Sulfur-assisted synthesis of nitride nanocrystals

Article abstract of DOI:10.1039/b920429f

A series of nitrides (TiN, ZrN, BN, AlN) were prepared by using the corresponding elements (Ti, Zr, B, Al), NaN₃ and sulfur as starting materials in a stainless steel autoclave at 250 °C. Sulfur was used to facilitate the exothermic reaction between NaN₃ and sulfur (at 250 °C) and the final formation of nitrides. The treatment temperature affected the growth of the nitride crystals, for example, diversified morphologies of TiN nanocrystals were formed in different temperature ranges: grain and truncated octahedron (250 °C), octahedron (>300 °C), and dendrite (>400 °C). Through similar processes, other nitrides (for example, TiN, AlN, Si₃N₄) could also be produced by employing NaNH ₂ and additives (such as iodine or N-aminothiourea instead of sulfur) in low temperatures.

Full text of DOI:10.1039/b920429f

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www.rsc.org/dalton | Dalton Transactions
Sulfur-assisted synthesis of nitride nanocrystals
Lishan Yang, Hongxiao Yu, Liqiang Xu,* Qiang Ma and Yitai Qian*a
a
b
a
a,b
Received 30th September 2009, Accepted 1st December 2009
First published as an Advance Article on the web 5th February 2010
DOI: 10.1039/b920429f
A series of nitrides (TiN, ZrN, BN, AlN) were prepared by using the corresponding elements (Ti, Zr, B,
◦
Al), NaN
3
and sulfur as starting materials in a stainless steel autoclave at 250 C. Sulfur was used to
◦
facilitate the exothermic reaction between NaN
3
and sulfur (at 250 C) and the final formation of
nitrides. The treatment temperature affected the growth of the nitride crystals, for example, diversified
morphologies of TiN nanocrystals were formed in different temperature ranges: grain and truncated
◦
◦
◦
octahedron (250 C), octahedron (>300 C), and dendrite (>400 C). Through similar processes, other
nitrides (for example, TiN, AlN, Si ) could also be produced by employing NaNH and additives
such as iodine or N-aminothiourea instead of sulfur) in low temperatures.
3
N
4
2
(
◦
Introduction
The whole reaction was started at 250 C, TiN, BN, AlN, ZrN
◦
were formed at a treatment temperature of 250 C. The possible
reaction processes and the effect of treatment temperature on the
shapes of the nitride crystals have also been discussed.
Nitrides are important technological materials due to their
properties including thermal, mechanical, and chemical stabilities,
wide band-gap, or electrical conductivity. Thus, nitrides can be
1
applied in many fields, for example, as abrasion-resistant coatings
2
3
and electroanalysis (TiN), lubricants and optical coatings (BN),
Experimental
4
light- and field-emitting devices (AlN), superconducting films
ZrN), high-temperature ceramics and catalysts.
5
6
(
Synthesis and purification
Traditionally, nitrides have been synthesized by direct
Ti powders (>99% purity, 200-300 mesh), B amorphous powders
7
◦
8
nitridation, carbothermal reduction and nitridation (> 1000 C),
(
>95.3% purity), Al powders (>99% purity, 100-200 mesh), Zr
powders (>92% purity, 300 mesh), sublimed sulfur (>99.5%
purity), NaN (>98% purity). The amorphous B powders were
◦
9
self-propagating high-temperature synthesis (800–1300 C),
◦
10
chemical vapor deposition (550–1500 C), and ammonolysis of
◦
11
3
sulfides (> 750 C). Several general synthesis methods have
purchased from Liaobin Chemical Co. Ltd. (Yingkou, China). All
the other reagents used in the experiments were purchased from
Shanghai Chemical Reagents Co.
