Hydrothermal synthesis of single-crystalline perovskite PbTiO3 nanosheets with dominant (001) facets

Article abstract of DOI:10.1021/ic501180g

Single-crystalline tetragonal perovskite lead titanate (PbTiO₃) nanosheets with dominant (001) facets have been successfully synthesized by employing layered K₂Ti₆O₁₃ nanofibers as titanium sources. The as-prepared PbTiO₃ nanosheets were characterized by means of X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and selected-area electron diffraction. In order to understand the formation mechanism of the PbTiO₃ nanosheets, a series of time-dependent experiments were performed. Because of the substitution of Pb²⁺ ions for K⁺ ions, the TiO₆ octahedral lamellas exfoliate from the layered K₂Ti₆O₁₃ crystal structure. Then the exfoliated TiO₆ octahedral lamellas as templates transform to lamellar PbTiO₃ species by reacting with the dehydrated Pb²⁺ ions. With hydrothermal treatment prolongation, the lamellar PbTiO₃ species crystallize to single-crystalline PbTiO₃ nanosheets. Moreover, the thickness of the synthesized single-crystalline PbTiO₃ nanosheets can be tailored in the range of 10-50 nm by controlling the hydrothermal treatment time. In addition, the band gap and the optoelectronic properties of the single-crystalline PbTiO₃ nanosheets are investigated by UV-vis absorption and photoluminescence. (Figure Presented).

Full text of DOI:10.1021/ic501180g

Article
pubs.acs.org/IC
Hydrothermal Synthesis of Single-Crystalline Perovskite PbTiO3
Nanosheets with Dominant (001) Facets
Shiqi Deng, Gang Xu,* Huiwen Bai, Lingling Li, Shan Jiang, Ge Shen, and Gaorong Han
State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, and Key Laboratory of Advanced
Materials and Applications for Battery of Zhejiang Province, Zhejiang University, Hangzhou 310027, China
S
* Supporting Information
ABSTRACT: Single-crystalline tetragonal perovskite lead
titanate (PbTiO3) nanosheets with dominant (001) facets
have been successfully synthesized by employing layered
K2Ti6O13 nanofibers as titanium sources. The as-prepared
PbTiO3 nanosheets were characterized by means of X-ray
diffraction, field-emission scanning electron microscopy, trans-
mission electron microscopy (TEM), high-resolution TEM,
and selected-area electron diffraction. In order to understand
the formation mechanism of the PbTiO3 nanosheets, a series
of time-dependent experiments were performed. Because of
the substitution of Pb2+ ions for K+ ions, the TiO6 octahedral lamellas exfoliate from the layered K2Ti6O13 crystal structure. Then
the exfoliated TiO6 octahedral lamellas as templates transform to lamellar PbTiO3 species by reacting with the dehydrated Pb2+
ions. With hydrothermal treatment prolongation, the lamellar PbTiO3 species crystallize to single-crystalline PbTiO3 nanosheets.
Moreover, the thickness of the synthesized single-crystalline PbTiO3 nanosheets can be tailored in the range of 10−50 nm by
controlling the hydrothermal treatment time. In addition, the band gap and the optoelectronic properties of the single-crystalline
PbTiO3 nanosheets are investigated by UV−vis absorption and photoluminescence.
reporting on the perovskite oxide 1D nanostructures was
proposed by Park and co-workers, in which the BaTiO3 and
SrTiO3 nanowires are realized by solution-phase decomposition
of bimetallic alkoxide precursors in the presence of
coordinating ligands.10 After a while, a more suitable method
was proposed in our previous research works.11 By virtue of the
preferential adsorption of the organic modifiers on the (100)
and (010) planes, PbZr0.52Ti0.48O3 nanorods and nanowires
were obtained. Using the layered alkali titanate nanofibers as
templates, BaTiO3 and PbTiO3 nanorods have also been
prepared by the hydrothermal route and low-temperature
molten salt method, respectively.12−14 With regard to the
ferroelectric perovskite oxide nanosheets, however, only
PbTiO3 nanosheets with a size of about 10 nm thickness
were reported in our more recent work.15 The preferential fast
combination of Na+ or K+ ions with the O2− ions situated on
the (001) or (111) surfaces results in PbTiO3 nanosheets with
dominant (001) or (111) facets. Nonetheless, accompanying
the formation of PbTiO3 nanosheets, a lot of PbTiO3
nanoparticles are also obtained.
