Discovering Synthesis Routes to Hexagonally Ordered Mesoporous Niobium Nitrides Using Poloxamer/Pluronics Block Copolymers

Discovering Synthesis Routes to Hexagonally Ordered Mesoporous Niobium Nitrides Using Poloxamer/Pluronics Block Copolymer

Media 1: Solving structure with sparse, randomly-oriented x-ray data

Originally published in Optics Express on 04 June 2012 (oe-20-12-13129

Monolithic Gyroidal Mesoporous Mixed Titanium–Niobium Nitrides

Mesoporous transition metal nitrides are interesting materials for energy conversion and storage applications due to their conductivity and durability. We present ordered mixed titanium–niobium (8:2, 1:1) nitrides with gyroidal network structures synthesized from triblock terpolymer structure-directed mixed oxides. The materials retain both macroscopic integrity and mesoscale ordering despite heat treatment up to 600 °C, without a rigid carbon framework as a support. Furthermore, the gyroidal lattice parameters were varied by changing polymer molar mass. This synthesis strategy may prove useful in generating a variety of monolithic ordered mesoporous mixed oxides and nitrides for electrode and catalyst materials

Synthesis and Formation Mechanism of Aminated Mesoporous Silica Nanoparticles

We report the room temperature formation of aminated mesoporous silica nanoparticles (NH₂-MSNs) by means of co-condensation of different molar ratios of tetraethyl orthosilicate (TEOS) and 3-aminopropyl triethoxysilane (APTES) in the synthesis feed. The resulting materials are characterized by a combination of transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and N₂ adsorption/desorption measurements. Analysis reveals that an increase in APTES loading (mol %) leads to structural transitions in the MSNs from hexagonal (0–49 mol %) to cubic <i>Pm</i>3̅<i>n</i> (54–64 mol %) to disordered at very high APTES amounts (69 mol %). Investigation of structural evolution during cubic <i>Pm</i>3̅<i>n</i> particle synthesis reveals early particle formation stages that are surprisingly similar to those discussed in recent literature on nonclassical single crystal growth. These include significant heterogeneities in particle density despite crystallographic orientation across the entire particle as well as particle growth via addition of preformed and prestructured silica clusters. Syntheses at varying pH reveal further details of the structure formation process. The results pose fundamental questions about the relation between formation mechanisms of classical crystalline materials and mesoscopically ordered, locally amorphous materials

Structural and Kinetic Effects on Changes in the CO₂ Binding Pocket of Human Carbonic Anhydrase II

This work examines the effect of perturbing the position of bound CO₂ in the active site of human carbonic anhydrase II (HCA II) on catalysis. Variants of HCA II in which Val143 was replaced with hydrophobic residues Ile, Leu, and Ala were examined. The efficiency of catalysis in the hydration of CO₂ for these variants was characterized by ¹⁸O exchange mass spectrometry, and their structures were determined by X-ray crystallography at 1.7–1.5 Å resolution. The most hydrophobic substitutions, V143I and V143L, showed decreases in the level of catalysis, as much as 20-fold, while the replacement by the smaller V143A mutation showed an only moderate 2-fold decrease in activity. Structural data for all three variants show no significant change in the overall position of amino acid side chains in the active site compared with the wild type. However, V143A HCA II showed additional ordered water molecules in the active site compared to the number for the wild type. To further investigate the decrease in the catalytic efficiency of V143I HCA II, an X-ray crystallographic CO₂ entrapment experiment was performed to 0.93 Å resolution. This structure revealed an unexpected shift in the CO₂ substrate toward the zinc-bound solvent, placing it ∼0.3 Ǻ closer than previously observed in the wild type in conjunction with the observed dual occupancy of the product bicarbonate, presumably formed during the acquisition of data. These data suggest that the Ile substitution at position 143 reduced the catalytic efficiency, which is likely due to steric crowding resulting in destabilization of the transition state for conversion of CO₂ into bicarbonate and a decreased product dissociation rate

Stimuli-Responsive Shapeshifting Mesoporous Silica Nanoparticles

Stimuli-responsive materials have attracted great interest in catalysis, sensing, and drug delivery applications and are typically constituted by soft components. We present a one-pot synthetic method for a type of inorganic silica-based shape change material that is responsive to water vapor exposure. After the wetting treatment, the cross-sectional shape of aminated mesoporous silica nanoparticles (MSNs) with hexagonal pore lattice changed from hexagonal to six-angle-star, accompanied by the loss of periodic mesostructural order. Nitrogen sorption measurements suggested that the wetting treatment induced a shrinkage of mesopores resulting in a broad size distribution and decreased mesopore volume. Solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy of samples after wetting treatment displayed a higher degree of silica condensation, indicating that the shape change was associated with the formation of more siloxane bonds within the silica matrix. On the basis of material characterization results, a mechanism for the observed anisotropic shrinkage is suggested based on a buckling deformation induced by capillary forces in the presence of a threshold amount of water vapor available beyond a humidity of about 50%. The work presented here may open a path toward novel stimuli-responsive materials based on inorganic components

Strain Mapping of Two-Dimensional Heterostructures with Subpicometer Precision

Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with subpicometer precision and the ability to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector–the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space data sets contain the full spatially resolved lattice information on the sample. By using this technique on tungsten disulfide (WS₂) and tungsten diselenide (WSe₂) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multimicron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally epitaxial structures

Formation of Periodically-Ordered Calcium Phosphate Nanostructures by Block Copolymer-Directed Self-Assembly

Structuring ionic solids at the nanoscale with block copolymers (BCPs) is notoriously difficult due to solvent incompatibilities and strong driving forces for crystallization of the inorganic material. Here, we demonstrate that elucidating pathway complexity in the BCP-directed self-assembly of an ionic solid, amorphous calcium phosphate (ACP), is a key component in obtaining nanostructured, <i>bulk</i> composite materials in which the nanostructure is the result of thermodynamically controlled BCP self-assembly, i.e., exhibiting sequences of bulk morphologies as known from typical equilibrium BCP phase diagrams. Specifically, we identify three critical pathway “decision points” for the evaporation-induced self-assembly of composites from ultrasmall, organosilicate-modified amorphous calcium phosphate nanoparticles (osm-ACP-NPs) and poly­(isoprene)-<i>block</i>-poly­(2-(dimethylamino)­ethyl methacrylate) (PI-<i>b</i>-PDMAEMA) block copolymers. Using this strategy enabled us to obtain composites with hexagonal, cubic network, and lamellar BCP morphologies, in addition to mesoporous, cellular materials and macrophase separated materials. The osm-ACP-NPs are synthesized via a two-step sol–gel process in which (3-glycidyloxypropyl)­trimethoxysilane (GLYMO) quenches the reaction, limits the particle size, and functionalizes the NP surface. Dynamic light scattering evidences a transition from BCP unimers to micellar aggregates with increasing amounts of sol solution, which is reflected by a corresponding switch from BCP-type morphologies to micellar/cellular morphologies of the nanocomposites. Nanostructured organic–inorganic composites with a continuous osm-ACP-NP matrix phase have indentation moduli (measured by nanoindentation) that are an order of magnitude larger than unstructured composites with similar compositions. Insights provided by this study have relevance to understanding the effects of pathway complexity in the assembly of organic–inorganic composites and may enable access to a broad range of hybrid nanostructures with potential applications in areas including dental repair and hard tissue engineering

High-Speed <i>in Situ</i> X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs

High-Speed <i>in Situ</i> X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs