All-Solid-State Mechanochemical Synthesis and Post-Synthetic Transformation of Inorganic Perovskite-type Halides

Article abstract of DOI:10.1002/chem.201705682

All-inorganic and hybrid perovskite type halides are generally synthesized by solution-based methods, with the help of long chain organic capping ligands, complex organometallic precursors, and high boiling organic solvents. Herein, a room temperature, solvent-free, general, and scalable all-solid-state mechanochemical synthesis is demonstrated for different inorganic perovskite type halides, with versatile structural connectivity in three (3D), two (2D), and zero (0D) dimensions. 3D CsPbBr₃, 2D CsPb₂Br₅, 0D Cs₄PbBr₆, 3D CsPbCl₃, 2D CsPb₂Cl₅, 0D Cs₄PbCl₆, 3D CsPbI₃, and 3D RbPbI₃ have all been synthesized by this method. The all-solid-state synthesis is materialized through an inorganic retrosynthetic approach, which directs the decision on the solid-state precursors (e.g., CsX and PbX₂ (X=Cl/Br/I) with desired stoichiometric ratios. Moreover, post-synthetic structural transformations from 3D to 2D and 0D perovskite halides were performed by the same mechanochemical synthetic approach at room temperature.

Full text of DOI:10.1002/chem.201705682

A Journal of
Accepted Article
Title: All-Solid-State Mechanochemical Synthesis and Post-Synthetic
Transformation of Inorganic Perovskite-type Halides
Authors: Provas Pal, Sujoy Saha, Ananya Banik, Arka Sarkar, and
Kanishka Biswas
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COMMUNICATION
All-Solid-State Mechanochemical Synthesis and Post-Synthetic
Transformation of Inorganic Perovskite-type Halides
Provas Pal, Sujoy Saha, Ananya Banik, Arka Sarkar, and Kanishka Biswas*
Abstract: All-inorganic and hybrid perovskite type halides are
generally synthesized by solution-based method with the help of long
chain organic capping ligands, complex organometallic precursors
and high boiling organic solvents. Herein, we demonstrate a room
temperature, solvent-free, general, and scalable all-solid-state
mechanochemical syntheses of different inorganic perovskite type
halides with versatile structural connectivity in three (3D), two (2D)
rapidity, reproducibility, and environment-friendly nature.[6] The
first example of a mechanochemical reaction is attributed to
Faraday who in 1820, reduced AgCl to Ag using Zn, Cu, Sn or Fe,
by grinding in a mortar-pestle, in the absence of a solvent.[
Recently, mechanochemical synthesis has been employed to
synthesize large scale pure metal clusters, supported
nanoparticles, metal-organic frameworks (MOFs) and covalent
organic frameworks (COFs).[7] Although the mechanochemical
syntheses of few organic-inorganic hybrid perovskite halides are
reported, [2c,8] till now there is no such report which has utilized
this method for the general synthesis of pure and variety of all-
inorganic perovskite halides. We thought it would be noteworthy
to explore mechanochemistry as a general synthetic protocol to
synthesize pure, gram scale and a large variety of inorganic
perovskite halides as they have immense technological
importance.
6]
and zero (0D) dimensions. We have synthesized 3D CsPbBr
CsPb Br 0D Cs PbBr , 3D CsPbCl , 2D CsPb Cl , 0D Cs PbCl
CsPbI and 3D RbPbI by this method. The all-solid-state synthesis is
materialized via an inorganic retrosynthetic approach, which directs to
decide on the solid-state precursors (eg. CsX and PbX (X = Cl/Br/I)
3
, 2D
2
5
4
6
3
2
5
4
6
, 3D
3
3
2
with desired stoichiometric ratios. Moreover, we have performed post-
synthetic structural transformations from 3D to 2D and 0D perovskite
halides by the same mechanochemical synthetic approach at room
temperature.
3
Recently inorganic perovskite type halides, e.g. CsPbX (X =
Cl/Br/I), have attracted immense attention for their superior
performances in photovoltaics and other optoelectronic
applications like light-emitting diodes, photodetectors and
lasing.[ Inorganic halides generally possess higher moisture and
temperature stability compared to that of the organic-inorganic
1]
3 3
hybrid halides (e.g. MAPbX , FAPbX
).[2] Although these
perovskite type halides exhibit high optoelectronic performances
but their efficiencies are extremely sensitive to the implemented
synthetic strategy. Traditionally, the inorganic perovskite type
halides are synthesized either by precipitation from solution[3] or
gas-phase
syntheses[
deposition.[2g,3]
and post-synthetic structural transformations
Generally,
solution-based
1,2]
[4,5]
have been carried out by using complex organometallic reagents,
long chain organic capping agents and high boiling solvents that
are not only expensive but also environmentally hazardous.