INTRODUCTION
■
Below the Curie temperature, ferroelectric oxides exhibit
spontaneous polarization that can be reoriented with the
external electric field. Because the two stable polarization states
correspond to the positive and negative electric bias, ferro-
electric oxides have received considerable attention in non-
volatile memory devices.1−5 Previous research works performed
by Park and co-workers have demonstrated that the perovskite
BaTiO3 nanowires with a diameter size as small as 10 nm still
retain ferroelectricity.6 Although 109 bit/cm2 can be achieved
based on the BaTiO3 nanowires, it is still necessary to pursue
nonvolatile ferroelectric random-access memories (NVFRAMs)
with much higher storage density to meet the practice
requirements of miniaturization. Whereas two-dimensional
(2D) nanostructures can have high tap density by overlapping
assembly compared to one-dimensional (1D) nanostructures, it
is a prospective to fabricate NVFRAMs with much higher
storage density by perovskite ferroelectric nanosheets.
However, synthesis of the perovskite oxide nanosheets is a
challenge because of the less anisotropic crystal lattice structure.
Because the semiconductor nanocrystals were found to have
excellent properties compared to their bulk counterpart,7,8
current worldwide intense researches on nanomaterials have
been initiated. As an important kind of ferroelectric material,
perovskite oxide nanostructures also have attracted consid-
erable attention. In spite of the weak anisotropism in the lattice
structure, a lot of perovskite oxide 1D nanostructures have been
realized by bottom-up chemical methods.9 An early paper
Since the discovery of graphenes,16,17 increasing attention
has been paid to 2D nanostructures because of their unique
properties and potential applications. Because the nanosheets
well reflect the intrinsic symmetry of the layered lattice
structures, to date a variety of layered metal oxides,18−20
Received: May 22, 2014
Published: September 29, 2014
© 2014 American Chemical Society
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chalcogenides,21,22 and hydroxides23 have been exfoliated into
nanosheets by chemical or physical methods. Potassium titanate
K2Ti6O13 is a layered lattice structure. Although synthesized
K2Ti6O13 is generally made of nanofibers,24−28 it is argued that
during the hydrothermal treatment the formed K2Ti6O13
species first crystallize to nanosheets, which reflect the intrinsic
symmetry of the layered lattice and then roll to nanofibers.
Moreover, many experiments have demonstrated that the K+
ions inserted among the TiO6 octahedral layers in the titanate
crystal structures are easily exchanged with H+ and other metal
ions.24,25 Therefore, the K2Ti6O13 nanofibers are widely used as
titanium sources to synthesize titanium-based compound
nanocrystals.12−14,29−33 For example, because of exchange of
Ba2+ and Pb2+ ions with the inserted K+ ions, the BaTiO312 and
PbTiO3 nanorods13,14 are synthesized by the hydrothermal
route and low-temperature molten salt method with the layered
potassium titanate nanofibers as templates, respectively. More
recently, in our group PbTiO3 perovskite nanoplates with a
thickness of above 100 nm are hydrothermally synthesized with
K2Ti6O13 nanofibers as titanium sources under the effect of the
NaOH high concentrations, in which the formed lead oxide
nanoplates in advance are argued to play an important role in
the formation of the PbTiO3 nanoplates.33 Nonetheless, it
should be noticed that all of the synthesized titanium-based
nanostructures are scarcely nanosheets.
