A Series of inorganic solid nitrogen sources for the synthesis of metal nitride clusterfullerenes: The dependence of production yield on the oxidation state of nitrogen and counter ion

Article abstract of DOI:10.1021/ic302436k

A series of nitrogen-containing inorganic solid compounds with variable oxidation states of nitrogen and counter ions have been successfully applied as new inorganic solid nitrogen sources toward the synthesis of Sc-based metal nitride clusterfullerenes (Sc-NCFs), including ammonium salts [(NH ₄)_xH_3-xPO₄ (x = 0-2), (NH ₄)₂SO₄, (NH₄)₂CO ₃, NH₄X (X = F, Cl), NH₄SCN], thiocyanate (KSCN), nitrates (Cu(NO₃)₂, NaNO₃), and nitrite (NaNO₂). Among them, ammonium phosphates ((NH₄) _xH_3-xPO₄, x = 1-3) and ammonium thiocyanate (NH₄SCN) are revealed to behave as better nitrogen sources than others, and the highest yield of Sc-NCFs is achieved when NH₄SCN was used as a nitrogen source. The optimum molar ratio of Sc₂O ₃:(NH₄)₃PO₄·3H₂O: C and Sc₂O₃:NH₄SCN:C has been determined to be 1:2:15 and 1:3:15, respectively. The thermal decomposition products of these 12 inorganic compounds have been discussed in order to understand their different performances toward the synthesis of Sc-NCFs, and accordingly the dependence of the production yield of Sc-NCFs on the oxidation state of nitrogen and counter ion is interpreted. The yield of Sc₃N@C₈₀ (I_h + D_5h) per gram Sc₂O₃ by using the N ₂-based group of nitrogen sources (thiocyanate, nitrates, and nitrite) is overall much lower than those by using gaseous N₂ and NH₄SCN, indicating the strong dependence of the yield of Sc-NCFs on the oxidation state of nitrogen, which is attributed to the in-situ redox reaction taking place for the N₂-based group of nitrogen sources during discharging. For NH₃-based group of nitrogen sources (ammonium salts) which exhibits a (-3) oxidation states of nitrogen, their performance as nitrogen sources is found to be sensitively dependent on the anion, and this is understood by considering their difference on the thermal stability and/or decomposition rate. Contrarily, for the N₂-based group of nitrogen sources, the formation of Sc-NCFs is independent to both the oxidation state of nitrogen (+3 or +5) and the cation.

Full text of DOI:10.1021/ic302436k

Article
pubs.acs.org/IC
A Series of Inorganic Solid Nitrogen Sources for the Synthesis of
Metal Nitride Clusterfullerenes: The Dependence of Production Yield
on the Oxidation State of Nitrogen and Counter Ion
Fupin Liu, Jian Guan, Tao Wei, Song Wang, Mingzhi Jiao, and Shangfeng Yang*
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion and
Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China
S
* Supporting Information
ABSTRACT: A series of nitrogen-containing inorganic solid
compounds with variable oxidation states of nitrogen and counter
ions have been successfully applied as new inorganic solid nitrogen
sources toward the synthesis of Sc-based metal nitride clusterfuller-
enes (Sc-NCFs), including ammonium salts [(NH4)xH3‑xPO4 (x =
0−2), (NH4)2SO4, (NH4)2CO3, NH4X (X = F, Cl), NH4SCN],
thiocyanate (KSCN), nitrates (Cu(NO3)2, NaNO3), and nitrite
(NaNO2). Among them, ammonium phosphates ((NH4)xH3‑xPO4, x
= 1−3) and ammonium thiocyanate (NH4SCN) are revealed to
behave as better nitrogen sources than others, and the highest yield of
Sc-NCFs is achieved when NH4SCN was used as a nitrogen source.
The optimum molar ratio of Sc2O3:(NH4)3PO4·3H2O:C and
Sc2O3:NH4SCN:C has been determined to be 1:2:15 and 1:3:15,
respectively. The thermal decomposition products of these 12 inorganic compounds have been discussed in order to understand
their different performances toward the synthesis of Sc-NCFs, and accordingly the dependence of the production yield of Sc-
NCFs on the oxidation state of nitrogen and counter ion is interpreted. The yield of Sc3N@C80 (Ih + D5h) per gram Sc2O3 by
using the N2-based group of nitrogen sources (thiocyanate, nitrates, and nitrite) is overall much lower than those by using
gaseous N2 and NH4SCN, indicating the strong dependence of the yield of Sc-NCFs on the oxidation state of nitrogen, which is
attributed to the “in-situ” redox reaction taking place for the N2-based group of nitrogen sources during discharging. For NH3-
based group of nitrogen sources (ammonium salts) which exhibits a (−3) oxidation states of nitrogen, their performance as
nitrogen sources is found to be sensitively dependent on the anion, and this is understood by considering their difference on the
thermal stability and/or decomposition rate. Contrarily, for the N2-based group of nitrogen sources, the formation of Sc-NCFs is
independent to both the oxidation state of nitrogen (+3 or +5) and the cation.
