Reversible transformation between CsPbBr3 and Cs4PbBr6 nanocrystals

Article abstract of DOI:10.1039/c8ce00911b

A reversible transformation between CsPbBr₃ and Cs₄PbBr₆ nanocrystals through a simple treatment with extra ligands or PbBr₂ (for the reverse direction) is reported. These transformations are so far the first report of direct reversibility, without a net change in morphology, between CsPbBr₃ and Cs₄PbBr₆ and then back to CsPbBr₃ nanocrystals, which provides a mechanism for flexible reorganization of the lead halide perovskite lattice.

Full text of DOI:10.1039/c8ce00911b

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Reversible transformation between CsPbBr and
3
Cs PbBr nanocrystals†
4
6
Cite this: DOI: 10.1039/c8ce00911b
Received 1st June 2018,
Accepted 30th July 2018
Yanxiu Li, He Huang,
*
Yuan Xiong,
Stephen V. Kershaw and Andrey L. Rogach
*
DOI: 10.1039/c8ce00911b
rsc.li/crystengcomm
3
6
A
reversible transformation between CsPbBr
nanocrystals through a simple treatment with extra ligands or
PbBr (for the reverse direction) is reported. These transformations
are so far the first report of direct reversibility, without a net
change in morphology, between CsPbBr and Cs PbBr and then
back to CsPbBr nanocrystals, which provides a mechanism for
3
and Cs
4
PbBr
6
chemical extraction of CsBr. CsBr can also be extracted via
37
an interfacial reaction with water in a two phase mixture.
The amine- and thiol-mediated extraction of PbBr2 from
CsPbX3 NCs has been reported to yield Cs PbBr6 NCs.
2
3
8,39
4
3
4
6
CsPbX
through insertion of PbBr2.
pathways between CsPbX3 and Cs PbX NCs and vice versa
3
2 5
to CsPb X conversion has been demonstrated
3
5,36
3
All the reported conversion
flexible reorganization of the lead halide perovskite lattice.
4
6
were so far processed in just one-direction, and necessitated a
follow-up purification process including centrifugation and
redispersion before further steps. Of direct relevance to the
present work, Udayabhaskararao et al. showed a phase trans-
formation between CsPbX and Cs PbX NCs driven by ligand
Since the first colloidal synthesis of lead halide perovskite
1
nanocrystals (NCs) was reported in 2014, the interest in this
2–13
class of materials has significantly increased.
Among
them, the all-inorganic CsPbX perovskite NCs introduced by
3
3
4
6
Kovalenko's group are one of the most promising candidates
owing to their high photoluminescence quantum yield (PL
QY) reaching almost 100% in solution, easy preparation, and
40
passivation. However and most notably, they transformed
2
,6,14–17
narrow emission peaks.
These outstanding properties
provide great potential for light-emitting devices with satu-
1
8–25
26–30
31,32
rated color output,
photodetectors,
and lasers.
There still remain limitations in the understanding of the
growth mechanism, formation, and surface properties of dif-
ferent lead halide perovskite NC polymorphs that requires ac-
1
5
3
tive exploration. For CsPbX NCs, the morphology changes
related to the dynamic surface ligands binding during the
synthesis have been systematically reported by Pan et al. and
3
3,34
De Roo et al.
The perovskite family of materials is not
solely restricted to ABX structures; Cs PbX and CsPb X are
3
4
6
2 5
examples of alternate perovskites, and these other modifica-
tions may not necessarily lend themselves to emissive
3
5–39
applications.
non-emissive Cs
rials can be converted to obtain CsPbX NCs by the addition
Manna's group reported the synthesis of
4
PbBr and demonstrated how those mate-
6
3
3
5
of PbBr2. The same group further reported post-synthetic
transformation of Cs PbBr NCs into CsPbBr by physical or
Fig. 1 (a) Absorption and PL spectra of the as-prepared CsPbBr
b) UV-vis absorption monitoring of the transformation progression
from CsPbBr NCs to Cs PbBr NCs every 5 min after adding additional
3
NCs,
4
6
3
(
3
4
6
OA/OLA (the sequence of spectra labelled “ligands-1 to ligands-7” cor-
responds to the time frame 5 to 35 min after addition); inset shows the
changes of relative absorbance at two spectral features (315 nm and
500 nm). (c) PL monitoring as in (b); inset shows changes of the PL
Department of Materials Science and Engineering, and Centre for Functional
Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon,
Hong Kong S.A.R. E-mail: hh@live.cn, andrey.rogach@cityu.edu.hk
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
intensity. (d) Absorption and PL spectra of CsPbBr
3
NCs transformed
c8ce00911b
back from intermediate Cs PbBr NCs by insertion of PbBr .
