Carbon based nanomaterials (CNMs) has drawn plenty of attention recently because of their outstanding mechanical and electrical properties. For instance, carbon nanotubes (CNTs), carbon nano fibers (CNFs) and graphene are ideal raw materials for various applications due to their excellent mechanical characteristics, including high tensile strength and high elastic modulus, as well as high thermal and electrical conductivities.
Thus, research for practical applications of CNMs has been actively pursued. In particular, metal composites incorporating these nanomaterials show promise as new materials offering improved and unique functionality. For instance, metal-CNF composites are expected to have high strength and high thermal conductivity. However, some defects and voids can be detected inside of the composites because of CNFs' poor wettability with molten metals such as tin alloy, aluminum and silver, which is also a common problem existing in all kinds of CNMs. Therefore, surface modification of CNMs is necessary before combining with a metal alloy.
Applying a metal coating on the surface is one of the most effective methods to improve their wettability with molten metals. Metal coating or deposition can also impart various beneficial properties to CNMs, such as high mean density, good solubility in various solvents and desirable magnetic and catalytic properties. A high mean density of metal coated CNMs can prevent them from being removed as impurities by solder flux after reflow. Homogeneous metal coating layers could also act as barriers to prevent nanomaterial from agglomeration thereby making it easier to obtain well-dispersed CNM-suspensions. CNMs coated with ferromagnetic metals can be controllably moved by magnetic fields, which may lead to CNM composites with highly controlled micro-textures. CNMs deposited with catalytic metal particles may provide superior catalysts, with applications such as improved fuel cell electrodes.
Therefore, a uniformly dispersed metal coating layer on the surface of CNMs becomes critical for the achievement of the aforementioned attractive properties. However, the inert nature of CNMs also leads to a weak interaction with the ambient, thus offering a challenge in terms of surface functionalization (including the uniform deposition of metal nanoparticles). Metallic coating processes on CNMs are known, but the normal sensitization process, like simply using Tin (II) ions as the sensitizer to pre-treat the CNMs, does not work very well because of the strong surface inertia of CNMs, and the traditional nitric acid treatment is unable to form enough activated points as the reaction between carbon and nitric acid cannot be precisely controlled. All these factors will have negative effects on the size distribution and uniformity of deposited metal nanoparticles.