Route to GaN and VN assisted by carbothermal reduction process

Article abstract of DOI:10.1021/ja055877i

A route to prepare nitrides, such as GaN, VN, and other nitrides, is reported. The reaction pathway involves a two-step process by using the as-synthesized a-C₃N_3.69 as precursor. The route is so potent that a series of nitrides can be directly synthesized from their oxides at moderate temperatures. A striking feature of this method lies in that a-C₃N_3.69 is found to play double roles as both carbonizing and nitridizing agent in these reactions. These results will greatly deepen our understandings of the mechanism for solid-state metathesis reactions. Copyright

Full text of DOI:10.1021/ja055877i

Published on Web 10/21/2005
Route to GaN and VN Assisted by Carbothermal Reduction Process
†
†
†
‡
,†
Huaizhou Zhao, Ming Lei, Xin’an Yang, Jikang Jian, and Xiaolong Chen*
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O.
Box 603, Beijing 100080, PR China, and Center for Condensed Material Physics, School of Sciences, Beihang
UniVersity, Beijing 100083, PR China
Received September 4, 2005; E-mail: xlchen@aphy.iphy.ac.cn
Metal nitrides are among the important materials that find wide
applications from optoelectronic devices to high-temperature coat-
1
ings. GaN, for example, has been recognized as one of the most
important semiconductors in the fabrication of blue LEDs and LDs.2
Conventionally, metal nitrides are made by the direct reactions of
metals with ammonia or nitrogen at high temperatures (g1000 °C).3
An effective and low energy consuming route, solid-state metathesis
(
SSM), has been developed for synthesizing nitrides over the past
4
decade. The precursors are usually the metal halides and the alkali
nitrides in these reactions. The driving force for the reactions comes
from the formation of the stable alkali or alkaline earth halides
that are very thermodynamically favored. The exothermic energy
makes the reactions self-propagate to completion. However, the
temperatures are usually as high as 1100 °C, leading to the
decomposition for some nitrides, such as GaN. Other existing
methods, such as metal amide decomposition, nonaqueous solvo-
thermal reactions, and carbothermal reduction processes, suffer
various drawbacks such as poorly crystallized, low yields, and high
energy consumption to some extent. Consequently, new solid-state
routes are still desired to prepare metal nitrides.
5
Figure 1. XRD pattern of the GaN product.
6
7
8
Carbon nitride materials have been the focus of materials research
for the past decades due to their possible high hardness comparable
9
to diamond. Recently, by a facile mechanochemical method, we
3
prepared the amorphous carbon nitride (a-C N3.69) with the atomic
ratio of 1.23 for N to C.10 In this communication, we report a solid-
state route to GaN based on a modified SSM reaction. Here,
3
however, we choose a-C N3.69 as a precursor with another one being
an oxide to form an SSM reaction. This reaction might be enhanced
by the formation of carbon monoxide and nitrogen byproducts that
are much more stable than the precursors. GaN and other nitrides
are expected to be obtained in a much softer condition by this route.
Our experiments show that the route allows one to nitridize the
gallium oxide into GaN at 650 °C. This route can be extended to
prepare other nitrides. VN nanocrystals are also synthesized by using
Figure 2. TEM image of the GaN nanocrystallites; inset is the HRTEM
image of the lattice.
2 5 3
V O and carbon nitride at 850 °C. A-C N3.69 is found to play double
roles as both carbonizing and nitridizing agents in these reactions.
The details of the preparation procedure are as follows:
of the ampule. Warning: appropriate quantity of the reactant
mixture should be adopted to avoid possible explosion due to high
pressures.
Energy-dispersive X-ray analysis (EDX) shows that the product
consists of Ga, N, and O elements, and the black particles on the
inner surface carbon element. The O element probably comes from
In a typical run, 0.17 g (1.94 mmol) of as-synthesized C
and 0.12 g (0.638 mmol) of Ga (the molar ratio is about 3:1 for
, here, we used 2-fold excess of the C 3.69 material
to ensure the complete conversion of Ga to GaN) are mixed
3 3.69
N