Lead titanate (PbTiO3) is a prototypical perovskite ferro-
electric oxide. Below a temperature of about 490 °C, PbTiO3
crystals display spontaneous polarization. Thus, PbTiO3 is a
promising candidate for NVFRAMs. Herein, we report a novel
approach to synthesizing the single-crystalline tetragonal
perovskite PbTiO3 nanosheets in high yield. In order to realize
the PbTiO3 nanosheets, the synthesized layered K2Ti6O13
nanofibers in advance are used as titanium sources. During
the hydrothermal treatment, the substitution of Pb2+ ions for
the K+ ions makes the TiO6 octahedral lamella exfoliated from
the K2Ti6O13 lattice structure and transformed to lamellar
PbTiO3 species. These special lamellar PbTiO3 species further
crystallize to the single-crystalline tetragonal perovskite PbTiO3
nanosheets with dominant (001) facets. The band gap and the
optoelectronic properties of the single-crystal tetragonal
perovskite PbTiO3 nanosheets are characterized by UV−vis
absorption and photoluminescence (PL).
dispersed in distilled water with vigorous magnetic stirring, followed
by the addition of Pb(NO3)2 and KOH pellets. After continuous
magnetic stirring for 6 h, the suspension was adjusted to 40 mL and
poured into a 50 mL stainless steel Teflon-lined autoclave for
hydrothermal treatment. In the final feedstock suspension, a Pb2+ ion
concentration of 0.2 mol/L, a Pb/Ti molar ratio of 1:1, and a KOH
concentration of 10 mol/L were created. The hydrothermal treatment
was performed by placing the sealed autoclave in an oven and keeping
it at 200 °C for 24 h, and then the autoclave was cooled to room
temperature in air. The products were filtered and washed several
times, first with distilled water and then with absolute ethanol, and
finally oven-dried in air at 60 °C for 24 h. In order to investigate the
formation mechanism of the single-crystalline PbTiO3 nanosheets, the
hydrothermal treatment time was adjusted in the range of 0.5−32 h,
and other hydrothermal treatment conditions were riveted.
Powder X-ray diffraction (XRD) patterns were obtained by a Rigaku
D/max-RA X-ray diffractometer with high-intensity Cu Ka (λ = 1.5418
Å) radiation with a step size of 0.02°. Scanning elelctron microscopy
(SEM) measurements and energy-dispersive spectroscopy (EDS) were
performed using a Hitachi S-4800 microscope (Japan). Transmission
electron microscopy (TEM and STEM) images were taken with a FEI
Tecnai G2 F30 transmission electron microscope at 160 kV, and high-
resolution TEM (HRTEM) images were caught at 200 kV. The optical
adsorption spectrum was measure by a Hitachi model U-4100 UV−vis
spectrometer. PL spectral measurements were done with a Shimadzu
RF-5301 spectrofluorimeter.
RESULTS AND DISCUSSION
Figure 1 shows the XRD pattern and SEM image of the
hydrothermally synthesized K2Ti6O13 nanofibers. All of the
■
Figure 1. (a) XRD pattern and (b) SEM image of the hydrothermally
synthesized K2Ti6O13 nanofibers.
diffraction peaks shown in Figure 1a are well indexed to the
monoclinic lattice of K2Ti6O13 (JCPDS no. 40-0403),
indicating that the obtained products after hydrothermal
treatment of the titanium hydroxide precipitate under the
effect of a high KOH concentration are pure K2Ti6O13 with a
layered lattice structure. The SEM image presented in Figure
1b reveals that the obtained K2Ti6O13 samples consist entirely
of nanofibers, agreeing well with that reported in lectures.24−28
Figure 2 shows the XRD pattern of the samples hydro-
thermally synthesized with the K2Ti6O13 nanofibers as titanium
sources. All of the diffraction peaks can be indexed to the
tetragonal PbTiO3 perovskite with the lattice parameters of a =
b = 0.3899 nm and c = 0.4153 nm, corresponding well with the
reported data of JCPDS no. 06-0452, implying that the as-
prepared samples are of pure crystalline PbTiO3 with tetragonal
perovskite structure. The strong and sharp reflection peaks
indicate that the as-prepared PbTiO3 products are well
crystallized. However, observed in detail, one can find that
the (001) diffraction peak becomes abnormally higher than the
(100) diffraction peak, which is opposite to the traditional
literature values (JCPDS no. 06-0425), suggesting that higher
EXPERIMENTAL SECTION
■
The hydrothermal treatment was carried out in a homemade Teflon-
lined stainless steel autoclave. All of the chemicals used in this work,
including tetrabutyl titanate [(C4H9O)4Ti], lead nitrate [Pb(NO3)2],
potassium hydroxide (KOH), ethanol, and ammonia, were of
analytical-grade purity and were used as received without further
purification. Distilled water was used in the preparation of all aqueous
solutions.