template” (TNT) process with the introduction of a small
INTRODUCTION
■
portion of gaseous nitrogen (N ) into the Kratschmer−
̈
2
Since the discovery of Sc3N@C80 as the first member of metal
nitride clusterfullerenes (NCFs) in 1999 via a modified
Huffman generator led to the discovery of Sc3N@C80 by
Dorn et al. in 1999.1 In 2003 Dunsch et al. invented the
“reactive gas atmosphere” method with gaseous ammonia
(NH3) used as nitrogen source, and realized the selective
synthesis of NCFs for the first time enabling NCFs as the main
products in the soot.18 Although these two routes appear to be
the most commonly used methods for NCF synthesis in
laboratories, an additional heating pretreatment up to 1000 °C
is generally required to “activate” the metal (or metal oxide)
before introducing the gaseous N2 or NH3 into chamber, and
an additional gas inlet may be required for the introduction the
gaseous nitrogen sources.19,20 Thus, it is highly desirable to
simplify the synthesis procedure without decreasing the
production yield and selectivity of NCFs.
Kratschmer−Huffman dc-arc discharging method,1 the family
̈
of endohedral fullerenes has been significantly enlarged by
clusterfullerenes, which are featured by the encapsulation of
metal clusters within the interior of fullerene cage.2−9 In
particular, NCF is the most rapidly growing branch of
clusterfullerenes because of its extraordinarily high yield and
unique electronic, physical, and chemical properties, which
promise potential applications in biomedicines and organic
photovoltaics, etc.2−8 Despite the considerable advance in the
synthesis of novel types of NCFs in the past decade,10−17 the
development of synthetic routes for NCFs has been quite
limited, which is crucial since the applications of NCFs put a
high demand on their yield.
Up to now only few synthetic routes for NCFs have been
developed with few nitrogen sources,2−9 and all are based on
the modifications of the standard Kratschmer−Huffman dc-arc
Received: November 7, 2012
Published: March 12, 2013
̈
discharging method. As the first success, the “trimetallic nitride
© 2013 American Chemical Society
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Figure 1. HPLC chromatograms of fullerene extracts obtained from discharging of mixture of Sc2O3 and 12 individual nitrogen-containing inorganic
compounds (10 mm × 250 mm Buckyprep column; flow rate 5.0 mL/min; injection volume 5 mL; toluene as eluent (mobile phase); 40 °C).
Fractions A−D mainly consist of Sc3N@C80 (Ih, D5h): Sc3N@C78 + C86, Sc3N@C70 + C84, Sc3N@C68 + C76 + C78, respectively.
The progress toward the facile synthesis of NCFs has
recently been achieved by the “solid nitrogen” routes developed
as the effective solution for simplification of the synthesis
procedure.21,22 So far only three inorganic or organic solid
nitrogen-containing compounds were applied as the nitrogen
sources of the “solid nitrogen” routes. In 2010 we established
the “selective organic solid” (SOS) route with guanidinium salts
such as guanidinium thiocyanate (CH5N3·HSCN) used as the
organic solid nitrogen sources, and achieved comparable yield
and selectivity of NCFs to those by using gaseous NH3 of the
“reactive gas atmosphere” route.21,23 More recently, we used
urea (CO(NH2)2) as a cheaper organic solid nitrogen source
for the synthesis of Sc-based NCFs, and found that the yield of
Sc3N@C80 (Ih + D5h) per gram Sc2O3 by using CO(NH2)2 is
almost identical to that by using gaseous N2 but much higher
than that by using NH3.22 Hence, urea and guanidinium
thiocyanate can be regarded as the alternative organic solid
nitrogen sources substituting gaseous N2 or NH3, respectively.
