Solvent-Free Mechanochemical Synthesis of a Systematic Series of Pure-Phase Mixed-Halide Perovskites MAPb(IxBr1?x)3 and MAPb(BrxCl1?x)3 for Continuous Composition and Band-Gap Tuning

Article abstract of DOI:10.1002/cplu.201900723

Hybrid perovskites have recently received much attention in optoelectronic applications. However, hybrid perovskites are unstable in a humid environment. Mixed halide perovskites (MHPs) show enhanced stability and band-gap tunability upon engineering of their halide composition. Here, MHPs are prepared through a solvent-free mechanochemical synthesis (MCS) route that allows superior control over halide compositions than the solvent synthesis routes (SS). The MCS route eliminates the problem in the preparation of MAPb(I_xBr_1?x)₃ with continuously varying x, while maintaining the material properties and suppressing phase segregation present in SS routes. UV-vis absorption and X-ray diffraction patterns confirm the production of the desired pure-phase MHPs. For MAPb(I_xBr_1?x)₃ (0≤x≤1), with increased ratio of halide (x), the cubic phase gradually transforms into the tetragonal phase and band-gap tunability is accomplished. The MCS route for the preparation of MHPs is a very promising and efficient technique for superior control in optoelectronic properties, leading to improved control in fabrication approaches.

Full text of DOI:10.1002/cplu.201900723

Accepted Article
Title: Solvent-Free Mechanochemical Synthesis of a Systematic Series
of Pure-Phase Mixed-Halide Perovskites MAPb(IxBr1−x)3and
MAPb(BrxCl1−x)3for Continuous Composition and Band-Gap
Tuning
Authors: Sheng Tang, Xinyu Xiao, Jing Hu, Bo Gao, Hunglin Chen,
Zongyang Peng, Jianchun Wen, Masanao Era, and Dechun
Zou
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.201900723
Link to VoR: http://dx.doi.org/10.1002/cplu.201900723
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ChemPlusChem
10.1002/cplu.201900723
Solvent-Free Mechanochemical Synthesis of a Systematic Series of Pure-Phase Mixed-
Halide Perovskites MAPb(I
and Band-Gap Tuning
x
Br1−x
)
3
and MAPb(Br
x
Cl1−x
3
) for Continuous Composition
Sheng Tang, Xinyu Xiao, Jing Hu, Bo Gao, Hunglin Chen, Zongyang Peng, Jianchun Wen,
Masanao Era* and Dechun Zou*
Dr. S. Tang, Dr. X. Xiao, Dr. J. Hu, Dr. B. Gao, Dr. H. Chen, Z. Peng, J. Wen, Prof. D. Zou
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Polymer Chemistry and Physics of Ministry of Education
Center for Soft Matter Science and Engineering
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871, China
E-mail: dczou@pku.edu.cn
Prof. D. Zou
Beijing Engineering Research Center for Active Matrix Display
Peking University
Beijing 100871, China
Prof. M. Era
Department of Chemistry and Applied Chemistry,
Saga University
Saga 840-8502, Japan
E-mail: era@cc.saga-u.ac.jp
Keywords:
Ball milling, band-gap tuning, mechanochemical synthesis, perovskites, solvent-free synthesis
Abstract
Hybrid perovskites have recently obtained much attention in optoelectronic applications.
However, hybrid perovskites are unstable in a humid environment. Mixed halide perovskites
(
MHPs) show enhanced stability and band-gap tunability upon engineering of their halide
composition. Here, MHPs are prepared through a solvent-free mechanochemical synthesis
MCS) route with superior control over halide compositions. It eliminates the problem in the
preparation of MAPb(I with continuously varying x, while maintaining the material
(
x
Br1−x
)
3
properties and suppressing phase segregation by solvent synthesis (SS) routes. UV-vis
absorption and X-ray diffraction patterns confirm the production of the desired pure-phase
MHPs. For MAPb(I
x
Br1−x
)
3
(0 ≤ x ≤ 1), with increased ratio of halide (x), the cubic phase
1
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ChemPlusChem
10.1002/cplu.201900723
gradually transforms into the tetragonal phase and band-gap tunability is accomplished. It is
obvious that the MCS route for the preparation MHPs is a very promising and efficient
technique for superior control in optoelectronic properties, leading to improved control in
fabrication approaches.
