Scalable Synthesis of Exfoliated Organometal Halide Perovskite

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Scalable Synthesis of Exfoliated Organometal Halide Perovskite Nanocrystals by Ligand-Assisted Ball Milling Seokjin Yun, Artavazd Gh Kirakosyan, Soon-Gil Yoon, and Jihoon Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04092 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Sustainable Chemistry & Engineering

Scalable Synthesis of Exfoliated Organometal Halide Perovskite Nanocrystals by Ligand-Assisted Ball Milling

Seokjin Yun1, Artavazd Kirakosyan1, Soon-Gil Yoon1, and Jihoon Choi1,*

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Department of Materials Science and Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 34134, Republic of Korea

Corresponding author: Email: [email protected] Phone: +82 42 821 6632; Fax +82 42 821 5850

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ABSTRACT The scalable synthesis of colloidal organohalide perovskite nanocrystals is essential with increasing demands of their use in many applications such as photovoltaic and light emitting devices. However, only sub-gram quantities of perovskite nanocrystals were produced in the typical precipitation synthesis involving excess amounts of non-solvent, limiting their high-yield production. In this contribution, the ligand-assisted ball milling represents a substantial improvement in the scalability of high quality perovskite nanocrystals as well as the corresponding optoelectronic properties (e.g. PLQYs) by the synergetic effect of chemical fragmentation (i.e. ligand exfoliation) and mechanical shearing force. In particular, the formation of perovskite nanocrystals as well as exfoliated nanoplatelets was preferentially developed in the presence of ligands (n-octylamine or octadecylamine) that protect their surfaces after the reaction of solid precursors. This procedure is facile and robust enough to produce kilogram quantities of perovskite nanocrystals with a controlled crystal size, which allows their usage in practical applications.

KEYWORDS: organometal halide perovskites, nanocrystals, ligands, ball milling, exfoliation, quantum yield, photoluminescence

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INTRODUCTION Organohalide perovskite materials with an ABX3 crystal structure, where A is an organic cation such as methylammonium (CH3NH3 = MA) or formamidinium (NH2CHNH2 = FA), B is a metal cation (Pb or Sn), and X is a halide element (Cl, Br, I), have considerable attentions for recent a few years due to the tunability to engineer their compositions in A and B cations as well as X halide elements that provides a novel opportunity for control of the energy band structure and the corresponding absorption (Abs) and photoluminescence (PL).[1-6] Indeed, the optoelectronic properties of hybrid organic-inorganic perovskites enable the use of these materials for photovoltaic devices with highly enhanced power conversion efficiencies approaching 20 % [7]. In addition, high photoluminescence (PL) yield and the emission color covering the entire visible spectrum allow them to be potentially valuable in light-emitting diodes (LEDs) [8-10]. Recently, organohalide perovskites in the form of colloidal nanocrystals have been extensively explored to understand their size-dependent quantum confinement effects, which can significantly change their bandgap energy, fluorescence decay time, and absorption crosssection, and so on [11-20]. Several synthetic approaches to produce perovskite nanocrystals have been proposed on a basis of either chemical method or mechanical exfoliation [14,15,22]. Schmidt et al. reported a non-template synthesis of 6 nm-sized methylammonium lead bromide (CH3NH3PbBr3) nanocrystals by precipitating a precursor solution of CH3NH3Br and PbBr2 in the presence of long chain alkyl ammonium bromide and oleic acid [12]. Also, highly luminescent colloidal nanocubes of fully inorganic cesium lead halide (CsPbX3) perovskites have been reported to exhibit a wide range of compositional modulations as well as quantum sizeeffects (i.e. bandgap energies and emission spectra tunable over the entire visible spectral region 3



of 410-700 nm) [16]. More recently, Hintermayr et al. showed the synthesis of perovskite microcrystals by grinding the solid precursors (i.e. CH3NH3Br and PbBr2) together in a mortar, and further solvent process under sonication in the presence of oleylamine (OlA) was employed to obtain exfoliated nanoplatelets [22]. However, particular interest was placed on describing how to tune their bandgap energy and the relevant PL emission color, and fail to meet the rising demands for scalable synthesis of the organohalide perovskite nanocrystals, which is necessary in many industrial scale application fields including photovoltaic devices, piezoelectric generator, and LEDs for information displays. To date, synthetic methods for exfoliated organohalide perovskite nanocrystals have relied on the differences in the solubility of component precursors [12-15]. This strategy has been successful in preparing several types of perovskite nanocrystals by limiting their precursor solubility in a non-solvent [12-15], while its quantity is limited to only sub-grams that needs to be scaled up for further process in such applications. Moreover, these procedures lead to the increase of processing time and cost for product purification and chemical waste. Thus, low-cost and facile route for mass production of exfoliated nano-scale perovskite crystals is the most prominent challenge to future advances in the organohalide perovskite materials. Here, we show the first report on a gram-scale synthesis of exfoliated perovskite nanocrystals by ligand-assisted ball milling via the synergetic effect of chemical exfoliation and mechanical shear forces to overcome the aforementioned limitations of the prior approaches. During ball milling, the shear force accommodates a solid-state reaction of the solid precursors, resulting in microscale perovskite crystals. In the presence of ligands (OA: n-octylamine or ODA: octadecylamine), the perovskite microcrystals are then peeled off and reduced to nanoscale crystals with a much enhanced PL quantum yields (PLQYs). This approach is also 4


