Solvothermal synthesis of cesium lead halide nanocrystals with controllable dimensions: A stoichiometry defined growth mechanism

Article abstract of DOI:10.1039/c9tc05559b

We presented here a solvothermal approach to synthesize CsPbX₃ nanostructures with precisely controlled dimensions. By simply varying the amount of Cs precursor, three-dimensional nanocubes, two-dimensional nanoplatelets/nanoribbons and one-dimensional nanorods with high uniformity in morphology and size were successfully synthesized. A stoichiometry defined growth mechanism has been proposed to explain the growth mechanism. This work not only adds another effective tool in the synthesis toolbox, but also helps to understand the formation mechanism of CsPbX₃ nanostructures.

Full text of DOI:10.1039/c9tc05559b

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Solvothermal synthesis of cesium lead halide
nanocrystals with controllable dimensions: a
stoichiometry defined growth mechanism†
Cite this: J. Mater. Chem. C, 2019,
7, 14493
Received 10th October 2019,
Accepted 12th November 2019
Min Chen,‡ab Huicheng Hu,‡b Nan Yao,b Xiaolei Yuan,*a Qixuan Zhong,b
b
b
Muhan Cao,*b Yong Xu
and Qiao Zhang
*
DOI: 10.1039/c9tc05559b
rsc.li/materials-c
We presented here a solvothermal approach to synthesize CsPbX3 inspire some new applications.21–27 For example, three-dimensional
nanostructures with precisely controlled dimensions. By simply (3D) CsPbX3 nanocubes with high PLQY and narrow emission
varying the amount of Cs precursor, three-dimensional nanocubes, linewidth have shown promising potential as building blocks for
two-dimensional nanoplatelets/nanoribbons and one-dimensional high-performance optoelectronic nanodevices, such as LEDs, solar
nanorods with high uniformity in morphology and size were success- cells, lasers and X-ray detectors.11,17,23,27 Yang et al. reported
fully synthesized. A stoichiometry defined growth mechanism has been that two-dimensional (2D) CsPbBr3 nanoplatelets could be ideal
proposed to explain the growth mechanism. This work not only adds candidates for blue LED devices due to the strong quantum
another effective tool in the synthesis toolbox, but also helps to confinement emission.26 Tong et al. demonstrated that one-
understand the formation mechanism of CsPbX3 nanostructures.
dimensional (1D) CsPbX3 nanowires exhibited strongly polarized
absorption and emission and thus can be integrated into nanoscale
optoelectronic devices.22 To date, the dimensionally controlled
synthesis of CsPbX3 nanocrystals with various nanostructures has
been achieved by adjusting the reaction parameters (such as time,
temperature, precursor and surface ligands) in direct synthesis or
post-modification approaches.26,28–35 However, it is quite challenging
to make monodisperse CsPbX3 nanocrystals with desired mor-
phology in a simple and effective way. Since this field is still in
its early stage, any new understanding or new findings will be
very helpful and may further lead to a series of innovation in
their practical application.