1. Introduction
Over the past few years, hybrid perovskites CH
3
NH
3
PbX
3
(X = Cl, Br, I, CH
3
NH = MA) have
3
emerged as strong competitors in photovoltaic and optoelectronic applications owing to their
superior characteristics, including high absorption coefficient, long carrier lifetime, high
balanced hole and electron mobility.[1–3] In 2009, Miyasaka et al. serendipitously discovered
that hybrid perovskites are useful for solar cells. After 10 years, numerous optimisations have
improved the power-conversion efficiency from 3.8% to 25.2%, which was higher than that of
commercialized crystalline silicon, CIGS, GaAs and other solar cells.[5] In addition to
photovoltaics, hybrid perovskites have been well applied in LEDs, photodetectors, transistors
and laser studies.[6–9] However, hybrid perovskites are unstable as they degrade rapidly in a
humid environment. On the other hand, mixed halide perovskites (MHPs) have shown enhanced
stability and a band-gap tunability upon engineering the halide (Cl, Br, or I) composition.[10–13]
Traditionally, hybrid perovskites are synthesised using a solvent synthesis (SS) route. When
MHPs are prepared from solution, solute–solvent interactions can vary under different
precursor component.[14] Consequently, solute–solvent interactions can alter the composition of
the final products.[15, 16] The synthesis of stoichiometric and homogeneous MHPs while
avoiding microscopic domains or inhomogeneous phase-separated solid mixtures remains a
challenge. Studies in which the ratio of halide (x) is varied continuously via a wet chemical
route are limited.
[4]
Nowadays, mechanochemical synthesis (MCS), including ball milling and manually hand
grinding, which is a kind of solid-state reaction, has been considered to be a candidate for such
abovementioned purpose, because high yields may be achieved in short times without utilizing
2
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10.1002/cplu.201900723
solvents or generating residuals.[17, 18] MCS has been successfully utilized for a long time in
other material systems, including metal-organic frameworks,[19, 20] polymorphs in drug
design,[21] and composite/metal oxide nanomaterials.[22] As recently reported for single-halide
perovskites prepared via MCS, these materials improved the device performance compared
with the SS counterparts.[23–29] MCS of perovskites has been reported not only for ternary halide
perovskites but for a variety of compositions, including mixed perovskites that has been
expanded into other A-sites (FA, Cs),[30–32] B-sites such as Sn[33–34] and X-sites (Cl, Br, I),[15, 16,
3
5]
even perovskites of different dimensionality.[36–39] MHPs are synthesized
mechanochemically by ball milling a mixture of AX and PbX , and ball milling, manually
2
grinding or thermally annealing a mixture of two single-halide perovskites.[39, 40] As a result,
MHPs prepared in the solid-state by solvent-free methods suppressed phase segregation and
resulted in fine composition control compared with their solution phase counterparts.[15, 16, 35]
However, to the best of our knowledge, the preparation of a systematic series of MAPb(I
MHPs by ball milling a mixture of MAPbI and MAPbBr has not been reported.
In this work, we explored the MCS route for the synthesis of MAPb(I MHPs and probed
their chemical structure via X-ray diffraction (XRD). For comparison, MAPb(Br MHPs
were also prepared. We found that MCS is an effective method for the preparation of
MAPb(I and MAPb(Br MHPs with superior control over halide stoichiometry,
x
Br1-
x
)
3
3
3
x
Br1−x
)
3
x
Cl1−x
)
3
x
Br1−x
)
3
x
Cl1−x
)
3
including the composition that is not previously accessible by using traditional SS methods. We
also compared samples of the same composition prepared by MCS and SS. The MCS route
described herein for a systematic series of pure-phase MHPs is a very promising technique for
optoelectronic applications.
