Vulcanized rubber, the main component of tires, is prized for its chemical durability and thermal stability. These properties, however, make disposal difficult and contribute to the increasing problem of polymer waste. To broaden the applications of end-of-life tires, we developed a method to chemically upcycle polybutadiene, a primary component of vulcanized rubber, via selenium-mediated allylic amination. We hypothesized that functionalizing the backbone of crosslinked polybutadiene with sulfonamide groups—without breaking their double bonds—would result in favorable thermal properties, creating a new life for the crosslinked polybutadiene. We used infrared spectroscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy to confirm the aminated crosslinked polybutadiene’s molecular structure and differential scanning calorimetry and thermogravimetric analysis to measure its changes in thermal properties. Our research has future implications for the reduction of tire waste and reprocessing of other end-of-life crosslinked polymers.

Solar energy is a promising form of renewable energy that will play a major role in reducing carbon emissions. Perovskite-based solar cells have attracted significant attention due to their high power conversion efficiency (PCE), which reached 26.1% this year, surpassing commercial silicon’s (23.3%). High PCE, low cost of materials, and ability to be solution processed make perovskite solar cells a prime candidate to replace silicon. However, efficiencies are still below the theoretical limit and these materials suffer from limited operational stability. To tackle these problems, scientists have focused on minimizing active layer and interfacial defects which act as barriers for charge extraction in a solar cell, lowering the device efficiencies. Defects also electronically dope the perovskite layer, changing the recombination kinetics in the sample. The goal of this project is to quantify how the electronic doping and defect concentration of the perovskite sample is affected by surface passivation treatements via fluence dependent photoluminescence (PL) and time resolved photoluminescence (TRPL) spectroscopy. By solving the kinetic equations at the basis of charge recombination, we can extract the rate constants that correspond to different charge recombination pathways. We pioneer a global fitting analysis to simultaneously fit TRPL and PL measurements for robust determination of these kinetic constants that are subsequently used to determine the doping density of films before and after passivation. We show that the electronic doping density is higher than previously reported in literature, and that this doping is reduced with a surface passivation treatment. We collaborate with the University of Arizona to correlate our measured electronic doping density to electrochemically measured defect densities on the same samples. This work will provide an implementable tool to quantitatively assess electronic doping and defect density values for various perovskite compositions which will be useful for optimizing future solar cell devices.

The synthesis of key organic molecules often requires toxic, expensive, non-reusable reduction agents and extreme conditions. In recent years, electrochemistry has emerged as a sustainable alternative to standard methods, but this approach is often hindered by high energy barriers for electron transfer to the substrate. Electrocatalysts address this challenge by shuttling charge between the electrode and dissolved substrates, accessing lower transfer barriers, and reducing the overall energy needed. Current electrocatalysts, however, cannot stabilize reactive intermediates, which often leads to harmful side reactions and degradation of the electrode. We hypothesize that redox-active supramolecular cages can address this limitation by both shuttling charge and providing unique microenvironments capable of stabilizing intermediates. Previously, we synthesized two tetrahedral supramolecular cages that incorporate redox-active perylene diimide (PDI) and pyromellitic diimide (PMDI) motifs. Using cyclic voltammetry, we then showed that both cages can lower the voltages required for the electroreduction of vicinal dihalides to alkenes, indicating electrocatalysis. To better understand these results, I used density-functional theory (DFT) calculations to obtain computer models of the PDI and PMDI cages. These DFT-optimized structures revealed significant differences in charge density between redox centers due to electron-donating functional groups, which may explain why the PMDI cage lowered the substrate reduction voltages more than the PDI cage. With these models, I have also studied the shape and volume of the cages’ internal cavities, thereby providing information about substrate compatibility. I am conducting additional DFT analysis to understand how modifications to the ligand motifs may alter the electrocatalytic behavior. By continuing to investigate supramolecular cages for reductive electrocatalysis, I aim to contribute to the development of low-waste synthetic strategies for the production of alkenes and other commercially significant organic compounds.

