Morphology control and large piezoresponse of hydrothermally synthesized lead-free piezoelectric (Bi0.5Na0.5)TiO3 nanofibres

Article abstract of DOI:10.1039/c7ra01293d

Lead-free piezoelectric bismuth sodium titanate (BNT) nanostructures were synthesised using a low-temperature hydrothermal technique. It is found that the phase and morphology of the products are strongly dependent on the composition and concentration of the precursors, as well as the processing conditions. Through optimising the synthesis parameters, well-crystallized BNT nanofibers with 150-200 nm diameters and ～5 μm length were obtained. The BNT fibres show a pure perovskite phase with (011) orientation along the length direction. A piezoelectric constant of d₃₃ = ～15 pm V^-1 in the diameter direction was observed for these BNT nanofibers.

Full text of DOI:10.1039/c7ra01293d

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Morphology control and large piezoresponse of
hydrothermally synthesized lead-free piezoelectric
Cite this: RSC Adv., 2017, 7, 15020
(
Bi Na )TiO nanofibres
0
.5
0.5
3
a
a
a
b
Mohammad Bagher Ghasemian, Qianru Lin, Esmaeil Adabifiroozjaei, Feifei Wang,
a
a
Dewei Chu and Danyang Wang*
Lead-free piezoelectric bismuth sodium titanate (BNT) nanostructures were synthesised using a low-
temperature hydrothermal technique. It is found that the phase and morphology of the products are
strongly dependent on the composition and concentration of the precursors, as well as the processing
conditions. Through optimising the synthesis parameters, well-crystallized BNT nanofibers with 150–
Received 31st January 2017
Accepted 23rd February 2017
2
00 nm diameters and ꢀ5 mm length were obtained. The BNT fibres show a pure perovskite phase with
DOI: 10.1039/c7ra01293d
ꢁ1
(011) orientation along the length direction. A piezoelectric constant of d33 ¼ ꢀ15 pm V in the
rsc.li/rsc-advances
diameter direction was observed for these BNT nanofibers.
The nanostructured materials present fascinating and
applicable properties because of quantum size and specic
Introduction
(
Bi1/2Na1/2)TiO3 (BNT), one of the most promising lead-free shape effects. Particularly, one-dimensional (1D) lead-free
piezoelectric materials was discovered in 1960 by Smolensky ferroelectric materials including nanobers, nanotubes and
1
et al. A-Site complex perovskite BNT has been recognised as nanoribbons are greatly desirable owing to their high surface to
a leading lead-free candidate owing to its marked remanent volume ratio, exceptional properties in energy harvesting,
polarization and piezoelectric coefficients, and highlighted by photocatalysis, electromechanical device and ferroelectric
2
–6
11
several research reviews. BNT-based materials have exhibited memories. The hydrothermal synthesis is a technique pre-
great potential for application in piezoelectric and pyroelectric senting remarkable advantages for fabrication of one-
7
devices. On the other hand, strong dependence of ferroelectric dimensional nanostructures. It offers controllable synthesis
ꢂ
properties on size, structure and morphology has attracted conditions and a cost-effective and low-temperature (ꢀ200 C)
much focus for controllable preparation of BNT nanostructures route without the need of substrate that promotes easy fabri-
8
12
such as nanospheres, nanocubes and nanobers. It has been cation. The hydrothermal synthesis provides many unique
reported that different precursors and morphologies have advantages which are not met by other synthetic techniques,
a signicant effect on energy transition and mass of piezoelec- such as direct precipitation of crystalline materials from solu-
tric material aer the sintering process which consequently tions, high purity, controllable rate and homogeneity of nucle-
affects the electrical properties and density of sintered piezo- ation, aging and growth, controllable size and morphology of
9,10
electric materials.
For example, sintered BNT nanobers crystals. In particular, low processing temperature can prevent
have higher piezoelectric constant (d ), planar electrome- undesirable changes in stoichiometry when compounds
3
3
chanical coupling factor (k ) and remnant polarization (P ) than contain volatile elements such as Bi and alkali elements in
p
r
sintered BNT nanocubes. In contrast, sintered BNT nanocubes many lead-free piezoelectrics. Recently, hydrothermal method
have higher mechanical quality factor (Q
) than that of nano- was used in making BNT nanostructures with different size and
m
9
13–19
13

ber counterparts. It is also reported that crystal morphology of shape.
