Effect of Organic–Cation Exchange Reaction of Perovskites in Water

Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4496−4503

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Effect of Organic−Cation Exchange Reaction of Perovskites in Water: H‑Bond Assisted Self-Assembly, Black Phase Stabilization, and Single-Particle Imaging 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), 50 Unist-gil, Ulsan 44919, Korea Downloaded via BUFFALO STATE on July 29, 2019 at 07:23:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Despite outstanding performance of organic− inorganic lead halide perovskite (OILHP)-based solar cells and light-emitting devices, they are still unusable for practical applications due to instability of OILHP in humid conditions. The extreme moisture sensitivity of these materials restricts all kinds of study in water media such as organic−cation exchange reaction, H-bonding assisted self-assembly, and phase stabilization in water. In order to address this issue, we report the full mechanistic details of nontemplate aqueousphase synthesis and band-gap engineering of formamidinium (FA) lead halide (FAPbX3; X = Cl/Br/I) as well as its conversion to fluorescent MAPbX3 (MA = methylammonium) in water. Our study is the first example of an organic cation exchange reaction of hybrid perovskites family in water. By this aqueous ionexchange synthetic approach, the yellow phase of FAPbI3 converts into black phase of MAPbI3 and a unique tube-structure of MAPbBr3−xIx self-assembles through hydrogen bonding, which is a well-known factor for degradation of OILHPs. The assynthesized tube-shaped perovskites monitored at the single-particle level exhibit shape-correlated fluorescence images. Our mechanistic synthetic details for ion-exchange reaction in water open an exciting path for synthesizing mixed organic cation perovskites in water for diverse applications such as solar-cells, light-emitting devices, and single-particle imaging/tracking. KEYWORDS: organic−cation exchange reaction, self-assembly, black phase, single-particle imaging, perovskite in water

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INTRODUCTION Despite superb performance, OILHPs based optoelectronic devices have not been launched in the market due to its extreme sensitivity toward water which stems from the intrinsic ionic nature of perovskite.1−5 Though water is a cheap, abundant, and greener solvent, still organic solvents are required for either synthesis or device fabrication of OILHPs.6,7 The most-studied OILHPs is APbX3 where A = FA/MA and X = chloride/bromide/iodide.8−15 But their aqueous chemistry16 has not been explored, such as organic cation exchange reaction or their transformation of one to another phase in water. All these phenomena are associated with the fabrication of efficient and stable optoelectronic devices, for example, black trigonal perovskite phase of FAPbI3 converts to yellow hexagonal non-perovskite phase in humid conditions, reducing the efficiency of optoelectronic properties.17−19 So there is a huge scientific gap between aqueous and nonaqueous chemistry of hybrid perovskites. Water-stable perovskites without any capping ligands and surface-passivating ligands will be superior for enhancement of charge transport in various optoelectronic devices. Previously, ligand exchange was successfully carried out in the case of different covalent chalcogenides nanocrystals, e.g., CdSe.20 However, the high degree of ionic character of perovskite does © 2019 American Chemical Society

not allow ligand exchange on the surface of perovskite, or at least such ligand exchange is not available in the literature. So for smooth charge transfer, the surface made of octahedral layers needs to be passivated by some other ways. However, it is a challenging task to control the layer of octahedral layers that are linked with covalent bonds. 2D materials like graphene21 have interlayers involved in weak van der Waals interaction and intralayers involved in strong covalent bonds. One can easily break the interlayers of these 2D materials by various chemical methods. This is not possible in the case of perovskites where all the octahedral layers are connected by covalent bonds. Thus, it requires treatment of the octahedral layers of perovskite in a much more sophisticated way. It was shown that metal oxides22−24 may be interfaced with lead halide perovskite layer. One possibility is to grow Pb(OH)2 or PbO on the interface of perovskite layer by reacting with base. This will bring extra stability to the perovskite without adding any capping ligand from outside. In this case, the crystallization procedure is also crucial. Only slow crystallization will be effective in controlling the layer and proper surface Received: April 13, 2019 Accepted: May 22, 2019 Published: May 22, 2019 4496