Alternatively, gas phase deposition involves high energy
processes and sophisticated instruments to control the thickness
of the films.[2g,3] A simple, energy efficient and universal synthetic
method to prepare inorganic perovskite halides is necessary,
which can produce gram-scale quantities of the materials without
using hazardous solvents, complex organometallic reagents and
capping ligands.
Figure 1.Crystal structure of (a) 3D CsPbBr
Cs PbBr . (d) All-solid-state mechanochemical synthesis of CsPbBr
and PbBr . (e) Physical appearance of as-synthesized CsPb Br , Cs
CsPbCl , CsPb Cl , CsPbI and RbPbI
3
, (b) 2D CsPb
2
Br
5
and (c) 0D
from CsBr
PbBr
4
6
3
2
2
5
4
6
,
3
2
5
3
3
.
Mechanochemical synthesis (i.e. all-solid-state synthesis) is
an attractive synthetic procedure with remarkable simplicity,
Herein, we report the all-solid-state mechanochemical
synthesis of several inorganic perovskite type halides with
different structural connectivity at room temperature via an
inorganic retrosynthetic approach. 3D CsPbBr
Cs PbBr , 3D CsPbCl , 2D CsPb Cl , 0D Cs
and 3D RbPbI with the reasonable optical properties are
synthesized via all-solid-state mechanochemical grinding and this
synthesis protocol can be extended to other inorganic halides.
The all-solid-state process is cost-effective, solvent-free and
scalable to obtain ~1g of pure crystalline inorganic halides. The
3
, 2D CsPb
2 5
Br , 0D
[*]
Dr. K. Biswas, Dr. P. Pal, Mr. S. Saha, Ms. A. Banik, and Mr. A.
Sarkar
New Chemistry Unit
Jawaharlal Nehru Centre for Advanced Scientific Research
4
6
3
2
5
4
PbCl , 3D CsPbI
6
3
3
(JNCASR)
Jakkur P.O., Bangalore, India
E-mail: kanishka@jncasr.ac.in
Homepage: http://www.jncasr.ac.in/kanishka/
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Chemistry - A European Journal
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COMMUNICATION
optical properties of as-synthesized inorganic halides were
studied by solid state UV-vis absorption spectroscopy and
photoluminescence. We have extended the present all-solid-state
synthesis to the post-synthetic structural transformation of 3D
Further, we have extended this synthesis to other halides such as
CsPbCl , CsPb Cl , Cs PbCl , CsPbI and RbPbI (Figure 1e).
3 2 5 4 6 3 3
Thus, the present synthesis demonstrates a simple and energy
efficient method, which can produce pure solid inorganic
perovskite type halides without using any sophisticated
instrument and it could be a general method to produce a library
of inorganic halides.
CsPbBr
3
to 2D CsPb
2
Br
5
and 0D Cs
4
PbBr
6
without using any
organic ligand or involving any complex step.
Figure 2a shows the powder X-ray diffraction (PXRD)
Retro
synthesis
Mechanochemistry
pattern of CsPbBr
indexed based on orthorhombic (space group Pbnm), tetragonal
space group I4/mcm) and rhombohedral (space group R-3ch)
3 2 4 6
, CsPb Br5, and Cs PbBr , which can be
X=Cl, Br, I
3
D
CsPbX3
CsX + PbX2
CsPbX
3
(
Retro
synthesis
Mechanochemistry
structures, respectively, without any impure phase within the
detection limit of PXRD. We have performed the profile
refinements of PXRD patterns to understand the phase purity
(Figure S2, SI) and to estimate cell parameters of the as-
synthesized samples (see Table S1, SI). The present synthesis
generally yields pure phase samples as confirmed by PXRD
refinement. The morphologies of the polycrystalline bulk materials
were studied by field emission scanning electron microscopy
2
D CsPb X
CsX + 2 PbX
CsPb X
2 5
2
5
2
X=Cl, Br
Retro
synthesis
Mechanochemistry
0
D Cs PbX
4 CsX + PbX
2
Cs PbX
4 6
4
6
Scheme 1.Retrosynthetic approach for the synthesis of inorganic perovskite
type halides using CsX and PbX as solid precursors.