On the other hand, inorganic solid nitrogen source has been
rarely reported. To our knowledge, until now only calcium
cyanamide (CaNCN) was applied as inorganic solid nitrogen
source for NCF synthesis by Dunsch et al. in 2004, resulting in
the synthesis of Sc3N@C80 NCF with a variable selectivity
ranging from 3% to 42%, and such a low reproducibility of yield
is presumably caused by traces of water and/or hydrocarbons in
the raw material.9 Therefore, this “inorganic solid nitrogen”
route using CaNCN could not be widely applied in the
synthesis of NCFs. Given that inorganic compounds typically
have a higher thermal stability and are more readily available
than organic compounds, to modify the “inorganic solid
nitrogen” route using other nitrogen-containing inorganic
compounds as nitrogen sources for NCF synthesis is of
practical importance but challenging still.
nitrogen changes for different categories, and within the same
category the counter ion varies. In this way the role of the
oxidation state of nitrogen and counter ion on the yield of Sc-
NCFs is addressed, shedding light on the correlation between
the NCF yield and the decomposition products of these new
inorganic nitrogen sources upon discharging.
RESULTS AND DISCUSSION
■
Different Nitrogen-Containing Inorganic Compounds
as Nitrogen Sources for the Synthesis of Sc-NCFs. To
vary the oxidation state of nitrogen, 12 nitrogen-containing
inorganic compounds falling into four categories were chosen
in this study for comparison. As the protonated form of
ammonia popularly used in the “reactive gas atmosphere”
method,18 ammonium salts were preferably considered, and 8
representative compounds with the same oxidation state of
nitrogen (−3) but different anions were investigated, including
triammonium phosphate hydrate ((NH4)3PO4·3H2O, hereafter
the water moiety is omitted for clarity), diammonium hydrogen
phosphate ((NH4)2HPO4), ammonium dihydrogen phosphate
(NH4H2PO4), ammonium sulfate ((NH4)2SO4), ammonium
carbonate ((NH4)2CO3), ammonium fluoride (NH4F), ammo-
nium chloride (NH4Cl), and ammonium thiocyanate
(NH4SCN). It is noteworthy that, for ammonium thiocyanate
+
(NH4SCN), both the cation (NH4 ) and anion (SCN−)
contain nitrogen with the same oxidation state of nitrogen
(−3). In order to investigate whether NH4 and SCN− behave
+
the same as the nitrogen sources, another thiocyanate,
potassium thiocyanate (KSCN), was selected and compared
with NH4SCN. On the other hand, among the known positive
oxidation states of nitrogen (+2 ∼ +5), the highest oxidation
state of nitrogen is +5 and is popularly found in the inorganic
nitrates. Thus, two nitrates including copper nitrate hydrate
(Cu(NO3)2·3H2O, hereafter the water moiety is omitted for
clarity) and sodium nitrate (NaNO3) were chosen and
compared so as to study the effect of cation. Finally, an
intermediate oxidation state of nitrogen (+3) is accessible in
nitrite, and sodium nitrite (NaNO2) was investigated and
compared with NaNO3 aiming to study further the effect of
In this Article, we report the successful application of a series
of nitrogen-containing inorganic solid compounds as new
nitrogen sources for the synthesis of Sc-based NCFs, including
12 compounds belonging to four categories: ammonium salts,
thiocyanates, nitrates, and nitrite. Among these 12 nitrogen-
containing inorganic compounds, the oxidation state of
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Figure 2. Positive-mode laser desorption time-of-flight (LD-TOF) mass spectroscopic (MS) spectra of fullerene extracts obtained from discharging
of mixture of Sc2O3 and 12 individual nitrogen-containing inorganic compounds. The inset shows the enlarged mass region of 1080−1120. The
intensity of the MS spectra were normalized by the most intense mass peak of the spectrum.
Figure 3. Comparison of the HPLC intensity of fraction A per gram Sc2O3 obtained from different nitrogen sources. See Supporting Information
(part S1) for the details of calculations.
oxidation state of nitrogen on their performance as nitrogen
sources.
Therefore, combining both HPLC and MS analysis results, we
conclude that all of the 12 nitrogen-containing inorganic
compounds can behave as new inorganic nitrogen sources
leading to the successful synthesis of Sc-NCFs.
Figure 1 presents the HPLC profiles of the fullerene extracts,
which were obtained from dc-arc discharges of one rod filled
with a mixture of Sc2O3 and 12 individual nitrogen-containing
inorganic compounds under the condition optimized, respec-
tively, as discussed below. Obviously all of the HPLC profiles of
these extracts are quite similar to those obtained by using the
reported gaseous nitrogen sources (N2 or NH3) or organic
solid nitrogen sources (CO(NH2)2 or CH5N3·HSCN) in terms
of the overall distribution of different fractions,21,22 suggesting
the formation of similar fullerene products in these extracts.
This is experimentally confirmed by laser desorption time-of-
flight (LD-TOF) mass spectroscopic (MS) analysis, indicating
clearly the existence of a series of reported Sc-NCFs Sc3N@C2n
(2n = 80, 78, 70, 68) in all of the extracts (see Figure 2).
