Orientadores: Cláudia Longo, Fernando Aparecido Sígoli / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-16T16:48:28Z (GMT). No. of bitstreams: 1 Silva_EversonThiagoSantosGeroncioda_M.pdf: 49355375 bytes, checksum: f552e7ceab4da2e918af08e239a6a01d (MD5) Previous issue date: 2010 / Resumo: O óxido de zinco é um semicondutor tipo-n que apresenta fotoatividade apenas sob radiação UV. Com o objetivo de aproveitar a radiação visível, investigou-se a incorporação de enxofre como dopante em amostras preparadas através da decomposição térmica de ZnS em atmosfera oxidante. O aquecimento de ZnS a 900°C por 30 minutos resultou em ZnO, um pó branco com estruttura de wurtzita, área de superfície de 6 m/g e energia de band gap Eg de 3,07 e 3,04 eV foram obtidas após 30 e 60 minutos a 620 °C; com coloração amarelada e estrutura de wurtzita, apresentaram área superficial de 17m/g. As propriedades eletroquímicas, investigadas em solução aquosa de Na2SO4 para eletrodos de filmes porosos depositados em vidro-FTO, indicaram comportamento de semicondutor tipo-n; sob irradiação com simulador solar, o potencial de circuito aberto negativo, VOC ~ -0,2 V, se manteve estável enquanto a fotocorrente positiva, inicialmente 120 mA cm, diminuiu gradativamente até 6 mA cm, diminuiu gradativamente até 6 mA cm após 4 horas sob irradiação. A baixa estabilidade pode ser atribuída à baixa adesão dos filmes no vidro-FTO e à fotocorrosão do semicondutor. Identificou-se a presença de íons Zn na solução, observou-se que a luz intensifica a dissolução dos óxidos em meio aquoso e que o ZnO é menos suscetível à fotocorrosão. Os eletrodos de ZnO e S:ZnO foram sensibilizados por corante de rutênio e utilizados na montagem de células solares. Os maiores valores de eficiência de conversão de luz em eletricidade, h, foram obtidos para células de 0,25 cm preparadas com eletrodos sensibilizados por 20 minutos e eletrólito líquido; as células de S:ZnO apresentaram corrente de curto circuito de 3,3 mA cm, VOC de 0,7 V e h ~ 1,0%, enquanto que as células de ZnO apresentaram valor similar de VOC, fotocorrente máxima de 0,76 mA/cm e h ~ 0,1%. Em comparação com as células de ZnO, a maior eficiência das células preparadas com ambas as amostras de S:ZnO pode estar relacionada à maior área de superfície e estabilidade mecânica destes filmes quando comparados aos de ZnO. Os estudos também foram realizados para células de S:ZnO preparadas sem o corante; o dispositivo apresentou VOC = 0,53 V, fotocorrente de 0,13 mA/cm e h ~ 0,04%. Embora o S:ZnO apresente Eg de 3,04 eV, com absorção em l < 410 nm, a eficiência de conversão é inferior à obtida na célula com do semicondutor sensibilizado com o corante. Os estudos revelaram que o S:ZnO pode ser utilizado em células solares, porém, devido à fotocorrosão, é necessário investigar meios para aumentar sua estabilidade para que não comprometa a durabilidade dos dispositivos para conversão de energia solar / Abstract: Zinc oxide is an n-type semiconductor that shows photoactivity under UV radiation. Aiming to decrease the zinc oxide band gap and consequently shift its absorption band to visible range of the electromagnetic spectrum, this work has investigated some physical chemical properties of sulfur containing zinc oxide (S:ZnO) samples. The S:ZnO samples were prepared by thermal decomposition of ZnS in oxidizing atmosphere. The heating of ZnS at 900°C for 30 minutes resulted in ZnO, a white powder with wurtzite structure with a surface area of 6 m/g and band gap energy Eg of 3.21 eV. Samples of S:ZnO, obtained by thermal treatment at 620°C for 30 and 60 minutes, have a band gap of 3.04 and 3.07 eV, respectively, yellowish color, wurtzite structure and surface area of 17 m/g. The electrochemical properties were investigated in Na2SO4 aqueous solution for porous film electrodes deposited on FTO-glass. All samples were characterized as an n-type semiconductor; under irradiation with solar simulator. The open potential circuit is negative, VOC ~ -0.2 V and remains stable while the positive photocurrent, initially 120 mA cm, gradually decreases to 6 mA cm after 4 hours of irradiation. The low stability may be attributed to poor adherence of the films on FTO-glass and to photocorrosion of the semiconductor in aqueous medium. The photocorrosion process was confirmed by presence of Zn ion in solution after the irradiation period. It was also observed that the light enhances the dissolution of S:ZnO samples in aqueous solution and that the sulfur free ZnO is less susceptible to the photocorrossion processes. The electrodes of ZnO and S:ZnO samples were sensitized by the ruthenium dye and tested as solar cells. The highest value of conversion efficiency of the light into electricity, h, was obtained for S:ZnO (Eg = 3,04 eV) solar cell that was prepared with 0.25 cm electrodes sensitized by 20 minutes using a liquid electrolyte. Solar cells