Browsing by Author "Wiley, Benjamin J"

The majority of consumer electronic devices, electric vehicles, and aerospace electronics are powered by lithium ion batteries because of their high energy and power densities. Commercially available lithium ion batteries consist of electrodes, separators and current collectors fabricated in multilayer rolls that are packaged in cylindrical or rectangular cases. The size and shape of the package as well as the composition of the electrode has a significant impact on the battery life and design of the products they power. For example, the battery life and shape of portable electronics such as cell phones or laptops, is governed by the volume that is dedicated to the battery. In the case of electric vehicles, decreasing the size and weight of the battery while increasing capacity is an engineering challenge that affects vehicle range and cost. Therefore, the of my dissertation consists of the development of a novel 3D printable lithium ion battery nanocomposites and the integration of conductive metal nanomaterials into conventional lithium ion anodes. Here, we report the development of PLA-anode, cathode, and separator materials that enable 3D printing of complete lithium ion batteries with a low-cost FFF printer for the first time. The most common 3D printing polymer polylactic acid (PLA) is an insulator. However, our work demonstrates that 3D printed PLA can be infused with a mixture of ethyl methyl carbonate, propylene carbonate, and LiClO4 provides an ionic conductivity of 2.3 x 10−4 S cm−1 which is comparable to that of polymer and hybrid electrolytes (10−3 to 10−4 S cm−1). It was found that up to 12-30 volume % of solids, depending on the filler morphology, could be mixed into PLA without causing it to clog during 3D printing. It was also found that not only is electrical conductivity crucial to the performance of a 3D printed lithium ion battery, but efficient electrical contact to the active materials is as well. To that effect, we investigated the effect of aspect ratio of silver-copper core-shell nanowires on the performance enhancement of a commercially fabricated graphite lithium ion anodes. Currently, carbon is the most common conductive filler used in commercial lithium ion battery anodes. We hypothesize that a more conductive, high aspect ratio would improve the performance of a lithium ion battery. We examined the effect of exchanging carbon with CuAg nanowires as the conductive filler in graphite lithium ion batteries. We tested 4 different aspect ratios and found that not only does aspect ratio matter, diameter and length have profound effect on capacity and energy of the anode at the same volume percent as carbon conductive filler.

The manipulation of microstructures within modern micro- and nanomaterials stands as a prevalent practice with extensive applications across diverse fields. The deliberate control of material microstructures empowers the fine-tuning of their distinctive physical and chemical properties, catering to specific requirements in various applications. This dissertation mainly explores the strategic utilization of materials endowed with controlled microstructures, particularly investigating their significance and applications in the field of electrochemistry and obscurants.

Finding ways to reduce reactor volume while increasing product output for electroorganic reactions would facilitate the broader adoption of such reactions for the production of chemicals in a commercial setting. The goal of the electrochemistry research is to investigate how the use of flow with different electrode structures impacts the productivity (i.e., the rate of product generation) of a TEMPO-mediated azidooxygenation reaction. Comparison of a flow and batch process with carbon paper (CP) demonstrated a 3.8-fold higher productivity for the flow reactor. Three custom carbon electrodes, sintered carbon paper (S-CP), carbon nanofiber (CNF), and composite carbon microfiber-nanofiber (MNC), were studied in the flow reactor to evaluate how changing the electrode structure affected productivity. Under the optimum conditions these electrodes achieved productivities 5.4, 6.5 and 7.8 times higher than the average batch reactor, respectively. Recycling the outlet from the flow reactor with the MNC electrode back into the inlet achieved an 81% yield in 36 minutes, while the batch reactor obtained a 75% yield in 5 hours. These findings demonstrate that the productivity of electroorganic reactions can be substantially improved through the use of novel flow-through electrodes. Further exploration on other type of electroorganic reaction with 3-D porous electrode, like electrochemical cross-electrophile coupling (XEC), got an extensively lower yield in the flow cell with different configurations, which was due to the pass of chemicals through membrane in divided cell and low residence time in undivided cell. Due to the time and funding limited, we did not dig deeper into this project.

