Facile synthesis of organic–inorganic hybrid perovskite CH3NH3PbI3 microcrystals

Article abstract of DOI:10.1016/j.jallcom.2017.07.154

The organic–inorganic hybrid perovskite CH₃NH₃PbI₃ is becoming an interesting material in the field of optoelectronic application. Most of the previous research focused on thin film and crystal growth of this material. Here we describe the rapid preparation of perovskite CH₃NH₃PbI₃ microcrystals using a facile synthesis method at room temperature. By using ultrasound assisted solutions of PbI₂ and CH₃NH₃I precursors in acid solvents, a single-phase perovskite was obtained. X-ray diffraction data reveal that the prepared CH₃NH₃PbI₃ crystals possess a tetragonal structure. FE-SEM and TEM images show the morphology and grain size of CH₃NH₃PbI₃ crystals. UV-VIS-NIR and PL measurements indicate that the perovskite crystals show a slight reduced bandgap, accompanying with red shift, compared with the perovskite polycrystalline films. DSC-TGA demonstrates that the perovskite microcrystals show the better thermal stability than that of the perovskite films, which suggests the wide potential application.

Full text of DOI:10.1016/j.jallcom.2017.07.154

Accepted Manuscript
Facile synthesis of organic–inorganic hybrid perovskite CH NH PbI microcrystals
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Xianyu Jia, Ziyang Hu, Yubin Zhu, Tianyao Weng, Jie Wang, Jing Zhang, Yuejin Zhu
PII:
S0925-8388(17)32530-6
10.1016/j.jallcom.2017.07.154
JALCOM 42570
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 11 April 2017
Revised Date: 14 July 2017
Accepted Date: 15 July 2017
Please cite this article as: X. Jia, Z. Hu, Y. Zhu, T. Weng, J. Wang, J. Zhang, Y. Zhu, Facile synthesis
of organic–inorganic hybrid perovskite CH NH PbI microcrystals, Journal of Alloys and Compounds
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(2017), doi: 10.1016/j.jallcom.2017.07.154.
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ACCEPTED MANUSCRIPT
Facile synthesis of organic–inorganic hybrid perovskite CH3NH3PbI3
microcrystals
Xianyu Jia, Ziyang Hu*, Yubin Zhu, Tianyao Weng, Jie Wang, Jing Zhang, Yuejin Zhu*
Department of Microelectronic Science and Engineering, Ningbo Collabrative Innovation Center
of Nonlinear Hazard System of Ocean and Atmosphere, Ningbo University, Ningbo 315211,
China
ABSTRACT: The organic–inorganic hybrid perovskite CH3NH3PbI3 is becoming an interesting
material in the field of optoelectronic application. Most of the previous research focused on thin
film and crystal growth of this material. Here we describe the rapid preparation of perovskite
CH3NH3PbI3 microcrystals using a facile synthesis method at room temperature. By using
ultrasound assisted solutions of PbI2 and CH3NH3I precursors in acid solvents, a single-phase
perovskite was obtained. X-ray diffraction data reveal that the prepared CH3NH3PbI3 crystals
possess a tetragonal structure. FE-SEM and TEM images show the morphology and grain size of
CH3NH3PbI3 crystals. UV-VIS-NIR and PL measurements indicate that the perovskite crystals
show a slight reduced bandgap, accompanying with red shift, compared with the perovskite
polycrystalline films. DSC-TGA demonstrates that the perovskite microcrystals show the better
thermal stability than that of the perovskite films, which suggests the wide potential application.
Keywords: Perovskite; Microcrystal; CH3NH3PbI3; Stability; Crystal structure
1. Introduction
Organic–inorganic hybrid perovskites (OHPs, CH3NH3PbX3, X = Cl, Br, I) are attracted great
attentions as one of the most promising semiconductor materials for low cost photovoltaic
devices[1–5]. Their unique optical properties also promise the potential utilization in the field of
photonics and optoelectronics[6–12]. Bulk OHPs exhibit a continuous adjustable wavelength from
green to infrared and narrow-band emission, which are indispensable for lasers and light emitting
diodes [13]. CH3NH3PbI3 single crystals have been synthesized using aqueous solutions of HI acid
containing Pb2+ ions and CH3NH3+ ions long before [14]. Recently, larger single crystals have
been grown by this method [15,16]. Also, other solution- processable methods have been
developed to grow centimeter-scale CH3NH3PbI3 single crystals.[17,18]. These perovskites show
extremely low trap density in the range of 109–1010 cm-3, high carrier mobility, and long diffusion
length. Recently, large crystals were fabricated by dissolving CH3NH3I and PbI2 into
γ-butyrolactone and then heating the solution up to 110 oC[19–21]. Regarding to perovskite films,
one advantage of OHPs is their versatile processibility. Perovskite films can be prepared by
chemical and physical deposition techniques, mainly including one-step solution processing[22],
two-step sequential deposition[23] ,vapor deposition and vapor assisted growth.[1, 24, 25] Since
*Corresponding author at: Department of Microelectronic Science and Engineering, Ningbo Collabrative
Innovation Center of Nonlinear Hazard System of Ocean and Atmosphere, Ningbo University, Ningbo 315211,
China.
