Science in China Ser. G Physics, Mechanics & Astronomy 2005 Vol.48 No.2 201—210
201
Synthesis and structural characterization of Zn3N2
powder
ZONG Fujian1, MA Honglei1, XUE Chengshan2, ZHUANG Huizhao2,
ZHANG Xijian1, MA Jin1, JI Feng1 & XIAO Hongdi1
1. School of Physics and Microelectronics, Shandong University, Jinan 250100, China;
2. Institute of Semiconductor, Shandong Normal University, Jinan 250014, China
Correspondence should be addressed to Zong Fujian (email: [email protected])
Received November 1, 2004
Abstract Zinc nitride (Zn3N2) powder has been synthesized through the nitridation
reaction of Zn powder with NH3 gas (at the flow rate of 500 ml/min) at the nitridation
temperature of 600℃ for 120 min. X-ray diffraction (XRD) indicates that Zn3N2 is cubic in
structure with the lattice constant being a = 0.9788 nm. Scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) show that Zn3N2 powder has surface
morphology of various types. X-ray photoelectron spectroscopy (XPS) shows the
differences in chemical bonding states between Zn3N2 and ZnO, and confirms the
formation of N–Zn bonds. Observation through high resolution transmission electron
microscopy (HRTEM) also indicates that the computer simulation of the structure of Zn3N2
is consistent with the structural model put forward by Partin.
Keywords: nitridation, Zn3N2 powder, structure, XRD, SEM, TEM, HRTEM, XPS.
DOI: 10.1360/ 04yw0095
1
Introduction
In the field of wide band gap semiconductors, many studies have been carried out
on zinc compounds. For example, ZnO, as a semiconductor material of n-type with a
—
wide direct band gap of 3.37 eV[1 3], can function as transparent conducting films of low
cost; ZnO, with an extremely large exciton binding energy of 60 meV and a strong ultraviolet (UV) stimulated emission at room temperature, has enormous potential for serving
as short-wave light devices[4], such as light-emitting diodes (LEDs), laser diodes (LDs),
and so on. Zn3P2, as a semiconductor material of p-type with a direct band gap of 1.51
eV[5], can be used for producing cheap solar cells. Here, we are expecting that Zn3N2
will also have excellent electric and optical properties.
Zn3N2 powder, black in colour and of anti-scandium oxide (Sc2O3) structure, was
first synthesized by Juza and Hahn[6] in 1940 and had remained relatively unstudied over
the following 50 years. However, it has drawn more and more attention in recent years.
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In 1993, polycrystalline Zn3N2 films were prepared by Kuriyama et al.[7] through the
direct reaction between NH3 and Zn which were evaporated onto quartz substrates.
These Zn3N2 films were cubic in structure with the lattice constant being a = 0.978(1)
nm and had a large optical band gap of 3.2 eV. In 1997, the structure of Zn3N2 was refined by Partin et al.[8] from neutron time-of-flight powder diffraction data. This compound has the antibixbyite structure, in which the metal atoms are at tetrahedral sites of
an approximately cubic closely packed array of N atoms. In 1998, the optical properties
of zinc oxynitride ZnxOyNz films were investigated by Futsuhara et al.[9]. ZnxNyOz films
were deposited onto glass substrates from a ZnO target in N2–Ar mixtures through reactive rf magnetron sputtering. The optical band gap decreased from 3.26 eV to 2.30 eV
with the increase of the nitrogen concentration density in the films. In the same year,
Futsuhara et al.[10] also investigated the structural, electrical and optical properties of
zinc nitride thin films prepared through reactive rf magnetron sputtering and found that
the polycrystalline Zn3N2 films showed a high electron mobility of about 100 cm2V-1s-1
at room temperature. The Zn3N2 films were determined to be an n-type semiconductor
with a direct gap of 1.23 eV.
This paper reports that Zn3N2 powder of high quality has been synthesized through
the nitridation reaction of Zn powder with NH3 gas under the optimum nitridation condition (in NH3 flow rate of 500 ml/min and at the nitridation temperature of 600 ℃ for 120
min). X-ray diffraction (XRD) technique was used for examining the structure; X-ray
photoelectron spectroscopy (XPS) technique for the chemical bonding states of Zn3N2
powders; scanning electron microscopy (SEM) for the surface morphology; transmission
electron microscopy (TEM) and high resolution transmission electron microscopy
(HRTEM) for the inner structure of Zn3N2 powder, which further tested the validity of
Partin’s model.
