Benzoyl halide as alternative precursor for synthesis of lead free double perovskite Cs3Bi2Br9 nanocrystals

Article abstract of DOI:10.1166/jnn.2020.17495

Ternary bismuth halides are interesting functional materials closely related to Pb halide perovskite photovoltaic material, and are widely sought after due to reduced toxicity of Bi compared to Pb. There are several reports on synthesis of Cs₃Bi₂Br₉ nanocrystals (NCs) due to its being relatively stable compared to lead perovskite. Cs₃Bi₂Br₉ nanocrystals have been synthesised using benzoyl bromide as an precursor using hot injection process at two different temperatures of 120 °C and 160 °C. Samples have been characterized for its structural, optical, microstructural and luminescent properties using X-ray diffraction, (XRD) UV-Vis spectroscopy, high resolution transmission electron microscopy and photoluminescent spectroscopy. XRD showed formation of Cs₃Bi₂Br₉ phase with mono-crystalline structure. UV-Vis showed two types of band gap in the visible region which shows that the material can be used for photovoltaic applications. HRTEM confined the particles to be composed of nanocrystals with ～5 nm particles in the samples grown at 120 °C and it the particles joined together yield various structures composed of nanoparticles. The time resolved photoluminescence shows average life times of 3.067 ns and 4.761 ns for samples grown at two different temperatures. To the best of our knowledge, this is the first report where benzoyl halide has been used as alternative precursor for the synthesis of lead free double perovskite Cs₃Bi₂Br₉ nanocrystals which have many applications.

Full text of DOI:10.1166/jnn.2020.17495

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Nanoscience and Nanotechnology
Vol. 20, 3802–3808, 2020
Copyright © 2020 American Scientific Publishers
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Printed in the United States of America
Benzoyl Halide as Alternative Precursor for Synthesis of
Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
Ashish Kumar1ꢀ2, S. S. Rawat1ꢀ2, Sanjay Kumar Swami1, Vidya Nand Singh1ꢀ2, and Ritu Srivastava1ꢀ∗
1CSIR-National Physical Laboratory, Dr. KS Krishnan Marg, New Delhi 110012, India
2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
Ternary bismuth halides are interesting functional materials closely related to Pb halide perovskite
photovoltaic material, and are widely sought after due to reduced toxicity of Bi compared to Pb.
There are several reports on synthesis of Cs3Bi2Br9 nanocrystals (NCs) due to its being relatively
stable compared to lead perovskite. Cs3Bi2Br9 nanocrystals have been synthesised using benzoyl
ꢀ
bromide as an precursor using hot injection process at two different temperatures of 120 C and
160 ꢀC. Samples have been characterized for its structural, optical, microstructural and luminescent
properties using X-ray diffraction, (XRD) UV-Vis spectroscopy, high resolution transmission elec-
tron microscopy and photoluminescent spectroscopy. XRD showed formation of Cs3Bi2Br9 phase
with mono-crystalline structure. UV-Vis showed two types of band gap in the visible region which
shows that the material can be used for photovoltaic applications. HRTEM confinedꢀthe particles
to be composed of nanocrystals with ∼5 nm particles in the samples grown at 120 C and it the
IP: 5.62.155.68 On: Mon, 06 Jan 2020 13:32:18
particles joined together yield various structures composed of nanoparticles. The time resolved
Copyright: American Scientific Publishers
photoluminescence shows average life times of 3.067 ns and 4.761 ns for samples grown at two
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different temperatures. To the best of our knowledge, this is the first report where benzoyl halide
has been used as alternative precursor for the synthesis of lead free double perovskite Cs3Bi2Br9
nanocrystals which have many applications.
Keywords: Double Perovskite, Nanocrystals, Photoluminescence and Quantum Yield.
1. INTRODUCTION
narrow emission and high photoluminescence quantum
yield (PLQY) compared to lead free perovskite nano-
crystals and quantum dots (QDs). In one of the report,
lead (Pb2+) was replaced by less toxic Tin (Sn2+) atom
as promising perovskite structure but tin based perovskite
nanocrystals and QD shave low PLQE of about 0.14%
and are very unstable. The Sn perovskite has 0.14% PLQE
due to the presence of intrinsic defect sites and tin have
two oxidation Sn2+ and Sn4+ [5]. The search for all inor-
ganic cesium (Cs) based double perovskite which usually
have less defect density compared with organic methyl
ammonium based perovskite and better stability is con-
tinued. All-inorganic and air-stable Cs3Bi2Br9 perovskite
nanocrystals show PLQY of only 0.2% when ligand free,
shows PLQY of 4.5% when adding oleic acid (OA) is
added. But bismuth halide based quantum dot show PLQE
of 22% when treated with extra surfactant octylammo-
nium bromide and oleic acid is added during the synthesis.