DOI: 10.1021/acsaem.9b00742 ACS Appl. Energy Mater. 2019, 2, 4496−4503

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ACS Applied Energy Materials passivation.25 Given that tremendous interest has been gained on both hybrid and fully inorganic perovskites, their severe instability issue should be resolved as early as possible. Herein, we report, for the first time, a nontemplate, aqueous synthesis of FAPbX3 (X = Cl/Br/I) at room temperature and its conversion to water-stable MAPbX3 with mechanistic details. The Pb(OH)2-coated MAPbX3 perovskites are stable in water for more than 1 year. Our present investigation is the first study of organic cation exchange reaction of OILHP in the one and only solvent water. The Br and Br/Cl based perovskites show bright fluorescence under UV light in both solid-state and water media, which is highly promising for various optoelectronic devices. In our simple and cost-effective synthetic approach the orthorhombic FAPbI3 converts into cubic, black phase MAPbI3 which is coated with Pb(OH)2, rendering it extremely stable in water. It is well-known that external H-bonding with polar solvents is detrimental for OIHPs. But we discovered that external H-bonding in mixed halide perovskites, MAPbBr3−xIx, gives a compact structure that can resist water and thus offers an excellent stability for a long time under water. This fundamental concept can be used for designing new OIHPs that would be stable in humid conditions. The single-particle imaging of all the synthesized fluorescent perovskites is also carried out successfully, and our present investigation will be useful for monitoring the perovskite NPs at the intracellular level.

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degradation of precipitate. The solution was decanted and the precipitate was washed three times with acetone. The red precipitate was dried at 60 °C. MAPbBr3−xIx: The vial containing acidic solution of MAPbBr3−xIx was kept in a bigger glass bottle for 10 days and left undisturbed. Next, the solution was decanted and the dark red precipitate was washed with water and dried at 60 °C. Synthesis of FAPbI3 and MAPbI3. FAPbI3: PbI2 (367 mg, 1 mmol) was dissolved in HI (2 mL, 8.9 mmol), and then FAOAc was added to it. Immediately, a gray colored precipitate appeared. The acidity was kept in acidic region to avoid degradation of precipitate. The solution was decanted, and the precipitate was washed three times with ether. The brownish colored precipitate was dried at 60 °C. MAPbI3: The acidic solution of FAPbI3 was kept inside the glass bottle filled with 25 mL of MAm. The solution was left undisturbed for 10 days, and after that the gray colored precipitate was isolated from the basic solution and washed with water several times. The product was dried at 60 °C. Characterization Methods. The powder X-ray diffraction was done using a D/MAX2500V/PC diffractometer, Rigaku, using Curotating anode. X-ray photoelectron spectra (XPS) were taken using a K-alpha model, ThermoFisher. The Bragg’s diffraction angle (2θ) range was set to 10−50° with scan rate of 2°/min. Scanning electron microscopy (SEM) images were collected using SU8220 Cold FE-SEM, Hitach High-Technologies, and the acceleration voltage was 10 kV. SEM energy-dispersive X-ray (EDX) study was carried out for characterizing the elements of all the samples. X-ray photoelectron spectroscopy (XPS) was performed using Kalpha (ThermoFisher) for analyzing the chemical compositions. The optical diffuse reflectance spectra of all solid samples were recorded using a Cary 5000 UV−vis−NIR spectrophotometer (Agilent) with integrated sphere in diffuse-reflectance mode and then converted to UV−vis diffuse reflectance spectra [F(R) vs wavelength (nm)] using Kubelka−Munk function, F(R). All the photoluminescence (PL) spectra were taken using Cary Eclipse fluorometer, (Varian) in solid state. Photoluminescence quantum yield (PLQY) of solid samples was evaluated experimentally using FP-8500ST spectrofluorometer (Jasco International). PLQY has been evaluated by integrating sphere, and the following equation has been used:

