Unimer suppression enables supersaturated homopolymer swollen micelles with long-term stability after glassy entrapment.

Micelle sizes are critical for a range of applications where the simple ability to adjust and lock in specific stable sizes has remained largely elusive. While micelle swelling agents are well-known, their dynamic re-equilibration in solution implies limited stability. Here, a non-equilibrium processing sequence is studied where supersaturated homopolymer swelling is combined with glassy-core ("persistent") micelles. This path-dependent process was found to sensitively depend on unimer concentration as revealed by DLS, SAXS, and TEM analysis. Here, lower-selectivity solvent combinations led to the formation of unimer-homopolymer aggregates and eventual precipitation, reminiscent of anomalous micellization. In contrast, higher-selectivity solvents enabled supersaturated homopolymer loadings favored by rapid homopolymer insertion. The demonstrated ∼40-130 nm core-size tuning exceeded prior equilibrium demonstrations and subsequent core-vitrification enabled size persistence beyond 6 months. Lastly, the linear change in micelle diameter with homopolymer addition was found to correlate with a plateau in the interfacial area per copolymer chain.

New approach for SANS measurement of micelle chain mixing during size and morphology transitions.

Chain exchange in amphiphilic block polymer micelles is measurable with time-resolved small-angle neutron scattering (TR-SANS) where contrast-matched conditions reveal chain mixing as reduced intensity. However, analyzing chain mixing on short time scales e.g. during micelle transformations remains challenging. SANS model fitting can quantify chain mixing during size and morphology changes, however short acquisition times lead to lower data statistics (higher error). Such data are unsuitable for form factor fitting, especially with polydisperse and/or multimodal scenarios. An integrated-reference approach, R(t), is compatible with such data by using fixed reference patterns for the unmixed and fully mixed states that are each integrated to improve data statistics (lower error). Although the R(t) approach is tolerant of low data statistics, it remains incompatible with size and morphology changes. A new shifting references relaxation approach, SRR(t), is proposed where reference patterns are acquired at each time point to enable mixed state calculations regardless of short acquisition times. The additional experimental measurements needed are described which provide these time-varying reference patterns. The use of reference patterns makes the SRR(t) approach size/morphology-agnostic, allowing for the extent of micelle mixing to be directly calculated without this knowledge. SRR(t) is thus compatible with arbitrary levels of complexity and can provide accurate assessment of the mixed state which could support future model analysis. Calculated scattering datasets were used to demonstrate the SRR(t) approach during multiple size, morphology, and solvent conditions (scenarios 1-3). The mixed state calculated from the SRR(t) approach is shown to be accurate for all three scenarios.

Enhanced Structural Control of Soft-Templated Mesoporous Inorganic Thin Films by Inert Processing Conditions.

Mesoporous thin films are widely used for applications in need of high surface area and efficient mass and charge transport properties. A well-established fabrication process involves the supramolecular assembly of organic molecules (e.g., block copolymers and surfactants) with inorganic materials obtained by sol-gel chemistry. Typically, subsequent calcination in air removes the organic template and reveals the porous inorganic network. A significant challenge for such coatings is the anisotropic shrinkage due to the volume contraction related to solvent evaporation, inorganic condensation, and template removal, affecting the final porosity as well as pore shape, size, arrangement, and accessibility. Here, we show that a two-step calcination process, composed of high-temperature treatment in argon followed by air calcination, is an effective fabrication strategy to reduce film contraction and enhance structural control of mesoporous thin films. Crucially, the formation of a transient carbonaceous scaffold enables the inorganic matrix to fully condense before template removal. The resulting mesoporous films retain a higher porosity as well as bigger pores with extended porous order. Such films present favorable characteristics for mass transport of large molecules. This is demonstrated for lysozyme adsorption into the mesoporous thin films as an example of enzyme storage.

High-χ, low-N micelles from partially perfluorinated block polymers.

