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High Surface Area Graphitized Mesoporous Carbons

By: W. Betz1, D. Shollenberger1, M. Keeler1, M. Buchanan1, L. Sidisky1  and K. Patel2*,
1 Supelco, 595 N Harrison Road, Bellefonte, PA 16823, USA.
2 Sigma-Aldrich Materials Science, 6000 N Teutonia, Milwaukee, WI 53210, USA

Introduction

Carbon black (CB) has been recognized and utilized since the 1960s both as a pigment and as a reinforcing material for rubber materials. It has also gained a reputation as an effective solid support for chromatographic separation applications.1 Recently, graphitized mesoporous carbons (GMCs), have been identified as promising materials for use in a wide range of applications such as electrode materials in aqueous supercapacitors.2 The structural features of GMCs include structural homogeneity with significant graphite-like domains and stacking heights, low amounts of surface functional groups, and low occurrences of imperfections such as twists, non-aromatic links, and carbon valencies.3 The GMC surface subsequently results in a narrow range of interaction energies with predictable chromatographic retention characteristics, in addition to Gaussian peak elution shapes and increased adsorbate mass balances.

In combination with the high surface area, the enhanced conductivities of GMCs have also been harnessed for their use in high performance components in electrochemical applications. Examples include electrodes for double layer capacitors,4 and binding materials. Their well-ordered pore structure as well as their uniform pore size also make them attractive as catalyst supports.5, 6

The efficient adsorption and desorption properties of GMCs have also allowed for their use in sample preparation applications, specifically where trace-level quantification of parts per trillion are required. Packed tubes filled with GMCs have been utilized7 for several decades to determine the concentration of adsorbates from air and liquid matrices. Desorption for subsequent analytical quantification is accomplished thermally by gas phase extraction or by use of a strong organic solvent for liquid phase extraction.

Variation in the particle size has yielded many GMCs that are suited for a wide range of sampling technologies and applications. Nanometer-sized GMCs for trace-level sample preparation applications have evolved to meet the needs of smaller sample preparation devices. Typically, coatings on substrates are configured for an appropriate micro sampling device.

In addition to their use as high surface area electrode materials and in small-scale separations applications, have also shown promise for specific adsorption and desorption of larger biomolecules in liquid sample prep applications. A 40µm GMC has been utilized in the micro-sample preparation of high molecular weight biomolecules such as oligosaccharides,8 glycopeptides and glycoproteins.9 

Structural Properties

We have found that the nanostructure of the GMC (Product No. 699624) possess a pentagonal configuration with an approximate diameter of 35 nm. These primary nano-particles form aggregates in the 175 nm size range. Further agglomeration of the material leads to particle agglomerates in the 400 nm range (see Figures 1 and 2).


Figure 1: TEM image of Graphitized Mesoporous Carbon
Figure 1. TEM image of Graphitized Mesoporous Carbon

Figure 2: TEM image of Purified Mesoporous Carbon
Figure 2. TEM image of Purified Mesoporous Carbon

 

This higher-ordered GMC provides for effective adsorption capacities and effective conductivity. Graphitization of the purified mesoporous carbon (PMC) intermediate (Product No. 699632) provides a higher degree of crystallinity (see Table 1), loss of surface imperfections, and increase in surface homogeneity.

Table 1. Physical Characteristics GMCs and PMCs

    Surface Area Total Pore Volume Average Pore Diameter Particle Size Distribution Absolute Density Crystallinity
  Cat. No. (m2/gram) (cm3/gram) (angstroms) (microns) (g/cm3) (% graphitic:turbostratic)
175 nm purified mesoporous carbon 699632  >200 0.324  64  0.175  1.887   1:99
330 nm graphitized mesoporous carbon 699624  70.0  0.240  137  0.330  1.8280 30:70 

 

As shown in the DFT plot (Figure 3), the pore structure also changes only a little as the degree of crystallinity increases. Therefore the introduction of graphitic structure to the nanocarbon framework does not significantly alter the micro and mesoporosity and as a result the pore volume is relatively maintained.

