Large-Scale Ligand-Free Synthesis of Homogeneous Core

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Large-Scale Ligand-Free Synthesis of Homogeneous Core−Shell Quantum-Dot-Modified Cs4PbBr6 Microcrystals Xiangfeng Wei,† Jiehua Liu,*,†,‡ Han Liu,† Xunyong Lei,§ Haisheng Qian,† Hualing Zeng,§ Fancheng Meng,† and Weiqiao Deng⊥

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†

Future Energy Laboratory, School of Materials Science and Engineering, School of Food and Biological Engineering, Hefei University of Technology, Hefei 230009, China § Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ⊥ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023 China ‡ Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei 230009, China S Supporting Information *

Cs4PbBr6 microcrystals also exhibited good PLQYs because of their shapes and dimensions.28,35,36 Samanta et al. reported an ambient-condition-controlled synthesis of Cs4PbBr6 microdisks, which exhibited strong green PLQYs up to 38% using cetyltrimethylammonium bromide as the structure-inducing agent.35 The PL intensity and lifetime distribution were attributed to trap-mediated nonradiative deactivation at the edges. Pure rhombohedral Cs4PbBr6 microdisks were developed by a one-step self-assembly solution method with a high PLQY of 52% Cs4PbBr6 and a full-width at half-maximum of 16.8 nm.28 Interestingly, Cs4PbBr6 single crystals exhibited PLQYs of higher than 40% and were investigated for the origins of their unique optical and electronic features by PL studies and density functional theory calculations.37 Very recently, nanosized CsPbBr3-embedded Cs4PbBr6 crystals with high PLQYs were grown by a HBr-assisted slow-cooling method.24 Cs4PbBr6 QDs exhibit a strong green PL performance, but high-boiling-point surfactants are hardly removed adequately (Scheme 1a). Cs4PbBr6 microcrystals and single crystals, which are more stable than QDs, also exhibit strong PLQYs, but thicker films are needed to obtain a good performance because their light absorbance is weaker than that of QDs (Scheme 1b). A proposed route is to synthesize Cs4PbBr6 with features of QDs and microcrystals (Scheme 1c) without organic structure-inducing agents. The desired QD-modified microcrystals may improve the light trapping and PL performance (Scheme S1). In this work, we present a chemical transformation and an organic ligand-free method to obtain homogeneous QDs (shell) joined with Cs4PbBr6 microcrystals (core) on a large scale. Cs4PbBr6 exhibits a high PLQY of 76%, higher than those of the most reported Cs4PbBr6 QDs and micro/nanocrystals. The QD-modified Cs4PbBr6 microcrystals were synthesized via a green ligand-free rapid-supersaturation route using NH4Br as a solubilizer and inducing agent at room temperature (Figure S1). The details are provided in the Supporting

ABSTRACT: An organic ligand-free solution method is developed for preparing homogeneous core−shell quantum-dot (QD)-modified pure Cs4PbBr6 microcrystals on a large scale (∼12 g) at room temperature. The ligand-free Cs4PbBr6 microcrystals show a high green photoluminescence quantum yield of 76% with 360 nm of excitation light, which is attributed to their unique microarchitecture, with several features including quantum confinement of the outer QDs, stability of the inner Cs4PbBr6 microcrystals, improved light trapping, and interfacial recombination. UV−vis−near-IR and photoluminescence analyses provide valued evidence to support the ligand-free Cs4PbBr6 with synergy between the QDs and microcrystals.

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ll inorganic perovskite materials are exciting subjects in optical,1−4 optoelectronics,5,6 and photovoltaic applications7−10 because of their tunable band gap, high photoluminescence quantum yield (PLQY), and optoelectronic features. Nanoarchitectures have provided strong power to accelerate the development of perovskite-based functional nanomaterials for advanced photoluminescence (PL) or optoelectronic devices in recent years. 11−14 Ultrafine CsnPbX2+n nanocrystals and quantum dots (QDs) can be easily synthesized by chemical and phase transformation methods, such as anion-exchange reactions,15,16 cationexchange reactions,17,18 ligand-mediated transformation.19,20 and other methods.21 Zero-dimensional Cs4PbBr6 as a lowcontent lead-based perovskite has demonstrated a good PLQY for both single crystals and powders.22−25 A higher PLQY of 65% was achieved by an ultrafine Cs4PbBr6 colloidal solution, which was synthesized by using an oil−N,N-dimethylformamide microemulsion with the aid of oleic acid and oleylamine.26 In general, high-purity and ultrafine Cs4PbBr6 nanomaterials exhibit higher PLQYs than their hybrids with impurity.27−29 However, organic ligands are hardly avoided and removed in the synthesis of ultrafine Cs4PbBr6 nanocrystals and QDs.29−34 © XXXX American Chemical Society

