The effect of starting precursors on size and shape modification of ZrB2 ceramic nanoparticles

Article abstract of DOI:10.1166/jnn.2015.11587

The formation of ZrB₂ nanoparticles through reaction of Zr(n-PrO)₄ with H₃BO₃ and carbon has been studied with different ligands by carbothermal reduction at 1500 °C. In the first step, by introducing N,N′-bis (salicylidene)-1,3-diaminopropane (H₂salpn) or salicylaldehyde (Hsal) species into reaction mixture, the reaction of the zirconium alkoxide using citric acid and boric acid yielded the zirconium diboride (ZrB₂) sol-gel precursors. In the second step, the mixture was heated by introducing the reactant compact into an argon furnace held at 1500 °C for 2 h to obtain the final pure phase ZrB₂ nanocrystallites with a diameter of about 50 nm. The kind of chelating agent used in the preparation of ZrB₂ nanoparticles plays the predominant role on the final product size. This demonstrates that the proper kind of donor atom and a very specific ligand structure are necessary for the reaction of Zr⁴⁺ complexes.

Full text of DOI:10.1166/jnn.2015.11587

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The Effect of Starting Precursors on Size and Shape
Modification of ZrB2 Ceramic Nanoparticles
Zahra Amirsardari1, Rouhollah Mehdinavaz Aghdam2ꢀ∗,
Masoud Salavati-Niasari1, and Mohammad Reza Jahannama2
1Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, I. R. Iran
2Department of Nano Technology, Space Transportation Research Institute, Tehran, P.O. Box 13445-75411, I. R. Iran
The formation of ZrB2 nanoparticles through reaction of Zr(n-PrO)4 with H3BO3 and carbon has
ꢀ
been studied with different ligands by carbothermal reduction at 1500 C. In the first step, by intro-
ducing NꢀNꢁ-bis (salicylidene)-1,3-diaminopropane (H2salpn) or salicylaldehyde (Hsal) species into
reaction mixture, the reaction of the zirconium alkoxide using citric acid and boric acid yielded the
zirconium diboride (ZrB2ꢁ sol–gel precursors. In the second step,ꢀthe mixture was heated by intro-
ducing the reactant compact into an argon furnace held at 1500 C for 2 h to obtain the final pure
phase ZrB2 nanocrystallites with a diameter of about 50 nm. The kind of chelating agent used in
the preparation of ZrB2 nanoparticles plays the predominant role on the final product size. This
demonstrates that the proper kind of donor atom and a very specific ligand structure are necessary
for the reaction of Zr4+ complexes.
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Keywords: ZrB2 Nanoparticles, Sol–Gel Precursor, Schiff-Base Compound, Synthesis.
Copyright: American Scientific Publishers
1. INTRODUCTION
including particle size control without using Schiff base
complexes. From the survey of past studies on the syn-
thesis of ZrB2 nanopowder, one may notice that the syn-
thesis of uniform crystalline nanoparticles of ZrB2 and
the fine control of their size by ligands has not been
attained. Recently, the effect of Schiff base compounds
on the size and morphology of ZrB2 nanoparticles has
been reported.9ꢁ10 However, it should be noted that the
chemistry of chelate complexes of Schiff base ligands is
widely used as analytical reagents since they allow sim-
ple and inexpensive preparation and the wide range of
applications.11 Thus it seems necessary to study in more
details the basic mechanism for the formation of ZrB2 par-
ticles by Schiff-base compounds, particularly in the nano-
size range, in order to control the size of this nanomaterial
with sufficient uniformity.