2 3
O
C N3.69 to Ga O
3 2 3
3
N
2 3
O
2 2
the O and H O absorbed on the surface of the sample. The X-ray
and grounded together, and then pressed to a pellet, followed by
diffraction (XRD) pattern (Figure 1) shows that the products are
hexagonal GaN without any trace of impurity phase under the
instrument’s resolution. The indexed pattern agrees well with the
ICDD-PDF #50-0792, only with 10% higher intensity for the (002)
peak, probably due to a small amount of O inclusions. The
broadening peaks indicate that the grain sizes are on the nanometer
scale, as confirmed by the TEM observations.
3
placing into a silica ampule with the volume of 10 cm . Second,
-
6
the ampule with the pellet is evacuated to 3 × 10 Pa and sealed
while heating at 100 °C. In succession, the ampule is kept at 650
°
C in an oven for 8 h. At last, the ampule is cooled naturally to
room temperature. Large-scale gray powder is obtained after the
reaction. In addition, black particles were found on the inner surface
The TEM image reveals the product consists of well-crystallized
nanoparticles with sizes ranging from 30 to 50 nm; see Figure 2.
†
Chinese Academy of Sciences.
Beihang University.
‡
15722
9
J. AM. CHEM. SOC. 2005, 127, 15722-15723
10.1021/ja055877i CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Ga O +C N f 2GaN + 3CO + N
2
2
3
3
4
3
V O + 5C N f 6VN + 15CO + 7N
2 5 3 4 2
2
Since the products, in particular, the nitrides and N , are all
thermodynamically stable, the above reactions can propagate to
completion according to the SSM principles.12 The possible reason
AlN cannot be formed from Al O might be that Al O is very
2 3 2 3
stable and the carbon-rich species cannot first react with it at
moderate temperatures. On the other hand, in our experiments, the
2 3 3
reaction of In O with a-C N3.69 only leads to metal In instead of
InN in the final products. Therefore, we can conclude that either
process can be the controlling step to the overall reaction.
To summarize, a new route for preparing nitrides GaN and VN
is reported here. The reaction pathway involves a two-step process
by using the as-synthesized a-C N3.69 as precursor. The route is so
3
potent that a series of nitrides can be directly synthesized from
their oxides at moderate temperatures. A striking feature of this
3
novel method is that a-C N3.69 serves as both the carbothermal
reduction agent and the nitridizing agent.
Figure 3. HRTEM image of the VN crystallite; inset is the amplified lattice
image.
Acknowledgment. This work is supported by the Chinese
Academy of Sciences and the National Natural Science Foundation
of China. We thank Prof. Chengchang Jia, Dr. Liliang Chen, and
Ting Zhou in the University of Science and Technology Beijing
Shown in the inset is the HRTEM image of the lattice. The lattice
spacings are 0.257 and 0.183 nm for (002) and (102) planes,
respectively, giving further evidence that the product is hexagonal
GaN. From their regular morphologies, we speculate that the
nanoscale GaN particles form in conditions close to the equilibrium
state in the synthesizing process. The growth speed is estimated to
be about 5 nm/h.
3 4
for their technical help in the synthesis of a-C N .
Supporting Information Available: SEM images of the GaN and
VN powder samples, XRD pattern of the VN product, EDX analysis
of GaN and VN powder products. This material is available free of
charge via the Internet at http://pubs.acs.org.
The route reported here is applicable to prepare other nitrides.
By a similar method, we synthesized the VN powders at the reacting
temperature of 850 °C, with the molar ratio of 8:1 for C
. The addition of more excess of C 3.69 is to prevent the
unreacted V in the product. For the VN product, besides the V,
N, and O elements, EDX shows a trace of C element; this is
probably due to the byproduct of the decomposed carbon nitride at
high temperatures.
XRD measurements indicate that the products are cubic VN (see
Supporting Information). Broadening peaks are also observed in
this case. The average size estimated by the Sherrer formula is about
3
N3.69 to
V
2
O
5
3
N
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11
C N
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below:
(
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