The catalytic ability of an industrial heterogeneous catalyst is determined by the interactions between the active sites, which are often transition metals, and the support. Insights into the interplay between the active sites and support during catalysis are difficult to gain because of the inherent complexity of heterogeneous surfaces. Alternatively, molecular catalysts are well-defined, and can be studied by a range of spectroscopic characterization techniques. To model multi-active site dynamics on a molecular scale, the Velian group has developed a system involving a cobalt selenide cluster with amido phosphine ligands that are used to tether transition metals that act as catalytically active sites onto the cluster surface. My project is probing the tri-metalated clusters’ (M₃Co₆Se₈L₆; M = Cr, Mn, Fe, Co, Cu, Zn; L = PPh₂N^-Tol, Ph = phenyl, Tol = 4-tolyl) ability to catalyze intramolecular carbon-hydrogen (C-H) amination. Previous work has shown that these clusters are remarkable catalysts for carbodiimide formation, but we have yet to compare reactivity among the tri-metalated clusters. I probed the transformation of aliphatic azides to pyrrolidines, a class of 5-membered-N-heterocycles with. This study seeks to understand how the reactivity of the clusters change as edge metal identity changes, and the role of the three active sites during catalysis. A substrate scope has shown how the steric and electronic profile of the azide affects the capability of the clusters for this reaction. This research provides insights into metal-support interactions that are important for heterogeneous catalysis. Development of next generation catalysts that can perform complex transformations benefits from the information these studies provide.

Nanopore sensing is a powerful electroanalytical technique used to analyze individual nanoparticles in an electrolyte solution. During the analysis process, a small direct current voltage is applied across the nanopore membrane, generating a stable ionic current whose magnitude is dependent on the applied voltage, size and shape of the nanopore, as well the ionic conductivity of the electrolyte solution. A nanoparticle is detected when it passes through the pore, which causes a transient decrease in the ionic current, due to a partial blockage of the nanopore's ionic pathway. One method of nanopore fabrication is by pulling borosilicate or quartz glass capillaries by laser or brown flaming pullers. However, due to the conical geometry of these pulled nanopipettes, the large volume of the pipette relative to the small volume displaced by the nanoparticle highly disfavors translocations. This study aims to resolve these complications by exploring an innovative method of fabricating nanopore sensors in an ultrathin glass membrane. These glass membranes are prepared by a bench-top microscale glassblowing process, which results in membranes as small as 100 nm with a tip of a spherical geometry. Individual nanopores are then created on the membrane surface utilizing focused ion-beam milling. Due to the thin glass walls, the nanopores have exhibited enhanced sensitivity and detection frequency for nanoparticle sensing. Additionally, they are less likely to experience clogging due to particle adsorption. We anticipate that our nanopore fabrication technique can be utilized in various research fields, such as cellular and DNA research, nanoparticle characterization, and fundamental electrochemistry.

Hydrostatic pressure can be used to tune the electronic properties of atomically thin layered materials by decreasing the interlayer spacing, thereby enhancing the strength of interlayer interactions. In moiré systems, pressure can be used to create flat bands in samples with twist angles away from the usual ‘magic angle’. Twisted trilayer graphene (tTLG) has a Dirac band superimposed atop a flat band. These two bands can hybridize in a finite displacement field making it possible to further tune the flat band. The flat band is host to a variety of flavor-polarized correlated states, which may be an important ingredient in generating the exotic superconducting phases seen in tTLG. Although pressure could provide a new avenue for tuning these correlated states, a high-pressure study has not previously been performed on tTLG owing to the challenges of applying pressure to layered 2D materials: limited sample space, difficulty mounting the sample, and challenges in establishing electrical contacts. In this talk, I will discuss advances we have made in addressing these issues via a custom printed circuit board (PCB). The PCB provides a sturdy platform for mounting samples, and has gold pads to enable wire bonding. I will also discuss ongoing high-pressure electrical transport measurements of tTLG nanodevices. This work could elucidate further the origin of the unusual superconducting phase seen in tTLG, and provide a blueprint for future high-pressure studies of 2D materials.