Bio-hydrogen production based on catalytic reforming of volatiles generated by cellulose pyrolysis: an integrated process for ZnO reduction and zinc nanostructures fabrication

The paper presents a process of cellulose thermal degradation with bio-hydrogen generation and zinc nanostructures synthesis. Production of zinc nanowires and zinc nanoflowers was performed by a novel processes based on cellulose pyrolysis, volatiles reforming and direct reduction of ZnO. The bio-hydrogen generated in situ promoted the ZnO reduction with Zn nanostructures formation by vapor–solid (VS) route. The cellulose and cellulose/ZnO samples were characterized by thermal analyses (TG/DTG/DTA) and the gases evolved were analyzed by FTIR spectroscopy (TG/FTIR). The hydrogen was detected by TPR (Temperature Programmed Reaction) tests. The results showed that in the presence of ZnO the cellulose thermal degradation produced larger amounts of H2 when compared to pure cellulose. The process was also carried out in a tubular furnace with N2 atmosphere, at temperatures up to 900 °C, and different heating rates. The nanostructures growth was catalyst-free, without pressure reduction, at temperatures lower than those required in the carbothermal reduction of ZnO with fossil carbon. The nanostructures were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The optical properties were investigated by photoluminescence (PL). One mechanism was presented in an attempt to explain the synthesis of zinc nanostructures that are crystalline, were obtained without significant re-oxidation and whose morphologies are dependent on the heating rates of the process. This route presents a potential use as an industrial process taking into account the simple operational conditions, the low costs of cellulose and the importance of bio-hydrogen and nanostructured zinc.

Nanoestruturas de GaN crescidas pelas técnicas de epitaxia por magnetron sputtering e epitaxia por feixe molecular

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Raman spectrum of group IV nanowires: influence of temperature

Group IV semiconductor nanowires are characterized by Raman spectroscopy. The results are analyzed in terms of the heating induced by the laser beam on the nanowires. By solving the heat transport equation one can simulate the temperature reached by the NWs under the exposure to a laser beam. The results are illustrated with Si and Si1-xGex nanowires. Both bundles of nanowires and individual nanowires are studied. The main experimental conditions contributing to the nanowire heating are discussed

Mechanical and electrical properties of single-walled carbon nanotubes synthesized by chemical vapor deposition

Despite the tremendous application potentials of carbon nanotubes (CNTs) proposed by researchers in the last two decades, efficient experimental techniques and methods are still in need for controllable production of CNTs in large scale, and for conclusive characterizations of their properties in order to apply CNTs in high accuracy engineering. In this dissertation, horizontally well-aligned high quality single-walled carbon nanotubes (SWCNTs) have been successfully synthesized on St-cut quartz substrate by chemical vapor deposition (CVD). Effective radial moduli (Eradial) of these straight SWCNTs have been measured by using well-calibrated tapping mode and contact mode atomic force microscopy (AFM). It was found that the measured Eradial decreased from 57 to 9 GPa as the diameter of the SWCNTs increased from 0.92 to 1.91 nm. The experimental results were consistent with the recently reported theoretical simulation data. The method used in this mechanical property test can be easily applied to measure the mechanical properties of other low-dimension nanostructures, such as nanowires and nanodots. The characterized sample is also an ideal platform for electrochemical tests. The electrochemical activities of redox probes Fe(CN)63-/4-, Ru(NH3) 63+, Ru(bpy)32+ and protein cytochrome c have been studied on these pristine thin films by using aligned SWCNTs as working electrodes. A simple and high performance electrochemical sensor was fabricated. Flow sensing capability of the device has been tested for detecting neurotransmitter dopamine at physiological conditions with the presence of Bovine serum albumin. Good sensitivity, fast response, high stability and anti-fouling capability were observed. Therefore, the fabricated sensor showed great potential for sensing applications in complicated solution.^

Single-crystal micro/nanostructures and thin films of lamellar molybdenum oxide by solid-state pyrolysis of organometallic derivatives of a cyclotriphosphazene

