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One-Pot Synthesis of Highly Stable CsPbBr3@SiO2 Core-Shell Nanoparticles Qixuan Zhong, Muhan Cao, Huicheng Hu, Di Yang, Min Chen, Pengli Li, Linzhong Wu, and Qiao Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04209 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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One-Pot Synthesis of Highly Stable CsPbBr3@SiO2 Core-Shell Nanoparticles Qixuan Zhong, Muhan Cao,* Huicheng Hu, Di Yang, Min Chen, Pengli Li, Linzhong Wu, and Qiao Zhang*

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.

E-mail: [email protected] (M.C.); [email protected] (Q.Z.)

KEYWORDS: CsPbBr3, nanoparticles, silica, core-shell, stability

ABSTRACT: The practical applications of CsPbX3 nanocrystals (NCs) have been limited by their poor stability. Although much effort has been devoted to making core-shell nanostructures to enhance the stability of CsPbX3 NCs, it is still very difficult to coat CsPbX3 NCs with another material on a single-particle level. In this work, we report a facile one-pot approach to synthesize CsPbBr3@SiO2 core-shell nanoparticles (NPs), in which each core-shell NP has only one CsPbBr3 NC. The formation process has been carefully monitored. It has been found that the formation rates, determined by reaction temperature, precursor species, pH value, etc., of both CsPbBr3 and SiO2 are critical for the successful preparation of core-shell NPs. Thanks to the protection of SiO2 shell, the product shows much higher long-term stability in 1 ACS Paragon Plus Environment

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humid air and enhanced stability against ultrasonication treatment in water than that of naked CsPbBr3 NCs. This work not only provides a robust method for the preparation of core-shell nanostructures, but also sheds some light on the stabilization and applications of CsPbX3 NCs.

Metal halide perovskites with a formula of ABX3 (A = CH3NH3+, HC(NH2)2+, C6H5CH2CH2NH3+, Cs+ or Rb+, B = Pb2+, Ge2+, or Sn2+, X = Cl-, Br-, or I-) have attracted great attention.1-4 When A is an organic cation, they are named as organic/inorganic hybrid perovskites,5-8 while the all-inorganic perovskite has an inorganic cation on the A site.9-13 The perovskite material has been considered as an emerging material because of their excellent photophysical properties, such as high photoluminescence quantum yield (PLQY), narrow full width at half maximum (FWHM), tunable emission spectra. As a result, they have shown promising applications in optoelectronics, including solar cells,8,14-17 light emitting diodes (LEDs),18-23 photodetectors,24-26 and lasers.2,27-29 Over the past several years, great progress has been achieved in the research of organic/inorganic hybrid perovskite materials. For example, the power conversion efficiency (PCE) of organic/inorganic hybrid perovskite solar cells has exceeded 22%.8,15,17 The external quantum efficiency (EQE) of organic/inorganic hybrid perovskite LEDs has achieved 11.7%.20,21 However, the poor stability of organic/inorganic hybrid perovskites against water, oxygen, photo- and thermal- treatment, has limited their practical applications.30

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In striking contrast, the all-inorganic cesium lead halide (CsPbX3) perovskites show dramatically improved stability because the Cs+ part is more stable than organo-groups.31,32 We have witnessed an explosive development in the synthesis and applications of CsPbX3 NCs since 2015.12,33,34 Numerous synthetic methods have been

developed,

(SR),11,36

including

solvothermal

hot-injection,9,12,35 reaction,37

supersaturated

post-treatment,38-42

recrystallization ultrasonication,43

microwave-assisted synthesis,44-46 and chemical vapor deposition (CVD).47,48 Various shaped NCs have been prepared, such as nanocubes,12,35,37,44 nanoplates,49,50 nanowires,37,51-53 and so on. The photophysical properties have been dramatically improved. For example, the PLQY of all-inorganic perovskite NCs can achieve near 100% after surface treatment.54,55 Despite the great progress, the stability of CsPbX3 against the deterioration of water and air is still a big challenge. A straightforward strategy to improve the stability of NCs is to coat the NCs with a robust and inert shell.56,57 Some examples have been successfully demonstrated, in which the stability of CsPbX3 NCs has been improved by encapsulating those NCs within SiO2,58-61 TiO2,62 Al2O3,63 or polyhedral oligomeric silsesquioxane.64 However, most of the products were large particles, in which multiple NCs were encapsulated into one shell. Because some important applications require the use of single particle rather than big aggregates,65 much effort has been devoted to exploring the surface modification of CsPbX3 NCs with oxides in a single particle level and some examples have been demonstrated.58,62 Our group recently developed an interfacial synthesis of CsPbX3/oxide Janus NCs by treating a mixture of Cs4PbX6 NCs and alkoxide with 3 ACS Paragon Plus Environment

