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Assembly of monoclinic ZrO2 nanorods: formation mechanism and

crystal phase control

CrystEngComm Accepted Manuscript

Received 00th January 20xx,
Accepted 00th January 20xx Aling Chen, Yan Zhou, Shu Miao, Yong Li* and Wenjie Shen*
DOI: 10.1039/x0xx00000x The crystal phase and shape of ZrO2 nanoparticles were finely tuned by mediating the hydrolysis rate of zirconium cation
and using sodium oleate as the capping agent under hydrothermal conditions. Pure monoclinic ZrO2 nanorods with the
www.rsc.org/
diameter of ~3 nm and the length of 30‐40 nm were obtained at a lower pH value of 9.4; whereas monodispersed ZrO2
particles of ~4 nm with mixed monoclinic and tetragonal phases were formed at a higher pH value of 11.4. Their formation
mechanism was discussed in terms of the hydrolysis rate of the zirconium cation and the structure‐directing role of the
oleate species. The monoclinic ZrO2 nanorods showed a prominent blue‐green fluorescence under the excitation of an
ultraviolet lamp (365 nm) because of the presence of a large number of oxygen‐vacancy defects.

nm, with the aid of tert‐butylamine and docosanoic acid; the

Introduction nanoparticles consisted of 90% tetragonal and 10% monoclinic
phases.15 As docosanoic acid was replaced by oleic acid or
The physical and chemical properties of zirconium dioxide
dodecanoic acid, however, ZrO2 nanorods with the diameter of
particles are closely linked with their size and shape at the
4.3 nm and the length of 12.8 nm (62% monoclinic and 38%
nanometer level.1‐5 For example, the shape of ZrO2
tetragonal) or branched ZrO2 nanorods with the diameter of 4
nanoparticles was found to alter their chemical properties
nm and the length of 20 nm (almost 100% monoclinic) were
significantly; ZrO2 nanorods with the diameter of ~8 nm and
produced, respectively.15 Hydrothermal treatment of ZrOCl2 in
the length of 22 nm exhibited quite promising sensitivity
aqueous alkaline solution yielded t‐ZrO2 particles of less than
towards moisture, which was caused by the enhanced vacancy
10 nm, but synthesis in a mixed N(CH3)4HCO3 and N(CH3)4OH
sites that favour water adsorption.6 The size/shape‐dependent
solution produced m‐ZrO2 particles of 5.4 nm.14 By using oleic
effects of ZrO2 nanoparticles have been extensively studied in
acid and oleylamine as the co‐capping agent, solvothermal
the past decades;6‐11 however, the impact of their crystalline
treatment of zirconyl oleate in octane yielded t‐ZrO2 particles
structure at specific size and/or shape has been less examined.
of 0.8‐3.0 nm; but m‐ZrO2 particles of 2.8‐8.0 nm were formed
This is primarily because the metastable phases, usually being
when the synthesis was conducted in ethanol; as the synthesis
more reactive, tend to convert into the thermodynamically
was performed in cyclohexane with oleylamine as the only
stable ones during the synthesis process. For example, the
capping agent, however, t‐ZrO2 nanowires with the diameter
inter‐transformation between monoclinic (m‐ZrO2) and
of 1.2 nm were produced.17 All these reports demonstrate that
tetragonal (t‐ZrO2) phases is intimately related to the particle
the mediation of the nanostructure of ZrO2 particles is
size, in particular around 10 nm.12 Bulk t‐ZrO2 is unstable at
generally performed in a narrow operation window with
ambient temperature and transfers into m‐ZrO2; as their size
precisely tuning the synthetic conditions. This was probably
lowers to 10 nm, however, t‐ZrO2 particles become more
due to the fast hydrolysis of Zr4+ in aqueous solution and the
stable than m‐ZrO2 particles.13,14 In fact, it still remains
low crystal symmetry of ZrO2 nanoparticles, which usually
challenging to simultaneously control the size/shape and
requires effective surfactants to modify the growth process
crystal phase of ZrO2 nanoparticles.
and the surface energy of specific crystal facets during
Hydrothermal and solvothermal synthesis,15‐20 sol‐gel
synthesis.
method,13,21,22 aqueous‐phase precipitation23 and pyrolysis of
In this work, we synthesized pure monoclinic ZrO2 nanorods
Zr‐organic precursors24,25 have been applied to prepare ZrO2
with the diameter of ~3 nm and the length of 30‐40 nm
nanoparticles; they were effective for controlling the size
through finely tuning the hydrolysis rate of zirconium cation
and/or shape, but often resulted in mixed crystal phases. For
and using sodium oleate as the capping agent under
example, solvothermal treatment of zirconium n‐propoxide in
hydrothermal conditions. The formation mechanism was
a toluene‐water mixture produced ZrO2 particles of about 3
discussed in terms of the synthetic parameters such as the pH
value and the type of capping agent.
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, China. E‐mail: yongli@dicp.ac.cn
shen98@dicp.ac.cn.