2
Experimental
Ten grams of pure Zn powder, taken in a quartz boat, were placed in a quartz reaction tube that had been equipped in the resistance heated horizontal tube furnace. The Zn
powder was nitrided in a flow of ammonia (NH3) at the rate of 500 ml/min under atmospheric pressure and at different temperatures (500℃—750℃) for 120 min. After the
process of nitridation was completed, the quartz boat was cooled down to room temperature in the flow of super pure N2 gas.
At a high temperature over 400 ℃, NH3 is gradually decomposed into NH2, NH, N2,
N, H2 and H[11, 12]. Therefore, NH3 can serve as the source of nitrogen in the process of
nitriding Zn powder, i.e. the reaction between Zn powder and NH3 gas that results in the
synthesis of Zn3N2 powder. The chemical reaction is as follows,
3Zn(s) + 2NH3 (g)→Zn3N2 (s)+ 3H2 (g).
The structural properties of the products were analyzed by using XRD (Rigaku
D/Max-γA with Cu-Kα radiation), the composition by using XPS, in which an Mg-Kα
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Synthesis and structural characterization of Zn3N2 powder
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line at 1253.6 eV was used as the X-ray source, and the surface morphology and crystal
structure of Zn3N2 powder by using SEM (JEOL Hitachi-800), TEM and HRTEM (Philips Tecnai 20u-TWIN).
3
Results and discussion
3.1
X-ray diffraction
Fig. 1 shows the XRD patterns of: (a) pure zinc powder, (b) powder of Zn3N2 synthesized by the nitridation reaction of Zn powder with NH3 gas in 500 ml/min at nitridation temperature of 500℃ for 120 min, (c) Zn3N2 powder synthesized at 550℃, (d)
Zn3N2 powder synthesized at 600℃, (e) Zn3N2 powder synthesized at 650℃, (f) Zn3N2
powder synthesized at 700℃, (g) Zn3N2 powder synthesized at 750℃, (h) pure ZnO
powder. The numbers above the peaks correspond to the values of crystal face indices
(hkl).
In fig. 1(a) and (b), only the peaks of Zn can be found, while in fig. 1(c) both of the
peaks of Zn and Zn3N2 can be found. We can get that the nitridation cannot occur below
500℃, while at 550℃ the nitridation is incomplete and about 50% of the Zn powder is
nitrided and changed into Zn3N2. Furthermore, in fig. 1(d), only the peaks of Zn3N2 can
be found, which suggests all Zn atoms have turned into Zn3N2 at 600℃ after 120 min.
As also clear in the figure, each peak fairly corresponds to the data of Zn3N2 powder
recorded in the JCPDS document1). Therefore, it can be concluded that Zn3N2 is cubic in
structure with the lattice constant being a = 0.9788 nm, which is in good agreement with
the published value (0.9777 nm) recorded in the JCPDS document.
Fig. 1(e) shows that when the nitridation temperature is 650℃, some weak ZnO
peaks can be found. While when it is 700℃, the ZnO peaks become stronger and the
Zn3N2 peaks become weaker. That is to say, when the nitridation temperature increases,
more and more Zn3N2 will be transferred into ZnO. Oxygen may come from the impurities of nitrogen and ammonia. The chemical reaction is as follows:
2Zn3N2 (s) +3O2 (g)→6ZnO (s)+ 2N2 (g).
Another possible source of oxygen may be the oxygen atoms of the SiO2 in the
quartz reaction tube, which is equipped in the resistance heated horizontal tube furnace.
Zinc is a metal, silicon is a semiconductor, and zinc has higher deoxidization. Comparing fig. 1(g) with fig. 1(h), we can discover that all Zn3N2 has been changed into ZnO
completely above 750℃.
The XRD patterns show that the nitridation cannot occur below 500℃ and is incomplete at 550℃, Zn3N2 can be decomposed partially at 650℃ and completely above
750℃, the optimum nitridation temperature is 600℃, and the optimum condition for
nitridation is in NH3 flow at the rate of 500 ml/min and at the nitridation temperature of
1) Powder Diffraction File Compiled by the Joint Committee on Powder Diffraction, Card No. 35-0762,
1985.
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600℃ for 120 min.
Fig. 1. XRD patterns of the pure zinc powder, Zn3N2 powder synthesized at different temperature and pure ZnO
powder. (a) Pure zinc powder; (b) 500℃; (c) 550℃; (d) 600℃; (e) 650℃; (f) 700℃; (g) 750℃; (h) pure ZnO powder.
