Episódios de contaminação envolvendo hidrocarbonetos de petróleo são relatados com bastante freqüência, principalmente em função dos acidentes envolvendo transporte e estocagem de combustíveis, dentre os quais se destacam o benzeno, tolueno, etilbenzeno e xileno (BTEX). Assim, não é surpreendente o grande número de trabalhos, atualmente disponíveis, relacionados à remediação de águas subterrâneas. Entretanto, é conhecido que a total mineralização de benzeno, na maioria das tecnologias utilizadas para remediação de solos e águas subterrâneas, não ocorre totalmente podendo formar compostos fenólicos altamente tóxicos. Por este motivo, este trabalho teve por finalidade a análise dos subprodutos formados a partir da degradação do benzeno, que são eles: hidroquinona, resorcinol, catecol, fenol, p-benzoquinona. Para este propósito, os compostos fenólicos formados foram medidos utilizando um eletrodo de carbono vítreo modificado com filme de nanotubos de carbono e ftalocianinas metalicas. Os compostos fenólicos também foram analisados por cromatografia liquida de alta eficiência com detector espectrofotométrico, espectrometria no UV/Visível. Os sensores eletroquímicos propostos mostraram-se altamente eletrocatalíticos e sensíveis na determinação dos derivados da oxidação do benzeno, obtendo-se um limite de detecção de 4,54 μmol L-1 para a hidroquinona, de 1,63 μmol L-1 para o resorcinol, 0,14 μmol L-1 para o catecol, 4,19 μmol L-1 para o fenol e 1,78 μmol L-1 para a p-benzoquinona. Observa-se que existem algumas diferenças nos limites de detecção para cada composto fenólico estudado e que o eletrodo GC/MWCNT/CoPc apresentou menores limites de detecção para o catecol, resorcinol e p-benzoquinona podendo analisar predominantemente esses compostos fenólicos a partir da eletro-oxidação do benzeno. A metodologia proposta comparada com os métodos oficiais de análise e foi observado que os sensores atingem os limites mínimos necessários de detecção, demonstrando que sensores eletroquímicos baseados em CNTs tornam-se uma alternativa no desenvolvimento de metodologias altamente sensíveis, rápidas e de baixo custo. / Contamination episodes involving petroleum hydrocarbons are reported quite frequently, mainly because of accidents involving transportation and storage of fuels, among which stand out as benzene, toluene, ethylbenzene and xylene (BTEX). Thus, it is not surprising the large number of papers currently available relating to remediation of groundwater. However, it is known that the complete mineralization of benzene, most of the technologies used for remediation of soil and groundwater, it doesn\'t occur and it can form highly toxic phenolic compounds. For this reason, this paper aims to analyze the products formed from the degradation of benzene, which are: hydroquinone, resorcinol, catechol, phenol, p-benzoquinone. For this purpose, the phenolic compounds formed were measured using a glassy carbon electrode modified with a film of carbon nanotubes and metallic phthalocyanines. Phenolic compounds were also analyzed by high performance liquid chromatography with UV detection, spectroscopy in UV / Visible. The proposed electrochemical sensors were highly sensitive and the electrocatalytic determination of the oxidation of benzene derivatives, obtaining a detection limit of 4.54 μmol L-1 for hydroquinone, 1.63 μmol L-1 for resorcinol , 0.14 μmol L-1 for catechol, 4.19 μmol L-1 for phenol and 1.78 μmol L-1 for p-benzoquinone. It is observed that there are some differences in detection limits for each phenolic compound studied and the electrode GC/MWCNT/CoPc had lower detection limits for catechol, resorcinol and p-benzoquinone can analyze these phenolic compounds predominantly from the electro- oxidation of benzene. The proposed methodology was compared with the official methods of analysis and it was observed that the sensors reach the required minimum limits of detection, demonstrating that electrochemical sensors based on CNTs become an alternative for the development of high sensitivity, rapid and low-cost. methodologies.

Infamous for “Mad hatter syndrome” and “Minamata disease”, mercury (Hg) is ranked high on the Agency for Toxic Substances and Disease Registry's priority list of hazardous substances for its potent neurologic, renal, and developmental toxicities. Most typical exposures are via contaminated water and food. Although regulations and advisories are exercised at various levels, Hg pollution from both natural and anthropogenic sources has remained a major public health and safety concern. Rapid detection of solvated aqueous Hg2+ ions at low levels is critical for immediate response and protection of those who are vulnerable (young children, pregnant and breast-feeding women) to acute and chronic exposures to Hg2+. Various types of sensors capable of detecting Hg in water have been developed. In particular, the novel use of engineered carbon nanotubes (CNTs) has garnered attention due to their specificity and sensitivity towards Hg2+ detection in solution. In this focused review, we describe the sensitivity, selectivity and mechanisms of Hg2+ ion sensing at trace levels by employing CNT-based various sensor designs, and appraise the open literature on the currently applied and “proof-of-concept” methods. Five different types of CNT-based sensor systems are described: potentiometric, DNA-based fluorescence, surface plasmon resonance (SPR), colorimetric, and stripping voltammetric assays. In addition, the recognized merits and shortcomings for each type of electrochemical sensors are discussed. The knowledge from this succinct review shall guide the development of the next generation CNT-based biochemical sensors for rapid Hg2+ detection in the environment, which is a significant first step towards human health risk analysis of this legacy toxicant.