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Experimental only 3 nm and the length of 30‐40 nm (Fig. 1b‐d). ViewMoreover,

Article Online
each nanorod had 2~5 short arms with DOI: 10.1039/C5CE02269J
the length of about 5
Materials synthesis nm, forming a branched structure. The continuous lattice
Monoclinic ZrO2 nanorods were hydrothermally synthesized fringes with a spacing of 0.26 nm on the junction boundary
using sodium oleate (NaOL) as the capping agent. In a typical between the nanorod and the short arm suggested that the
procedure, Zr(NO3)4•5H2O (0.117 mol L‐1, 30 mL) aqueous growth of the short arm on ZrO2 nanorod followed an oriented
26‐28 The two lattice spacings of 0.26 and 0.26 nm
solution were mixed with NaOL (0.233 mol L‐1, 15 mL) aqueous attachment.
o
solution ([NaOL]/[Zr] molar ratio of 1/1) under stirring at room with a dihedral angle of 90 , as viewed along [100] direction,
temperature, forming a white precipitant. Then, ammonia indicated the preferential exposure of the {020} and {100}
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4+ 2−
solution (25~28 wt %, 1 mL, [NH3•H2O]/[Zr] molar ratio of 30/7) planes; they were terminated by Zr and low‐coordinated O ,
was added into the mixture under stirring. After adding water respectively.In the STEM images with an atomic resolution of
(34 mL), the mixture with a pH value of 9.4 was transformed an individual nanorod (Fig. 1e, f), the atomic column appeared

CrystEngComm Accepted Manuscript

into a 100 mL autoclave, heated to 200 oC and kept at that as bright spots on a dark background. The distances of bright
4+
temperature for 20 h. After cooling down to ambient column (Zr ) with 0.23 and 0.28 nm were periodically
temperature naturally, the precipitant was centrifuged, alternated on the surface of the nanorod, which is very similar
washed thoroughly with cyclohexane and ethanol, and dried at
80 oC overnight. The key parameters for the hydrothermal
synthesis, such as the pH value (varied from 9.4 to 11.4 by
adjusting the amount of ammonia) and the type of surfactant,
were extensively investigated.

Characterizations
X‐ray powder diffraction (XRD) patterns were recorded on a
Rigaku D/MAX‐2500/PC diffractometer using a Cu Kα radiation
source that operated at 40 kV and 200 mA. Small‐angle XRD
patterns were recorded at 40 kV and 30 mA using the same
diffractometer. Transmission electron microscope (TEM)
images were recorded on a Philips Fei Tecnai G2 Sprit
instrument operated at 120 kV, and high‐resolution TEM
(HRTEM) images were taken on a Philips Fei Tecnai G2 F30 S‐
Twin instrument operated at 300 kV. Spherical aberration
corrected scanning transmission electron microscope (Cs‐
STEM) images were taken on a JEM‐ARM200F instrument
operated at 200 kV. The specimen was prepared by
ultrasonically dispersing the powder sample in cyclohexane,
depositing droplets of the suspensions on a carbon‐coated Cu
grid, and drying in air. Fourier transformation infrared
spectroscopy (FTIR) was recorded on a Bruker Tensor‐27 FTIR
spectrometer with a resolution of 4 cm−1. Samples were
uniformly mixed with KBr (sample/KBr mass ratio of 1/100) by
intensive grinding and pressed into a self‐supporting wafer.
Photoluminescence (PL) spectra were recorded on a FLS 920
fluorescence spectrophotometer with a resolution of 1.0 nm
using a Xe lamp as the excitation source with a wavelength of
365 nm. Electron paramagnetic resonance (EPR) spectra were
recorded at room temperature on a Bruker EMX A200
spectrometer equipped with a cylindrical cavity that operated
at 100 kHz field modulation.