Lencka et al. was the rst to synthesise BNT nano-
the BNT nanostructures, as a template, controls the grain spheres and their diameters can be effectively tuned through
15
growth and density of ceramics. Ceramics sintered from BNT controlling the pH values of precursor solutions. The mono-
nanobers have a large grain size which facilitates domain sized BNT nanospheres can be converted into uniform cubes
1
4,18
switching and subsequently results in a signicant improve- when using higher mineralizer concentration.
By increasing
9
ment of piezoelectric and ferroelectric properties.
the treatment temperature or NaOH concentration, nano-
whiskers of BNT were resulted. It is also suggested that growth
of BNT nanowires can be promoted by increasing the ratio of
17
a
School of Materials Science and Engineering, The University of New South Wales,
20
titanium to bismuth (Ti/Bi ¼ 2). Also, other similar 1D
Sydney, NSW 2052, Australia. E-mail: dy.wang@unsw.edu.au
b
perovskite materials such as acicular BaTiO nanostructures
3
Key Laboratory of Optoelectronic Material and Device, Department of Physics,
have been developed by controlling morphology through
Shanghai Normal University, Shanghai, 200234, China
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Table 1 Different applied precursors and controllable factors in synthesising BNT
ꢂ
Bi precursors
Ti precursors
NaOH (M)
4, 8, 12, 16
Time (h)
Temperature ( C)
1
2
3
Bi
BiCl
Bi(NO
2
O
3
TiO
Ti(OC
(C
2
10, 20, 40, 80
160, 180, 200
3
4 9 4
H )
3
)
3
$5H
2
O
6
H
20
N
2
O
6
)
2
Ti(OH)
2
changing the growth period and phase transition of the copre- XRD patterns for the obtained BNT nanostructures from
21
cipitated amorphous phase.
different starting chemicals. It is found a combination of
In this work, BNT nanobers were synthesised using a facile Bi(NO ) $5H O and Ti(OC H ) or BiCl and TiO result in the
3
3
2
4
9 4
3
2
hydrothermal technique. The effects of hydrothermal treatment pure perovskite phase under the aforementioned hydrothermal
time, temperature and chemicals' concentration on the conditions.
morphology of BNT nanostructures were investigated. Struc-
NaOH plays important roles in adjusting pH values of the
tural and piezoelectric properties of the as-grown nanobers solution and providing A-site cations for the perovskite struc-
1
3
were also characterised.
ture in the reaction process. The mineralizer effect of NaOH
on the structure and morphology of BNT nanostructures was
also studied through using different concentrations of NaOH (4,
Experimental
8
, 12 and 16 M). Fig. 2a demonstrates XRD patterns for hydro-
The hydrothermal synthesis of BNT nanostructures started with thermally produced BNT (by BiCl
analytical grade chemicals. Different Bi and Ti precursors were under the conditions of 200 C aer 80 h. At low NaOH
tested in order to nd out the optimal combination for fabri- concentration (e.g. 4 M), diffraction peaks of secondary phases
and TiO as precursors)
2
3
ꢂ
4 3 2
cating pure BNT phase. NaOH was used as both sodium source (Bi Ti O12 and TiO ) are superimposed on the typical perovskite
and mineralizer to control the size and morphology of the BNT pattern. The secondary phases diminish with the increase of
nanostructures. Table 1 shows different conditions in hydro- NaOH concentrations i.e. 8 and 12 M, leading to a pure BNT
thermal synthesis of BNT with various combinations of phase with perovskite structure as evidenced by the XRD pattern
precursors. Firstly, different combinations of Bi and Ti precur- in Fig. 2a. These sharp and strong peaks in XRD pattern of
sors (molar ratio of 1 : 2) were mixed in distilled water. Various nanostructures produced with high NaOH concentrations imply
amounts of NaOH were then introduced into the solution, a high crystallinity of BNT perovskite phase. Moreover, NaOH
which was stirred for one hour before transferring to a 50 ml concentration has a signicant impact on the morphology of
22
autoclave. The autoclave was held in an oven at different BNT nanostructures. When concentration of NaOH is 12 M,
temperatures and for different periods for hydrothermal treat- the resultant morphology is predominantly nanobers with
ment. It was naturally cooled down to room temperature aer ꢀ200 nm diameter and ꢀ5 mm length, as shown in Fig. 2b.
the completion of hydrothermal synthesis. As-grown products
were ltered and washed to natural pH with distilled water
ꢂ
before drying at 110 C.