2
. Results and Discussion
As shown in Scheme 1, the white, orange and black powders correspond to MAPbCl
MAPbBr , and MAPbI , respectively. Besides, a large scale of perovskites weighing up to 100 g
3
,
3
3
and even in those in kilogram scale can be prepared with almost no loss in short times by MCS
3
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10.1002/cplu.201900723
(
Figure S1, Supporting Information). The MAPb(I
x
Br1−x
)
3
and MAPb(Br
x
Cl1−x
)
3
MHPs were
(X = Cl, Br,
prepared by ball milling, starting with their respective parent perovskites, MAPbX
3
I) (see Supporting Information). For each series, 11 MHPs (x = 0, 0.1, …, 0.9, 1) were
synthesised. The structural and optical properties of the prepared materials were measured
precisely in detail.
XRD is a powerful tool for the characterization of crystalline materials. Figure 1A shows the
XRD patterns of MAPbX (X = Cl, Br, I) prepared by ball milling. Different reaction times
3
were employed, and the related products were characterized via XRD (Figure S2, Supporting
Information). The XRD patterns show that all reactants were completely transformed into
MAPbX
3
(X = Cl, Br, I) after mechanical ball milling for 3 h. Therefore, we set the reaction
belongs to a cubic system,
time to 3 h. XRD crystallographic analysis reveals that MAPbCl
3
and the diffraction peaks at 15.6°, 22.1° and 31.5°correspond to the (100), (110) and (200)
lattice planes, respectively.[41] MAPbBr
3
has similar diffraction peaks at 14.9°, 21.2° and 30.1°
corresponding to the (100), (110), and (200) lattice planes of the cubic structure, respectively.[42]
For X = I, the diffraction peaks at 14.1°, 28.4° and 31.9° correspond to the (110), (220) and
(
222) lattice planes of a tetragonal structure, respectively.[1, 43] Meanwhile, the clear split of the
220) and (004) peaks further confirm that the as-synthesised MAPbI powders adopt a
(
3
tetragonal phase with high crystalline quality.[44] These peaks shift to a higher diffraction angle
from I to Br to Cl, indicating that the lattice spacing decreases as expected.
As shown in Figure 1B, thermogravimetric analysis (TGA) was carried out under nitrogen flow
from room temperature to 600 °C to examine the thermal stability of the perovskite MAPbX
3
(
X = Cl, Br, I). For comparison, TGA measurements of CH NH X (X = Cl, Br, I) were also
3
3
presented (Figure S3, Supporting Information). The stability of perovskites increases at high
temperature, showing no change of weight up to 180 °C for X = Cl, to 226 °C for X = Br and
to 227 °C for X = I.
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10.1002/cplu.201900723
The optical properties of the as-synthesised powder were investigated using UV-vis and PL
spectrometer. Figure 1C shows the UV-vis absorbance spectra of MAPbX (X = Cl, Br, I),
showing the substitution from Cl to Br to I, causing dramatic changes in absorbance. The
absorption onsets shifted from 421 nm for MAPbCl to 563 nm for MAPbBr and to 813 nm
for MAPbI . All spectra show clear band edge cut-offs without excitonic signature or absorption
3
3
3
3
tails, in excellent agreement with previous literatures.[41, 42] The optical properties and ionic
radius with different halides (X) are summarised in Table 1. The band gap energy is reduced
by increasing the ionic radius of the halide in MAPbX
these perovskites, exhibiting narrow PL peaks at 417, 553 and 791 nm. Table 1 lists the
absorption onsets and corresponding PL peak values. The FTIR spectra of MAPbX were also
3
. Figure 1D shows the PL spectra of
3
measured (Figure S4, Supporting Information), and the characteristic peaks of different
perovskites are all listed in detail (Table S1, Supporting Information).
