Ligand Assisted Transformation of Cubic CsPbBr3 Nanocrystals into

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Ligand Assisted Transformation of Cubic CsPbBr3 Nanocrystals into 2-Dimensional CsPb2Br5 Nanosheets Subila K Balakrishnan, and Prashant V. Kamat Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04142 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Chemistry of Materials

Ligand Assisted Transformation of Cubic CsPbBr3 Nanocrystals into 2‐Dimensional CsPb2Br5 Nanosheets Subila K. Balakrishnan and Prashant V. Kamat* Radiation Laboratory Department of Chemistry & Biochemistry, University of Notre Dame Notre Dame, IN 46556 Address correspondence to this author: [email protected]

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Abstract: The interactive nature of metal halide perovskite nanocrystals makes them highly susceptible to the chemical environment, thus facilitating transformation into new derivatives with tailored functional properties. Ligand‐assisted transformation of CsPbBr3 nanocrystals (NCs) to highly crystalline two‐ dimensional (2‐D) CsPb2Br5 nanosheets has now been achieved via their exposure to dodecyl dimethyl ammonium bromide (DDAB) under ambient conditions. The formation of 2‐D CsPb2Br5 nanosheets was established using transmission electron microscopy (TEM) and X‐ray photoelectron spectroscopy (XPS) analysis. The changes in the absorption spectra indicated two‐step dissolution‐reorganization mechanism during the DDAB induced morphological transformations. TOC Graphics

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Owing to their excellent photoluminescence and optoelectronic properties, metal halide hybrid perovskite nanocrystals (NCs) are important for designing high efficiency solar cells and light emitting diodes.1‐3 The intrinsically soft structure and chemical instability renders these perovskite structures to undergo transformations with the exchange of cations or anions within the lattice. Such chemical exchanges have often been utilized to tune their photophysical properties.4‐8 For example, halide ion exchange reactions produce mixed halide perovskites with a tunable band gap covering the entire visible spectrum.4, 9‐11 Additionally, the cation exchange in methyl ammonium lead halide (MAPbX) perovskite nanocrystals (NCs) with long alkyl chain cations can produce hybrid two‐dimensional (2‐D) perovskites.12‐ 15

By controlling the concentration and composition of ligands (e.g., oleic acid (OA) and oleylamine (OM)),

it is also possible to alter the shapes of perovskite structure, from zero‐dimensional quantum dots to 2‐D nanoplatelets or nanosheets.16‐18 We now present dodecyl dimethyl ammonium bromide (DDAB) induced transformation of CsPbBr3 NCs into crystalline 2‐D CsPb2Br5 nanosheets and the mechanistic insights associated with morphological changes. CsPbBr3 perovskite nanocrystallites, exhibit three different phases: orthorhombic (Phnm), tetragonal (P4/mbm), and cubic (Pm3m).19‐20 The cubic phase obtained at higher temperatures (≥130 °C) exhibits good stability. We employed a hot injection method to obtain cubic CsPbBr3 NCs by reacting Cs‐ oleate with Pb(II)‐bromide at 170°C. The PbBr2 (0.19 mmol) was dissolved in octadecene (high boiling point solvent, 5 ml) containing OM (1.52 mmol) and OA (1.58 mmol) as ligands under an argon atmosphere. Cs‐oleate (0.046 mmol) was then quickly injected at 170o C and the formed NCs were precipitated using a butanol‐acetone (1:2) mixture followed by centrifugation. The purified NCs were then redispersed in the toluene (synthetic details are included in the supporting information, section 3). The absorption and emission spectra of colloidal CsPbBr3 in toluene are presented in Figure 1A. The CsPbBr3 NCs in toluene exhibit band edge (absorption shoulder) around 500 nm and emission wavelength maximum at 507 nm. The CsPbBr3 exhibit bright photoluminescence (Quantum yield= 70 %) with a full width at half maximum (FWHM) of 22 nm indicating the NCs to be monodisperse. The transmission electron microscopy (TEM) analysis further confirms that the NCs are monodisperse with an estimated size of 72 nm (Figure 2A). The d‐spacing analysis indicates that the nanocrystals possess (001) plane with a cubic structure. The elemental analyses by energy dispersive x‐ray (EDX) spectroscopy confirmed the 1:1:3 atomic ratios for the prepared CsPbBr3 sample (EDX spectra and atomic composition is included in the supporting information, Figure S1).