Received: July 4, 2019

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DOI: 10.1021/acs.inorgchem.9b01980 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

characterized using in situ X-ray diffraction (XRD) measurements. Figure 1b shows the powder XRD patterns of the samples obtained at 10, 20, and 60 min. The XRD patterns of the samples obtained in 60 min match well with the reported works.35 All of the peaks of the sample are well indexed to trigonal Cs4PbBr6 crystals (JCPDS 73-2478). That is, pure Cs4PbBr6 could be obtained in 1 h on a large scale of 12 g. By comparison, the XRD patterns of the samples obtained at 10 and 20 min show smaller lattices than that of the sample obtained at 60 min (Figure S3). We think the white Cs4−x(NH4)xPbBr6 was first formed by coprecipitation when CHCl3 assisted rapid supersaturation (eq 1). Because NH4Br has a high solubility in the above solution, the high-purity green Cs4PbBr6 could be further obtained by ion exchange of NH4+ by Cs+, which has a radius of 183 pm, larger than the 175 pm of NH4+ (eq 2). As a comparison task, the reference yellow Cs4PbBr6 sample was obtained by using oleic acid as a structure-inducing agent to replace NH4Br, and the XRD pattern shown is also trigonal Cs4PbBr6 crystals (Figure S4).

Scheme 1. (a) QDs with Organic Ligands, (b) a Microcrystal, and (c) the Proposed Organic Ligand-Free QD-Modified Cs4PbBr6 Microcrystal

PbBr2 + (4 − x)CsBr + x NH4Br → Cs4 − x (NH4)x PbBr6

Information. The photographs of the samples show the color changes at different reaction times in Figures 1a and S2. The color change from a white intermediate to a green product took place via a chemical transformation in the course of the reaction system. To illustrate the role of NH4Br, the samples were obtained at different reaction times and further

(1)

Cs4 − x(NH4)x PbBr6 + xCsBr → Cs4PbBr6 + x NH4Br (2)

UV−vis−near-IR (NIR) absorption spectra (Figure 1c) indicate that the green Cs4PbBr6 and reference materials have a favorable absorption capability. The UV−vis−NIR spectrum of green Cs4PbBr6 is improved significantly compared to that of the reference material. It should be noted that it is more than 144% higher than the light absorption of the reference material from 350 to 520 nm, as shown in the inset of Figure 1c. The core−shell Cs4PbBr6 can be used as the active layer for greenemitting materials because of the improved light trapping. Parts a and b of Figure 2 show the field-emission scanning electron microscopy (FESEM) images of QD-modified

Figure 1. (a) Chemical-transformation images of samples obtained at different times (5−60 min). (b) In situ XRD patterns of the obtained samples with different reaction times of 10, 20, and 60 min. (c) UV− vis−near-IR spectra of green and reference yellow Cs4PbBr6 samples.

Figure 2. FESEM images of (a and b) QD (shell)-modified Cs4PbBr6 microcrystals and (c) oleic acid-induced Cs4PbBr6 microcrystals. (d) TEM image of QD (shell)-modified Cs4PbBr6 microcrystals. (e) Proposed formation mechanism of a core−shell Cs4PbBr6. B

DOI: 10.1021/acs.inorgchem.9b01980 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

317 K, as shown in Figure 3b. The PL spectra of Cs4PbBr6 not only exhibit a temperature-sensitive reducing intensity but also show a small temperature-dependent red shift of emission peaks with increasing temperature. The exciton binding energy was fitted using eq 3:22

