Precursor solution volume-dependent ligand-assisted synthesis of CH3NH3PbBr3 perovskite nanocrystals

Article abstract of DOI:10.1016/j.jallcom.2018.09.054

Ligand-assisted reprecipitation (LARP) technique is a powerful approach for the synthesis of organometal halide perovskite nanocrystals (PNCs). The morphology and surface property of the formed PNCs which determine their optical properties are ultrasensitive to the synthetic parameters. To guarantee the batch-to-batch reproducibility of PNCs with excellent optical properties, it is of central importance to better understand the factors influencing the formation of PNCs during LARP process. Herein the dual-factor of the amount of perovskite precursor and the polarity of mixture solvent was modified by varying precursor solution volume (PSV) in the LARP system. The concentration, size, surface state and optical properties of the synthesized CH₃NH₃PbBr₃ PNCs as a function of PSV were systematically investigated aiming to understand the influence of the dual-factor on the nucleation and growth of PNCs. Experimental results revealed that few crystal nuclei was generated due to lower amount of precursor at lower PSV, which was favorable for the growth of large perovskite crystals. At the higher PSV, the inhibition of crystal growth with increased amount of precursor was compensated by increasing the polarity of mixture solvent, which led to the dissolution of surface ligands and eventually growth of large perovskite crystals. The obtained results on PSV-dependent synthesis of PNCs will be used as a guide to optimize the synthetic parameters in the LARP process.

Full text of DOI:10.1016/j.jallcom.2018.09.054

Accepted Manuscript
Precursor solution volume-dependent ligand-assisted synthesis of CH NH PbBr
3
3
3
perovskite nanocrystals
Yun Tang, Nan Yan, Zhiwei Wang, Hudie Yuan, Yalou Xin, Hongfeng Yin
PII:
S0925-8388(18)33292-4
10.1016/j.jallcom.2018.09.054
JALCOM 47473
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 30 April 2018
Revised Date: 31 August 2018
Accepted Date: 5 September 2018
Please cite this article as: Y. Tang, N. Yan, Z. Wang, H. Yuan, Y. Xin, H. Yin, Precursor solution volume-
dependent ligand-assisted synthesis of CH NH PbBr perovskite nanocrystals, Journal of Alloys and
3
3
3
Compounds (2018), doi: 10.1016/j.jallcom.2018.09.054.
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ACCEPTED MANUSCRIPT

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Precursor solution volume-dependent ligand-assisted synthesis of
CH3NH3PbBr3 perovskite nanocrystals
Yun Tang, Nan Yan, Zhiwei Wang, Hudie Yuan, Yalou Xin, Hongfeng Yin*
College of Materials & Mineral Resources, Xi’an University of Architecture & Technology, Xi’an,
Shaanxi, 710055, China
*Corresponding author. E-mail: yinhongfeng@xauat.edu.cn.
ABSTRACT
Ligand-assisted reprecipitation (LARP) technique is a powerful approach for the
synthesis of organometal halide perovskite nanocrystals (PNCs). The morphology and
surface property of the formed PNCs which determine their optical properties are
ultrasensitive to the synthetic parameters. To guarantee the batch-to-batch
reproducibility of PNCs with excellent optical properties, it is of central importance to
better understand the factors influencing the formation of PNCs during LARP process.
Herein the dual-factor of the amount of perovskite precursor and the polarity of
mixture solvent was modified by varying precursor solution volume (PSV) in the
LARP system. The concentration, size, surface state and optical properties of the
synthesized CH3NH3PbBr3 PNCs as a function of PSV were systematically
investigated aiming to understand the influence of the dual-factor on the nucleation
and growth of PNCs. Experimental results revealed that few crystal nuclei was
generated due to lower amount of precursor at lower PSV, which was favorable for the
growth of large perovskite crystals. At the higher PSV, the inhibition of crystal growth
with increased amount of precursor was compensated by increasing the polarity of
mixture solvent, which led to the dissolution of surface ligands and eventually growth
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of large perovskite crystals. The obtained results on PSV-dependent synthesis of PNCs
will be used as a guide to optimize the synthetic parameters in the LARP process.