Figure 3. Density Functional Theory Plot for Graphitized Mesoporous Carbon (699624) and Purified Mesoporous Carbon (699632) (Pore Width Plotted Against Incremental Pore Volume)

Figure 3. Density Functional Theory Plot for Graphitized Mesoporous Carbon (699624) and Purified Mesoporous Carbon (699632).
 

The surface area of this GMC is a function of internal mesoporosity and macroporosity, but also dictated by the external surface area of the primary particles. The d-spacing of a turbostratic carbon at 0.344 nm results in an increase in surface area relative to a graphitic carbon with a d-spacing value of 0.335 nm. Thus, a 30:70 ratio of graphitic:turbostratic carbon plays a role in defining the nitrogen BET surface area of the GMC (see Figure 4 for d-spacing illustration). Typically, the graphitic regime of the carbon resides at the surface, and the percentage of graphite layering results in a depth of graphitization relative to the particle size. This ordered graphitic carbon layering is responsible for the significant increase in conductivity of this carbon type.

d-spacing of Graphitic versus Turbostratic carbon

Figure 4. d-spacing of Graphitic versus Turbostratic carbon
 

The pore structure of the GMC is illustrated in Figure 5. This increased pore structure, with a peak apex at 100Å allows for efficient, selective adsorption of polar molecules from biological samples in the femtomole to micromole concentration range. These molecules include sugars, oligosaccharides, glycopeptides and glycoproteins.

DFT PLOT of 40 µm Graphitized Mesoporous Carbons

Figure 5. DFT PLOT of 40 µm Graphitized Mesoporous Carbons.

Conclusion

The use of nano-sized GMCs for trace-level sample preparation applications has evolved to meet the needs of smaller sample preparation devices, typically as coatings on substrates configured for an appropriate micro-device.
 
The up-regulated adsorption/desorption properties of GMCs (i.e., adsorbate mass balances) have also allowed for the use of GMCs in sample preparation applications. These nanoporous entities have also become promising carbon materials for applications requiring enhanced electrical conductivity such as electrode materials for supercapacitors and binding material components.  

The inert, ordered surface of GMC has been demonstrated to concentrate and analyze large, polar biomolecules such as sugars, oligosaccharides, glycopeptides and glycoproteins from extremely dilute solutions (down to femtomolar concentrations).

Applications of PMCs and GMCs are rapidly growing on account of their stable, well-defined frameworks, whose features can be dialed in during the production process, in order to create a whole host of GMC materials.

Table 2. A series of carbon black powders

Product Description Product
No.
Carbon Nanopowder, Mesoporous, >99.95% trace metal basis • 64 A average pore diameter.
• >200 m2/g specific surface area.
699632
Carbon Nanopowder, Mesoporous, Graphitized, >99.95% trace metal basis • 137 A average pore diameter
• 70 m2/g specific surface area.
699624
Carbon, Mesoporous, >99.95% trace metal basis • 100 A average pore diameter
• >200 m2/g specific surface area.
• Particle size – 45 µm ± 5
699640

 

 References

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  5. Kim, H.; Kim, P.; Joo, J. B.; Kim, W.; Song, I. K.; Yi, J. J. Power Sources 2005, 157, 196-200.
  6. Zeng, J.; Su, F.; Lee, J. Y. ; Zhao, X. S. ; Chen, J.; Jiang, X. J. Mater. Sci. 2007, 42, 7191-7197.
  7. Betz, W. R.;.Supina, W. R. Pure Appl. Chem. 1989, 61, 2047-2050.
  8. Koizumi, K.; Okada, Y.; Fukuda, M. Carbohydr. Res. 1991, 215, 67–80.
  9. Barroso, B.; Dijkstra, R.; Geerts, M.; Lagerwerf, F.; van Veelen, P.; de Ru, A. Rapid Commun. MassSpectrom. 2002, 16, 13201329.