Colloidal Synthesis of Air-Stable CH3NH3PbI3 Quantum Dots by

Article pubs.acs.org/cm

Colloidal Synthesis of Air-Stable CH3NH3PbI3 Quantum Dots by Gaining Chemical Insight into the Solvent Effects Feng Zhang,† Sheng Huang,† Peng Wang,‡ Xiaomei Chen,† Suling Zhao,‡ Yuping Dong,† and Haizheng Zhong*,† †

Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing 100081, China ‡ Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China S Supporting Information *

ABSTRACT: Because of the superior optical properties and potential applications in display technology, colloidal synthesis of halide perovskite quantum dots has been intensively studied. Although great successes have been made in the fabrication of green emissive CH3NH3PbBr3 quantum dots, the fabrication of stable iodide-based CH3NH3PbI3 quantum dots remains a great challenge because of their sensitivity to moisture in the open air. Even in a glovebox, the colloidal CH3NH3PbI3 quantum dots obtained from N,N-dimethylformamide suffer from instability caused by fast degradation within days to weeks. In this work, we investigated the interactions between perovskite precursors and various polar solvents as well as their influence on the crystallization of CH3NH3PbI3 in reprecipitation synthesis. By gaining chemical insight into the coordination effects, we can explain the degradation of CH3NH3PbI3 to the defective crystals with coordinated solvents on the surface and/or intrinsic inner iodine vacancies. On the basis of this understanding, we fabricated air-stable CH3NH3PbI3 quantum dots with a tunable size from 6.6 to 13.3 nm by selecting noncoordinated acetonitrile as a good solvent through ligand-assisted precipitation synthesis. The fabrication can be processed under ambient conditions, and the resulting CH3NH3PbI3 quantum dots exhibit tunable emission with high photoluminescence quantum yields (maximum of ∼46%) as well as good stability. Moreover, the quantum confinement effects in CH3NH3PbI3 quantum dots were discussed by correlating the size-dependent photoluminescence properties with theoretical calculations, which can be described by the infinite quantum well approximation model.

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INTRODUCTION

As an important feature of QDs, the quantum confinement effects in perovskite QDs have been a frequently discussed topic for understanding their dimensionally dependent properties.18−20 In particular, the comparison between experimental determinations and theoretical calculations of size-tunable QDs could derive a physical model to describe the influence of size and shape on their optical properties. Considering the quantum confinement effects in perovskite QDs, previous works have focused on the blue shifting in CH3NH3PbBr3 QDs.21,22 Yamauchil et al. discussed the quantum confinement effects of one-dimensional cylinder-like CH3NH3PbI3 nanocrystals eḿ bedded in mesoporous silica.23 Very recently, Anaya, Miguez, and co-workers investigated the quantum confinement effects of CH3NH3PbI3 nanocrystals embedded in thin metal oxide films.24 To avoid the interference of particle−particle interactions, size-tunable CH3NH3PbI3 QDs in colloidal solutions are in demand for the experimental determination of quantum confinement effects. Herein, we report the colloidal synthesis of air-stable CH3NH3PbI3 QDs with tunable size

Over the past three years, halide perovskite quantum dots (QDs) have newly emerged as QDs with superior photoluminescence (PL) properties that are inexpensive and have an easy fabrication process.1−4 These unique features make them promising candidates for lighting and display applications, including liquid crystal display (LCD) backlights and electroluminescence (EL) devices.5−9 The fabrication of stable and wavelength-tunable perovskites QDs is a prerequisite for achieving improved performance (color gamut, brightness, external quantum efficiency, etc.).10−12 However, current success give only stable green and blue emissive CH3NH3PbBr3−xClx or CH(NH2)2PbBr3−xClx (0 ≤ x ≤ 3) QDs.6,13,14 In contrast, iodide-substituted counterparts with PL emissions in the red and near-infrared regions are more sensitive to moisture and experienced rapid degradation under ambient conditions in the open air.5,6 The poor stability of CH3NH3PbI3 QDs hindered the fundamental spectroscopic study as well as the exploration of devices.15 Therefore, it has been of great interest to further explore the colloidal chemistry of perovskites to achieve air-stable CH3NH3PbI3 QDs.16,17 © 2017 American Chemical Society

