly of SNSs. X-ray diffraction (XRD) and nitrogen adsorption-desorption are used to characterize the degree of packing of the resulting 3D arrays. The assembled SNSs with large interparticle mesopores with the diameter of ca. 8.1 nm and low packing fraction of ca. 66.1% are observed upon solvent evaporation of as-synthesized sol in the presence of primary amine. This indicates that SNSs are loosely packed, compared with the packing fraction of 74% for a face-centered cubic array of ideal hard spheres. In contrast, with the aid of an organic buffer or lysine as additives, the assembly of SNSs having smaller mesopores (ca. 3.9 nm) and higher packing fraction of 70.5-71.5% are achieved. It is suggested that the chemical additives with the ability to maintain relatively strong repulsive interaction until the final stage of evaporation play a vital role in the fabrication of well-ordered SNSs arrays.

Biphasic synthesis of colloidal mesoporous silica nanoparticles using primary amine catalysts.

A new type of colloidal mesoporous silica nanoparticles (MSNs) is synthesized in liquid-liquid biphasic systems consisting of tetraethoxysilane (TEOS) and water in the presence of primary amines and cationic surfactants (cetyltrimethylammonium chloride, CTAC) under controlled pH conditions (pH 11.1-11.5). The obtained MSNs are characterized by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption. The results show that the colloidal MSNs with an average diameter in the range of 28-54 nm and a size polydispersity below ca. 15% have been obtained. Importantly, each MSN is composed of a number of tiny primary silica nanoparticles (PSNPs) forming 3D connected pore structure. The pore size of the MSNs can be tuned from 2.5 to 3.0 nm by changing the pH of catalyst stock solution, and larger pore sizes (3.1-4.5 nm) can be achieved by using pore swelling agent. The Brunauer-Emmett-Teller (BET) surface areas and total pore volumes vary from 550 to 750 m(2)g(-1) and from 1.2 to 1.7 cm(3)g(-1), respectively. Compared with conventional MCM-41-type MSNs, our new MSNs show outstanding colloidal and hydrothermal stabilities. They are colloidally stable at room temperature over 1 year, and their mesostructure was retained even after hydrothermal treatment at 120°C for 24h. Finally, based on the analysis of the morphology and structure of MSNs, a formation scheme based on the cooperative self-assembly of PSNPs and surfactant molecules is proposed.

Critical nuclei size, initial particle size and packing effect on the phase stability of sol-peptization-gel-derived nanostructured titania.

The influence of the initial particle size and packing of anatase crystallites on the phase stability of nanostructured titania was investigated. Dried anatase gels with different degrees of particle packing were prepared through the peptization-induced electrostatic stabilization of primary particles in the sol. The initial size of anatase primary particles was varied by precalcination prior to the anatase-rutile phase transformation that occurred during final calcination. In the case of well-packed titania, the initial size of anatase primary particles does not influence the phase-transformation behavior whereas loosely packed titania shows a strong initial anatase primary particle size dependence on the phase-transformation behavior.

Preparation of anisotropic silica nanoparticles via controlled assembly of presynthesized spherical seeds.

A facile solution process for the preparation of anisotropic silica nanoparticles (ASNPs) is presented. ASNPs are prepared via controlled self-assembly of spherical silica seeds (22 nm) in alcohol-water mixed media, followed by their in situ fixation and overgrowth with tetraethoxysilane (TEOS). Ethanol and L-arginine (Arg) are used to modify the dielectric constant and ionic strength of the reaction media, by which seed assembly is controlled through the adjustment of electrostatic interaction. Ethanol and Arg also serve as a cosolvent and a catalyst for hydrolysis and condensation of TEOS, respectively, which enables us to produce ASNPs in a simple one-pot process. In addition to ASNPs with wormlike structures, different kinds of NPs (bimodal spherical NPs, monodisperse spherical NPs, and spherical aggregates) have also been obtained by changing the concentrations of ethanol and Arg. The length, thickness, or both of ASNPs are controlled systematically by varying the concentrations of Arg, seed NPs, and TEOS. Other alcoholic cosolvents, such as methanol, 1-propanol, 2-propanol, and t-butanol, are also effective to give ASNPs when the dielectric constant of the alcohol-water mixed media is properly adjusted, showing the versatility of the present method.

Nanoparticle Vesicles with Controllable Surface Topographies through Block Copolymer-Mediated Self-Assembly of Silica Nanospheres

Silica nanoparticle vesicles (NPVs) with encapsulating capability and surface permeability are highly attractive in nanocatalysis, biosensing, and drug delivery systems. Herein, we report the facile fabrication of silica NPVs composed of a monolayer of silica nanospheres (SNSs, ca. 15 nm in diameter) through the block copolymer-mediated self-assembly of SNSs. The silica NPVs gain different surface topographies, such as raspberry- and brain coral-like topographies, under controlled heat treatment conditions. The vesicular assembly of SNSs is successful with a series of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) block copolymers, and the size of NPVs can be tuned by changing their molecular weight. The polymer is easily extracted from the NPVs with their colloidal dispersibility and structural integrity intact. The polymer-free silica NPVs further serve as a reaction vessel and host for functional materials such as tin oxide nanoparticles.

Chain-like nanostructures from anisotropic self-assembly of semiconducting metal oxide nanoparticles with a block copolymer

A facile method is reported for the preparation of chain-like nanostructures by anisotropic self-assembly of TiO(2) and SnO(2) nanoparticles with the aid of a block copolymer in an aqueous medium. Well-defined crystallographic orientations between neighbouring nanoparticles are observed in TiO(2) nanochains, which is important for tailoring the grain boundaries and thus enhancing charge transport.

Dendritic silica nanoparticles synthesized by a block copolymer-directed seed-regrowth approach

A facile seed regrowth method is presented for the preparation of a new type of colloidal dendritic silica nanoparticles (DSNPs) with unique Konpeito-like morphology and high surface area (∼400 m(2) g(-1)). Growth of silica nanoprotrusions on the surfaces of colloidal silica nanoparticles proceeds by hydrolysis and polycondensation of tetraethoxysilane (TEOS) in the presence of a PEO-PPO-PEO-type block copolymer (Pluronic F127) under controlled pH conditions. The polymers adsorbed on the seed surface play a crucial role in the formation of DSNPs. DSNPs with controllable size (28-85 nm) and narrow size distributions can be obtained by using monodisperse silica nanoparticles with various sizes as seeds. The surface morphology of DSNPs is tunable by changing the concentration of TEOS. Additionally, novel dendritic silica nanochains are prepared using one-dimensionally assembled silica nanoparticles as the seeds.

Synthesis of string-bean-like anisotropic titania nanoparticles with basic amino acids

An “assembly–aggregation–peptization” approach is reported for the preparation of colloidal dispersions of string-bean-like anisotropic titania nanoparticles using arginine. These titania nanoparticles have a single-crystalline anatase structure and possess high surface area, which is advantageous for potential functional materials.