Formation Mechanism of Flower-like Polyacrylonitrile Particles.

Flower-like polyacrylonitrile (PAN) particles have shown promising performance for numerous applications, including sensors, catalysis, and energy storage. However, the detailed formation process of these unique structures during polymerization has not been investigated. Here, we elucidate the formation process of flower-like PAN particles through a series of in situ and ex situ experiments. We have the following key findings. First, lamellar petals within the flower-like particles were predominantly orthorhombic PAN crystals. Second, branching of the lamellae during the particle formation arose from PAN's fast nucleation and growth on pre-existing PAN crystals, which was driven by the poor solubility of PAN in the reaction solvent. Third, the particles were formed to maintain a constant center-to-center distance during the reaction. The separation distance was attributed to strong electrostatic repulsion, which resulted in the final particles' spherical shape and uniform size. Lastly, we employed the understanding of the formation mechanism to tune the PAN particles' morphology using several experimental parameters including incorporating comonomers, changing temperature, adding nucleation seeds, and adjusting the monomer concentration. These findings provide important insights into the bottom-up design of advanced nanostructured PAN-based materials and controlled polymer nanostructure self-assemblies.

Autocatalysis and Oriented Attachment Direct the Synthesis of a Metal-Organic Framework.

Synthesis of porous, covalent crystals such as zeolites and metal-organic frameworks (MOFs) cannot be described adequately using existing crystallization theories. Even with the development of state-of-the-art experimental and computational tools, the identification of primary mechanisms of nucleation and growth of MOFs remains elusive. Here, using time-resolved in-situ X-ray scattering coupled with a six-parameter microkinetic model consisting of ∼1 billion reactions and up to ∼100 000 metal nodes, we identify autocatalysis and oriented attachment as previously unrecognized mechanisms of nucleation and growth of the MOF UiO-66. The secondary building unit (SBU) formation follows an autocatalytic initiation reaction driven by a self-templating mechanism. The induction time of MOF nucleation is determined by the relative rate of SBU attachment (chain extension) and the initiation reaction, whereas the MOF growth is primarily driven by the oriented attachment of reactive MOF crystals. The average size and polydispersity of MOFs are controlled by surface stabilization. Finally, the microkinetic model developed here is generalizable to different MOFs and other multicomponent systems.

Conductive, Large-Area, and Continuous 7,7,8,8-Tetracyanoquinodimethane@HKUST-1 Thin Films Fabricated Using Solution Shearing.

Most metal-organic frameworks (MOFs) have an insulating nature due to their porosity and redox-inactive organic components. The electrical conductivity of the prototypical MOF, HKUST-1, can be tuned by infiltrating a small-molecule organic semiconductor, 7,7,8,8-tetracyanoquinodimethane (TCNQ), into the HKUST-1 pores, creating TCNQ@HKUST-1. However, current processes of creating TCNQ@HKUST-1 films have many roadblocks such as slow crystallization rates, which limit high throughput production, and the formation of Cu(TCNQ) as a byproduct, which affects the electrical conductivity and degrades the chemical structure of HKUST-1. In this work, we show that HKUST-1 films can be rapidly synthesized over large areas with consistent thickness and no pinholes via a meniscus-guided coating technique called solution shearing. The subsequent pore activation process and TCNQ impregnation can be completed via solvent exchange to minimize the formation of the Cu(TCNQ) byproduct, and we obtain an increase in electrical conductivity of the solution-sheared TCNQ@HKUST-1 thin films of over 7 orders of magnitude, reaching a maximum value of 2.42 × 10-2 S m-1 when TCNQ is incorporated for 10 days. The conductivity of solution-sheared TCNQ@HKUST-1 is higher compared to films formed by high-pressure pelletization of TCNQ@HKUST-1. We show that solution shearing can produce large-area thin films rapidly and reduce the formation of grain boundaries better than pelletization, allowing for large-area electronics with both charge transport and porosity for applications as sensors and electronics.

Accelerated HKUST-1 Thin-Film Property Optimization Using Active Learning.

A flow-coating method termed solution shearing has been shown to grow large-area thin films with no void spaces. Attaining full coverage is one of the key prerequisites for the adoption of any metal-organic framework (MOF) thin film for a variety of practical applications, including separation, membranes and sensors. However, the solution-shearing process has multiple discrete and continuous parameters that can be varied, including the metal ion and linker concentrations, solvents, substrate temperature, coating speed, and the number of coating passes. Optimization of these parameters for full coverage is a time-consuming and daunting process due to vast parameter space. Here, we incorporate an active learning approach into the solution-sheared HKUST-1 thin-film-processing parameters to control the coverage and extend the approach to gain control over the thickness. The understanding of high-quality MOF thin-film formation using solution shearing is improved by correlating the processing parameter sets and their corresponding film coverage. A large area and fully covered HKUST-1 thin film with a minimized thickness of 2.2 µm is fabricated by using guidance from active learning. To confirm full coverage, a redox-active molecule, called 7,7,8,8-tetracyanoquinodimethane (TCNQ), is incorporated along with the HKUST-1 thin film. The TCNQ@HKUST-1 thin film with a minimized thickness has the same order of magnitude of electrical conductivity as that of the TCNQ@HKUST-1 thin film created previously while reducing the film thickness by 60%. We show that active learning has the potential to rapidly navigate the vast processing space in multicomponent systems, especially when experiments are expensive and traditional computational models are not readily available for process optimization.

Crystallization of high aspect ratio HKUST-1 thin films in nanoconfined channels for selective small molecule uptake.

We present the ability to create unique morphologies of a prototypical metal organic framework (MOF), HKUST-1, by carrying out its crystallization within a set of nano-confined fluidic channels. These channels are fabricated on cyclic olefin copolymer by the high-fidelity hot embossing imprinting method. The picoliter volume synthesis in the nanochannels is hypothesized to bias the balance between nucleation and growth rates to obtain high aspect ratio large-crystalline domains of HKUST-1, which are grown in defined morphologies due to the patterned nanochannels. Confined crystal growth is achieved in nanofluidic channels as shallow as 50 nm. HKUST-1 crystalline domains with aspect ratios greater than 2500, and lengths up to 144 µm are obtained using the nanochannels, exceeding values obtained using chemical modulation and other confinement methods. HKUST-1 crystals are characterized using optical microscopy and scanning electron microscopy with energy dispersive spectroscopy. Porosity of the MOF and selective molecular uptake is demonstrated through inclusion of anthracene and methylene blue within the HKUST-1 framework, and with exclusion of rhodamine B and riboflavin, characterized using a confocal fluorescence microscope. We attribute this selectivity to the analyte size and electrostatic characteristics. Nanoconfined crystallization of MOFs can thus yield control over crystalline morphology to create ideal MOF crystals for enabling selective molecular enrinchment and sensing.