The high-resolution transmission electron microscope (HRTEM) images in Figure 1E–G show
clear lattice fringes of MAPbX
3
(X = Cl, Br, I). For MAPbCl
3
and MAPbBr , inter-planar
3
spacings of 0.25 and 0.44 nm can be assigned to the (210) and (110) planes, respectively.[9, 45]
For MAPbI
3
, a typical inter-planar spacing of 0.31 nm was recorded, which can be assigned to
(
220) or (004) planes.[46] Moreover, an inter-planar spacing of 0.70 nm was recorded, which is
assigned to the (110) plane of PbI
that MAPbI underwent a degradation process towards PbI
In addition, the SEM images of MAPbX (X = Cl, Br, I) powders with the largest particle size
up to 10 μm, showing that the particles agglomerated (Figure S6, Supporting Information).
Figure 2A exhibits the XRD patterns monitored in the 2θ range of 10°–60° for MAPb(I
powders (0 ≤ x ≤ 1) prepared by MCS with increasing I ions. As I ions replaced Br ions in
MAPbBr , systematic shifts toward lower 2θ degrees were observed because the gradual
substitution of the smaller Br atoms with the larger I atoms increases the lattice spacing. As
2
(Figure S5, Supporting Information). This finding proves
under high voltage (200 keV).[47]
3
2
3
x
Br1−x
)
3
3
shown in Figure 2B, MAPbI
3
(x = 1) has two peaks located at 28.1° and 28.4°, which can be
5
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ChemPlusChem
10.1002/cplu.201900723
indexed to the (004) and (220) planes for the tetragonal I4/mcm phase.[44] The peak diffracted
by the (004) plane gradually disappeared upon decreasing x and finally merged into a single
peak corresponding to (200) at x = ~0.8 due to the increased symmetry. In other words, the
tetragonal phase of MAPbI
phase at x = ~0.8. Considering that the I4/mcm phase is transited from Pm3m phase in ideal
MAPbX perovskite structure by slight rotation of the PbX octahedrons along the ⟨001⟩ axis
3
was maintained up to x = 0.9 and then transformed into a cubic
3
6
on (00l) plane while maintaining their corner-sharing connectivity, the tetragonal phase can be
defined by a pseudocubic (pc) lattice. Therefore, the (100)pc spacing in a pseudocubic crystal
system can match the (110)
phase.[12] For comparison, we also prepared MAPb(I
Figure 2C shows the XRD patterns of MAPb(I
substrates prepared by SS. Phase segregation was observed in the x range of 0.2–0.9. The
t
spacing in a tetragonal (t) crystal system in the MAPb(I
MHPs via a wet chemical route.
films on mp-TiO /bl-TiO /FTO
x
Br1−x
)
3
x
Br1−x
)
3
x
Br1−x
)
3
2
2
residual impurities (peaks circled in Figure 2C) belongs to PbI
2
, and the reflection of FTO
substrates are labelled in Figure 2C.
In addition, the XRD patterns for MAPb(Br
x
Cl1−x
3
) MHPs prepared by MCS and SS confirmed
that all MHPs consist of a single phase that adopts the cubic Pm3m perovskite structure with
diffraction peak positions shifting to lower 2θ values with increasing Br content (Figure S7,
Supporting Information). Substitution of Cl anion with a large Br anion increased the lattice
parameter. Hence, no phase segregation has been observed, which indicates that the miscibility
of MAPbBr
3
and MAPbCl
3
in MAPb(Br
x
Cl1−x
3
) MHPs increased. Only the relative intensities
of the XRD peaks of MHPs synthesised through different routes are slightly different because
of the changes in their crystal orientation.