4
6
2
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cubic CsPbX NCs to rhombohedral Cs PbX particles, while
green emission steadily became more pronounced when
3
4
6
the opposite reaction recovered CsPbX nanoplatelets, instead
viewed under
a
UV lamp. After centrifugation and
3
of cubic CsPbX
3
nanocrystals, meaning that the morphology
redispersion in toluene, as shown in Fig. 1d, a large red shift
of the absorption spectrum (from displaying a distinct band
at 315 nm to showing a band edge step at 500 nm) took
place, and a strong and sharp PL peak at 514 nm emerged,
with a Stokes shift of 15 nm and FWHM of 22 nm, indicating
the transformation from Cs PbBr NCs to a luminescent
of the originally cubic NCs was not preserved after the trans-
formation cycle. To the best of our knowledge, the directly re-
versible and fully recovered transformation between the two
phases of perovskite NCs has not previously been realized. In
the present work, we demonstrate the completely reversible
4
6
post-synthetic transformation between CsPbBr
3
and Cs
4
PbBr
6
3
product, which, as we will show below, is CsPbBr NCs re-
NCs. The extraction of PbBr was trigged by introducing addi-
formed. Compared with pristine CsPbBr3 NCs, the
reproduced luminescent product showed slightly decreased
quality in the sharpness of the PL peak (FWHM of 22 nm) af-
ter undergoing the forward and reverse transformation pro-
cesses, but still maintained a high PL QY of 62%, which is
2
tional ligands to convert CsPbBr NCs to Cs PbBr NCs. What
3
4
6
4 6
is more, the obtained Cs PbBr NCs can be reverted to
CsPbBr NCs by reintroducing the PbBr .
3
2
The CsPbBr NCs were synthesized according to the hot-
3
injection strategy adopted from previous reports by
Kovalenko's group. As illustrated in Fig. 1a, the as-prepared
3
fairly close to the original CsPbBr NCs PL QY of 79%. The
4
whole transformation process can be observed in the videos
provided in the Supporting Information (Videos S1†). This
video clearly shows the direct and reversible transformation
from green-emitting CsPbBr3 NCs to transparent and color-
solution of CsPbBr NCs shows a sharp PL peak at 515 nm
3
(370 nm excitation wavelength), with a 15 nm Stokes shift
and a narrow full width half maxima (FWHM) of 18 nm.
Upon addition of the extra ligands, oleylamine (OA) and oleic
acid (OLA), the green emitting solution was gradually
quenched, eventually transforming into a transparent solu-
tion, colorless across the visible spectrum. Corresponding ab-
sorption and emission spectra were monitored during the re-
action process and are presented in Fig. 1b and c. When
extra OA/OLA ligands were introduced dropwise into the
4 6
less Cs PbBr NCs, as well as the following inverse transfor-
mation without any other treatment or separation. Once
transformation was completed, the reproduced CsPbBr3 NC
solution showed strong green emission again.
The TEM images in Fig. 2 illustrate the change of mor-
phology during the transformation process between CsPbBr3
4 6
NCs and Cs PbBr NCs, with high-resolution TEM (HRTEM)
CsPbBr
3
NCs, the absorbance at 500 nm decreased gradually,
images presented in the respective insets. As shown in
Fig. 2a, the as-prepared CsPbBr3 NCs are highly uniform,
monodisperse and possess a cubic shape with an average
edge size of 10 nm. After the addition of extra ligands, the
morphology of the samples dramatically changed to relatively
monodisperse, frequently hexagonal in cross-section, NCs
with an average edge length of 18 nm (Fig. 2b). If we approxi-
mate the NCs before and after transformation as discrete
indicating the decomposition/disappearance of the CsPbBr3
NCs. Meanwhile, a distinct absorption feature at 315 nm pro-
gressively emerged, which is indicative of Cs
tion, according to the previous reports on bulk Cs PbBr pow-
ders and films.
