From Nonluminescent Cs4PbX6 (X = Cl, Br, I) Nanocrystals to Highly

California 92521 United States. Nano Lett. , 2017, 17 (9), pp 5799–5804. DOI: 10.1021/acs.nanolett.7b02896. Publication Date (Web): August 14, 2...
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From Nonluminescent Cs4PbX6 (X = Cl, Br, I) Nanocrystals to Highly Luminescent CsPbX3 Nanocrystals: Water-Triggered Transformation through a CsX-Stripping Mechanism Linzhong Wu,† Huicheng Hu,† Yong Xu,†,‡ Shu Jiang,† Min Chen,† Qixuan Zhong,† Di Yang,† Qipeng Liu,† Yun Zhao,† Baoquan Sun,*,† Qiao Zhang,*,† and Yadong Yin*,‡ †

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu People’s Republic of China ‡ Department of Chemistry, University of California, Riverside, Riverside, California 92521 United States S Supporting Information *

ABSTRACT: We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent Cs4PbX6 (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX3 NCs. During the transformation, Cs4PbX6 NCs dispersed in a nonpolar solvent are converted into CsPbX3 NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of Cs4PbX6 NCs, and produces monodisperse and air-stable CsPbX3 NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as Cs4PbX6 NCs are converted into CsPbX3 NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX3 NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX3 NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals. KEYWORDS: Cesium lead halide, nanocrystal, chemical transformation, Cs4PbX6, CsPbX3, perovskite

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shaped nanoparticles that are usually difficult to be obtained by traditional direct-synthesis method.25−28 For example, CsPbX3 NCs with different halide composition can be prepared by an ion-exchange method.7,29 Recently, the chemical transformation between nonluminescent Cs4PbX6 and luminescent CsPbX3 has also been explored. Manna group converted Cs4PbBr6 NCs to CsPbBr3 NCs by treating presynthesized Cs4PbBr6 NCs with excess PbBr2.22,24 Alivisatos group reported that CsPbBr3 NCs could be converted to Cs4PbBr6 NCs through a “ligand-mediated transformation” process in the presence of oleylamine.21 In these previous reports, Cs4PbBr6 NCs were regarded as a PbBr2-deficient material, and the preparations were conducted in nonpolar solvents. In this work, we propose a CsX-stripping process that enables the preparation of monodisperse CsPbX3 NCs from nonluminescent Cs4PbX6 nanocrystalline precursors. Unlike previous reports, here we regard Cs4PbX6 as a CsX-rich structure with high ion-diffusion property. Because of the high solubility of CsX in water and the interface between water and

ll-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have been regarded as emerging materials in diverse fields due to their outstanding photophysical properties, such as high photoluminescence quantum yield,1 narrow emission width, and tunable band gap that covers the full visible range.2−4 Since the pioneering work by the Kovalenko group in 2015, considerable progress in preparation and application of CsPbX3 NCs has been made in a very short period.5 For example, CsPbX3 NCs with controllable shape and composition have been prepared by different methods, such as hot-injection,5−7 solvothermal,8 ultrasonication,9 room-temperature precipitation,10 and chemical vapor deposition (CVD).11 The prepared CsPbX3 NCs have already been utilized for many potential applications, including photovoltaics,10,12 lasing,13−15 light-emitting diode,16−18 and photodetector.19,20 In addition to CsPbX3 NCs, other types of cesium lead halide NCs, such as Cs4PbBr6 and CsPb2X5, have also gained considerable attention.21−24 While most existing methods to CsPbX3 NCs involve direct synthesis, some efforts have been made to prepare such nanocrystals through the chemical transformation of presynthesized NCs. As an effective and versatile tool, chemical transformation could be used for the preparation of variously © 2017 American Chemical Society

Received: July 8, 2017 Revised: August 10, 2017 Published: August 14, 2017 5799

DOI: 10.1021/acs.nanolett.7b02896 Nano Lett. 2017, 17, 5799−5804

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Figure 1. (a) Photographs showing the transformation process of Cs4PbBr6 NCs to CsPbBr3 NCs. The samples were illuminated with UV light (λ = 365 nm). In situ monitoring of (b) absorption and (c) emission spectra (λexc = 400 nm) during the reaction process. (d,e) Representative TEM images of (d) Cs4PbBr6 NCs and (e) CsPbBr3 NCs. Insets in (d,e) show the corresponding HRTEM image, respectively.