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water.58 Because the CsPbX3 NCs were partially covered with oxides, the long-term stability of product is still a problem. Zheng group reported the preparation of CsPbBr3@TiO2 core-shell NPs by encapsulating colloidal CsPbBr3 NCs with titanium precursor, followed by a calcination at high temperature.62 The result shows that stability of CsPbBr3@TiO2 core-shell NPs have been improved evidently. Despite the successful examples, some challenges are still remained, including tedious procedures, small volume production, high cost, and so on. It will be much better if one could develop a method that can achieve single-particle coating in a simple and cost-effective way. Herein, we report a facile one-pot synthesis of CsPbBr3@SiO2 core-shell NPs with highly improved stability. Monodisperse CsPbBr3@SiO2 core-shell NPs were obtained by injecting a mixture of CsBr, PbBr2, oleic acid (OA), oleylamine (OAm), dimethylformamide (DMF) and ammonia solution into a poor solvent (toluene in this work) containing tetramethoxysilane (TMOS). The growth process has been carefully monitored and a plausible growth mechanism has been proposed. Thanks to the fully coated silica layer, the as-prepared CsPbBr3 NCs are very stable in some harsh conditions, e.g., storing in humid air and ultrasonication in water.

RESULTS AND DISCUSSION


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Figure 1. (a) TEM image of synthesized CsPbBr3@SiO2 core-shell NCs. (b) HRTEM image of a single CsPbBr3@SiO2 NP. (c) Photograph of CsPbBr3@SiO2 core-shell NPs dispersed in toluene under sunlight (left) and UV light (right, λ = 365 nm) in a large scale (250 mL/batch). (d) (From left to right): HAADF-STEM image and elemental mapping images showing the elemental distribution of Si, Cs, Pb, Br and the overlapped image. The scale bars are 10 nm. (e) XRD pattern of CsPbBr3@SiO2 NCs (red) and the standard pattern for orthorhombic CsPbBr3 (ICSD #01-072-7929). (f) Absorption (black line) and photoluminescence (red dash line) spectra of CsPbBr3@SiO2 core-shell NPs. Monodisperse CsPbBr3@SiO2 core-shell NPs were obtained through a one-pot synthesis conducted at 30 oC using a modified supersaturated recrystallization method.36 In a typical experiment, CsBr, PbBr2, OA (final concentration in toluene = 5 ACS Paragon Plus Environment

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7.5 mM), OAm (final concentration in toluene = 2.5 mM), and ammonia solution were mixed in DMF. The mixture was then rapidly injected into an ultra-dry toluene solution containing certain amount of TMOS. Upon the injection, the colorless solution turned into light yellow immediately, suggesting the formation of CsPbBr3 NCs (Video S1, Supporting Information). The reaction system was kept at 30 oC for 2 hours under magnetic stirring. As shown in Figure 1a, uniform core-shell NPs with a core size around 10.5 nm and shell thickness about 7.7 nm were obtained. More importantly, only one core can be found in one shell. A clear lattice spacing of 0.42 nm can be observed in the high-resolution transmission electron microscopy (HRTEM) image (Figure 1b), suggesting that the core particle is CsPbBr3 and the light shell can be attributed to silica. It is worth pointing out that the CsPbBr3@SiO2 core-shell NPs could be obtained in a large scale. As shown in Figure 1c, we conducted an experiment in a 250 mL bottle, and 170 mg high-quality product was obtained in one batch (Figure S1 in Supporting Information). Further characterization using energy dispersive X-ray spectrometry (EDS) mapping (Figure 1d) confirmed the CsPbBr3@SiO2 core-shell nanostructure, in which Cs, Pb and Br elements were distributed in the central part and Si element was deposited around. Powder X-ray diffraction (XRD) was depicted in Figure 1e. The product can be indexed as orthorhombic CsPbBr3 (ICSD #01-072-7929), which is different from the monoclinic product reported by Zeng group.36 The different crystal structures might be attributed to the existence of NH4OH, which can change the pH value of reaction system and induce the phase change. To verify this hypothesis, the 6 ACS Paragon Plus Environment

