Template-free ordered mesoporous silicas by binary nanoparticle assembly

Preparation of periodic mesoporous organosilica with large mesopores using silica colloidal crystals as templates

Organosiloxane-based mesoporous materials with periodically ordered pores (periodic mesoporous organosilica, PMO) have many applications due to their various organic functions, high surface areas, and large pore volumes. Conventional methods using surfactant templates (soft templates) are limited in terms of the diversity of organosilane precursors and precise control over the pore size in a relatively large mesopore region (10-50 nm). This paper demonstrates the preparation of PMOs with precisely controlled pore sizes (>10 nm in diameter) and various organosiloxane frameworks, using colloidal crystals of monodisperse silica nanospheres as a template. An inverse opal structure with interconnected spherical mesopores was obtained through polycondensation of hydrolyzed organoalkoxysilanes [(EtO)₃Si-R-Si(OEt)₃, R = C₂H₄, CH[double bond, length as m-dash]CH, and C₆H₄; PhSi(OEt)₃], within the voids of silica colloidal crystals, followed by the preferential dissolution of silica under well-controlled basic conditions. The pore size varied depending on the size of the silica nanospheres. The versatility of this method will allow for the wide tuning of the physical and chemical properties of organosiloxane-based mesoporous materials.

One-Pot Aqueous and Template-Free Synthesis of Mesoporous Polymeric Resins

This work explores the novel one-pot aqueous phase synthesis of mesoporous phenolic-hyperbranched polyethyleneimine resins without the use of a template, and their utility as heterogeneous catalysts in batch reactors and continuous microreactors. Catalyst surface areas of up to 432 m2/g were achieved with a uniform Pd distribution and an interconnected, highly porous, network structure, confirmed through Brunauer–Emmett–Teller (BET) surface area measurements, scanning electron microscopes (SEM), X-Ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), and Energy-dispersive X-ray spectroscopy (EDS). The heterogeneous catalysts achieved a maximum 98.98 ± 1% conversion in batch Suzuki couplings, with conversions being dependent upon reaction conditions, reactant chemistries, Pd loading and catalyst surface area. The catalysts were shown to be recyclable with only a marginal loss in conversion achieved after five runs. Up to 62 ± 5% and 46.5 ± 8% conversions at 0.2 mL/s and 0.4 mL/s flow rates, respectively, were achieved in a continuous microreactor. Understanding the mechanism of action of this mesoporous resin is a future research area, which could help expand the application vistas for this catalyst platform.

Adaptive Shape Ripening and Interparticle Bridging of l-Arginine-Stabilized Silica Nanoparticles during Evaporative Colloidal Crystal Assembly

During evaporative self-assembly of colloidal crystal films, spherical l-arginine-stabilized silica colloids adapt to different close-packed geometries by faceting and forming bridge connections with their nearest neighbors. We systematically studied the morphological changes of 37 and 138 nm diameter colloids during evaporative assembly and compared them to 65 nm Stöber silica colloids prepared without l-arginine. Colloidal crystal films were grown from particles that had been dialyzed against water or l-arginine, and tetraethyl orthosilicate (TEOS) and/or l-arginine were added to solutions during colloidal film growth. Solid-state ²⁹Si NMR spectra showed the presence of l-arginine and incompletely condensed silica in colloids grown from silica seeds in l-arginine solutions. These colloids were especially susceptible to chemical ripening during the colloidal assembly process, adopting faceted shapes that reflected the packing symmetry of the colloidal crystal films. The addition of l-arginine and TEOS accelerated these shape changes by catalyzing the hydrolysis and olation of silica and by adding a source of silica to the solution, respectively. This chemistry provides a route to single-component and binary colloidal crystals composed of nonspherical silica building blocks.

