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Carbon-based Nano-materials Studied By X-ray Absorption Spectroscopy

Posted on:2011-11-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Q LiangFull Text:PDF
GTID:1101360305966586Subject:Synchrotron radiation and its application
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Nano-materials researches represent a hot topic in both theoretical and experimental physics. Due to their unique structures extraordinary physical and chemical properties, carbon-based materials will make possible new applications in the field of composite materials, microelectronic devices, biomedicine, sensors etc. However, the route toward real applications needs the control of functionalization and of the electronic properties of carbon-based nano-materials and additional investigations are required to learn how modify and dope these materials.In this thesis, a systematic study on the modification and purification of carbon nanotubes (CNTs) as well as on the doping and redox of graphene has been performed by means of X-ray absorption fine structure (XANES) spectroscopy. It shows the unique capabilities of the XANES spectroscopy to characterize structures and bonding of carbon-based nano-materials.a) XANES spectroscopy has been applied to investigate the modification of CNTs by nitric acid and the production of carbonaceous fragments during the process.1. This local spectroscopy has been used to identify changes induced on single-walled carbon nanotubes (SWCNTs) treated by mild nitric acid. Experimental results confirm the occurrence of carbonaceous fragments absorbed on SWCNTs, and these fragments would reduce the direct modification of SWCNTs by nitric acid.XANES indicate the carboxyl groups created by the nitric acid treatment have been found on both carbonaceous fragments and the side-walls of SWCNTs. Moreover, these carbonaceous fragments can be removed by a washing treatment with sodium hydroxide. SWCNTs walls are weakly oxidized by the nitric acid treatment although, after the fragments removal a direct oxidation process of SWCNTs has been observed. Data address the removal of carbonaceous fragments on SWCNTs as an efficient method for side-wall modification of SWCNT. XANES also indicate that carbonaceous fragments are the result of the synthesis process and/or of the nitric acid treatment.2. A comparative XANES study of the modification of both SWCNTs and multi-walled carbon nanotubes (MWCNTs) via concentrate nitric acid has been performed. Data indicate that SWCNTs are easily destroyed by concentrate nitric acid treatment generating the carbonaceous fragments that contain the majority of carboxyl groups. However, due to their higher structural stability and thicker tube walls, high sidewall modification was observed on MWCNTs. An effective modification on SWCNTs by mild nitric acid treatment has been studied before. It was suggested that a proper selection of treatment conditions and CNTs type may effectively control the modifications of these materials.3. XANES spectroscopy was also used to investigate the removal of oxidative carbonaceous fragments absorbed on SWCNTs. Data show that the annealing treatment in an inert gas atmosphere may effectively remove these fragments hardly removed by water washing. Results point out an alternative procedure to separate the tightly adsorbed fragments from SWCNTs respect to the previously reported NaOH washing.b. XANES spectroscopy was applied to carefully investigate covalent and non-covalent modifications of CNTs.1) We investigated the modification of MWCNTs via gas nitric acid vs. temperature. XANES indicate that MWCNTs can be effectively modified by gas nitric acid and the amount of oxygen-functional groups can be controlled by the modulation of the temperature. If compared to the liquid nitric acid treatment this method is easy and modified samples do not need washing and filtration. This gas-phase route is also suitable for mass production.2) We studied also the adsorption of phenylalanine and glycine on SWCNTs and the adsorption of amino acids has been confirmed by XANES analysis. An observed energy shift of the C 1s to C=OÏ€* peak for glycine absorbed on SWCNTs has been identified and assigned to the interaction between the amino acid and SWCNTs.c. The electronic and local structures of graphene after N-doping and redox have been also studied by XANES spectroscopy.1) In this study we combined XANES and X-ray photoelectron spectroscopy (XPS) to investigate the structural changes of N-doping graphene at different annealing temperatures. N-doping graphene samples were obtained by annealing the graphene oxide in an ammonia atmosphere. N K-edge XANES spectra indicate three different N-doping structures in the graphene layer:pyridine, amino and graphitic type. At low temperature, the doped nitrogens are of amino type and would decompose increasing the temperature. At high temperature, most of the doped nitrogens replace carbon atoms in the graphene layer forming both pyridine and graphitic doping structure. N 1 s XPS analysis is in good agreement with XANES data. In addition,O K-edge XANES spectra reveal that many oxygen-containing functional groups occur on the graphene oxide layer and are greatly reduced with temperature.2) The XANES spectroscopy has been also applied to investigate both chemical bonding and electronic structure of the graphene oxide layer after hydrogen and hydrazine reduction. Results indicate that both hydrogen and hydrazine may effectively reduce the graphene oxide. Hydrogen would react only at high temperature while hydrazine easily reacts with the epoxy groups. Actually, some N atoms are introduced in the graphene layers during the hydrazine reduction.
Keywords/Search Tags:Carbon nanotube, Graphene, XANES, Nitric acid treatment, Carbonaceous fragment, NaOH washing, Annealing, Amino acids, N-doping, Reduction
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