accompanied by a slight blue-shift and an increase in the
FWHM of the emission peak during the transformation pro-
cess. After 40 min, the green emission disappeared
4 6
PbBr NC forma-
4
6
3
5,36
The PL was gradually quenched and
completely, suggesting the full decomposition of the CsPbBr
3
NC structure; the trends in PL spectra matched well in time
scale compared with the absorbance changes. To emphasize
that the transformation is a cross-over in structures rather
than a two-step discrete destruction and replacement we have
presented the trends in the PL peak intensity and the evolu-
tion of the relative absorbances at two representative spectral
positions (315 nm for the characteristic peak of Cs PbBr
4
6
3
NCs, and 500 nm for the band edge of CsPbBr NCs) as a
function of time in the insets of Fig. 1b and c. The increase
of the absorbance at 315 nm is consistent with and compli-
mentary to the decrease of absorbance at 500 nm, as well as
the reduction of the PL peak intensity at 515 nm. It is worth
noting that here we choose a suitable concentration (4 mg
−
1
3
ml of the purified and dried CsPbBr NC powder) to illus-
trate the evolution of the transformation from CsPbBr
3
NCs
Fig. 2 TEM images of (a) as-prepared CsPbBr
transformed from CsPbBr NCs after addition of extra OA/OLA ligands,
c) and (d) CsPbBr NCs reshaped reversibly from Cs PbBr NCs (pic-
tures are taken on different parts of the grid and at two different mag-
nifications). Insets in (a) to (c) are HRTEM images of the respective sin-
gle particles.
3 4 6
NCs, (b) Cs PbBr NCs
to Cs PbBr NCs.
4
6
3
Afterwards, a solution of PbBr
toluene was introduced to the above obtained Cs
solution without any other treatment. Upon the addition of
the PbBr precursor, the solution started turning turbid and
2
dissolved in OLA, OA, and
(
3
4
6
4
PbBr NC
6
2
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spheres, and take Cs atom numbers per particle to remain
during and after treatment by X-ray diffraction (XRD). Fig. 3
shows the XRD patterns of as-prepared CsPbBr3 NCs, the
products collected during reaction of extra OA/OLA ligands
(measured every 3 min), and products collected after treat-
ment of PbBr . Peaks of the original CsPbBr NCs have been
the same before and after transformation, and take the Pb–
4−
Pb distance between neighboring [PbX
6
]
octahedra in
changing from corner-sharing in CsPbBr to isolated in
3
4
1
Cs PbBr , to shift from 5.9 Å to 10 Å, we can then calculate
4
6
2
3
4
2
the particle dilation to be 1.7 times, which is in good agree-
ment with our observed size change from 10 nm for CsPbBr3
NCs to 18 nm for Cs PbBr NCs. Markedly, as shown in
assigned to the cubic phase following our previous report
4
and that by Kovalenko's group. The structural evolution of
these NCs upon the addition of extra ligands was monitored.
4
6
Fig. 2c, the introduction of PbBr
2
results in the reshaping of
The characteristic peaks of CsPbBr
3
gradually disappeared,
the NCs from a hexagonoid shape back to regular cubic NCs.
Although the NCs underwent a shape transformation, the
overall size after reformation of the cubic NCs was almost
preserved, with an average edge size of 9 nm on completion
of the whole cycle. The slight emission quality loss, seen in
the PL spectra in Fig. 1d as a slightly extended short wave-
length tail, is correlated with the morphological transforma-
tion from hexagonal shape to cubic NCs, which leaves behind
some smaller debris, as shown in Fig. 2d. The overall shrink-
age in the size of the NCs from 10 nm to 9 nm edge lengths
following this treatment suggests the transformation may be
accompanied by considerable reorganization of the lattice
when going from CsPbBr to Cs PbBr , then back to CsPbBr
and some new peaks matching the reference #732478 pattern
3
5,36
for the hexagonal Cs PbBr phase became more obvious.