Keywords: Perovskite nanocrystals; Ligand-assisted reprecipitation; Dual-factor;
Nucleation; Growth; Optical property
1. Introduction
Organometal trihalide perovskites CH3NH3PbX3 (X=Cl, Br, I) materials are
attractive semiconductor materials with excellent optical and electrical properties and
great potential for a diverse range of applications in photovoltaics [1-6], light-emitting
diodes [7-9], laser [10,11], and photodetectors [12-14]. When reducing the dimension
of CH3NH3PbX3 from bulk to nanoscale, i.e., in the quantum confined space, the
CH3NH3PbX3 perovskite nanocrystals (PNCs) begin to display extraordinary optical
properties, including tunable emission over the entire visible spectrum, narrow
emission linewidth and high quantum efficiency [15]. Over the past few years, a vast
body of research has been carried out mainly on synthesis and optical properties of
PNCs. Since Galian and Perez-Prieto first reported the fabrication of CH3NH3PbBr3
PNCs using organic ammonium cations as capping ligands by hot-injection method in
2014 [16], solution-based approaches have been extensively investigated to synthesize
PNCs with various shapes, sizes and dimensions [17-23].
Ligand-assisted reprecipitation (LARP) technique is one of the most convenient
solution approaches, in which the formation of PNCs can be accomplished within a
few seconds without the needs of heating or protective atmosphere [9]. LARP is
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usually conducted by mixing the precursor solution consisting of a polar solvent,
perovskite precursor and organic ligand, with a nonpolar solvent under vigorous
+
stirring at room temperature. Due to the different solubility of CH3NH3 , Pb2+, X- in
polar and nonpolar solvent, the mixing produces a highly supersaturated state
immediately and then induces fast nucleation and growth of CH3NH3PbX3 PNCs.
Various synthetic parameters play a crucial role in the morphology and surface
property of resulting PNCs. For example, size-tunable CH3NH3PbBr3 PNCs were
synthesized through varying the temperature of the nonpolar solvent [18], the
precursor and ligand concentrations [19], or the ratio of CH3NH3Br to PbBr2 in the
perovskite precursor [20]. In addition, a variety of ligands, polar and nonpolar solvents
were employed to fabricate CH3NH3PbI3 and CH3NH3PbBr3 PNCs with different
shape and surface state [21]. Thus, in order to guarantee the batch-to-batch
reproducibility of PNCs achieve excellent properties, it is of central importance to
better understand the related synthetic parameters influencing the LARP process.
Although the influence of single factor on LARP process has been
well-established, understanding of the combined effect of several factors is also
important because practical synthesis of PNCs usually involves the variation of more
than one factor. The precursor and the polarity of the reaction system are two
important parameters during LARP process, while the combined effects of both factors
on the synthesis and optical properties of CH3NH3PbX3 PNCs are still poorly
understood. Herein, different precursor solution volume (PSV) was introduced to
modify the amount of precursor and the polarity of the mixture solvent at the same
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time. The concentration, size, surface state and optical properties of CH3NH3PbBr3
PNCs synthesized with different PSV were systematically investigated, aiming to
understand the influence of the dual-factor on the nucleation and growth of PNCs. The
experimental results will be used as a guide to optimize the related synthetic
parameters in the LARP process.
2. Experimental
2.1. Materials
Lead(II) bromide (PbBr2, 99%), methylamine (CH3NH2, 33 wt% in absolute
ethanol), hydrobromic acid (HBr, 48 wt% in water), N,N-Dimethylformamide (DMF,
analytical grade, 99.5%), oleic acid (OA, analytical grade) and n-octylamine (99%)
were purchased from Aladdin Reagent Company. Analytical grade toluene and
diethylether were purchased from Beijing Chemical Reagent Company. All the
chemicals were used without further purification.
2.2. Synthesis of CH3NH3Br and CH3(CH2)7NH3Br (OABr)
The CH3NH3Br or CH3(CH2)7NH3Br was synthesized through the reaction of
CH3NH2 or n-octylamine with HBr based on previous work [24, 25]. The synthesis of
CH3NH3Br is listed as follows: The reaction solution consisting of CH3NH2 and HBr
with the molar ratio of 1:1 was stirred for 2 h in an ice bath followed by vacuum
drying. The obtained slight yellow precipitates were washed three times with
diethylether and dried under vacuum condition to get purified white powders. OABr
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was prepared using the same method by mixing n-octylamine and HBr in a 1:1 molar
ratio.