Cs4PbBr6 microcrystals. In Figure 2a, the hexahedral Cs4PbBr6 microcrystals have sizes of 0.3−1.2 μm with a rough surface. The enlarged FESEM image shows ultrafine nanoparticles with sizes of 15−25 nm located on the surface of Cs4PbBr6 microcrystals in Figure 2b. QD-modified Cs4PbBr6 microcrystals coincide with our proposed structure (the inset of Figure 2b). In contrast, Cs4PbBr6 microcrystals with a smooth surface were obtained by replacing NH4Br with oleic acid as the ligand, as shown in Figure 2c. Transmission electron microscopy (TEM) also supports Cs4PbBr6 QDs on the surface of Cs4PbBr6 microcrystals (Figure 2d). We proposed the formation mechanism of core−shell Cs4PbBr6 in Figure 2e. Cs4PbBr6 QDs (outer) are formed by the defects as crystal seeds on the crystal surface, which are caused by volume swelling in chemical transformation. The X-ray photoelectron spectroscopy (XPS) survey spectrum of Cs4PbBr6 shows that the major components include Cs, Pb, and Br elements (Figure S5a). Their elemental ratios are close to 4:1:6, indicating that Cs4PbBr6 has high purity. Figure S5b shows that no XPS N 1s peak at 390−400 eV is detected, which is also a proof to support the high-purity analysis. The high-resolution XPS spectra of Cs 3d, Pb 4f, and Br 3d are provided in Figure S5c−e. No obvious change was observed for the binding energies of Cs 3d and Pb 4f. The peaks at 724.13 and 738.13 eV can be attributed to Cs 3d5/2 and Cs 3d3/2. The Pb 4f XPS spectrum also shows two peaks at 138.30 and 143.18 eV, which are assigned to Pb 4f7/2 and 4f5/2. In addition, the Br 3d5/2 peak can also be fitted into two peaks with binding energies of 68.13 and 69.16 eV for Cs−Br and Pb−Br, which have the corresponding area ratio of 2:1 shown in Figure S5e. Figure S6 shows that core−shell Cs4PbBr6 has a wide excitation spectrum with UV−vis light from 330 to 520 nm. Figure 3a presents that the core−shell Cs4PbBr6 perovskite has a steady-state PL peak centered at 520 nm at different wavelengths of 350−480 nm. To further explore the PL mechanism of core−shell Cs4PbBr6, temperature-dependent PL spectra were provided at temperatures ranging from 77 to

IT =

I0

( ) E

1 + A exp − k TB B

(3)

where IT is the integrated intensity at different temperatures (T), EB is the binding energy, and kB is the Boltzmann constant. Figure S7 shows that the corresponding exciton binding energy is ∼202 meV, which was calculated based on the integrated PL intensity by eq 3, indicating that exciton recombination is the key factor for PL behavior of Cs4PbBr6.38 Interestingly, Figure 3c shows that the emission peak has a significant shoulder peak and a main peak at 503 and 519 nm, which may be ascribed to QDs and microcrystals at 97 K. Time-resolved photoluminescence (TRPL) spectra are used to investigate the carrier dynamics changes of the core−shell Cs4PbBr6 and reference samples. Figure S8 shows that the exciton decay is nonlinear kinetics. The average lifetimes (τ1/e) are 0.65 and 0.83 ns for the core−shell Cs4PbBr6 and reference samples, respectively. τ1/e of the reference is close to the reported result.39 The shortened lifetime may be contributed by the fresh surface and rich interfacial defects rather than organic-ligand passivation. PLQYs of Cs4PbBr6 powders were obtained by using a spectrofluorometer equipped with an integrated sphere under an excitation light of 360 nm. A high PLQY of 76% was obtained for the core−shell Cs4PbBr6 powder, which is higher than those of QDs and microsheets in the most reported works.6,34,40,41 After storage for 1 year in dry air, it still has a PLQY of ∼70%, indicating that the core−shell microstructure has superior stability. As a comparison, the oleic acid-induced Cs4PbBr6 sample, which was synthesized by replacing NH4Br with oleic acid, has a PLQY of 56%, close to the reported result.26 If more NH4Br is added (twice), the as-synthesized sample has a PLQY of 72%, lower than the core−shell Cs4PbBr6 powder obtained at a low content of NH4Br but higher than the oleic acid-induced Cs4PbBr6 sample. The exciton migration in Cs4PbBr6 is due to tunneling and Dexter energy transfer and leads to nonradiative recombination at the intrinsic defects.42 We think the unique core−shell structure contributes to the enhanced PLQY owing to the unique core− shell microstructure with light trapping. Furthermore, Figure S9 shows that the core−shell Cs4PbBr6 has high green saturation at 360, 400, 440, and 480 nm, which can be used in the light-emitting devices and information encryption/decryption.39,43−45 We labeled the leaves and patterns with core−shell Cs4PbBr6, as shown in Figures 3d and S10. Figure 3d shows that the biomimetic images of a Cs4PbBr6-modified leaf can be clearly recognized, while the fresh leaf without Cs4PbBr6 becomes very vague under UV light at 365 nm. Cs4PbBr6-modified patterns can be displayed clearly under indoor light, indoor and UV light, and UV light only (Figure S10). Therefore, we think that the Cs4PbBr6 samples can be used as the green luminous coating for energyefficient smart buildings and traffic signs (Figure S11) as well as light-emitting devices. In summary, we have successfully prepared homogeneous core−shell QDs joined with Cs4PbBr6 microcrystals on a large

