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General Strategy for Rapid Production of Low-Dimensional AllInorganic CsPbBr3 Perovskite Nanocrystals with Controlled Dimensionalities and Sizes Wenna Liu,†,‡,∥ Jinju Zheng,‡,∥ Sheng Cao,§ Lin Wang,‡ Fengmei Gao,‡ Kuo-Chih Chou,† Xinmei Hou,*,† and Weiyou Yang*,‡ †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ‡ Institute of Materials, Ningbo University of Technology, Ningbo City 315016, People’s Republic of China § Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore S Supporting Information *

ABSTRACT: Currently, all-inorganic CsPbX3 (X = Br, I, Cl) perovskite nanocrystals (NCs) are shining stars with exciting potential applications in optoelectronic devices such as solar cells, light-emitting diodes, lasers, and photodetectors, due to their superior performance in comparison to their organic−inorganic hybrid counterparts. In the present work, we report a general strategy based on a microwave technique for the rapid production of low-dimensional all-inorganic CsPbBr3 perovskite NCs with tunable morphologies within minutes. The effect of the key parameters such as the introduced ligands, solvents, and PbBr2 precursors and microwave powers as well as the irradiation times on the production of perovskite NCs was systematically investigated, which allowed their growth with tunable dimensionalities and sizes. As a proof of concept, the ratio of OA to OAm as well as the concentration of PbBr2 precursor played important roles in triggering the anisotropic growth of the perovskite NCs, favoring their growth into 1D/2D single-crystalline nanostructures. Meanwhile, their sizes could be tailored by controlling the microwave powers and irradiation times. The mechanism for the tunable growth of perovskite NCs is discussed.

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INTRODUCTION In comparison to the organic−inorganic hybrid perovskites (e.g., CH3NH3PbX3, X = Br, I, Cl), their all-inorganic CsPbX3 counterparts exhibit higher stability against moisture and heat, more efficient and tunable photoluminescence (PL) across the whole visible spectrum, higher PL quantum yield, narrower emission width, shorter radiative lifetime, and faster carrier diffusion,1−3 which inspire their exciting applications in solar cells, light-emitting diodes, lasers, photodetectors, and so forth.4−9 To date, numerous efforts (e.g., hot-injection,10 anion exchange,11 room-temperature reprecipitation,12 and ultrasonication13) have been devoted to the growth of CsPbX3 perovskites. Due to their morphology-dependent optoelectronic properties,14,15 a further understanding of the constitutive relations between dimensionality and properties requires successful synthesis of various dimensional perovskites. Regardless of the fact that the perovskite NCs with different morphologies, including 0-dimensional (0D) quantum dots,16−18 one-dimensional (1D) nanowires/rods,10 and twodimensional (2D) nanoplatelets/sheets,19−23 have been obtained to some extent, a general strategy simultaneously © XXXX American Chemical Society

affording controlled dimensionalities and tunable size is highly desired. Microwave (MW) irradiation is recognized as one of the most popular and versatile strategies for fabricating materials in nearly all systems,24−28 owing to the following unique advantages:29,30 (i) homogeneous heating, (ii) high heating rate, (iii) selective activation of the target precursor, (iv) high reproducibility, (v) convenient and near-continuous process, and (vi) rapid production and high throughput. To the best of our knowledge, until most recently, just a few works have shed light on the synthesis of CsPbX3 perovskites31,32 based on the MW technique. Here, we report progress in the rapid production of lowdimensional all-inorganic CsPbBr3 perovskite NCs with tunable morphologies within minutes on the basis of a MW strategy. The effect of the key parameters such as the introduced ligands and solvents and irradiation powers as well as the reaction times on the growth of perovskite NCs has been systematically Received: November 21, 2017

A

DOI: 10.1021/acs.inorgchem.7b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

