DRDC-RDDC-2016-P080
Evidence For Plasma Synthesis Of An Amorphous Polynitrogen On Carbon Nanotubes
Thelma G. Manning 1, Viral Panchal1, Mounir Jaidann2, Hakima Abou-Rachid2, and Zafar
Iqbal3*
1
US Army RDECOM ARDEC, Picatinny Arsenal, New Jersey 07806, USA
2
Defence Research and Development Canada, 2459 De la Bravoure Road, Quebec (QC) G3J
1X5, Canada
3
Department of Chemistry and Environmental Science, New Jersey Institute of Technology,
Newark, New Jersey 07102,USA
*Address all correspondence to: Dr. Zafar Iqbal, E-mail: [email protected]
ABSTRACT Radio-frequency plasma synthesis of polynitrogen (PN) stabilized on single- and
multi-wall wall carbon nanotubes, has been carried out using nitrogen mixed with argon or with
hydrogen as precursors. Characterization of the samples produced was conducted by Raman
spectroscopy (Raman), attenuated total reflectance-Fourier transform infrared spectroscopy (ATRFTIR), scanning (SEM) and transmission (TEM) electron microscopy, energy-dispersive x-ray
spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry
(DSC). Raman, SEM, and TEM showed that an amorphous PN phase is formed on the sidewalls and
inside the carbon nanotubes (CNTs), which decomposes exothermally at approximately 300oC as
indicated by DSC measurements. Molecular modeling assessment of the energy performance was
carried out for the related nitrogen chain N8 molecule hosted inside a CNT and combined with the
double-base energetic material (nitrocellulose, nitroglycerin). Similar assessment was also carried out
for a promising nitrogen-rich molecule, 3,6-di(hydrazino)-1,2,4,5-tetrazine (DHT) in its double-base
form. The encapsulation of this system inside a nitrogen-doped CNT was considered for showing the
effect of CNTs on energetic performance.
KEY WORDS: plasma synthesis, carbon nanotube nanopaper sheets, Raman spectroscopy,
scanning and transmission electron microscopy, molecular modeling
1. INTRODUCTION
Developing environmentally friendly or “green” high energy density materials (HEDM) is
one of the most important current challenges for defense organizations. This explains the
growth of research interest in all nitrogen or nitrogen-rich energetic materials. The high
energy density of an all-nitrogen, crystalline polynitrogen (PN) consisting of N-N single
bonds is due to the fact that the strength of the N≡N triple bond in molecular nitrogen is one
of the strongest known chemical bonds (Kalescky et al., 2013), with an energy content of
about 4.94 eV.atom−1 compared to -0.83 eV.atom−1 for the N-N single bond (Huheey et al.,
1993). Therefore, a large amount of energy of about 2.3 eV.atom−1 (Eremets et al., 2008) is
expected to be released during the triple to single N-N bond transformation. Estimates for PN
predict up to a tenfold higher detonation pressure than that for one of the most powerful
explosives in use today: HMX (C4H4N8O8).
A crystalline PN phase was proposed some 40 years ago indicating the existence of an
extended structure beyond that of molecular nitrogen N2 and the molecular azide anion N3-.
Surprisingly, the most stable crystalline form of the proposed PN structure is an unusual one
corresponding to a cubic gauche (cg) lattice (Mailhiot et al., 1992). Although the
transformation from molecular nitrogen to the cubic gauche structure (abbreviated cg-PN)
was predicted to take place at pressures above 50 GPa that was within reach of diamond anvil
cells in use at that time, no successful synthesis was reported until 2004 when Eremets et al.
(2004) described its formation using a laser-heated diamond anvil cell at a pressure of 110
GPa and temperature near 2000 K. The cg-PN phase synthesized dissociated back to nitrogen
under ambient conditions and was shown to have the predicted cg structure by in-situ x-ray
diffraction. Raman spectroscopy provided further proof of the structure because the data
agreed with the theoretical predictions of Barbee (1993) and the later work of Caracas (2007).
Prior to that an important breakthrough was achieved in 1999 with the successful synthesis of
the pentazenium N5+ polynitrogen cation by Christe et al. (1999), where hydrazoic acid, HN3,
was reacted with N2F+ to eliminate HF and produce N5+ according to the reaction:
N2F+ + HN3 → N5+ + HF
(1)
The bent chain structure of the N5+ ion of C2v symmetry characterized by NMR, FTIR, and
Raman spectroscopy was found to be in excellent agreement with density functional theory
calculations.