2.3. Synthesis of CH3NH3PbBr3 PNCs
CH3NH3PbBr3 PNCs were fabricated following the reported LARP technique [9].
0.16 mmol CH3NH3Br and 0.2 mmol PbBr2 were dissolved in 5 mL DMF (polar
solvent) with 0.5 mL OA and 0.08 mmol OABr to form a clear precursor solution.
Then, variable volume (0.5, 1, 2 mL) of precursor solution was dropped into 10 mL
toluene as a nonpolar solvent under vigorous stirring at room temperature. After
centrifugation at 7000 rpm for 5 min to discard the precipitates, green or yellow-green
CH3NH3PbBr3 PNCs solutions were obtained.
2.4. Characterizations
The X-ray diffraction (XRD) patterns were taken on a X-ray powder
diffractometer (D/Max 2200V, Japan) using a CuKα (λ=1.5405981Å) radiation source.
The XRD samples were prepared by drop casting PNCs solutions onto quartz sample
holder and dried overnight. Liquid samples deposited on amorphous carbon-coated
copper grids were analyzed using a FEI Tecnai G2 F20 transmission electron
microscopy (TEM) instrument operating at an acceleration voltage of 200 kV. The
morphology of the precipitate was observed using scanning electron microscopy
(SEM, SU8010, Hitachi, Japan). The X-ray photoelectron spectroscopy (XPS) data
was collected on a X-ray photoelectron spectrometer (K-Alpha, Thermo Electron
Corporation, USA). The C 1s peak at 284.8 eV was set as a reference for all XPS peak
positions. The SEM and XPS samples were prepared by drop casting PNCs solutions
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on silicon wafers and also dried overnight. The UV-vis absorption spectra were
obtained on a spectrophotometer (U-3310UV, Hitachi, Japan) and photoluminescence
(PL) spectra were measured on a fluorescence spectrophotometer (F-4500, Hitachi,
Japan). The absolute PL quantum yield (PLQY) was determined by quantum yield
measurement system (C9920-02G, Hamamatsu, Japan) with an integrating sphere at
room temperature.
3. Results and discussion
3.1. LARP synthesis of CH3NH3PbBr3 PNCs
Different volumes of precursor solution containing a fixed concentration of
CH3NH3PbBr3 precursor (PbBr2 and CH3NH3Br) and organic ligands (OA and OABr)
were dropped into a vigorously stirred toluene solvent at room temperature. Once the
precursor solution was mixed with the toluene, the color of the mixture became bright
green immediately in all cases, which indicated the formation of perovskite crystals.
After removing the relatively larger particles by centrifugation, clear supernatant
solutions with different concentration of PNCs were obtained. It should be noted that
the amount of the precipitate was found to increase accompanying with higher PSV in
the reaction mixture (28, 42 and 60 mg per batch using 0.5, 1 and 2 mL precursor
solution, respectively). Fig. 1a and 1b present the CH3NH3PbBr3 PNCs solutions
synthesized with different PSV under normal indoor light and UV light illumination.
The solutions show bright luminescence under UV light using PSV of 0.5 or 1 mL. In
contrast, the luminescence intensity of PNCs solution obtained from 2 mL precursor
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solution is substantially lower, indicating a lower concentration of PNCs, which is also
consistent with the higher amount of the precipitate. The crystal structure of the
synthesized CH3NH3PbBr3 PNCs and the corresponding discarded precipitate was
characterized by XRD, as shown in Fig. 1c. Both the as-prepared CH3NH3PbBr3 PNCs
and the precipitate possess a cubic structure (space group, Pm-3m). The XRD peaks
located at 2θ of 14.9o, 21.2o and 30.6o can be assigned to the diffractions from (100),
(110) and (200) planes, respectively. The sharp diffraction peaks from the precipitate
indicate the formation of perovskite crystals with large particle size based on
Debye-Scherrer equation, as also observed in Fig. 1d. In contrast, the full width at half
maximum (FWHM) of the diffraction peaks become broader for PNCs, and the
crystallinity decreases, corresponding to the small particle size and the
superimposition of background peak originating from organic ligands [26].