On the basis of our earlier reports on Sc-NCFs synthesized
via other nitrogen sources, including N2, NH3, CH5N3·HSCN,
or CO(NH2)2, fractions A−D can be assigned to a series of Sc-
NCFs, Sc3N@C2n (2n = 80, 78, 70, 68), respectively, which are
coeluted with different empty fullerenes.21,22 Among them,
fraction A with the highest abundance is mainly composed of
the mixture of two isomers (Ih and D5h) of Sc3N@C80, and is
selected for further analysis. Figure 3 compares the HPLC
intensity of fraction A per gram Sc2O3 obtained via different
nitrogen sources, which visualizes the variation of the yield of
Sc3N@C80 (Ih + D5h) on different nitrogen sources. These
results reveal that the yield of Sc-NCFs depends strongly on
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Figure 4. HPLC chromatograms of Sc2O3/(NH4)3PO4·3H2O extracts obtained by using (NH4)3PO4·3H2O as the nitrogen source with different
molar ratio of Sc2O3/(NH4)3PO4·3H2O (10 mm × 250 mm Buckyprep column; flow rate 5.0 mL/min; injection volume 5 mL; toluene as eluent
(mobile phase); 40 °C). Fraction A mainly consists of Sc3N@C80 (Ih + D5h).
not only the nitrogen-containing group but also its counter ion
as discussed in more detail below. According to the comparison
of the HPLC intensity of fraction A, clearly ammonium
phosphates ((NH4)xH3‑xPO4, x = 1−3) and ammonium
thiocyanate (NH4SCN) work as better nitrogen sources than
others, and the highest yield is achieved when NH4SCN was
used as a nitrogen source (see Figure 3).
was still gaseous N2, and Cu(NO3)2 was believed to behave as
an additive, while the effectiveness of Cu(NO3)2 as a nitrogen
source independently was not studied.24 Therefore, our present
work demonstrates clearly that Cu(NO3)2 can work as an
independent nitrogen source. Interestingly, in addition to the
successful synthesis of Sc-NCFs using two nitrates (Cu(NO3)2
and NaNO3) as nitrogen sources, metal oxide clusterfullerene
such as Sc4O2@C80, which was discovered by Stevenson et al. in
2008 by introducing flowing air into the arc discharge
process,25 was also detected in the MS spectra of their extract
mixtures as a minor product with much lower yield than that of
Sc3N@C80 NCF (see Supporting Information, Figure S1),
suggesting that both nitrates could function as oxygen sources
for the synthesis of metal oxide clusterfullerenes as well.
Optimization of the Synthesis Conditions of Sc-NCFs.
Considering that NH4SCN and (NH4)3PO4 work as the best
nitrogen sources for the synthesis of Sc-based NCFs in terms of
the yield of Sc3N@C80 (Ih + D5h) as discussed above, we
optimized their synthesis conditions by varying the molar ratio
of Sc:N with the molar ratio of Sc:C fixed at 1:7.5.26 The HPLC
profiles of a series of fullerene extracts obtained under different
molar ratios of (NH4)3PO4:Sc2O3 ranging from 5:1 to 1:4.5
(i.e., molar ratio of N:Sc ranging from 7.5:1 to 1:3) are
compared in Figure 4, indicating the strong dependence of the
HPLC peak intensity of Sc3N@C80 (Ih + D5h) in fraction A as
well as the empty fullerenes (mainly C60 and C70) on the molar
ratio of (NH4)3PO4:Sc2O3. Such a dependence is illustrated in
Figure 5 and can be used to evaluate the dependence of the
yield of Sc3N@C80 (Ih + D5h) which is proportional to the
HPLC peak intensity of fraction A. Thus, the optimum molar
ratio of (NH4)3PO4:Sc2O3 is determined to be 2:1 (i.e., the
atomic molar ratio of N:Sc = 3:1). Besides, the effect of the
molar ratio of (NH4)3PO4:Sc2O3 on the intensity ratio of
Among these 12 new inorganic nitrogen sources, NH4SCN is
noteworthy because it affords the highest yield of Sc3N@C80 (Ih
+ D5h). Moreover, although NH4SCN works as an efficient
nitrogen source for the synthesis of Sc-NCFs, surprisingly our
previous attempts to use it as a nitrogen source for the synthesis
of Dy-NCFs was not successful as evidenced from both HPLC
and MS analysis.21 Replacing Dy (dysprosium) metal atom with
Sc (scandium) which has a much smaller ionic radius (0.75 Å
for Sc3+) than that of Dy3+ (0.91 Å),16 we now successfully
synthesized Sc-NCFs by using NH4SCN as nitrogen source. A
plausible reason might be the extremely low yield of Dy-NCFs
below the detection limit of MS spectroscopy under the
synthesis condition used in our previous work21 (no preheating
treatment, which seems however necessary for the yield
enhancement of Sc-NCFs as discussed below), which is to be
further verified experimetally later. Besides, Cu(NO3)2·2.5H2O
was previously used by Stevenson et al. as an additive for the
synthesis of Sc-NCFs via the TNT process with gaseous N2 as
the nitrogen source, resulting in the selective synthesis of Sc-
NCFs.24 Two roles of Cu(NO3)2·2.5H2O as additive were
proposed on the basis of the thermally decomposed products:
(1) the generated NOx vapor may suppress the formation of
empty fullerenes; (2) Cu moiety generated from a nonvolatile
route functions as a catalyst additive which may offset the
reactive plasma environment and enhance the yield of Sc-
NCFs. Nevertheless, in that report the major nitrogen source
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+ D5h) increased dramatically as demonstrated for several
nitrogen sources including NH4SCN, (NH4)3PO4, and
(NH4)2HPO4 (see Supporting Information, Figures S4−S6).
