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  1. Free, publicly-accessible full text available January 16, 2024
  2. Electrospray deposition (ESD) has shown great promise for manufacturing micro- and nanostructured coatings at scale on versatile substrates with complex geometries. ESD exhibits a broad spectrum of morphologies depending upon the properties of spray fluids. Among them are nanowire forests or foams obtained via the in-air gelation of electrospray droplets formed from methylcellulose (MC) solutions. In this study, we explored MC ESD loaded with nanoparticles of various shapes and uncovered the effects of particle fillers on morphology evolution using coarse-grained simulations and physical experiments. Utilizing electrostatic dissipative particle dynamics, we modeled the electrohydrodynamic deformation of particle-laden MC droplets undergoing in-flight evaporation. The simulations quantitatively predict the suppression of droplet deformation as the size or concentration of spherical nanoparticles increases. While small particles can be readily encapsulated into the nanowire body, large particles can arrest nanowire formation. The model was extended to nanoparticles with complex topologies, showing MC nanowires emerging from particle edges and vertices due to curvature-enhanced electric stress. In all cases, strong agreements were found between simulation and experimental results. These results demonstrate the efficacy of the coarse-grained model in predicting the morphology evolution of electrosprayed droplets and lay the groundwork for employing MC nanowires for developing nanostructured composites.
    Free, publicly-accessible full text available December 15, 2023
  3. Abstract High electrical conductivity is desired in MXene films for applications such as electromagnetic interference shielding, antennas, and electrodes for electrochemical energy storage and conversion applications. Due to the acid etching-based synthesis method, it is challenging to deconvolute the relative importance that factors such as chemical composition and flake size contribute to resistivity. To understand the intrinsic and extrinsic contributions to the macroscopic electronic transport properties, a systematic study controlling compositional and structural parameters was conducted with eight solid solutions in the Ti y Nb 2− y CT x system. In particular, we investigated the different roles played by metal (M)-site composition, flake size, and d -spacing on macroscopic transport. Hard x-ray photoemission spectroscopy and spectroscopic ellipsometry revealed changes to electronic structure induced by the M-site alloying. Consistent with the spectroscopic results, the low- and room-temperature conductivities and effective carrier mobility are correlated with the Ti content, while the impact of flake size and d -spacing is most prominent in low-temperature transport. The results provide guidance for designing and engineering MXenes with a wide range of conductivities.
    Free, publicly-accessible full text available November 11, 2023
  4. Abstract 2D material hydrogels have recently sparked tremendous interest owing to their potential in diverse applications. However, research on the emerging 2D MXene hydrogels is still in its infancy. Herein, we show a universal 4D printing technology for manufacturing MXene hydrogels with customizable geometries, which suits a family of MXenes such as Nb 2 CT x , Ti 3 C 2 T x , and Mo 2 Ti 2 C 3 T x . The obtained MXene hydrogels offer 3D porous architectures, large specific surface areas, high electrical conductivities, and satisfying mechanical properties. Consequently, ultrahigh capacitance (3.32 F cm −2 (10 mV s −1 ) and 233 F g −1 (10 V s −1 )) and mass loading/thickness-independent rate capabilities are achieved. The further 4D-printed Ti 3 C 2 T x hydrogel micro-supercapacitors showcase great low-temperature tolerance (down to –20 °C) and deliver high energy and power densities up to 93 μWh cm −2 and 7 mW cm −2 , respectively, surpassing most state-of-the-art devices. This work brings new insights into MXene hydrogel manufacturing and expands the range of their potential applications.
    Free, publicly-accessible full text available December 1, 2023
  5. Free, publicly-accessible full text available October 1, 2023
  6. Free, publicly-accessible full text available June 13, 2024
  7. Abstract MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti 2− y Nb y CT x , Nb 2− y V y CT x , and Ti 2− y V y CT x ) were investigated with scanning transmission electron microscopy and electron energy-loss spectroscopy to determine the localized valence states of metals at the nanoscale. The analysis demonstrates changes in the electronic configuration of V upon modification of the overall composition and within individual MXene flakes. These shifts of oxidation state can explain the nonlinear optical and electronic features of solid-solution MXenes. Vanadium appears to be particularly sensitive to modification of the valence state, while titanium maintains the same oxidation state in Ti–Nb and Ti–V MXenes, regardless of stoichiometry. The studymore »also explains Nb’s influential role in the previously observed electronic properties in the Nb–V and Nb–Ti systems.« less
  8. Abstract

    Driving the pseudocapacitive redox intercalation in 2DMXenes with neutral electrolytes is important for safer, more sustainable, and improved electrochemical charge storage. Single transition metal MXenes, such as Ti3C2, have shown great promise for energy storage, owing to their high conductivity and redox activity. Mixed metallic MXenes, such as out‐of‐plane ordered Mo2Ti2C3, have remained underexplored in energy storage because of the absence of redox activity in most of the electrolytes. Simultaneous structural modifications and instigating intercalation pseudocapacitance in neutral electrolytes could be a viable strategy for enhancing their electrochemical properties. Herein, a facile synthesis of partially oxidized Mo2Ti2C3MXene (PO‐Mo2Ti2C3) exhibiting improved charge storage capability is demonstrated. Optical, structural, and spectroscopic analyses indicate the formation of oxide nanostructures upon thermal oxidation of Mo2Ti2C3. This leads to an enhanced energy storage capability with remarkably improved cyclability as well as a high Coulombic efficiency in a neutral LiCl electrolyte. This work highlights the importance of structural modifications of MXenes to enhance their charge storage and shows the promise of less explored double transition metal MXenes in energy storage.

    Free, publicly-accessible full text available September 15, 2024