Síntese de resinas ligno-fenol-formaldeído para aplicação em painéis de média densidade.

bitstream/item/219766/1/TS2020-010-dis-MEPA.pdfDissertação (Mestrado em Química) - Universidade Federal do Ceará, Centro de Ciências, Fortaleza. Coorientador: Renato Carrhá Leitã

Polymer-Coated Mesoporous Carbon as Enzyme Platform for Oxidation of Bisphenol A in Organic Solvents

Bisphenol A (BPA) is not only a widely used chemical but also a toxic pollutant, and its biodegradation in an aqueous environment is hard due to its near insolubility in water. While the enzyme tyrosinase can oxidize BPA in organic solvents, it does so only very slowly. In the present study, we have found that in toluene the catalytic activity of tyrosinase deposited onto coated mesoporous carbon is significantly enhanced when the support is precoated with polyethylenimine. The resultant enzymatically formed o-quinone is both easily recoverable and potentially useful monomer. As a particular example, the o-quinone readily reacts with diamine in toluene to form poly­(amino-quinone) polymers, which are suitable for anticorrosion, energy storage, or biosensor applications

Growth Mechanism of Long and Horizontally Aligned Carbon Nanotubes by Chemical Vapor Deposition

The selective production of long, horizontally aligned carbon nanotubes (>1 mm) or short, randomly oriented carbon nanotubes (<50 μm) was achieved in a chemical vapor deposition process by influencing the catalyst pretreatment and reaction conditions. A detailed investigation was undertaken to elucidate the mechanism yielding the two different morphologies. It was found that the duration of the catalytic growth of a nanotube plays a vital role; that is, the actual growth period of long nanotubes is significantly higher (up to 15 min or more) compared to short nanotubes (10 s or less). Alignment with the gas flow occurs only when a nanotube reaches a critical length, which suggests that short growth durations limit not only the length of CNTs but also their alignment with the gas flow. Furthermore, it is concluded that differences in the nanoparticle's catalytic lifetime is the most probable factor determining the extension of growth duration and lengths to obtain long, horizontally aligned CNTs. This work represents a step forward toward the integration of CNTs in electronic applications

In-Situ Sample Rotation as a Tool to Understand Chemical Vapor Deposition Growth of Long Aligned Carbon Nanotubes

A new tool for studying the process of carbon nanotube chemical vapor deposition (CVD) synthesis is described. By rotating the substrate in situ during the CVD process, the orientation of floating nanotubes with respect to the substrate is changed by interaction with the gas stream. Nanotubes lying on the surface of the substrate, however, will maintain their relative orientation. As a result different nanotube alignment angles are observed. By defining a time window through multiple rotation steps it is possible to study carbon nanotube behavior during CVD growth in a time-resolved manner. As an example, the settling process (i.e., the sinking of the nanotube to the substrate) is investigated. The analysis of forces acting on a floating nanotube shows that a vertical gas stream due to thermal buoyancy over the sample can keep long nanotubes floating for extended times. A stochastic process, indicated by a constant settling rate over time, forces the nanotube to make contact with the substrate, and this process is attributed to flow induced instability. Additional information on the floating and settling process are revealed from our study. The settling velocity could be inferred from curved nanotubes. The clearance between a floating nanotube and the substrate was found to be several hunded micrometers

Photoswitching in Azafullerene Encapsulated Single-Walled Carbon Nanotube FET Devices

Photoswitching in Azafullerene Encapsulated Single-Walled Carbon Nanotube FET Device

Role of Kinetic Factors in Chemical Vapor Deposition Synthesis of Uniform Large Area Graphene Using Copper Catalyst

In this article, the role of kinetics, in particular, the pressure of the reaction chamber in the chemical vapor deposition (CVD) synthesis of graphene using low carbon solid solubility catalysts (Cu), on both the large area thickness uniformity and the defect density are presented. Although the thermodynamics of the synthesis system remains the same, based on whether the process is performed at atmospheric pressure (AP), low pressure (LP) (0.1−1 Torr) or under ultrahigh vacuum (UHV) conditions, the kinetics of the growth phenomenon are different, leading to a variation in the uniformity of the resulting graphene growth over large areas (wafer scale). The kinetic models for APCVD and LPCVD are discussed, thereby providing insight for understanding the differences between APCVD vs LPCVD/UHVCVD graphene syntheses. Interestingly, graphene syntheses using a Cu catalyst in APCVD processes at higher methane concentrations revealed that the growth is not self-limiting, which is in contrast to previous observations for the LPCVD case. Additionally, nanoribbons and nanostrips with widths ranging from 20 to 100 nm were also observed on the APCVD grown graphene. Interactions between graphene nanofeatures (edges, folds) and the contaminant metal nanoparticles from the Cu etchant were observed, suggesting that these samples could potentially be employed to investigate the chemical reactivity of single molecules, DNA, and nanoparticles with monolayer graphene