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shown to efficiently control the final composition of perovskite nanocrystals by adjusting their cationic and anionic precursor ratio.

EXPERIMENTAL Fabrication of perovskite nanocrystals by ball milling In a typical experiment stoichiometric amount of lead bromide, organic (methyl, ethyl, or formamidinium) ammonium bromide, surfactant species such as long alike chain amine, total corresponding to 1 gram of final product, was placed into plastic jar, which is 20 mL in volume and contains 25 g of zirconia ball (diameter is 1 mm). Octadecylamine and n-octylamine were used as a surfactant. The weight amount of starting materials for each type of perovskite is presented in the Table S1 (Supporting Information). Additionally 10 mL of toluene was added. Reactant to ball mass ratio was kept about 1 to 25 ratio. The jar was sealed under ambient atmosphere at room temperature and rotated at 150 rpm for one day. After the ball milling step the resulted product was separated from zirconia balls and subjected to ultrasonication step for 30 min. See the Supporting Information for more details.

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RESULTS AND DISCUSSION

Figure 1. (a-c) Schematic diagram and (d,e) the corresponding SEM micrographs and photographs (left panel: under day light, right panel: under 365 nm UV light) of the mechanism of scalable MAPbBr3 perovskite crystals formation and their fragmentation into nanoscale colloidal crystals. Total weight of the MAPbBr3 perovskite powder is 10 g.

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A simple solid-state reaction of conventional ball milling produced microscale-sized perovskite crystals (MAPbBr3) with an orange color as reported previously (Figure 1) [22,23]. The formation of a cubic crystal structure (space group: Pm3തm) with a lattice constant of 5.9334 Å was confirmed by XRD (Figure 2a), indicating that the precursors in a solid-state can efficiently react each other to produce the perovskites. However, PL intensity in toluene with OA was extremely low, resulting in ~ 8.5% of PLQY (Figure 2b,c) due to low absorption crosssection of bulk crystals with an average size of ~ 10 µm. Interestingly, the introduction of long chain alkyl ammonium such as OA and ODA into ball milling process significantly improved the formation of perovskite nanocrystals (Figure 1). SEM micrographs (Figure 1d,e) exhibit an obvious reduction of the crystallite size to submicrometer scales, but maintain a cubic crystal structure, regardless of the type of ligands (Figure 1a). See Supporting Information for more details (Figure S1). Also, largely enhanced PL intensity with a maximum at 533 nm was observed, leading to a quite higher PLQYs (~ 25%) (Figure 2b,c). This result can be interpreted as a consequence of the ligand-assisted exfoliation (i.e. osmotic swelling induced by the ligandscontaining solvent) combined by mechanical shearing force, which promotes the fragmentation of perovskite microcrystals during ball milling. Note that the gram-scale synthesis of organohalide perovskite nanocrystals can be achieved in a conventional procedure using a ball milling (Figure 1e), which enables them much more useful for commercial applications.

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Figure 2. Structural and optical characterization of the MAPbBr3 and FAPbBr3 perovskite nanocrystals. (a,d) X-ray diffraction patterns for MAPbBr3 and FAPbBr3 nanocrystals, respectively. Insets show the relevant SEM micrographs. Scale bars are 1 µm and 5 µm, respectively. Circles and diamonds denote PbBr2 impurities and nanoplatelets with the formula of [FAPbBr3]n-1PbBr4 (n = 2), respectively. (b,e) PL and (c,f) UV-Vis spectra for MAPbBr3 and FAPbBr3 nanocrystals, respectively. Insets of (b,e) show the corresponding photographs under day light and UV lamp (365 nm). Insets of (c,f) denote the relevant PLQY.