archives@tulane.edu / This dissertation utilizes molecular theory and simulations to study thermodynamics of ions in electrolyte solutions of practical interest. The first half of this work focuses on two important electrochemical energy storage systems: Lithium ion batteries and supercapacitors based on carbon nanotube (CNT) forests. In lithium ion batteries, the characteristics of Li+ transport are studied in the solid electrolyte interphase of batteries. This study has potential applications in the design and theoretical testing of novel fast-charging batteries. The work on CNT supercapacitor focuses on the dependence of capacitance on pore spacing and electrode potentials. In the second half, the hydration of halides (fluoride and chloride) are studied using Quasi-chemical theory (QCT). Here, refinements in the implementation of QCT are pursued, leading to free energies that are in excellent agreement with experiments. This advancement should be helpful to address issues such as Hofmeister effects and selectivity in ion channels. / 1 / Ajay Muralidharan

Carbon nanotubes (CNT) are a new form of carbon with exceptional electrical and mechanical properties. This makes them attractive as inclusions in nanocomposite materials with the potential to provide improvements in electrical and mechanical properties and allows for the creation of a new range of multifunctional materials. In this study single wall carbon nanotubes (SWCNT) were dispersed in polystyrene using a solution mixing method, with the aid of a surfactant. A good dispersion was achieved and the resulting nanocomposites were characterized for electrical conductivity and mechanical properties by 3 point flexural and fracture toughness tests. Results show a significant improvement in electrical properties with electrical percolation occurring between 0.1 and 0.2 wt%. A minor improvement was observed in the flexural modulus but the strength and fracture toughness values in the nanocomposites decreased relative to the neat material. Scanning electron microscopy (SEM) was performed to characterize the morphology and fracture surface of the specimens. The results of testing and microscopy show that the presence of the nanotubes has an adverse effect on the crazing mechanism in Polystyrene (PS) resulting in a deterioration of the mechanical properties that depend on this mechanism.

Modeling of single-walled carbon nanotubes, multi-walled nanotubes and nanotube reinforced polymer composites using both the Finite Element method and the Molecular Dynamic simulation technique is presented. Nanotubes subjected to mechanical loading have been analyzed. Elastic moduli and thermal coefficient of expansion are calculated and their variation with diameter and length is investigated. In particular, the nanotubes are modeled using 3D elastic beam finite elements with six degrees of freedom at each node. The difficulty in modeling multi walled nanotubes is the van der Waal's forces between adjacent layers which are geometrically non linear in nature. These forces are modeled using truss elements. The nanotube-polymer interface in a nano-composite is modeled on a similar basis. While performing the molecular dynamic simulations, the geometric optimization is performed initially to obtain the minimized configuration and then the desired temperature is attained by rescaling the velocities of carbon atoms in the nanotube. Results show that the Young's modulus increases with tube diameter in molecular mechanics whereas decreases in molecular dynamics since the inter-atomic potential due to chemical reactions between the atoms is taken into consideration in molecular dynamics unlike in molecular mechanics.

Carbon nanotubes (CNTs) have great potential in many applications due to their unique structure and properties. However, there are still many unsolved problems hampering their real applications. This thesis focuses on three important issues limiting their applications, namely: (1) direct growth of CNTs without additional catalyst, (2) secondary growth of carbon nanotubes on primary CNT bed without using extra catalyst, (3) and CNT alignment mechanisms during the growth.<p> The CNTs used in this thesis were prepared by hot filament chemical vapor deposition (CVD) reactor and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Field electron emission (FEE) properties of the CNTs were also tested.<p> Oxidation-reduction method was adopted in direct growth of CNTs on Inconel 600 plates and proved effective. The effect of oxidation temperature on the growth of CNTs was studied. It was found that the oxidation temperature had an influence on CNT height uniformity and FEE properties: the higher the treatment temperature, the more uniform the resultant CNTs, and the better the FEE properties of the resultant CNTs. The contribution of different oxides formed at different temperatures were investigated to explain the effect of oxidation temperature on the CNT height uniformity.<p> Secondary CNTs were grown on primary ones by simply changing the carbon concentration. No additional catalyst was used during the whole deposition process. It was found that synthesizing primary CNTs at extremely low carbon concentration is key factor for the secondary growth without additional catalyst. The CNT sample grown with secondary nanotubes exhibited improved field emission properties.<p> The effect of bias voltage on growth of vertically aligned carbon nanotubes was investigated. The CNTs grown at -500V shows the best alignment. At the early growth stage, simultaneous growth of randomly oriented and aligned carbon nanotubes was observed. This was consistent with the alignment mechanism involving stress that imposed on catalyst particles on tube tips. Through the observation of CNT growth on the scratched substrates, catalyst particle size was found as another determining factor in the alignment of CNTs. Big catalyst particles promoted aligned growth of CNTs.