Figure 2. (a) Schematic illustration of crystal structure change and transformation process from Cs4PbX6 to CsPbX3 after water treatment. XRD patterns of (b) original Cs4PbBr6 NCs (blue line) and product in the top hexane part (green line) and (c) product in the bottom water part (red line). The standard XRD patterns of Cs4PbBr6, CsPbBr3, and CsBr are also shown in (b,c). 5800

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rhombohedral phase (JCPDS card No. 73-2478). After water treatment, the characteristic peaks of Cs4PbBr6 disappear gradually, and some new peaks become more obvious (Figure S4). The final product can be indexed as pure cubic CsPbBr3 perovskite phase (JCPDS card No. 54-0752).The hexane part was removed after the reaction, and the aqueous solution at the bottom was then evaporated, leaving behind some white crystals. The XRD pattern of the white crystal is depicted in Figure 2c, demonstrating a major composition of cubic phase CsBr (JCPDS card No. 5-588) and some residue of rhombohedral Cs4PbBr6. The existence of Cs4PbBr6 might be attributed to the reprecipitation of some fully dissolved Cs4PbBr6. On the basis of the results discussed above, we can conclude that the transformation process can be written as

nonpolar solvent that can prohibit the further reaction with water, for the first time monodisperse and air-stable CsPbX3 NCs could be converted from nonluminescent Cs4PbX6 NCs by stripping CsX through an interfacial reaction with water. By simply tuning the halide composition of original Cs4PbX6 NCs, this process allows the formation of CsPbX3 NCs with tunable emission wavelengths covering the full visible range and narrow emission widths. Cs4PbX6 NCs were prepared in a Cs-rich environment through a hot-injection method (see more details in Supporting Information). As shown in Figures S1 and S2, monodisperse Cs4PbX6 NCs were obtained. We first use the transformation of Cs4PbBr6 NCs as the example. The prepared product could be dispersed in hexane to form a colorless and transparent solution. When the hexane solution was mixed with water, the colorless solution became green gradually, suggesting a transformation process. In order to clearly monitor the process, hexane solution was slowly and gently put on the top of the water. As shown in Figure 1a, under UV light irradiation, a greenish interface could be observed, indicating the initiation of reaction at the interface. With time prolonged, the greenish solution expanded to the top. Eventually, a green solution with more intense fluorescent emission can be obtained. The transformation process can be appreciated from the videos provided in the Supporting Information (Videos S1 and S2). The reaction process was further investigated by in situ monitoring the absorption and emission spectra. As plotted in Figure 1b, the original solution shows a sharp absorption peak at 314 nm, which is the characteristic absorption peak of Cs4PbBr6 NCs.30 When water was introduced into the reaction system, the intensity of sharp peak decreased gradually, suggesting the disappearance of Cs4PbBr6 NCs. Meanwhile, a small peak at 510 nm emerged, and its intensity increased steadily (inset in Figure 1b). Compared with the sharp absorption of Cs4PbBr6 NCs, the product showed less pronounced absorption intensity. Figure 1c displays the emission spectra recorded during the reaction. Upon the addition of water, a weak peak at 518 nm appeared and steadily became more pronounced, indicating the formation of a luminescent product. The photoluminescent quantum yield (PLQY) of the as-prepared product was calculated to close to 75%. To explore the reaction mechanism, we characterized the precursor and product carefully. Transmission electron microscopy (TEM) study showed that original Cs4PbBr6 NCs were quasi-spherical with an average edge length around 17.8 nm (Figure 1d). The high-resolution TEM image in the inset shows a clear lattice spacing of 0.31 nm, which is in good agreement with the (214) plane of rhombohedral Cs4PbBr6. After being treated with water, the product showed a cubelike structure with an edge length about 12.2 nm (Figure 1e). The measured d-spacing of 0.42 nm from the inset HRTEM image can be indexed as the (110) plane of cubic CsPbBr3. Figure 2a illustrates the change of crystal structure from Cs4PbX6 to CsPbX3 at the water/hexane interface. Cs4PbBr6 NCs are characterized as a zero-dimensional structure with lower symmetry in which the PbX64− octahedra are completely separated from each other.22,24 In contrast, CsPbX3 NCs can be regarded as a three-dimensional structure in which the PbX64− octahedra share all corners with Cs+ ions filling the voids. Figure 2b shows the X-ray diffraction (XRD) patterns of original Cs4PbBr6 NCs and the product collected from hexane after the transformation. Cs4PbBr6 NCs can be indexed as pure