Successful Computational Modeling of Isobornyl Chloride Ion-Pair Mechanisms

Along with the directly related Wagner−Meerwein camphene hydrochloride−isobornyl chloride rearrangement, the racemization of isobornyl chloride involves intermediate carbocation−anion ion pairs; both processes have become mechanistic icons in organic chemistry. The two known racemization pathways, involving either a hydride transfer or a methyl migration, are observed to be concurrent. However, prior quantitative computational modeling has not been able to reproduce the fine kinetic balance of these processes. We demonstrate that a density functional approach, which includes two explicit solvent molecules embedded in a continuum solvent field, coupled with full geometric optimization using smoothed solvent cavities and free energy calculation, yields results in accord with experiment. Alternative racemization routes also have been explored

Mechanism of Halogen Exchange in ATRP

Detailed mechanistic studies reveal that halogen exchange (HE) in ATRP can occur not only by a radical pathway (atom transfer) but also by an ionic pathway (SN2 reaction) because Cu(I)(L)X and Cu(II)(L)X2 complexes contain weakly associated halide anion that can participate in the SN2 reaction with alkyl halide (ATRP initiator). Both pathways were kinetically studied, and their contributions to the HE process were quantitatively evaluated for seven alkyl halides and three Cu(I)(L)Cl complexes. Radical pathway dominates the HE process for 3° and 2° alkyl bromides with more active complexes such as Cu(I)(TPMA)Cl. Interestingly, ionic pathway dominates for 1° alkyl bromides and less active ATRP catalysts. These studies also revealed that degree of association of alkyl halide anion depends on the structure of copper complexes. In addition, radical pathway is accompanied by the reverse reactions such as deactivation of radicals to alkyl bromides and also activation of alkyl chlorides, reducing the efficiency of halogen exchange

Electrical Control of Chemical Vapor Deposition of Graphene

Chemical vapor deposition (CVD) is widely used for the efficient growth of low-dimensional materials. The growth mechanism comprises mass and heat transport, gas-phase and surface chemical reactions, and the interaction between the product and the substrate/catalyst. Correspondingly, the controllable parameter space is conventionally focused on the mass flow of each component, the temperature of the reaction chamber and the substrate, and the material and structure of the substrate/catalyst. Here, we report that applying an electric field between the copper substrate and a counter electrode has significant impacts on the growth of graphene. Electrochemical effect and ionic collision effect are observed in different conditions. With the assistance of negative and positive voltages applied on the growth substrate, selective growth and rapid growth of clean graphene films are achieved, respectively. We anticipate such electrical control will open up new ways to assist the synthesis of two-dimensional (2D) materials

Graphene As Transparent Conducting Electrodes in Organic Photovoltaics: Studies in Graphene Morphology, Hole Transporting Layers, and Counter Electrodes

In this work, organic photovoltaics (OPV) with graphene electrodes are constructed where the effect of graphene morphology, hole transporting layers (HTL), and counter electrodes are presented. Instead of the conventional poly­(3,4-ethylenedioxythiophene)/poly­(styrenesulfonate) PEDOT:PSS HTL, an alternative transition metal oxide HTL (molybdenum oxide (MoO3)) is investigated to address the issue of surface immiscibility between graphene and PEDOT:PSS. Graphene films considered here are synthesized via low-pressure chemical vapor deposition (LPCVD) using a copper catalyst and experimental issues concerning the transfer of synthesized graphene onto the substrates of OPV are discussed. The morphology of the graphene electrode and HTL wettability on the graphene surface are shown to play important roles in the successful integration of graphene films into the OPV devices. The effect of various cathodes on the device performance is also studied. These factors (i.e., suitable HTL, graphene surface morphology and residues, and the choice of well-matching counter electrodes) will provide better understanding in utilizing graphene films as transparent conducting electrodes in future solar cell applications