Nanostructured Semiconductor Electrodes for Solar Energy Conversion and Innovations in Undergraduate Chemical Lab Curriculum

This dissertation presents the methodology and discussion of preparing nanostructured, high aspect ratio p-type phosphide-based binary and ternary semiconductors via “top-down” anodic etching, a process which creates nanostructures from a large parent entity, and “bottom-up” vapor-liquid-solid growth, a mechanism which builds up small clusters of molecules block-by-block. Such architecture is particularly useful for semiconducting materials with incompatible optical absorption depth and charge carrier diffusion length, as it not only relaxes the requirement for high-grade crystalline materials, but also increases the carrier collection efficiencies for photons with energy greater than or equal to the band gap. The main focus of this dissertation is to obtain nanostructured p-type phosphide semiconductors for photoelectrochemical (PEC) cell applications. Chapter II in the thesis describes a methodology for creating high-aspect ratio p-GaP that function as a photocathode under white light illumination. Gallium phosphide (GaP, band gap: 2.26 eV) is a suitable candidate for solar conversion and energy storage due to its ability to generate large photocurrent and photovoltage to drive fuel-forming reactions. Furthermore, the band edge positions of GaP can provide sufficient kinetics for the reduction of protons and carbon dioxide. The structure is prepared by anodic etching, and the resulting macroporous structures are subsequently doped with Zn by thermally driving in Zn from conformal ZnO films prepared by atomic layer deposition (ALD). The key finding of this work is a viable doping strategy involving ALD ZnO films for making functioning p-type GaP nanostructures. Chapter III compares the GaP nanowires grown from gold (Au) and tin (Sn) VLS catalysts in a benign solid sublimation growth scheme in terms of crystal structure and photoactivity. Sn is less noble than Au, allowing complete removal of Sn metal catalysts from the nanowires through wet chemical etching which found to be useful for subsequent thermal diffusion p-type doping without fear of contaminations like Au. The main finding of this work is Sn-seeded GaP nanowires although Sn was removed without any residues and the nanowires had less twin defects than Au-seeded GaP, the nanowires were degenerately n-doped. On the contrary, Au-seeded GaP nanowires exhibited n-type characteristics with orthogonalized light absorption and charge separation. Chapter IV describes the synthesis of zinc tin phosphide (ZSP), a ternary analog of GaP comprised of low-cost, earth-abundant elements in the nanowire form using Sn nanoparticles as the VLS growth seed. The as-prepared ZSP nanowire film is capable of sustaining stable cathodic photoresponse in aqueous electrolyte under white light illumination. The nanowires were crystalized in the stoichiometric sphalerite form and possessed a direct optical band gap of ~ 1.5 eV instead of the chalcopyrite structure that has comparable band gap energy to GaP. The Sn nanoparticles acted as the VLS seed as well as Sn source for the ZSP nanowires growth. Chapter V summarizes the progress and findings of p-GaP nanowire array films as well as a phase non-specific, persistent ALD dye attachment scheme that facilitates hole injection into p-GaP photocathodes, extending the photon absorption range beyond its band gap. Lastly, a separate work about undergraduate chemical education development is documented in Chapter VI of this thesis. Chapter VI details the efforts made in two distinct undergraduate laboratory coursework with the intention to introduce modern microfluidics and photovoltaic technologies including multidisciplinary research experience to the undergraduate students.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/138476/1/sudlee_1.pd

Use of graphene as protection film in biological environments

Corrosion of metal in biomedical devices could cause serious health problems to patients. Currently ceramics coating materials used in metal implants can reduce corrosion to some extent with limitations. Here we proposed graphene as a biocompatible protective film for metal potentially for biomedical application. We confirmed graphene effectively inhibits Cu surface from corrosion in different biological aqueous environments. Results from cell viability tests suggested that graphene greatly eliminates the toxicity of Cu by inhibiting corrosion and reducing the concentration of Cu(2+) ions produced. We demonstrated that additional thiol derivatives assembled on graphene coated Cu surface can prominently enhance durability of sole graphene protection limited by the defects in graphene film. We also demonstrated that graphene coating reduced the immune response to metal in a clinical setting for the first time through the lymphocyte transformation test. Finally, an animal experiment showed the effective protection of graphene to Cu under in vivo condition. Our results open up the potential for using graphene coating to protect metal surface in biomedical application

Use of graphene as protection film in biological environments

Corrosion of metal in biomedical devices could cause serious health problems to patients. Currently ceramics coating materials used in metal implants can reduce corrosion to some extent with limitations. Here we proposed graphene as a biocompatible protective film for metal potentially for biomedical application. We confirmed graphene effectively inhibits Cu surface from corrosion in different biological aqueous environments. Results from cell viability tests suggested that graphene greatly eliminates the toxicity of Cu by inhibiting corrosion and reducing the concentration of Cu2+ ions produced. We demonstrated that additional thiol derivatives assembled on graphene coated Cu surface can prominently enhance durability of sole graphene protection limited by the defects in graphene film. We also demonstrated that graphene coating reduced the immune response to metal in a clinical setting for the first time through the lymphocyte transformation test. Finally, an animal experiment showed the effective protection of graphene to Cu under in vivo condition. Our results open up the potential for using graphene coating to protect metal surface in biomedical application

Rational Design of Transparent Nanowire Architectures with Tunable Geometries for Preventing Marine Fouling

