Perovskite Nanocrystals at High Reaction Temperatures

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Cite This: J. Phys. Chem. Lett. 2018, 9, 6599−6604

Annealing CsPbX3 (X = Cl and Br) Perovskite Nanocrystals at High Reaction Temperatures: Phase Change and Its Prevention Anirban Dutta, Rakesh Kumar Behera, Sumit Kumar Dutta, Samrat Das Adhikari, and Narayan Pradhan* School of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India 700032

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S Supporting Information *

ABSTRACT: Annealing perovskite nanocrystals at high reaction temperature changes their crystal phase, shape, and optical properties. Carrying out reactions between 180 and 250 °C, the impact of thermal annealing for CsPbCl3 and CsPbBr3 nanocrystals in a reaction flask was investigated here. At higher temperature, a phase change was observed instantly, which could not be trapped even with ice-bath cooling. Interestingly, using a calculated amount of preformed alkylammonium halides as dual passivating agents, the nanocrystals of both CsPbCl3 and CsPbBr3 could even be stabilized for hours of annealing at 250 °C. CsPbCl3, which was reported to be a poor emissive nanocrystal in comparison to CsPbBr3, could sustain even more than 5 h of annealing at 250 °C and recorded ∼51% absolutely quantum yield. Details of the interface chemistry and the role of the used dual passivating agent for providing thermal stability are studied and reported in this Letter.

observed to be thermally stable even at 250 °C with hours (∼5 h) of annealing and absolutely retained their crystal phase, shape, and also the optical properties. With the observed optical changes and phase evolution for CsPbCl3 and CsPbBr3 being similar, the discussion of the thermal stability was also extended to CsPbBr3 nanocrystals. The record high-temperature reaction for CsPbCl3 also led to a 51% photoluminescence (PL) quantum yield (QY), which remained highest for the as synthesized materials in film without any postsynthesis surface treatments. However, for CsPbBr3, the PLQY remained ∼75%. In addition, the high-temperature reactions also helped in obtaining the phase-pure tetragonal CsPb2Cl5 nanostructures for the first time via such colloidal route. Hence, phase change might not be a serious issue but can be overcome by suitable surface-passivating agents, and hours of annealing could be carried out without losing the precious properties of the nanocrystals. As CsPbBr3 nanocrystals were widely studied, emphasis was given for high-energy emitting CsPbCl3 nanocrystals, and their thermal stability was first investigated. Carrying out the reaction at the optimized reaction temperature (180 °C),1 it was observed that the PL intensity reduced with prolonged annealing. Figure 1a,b presents annealing time-dependent absorbance and corresponding PL spectra, and the intensity of both were seen to be reduced with time. Time-dependent ex situ powder X-ray diffraction (XRD) patterns of the nanocrystals suggested that the initial cubic phase of the nanocrystals remained unstable and slowly transformed to

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mong emerging energy materials, lead halide perovskite nanocrystals with exciting optical properties remain in the forefront of current research.1−10 For light emission, these nanocrystals have proved to be a potential material for their unprecedented high quantum efficiency.11−18 From their architectural processes, it is revealed that annealing at high temperature changes the optical properties of the nanocrystals, and hence, ice-bath cooling was mostly adopted soon after nanocrystal formation.1,19−21 To date, the optimized 120−180 °C temperature synthesis originally developed by Kovalenko and co-workers,1 is still largely adopted following instant cooling soon after Cs precursor injection to harvest the nanocrystals.1,21−27 Further, while the effect of annealing for the most widely studied nanocrystal CsPbBr3 is reported, the fate of CsPbCl3 nanocrystals has been less explored.20,25 In addition, all of these studies were restricted within 200 °C, but what would happen at higher reaction temperatures along with prolonged annealing, and how to prevent the phase change during long-term thermal annealing has not been explored. To address these issues, at the outset, physical insights of the interface chemistry and the thermal resistivity of phase or shape transition of the nanocrystals need to be investigated. From different literature reports, it is revealed that lead halide perovskites are capped with alkylammonium ions, and these ions have significant influence on their shape and thermal stability.25,27−30 Keeping these in mind, herein, we studied the interface chemistry of perovskite nanocrystals that could sustain thermal annealing at even higher than conventional reaction temperatures for hours without changing their shape, phase, and optical properties. Importantly, in the presence of excess halide/ammonium ions, these nanocrystals were © XXXX American Chemical Society