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In a similar way, the ligand-assisted ball milling was adopted to synthesize formamidinium lead bromide (FAPbBr3) nanocrystals. Based on the XRD pattern and the optical characteristics (i.e. negligible PL and Abs), it was obvious that a simple mixing of solid precursors in the absence of ligands cannot form the FAPbBr3 crystals (Figure 2d-f). Although weak XRD peaks corresponding to the cubic crystal structure (100), (110), (200), (210) of FAPbBr3 are observed (Figure 2d), two main peaks indicate PbBr2 impurities [24], revealing that NH2CHNH2Br and PbBr2 are not allowed to react each other in the solid-state. Surprisingly, those PbBr2 phases in XRD patterns almost disappeared when the ligands of OA or ODA were employed during ball milling (Figure 2d). In particular, very strong peaks at about 12˚ denoted by diamonds are observed, which can be directly attributed to the formation of nanoplatelets with a formula of [FAPbBr3]n-1PbBr4 (n = 2) [25,26]. Weak (200) and (211) peaks correspond to the bulk unit cell of FAPbBr3, indicating that the ligand-assisted ball milling yields a mixture of thin nanoplatelets and bulk-like nanocrystals. PL spectra (Figure 2e) confirm that the mixture primarily consists of nanoplatelets (n = 2) and larger nanocrystals with a prominent PL peak at 440 nm and 530 nm, respectively. Furthermore, a sharp excitonic absorption is observed at 432 nm, which is 0.6 eV blue-shifted from that of the bulk FAPbBr3 phase. This observation is consistent with the previous results for two-dimensional layered perovskites (n = 1, 2, 3, and ∞) [25], which is attributed to the presence of quantum-confined nanoplatelets thinner than the exciton Bohr radius (i.e. 1.4-2.0 nm) [23]. Also, much higher PLQY (~ 20%) was observed for OA compared to that for ODA (~ 5%). Thus, the presence of the ligands under the shearing force on the solid precursors can effectively accommodate the formation of perovskite nanocrystals by chemical and mechanical exfoliation.

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Figure 3. Transmission electron micrographs and diffraction patterns of (a,c,e) MAPbBr3 and (b,d,f) FAPbBr3 perovskites prepared by (a,b) simple ball milling, (c-f) ligand-assisted ball milling with (c,d) OA and (e,f) ODA, respectively.

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Figure 3 depicts representative TEM images of typical MAPbBr3 and FAPbBr3 perovskites, demonstrating their morphological characteristics depending on the ligand-assisted exfoliation. For MAPbBr3 perovskites, the size of crystals was significantly reduced from hundreds to tens of nanometer scale (~ 20 nm) upon addition of the ligand species, which provide the colloidal stability and limit the growth of the nanocrystals (Figure 3a,c,e). We note that the use of ODA exhibits a well-defined square shape in crystalline (Figure 3e). More significant feature in structural changes was observed for FAPbBr3 perovskites during the ligandassisted ball milling (Figure 3d,f), which gives clear evidence of morphological evolution of these perovskite nanocrystals. As expected, both thin nanoplatelets (n ~ 2) and thicker nanocrystals (n >> 2) are found to be developed in the presence of OA and ODA (Figure 3d,f). The lateral dimension of nanoplatelets ranges from several hundred nanometers to a few micron scale. Nanocrystals (< ~ 100 nm) tended to form dark clusters with irregular shape and arrangement. Although it has been shown that scalable synthesis of exfoliated MAPbBr3 and FAPbBr3 perovskite nanocrystals can be achieved by ligand-assisted ball milling, the preferential development of FAPbBr3 nanoplatelets associated with formamidinium bromide is not fully understood. We explored this phenomenon by introducing formamidinium bromide into MAPbBr3 perovskites.

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Figure 4. (a) X-ray diffraction patterns where diamonds denote nanoplatelets with the formula of [FAPbBr3]n-1PbBr4 (n = 2). (b-g) Transmission electron micrographs of the MAPbBr3 perovskite nanocrystals with organic cation components of 1, 5, 10 wt% (left: formamidinium bromide, right: ethylammonium bromide) prepared by the ligand-assisted ball milling with OA, respectively. (h) PLQY depending on the amount of organic cation components (blue: FABr, red: EABr). (i) Octahedral and tolerance factor of the perovskite with different organic component.