These results suggest that water may indeed affect the dc-arc
discharging process sensitively and our preheating treatment
even at such a low dc current leads to the successful removal of
water contaminant in the raw mixture. Although this preheating
treatment adds an additional step prior to dc-arc discharging, it
can be made “in-situ” with a relatively short time; therefore,
such an “in-situ” preheating treatment was applied for all of 12
inorganic nitrogen sources in this work.
Decomposition Process of the New Inorganic Nitro-
gen Sources During Discharging. In order to understand
the different performances of these 12 nitrogen sources, their
difference on the decomposition products should be consid-
ered. The thermal decomposition products of 12 inorganic
compounds under conventional heating conditions are available
in the literature,27−39 and summarized in Scheme 1 (see also
Table 1).
Figure 5. Effect of the molar ratio of (NH4)3PO4:Sc2O3 on the HPLC
peak intensities of A (a) and its relative yield to C60 (b).
In general ammonia salts decompose with temperature below
300 °C and generate gaseous ammonia (NH3), which plays a
key role for the formation of Sc-NCFs. On the other hand, for
other three types of nitrogen sources including thiocyanate,
nitrates, and nitrite, obviously their decompositions lead to the
generation of gaseous nitrogen (N2) instead (see Table 1).
Assuming that the thermal decomposition behavior of these
inorganic compounds under rapid heating and extremely high
temperature during discharging (∼4000 K40) is similar to those
under the conventional heating conditions, it is reasonable to
attribute the difference on the decomposition products (NH3
or N2) between ammonia salts and other three types of
nitrogen sources to their difference on the performance in
terms of the yield of Sc-NCFs, and the detail is discussed below.
Dependence of the Yield of Sc-NCFs on the Oxidation
State of Nitrogen and Counter Ion. Since the different
performances of the 12 nitrogen sources in terms of the yield of
Sc-NCFs are proposed to be correlated with their decom-
position products (NH3 or N2) as the precursors for NCF
formation, these 12 nitrogen sources can be divided into two
groups based on their decomposition products: one is
ammonium salts which decompose to NH3 (NH3-based
group), and the decomposition products of another group
(thiocyanate, nitrates, and nitrite) are all based on N2 (N2-
based group). Figure 6 shows a close-up comparison of the
HPLC profiles and yield of Sc3N@C80 (Ih + D5h) of these two
fraction A (Sc3N@C80 (Ih + D5h)) to C60 is also plotted in
Figure 5, which is directly correlated to the relative yield of
(Sc3N@C80 (Ih + D5h)) to C60 as an indication of the selectivity
of Sc3N@C80 (Ih + D5h). Obviously the highest selectivity of
Sc3N@C80 (Ih + D5h) is achieved when the molar ratio of
(NH4)3PO4:Sc2O3 is set at 3:1, which is somewhat different to
the optimum one (2:1) in terms of the yield of Sc3N@C80 (Ih +
D5h). This discrepancy suggests that there is probably a trade-
off between the yield and selectivity of Sc-NCFs when
(NH4)3PO4 is used as the nitrogen source, and this phenomena
might be due to the influence of the (PO4)3‑ anion as discussed
below. Likewise, the optimum molar ratio of NH4SCN:Sc2O3 is
determined to be 3:1 (i.e., the atomic molar ratio of N:Sc =
3:1) (see Supporting Information, part S3).
