Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Facile Method for the Controllable Synthesis of CsxPbyBrz‑Based Perovskites Jichao Zhu,†,‡ Qian Di,† Xixia Zhao,† Xiaotong Wu,† Xiaokun Fan,† Qian Li,† Weidong Song,†,‡ and Zewei Quan*,† †
Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen, Guangdong 518055, P. R. China School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China
‡
S Supporting Information *
perovskite laser can be enhanced.12 It was also reported that the coexistence of another derivative of CsPbBr3 perovskite, CsPb2Br5, can enhance its performance in optoelectronic devices.13 However, there are still limited understandings about the physical properties of these CsPbBr3 perovskite derivatives, which is due to the difficulty in the controlled synthesis of these high-quality CsPbBr3 perovskite derivatives. It is necessary to find a versatile method to selectively synthesize all of these CsPbBr3 perovskite derivatives in order to fully understand the relationships between the CsPbBr3 perovskite and these derivatives and figure out the intrinsic properties of these derivatives for their proper applications. Herein, we report a delicate and versatile synthetic approach to controllably preparing CsPbBr3 perovskite and its derivatives, Cs4PbBr6 and CsPb2Br5. All of these perovskite products can be readily synthesized through a facile colloidal method, simply by increasing the amount of N,N-dimethylformamide (DMF; refer to the Supporting Information for details). Furthermore, postsynthetic treatment of CsPbBr3 NCs with a DMF solvent is also used to confirm the intriguing role of DMF in the selective synthesis of these perovskite derivatives, uncovering the relationships between CsPbBr3 and its derivatives. When a small amount of DMF is used in the synthesis, monoclinic phase CsPbBr3 NCs can be obtained. Figure 1a shows the XRD patterns of CsPbBr3 NCs prepared with 0.1−1 mL of DMF, as well as the standard X-ray diffraction (XRD) pattern of monoclinic phase CsPbBr3 (ICSD 18-0364). It should be noted that the XRD patterns of the monoclinic phase CsPbBr3 are similar to those of the cubic phase CsPbBr3 (Figure S1). The main difference between the standard powder XRD patterns of monoclinic and cubic phase CsPbBr3 is the split of several diffraction peaks in monoclinic phase CsPbBr3. From the XRD patterns in Figure 1a, the presence of double peaks at ∼15.1° and 30.5° confirms that the products obtained in this synthesis should be indexed to the monoclinic phase CsPbBr3 rather than the cubic phase CsPbBr3. It is also obvious that these diffraction peaks gradually become narrower with an increase of the DMF amount from 0.1 to 1 mL, which indicates that the size of asprepared CsPbBr3 NCs becomes larger accordingly from 6 to 13 nm (Figure S2). The band gap of the bulk CsPbBr3 crystal is 2.30 eV with a characteristic fluorescence emission wavelength of 539 nm,14 and the exciton Bohr radius of CsPbBr3 is calculated to be
ABSTRACT: We report a facile method to realize the selective synthesis of CsxPbyBrz-based perovskites, including CsPbBr3, Cs4PbBr6 and CsPb2Br5. The use of an appropriate amount of N,N-dimethylformamide (DMF) solvent is experimentally determined to play a critical role in the controlled formation of various perovskite products. With continuously increasing DMF concentration, first CsPbBr3 nanocrystals with tunable size can be achieved, and then the production of Cs4PbBr6 and CsPb2Br5 perovskite analogues is successively realized. Our findings present a novel path for the controlled synthesis of other perovskite analogues for specific applications.