prepared with S:ZnO (Eg = 3,04 eV) shows short-circuit current of 3.3 mA/cm, VOC of 0.7 V and h ~ 1.0%, while the cells of ZnO showed similar value of VOC, the maximum photocurrent of 0.76 mA/cm and h ~ 0.1%. Compared with the cells of ZnO, the better efficiency of the cells prepared from both S:ZnO samples may be related to the higher surface area and mechanical stability of these films when compared to undoped ZnO. Studies were also conducted for S:ZnO cells prepared without the dye. The prepared device shows VOC = 0.53 V, ISC = 0.13 mA/cmof photocurrent and h ~ 0.04%. Even though the S:ZnO presents a a low band gap value (Eg = 3.04 eV) and absorption at 410 nm, the conversion efficiency is lower than that obtained in dye sensitized cells. Studies revealed that the S:ZnO can be used in solar cells, however, an improvement of its photostability is necessary in order to enhances the durability of the devices applied in conversion of solar energy / Mestrado / Físico-Química / Mestre em Química

This thesis consists of two parts. The first part deals with the visible emission of ZnO Nanocrystals and its possible application in Resonance Energy Transfer (RET) studies. The second part of the thesis is on the magnetic properties of the layered transition metal Thiophosphates MPS3 (M = Mn, Fe), their solid solutions and intercalation compounds. Recent advances in semiconductor nanocrystals or quantum dots (QDs) as inorganic fluorophores have pioneered a new direction in the fluorescent based techniques to investigate fundamental processes in lifesciences. Their broad absorption spectra with narrow, Size-tunable emissions with high quantum e±ciency and stability under relative harsh environments have made inorganic QD's the fluorophores of choice in many applications. Among inorganic fluorophores the II-VI semiconductors based on cadmium chalcogenides are the front-runners. The cytotoxicity associated with these QDs is, however, a major drawback and has lead to the search for new nanocrystalline fluorophores that are non-toxic and possess the same favorable fluorescence properties as the Cd based QDs, viz, tunability and narrow spectral profile. ZnO Nano particles are known to exhibit two emission bands; a narrow emission band in UV around 380 nm (3.25 eV) at a wavelength just below the onset of the band gap excitation in the absorption spectra and a broad emission band in the blue-green part of the visible spectrum, with a maximum between 500 and 550 nm (2.5-2.2 eV). The UV Emission originates from the recombination of bound excitons - excited electrons in the Conduction band with holes in the valence band. The visible emission of ZnO nanocrystals is known to involve deep trap states that lie approximately midway between the Conduction and valence bands and surface defects that exist as shallow traps. In principle, visible-light-emitting ZnO nanocrystals would be ideal candidates as replacement for Cd-based fluorescent labels since they are nontoxic, less expensive, and chemically stable in air. Nanoscale ZnO, however, tends to aggregate and/or undergo Ostwald ripening be-Cause of high surface free energy resulting in an increase in crystallite size and consequent Disappearance of the visible emission. Most attempts to stabilize the ZnO nanocrystals by Capping has usually resulted in the quenching of the visible trap emission. The objective of the present study was to stabilize the visible light emission of ZnO nanocrystals, to Understand the origin and mechanism of the visible emission and to explore the possibility Of using the visible emission of ZnO in RET studies. The stabilization of visible light emission in ZnO nanocrystals was achieved by forming ZnO:MgO core-shell nanocrystals. The nanocrystals were synthesized by a sequential preparative procedure that involved formation of a ZnO core followed by an MgO shell. The Nanocrystals were characterized by using XRD, TEM, optical absorption and photoluminescence spectroscopy. These are described in Chapter 2 of the thesis. The ZnO: MgO Core-shell nanostructures exhibit stable emission in the visible for extended periods. Application of the ZnO: MgO nanocrystals either as fluorescent probes or RET studies require that they be dispersible in both polar and non-polar solvents. This as realized by appropriate choice of the capping agents (Chapter 3). ZnO: MgO nanocrystals with hydrophobic surface were obtained by capping the nanocrystals with oleic acid. The oleate capped ZnO: MgO nanocrystals are soluble in a variety of non-polar organic solvents with no change in their emission properties. Water-soluble ZnO: MgO nanocrystals were obtained by capping the ZnO:MgO nanocrystals with carboxymethyl-β-cyclodextrin (CMCD). The hydroxyl groups located at the rim of the cyclodextrin cavity renders the surface hydrophilic. The integrity of the CMCD molecules are preserved on capping