2
(
FESEM) (Figure S3, SI). Typically, FESEM images of CsPbBr
3
and CsPb Br revealed that the particles are generally
2
5
The general formula of 3D and 0D perovskite halides can
+
agglomerated and their sizes range from 80 to 350 nm (Figure S3,
SI). We have measured the average crystallite sizes of as-
synthesized samples (110-160 nm) by Scherrer formula using the
PXRD patterns, which are fairly consistent with the FESEM
analysis (Table S2, SI)
be written as A
n
BX2+n, where A is a monovalent cation, Cs ; B is
2
+
2+
a divalent metal, usually Pb or Sn ; and X is a halogen (Cl/Br/I)
anion. When n = 1 and 4, the perovskite are referred to as 3D and
0
D perovskites, respectively. However, 2D perovskites have the
generic chemical formula of AB 2n+1 and 2D CsPb is obtained
when n = 2. Figure 1a-c present the crystal structures of cesium
lead bromides with different structural connectivities. CsPbBr is
a 3D distorted perovskite (orthorhombic, Pbnm), where the crystal
structure is based on the corner-shared PbBr 4−
octahedra and
Cs+ ions occupy the cuboctahedral voids (Figure 1a). The
n
X
2 5
X
3
6
tetragonal phase (I4/mcm) of CsPb
2
Br
5
exhibits layered 2D
-
+
structure consisting of [Pb
2
Br
5
] slabs and intercalated Cs (Figure
1
b).[9] In the [Pb
2
Br
5
] layer, one Pb coordinate with four Br which
-
2+
-
form an elongated pentahedron. The structure of CsPb
2 5
Br
resembles layered double hydroxide. Perovskite derivative
4
−
octahedra which are isolated by Cs+
Cs
4
PbBr
6
consists of PbBr
6
bridges to form 0D perovskite (rhombohedral, R-3ch) (Figure 1c),
which promotes to strong exciton binding energy.[10] In Cs
4
6
PbBr ,
+
the Cs ions minimize the electronic overlap between adjacent
octahedra, thus the octahedra are spatially confined to generally
produce good emission properties.[10]
In order to design the synthesis protocol and decide the
starting precursors for mechanochemical synthesis of 3D, 2D and
0
(
D cesium lead bromides, we have taken retrosynthetic approach
Scheme 1), which is generally popular in organic chemistry.
Please note the ratio of CsBr and PbBr in CsPbBr , CsPb Br
and Cs PbBr are 1:1, 1:2 and 4:1, respectively, which actually
2
3
2
5
4
6
decide the mole ratio of the starting precursors for respective
mechanochemical grinding. Typically, a stoichiometric quantity of
Figure 2. (a) PXRD patterns of 3D CsPbBr
3 2 5 4 6
, 2D CsPb Br and 0D Cs PbBr .
(b) Solid state UV/Vis absorption (α/S) (black) and PL (blue) spectra of above-
CsBr and PbBr
pestle for 2 hours in a nitrogen-filled glove bag. The white CsBr
and PbBr converted to light yellow colored CsPbBr within 10
2
are mixed and ground mechanically in a mortar-
mentioned samples. Inset in (b) show images of solid samples under a 365 nm
UV lamp.
2
3
minutes (Figure 1d). Then the grinding was continued for 2 hours
to end up with the pure compound. We have synthesized pure
To assess temperature stability and phase transition
behavior of the as-prepared powders, we have performed
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) of cesium lead bromide samples. The TGA
phase and gram scale (Figure S1, SI) CsPbBr
Cs PbBr by the above simple and solvent-free synthesis with
only changing the mole ratio of CsBr and PbBr (Scheme 1).
3 2 5
, CsPb Br and
4
6
2
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COMMUNICATION
7
curve of as-synthesized 3D perovskite CsPbBr
3
powder shows no
p-type carrier concentration of 2.09 x 10 cm−3 and mobility of
significant weight loss up to 250 oC (Figure S4a, SI). In DSC
101.5 cm V s at room temperature. The electrical conductivity
of the 3D perovskite is superior than its 2D counterpart due to the
three-dimensional connectivity of PbBr octahedron. These
6
electrical transport data are comparable with the literature reports
(Table S4, SI).[10d,e]
Chloride-based compounds e.g., 3D CsPbCl
and 0D Cs PbCl , were also synthesized by mechanochemical
synthesis using CsCl and PbCl as starting precursors in the
stoichiometric ratio of 1:1, 1:2 and 4:1, respectively. The as-
synthesized materials were characterized by PXRD, which were
indexed based on orthorhombic (space group Pbmm), tetragonal
(space group I4/mcm) and rhombohedral (space group R-3ch)
2
-1 -1
heating curve of CsPbBr
3
, two well-defined endothermic peaks
o
o
were observed at 92 C and 132 C respectively (Figure S4b, SI).