The ultimate goal of the obscurants work is to create an engineered aerosol that acts as one-way smoke, i.e., it creates an asymmetric vision environment in which the ability to image objects depends on the viewing direction. To this end I developed a rapid, one-pot synthesis of copper-based microclubs that consist of a Cu2O octahedron attached to a Cu2O@Cu shaft. Millions of synthesized particles were analyzed in minutes with a FlowCam to provide a robust statistical analysis of their geometry, and rapidly elucidate the roles of the reaction constituents on the particle shape and yield. By utilizing Bayesian Optimization, the parameter space of the reaction conditions was fully explored, reducing the mean square error (MSE) between predicted and actual yield by 125 times after 14 iterations and achieving 64% yield of microclub production in 20 mL scale. With the slight modification on the optimized conditions, 67% yield was achieved under 2 L scale synthesis of microclub. The combination of asymmetry in both shape and composition introduces a 30% difference in scattering of light propagating parallel to the microclub axis from opposing directions. This work represents a first step toward the creation of an asymmetric imaging environment with an aerosol consisting of acoustically aligned microclubs.

In the field of electronic materials, metal nanowires have been extensively studied for both their syntheses and their properties in electronic composites and devices. This dissertation addresses challenges in the field of electronic materials development with the use of copper nanowires synthesized in gram-scale syntheses, as well as provides analysis of devices and composites that could only be feasibly manufactured thanks to the large-scale syntheses.

In the field of printed electronics, there has been research into the development of fully printed memories. One of the challenges has been developing a memory that has switching characteristics that are on par with existing commercial memories, such as Flash memory. This can be achieved with a composite of Cu-SiO2 nanowires dispersed in ethylcellulose, which acts as a resistive switch when between printed Cu and Au electrodes. A 16-cell crossbar array of these memristors was printed with an aerosol jet. The memristors exhibited moderate operating voltages (~3 V), no degradation over 104 switching cycles, write speeds of 3 µs, and extrapolated retention times of 10 years. The low operating voltage enabled the programming of a fully printed 4-bit memristor array with an Arduino. The excellent performance of these fully printed memristors could help enable the creation of fully printed RFID tags and sensors with integrated data storage. Thanks to the large-scale synthesis of copper nanowires, this can allow for the expanded production of high-quality, fully printed memories.

Materials that retain a high conductivity under strain are essential for wearable electronics. I describe a new conductive, stretchable composite consisting of a Cu-Ag core-shell nanowire felt infiltrated with a silicone elastomer. This composite exhibits a retention of conductivity under strain that is superior to any composite with a conductivity greater than 1000 S cm-1. This work also shows how the mechanical properties, conductivity, and deformation mechanisms of the composite changes as a function of the stiffness of the silicone matrix. The retention of conductivity under strain was found to decrease as the Young’s modulus of the matrix increased. This was attributed to void formation as a result of debonding between the nanowire felt and the elastomer. The nanowire composite was also patterned to create serpentine circuits with a stretchability of 300%. Composites of this scale and density could only be feasibly manufactured thanks to large-scale syntheses of copper nanowires and the silver coating of copper nanowires. With the advances made in the quality of stretchable conductive composites, alternate methods were employed as to manufacture new composites and structures, such as the cofiltration of nanowires and waterborne rubber to accelerate production, or the manufacturing of Cu-Ag nanowire aerogels with density tunable via the aspect ratio of the nanowires.

The demand for flat-panel televisions, e-readers, smart-phones, and touch-screens has been increasing over the past few years and will continue to increase for the foreseeable future. Each of these devices contains a transparent conductor, which is usually indium tin oxide (ITO) because of its high transparency and low sheet resistance. ITO films, however, are brittle, expensive, and difficult to deposit, and because of these problems, alternative transparent electrodes are being studied. One cheap and flexible alternative to ITO is films of randomly oriented copper nanowires. We have developed a synthesis to make long, thin, and well-dispersed copper nanowires that can be suspended in an ink and coated onto a substrate to make flexible transparent films. These films are then made conductive by annealing in a hydrogen atmosphere or by a solution processing technique that can be done in air at room temperature. The resulting flexible transparent conducting films display transparencies and sheet resistance values comparable to ITO.