EXPERIMENTAL SECTION

Materials. PbI2 (99.999%), PbBr2 (98%), PbCl2(98%), HBr (48% in water), HI (57 wt % in H2O, 99.9%), and formamidine acetate (FAOAc) were purchased from Sigma-Aldrich. Methylamine (MAm) (40% in methanol) was purchased from Tokyo Chemical Industry (TCI). Ether (99%) and acetone (99%) were taken from Samchun. Synthesis of FAPbBr3 and MAPbBr3. FAPbBr3 was synthesized according to the following procedure. In a 20 mL glass vial, HBr (2 mL, 12 mmol) was added to dissolved PbBr2 (367 mg, 1.3 mmol), and then FAOAc was added to it. Immediately, a bright orange colored precipitate (A) appeared. The solution was decanted in acidic condition, and the precipitate was washed with a copious amount of acetone. The bright orange precipitate was dried at 60 °C. MAPbBr3 was synthesized according to the Lewis base vapor diffusion method. The vial which contained acidic solution of bright orange colored precipitate (A) was kept in a larger glass bottle (250 mL) that was previously filled with 25 mL of MAm. The MAm vapor slowly diffused into solution of A. The color of the precipitate changed from orange to white to green. After 10 days, the solution was decanted and the green precipitate was washed with a copious amount of water. The precipitate was dried at 60 °C and kept for further use. Synthesis of FAPbBr3−xClx and MAPbBr3−xClx. FAPbBr3−xClx: PbCl2 (278 mg, 1 mmol) was dissolved in aqueous HBr (2 mL, 12 mmol), and then FAOAc was added to it portionwise. Immediately, a bright orange colored precipitate (B) appeared. The acidity was kept in acidic region to prevent undesired precipitation. The bright orange precipitate was isolated from acidic solution and was washed three times with acetone. The precipitate was dried at 60 °C. MAPbBr3−xClx: The vial that contained an acidic solution of bright orange colored precipitate (B) was kept in a larger glass bottle (250 mL) that was filled with 25 mL of MAm. The MAm vapor diffused into solution of B. The color of the precipitate changed from orange to greenish white. After 10 days, the solution was decanted and the green precipitate was washed with a copious amount of water. The precipitate was dried at 60 °C and kept for further use. Synthesis of FAPbBr3−xIx and MAPbBr3−xIx. FAPbBr3−xIx: PbBr2 (367 mg, 1 mmol) was dissolved in HI (2 mL, 8.9 mmol), and then FAOAc was added to it. Immediately, a dark red colored precipitate appeared. The acidity was kept in acidic region to avoid

quantum yield [%] =

S2 × 100 S0 − S1

S1 = area scattered from the sample, S2 = area emitted from sample, S0 = area from incident light. S0 was measured with nothing in the sample holder. Fourier transform infrared spectra (FTIR) were taken in FTIR instrument (670-IR, Varian) using attenuated total reflection detector. The photoluminescence single particle imaging of all the powder samples was done in LSM 780 NLO (maker, Carl Zeiss). Powder samples are spread in glass. The focus of the samples was adjusted mechanically using 10× air and 100× oil objective lenses. The laser used in the study was 405 nm. The detection range was set to 410− 700 nm (8.9 nm). GaAsP PMT detector (32 channels) was used for the present study. Thermogravimetric analysis (TGA) has been carried out using Q500 model, TA. Heating rate was 10°/ minute. Solid state 1H NMR was done in Varian 600 MHz FT-NMR instrument (VNMRS600) at 25 °C with a recycle delay time of 75 s. Spinning rate and acquisition time were 30 kHz and 0.1376 s, respectively.

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RESULTS AND DISCUSSION Syntheses, Characterization, and Formation Mechanism. Detailed experimental synthetic procedures of all the reactions are given in Experimental Section. The water-stable perovskites are prepared using Lewis base vapor diffusion 4497

DOI: 10.1021/acsaem.9b00742 ACS Appl. Energy Mater. 2019, 2, 4496−4503

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ACS Applied Energy Materials method25,26 in which methylamine vapor is first allowed to pass through the acidic solution of FAPbX3 to form MAPbX3 and then Pb(OH)2 is formed on the surface of perovskite by sacrificing few peripheral octahedral layers (Scheme 1 and

