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.