Since it is well known that copper oxidizes, we also developed a synthesis to coat the copper nanowires with a layer of nickel in solution. Our measurements indicated that copper nanowires would double their sheet resistance in 3 months, but the sheet resistance of cupronickel nanowire films containing 20 mole% nickel will double in about 400 years. The addition of nickel to the copper nanowires also gave the film a more neutral grey appearance. The nickel coating can also be applied to the copper nanowires after the film is formed via an electroless plating method.

To further optimize the properties of our transparent conductors we developed a framework to understand how the dimensions and area coverage of the nanowires affect the overall film properties. To quantify the effect of length on the sheet resistance and transmittance, wires with different lengths but the same diameter were synthesized to make transparent conducting films and finite-difference time-domain calculations were used to determine the effect of the nanowire diameter on the film's transmittance. The experimental data and calculations were then incorporated into random resistor network simulations that demonstrated that wires with an aspect ratio of 400 or higher are required to make a network that transmits >90% of visible light while maintaining a sheet resistance below 100 Ohm/sq.

These properties, and the fact that copper and nickel are 1000 times more abundant than indium or silver, make copper and cupronickel nanowire films a promising alternative for the sustainable, efficient production of transparent conductors.

This thesis explores the application of nanofibrous and microfibrous materials in the fields of cartilage repair and water electrolysis.

Articular cartilage lesions have a limited intrinsic ability to heal and are associated with joint pain and disability. The current treatment options suffer from high failure rates, prolonged rehabilitation times, and can be very costly. Therefore, an ideal solution is a low cost, mechanically strong, biocompatible replacement material with long lifetime.

To develop a cartilage replacement material, I first developed a two-step method to 3D print double network hydrogels at room temperature with a low-cost ($300) 3D printer. A first network precursor solution was made 3D printable via extrusion from a nozzle by adding a layered silicate to make it shear-thinning. After printing and UV curing, objects were soaked in a second network precursor solution and UV-cured again to create interpenetrating networks of poly(2-acrylamido-2-methylpropanesulfonate) and polyacrylamide. By varying the ratio of polyacrylamide to cross-linker, the trade-off between stiffness and maximum elongation of the gel can be tuned to yield a compression strength and elastic modulus of 61.9 and 0.44 MPa, respectively, values that are greater than those reported for bovine cartilage. The maximum compressive (93.5 MPa) and tensile (1.4 MPa) strengths of the gel are twice that of previous 3D printed gels, and the gel does not deform after it is soaked in water. By 3D printing a synthetic meniscus from an X-ray computed tomography image of an anatomical model, I demonstrate the potential to customize hydrogel implants based on 3D images of a patient’s anatomy.

On the basis of the previous work, I developed the first hydrogel with the strength and modulus of cartilage in both tension and compression, and the first to exhibit cartilage-equivalent tensile fatigue at 100,000 cycles. These properties were achieved by infiltrating a bacterial cellulose nanofiber network with a PVA-PAMPS double network hydrogel. The bacterial cellulose provided tensile strength in a manner analogous to collagen in cartilage, while the PAMPS provided a fixed negative charge and osmotic restoring force similar to the role of aggrecan in cartilage. The hydrogel has the same aggregate modulus and permeability as cartilage, resulting in the same time-dependent deformation under confined compression. The hydrogel is not cytotoxic, has a coefficient of friction 45% lower than cartilage, and is 4.4 times more wear-resistant than a polyvinyl alcohol hydrogel. The properties of this hydrogel make it an excellent candidate material for replacement of damaged cartilage.