The use of surfactant provides excellent thermal stability
of more than 30 days and high luminescent properties.
Organic and inorganic lead perovskite based materials
have several optoelectronic applications. But the biggest
issue with organic–inorganic lead based perovskites (LBP)
are that they are very toxic and unstable under envi-
ronmental conditions. Recently, several efforts have been
made to grow lead free double perovskite (replacing Pb
centre atom with Bi, Ag–Bi, Au, Ag–In, Sb, Ag–Sb etc.)
and other lead free other perovskite materials (Pb replaced
by Cu2+, Sn2+). Lately with focus on solving the stability
as well as toxicity issue there was a report on organo-
copper (environment friendly material) based perovskite
materials. There are several reports on lead free double
perovskite (elpasolite) based solar cell [1, 2]. The com-
putational studies on double perovskite (elpasolites) have
been carried out recently and growth of materials such as
Cs3Bi2X9ꢀ Cs2AgBiX6 (X = Cl, Br, I) and Cs2AgInCl6
have been reported [3–4]. The lead based perovskite has
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2020.17495

Kumar et al.
Benzoyl Halide as Alternative Precursor for Synthesis of Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
This is because of passivation of surface trap-states and
modification of intrinsic defect sites [6–9]. Cesium based
inorganic perovskite under good stability as well as very
less defect density compared with bulk organic–inorganic
based perovskite (MA Based) [10–13]. All inorganic based
perovskite has been reported that bismuth based bulk mate-
rials are highly crystalline nature with higher PLQE and
better optoelectronics performance compared to MA based
hybrid perovskite [14]. Therefore, in the present study,
double perovskite Cs3Bi2Br9 nanocrystals having size in
the range of 5–7 nm have been synthesized using the hot
injection method and use of benzoyl halide as alternative
precursor is an additional advantage. Benzoyl low coast
material and easy to use for hot injection method directly
injecting benzoyl bromide without any heating.
for 4 h and the mixture was continuously stirred. The mix-
ture converts from colourless to orange colour, indicating
formation of benzoyl bromide. It was cooled down to room
temperature (RT) and filtered using a PTFE membrane
with 0.45 ꢁm pore size [15].
2. Cs(OAc) (0.71 mmol), and bismuth neodecanoate
(1 mmol) were dissolved in the combined mixture of
octadecene (10 mL), oleic acid (2.8 mL) and oleylamine
(1.9 mL). The reaction mixture was heated to 100 ꢀC
in vacuum for 90 min. After that reaction temperature
ꢀ
ꢀ
was increased to 160 C (sample 2) and 120 C (sam-
ples 1) for 10 min in a nitrogen atmosphere. Neat 1 mL
benzoyl bromide diluted with 1 mL ODE was swiftly
injected. After 10 sec the reaction mixture was cooled
down to room temperature in an ice-water bath. The
yellow–green colloidal solution was obtained. Centrifuga-
tion of the reaction mixture was carried out to separate the
yellow powder and supernatant. Yellow powder was dis-
persed into toluene and precipitated with acetone/EA and
re-dispersed into toluene. The supernatant was purified by
acetone/EA with dispersed a small amount of toluene for
characterisation.
2. MATERIALS AND EXPERIMENTAL
METHODS
2.1. Materials
Materials used for the synthesis with their sources are
Bismuth neodecanoate (Sigma-Aldrich), Cesium acetate
(99.9% Aldrich), Oleic acid (OA, 90%, Sigma-Aldrich),
Oleylamine (Alfa Aesar), Octadecene (ODE), Toluene
(Sigma), acetone (analytical pure) Ethyl Acetate (EA).
Benzoyl Chloride, Lithium Bromide (LiBr). All materials
were used as received without further purifications.
3. RESULTS AND DISCUSSION
3.1. XRD Characterisation
The previous report of Cs Bi2Br9 single crystal was hexag-
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Copyright: American Socnieanl tsiftircucPtuurbeliswhiethrstrigonal P3m1 symmetry and crystal
2.2. Synthesis of Cs3Bi2Br9 Double Perovskite
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axis a = 0.796 nm = b and c = 0.984 nm. In the Cs3Bi2Br9
1. Conversion of benzoyl chloride into benzoyl bromide:
The reaction was performed in N2 atmosphere. 2 mL of
benzoyl chloride was added to 1.7369 g of LiBr in 25 mL
structure bismuth has body centred with Cs+ group is
surrounded by six metal halide octahedral [16, 17]. The
Cs3Bi2Br9 nanocrystal were synthesis by hot injection
ꢀ
three nick flask. The flak was kept on a hot plate at 75 C
(a)
(b)
Figure 1. (a) XRD patterns of Cs3Bi2Br9 QDs with temperature 120 ꢀC. (b) Result of the Rietveld refinement of XRD of Cs3Bi2Br9 NCs with data
calculated curve position of allowed Bragg reflection with temperature 160 ꢀC.