Kinetically trapped ("persistent") micelles enable emerging applications requiring a constant core diameter. Preserving a χN barrier to chain exchange with low-N requires a commensurately higher χcore-solvent for micelle persistence. Low-N, high-χ micelles containing fluorophobic interactions were studied using poly(ethylene oxide-b-perfluorooctyl acrylate)s (O45FX, x = 8, 11) in methanolic solutions. DLS analysis of micelles revealed chain exchange only for O45F8 while SAXS analysis suggested elongated core block conformations commensurate with the contour lengths. Micelle chain exchange from solution perturbations were examined by characterizing their behavior as templates for inorganic materials via SAXS and SEM. In contrast to the F8 analog, the larger χN barrier for the O45F11 enabled persistent micelle behavior in both thin films and bulk samples despite the low Tg micelle core. Careful measures of micelle core diameters and pore sizes revealed that the nanoparticle distribution extended through the corona and 0.52 ± 0.15 nm into the core-corona interface, highlighting thermodynamics favoring both locations simultaneously.

Coordination of Quantum Dots in a Polar Solvent by Small-Molecule Imidazole Ligands.

Colloidal quantum dots (QDs) are attractive fluorophores for bioimaging and biomedical applications because of their favorable and tunable optoelectronic properties. In this study, the native hydrophobic ligand environment of oleate-capped sphalerite CdSe/ZnS core/shell QDs was quantitatively exchanged with a set of imidazole-bearing small-molecule ligands. Inductively coupled plasma-optical emission spectroscopy and 1H NMR were used to identify and quantify three different ligand exchange processes: Z-type dissociation of the Zn(oleate)2, L-type association of the imidazole, and X-type anionic exchange of oleate with Cl-, all of which contributed to the overall ligand exchange.

Mesoporous TiO2 Microparticles with Tailored Surfaces, Pores, Walls, and Particle Dimensions Using Persistent Micelle Templates.

Mesoporous microparticles are an attractive platform to deploy high-surface-area nanomaterials in a convenient particulate form that is broadly compatible with diverse device manufacturing methods. The applications for mesoporous microparticles are numerous, spanning the gamut from drug delivery to catalysis and energy storage. For most applications, the performance of the resulting materials depends upon the architectural dimensions including the mesopore size, wall thickness, and microparticle size, yet a synthetic method to control all these parameters has remained elusive. Furthermore, some mesoporous microparticle reports noted a surface skin layer which has not been tuned before despite the important effect of such a skin layer upon transport/encapsulation. In the present study, material precursors and block polymer micelles are combined to yield mesoporous materials in a microparticle format due to phase separation from a homopolymer matrix. The skin layer thickness was kinetically controlled where a layer integration via diffusion (LID) model explains its production and dissipation. Furthermore, the independent tuning of pore size and wall thickness for mesoporous microparticles is shown for the first time using persistent micelle templates (PMT). Last, the kinetic effects of numerous processing parameters upon the microparticle size are shown.

Persistent Micelle Corona Chemistry Enables Constant Micelle Core Size with Independent Control of Functionality and Polyelectrolyte Response.

Polymer micelles have found significant uses in areas such as drug/gene delivery, medical imaging, and as templates for nanomaterials. For many of these applications, the micelle performance depends on its size and chemical functionalization. To date, however, these parameters have often been fundamentally coupled since the equilibrium size of a micelle is a function of the chemical composition in addition to other parameters. Here, we demonstrate a novel processing pathway allowing for the chemical modification to the corona of kinetically trapped "persistent" polymer micelles, termed Persistent Micelle Corona Chemistry (PMCC). Judicious planning is crucial to this size-controlled functionalization where each step requires all reagents and polymer blocks to be compatible with (1) the desired chemistry, (2) micelle persistency, and (3) micelle dispersion. A desired functionalization can be implemented with PMCC by pairing the synthetic planning with polymer solubility databases. Specifically, poly(cyclohexyl methacrylate-b-(diethoxyphosphoryl)methyl methacrylate) (PCHMA-b-PDEPMMA) was prepared to combine a glassy-core block (PCHMA) for kinetic control with a block (PDEPMMA) that is able to be hydrolyzed to yield acid groups. The processing sequence determines the resulting micelle size distribution where the hydrolyzed-then-micellized sequence yields widely varying micelle dimensions due to equilibration. In contrast, the micellized-then-hydrolyzed sequence maintains kinetically trapped micelles throughout the PMCC process. Statistically significant transmission electron microscopy (TEM) measurements demonstrate that PMCC uniquely enables this functionalization with constant average micelle core dimensions. Furthermore, these kinetically trapped micelles also subsequently maintain constant micelle core size when modifying the Coulombic interactions of the micelle corona via pH changes.

