Preparation of a soluble precursor by an aminolysis reaction of Zr(NEt2)4 and its pyrolytic conversion into ZrN

Article abstract of DOI:10.1246/cl.2000.138

A novel soluble precursor with a Zr-N-Zr backbone structure was prepared by an aminolysis reaction of Zr(NEt₂)₄ with PrⁱNH₂. Pyrolysis of the precursor under NH₃-N₂ atmospheres led to the formation of ZrN as a main crystalline phase with a ceramic yield of 45.2%. Aggregated particles consisting of very minute particles with diameters of 50-300 nm were obtained.

Full text of DOI:10.1246/cl.2000.138

138
Chemistry Letters 2000
Preparation of a Soluble Precursor by an Aminolysis Reaction of Zr(NEt2)4
and Its Pyrolytic Conversion into ZrN
Fei Cheng,† Yoshiyuki Sugahara,* † and Kazuyuki Kuroda†,††
†Department of Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo, Shinjuku-ku, Tokyo 169-8555
††Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda, Shinjuku-ku, Tokyo 169-0051
(Received October 6, 1999; CL-990854)
A novel soluble precursor with a Zr-N-Zr backbone struc-
ture was prepared by an aminolysis reaction of Zr(NEt2)4 with
PriNH2. Pyrolysis of the precursor under NH3-N2 atmospheres
led to the formation of ZrN as a main crystalline phase with a
ceramic yield of 45.2%. Aggregated particles consisting of
very minute particles with diameters of 50-300 nm were
obtained.
of a soluble product. The precursor was pyrolyzed under NH3-
N2 atmospheres, and the pyrolyzed product was characterized
by compositional analysis, X-ray powder diffraction analysis
(XRD), and scanning electron microscopy (SEM).
All the procedures were performed under a protective
nitrogen atmosphere using the standard Schlenk technique16 or
a glove box filled with nitrogen. Zr(NEt2)4 was prepared and
distilled (b.p. 120 °C at 0.1 mmHg) based on a previous
report.17 The IR and 1H NMR data were consistent with previ-
ous reports for Zr(NEt2)4;18 1H NMR (ppm): 1.11 (triplet, -
CH2CH3), 3.34 (quadruplet, -CH2CH3); IR (cm-1): 1000, 1152
(ν(NC2)), 550-578 (ν(Zr-N)). Elemental analysis exhibited an
empirical formula of ZrN4.3C15.6H40.2, which approximately
corresponds to Zr(NEt2)4.
Zirconium nitride (ZrN) exhibits beneficial properties,
such as high melting point, hardness, excellent chemical dura-
bility and interesting optical properties.1 Thin films of ZrN
have been used for various applications, including wear-resist-
ant coatings, diffusion barriers in integrated circuits and coat-
ing for solar control windows.2-4 ZrN powders have been pre-
pared commercially by direct nitridation of metallic zirconium
and carbothermal reduction of ZrO2 under nitrogen.1 Chemical
vapor deposition (CVD) process has been investigated for the
preparation of ZrN films.5
The precursor was prepared by the reaction of Zr(NEt2)4
with PriNH2 (Zr : PriNH2 = 1:5) according to an ideal reaction
expressed by Eq. (1).
Pyrolytic conversion of inorganic and organometallic
compounds into non-oxide ceramics provides an alternative
route for the preparation of films and fibers via liquid phase,
but it is required that the precursors are soluble or fusible for
such applications.2,6 Extensive work has been reported on the
preparation of carbides and nitrides of various main group ele-
ments, such as SiC, Si3N4, BN, and AlN by this chemical
route.2,6,7 This chemical route has also been developed for the
preparation of transition metal nitrides.2 Insoluble polymeric
precursors with highly cross-linked structures were prepared
by the ammonolysis of transition metal dialkylamides8 or via
an electrochemical route,9 and were converted into transition
metal nitrides via pyrolysis. Soluble precursors which were
converted into TiN or Ti(N,C) powders, fibers and films via
pyrolysis were prepared by the aminolysis of Ti(NMe2)4, since
the formation of highly cross-linked structures was suppressed
by the introduction of bulky organic groups into the
precursors.10-12 A soluble precursor which was converted into
δ-NbN via pyrolysis was also prepared by an aminolysis reac-
tion of Nb(NEt2)4.13 Bartlett investigated the aminolysis reac-
tions of Zr(NEt2)4 with excess primary alkylamines
(Zr(NEt2)4:RNH2=1:15, R=Bu, Pr, Et),14 but the resultant poly-