In the field of water electrolysis, I studied the effect of fiber dimensions to their performance in water electrolysis. Water electrolysis is a good way to convert excess renewable energy to hydrogen. The generation of renewable electricity is variable, leading to periodic oversupply. Excess power can be converted to hydrogen via water electrolysis, but the conversion cost is currently too high. One way to decrease the cost of electrolysis is to increase the maximum productivity of electrolyzers. I investigated how nano- and microstructured porous electrodes could improve the productivity of hydrogen generation in a zero-gap, flow-through alkaline water electrolyzer. Three nickel electrodes—foam, microfiber felt, and nanowire felt—were studied to examine the tradeoff between surface area and pore structure on the performance of alkaline electrolyzers. Although the nanowire felt with the highest surface area initially provided the highest performance, this performance quickly decreased as gas bubbles were trapped within the electrode. The open structure of the foam facilitated bubble removal, but its small surface area limited its maximum performance. The microfiber felt exhibited the best performance because it balanced high surface area with the ability to remove bubbles. The microfiber felt maintained a maximum current density of 25,000 mA cm-2 over 100 hrs without degradation, which corresponds to a hydrogen production rate 12.5- and 50-times greater than conventional proton-exchange membrane and alkaline electrolyzers, respectively.

This thesis explores how the change in material and the structure can better serve medicalfunctions. A 3D printed physical phantom and a hydrogel coated orthopedic implant were developed. The purpose of physical phantom work was to characterize and improve the ability of fused filament fabrication to create anthropomorphic physical phantoms for CT research. Specifically, we sought to develop the ability to create multiple levels of x-ray attenuation with a single material. CT images of 3D printed cylinders with different infill angles and printing patterns were assessed by comparing their 2D noise power spectra to determine the conditions that produced a minimal and uniform noise. A backfilling approach in which additional polymer was extruded into an existing 3D printed background layer was developed to create multiple levels of image contrast. A print with nine infill angles and a rectilinear infill pattern was found to have the best uniformity, but the printed objects were not as uniform as a commercial phantom. An HU dynamic range of 600 was achieved by changing the infill percentage from 40% to 100%. The backfilling technique enabled control of up to 8 levels of contrast within one object across a range of 200 HU, similar to the range of soft tissue. A contrast detail phantom with 6 levels of contrast and an anthropomorphic liver phantom with 4 levels of contrast were printed with a single material. In conclusion, this work improves the uniformity and levels of contrast that can be achieved with fused filament fabrication, thereby enabling researchers to easily create more detailed physical phantoms including realistic, anthropomorphic textures. The goal of the orthopedic implant work is to replace the damaged cartilage with a synthetic hydrogel. This requires a method for securing the hydrogel in a defect site with the same shear strength as the cartilage-bone interface. Bonding hydrogel to a titanium base that can in turn bond to bone could enable long-term fixation of the hydrogel, but current methods of forming bonds to hydrogels do not have the shear strength of the cartilage-bone interface. This thesis reports the first method for attaching a hydrogel to metal with the same shear strength as the cartilage-bone interface. The average shear strength of the junction between 1.2-mm-thick hydrogel and metal made in this manner exceeded the shear strength of porcine8 cartilage-bone interface. The shear strength of attachment increased with the number of bacterial cellulose layers and with the addition of cement between the bacterial cellulose layers. This new method of attachment will be useful to the creation of hydrogel-coated orthopedic implants for treatment of osteochondral defects. After creating the bonding between hydrogel and metal base, the thesis then introduces the work of a synthetic hydrogel. The goal of the work is to increase the mechanical strength of the hydrogel, which further increases the shear strength between the hydrogel and metal base. This work shows that reinforcement of annealed PVA with BC leads to a 3.2-fold improvement in the tensile strength (from 15.6 to 50.5 MPa) and a 1.7-fold increase in the compressive strength (from 56.7 to 95.4 MPa). The highly crystallized BC-PVA hydrogel that results from annealing is the first hydrogel with a tensile and compressive strength that exceeds that of cartilage. When tested against cartilage, annealed BC-PVA wore an opposing cartilage surface to the same extent as cartilage and was three times more resistant to wear than cartilage. The improved tensile strength of annealed BC-PVA enabled it to attach to a metal base with a shear strength 68% greater than the shear strength of cartilage on bone. The high strength, high wear resistance, and low COF of annealed BC-PVA make it an excellent material for replacing damaged cartilage. The wear performance of the BC-PVA hydrogel was further improved by doping nanoclay into original hydrogel network. By using a two-step infiltration, a tensile strength of 37.98 MPa was achieved. The wear of opposing cartilage against the hydrogel was much better compared to cartilage itself. Thus, the nanoclay doped hydrogel also has the potential to be used in actual cartilage repair.