The layered potassium titanate K2Ti6O13 nanofibers used as
titanium sources for the hydrothermal synthesis of single-crystalline
PbTiO3 nanosheets were first prepared by hydrothermally treating the
suspension of titanium hydroxide precipitates with a KOH
concentration of 10 mol/L at 200 °C for 16 h. The titanium
hydroxide precipitates were prepared by introducing the ammonia
solution to the ethanol/(C4H9O)4Ti solution under continuous
magnetic stirring. After filtering and washing with distilled water six
times, the titanium hydroxide precipitates were dispersed in distilled
water with vigorous magnetic stirring, followed by the addition of
KOH pellets, forming a titanium hydroxide precipitate suspension with
a high KOH concentration of 10 mol/L for the hydrothermal
treatment. In a typical procedure for the synthesis of single-crystalline
PbTiO3 nanosheets, the as-prepared K2Ti6O13 nanofibers were
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(100) diffraction peak of the as-prepared PbTiO3 nanosheets,
opposite to the traditional literature values (JCPDS no. 06-
0425; Figure 2).
In order to investigate the formation mechanism of the
single-crystalline tetragonal perovskite PbTiO3 nanosheets, a
set of time-dependent experiments were carried out by
changing the hydrothermal treatment time. The samples
obtained at different stages of the hydrothermal treatment
were characterized by XRD and SEM, respectively. Figure 4
Figure 2. XRD pattern of the samples hydrothermally synthesized
with the K2Ti6O13 nanofibers as titanium sources.
amounts of (001) planes contribute to the XRD in the as-
prepared PbTiO3 products.
The SEM image of the as-prepared PbTiO3 samples is shown
in Figure 3a. Evidently, the as-prepared PbTiO3 samples
Figure 4. XRD patterns of the samples obtained after hydrothermal
treatment for different times: (a) 0.5, (b) 2, (c) 6, (d) 16, (e) 24, and
(f) 32 h.
shows the XRD patterns of the samples obtained by the
hydrothermal treatment at 200 °C for 0.5, 2, 6, 16, 24, and 32
h. When the hydrothermal treatment time is less than 0.5 h, the
PbTiO3 perovskite is hardly detected from the obtained
samples; only PbO and K2Ti6O13 are identified out. With the
reaction time is prolonged to 2 h, the diffraction peaks of PbO
and K2Ti6O13 decline and simultaneously the PbTiO3 perov-
skite phase is found. When the hydrothermal treatment time is
further extended to over 6 h, all of the diffraction peaks of the
obtained samples can be indexed to the tetragonal PbTiO3
perovskite (JCPDS no. 06-0452) and without any PbO and
K2Ti6O13. Moreover, with prolongation of the hydrothermal
treatment time from 6 to 32 h, the diffraction peaks accordingly
become sharp and intense, implying that the synthesized
PbTiO3 perovskite crystals grow larger. Otherwise, when the
hydrothermal treatment time is extended to over 16 h, the
intensity of the (001) peak becomes abnormally higher than
that of the (100) peak, reflecting the higher amount of PbTiO3
nanosheet formation. There is an additional small peak at
around 35° in the XRD pattern of the samples synthesized for
32 h, revealing that a few impurities form, accompanying the
PbTiO3 nanosheets at this moment.