The stability of metal complexes as precursors depends
both on the metal ion and the ligand. The coordination of
the ligand at the surface of the metal is important since
this could be a factor related to the shape and size of the
particles.12 The ligand field effect employed in the forma-
tion of coordinate compounds plays the obvious role in the
stability of the complexes.13 One of the salicylaldehyde-
derived series is salpn ligand containing four binding sites
(N and O), with the flexibility of 1,3-propylenediamine
Zirconium diboride (ZrB2ꢀ is one of the most impor-
tant members of a family of ultra-high-temperatꢀure ceram-
ics (UHTCs) with high melting point (>3000 C).1 ZrB2
has low density (6.12 g/cm3ꢀ,2 low cost, high thermal,
electrical conductivity, and oxidation resistance.3 Recently,
because of their interesting properties, diborides based
ultra-high temperature ceramics, such as ZrB2 and HfB2
have attracted considerable attention. ZrB2 powder is one
of the particulate materials used for many purposes. The
combination of properties makes ZrB2 attractive for high
temperature applications, such as furnace elements.4
One common method used in ZrB2 nanoparticle produc-
tion is sol–gel reaction synthesis.5 The sol–gel process is
one of the leading techniques for producing nanostructures
with different morphologies of ceramic materials.6 The
applicable method shows promising potential for control
over particle size, high purity and desired chemical homo-
geneity at the nanometer level.7 Controlling the size and
size distribution of nanoparticles is still a challenging
problem, though great progress has been made in this
direction.8 Previous reports reveal that the most prepon-
derant strategies to achieve desired properties of ZrB2 are
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 12
1533-4880/2015/15/10017/005
doi:10.1166/jnn.2015.11587 10017

The Effect of Starting Precursors on Size and Shape Modification of ZrB2 Ceramic Nanoparticles
Amirsardari et al.
backbone compared with tetradentate salen and salophen
Schiff base ligands.14ꢁ15 On the other hands, different car-
bon sources have been extensively used for synthesis of
ZrB2 nanoparticles. Until now, ZrB2 nanostructures were
fabricated with the different carbon source such as citric
acid, carbon black, graphite16 and phenolic resin.17 Citric
acid was employed as both a chelating agent and a carbon
source in the present synthetic process which is a pro-
tective agent against particle growth. The citrate sol–gel
process with the mixing of ions at the atomic scale affects
the hydrolysis reactions, and finally leads to the formation
of the phase pure crystalline nanoparticle.
The present study is an approach that focuses on the
synthesis of ZrB2 nanopowder by using the advantages of
the sol–gel process such as its versatility and the possibil-
ity to obtain high purity materials, and study the influence
of conditions (starting materials) on the characteristics
of nanoparticles. The aim of this study is to produce
Zr(IV)(salpn) and Zr(IV)(sal) complexes as precursors for
the preparation of ZrB2 to explore the influence of ligand
on the ultimate structures. The conformational differences
between these two kinds of complexes result in marked
differences in the size of particles and different abilities to
induce nanoparticle assembly.
Figure 1. The schematic diagram of the sol–gel synthesis of ZrB2
nanoparticles.
Different characterization methods were adopted to
investigate the final particles. FT-IR spectra of compounds
were recorded on Bruker-Tensor 27 spectrophotometer in
the range of the 4000–400 cm−1. The microcapsule spec-
imens were prepared by grinding the sample with potas-
sium bromide (KBr). Thermogravimetric analysis (TG)
and derivative thermogravimetry (DTG) were performed
on a Linseys STA-1600 instrument under argon atmo-
ꢀ
sphere from room temperature up to 1000 C at a heating
2. EXPERIMENTAL SECTION
Zirconium n-propoxide (Aldrich), salicylaldehyde, dion-
ized water, boric acid, citric acid, acetic acid and
methanol were purchased from Merck. The production
of tetradentate ligand N,N ꢁ-bis(salicylidene)-1,3-diamino-
propane (H2salpn) was carried out by the reaction of
salicylaldehyde with 1,3-propylenediamine as reported
previously.18
ꢀ
rate of 10 C/min. X-ray powder diffraction analysis was
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conducted using a GNR.Co-Explorer with Cu-Kꢂ radia-
tion and a step size of 0.02ꢀ (2ꢃ). Data were digitally
recorded in a continuous scan mode in the angle range of
20–70. Transition electron microscopy (TEM) observation
was performed with a Philips-CM30 instrument, operating
at 150 kV. Nanoparticles were prepared by placing a drop
of the sample on a copper grid and allowing the ethanol
to evaporate. Mira2 Tescan field emission scanning elec-
tron microscopy (FESEM) was used to image the size and
morphology of the nanoparticle, and the specimens were
coated with gold via sputtering.