The solid-state pyrolysis of organometallic derivatives of a cyclotriphosphazene is demonstrated to be a new, simple and versatile solid-state templating method for obtaining single-crystal micro- and nanocrystals of transition and valve metal oxides. The technique, when applied to Mo-containing organometallics N3P3[OC6H4CH2CN·Mo(CO)5]6 and N3P3[OC6H4CH2CN·Mo(CO)4 py]6, results in stand-alone and surface-deposited lamellar MoO3 single crystals, as determined by electron and atomic force microscopies and X-ray diffraction. The size and morphology of the resulting crystals can be tuned by the composition of the precursor. X-ray photoelectron and infrared spectroscopies indicate that the deposition of highly lamellar MoO3 directly on an oxidized (400 nm SiO2) surface or (100) single-crystal silicon surfaces yields a layered uniphasic single-crystal film formed by cluster diffusion on the surface during pyrolysis of the metal-carbonyl derivatives. For MoO3 in its layered form, this provides a new route to an important intercalation material for high energy density battery materials.

Growth and characterization of III-V compound semiconductor nanostructures by metalorganic chemical vapor deposition

Planar <110> GaAs nanowires and quantum dots grown by atmospheric MOCVD have been introduced to non-standard growth conditions such as incorporating Zn and growing them on free-standing suspended films and on 10° off-cut substrates. Zn doped nanowires exhibited periodic notching along the axis of the wire that is dependent on Zn/Ga gas phase molar ratios. Planar nanowires grown on suspended thin films give insight into the mobility of the seed particle and change in growth direction. Nanowires that were grown on the off-cut sample exhibit anti-parallel growth direction changes. Quantum dots are grown on suspended thin films and show preferential growth at certain temperatures. Envisioned nanowire applications include twin-plane superlattices, axial pn-junctions, nanowire lasers, and the modulation of nanowire growth direction against an impeding barrier and varying substrate conditions.

Fabrication and optimizing of metal nano silicate as toxic metal absorbent from sea water

Pure Water, is a crucial demand of creature life. Following industrial development, extra amount of toxic metals such as chromium enters the environmental cycle through the sewage, which is considered as a serious threat for organisms. One of the modern methods of filtration and removal of contaminants in water, is applying Nano-technology. According to specific property of silicate materials, in this article we try to survey increased power in composites and various absorption in several morphologies and also synthesis of Nano-metal silicates with different morphologies as absorbent of metal toxic ions. At first, we synthesize nano zink silicate with three morphologies considering context and the purpose of this survey. 1) Nano synthesis of zink silicate hollow cavity by hydrothermal method in mixed solvent system of ethanol/glycol polyethylene. 2) Zink nano wires silicate in a water-based system by controlling the amount of sodium silicate. 3) Synthesis of nano zink silicate membrane. After synthesizing, we measured the cadmium ion absorbance by synthesized nano zink silicates. Controlling PH, is the applied absorption method. Next step, we synthesized nano zink-magnesium silicate composite in two various morphologies of nanowires and membrane by different precent of zink and magnesium, in order to optimize synthesized nano metal silicate. We used zink nitrate and magnesium nitrate and also measured cadmium absorption by synthesized nano metal silicates in the same way of PH control absorption. In the 3rd step, in order to determine the impact of the type of metal in nano metal silicate, we synthesized nano magnesium silicate and compared its absorption with nano zink silicate. Furthermore, we calculated the optimal concentration in one of synthesizes. Optimal concentration is the process which has the maximum absorption. While applying two methods of absorption in the test, finally we compared the effect of absorption method on the absorption level. Below you find further steps of synthesis: 1) Using IR, RAMAN, XRD spectroscopy to check the accuracy of synthesis. 2) Checking the dispersion of nano particles in ethanol solution by light microscope. 3) Measuring and observing particles with scanning electron microscope (SEM). 4) Using atomic absorption device for measuring the cadmium concentration in water-based solutions. The nano metal silicates were synthesized successfully. All of synthesized nano absorbents have the cadmium ion absorbency. The cadmium absorption via nano absorbents depend on various factors such as kind of metal in nano silicate and percent of metal in nano metal silicate composite. Meanwhile the absorption and PH control of medium containing the absorbent and solution would affect the cadmium absorption.