To check the optical properties in MHPs, we measured the UV-vis absorption spectra of
MAPb(I
MAPb(I
x
x
Br1−x
Br1−x
)
3
3
powders (0 ≤ x ≤ 1). As shown in Figure 3A, the onset absorption band of
)
powders can be tuned from 563 nm (2.21 eV) to 813 nm (1.53 eV), resulting
in color tunability. All spectra show clear band edge cut-offs without excitonic signature or
6
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10.1002/cplu.201900723
absorption tails. Figure 3C shows the corresponding colors of MAPb(I
Through compositional control, we can tune the colors from bright orange for MAPbBr
to brown/red for MAPb(I and then to black for MAPbI (x = 1) with increasing I content.
A systematic shift in the absorption band edge to a longer wavelength with increasing I content
in MAPb(I indicates that the band gap energy (E ) can be tuned by changing the
composition of the MHPs. The variation of E (estimated from the onset absorption band) with
I content in MAPb(I is plotted in Figure 3E. Empirically, the nonlinear variation of E
x
Br1−x
)
3
powders.
3
(x = 0)
x
Br1−x
)
3
3
x
Br1−x
)
3
g
g
x
Br1−x
)
3
g
with
a
composition can be expressed by following quadratic equation:
2
E
g
(MAPb(I
x
Br1−x
)
3
) = E
g
[MAPbBr
3
] + (E
g
[MAPbI
3
] − E
g
[MAPbBr
3
] − b)x + bx , where E
g
(x)
is the band gap, x denotes composition, and b is the bowing parameter.[48] A least-squares fit
2
(red line) of E
g
in Figure 3E transforms the equation into E
g
(x) = 2.19 − 0.99x + 0.33x , which
yields the bowing parameter of b = 0.33 eV. The extent of bowing is a measure of the degree
of fluctuations in the crystal field or the nonlinear effect arising from the anisotropic nature of
binding.[49] The relatively small bowing parameter (b = 0.33 eV) in MAPb(I
with those of other MHPs indicates that MAPbI and MAPbBr
have a good miscibility.[49] This
finding confirms that MAPb(I primarily forms the compound and consequently enables
convenient band gap tailoring by a simple MCS process.
Figure 3B shows the UV-vis absorption of MAPb(I
TiO /bl-TiO /FTO substrates. Interestingly, a progressive blue shift in the absorption edge is
Br1−x
x
3
) compared
3
3
x
Br1−x
)
3
x
Br1−x
3
) films prepared by SS on mp-
2
2
immediately occurred at the same ratio of halide (x) compared with that by MCS, which means
that the band gap energy of powders is less than that of the films. This observed shift is due to
the high Br content (phase segregation of residual PbI confirmed by XRD) compared with the
2
stoichiometric MCS composition. Hence, the sample synthesized according to the MCS route
showed a more efficient band gap energy than that processed via the traditional SS method used
in synthesis of perovskite materials. Smaller band gap energy corresponds to wider light
absorption range that will be beneficial for solar cells and many other optoelectronics. The
7
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10.1002/cplu.201900723
absorption onsets and band gaps for both samples are listed in detail (Table S2, Supporting
Information). Figure 3D shows the corresponding colors of MAPb(I films consistent
with that of powders. A least-squares fit (red line) of E in Figure 3F is also provided. The
bowing parameter (b = 0.41 eV) is slightly larger than 0.33 eV, which indicates that the
MAPb(I prepared by the MCS route has a better miscibility than that by the SS route.
In addition, the UV-vis absorption spectra and photographs of MAPb(Br (0 ≤ x ≤ 1)
x
Br1−x
)
3
g
x
Br1−x
)
3
x
Cl1−x
)
3
MHPs prepared by the MCS and SS routes are illustrated (Figure S8, Supporting Information),
and the optical properties are listed in detail (Table S3, Supporting Information). The smaller
bowing parameter (b = 0.23 eV and 0.33 eV) in MAPb(Br
MAPb(I indicates that MAPbBr and MAPbCl have a better miscibility for both
samples adopting the cubic Pm3m perovskite phase.