Reversible Molecular and Ionic Storage Mechanisms in High-Performance Zn0.1V2O5·nH2O Xerogel Cathode for Aqueous Zn-Ion Batteries.

The cathode is a critical component for aqueous Zn-ion batteries (ZIBs) to achieve high capacity and long stability. In this work, we demonstrate a dissolution-free, low-Zn-preinserted bilayer-structured V2O5 xerogel cathode, Zn0.1V2O5·nH2O (ZnVO), with excellent capacity and stability using a low-cost ZnSO4 electrolyte. Its discharge capacity reaches 463 mAh g-1 at 0.2 A g-1 and 240 mAh g-1 at 10 A g-1, while 93% and 88% of its capacity are retained at 0.2 A g-1 for 200 cycles and at 10 A g-1 for 20 000 cycles, respectively. We then show that the outstanding performance of ZnVO is derived from the enlarged gallery spacing by the solvent water intercalation and the water stable V2O5 bilayer structure. We further unveil via ab initio molecular dynamics that H+ is largely originated from the dissociation of the gallery water, while OH- moves out of the gallery to form Zn4(SO4)(OH)6·5H2O with ZnSO4 electrolyte on the surface of ZnVO; the intercalated Zn2+ forms aquo complex [Zn(H2O)6]2+ with the gallery water. Our theoretical analysis also suggests that the gallery water and solvent water in the electrolyte are statistically the same and functionally equivalent. Overall, this study shows the promise of ZnVO as a practical cathode for ZIBs and offers fundamental insights into the roles of gallery water, solvent water, bilayer V2O5 structure, and dual Zn2+/H+ intercalation mechanisms in achieving high capacity and long stability.

Effect of Membrane Properties on the Carbonation of Anion Exchange Membrane Fuel Cells.

Anion exchange membrane fuel cells (AEMFC) are potentially very low-cost replacements for proton exchange membrane fuel cells. However, AEMFCs suffer from one very serious drawback: significant performance loss when CO2 is present in the reacting oxidant gas (e.g., air) due to carbonation. Although the chemical mechanisms for how carbonation leads to voltage loss in operating AEMFCs are known, the way those mechanisms are affected by the properties of the anion exchange membrane (AEM) has not been elucidated. Therefore, this work studies AEMFC carbonation using numerous high-functioning AEMs from the literature and it was found that the ionic conductivity of the AEM plays the most critical role in the CO2-related voltage loss from carbonation, with the degree of AEM crystallinity playing a minor role. In short, higher conductivity-resulting either from a reduction in the membrane thickness or a change in the polymer chemistry-results in faster CO2 migration and emission from the anode side. Although this does lead to a lower overall degree of carbonation in the polymer, it also increases CO2-related voltage loss. Additionally, an operando neutron imaging cell is used to show that as AEMFCs become increasingly carbonated their water content is reduced, which further drives down cell performance.

Growth of Crystalline Bimetallic Metal-Organic Framework Films via Transmetalation.