J. Nanosci. Nanotechnol. 20, 3802–3808, 2020
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Benzoyl Halide as Alternative Precursor for Synthesis of Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
Kumar et al.
(a)
(c)
Figure 2. (a and b) Shows TEM, micrograph (c) shows SAED pattern and inset of figure (b) shows HRTEM lattice image of sample (1).
method. The yellow–green colloidal solution was obtained
after purification with acetone wash. The ligand free yel-
low powder dispersed into toluene was used for XRD. For
XRD characterisation film was made by drop casting the
solution on glass substrate kept on hot plate in the glove
higher purity. The diffraction peaks are related to trigonal
P3m1 symmetry with space group 164 (JCPDS card no
44-0714) [18].
ꢀ
From Figure it is clear that the sample grown at 120 C
shows Nanocrystalline nature and the crystallinity has
ꢀ
ꢀ
box at a temperature of 100 C. It was dried for 20 min.
improved in the sample grown at 160 C. The detailed
IP: 5.62.155.68 On: Mon, 06 Jan 2020 13:32:18
The results of XRD is shown in Figure 1(b) and analysis
analysis of XRD peaks shown in the supplementary infor-
Copyright: American Scientific Publishers
by Rietveld method is also shown. No other peakDseelixvceerpetd bymInagtieonn,tait is clear that the peaks are shifted towards lower
ꢀ
for Cs3Bi2Br9 is detected showing that the perovskite has
two theta value in the case of sample grown at 120 C
(a)
(c)
(b)
5 nm
(d)
Figure 3. (a–c) Shows TEM, micrograph (d) shows SAED pattern and inset of figure (c) shows HRTEM lattice image of sample (2).
J. Nanosci. Nanotechnol. 20, 3802–3808, 2020
3804

Kumar et al.
Benzoyl Halide as Alternative Precursor for Synthesis of Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
ꢀ
which indicates somewhat lattice expansion which is the
case with the nanocrystalline materials. The peaks for sam-
ple grown at 160 ꢀC shows very marginal shift which indi-
cates that the sample is crystalline.
From the TEM studies, it is clear that at 120 C small
particle are formed. When sample is made at 160 ꢀC these
particles coalesces and tetragonal structure is formed. The
different temperature have different shaped structure [19].
3.3. UV-Vis and Photolumanicence (PL)
3.2. Transmission Electron Microscopy (TEM)
Characterisation
The UV-Vis absorption spectrum of Cs3Bi2Br9 QDs is
shown in Figure 4(a). In the spectrum a band edge exciton
peak at 431 nm with tail up to 463 nm has been observed.
In the case of single crystal and thin film the exciton
peak were observed 459 and 440 nm. Thus, the exciton
peak of sample 1 got blue shifted by 28 and 9 nm com-
pared to single crystal and thin film values, respectively
as the absorption spectrum depend upon growth temper-
ature, exciton binding energy and quantum confinement
effect [20]. The PL spectroscopy is one of the very impor-
tant techniques to measure radiative recombination which
can be used for photovoltaic application [21]. PL in lead
free perovskite Cs3Bi2X9 (X = Cl, Br, I) with PL emis-
sion wavelength ranging from 400 to 500 nm has been
reported [22]. The ligand free Cs3Bi2Br9 perovskite shows
blue emission at 461 nm with FWHM (Full width Half
Maximum) of 50.53 nm. Ligands free double perovskite
does not show good PLQE as PL emission mostly arises
from transition between valance band to conduction band
through the fast trapping process. Although the raw data
shows single PL peak at 461 nm with FWHM 50.53 but
Figures 2(a)–(b) show the Transmission Electron
Microscopy (TEM) image of Cs3Bi2Br9 (sample 1) QDs.
The average size of spherical type particle is ∼3.0 .05.
The high-resolution transmission electron microscopic
(HRTEM) image is shown in the inset of Figure 2(b)
shows the lattice spacing distance ꢂdꢃ of 3.02 Å corre-
sponding to the (112) crystal plane. The selective area
electron diffraction (SAED) pattern shows polycrystalline
nature of sample and planes are marked on the Figure 2(c).
All the planes correspond to Cs3Bi2Br9.