As another strategy to optimize the synthesis conditions, we
noticed that some of the inorganic nitrogen sources studied in
the present work are hydrates (e.g., (NH4)3PO4·3H2O,
Cu(NO3)2·3H2O) or hygroscopic, hence an “in-situ” preheat-
ing treatment was carried out in order to remove the hydrated/
adsorbed water. Because of the relatively low decomposition
temperatures of all of 12 inorganic nitrogen sources (see Table
1), such an “in-situ” preheating treatment should be made by
contacting two electrodes for 30 min under a very low dc
current (ca. 10 A for the graphite rods of Φ 8 mm ×150 mm
used in our generator). Interestingly, applying such an “in-situ”
preheating treatment, we found that the yield of Sc3N@C80 (Ih
Table 1. Oxidation States and Decomposition Properties of Different Nitrogen Sources
a
entry
category
compd
oxidation state of nitrogen
decomp temp (°C)
decomp products
NH3, P2O5, H2O
refs
1
ammonium salt
ammonium salt
ammonium salt
ammonium salt
ammonium salt
ammonium salt
ammonium salt
ammonium salt
thiocyanate
(NH4)3PO4·3H2O
(NH4)2HPO4
NH4H2PO4
(NH4)2SO4
(NH4)2CO3
NH4F
−3
−3
−3
−3
−3
−3
−3
−3
−3
+5
+5
+3
100
100
200
213
20
27
28
28
2
NH3, P2O5, H2O
NH3, P2O5, H2O
NH3, H2O, SO2, N2
NH3, CO2, H2O
3
4
28, 29
28
5
6
145
200
227
450
109
600
620
NH3, HF
28
7
NH4Cl
NH3, HCl
28, 30
28, 31, 32
33
8
NH4SCN
NH3, CS2, C3H6N6 (melamine)
K2S, CS2, (CN)2, N2
CuO, Cu, O2, H2O, NOx, N2
Na2O, N2, O2
9
KSCN
10
11
12
nitrate
Cu(NO3)2·3H2O
NaNO3
34−36
37−39
37−39
nitrate
nitrite
NaNO2
Na2O, N2, O2
a
The temperature at which the decomposition reaction begins.
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Scheme 1. Decomposition Reactions of 12 Inorganic
Nitrogen Sources under Conventional Heating Conditions
a
Figure 6. (I) HPLC chromatograms of Sc2O3/(NH4)3PO4·3H2O and
Sc2O3/NH4SCN) extracts obtained by using (NH4)3PO4·3H2O and
NH4SCN as the nitrogen sources (copied from Figure 1). For
comparison, the chromatograms of Sc2O3/CO(NH2)2, Sc2O3/NH3,
and Sc2O3/CH5N3·HSCN (two rods) extracts reported previously
(refs 21 and 22) were also included. Note that the Sc2O3/CO(NH2)2
extract was obtained from the discharging of one rod of Sc2O3/
CO(NH2)2 (1:3 molar ratio) mixture with an “in-situ” preheating
treatment. Inset: Comparison of the HPLC intensity of fraction A per
gram Sc2O3 obtained from different nitrogen sources. (II) HPLC
chromatograms of Sc2O3/NH4SCN, Sc2O3/KSCN, Sc2O3/Cu(NO3)2,
Sc2O3/NaNO3, and Sc2O3/NaNO2 extracts obtained by using
NH4SCN, KSCN, Cu(NO3)2, NaNO3, and NaNO2 as the nitrogen
sources, respectively (copied from Figure 1). The chromatogram of
Sc2O3/N2 (three rods) extract is also plotted for comparison. (III)
Comparison of the HPLC intensity of fraction A per gram Sc2O3
obtained from different nitrogen sources.
a
These are presumed to be the possible decomposition processes
during discharging as well. In eq 18, the generated melamine
condenses into “melem” with the further release of NH3.22,32
groups. For the ammonia salts, only NH4SCN and (NH4)3PO4
as the best nitrogen sources are taken for clarity, which are
compared with the reported nitrogen sources based on NH3,
including gaseous NH3, guanidinium thiocyanate
(CH5N3·HSCN), and urea (CO(NH2)2) (Figure 6I). Ob-
viously the yield of Sc3N@C80 (Ih + D5h) per gram Sc2O3 by
using NH4SCN and (NH4)3PO4 surpasses those by using other
NH3-based nitrogen sources, indicating the advantage of these
two new inorganic nitrogen sources (see inset of Figure 6I and
Supporting Information, Table S1). Likewise, the HPLC
profiles and yield of Sc3N@C80 (Ih + D5h) of the N2-based
group of nitrogen sources, i.e., thiocyanate, nitrates, and nitrite,
are compared to those of gaseous N2 (Figure 6II). NH4SCN
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represents the best nitrogen source in terms of the yield of Sc-
NCFs in the NH3-based group and is thus also included for
comparison. Interestingly, it is found that the yield of
Sc3N@C80 (Ih + D5h) per gram Sc2O3 by using this N2-based
group of nitrogen sources is overall much lower than those by
using gaseous N2 and NH4SCN (see Figure 6III and
Supporting Information, Table S1). These results indicate the
strong dependence of the yield of Sc-NCFs on the nitrogen-
containing group in which the oxidation state of nitrogen is
different (see Table 1).