No description available.

Single-walled carbon nanotubes (SWCNTs) are promising for use in solar cells and photodetectors because of their strong optical absorption in most of the solar spectrum. There have been many reports about the photovoltaic effect in nanoelectronic devices based on individual SWCNTs, but they have been limited by complicated fabrication and miniscule absorption. There has been a growing trend for merging SWCNTs into micro-and macroscopic devices to provide more practical applications. Here we report the photoresponse of macroscopic SWCNT films with a p-n junction at room temperature. Photovoltage (PV) and photocurrent (PC) due to the photothermoelectric (PTE) effect were observed at the junction, and they were larger by one order of magnitude as compared with their values at the metal-SWCNT interfaces. Various factors affecting PV amplitude and response time have been studied, including junction length, substrate, and doping level. The maximal responsivity we observed was 1V/W with samples on Teflon tape, while a fast response time 80 S was observed with samples on AlN substrates. Hence an optimal combination of photoresponse time and amplitude can be found by proper choice of substrate. It was found that PV increased nonlinearly with increase in n-doping concentration, indicating the existence of an optimal doping level. This result also suggests the possibility to further improve photoresponse by changing p-doping level. Finally, we checked the photoresponse in wide wavelength range (360-900 nm), and PV was observed throughout, indicating that the device could potentially be used as a broadband photodetector.

Carbon nanotubes (CNTs) have great potential in many applications due to their unique structure and properties. However, there are still many unsolved problems hampering their real applications. This thesis focuses on three important issues limiting their applications, namely: (1) direct growth of CNTs without additional catalyst, (2) secondary growth of carbon nanotubes on primary CNT bed without using extra catalyst, (3) and CNT alignment mechanisms during the growth.<p> The CNTs used in this thesis were prepared by hot filament chemical vapor deposition (CVD) reactor and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Field electron emission (FEE) properties of the CNTs were also tested.<p> Oxidation-reduction method was adopted in direct growth of CNTs on Inconel 600 plates and proved effective. The effect of oxidation temperature on the growth of CNTs was studied. It was found that the oxidation temperature had an influence on CNT height uniformity and FEE properties: the higher the treatment temperature, the more uniform the resultant CNTs, and the better the FEE properties of the resultant CNTs. The contribution of different oxides formed at different temperatures were investigated to explain the effect of oxidation temperature on the CNT height uniformity.<p> Secondary CNTs were grown on primary ones by simply changing the carbon concentration. No additional catalyst was used during the whole deposition process. It was found that synthesizing primary CNTs at extremely low carbon concentration is key factor for the secondary growth without additional catalyst. The CNT sample grown with secondary nanotubes exhibited improved field emission properties.<p> The effect of bias voltage on growth of vertically aligned carbon nanotubes was investigated. The CNTs grown at -500V shows the best alignment. At the early growth stage, simultaneous growth of randomly oriented and aligned carbon nanotubes was observed. This was consistent with the alignment mechanism involving stress that imposed on catalyst particles on tube tips. Through the observation of CNT growth on the scratched substrates, catalyst particle size was found as another determining factor in the alignment of CNTs. Big catalyst particles promoted aligned growth of CNTs.

Due to entropy and Van der Waals¡¦ interaction, carbon nanotubes tend to aggregate degrading their excellent opto-electronic properties and limiting their applications. Chemical derivatizations were applied to the multi-wall carbon nanotube (MWCNT) by esterificating with different lengths of aliphatic pendants (COOC4H9, COOC10H21, and COOC18H37) to decrease the MWCNT aspect ratio to facilitate its dispersion, and to observe its percolation behavior. FTIR analysis revealed the more relevant absorption peaks of C-H at 2917 cm-1, 2846 cm-1 and C=O at 1733 cm-1 from the derivatization. H1-NMR showed that the aliphatic pendant functionalized MWCNT from the signals of OCH2 at £_ = 3.64 ppm, CH2 at £_ = 1.25 ppm, and CH3 at £_ = 0.88 ppm. Raman scattering indicated that esterification caused the ID/IG absorption peak area ratio to decrease. In applications, the electric conductivity was measured on thin-films of MWCNT:Poly-(3-hexylthiophene) (P3HT) as a function of nanotube content. Accompanied with nanotube doping concentration increased, the electric conductivity parallel to film surface (£m||) could range from an undoped value 1.4¡Ñ10-6 S/cm up to 1.2¡Ñ10-2 S/cm. The conductivity percolation threshold concentration decreased as the MWCNT aspect ratio increased due to the average distance between the nanotubes becoming sufficiently small for charges to hopping through P3HT. By incorporating [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), bulk heterojunction photovoltaic (PV) cells of ITO/PEDOT:PSS/MWCNT:[PC61BM:P3HT]/LiF/Al were fabricated. By varying the ratio of MWCNT to the PC61BM:P3HT (0.8:1) mixtures, the PV cells showed the maximum power conversion efficiency (£bp) close to 4 % with MWCNT-COOC4H9 at a doping concentration of 0.01 wt. %.