By low-dimensional carbon nanomaterials we refer to those with the dimension less than or equal to two and size in nanometer scale.
This thesis focuses on two types of such low dimensional carbon nanomaterials, namely carbon nanotubes, which are usually considered as one-dimensional, and higher fullerenes and their metal adducts, which can be regarded as zero-dimensional.
Chapter 1 outlines the general features of sp
2 carbon materials. The main techniques employed in this study are briefly introduced. The main objectives of this thesis are summarized.
Chapter 2 to 6 present the systematic study of carbon nanotubes (CNT). Chapter 2 gives the study of CNT by Raman scattering. We found that both singlewall and multiwall nanotubes display the similar Raman features as expected from their common curved tubular structure. The Raman spectra of multiwall nanotubes can be attributed to the average Raman signals of the inner and outer tubules, indicating that the symmetries of individual graphene layers govern their vibration properties. A new dispersion phenomenon, an observation not predicted by theory, has been observed in the Raman spectra of both singlewall and multiwall nanotubes with respect to that of graphite.
The splitting of the G band, only appearing in the Raman spectra of singlewall nanotubes before, has been observed in the Raman spectra of multiwall nanotubes using polymer-filament-guiding method. The present study reveals that the major factors affecting the observation of the splitting of the G band are (a) the diameter of nanotube including the inner and outer layers, (b) the assembly sample consisting of various nanotubes, (c) the impurity such as nanoparticles or graphitic sheets in the arc-discharged product. The detailed Raman scattering study indicates that there exist at least three types of carbon nanotubes in our sample.
Chapter 3 presents a comparison study of various sp
2 carbon materials. The dependence of the Raman features on the microscopic structure has been found. The physical meaning of 2G and 2D' bands has been further elaborated in the present study: they are associated with graphene (the single graphitic layer) and graphite structures, respectively. The relative intensities of both bands can be used to judge quality of three dimensional stacking of graphitic layers. The curved carbons, which occur in both nanotubes and nanoparticles, have their characteristic vibration bands which can be used to distinguish them from other carbon materials.
Chapter 4 studies the electrochemical behavior of nanotubes in concentrated perchloric acid. The electrochemical behavior of CNT is different from that of graphite. The major reaction in graphite is intercalation, whereas the oxidation is a major reaction in nanotubes. The difference is directly related to their structure. The tips of nanotubes can also be opened by electrochemical corrosion.
Chapter 5 summarizes our observation of nanotubes at various crystallization stages. Three types of carbon nanotubes, the well-graphitized, poorly-graphitized and non-graphitized, have been observed in the same arc-discharged product. Non-graphitized nanotubes are made of amorphous carbon with plastic-like behavior and are suggested to be the protoform of the well-graphitized nanotubes. A four-step model is proposed to account for the growth mechanism of the graphitized multiwall nanotubes.
Chapter 6 presents our exploration of the chemical applications of CNT. Nanofiber-like LiCoO
2 is synthesized at a low temperature (400 ℃, in air) by using multiwalled carbon nanotube as the template. The templating process is accomplished in two possible manners: the coating and the filling processes, respectively. The straight fiber-like LiCoO
2 can be observed in the sample with the phase structure of LT-LiCoO
2, whereas the distorted fiber-like LiCoO
2 appears in the sample with the phase structure of HT-LiCoO
2. Both XRD and Raman spectroscopy provide evidences for the gradual phase transition from LT-LiCoO
2 to HT-LiCoO
2. Furthermore, Raman spectra indicate that there exist two different environments around the octahedral Co
3+O
6 in LT-LiCoO
2 sample, and only one unique environment around the octahedral Co
3+O
6 in HT-LiCoO
2 sample.
Chapters 7 and 8 present the study of higher fullerenes and their adducts with rare-earth metals by means of electrochemical methods. The electrochemical properties of three higher fullerenes C
86, C
90 and C
92 are summarized in chapter 7. The first and the second reduction processes of C
92 are found to be overlapped, which is significantly different from electrochemical behaviors of other higher fullerenes. The reduction potentials of the fullerenes, from C
70 to C
90, are correlated to their sizes (the number of carbon atoms).
Chapter 8 summarizes the study of seven 4f-block metallofullerenes M@C
82, where M=Pr, Nd, Tb, Dy, Ho, Er and Lu, by CV and DPV. The first oxidation and the first reduction potentials of all seven M@C
82 were found to locate within a close vicinity to each other, suggesting that all the entrapped metals adopt a similar valence state, presumably a trivalent cation. Despite the proximity of the first oxidation and reduction potentials among different M@C
82, the subtle but visible variation caused by the different metals does exist. The plots of the first oxidation and reduction potentials of M@C
82 against the electronic configurations of the entrapped metal ion display a very interesting inverse-W shape which can be rationalized assuming the existence of a strong interaction between the entrapped metal cations and the unpaired electron on the carbon cage anion. A LMCT-like process is proposed to account for the nature of the interaction. The redox potentials of M@C
82 are well correlated to such physical properties of 4f lanthanide elements as their third ionization energy and ionic radii. Two isomers of Nd@C
82 were observed, one of which is dynamically stable while the other is thermodynamically stable.
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