Nanodiamond

People involved:
Vadym Mochalin (group leader)
Isabel Knoke
Ioannis Neitzel
Amanda Pentecost
Shruti Gour

Nanodiamond powder (ND) produced by detonation synthesis (http://en.wikipedia.org/wiki/ Detonation_nanodiamond) was first discovered in the Former Soviet Union in the 1960s and has an interesting and intricate history [1-3]. It remained essentially unknown to the Western World until the late 1980s when first successful synthesis of ND by detonation was carried out at Los Alamos National Laboratory in the U.S.A. [4]. Starting from the 1990s, ND becomes one of the hottest carbon nanomaterials due to its unique properties and a broad range of potential applications.

ND is composed of 5 nm diamond particles surrounded by amorphous and graphitic carbon bearing a large number of different functional groups on its surface (see molecular model of a nanodiamond particle). This structure provides many advantages over other carbon nanomaterials which usually feature a chemically inert bare graphitic surface (carbon nanotubes, graphene) or structureless mixture of different forms of carbon (amorphous and disordered carbon) with less controllable and less accessible external surface.

In order to get full advantage of the unique properties of ND, the material must be thoroughly characterized, purified and modified (functionalized) for each particular application (Fig. 1). Solving these three major problems constitutes main goals of our research.

Figure 1. A route to successful applications of nanodiamond.

Using a combination of UV/VIS Raman spectroscopy, XRD and XANES we were able to determine the content of diamond and non-diamond carbon phases in ND and to show that the products obtained from different manufacturers as well as different grades of ND produced by same manufacturer differ considerably in terms of content of diamond, graphitic and amorphous carbon (Fig. 2)

Figure 2. Different grades of nanodiamond with their corresponding diamond content as determined by XANES

FTIR analysis also shows differences in type and number of surface functional groups between NDs of different grades and different manufacturers. These differences are due to slight variations in synthesis (detonation) conditions and differences in subsequent purification schemes implemented by different manufacturers for different grades of ND. Thus, it is important to realize that ND from different sources may differ and while the material obtained from one manufacturer may show good performance in a particular application, the ND from another source (or different grade) may fail in the same application. ND is a material, not a substance; therefore it has no defined chemical formulae and must be fully characterized and purified before being used in any application.

Purification is aimed to remove non-diamond carbon and preserve the diamond phase. Current purification schemes utilize aggressive liquid oxidizers such as nitric, sulfuric acids, chromium anhydride, and hydrogen peroxide. Their use on a large commercial scale raises many environmental concerns. We explored a potential of air oxidation as an alternative purification technique [5]. By in situ monitoring of ND oxidation in air, we have found a narrow temperature window in which the non-diamond carbon can be efficiently oxidized to carbon oxides without any significant loss of the diamond phase. This observation provided a simple, ecologically benign purification technique utilizing air as the only reagent. The air oxidation is performed in a furnace at 425ºC and results in almost complete removal of non-diamond carbon (diamond content is increased from 12 - 85 % wt. in as-received powders to 92 - 97 % wt. in purified ND as shown in Fig. 2). Further treatment of the oxidized ND in aqueous HCl significantly reduces content of metals (such as Fe, Pb etc.) in the ND. As a side effect of the air oxidation, the variety of ND surface functional groups is converted into carboxyl and hydroxyl moieties. Purified ND with high content of diamond phase and known surface chemistry (Fig. 3) can be used "as is" or further subjected to surface modifications in order to better tailor it for particular applications.

Figure 3. High resolution TEM and optimized molecular models of as-received nanodiamond and nanodiamond purified by oxidation in air.