L
ead halide perovskites with the ABX3 formula have emerged as one novel class of functional materials because of their outstanding optical and electronic features.1 As for all-inorganic lead halide perovskites, they exhibit greatly improved stability at storage and operating conditions compared to hybrid perovskites and are promising for various applications such as solar cells,2 light-emitting diodes,3 lasers,4 photodetectors,5 memory devices,6 and photocatalysis.7 In recent years, a lot of research efforts have been focused on the structure modulation and/or band-gap engineering of these all-inorganic lead halide perovskites to achieve tunable optical and electronic properties. A series of currently available methods were developed for this purpose: the use of mixed halogens or cations or doping of other elements in perovskites to adjust the composition of the perovskite;3b,4,8 the synthesis of perovskite nanocrystals (NCs) with tunable dimensions to vary its band gap;2,3b,9 exposure of these perovskites to high pressure, inducing band-gap variations and phase transitions.10 Lead halide perovskite derivatives that have the same constituent elements but varied stoichiometric ratios compared to traditional ABX3 perovskites may possess different crystal symmetries and exhibit distinctly different chemical/physical properties. Therefore, the controlled synthesis of a series of novel all-inorganic lead halide perovskite derivatives is considered to be one alternative way to enhance the performance of current perovskite devices and/or explore their novel applications. CsPbBr3 with astonishing optical and electronic properties has been intensively studied in the form of a bulk solid, a thin film, and nanoparticles.3,11 Recent investigations reveal that, by embedding CsPbBr3 perovskite NCs in a Cs4PbBr6 perovskite derivative matrix, the thermal stability and device color purity of a © XXXX American Chemical Society
Received: March 15, 2018
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DOI: 10.1021/acs.inorgchem.8b00645 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
usually achieved with a high Cs+/Pb2+ molar ratio in precursors or a postsynthetic treatment with appropriate ligands.15 The formation of the Cs4PbBr6 derivative in a lead-rich environment (Pb2+/Cs+: 3/1) is believed to arise from the specific role of the DMF solvent during the synthesis. During these control experiments, the only variable is the amount of DMF that changes from 0.1 to 4 mL, while other experimental conditions are kept identical. It is reasonable to speculate that DMF is playing a critical role in the formation of different perovskite products. DMF has an oxygen group to readily coordinate with Pb2+ ions, therefore inhibiting the bromide coordination and stalling the crystallization of the CsPbBr3 perovskite and its derivative. As for the synthesis of CsPbBr3 perovskite NCs, the number of DMF ligands coordinating with Pb2+ ions increases with an increase in the DMF amount from 0.1 to 1 mL, and therefore the initial precipitation reaction to form CsPbBr3 nuclei is increasingly hindered, resulting in the formation of a smaller number of CsPbBr3 nuclei. In this case, there are more Pb2+ species available in solution to grow on these as-produced nuclei, leading to an increase in the sizes of CsPbBr3 NCs. When there is more DMF solvent (>1 mL) in the system, a higher percentage of Pb2+ ions are coordinated with DMF in this case, and less Pb2+ species in solution can easily react with relatively abundant Cs+ ions, resulting in the crystallization of a cesium-rich derivative: hexagonal Cs4PbBr6. The formation of these two perovskite products is illustrated in Scheme 1a,b. Additionally, the sizes of
Figure 1. (a) XRD spectra of pure monoclinic phase CsPbBr3 NCs synthesized with different amounts of DMF. (b) TEM image of CsPbBr3 NCs synthesized with 0.4 mL of DMF at different magnifications. (c) Emission spectra and (d) photographs of CsPbBr3 NCs synthesized with different amounts of DMF (from left to right: 0.1 to 1 mL) under ambient and 365 nm excitation, respectively.
around 7 nm.9b Therefore, all of these as-prepared CsPbBr3 NCs can exhibit a quantum confinement effect to some extent (Figure S3), and their photoluminescence (PL) emission wavelength can be tuned from 505 to 520 nm with an increase of the DMF amount (Figure 1c,d). As the DMF amount continues to increase above 1 mL, instead of bigger CsPbBr3 perovskite NCs, a new set of diffraction peaks begin to appear in the XRD patterns of the final products and the relative intensities of the diffraction peaks from the monoclinic phase CsPbBr3 decrease (Figure S4a,b). These new diffraction peaks can be indexed to one derivative of the CsPbBr3 perovskite, hexagonal Cs4PbBr6. When 4 mL of DMF is adopted during the synthesis, almost pure Cs4PbBr6 NCs with an average edge length of 50 nm could be successfully prepared (Figures 2 and S5). In previous experiments, the synthesis of Cs4PbBr6 was
Scheme 1. Proposed Growth Process of CsPbBr3, Cs4PbBr6, and CsPb2Br5 Perovskites Obtained with Different Amounts of DMF
the Cs4PbBr6 NCs continue to increase for the same reason, as mentioned above. It is experimentally determined that 4 mL of DMF is the optimum content to yield a pure phase Cs4PbBr6 perovskite derivative. These as-synthesized Cs4PbBr6 NCs are confirmed to be PL-inactive (Figure S4a). Interestingly, when the DMF amount is beyond 4 mL, an entirely different phenomenon appears. CsPb2Br5, with a tetragonal structure that is another derivative of CsPbBr3, can be produced rather than hexagonal Cs4PbBr6. With a continuous increase in the DMF amount, the percentage of the CsPb2Br5 phase in the final product is accordingly increased, and pure CsPb2Br5 is obtained when using more than 10 mL of DMF (Figures 2a and S4c). The formation of the CsPb2Br5 phase is due to the nucleation and subsequent crystallization of perovskites in a lead-rich environment, although the amounts of all precursors are kept identical in these control experiments.
Figure 2. (a) XRD patterns of perovskites synthesized with 0.1, 4, and 15 mL of DMF. The patterns for monoclinic phase CsPbBr3 with ICSD 18-0364, hexagonal Cs4PbBr6 with ICSD 73-2478, and tetragonal CsPb2Br5 with ICSD 25-0211 are provided for reference. (b) Structural models of monoclinic phase CsPbBr3, hexagonal Cs4PbBr6, and tetragonal CsPb2Br5 perovskites. B
DOI: 10.1021/acs.inorgchem.8b00645 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
investigate the relationships of the CsPbBr3 perovskite and its derivatives but also sheds new light on the precisely controlled synthesis of other perovskite analogues.
