1. Introduction

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% [5] to more than 20% [6] in the past years. Moreover, they also have potential applications in many other fields such as light-emitting devices (LEDs) [7], lasers [8], photodetectors [9], and so on.

Until now, CH₃NH₃PbX₃ (X = I, Br and Cl) materials have been prepared by several methods: spin coating [10], thermal evaporation [11], solid-state reaction [12], and intramolecular exchanging crystallization [6]. 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 [13]. 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 [14]. Subsequently, Zhao et al. [15] demonstrated a facile synthetic approach for preparing mixed halide perovskite crystals by the solvothermal growth. Recently, Zhang et al. [16] 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).

 

Fig. 1 XRD pattern (a), TEM image (b) and SAED pattern (c) of products obtained at 130°C for 6 h.

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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 [16].

 

Fig. 2 SEM images of products 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), respectively.

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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.

 

Fig. 3 SEM images of the products obtained with HI at 130°C ((a)-(d)) and 150 °C ((e)-(h)) for different reaction time: 1 h (a) and (e), 6 h (b) and (f), 10 h (c) and (g), 15 h (d) and (h), respectively.

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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 [17], 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 [18]), 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.

 

Fig. 4 XRD patterns of products obtained at 150°C for 6 h with different halogen sources: HI (a), HBr (b) and HCl (c).

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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 [16].

 

Fig. 5 SEM images of products obtained at 150°Cfor 6 h with different halogen sources: HI (a), HBr (b) and HCl (c).

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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 (E_g) of about 1.62 eV, 2.27 eV and 2.98 eV, respectively. According to Yan and co-author reported [21], 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 E_g 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.

 

Fig. 6 Photoluminescence spectra of CH₃NH₃PbI₃ (a), CH₃NH₃PbBr₃ (b) and CH₃NH₃PbCl₃ (c) crystals obtained at 150°C for 6 h with different halogen sources.

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4. Conclusions

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.