The phase structure of obtained materials was investigated
by X-ray diffraction (XRD Philips X'Pert pro MPD, Cu-Ka radi-
ꢀ
ation, l ¼ 1.54 A). The morphology of the synthesised BNT
nanostructures was imaged by a scanning electron microscopy
(SEM, Nova NanoSEM230/450). High resolution transmission
electron microscopy (HRTEM, CM200, FEI) images and selected
area electron diffraction (SAED) patterns were used to study the
crystal size, shape and lattice structure of products. A Raman
spectrometer (Reinshaw inVia) with an Ar-ion laser excitation
source (514 nm; continuous mode) was used to further conrm
the structures of BNT nanoparticles at room temperature in
ꢂ
1
80 backscattering geometry. Piezoelectric responses were
examined by piezoelectric force microscope (PFM, Asylum
Research).
Results and discussion
Fig. 1 XRD patterns of BNT nanostructures produced under the
Several different precursors were used as bismuth and titanium
sources to nd the optimal combination for synthesising the
pure BNT phase. All products were synthesised at 200 C with + (C
ꢂ
hydrothermal conditions of; 200 C, 20 h, NaOH ¼ 16 M by different Bi
and Ti precursors: (a) Bi(NO
3
)
3
$5H
; (d) BiCl
+ TiO
2
O + TiO
+ Ti(OC
2
; (b) Bi
2
O
3
+ TiO
2
; (c) BiCl
$5H O +
3
ꢂ
6
20 2
H N O
)
6 2
Ti(OH)
2
3
4
H )
9
4
; (e) Bi(NO
3
)
3
2
Ti(OC
4
H
9
)
4
; (f) BiCl
3
2
.
16 M NaOH and Bi : Ti molar ratio of 1 : 2 for 20 h. Fig. 1 shows
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The BNT particles are created aer the initial growth and
ripening process. Throughout the hydrothermal reaction,
precursors hydrolyse and initial crystals accumulate into bigger
particles to reduce surface free energy. The chemical reactions
occurred in the process of hydrothermal production of BNT can
14,23
be expressed as:
3
+
4+
ꢁ
1
2Bi + Ti + 40OH / Bi12TiO20 + 20H
2
O
(1)
(2)
+
4+
ꢁ
/
Bi12TiO20 + 12Na 23Ti + 104OH
3 2
4Bi0.5Na0.5TiO + 52H O
2
ꢁ
It is shown that the increase of hydroxyl (OH ) concentration
may lead to a higher growth rate. Various mechanisms for the
evolution of nanocubes and nanobers have been proposed,
9
including dissolution–recrystallization and oriented growth. In
the case of low NaOH concentration, the growth rate is different
in different directions producing plates and agglomerations.
The increase of mineralizer concentration might hasten the
22
kinetics of dissolution–recrystallization process. In supersat-
urated solutions, crystallization and nucleation happen on the
surface of aggregated particles, resulting in nanocubes, as
Fig. 2 XRD patterns and SEM images of hydrothermally synthesised
BNT nanostructures at 200 C for 80 h with different concentrations of
ꢂ
NaOH. (a) XRD patterns, (b) NaOH ¼ 12 M and (c) NaOH ¼ 16 M, (d) shown in Fig. 2c. In supersaturated solutions with concentra-
a typical nanocube on a nanofiber produced with NaOH ¼ 12 M.
