Aqueous Surface Chemistry of Diamond Grains

Lunar and Planetary Science XXIX 1961.pdf

AQUEOUS SURFACE CHEMISTRY OF DIAMOND GRAINS. A. W. Phelps, University of Dayton Research Institute, 300 College Park, Dayton, OH, 45469-0130, [email protected].

Introduction Some diamond grains extracted from Fig. 1. Electrophoretic mobility (µm s-1 V-1 cm-1) versus pH for meteorites and chondritic porous (CP) aggregates are be- fresh diamond powder (top) and diamond powder aged in water for 14 lieved to have survived since before the formation of the days (bottom). Adapted from Hartley and Shergold, 1993. solar system (Lewis et al., 1987; Anders and Zinner, 1993; Figure 1 shows that the isoelectric point for fresh dia- and Ott, 1993). Interstellar diamond is chemically extracted mond is pH 3.5 and much less for water aged (oxidized) from meteoritic and chondritic porous host material as the diamond. The surface charge on the aged diamond is nega- first step in preparing the material for isotopic analysis. tive due to the presence of hydroxyl. This fresh diamond The Murchison K process (Amari et al., 1994) relies heavily was natural diamond with metallic contaminants and had on wet chemical oxidative steps to remove all non-diamond significant amounts of silicon and zirconium impurities carbon from a concentrate. However, diamond is not inert (0.5 %). to attack by the oxidizing agents used in this process and skewed results will be obtained if current grain processing Discussion The surface chemistry and reactivity of methods destroy even a small amount of diamond. The very diamond powder will be a strong function of the crystallo- small size of interstellar diamond grains make them par- graphic structure, particle size and morphology, and surface ticularly sensitive to oxidative attack due to their large sur- area. Lewis et al. (1989) state that the grains they measured face to volume ratio. The work to be reported will examine had “an excellent fit to a log normal distribution with a the surface chemistry of both synthetic and natural nano- median diameter of 26 Å”. Small particles tend to be very diamonds. chemically reactive as the square-cubed relationship between surface area and volume becomes large at very small Background In order to develop more benign methods radii. The outermost layer of atoms will reorder and stretch of extraction and improved recovery yields, the surface their bonds to the next atomic layer down. This is not a chemistry of the diamond grains must be taken into account. problem with large samples but the next layer down for Natural diamond found in unaltered kimberlite (blue extremely small grains may represent a significant portion ground) is hydrophobic while diamond in hydrothermally of the total volume. altered kimberlite (yellow ground) and in alluvial deposits tends to be hydrophilic. In fact, hydrophilic diamond must The crystal structure of the grains will be an important be altered to hydrophobic in order to be processed in the factor in the chemical reactivity of the surface. Spear et al. same manner as other diamonds. Freshly cleaved diamond (1990) and Phelps et al. (1993) review the structures of surfaces are also hydrophobic. Interactions of diamond sur- cubic diamond and the other hexagonal and rhombohedral faces with gases and liquids have been examined by a num- polytypes of diamond including lonsdaleite (2H diamond). ber of groups (Hartley and Shergold, 1993; Kuznetsov et al., The diamond polytypes are characterized by the stacking 1991; Martynova and Nikitin, 1988; Pate, 1986; Pehrsson et arrangement of their carbon layers. Cubic diamond (3C) is al., 1994; Sappok and Boehm, 1968). Chemisorbed hydro- related to zincblende (ZnS) and beta (SiC) cubic phases. gen is believed to be responsible for the hydrophobic character of fresh diamond surfaces. Hydrogen has been shown A A to desorb only at high temperatures (>900°C) in vacuum but is easily replaced by oxygen containing species in aqueous B B solution. Hydrophobic diamond which has been aged in (111) C (0001) A water or oxidizing acids will become hydrophilic and have a surface saturated with a variety of oxygen containing surface 3C 2H groups. (Cubic Diamond) (Lonsdaleite)