Figure 3. (a) Excitation-dependent 3D PL spectra at wavelengths of 350−480 nm. (b) Temperature-dependent 3D PL spectra from 77 to 317 K. (c) PL spectrum with a wavelength of 405 nm at 97 K. (d) Photographs of leaves with/without Cs4PbBr6 coating under indoor and UV light (365 nm), respectively. C

DOI: 10.1021/acs.inorgchem.9b01980 Inorg. Chem. XXXX, XXX, XXX−XXX

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(8) Li, B.; Zhang, Y. A.; Fu, L.; Yu, T.; Zhou, S. J.; Zhang, L. Y.; Yin, L. W. Surface Passivation Engineering Strategy to Fully-Inorganic Cubic CsPbI3 Perovskites for High-Performance Solar Cells. Nat. Commun. 2018, 9, 1076. (9) Hoffman, J. B.; Zaiats, G.; Wappes, I.; Kamat, P. V. CsPbBr3 Solar Cells: Controlled Film Growth through Layer-by-Layer Quantum Dot Deposition. Chem. Mater. 2017, 29 (22), 9767−9774. (10) Panigrahi, S.; Jana, S.; Calmeiro, T.; Nunes, D.; Martins, R.; Fortunato, E. Imaging the Anomalous Charge Distribution Inside CsPbBr3 Perovskite Quantum Dots Sensitized Solar Cells. ACS Nano 2017, 11 (10), 10214−10221. (11) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal Halide Perovskites For Energy Applications. Nat. Energy 2016, 1, 16048. (12) Riedinger, A.; Ott, F. D.; Mule, A.; Mazzotti, S.; Knüsel, P. N.; Kress, S. J. P.; Prins, F.; Erwin, S. C.; Norris, D. J. An Intrinsic Growth Instability in Isotropic Materials Leads to Quasi-Two-Dimensional Nanoplatelets. Nat. Mater. 2017, 16, 743−748. (13) Pan, W.; Wu, H.; Luo, J.; Deng, Z.; Ge, C.; Chen, C.; Jiang, X.; Yin, W.-J.; Niu, G.; Zhu, L.; Yin, L.; Zhou, Y.; Xie, Q.; Ke, X.; Sui, M.; Tang, J. Cs2AgBiBr6 Single-Crystal X-Ray Detectors with a Low Detection Limit. Nat. Photonics 2017, 11 (11), 726−732. (14) Huang, H.; Polavarapu, L.; Sichert, J. A.; Susha, A. S.; Urban, A. S.; Rogach, A. L. Colloidal Lead Halide Perovskite Nanocrystals: Synthesis, Optical Properties and Applications. NPG Asia Mater. 2016, 8 (11), No. e328. (15) Akkerman, Q. A.; D’Innocenzo, V.; Accornero, S.; Scarpellini, A.; Petrozza, A.; Prato, M.; Manna, L. Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions. J. Am. Chem. Soc. 2015, 137 (32), 10276−10281. (16) Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M. J.; Kovalenko, M. V. Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). Nano Lett. 2015, 15 (8), 5635−5640. (17) van der Stam, W.; Geuchies, J. J.; Altantzis, T.; van den Bos, K. H. W.; Meeldijk, J. D.; Van Aert, S.; Bals, S.; Vanmaekelbergh, D.; de Mello Donega, C. Highly Emissive Divalent-Ion-Doped Colloidal CsPb1−xMxBr3 Perovskite Nanocrystals through Cation Exchange. J. Am. Chem. Soc. 2017, 139 (11), 4087−4097. (18) Huang, G.; Wang, C.; Xu, S.; Zong, S.; Lu, J.; Wang, Z.; Lu, C.; Cui, Y. Postsynthetic Doping of MnCl2 Molecules into Preformed CsPbBr3 Perovskite Nanocrystals via a Halide Exchange-Driven Cation Exchange. Adv. Mater. 