Solvothermal approach has proven to be an effective and
powerful method to synthesize nanomaterials with controllable
dimension and desired properties.36–40 Since our first report on
solvothermal synthesis of CsPbX3 nanocrystals in 2017,41 high-
quality CsPbX3 nanocrystals with various morphologies, such
as nanocubes, nanowires, nanoplatelets and dodecapod nano-
flowers, have been made through this approach.42–45 Different
from the fast nucleation and growth kinetics of traditional hot-
injection and re-precipitation methods, solvothermal method
performs a relatively slow reaction that can provide a great
opportunity to kinetically control the nucleation and growth
process in the synthesis of nanocrystals so that the morphologies
of product can be well defined. Moreover, solvothermal method
has been featured for some unique properties, such as high
crystallinity, high uniformity, precise control over the size and
shape of products, and simple and scalable synthesis. In this
study, we employed the solvothermal strategy to precisely control
the dimensions of CsPbBr3 nanostructures. By tuning the
Introduction
All inorganic perovskite CsPbX3 (X = Cl, Br, and I) nanocrystals
have been regarded as promising optoelectronic materials in
recent years owing to their excellent photophysical properties,
such as high photoluminescence quantum yield (PLQY), narrow
emission linewidth, tunable emission wavelength covering the
full visible range, high carrier mobility and long carrier diffusion
length.1–7 They have attracted tremendous attention in diverse
applications, including light emitting diodes (LEDs),8–12 solar
cells,13,14 lasers15–17 and photodetectors.18–20 Since the geometric
dimension of inorganic nanomaterials plays significant roles in
determining their physicochemical properties, how to precisely
control the dimensions of inorganic nanocrystals has been a hot
topic in the field of nanoscience and nanotechnology. Over the
past several years, researchers have demonstrated that CsPbX3
nanocrystals with different geometric dimensions could show
different photophysical and electronic properties which might
a School of Chemistry and Chemical Engineering, Nantong University, Nantong,
9 Seyuan Road, Nantong, 226019, Jiangsu, P. R. China.
E-mail: xlyuan@ntu.edu.cn
b Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key
Laboratory for Carbon-Based Functional Materials and Devices,
Soochow University, 199 Ren’ai Road, Suzhou, 215123 Jiangsu, P. R. China.
E-mail: mhcao@suda.edu.cn, qiaozhang@suda.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc05559b
‡ These authors contributed equally for this work.
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amount of cesium precursor, the dimension of CsPbBr3 nano-
crystals can be systematically manipulated from 3D nanocubes,
2D nanoplatelets and nanoribbons to 1D nanorods. Blue-shift of
PL emission peak has been observed in those low dimensional
CsPbBr3 nanostructures because of the strong quantum con-
finement. Due to the anisotropic shape of low dimensional
structures, the [0 0 n] (n = 1, 2, 3, 4,. . .) planes which parallel
to the lateral dimensions exhibited sharp peaks with strong
intensity in their XRD patterns. The atomic ratio of cesium
decreased gradually along with the shrinking of nanocrystals’
dimension. A plausible stoichiometry defined growth mechanism
has been proposed, in which the protonated octylamine can
occupy Cs+ sites on nanocrystals surface more efficiently in the
presence of lower concentration of Cs+, which can lower the
surface energy, making it possible to evolve to low dimensional
morphology. This proposed mechanism has been successfully
extended to the preparation of CsPbX3 nanocrystals with different
Fig. 1 (a–d) Low and (e–h) high magnification TEM images of CsPbBr3:
halide compositions. This work not only provides a new approach (a and e) nanocubes, (b and f) nanoplatelets, (c and g) nanoribbons and
(d and h) nanorods obtained by tuning the amount of Cs precursor:
in the controllable synthesis of CsPbX3 nanocrystals with tunable
dimension and morphology, but also sheds some light on
the mechanism understanding of CsPbX3 nanocrystals in the
120 mmol, 90 mmmol, 60 mmol and 30 mmol, while the other reaction
parameters were kept identical. Insets in (e) and (g) are HRTEM images of
CsPbBr3 nanocubes and nanoribbons, respectively. (i–l) Histograms of side
solvothermal synthesis approach.
length and width distributions of CsPbBr3 (i) nanocubes, (j) nanoplatelets,
(k) nanoribbons and (l) nanorods.
Results and discussion
(Fig. S1, ESI†). The corresponding size distribution histograms
The synthesis of CsPbX3 nanocrystals with controllable dimensions of different nanostructures were provided in Fig. 1i–l. The side
was performed through a solvothermal method. Briefly, certain lengths and side widths of products were 36.9 nm/15.0 nm (for
volume of Cs precursor (0.15 M, 0.8 mL to 0.2 mL, see details in nanoplatelets), 63.6 nm/10.2 nm (nanoribbons) and 94.7 nm/
Table S1, ESI†) was added into PbBr2 (0.025 M, 11 mL) solution 4.5 nm (nanorods), respectively (Table S1, ESI†). HRTEM images
containing oleic acid (OA), octylamine (OctAm) and 1-octadecene (insets in Fig. 1e and g) and their corresponding fast Fourier
(ODE) at room temperature. The mixtures were then transferred transform (FFT) patterns (Fig. S2 and S3, ESI†) confirmed a
into an autoclave and reacted at 120 1C for 60 minutes. The single-crystalline cubic feature of the nanocubes and nanoribbons.
products were finally separated by centrifugation and washed Furthermore, the lattice distances of nanocubes and nanoribbons
with hexane. The synthesis details could be found in Experi- were 0.42 nm, which can be assigned to the (110) planes of
mental section in ESI.† TEM images show the morphological cubic phase.