Electrophoretic Mobility of Fresh and Aged Diamond Fig. 2. The relationship between stacking arrangements in cubic diamond and lonsdaleite along the (111) and c-axis directions respec- 1 tively. 0 -1 The chemical reactivity of these two different structures -2 will be very different at very small sizes. The number of re- -3 entrant bonds present in the various stacking arrangements -4 will become a dominant factor in the oxidation of these -5 2 4 6 8 10 12 grains. The degree of hexagonal character of the polytype

pH will also control the polarity of the of the diamond surface and so alter the dielectric constant (ionicity etc.). Daulton et al (1996) demonstate that there is an admixture of hexagonal diamond in interstellar diamond extracted from meteorites. Lunar and Planetary Science XXIX 1961.pdf

AQUEOUS SURFACE CHEMISTRY OF DIAMOND GRAINS: A. W. Phelps

Gentler methods of powder separation are necessary Hartley, C. J. and H. L. Shergold, “The ageing of aque- which do not damage the as-formed diamond grains. Like- ous diamond suspensions.” Chemistry and Industry, 6, 244- wise, a method which does not alter the surface chemistry of 247, (1993). the diamond grains is desirable - thus allowing direct com- Kuznetsov, V. L., M. N. Aleksandrov, I. V. Zagoruiko, parison of the surface chemistry of the meteoritic diamond A. L. Chuvilin, E. M. Moroz, V. N. Kolomiichuk, V. A. with the synthetic diamond. Likholobov, P. M. Brylyakov and G. V. Sakovitch, “Study of ultradispersed diamond powders obtained using explo- This work will examine and compare the surface chem- sion energy.” Carbon, 29, 665-668, (1991). istry of synthetic and natural nano-diamonds. The synthetic Lewis, R. S., E. Anders and B. T. Draine, “Properties, nano-diamonds will be prepared in a CVD flowing stream detectability and origin of interstellar diamonds in meteor- system and the natural diamonds will be separated from ites.” Nature, 339, 117-121, (1989). kimberlite and lamproite host rock. Lewis, R. S., T. Ming, J. F. Wacker, E. Anders and E. Acknowledgment This work was supported in part by Steel, “Interstellar diamonds in meteorites.” Nature, 326, the NASA Office of Space Sciences under Grant no. # 160-162, (1987). NAG5-4575. Martynova, L. M. and Y. I. Nikitin, “Aggregate stability of aqueous synthetic diamond powder dispersions.” References Amari, S., R. S. Lewis and E. Anders, “In- Sverkhtverdye Materialy, 10, 37-41, (1988). terstellar grains in meteorites: I. Isolation of SiC, graphite, Ott, U., “Interstellar grains in meteorites.” Nature, 364, and diamond; size distributions of SiC and graphite.” Geo- 25-33, (1993). chimica et Cosmochimica Acta, 58, 459-470, (1994). Pate, B. B. “The diamond surface: Atomic and elec- Anders, E. and E. Zinner, “Interstellar grains in primi- tronic structure.” Surface Science, 165, 83-142, (1986). tive meteorites: Diamond, silicon carbide, and graphite.” Pehrsson, P. E., J. N. Russell, Jr., B. D. Thoms, J. E. Meteoritics, 28, 490-514, (1993). Butler, M. Marchywka, and J. M. Calvert, “Diamond sur- Daulton, T. L., D. D. Eisenhour, T. J. Bernatowicz, R. face chemistry.” In: 1994 NRL Review, Naval Research S. Lewis and P. R. Busek, “Genesis of presolar diamponds: Laboratory, Washington DC, 61-71, (1994). Comparitive high-resolution transmission electron micros- Sappok, R. and H. P. Boehm, “Chemie der oberflache copy study of meteoritic and terrestrial nano-diamonds.” des diamanten -II. Bildung, eigenschaften und structur der Geochimica et Cosmochimica Acta, 60, 4853-4872, (1996). oberflachenoxide.” Carbon, 6, 573-588, (1968). Hanneman, R. E., H. M. Strong and F. P. Bundy, “Hex- agonal diamonds in meteorites: Implications.” Science, 155, 995-997, (1967).