0.41 nm is consistent with the distance of (110) planes of orthorhombic CsPbBr3. The typical EDX spectrum and XRD pattern in Figure S5 in the Supporting Information further confirm that the obtained nanorods are also orthorhombic CsPbBr3 perovskites. With a further change in the amounts of OA and OAm to 6 and 4.5 mL, respectively, nanosheets with a lateral size up to 235.4 nm (Figure S6 in the Supporting Information) are obtained, as shown in Figure 1c1−c3. The 2D CsPbBr3 nanosheets are very thin and highly transparent to electrons. Similarly, the HRTEM image (Figure 1c3) and the EDX spectrum (Figure S7a in the Supporting Information) and XRD pattern (Figure S7b in the Supporting Information) evidence that the as-synthesized product is the all-inorganic orthorhombic perovskite CsPbBr3. Notably, the products of the three samples are highly pure in morphology. The PL performances of these three samples (i.e., nanocubes, nanorods, and nanosheets) are presented in Figure S8 in the Supporting Information, which disclose that they exhibit no significant difference, mainly due to their sizes exceeding the quantum confinement effect. Their corresponding digital photos are provided as top right insets in Figure 1a1−c1, respectively. On the basis of the experimental results as discussed above, it seems that the 0-dimensional (0D) nanocubes (denoted sample S0D) could be obtained with relatively more OA used. With an increase in OAm, the CsPbBr3 NCs would be changed into one-dimensional (1D) nanorods (denoted S1D) and subsequently grown into two-dimensional (2D) nanosheets (denoted sample S2D). These represent the fact that the dimensionality of the as-synthesized products could be tailored from 0D to 1D and finally to 2D, by controlling the ratios of OA to OAm in the raw materials. We attribute the dimensionality-controlled process mainly to the anisotropic growth of the crystals caused by the kinetic mechanism, in which the ligands are often selectively adhered to the NC facets, leading to the different growth rates in various crystal faces.38,39 The morphology evolution with the variation of OA and OAm confirms that the ammonium ions are effective for tuning the anisotropic growth of the perovskite NCs. In this case, with more OAm introduced, the longer-chain OAm would be electrostatically bound to the surface of the preformed monolayer nuclei to preclude the vertical oligomerization, thereby yielding nanosheet products via oriented attachment.40 In contrast, with a lower concentration of OAm introduced, the surface of the nuclei would not be entirely covered by the monomers, which would favor the growth of nanorods and nanocubes.13 In order to make this point more clear, a comparison experiment was performed, in which 10 mL of OA without OAm was utilized as the sole surfactant. As shown by the photographs of the PbBr2 precursor in this case (Figure S9 in the Supporting Information), it seems that, after being subjected to a series of experimental processes including drying at 120 °C for 1 h and heating at 150 °C for 0.5 h with stirring, PbBr2 just changed into a milky turbid liquid (Figure S9a in the Supporting Information), followed by a return to a white powder after natural cooling to room temperature (Figure S9b in the Supporting Information), as if nothing happened. However, the product was a clear, transparent, and homogeneous solution with just 1 mL of OAm used to replace the OA. This result confirms that it is OAm, rather than OA, that is bonded to the surface of the PbBr2, which rules the growth of perovskite NCs.41 Herein, it could be concluded that it is the OAm that facilitates the anisotropic growth of

investigated, showing their growth in tunable dimensionalities and sizes.

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RESULTS AND DISCUSSION It is well-known that the surfactants play an important role in the controlled growth of NCs.33−35 However, the effects of organic acids and amines on the growth of CsPbBr3 perovskite NCs are currently not agreed upon. For example, Liang36 and Lv21 proposed that the OA played an important role in controlling the morphology of perovskite NCs. However, De Roo et al.37 suggested that the OAm was responsible for the anisotropic growth of perovskite NCs. To understand this point, experiments were performed by varying the ratios of OA to OAm and fixing at 400 W with an irradiation time of 1 min, as shown in Figure 1. When the amounts of OA and OAm are 9

Figure 1. Typical SEM (a1−c1), TEM (a2−c2) and HRTEM (a3−c3) images of CsPbBr3 NCs obtained with different ratios of OA and OAm under irradiation power of 400 W for 1 min. The insets in a1−c1 are the corresponding photographs of the colloidal dispersion of the CsPbBr3 NCs in hexane under UV light (λ is 365 nm and slit width is 2.3 nm), respectively. The insets in a3−c3 are the corresponding SAED patterns. (a4−c4) Schematic illustrations for the formation of the perovskites in 0D nanocubes, 1D nanorods, and 2D nanosheets, respectively.

and 1 mL, respectively, the product is CsPbBr3 nanocubes (Figure 1a1−a3) with an average size of ∼10.8 nm (Figure S2 in the Supporting Information). Both the HRTEM image and SAED pattern (Figure 1a3 and the inset in a3) reveal that the asgrown CsPbBr3 nanocubes are single crystalline. The lattice fringe with an interplanar spacing of 0.29 nm (Figure 1a3) coincides with the (200) planes of orthorhombic CsPbBr3. The typical EDX spectrum confirms that the nanocubes are composed of Cs, Pb, and Br elements (Figure S3a in the Supporting Information). Figure S3b shows a representative XRD pattern of the product. The peaks at 2θ = 15.1, 21.5, 30.4° correspond to the diffractions from {001}, {110}, and {002} planes of orthorhombic CsPbBr3, evidencing the formation of orthorhombic CsPbBr3 nanocubes (No. 18-0364). Interestingly, by adjustment of the amounts of OA and OAm to 8 and 2 mL under otherwise similar conditions, respectively, nanorods with an average diameter of 24.6 nm (Figure S4a in the Supporting Information) and length of 216 nm (Figure S4b in the Supporting Information) are synthesized, as shown in Figure 1b1−b3. Figure 1b2,b3 gives the TEM and HRTEM images of the nanorods, respectively, in which the d spacing of B