Fig. 1e shows the UV-vis absorption and PL spectra of CH3NH3PbBr3 PNCs
synthesized with different PSV. The absorption onsets are all around 520 nm. The
optical band gaps, measured from the absorption data using Tauc plot ((αhν)2 vs hν for
a direct allowed transition, where α is absorption coefficient, h is Planck’s constant,
and ν is frequency of incident photons) are 2.3-2.35 eV. The PL emission spectra show
peak positions at 526-528 nm (FWHM=20 nm) with excitation at 350 nm. Thus, the
PNCs have a small Stokes shift (5-8 nm) between the absorption and the emission
peak, which is consistent with a direct exciton recombination process. Additionally,
the gradually reduced absorbance is observed with the increasing PSV, which could be
also associated with the decreased concentration of PNCs in solution.
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In order to further find out the relationship between PSV and PNCs
concentration, 0.1 mL precursor solution was also introduced in LARP system. After
mixing and centrifugation, a high amount of precipitate and a colorless supernatant
were obtained, indicating substantially low concentration of PNCs in solution. It is
known that the formation of perovskite particle is realized by supersaturation and
following rapid nucleation and growth processes. According to LaMer and Dinegar’s
model, the nucleation and growth of PNCs is highly dependent on the precursor
concentration, reaction temperature, etc [27]. Herein, the dual-factor of amount of
precursor and polarity of the mixture solvent becomes the key factor governing the
two processes. At the PSV of 0.1 mL, few crystal nuclei are generated in the mixture
due to the low amount of precursor, which is favorable for the growth of perovskite
crystals, and hence leads to low concentration of PNCs. With the increase of PSV, both
the amount of precursor and the polarity of the mixture solvent which relevant to the
ratio of DMF to toluene are increased. When increased the volume to 2 mL, on one
hand, a large amount of nuclei would generate in a short period as a result of
supersaturation, which could suppress the growth of perovskite particles. On the other
hand, the mixture solvent with high polarity could help to activate the particle surface
by dissolving the surface ligands, which in turn promote the growth of perovskite
particles. The obtained low concentration of PNCs in solution demonstrates the
dominant effect of high polarity on the growth of PNCs. In contrast, at the PSV of 0.5
mL, a large amount of nuclei are produced, most of the grains grow up to the
nanometer level. Besides, the low polarity of mixture solvent may have less effect on
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the surface ligands of the particles. Compared to 0.1 and 2 mL of PSV, the 0.5 mL
sample has the highest yield of PNCs concentration. Thus, in order to achieve high
concentration of PNCs, it is of importance to manipulate the PSV to the optimized
range in the LARP process.
Fig. 1. Photographic images of the CH3NH3PbBr3 PNCs solutions synthesized with different PSV
under (a) normal indoor light or (b) UV light illumination. (c) XRD patterns of PNCs synthesized
with 0.5 mL precursor solution and the corresponding precipitate. (d) SEM image of the precipitate.
(e) UV-vis absorption and PL spectra of the CH3NH3PbBr3 PNCs solutions and the inset shows the
comparison of absorbance of PNCs synthesized with different PSV.
3.2. Characterization of CH3NH3PbBr3 PNCs
TEM was employed to exam the structure and morphology of the prepared
CH3NH3PbBr3 PNCs. The corresponding TEM, high resolution TEM (HR-TEM)
images and size distribution (each obtained on 100 particles) are shown in Fig. 2. All
PNCs exhibit spherical shape. The HR-TEM images of CH3NH3PbBr3 PNCs present
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clear lattice fringes, which suggest a highly crystalline structure. The interplanar
distances of 2.91 and 2.92 Å in HR-TEM images are very close to that of (002) plane
measured by XRD (2.95 Å). The average diameter of CH3NH3PbBr3 PNCs is 7.08 ±
1.1 nm and 5.7± 0.5 nm for the 0.5 mL and 2 mL sample, respectively. It is worth
noting that PNCs obtained from 2 mL precursor solution possess smaller particle size,
which exactly results from the growth inhibition effect caused by the large amount of
crystal nuclei mentioned above.