Received: March 17, 2017 Published: April 5, 2017 3793

DOI: 10.1021/acs.chemmater.7b01100 Chem. Mater. 2017, 29, 3793−3799

Article

Chemistry of Materials through solvent engineering and illustrate their size-dependent PL emission. We recently developed the ligand-assisted reprecipitation synthesis (LARP) strategy for the fabrication of brightly luminescent colloidal CH3NH3PbX3 (X = Br, Cl, or I) QDs by mixing a pair of polar and nonpolar solvents.6 It was found that CH3NH3PbI3 QDs can be achieved only in a glovebox and are unstable in the open air. Solvent engineering has proven to be an effective way to optimize the morphology of perovskite thin films toward highly efficient solar cells.25−27 Encouraged by the enhanced performance in thin film-based device, we studied the solvent effects of CH3NH3PbI3 in the fabrication of perovskite QDs from colloidal solutions. In particular, we performed a detailed investigation of the interactions between CH3NH3PbI3 precursors and different polar solvents, including N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), γ-butyrolactone (GBL), and acetonitrile (ACN), using UV−vis absorption and 207Pb nuclear magnetic resonance (NMR). The coordination effects of solvents in LARP synthesis were discovered and discussed. By selecting ACN and toluene as a pair of solvents, we successfully fabricated air-stable colloidal CH3NH3PbI3 QDs with a tunable size from 6.6 to 13.3 nm. The correlation between size and PL emission peaks was theoretically and experimentally studied to demonstrate their quantum confinement effects in CH3NH3PbI3 QDs.

Figure 1. (a) Absorption spectra of iodide precursors (CH3NH3I and PbI2 at a molar ratio of 1:1) dissolved in solvents DMSO, DMF, THF, GBL, and ACN. (b) 207Pb NMR spectra of the precursor dissolved in DMSO, DMF, and ACN.

Table 1. Comparison of the Characterizations of the Precursor Solutions in DMSO, DMF, THF, GBL, and ACN

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RESULTS AND DISCUSSION In the previous works of LARP synthesis, DMF and toluene were usually chosen as a pair of good and poor solvents to introduce the crystallization process.28−32 However, the fabrication of CH3NH3PbI3 QDs through mixing DMF and toluene encounters a challenge of fast degradation in open air. The degradation usually results in white flocculent precipitates. Even using spectrograph DMF, the degradation process lasts for only several days to a week in glovebox. It has been learned that the interactions between CH3NH3PbI3 precursors and DMF result in the formation of CH3NH3PbI3·solvent intermediates, which play an important role in the subsequent crystallization process.33,34 To illustrate the solvent effects in colloidal CH3NH3PbX3 QDs, we first investigated the solubility of CH3NH3PbX3 precursors in various solvents. The qualitative results are summarized in Table S1, and a photograph of dissolved solutions is shown in Figure S1. The results show that most polar solvents with high polarity can act as good solvents for CH3NH3PbI3 precursors. Here, we selected five commonly used polar solvents (DMSO, DMF, THF, GBL, and ACN) to explore the solvent effects in the fabrication of CH3NH3PbI3 perovskite QDs. The solvent effects were first investigated by applying UV− vis absorption spectroscopy. Figure 1a shows the UV−vis absorption spectra of the precursor solutions in these five selected solvents, and Table 1 summarizes the identified absorption peaks. For comparison, the absorption spectrum of a PbI2 film deposited on quartz glass was also present. As shown in Figure 1a, the absorption spectra of PbI2 solutions in GBL and ACN are similar to that of the PbI2 film with two featured peaks centered at 325 and 370 nm. In contrast, the spectra of the PbI2 solutions in DMSO, DMF, and THF are different from that of the PbI2 film, implying the coordination effects between these solvents and CH3NH3PbI3. Similar results were also observed in the CH3NH3PbBr3 solutions (see Figure S2). PbI2 has an extended two-dimensional layered structure

absorption peak (nm)