Results and discussion

m‐ZrO2 nanorods
Fig. 1a shows XRD pattern of the sample prepared under the Fig. 1 XRD pattern (a), TEM/STEM images (b‐f) of the m‐ZrO2 nanorods
prepared under typical synthetic conditions; crystal structure (g) of m‐ZrO2
typical synthetic conditions. All diffraction lines were indexed
(P21/c) and projected atomic arrangement (h) of the (020) (left) and (100)
to monoclinic ZrO2 (JCPDS #65‐1022). TEM images of the (right) crystal planes viewed along [010] and [100] directions, respectively.
sample verified a rod‐like morphology with the diameter of

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to the Zr atomic arrangement of the projected {020} facets on nm. XRD pattern showed very weak diffraction lines Viewat 2 theta
Article Online
m‐ZrO2 as shown in Fig. 1h (left). This observation further degrees of 28.2 ° and 31.5 °, correspondingDOI: 10.1039/C5CE02269J
to monoclinic ZrO2 but
evidences that the m‐ZrO2 nanorod selectively exposed the with poor crystallinity. The diffraction line at the small angle
{020} facets.—m‐ZrO2 nanoparticles are usually terminated with diffraction zone further shifted to ~1.5°, equaling to an interlayer
the {111}, {111} and {001} facets, while the {020} facets is distance of 5.9 nm. At 12 h, the product was all branched ZrO2
rarely exposed because of its higher surface energy.29‐32 Here, nanorods of ~3 nm wide and ~27 nm long and well crystallized as
the preferential exposure of {020} planes on m‐ZrO2 nanorods monoclinic phase. As further extending the synthetic period to 36
was related with the unique synthetic process. or 48 h, the diameter of the m‐ZrO2 nanorods kept at about 3 nm
Information on the structural evolution of the m‐ZrO2 while the length slightly enlarged to 30‐40 nm; the short‐range
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nanorods during the hydrothermal synthesis was tracked by ordered nanostructure still maintained with a distance of 5.9 nm.
XRD patterns and TEM images (Fig. 2). The initially formed All these results demonstrate that the monoclinic ZrO2 nanorods
precipitate, just before hydrothermal treatment, showed an were produced directly under the hydrothermal conditions. It was

