Revealing the Formation Mechanism of CsPbBr3 Perovskite Nanocrystals Produced via a Slowed-Down Microwave-Assisted Synthesis

Article abstract of DOI:10.1002/anie.201713332

We developed a microwave-assisted slowed-down synthesis of CsPbBr₃ perovskite nanocrystals, which retards the reaction and allows us to gather useful insights into the formation mechanism of these nanoparticles, by examining the intermediate stages of their growth. The trends in the decay of the emission intensity of CsPbBr₃ nanocrystals under light exposure are well correlated with their stability against decomposition in TEM under electron beam. The results show the change of the crystal structure of CsPbBr₃ nanocrystals from a deficient and easier to be destroyed lattice to a well crystallized one. Conversely the shift in the ease of degradation sheds light on the formation mechanism, indicating first the formation of a bromoplumbate ionic scaffold, with Cs-ion infilling lagging a little behind. Increasing the cation to halide ratio towards the stoichiometric level may account for the improved radiative recombination rates observed in the longer reaction time materials.

Full text of DOI:10.1002/anie.201713332

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Accepted Article
Title: Revealing the formation mechanism of CsPbBr3 perovskite
nanocrystals produced via a slowed-down microwave assisted
synthesis
Authors: Yanxiu Li, He Huang, Stephen Kershaw, and Andrey L.
Rogach
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201713332
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Revealing the formation mechanism of CsPbBr perovskite
3
nanocrystals produced via a slowed-down microwave assisted
synthesis
[a]
*[a]
[a]
[a]
*[a]
Yanxiu Li, He Huang, Yuan Xiong, Stephen V. Kershaw, Andrey L. Rogach
Abstract: We developed microwave assisted slowed-down
synthesis of CsPbBr perovskite nanocrystals, which retards the
obtain and characterize intermediate products of their formation
and growth would be highly desirable and provide a useful guide
to improve existing synthesis methods.
3
reaction and allows us to gather useful insights into the formation
mechanism of these nanoparticles, by examining the intermediate
stages of their growth. The trends in the decays of the emission
Microwave assisted syntheses, utilizing microwave irradiation
as a relatively rapid input energy source to drive reactions, have
[10]
intensity of CsPbBr
3
nanocrystals under light exposure are well
become rather popular in synthetic chemistry.
The effect of
correlated with their stability against decomposition in TEM under
electron beam. The results point out on the change of the crystal
microwave irradiation is a combination of thermal effects, arising
from superheating or ‘‘hot spots’’, and the selective absorption of
[
10-11]
structure of CsPbBr
3
nanocrystals from a deficient and easier to be
irradiation by solvents
heating-up strategy,
.
Compared to
a
conventional
destroyed lattice to a well crystallized one. Conversely the shift in the
ease of degradation sheds light on the formation mechanism,
indicating first the formation of a bromoplumbate ionic scaffold, with
Cs ion infilling lagging a little behind. Increasing the cation to halide
ratio towards the stoichiometric level may account for the improved
radiative recombination rates observed in the longer reaction time
materials.
microwaves can
provide more
homogeneous heating, which is favorable for uniform nucleation
and the latter is essential for the formation of monodisperse
nanoparticles. Microwave assisted synthesis has already been
employed for preparation of high-quality semiconductor NCs:
quantum dots such as CdSe NCs with a strong emission were
successfully prepared via microwave treatment both in organic
[
12]
[13]
solvents , and in water. Microwave irradiation has been also
[
14]
[15]
recently employed by Long et. al
and Pan et. al
for the
Lead halide perovskites, both in the form of thin films and
colloidal nanocrystals (NCs), have recently attracted a lot of
synthesis of perovskite NCs, albeit just as a heating source for
conventional single pot reactions.