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reaction has been conducted under the same condition except the addition of NH4OH and monoclinic CsPbBr3 particles have been prepared (data not shown). It should be noted that there is a small but broad shoulder in the range of 2θ = 20 o ~ 30 o, which should be attributed to the existence of amorphous silica.59,66 From the absorption and photoluminescence spectra of CsPbBr3@SiO2 core-shell NPs (Figure 1f), an absorption peak at 480 nm was observed and the corresponding emission peak was located at 501 nm with a FWHM of 22 nm, confirming the high uniformity of prepared product. The PLQY of the obtained product was measured to be 90% by using an excitation wavelength of 365 nm and a calibrated sphere. The obtained average lifetime of as-prepared CsPbBr3@SiO2 is about 16.8 ns (Figure S2), which is longer than that of CsPbBr3 NCs prepared through the re-precipitation method.36 The enhancement in lifetime can be attributed to the presence of silica that can passivate the NCs via reducing surface dangling bonds.58

Figure 2. TEM images of products obtained by taking aliquots out from the reaction system after reacting for: (a) 2 min, (b) 30 min, (c) 60 min, and (d) 120 min. (e)


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Photoluminescence spectra of corresponding products. (f) FTIR spectrum of as-prepared CsPbBr3@SiO2 core-shell NPs. To better understand the formation mechanism of CsPbBr3@SiO2 core-shell NPs, the reaction system was studied systematically. First, the reaction process was carefully monitored by taking aliquots out during the reaction process and characterized by using TEM and photoluminescence spectroscopy. As described above, upon the mixing of precursors, the CsPbBr3 NCs were obtained immediately because of the fast recrystallization process. Figure S3 shows that the products were cube-like structure with an average size around 17.2 nm. 2 min later, the products were still cube-like structure (Figure 2a). But the surface became a little blurred in the TEM image, which might be attributed to the adsorption of some silica oligomers derived from the hydrolysis of TMOS. The average particle size was about 16.8 nm. The corresponding emission peak is located at 515 nm (Figure 2e.) Further prolonging the reaction to 30 min, an obvious shrinkage in particle size was observed. As shown in Figure 2b, the particle size was around 16.0 nm. Meanwhile, some amorphous silica can be clearly observed around the core part. A clear blue-shift of PL spectra (from 515 nm to 510 nm) was shown in Figure 2e, which was caused by the smaller size of CsPbBr3 NCs. The shrinkage of CsPbBr3 particle size and increase of silica thickness became more pronounced with longer reaction time, as depicted in Figure 2c and 2d. An obvious breakage of the core part can be observed in Figure 2c, which might be attributed to the dissolution of CsPbBr3 in the presence of ammonia solution. The final PL peak was located at 501 nm (Figure 2e). 8 ACS Paragon Plus Environment

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The final product has been characterized by using the Fourier transform infrared spectroscopy (FTIR), as depicted in Figure 2f. The existence of silica can be verified by the sharp peaks at 1085 cm-1 and 812 cm-1, which can be ascribed to the vibration and asymmetric stretching vibration of Si-O-Si, respectively.61 The C-H asymmetric and symmetric stretching vibrations (2923 and 2854 cm-1, respectively) and in-plane bending vibration (1465 cm-1) can be clearly observed, which come from OA and OAm. The peak at 1544 cm-1 can be assigned to the N-H bending vibration, which is a characteristic band for OAm, confirming the presence of OAm.67 The absorption at 1710 cm-1 can be assigned to the C=O stretching vibration of OA.68

Figure 3. TEM images of products obtained in the presence of different concentrations of OA and OAm: (a) [OA] = 7.5 mM, [OAm] = 2.9 mM; (b) [OA] = 7.5 mM, [OAm] = 2.1 mM; (c) [OA] = 6.8 mM, [OAm] = 2.5 mM; (d) [OA] = 8.4 mM, [OAm] = 2.5 mM.