and their by hydrophobic cavities available for host-guest chemistry. The visible emission of The ZnO: MgO nanocrystals are unaltered by the nature of the capping agent. The origin and mechanism of the visible emission from ZnO: MgO nanocrystals has been Investigated using time-resolved emission spectroscopy technique (Chapter 4). The time-evolution of the photoluminescence spectra show that there are, in fact, two features in the visible emission whose relative importance and efficiencies vary with time. These features originate from recombination involving trapped electrons and holes, respectively, And with efficiencies that depend on the occupancy of the trap density of states. The application of the visible emission of ZnO: MgO nanocrystals as resonance energy transfer (RET) donors in water and hydrophobic media are demonstrated. In aqueous media, the carboxymethyl β-cyclodextrin (CMCD) capped ZnO: MgO nanocrystals is able to accommodate the organic dye Nile Red by an inclusion in the anchored hydrophobic cyclodextrin cavity forming a 1:1 complex. Nile Red was chosen as the guest molecule because its absorption has appreciable overlap with ZnO: MgO visible emission, a prerequisite for RET to occur. The resonance energy transfer on the band gap excitation of The ZnO core to included Red molecules in the CMCD-ZnO: MgO-Nile Red supramolecular assembly is demonstrated in aqueous media. A similar RET process is shown to occur in the non-polar media in the oleate capped ZnO: MgO nanocrystals when Nile Red is partitioned from the solvent into hydrophobic anchored oleate chains. The wavelength dependent energy transfer in the system has been studied using time-resolved emission spectroscopic technique. The importance of trap states in giving rise to non-Forster distance dependence for the RET is highlighted. The second part of the thesis deals with magnetism in low dimensional layered transition metal thiophophates, MPS3 (M = Mn, Fe). Low dimensional magnetic systems continue to be a fertile ground for discovering new phenomena and properties. Among two-dimensional magnetic systems the insulating transition metal thiophosphates are one of the few known layered systems, in which both magnetic and crystallographic lattices are two dimensional (2D). In the metal chalcogenophosphates, the magnetic MPX3 layers are separated by a van der Waals gap that effectively rules out interlayer exchange and hence these systems are nearly perfect 2D magnetic systems, with the magnetic ions forming a honeycomb arrangement within the layer. Due to the crystallographic two-dimensional nature these materials may be intercalated by variety of molecules or ions leading to change in magnetic properties. The objective of this thesis work is to try and modify the magnetic properties of the transition metal thiophosphates either by forming solid solutions of the type, M1-xMxPS3, (M, M = Mn, Fe) or by intercalating hydrated metal ions within the layers. The structure, Bonding, reactivity and magnetic properties are briefly introduced in Chapter 7. The Scope and nature of the present work in presented towards the end of the chapter. MnPS3 and FePS3 have identical crystal structures and both order antiferromagnetically at low temperatures, TN. The in-plane magnetic structures of the antiferromagnetically ordered the Neel state in the two compounds are, however, different. In MnPS3 the spins Alternate up-down whereas in FePS3 the spins are arranged as ferromagnetic chains with Alternate chains coupled antiferromagnetically. Since the crystal structures are identical, These two compounds can form solid solutions, Mn1-xFexPS3(0≤x≥1) over the entire concentration range. The magnetic properties of the single crystals of the solid solutions was measured by using a SQUID magnetometer. This system is of interest since the contrasting Neel states of the end-members may give rise to new magnetic phenomena at intermediate composition. It is shown that the magnetic behavior falls into three distinct categories. The Mn-rich compositions behave like a dilute MnPS3 lattice, the Fe-rich compositions behave like dilute FePS3 and in the intermediate compositions a spin-glass like phase appears. The phase boundaries for these regime in Mn1-xFexPS3, 0≤x≥1 is shown to be related to the percolation threshold for a honeycomb lattice. MnPS3 is known to undergo a unusual ion-exchange intercalation reaction. Intercalation occurs by the inclusion of hydrated metal ions in the galleries of MnPS3 with charge neutrality maintained by loss of the Mn2+ ions from the layer (Equation). MnPS3 + 2xG+ (aq) → Mn1-xPS3 [G (H2O) y] 2x + xMn2+ (aq) Where G is a neutral guest species. This chemistry has been exploited to intercalate hydrated Mn2+ ions in the interlamellar space to give Mn1-xPS3[Mn(H2O)6]x. the magnetic properties of this 3D analogue of MnPS3 has been investigated in Chapter 9.