Mesoscale Optoelectronic Design of Wire-Based Photovoltaic and Photoelectrochemical Devices

The overarching theme of this thesis is mesoscale optical and optoelectronic design of photovoltaic and photoelectrochemical devices. In a photovoltaic device, light absorption and charge carrier transport are coupled together on the mesoscale, and in a photoelectrochemical device, light absorption, charge carrier transport, catalysis, and solution species transport are all coupled together on the mesoscale. The work discussed herein demonstrates that simulation-based mesoscale optical and optoelectronic modeling can lead to detailed understanding of the operation and performance of these complex mesostructured devices, serve as a powerful tool for device optimization, and efficiently guide device design and experimental fabrication efforts. In-depth studies of two mesoscale wire-based device designs illustrate these principles—(i) an optoelectronic study of a tandem Si|WO3 microwire photoelectrochemical device, and (ii) an optical study of III-V nanowire arrays.

The study of the monolithic, tandem, Si|WO3 microwire photoelectrochemical device begins with development and validation of an optoelectronic model with experiment. This study capitalizes on synergy between experiment and simulation to demonstrate the model’s predictive power for extractable device voltage and light-limited current density. The developed model is then used to understand the limiting factors of the device and optimize its optoelectronic performance. The results of this work reveal that high fidelity modeling can facilitate unequivocal identification of limiting phenomena, such as parasitic absorption via excitation of a surface plasmon-polariton mode, and quick design optimization, achieving over a 300% enhancement in optoelectronic performance over a nominal design for this device architecture, which would be time-consuming and challenging to do via experiment.

The work on III-V nanowire arrays also starts as a collaboration of experiment and simulation aimed at gaining understanding of unprecedented, experimentally observed absorption enhancements in sparse arrays of vertically-oriented GaAs nanowires. To explain this resonant absorption in periodic arrays of high index semiconductor nanowires, a unified framework that combines a leaky waveguide theory perspective and that of photonic crystals supporting Bloch modes is developed in the context of silicon, using both analytic theory and electromagnetic simulations. This detailed theoretical understanding is then applied to a simulation-based optimization of light absorption in sparse arrays of GaAs nanowires. Near-unity absorption in sparse, 5% fill fraction arrays is demonstrated via tapering of nanowires and multiple wire radii in a single array. Finally, experimental efforts are presented towards fabrication of the optimized array geometries. A hybrid self-catalyzed and selective area MOCVD growth method is used to establish morphology control of GaP nanowire arrays. Similarly, morphology and pattern control of nanowires is demonstrated with ICP-RIE of InP. Optical characterization of the InP nanowire arrays gives proof of principle that tapering and multiple wire radii can lead to near-unity absorption in sparse arrays of InP nanowires.

Modeling and analysis of thermoelectric energy conversion efficiency in nanostructures.

116 p.

Mechanical and Electrical Properties of Single-walled Carbon Nanotubes Synthesized by Chemical Vapor Deposition

Despite the tremendous application potentials of carbon nanotubes (CNTs) proposed by researchers in the last two decades, efficient experimental techniques and methods are still in need for controllable production of CNTs in large scale, and for conclusive characterizations of their properties in order to apply CNTs in high accuracy engineering. In this dissertation, horizontally well-aligned high quality single-walled carbon nanotubes (SWCNTs) have been successfully synthesized on St-cut quartz substrate by chemical vapor deposition (CVD). Effective radial moduli (Eradial) of these straight SWCNTs have been measured by using well-calibrated tapping mode and contact mode atomic force microscopy (AFM). It was found that the measured Eradial decreased from 57 to 9 GPa as the diameter of the SWCNTs increased from 0.92 to 1.91 nm. The experimental results were consistent with the recently reported theoretical simulation data. The method used in this mechanical property test can be easily applied to measure the mechanical properties of other low-dimension nanostructures, such as nanowires and nanodots. The characterized sample is also an ideal platform for electrochemical tests. The electrochemical activities of redox probes Fe(CN)63-/4-, Ru(NH3)63+, Ru(bpy)32+ and protein cytochrome c have been studied on these pristine thin films by using aligned SWCNTs as working electrodes. A simple and high performance electrochemical sensor was fabricated. Flow sensing capability of the device has been tested for detecting neurotransmitter dopamine at physiological conditions with the presence of Bovine serum albumin. Good sensitivity, fast response, high stability and anti-fouling capability were observed. Therefore, the fabricated sensor showed great potential for sensing applications in complicated solution.

