Tuning the Size of CsPbBr3 Nanocrystals: All at One Constant Temperature Anirban Dutta,† Sumit Kumar Dutta,† Samrat Das Adhikari, and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India
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S Supporting Information *
ABSTRACT: For varying the size of perovskite nanocrystals, variation in the reaction temperature and tuning the ligand chain lengths are established as the key parameters for hightemperature solution-processed synthesis. These also require sharp cooling for obtaining desired dimensions and optical stability. In contrast, using preformed alkylammonium bromide salt as the precise dimension-controlling reagent, wide window size tunable CsPbBr3 nanocrystals were reported without varying the reaction temperature or changing the ligands. The size tunability even with ∼1 nm step growth regimes was achieved as a function of only the concentration of added alkylammonium bromide salt. Not only the cube shape but also the width varied in the sheet structures. Because these nanostructures lose their optical stability and crystal phase on prolonged annealing, stabilizing these in high-temperature synthesis for all-inorganic lead halide perovskites is important and remains challenging. In this aspect, this method proved to be more facile because it does not require sharp cooling, and the nanocrystals retained their phase and optical properties even upon prolonged annealing.
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successfully controlled the size of cubes from the quantum confinement regime to bulk size just as a function of the amount of added salt without varying the reaction temperature or any other reaction parameters. The synthetic control remained robust, which could predict precisely the size of the nanocubes even in 1 nm step growth regimes. This approach does not require any sharp cooling strategy and retained the stable optical emission even upon prolonged annealing. Not only cube shapes but also the widths of sheets are tuned by varying the concentration of added salts. For the synthesis of CsPbBr3 nanocrystals, a stock solution of oleylamine−HBr (OLA−HBr) was first prepared (see experimental methods in the Supporting Information), and for controlling the size or shape, the required amount of this salt was introduced into the reaction system. Cs−oleate solution was injected at the desired reaction temperature for triggering the reactions following the procedure reported by Protesescu et al.4 When the reactions were optimized at different reaction temperatures, 140−180 °C was observed to be the ideal temperature window for obtaining wide size-tunable nanocrystals. Figure 1a−h (and Figures S1−S8) presents the TEM images of eight different sizes of CsPbBr3 nanocubes with introduction of different amounts of OLA−HBr at 160 °C. The
ead halide perovskites, which have emerged as a new class of functional energy materials for both lightemitting and photovoltaic applications, remain in the forefront of current research.1−16 Enormous efforts have been put forward in designing these nanocrystals and understanding their formation chemistry.4,6,8,17−57 The high-temperature synthesis of tunable monodisperse lead halide perovskite nanocrystals is typically achieved by modifying the surface ligands, ligands ratio, reaction temperature, precursors ratio, etc.,24,27,37,48,52,57 but not in a unique reaction setup with similar reagents and in a concise preprogrammable manner. Moreover, all such approaches required instant cooling for protecting the emission intensity, phase, and the dimensions of these nanocrystals. This indicates that these synthesis methods are indeed sensitive and demand more flexible processes for obtaining phase-stable, highly emissive nanocrystals. One of the most widely studied all-inorganic perovskite system is CsPbBr3 nanocrystals which have a room-temperature air stable highly emissive crystal phase. A close examination of the literature reports on high-temperature synthesis of lead halide perovskite nanocrystals reveals that their surfaces are coordinated or bonded with alkylamines (or ammonium ions) and acids (or carboxylate ions).24,27,58,59 A recent study also suggested that ammonium ions replaced surface Cs ions to passivate and stabilize the perovskite nanocrystals.60 Inspired by this concept, herein, alkylammonium bromide salt was introduced as a size-controlling additive reagent which © 2018 American Chemical Society
Received: December 6, 2017 Accepted: January 2, 2018 Published: January 2, 2018 329
DOI: 10.1021/acsenergylett.7b01226 ACS Energy Lett. 2018, 3, 329−334
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Cite This: ACS Energy Lett. 2018, 3, 329−334
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Figure 1. (a−h) TEM images of CsPbBr3 nanocubes obtained from the reaction with varying amount of OLA−HBr stock solution. The size of these cubes varied from ∼3.5 to ∼17 nm with introduction of 0.52, 0.44, 0.35, 0.26, 0.17, 0.09, 0.04, and 0.0 mmol of the salts. Scale bar in all cases is 100 nm. (i) Size distribution histograms of the set of nanocrystals (from a to h and Figures S1−S8). These are calculated considering 200 nanocrystals.