Fig. 2. TEM, HR-TEM images and size distribution histograms of the PNCs synthesized with 0.5 mL
(a and c) and 2 mL (b and d) precursor solution.
As for the PNCs synthesized with 2 mL precursor solution, an obvious color
change is observed upon dilution with more toluene. As shown in the photographs of
Fig. 3a, under UV light illumination, the change is immediately apparent, from light
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blue to bright green. The UV-vis absorption spectra in Fig. 3a also indicate an
increased absorbance after dilution. According to Beer-Lambert law, the absorbance is
proportional to the concentration at linear region. Therefore, we can conclude that the
concentration of CH3NH3PbBr3 PNCs in the 2 mL sample increases upon dilution.
Additionally, the PL spectrum of the 2 mL sample displays only one peak at 527 nm,
while an additional peak at shorter wavelength appears after dilution, which is not
observed in the dilution of the 0.5 mL or 1 mL sample. As mentioned above, the
polarity of the mixture solvent increases with the increase of PSV. The perovskite
precursor, and especially the ligands on the surface of PNCs may redissolved in such
solvent. A subsequent dilution of the 2 mL sample with toluene again leads to the
precipitation of CH3NH3PbBr3 PNCs. Furthermore, with the higher concentration of
the ligands stabilizing the surfaces, the newly formed PNCs possess a much smaller
size of 2-4 nm (see Fig. 3b), comparable or smaller than the exciton Bohr diameter of
bulk CH3NH3PbBr3 (~4 nm). The smaller PNCs with a higher degree of quantum
confinement eventually lead to the pronounced blue-shift of new peak as compared to
the original one in the PL spectrum [28].
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Fig. 3. (a) PL spectra and photographic images under normal indoor light (left) or UV light (right)
illumination of the 2 mL sample before and after dilution. The inset shows the comparison of UV-vis
absorbance of the 2 mL sample before and after dilution. (b) TEM image of the 2 mL sample after
dilution.
XPS analysis was also performed to investigate the surface state of PNCs
synthesized with 0.5 or 2 mL precursor solution. As shown in Fig. 4, in both cases, the
XPS spectrum of Pb 4f shows two symmetric peaks attributed to Pb 4f7/2 and Pb 4f5/2
level at binding energies of 138.7 eV and 143.6 eV, respectively. The spin orbit split
between the Pb 4f7/2 and Pb 4f5/2 levels is of 4.87 eV, which is in agreement with the
reported value (4.8 eV) in literature [29]. The Br 3d peak can be fitted into two peaks
centered at 68.8 and 69.8 eV, corresponding to the inner and surface Br ions,
respectively. The O 1s XPS spectrum can also be fitted into two peaks. The lower
energy peak at 532.5 eV results from two nonequivalent O atoms in oleic acid
(R-COOH), while the higher energy peak at 533.9 eV can be assigned to two
chemically equivalent O atoms in carboxylate species of deprotonated oleic acid
(R-COO−), which could bind to the surface Pb ions based on hard soft acid base
(HSAB) theory (see Fig. 4g). Although both R-COO- (borderline base) and Br-
(borderline base) can bond with Pb2+ (borderline acid), R-COO- can replace the Br- of
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PbBr2 for the binding constant of R-COO- with Pb2+ (lg k1=2.52) is much larger than
the second step binding constant for PbBr2 (lg k2=0.7) [30]. The N 1s spectrum
contains two peaks, implying the two existing chemical states of the N element. The
peak at 400 eV can be attributed to the methylamine salt [31], while the peak at 402
eV could be originated from octylamine salt. The octylammonium cation binds to the
surface Br presumably through hydrogen bonding or electrostatic interactions (see Fig.
4g) [32-34]. The difference of intensity ratio of these two peaks between the 0.5 mL
and 2 mL sample can be attributed to the smaller size and higher specific surface area,
thus more amount of the ligands on the surface of the 2 mL sample.
PLQY is an important parameter to evaluate the luminous efficiency of the PNCs.