Herein, we designed a new kind of microwave assisted
[
1]
[2]
attention as components of solar cells, light-emitting diodes ,
[
3]
and photodetectors . Size, shape, composition and thus optical
properties of perovskite NCs can be conveniently tuned by
controlling reaction parameters such as precursor concentration,
temperature, ratio of halide ions for mixed compositions, as well
3
synthesis of CsPbBr perovskite NCs, with an aim to address the
formation mechanism of these nanoparticles. Aiming to slow-
down the reaction rate and specifically to be able to obtain
intermediate products of the perovskite nanoparticle growth, we
employed the reaction set-up illustrated in Fig. 1. Two powdered
[4]
as via post-synthetic treatment. Apart from the direct synthetic
approaches, chemical transformation of pre-synthesized NCs
precursors PbBr
a quartz boat, and a mixture of 1-octadecene (ODE), oleic acid
OA), and oleylamine (OLA) is carefully transferred into the
2 2 3
and Cs CO are separated at the two ends of
[
5]
has also been reported. The understanding of the formation
mechanism of perovskite NCs is crucially important for the
(
[6]
further optimization of their synthesis. However, one of the
most widely employed methods to produce the so-called all-
inorganic (Cs cation based) lead halide perovskite NCs, namely
container without disturbing these powders. ODE provides the
solvent medium which is heated under microwave irradiation,
where initially separated precursors migrate and react on a
[
7]
the hot-injection synthesis introduced by Kovalenko et al.,
slower time scale than in the case of conventional CsPbBr
3
NC
makes it rather difficult to study their formation mechanism in
detail, as the NCs are obtained literally within a few seconds. To
overcome this issue, De Mello and Kovalenko’s groups
synthesis (several tens of minutes vs. few seconds for the latter).
+
OA reacts with Cs
2 3
CO to slowly generate the Cs source, while
both OA and OLA serve as capping co-ligands for the CsPbBr
3
employed
a microfluidic synthesis combined with on-line
NCs. We stress that here we did not stir or otherwise try to mix
the reactants during the synthesis. We found the oven’s full
power (700W) setting to give the most convenient reaction rates
such that intermediate reaction products could be collected.
To illustrate the changes occurring in the reaction mixture
during the microwave treatment for different time intervals,
corresponding optical images of the container were taken under
a UV lamp; they are presented in Fig. S1. After 6 min reaction,
small bright green spots start to appear in the middle part of the
container, signaling the formation of light-emitting perovskite
product. For further extended reaction times (10 to 25 min),
green emitting reaction products continue to accumulate in the
middle of the container. The reaction mixture becomes turbid
from 15 min on, mostly due to the diffusion of the unreacted
precursor powders, which can be easily removed from the NC-
absorption and photoluminescence measurements to study the
[
8]
formation process of CsPbX
3
NCs, while Manna’s group
addressed their in-situ decomposition under electron beam
[
9]
irradiation to obtain some related hints. A “slowed-down” wet
chemical synthesis of CsPbBr NCs which would allow us to
3
[a]
Y. Li, Dr. H. Huang, Y. Xiong, Dr. S. V. Kershaw, Prof. A. L. Rogach
Department of Materials Science and Engineering
Centre for Functional Photonics (CFP)
City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong
S.A.R.
E-mails: hh@live.cn, andrey.rogach@cityu.edu.hk
Supporting information for this article is given via a link at the end of
the document.
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containing solution by centrifugation. Compared to the
commonly used hot-injection method, our microwave assisted
approach allows us to execute better control over the time of
NCs formation, slowing down the reaction from a few seconds to
became a little narrower and sharper, reflecting the growth of the
crystal size (based on the Scherrer equation) as well as
improvement in the crystallinity.
UV−visible absorption and photoluminescence (PL) spectra
25 min. Taking the advantage of the slower reaction time, we
3
(370 nm excitation wavelength) of the three CsPbBr NC
analyzed the difference between the samples prepared at three
different reaction times, namely 8 min, 15 min, and 25 min,
which are denoted as such in the further discussion.
samples are shown in Fig. 2c. The PL peaks of 8 min, 15 min,
and 25 min NCs reach 515 nm, 517 nm, 520 nm, which
coincided well with the position of the absorption peaks at 511
nm, 513 nm, and 516 nm, respectively, each with a four nm
Stokes shift. The PL full width half maxima (FWHM) of the three
samples are 20 nm, 19 nm, and 18 nm, respectively, in the order
of increasing reaction time, which further indicates the
reasonable and improving monodispersity of these NCs. The PL
quantum yield (QY) of the 8 min, 15 min, 25 min NCs are 33%,
41%, and 56%, respectively.
3
Time-resolved PL decays of the CsPbBr NCs are presented
in Fig. 2d. They have been fitted to a tri-exponential function,
and average PL lifetimes were determined to be 119 ns, 72 ns,
and 67 ns for the 8 min, 15 min, and 25 min samples,
respectively. From the average PL lifetimes and PL QY values,
we calculated the radiative ( ) and effective nonradiative (
)
lifetimes of CsPbBr NCs, which are listed in the inset of Fig. 2d.