Tesis por compendio / [ES] La presente Tesis Doctoral se centra en el desarrollo de nanoestructuras de óxido de zinc (ZnO) mediante anodizado electroquímico en diferentes condiciones para su aplicación en el área energética, en particular, en la producción de hidrógeno mediante la rotura fotelectrocatalítica de la molécula de agua. El hidrógeno es un vector energético que se plantea como solución al problema asociado a la intermitencia diurna y estacional de la energía solar y a la variabilidad en la demanda de energía. Por otra parte, el ZnO es un material semiconductor prometedor como fotocatalizador para la producción de hidrógeno debido a sus características y propiedades. En este contexto, el ZnO es un material muy abundante, y por extensión, relativamente barato, no es tóxico y presenta una energía de banda prohibida de 3,37 eV, lo que le permite la absorción de fotones en la región UV del espectro solar. Asimismo, las posiciones de sus bandas de energía son apropiadas para llevar a cabo la fotoelectrólisis del agua. En la presente Tesis Doctoral la síntesis de nanoestructuras de ZnO se llevó a cabo mediante anodizado electroquímico, puesto que este método presenta múltiples ventajas frente a otros métodos de síntesis habituales. En general, el anodizado electroquímico constituye un método rápido, sencillo y eficaz de síntesis de nanoestructuras de ZnO mediante el que es posible diseñar las características superficiales de las nanoestructuras (tamaño y morfología) a través del control de sus parámetros. Como resultado de una revisión bibliográfica en profundidad, se analizó la influencia de los parámetros del anodizado en las características superficiales de las nanoestructuras. Además, se investigaron aquellos parámetros cuya influencia todavía no había sido analizada. Por una parte, se estudió la influencia de emplear diferentes condiciones hidrodinámicas de flujo (dadas por la variación de la velocidad de rotación del electrodo). Por otra parte, se estudió la influencia conjunta de modificar el electrolito con la adición de un disolvente orgánico (etanol o glicerol en distintas proporciones) y variar la velocidad de rotación del electrodo. Las muestras de ZnO sintetizadas se sometieron a una caracterización morfológica, estructural, electroquímica y fotoelectroquímica y se estudiaron sus propiedades para ser empleadas como fotocatalizadores en la producción de hidrógeno. De acuerdo con los resultados, las diferentes condiciones de anodizado dieron lugar a diversas nanoestructuras de ZnO con diferentes características superficiales y fotoelectrocatalíticas. Así, se obtuvieron nanoestructuras de elevada área superficial con morfologías de nanocables de distintos tamaños, nanotubos, nanoesferas y nanoesponjas. Asimismo, tras el calentamiento térmico las muestras presentaron una estructura cristalina hexagonal wurtzita con elevada cristalinidad y la presencia de defectos estructurales. Igualmente, las nanoestructuras sintetizadas presentaron una elevada fotoactividad, dada por los valores elevados de densidad de fotocorriente, presentando propiedades apropiadas para su utilización en la producción de hidrógeno. La muestra que presentó el valor de densidad de fotocorriente más elevado (0,34 mA/cm2) fue la muestra de nanocables de ZnO anodizada a 0 rpm en un electrolito con un contenido en etanol del 10 % en volumen. En la aplicación de dicha muestra en la producción de hidrógeno se obtuvo un volumen teórico de hidrógeno de 1,55 litros por hora de sol y metro cuadrado de ZnO. / [CA] La present Tesi Doctoral se centra en el desenvolupament de nanoestructures d'òxid de zinc (ZnO) mitjançant anoditzat electroquímic en diferents condicions per a l'aplicació en l'àrea energètica, en particular, en la producció d'hidrogen mitjançant el trencament fotelectrocatalític de la molècula d'aigua. L'hidrogen és un vector energètic que es planteja com a solució al problema associat a la intermitència diürna i estacional de l'energia solar i a la variabilitat en la demanda d'energia. D'altra banda, el ZnO és un material semiconductor prometedor com a fotocatalitzador per a la producció d'hidrogen degut a les seues característiques i propietats. En aquest context, el ZnO és un material molt abundant, i per extensió, relativament barat, no és tòxic i presenta una energia de banda prohibida de 3,37 eV, la qual cosa li permet l'absorció de fotons a la regió UV de l'espectre solar. Així mateix, les posicions de les seues