E-mail address: huziyang@nbu.edu.cn (Z. Hu), zhuyuejin@nbu.edu.cn (Y. Zhu)
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the film morphology is very susceptible to technological conditions, [26] it is importance to
explore optimal approach, which enables high-quality film for high device performance.
Current researches are focusing on exploring efficient methods to make high-performance
optoelectronic devices based on perovskite materials. However, the time-consuming growth of
single crystal and the susceptible solution-processed film fabrication only allow small-batch
product. This material will be one of the massive components in optoelectronic field in the future.
The preparation of perovskite crystals is rarely reported ,[16, 27] which needs to further explore
and characterization. The currently most common solvents for dissolving PbI2 are
dimethylformamide (DMF) and dimethylsulfoxide (DMSO), or usually a mixture of both. The
toxicity of these solvents used may be a trouble in large-scale production. Exploring alternative
solvents for the production is important. In this work, we describe the rapid preparation of
perovskite CH3NH3PbI3 crystals using a facile synthesis method at room temperature. Using the
environmentally friendly glacial acetic acid as a solvent, perovskite CH3NH3PbI3 was
instantaneously generated by mixing CH3NH3I and PbI2 powder in a one-step process. Studies
into the tetragonal CH3NH3PbI3 crystal structure are conducted. The experimental results indicate
that the absorption edge of the tetragonal CH3NH3PbI3 in the UV-vis spectrum is at 850 nm, and
the band gap is approximately 1.49 eV. It exhibits a wide absorption window and relatively good
thermal stability compared with the perovskite films. These behaviors make the CH3NH3PbI3
microcrystals a promising candidate for optical and electronic applications.
2. Experimental section
2.1 Materials
All starting materials were of high pure grade from commercial sources. Methylamine
(CH3NH2) solution and HI solution were purchased from Sigma-Aldrich and used without further
treatment. A slight excess CH3NH2 solution was reacted with HI in an ice bath under ambient
atmosphere. The crystallization of methyl-ammonium iodide (CH3NH3I) was achieved using a
rotary evaporator. White CH3NH3I microcrystals were washed with diethyl ether several times,
and finally dried at 60 °C in a vacuum oven overnight.
2.2 Growth of CH3NH3PbI3 microcrystals and films
The CH3NH3PbI3 microcrystals were synthesized as follows. PbI2 (462 mg) and excess
CH3NH3I (477 mg) were mixed and grinded for 5 min. After that, the mixture was gradually added
into glacial acetic acid solution and simultaneously ultrasonic stirring for 10 min in order to react
completely. The black precipitate was collected by filtrating, and then repeatedly washed with
ethanol in order to remove the residual CH3NH3I. The collection was finally dried at 60 °C in a
vacuum. The perovskite CH3NH3PbI3 films were synthesized by the one-step solution process. To
deposit the perovskite layer, a 1:3 ratio of PbCl2/CH3NH3I (0.8 M and 2.4 M) was mixed in DMF.
The solution was spin-coated on the clean FTO substrates at 2000 rpm for 40-50 s and then treated
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with a multi-step slow annealing method.[28, 29]
2.3 Characterizations
X-ray powder diffraction (XRD) patterns of nanocrystal material were collected using a
Bruker-AXS D8 Advance X-ray diffractometer with Cu Kαradiation (λ = 1.54186 Å) in the range
of 10–70° (2θ). No impurity peaks were observed. The powder XRD patterns are in good
agreement with the calculated XRD patterns from the single-crystal models. The UV-VIS-NIR
absorption spectra of the perovskite materials were recorded with a VARIAN Cary 5000
UV-Vis-NIR spectrophotometer. For thermal Analysis, thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) are performed using Q600 SDT (TGA-DSC, TA
Instruments-Waters LLC), heating at a steady constant rate of 10 °C min−1 under a constant 20 mL
min−1 N2 gas flow. The investigated temperature intervals were between a minimum of 30 °C to a
maximum of 800 °C. Ceramic crucibles and ~5−15 mg of sample material were employed for
each measurement. The data were recorded and analyzed using TA Instruments Universal Analysis
2000 software. The surface morphology and element analysis of the nanocrystals were observed
by scanning electron microscopy (SEM) using Hitachi SU8010. Steady-state photoluminescence
spectroscopy (PL) measurements were acquired using an Edinburgh Instruments FLS 920
fluorescence spectrometer with an excitation wavelength of 460 nm.