CrystEngComm Accepted Manuscript

amorphous phase. The weak diffraction line at 2 theta value of previously observed that tetragonal ZrO2 nanoparticles were
3.0 ° at the small angle diffraction zone indicated the initially formed as the primary product and gradually transferred
formation of a short‐range ordered structure with an into monoclinic ZrO2 nanorods with prolonging the hydrothermal
interlayer distance of 2.9 nm;33 this is very close to the end‐to‐ time, following the typical martensitic transformation mechanism;
end distance of two oleate chains (CH3(CH2)7CH=CH(CH2)7COO−, the resulting ZrO2 nanorods consisted of 64% monoclinic and 36%
1.9 nm)34. Therefore, it could be assumed that the oleate tetragonal phases.15 Therefore, the direct formation of m‐ZrO2
species strongly bonded to zirconium cations and formed a nanorods is based on the precise control in hydrolysis rate of
ZrO‐oleate complex, in which the overlapping C=C bond zirconium cation and the strong adsorption of oleate species that
between two adjacent oleate molecules favored the assembly lowered the surface energy of the monoclinic phase and induced
in a bilayer model.35 After hydrothermal synthesis for 3 h, the the growth of ZrO2 crystals along one‐dimensional direction.
product presented as amorphous floccules mainly, but small Further, the electrostatic interaction between oleate anion and Zr4+
part of rod‐like nanoparticles with the length of 21 nm and the on the m‐ZrO2 nanorods drastically lowered the surface energy of
diameter of 3 nm appeared as well. The diffraction line at 2 the {020} facets and thus favored their preferential exposure.
theta value of 1.9 ° indicated that the interlayer distance was The role of the oleate species on the growth of m‐ZrO2 nanorods
enlarged under the hydrothermal conditions. At 6 h, the was further examined through control experiments. As shown in Fig.
obtained sample had the shape of branched nanorod (2~5 3, without the use of capping agent, irregular ZrO2 nanoparticles of
short arms) with the diameter of ~3 nm and the length of 25 ~8 nm were produced; the nanoparticles consisted of 59%
tetragonal and 41% monoclinic phases, estimated from the
—
diffraction intensities of the (111) and (111) lines.15 As sodium
citrate was applied as the capping agent, ZrO2 nanoparticles of ~11
nm were obtained; they contained 89 % tetragonal and 11%
monoclinic phases. As sodium stearate was used as the capping
agent, branched ZrO2 nanorods with pure monoclinic phase were
produced, but the size widely ranged from 5 to 55 nm. By using
sodium oleate as the capping agent, uniform m‐ZrO2 nanorods were
obtained (Fig. 1). These results suggest that oleate and stearate
species with same carbon chain (C18) benefited the crystallization
of monoclinic ZrO2 nanorods; but oleate with a C=C bond
favored a more densely packed layer on the surface of ZrO2
crystals and thus regulated a uniform size distribution of m‐
ZrO2 nanorods.
The interaction between sodium oleate and zirconium
cation was investigated by FTIR measurement on the initially
formed precipitates before hydrothermal treatment. As shown
in Fig. 3f, the bands at 2925 and 2855 cm−1 represented the
asymmetric and symmetric stretching vibra ons of the −CH2−
group; the very weak band at 1700 cm−1 showed C=O
stretching vibration; and the two intense bands at 1545 and
1454 cm−1 corresponded to the asymmetric and symmetric
stretching of the COO− group.36,37 Notably, the wavenumber
separation (Δ) of 91 cm−1 for the functional −COO− group
Fig. 2 XRD patterns (a, b) and TEM images (c‐h) of the samples obtained at indicated that oleate species strongly chemisorbed on the
different intervals of 0, 3, 6, 12, 36, 48 h during the typical hydrothermal surface of the precipitate through the chelating bidentate.37
synthesis.
Together with the XRD patterns in the small angle diffraction
zone (Fig. 2), it could be deduced that oleate strongly bonded

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acid as co‐surfactant in a toluene‐water mixed solution; 15 the

View Article Online
DOI: 10.1039/C5CE02269J
shorter hydrocarbon chain (C12) of dodecanoic acid facilitated
the diffusion process and enhanced the nucleation rate. Here,
pure monoclinic ZrO2 nanorods crystallized in aqueous solution
by using sodium oleate as the capping agent. Kinetically, the
oleate species bonded with zirconium cation, forming a
zirconium‐oleate complex. This critical intermediate not only
acted as ZrO2+ reservoir to control the hydrolysis rate,38 but
also promoted the diffusion rate of the nuclei and induced the
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anisotropic growth of the crystals into a rod‐shaped structure.

Thermodynamically, the oleate species selectively adsorbed on
the {020} and {100} facets of the growing m‐ZrO2 nanorods and

CrystEngComm Accepted Manuscript

effectively lowered the surface energy of the monoclinic phase
under the synthetic conditions.

t/m‐ZrO2 nanoparticles
The shape and crystal phase of ZrO2 nanoparticles during the
hydrothermal synthesis was very sensitive to the pH value of the
synthetic solution. As the pH value was increased from 9.4 to
11.4 through increasing the amount of ammonia in the synthetic
solution, ZrO2 nanoparticles consisting of 62% monoclinic and 38%
tetragonal phases were produced (Fig. 4). TEM images identified

Fig. 3 TEM images (a‐d) and corresponding XRD patterns (e) of the ZrO2
samples prepared without using capping agent (a) and with using sodium
citrate (b), sodium stearate (c), and sodium oleate (d) as the capping agent
during the typical hydrothermal synthesis; FTIR spectra (f) of the initially
formed precipitates with the use of sodium oleate and without using
capping agent.

