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Assembly of and Ion Transport Through Porous Nanocrystal Thin Films


Of all the defining characteristics of a material, there are probably none more important than structure. Through a simple change in structure, materials can exhibit vastly different properties due to its influence at length scales from atomic crystal structure to microstructure. In fact, structure is so important in the study of materials science that it is given one of the four coveted spots on the materials science tetrahedron.

From advances in colloidal nanocrystals, materials with well-defined intrinsic characteristics such as composition and phase can now be synthesized reproducibly. However, these materials are often orders of magnitude smaller than actual device length scales. This disparity in length scales, however, is a fertile opportunity space where structural control can be used both to augment the intrinsic properties of nanocrystals and to bridge the length scales between nanocrystal building blocks and that of an actual device. More specifically, it may actually allow independent imposition of a structural motif separate from other parameters like composition and phase: an almost impossible feat from the standpoint of bulk materials processing.

Recent developments in nanocrystal surface chemistry have generated a sub-class of nanocrystals, called ligand-stripped nanocrystals, which are colloidally stable even in the absence of stabilizing ligands. This advancement opens both opportunities to access properties that require access to the nanocrystal surface, and new avenues for assembly that capitalizes on interactions with the nanocrystal surface. In assembly, it opens the question of how one might direct the arrangement of these nanocrystals through the use of a structure-directing agent such as a block copolymer. Initial work in 2012 demonstrated the first assembly of these nanocrystals using an artisanal polystyrene-b-polydimethylacrylamide (PS-PDMA) block copolymer of which the latter block is hypothesized to interact strongly with the nanocrystal surface. Chapter 2 expounds this discovery by investigating the assembly of ligand stripped nanocrystals using PS-PDMA micelles with emphasis on the influence of nanocrystal size and volume fraction on the overall ordering of the assembled structures. Grazing incidence small angle x-ray scattering is employed to quantitatively characterize ordering both at the block copolymer and nanocrystal length scale. The nanocrystal size dependence of ordering is shown such that ordering decreased dramatically for nanocrystal sizes bigger than the PDMA domain size. Similarly, nanocrystal ordering also decreased for nanocrystal volume fractions exceeding the volume fraction of PDMA in the system. Finally, the extreme limits of assembly using PS-PDMA micelles is demonstrated whereby single nanocrystal networks or networks with two length scales of ordering can be generated either at low volume fractions of large nanocrystals or at high volume fractions of small nanocrystals.

Chapter 3 extends the assembly of ligand stripped nanocrystals into block copolymer microphase-separated morphologies using PS-PDMA. Here, the phase separation behavior of PS-PDMA with and without nanocrystals is shown alongside methods used to achieve the final morphologies. Both volume fraction and size studies mirroring the studies in Chapter 2 is conducted to arrive at the maximal nanocrystal size and volume fractions after which assembly is kinetically arrested. Morphological control to access the hexagonal and lamellae phases is demonstrated with either a change in relative block copolymer block lengths or through a co-swelling approach using mixed solvents. Then, the compositional diversity of this assembly paradigm is demonstrated with the successful assembly of different metal oxide, metal chalcogenide, and gold nanocrystals. The nature of this diversity is expanded upon with a Fourier Transform Infrared Spectroscopy (FTIR) study that ultimately suggests that the nature of the interaction between PDMA and the nanocrystal surface is based upon hydrogen bonding. Finally, Chapter 4 discusses future work based on the co-assembly of nanocrystal mixtures, the control of PS-PDMA morphology in solution, and the use of block copolymers beyond PS-PDMA for the directed assembly of ligand stripped nanocrystals.

Moving beyond the context of assembly towards the arena of ion transport properties, ligand free nanocrystal thin films are applied as model systems to investigate the phenomena of intermediate temperature proton conduction between 250 °C and 100 °C: an anomalous phenomenon where porous metal oxide structures exhibit significant protonic conductivity that are traditionally absent in their bulk counterpart. Chapter 5 explores this phenomenon using porous nanocrystal thin films of cerium oxide or titanium oxide. The study establishes the viability of nanocrystals as model systems by demonstrating the influence of nanocrystal size on protonic conductivity for cerium oxide holding other variables such as porosity comparable. Then, capillary condensation is ruled out as the cause of the phenomenon, and an alternate hypothesis built upon metal oxide surface defect chemistry is proposed. This influence of defect chemistry is preliminary studied with emphasis on the oxygen partial pressure dependence of intermediate temperature protonic conductivity. The observed non-dependence of conductivity on oxygen partial pressure for cerium oxide is consistent with prior observations of the poor dependence of cerium oxide surface defect chemistry on oxygen partial pressure. This is in contrast with the clear oxygen partial pressure dependence observed for titanium dioxide. Holding porosity constant, the higher proton conductivity observed for 4 nm cerium oxide compared to that of 9 nm cerium oxide is rationalized by an enrichment of Ce3+ on the surface and corresponding oxygen vacancies for ultra small cerium oxide nanocrystals. Similarly, the higher proton conductivity observed for cerium oxide compared to titanium dioxide is rationalized by the lower enthalpy of formation of oxygen vacancies for cerium oxide. Then, the link between surface defect chemistry and protonic conductivity is proposed: dissociate water adsorption in surface oxygen vacancies may be responsible for the generation of mobile protons on the surface of the metal oxide.

Chapter 6 continues the investigation of intermediate temperature proton conductivity but addresses the stability of the phenomena. Here, time dependent conductivities at all temperatures is presented where a general decrease in conductivity under humidified conditions at temperatures lower than 200 °C is observed. Extended time dependent conductivity measurements at 100 °C show a gradual decrease in conductivity over 2 orders of magnitude over 48 hours for cerium oxide. Detailed FTIR studies reveal the nature of the decrease as passivation of the metal oxide surface due to the formation of cerium hydroxycarbonate consistent with the characteristic instability of rare-earth oxides under ambient or humidified conditions. Thermodynamic analysis further reveal a transition point of 575 °C after which the formation of cerium hydroxycarbonate becomes thermodynamically unfavorable. A reaction for the formation of cerium hydroxycarbonate from cerium oxide, CO2 and H2O is proposed and tested with a time, temperature and oxygen partial pressure dependent conductivity measurement. The results show that the rate of decrease in conductivity is significantly slower for pure oxygen environments. Gallium doping of cerium oxide to reduce the surface affinity toward hydroxycarbonate formation was tested but was found to have little efficacy in enhancing the stability. Thus, an alternate materials selection criteria based upon mineralogy that ultimately suggest titanium dioxide as a stable material under humidified conditions is tested. While the absolute conductivity of porous titanium dioxide nanocrystal systems start lower than that of cerium oxide nanocrystal systems, titanium dioxide appears stable over the tested 48-hour period thus showing the merit of using titanium dioxide over cerium oxide in actual applications due to gains in system stability. The study for titanium dioxide is completed with another detailed FTIR study that shows the formation of bicarbonate species on the surface of titanium dioxide under humidified conditions though the species do not hinder protonic conductivity. The stability of the phenomena for titanium dioxide under pure oxygen environments is also demonstrated. Finally, Chapter 7 discusses future work utilizing in situ FTIR studies to identify the spectroscopic signatures of acidic protons on the oxide surface that result from the aforementioned dissociative water adsorption on surface oxygen vacancies, and tuning of conductivity through manipulation of surface defect concentrations either by acceptor doping or tuning of surface facet termination.

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