3
There was a decrease in the average PL lifetime as the reaction
time increased, and similar and consistent trends were also
found for both radiative and nonradiative lifetimes, though the
more substantial impact was initially on the radiative
recombination rate. Between 8 and 15 minutes the effective non-
radiative rate increased significantly, but then declined once
Fig. 1 Schematic diagram of the microwave assisted synthesis of green-
emitting CsPbBr NCs. Powdered precursors PbBr and Cs CO are carefully
3 2 2 3
placed at the two opposite ends of a quartz boat, and a mixture of 1-
octadecene, oleic acid, and oleylamine is placed on top avoiding any
disturbance of the precursors, followed by treatment in a microwave oven for
different time intervals. The reactants were not stirred or mixed directly during
the reaction.
more by 25 mins presumably indicating
a
prevailing
improvement in the NC quality as the NCs became better
defined. We stress here that the calculated non-radiative rate
includes the effect of dark dots that are not active during the
decay lifetime sampling window and as such will overestimate
[
16]
the true non-radiative rate by some factor between 0 and 1.
The large shift in radiative recombination rates between the 8
min and later samples most likely correlates with the shift from
quasi-spherical to cubic particles that are more well defined in
both morphology and stoichiometry. At longer times the radiative
recombination rate was higher and is reflected in the higher
PLQYs. The modest red shift over the course of the 8-25 min
reaction times (around 5nm) is not enough, according to the
Fermi golden rule, to account for the three-fold increase in the
radiative recombination rate. The change in the latter rate is not
driven by the change in size alone, but also by changes in the
stoichiometry, which may to some extent also mask the size
Transmission Electron Microscopy (TEM) images of 8 min and
1
5
min CsPbBr
3
NCs are shown in Fig.S2a and S2b,
respectively. For 8 min NCs, predominantly spherical particles
with an average diameter of 6 nm were observed, alongside
some number of larger, cubic-shaped NCs. In the 15 min sample,
cubic-shaped particles became most prevalent, and the relative
number of spherical particles decreased. In the 25 min sample
Fig. 2a), NCs were highly uniform and monodisperse, and
possessed a cubic shape with an average edge size of 10 nm.
The HRTEM image of the 25 min sample (inset Fig. 2a)
(
demonstrates the high crystallinity of CsPbBr
characteristic lattice plane distance of 0.58 nm.
3
NCs with
[
7]
dependent red shift that is reported by Protesescu et al.
in
higher temperature grown material, where around 35 nm red
shift would be expected in this size range. These factors, i.e. the
competition between size and stoichiometric effects influencing
the emission peak positions, are also discussed elsewhere in
X-ray diffraction (XRD) patterns of three NC samples are
shown in Fig.2b, where the peaks have been assigned to the
cubic phase of CsPbBr
and that by Protesescu et al.
3
perovskite following our previous report
[
2f, 7]
We note that the orthorhombic
[
4j, 17]
literature.
phase would have differing lattice plane spacings, and would
also have a wider optical bandgap than the cubic phase, further
supporting our assignment. The diffraction peaks in all three
3
CsPbBr samples were rather broadened compared with the
corresponding bulk material, indicating their nano-scale
diameters. Additionally, as the reaction time increased more
signal peaks, which were unnoticeable in the early stage
samples, appeared in the later samples. The diffraction peaks
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Fig.2 Characterization of CsPbBr
The inset shows a representative HRTEM image of a CsPbBr
characteristic lattice plane distance of 0.58 nm. (b) XRD patterns of the three
samples, as indicated on the frame. Reference card ICSD 29073 indexes for
3
the cubic phase of CsPbBr are given for reference. (c) Absorption and PL
spectra of the three samples, as indicated. (d) Time-resolved PL decays of the
three samples fitted with tri-exponential functions; the inset provides average
decay lifetimes (ꢀꢁꢂꢃ), PL quantum yields (QY), radiative recombination
3
NCs. (a) TEM images of the 25 min sample.
NC with a
3
Fig.3 Changes of relative PL intensity of CsPbBr NC samples (8 min, 15 min
and 25 min microwave treatment) under room light exposure under ambient
conditions. Each curve is normalized to 100% at time=0 min. The inset shows
photographs of CsPbBr NCs samples prepared at 8 min, 15 min and 25 min
3
taken under a UV lamp before and after room light exposure for 30 min in
ambient conditions.