3. Results and discussion
CH3NH3PbI3 was formed rapidly as a product on addition of CH3NH3I and PbI2 mixture into
the glacial acetic acid solution, following with ultrasonic treatment. The change in color of the
reaction mixture from yellow to black indicates the formation of perovskite precipitate. In fact,
CH3NH3I and PbI2 in glacial acetic acid are insoluble, but both are milled and mixed in glacial
acetic acid, there will be generated perovskite crystals. Glacial acetic acid provides a good
environment for the organic part (CH3NH3I) due to solvent-solute hydrogen bond interactions,
which results in CH3NH3+ and I- ions. Since PbI2 is difficult to dissolve in glacial acetic acid, the
asymmetric effect between their atoms will be reduced. I- will interact strongly with the MA+ to
break the hydrogen bonds in the MAI. The PbI2 crystal structure has a layered structure allowing
+
intercalation of CH3NH3 , leading to high accessible surface for reactions and high reaction
rate[30].
X-ray diffraction (XRD) was used to characterize the crystallinity of the prepared CH3NH3PbI3
powder. Fig. 1 shows the powder XRD patterns of the CH3NH3PbI3 powder. For comparison, we
also presented XRD patterns of the standard CH3NH3PbI3 films. No peak can be observed at 2θ =
12.8o, which indicates the completion of the PbI2 reaction. It shows that the main peaks for the
prepared CH3NH3PbI3 powder are at 2θ = ~14 o, ~28 o and 43.56o. The positions of the main peaks
reveal that the prepared CH3NH3PbI3 crystals possess a tetragonal structure. Both the clear peaks
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of (002) and (110) at 2θ = 14.03o and 2θ= 14.13o, splitting from the (100) peak of the cubic phase,
and the clear peaks of (004) and (220), splitting from the (200) peak of the cubic phase at 2θ =
28.50o and 2θ = 28.74o confirm that the crystal adopts a tetragonal phase with high crystalline
quality as shown in Fig 2(c, d). The diffraction peaks are in good agreement with the calculated
diffraction [15]and other detailed studies of the diffraction of the perovskites[31, 32]
In Fig. 2(a), the FE- SEM image shows that the CH3NH3PbI3 crystals are gathered closely. Their
corresponding higher-magnification SEM image presented the real size of one crystal particle as
shown in Fig. 2(b). In order to evaluate the exact particle size of powder CH3NH3PbI3, the TEM
images were conducted in Fig. 3. It was clear that the CH3NH3PbI3 crystal grains are clearly seen
in the TEM images. The grain size of powder CH3NH3PbI3 and crystal CH3NH3PbI3 perovskite
lies in between 500–1000 nm. Some small particles agglomerate into larger grains as shown in Fig.
3(b). High resolution TEM of a single grain confirms high crystalline quality of CH3NH3PbI3
microcrystal.
(a)
(b)
(c)
Fig. 1 (a) XRD patterns recorded on the CH3NH3PbI3 powder and film, (b) the clear peaks of (002)
and (110) at 2θ = 14.03o and 2θ = 14.13o splitting from the (100) peak of the cubic phase, (c) the
clear peaks of (004) and (220) at 2θ = 28.50o and 2θ = 28.74 o splitting from the (200) peak of the
cubic phase.
Fig. 2 (a) FE-SEM image of as synthesized perovskite, (b) the corresponding higher-magnification
SEM image.
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(a)
(b)
(c)
Fig. 3(a) TEM images of crystal grains of the structured perovskite, (b) smaller particles
agglomerate into larger grains, and (c) high resolution TEM of a single hexagonal grain.