Received: September 13, 2018 Accepted: October 31, 2018

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DOI: 10.1021/acs.jpclett.8b02825 J. Phys. Chem. Lett. 2018, 9, 6599−6604

Letter

The Journal of Physical Chemistry Letters

Figure 1. Time-dependent (a) absorption spectra and (b) PL spectra of cesium lead chloride nanocrystals obtained at different time intervals carried out at a 180 °C reaction temperature. (c) Ex situ powder XRD pattern of the nanocrystals obtained at different reaction times. PL spectra were recorded at 350 nm excitation, and the optical density was corrected at the excitation wavelength. (d,e) Atomic models of cubic and tetragonal phases of cesium lead chlorides, respectively. (f,g) TEM images of cube-shaped CsPbCl3 nanocrystals and sheets of CsPb2Cl5.

Figure 2. (a) Absorption spectra of cesium lead chloride samples collected from the reaction carried out at 250 °C following instant cooling and after 1 min of annealing. (b) Corresponding PL spectra of respective samples. (c) Powder XRD pattern of the sample collected after 1 min of the reaction at 250 °C. (d) TEM image of the respective sample; 1 min in this case is referred to as the time after Cs precursor injection at 250 °C.

the nonemissive tetragonal phase of CsPb2Cl5(Figure 1c). Figure 1d,e presents typical unit cells of cubic CsPbCl3(COD: 1530111) and tetragonal CsPb 2 Cl 5 (COD: 4302867) respectively. The tetragonal CsPb2Cl5 unit cell is quite different than that of cubic CsPbCl3. In the cubic CsPbCl3 unit shell, [PbCl6]4−octahedra occupy all eight corners of a cube, and Cs occupies the central voids, whereas in tetragonal CsPb2Br5, Cs is sandwiched between two Pb−Cl polyhedra layers. Interestingly, this phase change was even noticed within 1 min of annealing at 180 °C. Transmission electron microscopy (TEM) images revealed that the initial (5 s) cube-shaped nanocrystals (Figures 1f and S1) were transformed to sheetlike structures (Figures 1g and S2). Hence, to retain the semiconducting phase of the nanocrystals, these reactions were optimized with instant or ice cooling soon after nanocrystal formation. Further, the reaction temperature was increased to 230 °C, and it was observed that only instant cooling after Cs precursor injection retained the optical emission and even with 1 min of annealing completely quenched the emission and transformed

the phase to tetragonal CsPb2Cl5 (Figure S3). Going further to 250 °C, while the Cs precursor was injected, these nanocrystals were observed to instantly lose the optical emission. Even with ice-bath cooling, these could not retain their cubic phase. These observations suggested that the light-emitting cubic phase of CsPbCl3 nanocrystals is unstable to sustain or might not be formed at higher reaction temperature. Figure 2a,b shows the UV−vis absorbance and PL spectra of the samples obtained soon after the injection and after 1 min of annealing for the reaction carried out at 250 °C. These spectra showed no band edge absorption or emission, as expected for CsPbCl3 nanocrystals. Figure 2c shows the powder XRD pattern of the sample obtained after 1 min of annealing, and the peak positions were overlapped with those of the bulk tetragonal CsPb2Cl5 phase. The TEM image of the corresponding sample (Figures 2d and S4) showed a micrometer length sheet-like structure. These results confirmed that higher reaction temperature and prolonged annealing transformed the cubic semiconducting CsPbCl3 nanocrystals into indirect band gap tetragonal CsPb2Cl5 microcrystals. Hence, the reported 6600

DOI: 10.1021/acs.jpclett.8b02825 J. Phys. Chem. Lett. 2018, 9, 6599−6604

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The Journal of Physical Chemistry Letters

Figure 3. (a) Optical absorption and (b) PL spectra of CsPbCl3 nanocrystals obtained from a 250 °C reaction at 1 min and 5 h of annealing in presence of ammonium chloride salt. Inset shows digital image of illuminated nanocrystals in solution. (c) Powder XRD pattern and (d) corresponding TEM image of CsPbCl3 nanocrystals synthesized at 250 °C with 5 h of annealing in the presence of ammonium chloride salt (Figure S5).