Crystalline films of the Cu3(BTC)2 (BTC3- = 1,3,5-benzenetricarboxylate) metal-organic framework (MOF) have been grown by dip-coating an alumina/Si(111) substrate in solutions of Cu(II) acetate and the organic linker H3BTC. Atomic force microscopy (AFM) experiments demonstrate that the substrate is completely covered by the MOF film, while grazing incidence wide-angle X-ray scattering (GIWAXS) establishes the crystallinity of the films. Forty cycles of dip-coating results in a film that is ∼70 nm thick with a root mean squared roughness of 25 nm and crystallites ranging from 50-160 nm in height. Co2+ ions were exchanged into the MOF framework by immersing the Cu3(BTC)2 films in solutions of CoCl2. By varying the temperature and exchange times, different concentrations of Co were incorporated into the films, as determined by X-ray photoelectron spectroscopy experiments. AFM studies showed that morphologies of the bimetallic films were largely unchanged after transmetalation, and GIWAXS indicated that the bimetallic films retained their crystallinity.

A High Performing Zn-Ion Battery Cathode Enabled by In Situ Transformation of V2 O5 Atomic Layers.

Developing high capacity and stable cathodes is a key to successful commercialization of aqueous Zn-ion batteries (ZIBs). Pure layered V2 O5 has a high theoretical capacity (585 mAh g-1 ), but it suffers severe capacity decay. Pre-inserting cations into V2 O5 can substantially stabilize the performance, but at an expense of lowered capacity. Here we show that an atomic layer deposition derived V2 O5 can be an excellent ZIB cathode with high capacity and exceptional cycle stability at once. We report a rapid in situ on-site transformation of V2 O5 atomic layers into Zn3 V2 O7 (OH)2 ⋅2 H2 O (ZVO) nanoflake clusters, also a known Zn-ion and proton intercalatable material. High concentration of reactive sites, strong bonding to the conductive substrate, nanosized thickness and binder-free composition facilitate ionic transport and promote the best utilization of the active material. We also provide new insights into the V2 O5 -dissolution mechanisms for different Zn-salt aqueous electrolytes and their implications to the cycle stability.

A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor-Acceptor Fulleretic Covalent-Organic Materials.

The effect of donor (D)-acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for "static" changes in materials properties, an external stimulus was applied for "dynamic" control of the electronic profiles. Overall, the presented D-A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.

Supramolecular Assembly of Oriented Spherulitic Crystals of Conjugated Polymers Surrounding Carbon Nanotube Fibers.

The directed assembly of conjugated polymers into macroscopic organization with controlled orientation and placement is pivotal in improving device performance. Here, the supramolecular assembly of oriented spherulitic crystals of poly(3-butylthiophene) surrounding a single carbon nanotube fiber under controlled solvent evaporation of solution-cast films is reported. Oriented lamellar structures nucleate on the surface of the nanotube fiber in the form of a transcrystalline interphase. The factors influencing the formation of transcrystals are investigated in terms of chemical structure, crystallization temperature, and time. Dynamic process measurements exhibit the linear growth of transcrystals with time. Microstructural analysis of transcrystals reveals individual lamellar organization and crystal polymorphism. The form II modification occurs at low temperatures, while both form I and form II modifications coexist at high temperatures. A possible model is presented to interpret transcrystallization and polymorphism.

Widely tunable persistent micelle templates via homopolymer swelling.

The combination of precision control with wide tunability remains a challenge for the fabrication of porous nanomaterials suitable for studies of nanostructure-behavior relationships. Polymer micelle templates broadly enable porous materials, however micelle equilibration hampers independent pore and wall size control. Persistent micelle templates (PMT) have emerged as a kinetic controlled platform that uniquely decouples the control of pore and wall dimensions. Here, chain exchange is inhibited to preserve a constant template dimension (pore size) despite the shifting equilibrium while materials are added between micelles. Early PMT demonstrations were synthesis intensive with limited 1-1.3× pore size tuning for a given polymer. Here we demonstrate PMT swelling with homopolymer enables 1-3.2× (13.3-41.9 nm) pore size variation while maintaining a monomodal distribution with up to 250 wt% homopolymer, considerably higher than the ∼90 wt% limit found for equilibrating micelles. These swollen PMTs enabled nanomaterial series with constant pore size and precision varied wall-thickness. Kinetic size control here is unexpected since the homopolymer undergoes dynamic exchange between micelles. The solvent selection influenced the time window before homopolymer phase separation, highlighting the importance of considering homopolymer-solvent interactions. This is the first PMT demonstration with wide variation of both the pore and wall dimensions using a single block polymer. Lastly this approach was extended to a 72 kg mol-1 block polymer to enable a wide 50-290 nm range of tunable macropores. Here the use of just two different block polymers and one homopolymer enabled wide ranging pore sizes spanning from 13.3-290 nm (1-22×).