HPbX3 + [(H 2NCH(NH 2)]X = [(H 2NCH(NH 2)]PbX3 + HX

[(H 2NCH(NH 2)]PbX3(aq) = [(H 2NCH(NH 2)]+

Scheme 1. Diagram for Synthesis of Water-Stable Pb(OH)2Coated MAPbX3a

+ X−(aq) + PbX 2

Figure S1). All the reactions for the formation of FAPbX3 are formulated as follows (A−D): (ref 17)

(D)

In a typical synthesis, PbX2 is dissolved in halide acids [reaction A] and then formamidinium acetate (FAOAc) is added to it. In the presence of HX, FAOAc first converts to FAX [reaction B]. Immediately, a colorful precipitate is observed in a vial, which indicates the formation of FAPbX3 [reaction C]. These perovskites are unstable in neutral water [reaction D]. Compounds FAPbBr3, FAPbBr3−xClx, and FAPbBr3−xIx adopt the cubic crystal structure,28 whereas FAPbI3 exhibits the orthorhombic structure29 as observed from the powder X-ray diffraction (PXRD) pattern (Figure 1a). From scanning electron microscopy (SEM) images we confirm that all these structures have deformed cubic morphology (Figure S2a−d). The energy-dispersive X-ray spectroscopy (EDS) measurements reveal that the Br/Pb or I/ Pb ratio is very high, which indicates that the surface of FAPbX3 is highly Br- or I-rich (Figure S3a−d). This is due to the fact that all the FAPbX3 compounds are isolated from acidic media. Then, all the reactions involved in the transformation of FAPbX3 into water-stable MAPbX3 are formulated as follows (E−J):

a PbX2 is dissolved in HX in a vial, and then FAOAc is added and the colored FAPbX3 appeared immediately. The vial is kept in a large bottle prefilled with methylamine (MAm) which reacts with HX to form MAX. MAX replaced FA+ to form MAPbX3. In basic conditions, OH− attacks MAPbX3 leading to Pb(OH)2-coated MAPbX3 which is highly stable in water. Total synthesis is carried out in water. Black and red arrows indicate the main and side reactions, respectively. See text for A−J.

PbX 2 + HX = HPbX3 (X = Cl, Br, I)

(C)

(A)

CH3NH 2 + HX = CH3NH3X

17

(ref 30)

(E)

30

[(H 2NCH(NH 2)]OAc + HX = [(H 2NCH(NH 2)]X + HOAc

(ref 27)

[(H 2NCH(NH 2)]PbX3 + CH3NH3X = CH3NH3PbX3

(B)

+ [(H 2NCH(NH 2)]X

27

(F)

Figure 1. Characterizations of FAPbX3 (a−c) and MAPbX3 (d−f): (a, d) powder X-ray diffraction (PXRD) pattern; (b, e) attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy; (c, f) solid-state magic angle spin 1H NMR of FAPbX3 and MAPbX3. The prominent peaks of FAPbI3 at 11.71° and 25.34° corresponding to the (010) and (012) planes in (a) indicate that FAPbI3 exists mainly in orthorhombic phase. MAPbI3 adopts cubic structure (a) where XRD peaks disappear. In (b), quadruple IR peaks of FA are observed between 3400 and 3160 cm−1, while in (c), those peaks vanish and there appear the O−H stretching frequency around 3500 cm−1 corresponding to Pb(OH)2 and a new peak around 1400 cm−1 attributed to MA+ ion in (e) in all the final products. The 1H NMR peaks of FAPbX3 in (c) between 7 and 9 ppm disappear in all the final products (f), while two new peaks appear between 3 and 7 ppm in MAPbX3. Together, PXRD, FTIR, and solid-state 1H NMR give the full evidence for complete conversion of FAPbX3 to MAPbX3 in water. 4498