IP: 201.20.127.180 On: Sat, 02 Jan 2016 09:20:24
Copyright: American Scientific Publishers
The preparation of ZrB2 nanopowder was carried out as
follows: 0.02 mol of zirconium n-propoxide was added to
25 ml of methanol followed by 4 ml of dionized water and
0.01 mol of salicylaldehyde. This mixture was shaken well
for a couple of minutes and then distinguished volumes of
citric acid and boric acid/acetic acid were added to the sol,
and mixed well until the solution became homogeneous for
3 h at 65 ꢀC. During formation, temperature of the solution
was constantly monitored with the help of a thermometer
placed within the solution through a glass breaker. After
drying at 120 ꢀC for 3 h, the obtained precursor was heated
3. RESULTS AND DISCUSSION
The zirconium diboride nanoparticle precursor was pre-
pared by sol–gel methods, from Zr(IV) n-propoxide with
the in situ one-step addition of boric acid (H3BO3ꢀ dur-
ing the gelification process. The molar ratio of the carbon
and boron to metal were 5 and 4, respectively. Zirconium
n-propoxide was hydrolyzed in methanol with some water
ꢀ
in an alumina furnace to 1500 C at argon atmosphere.
ꢀ
The heating rate was maintained at 2 C/min, and ZrB2
nanoparticles were synthesized at 1500 C for 2 h. The
ꢀ
ꢀ
in a low-temperature synthesis (at 65 C). The reactions
flow rate of argon gas was 100 L/min. The experimen-
tal procedure used to prepare ZrB2 nanosized particles is
illustrated in Figure 1. The synthesis methods of zirco-
nium complexes were investigated in the same conditions
of solvent, temperature and argon pressure, but H2 salpn
was used instead of Hsal to synthesize Zr(IV)(salpn) pre-
cursor. The ZrB2 nanostructures synthesized by using sal
and salpn were denoted as A and B samples, respectively.
can be carried out in the presence of Schiff-base com-
pound and a controlled amount of water, and then crys-
talline nanoparticles can be synthesized conveniently at
high temperatures.
The reaction of chelating agents with the zirconium
alkoxide is an equilibrium reaction, and the identity of
the species generated is sometimes difficult to predict.
The stability of the sol strongly depends on the molecular
10018
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Amirsardari et al.
The Effect of Starting Precursors on Size and Shape Modification of ZrB2 Ceramic Nanoparticles
structure and type of atom in chelating agent. Differences
in the structure of the chelating agents led to main effects
on gelation time and condensation degree. The intermedi-
ate structures of precursors have been depicted to empha-
size these differences (Fig. 2).
The process of calcination of the dried gel was studied
by thermal analysis (TG-DTG) of two samples. Two pre-
cursor powders behave similarly, since the citric acid and
boric acid quantity are the same and the peaks correspond-
ing to ligand completely overlap with together in both
samples. Fiꢀgure 3 shows that the main weight loss occurs
below 500 C, and the total mass loss of the samples was
67.5%. The subdivision of the carbothermal reduction of
ZrB2 in some reaction steps is clearly demonstrated by the
results of direct monitoring by DTG. The curve shows a
Hsal and H2salpn can be added to replace water in
a chemical process. The zirconium atom has a different
geometry with the N and O donor atoms coming from
the tetra-dentate ligand (salpn) compared with an O donor
atom of sal. The two nitrogen atoms and each phenoxo
oxygen atom of the salpn ligand can bond to the zirco-
nium(IV). The H2salpn ligand occupies the four equatorial
positions, whereas the two propoxide ligands are trans-
bonded. Nitrogen is less electronegative than oxygen and
has a more available lone pair that led to a higher electron
density at the zirconium atom, consequently higher inter-
electronic repulsion by raising the energy of the d orbitals
and less effective interaction with others. However, our
results indicate that the N-donor ligand provokes a defor-
mation of the Zr atom, which can be explained in terms
of different hybridizations of the metal center that the
N-donor ligand is less bulky than other types. The result-
ing sal ion becomes coordinated as a bidentate ligand to
the zirconium(IV). In stoichiometric reaction an exchange
of sal ion with the propoxy group in one molecule of zirco-
nium(IV) is envisaged to occur. Prediction of coordination
would place an O:O-coordinated sal ligand at a Zr(IV) site
ꢀ
process about 100 C which takes place with low speed.
