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Nanoporous electrically conducting materials can be prepared with high specific pore volumes and surface areas which make them well-suited for a wide variety of technologies including separation, catalysis and owing to their conductivity, energy related applications like solar cells, batteries and capacitors. General synthetic methods for nanoporous conducting materials that exhibit fine property control as well as facility and efficiency in their implementation continue to be highly sought after. Here, general methods for the synthesis of nanoporous conducting materials and their characterization are presented. Antimony-doped tin oxide (ATO), a transparent conducting oxide (TCO), and nanoporous conducting carbon can be prepared through the step-wise synthesis of interpenetrating inorganic/organic networks using well-established sol-gel methodology. The one-pot method produces an inorganic gel first that encompasses a solution of organic precursors. The surface of the inorganic gel subsequently catalyzes the formation of an organic gel network that interpenetrates throughout the inorganic gel network. These mutually supporting gel networks strengthen one another and allow for the use of evaporative drying methods and heat treatments that would usually destroy the porosity of an unsupported gel network. The composite gel is then selectively treated to either remove the organic network to provide a porous inorganic network, as is the case for antimony-doped tin oxide, or the inorganic network can be removed to generate a porous carbon material. The method exhibits flexibility in that the pore structure of the final porous material can be modified through the variation of the synthetic conditions. Additionally, porous carbons of hierarchical pore size distributions can be prepared by using wet alumina gel as a template dispersion medium and as a template itself. Alumina gels exhibit thixotropy, which is the ability of a solid to be sheared to a liquid state and upon removal of the shear force, return to a solid gel state. Alumina gels were prepared and blended with monomer solutions and sacrificial template particles to produce wet gel composites. These composites could then be treated to remove the alumina and template particles to generate hierarchically porous carbon.
This thesis focused on physicochemical and electrochemical projects directed towards two electrolyte types: 1) class of ionic liquids serving as electrolytes in the catholyte for alkali-metal ion conduction in batteries and 2) gel membrane for proton conduction in fuel cells; where overall aims were encouraged by the U.S. Department of Energy.
Large-scale, sodium-ion batteries are seen as global solutions to providing undisrupted electricity from sustainable, but power-fluctuating, energy production in the near future. Foreseen ideal advantages are lower cost without sacrifice of desired high-energy densities relative to present lithium-ion and lead-acid battery systems. Na/NiCl2 (ZEBRA) and Na/S battery chemistries, suffer from high operation temperature (>300ºC) and safety concerns following major fires consequent of fuel mixing after cell-separator rupturing. Initial interest was utilizing low-melting organic ionic liquid, [EMI+][AlCl4-], with well-known molten salt, NaAlCl4, to create a low-to-moderate operating temperature version of ZEBRA batteries; which have been subject of prior sodium battery research spanning decades. Isothermal conductivities of these electrolytes revealed a fundamental kinetic problem arisen from "alkali cation-trapping effect" yet relived by heat-ramping >140ºC.
Battery testing based on [EMI+][FeCl4-] with NaAlCl4 functioned exceptional (range 150-180ºC) at an impressive energy efficiency >96%. Newly prepared inorganic ionic liquid, [PBr4+][Al2Br7-]:NaAl2Br7, melted at 94ºC. NaAl2Br7 exhibited super-ionic conductivity 10-1.75 Scm-1 at 62ºC ensued by solid-state rotator phase transition. Also improved thermal stability when tested to 265ºC and less expensive chemical synthesis. [PBr4+][Al2Br7-] demonstrated remarkable, ionic decoupling in the liquid-state due to incomplete bromide-ion transfer depicted in NMR measurements.
Fuel cells are electrochemical devices generating electrical energy reacting hydrogen/oxygen gases producing water vapor. Principle advantage is high-energy efficiency of up to 70% in contrast to an internal combustion engine <40%. Nafion-based fuel cells are prone to carbon monoxide catalytic poisoning and polymer membrane degradation unless heavily hydrated under cell-pressurization. This novel "SiPOH" solid-electrolytic gel (originally liquid-state) operated in the fuel cell at 121oC yielding current and power densities high as 731mAcm-2 and 345mWcm-2, respectively. Enhanced proton conduction significantly increased H2 fuel efficiency to 89.7% utilizing only 3.1mlmin-1 under dry, unpressurized testing conditions. All these energy devices aforementioned evidently have future promise; therefore in early developmental stages.