DOI: 10.1021/acsaem.9b00742 ACS Appl. Energy Mater. 2019, 2, 4496−4503

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ACS Applied Energy Materials

ppm) broad peaks39,40 of FA+ are observed in FAPbX3, but those peaks disappear in MAPbX3, and the CH3 (4.15−4.22 ppm) and NH3 (6.67−6.88 ppm) broad peaks41 of MA+ appear at different positions. This indicates that MA+ ions fully replace the FA+ ions from the crystal lattice, producing MAPbX3. The NMR peaks are broad due to the orientationdependent interactions. Appearance of spinning sidebands in the NMR spectra (Figure S4) is attributed to the first-order anisotropy that is not fully eliminated by magic angle spinning. The presence of FA+ in FAPbX329 (Figure 1b) and MA+ in MAPbX3 (Figure 1e) is confirmed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The observed quartet splitting is attributed to the four short charged hydrogen bonds (N−H···X−)42,43 formed between the FA+ and X− ions. The strong band around 1710 cm−1 is associated with strong CN symmetric stretching frequency. In all the final products, the characteristic IR peaks of FA+ vanish. This indicates that FA+ is completely replaced by MA+, forming MAPbX3. The other peak around 3500 cm−1 is also observed, and this peak confirms the presence of Pb(OH)2 on the surface of all the final products. An exciting phenomenon is observed during the transformation from FAPbBr3−xIx to MAPbBr3−xIx. Upon the insertion of MA+, FAPbBr3−xIx gradually changes its structure to microtube structure which converts to nanorod structure upon further inclusion of MA+ having Pb(OH)2 on the surface of nanorods (Figure 2). The presence of an −OH group is

CH3NH 2 + CH3NH3X = CH3NH3X + CH3NH 2

(equilibration)

(G)

CH3NH 2 + [(H 2NCH(NH 2)]X = CH3NH3X + [(HNCH(NH 2)]

(equilibration) (H)

CH3NH 2 + H 2O = CH3NH3+ + OH− (ref 31)

(I)

31

2CH3NH3PbX3 + 2OH− + 2CH3NH3+ = Pb(OH)2 −CH3NH3PbX3 + 3CH3NH3X

(J)

Organic cation exchange reaction is done by keeping the acidic solution of FAPbX3 in methylamine vapor without isolation from the acidic reaction mixture. Methylamine reacts with the remaining halide acid to form the methylammonium halide [reaction E]. As the acidic solution possesses high concentration of MA+, MA+ ions insert into the FAPbX3 structure to replace FA+ ions, and after a while all the FA+ ions are replaced by MA+ ions [reaction F]. Inorganic moiety [PbX6]4− plays a vital role during the transformation reaction. It strongly holds the overall geometry during the “in−out” process of organic cations. When there is no free H+ in the solution, methylamine exists in dynamic equilibrium with MA+ and FA+ ions [reactions G and H, respectively]. Now, upon changing of the pH from the acidic to basic region, methylamine reacts with water to produce hydroxide ions [reaction I] and then the concentration of MA+ ions reduces due to the presence of hydroxyl ions which react with MA+ due to higher pKa of MA+ (10.63) at 25 °C than the water pKa (14), where Ka is acidity constant. After 10 days, the color changes and ends up with water-stable MAPbX3 [reaction J]. The hydroxides easily replace halide ions which have lower nucleophilicity than OH− ions. Thus, outer layers convert to Pb(OH)2 which protects the subtle perovskite moiety from water. From PXRD, it is observed that MAPbBr3, MAPbBr3−xClx, MAPbBr3−xIx, and MAPbI3 adopt the cubic geometries and all these perovskites are coated with water-proof Pb(OH)2 (Figure 1d). During the conversion of FAPbX3 to MAPbX3 (X = Cl/Br or mixture of Br/I), the cubic motif remains intact, but in the case of FAPbI3, the orthorhombic phase changes to cubic phase by forming MAPbI3. The X-ray diffraction peak corresponding to the (200) plane of Pb(OH)2 around 17.6° is observed in all the final products, indicating the presence of Pb(OH)2 on the surface of Pb(OH)2.25 As the amounts of chloride and iodide in the mixtures are very small and Pb(OH)2 is present on the surface, the shift of PXRD peaks does not occur significantly. Generally halides have higher radius (Cl− = 167 pm, Br− = 196 pm, I− = 206 pm) than hydroxide ion (OH− = 110 pm). Iodide expanded the lattice, whereas hydroxide contracted the crystal lattice in bromiderich perovskite. All the final products have tube or rod structures (Figure S2e−h) as observed in SEM images. Previously, microrod and nanorods of CH3NH3PbBr3 and CH3NH3PbI3 structures were also reported (Table S1).32−38 To gain more mechanistic insight into the conversion from FAPbX3 to MAPbX3, we have done the solid-state magic angle spinning (MAS) 1H NMR of all the compounds (Figure 1c,f). The characteristic CH (8.06−8.10 ppm) and NH2 (8.76−9.04