The demand for printable electronics has sharply increased in recent years and is projected to continue to rise. Unfortunately, electronic materials which are suitable for desired applications while being compatible with available printing techniques are still often lacking. This thesis addresses two such challenging areas.

In the realm of two-dimensional ink-based printing of electronics, a major barrier to the realization of printable computers that can run programs is the lack of a solution-coatable non-volatile memory with performance metrics comparable to silicon-based devices. To address this deficiency, I developed a nonvolatile memory based on Cu-SiO2 core-shell nanowires that can be printed from solution and exhibits on-off ratios of 106, switching speeds of 50 ns, a low operating voltage of 2 V, and operates for at least 104 cycles without failure. Each of these metrics is similar to or better than Flash memory (the write speed is 20 times faster than Flash). Memory architectures based on the individual memory cells demonstrated here could enable the printing of the more complex, embedded computing devices that are expected to make up an internet of things.

Recently, the exploration of three-dimensional printing techniques to fabricate electronic materials began. A suitable general-purpose conductive thermoplastic filament was not available, however. In this work I examine the current state of conductive thermoplastic filaments, including a newly-released highly conductive filament that my lab has produced which we call Electrifi. I focus on the use of dual-material fused filament fabrication (FFF) to 3D print electronic components (conductive traces, resistors, capacitors, inductors) and circuits (a fully-printed high-pass filter). The resistivity of traces printed from conductive thermoplastic filaments made with carbon-black, graphene, and copper as conductive fillers was found to be 12, 0.78, and 0.014 ohm cm, respectively, enabling the creation of resistors with resistances spanning 3 orders of magnitude. The carbon black and graphene filaments were brittle and fractured easily, but the copper-based filament could be bent at least 500 times with little change in its resistance. Impedance measurements made on the thermoplastic filaments demonstrate that the copper-based filament had an impedance similar to a conductive PCB trace at 1 MHz. Dual material 3D printing was used to fabricate a variety of inductors and capacitors with properties that could be predictably tuned by modifying either the geometry of the components, or the materials used to fabricate the components. These resistors, capacitors, and inductors were combined to create a fully 3D printed high-pass filter with properties comparable to its conventional counterparts. The relatively low impedance of the copper-based filament enable its use to 3D print a receiver coil for wireless power transfer. We also demonstrate the ability to embed and connect surface mounted components in 3D printed objects with a low-cost ($1,000 in parts), open source dual-material 3D printer. This work thus demonstrates the potential for FFF 3D printing to create complex, three-dimensional circuits composed of either embedded or fully-printed electronic components.

Research on the unique properties of metal nanocrystals sparked the advent of powerful nanomaterial-based technologies that improve the performance of catalysts, electronics, and cancer therapies, to name a few. Due to their promise, extensive progress has been made in tailoring the properties of metal nanocrystals by controlling their size, shape, composition, and structure. However, these nano-scaled materials remain largely within the confines of research laboratories. One of the biggest hindrances to the widespread use of metal nanocrystals is cost. Precious metals such as Ag and Au exhibit compelling properties in the nano scale, yet the astronomical cost of bulk materials and low throughput of synthetic methods make them impractical for applications that require more than a few milligrams of nanocrystals. This work addresses this issue by furthering our knowledge of Cu nanocrystal synthesis and presenting synthetic methods that lower the cost of its production.

