Matching Items (5)
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- All Subjects: nanotechnology
- Creators: Chan, Candace
- Resource Type: Text
Description
Dealloying, the selective dissolution of an elemental component from an alloy, is an important corrosion mechanism and a technological significant means to fabricate nanoporous structures for a variety of applications. In noble metal alloys, dealloying proceeds above a composition dependent critical potential, and bi-continuous structure evolves "simultaneously" as a result of the interplay between percolation dissolution and surface diffusion. In contrast, dealloying in alloys that show considerable solid-state mass transport at ambient temperature is largely unexplored despite its relevance to nanoparticle catalysts and Li-ion anodes. In my dissertation, I discuss the behaviors of two alloy systems in order to elucidate the role of bulk lattice diffusion in dealloying. First, Mg-Cd alloys are chosen to show that when the dealloying is controlled by bulk diffusion, a new type of porosity - negative void dendrites will form, and the process mirrors electrodeposition. Then, Li-Sn alloys are studied with respect to the composition, particle size and dealloying rate effects on the morphology evolution. Under the right condition, dealloying of Li-Sn supported by percolation dissolution results in the same bi-continuous structure as nanoporous noble metals; whereas lattice diffusion through the otherwise "passivated" surface allows for dealloying with no porosity evolution. The interactions between bulk diffusion, surface diffusion and dissolution are revealed by chronopotentiometry and linear sweep voltammetry technics. The better understanding of dealloying from these experiments enables me to construct a brief review summarizing the electrochemistry and morphology aspects of dealloying as well as offering interpretations to new observations such as critical size effect and encased voids in nanoporous gold. At the end of the dissertation, I will describe a preliminary attempt to generalize the morphology evolution "rules of dealloying" to all solid-to-solid interfacial controlled phase transition process, demonstrating that bi-continuous morphologies can evolve regardless of the nature of parent phase.
ContributorsChen, Qing (Author) / Sieradzki, Karl (Thesis advisor) / Friesen, Cody (Committee member) / Buttry, Daniel (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2013
Description
Photocatalytic water splitting has been proposed as a promising way of generating carbon-neutral fuels from sunlight and water. In one approach, water decomposition is enabled by the use of functionalized nano-particulate photocatalyst composites. The atomic structures of the photocatalysts dictate their electronic and photonic structures, which are controlled by synthesis methods and may alter under reaction conditions. Characterizing these structures, especially the ones associated with photocatalysts’ surfaces, is essential because they determine the efficiencies of various reaction steps involved in photocatalytic water splitting. Due to its superior spatial resolution, (scanning) transmission electron microscopy (STEM/TEM), which includes various imaging and spectroscopic techniques, is a suitable tool for probing materials’ local atomic, electronic and optical structures. In this work, techniques specific for the study of photocatalysts are developed using model systems.
Nano-level structure-reactivity relationships as well as deactivation mechanisms of Ni core-NiO shell co-catalysts loaded on Ta2O5 particles are studied using an aberration-corrected TEM. It is revealed that nanometer changes in the shell thickness lead to significant changes in the H2 production. Also, deactivation of this system is found to be related to a photo-driven process resulting in the loss of the Ni core.
In addition, a special form of monochromated electron energy-loss spectroscopy (EELS), the so-called aloof beam EELS, is used to probe surface electronic states as well as light-particle interactions from model oxide nanoparticles. Surface states associated with hydrate species are analyzed using spectral simulations based on a dielectric theory and a density of states model. Geometry-induced optical-frequency resonant modes are excited using fast electrons in catalytically relevant oxides. Combing the spectral features detected in experiments with classical electrodynamics simulations, the underlying physics involved in this excitation process and the various influencing factors of the modes are investigated.
Finally, an in situ light illumination system is developed for an aberration-corrected environmental TEM to enable direct observation of atomic structural transformations of model photocatalysts while they are exposed to near reaction conditions.
Nano-level structure-reactivity relationships as well as deactivation mechanisms of Ni core-NiO shell co-catalysts loaded on Ta2O5 particles are studied using an aberration-corrected TEM. It is revealed that nanometer changes in the shell thickness lead to significant changes in the H2 production. Also, deactivation of this system is found to be related to a photo-driven process resulting in the loss of the Ni core.
In addition, a special form of monochromated electron energy-loss spectroscopy (EELS), the so-called aloof beam EELS, is used to probe surface electronic states as well as light-particle interactions from model oxide nanoparticles. Surface states associated with hydrate species are analyzed using spectral simulations based on a dielectric theory and a density of states model. Geometry-induced optical-frequency resonant modes are excited using fast electrons in catalytically relevant oxides. Combing the spectral features detected in experiments with classical electrodynamics simulations, the underlying physics involved in this excitation process and the various influencing factors of the modes are investigated.
Finally, an in situ light illumination system is developed for an aberration-corrected environmental TEM to enable direct observation of atomic structural transformations of model photocatalysts while they are exposed to near reaction conditions.
