Assembly of Linked Nanocrystal Colloids by Reversible Covalent Bonds

The use of dynamically bonding molecules designed to reversibly link solvent-dispersed nanocrystals (NCs) is a promising strategy to form colloidal assemblies with controlled structure and macroscopic properties. In this work, tin-doped indium oxide NCs are functionalized with ligands that form reversible covalent bonds with linking molecules to drive assembly of NC gels. We monitor gelation using small angle X-ray scattering and characterize how changes in the gel structure affect infrared optical properties arising from the localized surface plasmon resonance of the NCs. The assembly is reversible because of the designed linking chemistry, and we disassemble the gels using two strategies: addition of excess NCs to change the ratio of linking molecules to NCs and addition of a capping molecule that displaces the linking molecules. The assembly behavior is rationalized using a thermodynamic perturbation theory to compute the phase diagram of the NC–linking molecule mixture. Coarse-grained molecular dynamics simulations reveal the competition between loop and bridge linking motifs essential for understanding NC gelation. This combined experimental, computational, and theoretical work provides a platform for controlling and designing the properties of reversible colloidal assemblies that incorporate NC and solvent compositions beyond those compatible with other contemporary (e.g, DNA-based) linking strategies.We would like to acknowledge the UT Mass Spectrometry Facility for their instrumental help and the UT NMR facilities for equipment use and assistance: NIH Grant Number 1 S10 OD021508-01. This work was primarily supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF Materials Research Science and Engineering Center (NSF MRSEC) under Cooperative Agreement DMR-1720595. This work was also supported by NSF Graduate Research Fellowships DGE-1610403 (M.N.D. and S.V.), an Arnold O. Beckman Postdoctoral Fellowship (Z.M.S.), NSF (CHE- 1905263), and the Welch Foundation (F-1848 and F-1696). E.V.A. acknowledges support from the Welch Regents Chair (F-0046). We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources.Center for Dynamics and Control of Material

Structural and chemical characterization of responsive nanocrystalline materials

Nanocrystalline materials have interesting applications in many technological arenas, including catalysis, smart windows, and sensing. These crystals, with characteristic dimensions on the order of hundreds of nanometers or less, offer some key advantages over other materials for approaching these technologies. For heterogeneous catalysis, this small characteristic dimension translates to a very high surface-area-to-volume ratio. This extremely high specific surface area offers more chemically active sites than the same mass of material in a bulk form. Nanocrystals (NCs) can also exhibit unique chemical and physical properties in their very small form. For example, NCs of metallic materials can exhibit localized surface plasmon resonance (LSPR), a physical phenomenon not seen for bulk metals, that can make the material more useful for chemical sensing and electronic applications. Because they can be synthesized and processed using colloidal methods, NCs have also been investigated and employed as smart window materials, where solution-based processing keeps the cost of manufacturing window coatings much less expensive than the alternative low-pressure, high-temperature methods. During my PhD, I have had the privilege of working on many classes of nanocrystalline materials, with a broad variety of potential applications. In chapter 1 of this dissertation I describe my work synthesizing vanadium sesquioxide (V2O3) NCs doped with transition metal ions. These NCs reversibly absorb oxygen, with a remarkably low oxygen uptake onset temperature, which we propose for use in solving the cold start problem in automotive catalysis. Dopants added to the NC increase the initial oxygen storage capacity of the material, and have a substantial effect on the degradation of the material over ten oxidation and reduction cycles. In chapter 2, I discuss my collaboration with researchers from Brian Korgel's group. In one such collaboration, the reversible thermochromic phase transition of nickel iodide specific to thin films is explored. This thermochromism is found to be deliquescent, meaning it depends on the availability of humid air. These two properties give the material potential applications in both smart windows, where the phase transition causes a controllable color change on a window surface, and in humidity sensing, where the color change would indicate the presence of air with a relative humidity surpassing the critical point of the material. Using the same in situ heating techniques, the irreversible phase transition of perovskite cesium lead iodide (CsPbI3) NCs from the metastable black CsPbI3 phase to the yellow delta-orthorhombic phase is found to occur upon heating the NC films. This transition is destructive to the material, since the metastable gamma-phase has excellent electronic structure for photovoltaic applications, while the thermodynamic delta-orthorhomic phase is inactive. We found that this irreversible transition depended weakly on the humidity, but also proceeded when heated in dry nitrogen environments. In chapter 3, the design of a transparent conductive composite material is described. The goal material will be designed and engineered to be applied to the outside surface of aircraft windshields, where the conductive overlayer can shuttle charge built up by friction with ice and dust within the atmosphere to the aircraft body. This will prevent the build up of static charge on the windshield, which can interfere with electronics onboard the aircraft and even cause dielectric fracture of the windshield in severe cases. By incorporating cerium-doped indium oxide NCs and the conducting polymer PEDOT:PSS into an insulating, mechanically robust polymer matrix, we intend to create a thick material with high enough conductivity to be suitable for this anti-static application. By incorporating cerium-doped indium oxide NCs, the visible transparency of the resulting coating can be kept high despite the presence of visibly dark PEDOT:PSS. Lastly, in chapter 4, I describe my supporting contributions to work in my research group involving indium oxide and tin-doped indium oxide NCs. Chapter 4 is broken into three parts. First, the independent synthetic control of size and shape of indium oxide NCs was achieved through the addition of spectating alkali ions Na+ and K+. Since size and shape are critical parameters for controlling the sensing, catalytic, and assembly properties of the resulting NCs, this is an enabling discovery for many studies in the future. Next, I describe my contributions to the demonstration of depletion effects in degenerately doped tin-doped indium oxide (Sn:In2O3) NCs. A well-known effect in other semiconducting materials, depletion causes noticeable changes in the LSPR that can be controlled using electrochemical charging, and depends strongly on the NCs size and dopant concentration. In the last part of chapter 4, I discuss my work using X-ray photoemission spectroscopy (XPS) to evaluate Sn:In2O3 NCs before and after use in electrochemical CO2 reduction. This reaction has received interest for its ability to convert the greenhouse gas CO2 into more industrially relevant chemicals. Sn:In2O3 NCs show high selectivity for formate and CO, and our analysis show the NCs are stable during the electrochemical reaction, with no change in particle size or dopant concentration. This dissertation describes work on a broad range of nanocrystalline materials, including metal oxides, inorganic perovskites, and nickel iodide thin films. In evaluating their characteristics for their varied applications, I gained experience with a broad range of analytical tools, including electron microscopy, optical microscopy, UV-visible spectroscopy, and X-ray diffraction, among others. With an eye toward technological applications, fundamental studies help de fine the properties and limitations of the materials at our disposal for solving a wide range of problems.Chemical Engineerin