Figure 2. SEM images and H-bonding scheme. SEM images of MAPbBr3−xIx at different magnification: (a) 500 (few rods), (b) 5K (one rod), and (c) 70K (small area of single rod). (d) O−H bond (dotted line) of Pb(OH)2 present on the surface. H-bonding between the interlayers of nanorods (blue region) holds the tube structure (red ball = oxygen, small pale gray ball = hydrogen, and dark gray ball = lead). At low magnification the structure seems to be tube structures, while at high magnification, it is observed that the tube structure consists of nanorods that are attached together by H-bonding.

confirmed by FTIR and XPS analyses. A peak around 3508 cm−1 in FTIR (Figure 1e) and a peak around 530.31 eV in XPS (Figure S5) support the presence of Pb(OH)2 on the surface of MAPbBr3−xIx. From SEM images, it is found that the nanorods are attached together. This is due to the fact that the O−H bond between the two interlayers of Pb(OH)2 (Figure 2d) which is present on the surface of nanorods makes strong H-bonding among nanorods in such a way that the overall 4499

DOI: 10.1021/acsaem.9b00742 ACS Appl. Energy Mater. 2019, 2, 4496−4503

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ACS Applied Energy Materials structure looks like a hollow tube structure. The hollow structure was formed within 10 days. Previously, the syntheses of spherical and hollow nanoparticles44,45 through hydrogen-bonding-assisted self-assembly process were also reported. This unique feature in selfassembly process has never been reported in perovskite materials. The H-bonding assisted different morphology could be created for perovskite materials which can be used for different practical applications. Generally, in perovskite nanocrystals the self-assembly process occurs through the intercalation of the alkyl chains.46 H-bonding with polar protic solvents is detrimental toward the stability of perovskite, and that is why hybrid perovskite degrades in polar protic solvents within a fraction of second. But in our synthesized perovskite, H-bonding not only stabilizes the structure in water but also generates tube-like morphology. The conversion of yellow phase FAPbI3 to black phase MAPbI3 in water is investigated carefully. Initially, FAPbI3 is formed by the reaction of acidic solution of PbI2 and FA. FAPbI3 is stable in acidic water media but not in neutral water. This is due to the fact that in highly acidic solution the concentrations of H+ and I− are very high, which is the reason why the structural motif remains intact in acid media. However, when the acidic FAPbI3 solution is kept in methylamine environment, there occurs incorporation of MAI which allows the octahedrons to form octahedral layer structures more closely. In basic condition, these peripheral octahedral layers react with hydroxide ions, convert to Pb(OH)2, and form Pb(OH)2-coated MAPbI3. An FTIR peak at 3491 cm−1 (Figure 1e) and an XPS peak around 530.09 eV (Figure S6) confirm the presence of Pb(OH)2 on the surface of MAPbI3. The other benefit of this transformation is the formation of black phase of MAPbI3 which is known to be one of the best light active materials for solar cells. The bond length in yellow phase FAPbI3 is higher than that of black MAPbI3 due to higher radius of FA+ (diameter = 2.53 Å) than MA+ (diameter = 2.17 Å).7 Thus, instead of breaking the yellow phase FAPbI3, MA+ is inserted into the structure and forms the cubic structure of MAPbI3. Optical Properties. We study the optical properties of both FA (Figure 3a,c) and MA (Figure 3b,d) based perovskites (Table S2). FAPbBr3, MAPbBr3−x, FAPbBr3−xClx, and MAPbBr3−xClx show fluorescence in the solid state, but only MAPbBr3−x and MAPbBr3−xClx show bright fluorescence in water under UV light due to its stability in water (Figure S7). In FAPbBr3 and FAPbBr3−xClx, two absorbance peaks appear at (519, 550 nm) and (520, 548 nm), respectively. But after conversion to the MA congener perovskite, those peaks vanish and new blue-shifted absorption peaks appear at 525 and 502 nm in MAPbBr3 and MAPbBr3−xClx, respectively, exhibiting green and sky-blue fluorescence under UV light. FAPbBr3−xIx shows two absorbance peaks at 544 and 570 nm, while these peaks disappear in MAPbBr3−xIx and a new peak is observed at 428 nm. This is attributed to the formation of nanosized rods (Figure 2c). In the case of FAPbI3, a dramatic optical feature is noted. FAPbI3 exhibits absorption peaks at 373, 424, 495, 551, and 594 nm. Here, red-shift occurs from 594 to 754 nm with Stokes shift of almost 160 nm. This is due to the fact that FAPbI3 exists in yellow phase, whereas the structure converts to black phase of MAPbI3. FAPbBr3, FAPbBr3−xClx, and FAPbBr3−xIx show the PL maxima at 567, 565, and 597 nm, respectively, whereas the yellow phase of FAPbI3 exhibits peaks at 486 and 519 nm. All the PL peaks in the first three