4
6
Both phases coexist from 0 min to 9 min. Finally, after 12
min, the peaks corresponding to the CsPbBr NCs vanished
3
and only the Cs PbBr phase remained. Then, following the
4
6
introduction of PbBr
2
, the crystal structure transformed back
to CsPbBr NCs. Peaks can be attributed to reference pattern
3
#29073 for the cubic phase of CsPbBr3 NCs, although with
broader peaks compared to the pristine CsPbBr
cating slightly smaller NC sizes. Additionally, XRD reference
patterns of the precursors (#310679 for PbBr and #050588
3
NCs, indi-
2
for CsBr) and the absence of their characteristic lines in
intermediate patterns can further rule out the consideration
of decomposition or degradation of NCs back to precursors
during the transformation process. As a whole, the XRD pat-
terns confirm the crystal phase of CsPbBr NCs and Cs PbBr
6
3
4
6
3
resulting in some loss of the original material.
To confirm the crystal structure and phase purity during
the transformation, we characterized the products before,
3
4
NCs during the reversible transformation. Both the as-
prepared CsPbBr NCs and the finally obtained CsPbBr NCs
were in the cubic phase and the intermediate Cs PbBr NCs
3
3
4
6
were in the hexagonal phase.
The transformation in morphology and crystal structure
are evidenced by TEM and XRD, and these are coincident
with the evolution of absorption and PL spectra. From these
data, we can infer the reversible transformation process be-
tween CsPbBr NCs and Cs PbBr NCs, which is outlined in
3
4
6
Fig. 4 Structural models demonstrating the reversible transformation
between luminescent CsPbBr and non-luminescent Cs PbBr NCs.
Cs PbBr NCs are obtained after addition of extra ligands (OA/OLA)
into the original CsPbBr NCs solution. Reversibly, the Cs PbBr NCs
are transformed back to strongly fluorescent CsPbBr NCs via an inser-
NCs structure is char-
3
4
6
Fig. 3 XRD pattern evolution during the reversible transformation
4
6
from as-prepared CsPbBr
formed back to CsPbBr NCs. Both the as-prepared CsPbBr
the finally obtained CsPbBr NCs were in the cubic phase, the
intermediate Cs PbBr NCs were in an hexagonal phase. The XRD pat-
3
NCs to Cs
4
PbBr
6
NCs, and then trans-
3
4
6
3
3
NCs and
3
3
tion reaction with additional PbBr
2
. The CsPbBr
4−
3
+
4
6
acterized by corner sharing [PbX
6
]
octahedra with Cs cations filling
4
−
terns during the transformation process were monitored every 3 mi-
nutes upon adding extra OA/OLA ligands and revealed the coexistence
the voids created by four neighboring [PbX
6
]
octahedra. In the
octahedra do not share any
corners and are completely decoupled in all directions. Thus, excess
PbBr can be integrated into the crystal structure of Cs PbBr NCs to
form compactly arranged CsPbBr NCs.
4
−
4 6 6
Cs PbBr NCs structure, adjacent [PbBr ]
and peak change of both CsPbBr
3
NCs and Cs
4
PbBr
6
NCs. XRD refer-
are
ences patterns #732478 for Cs
shown as line spectra.
4
PbBr and #29073 for CsPbBr
6
3
2
4
6
3
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Fig. 4. The forward reaction is triggered by extra ligands OA
and OLA. Upon the addition of these ligands, stripping of
PbBr occurs because they can break the equilibrium of the
2
dynamic surface ligand shell, offering the opportunity for li-
gand desorption, leading to strong binding with surface
lution was swiftly injected. After 5 s, the reaction was
quenched by placing the mixture into an ice water bath. The
NCs were precipitated by centrifugation at 5000 rpm and re-
dispersed in 5 mL toluene, followed by a subsequent centrifu-
gation at 10 000 rpm and re-dispersion in toluene for further
use.
PbBr
vious cubic structure.
tion, the formation of Cs PbBr NCs is achieved. TEM and
2
and subsequent extraction of these units from the pre-
3
8,39
Eventually, after sufficient extrac-
Transformation of CsPbBr NCs to Cs PbBr NCs via extra
4
6
3
4
6
X-ray diffraction indicate that all the NCs in the final forward
product have the hexagonal Cs PbBr phase and no
ligand addition
4
6
For a typical reaction, 100 μL OA and 100 μL OLA were added
dropwise into 2 mL of CsPbBr
der stirring. The solution was quenched totally within 30
min, transforming into a transparent non-luminescent solu-
tion. The transformation time was dependent upon the con-
remaining CsPbBr3 phase was present. Cs PbBr NCs have
−1
4
6
3
NC solution (4 mg mL ) un-
2 2
been previously regarded as a PbBr -deficient or PbBr -de-
3
8
pleted structure, which supports our hypothesis of extrac-
tion of PbBr2 from another aspect. For the backwards reac-
4 6
tion, the hexagonal Cs PbBr NCs are converted back to cubic
centration of the CsPbBr NC solution, as well as the amount
of extra ligand added.