Transmission Electron Microscopy (TEM) image
of Cs3Bi2Br9 (sample 2) nanocrystal is shown in
Figures 3(a)–(b). The average size of tetragonal type par-
ticle is in the range of 10 to 70 nm. These tetragonal
structures are made of 2 to 5 nm range spherical parti-
cle. The high-resolution transmission electron microscope
(HRTEM) shown in inset of Figure 3(c) shows the lat-
tice spacing distance ꢂdꢃ of 4.10 Å corresponding to the
(102) crystal plane. The SAED paten shown in Figure 3(d)
shows polycrystalline nature of the Cs3Bi2Br9 nanocrystal
IP: 5.62.155.68 On: Mon, 06 Jan 2020 13:32:18
and the planes are marked in the Figure 3(c).
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(b)
(a)
(d)
(c)
Figure 4. For sample 1 (a), UV-Vis and photoluminescence (PL). (b) PL fitting dat. For sample 2 (c), UV-Vis and photoluminescence (PL). (d) PL
fitting dat.
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Benzoyl Halide as Alternative Precursor for Synthesis of Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
Kumar et al.
the fitted PL data shows red shifted peak at 466 nm with
FWHM 57 nm as shown in Figure 4(b).
NCs, both band-edge emission and red shifted has been
observed in defect-related emission.
The room temperature UV-Vis absorption spectrum of
Cs3Bi2Br9 perovskite NCs are shown in Figure 4(c). The
broad absorption peak are illustrated in the absorption
stared approximately 630 nm of ligand free perovskite
material with the peak 454 nm (perfect blue region) the
broad peak arise due to the spherical crystal with range of
size 2 to 5 nm and the spherical crystals converted into
the tetragonal shape of nanocrystal with size 10 to 70 nm
it is confirmed by TEM.
3.4. Optical Band GAP
Both indirect and direct nature of band gap of bismuth
based materials due to exciton recombination process, tem-
perature dependence has been reported using density func-
tional theory (DFT).
The equation for calculating the optical band gap is
given below
ꢀ
ꢄhꢅ = hꢅ −Eg
The PL of synthesized inorganic Cs3Bi2Br9 perovskite
material cover broad wavelength (400 to 560 nm) as shown
in Figures 4(c) and (d). Ligand free Cs3Bi2Br9 (sample 1)
exhibit one peak due to presence of mono-shaped crys-
tal, the peak was blue-shifted compared to sample 2. For
sample 2 grown at higher temperature three peak were
observed due to formation of various forms of crystal at
higher temperature. In the case of sample 1, a sharp narrow
peak was observed in UV-Vis spectra at 431 nm and PL
was observed at 460 nm as shown in Figure 4(a). But in
case of sample 2, in the UV-Vis broad peak was observed
at 445 nm and three peaks at 407, 431 and 460 nm were
observed as shown in Figure 4(d). The lead and lead free
double perovskite bulk have shown difference properties
with their nanocrystals. For example CsPbX3 NCs shows
strong quantum confinement effect [23–25]. In Case of
Where ꢄ is the absorption coefficient and hꢅ is the energy
of photon with frequency (h) Planck constant and fre-
quency ꢂꢅꢃ and Eg is the optical bandgap energy.
Using this equation we can find out the band gap and
the nature of direct and indirect band gap can be deduced
using the formula.
Direct bandgap = ꢂꢄhꢅꢃ2
Indirect bandgap = ꢂꢄhꢅꢃ0ꢆ5
1240ꢆ59
hꢅ =
ꢇ
Where ꢇ is the wavelength.
Figure 5 shows the Tauc’s plot for samples. Both bis-
muth based samples shows absorbance tails lower ener-
gies, which have been shows to indirect band gap [29].
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double perovskite the band-edge emission of Cs2Sb3I9 and
Cs Bi I are shows differ with their respective NCs coun-
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2
3 9
Tauc plot and estimated the optical direct and indirect band
gap for samples 1 are 2.78 eV and 2.67 eV respectively.
But the optical direct and indirect band gap for samples
2 (higher temperature reaction) are 2.31 eV and 2.55 eV
respectively, which are similar to their bulk counterpart.
Therefore neither of both samples shows strong quantum
confinement effects.
terpart [26]. When the size of crystal increase (Red side
shifted peak) as the absorbance edge of sample 2 is pushed
to low energy as a result of decrease the band gap and
the PL shows the higher energy peak arise due to indirect
band nature. When the PL data fitting the three peaks are
407, 430, 447 respectively, with full range of blue region
shown in Figure 4(d) dotted line. In many semiconductors
have shown both dopant emission and band-edge emis-
sion [27, 28]. At higher temperature the absorbance, a red
shift arises with trap states below the first electronic state
(on conduction band). Moreover in lead free and lead base
3.5. Time-Resolved Photoluminescence
The Tri-exponential function has been used for investiga-
tion the size dependent improve the trap state of upper
(a)
(b)
Figure 5. Sample 1 (a) and sample 2 (b). Direct and indirect band gap from Tauc plot.