Figure 3. Photophysical properties and images of perovskites. Solidstate UV−vis diffuse reflectance spectra [F(R) vs wavelength (nm)] of (a) FAPbX3 and (b) MAPbX3 (K-M denotes Kubelka−Munk). Solidstate PL spectra of (c) FAPbX3 and (d) MAPbX3.

compounds blue-shift, but the PL peak red-shifts in FAPbI3 upon the incorporation of MA+ into the [PbX6]4− moiety. The new PL peaks appear at 567, 565, and 597 nm in FAPbBr3, FAPbBr3−xClx, and FAPbBr3−xIx, respectively. The PL peak for MAPbI3 appears at 763 nm. FAPbBr3 and FAPbBr3−xClx have narrow emission features with full width at half-maximum (fwhm) of 25 and 28 nm, respectively, and Stokes shift is 17 nm in both cases. For FAPbBr3−xIx, the Stokes shift is 27 nm. This is due to the fact that the mixed halide composition creates more surface defects than the pure one due to mismatch of their radius. MAPbI3−xBrx and FAPbI3 exhibit the PL peaks at similar positions, but their absorption features are different. The PL quantum yield (QY) increases upon the insertion of MA+ into the structures, e.g, FAPbBr3 and FAPbCl3−xBrx exhibit PL-QY 1.6% and 1%, respectively, whereas MAPbBr3 and MAPbCl3−xBrx show 6.5 and 7.5%, respectively. The PL-QY of MAPbI3 is 3.2% in the red region. It is found that all the samples retain its original PL-QY (Table S3) as well as bright intensity in water, indicating that MAPbX3 is highly47,48 stable. As the size of final products are large (50−100 μm), the probability of nonradiative recombination is high due to high crystalline defects and charge carrier traps on crystal surfaces, and consequently, PLQY is low as compared to the reported nanocrystals.32 The improvement of PLQY could be made by surface modification or new modified synthetic route. Our as-synthesized Pb(OH)2-coated MAPbX3 perovskites are stable more than one year which is higher than any other organic−inorganic lead halide perovskites (Table S4).49−53 Single-Particle Imaging and PL Study. We have carried out single-particle imaging and PL study to obtain shape dependent PL images of individual particles and PL spectra of those particles using 405 nm laser (Figure 4 and Figure S8). The PL images of Pb(OH)2-coated MAPbBr3, MAPbBr3−xClx, and MAPbI3 reveal that the particles are tube-shaped and these perovskites exhibit single color, e.g., green, sky-blue, and red, respectively, corresponding to their emission wavelengths; for example, individual particles of MAPbBr3 and MAPbBr3−xClx 4500