DOI: 10.1021/acs.inorgchem.7b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and 23 nm, respectively (Figure S11 in the Supporting Information). Their UV/vis spectra exhibited a single and steep absorption onset, which are slightly red-shifted from the PL maximum. The small Stokes shift suggests that the PL emission of the CsPbBr3 nanocubes originates from the exciton recombination.37 The experiments as discussed above show that the tunable growth in dimensionality of the perovskite NCs via the MW strategy could be accomplished by tailoring the ratios of OA to OAm and the precursor concentrations within the solutions. Now we come to the point of controlling the sizes of the assynthesized perovskite NCs of samples S0D, S1D, and S2D by adjusting the irradiation powers to 400, 600, and 800 W with an identical irradiation time of 1 min, respectively, which is shown in Figure 3. It seems that the irradiation powers have little

perovskite NCs, making their growth in a tunable manner in dimensionalities. It is well recognized that the concentration of the precursor has a fundamental effect on the growth of the NCs. With regard to this point, we chose the nanorod perovskites (i.e., sample S1D, the product of Figure 1b1−b3) as an example to investigate the precursor concentration dependent growth of the perovskite NCs. On the basis of sample S1D, the concentrations of the PbBr2 precursor were fixed at double, half, and one-third, respectively, with irradiation at 400 W for 1 min. The resultant samples are denoted D400, H400, and T400, respectively. As shown in Figure 2a1−a3, D400 still

Figure 2. Representative SEM (a1−c1), TEM (a2−c2), and HRTEM (a3−c3) images of the samples obtained at different PbBr2 precursor concentrations under 400 W for 1 min. The samples D400, H400, and T400 denote precursor concentrations fixed at double, half, and onethird in comparison to that of sample S1D.

maintains the nanorod morphology with orthorhombic crystal structure, while the length (274 nm, Figure S10a in the Supporting Information) is greater than that of the sample S1D (i.e., 216 nm, Figure S4 in the Supporting Information). Different from the case for the sample D400, the products of the samples H400 and T400 are completely changed into nanocubes, as shown in Figure 2b1−b3,c1−c3), suggesting that the dimensionalities of the perovskite NCs could be tailored by changing the concentrations of PbBr2 precursors. With a higher irradiation power of 600 W (denoted samples D600, H600, and T600), an almost similar growth of perovskite NCs is observed, which is shown in Figures S12 and S13 in the Supporting Information. Accordingly, the conclusion can be drawn that a higher concentration of the PbBr2 precursor would favor the anisotropic growth of 1D perovskite NCs. This is mainly attributed to the fact that the shape of NCs is strongly dependent on the concentration of monomer within the solution. Commonly, a higher concentration of the precursor results in a correspondingly higher concentration of monomer, which would facilitate the growth of 1D nanorods.42,43 The TEM and HRTEM characterizations disclose that these nanocubes are all single crystals with average sizes of 8.1 and 15.0 nm for H400 and T400 (Figure S10b,c in the Supporting Information), respectively. They exhibit narrow and single-peak PL emission with full widths at half-maximum (fwhm) of 20

Figure 3. (a1−a3) Typical SEM images showing the obtained NCs based on sample S0D with the variation of irradiation powers of 400, 600, and 800 W for 1 min. (b1−b3) Typical SEM images showing the obtained NCs based on sample S1D with the variation of irradiation powers of 400, 600, and 800 W for 1 min. (c1−c3) Typical SEM images showing the obtained NCs based on sample S2D with the variation of irradiation powers of 400, 600, and 800 W for 1 min. The inset diagrams under the SEM images illustrate schematically the size change of the NCs with the variation of the MW powers. (d1−d3) Typical SEM images showing the obtained NCs based on sample D400 with the variation of irradiation times of 1, 3, and 5 min under a MW power of 400 W.

influence on the shapes of perovskite NCs (Figure S13 in the Supporting Information). That is to say, the dimensionality of the perovskites is stable, regardless of the various irradiation powers applied. Meanwhile, these experiments suggest that the irradiations at various powers do not change the phase compositions of the obtained perovskite NCs (Figure S14 in the Supporting Information). However, the average sizes C

DOI: 10.1021/acs.inorgchem.7b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Relationship between the Structures of As-Synthesized CsPbBr3 NCs and the Experimental Processa reagent in use

a

sample

dimension

PbBr2 (mg)

OA (mL)

OAm (mL)

Cs-oleate (mL)

power (W)

time (min)

av size/length (nm)

shape

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0

690 690 690 345 345 230 230 690 690 690 1380 1380 1380 1380 1380 1380 1380 690 690 690

