Synthesis of Carbon Nanotube-Metal Oxides Composites; Adsorption and Photo-degradation

The unique chemical, physical and mechanical properties of carbon nanotubes (CNTs) have stimulated extensive investigation since their discovery in the early 1990s (Iijima, 1991). The primary research interests focus on the development of simple and cost effective methods for the synthesis of CNTs. To synthesize single wall carbon nanotubes (SWCNT) and multi wall carbon nanotubes (MWCNT), various techniques have been developed. Examples are laser ablation, arc discharge, and chemical vapor deposition (CVD). However, CVD is considered to be more productive method to produce CNTs with acceptable price. Therefore, the extraordinary properties of CNTs and the continuous decrease in their price make them potentially useful in a wide range of applications like in catalysis, photocatalysis and adsorption. The adsorption properties of CNTs depend on different factors such as their large specific surface area, highly porous and hollow structure, light mass density and strong physical interaction ability. Recently, the applicability of CNTs as adsorbent has been extensively investigated. The applications of CNTs for the removal of pollutants from aqueous solutions have been studied. Numerous experimental studies have already been carried out on the adsorption of heavy metals and hazardous organic materials (Li, et al., 2002; Li, et al., 2003; Chen, et al., 2006; Hyung and Kim, 2008; Goering, et al., 2008). For further enhancement the adsorption ability of CNTs, purification and activation or functionalization of CNTs is essential and can be performed via oxidation treatment (Cho, et al., 2010; Shao, et al., 2010). Purification is necessary to remove the impurities, like catalyst particles, soot, amorphous carbon and other forms, which are often found, mixed with CNTs. Impurities can reduce the adsorption by their coating on the surfaces of CNTs, and as a result influencing the adsorption on the external surface of the nanotubes (Agnihotri et al., 2006). The enhancement in the adsorption ability of CNTs via oxidation treatment with oxidizing agent, such as nitric acid, hydrogen peroxide, potassium permanganate, or a mixture of nitric acid and sulfuric acid, is most probably ascribed due to an increase in hydrophilicity of CNTs. Oxidation treatment of CNTs results in creating new functional groups on the surface of nanotubes.