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a, Abs b, PL

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400 500 Wavelength, nm

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CsPbBr3 +DDAB

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550

400 500 Wavelength, nm -

[PbBr3]

CsPbBr3 +DDAB

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b

-

[Pb2Br5]

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k 0.3 300


360

Figure 1. (A) Absorption (a, black trace) and emission (b, red trace) spectra, of CsPbBr3 nanocrystals in toluene (3 ml). (B) Absorption and (C) normalized emission spectra (excitation wavelength 380 nm) recorded with a time interval of 1 minute after the addition of DDAB (250 toCsPbBr3 NCs in toluene: a) before adding DDAB and (b)‐(k) after addition of DDAB. The magnified region of absorption at the band edge is shown in the inset of B. (D) The zoomed section of absorption spectra in the 300 ‐ 360 nm wavelength region during the reaction with DDAB. The chemical transformation of perovskite NCs through the ligand exchange was initiated with the addition of DDAB (250 µM) to CsPbBr3 NCs (100 µM) dispersed toluene. The changes in the absorption and emission spectra were monitored for 10 minutes (Figures 1B & C). Within the first minute, a marked change is seen in the absorption spectrum with decreased absorbance in the 400‐520 nm region and increased absorbance at 320 nm. This initial transformation seen upon addition of DDAB represents the first step of ligand exchange. This change is then followed by gradual blue shift in the absorption shoulder (500 to 490 nm) with time (the magnified view of the absorption change can be seen in the inset of Figure 1B). Concurrently a blue‐shift was also observed in the emission spectra (Figure 1 C) accompanied by a significant decrease in the emission quantum yield (from 70% to 4%). A closer look at the absorption changes in the 300‐380 nm wavelength region reveals the existence of two distinct species (Figure 1D). In addition to the 320 nm band observed immediately after

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the addition of DDAB, a new band at 345 nm emerges with increasing time. The absorption at 320 nm band corresponds to [PbBr3]‐ complex21 and represents initial product of chemical transformation induced by DDAB within the first minute. Thus, the interaction between DDAB and CsPbBr3 results in quick exfoliation as Cs+ ions are exchanged with long cationic alkyl chain leading to the formation of Pb‐ complexes. With increasing time we observe the formation of [Pb2Br5]‐ complex with its broad absorption at around 340 nm.22,23 As the system equilibrates, the lead halide complex reorganizes to form 2‐D CsPb2Br5 perovskite in solution. A blue shift was observed in the emission spectra with time during the same time frame of system equilibration. It is known that the position of valance band maximum (VBM) and conduction band minimum (CBM) in lead halide perovskite (ABX3) structure is governed by the corner sharing BX6 (eg: PbBr6 ) in the perovskite structure.24, 25 The extended sharing of PbBr6 octahedra in the lattice of perovskite contributes to the band structure. As the DDAB ligand exfoliates the PbBr6 octahedra of CsPbBr3 we observe a blue shift in the absorption and band edge emission. At longer times we observe an evolution of second emission band with maximum at 523 nm (Figure 1C, k). This lower energy emission band seen at 523 nm is rather weak making a smaller contribution to overall quantum yield of 4%. The existence of dual emissions have been noted for other metal halide nanocrystals. Jiang and co‐workers reported CsPb2Br5 nanosheets with large indirect band gap and a PL inactive behavior26. On the other hand Han et al synthesized green emissive CsPb2Br5 nanoplatelets27. Although the emission properties of CsPb2Br5 yet to be established fully because of the existence of direct and indirect transitions28‐29. We have earlier studied the complexation of Pb2+ and halide ion in the precursor solution to understand the evolution of the overall perovskite structure in achieving desired electronic as well as optical properties.23, 30‐32 The blank experiments showing the ability of Pb2+ to complex with Br− in a solution containing DDAB and spectral changes associated with the formation of [PbBr3]‐ and [PbBr4]2− complexes are shown in the supporting information (Figure S4). A decrease in the 285 nm band of PbBr2 (1 mM) with a concurrent increase in the absorption at 310 nm indicated the formation of PbBr3 − complex following the addition of 250 µM of DDAB. Upon increasing the concentration of DDAB to 500 , a new peak at 360 nm corresponding to [PbBr4]2− emerges. These control experiments indicate the formation of lead bromide complex in a Pb2+ system containing excess bromide (DDAB). However when DDAB is added to presynthesised CsPbBr3 NCs we initially observe formation of PbBr3‐ complex. But this complex soon reorganizes to form a stable [Pb2Br5]‐ species with absorption at wavelength ~345 nm. The presence of isosbestic points at 320 and 345 nm further indicate evolution of CsPb2Br5 from initially formed DDA+[PbBr3‐] complex (Reactions 1 & 2). nCsPbBr3 + DDA+ (n‐x)CsPbBr3 + xDDA+[PbBr3]‐ +xCs+