3
ꢄ
lifetimes (ꢀ ), and effective nonradiative recombination lifetimes (ꢀꢅꢄ).
A number of previous studies addressed the degradation of
[
9, 18]
perovskite NCs under electron beam irradiation,
in the hope
3
We have examined the stability of the three CsPbBr NC
of obtaining more insights into the structural characteristics of
[
19]
samples in terms of their PL quenching under exposure to
ambient room light, which can be related to structural differences
of the nanoparticles formed at different reaction times. Quartz
cuvettes containing diluted toluene solutions of NCs with the
same band edge optical density (0.1 at the respective band
edge absorption peaks), were subjected to room-light exposure
for different times (up to 30 min) under the same ambient
conditions, with an average relative humidity of 70%. The
changes in the PL spectra of these solutions are presented in
Fig. S3 and the trends for the relative PL intensity are
summarized in Fig. 3. Photographs of the three samples before
and after room light exposure for 30 min under ambient
conditions, taken under UV light illumination, illustrating their
different stabilities in terms of PL quenching are shown as an
inset to Fig. 3. There are obvious differences in the PL intensity
changes for the three samples. The 8 min NCs experienced
steady PL decay, and its emission was completely quenched
after 30 min exposure to room light. The 15 min sample was
rather stable for the first 10 min of the room-light exposure, but
then also experienced a PL decay which dropped to about 65%
of the initial value over 30 min. The 25 min sample was the most
stable, and its PL intensity was preserved at 90% of the initial
value.
these materials.
We conducted similar studies for our three
CsPbBr NC samples, as well. The 8 min sample was extremely
3
vulnerable under electron beam irradiation and the structure was
destroyed rapidly within a few seconds. The 25 min sample was
comparatively quite stable, as there were no obvious changes
even though we kept the sample under the electron beam for 10
min. The most interesting information could be obtained for the
15 min NCs; Fig. 4 shows their TEM images acquired under
continuous irradiation with an electron beam for an increasing
exposure time. At the very beginning of electron beam irradiation
(
Fig 4a), the CsPbBr
sharp and clear edges. Then, as the exposure continued (to 30
s), a few of these cubic-shaped CsPbBr NCs started to turn
3
NCs had uniform cubic morphologies with
3
from solid into hollow particles (Fig. 4b). As the irradiation time
extended to 50 s (Fig. 4c), progressively more cubic particles
were transformed into hollow particles with blurred edges, and
the volume of the voids inside the particles enlarged. As the
irradiation time continued to 70 s (Fig. 4d), the empty areas
inside these NC particles further enlarged, even splitting cubes
into fragmented domains. Eventually most of the particles
collapsed and were substantially damaged under continuous
electron beam exposure. More detailed partial enlarged TEM
images of this transformation process are also presented in
Fig.4.
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quasi spherical field of particles seen in Fig. 2a. The debris in
the longer synthesis time samples may be more stable than the
initial quasi spherical and later time cubic particles but may well
differ in composition from the former. As such the debris may
simply be vestigial particles (mainly PbBr ) that could not acquire
x
very much, if any, Cs ions to form the full perovskite structure.
Based on the experimental findings of our work, and taking
[
8, 20]
into account related literature results,
we propose the model
NCs, schematically
depicting the formation process of CsPbBr
3
[
20a]
[20b]
illustrated in Figure 5. Manser et al. , Yang et al.
and Guo
[
20c]
et al.
all emphasize the leading role that the coordination
chemistry of the lead halide ions has in the formation of
extended 1-D and 2-D sheets that may act as superstructures
for the eventual perovskite network. At the initial stage, corner-
sharing PbBr
6
octahedra form a framework structure which
gradualy becomes further stabilized by Cs ions filling some of
the interstitial voids. It would appear that the structure does not
have to be completely populated with Cs ions to start to form
periodic structures with the CsPbBr lattice spacings, hence the
3
observation of the XRD pattern at 8 mins in Fig. 2b for the quasi
spherical particles. This pattern has the main features of the
3
CsPbBr lattice but some of the weaker indices are less evident
and far less sharp than for the bulk material. As the growth
continues and further Cs ions pack the gaps in the framework
the subsidiary indices in the XRD patterns become more clearly
defined. The increase in NC size and more cubic habit of the
particles also contributes to the sharpening of the diffraction
peaks. Finally, the NC structure achieves a more complete
general perovskite structure, in which Cs ions occupy the
6
vertexes of cubes in each unit cell, while PbBr octahedra are
arranged in the center of the framework formed by Cs ions.
