Water-Stable, Fluorescent OrganicâInorganic Hybrid and Fully

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Water-Stable, Fluorescent Hybrid and Fully Inorganic Perovskites Atanu Jana, and Kwang S. Kim ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01394 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Energy Letters

Water-Stable, Fluorescent Hybrid and Fully Inorganic Perovskites Atanu Jana and Kwang S. Kim* Center for Superfunctional Materials, Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea. *Corresponding Author: [email protected]

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ABSTRACT

Stabilization of perovskites without addition of foreign surface passivating ligands in aqueous media is essential for their applications in optoelectronics, biomedical science, and catalysis. However, these materials instantly degrade in water due to its intrinsic ionic nature. By controlling the peripheral layer of octahedral perovskite geometry, we reproducibly synthesized a series of rod shape fluorescent hybrid perovskite in both acidic and basic media at ambient condition in large scale without capping ligands. The band gap is tunable from red to sky-blue region with sharp emission. The lead bromide perovskites are stable more than six months in water without structural change. Our simple synthetic route has resolved the longstanding problems for its practical application in aqueous environment.

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Fully inorganic perovskites and hybrid lead halides have been the essence of material research community, especially in solar cells,1–4 solid state light-emitting diodes,5–7 and many more optoelectronic devices.6,8,9 However, intrinsic ionic properties of both perovskites make them extremely unstable in water.10 This holds back all the possible applications where water is present. Several efforts have been made to passivate metal halide perovskites by Zwitterions,11 organic-inorganic hybrid ion pairs,12 organic Lewis bases such as thiophene and pyridine,13 and several metal oxides such as SiO2,14 Al2O3,15 Ta2O5.16 These methods are highly appreciable towards solving the stability issue in water. Furthermore, in some cases water was introduced during the synthesis of perovskite.17–20 This indicates that perovskite can be synthesized in water media21 by choosing appropriate conditions such as controlling the pH, appropriate metal halide salts, different phase of organic components, etc. In this letter, we report an easy, facile and cost-effective aqueous synthesis of a series of water-stable hybrid and fully inorganic perovskites (1-11) following the Lewis base vapor diffusion (LBVD) method except 1 and 10 (Scheme 1 and Figure 1). (See supporting information for synthesis of all the compounds and sample colors in Figure S1). 3, 5, 7, 9 and 11 were waterstable and exhibited rod shape morphology, which has a great potential in device applications as well as exciting optoelectronic properties.22 In synthetic condition, MAPbBr3 (3) (MA = CH3NH3+) and CsPbBr3 (11) exhibited bright green fluorescence, and MAPbBr3-xClx (5) showed cyan blue color under UV light (365 nm) in highly basic solution (Figure 2). The photoluminescence (PL) maximum of hybrid perovskite was tuned from 763 nm to 491 nm. The highest internal PL quantum yield (PLQY) of the solid sample was achieved for CsPbBr3 (53.9%). Dried MAPbBr3 (3) and MAPbBr3-xClx (5) and CsPbBr3 (11) were highly stable in water for more than six months retaining its bright fluorescence.


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Scheme 1. General synthetic procedure for different hybrids and fully inorganic perovskites following the Lewis base vapor diffusion (LBVD) method. The small inner vial contains halide acid-metal halide precursor salt, while the big outer vial contains methylamine (MAm) solution. MAm vaporizes and goes into the small vial. Blue balls indicate the MAm vapor and the green rod indicates the water-stable perovskite.