For ammonium salts as the NH3-based group of nitrogen
source, the oxidation states of nitrogen (−3) keep constant in
the decomposed product: NH3. However, within the N2-based
group of nitrogen sources, thiocyanate, nitrite, and nitrates
adopt −3, +3, and +5 oxidation states of nitrogen, respectively,
which change upon the formation of the decomposed product:
N2. Hence, an “in-situ” redox reaction takes place along with
the decomposition reaction of these N2-based group of
nitrogen sources during the discharging, which is however
not required for the ammonium salts. Such an “in-situ” redox
reaction may be regarded as a side reaction and partially
dissipate the energy generated from dc-arc discharging,
consequently affecting the fullerene growth and lowering the
yield of Sc-NCFs. Hence, the dependence of the yield of Sc-
NCFs on the oxidation state of nitrogen may be attributed to
the “in-situ” redox reaction of the nitrogen sources.
NaNO2, the yield of Sc-NCFs is found to be quite comparable
(see Figure 6III and Supporting Information, Table S1),
suggesting that in this case the formation of Sc-NCFs is
independent to both the oxidation state of nitrogen (+3 or +5)
and the cation. This phenomenon is obviously different from
cases of NH3-based group of nitrogen source. A plausible
interpretation is that the “in situ” generated gaseous N2 in the
decomposed products plays a dominant role in the formation of
Sc-NCFs, whereas other “in situ” generated species, which are
variable with the change of cation, affect little of this formation
process. On the other hand, in terms of the relative yield of
Sc3N@C80 (Ih + D5h) to C60, interestingly Cu(NO3)2 exhibits a
much higher selectivity of Sc-NCFs than those obtained by
NaNO3 and NaNO2, suggesting Cu2+ is more beneficial to the
formation of Sc-NCFs compared to Na+. According to a
previous study by Stevenson et al. reporting the use of
Cu(NO3)2·2.5H2O as additive for the synthesis of Sc-NCFs via
the TNT process with gaseous N2 as the nitrogen source, the
generated NOx vapor as the decomposed product of Cu(NO3)2
may suppress the formation of empty fullerenes, and
concurrently the Cu moiety generated from a nonvolatile
route may enhance the yield of Sc-NCFs by offsetting the
reactive plasma environment. As a result the selective synthesis
of Sc-NCFs was accomplished.24 In our case, a similar
mechanism may be applicable to interpret the observed higher
selectivity achieved by Cu(NO3)2 than those by NaNO3 and
NaNO2.
The yield of Sc3N@C80 (Ih + D5h) by using different nitrogen
sources within the same NH3- or N2-based group is then
compared and analyzed in detail so as to study the effect of
counter ion. For ammonium salts, when the anion changes
CONCLUSIONS
■
In summary, 12 nitrogen-containing inorganic solid compounds
with variable oxidation states of nitrogen and counter ions,
including ammonium salts ((NH4)xH3‑xPO4 (x = 0−2),
(NH4)2SO4, (NH4)2CO3, NH4X (X = F, Cl), NH4SCN),
thiocyanate (KSCN), nitrates (Cu(NO3)2, NaNO3), and nitrite
(NaNO2), have been successfully applied as new inorganic solid
nitrogen sources toward the synthesis of Sc-NCFs. Among
them, triammonium phosphate hydrate ((NH4)3PO4·3H2O)
and ammonium thiocyanate (NH4SCN) are revealed to behave
as better nitrogen sources than others, and the highest yield of
Sc-NCFs is achieved when NH4SCN was used as a nitrogen
source. The optimum molar ratio of Sc2 O3 :
(NH4)3PO4·3H2O:C and Sc2O3:NH4SCN:C has been deter-
mined to be 1:2:15 and 1:3:15, respectively. An “in-situ”
preheating treatment by contacting two electrodes under a very
low dc current has been also developed as another strategy to
optimize the synthesis conditions in order to remove the
hydrated/adsorbed water of these inorganic compounds.