2017, 29 (29), 1700095. (19) Liu, Z.; Bekenstein, Y.; Ye, X.; Nguyen, S. C.; Swabeck, J.; Zhang, D.; Lee, S.-T.; Yang, P.; Ma, W.; Alivisatos, A. P. Ligand Mediated Transformation of Cesium Lead Bromide Perovskite Nanocrystals to Lead Depleted Cs4PbBr6 Nanocrystals. J. Am. Chem. Soc. 2017, 139 (15), 5309−5312. (20) Zhu, J. C.; Di, Q.; Zhao, X. X.; Wu, X. T.; Fan, X.; Li, Q.; Song, W. D.; Quan, Z. W. Facile Method for the Controllable Synthesis of CsxPbyBrz-Based Perovskites. Inorg. Chem. 2018, 57 (11), 6206−6209. (21) Bao, Z.; Wang, H. C.; Jiang, Z. F.; Chung, R. J.; Liu, R. S. Continuous Synthesis of Highly Stable Cs4PbBr6 Perovskite Microcrystals by a Microfluidic System and Their Application in WhiteLight-Emitting Diodes. Inorg. Chem. 2018, 57 (21), 13071−13074. (22) Saidaminov, M. I.; Almutlaq, J.; Sarmah, S.; Dursun, I.; Zhumekenov, A. A.; Begum, R.; Pan, J.; Cho, N.; Mohammed, O. F.; Bakr, O. M. Pure Cs4PbBr6: Highly Luminescent Zero-Dimensional Perovskite Solids. ACS Energy Lett. 2016, 1 (4), 840−845. (23) Chang, S.; Bai, Z.; Zhong, H. In Situ Fabricated Perovskite Nanocrystals: A Revolution in Optical Materials. Adv. Opt. Mater. 2018, 6 (18), 1800380. (24) Chen, X.; Zhang, F.; Ge, Y.; Shi, L.; Huang, S.; Tang, J.; Lv, Z.; Zhang, L.; Zou, B.; Zhong, H. Centimeter-Sized Cs4PbBr6 Crystals with Embedded CsPbBr3 Nanocrystals Showing Superior Photoluminescence: Nonstoichiometry Induced Transformation and LightEmitting Applications. Adv. Funct. Mater. 2018, 28 (16), 1706567. (25) Almutlaq, J.; Yin, J.; Mohammed, O. F.; Bakr, O. M. The Benefit and Challenges of Zero-Dimensional Perovskites. J. Phys. Chem. Lett. 2018, 9 (14), 4131−4138.

scale by a facile ligand-free method. Cs4PbBr6 has strong green luminescence with a PLQY of 76% owing to improved light trapping by the synergy of quantum confinement of the QDs and stability of Cs4PbBr6 microcrystals. Importantly, the Cs4PbBr6 product also exhibits excellent stability. We believe that core−shell Cs4PbBr6 is a desired green-emitting material for biomimetic images, energy-efficient smart buildings, and traffic signs when matched with other luminescent materials.

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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.9b01980.

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Experimental details, photographs, XRD patterns, PL intensities, TRPL spectra, and photographs of the school badge and traffic sign (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.). ORCID

Jiehua Liu: 0000-0003-4958-5137 Haisheng Qian: 0000-0003-4903-3447 Hualing Zeng: 0000-0001-5869-9553 Weiqiao Deng: 0000-0002-3671-5951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants U1832136 and 21303038) and the Fundamental Research Fund for the Central Universities (Grants WK2340000082 and JZ2016HGTA0690).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b01980 Inorg. Chem. XXXX, XXX, XXX−XXX