Elemental composition was analyzed via energy dispersive x-ray (EDX) elemental mapping
Figure S9 and S10, Supporting Information). Atomic percentage follows closely the desired
stoichiometry, demonstrating that fine control over composition is possible through MCS. The
detailed quantitative results for MAPb(I (Table S4, Supporting Information) and
MAPb(Br (Table S5, Supporting Information) are summarised. However, a small
x
Cl1−x
)
3
compared with
x
Br1−x
)
3
3
3
(
x
Br1−x
)
3
x
Cl1−x
)
3
deviation in composition may be caused by incomplete/partial reaction in the corner area of the
grinding vial or measurement errors. TGA-DSC measurements show that MHPs decomposed
before melting at the temperature range of 218–230 °C for MAPb(I
MAPb(Br prepared by MCS with good thermal stability (Figure S11, Supporting
Information).
. Conclusion
x
Br1−x
3
) and 180–226 °C for
x
Cl1−x
)
3
3
In summary, we have demonstrated MCS route as an efficient method for the preparation of
MHPs with superior control over halide compositions, including some MHPs that are not
accessible via the traditional SS method. It eliminates the problem in the preparation of
MAPb(I
x
Br1−x
)
3
with continuously varying x, while maintaining the material properties and
8
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10.1002/cplu.201900723
suppressing phase segregation by SS routes. Related analysis of XRD data showed that the
MHPs prepared by ball milling are highly crystalline. The lattice structure of MAPb(I
x
Br1−x
)
3
changes from more symmetric cubic structure to less symmetric tetragonal phase as x increased.
Absorption spectra study indicated that the absorption onset spans the whole visible and NIR
region of the solar spectrum, resulting in colour tunability. Thus, the optical band gaps can be
tuned easily by compositional control. The E of MHPs has nonlinear relationships with halide
g
content (x). Hence, this ‘green’ method via MCS for the precise large-scale preparation of
MHPs can provide superior control in optoelectronic properties, leading to improved control in
fabrication approaches and facilitating the development of highly efficient and stable devices
in the future. Accordingly, further studies are being carried out.
4
. Experimental Section
Materials: Methylamine (CH
in water), hydroiodic acid (HI) (55.0 wt%–58.0 wt% in water, stable with 1.5 wt%
hypophosphorous) and PbCl (99.9%) were purchased from Aladdin Reagent Ltd. (China).
PbBr (99.0%) and PbI (99.0%) were purchased from Macklin Reagent Ltd. (China).
CH NH Cl (99.0%) and other basic materials were purchased from Shanghai Reagent Ltd.
China). All materials were used as received without further purification.
Synthesis of CH NH X (X = Br, I): CH NH I was synthesized by mixing CH
the molar ratio of 1.2:1. HI was added dropwise into the CH NH solution in a flask under
nitrogen atmosphere in an iced bath for 2 h. The resulting solution was evaporated at 55 °C in
a rotary evaporator for approximately 2 h to remove the solvent. CH NH Br was synthesized
with the same procedure as above except by replacing HI with HBr. The white CH NH
X = Br, I) crystalline powders were collected using Büchner funnel filtration after being
3
NH ) (40.0 wt% in ethanol), hydrobromide acid (HBr) (48.0 wt%
2
2
2
2
3
3
(
3
3
3
3
3
NH and HI with
2
3
2
3
3
3
3
X
(
washed thrice with anhydrous ethanol, recrystallized in anhydrous diethyl ether, and then dried
in a vacuum oven at 60 °C for overnight.
9
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10.1002/cplu.201900723
Synthesis of MAPbX
3
(X = Cl, Br, I): MAPbX
NH
3
(X = Cl, Br, I) powders were synthesized by
mixing the desired amount of CH
3
3
X (X = Cl, Br, I) with PbX (X = Cl, Br, I) (1:1 n/n,
2
molar ratio) in an agate jar and reacted via mechanical ball milling at RT under ambient
conditions (speed, 300 rpm; humidity, 20%–30%).