meric products (proposed formula: [Zr(NR)(NHR)2]n) were
insoluble in organic solvents. Only one soluble ZrN precursor
[(CH3)3Si]2NH·ZrCl4 was reported, but a corrosive HCl by-
product evolved during the pyrolysis.15
Zr(NEt2)4 was dissolved in toluene in a three-neck flask, and
PriNH2 in toluene was slowly dropped into the Zr(NEt2)4 solu-
tion at about 5 °C with stirring. The mixed solution was stirred
for 2 h at about 5° C. Then the resultant solution was gradual-
ly heated, and finally refluxed for 16 h. An orange solid,
which was soluble in benzene and toluene, was obtained after
the separation of a small amount of insoluble yellow solid by
filtration and removal of toluene under a reduced pressure.
1
The H NMR spectra of the precursor, Zr(NEt2)4, and
PriNH2 are shown in Figure 1. In the 1H NMR spectrum of the
precursor, a quadruplet at 3.34 ppm due to the -CH2CH3 of
Zr(NEt2)4 is hardly detected, and many weak signals appear at
3.00-4.80 ppm. In the region for a triplet at 1.11 ppm (for
Zr(NEt2)4) and a doublet at 0.90 ppm (for PriNH2), some dou-
blets and unresolved peaks are observed at 1.12-1.86 ppm. The
Here we report the preparation of a precursor possessing a
Zr-N-Zr backbone structure by an aminolysis reaction of
tetrakis(diethylamino)zirconium (Zr(NEt2)4) with isopropy-
lamine (PriNH2). Since isopropyl groups are branched, it is
expected that the aminolysis reaction will lead to the formation
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
139
parameters of 0.4577, 0.4693 and 0.4620 nm, respectively.20
ZrN can form solid solution with ZrC,21 and the formation of a
Zr(N,C,O) solid solution with an NaCl-type structure was also
reported.22 However, the lattice parameter of the NaCl-type
compound in the present study is 0.4576 nm, very close to that
of ZrN, indicating that the crystallized NaCl-type compound
should be cubic ZrN.
The morphology of the product was investigated using
SEM (Figure 3). Aggregated particles consisting of very
minute particles with diameters of 50-300 nm are observed on
the surface of the pyrolyzed product. The calculated mean
crystallite diameter based on the XRD pattern using the
Scherrer equation was 53 nm, consistent with the SEM obser-
vation.
integrated intensity ratio of the signals at 1.12−1.86 ppm to
those at 3.00−4.80 ppm is about 6, corresponding to the proton
number ratio of CH3 to CH in the -CH(CH3)2 group. All of
these observations indicate that the aminolysis reaction of
Zr(NEt2)4 with PriNH2 occurs and isopropyl groups with vari-
ous environments are present in the precursor. Since no ν(N-
H) and δ(N-H) bands were observed in the IR spectrum of the
precursor, it is reasonable to consider that the nitrogen atoms
in the NPri groups of the precursor are mainly present as Zr-
N(Pri)-Zr imido-bridges, which is different from the assumed
environment of nitrogen in the polymeric compound
[Zr(NR)(NHR)2]n reported by Bartlett.14 In addition, although
the presence of Et groups can not be confirmed in the 1H NMR
spectrum of the precursor due to overlapping of signals, a
week shoulder band assignable to νs(CH2) mode of the ethyl
groups at around 2829 cm-1 19 was present in the IR spectrum
obtained with a hexachloro-1,3-butadiene technique, suggest-
ing that a small amount of NEt2 groups remains in the precur-
sor (The other three ν(C-H) bands due to ethyl groups show
wavenumbers similar to those of isopropyl groups).
Pyrolysis of the precursor under NH3 flow at 600 °C for 2
h and subsequently under N2 at 1350 °C for 8 h gave a yellow-
gray residue with a ceramic yield of 45.2%. The characteris-
tics of the product are shown in Table 1. The amount of carbon
is very small (0.4 mass%), while 3.1% of oxygen, which was
probably introduced during the pyrolysis, are present in the
product.
The pyrolysis of the precursor under NH3-N2 leads to the
formation of a well-crystallized NaCl-type compound (Figure
2). However, some weak peaks assignable to Zr7O11N2 and
ZrO2 are also observed,20 which is consistent with the presence
of oxygen in the product. It has been reported that ZrN, ZrC
and ZrO all possess NaCl-type structures with the lattice
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