207 Pb chemical shift (ppm)

category

solvent

coordinated

DMSO DMF

295 323

805, 397 803, 367

THF GBL ACN

300, 368 325, 370 325, 370

− − 779, 335

noncoordinated

precipitate none CH3NH3PbI3· H2O PbI2 CH3NH3PbI3 CH3NH3PbI3

with planes of Pb2+ ions sandwiched between adjacent layers of hexagonally arranged iodide ions.35 The good consistency of absorption spectra of the precursor solutions in GBL and ACN with the PbI2 film indicates that the intrinsic structure of PbI2 is maintained when it is dissolved in GBL and ACN, while the distinct absorption spectra of precursor solutions in DMSO, DMF, and THF confirm the formation of CH3NH3PbI3·solvent intermediates, which is consistent with the literature reports on thin film fabrications.36−38 To understand the coordination between precursors and solvents in the CH3NH3PbI3·solvent intermediates, we then determined the chemical shift by 207Pb NMR spectra. Figure 1b shows the 207Pb NMR spectra of the precursor dissolved in deuterated DMSO, DMF, and ACN. The precursor solution in ACN exhibits obvious chemical shifts of 335 and 779 ppm, while the chemical shifts more downfield to 367 and 803 ppm for the DMF solution and 397 and 805 ppm for the DMSO solution, respectively. According to the calculated chemical shifts in lead dehalides, PbI2 in the fragment of [PbI6]4− can exhibit chemical shifts ranging from 2 to 800 ppm with varied crystallographic data.39 The substitution of an iodine atom with an oxygen atom leads to a downfield shift in 207Pb spectra due to the deshielding effect of the oxygen atom.40,41 On the basis of Lewis acid−base theory, the observed downfield-shifted chemical shifts in DMF and DMSO precursor solutions can be explained by the formation of the Pb−O bond. DMSO, DMF, and THF have coordination abilities that are stronger than those of GBL and ACN to coordinate with the precursors of CH3NH3PbI3.42,43 As summarized in Table 1, the interactions between polar solvents and CH3NH3PbI3 precursors can be divided into coordination and noncoordination. To explore the solvent effects in the fabrication of CH3NH3PbI3 perovskite 3794


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Chemistry of Materials

Figure 2. Schematic illustrations of the transformation process from a precursor (CH3NH3I and PbI2) to CH3NH3PbI3 perovskite in coordinated solvents (top) and noncoordinated solvents (bottom).

Figure 3. TEM images of resulting CH3NH3PbI3 QDs with average sizes of (a) 13.3 ± 1.2 nm, (b) 11.7 ± 1.0 nm, (c) 10.6 ± 1.1 nm, (d) 9.8 ± 1.0 nm, and (e) 6.6 ± 0.7 nm. The scale bar is 50 nm. (f) HRTEM image of a typical CH3NH3PbI3 QD. The inset is the corresponding FFT image.

On the basis of the results described above, we proposed a model to illustrate the degradation of CH3NH3PbI3 QDs from the precursors in DMF. As shown in Figure 2, the precursor dissolved in coordinated solvents forms PbI2·solvent intermediates. In the precipitation process, PbI2·solvent intermediates and methylamine coprecipitated to form CH3NH3PbI3. Because of the strong bonding between PbI2 and coordinated solvents, the CH3NH3PbI3 that crystallized from PbI2·solvent intermediates contains residual solvent molecules on the surface. Meanwhile, the removal of coordinated solvents may also generate intrinsic iodine vacancies in the inner of the resulting CH3NH3PbI3 crystals. The resulting CH3NH3PbI3 with coordinated solvents on the surface or intrinsic iodine vacancies are defective crystals and are likely to transform into the CH3NH3PbI3·H2O intermediate in the presence of water from solvent or air. This assumption is supported by the larger formation energy of 5.87 eV for DMF substitutions and 1.85 eV for I vacancies through first-principles calculations (see the Supporting Information).46 In comparison with the defective crystals from coordinated solvents, the PbI2 units in the noncoordinated solvents can be well kept and experienced an in situ crystallization into defect free CH3NH3PbI3 with improved stability under ambient conditions.