3
structured CsPbBr3 NCs through the insertion reaction of
PbBr , which is supported by the ionic nature of Cs PbBr . In
2
4
6
4−
Cs
4
6 6
PbX , the [PbX ] octahedra are completely decoupled in
Transformation of Cs PbBr NCs back to CsPbBr NCs with
4
6
3
all lattice directions. In contrast, CsPbBr possesses a higher
3
added PbBr
2
precursor
symmetry, simple cubic structure with corner sharing
4
−
+
100 μL of PbBr2 precursor was injected (1 mmol PbBr2
dissolved in 0.5 mL toluene, 0.25 mL OA and 0.25 mL OLA)
into 2 mL of Cs PbBr NC solution heated at 80 °C under stir-
4 6
ring. Upon injection, the solution started turning green and
turbid. After 30 min, the resulting solution was cooled down
with an ice water bath and subsequently centrifuged at 5000
rpm and re-dispersed in 5 mL toluene for further analysis.
[
PbX
ated by four neighboring [PbX6] octahedra. Thus, excess
PbBr can be integrated into the crystal structure of Cs PbBr
NCs to form compactly arranged CsPbBr NCs.
In summary, we have realized a reversible transformation
between CsPbBr and Cs PbBr NCs through simple treat-
ments with extra OA/OLA ligands or PbBr , which is governed
by an extraction or insertion of PbBr . This transformation is
6
]
octahedra with the Cs cations filling the voids cre-
4−
2
4
6
3
3
4
6
2
2
Characterization
yet the first report of direct reversibility between CsPbBr
NCs and Cs PbBr NCs where the cubic morphology of the
3
4
6
PL spectra were measured on a Cary Eclipse (Varian) spectro-
meter. Absorption spectra were recorded using a Cary 50 UV-
vis spectrophotometer (Varian). Powder X-ray diffraction
(XRD) patterns were taken on a Philips X-Pert X-ray diffrac-
tometer using Cu Kα radiation (λ = 1.5418 Å). Transmission
electron microscopy (TEM) was carried out on a Philips CM-
20 instrument. The PL quantum yield, defined as the ratio
between photons emitted and absorbed by the sample, was
determined by an absolute method using an integrating
sphere with its inner face coated with BENFLEC® (Edinburgh
Instruments), fitted to an Edinburgh Instruments FLS 920
spectrofluorimeter.
CsPbBr3 NCs before and after the whole cycle is preserved.
The study not only allows for understanding the detailed
structural characteristics of CsPbBr NCs and Cs PbBr NCs,
3 4 6
but also for the flexible reorganization of lattices in perov-
skite systems. The possibility of reversible transformation in-
dicates that the perovskite crystal structure may potentially
withstand multiple reorganization cycles tailored to the de-
sired application.
Experimental section
Materials
PbBr
2
(98%, Aldrich), Cs
2 3
CO IJ99.99%, Aldrich), oleylamine
Conflicts of interest
(98%, OA, Aldrich), oleic acid (90%, OLA, Aldrich),
1
-octadecene (90%, Aldrich), toluene (99.7% GC, Sigma-Al-
There are no conflicts to declare.
drich). OA and OLA were dried for 1 h under vacuum at 100
C before use. The other chemicals were used as received.
°
Acknowledgements
This work was supported by the Research Grants Council of
Hong Kong S.A.R. (CityU11337616).
CsPbBr
3
NCs synthesis and purification
Cs-oleate was synthesized by reaction of Cs CO (0.16 g) with
2
3
OA (0.5 mL) in octadecene (10 mL) and pre-heated to 100 °C
before injection. PbBr2 (0.282 mmol) and 7.5 mL of
octadecene were loaded into a 100 mL 2-neck flask, dried un-
der vacuum at 120 °C for 1 h, and mixed with vacuum dried
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