3.2
Scanning electron microscopy
Fig. 2 shows SEM images of Zn3N2 powder synthesized through the nitridation reaction of Zn powder with NH3 gas under the optimum nitridation condition. Fig. 2(a)
shows the micrograph of the surface morphology of Zn3N2 powder at some random position. Fig. 2(b) presents the magnification of part of fig. 2(a), in which spherical particles about 7 µm in diameter can be clearly observed. Fig. 2(c) presents some hexastylos
structures about 0.5—1 µm in border width and a nanowire structure about 100 nm in
diameter. Besides, we can also discover Zn3N2 powders of other irregular shapes, such
as flakes, spherical hollow shell particles, and different segments of spherical hollow
shell particles. In a word, Zn3N2 powders of various shapes can be found.
Under standard atmosphere pressure, the melting point of Zn is 420 ℃ and the
boiling point of it is 907 ℃. However, at 600 ℃ that is high above the melting point, the
liquated state of Zn cannot be found. That is to say, now the surface of the Zn powder is
in instability, with plenty of Zn steaming, sublimating, or spurting out at any time. When
Zn of nanowire shape sprays out and reacts with NH3, Zn3N2 of nanowire shape will be
synthesized; when Zn of round global shape sprays out and reacts with NH3, Zn3N2 of
solid spherical particle shape will be synthesized; when Zn in hollow bubble state sprays
out and reacts with NH3, Zn3N2 of hollow spherical shell particle will be attained. In a
word, Zn3N2 powders of various shapes can be synthesized.
3.3
Transmission electron microscopy
Fig. 3 presents the TEM images of Zn3N2 powders synthesized through the nitrida-
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Fig. 2. SEM images of Zn3N2 powders synthesized at 600℃ for 120 min.
tion reaction of Zn powder with NH3 gas at the optimum nitridation condition. Fig. 3(a)
is the micrograph of a flake 0.3 µm in diameter, fig. 3(b) shows the shape of a solid
spherical particle 1.5 µm in diameter, and fig. 3(c) is the micrograph of a nanowire 20—
40 nm in diameter. In TEM images, various shapes of Zn3N2 powder are presented
clearly.
3.4
High resolution transmission electron microscopy
The SAED patterns and HRTEM images of Zn3N2 powders synthesized at the optimum nitridation condition have been surveyed. Fig. 4 presents the HRTEM micrographs and SEAD pattern of Zn3N2 powder.
Fig. 4(a) and (b) clearly show the arrangement of atoms inside the crystal, with the
bright points being the interspaces and the dark thread being the shadow of the atoms.
Through fig. 4(a) we can precisely measure that the interplanar distance of (321) is
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Fig. 3. TEM images of Zn3N2 powders synthesized at 600℃ for 120min. (a) Flake; (b) solid spherical particle; (c)
nanowires.
0.261 nm and its XRD peak is the strongest among peaks of Zn3N2 powder. Through fig.
4(b), we can reckon that the interplanar distance of (222) is 0.282 nm and that of (004) is
0.244 nm. Fig. 4(c) is the SEAD pattern of fig. 4(b). As indicated by the above data, the
structural integrity of Zn3N2 powder is perfect.
3.5
X-ray photoelectron spectroscopy
Fig. 5 provides the typical XPS wide scan spectrum and Zn2p peaks of pure Zn
powder, pure ZnO powder and Zn3N2 powder synthesized under the optimum nitridation
condition. Fig. 5(a) presents the peaks of Zn, N, O and C, among which the peaks of O
and C come from adsorbed CO2 and O2 from air. Fig. 5(b)—(d) presents the binding
energy of the Zn2p peaks of Zn, ZnO and Zn3N2 powder. Fig. 5(d) tells the binding energy of Zn2p 3/2 is 1021.9 eV, which is almost identical with 1022.0 eV provided by
Futsuhara et al.[10].
Fig. 6 presents N1s and O1s peaks of Zn, ZnO and Zn3N2 samples. Fig. 6(a) shows
N1s peak, which can be seen as main N1s peak (395.9 eV) plus lesser N1s peak (398.6 eV).
Compared with the N1s (398.8 eV) of free amine, main N1s peak has a large chemical
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Fig. 4. HRTEM micrographs ((a), (b)) and SAED pattern (c) of Zn3N2 powders.
Fig. 5. X-ray photoelectron spectroscopy. (a) The typical XPS wide scan spectrum of Zn3N2; (b) Zn2p peak of Zn;
(c) Zn2p peak of ZnO; (d) Zn2p peak of Zn3N2 powder.
