Nanodiamonds functionalization

The ability to synthesize carbon nanostmctures, such as fullerenes, carbon nanotubes, nanodiamond, and mesoporous carbon functionalize their surface or assemble them into three-dimensional networks has opened new avenues for material design. Carbon nanostructures possess tunable optical, electrical, or mechanical properties, making them ideal candidates for numerous applications ranging from composite structures and chemical sensors to electronic devices and medical implants. 

Mochalin V, Osswald S, Gogotsi Y (2009) Contribution of functional groups to the Raman spectrum of nanodiamond. Chem Mater 21(2) 273-279... 

Mochalin VN, Osswald S, Portet C, Yushin G et al (2008) High temperature functionalization and surface modification of nanodiamond powders. Mater Res Soc Symp Proc 1039 1039-P11-03... 

However, the particles small dimensions give rise to one of the characteristic structural features of nanodiamond The large portion of surface atoms causes strain within the particles that shows in an altered bonding situation close to the surface. In NMR-examinations, for instance, the carbon atoms on the surface exhibit another chemical shift than those situated in the core of the particle. This is due to the attachment of functional groups or due to sp -hybridized atoms arising from a reconstruction of the surface (Section 5.2.2). 

In some cases, the oxidative purification is followed by a thermal treatment that is, heating the sample to ca. 700 C in an atmosphere of argon. It helps annealing surface defects and, partially, causes a thermal removal of functional groups from the surface, too. However, it also leads to a further graphitization of the surface. Hence careful consideration is required whether graphitized or functionalized nanodiamond is more advantageous for a certain experiment or application. 

A very informative picture of the respective nanodiamond s structure can be obtained from summarizing the observed signals (diamond peak, G-band, D-band, T-band) and the bands of functional groups. In doing so, however, one must be aware that for particles bearing unordered graphitic material in their shell, the... 

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. 

As discussed in Section 5.2.2, nanodiamond particles produced by detonation or shock wave synthesis exhibit a primary surface functionalization right from the preparation. It comprises a multitude of different groups, so it should be possible to make use of these functional groups. To ensure a reproducible quality of the secondary products obtained, however, a homogenization of the primary functionalities is required. There are several strategies to achieve this (Figure 5.29). 

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. 

Upon treating nanodiamond with alkoxy silanes, a condensation occurs between one or more alkoxy functions of the silane and the hydroxyl groups situated on the diamond surface. The resulting compounds feature a C-O-Si-bonding of the organic residues to the diamond surface (Figure 5.35). 

In many applications of functionalized nanodiamond, a very tight attachment of the functional groups to the surface is required. For silanized nanodiamond this is given only up to a point. Especially in an acidic environment, the C-O-Si-bonding is easily hydrolyzed and the starting material is recovered. Hence sUan-ized nanodiamond can only be employed at operating conditions that ensure the stabihty of the C-O-Si-bond. 

A modification of functional groups already attached to the nanodiamond surface is of considerable interest for the development of new diamond materials for biomedical or mechanical applications. 

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