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Figure 1. All the perovskites including intermediates and their morphology analyses. SEM images of (1) MAPbBr3 synthesized by direct addition of MAm solution, (2) MAPbBr3intermediate before forming 3, (3) MAPbBr3 (dried end-product), (4) MAPbBr3-xClxintermediate before forming 5, (5) MAPbBr3-xClx (dried product), (6) MAPbBr3-xIx-intermediate before forming 7, (7) MAPbBr3-xIx (dried product), (8) (MA)4PbI6.2H2O, (9) MAPbI3 (dried product), (10) CsPbBr3 obtained after addition of Cs2CO3 into the PbBr2 + HBr solution, and (11) dried CsPbBr3 (scale bar 10 µm) isolated after one day. During the synthesis, we observed that cubic shape perovskites were stable in acid medium, while rod shape ones (specially micrometer sized rods) were preferably formed in basic medium. CsPbBr3 was first discovered by Wells in 189323 and hybrid perovskites in 197824 by Weber. Yet, there has been no report on long-time stabilization of these materials in neutral water. In a typical synthesis, metal halides were dissolved individually in halide acids in a 20 ml vial. The vial without cap was kept in MAm bottle in capped condition (Scheme 1). MAm was slowly diffused into the solution of metal halide precursor. In the case of MAPbBr3 synthesis, orange-colored precipitate was observed immediately. At that time, the pH was below 1. The precipitation was completed within 1 h. Then, the color of the precipitate changed to greenish white, and it showed bright green fluorescence under UV light (Figure 2). The precipitate was kept for 10 days in MAm environment without removing from the solution. Then, the pH of the solution was again checked after a week and found to be above 12. The precipitate was removed from the solution and washed with water several times to remove the basic MAm solution and dried at 60 °C to obtain 3. 3 was much more stable than intermediate 2 which was degraded in


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neutral water within a fraction of a second. The greenish white precipitate (3) was stable in water more than 6 months. In a similar way, MAPbBr3-xClx (5), MAPbBr3-xIx (7), and MAPbI3 (9) were also synthesized successfully. The dried sample 5 showed ultrastablity in water, exhibiting bright cyan blue fluorescence.

Figure 2. Images under synthetic condition. 3, 5 and 11 exhibited bright green, cyan blue, and cyan green in basic media during synthesis under UV light. Synthesis of fluorescent CsPbBr3 (11) was carried out in a different way from that of hybrid perovskite. We synthesized CsPbBr3 by dissolving PbBr2 in HBr and the subsequent addition of Cs2CO3 yielded yellow colored bulk CsPbBr3 precipitate (10) which showed no fluorescence under UV light. The product was not isolated from the acidic solution. This was kept in MAm vapor environment to synthesize water-stable fluorescent CsPbBr3. After a few minutes, the precipitate changed to light orange color. After one day, the color again changed to greenish white which showed bright cyan green fluorescence under UV light (365 nm) illumination (Figure 2).


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The crystal structures of perovskite powders were confirmed by powder X-ray diffraction (PXRD) measurement (Figure 3 and S2). All the compounds except 11 exhibited cubic phase along with some other peaks in final products. 11 exhibited orthorhombic phase.25 The diffraction peaks of 2 revealed its cubic phase.26,27 Some extra peaks which were assigned for Pb(OH)2 [mp-690727, collected from Materials Project data repository] were also present in 3, 5, 7, and 9. Pb(OH)2 was interfaced with perovskites, which was confirmed from the Braggs diffraction angle shift. As the radius of oxide ion is less than that of bromide ion, there occurred shrinkage in the cubic lattice, but as the hydroxide layer was present at the interface, the shrinkage effect was very small and the Braggs diffraction angle of 3 shifted to a little bit higher angle as compared to 2 [Figure S3].


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Figure 3. Structural analysis. X-ray diffraction patterns for hybrid perovskites (a, b) and fully inorganic perovskites (c, d). b and d are zoomed versions of a and c which exhibit the peak shifts in Pb(OH)2 coated perovskites. Pb(OH)2 cif [mp-690727] was collected from Materials Project data repository].