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shift and presents the formation of N-Zn bonds. The decrease in binding energy is related to the increase in valence electron density. Therefore, the large chemical shift suggests that the N–Zn bond has more ionicity. The lesser N1s peak suggests the existence of
N-H bonds in this material. Fig. 6(b)—(d) shows the O1s peaks of Zn, ZnO and Zn3N2
powders. Comparing fig. 6(b) with fig. 6(c) and (d), we can see the difference between
the O1s peaks of Zn, ZnO and that of Zn3N2 samples, the binding energy of O1s peak of
Zn sample being 531.5 eV, while that of ZnO sample being 530.0 eV, and that of Zn3N2
sample being 531.0 eV. Since it is the strongest among those of ZnO samples, the peak
(530.0 eV) is O-Zn peak. Therefore, Zn3N2 exposed in the air will be hydrolyzed by absorbing H2O, thus resulting in the formation of N-H bonds and O-Zn bonds. This reaction can be written as
Zn3N2 + 6H2O→3Zn(OH)2+ 2NH3.
Hence, the O–Zn bond in the Zn3N2 is attributed to zinc hydroxide, while the N–H
bond may be associated with the ammonium salt, NH4OH, which is formed by the reaction between NH3 and H2O.
Fig. 6. X-ray photoelectron spectroscopy. (a) N1s peaks of Zn3N2 powder; (b) O1s peak of Zn; (c) O1s peak of ZnO;
(d) O1s peak of Zn3N2 powder.
3.6
The Partin’s model
In 1997, the structure of Zn3N2 was refined by Partin et al.[8] from neutron
time-of-flight powder diffraction data. This compound has the antibixbyite structure, in
which the metal atoms are in tetrahedral sites of an approximately cubic close packed
array of N atoms. The space group is Ia 3 and a = 0.97691(1) nm. In this cubic structure,
every crystal unit cell has 80 atoms (48 Zn atoms and 32 N atoms), i.e. 16 Zn3N2 moleCopyright by Science in China Press 2005
Synthesis and structural characterization of Zn3N2 powder
209
cules.
The accurate positions of every atom in the crystal unit cell were also indicated.
According to Partin’s model, the metal atoms are in general positions, 48e (x, y, z; etc.)
and there are two kinds of N atoms, N1 in position 8b (1/4, 1/4, 1/4; etc.) and N2 in positions 24d (x, 0, 1/4; etc.). The coordinate of Zn is (0.3957, 0.1489, 0.3759) including
48 atoms, that of N1 is (0.25, 0.25, 0.25) including 8 atoms, and that of N2 is (0.9784, 0,
0.25) including 24 atoms. Here, we further calculate the exact position of each atom inside the Zn3N2 crystal and present the computer simulation model of the structure of
Zn3N2. Fig. 7 shows the computer simulation of the structure of Zn3N2 and HRTEM images. Fig. 7(b) presents the HRTEM image observed along the (1, −1, 0) direction,
Fig. 7. Computer simulation of structure of Zn3N2 (a) and HRTEM images (b).
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when the yoz plane is observed along the x-axis with the crystal revolving around z-axis
anticlockwise for 45 degrees. Fig. 7(b) and fig. 4(b) are basically identical which proves
the validity of Partin’s model with the HRTEM image.
4
Conclusion
Zn3N2 powders of high quality can be synthesized through the nitridation reaction
of Zn powders with NH3 gas in 500 mL/min at a nitridation temperature of 600℃ for
120 min. The nitridation cannot occur below 500 ℃ and is incomplete at 550 ℃, Zn3N2
can be decomposed partially at 650 ℃ and completely above 750 ℃, and the optimum
nitridation temperature is 600 ℃. X-ray diffraction (XRD) indicates that Zn3N2 powder
has a cubic structure with lattice constants being a = 0.9788 nm. Various shapes of
Zn3N2 powders can be found through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) tells the
difference in chemical bonding states between Zn3N2, ZnO and Zn powders, which confirm the formation of N–Zn bonds. By using high-resolution transmission electron microscopy (HRTEM), we can reckon that the interplanar distance of (222) is 0.282 nm
and that of (004) is 0.244 nm. We further calculate the exact position of each atom inside
the Zn3N2 crystal and present the computer simulation of the structure of Zn3N2, which
in turn proves the validity of Partin’s model with HRTEM image.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos.
90201025 and 90301002), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP)
(Grant No. 20020422056).
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