This can likely due to water elimination and decomposi-
tion of boric acid. A single peak with significant intensity
between 170 and 260 ꢀC was observed that appeared to be
due to the initiation of citric acid decomposition. As shown
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but would not be strict for the remaining groups. However,
IP: 201.20.127.180 On: Sat, 02 Jan 2016 09:20:24
the ZrO2 was firstly prepared, and secConodplyyriZghrBt: Apmowedriecran Scientific Publishers
2
was successfully synthesized via carbothermal reduction
by boron and carbon sources.19 The reactions showed that
the carbothermal reduction of ZrO2 with carbon could be
completed with an excess of H3BO3 to compensate the
loss of B2O3. The B2O3 product in during reaction played
an important role in synthesizing ZrB2. In fact, B2O3 has a
ꢀ
low melting point about 450 C, that a rapid vaporization
is obtained at temperatures above 1100 C.20 B2O3 and
ꢀ
ZrO2 tended to react with carbon to form ZrB2.16ꢁ19 The
amount of carbon is dictated by the decomposition of cit-
ric acid with three moles of C. During the heat-treatment
process, a great deal of gases including CO as an efficient
reducing agent was produced and escaped away. Finally, a
single phase of ZrB2 without residual ZrO2 was achieved
ꢀ
at 1500 C.
Figure 2. The structure of zirconium complexes obtained from H2salpn
(a) and Hsal (b).
Figure 3. TG-DTG curve of calcination process for sample B.
J. Nanosci. Nanotechnol. 15, 10017–10021, 2015
10019

The Effect of Starting Precursors on Size and Shape Modification of ZrB2 Ceramic Nanoparticles
in DTG curve, decomposition of citric acid is a single-step
Amirsardari et al.
(101)
process,21 also confirmed by the mass losses indicated on
ꢀ
ꢀ
(100)
the TG curve. At 800 C to 900 C, the thermal inter-
actions of ZrO2 occurred, and when the temperature was
ꢀ
1200 C, ZrO2 reacted with B2O3 that the weight loss
(001)
accelerated again and tended to stabilize, which is believed
to be the process of the carbothermal reduction reaction.
Both ZrB2 precursor particles prepared via sol–gel
method were characterized by FTIR analysis primarily.
Figure 4 shows the FTIR spectra of the powders produced
(110)
(102)
(111)
(200)
(002)
(c)
(b)
(a)
ꢀ
at 120 C. The very strong absorption at 3260 cm−1 and
the weak peak at 1650 cm−1 are in the range for the
absorption band of O–H stretching and bending vibrations,
respectively. It is obvious that this spectrum exhibits peaks
20
30
40
50
60
70
2theta
at 1197, 1470 cm−1 and another small peak at 1633 cm−1
,
Figure 5. XRD patterns of (a) precursor, (b) sample
A and
implying the presence of the phenolic C–O, C C and
(c) sample B.
C
N in the Schiff base ligand (salpn), respectively.18
However, another small band located at 930–710 cm−1
(asymmetric stretching) and a band at 1190 cm−1 (B–OH
bending) are in the range for the absorption band of
B–O–H.14
ꢀ
been completed at 1500 C for 2 h, and the molar ratios
of C and B in the pyrolyzed gel were sufficient to ZrB2
synthesis.
