Synthesis of nanocrystalline CrN from Cr[OC(NH2) 2]6Cl3 coordination compound

Article abstract of DOI:10.1016/S0025-5408(03)00151-X

Nanocrystalline chromium nitride (CrN) particles were prepared by the pyrolysis of Cr[OC(NH₂)₂]₆Cl₃ coordination compound in ammonia gas. The compound was characterized by infrared spectrum analysis and thermogravimetry-differential scanning calorimetry (TG-DSC). X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the phase and morphology of the CrN powders. The CrN particle size obtained at 600°C was as small as 7nm. The influence of reaction temperature and time on particle sizes was also studied. The formation of CrN nanoparticles was proposed following nucleation and growth mechanism on the basis of the experimental results.

Full text of DOI:10.1016/S0025-5408(03)00151-X

Materials Research Bulletin 38 (2003) 1551–1557
Synthesis of nanocrystalline CrN from
Cr[OC(NH2)2]6Cl3 coordination compound
Yu Qiu, Lian Gao*
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, PR China
Received 8 January 2003; received in revised form 20 May 2003; accepted 27 May 2003
Abstract
Nanocrystalline chromium nitride (CrN) particles were prepared by the pyrolysis of Cr[OC(NH2)2]6Cl3
coordination compound in ammonia gas. The compound was characterized by infrared spectrum analysis and
thermogravimetry-differential scanning calorimetry (TG-DSC). X-ray diffraction (XRD) and transmission
electron microscopy (TEM) were used to investigate the phase and morphology of the CrN powders. The CrN
particle size obtained at 600 8C was as small as 7 nm. The influence of reaction temperature and time on particle
sizes was also studied. The formation of CrN nanoparticles was proposed following nucleation and growth
mechanism on the basis of the experimental results.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Nitrides; B. Chemical synthesis; C. Thermogravimetric analysis; C. X-ray diffraction; C. Electron microscopy
1. Introduction
Chromium nitride (CrN) with the cubic rock-salt structure has been known as a technologically
important material because of its hardness, excellent corrosion, oxidation, and wear resistance [1,2]. In
fact, CrN has been successfully used as a protective coating material for many applications, e.g. wear
and corrosion-resistant coatings for various materials, tools, machine parts, and so on [3]. It is also the
only member to show a paramagnetic to antiferromagnetic transition in the family of mononitrides,
´
with a Neel temperature TN of 273–290 K [4]. Recently, there was increasing interest in synthesis of
nanocrystalline CrN [1,5–7], which may provide us another kind of material with different
characteristics. Hirota et al. [8] studied CrN ceramic and found that it had a higher fracture toughness
among ceramics except for ZrO2-based materials. Nanocrystalline CrN provides a possibility of
*
Corresponding author. Tel.: þ86-21-52412718; fax: þ86-21-52413122.
E-mail address: Liangaoc@online.sh.cn (L. Gao).
0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0025-5408(03)00151-X

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fabrication of nanocrystalline ceramics with higher fracture toughness and other superior mechanical
´
properties, compared with conventional ones. Large reductions in the Neel temperatures have also been
reported for nanocrystalline materials including chromium [9].
In our previous publication [5] we reported the synthesis of nanocrystalline CrN with particle sizes in
the range of 40–80 nm, by direct nitridation of nanosized Cr2O3 powders in ammonia flow at 800 8C for
8 h. The reaction time was drastically shortened, compared with that of the industrial preparations (2 or
3 weeks for fine powders), where the reactions of chromium metal, halide or hydride with nitrogen or
ammonia were used [1]. In the present work, Cr[OC(NH2)2]6Cl3 coordination compound is reported for
the first time as the precursor to synthesize nanocrystalline CrN. Uniform CrN nanoparticles with
average size of 7 nm can be obtained by reaction for 1 h at 600 8C. Especially considering small
particle size, narrow size distribution, low reaction temperature and short reaction time, this method
should be promising for the fabrication of nanocrystalline CrN and other metal nitrides.
2. Experimental procedure
Chromium chloride (CrCl3Á6H2O) was dissolved in absolute ethanol to get concentrated solution.
Then the solution was added dropwise into saturated urea/ethanol solution at 75–80 8C with stirring,
until the final salt/urea molar ratio reached 1:6. The reaction gave water-soluble green precipitation of
Cr–urea coordination compound, which was separated by filtering and dried at 80 8C. It was
characterized, and used directly to synthesize CrN without further purification. 5.0 g of the coordination
compound for each batch was loaded in a quartz boat and heated in flowing NH3 (0.5 l/min) at 500–
800 8C for 1–3 h in a tube furnace. NH3 gas was allowed to pass through the furnace for 1 h to remove
the air in it before the furnace was turned on, and the heating rate was kept at 10 8C/min before
reaching the set temperature. After the furnace was cooled down in the flow of NH3 gas, dark-gray
residues were collected at a yield of 11–12% (except for the case at 500 8C) and pulverized. At 500 8C,
the solids yields were 20% for 1 h and 14% for 3 h, respectively, indicating the existence of another
phase besides CrN in the final residues (see XRD analysis in Section 3).
Cr–urea coordination compound was characterized by Fourier transformation infrared spectroscopy
(FTIR; NEXUS, Nicolet, USA). Its pyrolysis behavior was monitored via simultaneous thermo-
gravimetry-differential scanning calorimetry (TG-DSC; STA 449C, NETZSCH, Germany) in an inert
argon flow (20 ml/min) at a heating rate of 10 8C/min up to 800 8C. The X-ray diffraction (XRD)
patterns of the pulverized products were obtained by powder X-ray diffraction (D/Max 2550V, Rigaku,
˚
Japan), using Cu Ka radiation (l ¼ 1:5418 A). In addition to phase identification, powder XRD was
also used to measure the average crystallite size using the Scherrer formula [10]
0:9l
D ¼
B cos y
where l is the wavelength and B is the broadening of a diffraction line due to the particle size effect
measured at half of its maximum intensity
2
2
B2 ¼ Bm À Bs
where Bm was the measured breadth of the diffraction line and Bs was the measured breadth of the
˚
standard. Micron-sized CrN was used as the external standard and Bs was measured as 0.1501 A.

