CH3NH3PbX3 (X = I, Br and Cl) crystals have been successfully synthesized via a facile solvothermal method with different acids as halogen sources. Reaction conditions including concentration of the precursor, reacting temperature, time, and halogen sources, have been investigated. The results showed that CH3NH3PbI3 crystals exhibit tetragonal phase and the crystals change from normal cuboids to concave-faced cuboids by different reaction conditions. The CH3NH3PbBr3 and CH3NH3PbCl3 crystals show cubic phase structure and normal cuboid. Moreover, the photoluminescence spectra of CH3NH3PbX3 (X = I, Br and Cl) crystals obtained with different halogen sources also were discussed in detail.
© 2017 Optical Society of America
CorrectionsFuqiang Guo, Baohua Zhang, Junjun Wang, Haineng Bai, Renqing Guo, Yineng Huang, and Pinyun Ren, "Facile solvothermal method to synthesize hybrid perovskite CH3NH3PbX3 (X = I, Br, Cl) crystals: publisher’s note," Opt. Mater. Express 8, 210-210 (2018)
21 December 2017: Typographical corrections were made to the author listing and funding section.
The perovskite materials have an ABX3 crystal structure (A: organic ammonium cation, B: metal cation and X: halide anion). Among of perovskite crystals, the CH3NH3PbX3 (X = I, Br, Cl) have attracted much attention ascribe to large absorption coefficients [1-2], outstanding charge transport characteristics of holes and electrons [3-4] etc., especially in the field of solar cells with photoelectric conversion efficiency (PCE) increasing from 3.81%  to more than 20%  in the past years. Moreover, they also have potential applications in many other fields such as light-emitting devices (LEDs) , lasers , photodetectors , and so on.
Until now, CH3NH3PbX3 (X = I, Br and Cl) materials have been prepared by several methods: spin coating , thermal evaporation , solid-state reaction , and intramolecular exchanging crystallization . However, these methods require lots of conditions such as higher vacuum, many kinds of organic solvents, long time for the reaction, careful adjustment of reaction condition, resulting in difficulty to obtain high-purity materials. To date, a facile and rapid method is still needed to synthesize organometal halide perovskite materials.
The facile solvothermal method has been considered as the most promising route due to its advantages of low temperature, single-step process, and high reproduction . High-quality and uniform cuboid-shaped CH3NH3PbI3 perovskite single crystals has been successfully synthesized via solvothermal method in 2015, and found the dissolution phenomenon from specific facets for the first time . Subsequently, Zhao et al.  demonstrated a facile synthetic approach for preparing mixed halide perovskite crystals by the solvothermal growth. Recently, Zhang et al.  systematically synthesized CH3NH3PbI3 crystals using solvothermal process, and the reaction conditions such as lead source etc. have been comprehensively investigated to obtain shape-controlled CH3NH3PbI3 crystals. However, above work rarely discussed the synthesis on CH3NH3PbX3 (X = I, Br, Cl) crystal by solvothermal method.
In this work, CH3NH3PbX3 (X = I, Br, Cl) perovskite materials were synthesized by a facile solvothermal process with different acid such as hydroiodic acid (HI), hydrobromic acid (HBr) and hydrochloric acid (HCl) as halogen sources. Moreover, the photoluminescence spectra of CH3NH3PbX3 (X = I, Br, Cl) crystals have been discussed in detail. These results demonstrated that our work offers an effective way to synthesize hybrid perovskite CH3NH3PbX3 (X = I, Br, Cl) crystals for various applications.
2. Experimental methods
All chemical reagents (analytical grade) were directly used without further purification. In a typical experimental process, 30 mg Pb(Ac)2•3H2O (Ac- = COO-, 99.9%) was completely dissolved in 1 mL of HI (45 wt% in water), then 30 mL of isopropanol (IPA) was added and stirred for 5 min, accompanied with a color change. 0.3 mL of methylamine solution (30 wt% in water) was then added dropwise. The mixture was further stirred for 10 min and then put into 50 mL stainless steel Teflon-lined autoclave, and this autoclave was sealed and heated in a furnace at 130°C for 6 h, and cooled to room temperature. The precipitates were collected and washed with isopropanol twice by centrifugation in ambient temperature, and then dried in vacuum at 60°Cfor 4 h. Moreover, HI was substituted with the HBr and HCl as halogen sources to synthesize precipitates.
