Synthesis and systematic study of Co₃O₄-based catalysts for oxygen reduction and oxygen evolution reactions

textCo₃O₄-based composite materials are good electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in alkaline solutions. Here, this thesis first investigated the individual functionality of Co₃O₄ and the N-doped carbon nanoweb (CNW) in ORR and OER. The Co₃O₄/CNW bifunctional catalysts were synthesized by an in situ growth of Co precursors onto CNW followed by a controlled heat treatment. Rotating disk electrode measurements were utilized to provide insight into the specific functions of Co₃O₄ and CNW in the composite material during catalysis. It was found that Co₃O₄ alone exhibited poor ORR catalytic activity. However, in the presence of CNW, Co₃O₄ assisted the selective four-electron oxygen reduction over the two-electron pathway. Co₃O₄ acted as the primary catalytic site for OER and CNW improved the electronic conduction between Co₃O₄ and the current collector. CNW underwent serious degradation at the high potential of the OER, but its stability improved greatly upon the deposition of Co₃O₄. Two possible mechanisms for the improved catalytic stability are discussed. The findings demonstrate the specific functions of Co₃O₄ and CNW in catalyzing the OER and ORR and further establish an understanding of the synergy of the composite in electrocatalysis. Based on the critical functionality of Co₃O₄ in stabilizing carbon materials in the OER potential region, it is of interest to investigate novel synthesis methods to prepare nano-sized Co₃O₄ that can provide more active sites for catalytic reactions and thus, improve the OER kinetics. Here, in situ electrochemical generation of 2-dimensional Co₃O₄ (2D-Co₃O₄) nanoplates were achieved by scanning CoO[subscript x]/Co precursors in 1 M KOH solution. X-ray diffraction characterization suggested that CoO[subscript x]/Co precursors were oxidized to Co₃O₄ before the onset potential of OER. Scanning electron microscopy showed that oxidation from CoO[subscript x]/Co to 2D-Co₃O₄ was associated with the formation of hexagonal nanoplates. The 2D-Co₃O₄ exhibited excellent OER catalytic activity and stability probably due to the effective mass transfer through the 2D structure.Materials Science and Engineerin

Generalized frieze varieties and Gr\"obner bases

We study properties of generalized frieze varieties for quivers associated to cluster automorphisms. Special cases include acyclic quivers with Coxeter automorphisms and quivers with Cluster DT automorphisms. We prove that the generalized frieze variety X of an affine quiver with the Coxeter automorphism is either a finite set of points or a union of finitely many rational curves. In particular, if dim X=1, the genus of each irreducible component is zero. We also propose an algorithm to obtain the defining polynomials for each irreducible component of the generalized frieze variety for affine quivers. Furthermore, we give the Gr\"obner basis with respect to a given monomial order for each irreducible component of frieze varieties of affine quivers with given orientations, and show that each component is a smooth rational curve.Comment: 41 page

The Effect of Plant Elicitor Peptide on Nematode Infected Soybean

Soybean (Glycine max), an important economic crop native to East Asia, is grown worldwide for its edible beans. Soybean plants are vulnerable to a wide range of diseases, and nematodes are among the worst pests of soybean. The southern root-knot nematode (Meloidogyne incognita) and soybean cyst nematode (Heterodera glycines) are both notable nematode pests of soybean, with the soybean cyst nematode causing more than 30% of total yield loss in the US. Numerous attempts have been made to control soybean nematodes over the past several decades, and genetic engineering technology has received a lot of attention in recent years. Plant elicitor peptides (Peps) are endogenous polypeptides consisting of amino acids that can trigger innate immune defense in plants and are present in various plant species, including soybean. GmPep3 are plant elicitor peptides present in soybean and the gene GmPROPEP3 regulates the synthesis of GmPep3 in soybean. In this study, two independent nematode bioassays were conducted to investigate whether GmPep3 treatment and the insertion of GmPROPEP3 in susceptible soybean cultivar Magellan could enhance soybean defense towards the southern root-knot nematode and the soybean cyst nematode, and whether different treatments on soybean have any effect on soybean growth. According to the results, the GmPep3 treatment and the insertion of GmPROPEP3 in Magellan have no negative effect on soybean growth. However, whether the treatments have any positive effects on soybean growth needs further investigation. The root-knot nematode bioassay results suggested that there were no significant differences in egg number between treatment groups, while the soybean cyst nematode result indicated that both transgenic lines were resistant to soybean cyst nematodes. However, the data presented in this study is insufficient, and future experiments are necessary in order to support the current conclusion

Resolving the Two impasses in the Global Justice Debate through a Reconstruction of David Miller’s Account of Deliberative Democracy on the Basis of Habermas’s Conception of Communicative action