The first peak represents a phase transition from orthorhombic to
tetragonal phase and the second peak occurring at 132 °C
indicating a phase transition from tetragonal to cubic phase.[4a]
These phase transitions are reversible. Thus the products stay
stable throughout the consecutive two cycles of DSC run. The
DSC data in the first cycle is somewhat distorted due to enthalpy
relaxation. Whereas, the 2D perovskite shows less thermal
3 2 5
, 2D CsPb Cl
4
6
2
stability than its 3D counterpart. In DSC analysis of CsPb
2
Br
5
, the
o
data shows stability up to 195 C. An endothermic peak appears
o
near 220 C for CsPb
2
Br
5
(Figure S4d, SI), which is due to the
into PbBr and CsPbBr which is the
structures for CsPbCl
3
, CsPb
2
Cl5, and Cs
4
PbCl
6
(see Figure 3a
decomposition of CsPb
most stable form at high temperatures.
2
Br
5
2
3
and Figure S6, SI). CsPbCl
3
and CsPb
2
Cl
5
show absorption edge
[
4]
at 425 nm (2.92 eV) and 418 nm (2.97 eV) (see Figure 3b and
Figure S6, SI) respectively, which are due to the presence of the
X-ray photoelectron spectroscopy (XPS) was carried out to
detect the surface composition and the oxidation states of the
elements. For example, XPS survey scan and high-resolution
interconnected [PbCl
CsPbCl shows a broad PL band with a maximum at 457 nm
(Figure 3b), whereas 2D CsPb Cl shows a PL band edge
6
]4- octahedra. Upon excitation at 325 nm,
3
Cs3d, Pb4f and Br3d spectra of CsPb
2
Br
5
are shown in Figure S5,
2
5
SI. Peaks for Cs3d and Pb4f are observed in the range of 720-
emission at 446 nm while excited at 300 nm (Figure S6. SI). In
both the cases, a low intense additional lower energy peak
appears at ~500 nm, which is mainly due to the presence of
surface traps states. However, a blue-shifted absorption edge
7
50 and 120-145 eV, respectively, which are well matched with
[1d,1e]
the previous literature reports.
This clearly indicates the
+
2+
presence of Cs and Pb . The Br3d peaks can be fitted into two
peaks at binding energies of 70.5 and 73.5 eV, which correspond
to the core and surface Br ions.
appears at 355 nm (3.49 eV) for Cs
4 6
PbCl (Figure S6, SI), which
is due to isolated [PbCl
6
]4- unit in 0D structure, where the excitons
Figure 2b shows the electronic absorption spectra and
are bound in an isolated domain more strongly than that in the
corresponding 3D and 2D structures.[
12]
photoluminescence (PL) spectra of CsPbBr
Cs PbBr measured in solid state. 3D CsPbBr
absorption edge at 546 nm (2.27 eV) which is due to the presence
of the interconnected [PbBr
]4- octahedra.[11] Whereas, a blue-
shifted absorption edge appears at 530 nm (2.33 eV) for Cs PbBr
which is due to isolated [PbBr
]4- in 0D structure where the
excitons are bound in an isolated domain more strongly than that
3
, CsPb
2
Br
5
and
4 6
Cs PbCl shows an
intense broad PL band with a maximum at 357 nm when excited
at 300 nm (Figure S6, SI).
4
6
3
shows an
6
4
6
,
6
in the 3D structure.[10] In the 2D CsPb
Br , the layers are
2 5
separated by Cs+ and the excitons are retarded by layered
structure. As a result, a blue shift of absorption edge occurs at
5
37 nm (2.30 eV) compared to that in CsPbBr
at 425 nm, CsPbBr shows a PL band with a maximum at 570 nm.
Similarly, 0D Cs PbBr
3
. Upon excitation
3
4
6
shows an intense PL with a maximum at
35 nm when excited at 350 nm.[ In case of 2D CsPb
10]
Br , we
2 5
5
observe PL band edge emission at 540 nm while excited at 400
nm. We also observe a low energy PL peak at 590 nm, which is
probably due to the surface defects. The orthorhombic CsPbBr
and rhombohedral Cs PbBr demonstrate an apparent strong
3
4
6
green emission under a 365 nm UV lamp (insets Figure 2b). We
have measured the absorption coefficient of the solid powders of
cesium lead bromide samples and the values are comparable with
the literature reports.[10c] (Table S3, SI). This indicates that the
synthesized materials are good absorber of visible light.