Cu is significantly cheaper and more abundant compared to other metals, and much like Au and Ag nanocrystals, Cu nanocrystals exhibit conductive and catalytic properties. In this work, we present efforts to increase the impact of Cu nanowires by first developing a multigram-scale synthesis of stable Ag-coated Cu nanowires (Cu-Ag nanowires). The two-step process starts with the production of 4.4 g of Cu nanowires in 1 h, followed by a Ag-coating procedure that yields 22 g of Ag-coated Cu nanowires (Cu-Ag nanowires) in 1 h. Due to the large diameters of Cu nanowires (≈240 nm) produced by this synthesis, a Ag:Cu mol ratio of 0.04 is sufficient to coat the nanowires with a protective, oxidation-resistant shell. This multigram synthesis of Cu and Cu-Ag nanowire production process enabled the development of the first nanowire-based conductive polymer composite for 3D printing with a resistivity of 0.002 Ω cm, which is >100 times more conductive than commercially available graphene-based 3D printing filaments. Furthermore, a felt-like material consisting of Cu-Ag nanowires was also used to create a stretchable conductor that has a conductivity of 1000 S/cm and is stretchable up to 300%. Finally, Cu nanowires were annealed to create a porous flow-through electrode (FTE). Compared to other commercially available FTEs, Cu nanowires are significantly thinner, resulting in a 15x increase in surface area compared to carbon paper. This translates to a 4.2x increase in the productivity of an electroorganic intramolecular cyclization reaction.

We also developed a self-heating synthesis of Cu nanowires, which produced shorter, thinner nanowires than was previously possible. Self-heating is accomplished through NaOH dilution and glucose degradation, which allows us to tune the length of the Cu nanowires in the range of 1.0 to 5.5 µm by simply changing the concentration of glucose, and thereby the temperature of the reaction. The self-heating aspect of this synthesis decreases the energy cost associated with producing Cu nanowires, simplifies the reaction procedure, as well as increases its scalability.

Finally, we employed single-crystal electrochemistry to elucidate the growth mechanism of Cu nanoplates. Cu nanoplates have also been used in electronic and catalytic applications, yet their growth is not as well-understood as Cu nanowires. Thus, before they can be produced at quantities similar to that of Cu nanowires, we need to understand how they grow. Single-crystal electrochemistry allows us to replicate the crystal structure on the surfaces of Cu nanoplates using single crystal electrodes. The current density (jmp) of Cu(111) and Cu(100) single crystal electrodes were measured in the nanoplate growth solution, which showed that Cu reduction is 2.6x higher on Cu(100) compared to Cu(111) in the presence of iodide ions (I-) in a solution containing a CuCl2 precursor and hexadecylamine (HDA). Increasing the concentration of I- results in a higher ratio jmp(100)/jmp(111), which alludes to the passivation of the {111} basal surfaces of Cu nanoplates, resulting in lateral growth. This work clarifies the role of halide ions on the growth of Cu nanoplates, giving us insight into how size-control can be achieved.

As a whole, this work aims to increase the impact of Cu nanocrystal-based innovations by increasing throughput, simplifying reaction procedures, and achieving control over the size and dimensions of the synthesized product.

Printed electronics, including transparent conductors, currently rely on expensive materials to generate high conductivity devices. Conductive inks for thick film applications utilizing inkjet, aerosol, and screen printing technologies are often comprised of expensive and rare silver particles. Thin film applications such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs) predominantly employ indium tin oxide (ITO) as the transparent conductive layer which requires expensive and wasteful vapor deposition techniques. Thus an alternative to silver and ITO with similar performance in printed electronics warrants considerable attention. Copper nanomaterials, being orders of magnitude cheaper and more abundant than silver or indium, solution-coatable, and exhibiting a bulk conductivity only 6 % less than silver, have emerged as a promising candidate for incorporation in printed electronics.

First, we examine the effect of nanomaterial shape on the conductivity of thick films. The inks used in such films often require annealing at elevated temperature in order to sinter the silver nanoparticles together and obtain low resistivities. We explore the change in morphology and resistivity that occurs upon heating thick films of silver nanowires (of two different lengths, Ag NWs), nanoparticles (Ag NPs), and microflakes (Ag MFs) deposited from water at temperatures between 70 and 400 °C. At the lowest temperatures, longer Ag NWs exhibited the lowest resistivity (1.8 × 10-5 Ω cm), suggesting that the resistivity of thick films of silver nanostructures is dominated by the contact resistance between particles.