On the other hand, the crystal lattice of Pb(OH)2 was expanded due to the bonding of lead with bromide ions, and eventually the diffraction peak corresponding to (200) plane of Pb(OH)2 shifted from 17.98° to 17.72°. The (200) peaks in 5, 7 and 9 appeared at 17.73°, 17.71° and 16.98°, respectively. The largest shift of (200) peak was observed in the case of Pb(OH)2-coated MAPbI3 due to the large ionic radius of iodide, as compared to the bromide, chloride, and oxide ions. The PXRD peaks of 8 supported the (MA)4PbI6.2H2O crystal structure,28 whereas 9 showed the cubic phase geometry coated with Pb(OH)2. To explain the presence of Pb(OH)2 at the interface of 11, we prepared CsPbBr3 and Cs4PbBr6 single crystals via anti-solvent diffusion method.29 The diffraction peaks for both CsPbBr3 and Cs4PbBr6 were present along with the Pb(OH)2 peaks. The lower angle peaks for Cs4PbBr6 at 12.61° and 12.95° shifted to 12.71° and 12.09°, respectively. On the other hand, the diffraction peak at 15.17o shifted to 14.98o, whereas the peak at 17.98° for pure Pb(OH)2 shifted to 17.78°. This is beacuse iodide ions expanded the crystal lattice and oxide ions shrank the perovskite crystal lattice. This subtle change in PXRD pattern indicates that 11 interfaced with the Pb(OH)2 renders ultrastable in water. Scanning electron microscopy (SEM) images are shown in Figures 1, S4 and S5. 1 which was synthesized by adding MAm solution directly to the acidic PbBr2 solution gives the kinetically controlled product having cube-shaped morphology with average size 4-5 µm. 2 exhibited distorted cubic structure, whereas 3 showed thermodynamically controlled rod-shaped


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crystals of 4-10 µm. SEM-EDS analysis of these samples revealed that the Br/Pb ratios in 1 and 2 were 3.38 and 3.4, respectively and that in 3 was 1.05. Since 1 and 2 were isolated from hydrobomic acidic solution, these perovskite surfaces were bromide-rich. However, 3 was isolated from basic solution and the Br/Pb ratio deviated from its ideal value of 3 due to the presence of Pb(OH)2 on the surface of MAPbBr3 rods. A similar case was also observed for MAPbI3. 8 and 9 have the I/Pb ratios of 4.38 and 1.26, respectively. This is because 8 was isolated as (MA)4PbI6.2H2O from hydroiodic solution, but Pb(OH)2-coated 9 was separated from highly basic solution. 3 exhibited the Br/Pb and Cl/Pb ratios as 3.56 and 0.039, rspectively. The Cl content in 3 was very small due to the high concentration of bromide ion in solution and these bromide ions replaced chloride ions. The Br/Pb and Cl/Pb ratios in 4 were 0.92 and 0.001, respectively, due to the presence of Pb(OH)2. At the initial stage, 10 exhibited the Br/Pb ratio of 3.68, indicating highly Br-rich surface as it was isolated from hydrobromic acid. 11, isolated after one day from the starting point, exhibited two different colors in solid state: green and orange which are named as Pg and Po , respectively. The hydrated product (Pg) changed its color from greenish to orange (Po) upon drying the sample at 60 °C and regained its green color upon the addition of water. We observed this phenomenon reversibly and tested it for three cycles after which Pg still exhibited bright cyan blue fluorescence [Figure S6]. This is due to the fact that the surface was not properly passivated by Pb(OH)2 within a short time. Water was present in the crystal, as confirmed from IR (discussed later). Upon heating, these water molecules were removed, leaving some void spaces in crystals. Those void spaces took water molecules and regained its green color. To address this issue, we have done the PXRD, IR (discussed later) and SEM analyses for Pg and Po. Both CsPbBr3 and Cs4PbBr6 were present in Pg as indicated by the characteristic PXRD peaks (Figure S7). But, the peaks of Cs4PbBr6 were more prominent upon