Synthesis of MAPb(I
x
Br1−x
)
3
and MAPb(Br
x
Cl1−x
)
3
(0 ≤ x ≤ 1) powders: MAPb(I
x
Br1−x
)
3
(0 ≤ x
≤
1) powders were synthesized by mixing a desired amount of MAPbI
3
with MAPbBr
3
in an
agate jar and reacted via mechanical ball milling for 3 h at RT under ambient conditions (speed,
00 rpm; humidity, 20%–30%). MAPb(Br (0≤ x ≤1) powders were synthesized with the
same procedure as above except by replacing MAPbI with MAPbCl
Preparation of MAPb(I and MAPb(Br (0 ≤ x ≤ 1) films: A dense blocking layer
of TiO (bl-TiO ) was deposited onto the FTO substrate. The FTO substrate, pretreated with
UV ozone for 45 min, was treated with an acidic ethanol solution containing tetrabutyl titanate
Sigma-Aldrich) by spin-coating at 2,000 rpm for 45 s, followed by sintering at 500 °C for 30
min. The mesoporous TiO (mp-TiO ) film was then spin coated onto the bl-TiO /FTO substrate
3
x
Cl1−x
)
3
3
3
.
x
Br1−x
)
3
x
Cl1−x
)
3
2
2
(
2
2
2
using diluted commercial paste (Dyesol DSL 18NR-T in ethanol, 1:6.5 w/w) and were calcined
at 500 °C for 1 h in air to remove the organic portion. The perovskite solution was prepared by
dissolving 1 mmol MAPb(I
The perovskite precursor solution (C[MAPb(I
TiO /bl-TiO /FTO substrate. After spin-coating at 2,000 rpm for 30 s, the substrate was placed
on a hot stage at 100 °C for 60 min, with all operations under the protection of nitrogen.
MAPb(Br (0 ≤ x ≤ 1) films were prepared with the same procedure as above except by
replacing MAPb(I with MAPb(Br
x
Br1−x
)
3
powder in 1 ml of DMF/DMSO (1:1 v/v) mixed solvent.
x
Br1−x
)
3
] = 1 M) was dropped onto the mp-
2
2
x
Cl1−x
)
3
x
Br1−x
)
3
x
Cl1−x) .
3
Characterization: Powder X-ray diffraction (XRD) patterns of the products were collected
using a Philips X’ Pert Pro with Cu Kα1 line (λ = 1.54186 Å) at V = 40 kV and I = 40 mA. The
morphology and structure of the products were observed by a scanning electron microscope
(SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEM-2100F). Elemental
1
0
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10.1002/cplu.201900723
mapping images were observed by energy dispersive x-ray (EDX, EDAX Apollo). UV-vis-NIR
diffuse reflectance spectrum was measured at RT using a UV-vis-NIR spectrophotometer (UV-
vis-NIR, Shimadzu UV-3600 Plus) with an integrating sphere attachment operating in the 300–
1
,200 nm region. A highly refined BaSO powder plate was used as the standard (100%
4
reflectance). The photoluminescence (PL) of the perovskite powder was measured with a
lifetime and steady state spectrometer (Edinburgh FLS980). The Fourier transformation
infrared (FTIR) spectra were recorded on a Bruker Tensor 27 FTIR spectrometer using KBr
–
1
disks in the range of 4000–400 cm . Thermogravimetric analysis (TGA-DSC) was performed
on a TA Q600 SDT. The sample was placed in an Al crucible and heated in an interval from
room temperature to 600 °C at a ramp rate of 10 °C min under flowing nitrogen gas with a
2
O
3
−
1
−
1
flow rate of 100 ml·min . Approximately 5 mg of the powder was used for each experiment.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This study is jointly supported by the National Natural Science Foundation of China (Grant
Nos. 51573004, 51773003, and 51711540302) and the Beijing Natural Science Foundation
(Grant No. 2184134).