average size of ∼17 nm in the standard case obtained without salt addition was reduced to ∼3.5 nm with increasing the amount of OLA−HBr from 0 to 0.6 mL (0 to 0.52 mmol). The size tuning window of these nanocubes is further represented in their size distribution histograms shown in Figure 1i, which clearly suggested the effect of the concentration of OLA−HBr on the particles size. A representative high-resolution TEM image for ∼9 nm nanocubes is shown in Figure S9. Similarly, nanocrystals obtained in 180 °C reactions also showed that with changing the amount of added salt, the size was tuned. TEM images and corresponding size distribution histograms of these nanocrystals are presented in Figure S10. Interestingly, with an increase in the OLA−HBr amount, monodispersity of the nanocubes was also enhanced. Smaller nanocubes that were obtained with higher amounts of salt were highly monodisperse, and this result was reflected in their twodimensional self-assemblies on the TEM grid (Figures 1a−d and S1−S4). To correlate further the size of nanocubes as a function of added ammonium salts, a plot of size versus the amount of introduced salt for both 160 °C and 180 °C are presented in Figure 2. These size variations were also highly reproducible with ∼6−10% error range. Figure 3a shows the powder X-ray diffraction (XRD) patterns of four different sizes of nanocubes obtained from four different concentrations of the salt. The peak positions resembled the orthorhombic phases of CsPbBr3. However, several studies showed that the orthorhombic phase is the stable phase, and we have also found the same phase (COD: 4510745).54,61−65 Figure 3b presents the absorption spectra of all eight samples where the tunability is clearly observed from below the quantum confinement regime to bulk size. Corresponding photoluminescence (PL) spectra of all these samples are shown in Figure 3c where the emissions were observed to be tunable from 465 to 510 nm. The quantum yield of these nanocrystals (varied from 70 to 80%) remained similar to its best reports, and the excited-state decay lifetime was also observed in nanoseconds (4.9−5.24 ns; see Figure S11).24 The interesting observation noticed here is the PL stability of these nanocrystals (Figure 3d). For the standard reaction without the salt carried out at 160 °C, the PL intensity is significantly reduced, and it is completely quenched for 180 °C
Figure 2. Plot of amount of added OLA−HBr vs obtained size of nanocubes carried out at 160 °C (black squares) and 180 °C (red triangles). Reaction conditions for all cases remain identical in both temperatures. The error bars remain ∼6−10%.
reaction while annealing. However, in the presence of salt, the intensity remained unchanged. The comparison PL spectra are shown in Figure 3d, and corresponding ultraviolet−visible (UV−vis) spectra are presented in Figure S12. The decrease in the optical property might be due to the possible phase change as reported in the literature.66 This suggests that this approach not only results in a precise, tunable size but also remained thermally stable. However, we also observed a limiting stage for the density of OLA−HBr for retaining the cube shape at 160 °C reaction. Introducing excess salt changed the shape to square platelets. UV−vis spectra, PL spectra, powdered XRD pattern, and TEM images of these platelets are shown in Figure S13. Hence, our entire study was limited within the permitted amount of OLA− HBr for retaining the cube shape. It is widely established that the surface is bound with both ammonium and carboxylate ions. Though, carboxylate binds strongly to the nanocrystal surface, the ammonium ion is more effective in tuning the growth of anisotropic platelet structure.24,48,62 While oleylamine leads to exclusively nano330
DOI: 10.1021/acsenergylett.7b01226 ACS Energy Lett. 2018, 3, 329−334
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Figure 3. (a) XRD patterns of CsPbBr3 nanocrystals with variation of amount of OLA−HBr. (b) Absorption and (c) corresponding PL spectra of CsPbBr3 nanocrystals with variations of OLA−HBr salts. (d) Annealing time-dependent successive PL spectra of nanocrystals obtained at 160 and 180 °C reactions in presence and absence of the salt. In each case the amount of salt is written alongside each spectrum. Excitation wavelength is 350 nm.