Figure 5 presents the SEM images of the obtained samples
after hydrothermal treatment for different times. One can find
that, with formation of the tetragonal perovskite PbTiO3, the
morphology of the obtained samples gradually changes from
nanofibers to nanosheets. With regard to the samples obtained
by hydrothermal treatment for 0.5 h, predominant nanofibers
are observed, reflecting the morphology of the K2Ti6O13
precursors (Figure 5a). Wheres at this time a small amount
of PbO is identified by XRD in the obtained samples and PbO
easily crystallizes to nanosheets in aqueous solution, it can be
Figure 3. (a) SEM image, (b) EDS spectrum, (c) TEM image, and (d)
HRTEM image of the hydrothermally synthesized PbTiO3 nanosheets.
The inset in part a is the STEM image of one standing PbTiO3
nanosheet, and the inset in part c shows the SAED pattern caught
from the corresponding whole single PbTiO3 nanosheet.
entirely consist of nanosheets with a thickness of ca. 20 nm (see
the inset in Figure 3a) and a lateral size of 500−800 nm. The
corresponding EDS spectrum shown in Figure 3b indicates that
the nanosheets consist of lead, titanium, and oxygen with a ratio
of about 1:1:3, agreeing well with the nominal chemical
composition of the PbTiO3 perovskite. The PbTiO3 nanosheets
were further characterized by TEM and HRTEM. Representa-
tive TEM and HRTEM images of a single PbTiO3 nanosheet
are presented in parts c and d of Figure 3, respctively. Two sets
of lattice fringes with 0.390 intervals, which agree well with the
spacing of the (100) and (010) planes, are identified from the
HRTEM image shown in Figure 3d. Moreover, the selected-
area electron diffraction (SAED) pattern (the inset in Figure
3c) caught from the corresponding whole single PbTiO3
nanosheets is regular and can be indexed as the incident
electron beam parallel to the [001] direction. It is argued that
the as-prepared PbTiO3 nanosheets are single-crystalline and
dominated with (001) facets. Therefore, the intensity of the
(001) diffraction peak enhances abnormally higher than the
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important role in synthesis of the singe-crystal tetragona
perovskite PbTiO3 nanosheets. The structural model of the
layered K2Ti6O13 compound is shown in Figure 6. In a
Figure 6. Structural models of K2Ti6O13.
K2Ti6O13 crystal, TiO6 octahedra compose 2D layers by sharing
edges and corners. The 2D TiO6 octahedral layers link together
by connecting the corners of the opposing octahedra. K+ ions
insert in the tunnels formed between the 2D TiO6 octahedral
layers. Many research reports have demonstrated that the alkali
ions in titanate between the TiO6 octahedral layers can be
exchanged.24,25 On the other hand, as the Pb(NO3)2 and KOH
pellets are dissolved, in turn, in the suspension, in which the
K2Ti6O13 nanofibers were well dispersed, a lot of soluble
−
Pb(OH)3 ions form because of the high KOH concen-
Figure 5. SEM images of the samples obtained after hydrothermal
treatment for different times: (a) 0.5, (b) 2, (c) 6, (d) 16, (e) 24, and
(f) 32 h.