bandes d'energia són apropiades per a dur a terme la fotoelectròlisi de l'aigua. En la present Tesi Doctoral la síntesi de nanoestructures de ZnO es va dur a terme mitjançant anoditzat electroquímic, ja que aquest mètode presenta múltiples avantatges enfront d'altres mètodes de síntesi habituals. En general, l'anoditzat electroquímic constitueix un mètode ràpid, senzill i eficaç de síntesi de nanoestructures de ZnO mitjançant el qual és possible dissenyar les característiques superficials de les nanoestructures (grandària i morfologia) a través del control dels seus paràmetres. Com a resultat d'una revisió bibliogràfica en profunditat, es va analitzar la influència dels paràmetres de l'anoditzat en les característiques superficials de les nanoestructures. A més, es van investigar aquells paràmetres la influència dels quals encara no havia sigut analitzada. D'una banda, es va estudiar la influència d'emprar diferents condicions hidrodinàmiques de flux (donades per la variació de la velocitat de rotació de l'elèctrode). D'altra banda, es va estudiar la influència conjunta de modificar l'electròlit amb l'addició d'un dissolvent orgànic (etanol o glicerol en diferents proporcions) i variar la velocitat de rotació de l'elèctrode. Les mostres de ZnO sintetitzades es van sotmetre a una caracterització morfològica, estructural, electroquímica i fotoelectroquímica i es van estudiar les seues propietats per a ser emprades com fotocatalitzadors en la producció d'hidrogen. D'acord amb els resultats, les diferents condicions d'anoditzat van donar lloc a diverses nanoestructures de ZnO amb diferents característiques superficials i fotoelectrocatalítiques. Així, es van obtindre nanoestructures d'elevada àrea superficial amb morfologies de nanocables de diferents grandàries, nanotubs, nanoesferes i nanoesponges. Així mateix, després del calfament tèrmic les mostres van presentar una estructura cristal·lina hexagonal wurtzita amb elevada cristallinitat i la presència de defectes estructurals. Igualment, les nanoestructures sintetitzades van presentar una elevada fotoactivitat, donada pels valors elevats de densitat de fotocorrent, presentant propietats apropiades per a la seua utilització en la producció d'hidrogen. La mostra que va presentar el valor de densitat de fotocorrent més elevat (0,34 mA/cm²) va ser la mostra de nanocables de ZnO anoditzada a 0 rpm en un electròlit amb un contingut en etanol del 10% en volum. En l'aplicació d'aquesta mostra en la producció d'hidrogen es va obtindre un volum teòric d'hidrogen de 1,55 litres per hora de sol i metre quadrat de ZnO. / [EN] This Doctoral Thesis focuses on the development of zinc oxide (ZnO) nanostructures by electrochemical anodization under different conditions for its application in the energy area, in particular, in the production of hydrogen through photoelectrochemical water splitting. Hydrogen is an energy vector that is proposed as a solution to the problem associated with the diurnal and seasonal intermittency of solar energy and the variability in the energy demand. On the other hand, ZnO is a promising semiconductor material as a photocatalyst for hydrogen production due to its characteristics and properties. In this context, ZnO is a very abundant material, and by extension, relatively cheap, it is non-toxic and has a band-gap energy of 3.37 eV, which allows it to absorb photons in the UV region of the solar spectrum. Besides, the positions of ZnO energy bands are appropriate to carry out photoelectrochemical water splitting. In the present Doctoral Thesis, the synthesis of ZnO nanostructures was carried out by electrochemical anodization, since this method has multiple advantages compared to other common synthesis methods. In general, electrochemical anodization constitutes a fast, simple, and effective method of synthesis of ZnO nanostructures by means of which it is possible to design the surface characteristics of the nanostructures (size and morphology) by controlling anodization parameters. As a result of an in-depth bibliographic review, the influence of anodization parameters on the surface characteristics of nanostructures was analyzed. In addition, those parameters whose influence had not yet been analyzed were investigated. On the one hand, the influence of using different controlled hydrodynamic conditions (given by the variation of the rotation speed of the electrode) was studied. On the other hand, the influence of both modifying the electrolyte