New sol-gel routes were developed to fabricate transparent conducting oxide coatings for energy applications. Sol-gel synthesis was chosen because the metal oxide products have high surface area and porosity. Titanium sol-gel chemistry was the main focus of the studies, and the synthesis of macroporous antimony-doped tin oxide was also explored. The surface chemistry and band characteristics of anatase TiO2 show promise for solar energy purposes as photoelectrodes in DSSCs and as photocatalysts to degrade organic dyes and to split water. Modifying the band structure by increasing the conduction band edge energy is specifically of interest for reducing protons in water. To this end, a new sol-gel method was developed for incorporating Zr-dopant in nanoporous anatase TiO2. The products follow Vegard’s law up to 20 atom%, exhibiting surface area of 79 m2/g and pore volume of 0.20 cm3/g with average pore diameter of 10.3 nm; the conduction band edge energy increased by 0.22 eV and the band gap increased by 0.1 eV.
In pursuit of a greener sol-gel route for TiO2 materials, a solution of TiOSO4 in water was explored. Success in obtaining a gel came by utilizing hydrogen peroxide as a ligand that suppressed precipitation reactions. Through modifying this sol-gel chemistry to obtain a solid acid, the new material hydrogen titanium phosphate sulfate, H1-xTi2(PO4)3-x(SO4)x, (0 < x < 0.5) was synthesized and characterized for the first time. From the reported synthetic route, this compound took the form of macroscopic agglomerates of nanoporous aggregates of nanoparticles around 20 nm and the product calcined at 600 °C exhibited surface area of 78 m2/g, pore volume of 0.22 cm3/g and an average pore width of 11 nm. This solid acid exhibits complete selectivity for the non-oxidative dehydrogenation of methanol to formaldehyde and hydrogen gas, with >50% conversion at 300 °C.
Finally, hierarchically meso-macroporous antimony doped tin oxide was synthesized with regular macropore size around 210 nm, determined by statistical dye trajectory tracking, and also with larger pores up to micrometers in size. The structure consisted of nanoparticles around 4 nm in size, with textural mesopores around 20 nm in diameter.
Geopolymers, a class of X-ray amorphous, ceramic-like aluminosilicate materials are produced at ambient temperatures through a process called geopolymerization. Due to both low energy requirement during synthesis and interesting mechanical and chemical properties, geopolymers are grabbing enormous attention. Although geopolymers have a broad range of applications including thermal/acoustic insulation and waste immobilization, they are always prepared in monolithic form. The primary aim of this study is to produce new nanostructured materials from the geopolymerization process, including porous monoliths and powders.
In view of the current interest in porous geopolymers for non-traditional applications, it is becoming increasingly important to develop synthetic techniques to introduce interconnected pores into the geopolymers. This study presents a simple synthetic route to produce hierarchically porous geopolymers via a reactive emulsion templating process utilizing triglyceride oil. In this new method, highly alkaline geopolymer resin is mixed with canola oil to form a homogeneous viscous emulsion which, when cured at 60 °C, gives a hard monolithic material. During the process, the oil in the alkaline emulsion undergoes a saponification reaction to decompose into water-soluble soap and glycerol molecules which are extracted to yield porous geopolymers. Nitrogen sorption studies indicates the presence of mesopores, whereas the SEM studies reveals that the mesoporous geopolymer matrix is dotted with spherical macropores. The method exhibits flexibility in that the pore structure of the final porous geopolymers products can be adjusted by varying the precursor composition.
In a second method, the geopolymerization process is modified to produce highly dispersible geopolymer particles, by activating metakaolin with sodium silicate solutions containing excess alkali, and curing for short duration under moist conditions. The produced geopolymer particles exhibit morphology similar to carbon blacks and structured silicas, while also being stable over a wide pH range.
Finally, highly crystalline hierarchical faujasite zeolites are prepared by yet another modification of the geopolymerization process. In this technique, the second method is combined with a saponification reaction of triglyceride oil. The resulting hierarchical zeolites exhibit superior CO2-sorption properties compared to equivalent commercially available and currently reported materials. Additionally, the simplicity of all three of these techniques means they are readily scalable.