Absence of ferromagnetic order in high quality bulk Co-doped ZnO samples

Bulk Zn(1-x)Co(x)O samples were synthesized via standard solid-state reaction route with different Co molar concentrations up to 21%. A detailed microstructural analysis was carried out to investigate alternative sources of ferromagnetism, such as secondary phases and nanocrystals embedded in the bulk material. Conjugating different techniques we confirmed the Zn replacement by Co ions in the wurtzite ZnO structure, which retains, however, a high crystalline quality. No segregated secondary phases neither Co-rich nanocrystals were detected. Superconducting quantum interference device magnetometry demonstrates a paramagnetic Curie-Weiss behavior with antiferromagnetic interactions. We discuss the observed room temperature paramagnetism of our samples considering the current models for the magnetic properties of diluted magnetic semiconductors. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3459885]

Modification des propriétés optiques de nanofils à base de GaN par plasma N2/O2

Une sonde électrostatique de Langmuir cylindrique a été utilisée pour caractériser une post-décharge d’un plasma d’ondes de surface de N2-O2 par la mesure de la densité des ions et électrons ainsi que la température des électrons dérivée de la fonction de distribution en énergie des électrons (EEDF). Une densité maximale des électrons au centre de la early afterglow de l’ordre de 1013 m-3 a été déterminée, alors que celle-ci a chuté à 1011 m-3 au début de la late afterglow. Tout au long du profil de la post-décharge, une densité des ions supérieure à celle des électrons indique la présence d’un milieu non macroscopiquement neutre. La post-décharge est caractérisée par une EEDF quasi maxwellienne avec une température des électrons de 0.5±0.1 eV, alors qu’elle grimpe à 1.1 ±0.2 eV dans la early afterglow due à la contribution des collisions vibrationnelles-électroniques (V-E) particulièrement importantes. L’ajout d’O2 dans la décharge principale entraîne un rehaussement des espèces chargées et de la température des électrons suivi d’une chute avec l’augmentation de la concentration d’O2. Le changement de la composition électrique de la post-décharge par la création de NO+ au détriment des ions N2+ est à l’origine du phénomène. Le recours à cette post-décharge de N2 pour la modification des propriétés d’émission optique de nanofils purs de GaN et avec des inclusions d’InGaN a été étudié par photoluminescence (PL). Bien que l’émission provenant des nanofils de GaN et de la matrice de GaN recouvrant les inclusions diminue suite à la création de sites de recombinaison non radiatifs, celle provenant des inclusions d’InGaN augmente fortement. Des mesures de PL par excitation indiquent que cet effet n’est pas attribuable à un changement de l’absorption de la surface de GaN. Ceci suggère un recuit dynamique induit par la désexcitation des métastables de N2 suite à leur collision à la surface des nanofils et la possibilité de passiver les défauts de surface tels que des lacunes d’azote par l’action d’atomes de N2 réactifs provenant de la post-décharge. L’incorporation d’O2 induit les mêmes effets en plus d’un décalage vers le rouge de la bande d’émission des inclusions, suggérant l’action des espèces d’O2 au sein même des nanostructures.

Ion-sensing properties of 1D vanadium pentoxide nanostructures

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Study of ITO@PMMA composites by transmission electron microscopy

ITO nanowires were synthesized by carbothermal reduction process, using a co-evaporation method, and have controlled size, shape, and chemical composition. The electrical measurements of nanowires showed they have a resistance of about 102 Ω. In order to produce nanocomposites films, nanowires were dispersed in toluene using an ultrasonic cleaner, so the PMMA polymer was added, and the system was kept under agitation up to obtain a clear suspension. The PMMA polymer was filled with 1, 2, 5 and 10% in weight of nanowires, and the films were done by tape casting. The results showed that the electrical resistance of nanocomposites changed by over 7 orders of magnitude by increasing the amount of filler, and using 5 wt% of filler the composite resistance decreased from 1010 Ω to about 104 Ω, which means that percolation threshold of wires occurred at this concentration. This is an interesting result once for nanocomposites filled with ITO nanoparticles is necessary about 18% in weight to obtain percolation. The addition of filler up to 10 wt% decreased the resistance of the composite to 103 Ω, which is a value close to the resistance of wires. The composites were also analyzed by transmission electron microscopy (TEM), and the TEM results are in agreement with the electrical ones about percolation of nanowires. These results are promising once indicates that is possible to produce conductive and transparent in the visible range films by the addition of ITO nanowires in a polymeric matrix using a simple route. © 2011 Materials Research Society.

Síntese de nanofios de óxidos semicondutores para aplicações em dispositivos ópticos e eletrônicos

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)