also been developed. For example, the solid-state metathesis
(
SSM) route can be used to synthesize nitrides by using metal
◦
12
halides and Li
3
N or NaN (440–850 C), or by employing
3
1
In a typical synthesis procedure for TiN, Ti powder (0.01 mol),
3a
13b
13c
metal oxides with a-C
3
N
3.69
,
or cyanamide/urea, or Li N,
600–1200 C). Ammonolysis of organometallic precursors is
3
NaN (0.042 mol), and sublimed sulfur (0.02 mol) were mixed
3
◦
(
and put into a 20 mL stainless-steel autoclave. Then the autoclave
applied to synthesize nitrides in relatively low temperatures: TiN
◦
was sealed and heated from room temperature to 250 C at a
◦
14a
◦
14b
◦
14c
(
(
150–450 C), ZrN (>300 C), AlN (>400 C), and BN
◦
-1
rate of 10 C min and kept for 4 h. After it was naturally
cooled to room temperature, the raw product was washed in turn
with ethanol, distilled water, hydrochloric acid (0.5 M), distilled
◦
14d
>800 C). Nitrides synthesized via this method often have a
certain proportion of hydrogen content. Solvothermal synthesis is
◦
15a
an effective method for the preparation of nitrides: TiN (350 C),
◦
water and ethanol, finally dried in a vacuum oven at 60 C for
◦
15b
◦
15c
ZrN (380 C),
AlN (250 C), hexagonal and cubic BN
8
h. By similar procedures, other nitrides (BN, AlN, ZrN) were
◦
15d
(
480 C). However, it is still desired to develop general and
synthesized starting from the corresponding elemental powders at
simple methods to synthesize nitrides in lower temperatures.
Very recently, a sulfur-assisted approach was developed to
◦
2
50 C.
To study their effects on the morphology diversifications of the
◦
16
synthesize Si
3
N nanocrystals (250 C). In this study, a series of
4
nitrides (TiN, BN, AlN, ZrN), different treatment temperatures
nitrides were prepared from the corresponding element powders
via a sulfur-assisted synthesis process (SAS) following [eqn (1)]:
◦
(
250–500 C) were used. Details of the experiments and the
morphologies of the nitrides products are listed in Table 1.
xM + 2yNaN
3
+ yS → M
x
N
z
+ yNa
2
S + (6y - z)/2N
2
(M =
Ti, B, Al, Zr)
(1)
Characterization
The final products were characterized by an X-ray powder diffrac-
tometer (XRD; Bruker D8 with Cu-Ka radiation), a scanning
electron microscopy (SEM; JSM-6700F, 10 kV), a high-resolution
transmission electron microscope (HRTEM; JEOL 2100; 200 kV)
and X-ray photoelectron spectroscopy (XPS; VG ESCALAB 250
with monochromatized Al KR radiation).
a
Key Laboratory of Colloid and Interface Chemistry (Shandong University),
Ministry of Education, and School of Chemistry and Chemical Engineering,
Shandong University, Jinan, 250100, PR China. E-mail: xulq@sdu.edu.cn;
Fax: +86-531-8836-6280; Tel: +86-531-8836-6280
b
China Tobacco Shandong Industrial Corporation, Jinan, 250100, PR China
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Table 1 Experimental conditions, structural information and morphologies of the as-obtained nitrides
a
◦
Starting metals, Molar ratio
T/ C, time/h
Products, Space groups, JCPDS card No.
Morphologies (most are in the nanometre scale)
¯
1
2
3
Ti, 1 : 2 : 4
250, >4
300, >0.2
400, >0.2
TiN, Fm3m, 38-1420
grains, truncated octahedrons
octahedrons, truncated octahedrons, grains
octahedrons, dendrites, truncated octahedrons,
grains
4
5
6
7
8
9
B, 1 : 4 : 8
Al, 1 : 2 : 4
Zr, 1 : 4 : 8
250, >4
≥300, >2
250, >4
≥300, >2
250, >4
≥300, >0.5
BN, F6
AlN,F6
ZrN, Fm3m, 65-9411
3
/mmc, 34-0421
membranes
membranes, quasi-hollow spheres
particles
particles, short-rods, dendrites
grains
grains, octahedrons
3
/mc, 25-1133
a
Refers to metal : sulfur : sodium azide.
Results and discussion
Undeveloped flat faces (Fig. 2a f1–f5) and edges (dashed lines)
were found on some of the nanograins, which partly agreed with
the nature of the truncated octahedron as reported previously.