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It is well known that capping ligands play important roles in the synthesis of nanomaterials. In this work, we have carefully studied the influence of capping ligands, i.e., OA and OAm. In the absence of both OA and OAm, only large aggregates with very weak fluorescence could be obtained (Figure S4, Supporting Information), which can be attributed to the lack of surface passivation. Additionally, we found that both OA and OAm were necessary in this work. In the absence of OAm and keeping other parameters identical, the solubility of CsBr and PbBr2 in DMF was very low. Most of the precursors were kept undissolved and yellow solid was observed in DMF (Figure S5a). The products precipitated and formed large aggregates with very weak fluorescence, as shown in Figure S5b-c. In the absence of OA while other parameters were kept constant, CsBr and PbBr2 powder could not be dissolved completely (Figure S6a). As shown in Figure S6b-c, the product was big particles with many CsPbBr3 particles encapsulated in one big SiO2 layer, which might be caused by the weak protection of OAm. We have further varied the concentrations of OA and OAm to see the influence of each ligand. When the concentration of OA was fixed at 7.5 mM and the concentration of OAm was increased from 2.5 mM to 2.9 mM, CsPbBr3@SiO2 core-shell NPs were obtained with a core size around 13.0 nm and shell thickness of 10.0 nm (Figure 3a). Meanwhile, some free silica spheres were observed, which might be attributed to the excessive OAm that can promote the formation of free silica because of the existence of amine groups.69 The product shows a broad emission peak at 500 nm (Figure S7a), indicating poor uniformity of products. When less OAm (2.1 10 ACS Paragon Plus Environment

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mM) was added, CsBr and PbBr2 cannot be dissolved completely in the DMF precursor (Figure S8). Due to the lack of effective protection from less OAm, CsPbBr3 NCs and SiO2 shell were not uniform, as evidenced by the TEM image (Figure 3b). It should be noted that the majority of products were Janus particles, suggesting that sufficient OAm is critical for the formation of SiO2 on the CsPbBr3 surface. Very few free silica NPs were observed in the final product, further confirming the effect of OAm. The effect of OA has also been studied. When the concentration of OA was decreased from 7.5 mM to 6.8 mM and all the other conditions were kept unchanged, CsPbBr3@SiO2 core-shell NPs were still obtained but serious aggregation happened. As shown in Figure 3c, all NPs aggregated together. The product showed a broad emission peak at 499 nm (Figure S7c). This result suggested that OA could act as a capping ligand to separate the NPs from each other. In the presence of excessive OA (8.4 mM in the solution), a mixture of CsPbBr3@SiO2 core-shell NPs and free silica particles were obtained (Figure 3d). This phenomenon can be explained by the relatively low concentration of OAm. From the above-mentioned results, we can get a conclusion that the concentration of capping ligands as well as the ratio between OA and OAm should be kept in a suitable range to get uniform CsPbBr3@SiO2 core-shell nanostructures. To investigate the influence of capping ligands, different kinds of ligands were introduced. Octanoic acid (OctA) and octylamine (OctAm) were employed to act as the capping ligands. The concentration of OctA and OctAm was kept as 7.5 mM and 2.5 mM, respectively. From Figure S9a, it can be found that non-uniform sphere-like 11 ACS Paragon Plus Environment

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CsPbBr3 NCs were obtained, along with large-area SiO2 matrix, which caused by the weak protection of shorter-chain ligands. For the system involving OA and OctAm, TMOS preferred to rapidly hydrolysis, nucleate and grow into large-sized free SiO2 spheres (ca. 25 nm). At the same time, very tiny CsPbBr3 NCs with size of about 3.2 nm were obtained, as shown in Figure S9b. By using shorter-chain amine, the size of NCs decreased obviously, which was consistent with previous report.67 When OA was replaced by OctA, large CsPbBr3 cubes were obtained. This phenomenon was also found when shorter acid was employed.67 Since larger perovskite nanocubes have relatively lower surface energy than smaller particles, SiO2 oligomers would remain in solution rather than grow on the surface of large nanocubes, resulting in free silica, as shown in Figure S9c.

Figure 4. TEM images of product obtained at different reaction temperatures: (a) 20 o

C; (b) 25 oC; and (c) 35 oC. (d) Corresponding UV-vis absorption (solid line) and PL

spectra (dash line) of the as-prepared CsPbBr3/SiO2 NPs.