ContributorsLiu, Qianlang (Author) / Crozier, Peter A. (Thesis advisor) / Chan, Candace (Committee member) / Buttry, Daniel (Committee member) / Liu, Jingyue (Committee member) / Nemanich, Robert (Committee member) / Arizona State University (Publisher)
Created2018
Description
In this dissertation, micro-galvanic corrosion effects and passivation behavior of single-phase binary alloys have been studied in order to formulate new insights towards the development of “stainless-like” lightweight alloys. As a lightweight material of interest, Mg-xAl alloys were studied using aqueous free corrosion, atmospheric corrosion, dissolution rate kinetics, and ionic liquid dissolution. Polarization and “accelerated” free corrosion studies in aqueous chloride were used to characterize the corrosion behavior and morphology of alloys. Atmospheric corrosion experiments revealed surface roughness and pH evolution behavior in aqueous environment. Dissolution in absence of water using choline-chloride:urea ionic liquid allowed for a simpler dissolution mechanism to be observed, providing additional insights regarding surface mobility of Al. These results were compared with commercial alloy (AZ31B, AM60, and AZ91D) behavior to better elucidate effects associated with secondary phases and intermetallic particles often present in Mg alloys. Aqueous free corrosion, “accelerated” free corrosion and ionic liquid dissolution studies have confirmed Al surface enrichment in a variety of morphologies, including Al-rich platelet and Al nanowire formation. This behavior is attributed to the preferential dissolution of Al as the more “noble” element in the matrix. Inductively-coupled mass spectroscopy was used to measure first-order rate reaction constants for elemental Mg and Al dissolution in aqueous chloride environment to be kMg= 9.419 x 10-6 and kAl = 2.103 x 10-6 for future implementation in kinetic Monte Carlo simulations. To better understand how “stainless-like” passivation may be achieved, Ni-xCr alloys were studied using polarization and potential pulse experiments. The passivation potential, critical current density, and passivation current density were found to decay with increasing Cr composition. The measured average number of monolayers dissolved during passivation was found to be in good agreement with percolation theory, with a fitted 3-D percolation threshold of p_c^3D=0.118 compared with the theoretical value of 0.137. Using these results, possible approaches towards achieving passivation in other systems, including Mg-Al, are discussed.
ContributorsAiello, Ashlee (Author) / Sieradzki, Karl (Thesis advisor) / Buttry, Daniel (Committee member) / Chan, Candace (Committee member) / Crozier, Peter (Committee member) / Arizona State University (Publisher)
Created2018
Description
Hydrogen is considered one of the most potential fuels due to its highest gravimetric energy density with no pollutant emission during the energy cycle. Among several techniques for hydrogen generation, the promising photoelectrochemical water oxidation is considered a long-term solar pathway by splitting water. The system contains a photoanode and a cathode immersed in an aqueous electrolyte where charge separation takes place in the bulk of the semiconducting material on light absorption, leading to water oxidation/reduction at the surface of the photoelectrodes/cathode. It is imperative to develop materials that demonstrate high light absorption in the wide spectrum along with photoelectrochemical stability. N-type Monoclinic scheelite bismuth vanadate (BiVO4) is selected due to its incredible light absorption capabilities, direct bandgap (Eg ∼ 2.4-2.5 eV) and relatively better photoelectrochemical stability. However, BiVO4 encounters huge electron-hole recombination due to smaller diffusion lengths and positive conduction bands that cause slow charge dynamics and sluggish water oxidation kinetics. In order to improve the illustrated drawbacks, four strategies were discussed. Chapter 1 describe the fundamental understanding of photoelectrochemical cell and BiVO4. Chapter 2 illustrates details of the experimental procedure and state-of-the-art material characterization. Chapter 3 provide the impact of alkali metal placement in the crystal structure of BiVO4 systematically that exhibited ~20 times more performance than intrinsic BiVO4, almost complete bulk charge separation and enhancement in the diffusion length. Detailed characterization determined that the alkali metal getting placed in the interstitial void of BiVO4 lattice and multiple interbands formation enhanced the charge dynamics. Chapter 4 contains stoichiometric doping of Y3+ or Er3+ or Yb3+ at the Bi3+ site, leading to an extended absorption region, whereas non-stoichiometric W6+ doping at the V5+ site minimizes defects and increased charge carriers. To further enhance the performance, type-II heterojunction with WO3 along p-n junction with Fe:NiO enhance light absorption and charge dynamics close to the theoretical performance. Chapter 5 provides a comprehensive study of a uniquely developed sulfur modified Bi2O3 interface layer to facilitate charge dynamics and carrier lifetime improvement by effectively passivating the WO3/BiVO4 heterojunction interface. Finally, chapter 6 summarized the major findings, conclusion and outlook in developing BiVO4 as an efficient photoanode material.
ContributorsPrasad, Umesh (Author) / Kannan, Arunachala Mada (Thesis advisor) / Azeredo, Bruno (Committee member) / Chan, Candace (Committee member) / Segura, Sergio Garcia (Committee member) / Arizona State University (Publisher)
Created2021
Description
Heavy metals such as selenium can be especially important to limit because they can cause serious health problems even at relatively low concentrations. In an effort to selectively remove selenium from solution, a PAABA (poly(aniline-co-p-aminobenzoic acid) conductive copolymer was synthesized in a selenic acid solution, and its ability to remove selenium was studied. Analysis of the Raman spectra confirmed the hypothesized formation of PAABA polymer. Constant voltage cycles showed success in precipitating the selenium out of solution via electroreduction, and ICP-MS confirmed the reduction of selenium concentrated in solution. These results indicate the PAABA synthesized in selenic acid shows promise for selective water treatment.
ContributorsSulzman, Serita Lynne (Author) / Wang, Qing Hua (Thesis director) / Chan, Candace (Committee member) / Materials Science and Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2020-05