High-magnification TEM images (inset) of the TiN nanograins
show the distance of neighboring lattice fringes was 0.245 nm,
corresponding to the (111) planes of TiN (JCPDS #38-1420).
Compared to the nanograins, the octahedra are larger in diameter
Fig. 1 shows the typical XRD patterns of the nitrides prepared via
this sulfur-assisted synthesis (SAS): the raw product is a mixture
of TiN (JCPDS #38-1420) and Na
being washed (Fig. 1a); the following are patterns of pure nitrides:
17a
2
S (JCPDS #65-0525) before
(
(
b) TiN (JCPDS #38-1420), (c) BN (JCPDS #34-0421), (d) AlN
JCPDS #25-1133), (e) ZrN (JCPDS #65-7723). All the nitrides in
(100–300 nm) and have regular flat faces with clear edges (Fig. 2
◦
Fig. 1 were obtained at 250 C. The calculated cell parameters of
these nitrides (listed in the caption of Fig. 1) are in agreement with
their known values. In the contrast experiments, no significant
peak shift was observed when the treatment temperature (250 C)
and time (4 h) were increased.
b1). Also, some truncated or imperfect octahedra were formed,
which might be as a result of the fast cooling rate during the
17b,17c
reaction.
The HRTEM image (Fig. 2 b2) taken from one
◦
corner of a TiN octahedron shows well defined 2D lattice planes
and the distances of neighboring lattice fringes coincide with
the (111) and (200) lattice spacings of TiN (JCPDS #38-1420).
The corresponding fast Fourier transform (FFT) pattern (inset in
Fig. 2b) can be assigned to face-centered cubic TiN, indicating
that the octahedron is a single crystal with its [011] orientation
parallel to the electron beam. Raising the treatment temperature
would increase the yield of octahedral TiN nanocrystals.
TiN dendrites were observed in the as-synthesized products
◦
when the treatment temperature was increased above 400 C. The
TEM image (Fig. 2c) reveals the dendrites have hierarchical
treelike branches, the SEM image (Fig. 2 d6) clearly shows the
TiN three-dimensional (3D) dendritic structures: the secondary
branches are parallel to each other in the same plane and
perpendicular to the primary trunks. The lengths of the longest
primary trunk and the secondary branches could reach 600 nm and
2
50 nm, respectively. The EDS spectrum (inset of Fig. 2c) indicates
that the dendrites consist of Ti and N. Fig. 2d shows various
TiN nanostructures obtained at 400 C: grains (d1); octahedra
Fig. 1 Typical XRD patterns of the synthesized nitrides: (a) TiN before
and (b) after being washed [the ᭹ and ᭢ indicate the diffraction peaks
◦
(d2), overdeveloped octahedra (d3), undeveloped dendrites (d4),
from TiN (JCPDS #38-1420, a
respectively]; (c) ZrN (JCPDS #65-7723, a
25-1133, a
= 3.108 A˚ , b = 4.980 A˚ ); (e) BN (JCPDS #34-0421, a
.511 A˚ , b
= 6.662 A˚ ). All the nitrides were obtained at 250 C for 4 h.
1
= 4.243 A˚ ) and Na
2
S (JCPDS #65-0525),
= 4.577 A˚ ); (d) AlN (JCPDS
and developed dendrites (d5 and d6). It is thought that higher
temperatures have more energy available to overcome activation
barriers, and the product crystal growth will continue as long
2
#
2
3
3
4
=
◦
2
18
as the byproduct salt remains molten due to Ostwald ripening.