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The reaction temperature is another important parameter for the successful preparation of CsPbBr3@SiO2 core-shell NCs. When the reaction was conducted at 20 o

C, big CsPbBr3 particles could be prepared. As shown in Figure 4a, the particle size

was about 31.3 nm, which is much larger than that shown in Figure 1. Some very rough and isolated silica particles could be identified around the big CsPbBr3 particles. It is well known that the particle size can be controlled by varying the reaction kinetics. At lower reaction temperature, the nucleation process will be much slower than that at higher temperature, resulting in fewer nuclei and bigger particle size.70,71 At the same time, the hydrolysis and condensation of silica would be much slower at lower temperature, leading to the formation of less condensed silica. Meanwhile, because the surface energy of big CsPbBr3 particles is relatively lower than that of smaller ones, separated CsPbBr3 and silica particles were obtained rather than the formation of core-shell particles. The product has an emission peak at 505 nm (Figure 4d). When the reaction was carried out at 25 oC, the nucleation process of CsPbBr3 was accelerated, as evidenced by the smaller size of CsPbBr3 NCs. The hydrolysis and condensation rates of silica have also been raised, as confirmed by the free silica spheres with higher contrast than that in Figure 4b. Some CsPbBr3@SiO2 core-shell NPs have been prepared, but the majority of products were still separated particles. It is worth pointing out that the core-shell NPs were eccentric structure rather than concentric structure. As shown in Figure S10, the eccentric core-shell structure has been confirmed by the EDS mapping analysis. This phenomenon might be interpreted 13 ACS Paragon Plus Environment

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by the relatively different formation rates of CsPbBr3 and SiO2. The product shows an emission at 502 nm (Figure 4d). As we discussed in Figure 1, concentric CsPbBr3@SiO2 core-shell NPs were obtained when the reaction was conducted at 30 o

C. Further raising the reaction temperature to 35 oC led to the formation of big

aggregates, as shown in Figure 4c. TEM characterization shows that multiple CsPbBr3 particles were encapsulated in a big SiO2 layer. This result can be explained by the fast reaction rates of precursors and weak protection from OA and OAm.35 A broad emission peak in the range of 500 nm and 510 nm could be observed (Figure 4d), indicating the broad size distribution of products. Because of the weak protection of capping ligands, higher temperature (above 40 oC) resulted in serious aggregates (Figure S11). From the above phenomena, it can be concluded that the optimum reaction temperature is 30 oC.

Scheme 1. The proposed formation process of CsPbBr3@SiO2 core-shell NPs. On the basis of above mentioned results and observations, we have proposed a plausible growth mechanism, as illustrated in Scheme 1. In the presence of OA and OAm, CsBr and PbBr2 were dissolved in a good solvent DMF. Upon the injection 14 ACS Paragon Plus Environment

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into a poor solvent, toluene, a fast precipitation process occurred and CsPbBr3 NCs formed immediately. Due to the protective effect provided by OA and OAm, the as-formed CsPbBr3 NCs could be well dispersed in the solvent. Meanwhile, the hydrolysis of TMOS occurred accordingly and produced a lot of silica oligomers. In the presence of OA and OAm, silica oligomers would tend to adsorb on the surface of CsPbBr3 NCs. Further hydrolysis of TMOS and condensation of silica species led to the formation of core-shell nanostructures. It should be noted that the stability of CsPbBr3 NCs against water and ammonia solution is not very high. As a result, shrinkage of CsPbBr3 NCs was observed during the growth process. To confirm the proposed mechanism and investigate the effect of hydrolysis rate, some control experiments have been carried out. For example, the silica precursor, TMOS, was replaced by tetraethylorthosilicate (TEOS). Only big aggregates were obtained, in which many CsPbBr3 NCs were covered with a layer of silica (Figure S12). It is well-known that the hydrolysis rate of TEOS is much slower than that of TMOS under the same reaction condition. As a result, big aggregates were formed. Since the growth rate of silica is important for the formation of core-shell NPs, the concentration of ammonia solution was varied. In the absence of ammonia, serious aggregation and large amount of CsPbBr3 NCs encapsulated in SiO2 oligomers can be found (Figure S13a). It indicated that ammonia was not necessary for the hydrolysis of TMOS but would significantly affect the morphology and size of the resulting product. When the concentration of ammonia was 0.3 mM, big aggregate was also observed (Figure S13b), which is similar to the TEOS case. This result can be also 15 ACS Paragon Plus Environment

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interpreted as the slow hydrolysis of TMOS and less condensed silica, leading to the formation of sticky silica species. However, the quality of SiO2 was higher than that prepared in the absence of silica. In the presence of much higher concentration of ammonia (1.2 mM), many free silica particles and naked CsPbBr3 NCs were observed in the TEM image (Figure S13c), caused by the fast formation of silica. Therefore, the formation rates of SiO2 should be in a suitable level.