This result supported previous research showing that junction resistance between Ag NWs in thin film conductors also dominates optoelectronic performance. Since the goal is to replace silver with copper, we perform a similar analysis by using a pseudo-2D rod network modeling approach that has been modified to include lognormal distributions in length that more closely reflect experimental data collected from the nanowire transparent conductors. In our analysis, we find that Cu NW-based transparent conductors are capable of achieving comparable electrical performance to Ag NW transparent conductors with similar dimensions. We also synthesize high aspect ratio Cu NWs (as high as 5700 in an aqueous based synthesis taking less than 30 minutes) and show that this increase in aspect ratio can result results in transparent conducting films with a transmittance >95% at a sheet resistance <100 Ω sq−1, optoelectronic properties similar to that for ITO.

Two of the major barriers preventing the further use of Cu NWs in printed electronics are the necessity to anneal the nanowires under H2¬ at higher temperatures and copper’s susceptibility to oxidation. The former issue is solved by removing the insulating oxide along the Cu NWs with acetic acid and pressing the nanowires together to make H2 annealing obsolete. Finally, several methods of preventing copper oxidation in the context of transparent conductors were successfully developed such as electroplating zinc, tin, and indium and electrolessly plating benzotriazole (BTAH), nickel, silver, gold, and platinum. While all of the shells lessened or prevented oxidation both in dry and humid conditions, it was found that a thin layer of silver confers identical optoelectronic properties to the Cu NWs as pure Ag NWs. These results are expected provide motivation to replace pure silver and ITO in printed electronics.

The synthesis of various anisotropically assembled metal structures has increased at a rapid pace in the past two decades in order produce nanomaterials with unique properties that can meet the demands of future electronics and materials. However, the growth of many nanomaterials is still poorly understood. A key insight for the future of current anisotropic nanomaterials is to understand both their mechanism of growth as well as how to manipulate their dimensions, which controls their properties, for full utilization in different applications. One nanomaterial of interest are copper nanowires because of their favorable material characteristics for various applications.

Here I investigate the growth of copper nanowires in an EDA-NaOH and an alkylamine driven synthesis, the two most common types of solution phase syntheses of copper nanowires. The first synthesis I investigated was a seeded EDA-NaOH synthesis of copper nanowires where the nanomaterials were grown photocatalytically using visible light. We discovered that when exposed to visible light with an energy greater than the band gap of Cu2O, electrons excited from the valence band to the conduction band within the Cu2O octahedra seed reduce Cu(OH)2- onto the octahedra surface to form copper nanowires. This phenomenon was used to turn nanowire growth on and off with visible light and control the length of the copper nanowires. The phenomenon was also used to pattern the growth of nanowires on a substrate.

The second synthesis I investigated was the alkylamine driven copper nanowires synthesis, where I examined the role of alkylamine chain length on the dimensions and yield of the copper nanowires in the synthesis. Testing of 10 alkylamines (C6H15N-C18H35N) revealed that only those with 12–18 carbons induce anisotropic growth of copper nanowires. Tetradecylamine produced the highest yield of nanowires (54%) and octadecylamine the lowest (6%). The length of the Cu nanowires generally increased with decreasing alkylamine chain length and decreasing alkylamine concentration, whereas the diameter remained generally the same. To explain these phenomena, we explored the effect of the alkylamine chain length on the generation of reducing agent and the reduction of Cu. Alkylamine chain length has a negligible effect on the conversion of glucose to reductones, i.e. the reducing agents. In situ visualization of nanowire growth and electrochemical measurements indicate the growth of nanowires is kinetically-limited rather than mass transport-limited. In situ visualization of nanowire growth rates corroborate these measurements, with nanowires growing 14 times faster with tetradecylamine relative to octadecylamine. Alkylamine chain length also affects the adsorption of alkylamines on the Cu surface and the rate of Cu oxidation. These results indicate that the binding of alkylamines to Cu ions and their adsorption on Cu surfaces both likely play a role in modulating the rate of nanowire growth, and thus the yield and aspect ratio of nanowires.