tion of NaOH higher than 12 M, hydrothermal process is
controlled by dissolution–crystallization mechanism and BNT
particles are produced by a short and fast nucleation cycle
24
When the concentration of NaOH increases to 16 M, nanocubes according to Ostwald ripening phenomenon. Therefore,
become the dominant morphology and their size is around reasonably high mineralizer concentration is desirable for
400 nm (Fig. 2c). Fig. 2d compares a typical BNT nanober and synthesising of uniform BNT nanocubes. On the other hand,
cube produced at a NaOH concentration of 12 M, illustrating kinetically, crystal growth also strongly depends on degree of
25
the importance of NaOH concentration on the size of nano- oversaturation. The solution with low concentration of
bers and nanocubes.
mineralizer is prone to the assembly of nanobers. Due to the

Fig. 3 XRD patterns and SEM images of hydrothermally synthesised
BNT nanostructures at different temperatures with 20 h treatment Fig. 4 XRD patterns and SEM images of BNT nanostructures syn-
time and NaOH ¼ 12 M. (a) XRD patterns, (b) 160 C, (c) 180 C and (d) thesised at 200 ꢂC with NaOH ¼ 12 M. (a) XRD patterns, (b) 10 h, (c)
ꢂ
ꢂ
ꢂ
2
00 C.
20 h, (d) 60 h reaction time.
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Table 2 Different hydrothermal conditions and products in synthesising BNT by BiCl
3
, TiO
2
and NaOH as precursors
ꢂ
Temperature ( C)
NaOH (M)
Time (h)
Phase
Morphology
200
200
200
200
200
4
8
12
12
12
80
80
10
20
40
BNT, Bi
BNT, Bi
BNT, Bi
BNT
4
4
4
Ti
3
Ti
3
Ti
3
O
O
O
12, TiO
2
—
—
12
12
Cubic, nanober
Nanober
Nanober,
BNT
agglomeration
Nanober,
2
00
12
60
BNT
agglomeration
Cubic, agglomeration
Cubic
200
200
200
200
180
16
16
16
16
12
10
20
60
80
20
BNT, Bi
BNT
BNT
BNT
BNT
4
Ti
3
O
12
Cubic
Cubic, agglomeration
Agglomeration,
nanober
1
80
12
40
BNT
Agglomeration,
nanober
1
1
60
60
12
12
20
40
BNT, Bi
BNT, Bi
4
4
Ti
Ti
3
O
O
12
12
Agglomeration
Agglomeration,
nanober
3
reduction of surface free energy, nanocubes attach one another structures can be detected if the treatment time is longer than
orientally creating nanobers. 20 h. However, the change of treatment time results in signi-
The effect of hydrothermal treatment temperature on the cantly different shape and morphology of the nanostructures.
structure and morphology of the produced BNT nanostructures As shown in Fig. 4b, only the 20 h-treated samples render
was also studied. Fig. 3a shows XRD patterns of BNT nano- a nanober dominant morphology. All the other treatment
ꢂ
structures fabricated at 160, 180 and 200 C aer 20 h hydro- periods lead to only a small population of bres embedded into
thermal treatment with NaOH ¼ 12 M. In the case of low
ꢂ
temperature (160 C) synthesized BNT, characteristic peaks of
BNT are clearly seen in the XRD spectrum. The broadness of the
peaks suggests a rather poor crystalline quality of the BNT.
ꢂ
Nevertheless, the appearance of a shoulder at around 2q ¼ 32
implies the presence of Bi
4
Ti
3
O
12 secondary phase. With the
ꢂ
increase of hydrothermal temperature to 180 and 200 C, BNT
peaks become sharper and stronger, indicating a higher degree
of crystallization. Particularly, almost no secondary phase is
ꢂ
visible in the XRD pattern of BNT produced at 200 C.
The hydrothermal treatment temperature also leads to
substantially different morphologies as shown in Fig. 3b–d.
ꢂ
When treated at 160 C, BNT manifests agglomeration of
nanoparticles with very ne whiskers of ꢀ50 nm diameter
(Fig. 3b). As evidenced by Fig. 3c and d, higher treatment
temperatures can effectively promote the growth of BNT nano-
bers. Although a considerable amount of agglomeration is still

present, bers of 150 nm diameter and several microns length
can be clearly seen in the SEM image (Fig. 3c). If the treatment
ꢂ
temperature rises to 200 C, the agglomerations are almost gone
and the dominant morphology is bre with some nanocubes at
local regions.