QDs, the precursor solutions were added dropwise to toluene (the preparation of the precursor solutions is described in the Experimental Section). Then, 1 mL of the precursor solution was slowly added to 10 mL of toluene to induce the precipitation process. As shown in Figure S3, the results varied with the solvent, and the phenomenon was summarized as follows. (i) No precipitate was observed for the precursors in DMSO. (ii) The precipitation from the precursor solution in DMF results in white flocculent precipitates. (iii) The precursor solution in THF results in orange yellow-colored precipitates. (iv) The precursor solutions in GBL and ACN produced black precipitates. The precipitates were analyzed by applying XRD determinations, and the results are shown in Figure S4. The white flocculent precipitates from the precursors in DMF can be attributed to the CH3NH3PbI3·H2O intermediate,44 while the precipitates from precursors in THF exhibit obvious features of PbI2.45 The XRD spectra of the precipitates from the precursors in GBL and ACN match well with the standard pattern of CH3NH3PbI3 perovskite in the tetragonal phase (I4/ mcm). The difference in the precipitation process from CH3NH3PbI3 precursor solutions shows the importance of the interactions between solvents and precursors. 3795


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emission peaks in the PL spectra from 740 to 650 nm with their size decrease from 13.3 to 6.6 nm. Figure 4b is a photograph of the resulting CH3NH3PbI3 QDs in solution under ambient light and 365 nm UV radiation. With the size varied from 6.6 to 13.3 nm, the color of the QD solution gradually evolves from red to dark red. Quantitative absolute PL QY measurements show that the PL QYs of these samples are 21% (13.3 nm), 26% (11.7 nm), 30% (10.6 nm), 37% (9.8 nm), and 46% (6.6 nm). In addition, we also measured the photostability of the resulting CH3NH3PbI3 QDs by recording the evolution of the PL intensity under 6 W UV lamp radiation for 24 h in the open air. A slight decrease of 20% indicates good stability against UV radiation (see Figure S8). Time-resolved PL measurements were taken to study the size-dependent properties. As shown in Figure 4c, the CH3NH3PbI3 QDs with a tunable size exhibit a varied time dependency. These decay curves were fitted by using multiexponential functions. The fitting results illustrate that the average lifetimes are 11, 18, 32, 43, and 143 ns for 6.6, 9.8, 10.6, 11.7, and 13.3 nm CH3NH3PbI3 QDs, respectively (Supporting Information). The observed decrease in PL QYs and the shortness of the PL lifetime with a decrease in size corresponds to the transformation from exciton recombination to free electron−hole pair recombination.51,52 This conclusion is also supported by the spectroscopic study in large grain thin films.53 To understand the size-dependent PL properties of CH3NH3PbI3 QDs, we tried to correlate the experimental values with the calculation results by adapting Brus’s equation of infinite and finite quantum well (QW) model (see the Supporting Information).54,55 Figure 5 plots the calculated

ACN was then selected for the fabrication of CH3NH3PbI3 QDs because of its low boiling point and easy removal process.47 The experimental details can be found in the Experimental Section. Generally, PbI2, CH3NH3I, OA, and a certain amount of octylamine were dissolved in ACN to form a clear solution, which was added dropwise to toluene under a UV lamp (365 nm). With the addition of the precursor solution, black-brown precipitates immediately appeared in the toluene solution, with strong red PL emission implying the crystallization of CH3NH3PbI3. After the addition, the toluene solution was in a turbid state. Then the turbid solution was centrifuged at 7000 rpm for 5 min to separate CH3NH3PbI3 QDs. As reported in the literature, surface ligands play a vital role in the contribution to the formation of nanocrystals.48−50 Here, the amount of octylamine was adjusted from 10 to 20 μL to obtain size-tunable CH3NH3PbI3 QDs. Figure 3 and Figure S5 show the transmission electron microscopy (TEM) images and statistical analysis of CH3NH3PbI3 QD samples with tunable sizes of 13.3 ± 1.2, 11.7 ± 1.0, 10.6 ± 1.1, 9.8 ± 1.0, and 6.6 ± 0.7 nm that were fabricated with varying amounts of octylamine. This method can also be extended for the fabrication of CH3NH3PbBrxI3−x QDs (see Figure S6). The resulting CH3NH3PbI3 and CH3NH3PbBrxI3−x QDs are stable in the open air for more than 3 months. Figure 3f shows the high-resolution TEM (HRTEM) image of a typical CH3NH3PbI3 QD. From the HRTEM image and corresponding Fourier transform (FFT) image (inset of Figure 3f), interplanar distances of 3.63 and 3.09 Å were identified, corresponding to the (202) and (220) planes of CH3NH3PbI3, respectively. The phase structure of these resulting CH3NH3PbI3 QDs was further confirmed by applying XRD measurements. As shown in Figure S7, the main diffraction peaks for these samples can be assigned to a tetragonal CH3NH3PbI3 crystal structure (space group I4/mcm). The resulting size-tunable CH3NH3PbI3 QDs are of high stability, allowing us to study their size-dependent optical properties. Figure 4a shows the steady state absorption and PL spectra. The absorption spectra show the band edges in the absorption spectra blue-shifted in comparison to that of their bulk counterpart. Corresponding to the absorption spectra, the