Figure 4. (a) Absorption spectra, (b) PL spectra, and (c) powder XRD patterns of CsPbBr3 nanosheets obtained with introduction of 0.44 mmol and without OCTA−HBr salt. TEM images of nanosheets in different resolutions obtained in (d, e) absence and (f, g) presence of OCTA−HBr, respectively.
to be ∼5 and ∼15 nm, respectively (also visible from the TEM image of marked area in Figure 4e) (Figure S14). This confirmed that alkylamine−HBr not only reduced the size of the nanocubes of CsPbBr3 but also reduced the thickness of sheets obtained in different amine media. All the above results either for the size of the cubes or width of sheets suggested that the preformed ammonium bromide salt played an important role for such dimension variations. From NMR and Fourier transform infrared spectroscopic data analysis, it is observed that ammonium ions are present on the surfaces (Figures S15 and S16). The results also overlapped with previous reports in oleylamine and oleic acid mixed systems.24,60 These ions typically replace surface Cs+ ions and act as integrated bonded ions on the surface of perovskite crystals.60 Accordingly, with increase of their density or concentration on the reaction mixture, the dimension of the nanocrystals became smaller because of instant capping. Schematic presentations for the surface bonding with ammonium ions in replacement of Cs+ ions are shown in Figure S17, which is the most plausible condition expected
cubes, octylamine (OCTA) forms nanosheets under identical reaction conditions.4,48 It is interesting to note that when these nanosheet formations are carried out in the presence of octylamine−HBr (OCTA−HBr), the thickness of the sheets was observed to decrease. Figure 4 presents the optical spectra, XRD patterns, and TEM images obtained with and without OCTA−HBr. The appearance of a new absorption peak (∼440 nm) (Figure 4a) suggested that the length of one of the confinement directions (thickness) of the sheets was reduced. PL spectra (Figure 4b) further supported with dual emissions, where one was controlled by quantum confinement regime and another by the bulk. These are typically observed for sheetlike structures for their dual modes of confinement.48 Powder XRD patterns confirmed the same orthorhombic phase of these nanosheets. Figures 4d,e and 4f,g present TEM images obtained without and with OCTA−HBr (0.5 mL) respectively. The contrast differences clearly indicate sheets obtained with OCTA−HBr were thinner. From atomic force microscopy (AFM) images, the widths of sheets obtained with and without OCTA−HBr were observed 331
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here. Furthermore, it is also observed that PbBr2, which is widely known as bromide source, does not control the dimensions. While reactions were carried out using PbO for the synthesis of CsPbBr3, similar results were obtained (Figures S18 and S19 and Supporting Information). The bromide source of this reaction was also the ammonium bromide. In a typical case under these reaction conditions, more precursor should result in larger particle size, but the case remained the opposite. Hence, the sole reason for the size restrictions with more alkyl ammonium bromide assumed here is the strong interface binding, which restricts the size. This alkyl ammonium halide approach was extended to the chloride system at 160 °C; a mixture of differently shaped particles was formed, probably because of less solubility of PbCl2 at this temperature. However, a wide size tunable window was observed for CsPbCl3 at 180 °C, ranging from 25 to 6 nm. The TEM images of the nanocubes obtained on varying the amount of salt and the corresponding size histogram are presented in Figure S20. The method also did not function for iodides, possibly because of the phase instability of the nanocrystals. In conclusion, a fixed reaction temperature synthetic approach for wide window size tunable CsPbBr3 perovskite nanocrystals is reported. This has been achieved by introducing alkylammonium bromide salts, and with the increase of its concentration, the size of the cubes and width of sheets were reduced. The size window tunability also varied while the reaction temperature varied. Importantly, this approach requires no instant cooling, and the stable emission, size, shape, and phase of the nanocrystals remained intact even on prolonged annealing, which were typically not observed in standard synthetic routes. Although this approach may lead to more widespread synthesis of these materials, the chemistry of controlling the dimensions of ionic perovskites is still not clearly understood, and such an understanding is necessary for better control of the synthesis of these nanostructures.
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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/acsenergylett.7b01226. Experimental methods and supporting figures (lifetime decay plots, additional TEM images, AFM images, etc.) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Anirban Dutta: 0000-0001-9915-6985 Narayan Pradhan: 0000-0003-4646-8488 Author Contributions †
Authors A.D. and S.K.D. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.D., S.K.D., and S.D.A. acknowledge CSIR for fellowships. DST (SR/NM/NS-1383/2014(G)) of India is acknowledged for Funding. 332
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DOI: 10.1021/acsenergylett.7b01226 ACS Energy Lett. 2018, 3, 329−334
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DOI: 10.1021/acsenergylett.7b01226 ACS Energy Lett. 2018, 3, 329−334