We have developed a set of surface modification techniques for ND including oxidation, graphitization, hydrogenation, chlorination, amination [6], and hydrophobization [7]. Performance of the modified NDs has been tested in different applications. For example, graphitized ND has been studied for batteries and supercapacitors [8]; oxidized and HCl purified nanodiamond can be well dispersed in polyacrilonitrile and polyamides and has been used to produce the ND-polymer nanofibers by electrospinning [9]; aminated ND has a great potential for biomedical and polymer composites applications. Hydrophobization of ND via covalent linking of octadecylamine (ODA) [7] results in a material which is absolutely immiscible with water and hydrophilic organic solvents but has high affinity toward the hydrophobic solvents such as benzene, toluene, chlorophorm etc. Thus, the ODA modified ND is considered as a material of choice for oil and fuel additives, polymer composites and other application where the ND must be dispersed in a hydrophobic environment. Surprisingly, suspensions of the ODA modified ND demonstrate bright blue fluorescence under UV light illumination [7], same as selected natural diamonds (Fig. 4). This feature can be used for in vivo biomedical imaging where the non-toxic fluorescent ODA modified nanodiamond could be advantageous compared to toxic semiconductor quantum dots which are now successfully used in vitro.

Figure 4. Suspensions of octadecylamine-modified nanodiamond (in the center) show bright blue fluorescence under UV light illumination similar to that of selected natural diamonds.

We also work on deaggregation of ND to produce single ND particle dispersion in aqueous and non-aqueous media; and on fundamental problems of spectroscopy of ND. In particular, we have found that functional groups either chemically attached to the surface of ND or present in molecules adsorbed onto the surface, strongly contribute to the Raman spectra of ND in the range 1600-1800 cm^-1 [10]. This contribution, if not accounted properly, significantly distorts the diamond / graphitic carbon ratio, that is commonly determined by Raman spectroscopy. Our study not only emphasized the importance of proper account of this contribution but allowed for discrimination of various previous hypotheses regarding the origin of the peaks in the range 1600-1800 cm^-1 in Raman spectrum of ND, and resulted in correct assignment of these peaks to bond vibrations of the functional groups on the surface of ND. Another study in Raman spectroscopy of ND being carried out in our group is aimed to refine and improve phonon confinement model, which relates the parameters of Raman peak to the size of the diamond crystal. The improved model will incorporate not only a single particle size but rather the particle size distribution, multiple phonon dispersion curves rather than a single averaged curve, and it will also take into account the presence of defects in nanodiamond, which are just ignored in current phonon confinement models. We believe that the improved phonon confinement model will serve both to a better understanding of the effects influencing the Raman spectrum of nanodiamond and to a practical purpose of measurement of diamond crystallite sizes at nanometer scale with subnanometer precision.

REFERENCES

1. Danilenko, V. V. Phys. Solid State 2004, 46, 595-599.

2. Shenderova, O. A.; McGuire, G. In Nanomaterials handbook; Gogotsi, Y., Ed.; CRC Taylor and Francis Group: Boca Raton London New York, 2006, p 203-237.

3. Ultrananocrystalline Diamond: Synthesis, Properties, and Applications; Shenderova, O. A., Gruen, D. M. Eds., William Andrew Publishing, 2006.

4. Greiner, N. R.; Phillips, D. S.; Johnson, J. D.; Volk, F. Nature 1988, 333, 440-442.

5. Osswald, S.; Yushin, G.; Mochalin, V.; Kucheyev, S. O.; Gogotsi, Y. J. Am. Chem. Soc. 2006, 128, 11635-11642.

6. Mochalin, V. N.; Osswald, S.; Portet, C.; Yushin, G.; Hobson, C.; Havel, M.; Gogotsi, Y. in Materials Research Society Fall Meeting, Boston, MA, USA, 2007; 1039, p 1039-P11-03.

7. Mochalin, V. N.; Gogotsi, Y. J. Am. Chem. Soc. 2009, 131, 4594-4595.

8. Portet, C.; Yushin, G.; Gogotsi, Y. Carbon 2007, 45, 2511-2518.

9. Behler, K. D.; Stravato, A.; Mochalin, V.; Korneva, G.; Yushin, G.; Gogotsi, Y. ACS Nano 2009, 3, 363-369.

10. Mochalin, V.; Osswald, S.; Gogotsi, Y. Chem. Mat. 2009, 21, 273-279.