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heating the sample. This indicated that the crystallinity of Cs4PbBr6 increased to a great extent upon heating. From SEM analysis of Pg, it was found that still cubic shaped CsPbBr3 was present along with the rod shaped structure, indicating that the reaction was not completed within this short time (Figure S8a). The cubic shape of Pg was broken in Po, as indicated by SEM images (Figure S8b). We hypothesize that there might also exist a dynamic equilibrium between CsPbBr3 and Cs4PbBr6 which can be achieved through the reaction: Cs4PbBr6.2H2O = CsPbBr3 + 3CsBr + 2H2O.17,18 The surface chemistry of 3, 5, 7, 9 and 11 was explored by the X-ray photoelectron spectroscopy (XPS) analysis (Figure S9). 3 exhibited two peaks for oxygen at 529.73 eV and 533.05 eV corresponding to the PbO and Pb(OH)2, respectively. 5, 7, and 9 had only one peak for oxygen at 530 eV, 530.46 eV, and 530.32 eV, respectively, which represented the PbO peak. The peak at 531.19 eV of 11 indicated that PbCO3 was present in the sample. In the presence of water, CsPbBr3 or other perovskite degraded into PbCO3 and Pb(OH)2, and Pb(OH)2 might further break into PbO and H2O. The stoichiometric reaction for CsPbBr3 can be formulated as follows: 2CsPbBr3.H2O + O2 + CO2 = 2CsBr + PbCO3 + Pb(OH)2 + 2HBr + Br2 ; Pb(OH)2 = PbO + H2O.30 We observed no diffraction peak of PbO and PbCO3 in the PXRD. This indicates that these were amorphous in nature or they were present in low quantity which was beyond the detection limit. The Pb 4f peaks of 11 at 137.15 and 141.96 eV corresponded to Pb 4f7/2 and Pb 4f5/2, respectively.30 The PbCO3 peaks appeared at 139.3 eV and 144.2 eV, while the Br 3d5/2 and 3d3/2 peaks appeared at 67.09 and 67.93 eV, respectively.31 To confirm the presence of Pb(OH)2 in all the final products, we have also done the Fourier-transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) measurements (Figure S10). All the intermediates (2, 4, 6 and 7) showed IR frequencies of MA+ (Tables S1 and


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S2).28,32 As the final compounds (3, 5, 7, 9 and 11) were coated with Pb(OH)2, we observed the strong O-H stretching frequency around 3500 cm-1 (Figure S10) but the intensity of other IR peaks corresponding to MA+ were either very small or invisible (in some cases). This is due to the fact that in IR spectroscopy, the impinging photons interact with the surface of a compound and reflect from the surface. In FTIR, we observed that IR peaks for MA+ were present in both Pg and Po. Moreover, the IR peak of H2O was also present in the full orange intermediate sample (Pint) obtained during the formation of 11, and the IR peak of water in Pg appeared at 3418 cm-1. However, 12 isolated after 10 days, showed no IR peak of H2O or MA+ due to proper suface passivation by Pb(OH)2 (Figure S10). In 8, we observed O-H stretching frequency which indicated the presence of water molecules as 8 required two water molecules for its crystallization.28 We have carefully studied the mechanism for formation of water-stable hybrid and fully inorganic perovskites. To invesigate the role of HBr in the synthesis, we performed two control experiments where bulk MAPbBr3 (100 mg) was taken in 2 ml toluene with 100 µl aqueous HBr (i) and without aqueous HBr (ii), and then kept in MAm environment. For (i), we observed fluorescence under UV light, while for (ii), the MAPbBr3 structure was broken, yielding white color precipitate, which showed no fluorescene under UV light (Figure S11). For (i), we envisaged that MAm vapor diffused into the highly concentrated acidic HBr solution. In contact with acid, MAm formed MABr which reacted with the acidic metal halide solution. If there was no acid in the precursor solution, only MAm diffused into the crystal, breaking the crystal structure as observed in (ii). Thus, we conclude that HBr was required for the synthesis of perovskite. Water was also necessary in the reaction mixture to produce hydroxide in the presence of methylamine in the solution. After the completion of stoichiometric reaction of acid