The phase of the products was investigated by X-ray diffraction (XRD, X'TRA) using Cu Kα radiation (40 kV and 40 mA, 0.02°/step from 10 °to 50°). The field-emission scanning electron microscope (FE-SEM, JSM-7000F) was used to investigate the morphological features. A high-resolution transmission electron microscope (HR-TEM, JEM-200 CX) was used to confirm the crystallinity and microstructure of products, and selected area electron diffraction (SAED) patterns were obtained. The photoluminescence (PL) were recorded on a HORIBA iHR 320 type fluorescence spectrophotometer with an excitation wavelength of 375 nm at room temperature.
3. Results and discussion
The X-ray diffraction (XRD) pattern of products obtained with HI at 130°C for 6 h was shown in Fig. 1(a). Sharp and strong diffraction pattern indicate the fine crystallinity of the samples, and the main diffraction peaks were assigned to the (110), (220), (310), (224), and (314) at 2θ = 14.15°, 28.45°, 31.85°, 40.48°, and 43.05°, respectively. The reflection peaks can be indexed as tetragonal phase CH3NH3PbI3 with lattice parameters of a = b = 8.854 Å and c = 12.672 Å. Moreover, no peaks of impurities were detected, indicating the high purity of the synthesized products. The microstructure of CH3NH3PbI3 crystals was also characterized with TEM (Fig. 1(b)), which further confirmed the regular tetragonal shape. The single-crystal characteristic was illustrated by SAED patterns in Fig. 1(c).
The concentration of precursor has great impact on the growth of crystals in solvothermal method. As shown in Fig. 2, the SEM images of CH3NH3PbI3 obtained at 130°C for 6 h with different amounts of Pb(Ac)2: 10 mg (a), 20 mg (b), 30 mg (c), 40 mg (d), 50 mg (e) and 60 mg (f) were performed. An interesting shape-evolution process from out-of-shape to concave-face cuboid is observed with increasing the concentration of Pb(Ac)2 from 10 mg to 60 mg. CH3NH3PbI3 crystals synthesized at 10 mg constitute irregular aggregates consisting of primary particles (Fig. 2(a)). With increasing the concentration to 20 mg, the some crystals appear cubic morphology (Fig. 2(b)). High-quality cuboid crystals are obtained with 30 mg Pb(Ac)2 as shown in Fig. 2(c). Moreover, it can be clearly observed that part of the cuboid crystals appear concave-faced cuboids at the centers of the (001) faces of the crystals with increasing the concentration from 40 mg to 60 mg (Fig. 2 (d)-2(f)), which correspond to the literature reports .
In order to understand the growing mechanism of CH3NH3PbI3 crystals, the microstructures of products obtained at two typical reacting temperatures for different reacting time were studied. As shown in Fig. 3, all of the products obtained at 130°C and 150°C exhibit hexahedron structures, and the microstructure of products obviously change from normal cuboid to concave-face cuboid prolonging the reacting time from 1 h to 15 h. Meanwhile, products obtained less than or equal to 6 h at 130°C show cuboid shape (Fig. 3(a), 3(b)) and more than 10 h show concave-face cuboid (Fig. 3(c) and 3(d)). This difference were found at 150°Cwith 6 h (Fig. 3(f)), and the degree of the concave become stronger by increasing reaction time as shown in Fig. 3(g) and 3(h). Therefore, reacting temperature has more influence on the morphologies than reacting time. The higher the temperature, the concave-face cuboid morphology of products occurs with shorter reacting time.
According to above experiment results, crystals will transfer from cuboid shape into concave-face cuboid morphology when increase the concentration of Pb(Ac)2, elevate the reaction temperature, or prolong the reaction time. In opinion of the Ichiro Sunagawa about micro-step morphology , the concave-face cuboid morphology is called “hopper” morphology and occurs at relatively high supersaturations which favor two-dimensional nucleation of steps and edges and corners of flat faces (F face ), because that supersaturation at the center of a face on growing crystal is much lower than that at the edges and corners, as so-called Berg effect. When the crystal is bounded by stepwisely depressed faces, the hopper crystal finally appears.