Full Gamut Wall Tunability from Persistent Micelle Templates via Ex Situ Hydrolysis.

The predictive self-assembly of tunable nanostructures is of great utility for broad nanomaterial investigations and applications. The use of equilibrium-based approaches however prevents independent feature size control. Kinetic-controlled methods such as persistent micelle templates (PMTs) overcome this limitation and maintain constant pore size by imposing a large thermodynamic barrier to chain exchange. Thus, the wall thickness is independently adjusted via addition of material precursors to PMTs. Prior PMT demonstrations added water-reactive material precursors directly to aqueous micelle solutions. That approach depletes the thermodynamic barrier to chain exchange and thus limits the amount of material added under PMT-control. Here, an ex situ hydrolysis method is developed for TiO2 that mitigates this depletion of water and nearly decouples materials chemistry from micelle control. This enables the widest reported PMT range (M:T = 1.6-4.0), spanning the gamut from sparse walls to nearly isolated pores with ≈2 Å precision adjustment. This high-resolution nanomaterial series exhibits monotonic trends where PMT confinement within increasing wall-thickness leads to larger crystallites and an increasing extent of lithiation, reaching Li0.66 TiO2 . The increasing extent of lithiation with increasing anatase crystallite dimensions is attributed to the size-dependent strain mismatch of anatase and bronze polymorph mixtures.

Atomic Layer Deposition of Space-Efficient SnO2 Underlayers for BiVO4 Host-Guest Architectures for Photoassisted Water Splitting.

Bismuth vanadate (BiVO4 ) is promising for solar-assisted water splitting. The performance of BiVO4 is limited by charge separation for >70 nm films or by light harvesting for <700 nm films. To resolve this mismatch, host-guest architectures use thin film coatings on 3D scaffolds. Recombination, however, is exacerbated at the extended host-guest interface. Underlayers are used to limit this recombination with a host-underlayer-guest series. Such underlayers consume precious pore volume where typical SnO2 underlayers are optimized with 65-80 nm. In this study, conformal and ultrathin SnO2 underlayers with low defect density are produced by atomic layer deposition (ALD). This shifts the optimized thickness to just 8 nm with significantly improved space efficiency. The materials chemistry thus determines the dimension optimization. Lastly, host-guest architectures are shown to have an applied bias photon-to-charge efficiency of 0.71 %, a new record for a photoanode absorber prepared by ALD.

QCM detection of molecule-nanoparticle interactions for ligand shells of varying morphology.

Nanoparticles (NP) have widespread applications from sensing to drug delivery where much behavior is determined by the nature of the surface and the resulting intermolecular interactions with the local environment. Ligand mixtures enable continuously tunable behavior where both the composition and morphology influence molecular interactions. Mixed ligand shells form multiple morphologies ranging from Janus to patchy and stripe-like with varying domain dimensions. Solvent-NP interactions are generally measured by solubility measures alone. Here we develop a quartz crystal microbalance (QCM) approach to more broadly quantify molecule-NP interactions via vapor phase uptake into solid NP-films independent from solvation constraints. The composition and morphology of mixed ligand shells were found to exhibit pronounced non-monotonic behavior that deviated from continuum thermodynamics, highlighting the influence of ligand morphology upon absorption/adsorption. Alkyl and perfluorinated thiols were used as a model case with constant core-size distribution. The ligand morphology was determined by 19F NMR. Molecule uptake into NPs was measured with five benzene derivatives with varied degree of fluorination. For the cases examined, QCM measurements revealed enhanced uptake for patchy morphologies and suppressed uptake for stripe-like morphologies. These results contrast with insights from solubility measures alone where QCM sometimes identified significant molecular uptake of poor solvents. This QCM method thus provides new insights to molecule-NP interactions independent of the solvation shell.

