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1.3 PHYSICAL SCIENCES
As prepared SWNTs are obtained in bundles with a mixture of both metallic and semi conducting tubes. For many specific applications, electronically separated single individual tubes are required and in order to reach these application criteria, the tubes should be processed. This thesis reports on a systematic exploration of methods to routinely process, electronically separate and characterise commercially available single walled carbon nanotubes (SWNTs). Commercially available HiPco SWNTs were dispersed in water with the assistance of 1 % by weight sodium dodecyl benzene sulphate (SDBS). The tubes were dispersed in the water surfactant system through the aid of sonication and centrifugation. The concentration dependent properties were studied in order to establish the aggregation state of the sample. The starting concentration was 5 mg/ml and this was sequentially diluted by a factor of two down to 1.2 x 10-3 mg/ml. UV/Vis/NIR absorption, Raman spectroscopy and atomic force microscopy (AFM) were performed. Using the Lambert Beer law the critical debundling point (CDP) was found to be 0.07±0.03 mg/ml and the extinction coefficient at 600 nm was found to be 215 mL mg-1 m-1 for low concentrations (individual tubes) and 99.5 mL mg-1 m-1 for higher concentrations (bundles). Once establishing the CDP, chiral indices of the tubes were determined by curve fitting the UV/Vis/NIR absorption spectrum and the Radial Breathing Modes (RBM) of the Raman spectrum. Though the chiral indices of the tubes were obtained from two different methods, the values converged enabling UV/Vis/NIR and Raman spectroscopy to be used for routine characterisation. A quantitative analysis of doping of individual tubes at the CDP was conducted by varying the pH from 1 to 13 and monitoring the Raman spectrum. A drastic change in RBM of the Raman spectrum was observed. Mixed Gaussian and Lorentzian line shapes were fitted, and the doping behaviour was modelled using a simple protonation model. The protonation rate was found to relate to the optical band gap and the number of protons acting per SWNT was also calculated. It was concluded that metallic tubes can be more readily protonated than their semi conducting counter parts. Functionalization of the pristine tubes (both HiPco and arc discharge) was carried out in an attempt to separate the nanotubes according to electronic character. This functionalization method resulted in significant damage to the SWNT and the separation process was deemed not suitable for routine processing. One of the most straight forward techniques and an alternative to functionalization method is based on microwave treatment, whereby exposing the tubes to radiation preferentially destroys the metallic SWNT due to their high dielectric constant, leaving behind the semi conducting tubes. Microwave treated SWNT were dispersed in water surfactant solution and were studied using Raman spectroscopy both at 514 nm and 633 nm excitation wavelength. The ratio of the G-/G+ mode and the RBM ratios were plotted against the exposure time. Curve fitting was performed to obtain the rates of the reaction and the optimum irradiation conditions were established. A batch of SWNTs treated under these conditions was produced and solutions in water surfactant were prepared and concentration dependent studies were conducted enabling enriched semi conducting tubes to be dispersed at a certain concentration. These studies thus enabled a clear characterisation of the Raman and UV/Vis/NIR absorption characteristics of semi conducting bundles and the isolated tubes as well as the debundling process.
Rao, Priya Baskar. 92009). Enhanced absorption metal oxides for photocatalytic applications. Technological University Dublin. doi:10.21427/D76W2S