Figure 5. XRD patterns showing the stability of (a) CsPbBr3 NCs and (b) CsPbBr3@SiO2 core-shell NPs stored in air (25 oC and humidity of 75%). It has been widely accepted that the stability of CsPbBr3 NCs against water treatment is very poor. To examine the stability of CsPbBr3 NCs before and after silica coating, the CsPbBr3 and CsPbBr3@SiO2 powders were placed in air with a humidity of 75% and a temperature of 25 oC. To ensure a fair comparison, CsPbBr3 NCs were obtained by using the same method except the absence of TMOS. As depicted in Figure 5, the XRD patterns of both samples can be indexed as orthorhombic phase of CsPbBr3 (ICSD #01-072-7929). The stability of uncoated CsPbBr3 NCs was very poor. After stored in the humid air for 3 days, the XRD signal almost disappeared due to the deterioration of CsPbBr3. As shown in Figure 5a, after 16 ACS Paragon Plus Environment

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multipled by 10 times, the intensity of XRD signal is still very weak. In contrast, the CsPbBr3@SiO2 core-shell NPs showed much higher stability. As depicted in Figure 5b, almost no change in the XRD pattern could be identified under the same conditions, suggesting the improved stability of the core-shell structure. After stored for 4 weeks under the same condition, although the intensity of XRD peaks decreased slightly, the characteristic peaks can be clearly identified, confirming the outstanding long-term stability.


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Figure 6. Photographs showing the stability against water: (a-c) CsPbBr3 NCs, (d-f) CsPbBr3@SiO2 core-shell NPs. 4 mg samples were dispersed in 2 mL of DI water and treated by ultrasonication. (g) The change of PL intensity under ultrasonication.

The stability of products has been further confirmed by treating the samples in water under a very harsh condition, i.e., an ultrasound irradiation. The treatment was carried out in the ultrasonic equipment (KQ-100KDB, Kunshan Ultrasonic Instruments Co., Ltd., China) with a frequency of 40 kHz and output power of 100 W. The temperature of the water bath was maintained at 25 oC. As shown in Figure 6a-c, 4 mg CsPbBr3 NCs prepared through the room temperature re-precipitation method was put in a glass vial, followed by the addition of 2 mL deionized (DI) water. Because the capping ligands were hydrophobic OA and OAm, these NCs could not be well dispersed in water. A green emission could be observed under UV light irradiation (Figure 6b-c). The glass vial was then put into an ultrasonication machine. It can be seen that the green emission dropped rapidly under ultrasonication. The bright emission of CsPbBr3 NCs became very weak after 8 min and disappeared completely after 16 min. In a striking contrast, the CsPbBr3@SiO2 core-shell nanostructures showed much higher stability. As shown in Figure 6d-f, bright emission could still be observed clearly after the same ultrasonication treatment for 40 min, confirming the improved stability. It should be noted that the green emission became brighter after the ultrasonication treatment for 8 min, which is different from the uncoated CsPbBr3 NPs that shows only declined emission during the treatment process. 18 ACS Paragon Plus Environment

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A quantitative study has been carried out by measuring the PL intensity. As plotted in Figure 6g and S14, the PL intensity of the uncoated CsPbBr3 NCs declined dramatically upon ultrasonication treatment. The PL intensity dropped to 0% after 16 min. For the CsPbBr3@SiO2 core-shell NPs, a clear increase of the PL intensity from 100% to 177% in the first 16 min can be identified. This phenomenon can be interpreted by the improved dispersity of NPs in water under ultrasonication treatment. Because the surface of products was covered with OA and OAm, the dispersity of CsPbBr3@SiO2 core-shell NPs was poor. Under ultrasonication treatment, the CsPbBr3@SiO2 core-shell NPs can be dispersed better in water, resulting in higher PL intensity. With prolonged ultrasonication time, a slight decrease in the PL intensity can be observed. As plotted in Figure 6g, the PL intensity is about 112% and 63% compared with the initial (t = 1 min) and maximum (t = 16 min) intensities, respectively. This dramatically improved stability should come from the protection of SiO2 shell that can prevent the damage of water. Currently, we are trying to extend this method to prepare other oxide coated CsPbBr3 NCs and explore their potential applications in various fields, such as LED, photocatalysis, photodetection, and so on.