(1)

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2x DDA+[PbBr3]‐ + xCs+ x[CsPb2Br5] + xDDA+Br‐ + xDDA+

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(2)

The selection of the ligand cation is important for inducing exfoliation of the perovskite lattice. If we replace dodecyl ammonium bromide (DDAB) with dodecyl amine (DDA) we fail to observe similar transformations. The spectral behavior monitored over 10 minutes upon addition of DDA to CsPbBr3 nanocrystal suspension did not show any significant changes in the band edge absorption (supporting information, Figure S5). The perovskite nanocrystals failed to exhibit morphological changes as evident from the TEM analysis (supporting information, Figure S6). The above results indicate that the ammonium cation of the long alkyl chain is essential to trigger the ligand‐assisted exfoliation. The morphological changes associated with the DDAB interaction with perovskite NCs was analyzed through the TEM analysis. Figure 2 shows the TEM images of NCs, before and after 10 minutes of DDAB treatment. Transformation of cubic CsPbBr3 NCs into 2‐dimensional nanosheets with lateral sizes on the micrometer scale could be seen in these images. The shape and size of these 2‐D crystallites are significantly different than the CsPbBr3 NCs (7 nm) prior to the DDAB treatment. Owing to their large lateral dimension (in microns), these nanosheets tend to lie flat on the TEM grid. On the other hand, the TEM images of the intermediate sample confirm dissolution of perovskite NCs (supporting information Figure S1 B). The TEM images of samples corresponding to 1 minute DDAB treatment did not show any NCs or 2‐D sheets. These results suggest that the transformation to obtain 2‐D structure is not instantaneous but requires additional time (~10 min) to achieve the stabilized configuration. The time dependent TEM analysis and spectral changes presented here confirm the two step mechanism of dissolution and reorganization of perovskite nanocrystals (reactions 1 and 2) in to 2‐D nanosheets. The selected area diffraction (SAED) pattern obtained for nanosheet (Panel F, Figure 2) exhibits a crystalline nature of the 2‐D structure. The diffraction spots associated with the (220) and (200) crystal planes could be indexed to the [001] zone axis of tetragonal CsPb2Br526, 33. The d‐spacing analysis carried out on the HRTEM images of nanosheets show a value of 0.306, matching with the value of (220) plane of tetragonal structure. Note that this value differs from the value of 0.576 nm, corresponding to (001) plane of cubic perovskite. The HRTEM analysis indicates a tetragonal structure (supporting information, Figure S2 & S3) for the newly derived 2‐D nanosheets. The XRD spectrum of CsPbBr3 nanocrystals (Supporting Information Figure S9) showed a peak at 2 14.9o and 30.2o corresponding to (100) and (200) planes respectively as per the cubic crystal structure (ICSD 29073). The DDAB treated sample shows disappearance of XRD peak at 2 14.9o (100) and appearance of new diffraction peaks at 2θ=12, 24, 29.3. These peaks are assigned to (002), (202) and (213), planes, consistent with the standard PXRD patterns of tetragonal CsPb2Br5 (PDF#25‐0211). 6