Figure 5. Calculated curves from the infinite QW (green dotted curve) and finite QW (red dotted curve) model, along with the experimental results of CH3NH3PbI3 QDs of various sizes (gray spheres).

curves of the infinite QW and finite QW model. The experimental data were also marked. Unlike the reported sizedependent PL emissions in perovskite nanocrystals embedded in mesoporous silica, our experimental data match well with the infinite QW model.23 The difference can be explained to the strong Coulombic interactions due to tight packing of the NCs in silica pores. In our case, the resulting CH3NH3PbI3 QDs are capped with ligands and well-dispersed in toluene; the interactions between these QDs can be greatly avoided to show their inherent quantum confinement effects.

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Figure 4. (a) UV−vis absorption and PL emission spectra. (b) Photographs of CH3NH3PbI3 QDs with average diameters of 6.6, 9.8, 10.6, 11.7, and 13.3 nm in ambient light and under UV 365 nm radiation. (c) Time-resolved PL decay and fitting curve of typical CH3NH3PbBr3 QDs in toluene.

CONCLUSIONS In summary, we investigated the interactions between perovskite precursors and solvents during the crystallization of 3796


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solutions were filtered with polytetrafluoroethylene (pore size of 220 nm) prior to characterization. XRD patterns were measured on a Bruker/D8 FOCUS X-ray diffractometer with a Cu Kα radiation source (wavelength of 1.5406 Å). The samples were scanned from 3° < 2θ < 60° at an increment of 2°/min. Liquid samples deposited on amorphous carbon-coated copper grids were analyzed using a JEOLJEM 2100F TEM instrument operating at an acceleration voltage of 200 kV. PL spectra were recorded using a F-380 fluorescence spectrometer (Tianjin Gangdong Science & Technology Development Co., Ltd.). Time-resolved PL was collected using a fluorescence lifetime measurement system (C11367-11, Hamamatsu Photonics) with an excitation wavelength of 405 nm. The absolute PL QYs of diluted QD solutions were determined using a fluorescence spectrometer with an integrated sphere (C9920-02, Hamamatsu Photonics) under blue light-emitting diodes excited at a wavelength of 450 nm. The solubility of the CH3NH3PbI3 precursor in different solvents was tested by adding equimolar amounts of CH3NH3I and PbI2 (0.1 mmol) to a certain amount of solvent (2 mL) followed by centrifugation for 10 min.