There are both substantive and methodological disagreements in the global justice debate between cosmopolitanism on the one hand, and statism and liberal nationalism on the other. The substantive disagreement avers that statists and liberal nationalists restrict the regulation of substantive inequality to the domestic sphere, whereas cosmopolitans extend it to the global level. The methodological disagreement is based on the fact that statists and liberal nationalists ground concern for distributive justice in the institutions of citizenship, whereas cosmopolitans focus on the moral equality of human beings. David Miller’s liberal nationalist account of distributive justice differs from the statist account by drawing on a premise about human nature: that a common nationality is a powerful motivational force in supporting the institutions of citizenship within a nation-state. This provides more ethically acceptable justification for restricting the regulation of substantive inequality in the face of the cosmopolitans’ insistence on respecting the moral equality of human beings. However, Miller’s account is incomplete because it fails to explain why human beings’ common moral intuitions and political emotions would justify restricting the regulation of substantive inequality to the domestic level. In this thesis, I will propose a way of extending Miller’s account based on the idea of deliberative democracy at the global level. I will argue that the regulation of substantive inequality could be grounded in institutions of global citizenship based on public deliberation at the global level that would adjudicate among different distributive principles and that this could help to resolve the two disagreements in the global justice debate

Optoelectronic Properties of Graphene/Oxide/Semiconductor Structure with Nano-Channels

Metal-oxide-semiconductor (MOS) structure is a basic building block of silicon CMOS electronics. The silicon surface, when passivated by thermally grown oxide, can harbor a good quality two-dimensional electronic system (2DES). We have developed a graphene/oxide/nano-channel-etched Si (GOS) capacitor structure and investigated the photodetection properties. In this structure photogenerated carriers are extracted through a thin oxide layer via tunneling process and a monolayer graphene is used as a transparent conducting electrode. Nano-channels of high aspect ratio (length to diameter ratio of ~100 with diameter < 100nm) are formed in Si using a non-lithographic electrochemical process, metal-assisted-chemical-etching (MACE). We introduced a thin oxide layer in the MACE process for controlled formation of nano-channels. The nano-channel-etched Si demonstrates an antireflection effect. The current-versus-voltage (I-V) characteristic of GOS structure is measured both in dark and under illumination at 633 nm wavelength. Tunneling of carriers through the thin (2-4nm) oxide layer as well as ballistic transport through nano-channels is discussed as underlying mechanisms of carrier transport. A photo responsivity of 0.33 A/W and internal quantum efficiency of 84 % is observed on graphene/SiO2/nano-channeled n-Si structure

Electrically-triggered Atomic Emission Spectroscopy on Graphene/oxide/silicon Structure

We present a device technology that promises chip-scale atomic emission spectroscopy operating in air ambient at room temperature with low voltage pulses. Analytes are placed on top of a graphene/SiO2/Si (GOS) substrate and are atomized for atomic luminescence under electrical excitation. Here the graphene is designed to serve as an electron-transparent conducting electrode. When applying proper voltage pulses, the thin insulating layer (10-nm thermal grown SiO2) breaks down inducing high local leakage current flow. Injection of kinetic electrons induces explosions, atomizing all the material nearby as well. This explosive fragmentation produces atoms in various excited states. The excited atoms then relax producing characteristic luminescence. We have investigated the mechanisms of oxide breakdown in a GOS capacitor structure under high-field pulsed voltage drive. Four different configurations are analyzed and compared in terms of bias polarity and substrate conductivity type: inversion or accumulation bias on a GOS structure formed on n-Si or p-Si substrate. Electric field distributions in the GOS structure are analyzed under strong bias near the breakdown field regime, and the resulting quantum yield of electron impact ionization is calculated for SiO2 and Si regions. Oxide breakdown is found to occur more readily in inversion bias than in accumulation bias due to the existence of depletion region. In the case of n-Si GOS under inversion bias, a cascade of impact ionization occur, first in SiO2 and then in Si, resulting in explosive melting of Si in the depletion region. In the p-Si GOS case, impact ionization occurs mostly in SiO2 and near SiO2/Si interface. Post-AES study reveals significantly different breakdown damages caused by lateral propagation of oxide breakdown in GOS device under inversion high-field: highly localized, circular, protruding/deep melt explosion of Si for the n-Si GOS case; shallow, irregular, widely spread, meandering eruptions in SiO2/Si for the p-Si GOS case. These very different damage morphologies are explained by the different carrier-multiplication processes: a cascade of electron impact ionization, escalating towards the Si depletion region for the n-Si case; carrier multiplication accumulating at the graphene side for the p-Si case