onto ZrO2+ through the COO− group in the precipitate, and the
formed zirconium‐oleate complex with a lamellar structure
acted as the key precursor or intermediate for the production
of m‐ZrO2 nanorods.
Oleic acid with oleylamine or other fatty acids (C10 to C22)
has been commonly applied as co‐surfactant to fabricate ZrO2
nanoparticles under hydrothermal or solvothermal conditions,
but the products crystallized mostly in tetragonal phase or
mixed tetragonal and monoclinic phases, such as t‐ZrO2
spherical particles of 0.8‐3.0 nm,17 t‐/m‐ZrO2 nanorods with a
diameter of 4.3 nm and a length of 12.8 nm.15 The difficulty
lies in the slower nucleation rate under the synthetic
conditions, which limited the martensitic phase transformation
from tetragonal to monoclinic. Therefore, pure monoclinic
ZrO2 nanoparticles were usually fabricated by accelerating the
nucleation rates. For example, m‐ZrO2 spherical particles of
2.8‐8 nm were obtained by solvothermal treatment of zirconyl
oleate in ethanol with the aid of oleic acid and oleylamine, in
which ethanol expedited the esterification and hydrolysis of
zirconyl oleate and hence facilitated the formation of m‐ZrO2 Fig. 4 XRD pattern (a) and TEM images (b‐f) of monodispersed ZrO2
nanoparticles obtained at the pH value of 11.4.
nanoparticles.17 Almost 100% monoclinic ZrO2 nanorods were
also fabricated by applying tert‐butylamine and dodecanoic

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that the ZrO2 particles had a spherical shape and an average View Article Online
size of 4 nm with a very narrow distribution. Some ZrO2 DOI: 10.1039/C5CE02269J
particles further assembled into two‐dimensional stacking,
indicating the surface‐functionalization of oleate species on
the particles.39 That is, the hydrophobic character of C18 chains
in oleate favored the assembly of the ZrO2 nanoparticles into the
two‐dimensional nanostructure.
Statistic analysis of about 200 particles suggested that about 60%
nanoparticles had a lattice spacing of 0.36 nm that corresponded to
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the (011) facets of m‐ZrO2; while the remaining 40% nanoparticles

presented lattice spacings of 0.29 and 0.29 nm with a dihedral angle
of 71 o, which could be ascribed to the (011) and (101) planes of t‐