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Cation

exchange

of

MAPbBr3

perovskites

with

formamidinium

(FA+)

and

ethylammonium (EA+) was carried out to further investigate the role of the organic amine (i.e. MA+, FA+, and EA+) on the morphological evolution of the perovskite nanocrystals (Figure 4). Figure 4a shows XRD patterns of (FA)x(MA)1-xPbBr3 and (EA)x(MA)1-xPbBr3 perovskite nanocrystals by varying the ratio of FABr/MABr or EABr/MABr. Regardless of the type of organic amine and their ratio up to 10 wt.%, the diffraction angles corresponding to (100), (110), (200), and (210) planes of the bulk unit cell as well as nanoplatelet stacks were observed. Interestingly, only 1 wt.% of FA+ and EA+ could yield a significant deviation from the typical XRD pattern of pure MAPbBr3 perovskite, yielding strong peaks at about 12˚ and 27 ˚ relevant to the nanoplatelet stacks as we discussed in Figure 2 and 3 [25]. The overall characteristics are very similar each other, indicating the independence of the crystal structure on the amounts of organic amine [27]. Electron micrographs of (FA)x(MA)1-xPbBr3 also exhibit a similar trend on their morphology and size (Figure 4b-d). The size of nanocrystals is about tens of nanometer, revealing no significant change with the amount of FA+ (1 – 10 wt.%) on their morphology. In contrast, (EA)x(MA)1-xPbBr3 perovskite shows a more notable reduction of the crystal size with increasing EA+ ratio (Figure 4e-g). For 1 wt.% of EA+, large perovskite crystals (about 100 ~ 200 nm) are frequently observed with smaller nanoplatelets in Figure 4e. Upon addition of more EA+, the large crystals disappeared and only nanoplatelets with much smaller size (< ~100 nm) were observed (Figure 4f,g). It is thus natural to expect the dependence of their PLQY associated with their absorption cross-section on the organic amine (i.e. MA+, FA+, and EA+). Figure 4h depicts PLQYs as a function of the amount of FA+ and EA+. Addition of EA+ into MAPbBr3 perovskites substantially increases their PLQYs up to ~ 80 % at 10 wt. %, revealing a correlation

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between the PLQY and EA+ amounts while (FA)x(MA)1-xPbBr3 does not show any dependence. This can be interpreted as a consequence of the fragmentation of the large crystals due to EA+ ions (0.23 nm) with larger ionic radius than MA+ (0.18 nm) [28], which cannot fit into the space in between the corner sharing PbBr6 octahedra because a tolerance factor of MAPbBr3 (~ 0.84) is much smaller than that of EAPbBr3 (~ 0.96) (Figure 4i) [29]. However, FA+ ions (0.19-0.22 nm) are relatively easier to form the perovskite nanocrystals, exhibiting constant PLQYs for varying the ratio of FA+. Thus, on the basis of the correlation in Figure 4, we may conclude that the preferential development of the fragmented perovskite nanocrystals and their enhanced PLQYs are closely attributed to not only the ligand-assisted ball milling process (i.e. osmotic swelling and mechanical shearing force), but also the relative size of organic cations in precursor materials.

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CONCLUSIONS In conclusion, we have developed a simple, scalable route to synthesize exfoliated organometal halide perovskite nanocrystals by the synergetic combination of ligand exfoliation and mechanical shearing force in a conventional ball milling process, which leads to an improved yield of the nanoscale perovskite crystals and their optoelectronic properties. Ligandassisted mechanical exfoliation accelerates the formation of the MAPbBr3 perovskite nanocrystals with a much higher PLQY, and the preferential development of FAPbBr3 nanoplatelets was achieved when formamidinium bromide was employed during the process. The formation of [FAPbBr3]PbBr4 nanoplatelets and bulk-like nanocrystals indicates that this exfoliation process is not only easily scalable, but also applicable to similar types of layered materials. Further experimental study on cation exchange of perovskite nanocrystals was investigated to identify the fragmentation of the large crystals into nanocrystals and nanoplatelets, which shows a good correlation between the enhanced PLQYs and the preferential development of fragmented nanocrystals associated with the relative size of organic cation radii.

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ACKNOWLEDGEMENTS This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (No. NRF-2015R1C1A1A01052865, NRF-2016R1D1A1B03933212, NRF-2013R1A4A1069528).

SUPPORTING INFORMATION The detailed experimental procedures (Table S1) including the precursor preparation and the characterization (XRD, SEM, TEM, PL, Abs), the TEM images (Figure S1).

Competing Financial Interest Statement The authors declare no competing financial interests.

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TABLE OF CONTENTS

Ligand-assisted ball milling process provides a large-scale synthesis of organometal halide perovskite nanocrystals via the synergetic effect of chemical exfoliating and mechanical shear forces.

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