Development of A New Methodology for Both Qualitative and Quantitative Surface Characterization of Carbon Nanomaterials

Abstract

This thesis describes a titration methodology based on a universal titration equation for the surface characterization of carbon nanomaterials and their applications. Carbon nanomaterials are expected to become a next-generation functional material due to their extraordinary properties. Functionalization is one of the key procedures leading to viable applications of carbon nanomaterials as this process generates desired functional groups on their surfaces. These functional groups adjust the surface properties of carbon nanomaterials to enhance their dispersibility, adsorption properties, and reactivity levels, thereby potentially broadening the areas in which they may be used. With regard to these procedures, the precise elucidation of surface functional groups is significant for the proper utilization of functionalized carbon nanomaterials. Among the various characterization techniques which have been developed thus far, the titration method has been widely adopted due to its simple operating principles and for the useful information it provides. Titration methods of carbonaceous materials are largely categorized into indirect and direct methods. While direct titration provides information on the population of the acidic groups in specific pKa ranges expressed in terms of the pKa distribution function, indirect titration simply provides the concentrations of practical functional groups which are directly applicable regarding the use of carbon materials. Hence, indirect titration has been widely adopted for the surface characterization of various carbonaceous materials. However, for the easy adoption of indirect titration to carbon nanomaterials, complicated and inaccessible procedures compared to direct titration and sophisticated issues originating from the specific properties of carbon nanomaterials should be overcome. Therefore, the development of a titration method with a combination of the aforementioned advantages, i.e., the convenience of direct titration and the practical information of indirect titration, is crucial. The aim of the present study is to develop a titration methodology which utilizes the principles of the direct and indirect titration methods for the straightforward determination of the surface properties, and applications, of carbon nanomaterials. Part I describes the general concept and definition of acidities of functionalized carbon nanomaterials. This is followed by an introduction to the basics of conventional titration methods. The contribution of the present study stems from theoretical considerations of the drawbacks of conventional titration methods and state-of-the-art works. In Part II, a universal titration equation for the development of a titration methodology is theoretically derived and its validity is experimentally demonstrated. The derived equation is adapted to indirect titration conditions in which simple acidic molecules such as acidic carbon compounds (ACCs) are formed during the functionalization of carbon nanotubes, or where atmospheric carbon dioxide (CO2) is involved in the standardization of readily adoptable indirect titration methods. The effects of ACCs and CO2 are clearly elucidated on the basis of a universal titration equation. More importantly, this critical revisit of indirect titration shows that the conventional CO2-removal process is completely unnecessary. This makes indirect titration simpler and more accessible, with high precision in the results as well. Part III develops a one-pot titration methodology by altering the conventional indirect and direct titration methods. In this method, the pKa distribution functions of nitric acid-oxidized carbon nanotubes (CNTs) from direct titration are reconstructed into the concentrations of practical functional groups obtainable from indirect titration. The one-pot titration results were fairly comparable to the well-established indirect titration results, implying that the titration methodology developed in this study is universally applicable. Part IV applies the titration methodology for an analysis of the dispersion behaviors of mixed acid-oxidized CNTs in neutral water. It is shown here that highly carboxylated ACCs on CNTs are easily ionized in neutral water and that they play a crucial role in the high-quality stable aqueous dispersion of CNTs. In addition, the mechanism of graphene oxide cross-linking aided by diamine ion bridges is proposed based on the titration methodology for the effective fabrication of GO fibers. These investigations and analyses of practical functional groups clearly show that the developed titration methodology is applicable for the actual utilization of carbon nanomaterials.