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Transmission electron microscopy (TEM; 2010CX, JEOL) was performed to observe the micro-
structure of the powders.
3. Results and discussion
The reaction of CrCl3Á6H2O with urea in ethanol solution gave a green precipitation of Cr–urea
coordination compound very fast. The IR characteristics of the compound are shown in Fig. 1, compared
to those of urea. IR spectra of the compound is consistent with that of the reported Cr[OC(NH2)2]6Cl3
[11]. Especially, the absence of the carbonyl absorption at 1684 cmÀ1, and the decrease in wave number
of dðNH2Þ þ nðCOÞ vibration from 1600 to 1572 cmÀ1 relative to urea indicates the formation of
oxygen-to-chromium coordinate bonds (C¼O ! Cr) [11,12]. Thus, the reaction can be written as
CrCl3 Á 6H2O þ 6ðNH2Þ2CO ! Cr½OCðNH2Þ2ꢀ6Cl3 þ 6H2O
(1)
Simultaneous TG-DSC analysis of Cr[OC(NH2)2]6Cl3 (Fig. 2) shows that this coordination
compound decomposes gradually in an wide range of temperatures between 170 and 700 8C with a total
weight loss of 79.3%. The pyrolysis takes place in two stages: (i) from 170 to 350 8C and (ii) from 350
a
b
4000 3500 3000 2500 2000 1500 1000 500
Wavenumbers (cm-1)
Fig. 1. IR spectra of urea (a) and Cr(urea)6Cl3 (b).
110
100
90
80
70
60
50
40
30
20
3
2
1
0
TG
DSC
exo
-1
-2
-3
100 200 300 400 500 600 700 800
Temperature (oC)
Fig. 2. TG and DSC curves of Cr(urea)6Cl3 in argon at a heating rate of 10 8C/min.

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to 700 8C. The stage (i) begins at 170 8C, with two major strong endothermic processes around 193 and
323 8C, respectively. The former is considered associated with the cleavage of oxygen-to-chromium
coordinate bonds (reaction (2)) and the latter is due to the degradation of urea (reaction (3)). The weight
loss is 51.8% at the end of this stage, not 70.47% on the assumption that the product is CrCl3 after this
stage. Our previous study of Al[OC(NH2)2]6Cl3 indicated that AlCl3ÁxNH3 adduct and species such as
(ClAlNH)n formed during the decomposition process due to the reaction between AlCl3 and NH3,
which is released by the degradation of urea [13]. We believe that similar adduct CrCl3ÁxNH3 (x ¼ 6
when weight loss is 49.77%) is the solid remaining after this stage. This is possible because the
structure of the valence shell of Cr3þ is 3d34s04p0, and six d2sp3 hybrid orbitals can be formed, thus it is
easy for Cr3þ to form coordination compound with six Lewis base molecular, including NH3. The total
weight loss after the stage (ii) is not 70.47%, indicating CrCl3 is not the final pyrolysis product, either.
4-Coordinated compounds (ClAlNH)4 and (ClAlNH)6 were the most important species in the chemical
vapor deposition of AlN from the Cl3AlNH3 adduct according to reference [14]. Jegier et al. [15] also
proposed a solid remaining (HGaNH)n after the heat-treatment of (H2GaNH2)3. Species like (ClCrNH)n
is also likely to be the final solid remaining with the releasing of volatile HCl and the calculated total
weight loss is 80.25%, very close to the experimental value 79.3%. Based on the discussion above, the
possible pyrolysis process is presented as below
Stage ðiÞ : Cr½OCðNH2Þ2ꢀ6Cl3 ! CrCl3þ6ðNH2Þ2CO
ðNH2Þ2CO ! NH3þCONH
Stage ðiiÞ : CrCl3þ6NH3 ! CrCl3 Á 6NH3
CrCl3 Á 6NH3 ! 1=nðClCrNHÞnþ5NH3þ2HCl
(2)
(3)
(4)
(5)
Thus, the reaction temperature of 500–800 8C has been chosen in the experiments when species like
CrCl3ÁxNH3 or (ClCrNH)n can be generated at stage (ii).
Fig. 3 shows the XRD pattern of typical products at different reaction temperature. It shows that
powders synthesized at 500 8C for 1 h display the diffraction peaks of CrN, accompanied by several
unknown peaks. These unmarked diffraction peaks (Fig. 3) can not be assigned as any of the possible
known phases, such as Cr2N, chromium oxide, chromium carbide, Cr–O–C or Cr–C–N, etc. When the
reaction is conducted at 600 8C for 1 h, pure cubic CrN is obtained. The CrN powders as synthesized
are stable up to 1100 8C and disproportionate to Cr2N above 1150 8C in N2 atmosphere [16].
Fig. 3. Powder X-ray diffraction patterns of the pyrolysis products in ammonia at different temperatures for 1 h.

