Chapter 8
Molten Salt-Assisted Preparation
of Nanodiamonds at Atmospheric
Pressure
Abstract Graphite and diamond are well-known allotropes of carbon. Since the
latter is far more valuable than the earlier, the transformation of graphitic structures
to diamond has been subjected to a large number of studies. The industrial production
of diamond crystallites is currently achievable by applying either a combination of
enormous external pressure and heat on graphite or graphite–catalyst systems, or
detonation of explosive carbonaceous materials. In comparison with graphite, the
conversion of carbon nanostructures into diamond can be more favorable due to
the effect of the surface tension brought about by the nanometer-sized curvature of
carbon nanomaterials. A more facile synthesis of diamond is based on the molten
salt preparation of core–shell carbon nanostructures in molten salts using either
graphite or CO 2 as the feed materials, and subsequent heat treatment of the fabricated
nanostructured materials at atmospheric pressures, far less severe conditions than
conventional processes. This chapter reviews some of the main features of these
processes.
Keywords Nanodiamonds · Molten salt · Carbon encapsulation · Nucleation ·
Growth · Atmospheric pressure · CO 2 capture
Elemental carbon can exist in diverse forms such as graphite, diamond and fullerene
owing to its ability to form sp, sp
2 and sp
3 hybridized bonds. Figures 8.1a, b show
the atomic and electronic structure of graphite and diamond, respectively. A carbon
atom has six electrons; four of which are in the outer shell and two in the inner shell.
In an individual carbon atom, the four outer shell electrons are available for chemical
bonding, enabling the formation of four possible strong covalent bonds. In graphite,
which is the thermodynamically stable form of carbon at ambient conditions, carbon
atoms are sp
2 hybridized, so that each carbon atom uses three of its valence electrons
to form simple bonds to its three close carbon neighbors, leaving the fourth electron in
the bonding level as delocalized electron over the whole of the sheet of atoms in one
layer. In the hexagonal structure of graphite, the sp
2 hybridized carbon layers stick
together due to the van der Waals interaction, which is much weaker than the in-plane
covalent bonds. As a result, graphite has great mechanical properties and a reasonable
electrical and thermal conductivity only along the in-plane crystalline directions. The
mechanical and physical properties are rather poor in the direction perpendicular to
© Springer Nature Singapore Pte Ltd. 2020
A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts
and Applications, https://doi.org/10.1007/978-981-15-2373-1_8
141
Molten Salt-Assisted Preparation
of Nanodiamonds at Atmospheric
Pressure
Abstract Graphite and diamond are well-known allotropes of carbon. Since the
latter is far more valuable than the earlier, the transformation of graphitic structures
to diamond has been subjected to a large number of studies. The industrial production
of diamond crystallites is currently achievable by applying either a combination of
enormous external pressure and heat on graphite or graphite–catalyst systems, or
detonation of explosive carbonaceous materials. In comparison with graphite, the
conversion of carbon nanostructures into diamond can be more favorable due to
the effect of the surface tension brought about by the nanometer-sized curvature of
carbon nanomaterials. A more facile synthesis of diamond is based on the molten
salt preparation of core–shell carbon nanostructures in molten salts using either
graphite or CO 2 as the feed materials, and subsequent heat treatment of the fabricated
nanostructured materials at atmospheric pressures, far less severe conditions than
conventional processes. This chapter reviews some of the main features of these
processes.
Keywords Nanodiamonds · Molten salt · Carbon encapsulation · Nucleation ·
Growth · Atmospheric pressure · CO 2 capture
Elemental carbon can exist in diverse forms such as graphite, diamond and fullerene
owing to its ability to form sp, sp
2 and sp
3 hybridized bonds. Figures 8.1a, b show
the atomic and electronic structure of graphite and diamond, respectively. A carbon
atom has six electrons; four of which are in the outer shell and two in the inner shell.
In an individual carbon atom, the four outer shell electrons are available for chemical
bonding, enabling the formation of four possible strong covalent bonds. In graphite,
which is the thermodynamically stable form of carbon at ambient conditions, carbon
atoms are sp
2 hybridized, so that each carbon atom uses three of its valence electrons
to form simple bonds to its three close carbon neighbors, leaving the fourth electron in
the bonding level as delocalized electron over the whole of the sheet of atoms in one
layer. In the hexagonal structure of graphite, the sp
2 hybridized carbon layers stick
together due to the van der Waals interaction, which is much weaker than the in-plane
covalent bonds. As a result, graphite has great mechanical properties and a reasonable
electrical and thermal conductivity only along the in-plane crystalline directions. The
mechanical and physical properties are rather poor in the direction perpendicular to
© Springer Nature Singapore Pte Ltd. 2020
A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts
and Applications, https://doi.org/10.1007/978-981-15-2373-1_8
141