Marine biofouling is a sticky global problem that hinders maritime industries. Various microscale surface structures inspired by marine biological species have been explored for their anti- fouling properties. However, systematic studies of anti- marine- fouling performance on surface architectures with characteristic length- scales spanning from below 100 nm to greater than 10 µm are generally lacking. Herein, a study on the rational design and fabrication of ZnO/Al2O3 core- shell nanowire architectures with tunable geometries (length, spacing, and branching) and surface chemistry is presented. The ability of the nanowires to significantly delay or prevent marine biofouling is demonstrated. Compared to planar surfaces, hydrophilic nanowires can reduce fouling coverage by up to - 60% after 20 days. The fouling reduction mechanism is mainly due to two geometric effects: reduced effective settlement area and mechanical cell penetration. Additionally, superhydrophobic nanowires can completely prevent marine biofouling for up to 22 days. The nanowire surfaces are transparent across the visible spectrum, making them applicable to windows and oceanographic sensors. Through the rational control of surface nano- architectures, the coupled relationships between wettability, transparency, and anti- biofouling performance are identified. It is envisioned that the insights gained from the work can be used to systematically design surfaces that reduce marine biofouling in various industrial settings.Core- shell nanowire architectures with tunable geometries (length, spacing, and branching) and surface chemistry are shown to significantly delay marine biofouling. The fouling reduction mechanism is mainly due to the two effects: reduced effective settlement area and mechanical biocide. The insights gained from the work can be used to systematically design surfaces that reduce marine biofouling in various industrial settings.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162819/3/admi202000672-sup-0001-SuppMat.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162819/2/admi202000672_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162819/1/admi202000672.pd

Multiphase Power Converter Drive for Fault-Tolerant Machine Development in Aerospace Applications

This paper describes an experimental tool to evaluate and support the development of fault-tolerant machines designed for aerospace motor drives. Aerospace applications involve essentially safety-critical systems which should be able to overcome hardware or software faults and therefore need to be fault tolerant. A way of achieving this is to introduce variable degrees of redundancy into the system by duplicating one or all of the operations within the system itself. Looking at motor drives, multiphase machines, such as multiphase brushless dc machines, are considered to be good candidates in the design of faulttolerant aerospace motor drives. This paper introduces a multiphase two-level inverter using a flexible and reliable fieldprogrammable gate-array/digital-signal-processor controller for data acquisition, motor control, and fault monitoring to study the fault tolerance of such systems. Index Terms—Control of drive, converter control, digital signal processor (DSP), multiphase drive, reliability

Selection of Ankyrin Targeting HIV-1 Matrix and Identification of Its Binding Domain

Ankyrin repeat protein is a novel class of non-antibody binding protein that can be applied as an alternative antiretroviral agent. Engineered ankyrin targeting the HIV-1 matrix (MA) would be a promising agent to interfere with HIV replication, since MA plays a major role in multiple processes of the viral life cycle. In this study, MA-specific ankyrin (AnkGAGG31) was isolated from an artificial ankyrin library using a semi-automated selection process with biotinylated MA-streptavidin magnetic beads. The AnkGAGG31-recognition site on MA was determined using both indirect and competitive ELISAs with overlapping MA tri-helical fragments and pentadecapeptides. The AnkGAGG31 showed the highest binding signal to the MA-fragments covering helices 2-3-4 and peptides corresponding to helix 2 (residues 25-43), which were found as the target epitope. This finding was further analyzed by molecular modeling and docking. The rational models of AnkGAGG31-MA complex indicated that the strong binding interaction was shown on helix 2 at key residues K27MA, K30M, and K32MA. Taken together, the identification of the binding domain on the MA target improves our understanding of the AnkGAGG31-MA interaction and provides the information necessary to design innovative protein targeting of the MA protein

Fault tolerant power converter topologies for PMSM drives in aerospace applications

This paper proposes two fault tolerant power converter permanent magnet synchronous motor (PMSM) drive topologies. A reconfigured conventional DC link/two-level VSI converter and a reconfigured conventional matrix converter are presented to improve the reliability of the drive system under open phase fault operating condition. Additional phase redundancy and its control technique are included. Simulation and experimental results are shown to verify the effectiveness of the proposed topologies

Macroporous p‑GaP Photocathodes Prepared by Anodic Etching and Atomic Layer Deposition Doping

P-type macroporous gallium phosphide (GaP) photoelectrodes have been prepared by anodic etching of an undoped, intrinsically n-type GaP(100) wafer and followed by drive-in doping with Zn from conformal ZnO films prepared by atomic layer deposition (ALD). Specifically, 30 nm ALD ZnO films were coated on GaP macroporous films and then annealed at <i>T</i> = 650 °C for various times to diffuse Zn in GaP. Under 100 mW cm^–2 white light illumination, the resulting Zn-doped macroporous GaP consistently exhibit strong cathodic photocurrent when measured in aqueous electrolyte containing methyl viologen. Wavelength-dependent photoresponse measurements of the Zn-doped macroporous GaP revealed enhanced collection efficiency at wavelengths longer than 460 nm, indicating that the ALD doping step rendered the entire material p-type and imparted the ability to sustain a strong internal electric field that preferentially drove photogenerated electrons to the GaP/electrolyte interface. Collectively, this work presents a doping strategy with a potentially high degree of controllability for high-aspect ratio III–V materials, where the ZnO ALD film is a practical dopant source for Zn