Attempts to obtain a PN phase by quenching to ambient temperature and low
pressures have been previously made by Eremets et al. (2001, 2008) and Lipp et al. (2007) to
form opaque and red-colored amorphous phases, respectively. Direct evidence for PN could
not be obtained since the Raman lines from the samples produced were very broad.
Theoretical modeling using first-principles molecular dynamics by Nordlund et al. (2004)
showed that the amorphous phase has predominantly 3-fold bonding with both single and
double bonded nitrogen atoms, but there is some short-range order with preferred bonding
angles around 90o. Decomposition simulations indicated that the amorphous form can, when
suitably confined, be stable at low temperature. Another amorphous form of PN with a
porous structure that is stable at zero pressure and low temperatures was predicted using
dynamic shock simulations by Beaudet, Mattson, and Rice (2013).
A PN phase with a so-called chaired web arrangement, which has an electrically
insulating rhombohedral structure was proposed by Zahariev et al. (2007) to be
thermodynamically more stable by 20 meV than the electrically non-conductive cubic gauche
PN phase at zero pressure, but this phase has not been experimentally observed to date. In a
recent theoretical study (Hirshberg et al., 2014), an interesting molecular crystal phase
containing N8 chains was predicted to be metastable at ambient pressures, with energies
lower than that of the cubic gauche PN phase.
N8 and N24 chains were proposed to be stabilized inside single wall carbon nanotubes
(SWNTs) by Abou-Rachid et al. (2008) based on high level theoretical calculations. Carbon
nanotubes have the potential to be ideal hosts to confine the metastable PN structure by
providing stability through bonding between the CNTs and PN. This prediction is consistent
with the work of Wu et al. (2014) who showed the formation of a N8ˉ anion stabilized on the
positively charged carbon nanotube working electrode by electrochemical cyclic voltammetry
in sodium azide solution. More recent cyclic voltammetry (Hu et al., 2015) in the presence of
palladium oxide (PdO) on the carbon nanotube working electrode showed enhanced
formation of N8ˉ anions as a result of reduction reactions to form hydrated [Pd(H2O)4]2+
cations (Polotnyanko and Khodakovskii, 2013) which provides greater stability to the N8ˉ
anions than the positively charged CNT sidewalls. Earlier work (Manning and Iqbal, 2012)
has shown that analogous to the plasma synthesis of the high pressure-high temperature
crystalline diamond phase on silicon from methane, a nitrogen-hydrogen or argon plasma
reaction can form a PN structure on a CNT substrate. Here, a radio-frequency (RF) plasmaenhanced chemical vapor deposition technique using nitrogen combined with either argon or
hydrogen, has been employed to produce a PN coating on the sidewalls and inside single- and
multi-wall carbon nanotube (SWNT and MWNT, respectively) substrates, which was shown
by Raman and selected area electron diffraction to be amorphous. Molecular modeling
assessments of the energetic performance of a N8 cluster molecule inside a SWNT and pure
nitrogen structures combined with double-base nitrocellulose, nitroglycerin formulations
inside SWNTs, were also carried out.
2. EXPERIMENTAL AND COMPUTATIONAL METHODS
2.1 Plasma synthesis
A schematic of the setup for carrying out the plasma reactions is shown in Fig. 1 (left) and
the panel on the right shows a digital photograph of the reactor with the plasma turned on.
Nitrogen and argon or hydrogen were fed into the quartz tube through needle valve flow
meters allowing control of the flow rate of the gases. An adjustable radio frequency generator
delivering up to 500 Watts attached to an adjustable impedance matching box was used to
generate the plasma. The actual plasma power used was the difference between the incident
power from the generator and the reflected power. Plasma powers up to 300 Watts were used.
A vacuum pump was used to evacuate the deposition chamber to pressures below 1 Torr.
Flow rates of 10-15 sccm (standard cubic centimeters per minute) were maintained for each
gas during the reactions. Gas mixtures of 50%-75% nitrogen and 50%-75% argon or
hydrogen were introduced into the deposition chamber where a quartz boat was used to hold
2cm×2cm in area CNT nanopaper sheets comprised of either SWNTs or MWNTs. Carbon
nanotube nanopaper sheets were fabricated by suspending either single- or multi-wall CNT
powders (obtained from Cheap Tubes Inc.) in de-ionized water using 0.5 wt.% sodium
dodecyl sulfate surfactant and horn sonication. The SWNTs used have bundle and individual
tube average diameters of 12 nm and 1.4 nm, respectively, and average length of a few
micrometers. The MWNTs have an average individual tube diameter of 15 nm and average
length of a few micrometers. Both SWNTs and MWNTs used are greater than 95% pure.