The yields of TiN nanocrystals with different morphologies
Additionally, hotter reactions take longer to cool, more time
will be available for crystal growth, generally leading to better
crystallinity and greater yields.
varied with the treatment temperature (Table 1; Fig. 2). TiN quasi-
spherical nanograins and truncated octahedrons (with a diameter
range of 50–150 nm) were the main products under a treatment
The shape of nanocrystals is closely related to the crystallo-
graphic surfaces, because the crystal faces with higher growth rates
will shrink and those with lower growth rates will expand during
◦
temperature of 250 C (Fig. 2a). TiN octahedra were formed above
◦
3
00 C, and the ratio of octahedra and truncated octahedra would
◦
17a
reach ~80% at 400 C (Fig. 2b). And TiN dendrites were found
the crystal growth. Low-index crystallography planes ({100}
◦
in the products above 400 C, and its ratio would reach ~8% at
and {111}) are preferentially formed on the nanoparticle surface
◦
5
00 C (Fig. 2c).
to lower the particle’s surface energy. Thus, octahedra are the result
2
856 | Dalton Trans., 2010, 39, 2855–2860
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Fig. 2 (a) SEM images of TiN nanograins. Insets: a typical nanograin with undeveloped flat faces (f1–f4) and edges (dashed lines); a high-magnification
TEM image taken from the top of a nanograin. (b) SEM images of TiN octahedra and truncated octahedra (b1); a HRTEM image taken from one corner
of an octahedron (b2), inset: related fast Fourier transform pattern. (c) A typical TEM image of the TiN dendrites. Insets: a HRTEM image taken from
the designated area (left) and the EDS spectrum (right). (d) SEM images with TiN nanostructures with different shapes: grain (d1); octahedron (d2);
◦
◦
overdeveloped octahedron (d3); undeveloped dendrite (d4); developed dendrite (d5 and d6). The products were collected at 250 C (a), 350 C (b), and
◦
4
00 C (c) for 4 h, respectively.
1
9
of a high growth rate along the {100} faces, while dendritic or
from Ti powders and undergo a fast growth in all directions,
which would produce quasi-spherical TiN nanograins. Because
of the preferential growth of the {100} facets, TiN nanocrystals
undergo a shape evolution from grains to truncated octahedra,
then octahedra, and finally dendrites.
star-like structures are often formed through preferential growth
2
0
of the [100] directions of truncated octahedra. On the basis of the
above theories, we could hypothesize the formation and the shape
evolution of TiN nanocrystals as illustrated in Fig. 3. In the SAS,
the system temperature was increased by the exothermic reaction
Fig. 4 shows the XPS spectra of the TiN products obtained at
◦
of S/NaN
3
abruptly and decreased naturally to the heating
400 C. The binding energy of N1s matches well with the literature
temperature under a rapid cooling rate. TiN nuclei were formed
value of 397.0 eV (upper-left in Fig. 4), and the binding energies
◦
Fig. 3 A schematic of a proposed mechanism for the formation and shape
Fig. 4 XPS spectra of TiN nanocrystals synthesized at 400 C for 4 h.
evolution of TiN nanocrystals by SAS.
Inset: N1s spectrum (upper-left); and Ti2p spectrum (upper-right).
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of the Ti 2p doublet lines for TiN nanocrystals are 455.2 eV and
61.0 eV (upper-right in Fig. 4), which are in agreement with
the published literature values for Ti2p3/2 and Ti2p1/2 spectra
(insets of Fig. 5c and 5d) indicated the AlN and ZrN nanocrystals
are pure and well crystallized.
4
21
of titanium nitride, respectively. Quantitative analysis gives the
average Ti : N atomic ratio of 1.05 : 1, this value is close to that
of TiN. The appearance of carbon and oxygen signals in the
survey spectrum (Fig. 4) might be attributed to surface oxidation
Chemical reactions of SAS
Previous works of SSM found that similar exothermic reactions
often experience an intermediate state before product formation,
which would change the original energy barrier (from precursor to
product) into two smaller energy barriers: DEa1 (from precursor to
intermediate) and DEa2 (from intermediate to product), then the
entire reactions would occur in relatively low temperatures. (see
Fig. 8 in Ref. 12c). Furthermore, the formation of thermodynam-
ically favored by-products (such as H S, Li S, Na S, (NH ) CS,
21b
or absorption.