Synthetic control of metal nanocrystal shape is a common strategy to control their properties. Shape control is often achieved by controlling the crystal structure of the seed crystals, as well as through the use of additives which are thought to block atomic addition to certain facets. However, the effect of crystal structure or additives on the rate of atomic addition to a specific facet is not usually quantified, making it difficult to design nanocrystal syntheses. This work combines seed-mediated growth, single-crystal electrochemistry measurements and Raman spectroscopy to understand the roles of capping agents and planar defects in the anisotropic growth of silver nanocrystals. The roles of citrate, polyvinylpyrrolidone (PVP), and halides have been investigated. Synthetic results show citrate is a {111} capping agent, PVP is a weak {111} capping agent, chloride and bromide are weak {100} capping agents. However, when chloride or bromide is added with PVP, they become strong {100} capping agents. Electrochemical measurements show the anisotropic growth is caused by capping agents selectively suppressing the oxidation of ascorbic acid (a reducing agent) on a specific crystal facet. The effect of capping agents on silver ion reduction is not facet-selective. Further comparison between the growth of single-crystal seeds and seeds with planar defects indicates defects can catalyze silver atom deposition by up to 100 times and cause greater anisotropic growth than can be explained by facet-selective passivation. Overall this work advances our understanding of nanocrystal chemistry, and informs the design of nanocrystal synthesis to obtain a desired nanocrystal morphology with a desired set of properties.

Heterogeneous catalysts are a major energy-saving tool in industrial chemical processes, both by reducing energy input requirements for reactions and also by reducing waste disposal costs. Currently, transition metals and platinum group metals in particular are the industry standard in terms of performance and roustness. Unfortunately, the most valuable catalytic materials of our time are also the most costly and scarce, and as such it is of considerable importance to find ways to optimize catalysts to be as effective as possible and to use a minimum of precious metal. One method of doing this is by constructing bimetallic nanoparticles, which, in addition to lessening the amount of expensive active metal required per gram of catalyst, has been shown in many cases to improve the performance of the catalyst. Our current understanding of the mechanisms by which the introduction of a second element to a first alters its catalytic properties is limited by how well we can characterize the structure of a catalysts, and this has been a considerable challenge. By developing uniform bimetallic nanoparticle catalysts that can be tuned to discretely change either the composition of the particle or the morphology of the particle, we can systematically study how the two metals interact to change the pathway of catalysis. The gold-palladium (AuPd) system is employed, because of its simplicity and ubiquity in the literature, to demonstrate this concept, which may be applied to other combinations of metals. By altering the amounts of Pd precursor, reducing agent, and reaction quenching agent, as well as varying the rate of introduction of Pd ion to the reaction solution, 4.3 nm Au core particles were coated with Pd shells determined to be 0.7 ± 0.2, 1.9 ± 0.3, and 3.8 ± 0.8 atomic monolayers thick. Upon immobilization on silica and calcination and reduction treatments at 300 °C, the particles transformed to alloys containing 10, 20.2, and 28.5% by weight Pd per particle. A variety of spectroscopic and microscopic characterization techniques were used to investigate the bimetallic particles' ability to resist sintering during heating, as well as their shell-thickness dependent propensity to absorb hydrogen into their bulk. Testing the catalysts in the conversion of limonene to cymene demonstrated that the resistance to sintering imparted by the inclusion of Au allowed the 20 and 28.5% AuPd/SiO₂ catalysts to achieve higher selectivity to the desired product while minimizing the amount of Pd in the catalyst. The core-shell and alloy particles were compared to one another in the low temperature hydrogenation of ethylene, and it is shown that conversion of ethylene over the 1.9 Pd@Au/SiO₂ catalyst is structure sensitive, with its rate of conversion before calcination and reduction measured to be ten times higher than it is afterwards, likely because of its unique ability to store hydrogen near its surface. In general, these reactions demonstrate that the interactions between Au and Pd can be described in terms of experimentally observable effects, which occur as a consequence of the traditionally described geometric and electronic effects under a given set of experimental conditions. Using composite effects such as these, rather than ones that are difficult to isolate, will render our heightened understanding of the effect of structure on catalytic function more directly practicable in developing better catalysts for specific reactions.