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, phelps@saber.udayton.edu.
Introduction Some diamond grains extracted from
meteorites and chondritic porous (CP) aggregates are believed to have survived since before the formation of the
solar system (Lewis et al., 1987; Anders and Zinner, 1993;
and Ott, 1993). Interstellar diamond is chemically extracted
from meteoritic and chondritic porous host material as the
first step in preparing the material for isotopic analysis.
The Murchison K process (Amari et al., 1994) relies heavily
on wet chemical oxidative steps to remove all non-diamond
carbon from a concentrate. However, diamond is not inert
to attack by the oxidizing agents used in this process and
skewed results will be obtained if current grain processing
methods destroy even a small amount of diamond. The very
small size of interstellar diamond grains make them particularly sensitive to oxidative attack due to their large surface to volume ratio. The work to be reported will examine
the surface chemistry of both synthetic and natural nanodiamonds.
Background In order to develop more benign methods
of extraction and improved recovery yields, the surface
chemistry of the diamond grains must be taken into account.
Natural diamond found in unaltered kimberlite (blue
ground) is hydrophobic while diamond in hydrothermally
altered kimberlite (yellow ground) and in alluvial deposits
tends to be hydrophilic. In fact, hydrophilic diamond must
be altered to hydrophobic in order to be processed in the
same manner as other diamonds. Freshly cleaved diamond
surfaces are also hydrophobic. Interactions of diamond surfaces with gases and liquids have been examined by a number of groups (Hartley and Shergold, 1993; Kuznetsov et al.,
1991; Martynova and Nikitin, 1988; Pate, 1986; Pehrsson et
al., 1994; Sappok and Boehm, 1968). Chemisorbed hydrogen is believed to be responsible for the hydrophobic character of fresh diamond surfaces. Hydrogen has been shown
to desorb only at high temperatures (>900°C) in vacuum but
is easily replaced by oxygen containing species in aqueous
solution. Hydrophobic diamond which has been aged in
water or oxidizing acids will become hydrophilic and have a
surface saturated with a variety of oxygen containing surface
groups.
1
0
-1
-2
-3
-4
-5
4
6
8
pH
10
Figure 1 shows that the isoelectric point for fresh diamond is pH 3.5 and much less for water aged (oxidized)
diamond. The surface charge on the aged diamond is negative due to the presence of hydroxyl. This fresh diamond
was natural diamond with metallic contaminants and had
significant amounts of silicon and zirconium impurities
(0.5 %).
Discussion The surface chemistry and reactivity of
diamond powder will be a strong function of the crystallographic structure, particle size and morphology, and surface
area. Lewis et al. (1989) state that the grains they measured
had “an excellent fit to a log normal distribution with a
median diameter of 26 Å”. Small particles tend to be very
chemically reactive as the square-cubed relationship between surface area and volume becomes large at very small
radii. The outermost layer of atoms will reorder and stretch
their bonds to the next atomic layer down. This is not a
problem with large samples but the next layer down for
extremely small grains may represent a significant portion
of the total volume.
The crystal structure of the grains will be an important
factor in the chemical reactivity of the surface. Spear et al.
(1990) and Phelps et al. (1993) review the structures of
cubic diamond and the other hexagonal and rhombohedral
polytypes of diamond including lonsdaleite (2H diamond).
The diamond polytypes are characterized by the stacking
arrangement of their carbon layers. Cubic diamond (3C) is
related to zincblende (ZnS) and beta (SiC) cubic phases.
A
B
C
(111)
3C
(Cubic Diamond)
A
B
(0001)
A
2H
(Lonsdaleite)
Fig. 2. The relationship between stacking arrangements in cubic
diamond and lonsdaleite along the (111) and c-axis directions respectively.
Electrophoretic Mobility of Fresh and Aged Diamond
2
Fig. 1. Electrophoretic mobility (µm s -1 V-1 cm-1) versus pH for
fresh diamond powder (top) and diamond powder aged in water for 14
days (bottom). Adapted from Hartley and Shergold, 1993.
12
The chemical reactivity of these two different structures
will be very different at very small sizes. The number of reentrant bonds present in the various stacking arrangements
will become a dominant factor in the oxidation of these
grains. The degree of hexagonal character of the polytype
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
which do not damage the as-formed diamond grains. Likewise, a method which does not alter the surface chemistry of
the diamond grains is desirable - thus allowing direct comparison of the surface chemistry of the meteoritic diamond
with the synthetic diamond.
This work will examine and compare the surface chemistry of synthetic and natural nano-diamonds. The synthetic
nano-diamonds will be prepared in a CVD flowing stream
system and the natural diamonds will be separated from
kimberlite and lamproite host rock.
Acknowledgment This work was supported in part by
the NASA Office of Space Sciences under Grant no. #
NAG5-4575.
References Amari, S., R. S. Lewis and E. Anders, “Interstellar grains in meteorites: I. Isolation of SiC, graphite,
and diamond; size distributions of SiC and graphite.” Geochimica et Cosmochimica Acta, 58, 459-470, (1994).
Anders, E. and E. Zinner, “Interstellar grains in primitive meteorites: Diamond, silicon carbide, and graphite.”
Meteoritics, 28, 490-514, (1993).
Daulton, T. L., D. D. Eisenhour, T. J. Bernatowicz, R.
S. Lewis and P. R. Busek, “Genesis of presolar diamponds:
Comparitive high-resolution transmission electron microscopy study of meteoritic and terrestrial nano-diamonds.”
Geochimica et Cosmochimica Acta, 60, 4853-4872, (1996).
Hanneman, R. E., H. M. Strong and F. P. Bundy, “Hexagonal diamonds in meteorites: Implications.” Science, 155,
995-997, (1967).
Hartley, C. J. and H. L. Shergold, “The ageing of aqueous diamond suspensions.” Chemistry and Industry, 6, 244247, (1993).
Kuznetsov, V. L., M. N. Aleksandrov, I. V. Zagoruiko,
A. L. Chuvilin, E. M. Moroz, V. N. Kolomiichuk, V. A.
Likholobov, P. M. Brylyakov and G. V. Sakovitch, “Study
of ultradispersed diamond powders obtained using explosion energy.” Carbon, 29, 665-668, (1991).
Lewis, R. S., E. Anders and B. T. Draine, “Properties,
detectability and origin of interstellar diamonds in meteorites.” Nature, 339, 117-121, (1989).
Lewis, R. S., T. Ming, J. F. Wacker, E. Anders and E.
Steel, “Interstellar diamonds in meteorites.” Nature, 326,
160-162, (1987).
Martynova, L. M. and Y. I. Nikitin, “Aggregate stability
of aqueous synthetic diamond powder dispersions.”
Sverkhtverdye Materialy, 10, 37-41, (1988).
Ott, U., “Interstellar grains in meteorites.” Nature, 364,
25-33, (1993).
Pate, B. B. “The diamond surface: Atomic and electronic structure.” Surface Science, 165, 83-142, (1986).
Pehrsson, P. E., J. N. Russell, Jr., B. D. Thoms, J. E.
Butler, M. Marchywka, and J. M. Calvert, “Diamond surface chemistry.” In: 1994 NRL Review, Naval Research
Laboratory, Washington DC, 61-71, (1994).
Sappok, R. and H. P. Boehm, “Chemie der oberflache
des diamanten -II. Bildung, eigenschaften und structur der
oberflachenoxide.” Carbon, 6, 573-588, (1968).