CONCLUSIONS

In summary, monodisperse CsPbBr3@SiO2 core-shell NPs with only one CsPbBr3 core encapsulated in one SiO2 shell were successfully prepared through a simple 19 ACS Paragon Plus Environment

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one-pot synthesis method. A systematic study has been carried out, and a plausible growth mechanism has been proposed. It was found that the successful generation of SiO2 oligomers at early stage led to the growth of SiO2 shell in the presence of ammonia and TMOS. With the coating of SiO2 shell, the as-prepared core-shell nanostructures showed outstanding stability against a long-term storage in air and even ultrasonication treatment in water. This simple but effective approach to prepare the core-shell CsPbBr3@SiO2 nanostructures might be extended to other materials with various components, and may inspire the synthesis of well-defined morphologies in the inorganic-perovskite filed. This work not only provides a robust method for the preparation of core-shell nanostructures, but also sheds some light on the stabilization and applications of CsPbX3 NCs.

METHODS

Materials: Cesium bromide (CsBr, 99.999%, Alfa Aesar), PbBr2 (99.999%, Alfa Aesar), OA (90% tech.), OAm,(80-90%, Alfa Aesar), DMF (99.5%, Sigma-Aldrich), TMOS (99%, Energy Chemical), hexane (99.5%, Sigma-Aldrich)

are used directly without further purification. Toluene (99.95%) was purchased from Fisher and dried before use. Synthesis of CsPbBr3 NCs: 0.1468 g PbBr2, 0.0851 g CsBr, 0.6 mL OAm and 1.8 mL OA were added in 10 mL DMF. Then the mixture was stirred at 90 oC for 2 h to obtain a clear solution. Ammonia solution (40 µL, 2.8%) was added into 2 mL of the


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precursor solution. After that, 0.2 mL of the precursor solution was quickly added into 10 mL dry toluene under vigorous stirring (1500 rpm). Synthesis of CsPbBr3@SiO2 core-shell NPs: 0.2 mL of the precursor solution was quickly added into 10 mL dry toluene containing 5 µL TMOS under vigorous stirring (1500 rpm). After 10 s, the stirring speed was adjusted to 150 rpm and kept for 120 min. The final products were collected by centrifugation at 9000 rpm for 5 min. Characterization methods: UV-Vis absorption spectra were recorded in a range of 300-800 nm by using an Evolution 220 spectrophotometer in transmission mode. The photoluminescence spectra and PLQY were obtained by a FLUOROMAX-4 spectrofluorometer equipped with a Xenon lamp. The PL lifetime measurements were carried out by using the HORTB-FM-2015 spectrofluorometer and fitted with a triexponential decay. Powder X-ray diffraction was performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54056 Å). TEM images were collected by a TECNAI G2 F20 transmission electron microscope with an accelerating voltage 200 kV and a Gatan SC200 CCD camera. FTIR spectra of the sample was recorded with a Bruker Vertex 70 spectrometer in the range from 4000 to 600 cm-1. The ultrasonication test experiments were carried out in an ultrasonication equipment (KQ-100KDB, Kunshan Ultrasonic Instruments Co., Ltd., China) with a frequency of 40 kHz and output power of 100 W. ASSOCIATED CONTENT

Supporting Information.


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Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

Additional data about the weight of product obtained from one batch; PL lifetime of product; TEM images, size distributions, photographs, UV-vis absorption and PL spectra, of product obtained by varying reaction parameters, such as concentration of precursors and reaction temperature; HAADF-STEM and EDS mapping

images

of

eccentric

CsPbBr3@SiO2

NPs

core-shell

structure,

time-dependent PL spectra of product under ultrasonication treatment (PDF).

Video S1: The solution color change during the formation of CsPbBr3@SiO2 NCs. (AVI)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (M.C.);

* E-mail: [email protected] (Q.Z.)

ORCID Qiao Zhang: 0000-0001-9682-3295 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT


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This work is supported by the Ministry of Science and Technology of the People's Republic of China (2016YFE0129600), National Natural Science Foundation of China (21673150, 21611540336, 21703146) and the Postdoctoral Science Foundation of China (2016M591909). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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ACS Nano

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SYNOPSIS Monodisperse CsPbBr3@SiO2 core-shell nanoparticles have been prepared through a facile and effective method on a single particle level. The obtained samples exhibited great optical properties and well-defined morphology. With the coating of SiO2 shell, the product showed significantly enhanced stability against a long-term storage in humid air and even under very harsh condition, e.g., ultrasonication in water. ToC figure (For Table of Contents Only)


One-Pot Synthesis of Highly Stable CsPbBr3 ... - ACS Publications

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