DOI: 10.1021/acsaem.9b00742 ACS Appl. Energy Mater. 2019, 2, 4496−4503

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ACS Applied Energy Materials

that of FAPbX3, the thermal stability follows the reverse trend. This is because FAX possesses higher thermal stability than MAX, and hence, the first weight loss of MAPbX3 occurs at lower temperatures (193−220 °C) as compared to FAPbX3 (289−324 °C). In both FAPbX3 and MAPbX3, the second weight loss occurs due to degradation of PbX2. As PbBr2 is more stable than PbI2, all the bromide rich perovskites degrade at higher temperature than iodide rich perovskites. In all the final products, MAPbX3 around 34−38 wt % remains undegradable. This is attributed to the formation of PbO.54

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CONCLUSIONS In summary, we have shown the aqueous conversion of FAPbX3 to MAPbX3 by nontemplate sacrificial approach and this MAPbX3 is stable in water more than one year. This work represents the first example of organic cation exchange reaction of hybrid perovskites family in water. By use of this cation exchange reaction, the phase transformation of yellow phase FAPbI3 to black phase MAPbI3 and the formation of tube structure of MAPbI3−xBrx through H-bonding mediated selfassembly process in water, for the first time, have been explored. The single-particle imaging study reveals the color purity as well as shape-correlated images of water-stable perovskites. Our detailed mechanistic study of organic cation exchange reaction in water, reaction chemistry, and physical processes in hybrid perovskites would indeed help in making widespread their applications in humid conditions.

Figure 4. Single-particle imaging and PL spectra: (a) MAPbBr3; (b) MAPbBr3−xClx; (c) MAPbI3. (d) PL spectra of MAPbBr3 and MAPbBr3−xClx. The measured points are shown as red “+” in (a) and (b). Due to limitation of the instrument, the PL spectrum of MAPbI3 is not shown here.

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show PL maxima at 525 and 502 nm, respectively. The phase segregation of mixed halide perovskites is a well-known problem which shifts the resulting PL signal. However, here, in the case of FA and MA based Br−Cl systems, we find no blue-shifted PL peaks and fluorescence images (Figure 4 and Figure S9), ruling out the phase segregation phenomena. The pure color of these perovskites indicates that all the particles of each individual perovskite are made of similar kinds of particles that emit PL in the same position. All these powder perovskites also show the same PL peak positions with those of single particles, indicating the high color purity of those samples. The similar cases are also observed for FAPbBr3, FAPbBr3−xClx, and FAPbBr3−xIx (Figure S9). FAPbBr3, FAPbBr3−xClx, and FAPbBr3−xIx show PL peaks around 557, 560, and 592 nm, respectively. However, a different situation is observed for Pb(OH)2-coated MAPbBr3−xIx which emits blue, green, yellow, and red color (Figure S10). This is due to the fact that this compound is the mixture of several kinds of halide perovskites, e.g., pure bromide with different layers, bromide− iodide, or iodide. From the image, it is observed that green particles are present in most of the area, due to the presence of MAPbBr3. Water and Thermal Stability Test. The longevity of Pb(OH)2-coated MAPbX3 in water is tested by keeping all the final compounds in water for 1 year, and then PL-QY is checked. It is found that all the samples retain its original PLQY (Table S3) as well as bright intensity in water, indicating that MAPbX3 is highly stable. As the size of final products is large (50−100 μm), the probability of nonradiative recombination is high due to high crystalline defects and charge carrier traps on crystal surfaces, and consequently, PLQY is low as compared to the reported nanocrystals.32 The improvement of PLQY could be made by surface modification or new modified synthetic route. Thermal stability of all the samples are also tested by thermogravimetric analyses (TGA) (Figures S11 and S12). Though the stability of MAPbX3 in water is higher than

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00742. SEM images, NMR spectra, XPS results, single particle imaging and PL spectra, comparison of photophysical properties, water stability results, and TGA curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Atanu Jana: 0000-0001-6566-0438 Kwang S. Kim: 0000-0002-6929-5359 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NRF (National Honor Scientist Program, Grant 2010-0020414). REFERENCES

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