Cs4PbBr6(hexane) → CsPbBr3(hexane) + 3CsBr(water)

(1)

A CsBr-stripping mechanism is therefore proposed to account for the transformation process, as illustrated in Figure 2a. While Cs4PbX6 NCs have been previously regarded as a PbBr2deficient structure, it can also be viewed as a CsBr-rich perovskite structure. Upon water treatment, stripping of CsBr occurs because of the ionic nature of Cs4PbBr6 and the very high solubility of CsBr in water (1243 g/L at 25 °C),31 which drive the decomposition of Cs4PbBr6 and the formation of CsPbBr3 NCs. During the process, the rhombohedral Cs4PbX6 NCs are converted to simple cubic structured CsPbBr3 NCs. Meanwhile, CsBr-stripping can be confirmed by the shrinkage of particle size from 17.8 to 12.2 nm. It is worth pointing out that the size shrinkage is consistent with the theoretical values calculated based on stoichiometric relation (Supporting Information). Because the solubility of hexane in water is very low (only 9.5 mg/L), further dissolution of CsPbBr3 NCs is prohibited. As a result, the greenish color was observed first at the interface. When the as-formed CsPbBr3 NCs diffused to the top, more Cs4PbBr6 NCs were converted to CsPbBr3 NCs at the interface. This result suggests that the as-prepared CsPbBr3 NCs are more stable than Cs4PbBr6 NCs against water treatment, which may be an important feature for future applications. To further verify the stability of as-prepared CsPbBr3 NCs, a control experiment has been conducted. CsPbBr3 NCs obtained through the hot-injection method were dispersed in hexane, which was then placed on the top of the water. As shown in Figure 3a, the original transparent solution in the top layer shows a light yellowish color. With time prolonged, the yellowish color became colorless gradually, implying the low stability. When the product was irradiated with UV light, a clear decay in photoluminescence can be observed. As shown in Figure 3b, after stored for 36 h the bright green emission became bluish, and the intensity became much lower. In striking contrast, CsPbBr3 NCs obtained through this watertriggered transformation process showed much higher stability. After stored for 36 h, the light yellowish solution kept almost the same color (Figure 3c). Under UV light irradiation, no decay in photoluminescence has been detected (Figure 3d), suggesting excellent stability against moisture. The improved stability might be attributed to the surface passivation: since the NCs are obtained through the water-triggered transformation, the surface might be passivated during the process.32 More research is currently being conducted to figure out the mechanism. 5801