with the addition of an organic solvent (ethanol or glycerol in different proportions) and varying the electrode rotation speed was studied. The synthesized ZnO samples were subjected to a morphological, structural, electrochemical and photoelectrochemical characterization and their properties were studied to be used as photocatalysts in hydrogen production. According to the results, the different anodization conditions gave rise to various ZnO nanostructures with different surface and photoelectrocatalytic characteristics. Thus, high surface area nanostructures were obtained with morphologies of nanowires of different sizes, nanotubes, nanospheres and nanosponges. Likewise, after thermal annealing the samples presented a wurtzite hexagonal crystalline structure with high crystallinity and the presence of structural defects. Likewise, the synthesized nanostructures presented high photoactivity, given by the high values of photocurrent density, presenting appropriate properties for their use in the production of hydrogen. The sample that presented the highest photocurrent density value (0.34 mA / cm2) was the ZnO nanowires anodized at 0 rpm in an electrolyte with an ethanol content of 10 % by volume. In the application of this sample in the hydrogen production, a theoretical volume of hydrogen of 1.55 liters per hour of sun and square meter of ZnO was obtained. / Authors would like to express their gratitude for the financial support to the Generalitat Valenciana and to the European Social Fund within the subvention to improve formation and employability of technical and management staff of I+D (GJIDI/2018/A/067) and for its financial support through the project: IDIFEDER/018/044. Authors also thank for the financial support to the Ministerio de Economía y Competitividad (Project Code: CTQ2016-79203-R) and to the Ministerio de Ciencia e Innovación-Agencia Estatal de Investigación (Project Code: PID2019-105844RB-I00) for its help in the Laser Raman Microscope acquisition (UPOV08-3E- 012) and for the co-finance by the European Social Fund. / Batista Grau, P. (2021). Desarrollo de nanoestructuras de ZnO mediante anodizado electroquímico en diferentes condiciones para su aplicación en el área energética [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/172606 / Compendio

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<p>Over the past decade, the study of zinc oxide (ZnO) II-VI semiconducting nanostructures has been a burgeoning research area because of this material's unique electrical and optical properties. Despite the promise of its characteristics for numerous applications, usage of ZnO in the fabrication of nanoscale devices on a commercial scale remains a challenge because of our lack of knowledge of the underlying physics and chemistry of nanostructures. Sustainable progress in nanowire manufacturing techniques requires that we first undertake basic studies to address these poorly understood underlying concepts before we embark on applied engineering. If these fundamental studies prove successful, then characterization, fabrication, and large-scale integration of nanostructures that use ZnO could be applied to a range of engineering fields. This doctoral dissertation is primarily concerned with the synthesis and doping required for the creation of novel ZnO nanostructures and the growth mechanisms of such structures. Numerous studies have been made of various kinds of ZnO nanostructures. However, no studies have been reported of systematic theoretical modeling that uses both density functional theory and as-synthesized nanostructures to explain the growth mechanisms involved in these devices. First, sulfur-doped ZnO nanostars, synthesized through a hydrothermal method, will be discussed. This section uses ab initio simulations in discussing the synthesis of novel ZnO nanostructures and their proposed growth mechanisms. Moreover, this discussion also addresses the optical properties of ZnO structures that cause sulfur doping to enhance their emission of green light. The next section introduces a novel synthetic methodology to reliably produce well-aligned vertical ZnO nanowire arrays on amorphous substrates. Vertical alignment of nanowires significantly improves the performance of devices like LEDs and solar cells. Because these vertically aligned arrays have historically been made using sapphire substrates that hinder their commercialization, substantial effort has been invested in using ZnO