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Figure 2: TEM and HRTEM images of CsPbBr3 NCs (Panls A, B, C) and DDAB treated CsPbBr3 sample (Panels D, E, F) at different magnifications. The d‐spacing value is inserted in the HRTEM image of a single CsPbBr3. d‐space analysis was carried out with single crystal of CsPbBr3 (Panel C) and a small area of 2‐D sheet (Panel F). The selected area diffraction pattern collected during HRTEM analysis is included in the inset of panels B and E. We also probed the influence of alkyl chain length on the selective transformation of CsPbBr3 nanocrystals. The absorption spectra recorded following the addition of butylammonium bromide (BAB) and methyl ammonium bromide (MAB) to CsPbBr3 NCs suspension are shown in the supporting information (Figure S10). The butyl ammonium cation (BAB, 250 ) initiates the exfoliation of perovskite NCs by forming PbBr3‐ complex as seen from the appearance of the absorption band at 320 nm. However, additional spectral changes that can be associated with CsPb2Br5 formation could not be seen upon leaving the solution for equilibration. The TEM analysis also failed to show the presence of 2‐D nanosheets in BAB treated samples. The treatment of CsPbBr3 NCs with MAB (500 failed to exhibit noticeable spectral changes, an indication that CsPbBr3 is not susceptible to ligand exchange with MAB. Thus, the length of the alkyl chain becomes important as it drives the reorganization to form 2D structures. This process is analogous to the formation of layered double hydroxides.36 Thus, we need ammonium salt with long alkyl chain (e.g., DDAB) to induce the exfoliation followed by reorganization (Figure 3). The structural sensitivity to different chemical environment have been tested by other research groups. The synthesis of green luminescent tetragonal CsPb2Br5 nanoplatelets was reported by Guopeng Li et al who studied the formation of CsPb2Br5 nanosheets from the CsPbBr3 nanocubes by reacting with 7



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the excess PbBr2 precursor.26 They showed that it is thermodynamically favorable to undergo reorganization of PbBr6 octahedrons to PbBr8 capped‐triangular prisms and forming metastable tetragonal CsPb2Br5 structures. Recently, Alivisatos and coworkers studied the role of ligands such as amines and thiols in transforming CsPbBr3 NCs to Cs4PbBr6 NCs.34‐35

Figure 4: Schematic representation of ligand‐assisted exfoliation and transformation of cubic CsPbBr3 perovskite nanocrystals to tetragonal CsPb2Br5 nanosheets. The atomic ratio of the nanocrystals was calculated from the XPS analysis (details are included in the supporting information (Figure S7 & S8, Table S1)). The ratios of Pb /Cs and Br/Pb obtained for CsPbBr3 nanocrystals were 1.1 and 2.85 respectively, and they are in good agreement with the expected empirical ratio of 1: 1: 3. Interestingly, the ratios of Pb /Cs and Br/Pb for 2D nanosheets (viz., after DDAB treatment) were 2.12 and 2.48 respectively indicating a composition of 1: 2: 5. The atomic weight percentage estimated from TEM‐EDS analysis of the nanosheets also shows the ratio of Pb: Cs to be 2 (EDS data is tabulated in the supporting information). These results further support the transformation of CsPbBr3 NCs to CsPb2Br5nanosheets. In summary, ligand‐assisted transformation of CsPbBr3 NCs to 2‐D CsPb2Br5 nanosheets offers a simple technique to alter the morphology of perovskite structures. Such structural transformations are likely to be important for surface treatment to passivate defect sites and designing 3‐D/2‐D interfaces37. By subjecting CsPbBr3 films to controlled surface treatment with DDAB it should be possible to create such interfaces. As demonstrated recently creation of 3‐D/2‐D interface improves the photovoltaic and optoelectronic properties of perovskite solar cells.38‐39

Supporting Information. Experimental details of CsPbBr3NC synthesis, Instruments and techniques employed, TEM images and EDX Analysis, X‐ray Photoelectron Spectroscopy (XPS) & XRD Analysis Control experiment with BAB and MAB are included. This material is available free of charge via the Internet at http://pubs.acs.org 8


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Acknowledgements. The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, U.S. Department of Energy through grant no. DE‐FC02‐04ER15533. This is document no. NDRL 5188 from Notre Dame Radiation Laboratory. We would like to thank Dr. Sergei Rouvimov of Notre Dame Integrated Imaging Facility Advanced Electron Microscopy Core for his assistance in TEM and EDX measurements and Steven Kobosko for his help in XPS measurements References

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Ligand Assisted Transformation of Cubic CsPbBr3 Nanocrystals into

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