This was followed by vacuum filtration of the suspension through a 10 μm pore PTFE
membrane from Millipore, washing with methanol and deionized water, drying at 50oC in a
vacuum oven, and then peeling off from the membrane as a free-standing nanopaper sheet.
2.2 Characterization
Raman spectra were obtained using a DXR micro-Raman spectrometer (Thermo Scientific)
in the spectral range between 50 and 3500 cm-1 using a 780 nm laser with a power of 2 mW
and a 10x microscope objective. Fourier transform infrared (FTIR) spectroscopy was
conducted using a Magna Model 560 instrument (Nicolet) attached to an attenuated total
reflectance (ATR) accessory with a single reflection ZnSe crystal (MIRacle, Pike
Technologies).
SEM images were obtained with a VP-1530 Carl Zeiss LEO field emission scanning
electron microscope. The samples were mounted on aluminum stubs using double-sided
carbon tape. TEM imaging and selected area electron diffraction was carried out using a
Philips field emission CM20 TEM/STEM instrument with Gatan 792 Multi-scan CCD (1k
x1k) camera, Schottky field emitter and TWIN pole pieces, objective lens: 0.282 nm point
resolution, 0.144 nm line resolution, and 0.180 nm information limit at 200 kV (TEM mode),
and 1.0 nm BF/DF STEM resolution.
Differential scanning calorimetry (DSC) measurements were carried out using a TA
Instruments Q100 V9.8 Build 296 instrument. The XPS data were collected using a Kratos
Axis Ultra DLD spectrometer. The source was monochromatic AlKα radiation (1486.6 eV)
operating at 225 W. The spectra were collected using a combination of electrostatic and
magnetic lens (hybrid mode) for large area acquisition (700 mm by 300 mm). Pass energies
of 160 and 20 eV were used for survey and high-resolution scans, respectively. All data
analysis was accomplished using the commercial software CasaXPS, version 2.2.14.
2.3 Computational methods
The heats of formation were predicted using quantum mechanical calculations and
CHEETAH 2.0 software to generate the BLAKE code output and determine how the heats of
formation of the compounds are directly related to the energetic performance of explosive
formulations. To optimize the desired improved properties of the explosive formulations,
various ingredients and compositions have been considered. To predict gunpowder
performance, a combination of a double-base [NC (12.6% Nitrogen), NG] with nitrogen-rich
energetic molecules has been studied. Nitrogen-rich compound, 3,6-di(hydrazino)-1,2,4,5tetrazine (DHT) and N8 hosted inside a carbon nanotube [N8@CNT(5,5)], are shown in Fig.2.
The Constant Pressure Model (CPM) was used to estimate the muzzle velocity VPM for
gunpowder (Robbins, 2007). This process provides the highest velocity which could be
reached at the loading density for a gun system following Equation 2:
2E
VPM = PM
mP
(1)
where EPM is the energy of the projectile when the propellant is completely burned and mp is
the projectile mass. Using the BLAKE thermodynamic code from the U.S. Army Ballistic
Research Laboratory (BRL) (Freedman, 1998) included in the CHEETAH2.0 software (Fried
et al., 1998), the specific impetus is defined as:
I s = RT M
(2)
where R is the gas constant, T is the temperature, and M is the average molecular weight of
the gaseous compounds. For the DHT molecule, experimental heat of formation is available
(Chavez and Hiskey, 1998) and I s was estimated from empirical values of DHT. For
N8@CNT(5,5), no experimental data are available. Heat of formation for this system was
predicted by quantum chemistry calculations, such as density functional theory (DFT) (Nagy,
1998) and the Parameterized Model number 6 (PM6) methods (Stewart, 2007). To assess
heats of formation for the studied molecules, calculations were carried out using Gaussian 03
(Frisch et al., 2003) and the Molecular Orbital PACkage (MOPAC) 2009 (Stewart, 2009)
software.