The UV-VIS-NIR spectrum for CH3NH3PbI3 is shown in Fig. 4(a). The result of the band
structure calculation implies that tetragonal CH3NH3PbI3 has a direct band gap. The band gap can
be determined by fixing the tangent line of the curve and the (hν) axis. The UV-VIS-NIR spectrum
shows that the absorption edge is at about 850 nm for CH3NH3PbI3 microcrystal, larger than that
of CH3NH3PbI3 film (800 nm). In Fig. 4(b), the band gap of CH3NH3PbI3 microcrystal is
approximately 1.49 eV, which is close to the theoretical results and smaller than those (1.58 eV)
derived from polycrystalline and thin-films.[33, 34] Based on this data, we infer that CH3NH3PbI3
exhibits a relatively narrow band gap semiconductor range. Finally, the optical properties of the
CH3NH3PbI3 microcrystal were studied by photoluminescence (PL) spectroscopy. As shown in
Fig. 4(c), in comparison with the CH3NH3PbI3 thin film, the PL peak position of the microcrystal
shows a slight red shift f from 774 nm to 783 nm, which also agrees well with the fitting results.
(a)
(b)
(c)
Fig. 4 (a) UV-VIS-NIR spectrum, (b) Eg from the fitting results, (c) steady-state PL spectra for
CH3NH3PbI3 nanocrystal and film.
In order to examine the thermal stability of the CH3NH3PbI3 microcrystals, DSC-TGA was used
under nitrogen flow from room temperature up to 700 oC. Fig. 5(a) shows the TGA results for the
CH3NH3PbI3 microcrystal and film. At the beginning of ~ 200 °C, the mass loss of CH3NH3PbI3
film occurs. However, the CH3NH3PbI3 microcrystal is thermally stable up to 250 °C, when it then
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starts losing weight. The initial undergoes ~26% mass loss when the temperature reaches to
~310 °C for CH3NH3PbI3 film and ~350 °C for CH3NH3PbI3 microcrystal, respectively. The
compound continues to lose weight until about 70% is left. This implies that the residue is mainly
light yellow PbI2, which is consistent with the previous report.[20, 35] In Fig. 5(a), the two
inflection points corresponding to the two extreme points in the TGA first derivative imply that
two materials undergo thermal decomposition. DSC was used to gain further insight into the
thermal behavior of the CH3NH3PbI3 microcrystal and film in Fig. 5 (b). The thermograms of
CH3NH3PbI3 film show the first peak at ~310 °C, while at ~350 °C for CH3NH3PbI3 microcrystal.
The first peak is corresponding to the CH3NH3PbI3 perovskite converting into organic gas and
solid PbI2. The CH3NH3PbI3 microcrystal is not hygroscopic and has relatively good stability after
being exposed to air for several days. As for the reasons, the better thermal stability of perovskite
microcrystal is due to the higher crystallinity and the reduced bulk defects compared with the
perovskite films. As reported previous, the single crystal of perovskite materials shows better
thermal stability due to the low concentration defect states or gap states[20]. Thermally stimulated
current (TSC) technique also demonstrates that the formation of carrier traps is strongly related to
the stability[36]. Except from the thermal test, the stability of the newly made CH3NH3PbI3
microcrystals and CH3NH3PbI3 films were stored in air condition was also tested and compared.
The experiment showed that CH3NH3PbI3 microcrystal was stable for one month, while the
CH3NH3PbI3 film was serious decomposed into PbI2.
(a)
(b)
Fig. 5 (a) TGA heating curve and corresponding first derivative for CH3NH3PbI3 microcrystal and
film, (b) DSC heating curve for the CH3NH3PbI3 microcrystal and film.
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4. Conclusions
In summary, we succeeded in growing CH3NH3PbI3 microcrystals of high-quality using a facile
method based on the use of an acid organic solvent at room temperature. The XRD analysis and
SEM images confirm that the prepared CH3NH3PbI3 powders are microcrystals with a tetragonal
structure. Moreover, its band gap is approximately 1.49 eV, and its PL peak is red-shifted, which
indicate a relatively narrow band gap semiconductor compared with the counterpart of
CH3NH3PbI3 films. Meanwhile, the TGA-DSC results suggest that the CH3NH3PbI3 microcrystal
possesses better thermal stability than the common thin films. These properties make the
CH3NH3PbI3 microcrystal a promising candidate for both fundamental physical studies and
electronic applications.
Acknowledgements
This work was supported by the National Science Foundation of China (Grant Nos. 11304170,
11374168, and 51302137). The authors would like to thank the sponsor by K.C. Wong Magna
Fund in Ningbo University.
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Highlights
1. A facile synthesis of perovskite microcrystals was demonstrated.
2. A tetragonal crystal structure was tested by XRD and FE-SEM.
3. Red shift and thermal stability were investigated by PL and DSC-TGA, respectively.
4. CH3NH3PbI3 microcrystal promises a candidate for optoelectronic applications.