The transformations of B, Al and Zr powders to the correspond-
ing BN, AlN and ZrN nanocrystals by SAS offer more examples to
investigate the influence of the treatment temperature on the final
shapes of the nitrides. BN membranes (Fig. 5a) with a thickness of
◦
~
2 nm are the dominating shape at 250 C, while BN quasi-hollow
2
2
2
2 2
◦
11,12e,15e
spheres (Fig. 5b) were the main morphology above 400 C. The
honeycomb sp -like bonded layers and a similar layered structure
etc.) assists the low-temperature synthesis of the nitrides.
2
Without the addition of sulfur, no binary nitride was formed
to graphite are the intrinsic reasons why membranous, tubular and
vesicle-like BN could be synthesized. The typical morphologies
from the reactions of the element powders and NaN even at
3
3
◦
600 C. While the exothermic reaction between NaN and S
3
◦
16
of the obtained AlN were particles with a diameter of ~100 nm
occurred at ~250 C for 4 h from eqn (2). Then, the nitridation
◦
◦
at 250 C, and a few short-rods and dendrites could be formed
reactions were initiated. Taking Ti powders as an example, the
at 400 C (Fig. 5c). In the synthesis of ZrN, a low treatment
energy released from the reaction between NaN and S would
3
◦
temperature (250 C) or a high Zr molar ratio of the reagents
support NaN to decompose into Na and N , and release lots
3
2
22
(
Zr : S > 1 : 4) often results in the incomplete nitridation of Zr
of energy [eqn (3)]. All that energy released might support the
2
3
powders. ZrN particles with a diameter of 30 nm were the main
transformation of Ti to TiS (0.5 ≤ x ≤ 3) from eqn (4). And
x
◦
products at 250 C, while ZrN octahedra were formed above
TiN nanocrystals would be formed from a TiS /Na/N mixture
x
2
◦
11
4
00 C (Fig. 5d). The EDS measurements and the HRTEM images
system [eqn (5)] similar to the SSM model.
S + 2NaN S + 3N
3
→ Na
2
2
(2)
Fig. 5 TEM and related HRTEM images of nitride nanostructures: (a) BN membranes; (b) BN quasi-hollow spheres; (c) AlN particles and short rods;
◦
◦
(
(
d) ZrN particles and octahedra. Insets in c and d: SEM images and the corresponding EDS spectra. The products were collected at 250 C (a) and 400 C
b–d), respectively.
2
858 | Dalton Trans., 2010, 39, 2855–2860
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NaN
3
→ 2Na + 3N
2
(3)
(4)
(5)
reason why GaN/InN could not be formed by this method.
However, the exact reasons still need further investigation.
Ti + xS → TiS
x
NaN
3
/NaNH
2
+ additives → N
2
/NH
3
/H + by-products (8)
2
TiS
x
+ 2xNa + 1/2N
2
→ xNa
2
S + TiN
M + NaN
3
/NaNH
2
+ additives → MN + N
2
/NH
3
/H
2
+
by-products (additive = S, I , N-aminothiourea)
2
(9)
Meanwhile, another possible reaction pathway might have
occurred. The reaction between NaN and sulfur will produce
a large amount of heat energy [eqn (2)], which might support the
direct transformation from Ti to TiN through a progress similar
3
It was found that other additives such as iodine (I
N-aminothiourea could also react acutely with NaN
relatively low temperature, and some thermodynamically favored
by-products (Na S, NaI) would form [eqn (8)]. If iodine or N-
aminothiourea was used instead of sulfur in the SAS process
) or
2
3
or NaNH at
2
1
0
to the SHS model:
2
Ti + 1/2N
2
→ TiN
(6)
2
8
similar to the synthesis of Si
3
N
4
,
nitrides (TiN, AlN) could also
◦
However, both equations of these reaction models can be
expressed as:
be obtained under a treatment temperature below 200 C. When
NaN was substituted by NaNH in the SAS process, nitrides
3
2
◦
(
TiN, AlN, Si
3
N ) could also be synthesized at 380 C (above
4
Ti + 4NaN
3
+ 2S → TiN + 2Na
2
S + 11/2N
2
(7)
29
the decomposition temperature of NaNH
general additive-assisted method [eqn (9)] has successfully been
applied to synthesize eight nitrides (TiN, AlN, BN, ZrN, Si N ,
2
). Presently, this
The calculated thermodynamic factor values can also support
24
the above assumption: reactions [eqn (2)–eqn (7)] are thermody-
3
4
◦
-1
◦
namically spontaneous (e.g. DG (6) = -287.3 kJ mol , DG (7) =
1467.2 kJ mol ) and exothermic (e.g. DH (6) = -338.4 kJ mol ,
VN, CrN, WN). The exact reaction mechanism requires further
investigation. More chemicals will be investigated to replace sulfur
-
1
◦
◦
-1
-
◦
◦
-1
◦
DH (7) = -1165.6 kJ mol ). Because DG (7) ꢀ DG (6) < 0, and
or NaN in preparing nitrides by similar additive-assisted methods.