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50
40
30
20
10
0
1 h
3 h
550
600 650
700 750 800 850
Temperature (oC)
Fig. 4. Comparison of particle size between the CrN powders synthesized at different reaction temperatures for 1 h (&) and
3 h (&).
In addition, we calculated the average crystallite size of CrN from Bm of its diffraction peaks. The
diffraction peaks for the samples obtained at lower temperatures or shorter time are broader, due to low
crystallinity and small particle sizes. The peaks become sharp gradually with the increasing reaction
temperature and the extending reaction time, which means the particles grow larger. Fig. 4 shows the
average crystallite size as a function of the temperature for different reaction time. Crystallite size
increases with the increasing reaction temperature. The minimum crystallite size of CrN is found at
600 8C for 1 h, which is about 6.5 nm. The crystallite size is 15 nm for CrN obtained at 700 8C for 1 h,
and is 41 nm at 800 8C. The crystallite size also increases as the reaction time is prolonged. At 600 8C,
the crystallite size grows from 6.5 to 10 nm when the time is prolonged from 1 to 3 h, whereas the
growth is 5 and 6 nm at 700 and 800 8C, respectively.
The small crystallite size is further confirmed by the TEM observation. The average particle size,
estimated by measuring the maximum diameter of more than 200 particles, is very close to that
calculated from XRD analysis. Some typical TEM micrographs of the samples are shown in Fig. 5. The
sample prepared at 600 8C for 1 h has a small size less than 10 nm for most of its particles (Fig. 5a).
The average diameter is about 7 nm. The particles are near spherical and uniform, with some
agglomeration because of their small sizes. Fig. 5b shows a particle growth with the increase of the
reaction temperature. The mean particle size of sample prepared at 700 8C for 1 h is measured to be
approximately 16 nm, in agreement with XRD analysis. Several flaky particles are observed and some
particles tend to link with each other, thus the size distribution becomes wider. Similar particle growth
is observed when the reaction time is extended.
From the thermal analysis of Cr(urea)6Cl3 precursor and the synthetic process, we speculate that the
formation of CrN nanocrystals may result from the following chemical reaction
1=nðClCrNHÞn ! CrN þ HCl
It can also be written as (7) in ammonia gas on further reaction of HCl with ammonia
(6)
1=nðClCrNHÞnþNH3 ! CrN þ NH4Cl
(7)
Similar decomposition reactions of (ClAlNH)n have been reported by Riedel et al. [17] and Li et al.
[18]. The formation of AlN particles from (ClAlNH)n was through nucleation and growth mechanism
[18]. AlN has also been reported to precipitate from Cl3AlNH3 in liquid ammonia [19]. In the present

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Fig. 5. TEM images of CrN particles synthesized at (a) 600 8C and (b) 700 8C for 1 h.
preparation of nanocrystalline CrN through the pyrolysis of Cr[OC(NH2)2]6Cl3 in ammonia gas, the
formation of CrN particles is proposed to follow the same mechanism. At a low reaction temperature
(<500 8C) intermediate solid adducts such as CrCl3ÁxNH3 and/or (ClCrNH)n are generated, which form
initial solid particles by nucleation and growth. The unidentified diffraction peaks in Fig. 3 may be
attributed to these intermediate compounds. At higher temperatures (ꢁ500 8C), these initial particles
release HCl and/or NH3 to form CrN nanoparticles, which grow larger with the increase of the reaction
temperature or time.
4. Conclusion
A new method for the preparation of nanocrystalline CrN has been developed by pyrolysis of
Cr[OC(NH2)2]6Cl3 coordination compound in ammonia gas. In a temperature range of 600–800 8C, the

Y. Qiu, L. Gao / Materials Research Bulletin 38 (2003) 1551–1557
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powders had a particle size of 7–50 nm. The CrN particles grow with the increase of reaction
temperature or time. As a possible reaction mechanism, CrN particles may form by nucleation and
growth processes, via intermediate solid compound CrCl3ÁxNH3 and/or (ClCrNH)n.
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