Oxygen Storage in Transition Metal-Doped Bixbyite Vanadium Sesquioxide Nanocrystals

Bixbyite vanadium sesquioxide (V2O3) is a metastable polymorph of vanadium oxide that has been shown to have a significant oxygen storage capacity with very low temperature oxidation onset. In this work, bixbyite V2O3 nanocrystals were synthesized with titanium and manganese dopants. Doped materials with varied dopant concentration were synthesized, and all were incorporated as aliovalent metal ions. The oxygen storage capacity of these nanocrystal materials was evaluated over ten oxidation and reduction cycles. It was found that over these ten cycles, the oxygen storage capacity of all the materials fell drastically. In situ X-ray diffraction evidence shows that manganese-doped materials degrade into an amorphous manganese-containing vanadate, while titanium-doped materials form crystalline degradation products. In all cases, this degradation causes an increase in the minimum mass achieved during oxygen release, indicating irreversible oxidation. </p

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

Nanocrystalline anatase TiO2 is a robust model anode for Li-insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li-insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li-insertion and electron accumulation, respectively, and these optical signals are used to deconvolute Li-insertion from other electrochemical responses, such as double-layer capacitance and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO2 systematically tunes Li-insertion potentials with particle size. However, particle size does not affect the kinetics of Li-insertion in ensemble electrodes. Rather, Li-insertion rates depend on applied overpotential, electrolyte concentration, and initial state-of-charge. We conclude that Li diffusivity and phase propagation are not rate-limiting during Li-insertion in TiO2 nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO2 particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute non-equilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations and spectroelectrochemistry.<br /

Data from: Falling with style: bats perform complex aerial rotations by adjusting wing inertia

The remarkable maneuverability of flying animals results from precise movements of their highly specialized wings. Bats have evolved an impressive capacity to control their flight, in large part due to their ability to modulate wing shape, area, and angle of attack through many independently controlled joints. Bat wings, however, also contain many bones and relatively large muscles, and thus the ratio of bats’ wing mass to their body mass is larger than it is for all other extant flyers. Although the inertia in bat wings would typically be associated with decreased aerial maneuverability, we show that bat maneuvers challenge this notion. We use a model-based tracking algorithm to measure the wing and body kinematics of bats performing complex aerial rotations. Using a minimal model of a bat with only six degrees of kinematic freedom, we show that bats can perform body rolls by selectively retracting one wing during the flapping cycle. We also show that this maneuver does not rely on aerodynamic forces, and furthermore that a fruit fly, with nearly massless wings, would not exhibit this effect. Similar results are shown for a pitching maneuver. Finally, we combine high-resolution kinematics of wing and body movements during landing and falling maneuvers with a 52-degree-of-freedom dynamical model of a bat to show that modulation of wing inertia plays the dominant role in reorienting the bat during landing and falling maneuvers, with minimal contribution from aerodynamic forces. Bats can, therefore, use their wings as multifunctional organs, capable of sophisticated aerodynamic and inertial dynamics not previously observed in other flying animals. This may also have implications for the control of aerial robotic vehicles

EulerAngles

Measured and Simulated Euler angles as a function of time for all 12 trials discussed in the manuscrip

Synthetic control of intrinsic defect formation in metal oxide nanocrystals using dissociated spectator metal salts

Crystallographic defects are essential to the functional properties of semiconductors, controlling everything from conductivity to optical properties and catalytic activity. In nanocrystals, too, defect engineering with extrinsic dopants has been fruitful. Although intrinsic defects like vacancies can be equally useful, synthetic strategies for controlling their generation are comparatively underdeveloped. Here we show that intrinsic defect concentration can be tuned during synthesis of colloidal metal oxide nanocrystals by the addition of metal salts. Although not incorporated in the nanocrystals, the metal salts dissociate at high temperature, promoting the dissociation of carboxylate ligands from metal precursors, leading to introduction of oxygen vacancies. For example, the concentration of oxygen vacancies can be controlled up to 9% in indium oxide nanocrystals. This method is broadly applicable as we demonstrate by generating intrinsic defects in metal oxide nanocrystals of various morphologies and compositions

minimal_simulation

MATLAB codes to perform minimal model simulation