3. RESULTS AND DISCUSSION
Nitrogen mixed with hydrogen or argon was reacted on CNT sheets in a radio-frequency
plasma at powers ranging from 65 to 300 W for 30 minutes to an hour. The reactions resulted
in the formation of coatings deposited on the CNT sidewalls as evident from the SEM images
in Fig. 3B compared with that of the pristine CNT sheet in Fig. 3A.. The coatings were fairly
smooth after reaction in a nitrogen-hydrogen plasma but showed fairly large bulb-like
features after reaction in a nitrogen-argon plasma as evident from Figs. 3B(c) and 3B(e),
respectively. This difference in morphology may be due to etching to form a smooth film
from intermediate N-H and H species produced in the nitrogen-hydrogen plasma. The EDX
spectrum in Fig. 3C (left panel) clearly shows the presence of nitrogen together with oxygen
and Fe catalyst from the CNT substrates. The XPS spectra shown in Fig. 3C (right panel)
were taken from plasma-reacted SWNT and MWNT substrates. On the MWNT substrate, a
N1s line at a binding energy of 398 eV together with a high energy shoulder near 399 eV are
observed. On the SWNT substrate, the intensities are reversed – the stronger line is at a
binding energy of 399 eV, whereas the shoulder with lower intensity is at 398 eV. Moreover,
on the SWNT substrate a weak, broad feature is seen at 405.5 eV, which can be attributed to
bonding interactions between the carbon-based substrate and nitrogen.
The Raman spectrum of a nitrogen-argon plasma-reacted MWNT sheet is displayed in
Fig. 4 showing broad bands centered near 800 and 2000 cm-1 indicating an amorphous
network. Similar broad features are observed on SWNT sheets reacted in a nitrogen-hydrogen
plasma. The band centered at 800 cm-1, can be assigned to N-N stretching modes in
agreement with the calculations of Barbee (1994) which is supported by the later work of
Caracas (2007), and the experimental data of Eremets et al. (2004) for high pressure and
temperature synthesized crystalline cubic gauche PN, which has only N-N bonds. The band
centered at 2000 cm-1 can be assigned to stretching modes of conjugated N= N bonds
(Bellamy, 1975) in an amorphous network. The amorphous structure is likely to be related to
a semiconducting disordered PN structure synthesized by Eremets et al. (2001) under high
pressure conditions that could be recovered at low temperatures, and has been simulated by
Nordlund et al. (2004) by first-principles molecular dynamics.
A typical ATR-FTIR spectrum shown in Fig. 5 provides further information on the
chemical structure of the PN phase. Lines at 800 and 2100 cm-1 complements the broad
features in the same region in the Raman spectrum assigned to N-N and conjugated N= N
stretching modes, respectively. In addition, lines observed at 1100, 1300, 1500, and 1700 cm1
correspond to those seen in a carbon nitride by Wu et al. (2014) and can therefore be
assigned to C-N vibrations associated with bonding interactions of the nitrogen structure with
the carbon structure of the CNTs. The line at 2800 cm-1 is due to a C-H stretching mode
suggesting that hydrogen ion are generated during the plasma process.
Selected area electron diffraction taken from particles in a nitrogen-argon plasma
reacted sample shown in Fig. 6(b) showed broad rings with some spots consistent with that of
an amorphous nitrogen network supported on MWNTs. The TEM images in Fig. 6 show
lattice fringes only for the nanotube supports and disordered networks of PN both inside and
outside the nanotubes. Strong experimental support for the formation of an energetic PN network
comes from the highly exothermic broad DSC peak at 317oC shown in Fig. 7 for a SWNT substrate
reacted in a nitrogen-hydrogen plasma. The exotherm is similar to that seen in HMX with a peak at
286.6oC by Li, Jiang and Yu (2002).
Computations were used to shed light on some properties of materials prior to their
experimental testing. Specific energetic compounds have been considered, especially related
nitrogen-rich molecules, such as 3,6-di(hydrazino)-1,2,4,5-tetrazine (DHT), and pure nitrogen
structures inside carbon nanotubes combined with double-base [Nitrocellulose (NC),
Nitroglycerin (NG)]. The specific impetus and the muzzle velocity for different formulations
which provide better performance are listed in Table 1 for a charge density of 0.2 g/cc. A
quantity of diphenylamine (DPA) around 1% was added. The DPA compound is one of the
most common stabilizers used to prevent the deterioration of the products. The reference
Hodgdon H50BMG propellant (double-base) formulation for which experimental data is
available was also investigated to validate the computational approach. Its muzzle velocity
was estimated for a 0.50 Calibre for the Tactical Sniper System.
Muzzle velocities increase for nitrogen-rich molecule DHT by approximately 15% as
compared to the Hodgdon H50BMG propellant. For Hodgdon H50BMG, the calculated value
is in good agreement with the experimental data reported in Ref "(Calibre.50, 2000)". DHT is
a highly promising molecule as an energetic material in composite gun propellants due to its
high
heat
of
formation
(128
kcal/mol)
"(Chavez
and
Hiskey,
1998)".