3
◦
DH (7) ꢀ DH (6) < 0, the reaction in eqn (7) (SAS) is more prone
to occur than that in eqn (6) (SHS).
[Caution: the reaction between NaNH
2
and I can occur below
2
◦
100 C, and I
2
can react intensively with extra Na (decomposed
In this SAS process, TiN could not be obtained if the treatment
time was shorter than 4 h at 250 C, while TiN could be formed
from NaN
3
or NaNH ) during the purification of the raw products.]
2
◦
within 10 min when the treatment temperature was increased
Conclusions
◦
above 300 C. Other nitrides (BN, AlN, ZrN) could also be
◦
obtained through SAS at 250 C. While elements like vanadium(V),
The SAS process offers a general and simple method for the
synthesis of nitrides (TiN, AlN, BN, ZrN, Si N , VN, CrN, WN)
from element powders. The exothermic reaction between NaN3
and S was favorable for the final formation of nitrides at relatively
low temperature. Nitride nanocrystals with regular shapes could
chromium (Cr) and tungsten (W) need higher temperatures (VN:
3
4
◦
◦
◦
4
00 C; CrN: 450 C; WN: 450 C) to be converted into related
nitrides by a similar method; metals like Cr and W are often used
as additives to enhance the corrosion resistance and mechanical
25
◦
strength of low-alloy steels. The average yields (calculated
according to the amount of elements) of each product are: 52%
be formed at higher treatment temperatures (>400 C). Instead
of sulfur and NaN , other additives (I or N-aminothiourea) and
3
2
(
TiN), 41% (BN), 44% (AlN), 45% (ZrN), 37% (VN), 32% (CrN),
NaNH were successfully used to synthesize several nitrides (TiN,
2
3
0% (WN).
AlN, Si N ) at extremely low temperatures.
3
4
Up to now, SHS (or combustion synthesis) has been successfully
9
applied to prepare lots of nitrides (e.g. Si
3
4
N , TiN, AlN, etc.),
Acknowledgements
however, to the best of our knowledge, GaN/InN has not been
synthesized through an SHS method (or combustion synthesis)
by using metallic Ga/In as the gallium or indium source. Our
experimental results are consistent with them to some extent: no
TiN/AlN has been obtained by using Ti/Al ribbons (1 cm long,
This work was financially supported by the National Natural Sci-
ence Found of China (Nos. 20871075, 20971079), the Ph.D. Pro-
grams Foundation of Ministry of Education of China (No.
20070422046), and the 973 Project of China (No. 2005CB623601),
and Independent Innovation Foundation of Shandong University.
0
.5 cm wide, and 0.2 mm thick) instead of Ti/Al powders (~200
mesh); Neither GaN nor InN has been obtained from metallic
◦
Ga/In by this method. As the melting points of Ga (29.89 C)
Notes and references
◦
◦
and In (156.40 C) are lower than 250 C, but the boiling points
of Ga (2203 C) and In (2072 C) are markedly higher than
50 C. Thereby, the Ga and In would melt into liquids and
congregate at 250 C. And we found only Ga/In bulk solids
but not GaN/InN nanocrystals in the final products. According
to previous reports, crystals with a smaller size would have a
higher specific surface area along with a higher specific surface
energy, which would result in some different physical properties
and chemical reactivities in contrast to the bulk materials. It is
considered that the preferential formation of Ga/In bulk molten
◦
◦
1
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