XRD patterns in Fig. 4a depict the effect of hydrothermal
treatment time on the phase structure of the BNT nano-
ꢂ
structures synthesized at 200 C with NaOH ¼ 12 M. Weak peak
of secondary phase is seen from the XRD pattern of 10 h-treated
BNT. When the treatment time is $20 h, the secondary phase is
gone and pure perovskite phase is obtained. According to the
XRD patterns, no conspicuous changes on the BNT phase
Fig. 5 EDS spectra of (a) BNT nanofibers and (b) BNT nanocubes.
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a matrix of spherical and agglomeration particles. Surprisingly,
prolongation of the reaction time doesn't promote the amount
of nanobres. Therefore, the hydrothermal treatment time is
critical to obtain the desired BNT morphology, i.e. nanobres.
As discussed, the formation of BNT particles is a two-step
process including dissolution and recrystallization. This
suggests, by an oriented attachment process, the larger particles
are produced from smaller initial nanoparticles. Then, the
neighbouring nanocubes join together through a self-assembly
mechanism at a planar interface and a same crystallographic
26
direction to form BNT nanobers. Thermodynamically, the
elimination of high energy surfaces which leads to a signicant
decrease in surface free energy is the motivation for this natural
27
oriented attachment. Meanwhile, for long hydrothermal treat-
ment period, adjacent particles with different size and
morphology can be self-assembled and produce bulks of cubes
28
and nanobers or massive agglomerations as shown in Fig. 4d.
Table 2 summarizes the different products and morphol-
ogies of BNT nanostructures produced in various hydrothermal
conditions which might be referable to other lead-free piezo-
ꢂ
Fig. 7 (a) TEM bright field micrograph, (b) SAED pattern and (c) HR-
TEM image of the BNT nanocubes.
electric materials. It can be concluded that 200 C, NaOH ¼
12 M and the hydrothermal treatment time of 20 h are the
optimal conditions for synthesising BNT nanobers whilst BNT
nanocubes can be produced at 200 ꢂC, NaOH ¼ 16 M and
treatment period between 20 and 60 h.
optimal conditions. Fig. 6b and c show selected area electron
ꢁ
diffraction (SEAD) pattern around the [111] zone axis and HR-
Fig. 5 shows the energy dispersive X-ray spectrometry (EDS)
spectra which reveal the detailed elemental compositions in
BNT nanobers and cubes. These analyses prove that both
morphologies include Na, Bi, Ti and O with the approximate
atomic ratio of 1 : 1 : 2 : 6, which is in good agreement with the
TEM image of a single BNT nanober, indicating the single
crystal nature of the nanobers. TEM characterization on the

bers showed an orthorhombic structure and the growth plane
is (011) with interplanar spacing of 0.44 nm, indicating the
growth direction is [011].
29
nominal stoichiometric ratio of BNT material.
Fig. 7 shows the TEM results of the BNT nanocubes, as the
Fig. 6a shows the bright eld TEM image of a single BNT
nanober with a diameter about 150 nm synthesised under the
ꢂ
by-products of the nanobres, synthesized at 200 C for 20 h
with NaOH concentration of 12 M. Nanocubes have a size range
of 80–150 nm, as shown in Fig. 7a, in which a number of
nanocubes overlapped. SEAD pattern (Fig. 7b) and the clear
ꢂ
Fig. 6 (a) Bright field TEM micrograph, (b) SAED pattern and (c) HR- Fig. 8 Raman spectrum of BNT nanofibres synthesized at 200 C for
TEM image of a BNT nanofiber.
20 h with NaOH concentration of 12 M.
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lattice fringes in the HR-TEM image of a single cube (Fig. 7c) Table
3
Piezoelectric coefficient of some hydrothermally-syn-
thesised 1D piezoelectric nanostructures
indicate that cubes are also highly crystalized. TEM character-
ization on the nanocubes indicates a cubic structure that the
growth plane is (110) with interplanar spacing of 0.27 nm.