For
DHT@N@CNT(7,7) and N8@CNT(5,5), specific impetus and muzzle velocity values are not
very high, even though their heats of formation are, with values of 918 and 676 kcal/mol for
DHT@N@CNT(7,7) and N8@CNT(5,5), respectively.
This observation for DHT@N@CNT(7,7) and N8@CNT(5,5) is due to the presence
of solid carbon residues in the two sets after the combustion process. More than 34 mol% of
carbon and less than 31 mol% of oxygen were present in the reactants, which contributes to
the increase of solid carbon residues in the product. The solid carbon concentrations (which
have in fact an abrasion effect) after the combustion process were around 2.12 and 3.1 mol/kg
for DHT@N@CNT(7,7) and N8@CNT(5,5), respectively (see Table 2). However, carbon
residues can be alleviated by increasing the ratio of oxygen to carbon shown in the
combustion products. This will lead to the energy performance increase for these promising
nitrogen rich compounds.
4. CONCLUSIONS
Spectroscopic data, and both scanning and transmission electron microscopy have been used
to show the formation of an amorphous polynitrogen (PN) structure deposited on and inside
single- and multi-wall CNT nanopaper substrates from radio-frequency plasma reaction of
nitrogen in argon, and nitrogen in hydrogen. High resolution TEM provides evidence for the
formation the amorphous PN phase within the CNTs and on the CNT sidewalls. Strong
experimental support for the formation of an energetic PN network comes from the highly
exothermic broad DSC peak at 317oC. The energy performance was assessed by molecular
modeling for the nitrogen chain N8 molecule hosted inside a CNT and combined with the
double-base energetic material, (Nitrocellulose, Nitroglycerin). Similar modeling assessment
was carried out for a promising nitrogen-rich molecule, 3,6-di(hydrazino)-1,2,4,5-tetrazine
(DHT) in double-base form.
ACKNOWLEDGEMENTS Most of this work was carried out under US Army Novel
Energetics Tech Base Program and contract W15QKN-10-C-0110. The authors would like to
thank Dr. Alex Chou at the Stevens Institute of Technology, Hoboken, New Jersey and Dr.
Chi Yu at the New Jersey Institute of Technology for the TEM data, and Dr. Hong Piao, GE,
Niskayuna, New York, for the XPS data.
FIG. 1. Schematic drawing showing radio-frequency (RF) plasma system used in this work (left).
Photograph showing the plasma reactor during operation. Arrow indicates the back region of the
reactor from which the plasma-reacted samples were taken for characterization (right).
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FIG. 7. (a) Differential scanning calorimetry (DSC) scan in air for pristine SWNT
nanopaper; and (b) DSC scan in air from SWNT nanopaper plasma-reacted in 25%
Nitrogen:75% Hydrogen. Both scans were taken at a scanning rate of 20oC/minute.
(a)
(b)
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TABLE 1: Predicted performance data of propellant powders (charge density 0.2 g/cc, using
CHEETAH 2.0 and the CPM code).
Formulations
Is
(J/g)
VPM
(m/s)
10% NC12.6/40% NG/1% DPA/49% DHT
1265
979
30% NC12.6/40% NG/1% DPA/29% DHT
1245
976
10% NC12.6/30% NG/1% DPA/59% DHT
1191
962
29% NC12.6/ 20% NG/1% DPA/50% DHT
1137
948
10% NC12.6/60% NG/1% DPA/
29%DHT@N@CNT(7,7)
893
876
10% NC12.6/ 60% NG/1% DPA/
29%N8@CNT(5,5)
917
902
Hodgdon H50BMG
920
888
TABLE 2: Product concentration predicted after the combustion process of propellant
powder based on DHT@N@CNT(7,7) and N8@CNT(5,5).
Name
State
CO
Concentrations (mol/kg)
DHT@N@ CNT(7,7)
N8@CNT(5,5)
Gas
26.34
26.96
H2
Gas
6.78
6.82
N2
Gas
6.06
5.47
HCN
Gas
0.64
0.74
C2H2
Gas
0.14
0.20
CH4
Gas
0.46
0.24
H2O
Gas
0.50
0.22
CO2
Gas
0.26
0.12
CH3
Gas
0.01
0.02
NH3
Gas
0.06
0.03
C(S)
Solid
2.12
3.10