Process of Purifying Nanodiamond Compositions Developed by Drexel University Nanomaterials Group

Ultrananocrystalline diamond ("nanodiamond") is a unique nanomaterial which can be easily produced in hundred of kilograms by detonation synthesis. Unfortunately there are no detonation synthesis techniques known at the present time which would give pure nanodiamond product ready for commercial applications. Diamond-bearing soot is obtained as a result of detonation synthesis which consists of nanodiamond particles (for example, having an average diameter of about 5 nm) contaminated by different kinds of non-diamond carbon as well as metals and metal oxides particles coming from the material of the detonation chamber. Accordingly, purification of the nanodiamond detonation product is needed. The purification stage is considered to be the most complicated and expensive stage in producing nanodiamonds.

The presence of large amounts of non-diamond carbon in detonation synthesized nanodiamond (ND) severely limits applications of this exciting nanomaterial.  Drexel University nanomaterials researchers developed an environmentally-friendly process to selectively remove sp²-bonded carbon from ND. The content of up to 96% of sp³-bonded carbon in the oxidized samples is comparable to that found in microcrystalline diamond and is unprecedented for ND powders, say Drexel University Materials Science Engineering Professor Yury Gogotsi, with Vadym Mochalin, Sebastian Osswald and Gleb Yushin in U.S. Patent Application 20100028675.

The researchers demonstrated the use of ambient air for the oxidative purification of diamond-bearing detonation soot, eliminating the need for any additional oxidizers, catalysts or inhibitors. Moreover, the presented techniques are also capable of significantly improving the quality of nanodiamond samples which underwent prior acid purification treatments without appreciable loss of the diamond phase. These results open avenues for numerous new applications of nanodiamond.

Transmission electron microscopy and Fourier transform infrared spectroscopy studies show high purity 5-nm ND particles covered by oxygen-containing surface functional groups. The surface functionalization can be controlled by subsequent treatments. In contrast to current purification techniques, the disclosed process does not require the use of toxic or aggressive chemicals, catalysts or inhibitors and opens avenues for numerous new applications of nanodiamond.  

FIG. 5 provides HRTEM images of (a, b) UD50 and (c, d) UD90 before and after oxidation for 5 h at 425.degree. C. in air; (e, f) HyperChem molecular models of ND (e) before and (f) after oxidation;

FIG. 6 depicts low resolution TEM image of nanodiamond particles of the oxidized UD90 on a lacey carbon film.

FIG. 7 depicts optical images of UD50, UD90, and UD98 before and after oxidation for 5 h at 425.degree. C. in air;

FIG. 8 depicts Table 1 illustrating carbon nanostructures and listing selected physical properties of the investigated samples

Methods of decreasing the degree of aggregation in a nanodiamond composition are also provided which provide a nanodiamond aggregate composition with an average diameter in the range of from about 10 nm to about 100,000 nm.  The aggregates are comprised of nanodiamond particles with an average diameter in the range of from about 1 nm to about 30 nm.   By heating the nanodiamond aggregate composition in the presence of gaseous molecular oxygen to a temperature in the range of from about 375 degree C  to about 630 degree C to give rise to a reduction in the average diameter of the aggregates. ND aggregates are suitably characterized using any type of particle sizing instrument, such as laser light scattering.

The degree of aggregation of NDs can also be controlled. Methods of increasing the degree of aggregation in a nanodiamond composition include: heating a nanodiamond composition in the presence of gaseous molecular oxygen to a temperature in the range of from about 375.degree. C. to about 630.degree. C. to give rise to production of oxygen-containing bridge structures of the type nanodiarnond-C(.dbd.O)O-nanodiamond, nanodiamond-O-nanodiamond, nanodiarnond-C(.dbd.O)-nanodiamond, or any combination thereof.  The bridge structures give rise to nanodiamond aggregates having an average diameter in the range of from about 10 nm to about 100,000 nm, the aggregates comprising a plurality of nanodiamond particles having an average diameter in the range of from about 1 nm to about 30 nm.  

Research in the Drexel Nanomaterials Group (NMG) is focused on the fundamental and applied aspects of synthesis and characterization of carbon nanomaterials (nanotubes, nanodiamond and nanoporous carbons), ceramic nanoparticles (whiskers, nanowires, etc) and composites. NMG develops and studies new nanomaterials and works closely with industry with the goal to significantly decrease the time from discovery to commercial application of new materials.