3806
J. Nanosci. Nanotechnol. 20, 3802–3808, 2020

Kumar et al.
Benzoyl Halide as Alternative Precursor for Synthesis of Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
shows much shorter PL lifetime corresponding to the bulk
single crystal counterpart because of the stronger over-
lap between electron–hole wave function in a dimension
on confined space and electron–phonon commonly shows
band gap behaviour of increasing to temperature [30–32].
The small size of quantum dots make them almost to be
defect free, dominating to excitions being more level to
radiative recombination. The fast decay component can
be naturally attributed to quenching of PL and is affected
by particle size. The ꢈ3 component can be attributed to
edge state associated with decay pathway. The difference
in average time for samples 1 and 2 are arises with trap
states below the first electronic state and may be excessive
trap in material as a result non-radiative decay of excitons
exceed the way.
4. CONCLUSION
Figure 6. TRPL decay and fit profile sample 1 and 2 emission wave-
First time benzoyl Bromide precursor is use for colloidal
synthesis of lead free double perovskite nanocrystal of
Cs3Bi2Br9. Benzoyl Bromide was worked as a bromide
precursor with differ temperature injected it. The Colloidal
of Cs3Bi2Br9 material with spherical and tetragonal struc-
ture as synthesized by using hot injection method. The
optical and structural properties of Cs3Bi2Br9 have been
studied. There are two features in both absorption and
emission spectra of Cs3Bi2Br9. The UV-Vis spectrum of
Cs3Bi2Br9 shows excitonic band gap with direct band gap
of sample 1 and 2 are 2.74 and 2.31 eV respectively.
lengths 460 nm (2.69 eV).
band and quantum confinement in Cs3Bi2Br9. Two type of
Cs3Bi2Br9 with average size of 3.0 0.5 nm and the range
size 10 to 70 nm were investigated using TRPL (Fig. 6).
All the TRPL decay profiles were fitted by using Tri expo-
nential function with the help of DAS 6 Software.
Iꢂtꢃ = ꢄ1 expꢂ−t/ꢈ1ꢃ+ꢄ2 expꢂ−t/ꢈ2ꢃ+ꢄ3 expꢂ−t/ꢈ3ꢃ
(1)
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ꢁ
ꢄiꢈi2
ꢄiꢈi
Copyright: American Scientific Publishers
ꢈavg = ꢁ
(2)
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Acknowledgments: One of the author Ashish Kumar,
acknowledges CSIR for grant of JRF. Sanjay Kumar
Swami thanks Department of Science and Technology
(DST), New Delhi for Inspire Faculty Award. Authors are
grateful to Director, CSIR-NPL for granting permission to
publish this result.
Where the Time Constant ꢂꢈ1ꢀꢈ2ꢀꢈ3ꢃ and exponential
function ꢂꢄ1ꢀꢄ2ꢀꢄ3ꢃ represent decay time and the relative
amplitude each component, respectively.
The Time-Resolved photoluminescence (TRPL) mea-
surement on both sample 1 and 2 were carried out using
correlated single photo counting (TCSPC) system. The
decay were fitted by Tri exponential function with short
lived component ꢈ1 (1.44×10−11 s), (8.87×10−11 s) a mid-
dle component ꢈ2 (1.274 × 10−9 s), (1.649 × 10−9 s) and
final ultra-long lived component (edge state) ꢈ3 (5.488 ×
10−9 s), (6.164×10−9 s) respectively.
References and Notes
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3.067 ns and 4.761 ns respectively. The major difference
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of sample 1 and 2 with ꢉ2 (XSQ) value.
Cs3Bi2Br9 ꢄ1% ꢈ1 (ns) ꢄ2% ꢈ2 (ns) ꢄ2% ꢈ3 (ns) ꢈavg (ns)
Sample 1 92ꢆ87 0ꢆ014
Sample 2 64ꢆ37 0ꢆ088 20ꢆ43 1ꢆ649 15ꢆ20 6ꢆ164
5ꢆ83 1ꢆ274
1ꢆ30 5ꢆ488
3ꢆ067
4ꢆ761
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Benzoyl Halide as Alternative Precursor for Synthesis of Lead Free Double Perovskite Cs3Bi2Br9 Nanocrystals
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Received: 27 August 2018. Accepted: 20 March 2019.
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