Figure 3.(a) PXRD patterns of 3D CsPbCl3 and CsPbI . (b) Solid state UV/Vis
3
We have measured the room temperature electrical
conductivity of solid cesium lead bromide samples by the use of
four probe method. The room temperature electrical conductivity
absorption (α/S) (black) and PL (blue) spectra of the above samples.
values of CsPbBr
3
3
and CsPb
2
Br
5
were found to be 1.99x10-7 and
3 3
Further, 3D CsPbI and RbPbI were synthesized by similar
mechanochemical grinding and PXRD patterns of both the
compounds were indexed based on the orthorhombic (space
.41x10-10 Scm , respectively. Carrier concentration and type of
-1
the carrier was measured by Hall coefficient measurement. The
orthorhombic 3D CsPbBr
concentration of 8.36 x 10 cm and mobility of 149.2 cm V s
at room temperature. Whereas, the tetragonal 2D CsPb Br has a
2 5
3
possesses
a
p-type carrier
group Pnma) structure (Figure 3a and Figure S7, SI). CsPbI
RbPbI show absorption edges at 546 nm (2.27 eV) and 530 nm
(2.34 eV) respectively (Figure 3b and Figure S7, SI). Upon
3
and
9
−3
2
-1 -1
3
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Chemistry - A European Journal
10.1002/chem.201705682
COMMUNICATION
excitation with 400 nm light, CsPbI
centered at 545 nm and 570 nm respectively (Figure 3b and
Figure S7, SI).
3
and RbPbI
3
show PL bands
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to corresponding 2D and 0D
structures have earlier been carried out in organic solvents with
the help of long-chain capping ligands.[4,5] Here, we have
performed the post-synthetic solid state transformations of
1
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CsPbBr
single step mechanochemical grinding with stoichiometric
quantities of CsPbBr with either PbBr or CsBr (Scheme 2 and
Figure S8, SI). The PXRD patterns of the transformed product are
well matched with the pre-synthesized CsPb Br (space group
I4/mcm) and Cs PbBr (space group R-3ch). Further, we have
performed solid state transformations of Cs PbBr toCsPbBr and
CsPb Br to CsPbBr by the addition of appropriate concentration
of PbBr
3 2 5 4 6
to CsPb Br and Cs PbBr by solvent and ligand-free
3
2
2
5
4
6
7101–7108; j) I. Chung, J.-H. Song, J. Im, J. Androulakis, C. D. Malliakas,
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2
5
3
and CsBr respectively (Scheme S1, SI).
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3
D
CsPbBr3 + PbBr2
CsPb Br5
2D
0D
2
Mechanochemistry
3
D CsPbBr3 + 3CsBr
Cs PbBr
4
6
Scheme 2. Post-synthetic solid state transformations of CsPbBr
3 2 5
to CsPb Br
and Cs PbBr
4
6
.
In summary, we have demonstrated a simple all-solid-state
mechanochemical synthesis of pure and crystalline inorganic
perovskite-related halides, e.g. 3D CsPbBr , 2D CsPb Br 0D
Cs PbBr , 3D CsPbCl , 2D CsPb Cl , 0D Cs PbCl , 3D CsPbI
and 3D RbPbI . The reaction path and molar ratios of the starting
binary halides (eg. CsBr and PbBr ) are decided based on the
3
2
5
395–398.
4
6
3
2
5
4
6
3
[
[
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inorganic retrosynthesis scheme. Further, post-synthetic
structural transformations from 3D to 2D and 0D halides are
shown by the same all-solid-state synthesis. The
mechanochemical syntheses and transformation reactions of
halide perovskites can be scaled up to ~1g of pure product in a
single lab scale reaction. Mechanochemical grinding is a solvent-
free, cost-effective, scalable and energy efficient method as
compared to the other existing methods such as ultra-sonication,
CVD, and organic solvent assisted growth for the synthesis of
inorganic halides. The present mechanochemical synthesis can
be extended to other Pb-free halides and complex inorganic
halides such as Ruddlesden-Popper perovskite phases.
4
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Keywords: Mechanochemistry • Inorganic halide perovskites •
Solid state synthesis • Structure• Post-synthetic transformation
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Chemistry - A European Journal
10.1002/chem.201705682
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Chemistry - A European Journal
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Inorganic perovskite-type halides with
different structural connectivity were
synthesized by a simple, room
Provas Pal, Sujoy Saha, Ananya Banik,
Arka Sarkar, and Kanishka Biswas *
temperature and scalable all-solid-
state mechanochemical grinding. Post
synthetic structural transformations
from 3D to 2D and 0D halides and
vice-versa have been achieved by
same all-solid-state synthesis.
Page No. – Page No.
All-Solid-State Mechanochemical
Synthesis and Post-Synthetic
Transformation of Inorganic
Perovskite-type Halides
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