Such a lead-rich environment is believed to originate from the different coordination effects of DMF toward both Cs+ and Pb2+ cations. According to the hard−soft acid−base theory, DMF is a hard base and therefore prefers to coordinate with hard acid (Cs+) compared to the borderline Lewis acid (Pb2+).16 Therefore, when there is abundant DMF present (>4 mL) in the solution, all Pb2+ ions are saturatedly coordinated with DMF, and excess DMF molecules would readily coordinate to Cs+ ions as soon as a cesium oleate precursor solution is injected. As a consequence, coordinated Pb2+ and Cs+ ions follow a different precipitation reaction pathway, in which the formation of CsPb2Br5 NCs is favored in an original lead-rich phase (the raw material ratio of Pb2+/Cs+ is about 3:1). Scheme 1c illustrates the possible growth mechanism of CsPb2Br5 NCs when using an abundant DMF solvent. Similar to the tendency in the preparation of CsPbBr3 and Cs4PbBr6 NCs, the sizes of CsPb2Br5 NCs become further larger with an increase in the DMF amount. As shown in Figure S5d−f, pure phase CsPb2Br5 crystals obtained with 15 mL of DMF have an average width of 80 nm and length of 150 nm, which are composed of multiple tiny CsPb2Br5 domains. These as-prepared CsPb2Br5 crystals are also determined to be PL-inactive (Figure S4a). Fourier transform infrared (FTIR) spectra of three representative perovskite products are shown in Figure S6. It is clearly shown that the characteristic stretching vibrations of CO (1658 cm−1) and C−N (1417 cm−1) of DMF are detected in all spectra, together with the characteristic stretching vibration of C−N (1340 and 1070 cm−1) and bending vibration of N−H (1633 and 725 cm−1) of OAm and the characteristic stretching vibrations of CO (1710 cm−1) and C−O (1282 cm−1) and bending vibration of O−H (939 cm−1) of OA. Such results confirm that, similar to versatile OAm and OA ligands, DMF is actively involved in the precipitation reaction to produce CsPbBr3, Cs4PbBr6, and CsPb2Br5 and is finally capped on the surface of these perovskite NCs. In order to support the mechanism proposed above, we further demonstrate the postsynthetic modification of as-prepared CsPbBr3 NCs by using a DMF solvent. When DMF is introduced into a CsPbBr3 NC solution, color changes from green to transparent/turbid could be immediately observed, and even a very small amount of DMF can induce the transformation from CsPbBr3 to CsPb2Br5 with high crystallinity (Figure S7). When the amount of DMF reaches up to 0.3 mL, pure CsPb2Br5 NCs can be obtained, as shown in the XRD patterns (Figure S8). Meanwhile, the FTIR spectrum of the final CsPb2Br5 NCs (Figure S6) also shows the characteristic stretching vibrations of CO (1658 cm−1) and C−N (1417 cm−1) of DMF. These postsynthetic treatments indicate that the presence of DMF favors the formation of a lead-rich (or cesium-deficient) CsPb2Br5 phase, which is consistent with the direct chemical synthesis of CsPb2Br5 NCs in the presence of excess DMF. Such a transformation is believed to originate from the preferential coordination of DMF to Cs+ ions compared to Pb2+ ions as mentioned above, resulting in the recrystallization of a lead-rich CsPb2Br5 perovskite derivative. In conclusion, we have developed a facile approach to selectively synthesize CsPbBr3, Cs4PbBr6, and CsPb2Br5 perovskites and also realized the size-controllable synthesis of CsPbBr3 NCs by only adjusting the amount of DMF. The intriguing role of DMF during this synthesis is attributed to the coordination ability of DMF with Cs+ and Pb2+ ions to construct different reaction conditions at varied amounts of DMF. More importantly, this method not only provides a new path to
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00645. Detailed experimental procedures, XRD patterns, sizedistribution histograms, UV−vis absorption spectra, Tauc plots, TEM images, and FTIR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qian Li: 0000-0002-4847-4892 Zewei Quan: 0000-0003-1998-5527 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 11604141 and 51772142), Shenzhen Science and Technology Innovation Committee (Grants JCYJ20170412152528921 and JCYJ20160530190842589), Development and Reform Commission of Shenzhen Municipality (Novel Nanomaterial Discipline Construction Plan), and start-up and Presidential funds from SUSTech. This work was also supported by the Pico Center at SUSTech, which receives support from the Presidential fund and Development and Reform Commission of Shenzhen Municipality.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b00645 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
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DOI: 10.1021/acs.inorgchem.8b00645 Inorg. Chem. XXXX, XXX, XXX−XXX
