Reactivity of Nanodiamond

In comparison to bulk diamond, nanodiamond particles are distinctly more reactive. This may be explained by the larger number of defects and by a markedly enlarged surface. Both effects increase the number of potential sites for the attack of a reagent, thus facihtating chemical modifications of nanodiamond particles. These include not only a functionalization of the surface, but also a conversion into other forms of carbon as discussed in Section 5.5.3. Due to the defective structure and to the presence of small graphitic domains on the particle surface, these transformations as well proceed much easier here than with macroscopic diamond particles. 

As mentioned in Section 5.2 on the structure of nanodiamonds, they possess certain, but not very large number of unsaturated bonds owing to the saturation of free valencies by n-bond formation. The residual radical centers are normally surrounded by sp -structures, so eventual reagents caimot access them freely. Hence this approach to surface functionalization does not bear the desired results. 

The sp -hybridized domains on the surface exist in parts as bent, condensed aromatic structures, and in parts as isolated double bonds. Therefore, a possible strategy might comprise the application of typical reachons of olefins or aromatic compounds like, for instance, the Diels-Alder reachon or other cycloadditions, the alkylation, or the halogenation of aromatic compounds. However, there is a certain drawback to this approach usually it is only van der Waals forces connecting the graphitic structures situated on the surface to the particle s actual core. Hence, the functionality is only attached to a kind of shell, and stability problems may arise, especially if the respective material is to be employed in mechanically demanding applications. 

A suitably conducted thermal treatment, for instance, removes not only adsorbates, but also functional groups. At sufficient temperatures (usually 800 °C) in vacuo, the surface looses its functionalization, and a graphitization of the nanodiamond s outermost shell occurs. However, a thermal treatment still increases agglomeration, so a functionalization of single primary particles cannot be achieved in this manner so far. 

Apart from the thermal homogenization, various chemical methods may serve as well to modifying the groups on the surface. Basically they can be classified to be reductive or oxidative reactions. In both cases the aim is to cover the particle surface as uniformly as possible with just one kind of functional group, so homogeneous products may be obtained upon further reaction. 

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