tration.36 According to the oxide formation mechanism under
hydrothermal conditions,37 the soluble Pb(OH)3 ions
−
dehydrate and condense to PbTiO3 perovskite crystals by
reacting with the K2Ti6O13 nanofibers. Whereas the radius of
the Pb2+ ions (119 pm) is smaller than that of the K+ ions (138
pm), it can be inferred that during the hydrothermal treatment
the Pb2+ ions tend to diffuse into the tunnels of the K2Ti6O13
structure and exchange with the K+ ions under the effect of the
high KOH concentration. When the substitution of Pb2+ ions
for K+ ions achieves a moderate extent, the layered titanate
structure becomes unstable and some TiO6 octahedral lamella
exfoliate from the layered titanate structure by breaking the
connecting corners of the opposing octahedra, which belong to
the opposite TiO6 octahedral layers. The exfoliated TiO6
octahedral lamella as templates further react with the soluble
suggested that the observed few nanosheets correspond to PbO
formation. When the hydrothermal treatment time increases to
2 h, a higher number of nanosheets are observed but for the
predominant K2Ti6O13 nanofibers (Figure 5b). In view of
formation of the PbTiO3 perovskite and the decline of PbO
(Figure 4b), the observed nanosheets at this time can be
ascribed to the tetragonal perovskite PbTiO3 nanosheets. As
the hydrothermal treatment time extends to 6 h, the tetragonal
perovskite PbTiO3 nanosheets predominate over the obtained
samples, while a few of the K2Ti6O13 nanofibers are still found
(Figure 5c). Nonetheless, because the diffraction intensity of
K2Ti6O13 is far smaller than that of PbTiO3, some of titanate
K2Ti6O13 is not checked out by XRD (Figure 4c). With
prolongation of the hydrothermal treatment time from 6 to 32
h, the K2Ti6O13 nanofibers gradually disappear and pure
tetragonal perovskite PbTiO3 nanosheets are obtained (Figure
5c−f). Moreover, the PbTiO3 nanosheets grow larger and
thicker. Thus, the diffraction peaks of the PbTiO3 perovskite
become sharper and higher (Figure 4c−f). The abnormal
enhancement of the (001) diffraction peak in the XRD pattern
(Figure 4d−f) of the obtained samples after hydrothermal
treatment for over 16 h can be ascribed to the predominant
PbTiO3 nanosheets.
−
Pb(OH)3 ions to form lead titanate lamellar species by
undergoing a dehydration and condensation process, in which
the edge-shared TiO6 octahedral lamella transform to corner-
shared TiO6 lamella in pursuit of more stability. Subsequently,
the lead titanate lamellar species crystallize to single-crystal
tetragonal perovskite PbTiO3 nanosheets. Moreover, because
the limitation along the normal direction of the nanosheets is
less than that along the plane, the single-crystal tetragonal
perovskite PbTiO3 nanosheets are dominated with (001) facets
(Figure 3c,d). Therefore, after hydrothermal treatment for 2 h,
in the obtained samples, a few tetragonal perovskite PbTiO3
nanosheets are observed, accompanying the predominant
K2Ti6O13 nanofibers (Figure 5b). With prolongation of the
hydrothermal time from 2 to 16 h, the PbTiO3 nanosheets
gradually increase and grow larger; meanwhile, the K2Ti6O13
nanofibers decrease (Figure 5b−d). When the hydrothermal
time increases to 16 h, the obtained samples are only composed
of PbTiO3 nanosheets, whereas the precursor K2Ti6O13
nanofibers are hardly observed. At this time, the synthesized
PbTiO3 nanosheets are irregular and very thin with a thickness
of less than 10 nm (Figure 5d). After this moment, because the
Our previous research works have demonstrated that, with
titanium hydroxide precipitates or commercial TiO2 powders as
titanium sources, the hydrothermally synthesized tetragonal
perovskite PbTiO3 samples are cubic particles, reflecting the
intrinsic symmetry of the lattice.34,35 In the present experiment,
because of the use of the titanate K2Ti6O13 nanofibers as
titanium sources, the synthesized PbTiO3 crystals change from
cubic particles to single-crystal nanosheets. Therefore, it is
believed that the used titanate K2Ti6O13 nanofibers play an
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Figure 7. Schematic illustration of (a−d) the formation mechanism and (e) the hydrothermal synthesis process of the single-crystalline tetragonal
perovskite PbTiO3 nanosheets with layered K2Ti6O13 nanofibers as titanium sources. (a) Pb2+ ions diffuse into the K2Ti6O13 crystal structure and
substitute for K+ ions. (a and b) Because of the extensive substitution of Pb2+ ions for K+ ions, a lot of edge-shared TiO6 octahedral lamella exfoliate
from the layered K2Ti6O13 crystal structure. (b and c) In order to be more stable, the exfoliated TiO6 octahedral lamella transform to lamellar lead
titanate species with corner-shared TiO6 octahedra by reacting with the dehydrated Pb2+ ions. (c and d) The lamellar lead titanate species crystallize
to single-crystalline tetragonal perovskite PbTiO3 nanosheets.