CH3NH3PbI3 from a colloidal solution. By analyzing UV−vis absorption spectra and 207Pb NMR spectra, we demonstrated that the precursor of CH3NH3PbI3 formed intermediates in coordination solvents (DMSO, DMF, and THF) but maintained intrinsic PbI2 units in noncoordination solvents (GBL and ACN). The CH3NH3PbI3 resulting from coordinated solvents could be a defective crystal with residual solvents on the surface and intrinsic iodine vacancies, which account for their fast degradation. On the basis of our understanding described above, we chose ACN and toluene as a good and poor solvent to fabricate air-stable CH3NH3PbI3 QDs with a tunable size from 6.6 to 13.3 nm. This fabrication can be performed under ambient conditions, and the resulting CH3NH3PbI3 QDs show strong emission with a maximal PL QY of 46%. Moreover, the quantum confinement effects in CH3NH3PbI3 QDs were illustrated by analysis of their sizedependent optical properties and theoretical calculations. It was found that the size-dependent optical bandgap can be described using the infinite QW model. This provides a good model for predicting the size-dependent properties of CH3NH3PbI3 QDs. Moreover, the available stable CH3NH3PbI3 QDs will pave the way to achieving efficient 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.chemmater.7b01100. Additional information about the precipitates, XRD patterns, DLS results, calculation of the binding energy between CH3NH3PbI3 and DMF, analysis of PL lifetimes, and calculation of the quantum confinement model (PDF)

EXPERIMENTAL SECTION

Materials. PbI2 [lead(II) iodide, 98.5%, Alfa Aesar], methylamine (CH3NH2, 33 wt % in absolute ethanol, Aladdin), n-octylamine (≥99%, Aladdin), hydriodic acid (HI, 57 wt % in water, Alfa Aesar), oleic acid (≥90%, Alfa Aesar), DMF (analytical grade, Beijing Chemical Reagent Co., Ltd.), DMSO (analytical grade, Beijing Chemical Reagent Co., Ltd.), THF (analytical grade, Beijing Chemical Reagent Co., Ltd.), GBL (analytical grade, Beijing Chemical Reagent Co., Ltd.), ACN (analytical grade, Beijing Chemical Reagent Co., Ltd.), toluene (analytical grade, Beijing Chemical Reagent Co., Ltd.) were used in this study. Deuterated solvents (DMSO-d6, DMF-d7, and ACN-d3) were purchased from Aladdin. All the reagents were used as received without further purification. Preparation of Precursor Solutions. CH3NH3I was synthesized as described in the literature.6 A fixed amount of PbI2 (0.1 mmol, 0.0459 g) and CH3NH3I (0.1 mmol, 0.0159 g) were separately dissolved in 2 mL of different solvents (DMSO, DMF, THF, GBL, and ACN). The obtained solutions were sonicated for 10 min followed by centrifugation at 7000 rpm for 5 min. Then the supernatant was collected for further characterization. Fabrication of Hybrid CH 3 NH 3 PbI 3 QDs. Colloidal CH3NH3PbI3 QDs were fabricated following the reported LARP technique.6 In a typical synthesis, 0.1 mmol of PbI2 (0.0459 g), 0.08 mmol of CH3NH3I (0.0127 g), 0.2 mL of oleic acid, and a certain amount of n-octylamine (10, 12, 14, 16, or 20 μL) were dissolved in 2 mL of ACN. Then the mixed solution was sonicated for 15 min to obtain a clear precursor. Under the UV lamp, the precursor solution was added dropwise to 10 mL of toluene while being vigorously stirred. The injection speed was varied between 20 and 100 μL/min. Along with the precursor precipitating, the solution gradually became cloudy. After centrifugation at 7000 rpm for 10 min and after precipitates had been discarded, a bright red colloidal solution was obtained. The colloidal solution was further treated by bubbling it nitrogen gas to remove the residual ACN. CH3NH3PbI3 QD powder can be obtained by eliminating the organic solvents. Characterizations. UV−vis absorption spectra were recorded on a UV-6100 UV−vis spectrophotometer (Shanghai Mapada Instruments Co., Ltd.). 207Pb NMR experiments were performed on a Bruker Ascend 700 spectrometer, using a 5 mm quartz tube and deuterated solvents (DMSO-d6, DMF-d7, and ACN-d3). The concentration of the CH3NH3PbI3 precursor was adjusted to saturation to obtain relatively strong signals. All of the 207Pb NMR chemical shifts were referenced to tetramethyllead (TML; δ = 0 ppm). Pb(NO3)2 was used as an external standard (δ = −2990 ppm, 1.0 M in D2O, 25 °C). The prepared

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suling Zhao: 0000-0003-0310-5628 Haizheng Zhong: 0000-0002-2662-7472 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573018) and the Beijing Nova program (xx2014B040).

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REFERENCES

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