Even though the solubility of carbon nanoforms (CNFs) in common solvents is very low, the dominant methods for transforming them have involved the use of traditional chemical techniques such as refluxing and sonication in the presence of a large amount of inorganic acids or organic solvents over long time. But if these structures are not soluble what happens if we don’t use solvents for their modification? Are solvents really needed? Solvent-free reactions have many benefits: reduced pollution, low costs and simplicity in process and handling. These factors are especially important for scalable production. The MSOC Nanochemistry Groupgroup has used non-conventional techniques such as microwaves and mechanochemical treatments for the modification of CNFs. Our research goes from the preparation of graphene to the functionalization of different carbon nanostructures by covalent and non-covalent approaches. Moreover, our group is also focussing in the development of new stimuli-responsive materials, more concretely in smart hybrid hydrogels. This awesome materials can be applied in a wide range of fields such as controlled drug delivery or tissue engineering, among others.
Ball milling treatments
Another stimulating possibility is the mechanochemical approach where chemical bonds are activated by the presence of an external mechanical force. Mechanochemical treatments have shown to be very useful for the preparation graphene. We have reported the easy and practical exfoliation of graphite by a new method that avoids structural changes of graphene. Large quantities of inexpensive materials like graphite and melamine can be used for massive and fast production of few layer graphenes with a low concentration of defects. The methodology opens the way for an alternative and efficient processing of graphene materials, such as film deposition and chemical functionalization.
But the mechanochemical approach is also useful in CNF modifications. For some applications, such as gas storage or medicine, the length of as-synthesized carbon nanotubes (CNTs) is a drawback. So far, one of the dominant methods to cut CNTs involves acid treatments, hindering the control of the damage on the CNT surface. Recently, we have shown that solid phase techniques together with mechanical activation by milling processes can be used to prepare high-quality, short CNTs. In addition, pristine CNTs can be functionalized and cut at the same time using a reactive nitrene derivative during the milling/cutting process. Finally, the purification/cutting can be easily scaled up to obtain large amounts of purified and relatively homogeneous short/functionalized carbon nanotubes, avoid the use of complicated techniques and toxic processes or harmful to the environment.
It has been shown that carbon nanostructures display strong microwave absorbing properties and this behavior has been used for their purification and functionalization. In the absence of solvents, CNFs absorb directly the radiation and it is possible to take full advantage of the strong microwave absorption typical of these structures. In this way, very high temperatures are obtain in just seconds providing extremely time-efficiently reactions but also making possible new transformations. The methodology has been applied to produce multifunctionalized carbon nanotubes and carbon nanohorns, using a combination of two different addition reactions, the 1,3-dipolar cycloaddition of azomethine ylides and the addition of diazonium salts, both via a simple, fast and environmentally friendly method. And also for the covalent modification of graphenes Different derivatives have been prepared with applications in materials and biomedical chemistry.