CrystEngComm Accepted Manuscript

ZrO2. This is in accordance with the crystal phase composition
estimated from the XRD pattern in Fig. 4a. Detailed analysis on the
monodispersed ZrO2 nanoparticles revealed that there were no
mixed crystal phases and/or twinning structures in the individual
particles. This result suggests that the t‐ZrO2 and m‐ZrO2
nanoparticles crystallized in parallel, excluding the occurrence of
martensitic transformation. It is generally proposed that martensitic
transformation from t‐ZrO2 to m‐ZrO2 nanoparticles was controlled
by the nucleation kinetics and accompanied with the formation of
twinning structures on an individual nanoparticle.15,40‐42 Under the
current hydrothermal conditions, however, such a kind of twinning
structure was not observed. It is most likely that the higher
concentration of ammonia caused very fast hydrolysis of ZrO2+ Fig. 5 (a) Small‐angle XRD patterns of precursors obtained at different pH
values and in the absence of sodium oleate; (b) Schematic illustration of
species and produced larger amounts of crystallite nuclei before
the formation routes of m‐ZrO2 nanorods and monodispersed t/m‐ZrO2
hydrothermal treatment. Moreover, the higher pH value in the nanoparticles (OL‐ represented oleate ions).
synthetic solution promoted the rapid growth of nuclei into well‐
crystallized ZrO2 nanoparticles, which avoids the occurrence of directed the formation of monodispersed ZrO2 particles. Because
martensitic transformation. On the other hand, however, the higher the adsorption of oleate species on ZrO2 nanoparticles was not so
basicity also weakened the adsorption of oleate species on the uniform under such a strong basic environment, their role in
surface of ZrO2 nanoparticles for lowering the surface energy of the stabilizing the monoclinic phase was weakened considerably. As a
monoclinic phase, leading to the formation of mixed crystal phases. consequence, monodispersed ZrO2 particles of ~4 nm, containing
Accordingly, the fabrication of ZrO2 nanoparticles under the 62% monoclinic and 38% tetragonal phases, were produced.
current hydrothermal conditions could be viewed as the
combination of an initial hydrolysis of ZrO2+ and a subsequent Luminescence properties of the m‐ZrO2 nanorods
oxolation to ZrO2.43 As illustrated in Fig. 5, sodium oleate was The photoluminescence (PL) character of ZrO2 was usually
initially coordinated with ZrO2+ through its carboxyl anions group ascribed to the presence of significant amount of surface
and yielded a Zr‐oleate complex, in which the C18 chain in oleate lattice defects, which is closely associated with their size/shape
directed the assembly of the interlayer structure. Upon ammonia and crystal phase.44‐47 For example, polyvinyl alcohol stabilized
addition, hydrolysis of the Zr‐oleate complex yielded oleate–[Zr‐ t‐ZrO2 particles of 2 nm exhibited emissions at 402, 420 and
(OH)2‐Zr]x–oleate species, which gradually transferred into –[Zr‐O‐ 459 nm under 254 nm excitation.48 Oleic acid and oleylamine
Zr]x– species during the subsequent hydrothermal treatment and coated t‐ZrO2 particles of 2‐3 nm showed emission at 396 nm
finally formed ZrO2 nanoparticles. The hydrolysis rate of the layered under 280 nm excitation; whereas t‐ZrO2 nanowires of 1‐2 nm
Zr‐oleate complex largely relies on the pH value of the synthetic thick (coated by oleylamine) shifted the PL spectrum to 337
solution. At the pH value of 9.4, the relatively slow hydrolysis of the nm under the same excitation resource. As the size of m‐ZrO2
Zr‐oleate precursor retained its closely packed layered structure nanoparticles, stabilized by oleic acid and oleylamine, slightly
with an interlayer distance of 2.9 nm, which favored the growth of increased from ~4 to ~5 nm, the PL peak shifted from 402 to
ZrO2 crystals along the one‐dimensional direction during the 442 nm under 260 nm excitation.17 ZrO2 particles of 6.1 nm,
hydrothermal synthesis. Meanwhile, the strong adsorption of predominately in monoclinic phase, presented emissions at
oleate species efficiently lowered the surface energy of monoclinic 311, 344, 401 and 422 nm under 285 nm excitation.49
phase and thus facilitated the formation of pure monoclinic ZrO2 Apparently, the photoluminescence of ZrO2 nanoparticles was
nanorods. At the pH value of 11.4, however, hydrolysis of the Zr‐ previously observed only under low UV excitation wavelength
oleate precursor occurred very fast, producing a large amount of (< 300 nm).
crystallite nuclei. This fast hydrolysis process partially destroyed the The photoluminescent properties of the m‐ZrO2 nanorods and
interlayer structure of the Zr‐oleate complex. The released oleate t/m‐ZrO2 nanoparticles were measured under a 365 nm excitation
species might be re‐chemisorbed on the surface of ZrO2 nuclei and source. As shown in Fig. 6, the m‐ZrO2 nanorods gave a strong