nanocrystal seeds to grow vertically aligned ZnO nanowires on silicon substrates. Well-known synthetic methods, such as zinc acetate dissolved in methanol or zinc acetate combined with sodium hydroxide (or potassium hydroxide), have typically been used in pursuit of this goal without a detailed understanding of the mechanisms of seed creation. The consequence of this lack of knowledge has been inconsistent reproducibility in growing vertically aligned nanowires on silicon substrates. This discussion includes the details of mechanisms that explain the why and how of creation of vertical/misoriented ZnO nanocrystal seeds on silicon substrates. In addition, a preferential c-axis-oriented ZnO nanocrystal seed has been successfully synthesized using a solution composed of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2). Lastly, the synthesis of sea urchin-like microstructures known as ZnO sea urchins will be introduced. Among the various kinds ZnO structures, the ZnO sea urchin is a integrated structure composed of a 3-D microsphere and 1-D nanowires. Dye-sensitized solar cells (DSSCs) made of ZnO sea urchins have shown a higher power conversion efficiency than planar nanowires. This is because ZnO sea urchins have a higher surface area per unit of volume than planar nanowire arrays. This larger surface area allows larger amounts of dye to access the semiconducting nanowires. We have synthesized the sea urchin structures composed of ZnOxPy microspheres, a mixed of zinc phosphide (Zn3P2) and ZnO phase, encapsulated in an array of ZnO nanowires. Synthesis of these interesting structures was achieved without resorting to the prefabricated 3-D microsphere templates that other groups used in previous studies. This new approach to the synthesis of ZnO sea urchin structures was accomplished by simply adding Zn3P2 powder to the C (graphite) and ZnO source powders in a chemical vapor transport method. The ZnO sea urchin's material properties and growth mechanism will be characterized and discussed in detail.</p> / Dissertation

In recent decades, there has been a rapid expansion in the use of manufactured nanoparticles (MNPs). Experimental evidence and material flow models predict that MNPs enter wastewater treatment plants and partition to sewage sludge and majority of that sludge is land applied as biosolids. During wastewater treatment and after land application, MNPs undergo biogeochemical transformations (aging). The primary transformation process for silver MNPs (Ag-MNPs) is sulfidation, while zinc oxide MNPs (ZnO-MNPs) most likely undergo phosphatation and sulfidation. Our overall goal was to assess bioavailability and toxicogenomic impacts of both pristine, defined as-synthesized, and aged Ag- and ZnO-MNPs, as well as their respective ions, to a model organism, the soil nematode Caenorhabditis elegans. We first investigated the toxicity of pristine Ag-MNPs, sulfidized Ag-MNPs (sAg-MNPs), and AgNO3 to identify the most sensitive ecologically relevant endpoint in C. elegans. We identified reproduction as the most sensitive endpoint for all treatments with sAg-MNPs being about 10-fold less toxic than pristine Ag-MNPs. Using synchrotron x-ray microspectroscopy we demonstrated that AgNO3 and pristine Ag-MNPs had similar bioavailability while aged sAg-MNPs caused toxicity without being taken up by C. elegans. Comparisons of the genomic impacts of both MNPs revealed that Ag-MNPs and sAg-MNPs have transcriptomic profiles distinct from each other and from AgNO3. The toxicity mechanisms of sAg-MNPs are possibly associated with damaging effects to cuticle. We also investigated the effects pristine zinc oxide MNPs (ZnO-MNPs) and aged ZnO-MNPs, including phosphatated (pZnO-MNPs) and sulfidized (sZnO-MNPs), as well as ZnSO4 have on C. elegans using a toxicogenomic approach. Aging of ZnO-MNPs reduced toxicity nearly 10-fold. Toxicity of pristine ZnO-MNPs was similar to the toxicity caused by ZnSO4 but less than 30% of responding genes was shared between these two treatments. This suggests that some of the effects of pristine ZnO-MNPs are also particle-specific. The genomic results showed that based on Gene Ontology and induced biological pathways all MNP treatments shared more similarities than any MNP treatment did with ZnSO4. This dissertation demonstrates that the toxicity of Ag- and ZnO-MNPs to C. elegans is reduced and operates through different mechanisms after transformation during the wastewater treatment process.