Binary colloidal crystals (BCCs): Interactions, fabrication, and applications

The organization of matter into hierarchical structures is a fundamental characteristic of functional materials and living organisms. Binary colloidal crystal (BCC) systems present a diversified range of nanotopographic structures where large and small colloidal particles simultaneously self-assemble into either 2D monolayer or 3D hierarchical crystal lattices. More importantly, understanding how BCCs form opens up the possibility to fabricate more complex systems such as ternary or quaternary colloidal crystals. Monolayer BCCs can also offer the possibility to achieve surface micro- and nano-topographies with heterogeneous chemistries, which can be challenging to achieve with other traditional fabrication tools. A number of fabrication methods have been reported that enable generation of BCC structures offering high accuracy in growth with controllable stoichiometries; however, it is still a challenge to make uniform BCC structures over large surface areas. Therefore, fully understand the mechanism of binary colloidal self-assembly is crucial and new/combinational methods are needed. In this review, we summarize the recent advances in BCC fabrication using particles made of different materials, shapes, and dispersion medium. Depending on the potential application, the degree of order and efficiency of crystal formation has to be determined in order to induce variability in the intended lattice structures. The mechanisms involved in the formation of highly ordered lattice structures from binary colloidal suspensions and applications are discussed. The generation of BCCs can be controlled by manipulation of their extensive phase behavior, which facilitates a wide range potential applications in the fields of both material and biointerfacial sciences including photonics, biosensors, chromatography, antifouling surfaces, biomedical devices, and cell culture tools.

Binary Superlattice Design by Controlling DNA-Mediated Interactions

Most binary superlattices created using DNA functionalization rely on particle size differences to achieve compositional order and structural diversity. Here we study two-dimensional (2D) assembly of DNA-functionalized micron-sized particles (DFPs), and employ a strategy that leverages the tunable disparity in interparticle interactions, and thus enthalpic driving forces, to open new avenues for design of binary superlattices that do not rely on the ability to tune particle size (i.e., entropic driving forces). Our strategy employs tailored blends of complementary strands of ssDNA to control interparticle interactions between micron-sized silica particles in a binary mixture to create compositionally diverse 2D lattices. We show that the particle arrangement can be further controlled by changing the stoichiometry of the binary mixture in certain cases. With this approach, we demonstrate the ability to program the particle assembly into square, pentagonal, and hexagonal lattices. In addition, different particle types can be compositionally ordered in square checkerboard and hexagonal-alternating string, honeycomb, and Kagome arrangements.

Binary Colloidal Crystal Films Grown by Vertical Evaporation of Silica Nanoparticle Suspensions

Despite intensive research efforts in the synthesis of binary colloidal crystals, the production of well ordered binary colloidal crystal films over large areas continues to be synthetically challenging. In this paper, we investigate the phase behavior of binary mixtures of l-arginine-stabilized 36 and 22 nm silica nanoparticles deposited as centimeter-scale thin films onto a vertical substrate via evaporative assembly. By adjusting the temperature and relative colloid composition under high humidity conditions, we controlled the order of the resultant colloidal crystal films. The domain size of the AB₂ binary crystalline phase increased with an excess of small (B) particles and a very slow evaporation rate below 45 °C, with the best results obtained at 30° and 35 °C.

Template-Induced Structuring and Tunable Polymorphism of Three-Dimensionally Ordered Mesoporous (3DOm) Metal Oxides

Convectively assembled colloidal crystal templates, composed of size-tunable (ca. 15-50 nm) silica (SiO₂) nanoparticles, enable versatile sacrificial templating of three-dimensionally ordered mesoporous (3DOm) metal oxides (MO_x) at both mesoscopic and microscopic size scales. Specifically, we show for titania (TiO₂) and zirconia (ZrO₂) how this approach not only enables the engineering of the mesopore size, pore volume, and surface area but can also be leveraged to tune the crystallite polymorphism of the resulting 3DOm metal oxides. Template-mediated volumetric (i.e., interstitial) effects and interfacial factors are shown to preserve the metastable crystalline polymorphs of each corresponding 3DOm oxide (i.e., anatase TiO₂ (A-TiO₂) and tetragonal ZrO₂ (t-ZrO₂)) during high-temperature calcination. Mechanistic investigations suggest that this polymorph stabilization is derived from the combined effects of the template-replica (MO_x/SiO₂) interface and simultaneous interstitial confinement that limit the degree of coarsening during high-temperature calcination of the template-replica composite. The result is the identification of a facile yet versatile templating strategy for realizing 3DOm oxides with (i) surface areas that are more than an order of magnitude larger than untemplated control samples, (ii) pore diameters and volumes that can be tuned across a continuum of size scales, and (iii) selectable polymorphism.

Superwettability Strategy: 1D Assembly of Binary Nanoparticles as Gas Sensors

Binary 1D nanowires consisting of both SnO₂ nanoparticles and Au nanorods are fabricated through a "substrate-particle solution template" assembling method, which shows highly enhanced gas sensitivity toward acetone under ambient conditions.