lead titanate species concentration around the lateral of the
nanosheets is higher than that around the planes, an Ostwald
ripening process occurs. With prolongation of the hydrothermal
treatment time to 24 and 32 h, as a result, the irregular PbTiO3
nanosheets grow thicker and of regular rectangular shape
(Figure 5e,f). To summarize the discussion above, a scheme is
simply depicted in Figure 7 for illustrating the formation
mechanism and the formation process of the single-crystal
tetragonal perovskite PbTiO3 nanosheets. The above-proposed
formation mechanism of the PbTiO3 nanosheets is also
confirmed by the another experimental research work (see
the Supporting Information), in which, because of formation of
the layered K2Ti6O13 in advance under the high KOH
concentration, the PbTiO3 nanosheets form and self-assemble
into three-dimensional flowerlike PbTiO3 nanostructures. This
research work will be reported elsewhere in the future.
However, at the initial stage of the hydrothermal treatment,
because the Pb2+ ion diffusion and substitution for K+ ions in
the layered K2Ti6O13 structure delay the formation of PbTiO3,
a lot of PbO forms by dehydration and condensation of some
−
Pb(OH)3 ions. Thus, in the samples obtained by hydro-
thermal treatment for 0.5 h, a few PbO are identified (Figure
−
4a). Whereas the dehydration of Pb(OH)3 ions to PbO is
−
Figure 8. (a) UV−vis absorption spectra of as-prepared PbTiO3
nanosheets and nanoplates. (b) Plots of (αhν)1/2 as a function of
the photon energy for PbTiO3 nanosheets and nanoplates.
reversible, because of the expense of Pb(OH)3 ions for the
formation of lead titanate, with prolongation of the hydro-
thermal treatment, the PbO formed gradually reverses to the
Pb(OH)3− ion and disappears (Figure 4a−c). Otherwise, upon
prolongation of the hydrothermal treatment time to 32 h,
because the dissolvability of lead hydroxide is larger than that of
titanium hydroxide, a higher amount of Pb2+ ions dissolve in
the reaction medium solvent from the formed PbTiO3
perovskite lattice than titanium. As a consequence, a few
titanium-rich lead titanate forms in the samples obtained after
hydrothermal treatment for 32 h and is checked out by XRD
(Figure 4f).
UV−vis absorption spectra measured from the as-prepared
single-crystalline PbTiO3 nanosheets. As a control, the typical
UV−vis absorption spectra were also measured from the
PbTiO3 nanoplates,33 the thickness of which is over 100 nm,
much larger than that of the as-prepared single-crystalline
PbTiO3 nanosheets. A strong absorption in the UV region
associated with the band−band transition is clearly observed
from both samples. However, an obvious red shift of the
absorption edge occurs for the PbTiO3 nanosheets compared
to the PbTiO3 nanoplates. The absorption edge for the PbTiO3
nanosheets is about 412 nm, whereas that for the PbTiO3
UV−vis absorption measurement was carried out to
understand the energy structures and optical properties of the
as-prepared PbTiO3 nanosheets. Figure 8a shows the typical
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nanoplates is about 403 nm. Because the tetragonal perovskite
PbTiO3 is an indirect semiconductor,38 the band gap of as-
prepared PbTiO3 nanosheets can be estimated from the
intercept of the tangents to the plots of (αhν)1/2 versus
photon energy (hν). Figure 8b shows the plots of (αhν)1/2
versus hν, which is obtained on the basis of the UV−vis
absorption spectra shown in Figure 8a. The estimated band gap
of the single-crystalline PbTiO3 nanosheets is about 2.84 eV,
whereas that of the PbTiO3 nanoplates is about 2.90 eV.