In order to prepare other organometal halide perovskite crystals by solvothermal method, HI were substituted with other acid including HBr and HCl as halogen sources. Figure 4 shows the XRD patterns of the products obtained at 150°C for 6 h with different acid: HI (a), HBr (b) and HCl (c). The series of peaks of sample in Fig. 1(a) can be indexed as tetragonal phase CH3NH3PbI3 as same as the results in Fig. 1. Moreover, all of peaks of samples in Fig. 1(b) and 1(c) can be clearly assigned to (100), (110), (200), (210), (220), and (300), and indexed the cubic phase CH3NH3PbBr3 and CH3NH3PbCl3 in accordance with the literatures [19-20]. Sharp and strong diffraction patterns indicated the fine crystallinity and high purity of all the obtained products with HBr (b) and HCl (c). However, we clearly found that the peaks position of CH3NH3PbCl3 slightly shift to high angle comparing with the peaks of CH3NH3PbBr3, indicating that crystals have different lattice or lattice plane spacing. These results illustrated that the solvothermal method is an effective process to synthesize organic-inorganic hybrid perovskite CH3NH3PbX3 (X = I, Br, Cl) crystals.
To better understand the effect of different acids on morphology and structure of crystals, the SEM images of products were displayed in Fig. 5. The CH3NH3PbI3 obtained with HI appear concave-face cuboid as shown in Fig. 5(a). In comparison, the CH3NH3PbBr3 and CH3NH3PbCl3 obtained with HBr and HCl shows smaller normal cuboid. The results indicated that the CH3NH3PbI3 crystals can be etched easily on surface center of cuboid comparing with CH3NH3PbBr3 and CH3NH3PbCl3 crystals, because impurities (undesirable byproducts and excess components like HI or CH3NH2) probably presented during crystal growth can result in concave-face growth .
In order to furtherly understand the optical characteristics of CH3NH3PbX3 (X = I, Br, Cl) crystals, Fig. 6 shows the PL spectra of products using 30 mW power of laser with a wavelength of 375 nm. The PL peaks of crystals obtained with HI, HBr and HCl for pure CH3NH3PbI3(a), CH3NH3PbBr3 (b) and CH3NH3PbCl3 (c) are 765.6 nm, 546.9 nm and 416.3 nm, corresponding to the bandgap (Eg) of about 1.62 eV, 2.27 eV and 2.98 eV, respectively. According to Yan and co-author reported , the intensity of peaks is related to the number of defects, and the position of peaks is related to the nature of defects. This experimental value Eg of CH3NH3PbI3 crystal is higher than the theoretical value of 1.5 eV [22-23], the blue shift of about tens of nanometers may be caused by different defects associated with the absorption edge, which probable are iodide defects. Moreover, the PL intensity is different for CH3NH3PbX3 (X = I, Br, Cl) crystals, because the intensity of peaks is related to the number of defects. In our experiment, we find a relatively weak PL emission intensity for CH3NH3PbI3 crystals in Fig. 6(a) ascribe to a low concentration of defects in the structure, and the CH3NH3PbCl3 crystals in Fig. 6(c) have a high concentration of defects comparing with CH3NH3PbBr3 crystals in the structure.
In summary, CH3NH3PbX3 (X = I, Br and Cl) crystals were synthesized via facile solvothermal method with different acid including HI, HBr and HCl as halogen sources. The XRD showed that CH3NH3PbI3 crystals exhibit the tetragonal phase structure and high crystal quality. CH3NH3PbBr3 and CH3NH3PbCl3 have cubic phase structure. The SEM displayed that morphology of the CH3NH3PbI3 crystals easily change from normal cuboids to concave-faced cuboid under higher concentration of precursor and higher temperature for shorter reacting time. Moreover, the PL spectra of CH3NH3PbI3 crystals shows a blue shift due to the nature of defects, and intensity of peaks of CH3NH3PbX3 (X = I, Br, Cl) crystals is different ascribe to the number of defects. These results demonstrated that our work may be offers a facile way to develop synthesis of organometal halide perovskite CH3NH3PbX3 (X = I, Br and Cl) crystals for various applications.
National Natural Science Foundation of China (NSFC) (21502007, 61604099); Science and Technology Personnel Training Project of Xinjiang Uygur Autonomous Region of China (qn2015bs014); Science and Technology Innovation Team Projects of Xinjiang Uygur Autonomous Region (2014751001, 2015BSQD001); Scientific Research Program of the Higher Education Institution of Xinjiang (XJEDU2016048); the Thirteenth Five-year Key Disciplines of Xinjiang Uygur Autonomous Region (Materials Science and Engineering); Natural Science Foundation of Jiangsu Province (BK20160883); the Zhejiang Provincial Natural Science Foundation of China (LY15E020002); and Scientific Research Fund of Sichuan Provincial Education Department (16ZA0245).
The authors declare that there are no conflicts of interest related to this article
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