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and MAm, the solution became neutral and changed to basic medium, producing hydroxide ions. The hydroxide anions reacted with the peripheral layer of perovskite and formed Pb(OH)2 at the surface of perovskite, breaking the labile surface layer of octahedral [PbX6]4-. The metal hydroxide layers acted as strong diffusion barriers for water. The reductuion of surface layer rendered the cubic structure into a rod structure, as evidenced from the SEM analysis (Figure S4). During the synthesis, we observed that bare perovskites were stable in acidic condition but these isolated and dried products were not stable in neutral water. This is due to the fact that methylamine and water remained protonated in the highly acidic solution where the concentration of halide ions was also high. However, in neutral water media, water diffused into the structure and both bromide and MA+ got hydrated and so these sizes increased to a great extent, compelling them to go outside the perovskite and broke the moiety.32 The formation mechanism of water-stable CsPbBr3 is a little bit different from that of the water-stable hybrid perovskite growth mechanism. The formation of water-stable CsPbBr3 from its precursor salts occurred through several steps (Figure 4). Intermediate products were characterized by PXRD, SEM, and IR analyses. First, CsPbBr3 was formed by the reaction of HBr solution of PbBr2 and Cs2CO3, and then MAm was allowed to pass through the solution. After formation of MA by the reaction MAm and HBr, MA was incorporated inside the CsPbBr3 crystal structure by diffusion and the color changed gradually from yellow to orange. The orange product showed no fluorescence under UV light and the overall structure still existed in cubic form (Figure S12). The presence of CH3NH3 was confirmed by IR analysis (Figure S10). Over the time, the solution became basic, as more and more MAm vapor diffused into the solution producing hydroxide ions in the solution.


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Figure 4. All the steps for the formation of water-stable 11 (a-f). The color changed from yellow to orange to white to greenish white. (a) CsPbBr3 (10), (c) full orange intermediate product (Pint), (f) 12.

Those hydroxide ions had strong nucleophilicity and reacted with the peripheral layer of octahedral moiety, [PbX6]4- where both Cs+ and MA+ were present. Thus, there occurred a solidsolution interaction at the surface of fully inorganic perovskite. This time, the color changed from orange to greenish white which was stable in water. The dried product isolated after 10 days, did not change its color and was highly stable in water. To address this issue, we have done the PXRD, SEM and FTIR-ATR analyses for 10, full orange intermediate product (Pint) (Figure 4(c)), and 12 (isolated after 10 days). From PXRD, we observed that the peak at 15.16˚ gradually


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shifted to lower angle and reached at 14.83˚. This may be due to the fact that the radius of MA+ (2.70 Å) is higher than that of Cs+ (1.81 Å)26. As more and more MA+ was incorporated in the crystal structure of CsPbBr3, forming CsxMA1-xPbBr3 (Pint) and finally CsxMA1-xPbBr3 (12), the Bragg’s diffraction angle was shifted to lower angle (Figure S13). From SEM-EDS analysis, we have confirmed the presence of Cs+ in the rod morphology of 12 (Figure S12). As FTIR analysis gave the surface properties of a compound, we did not observe the characteristic IR peaks of MA in the final product-Pb(OH)2 coated 12 (Figure S10(e)), but the O-H peak at 3514 cm-1 was present. The IR peaks of MA+ was observed in Pint which confirmed the presence of MA+ in the CsPbBr3 crystal. The procedure for conversion of CsPbBr3 to MAPbBr3 in a more control way is currently under investigation. To check the ultrastability of all the final products (3, 5, 11), we grinded them for a long time and added water, by which the bright fluorescence was still observed as before grinding. (Figure S14). Even after a long time sonication of 3, 5, 11 in water we observed bright fluorescence in water (Figure S15). This indicated that all the final products were efficiently interfaced with Pb(OH)2 which rendered them ultra-stable in water. The UV−vis absorption and PL studies were done only for 3, 5, 9 and 11 because of their considerable fluorescence (Figure 4). The absorbance maxima of intermediates 2, 4, 7, 10 appeared in green region, whereas that of 8 appeared in red region (Figure S16). 3, 5 and 11 exhibited bright fluorescence in solid state as well as in water under UV light (Figures 5c, d and S17). The absorbance maxima of MAPbI3 (9), MAPbBr3-xIx (7), MAPbBr3 (3), MAPbBr3-xClx (5), and CsPbBr3 (11) appeared at 747, 546, 525, 425, and 431 nm, respectively. 3 and 5 showed bright green and cyan blue fluorescence, respectively, under UV light. From Kubelka-Munk plot, we have determined the band gaps of 3, 5, 9 ans 11 which were evaluated as 2.31, 2.52, 1.61, and