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CsPbBr3 NCs, a similar phenomenon to the hexane case can be observed. The colorless solution became green upon the addition of water. Monodisperse CsPbBr3 nanocubes have been successfully prepared (Figure 4d). When toluene was used as the solvent, highly luminescent CsPbBr3 nanocubes can also be prepared (Figure 4b,e). Because the solubility of toluene in water is relatively higher (0.52 g/L at 20 °C) than that of hexane and cyclohexane, the luminescent product should be separated from the water after the transformation to avoid the damage caused by excessive water. Consistently, because chloroform has a much higher solubility in water (8.09g/L at 20 °C) than toluene, although the transformation process can be initiated (Figure 4c) only large aggregates were obtained (Figure S6). Therefore, while the interface between nonpolar solvent and water is critical for successful transformation, the solubility of nonpolar solvent in water needs to be low to avoid decomposition of the formed CsPbBr3 NCs by water. CsPbX3 NCs are featured by their excellent compositiondependent luminescence properties. Here we show that CsPbX3 NCs with controllable halide composition can be obtained by exposing the corresponding Cs4PbX6 NCs to water at the interface. The transformation process can also be attributed to the high solubility of cesium halide in water (CsCl is 1865 g/L and CsI is 440 g/L)31 and ionic nature of Cs4PbX6 NCs. Cs4PbX6 (X = Cl, Cl/Br, Br/I, and I) NCs of pure and mixed halides were prepared using the same protocol (see more details in SI). It is worth pointing out that the reaction rate of the transformation process depended on the halide composition. The transformation process of Cs4PbCl6 NCs started immediately with the addition of water, while it took several minutes to gradually see the color change in the case of Cs4PbI6 NCs (Figure S7). These phenomena are consistent with the high solubility of CsCl and relatively low solubility of CsI in water. Again, these results further confirmed our proposed CsX-stripping mechanism. Figure 5a,b shows the photographs of CsPbX3 NCs obtained by treating Cs4PbX6 NCs dispersed in hexane with water under ambient light (Figure 5a) and UV light illumination (Figure 5b). One can clearly see that CsPbX3 NCs of different halides emit different colors. A redshift of the PL peak position can be observed in the mixed halide perovskite NCs, which can also be attributed to the different solubility of CsX. Sharp absorption and emission spectra can be obtained (Figure S8). As shown in Figure 5c, the emission of CsPbX3 NCs covers the full visible range (from 408 to 695 nm). All samples showed relatively sharp peaks with the full width at half-maximum in the range of 13 to 34 nm, suggesting excellent luminescent properties. In conclusion, we report here an interesting water-triggered chemical transformation of cesium lead halide perovskite NCs from nonluminescent Cs4PbX6 to highly luminescent CsPbX3 with great stability in air and tunable optical properties. The transformation occurs at the interface of water and a nonpolar solvent, making it a clean synthesis by leaving the product of CsPbX3 NCs in the organic phase and the byproduct CsX in the water phase. A plausible CsX-stripping mechanism has been proposed, highlighting the critical roles of the ionic nature, high solubility of CsX in water, and interface between nonpolar solvent and water to the transformation process. The asprepared NCs show greater stability against moisture than those obtained through the hot-injection method. The composition of CsPbX3 NCs could be manipulated by starting with Cs4PbX6 NCs of mixed halides, thus enabling convenient control of luminescent properties covering the full visible range.

Figure 3. Photographs showing the stability of CsPbBr3 NCs obtained through (a,b) hot-injection method and (c,d) water-triggered transformation process. Top layer, CsPbBr3 NCs dispersed in hexane; bottom layer, water. Photographs are taken under (a,c) daylight and (b,d) UV light (λ = 365 nm) illumination.

From the proposed mechanism, this reaction should happen at the interface between water and other nonpolar solvents that are immiscible with water. Several nonpolar solvents, such as cyclohexane, toluene, and chloroform, were used to test the hypothesis. As shown in Figure 4a, when cyclohexane (immiscible with water) was used as the solvent to disperse

Figure 4. Photographs showing CsPbBr3 NCs obtained when different solvents are used to replace hexane: (a) cyclohexane, (b) toluene, and (c) chloroform (UV light, λ = 365 nm). (d,e) TEM images showing CsPbBr3 NCs obtained by using (d) cyclohexane and (e) toluene as the solvent, respectively. 5802

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Qiao Zhang: 0000-0001-9682-3295 Yadong Yin: 0000-0003-0218-3042 Author Contributions

The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript. L.W., H.H., and Y.X. contributed equally to this work. 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 (21401135, 21673150), Ministry of Science and Technology of China (2016YFE0129600), and Natural Science Foundation of Jiangsu Province (BK20140304). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO−CIC), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and SWC for Synchrotron Radiation Research. Acknowledgment is also made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research.

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Figure 5. (a,b) Photographs of CsPbX3 NCs dispersed in hexane under (a) daylight and (b) UV light (λ = 365 nm) illumination. From left to right, the NCs are obtained using Cs4PbCl6, Cs4PbCl4.5Br1.5, Cs4PbCl3Br3, Cs4PbBr4.5I1.5, Cs4PbBr3I3, and Cs4PbI6 NCs as the precursors. (c) PL emission spectra (λexc = 400 nm for all but 350 nm for CsPbCl3 sample) of the corresponding CsPbX3 NCs dispersed in hexane. X is Cl, Cl/Br, Br, Br/I, or I.

We believe our finding is important because it not only provides a new synthesis approach to highly luminescent CsPbX3 NCs with tunable emission wavelength but also sheds light on the transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.

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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.nanolett.7b02896. Synthesis and material characterization, additional TEM images and photographs (PDF) Transformation process showing green solution with more intense fluorescent emission (AVI) Transformation process showing green solution with more intense fluorescent emission (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.S.). *E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (Y.Y.). ORCID

Baoquan Sun: 0000-0002-4507-4578 5803

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