Figure 4. (a) Absorption spectra of cesium lead bromide nanostructures obtained from the reactions without and with added oleylammonium chloride salts and (b) corresponding PL spectra. The excitation wavelength was 350 nm. (c, d) Powder XRD patterns for tetragonal and orthorhombic phases of cesium lead bromide nanostructures obtained in the absence and presence of added salts. (e, f) TEM images of the nanostructures obtained without salt (1 min) and with salt (2 h).

reaction was optimized at below 180 °C and followed instant cooling to retain the semiconducting CsPbCl3 cubic phase of the nanocrystals. As the crystal phase turned to tetragonal CsPb2Cl5, it could be assumed here that the amount of Cs was reduced, did not participate in, or leached out of the excepted CsPbCl3 nanocrystals. Hence, reactions were planned with excess Cs salt to stabilize the nanocrystals at elevated temperature. However, no positive impact was observed irrespective of Cs and Pb precursor composition variations. Typically, in such reactions, the Cs precursor remains the limiting reagent, where excess PbX2 is used to supply Pb as well as being the halide source for fulfillment of a 1:3 ratio of Pb to halide in the nanocrystals, and ammonium ion acts as the capping agent.26,27 It was reported that an alkylammonium ion occupies the surface Cs atom of the nanocrystals and is bound through Hbonding with the surface halides.26,27 Importantly, halide ions were expected to protect the nanocrystal optical properties.28,29 Accordingly, compensating these effects, the salt of

alkylammonium chloride having both cations and halide ions was introduced as a dual passivating agent. To understand the extent of passivation in more adverse conditions, reactions were set at 250 °C (much higher than the traditional reaction temperature) in the presence of an appropriate amount of the ammonium salts. Interestingly, it was observed that the nanocrystals retained their optical properties and had intense violet emission. More importantly, these nanocrystals were observed to be stable even for 5 h of constant annealing at 250 °C. Previously, even though we had carried out reactions with these ammonium salts (below 200 °C), their impact on reaction temperature and thermal annealing beyond the optimized temperature was not explored.22,25 Figure 3a shows the absorption spectra of the obtained nanocrystals after 1 min and 5 h of annealing, and their nature remained almost identical. Similarly, the PL spectra presented in Figure 3b also showed almost overlapping emission. The powder XRD pattern (Figure 3c) confirmed that the semiconducting cubic phase of CsPbCl3 and corresponding TEM image 6601