Whereas the band gap of the single-crystalline PbTiO3
nanosheets is much smaller than that reported (about 3.45
eV) for the PbTiO3 crystals,39 it can be argued that the density
of the surface states in the PbTiO3 nanosheets is relatively high
and the interaction between the exciton and lattice phonon is
very strong.40 With increasing thickness, the density of the
surface states would decrease. Thus, the band gap of the
PbTiO3 nanoplates is larger than that of the PbTiO3
nanosheets. Whereas the electron and hole can be effectively
trapped on the surface states and the charge separation would
be effective,41 the high PL emission of the PbTiO3 nanosheets
is prospective.
crystal tetragonal perovksite PbTiO3 nanosheets can be tailored
in the thickness range of 10−50 nm by controlling the
hydrothermal treatment time. During the hydrothermal treat-
ment process, the Pb2+ ions diffuse into the layered K2Ti6O13
structure and substitute for the K+ ions under the effect of the
high KOH concentration. As substitution of the Pb2+ ions for
the K+ ions accumulates to a moderate extent, the edge-shared
TiO6 octahedral lamella exfoliate from the layered K2Ti6O13
lattice structure and the template transforms to lamellar
PbTiO3 species by reacting with the dehydrated Pb2+ ions.
Then, with prolongation of the hydrothermal treatment, the
lamellar PbTiO3 species crystallize to single-crystalline PbTiO3
nanosheets. Because of less limitation along the normal
direction of the nanosheets, moreover, the synthesized single-
crystalline PbTiO3 nanosheets are dominated with (001) facets.
In addition, the single-crystalline PbTiO3 nanosheets with
dominant (001) facets are a wide-band-gap semiconductor with
a band gap of about 2.84 eV and exhibit strong PL emission in
the visible range at room temperature. Because of the high tap
density, the PbTiO3 nanosheets could be a prospective
candidate for nonvolatile memory devices, optoelectronics,
piezoelectric sensors, and other microelectronic functional
devices. From a more fundamental point of view, it is believed
that the present reported study proposes a promising technique
to prepare a 2D nanostructure with perovskite structure and
may bring a lot of perovskite oxide ferroelectric nanosheets.
Figure 9 shows the PL emission spectra of the PbTiO3
nanosheets and nanoplates with an excitation wavelength of
ASSOCIATED CONTENT
* Supporting Information
■
S
Details of our other experimental research work, confirming our
proposed formation mechanism of the PbTiO3 nanosheets.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: msegxu@zju.edu.cn.
■
Figure 9. Room temperature PL spectra of PbTiO3 nanosheets and
nanoplates excited at 280 nm.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
280 nm at room temperature. The PbTiO3 nanosheets exhibit a
strong structure surface-emission band around 467 nm with
broad bands from 450 to 500 nm, confirming the presence of
several surface states. Whereas the research work performed by
Leite and co-workers has demonstrated that the PbTiO3
powders with perfect crystal structure do not display any
surface emmission, the strong emission of the PbTiO3
nanosheets should be attributed to the recombination of
electron−hole pairs trapped on the surface states. As for the
surface-emission band around 570 nm, this can be ascribed to
the recombination of the trapped electrons and holes located in
the interfacial region between the crystalline and amorphous
PbTiO3 of the PbTiO3 nanosheets.42 Because of the decrease in
the density of the surface states, the PbTiO3 nanoplates with a
size of above 100 nm thickness do not display any surface
emission, similar to the PbTiO3 powders with perfect crystal
structure.42
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China, under Grants 61274004 and 51232006,
and the Zhejiang Natural Science Foundation, China, under
Grant LY12B07007.
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