ꢁ1
Material
d33 (pm V
)
Ref.
Fig. 8 shows Raman spectrum of BNT nanobres measured BNT
at room temperature. The samples were dried at 110 C to bake KNbO
15
10.8
12
12
4
31.1
25
11.6
This work
ꢂ
3
36
39
42
44
47
48
49
SbSI
out the absorbed moisture prior to Raman measurement. Three
Na
NaNbO
BaTiO
at 789 cm . Intense band at 128 cm is assigned to Na–O LiNbO3
6
(W
6
O
19)(SeO
3
)
2
1
A (TO) absorption bands related to perovskite structure
3
ꢁ
1
1
appeared at 128, 267 and 529 cm while A ,E(LO) band appears
3
ꢁ1
ꢁ1
ꢁ1
vibrations while bands at 267 and 529 cm are associated with KNbO
3
30–33
TiO octahedral vibrations.
6
While the piezoelectric properties of many one-dimensional
3
4–49
materials have been reported,
the electrical properties of loop and amplitude-voltage buttery loop are observed as shown
ꢂ
BNT nanobers are rarely investigated. In this work, piezores- in Fig. 9e. The 180 piezoresponse phase reversal suggests the
ponse of BNT nanobers was measured by PFM with Dual AC ferroelectric nature of the BNT nanober. A piezoelectric coeffi-
ꢁ1
Resonance Tracking (DART) mode. The sample was prepared by cient (d33) of ꢀ18 pm V was estimated through measuring the
dispersing BNT nanobers in ethanol and distributing on a Pt- slop of linear part of amplitude-voltage buttery loop. Table 3
ꢂ
coated silicon wafer followed by drying at 50 C in oven. Fig. 9a shows piezoelectric coefficient for some 1D piezoelectric mate-
describes a schematic of the direction of a nanober of interest rials synthesised with hydrothermal routes. These values indicate
against the PFM tip. For PFM measurements, a Ti/Ir-coated that our hydrothermally synthesised BNT nanober is highly
ꢁ
1
silicon tip was used and a force contact of 3 N m
was comparable with other 1D piezoelectric materials, suggesting the
applied. Topography, piezoresponse amplitude and piezores- potential of our BNT nanobres for applications in nano-
ponse phase images of a 200 nm-diameter BNT nanober of generators and nanoactuators.
interest are shown in Fig. 9b–d, respectively. Local piezoresponse
of the BNT nanober was characterized by applying an AC voltage
Conclusions
of 4 V between Pt bottom electrode and the conductive PFM tip
under a DC voltage of ꢃ45 V. The typical phase-voltage hysteresis In conclusion, a morphology-optimized synthesis of BNT
nanobers has been accomplished by a simple and feasible
hydrothermal method. The best combination of precursors was
identied and the effect of several controllable factors on
structure and morphology of BNT was investigated to increase
the production of nanobers. Effective control of the BNT
morphology can be achieved through optimizing the mineral-
izer (NaOH) concentration, hydrothermal treatment time and
temperature. It is found that the concentration of mineralizer is
the dominant factor that determines the structure and
morphology of BNT nanostructures, while reaction time has
a more signicant impact on morphology than treatment
temperature. Large volume of highly crystallized BNT nano-

bres with diameter of 150–200 nm was produced under the
optimized hydrothermal conditions. Good piezoelectric
ꢁ1
response of d33 ¼ ꢀ18 pm V was conrmed by the PFM
measurements, implying the potential applications of our BNT
nanobres in next generation nanoelectronic devices.
Acknowledgements
This work was partially supported by the Australian Research
Council Discovery Project (Grant No. DP170103514). D. Y. Wang
also acknowledges the nancial support of Silverstar Award,
UNSW Australia (Grant number PS24402).
ꢂ
Fig. 9 PFM results of a BNT nanofibres synthesized at 200 C for 20 h
Notes and references
with NaOH concentration of 12 M. (a) Schematic of a nanofiber on Pt/
Si substrate for PFM measurement; (b) topography, (c) piezoresponse
amplitude image, (d) piezoresponse phase image, (e) phase hysteresis
and amplitude butterfly loops versus voltage.
1
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