DOI: 10.1021/acs.jpclett.8b02825 J. Phys. Chem. Lett. 2018, 9, 6599−6604

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The Journal of Physical Chemistry Letters (Figures 3d and S5) of the sample showed cube-shaped nanocrystals with a size of ∼15 nm. These results confirmed that in the presence of ammonium and chloride ions the semiconducting phase sustained even at high reaction temperature, and no agglomeration or shape change was observed during annealing. Hence, with proper passivation, thermal stable nanocrystals were successfully obtained. Moreover, while we observed the QY for as-synthesized CsPbCl3 nanocrystals remained 1−3% after purifications,1 it was observed here ∼51% in film, which we believed to be the highest among all such reports. High-temperature reactions were extended to not only CsPbCl3 but also to CsPbBr3 nanocrystals. In the traditional approach, CsPbBr3 nanocubes were also converted to nonemitting tetragonal CsPb2Br5 nanosheets31 (Figure S6), and similar to chloride, the phase change remained more pronounced at high temperature. However, in the presence of oleylammonium bromide, no shape, phase, and optical property change was observed even at 250 °C for 2 h. Figure 4a shows the optical absorption spectra in the presence and absence of salts, and Figure 4b represents corresponding PL spectra. It was observed that the emission was retained only for the reaction with added ammonium bromide salts. The powder XRD pattern in Figure 4c,d shows that the phase was transformed to tetragonal for the reaction without salt but retained an orthorhombic phase in the presence of oleylammonium salt. TEM images (Figure 4e) confirmed sheet-like structure for the sample obtained from the without salt reaction, but in the presence of salt, as expected, cubes (Figure 4f) of CsPbBr3 remained even after prolonged annealing (2 h). These results confirmed that ammonium bromide salts in the reaction flask also helped bring thermal stability to CsPbBr3 nanocrystals and retained the shape, phase, as well as optical properties even when annealed at elevated reaction temperature. The QY for these nanocrystals remained 76% without any postsynthesis surface treatment, and this remained constant throughout annealing (Figure S7). Further, to quantify the amount of salt for thermal stability, reactions were carried out at various salt concentrations. In both chloride and bromide cases, optimized amounts of salts are required to prevent annealing-induced phase change. Figure 5a, b presents the powder XRD patterns of the reaction products carried out with various salt concentrations, and for our standard reactions, 0.55 mmol salts were observed to protect the phase. For both cases, reactions were carried out at 250 °C, and annealing was performed for 30 min. These results further confirm that the solution should have a certain amount of free ammonium ions that would assist the equilibrium of the surface-bonded ammonium ligands. To understand the surface ligands of these nanostructures, nuclear magnetic resonance (NMR) spectra of the purified nanocrystals were obtained in CDCl3 solvent (Figure S8). The sharp multiplet at 7.18 ppm suggests the presence of ammonium ion as capping agent. Interestingly, in comparison to the previous reports on broad spectra of ammonium protons,26 here multiplet spectra were observed.11 This indicates that the nanocrystals were well passivated by ammonium ions in the reactions with added preformed alkylammonium halide salt. From the above observations, it could be concluded here that alkylammonium halide played a crucial role in bringing thermal stability to the perovskite nanocrystals. For a particular reaction, an optimized amount (see experimental) of this dual

Figure 5. Ex situ powder XRD patterns of (a) cesium lead chloride and (b) cesium lead bromide nanocrystals with various oleylammonium ion concentrations at 250 °C after 30 min annealing.

passivating organic−inorganic salt was required to obtain the phase-pure nanocrystals. The presence of excess halide ions also protected the nanocrystals from halide leaching during annealing. The prolonged annealing (1 to 5 h) was done at 250 °C, the stability was also tested at 280 °C, and no instant cooling was required to retain the semiconducting phase of the nanocrystals. However, more annealing time (∼10 h) led to a mixture of sheets and cubes, suggesting phase change to some extent (Figure S9). We skipped here the discussion for CsPbI3, although these nanocrystals were also observed to be stable with ammonium ion capping but followed a different mechanism for thermal degradation.12 In summary, physical insights at the interface of lightemitting CsPbX3 (X = Cl and Br) nanocrystals for sustaining thermal stability were investigated. The quantitative analysis of the change in the crystal structure for the case of high-energy emitting CsPbCl3 was carried out, and using an ammonium ion as the surface capping agent, intense emission of as-synthesized nanocrystals having QY ≈ 51% is reported. Similarly, the impact of reaction temperature and thermal annealing of CsPbBr3 was studied, and the stability beyond the reported 200−220 °C reaction temperature was investigated. Hence, the surface chemistry with strong ligand binding indeed provided new physical insights for enabling the stability at higher temperature. In addition, the results also led to thermally stable nanocrystals for further investigations of new materials properties.

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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.jpclett.8b02825. Experimental details, additional TEM images, XRD patterns, NMR, and optical spectra (PDF) 6602

DOI: 10.1021/acs.jpclett.8b02825 J. Phys. Chem. Lett. 2018, 9, 6599−6604

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anirban Dutta: 0000-0001-9915-6985 Sumit Kumar Dutta: 0000-0002-9228-1916 Narayan Pradhan: 0000-0003-4646-8488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by DST, Govt. of India (SERB/F/ 7159/2016-17). A.D., R.K.B., S.K.D., and S.D.A. acknowledge CSIR for fellowships.

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