Increasing concentrations of carbon dioxide in the atmosphere will inevitably lead to long-term changes in climate that can have serious consequences. Controlling anthropogenic emission of carbon dioxide into the atmosphere, however, represents a significant technological challenge. Various chemical approaches have been suggested, perhaps the most promising of these is based on electrochemical trapping of carbon dioxide using pyridine and derivatives. Optimization of this process requires a detailed understanding of the mechanisms of the reactions of reduced pyridines with carbon dioxide, which are not currently well known. This thesis describes a detailed mechanistic study of the nucleophilic and Bronsted basic properties of the radical anion of bipyridine as a model pyridine derivative, formed by one-electron reduction, with particular emphasis on the reactions with carbon dioxide. A time-resolved spectroscopic method was used to characterize the key intermediates and determine the kinetics of the reactions of the radical anion and its protonated radical form. Using a pulsed nanosecond laser, the bipyridine radical anion could be generated in-situ in less than 100 ns, which allows fast reactions to be monitored in real time. The bipyridine radical anion was found to be a very powerful one-electron donor, Bronsted base and nucleophile. It reacts by addition to the C=O bonds of ketones with a bimolecular rate constant around 1* 107 M-1 s-1. These are among the fastest nucleophilic additions that have been reported in literature. Temperature dependence studies demonstrate very low activation energies and large Arrhenius pre-exponential parameters, consistent with very high reactivity. The kinetics of E2 elimination, where the radical anion acts as a base, and SN2 substitution, where the radical anion acts as a nucleophile, are also characterized by large bimolecular rate constants in the range ca. 106 - 107 M-1 s-1. The pKa of the bipyridine radical anion was measured using a kinetic method and analysis of the data using a Marcus theory model for proton transfer. The bipyridine radical anion is found to have a pKa of 40±5 in DMSO. The reorganization energy for the proton transfer reaction was found to be 70±5 kJ/mol. The bipyridine radical anion was found to react very rapidly with carbon dioxide, with a bimolecular rate constant of 1* 108 M-1 s-1 and a small activation energy, whereas the protonated radical reacted with carbon dioxide with a rate constant that was too small to measure. The kinetic and thermodynamic data obtained in this work can be used to understand the mechanisms of the reactions of pyridines with carbon dioxide under reducing conditions.
Deoxyribonucleic acid (DNA) has been treated as excellent building material for nanoscale construction because of its unique structural features. Its ability to self-assemble into predictable and addressable nanostructures distinguishes it from other materials. A large variety of DNA nanostructures have been constructed, providing scaffolds with nanometer precision to organize functional molecules. This dissertation focuses on developing biologically replicating DNA nanostructures to explore their biocompatibility for potential functions in cells, as well as studying the molecular behaviors of DNA origami tiles in higher-order self-assembly for constructing DNA nanostructures with large size and complexity. Presented here are a series of studies towards this goal. First, a single-stranded DNA tetrahedron was constructed and replicated in vivo with high efficiency and fidelity. This study indicated the compatibility between DNA nanostructures and biological systems, and suggested a feasible low-coast method to scale up the preparation of synthetic DNA. Next, the higher-order self-assembly of DNA origami tiles was systematically studied. It was demonstrated that the dimensional aspect ratio of origami tiles as well as the intertile connection design were essential in determining the assembled superstructures. Finally, the effects of DNA hairpin loops on the conformations of origami tiles as well as the higher-order assembled structures were demonstrated. The results would benefit the design and construction of large complex nanostructures.
Thiol functionalization is one potentially useful way to tailor physical and chemical properties of graphene oxides (GOs) and reduced graphene oxides (RGOs). Despite the ubiquitous presence of thiol functional groups in diverse chemical systems, efficient thiol functionalization has been challenging for GOs and RGOs, or for carbonaceous materials in general. In this work, thionation of GOs has been achieved in high yield through two new methods that also allow concomitant chemical reduction/thermal reduction of GOs; a solid-gas metathetical reaction method with boron sulfides (BxSy) gases and a solvothermal reaction method employing phosphorus decasulfide (P4S10). The thionation products, called "mercapto reduced graphene oxides (m-RGOs)", were characterized by employing X-ray photoelectron spectroscopy, powder X-ray diffraction, UV-Vis spectroscopy, FT-IR spectroscopy, Raman spectroscopy, electron probe analysis, scanning electron microscopy, (scanning) transmission electron microscopy, nano secondary ion mass spectrometry, Ellman assay and atomic force microscopy. The excellent dispersibility of m-RGOs in various solvents including alcohols has allowed fabrication of thin films of m-RGOs. Deposition of m-RGOs on gold substrates was achieved through solution deposition and the m-RGOs were homogeneously distributed on gold surface shown by atomic force microscopy. Langmuir-Blodgett (LB) films of m-RGOs were obtained by transferring their Langmuir films, formed by simple drop casting of m-RGOs dispersion on water surface, onto various substrates including gold, glass and indium tin oxide. The m-RGO LB films showed low sheet resistances down to about 500 kΩ/sq at 92% optical transparency. The successful results make m-RGOs promising for applications in transparent conductive coatings, biosensing, etc.