ZnO nanowires are a promising material with great semiconductor properties. ZnO nanowires were prepared by carbothermal reduction and vapor-liquid-solid growth mechanism. Altering a variety of parameters ranging from mole to mole ratio of ZnO to C all the way to gas flow rate was examined. The nanowires were then characterized and their morphology examined under a SEM to observe what effect the parameter had on the morphology of the nanowires. From the experiments and the parameters tested it was observed that in order to produce the highest quality straight nanowires one should use a mole to mole ratio of ZnO to C graphite of 1 to 3. With a dwell temperature and time of 900 °C for 3 hours. A gold seed catalyst of 4nm and a gas flow rate of 50 to 100sccm of Ar provides the straightest nanowires. Understanding the effect of each parameter on the morphology of ZnO nanowires is vital for the current research. This will only lead to further the research and provide a better understanding of the growth mechanism of these wires and how the production of specific wires with certain morphologic features and characteristics can be achieved.

published_or_final_version / Physics / Master / Master of Philosophy

Work on individually constructed devices has demonstrated that nanowires (NWs) offer great promise for applications such as sensing and optoelectronics. Despite this work, reliable large scale alignment and integration of these individual nanostructures into a lithographically defined process remains a challenge. Dielectrophoresis (DEP) is a promising alignment method in which a nonuniform electric field is used to exert force on and manipulate NWs in solution. DEP offers the possibility of rapid, large area room-temperature assembly of NWs across opposing electrodes. DEP structures were fabricated on Si substrates and consisted of pairs of parallel Al electrodes on a 100nm insulating SiO2 film. ZnO NWs were suspended in isopropyl alcohol (IPA) and flowed across the electrodes. Alignment yield and angle of alignment were investigated as a function voltage and frequency. A method to remove excess nanowires through frequency tuning and IPA flushing is also investigated. The electrical properties of the formed ZnO NW devices will be reported. / Graduation date: 2010

Die vorliegende Arbeit befasst sich mit der Untersuchung von weitbandlückigen Halbleitern mittels Raman-Spektroskopie. Diese wurde vorwiegend unter Verwendung von Licht einer Wellenlänge von 325 nm im ultravioletten Spektralbereich angeregt. Damit konnten zum einen aufgrund eines erhöhten Streuquerschnittes Messungen zur Probencharakterisierung durchgeführt werden, die mit Anregung im sichtbaren Spektralbereich nicht möglich gewesen wären. Zum anderen wurden bei dieser Anregungswellenlänge auftretende Resonanzeffekte untersucht. Dabei werden zwei verschiedene Materialsysteme behandelt: zum einen Kristalle mit Wurtzitstruktur und zum anderen binäre und ternäre Sesquioxide mit Metallionen der III. Hauptgruppe. An den Kristallen mit Wurtzitstruktur wurde die Streuung des Anregungslichts mit Energie oberhalb der Bandlücke an longitudinal-optischen (LO) Phononen untersucht. Die Streuung an einzelnen LO-Phononen wird unter diesen Anregungsbedingungen von einem Prozess dominiert, der eine elastische Streuung beinhaltet, durch die die Impulserhaltung verletzt wird. Es wurde ein Modell aufgestellt, dass zwischen einer elastischen Streuung an der Oberfläche und an Punktdefekten unterscheidet, und mit Hilfe von Experimenten verifiziert. Weiterhin wurde der Einfluss von Ladungsträgern auf die Energie der LO-Phononen untersucht und es wird eine Anwendung dieser Erkenntnisse zur Charakterisierung der Oberfläche von Zinkoxid vorgestellt. An den binären Oxiden des Galliums und Indiums wurden die Energien der Phononenmoden ermittelt und die resonante Verstärkung bei der verwendeten Anregungswellenlänge untersucht. Für das Galliumoxid wurde dabei insbesondere die Anisotropie des Materials berücksichtigt. Für das Indiumoxid wird dargestellt, dass durch die resonante Anregung alle Phononenmoden beobachtet werden können, was insbesondere auch die Bestimmung der Phononenmoden von Dünnschichtproben ermöglicht. Weiterhin waren Mischkristalle des Galliumoxids Untersuchungsgegenstand, in denen das Gallium teilweise durch Indium oder Aluminium ersetzt wurde. Die Phononenenergien wurden in Abhängigkeit der Zusammensetzung ermittelt und der Einfluss von strukturellen Eigenschaften darauf sowie das Auftreten von Phasenübergängen untersucht.