9 9 9 4 4 2.67 2.67 8 8 8 16 16 16 16 16 16 16 6 6 6

1 1 1 1 1 0.67 0.67 2 2 2 4 4 4 4 4 4 4 4.5 4.5 4.5

0.4 0.4 0.4 0.2 0.2 0.133 0.133 0.4 0.4 0.4 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.4 0.4 0.4

400 600 800 400 600 400 600 400 600 800 100 100 100 400 400 400 600 400 600 800

1 1 1 1 1 1 1 1 1 1 1 3 5 1 3 5 1 1 1 1

10.8 13 15.2 8.1 8.5 15 17.4 216 241 295 31 254 456 280 679 763 345 235.4 248.8 312.3

nanocube

1

2

nanorod

nanosheet

The amounts of both ODE and DGBE were fixed at 25 mL.

skite NCs within minutes with tunable morphologies in dimensionalities and sizes.

become larger and larger with an increase in the irradiation powers, namely, from 10.8 to 13.0 nm and then to 15.2 nm for the sample S0D, once the powers are fixed at 400, 600, and 800 W, respectively (Figures S15 and S16 in the Supporting Information). Similar phenomena are observed for the samples S1D and S2D, as shown in Figure 3b1−b3,c1−c3) and Figure S17 in the Supporting Information. For sample S1D, the NC lengths are tailored to 216, 241, and 295 nm, with irradiation powers fixed at 400, 600, and 800 W, respectively. For sample S2D, the lateral sizes of the NCs change to 235.4, 248.8, and 312.3 nm, once the powers are fixed at 400, 600, and 800 W, respectively (Figure S18 in the Supporting Information). These experimental results evidence that the sizes of the as-grown perovskite NCs could be tailored by controlling the MW irradiation powers. This could be mainly ascribed to the fact that a higher irradiation power shortens the time to trigger the growth of the NCs.44 In addition to the MW power, the irradiation time is also crucial to the growth of the NCs.45 For the sample S1D at a MW power of 400 W, we changed the irradiation times to 1, 3, and 5 min, leading to the growth of the perovskite nanorods with average sizes of 280, 697, and 763 nm, respectively (Figure 3d1−d3 and Figure S19 in the Supporting Information). Interestingly, the diameters of the nanorods exhibit no obvious change. Similar experiments were carried out at a lower power of 100 W, and the products are shown in Figure S20 in the Supporting Information, showing that their lengths average 31, 254, and 456 nm on irradiation for 1, 3, and 5 min, respectively. These experimental results clarify that, in comparison to the irradiation power, the irradiation times are more effective in tailoring the lengths of perovskite NCs. Briefly, Table 1 provides a summary to show the relationship between the MW parameters and the as-fabricated perovskite NCs (their corresponding SEM images are shown in Figure S21 in the Supporting Information), confirming that the microwave technique could be a general strategy for the rapid production of low-dimensional all-inorganic CsPbBr3 perov-

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CONCLUSIONS In summary, we have demonstrated a general strategy based on the microwave technique for the rapid production of lowdimensional all-inorganic CsPbBr3 perovskite nanostructures with tunable morphologies within minutes. The dimensionalities of the perovskite NCs could be tailored by adjusting the ratios of OA to OAm and of ODE to DGBE and the concentration of the PbBr2 precursors. Accordingly, a higher amount of OAm and DGBE introduced in the raw materials might favor the growth of the perovskite NCs from 0D to 1D and then to 2D, and a higher concentration of the PbBr2 precursor would facilitate the anisotropic growth of 1D perovskite NCs. Meanwhile, the sizes of the perovskite NCs could be tuned by controlling the microwave powers and irradiation times, in which the latter might be more effective. The present work offers some valuable insights in directing the controlled growth of low-dimensional all-inorganic perovskite NCs with high purity in morphologies, which could push them forward to be potentially applied in optoelectronic devices.

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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.7b02941. Experimental procedures, characterizations of EDX, XRD, SEM, and TEM, size distribution, and PL performance of the as-synthesized CsPbBr3 perovskite nanocrystals (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for X.H.: [email protected]. D

DOI: 10.1021/acs.inorgchem.7b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry *E-mail for W.Y.: [email protected].

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ORCID

Weiyou Yang: 0000-0002-3607-3514 Author Contributions ∥

W.L. and J.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation for Excellent Young Scholars of China (Grant No. 51522402), the National Natural Science Foundation of China (NSFC, Grant Nos. 51572133, 51672137, and 51702175), the Zhejiang Provincial Nature Science Foundation (Grant No. LQ17E020002), the Natural Science Foundation of the Ningbo Municipal Government (Grant No. 2016A610103 and 2016A610104), and the Special Fund of the National Excellent Doctoral Dissertation (No. 201437).

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DOI: 10.1021/acs.inorgchem.7b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02941 Inorg. Chem. XXXX, XXX, XXX−XXX