The discovery of DNA helical structure opened the door of modern molecular biology. Ned Seeman utilized DNA as building block to construct different nanoscale materials, and introduced a new field, know as DNA nanotechnology. After several decades of development, different DNA structures had been created, with different dimension, different morphology and even with complex curvatures. In addition, after construction of enough amounts DNA structure candidates, DNA structure template, with excellent spatial addressability, had been used to direct the assembly of different nanomaterials, including nanoparticles and proteins, to produce different functional nanomaterials. However there are still many challenges to fabricate functional DNA nanostructures. The first difficulty is that the present finite sized template dimension is still very small, usually smaller than 100nm, which will limit the application for large amount of nanomaterials assembly or large sized nanomaterials assembly. Here we tried to solve this problem through developing a new method, superorigami, to construct finite sized DNA structure with much larger dimension, which can be as large as 500nm. The second problem will be explored the ability of DNA structure to assemble inorganic nanomaterials for novel photonic or electronic properties. Here we tried to utilize DNA Origami method to assemble AuNPs with controlled 3D spacial position for possible chiral photonic complex. We also tried to assemble SWNT with discrete length for possible field effect transistor device. In addition, we tried to mimic in vivo compartment with DNA structure to study internalized enzyme behavior. From our results, constructed DNA cage origami can protect encapsulated enzyme from degradation, and internalized enzyme activity can be boosted for up to 10 folds. In summary, DNA structure can serve as an ideal template for construction of functional nanomaterials with lots of possibilities to be explored.
Global warming resulted from greenhouse gases emission has received widespread attention. Meanwhile, it is required to explore renewable and environmentally friendly energy sources due to the severe pollution of the environment caused by fossil fuel combustion. In order to realize a substantial adsorption process to resolve the environmental issues, the development of new adsorbents with improved properties has become the most critical issue. This dissertation presents the work of four individual but related studies on systematic characterization and process simulations of novel adsorbents with superior adsorption properties.
A perovskite oxide material, La0.1Sr0.9Co0.9Fe0.1O3-δ (LSCF1991), was investigated first for high-temperature air separation. The oxygen sorption/desorption behavior of LSCF1991 was studied by thermogravimetric analysis (TGA) and fixed-bed breakthrough experiments. A parametric study was performed to design and optimize the operating parameters of the high-temperature air separation process by pressure swing adsorption (PSA). The results have shown great potential for applying LSCF1991 to the high-temperature air separation due to its excellent separation performance and low energy requirement.
Research on using nanostructured zeolite NaX (NZ) as adsorbents for CO2 capture was subsequently conducted. The CO2/N2 adsorption characterizations indicated that the NZ samples lead to enhanced adsorption properties compared with the commercial zeolites (MZ). From the two-bed six-step PSA simulation, NZ saved around 30% energy over MZ for CO2 capture and recovery while achieving a higher CO2 purity and productivity.
A unique screening method was developed for efficient evaluation of adsorbents for PSA processes. In the case study, 47 novel adsorbents have been screened for coal bed methane (CBM) recovery. The adsorbents went through scoring-based prescreening, PSA simulation, and optimization. The process performance indicators were correlated with the adsorption selectivity and capacities, which provides new insights for predicting the PSA performance.
A new medium-temperature oxygen sorbent, YBaCo4O7+δ (YBC114), was investigated as an oxygen pumping material to facilitate solar thermochemical fuel production. The oxygen uptake and release attributes of YBC114 were studied by both TGA and a small-scale evacuation test. The study proved that the particle size has a significant effect on the oxygen pumping behavior of YBC114, especially for the uptake kinetics.
The movement of energy within a material is at the heart of numerous fundamental properties of chemistry and physics. Studying the process of photo-absorption in real time provides key insights into how energy is captured, stabilized, and dissipated within a material. The work presented in this thesis uses ultrafast time-of-flight mass spectrometry and computational modeling to observe and understand the properties of photo-excited states within molecules and clusters. Experimental results provide direct measurement of excited state lifetimes, while computational modeling provides a more thorough understanding of the movement of energy within an excited state. Excited state dynamics in covalent molecules such as n-butyl bromide (C4H9Br), presented in Chapter 4, demonstrate the significance of IVR of photo-excited states. Exciting to the high energy Rydberg manifold leads to predissociation into fragments of various lengths and degrees of saturation but the predissociation process is disrupted by energy redistribution into hot vibrational states. Experimental lifetimes show that IVR occurs over rapidly (~ 600 fs) leaving less energy for bond dissociation. Additionally, a long-lived feature in the dynamics of C4H9+ shows evidence of ion-pair formation – a known phenomenon which creates a stable A+/B- pair separated by several angstroms and occurring at energies lower than direct ionization. The results of this research show the dynamics of energy transfer into bond fragmentation, kinetic energy, and vibrational motion.
Metal-oxide clusters are unique materials which are representative of bulk materials but with quantized excited states instead of bands and as such can be used as atomically precise analogs to semiconducting materials. Excited state lifetimes and theoretical descriptors of electron-hole interactions of titanium oxide clusters, presented in Chapter 5, shows the significance of structure and oxidation of charge-transfer materials. Modeling the excited states of the photo-generated electrons and holes provides a window into the dynamics of charge-transfer and electron-hole separation and recombination in bulk materials. Furthermore, changes in the oxidation of the cluster have a dramatic impact on the nature of excited states and overall cluster properties. Such changes are analogous to oxygen defects in bulk materials and are critical for understanding reaction chemistry at defect sites.