structures of CsPbBr3 nanocrystals obtained by tuning the
UV-Vis absorption and PL emission spectra were used to
amount of Cs precursor. As depicted in Fig. 1 and Fig. S1 (ESI†), investigate the optical properties of as-prepared CsPbBr3 nano-
all products exhibited uniform morphology and size distribution, crystals. As depicted in Fig. 2a, when 120 mmol of Cs precursor
suggesting the high yield and high quality of these products. As was employed, the as-obtained 3D CsPbBr3 nanocubes showed
shown in Fig. 1a and e, when 120 mmol Cs precursor was added, a green color and a sharp emission peak at 516 nm, which was
uniform 3D nanocubes with an average size of 12.3 nm were consistent with the previous reports.46 In addition, CsPbBr3
obtained. As the amount of Cs precursor decreased to 90 mmol nanocubes showed an excitonic absorption peak at around
and 60 mmol while the other parameters kept identical, mono- 510 nm. As the amount of Cs precursor decreased, a remarkably
disperse 2D nanoplatelets (Fig. 1b and f) and nanoribbons large blue-shift in both absorption and emission peak was
(Fig. 1c and g) with low contrast were obtained, respectively. observed. This result could be attributed to the quantum
The low contrast suggested the thin nature of products, which confinement effect, in which the thickness of low dimensional
was confirmed by TEM images of nanocrystals assembled in a structures (nanorods/nanoribbons/nanoplatelets) was smaller
face-to-face manner (Fig. S1, ESI†). From their assembled mor- than their Bohr diameter (B7.0 nm).21 As shown in Fig. 2a, 2D
phology, it could be found that the thickness was around 3.0 nm. CsPbBr3 nanoplatelets and nanoribbons and 1D nanorods
Since the unit cell of cubic CsPbBr3 is about 0.58 nm, the displayed very similar sharp PL emission peaks at around
thicknesses of 3.0 nm can be corresponded to 5 unit cells. 460 nm, which can be corresponded to 5 perovskite unit cells
Further decreasing the amount of Cs precursor to 30 mmol, 1D as reported in previous works.26,47 Their excitonic absorption
nanorods with longer length were obtained (Fig. 1d and h). peaks were located at the same position ca. 446 nm. The sharp
Interestingly, CsPbBr3 nanorods had a thickness of 3.0 nm, and narrow absorption peak indicated high uniformity of the
which was consistent with 2D nanoribbons and nanoplatelets product. The PL decay curves of all samples can be triexponential
14494 | J. Mater. Chem. C, 2019, 7, 14493--14498
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morphologies. The analysis of Cs 3d, Pb 4f and Br 3d peaks for
each sample was shown in Fig. 2c–e, respectively. Based on the
XPS analysis, the relative atomic ratios of Cs : Pb : Br for
all samples were calculated and summarized in Table 1. The
Cs : Pb ratio was found to be 0.99 : 1 in CsPbBr3 nanocubes,
which was close to the ideal 1 : 1 ratio in CsPbBr3 structure.
When the amount of Cs precursor decreased, the Cs : Pb ratio
was gradually decreased and was lower than 1 : 1. For CsPbBr3
nanoplatelets, nanoribbons and nanorods, the Cs : Pb : Br ratios
were 0.90 : 1 : 2.60, 0.85 : 1 : 2.50 and 0.81 : 1 : 2.45, respectively.
The amount of Cs precursor was found to significantly affect
the surface component of as-obtained nanocrystals. Furthermore,
the result suggested that our low dimensional nanocrystals had
Cs deficiency on their surface which might be originated from
the replacement of Cs+ by octylammonium ions. Previous
literatures demonstrated that Cs cations on the surface of
perovskites would be replaced by oleylammonium ions in the
Fig. 2 (a) UV-Vis (solid) and PL emission (dash) spectra of CsPbBr3 nano-
crystals. The excitation wavelength is 365 nm. Bright digital images of
colloidal CsPbBr3 nanocrystals solution dispersed in hexane under UV light
(365 nm) were shown in the insets in (a). (b) XRD patterns and (c–e) XPS
spectra of CsPbBr3 nanocrystals. The binding energies were calibrated with synthesis of 2D nanoplatelets and 1D nanowires, resulting in
the Cs deficiency on their surface.24,47,49,50 The content of Cs,
the C 1s peak of the free carbon (284.6 eV).