It can be found that the PLQY of as-prepared CH3NH3PbBr3 PNCs exceed 70% in the
case of 0.5 mL precursor solution, whereas the PLQY of CH3NH3PbBr3 PNCs
synthesized with 2 mL precursor solution reach as low as 20-30%. It is known that the
PLQY represents the ratio between radiative and non-radiative recombination in the
PNCs, and the enhancement of non-radiative recombination is usually generated by
the surface trap states in PNCs. In addition, the presence of OA/OABr ligands serving
as surface binding species could passivate the surface defects and prevent nonradiative
recombination at surface trap states. In our case, the 0.5 mL sample has more efficient
surface ligands coordination for the relatively lower polarity of mixture solvent.
Therefore, compared to the 2 mL sample, PNCs synthesized with 0.5 mL precursor
solution possess a relatively inert, well passivated particle surface and thus higher
PLQY.
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Fig. 4. XPS survey scans of PNCs synthesized with 0.5 mL (a) and 2 mL (b) precursor solution. (c-f)
High resolution XPS spectra of Pb 4f (c), Br 3d (d), O 1s (e) and N 1s (f) for the 0.5 mL and 2 mL
samples. (g) Schematic representation of the ligand binding modes on the CH3NH3PbBr3 PNC
surface, including octylammonium cation binding to surface Br, and oleate anion binding to the
surface Pb ions.
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3.3. Stability of CH3NH3PbBr3 PNCs
The stability of PNCs is vital for application in optoelectronic devices. To
evaluate the stability of the synthesized CH3NH3PbBr3 PNCs, the PNCs solutions were
placed under the dark at room temperature, and the PL spectra were recorded after
different storage periods in the open air as shown in Fig. 5. The absolute PLQYs were
also measured and summarized in Table 1. For the 0.5 mL sample, the PL spectrum
keeps almost constant during the test. After 30 days, no detectable decrease of PL
intensity is observed. In addition, the initially observed 85% PLQY for the
as-synthesized 0.5 mL sample shows only slight decrease with time and reaches a
value of 78% after 15 days and 67% after 30 days, suggesting a high stability of the
PNCs. The excellent stability is assigned to the low polarity of colloidal system which
could protect the surface of PNCs and inhibit the agglomeration and degradation of
particles. However, the 2 mL sample presents a fast decay of PL intensity with
increase in time, e.g. the PL vanishes after 30 days. The concurrent decrease in PLQY
is also observed. The initially observed 20% PLQY for 2 mL sample shows a
considerable decrease with time and reaches a value of 5% after 15 days and totally
quenches after 30 days, which is likely due to a complete degradation of the PNCs in
the more polar mixture system.
Table 1. PLQYs obtained for the PNCs in different time intervals
PLQY(%)
(As-synthesized)
PLQY(%)
(15 days)
PLQY(%)
(30 days)
Sample
0.5 mL
1 mL
2 mL
85
58
20
78
53
3
67
12
2
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Fig. 5. PL intensity variation of CH3NH3PbBr3 PNCs solution versus the storage time in the open air,
and the inset shows the PL spectra of the PNCs measured after different days: (a) the 0.5 mL sample;
(b) the 2 mL sample.
4. Conclusions
In summary, CH3NH3PbBr3 PNCs were synthesized with variable PSV by LARP
approach. All the PNCs exhibited spherical shape with narrow size distribution, green
emission with narrow FWHM and small Stokes shift. 0.5 mL of PSV was found to
yield highest PNCs concentration and excellent optical properties including high
PLQY and good stability. Both the amount of precursor and the polarity of reaction
system play important roles in the nucleation and growth of PNCs. At the lower PSV,
few crystal nuclei are generated due to lower amount of precursor, which is favorable
for the growth of large perovskite crystals. At the higher PSV, the inhibition of crystal
growth with increased amount of precursor is compensated by increasing the polarity
of mixture solvent, which leads to the dissolution of surface ligands and eventually
growth of large perovskite crystals. In addition, PNCs obtained from high polar
mixture exhibit substantially lowered PLQY and stability in air. The obtained results
on dual-factor effect on nucleation and growth of PNCs will be very instructive for
further studies in the LARP process.
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Acknowledgements
The work was supported by the National Natural Science Foundation of China
(NSFC Grants 51704225).
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Highlights
ꢀ PSV was varied to modify precursor amount and polarity of mixture at the same time
ꢀ Concentration and optical properties of PNCs as a function of PSV were investigated
ꢀ The influence of the dual-factor on the nucleation and growth of PNCs was revealed