Controlling Self-Assembly in Gyroid Terpolymer Films By Solvent Vapor Annealing.

The efficacy with which solvent vapor annealing (SVA) can control block copolymer self-assembly has so far been demonstrated primarily for the simplest class of copolymer, the linear diblock copolymer. Adding a third distinct block-thereby creating a triblock terpolymer-not only provides convenient access to complex continuous network morphologies, particularly the gyroid phases, but also opens up a route toward the fabrication of novel nanoscale devices such as optical metamaterials. Such applications, however, require the generation of well-ordered 3D continuous networks, which in turn requires a detailed understanding of the SVA process in terpolymer network morphologies. Here, in situ grazing-incidence small-angle X-ray scattering (GISAXS) is employed to study the self-assembly of a gyroid-forming triblock terpolymer during SVA, revealing the effects of several key SVA parameters on the morphology, lateral order, and, in particular, its preservation in the dried film. The robustness of the terpolymer gyroid morphology is a key requirement for successful SVA, allowing the exploration of annealing parameters which may enable the generation of films with long-range order, e.g., for optical metamaterial applications.

Deciphering magnesium stearate thermotropic behavior.

Magnesium stearate (MgSt) is the most commonly used excipient for oral solid dosage forms, yet there is significant commercial physicochemical variability that can lead to variable performance of critical product attributes. Differential scanning calorimetry (DSC) is often used as a quality control tool to characterize MgSt, but little data is available regarding the physicochemical relevance for the DSC thermograms. The main aim of this study was to decipher MgSt's complex thermotropic behavior using DSC, thermogravimetric analysis, capillary melting point, polarized hot-stage microscopy, and temperature dependent small-angle X-ray scattering (SAXS) and assign physicochemical relevance to the DSC thermograms. Several DSC thermal transitions are irreversible after the first heating cycle of a heat-cool-heat-cool-heat cycle. Interestingly, after the first heat cycle, the complex cool-heat-cool-heat DSC thermograms were highly reproducible and exhibited 6 reversible exothermic-endothermic conjugate pairs. SAXS identified 5 distinct mesophases at different temperatures with Phase C' persisting to 250 °C. MgSt maintained molecular ordering beyond 276 °C and did not undergo a simple melting phenomena reported elsewhere. This research serves as a starting point to design heat-treatment strategies to create more uniform MgSt starting material.

2D matrix engineering for homogeneous quantum dot coupling in photovoltaic solids.

Colloidal quantum dots (CQDs) are promising photovoltaic (PV) materials because of their widely tunable absorption spectrum controlled by nanocrystal size1,2. Their bandgap tunability allows not only the optimization of single-junction cells, but also the fabrication of multijunction cells that complement perovskites and silicon 3 . Advances in surface passivation2,4-7, combined with advances in device structures 8 , have contributed to certified power conversion efficiencies (PCEs) that rose to 11% in 2016 9 . Further gains in performance are available if the thickness of the devices can be increased to maximize the light harvesting at a high fill factor (FF). However, at present the active layer thickness is limited to ~300 nm by the concomitant photocarrier diffusion length. To date, CQD devices thicker than this typically exhibit decreases in short-circuit current (JSC) and open-circuit voltage (VOC), as seen in previous reports3,9-11. Here, we report a matrix engineering strategy for CQD solids that significantly enhances the photocarrier diffusion length. We find that a hybrid inorganic-amine coordinating complex enables us to generate a high-quality two-dimensionally (2D) confined inorganic matrix that programmes internanoparticle spacing at the atomic scale. This strategy enables the reduction of structural and energetic disorder in the solid and concurrent improvements in the CQD packing density and uniformity. Consequently, planar devices with a nearly doubled active layer thicknesses (~600 nm) and record values of JSC (32 mA cm-2) are fabricated. The VOC improved as the current was increased. We demonstrate CQD solar cells with a certified record efficiency of 12%.