Growth and physical-chemical properties of carbon nanotube arrays for energy conversion devices.

Abstract

Energy demand has been on the increase globally whilst there has been continuous depletion of energy sources. This has prompted investigative research towards sustainable energy through the synthesis of carbon nanotubes (CNTs) for energy storage and conversion devices. Multiwalled carbon nanotubes (MWCNTs) were synthesised using two methods, the thermal chemical vapour deposition (CVD) method and purpose built non-equilibrium plasma-enhanced chemical vapour deposition (PECVD) method. The synthesis temperatures were 850 and 200 °C for CVD and PECVD. In non-equilibrium PECVD the low temperatures used retained the properties of indium tin oxide (ITO) coated glass substrate. MWCNTs synthesis involved the use of either, commercially available ferrocene or synthesised metal nanoparticle catalysts such as iron (Fe), cobalt (Co), nickel (Ni), nickel ferrite (NiFe), nickel cobaltite (NiCo) and cobalt ferrite (CoFe). The metal nanoparticles (MNPs) were synthesised using the co-precipitation method in the presence of hexadecylamine (HDA) as a surfactant. The MNPs and the MWCNTs were characterised using transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and Raman spectroscopy. Growth, physical and chemical properties of the MWCNTs were studied. The synthesised MWCNTs were used as part of the electrode material in organic solar cells (OSC), where poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PDOT: PSS) was used as an electron transporter and poly-3-hexyl thiophene (P3HT) as an electron donor. The OSC performance was tested in a solar simulator. Mono-dispersed MNPs, in the diameter range 3-10 nm, were successfully synthesised. HDA was suitable as both surfactant and reducing agent and it aided the formation of mono-dispersed MNPs. EDX confirmed the presence of typical metals such as Fe, Ni and Co, and the oxygen peak which correlated with the FTIR characteristic metal oxide bonds. The presence of the carbon peak correlated with TGA which showed the decomposition profile of the organic coating. All the CVD methods produced MWCNTs with non-equilibrium PECVD producing vertically aligned MWCNTs directly on the substrate. In non-equilibrium PECVD, liquefied petroleum gas (LPG) and acetylene were successfully used to synthesise MWCNTs at low temperatures. Typical hollow tubular structures of MWCNTs were observed using TEM. These observations correlated morphology from SEM which showed “spaghetti like” structures in the case of thermal CVD and vertical tubular structures in the case of non-equilibrium PECVD. This correlated well with the thermal stability studies of the MWCNTs which showed the characteristic peak for MWCNTs. In addition, Raman spectroscopy showed typical MWCNTs bands, G-band and D-band due to graphitic carbon vibrations and defects respectively and the graphitic nature of the synthesised MWCNTs. The non-equilibrium PECVD, LPG synthesised, MWCNTs were tested in OSC. Positive results that showed dependency on the metal catalyst used during synthesis were observed. The Fe synthesised MWCNTs gave the highest efficiency, 0.116%, among the single metal catalysed MWCNTs followed by Co (0.012%) and Ni with 0.003%. CoFe synthesised MWCNTs also gave the best efficiency (0.009%) among mixed metal catalysed MWCNTs. Therefore, the synthesised MWCNTs gave positive results as part of the electrode material, however, with low efficiencies