Treatments and Analysis of C-sp(2) nanomaterials

Resumen

Carbon can be found or synthesised in different arrangements, each one with its particular properties. Curiously, the main building block of the majority of these structures (graphene) was first isolated in 2004 by A.K Geim and K. Novoselov. Using graphene layers as a base is possible to derive almost all the other carbon structures. Bucky balls, carbon nanotubes and nanofibers or graphite, all these are examples of different configurations of graphene layers. The properties and applications of each nanomaterial vary depending of this configuration. For example, carbon nanotubes/nanofibers aspect ratio are ideal for create polymer composites using low charges, obtaining new materials with increased mechanical, electrical or thermal properties. On the other side, graphene may be very useful for the fabrication of protective coatings or transparent electrodes. Carbon nanofibers are an interesting nanostructure for using in gas sensing applications. It is known that pure carbon sp2 surfaces are chemically inert because the nature of the bonding. This is the reason that pure carbon nanotubes need a surface treatment for generate defects over its surface in order to create bonding sites for achieving the efficient adsorption of the environmental molecules. On the other hand, the carbon nanofibers have more sp3 bonds exposed due its natural structures allowing an easier natural adsorption of molecules. Moreover, this property allows to the nanofibers being stable in different polar solvents making easier its manipulation. In this work two types of stacked-up carbon nanofibers were used: the bare carbon nanofibers (CNF) and the graphitized carbon nanofibers (CNFG), that are nanofibers treated with a high temperature process in order to eliminate all its impurities and obtain a rearrangement of the crystal structure. The critical role of the surface of the nanofiber in its sensing characteristics justifies the study of the surface chemical properties. In order to do so, the nanofibers were studied using the X-ray Photoelectron Spectroscopy (XPS) technique in order to obtain information of the chemical composition of its surface. It has been found that the graphitication process eliminates all the impurities of the surface, leaving only carbon and oxygen. Oxygen is a surprising finding, as although the amount is low (around 1%), this means that despite the graphitization process there are some functional groups that can survive the treatments although the high temperatures and the reducing atmosphere. Basically, seems that the surviving functional groups are single bonded oxygen and hydroxyl groups, meanwhile the carbonyl groups are the ones that suffers a higher degree of reduction. In order to fabricate the sensor is necessary to find a way to manipulate the carbon nanofibers. The easiest way is using a solvent as a carrier, but the treated CNFG aggregates rapidly in polar solvents. XPS measurements show that the surface becomes more ordered (increase of the sp2 bonds). A close inspection using Transmission Electron Microscopy (TEM) shows that the edges of the carbon nanofibers close itself during the graphitization process, exposing mainly the sp2 bonds thus making the surface less reactive. It has been found that applying a thermal treatment in a oxygen atmosphere reopen these edges without increasing the surface oxygen functional groups, making the surface more reactive and allowing the formation of stable solutions of carbon nanofibers in polar solvents in concentrations as high as 1 mg/ml. With these solutions was possible to fabricate a gas sensor using the carbon nanofibers as a sensing layer. As the objective is to fabricate a flexible gas sensor, the interdigitated electrodes were ink-jet printed using Ag ink over a kapton substrate. Then, using a modified electrospray method the carbon nanofibers were deposited over the interdigitated electrodes, forming the sensing layer. In addition of increase the reactivity and stability in solution of the nanofibers, the possibility of decorate its surface with noble metal nanoparticles is studied. The direct mixing of precursor salts (AuCl3 and PdCl2) is used, taking advantage of a ball milling process in order to increase the wetting of the nanofiber surface by the metal salts. After the impregnation of the surface, the product is annealed in an oxygen atmosphere for decomposing the salt and obtaining the metal nanoparticles. Depending the salt used it was found different behaviour on the formation of the clusters with the same thermal treatment. For the Pd case, for low PdCl2 concentrations results in no visible clusters albeit the XPS measurements show the presence of Pd in the surface. This means that its surface is covered by Pd atoms that did not reach the critical surface concentration for coalesce. Increasing the concentration of PdCl2 promotes the growing of Pd nanoparticles that with greater diameter with higher concentrations of salt. For the case of AuCl3 the behaviour is more or less the contrary. For low salt concentrations disperse relatively big clusters of Au can be seen. If the salt concentration is increased, the size of the clusters decrease meanwhile its number increase. Finally, at a critical salt concentration (around 50% respect the carbon nanofiber concentration) the surface of the nanofibers is covered by 1nm Au nanoparticles. The response of the different nanofibers (bare, Au decorated and Pd decorated) to humidity, NH3 and NO2 is studied. It has been found that the bare carbon nanofibers responses quite well to humidity even at room temperature. In addition, is possible to detect NH3 and NO2 gases in a 50% RH ambient at room temperature with good responses and recovery times, although some poisoning is detected. The decoration with metal nanoparticles modifies the response, in particular for NH3 the Au enhance the response meanwhile Pd decreases it, meanwhile for NO2 the two types of cluster decreases the response. The properties of the sensor (response time, recovery time,…) can be enhanced by applying heat in order to clean the surface and increase the adsorption and desorption rates. It was found that the best operating temperature is around 110ºC in wich there is a compromise between the best response time and lower recovery time. Graphene is one of the promising materials that could be applied in a wide range of applications. Because its planar structure and transparency is very adequate for the fabrication of transparent electrodes, although also is planned to be used for example in transistors, supercapacitors, protective coatings. One of the main issues is its synthesis and manipulation at industrial level. Although the great efforts invested still there no is a standardized method to synthesise and coat wide surfaces with good quality graphene. One of the possibilities is the chemical route, where carbon layers are separated by a strong oxidation and separated by sonication. Then the resulting graphene oxide (GO) is reduced for obtain reduced graphene oxide (rGO), a material similar to graphene. All these treatments need to be monitored in order to obtain the best material possible and if is it possible to be compatible in an industrial process. Raman spectroscopy meets with these requirements, as is a fast technique and easy to use. Moreover, is non-destructible, opening the possibility to use this characterization technique in an industrial process directly over the final devices. In this study is proposed the use of the rarely used D’’ peak to analyse the GO and its reduction products in a thermal reduction process. We found that the D’’ can be used as a marker of the reduction state of the product as its position depend on it. Moreover, is demonstrated that with the use of the D’’ peak the other peaks (D, G, and D’) follows the relations presented for different authors for disordered or activated carbons, meaning that the D’’ also helps to obtain the real disorder degree of the sample.