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emission ranging from 400 to 700 nm centered at 470 nm; but the terminated by the low‐coordinated surface O2−View anion; this
Article Online
monodispersed ZrO2 nanoparticles only showed a minor and weak medium‐strength basic site may abstract DOI:
a 10.1039/C5CE02269J
proton from the
emission. The m‐ZrO2 nanorods were completely dispersed into adsorbed oleate and yield a carbanion, especially with the aid
cyclohexane (~30 mg/ml), whereas the monodispersed ZrO2 of the C=C bond. Meanwhile, the {020} facets on the m‐ZrO2
nanoparticles were only partially dispersed in cyclohexane. nanorods was terminated by Zr4+ cation only; this low‐
This further evidence that the oleate species adsorbed on the coordinated Zr4+ might act as Lewis acid to stabilize the
surface of the m‐ZrO2 nanorods promoted the dispersion reactive carbanion species.53 Under excitation by a UV lamp,
through their hydrophobic carbon chain. Under excitation by a the carbanion readily transfers an electron to molecular
UV lamp with a wavelength of 365 nm, the well dispersed m‐ oxygen, forming superoxide O2− species. Therefore, the
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ZrO2 nanorods showed emission of blue‐green fluorescence simultaneous exposure of the {100} and {020} facets over the
(inset of Fig. 6a), whereas the partially dispersed t/m‐ZrO2 m‐ZrO2 nanorods jointly promoted the formation of
nanoparticles showed very weak fluorescence. superoxide O2− species. Surface oxygen defects on ZrO2

CrystEngComm Accepted Manuscript

The origin of the photoluminescent properties of the m‐ZrO2 nanoparticles generally showed a violet‐blue emission as being
nanorods were investigated by EPR in Fig. 6b. The t/m‐ZrO2 excited by a UV lamp.17,46,56 Here, the m‐ZrO2 nanorods
nanoparticles exhibited a weak signal at g=2.0070, emitted blue‐green luminescence under a 365 nm excitation
corresponding to the electrons trapped in surface oxygen source, indicating the chemical variations in surface oxygen
vacancies.50 This kind of defect typically involves electron defects. Both experimental studies and theoretical calculations
transfer from valence band to the local mid‐gap state, showing have indicated that the violet‐blue luminescence of ZrO2
weak luminescence emission.51,52 The m‐ZrO2 nanorods originates from the single oxygen vacancy,45,57,58 while the
presented characteristic signal of superoxide O2− species with green emission requires di‐vacancies, i.e., the combination of
gx, gy, and gz values of 2.0038, 2.0109 and 2.0349, single oxygen vacancy.59 The formation of such a kind of di‐
respectively.50,53,54 The formation of superoxide O2− species vacancy site on the m‐ZrO2 nanorods might be linked to the
might be related to the surface intermolecular electron {020} facets that are enclosed with Zr4+ only; this unsaturatedly
transfer between the adsorbed oleate and the low‐ coordinated Zr4+ site with a coordination number of 7
coordinated surface O2− anions on ZrO2.53,55 As mentioned principally favours the generation of oxygen vacancies. In
above, the {100} facets exposed on the m‐ZrO2 nanorods was addition, the ordered boundary of the rod‐shape structure
facilitates the migration and combination of surface oxygen
vacancy to form di‐vacancy site. As a result, the enriched
oxygen defects on the m‐ZrO2 nanorods lead to the blue‐green
photoluminescence.

Conclusions
Simultaneous control of size/shape and crystal phase of ZrO2
nanoparticles under hydrothermal conditions largely
depended on the pH value of the synthetic solution and the
type of the capping agent. With the aid of sodium oleate and
at the pH value of 9.4, the relatively slower hydrolysis of the
interlayered Zr‐oleate complex favored the formation pure
monoclinic ZrO2 nanorods with the diameter of ~3 nm and the
length of 30‐40 nm, which preferentially exposed the {020}
and {100} facets. As slightly increasing the pH value to 11.4,
the rapid hydrolysis of the Zr‐oleate complex destroyed the
interlayered structure and formed monodispersed ZrO2
particles of ~4 nm but with mixed monoclinic and tetragonal
phases. This observation demonstrates that mediating the
hydrolysis rate of zirconium cation and simultaneously
modifying the surface energy of ZrO2 crystals through selective
adsorption of proper surfactant could efficiently tune the
crystal phase and size/shape of ZrO2 nanoparticles. The m‐ZrO2
nanorods showed prominent blue‐green fluorescence, being
originated from the enriched surface oxygen defects on the
Fig. 6 PL (a) and EPR (b) spectra of the m‐ZrO2 nanorods, t/m‐ZrO2 rod‐shaped nanostructure.
nanoparticles. Insets are the photos of the samples dispersed in
cyclohexane (recorded by a CCD camera in a dark room under an ultraviolet
lamp with an excitation wavelength of 365 nm).

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