The thermal decomposition products of these 12 inorganic
compounds have been discussed in order to understand their
different performances toward the synthesis of Sc-NCFs, and
accordingly the dependence of the production yield of Sc-
NCFs on the oxidation state of nitrogen and counter ion is
interpreted. These 12 nitrogen sources can be divided into two
groups: one is ammonium salts which decompose to NH3
(NH3-based group), and the decomposition products of
another group (thiocyanate, nitrates, and nitrite) are all based
on N2 (N2-based group). The yield of Sc3N@C80 (Ih + D5h) per
gram Sc2O3 by using the N2-based group of nitrogen sources is
overall much lower than those by using gaseous N2 and
NH4SCN, indicating the strong dependence of the yield of Sc-
NCFs on the oxidation state of nitrogen, which is attributed to
the “in-situ” redox reaction taking place for the N2-based group
of nitrogen sources during discharging. For NH3-based group
2‑
2‑
from the oxyacidic groups ((H3‑xPO4)x‑, SO4 , CO3 ) to
halogen, the yield of Sc3N@C80 (Ih + D5h) decreases
dramatically (see Figure 3), suggesting that the performance
of ammonium salts as nitrogen sources is sensitively dependent
on the anion despite the gaseous NH3 generating “in situ”
directly leads to the formation of Sc-NCFs. A possible reason
for this phenomena is that such compounds like (NH4)2CO3,
(NH4)2SO4, NH4F, and NH4Cl are more easily decomposed at
the beginning of discharging because of their much lower
thermal stabilities as indicated by their lower decomposition
temperatures and/or larger decomposition rate, resulting in the
inevitable loss of reactants. Indeed, we found that the
decompositions of (NH4)2CO3, (NH4)2SO4, NH4F, and
NH4Cl were so violent that some mixture jetted from the
filled graphite rod at the beginning of discharging and became
far away from the arc center. Besides, the existence of the
nonmetal atom other than oxygen atom (P, S, F, Cl, etc.) might
affect the formation of Sc-NCFs as well. Contrarily, the
decomposition temperature of NH4SCN is even higher than
(NH4)xH3‑xPO4), so the loss of reactant at the beginning of
discharging could be suppressed; thus, the higher yield of
Sc3N@C80 (Ih + D5h) is achieved.
In the N2-based group of nitrogen sources including
thiocyanate, nitrates, and nitrite, nitrogen atom is a component
of anion, and we vary the cation so as to study its effect on the
yield of Sc-NCFs. When the cation of NH4SCN is substituted
by K+, the yield of Sc-NCFs decreases dramatically (see Figure
6II and Supporting Information Table S1). This suggests that
+
for NH4SCN the ammonium cation (NH4 ) contributes
overwhelmingly to the formation of Sc-NCFs instead of the
thiocyanate anion (SCN−).
For the three nitrates (nitrite) which also belong to the N2-
based group of nitrogen sources, including Cu(NO3)2, NaNO3,
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Article
of nitrogen sources which exhibits a (−3) oxidation states of
nitrogen, their performance as nitrogen sources is found to be
sensitively dependent on the anion, and this is understood by
considering their difference on the thermal stability and/or
decomposition rate. Contrarily, for the N2-based group of
nitrogen sources, the formation of Sc-NCFs is independent to
both the oxidation state of nitrogen (+3 or +5) and the cation.
The new synthesis method by using a series of inorganic solid
nitrogen sources is expected to be developed as a general route
to the high yield synthesis of NCFs, and the versatility of the
nitrogen sources revealed in this study provides a new insight
into the chemical reactions involving NCF formation during
arc-discharging.
ACKNOWLEDGMENTS
■
We thank the financial support from the National Natural
Science Foundation of China (90921013, 21132007), “100
Talents Programme of CAS” from Chinese Academy of
Sciences, and National Basic Research Program of China
(2010CB923300, 2011CB921400).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Comparison of yield of Sc3N@C80 (Ih + D5h) per gram Sc2O3
by using different nitrogen sources, enlarged MS spectra of
fullerene extracts obtained by using Cu(NO3)2·3H2O or
NaNO3 as nitrogen sources, optimization of molar ratio of
Sc:N by using NH4SCN as nitrogen source, and effect of the
“in-situ” preheating treatment on the synthesis of Sc-NCFs, etc.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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ratio of Sc:N:C is fixed at 1:3:7.5 which is selected according to the
optimized conditions for NH4SCN or (NH4)3PO4 (see Table S1).
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sfyang@ustc.edu.cn.
Notes
The authors declare no competing financial interest.
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