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Scheme 1. Preparation process of MAPb(I
x
Br1−x
)
3
and MAPb(Br
x
Cl1−x
)
3
(0 ≤ x ≤ 1) MHPs by
ball milling a mixture of MAPbX (X = Cl, Br, I).
3
1
6
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10.1002/cplu.201900723
A
C
D
1
00
1
10
2
00
2
00
1
00
1
10
1
10
220
04
0
B
E
F
G
0
.44 nm
110)
0
.25 nm
210)
(
(
0
.31 nm
220)
(
Figure 1. A) XRD comparison of MAPbX
and TGA curves of three kinds of perovskites; C) UV-vis absorption spectrum, D)
Photoluminescence (PL) properties of MAPbX ; High-resolution transmission electron
microscope (HRTEM) images of MAPbX , E) X = Cl, F) X = Br, G) X = I.
3
(X = Cl, Br, I); B) Thermal stability of the products,
3
3
1
7
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10.1002/cplu.201900723
A
B
C
(004)T
(220)T
(
Tetragonal
200)C
Cubic
#
#
#
#
Figure 2. XRD analysis of MAPb(I
x
Br1−x
3
) (x = 0, 0.1, …, 0.9, 1.0). A) XRD patterns of
MAPb(I powders prepared by MCS; B) Magnified view of the selected region of XRD
x
Br1−x
)
3
patterns at 27.5°–31°; C) XRD patterns of MAPb(I
x
Br1−x
3
) films prepared by SS. Reflection of
FTO substrates are labelled with “#”.
1
8
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10.1002/cplu.201900723
A
B
C
E
D
F
Figure 3. UV-vis absorption spectra and photographs of MAPb(I
UV-vis absorption spectra of A) MAPb(I
films prepared by SS; Photographs of C) powder, D) films on mp-TiO
top: x = 0, 0.1, …, 0.5; down: x = 0.6, …, 1.0); Quadratic relationship of the band gaps of
MAPb(I as a function of Br composition (x), E) powders, F) films.
x
Br1−x
)
3
(0 ≤ x ≤ 1) MHPs.
x
Br1−x
)
3
powders prepared by MCS, B) MAPb(I
x
Br1−x
)
3
2
/bl-TiO /FTO substrates
2
(
x
Br1−x
)
3
1
9
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10.1002/cplu.201900723
Table 1. Optical properties of MAPbX
3
(X = Cl, Br, I).
PL peak
[nm]
X
Ionic radius
Å]
λ
onset
E
[eV]
2.95
g
[
[nm]
421
Cl
Br
I
1.81
1.96
2.20
417
563
813
2.21
1.53
553
791
2
0
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Mixed halide hybrid perovskites (MHPs) are prepared through a solvent-free mechanochemical
synthesis (MCS) route with superior control over halide compositions. It eliminates the problem
in the preparation of MAPb(I
x
Br1−x
3
) with continuously varying x, while maintaining the
material properties and suppressing phase segregation by solvent synthesis (SS) routes. Thus,
the optical band gaps can be tuned easily by compositional control.
Keyword
Perovskites
Sheng Tang, Xinyu Xiao, Jing Hu, Bo Gao, Hunglin Chen, Zongyang Peng, Jianchun Wen,
Masanao Era* and Dechun Zou*
Solvent-Free Mechanochemical Synthesis of a Systematic Series of Pure-Phase Mixed-
Halide Perovskites MAPb(I
and Band-Gap Tuning
x
Br1−x
)
3
and MAPb(Br
x
Cl1−x
3
) for Continuous Composition
ToC figure
MAPb(I Br1-x)3
x
MAPbI3
Ball
Milling
MAPbBr3
ü MCS Route
ü Solvent-Free
ü Pure-Phase
ü Band-Gap Tuning
2
1
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