However, we emphasize that the linked bromoplumbate
octahedra provide the scaffold first for the Cs ions to add to. The
overall structural integrity, which is responsible for the stability
and crystallinity of the resulting NCs, results from the
arrangement of the framework and filling with Cs ions. This
model illustrates an evolution from a Cs deficient towards the
Fig.4 TEM images illustrating gradual decomposition of the 15 min CsPbBr
3
NCs from inside to outside under electron beam exposure for 10 s (a), 30 s (b),
0 s (c) and 70 s (d), resulting in the formation of the hollow cubes. The beam
5
energy during imaging and irradiation stages was 200kV.
3
complete and more robust crystal structure of CsPbBr NCs.
The inside-out mode of the decomposition of 15 min CsPbBr
3
NCs in the electron beam, which we observed under TEM
examination, indicates that the crystal structure at short
synthesis times inside these NCs is rather fragile/incomplete,
which makes them unstable under electron beam irradiation.
Voids would allow rapid ingress and egress of Cs cations for
example. Another possible explanation of the observed
phenomenon would be that ligands attached at the surface are
able to better stabilize the outside skin of the perovskite NCs.
However, for the 25 min sample, no obvious decomposition or
degradation was observed under electron beam irradiation for
comparable times, which mitigates against the ligand
stabilization explanation but supports the incomplete filling of the
framework-like crystal structure at short times. In addition, TEM
images of 25 min NCs (Fig.2a) clearly demonstrate the
formation of predominantly cubic-shaped particles which could
again be attributed to a relatively complete crystal structure. In
Fig. S2b and Figs. 4a to 4d there are some non-cubic (or in
some cases only vaguely cubic) debris scattered peripheral to
the clearly cubic main blocks. This debris is quite unlike the
more monodisperse and more densely packed and ordered
Fig.5 Schematic representation of the growth model depicting the formation of
CsPbBr3 NCs, gradually progressing from deficient to complete crystal
structure.
In summary, we have introduced the synthetic process of
CsPbBr
3
NC formation via a microwave irradiation assisted
method with separated precursors, instead of the conventional
one-flask method utilizing a fully mixed precursor solution. This
approach retards the reaction and allows us to gather useful
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insights into the formation mechanism of perovskite NCs, by Acknowledgements
examining the intermediate stages of their growth. The trends in
3
the decays of the PL intensity of CsPbBr NCs under light
This work was supported by the Research Grant Council of
Hong Kong S.A.R. (GRF project CityU11337616).
exposure (Fig. 3) are well correlated with the stability of the NCs
against decomposition in TEM under electron beam (Fig. 4),
reflecting the change of the crystal structure from a deficient and
easier to be destroyed lattice to a well crystallized one.
Conversely the shift in the ease of degradation sheds some light
on the formation mechanism for the NCs, indicating first the
formation of a bromoplumbate ionic scaffold, with Cs ion infilling
lagging a little behind. Changes in the halide to metal ratio will
influence the ease of charge transfer between the two types of
3
Keywords: perovskite nanocrystals • CsPbBr • formation
mechanism • microwave synthesis
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3
Isolation and purification of CsPbBr NCs
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
*
Schematic diagram of the microwave
assisted synthesis of green-emitting
Yanxiu Li, He Huang, Yuan Xiong,
*
Stephen V. Kershaw, Andrey L. Rogach
CsPbBr
precursors PbBr
3
nanocrystals. Powdered
and Cs CO are
Page No. – Page No.
2
2
3
placed at the opposite ends of a
quartz boat, and a mixture of ligands
is placed on top, followed by treatment
Revealing the formation mechanism
of CsPbBr perovskite nanocrystals
3
produced via a slowed-down
microwave assisted synthesis
in
a
microwave oven. Such an
arrangement retards the reaction and
allows us to gather insights into the
formation mechanism of perovskite
nanocrystals, by examining the
intermediate stages of their growth.
Keywords: lead halide perovskite
nanocrystals; microwave synthesis;
formation mechanism; crystal structure
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