Pb and Br within the resulting products were further confirmed
by energy dispersive X-ray (EDX) spectroscopy analysis. As
fitted well, as displayed in Fig. S4 (ESI†). The average lifetimes shown in Table 1 and Fig. S6 (ESI†), the Cs : Pb : Br ratios were
were determined to be 2.6 ns, 4.2 ns, 5.1 ns and 12.4 ns for 1.00 : 1 : 2.83 (nanocubes), 0.91 : 1 : 2.62 (nanoplatelets), 0.85 : 1 : 2.60
nanorods, nanoribbons, nanoplatelets and nanocubes, respectively (nanoribbons) and 0.82 : 1 : 2.50 (nanorods), respectively, which was
(Fig. S4 and Table S1, ESI†). The phase structures of all samples in line with the XPS results. It should be noted that the low
were confirmed by X-ray diffraction (XRD) analysis. As shown in dimensional nanocrystals had a halide-deficient surface as well,
Fig. 2b, the XRD signals of CsPbBr3 nanocubes fitted well with the which could be attributed to the increased surface-to-volume
standard XRD patterns of cubic phase (JCPDS card# 054-752). The and decreased formation energy.25,49,51,52
broad signal could be ascribed to the small particle size. Due to
To further investigate the surface chemistry of our products,
the anisotropic shape of the CsPbBr3 nanoplatelets, the [0 0 n] the aspect ratio (length/width) and surface-to-volume ratios for
(n = 1, 2, 3, 4,. . .) planes which parallel to the lateral dimensions all samples were calculated. As shown in Fig. 3a, with the
exhibited narrow peaks with high intensity. In contrast, the decreasing amount of Cs precursor, the aspect ratios of nano-
other peaks were much broader and less intense (see the crystals increased from 1.0 to 21.0. Additionally, as the Cs
enlarged XRD patterns in Fig. S5, ESI†). Interestingly, the peaks precursor decreased, the surface-to-volume ratio of nanocrystals
were slightly shifted toward low angles with respect to the increased. Thus, we can conclude that OctAm capping on the
standard diffraction pattern of cubic CsPbBr3 phase, indicating a surface of perovskites can promote the anisotropic growth of
slight expansion of the overall lattice.32,47,48 The expanded lattice nanocrystals during the synthesis of perovskites. Based on the
may come from the replacement of Cs+ ions by oleylammonium above results, a possible formation mechanism was proposed
ions, which seems like an alkylammonium lead halide type (Fig. 3b). It has been widely accepted that the protonated OctAm
like perovskite that could further slightly expand the lattice of by OA would compete with Cs+ cations in the formation of
nanocrystals.47,48 Similar phenomena were observed in CsPbBr3 CsPbX3 nanocrystals.24,47,53,54 When the concentration of Cs
nanoribbons and nanorods. Only two XRD peaks at 2y = 14.81 precursor was high, few Cs+ cations were replaced by octyl-
and 29.81 with high intensity were observed.
ammonium cations, resulting in the formation of isotropic
The surface chemistry of metal halide perovskites plays shape of CsPbBr3 nanocubes. When the amounts of Cs+ cations
an important role on their morphology and size. X-ray photo- decreased, more protonated OctAm would locally mimic an
electron spectroscopy (XPS) analysis was performed to elucidate octylammonium lead halide type cell. On the other hand,
the surface properties of CsPbBr3 nanocrystals with different OctAm on the surface of nanocrystal would slow down the
Table 1 The relative atomic ratios of Cs : Pb : Br obtained from XPS and EDX, respectively
Nanocubes
XPS
Nanoplatelets
XPS
Nanoribbons
XPS
Nanorods
XPS
Elements
EDX
EDX
EDX
EDX
Cs(At%)
Pb(At%)
Br(At%)
Cs : Pb : Br
21.10
21.32
57.58
0.99 : 1 : 2.70
20.70
20.72
58.58
1.00 : 1 : 2.83
20.01
22.22
57.77
0.90 : 1 : 2.60
20.10
22.08
57.82
0.91 : 1 : 2.62
19.55
23.00
57.45
0.85 : 1 : 2.50
19.10
22.47
58.43
0.85 : 1 : 2.60
19.02
23.47
57.51
0.81 : 1 : 2.45
18.98
23.15
57.87
0.82 : 1 : 2.50
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Fig. 3 (a) Aspect ratio (length/width) (red) and surface-to-volume (blue)
relations as a function of the volume of Cs precursor added. (b) The
possible proposed growth mechanism.