The produced nanopowders were visualized by field
emission scanning electron microscopy. The effect of lig-
and on particle size is clearly is observed in FESEM
images (Fig. 6). With adding sal compared to salpn, the
aspect ratio of the particles was increased, but the particles
were not agglomerated in Figure 6(a). Conversely, salpn
ligand led to produce smaller nanoparticles, but they hav-
XRD patterns of the powders calcined at 1500 ꢀC show
the characteristic XRD peaks of single phase zirconium
diboride in Figure 5. It is evident that no obvious Bragg
diffraction peaks have been detected in the precursor, as
shown in Figure 5(a), indicating that the sample is amor-
phous in structure. Figures 5(b) and (c) display the XRD
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spectrum of sample A and B, respectively. The resulting
ing practically nonuniformity diameters are not clearly dis-
IP: 201.20.127.180 On: Sat, 02 Jan 2016 09:20:24
Copyright: American Scientific Publishers
materials from both processes can be identified the crys-
tinguishable, and appear to adhere to each other as shown
in Figure 6(b). Nitrogen is less electronegative than oxy-
gen, and it has a more available lone pair, one might guess
a higher electron density at the zirconium atom. Hence,
the nitrogen interaction is strong enough to make break-
down of the crystal structure. This molecular geometry of
the H2salpn is in relation to the smaller size of crystalline
nanoparticles that are embedded in a glassy phase. On the
other hands, Hsal produces chelating species that they are
different compared with H2 salpn. The reaction of sal ion
involved the interaction of two O groups with Zr ions and
complexation, which formed a complex through a Zr atom
binding to O atoms.
tal structure of ZrB2 that is hexagonal. If sufficient carbon
was provided, this would logically result in only zirco-
nium diboride without carbide and oxide after carbother-
mal reaction. In a stoichiometric reaction, the gel required
5C/Zr for the carbothermal reduction. According to the
absence of any peak corresponding to ZrO2 and ZrC in
the XRD pattern of both samples,22 a conclusion can be
drawn that the carbothermal reduction of two samples has
(b)
Figures 7(a) and (b) show typical TEM images of sam-
ple A and B, respectively. It indicates that the particles
(a)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm–1
)
Figure 4. FTIR spectra of the ZrB2 precursors for synthesis of (a) sam-
ple A and (b) sample B.
Figure 6. FESEM micrographs of (a) sample A and (b) sample B.
J. Nanosci. Nanotechnol. 15, 10017–10021, 2015
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Amirsardari et al.
The Effect of Starting Precursors on Size and Shape Modification of ZrB2 Ceramic Nanoparticles
uniformity compared with other literatures. According to
SEM images, it was observed that ligand plays a key role
in controlling the growth of ZrB2 nanostructures as well as
providing an in depth understanding of Zr(IV) complexes
and how ligand work.
Acknowledgments: The authors are grateful to the
Space Transportation Research Institute of Iran for finan-
cial support.
Figure 7. TEM micrographs of (a) sample A and (b) sample B.
are nearly spherical (Fig. 7(a)), and the average particle
size is 30 nm. A variable distribution of particle size is
clearly observed in Figure 7(b), showing the different form
of nanoscale particles in sample B. It is interesting to note
that Schiff-base compounds play the role of a stabilizer of
Zr(IV) ion against hydrolysis. The rate-determining step
for the formation of ZrB2 can be due to the inhibition
of the nucleation or the growth of the generated parti-
cles by the adsorption of ligand thereon.23 Accordingly,
the particle-size distribution of ZrB2 indicated that the
average diameter of nanoparticles counted from the TEM
image is about 30–50 nm. Such investigations can increase
our knowledge on synthesis of refractory and hard mate-
rials, and offer a guide for controlling the structure of the
products.
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Synthesis of ZrB2 nanoparticles by sol–gel process has
proved to be very efficient in controlling the size of the
particles obtained from the ligands derived from sali-
cylaldehyde. In this synthesis, investigations on reaction
mechanisms are necessary for processing optimization to
improve the properties of the products. The zirconium
ion is coordinated through the azomethine (salpn) nitro-
gens and phenolic oxygen atoms that the nitrogen donor
group of salpn ligand leads to the unusual properties of
nanoparticles, which should be taken careful considera-
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Received: 15 October 2014. Accepted: 6 February 2015.
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10021