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2.44 eV, respectively (Figure S18) and it reveals that the band gaps of 3, 5, 9 and 11 were blue shifted as compared to their respective bulk band gaps. This is attributed to the fact that initially nanocrystals were formed in basic condition and then nanocrystals or nanocrystalline domains spread throughout the larger crystals. It is also to be mentioned that varying sizes with varying degrees of quantum confinement might also occur as there was no control over the vaporization of MAm. This could be a reason why we observed relatively large PL full width at half maximum (fwhm), 40-50 nm in all the final products.

Figure 5. Optical study: (a) (a) UV-Vis diffuse reflectance spectra (Kubelka–Munk function vs. wavelength), (b) room-temperature fluorescence taken for solid samples 3, 5, 9, and 11. Images of 3, 5 and 11 in neutral water: (c) visible light and (d) UV light.


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Table 1. Optical properties of 3, 5, 9, and 11 λabs

λem

fwhma

(nm)

(nm)

(nm)

MAPbBr3 (3)

518

518

MAPbCl3-xBrx (5)

425, 492

MAPbI3 (9) CsPbBr3 (11)

Perovskite

a

EQYb (%)

IQYc (%)

50

3.6

11.7

493

44

4.3

8.3

747

763

50

0.3

5.1

431, 508

508

45

12.1

53.9

Full width at half maximum (fwhm), bExternal quantum yield, cInternal quantum yield

The PL maxima for MAPbI3 (9), MAPbBr3 (3), CsPbBr3 (11), and MAPbBr3-xClx (5) appeared at 763, 518, 508, and 491 nm, respectively. The band gap determined from Kubelka-Munk plot [(F(R)hν]2 vs energy (eV)] and PL band gap were almsot same in 3, 5, 9, 11, which indicates that the Stokes shifts are very small or not at all due to the high crystallinity of all the compounds. The internal PLQYs of MAPbBr3, MAPbBr3-xClx, and CsPbBr3 were 11.7 %, 8.3 %, and 53.9 %, respectively (Table 1). This indicated that radiative recombination occurred much more efficiently in the case of rod morphology as compared to their bulk counterpart. All these perovskites were stable in water for more than six months, retaining their bright fluorescent intensities [Table S3] which indicated that Pb(OH)2 coated perovskite were highly stable in water. In summary, we demonstrated the aqueous synthesis of various hybrid and fully inorganic halide perovskites in acidic and basic media. The rod perovskites were stable in water for more than six months and these were highly fluorescent in both solid state and solution. We


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find that there exists a very rich chemistry of perovskite under water, yet to be explored. Our new synthetic approach will open up a new research area for perovskite materials. ASSOCIATED CONTENT Supporting Information. Synthetic procedure, structural and optical characterization of perovskites. AUTHOR INFORMATION E-mail: [email protected] ACKNOWLEDGMENT This work was supported by NRF (National Honor Scientist Program: 2010-0020414). REFERENCES (1)

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