growth in vertical dimension and then limit the growth of vertical
planes. As a result, thinner 2D nanoplatelets and nanoribbons
could be obtained. When the concentration of Cs+ cations was
very low, Cs+ cations were occupied by octylammonium cations
more efficiently, which could lower the surface energy and
further increase the specific surface area, resulting in the shrink-
ing of dimension and formation of 1D nanorods. In order to
further confirm the proposed mechanism, a series of experi-
ments were performed (Table S2, ESI†). Firstly, when the amount
of PbBr2 was increased from 270 mmol to 406 mmol with fixed
amount of Cs precursor (60 mmol), Cs+ cations can be replaced
easier by octylammonium cations, which would limit the growth
of nanocrystals in vertical dimension, leading to the morphology
evolution from 2D nanoribbons to 1D nanorods (Fig. S7, ESI†).
Furthermore, when the amount of PbBr2 was decreased to
187 mmol, fewer Cs+ cations were replaced by octylammonium
cations, resulting in the morphology evolution from 2D nano-
ribbons to CsPbBr3 nanocubes (Fig. S8, ESI†).
Fig. 4 (a–f) TEM images of low dimensional CsPbX3 nanocrystals with
different halide (X = Cl, Cl/Br and Br/I) compositions. (g) Digital photo-
graphs of colloidal dispersions of CsPbX3 nanocrystals in hexane under UV
light (365 nm) and (h) corresponding UV-vis (solid) and PL emission (dash)
spectra of CsPbX3 nanocrystals (lexc = 365 nm for all samples but 320 nm
for CsPbCl3).
nanostructure. Moreover, a slight blue-shift of the XRD peaks
was observed with the halide changed from ClÀ to IÀ (see enlarge
XRD peaks in Fig. S9, ESI†), indicating the successful diffusion of
halide ions in the lattice structures.
In addition, the emission of low dimensional CsPbX3 nano-
structures can be tuned by a fast anion exchange reaction at
room temperature. Here we used CsPbX3 nanoribbons as the
example. As shown in Fig. 5a, 2D CsPbX3 nanoribbons solution
Low dimensional CsPbX3 nanostructures can be obtained by
simply adding other lead halides or their mixtures (Cl, Cl/Br, or
Br/I) in appropriate ratios (see more details in ESI†). On the
surface of CsPbX3 nanocrystals, octylammonium cations can
occupy Cs+ cations and coordinate with halides on the surface.
Since the binding energy of halides with octylammonium is
different (BEClÀ 4BEBrÀ 4BElÀ),55 octylammonium cations can
replace Cs+ cations more easily on the surface of CsPbCl3
nanocrystals in comparison to CsPbBr3 and CsPb(Br/I)3 nano-
crystals, leading to the generation of final morphologies from
1D nanorods, 2D nanoribbons to 2D nanoplatelets (Fig. 4a–f).
The low dimensional CsPbX3 nanocrystals dispersed in hexane
showed bright emission from violet to green (Fig. 4g). The
absorption and PL emission spectra showed slight red-shift
with different halide compositions from chloride to bromide to
iodide (Fig. 4h). It was worth pointing that, compared with
CsPbCl3 nanocubes emitting around 410 nm, 1D CsPbCl3
nanorods exhibited a large blue-shift (B25 nm), which might
be a good candidate for UV detector. The powder XRD patterns
of low dimensional CsPbX3 nanocrystals were shown in Fig. S9
(ESI†). The diffraction peaks of all samples can be indexed to
cubic phase. In addition, two sharp and strong diffraction
peaks correspond to (0 0 n) planes of cubic phase for all
samples were observed, further confirming their anisotropic
Fig. 5 (a) Photographs of CsPbX3 nanoribbons dispersed in hexane pre-
pared by mixing CsPbBr3 nanoribbons solution with various concentrations
of PbCl2 or PbI2 solution under UV light (l = 365 nm) and (b) corresponding
PL emission spectra of the halide-anion exchanged samples (lexc = 365 nm
for all but 320 nm for CsPbCl3, 400 nm for CsPbI3).
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Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (21673150, 21703146, 51802206, 51922073),
Natural Science Foundation of Jiangsu Province (BK20180097,
BK20180846). We acknowledge the financial support from the
111 Project, Collaborative Innovation Center of Suzhou Nano
Science and Technology (NANO-CIC), the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD), and Postgraduate Research & Practice Innovation Program
of Jiangsu Province (KYCX19-1920).
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Communication
Journal of Materials Chemistry C
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14498 | J. Mater. Chem. C, 2019, 7, 14493--14498
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