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C: Physical Processes in Nanomaterials and Nanostructures

In-situ Investigation of Water Interaction with Lead Free All Inorganic Perovskite (CsSnICl ) 2

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6-x

Weiguang Zhu, Tiankai Yao, Junhua Shen, Wenqian Xu, Bowen Gong, Yachun Wang, and Jie Lian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00720 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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The Journal of Physical Chemistry

In-situ Investigation of Water Interaction with Lead Free All Inorganic Perovskite (Cs2SnIxCl6-x) Weiguang Zhu†, Tiankai Yao†, Junhua Shen‡, Wenqian Xu§, Bowen Gong†, Yachun Wang†, Jie Lian†,* †Department

of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic

Institute, Troy, New York 12180, USA ‡Department

of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New

York 12180, USA §X-ray

Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont,

Illinois 60439, USA

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

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ABSTRACT Hybrid halide perovskites display a great tunability of optoelectronic properties and environmental stability through changing the halogen anions (e.g., I, Cl and Br). However, their water interaction and degradation mechanisms are not fully elucidated. In this work, the interaction of Cs2SnCl6 and Cl-enriched solid solution Cs2SnI0.9Cl5.1 with water was systematically studied by in-situ synchrotron X-ray diffraction and micro-Raman spectra, comparing with the isostructural Cs2SnI6. Unlike Cs2SnI6, which experiences a direct dissolution in water, Cs2SnCl6 displays enhanced stability and remains crystalline with minor amorphous precipitation during dehydration. Under controlled dehydration conditions, 2D Cs2SnCl6 flakes can be precipitated out from water solution. Furthermore, the mixed halide perovskite (Cs2SnI0.9Cl5.1) experiences fast iodide dissolution in water solution and transforms to a more chloride-enriched perovskite which shows a behavior similar to Cs2SnCl6. The mechanistic understanding of the dissolution-precipitation process of Cs2SnIxCl6-x perovskites is useful for developing new perovskites with varied halogen and controlled environmental stability.

Table of Contents Graphic

KEYWORDS: Hybrid halide perovskites, water interaction, iodide dissolution, recrystallization


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INTRODUCTION The organic-inorganic perovskites (MAPbX3) have attracted significant research interests due to their high light absorption coefficient,1 long carrier diffusion length2-4 and extraordinary optoelectronic properties,4 as well as their solution processability,5-7 and have already been demonstrated as excellent light absorber materials for solar cells.8-14 However, there are two major challenges of these materials to impede their further development and commercialization, including the toxicity of lead and humidity instability.1,15-17 For the moisture instability of MAPbI3, water molecules can be incorporated into the crystal structure, forming monohydrate perovskite (MAPbI3·H2O).18-20 Further exposure to water vapor can lead to the formation of dihydrates (MA4PbI6·2H2O).18,20 Water intercalation induces the distortion of the 3D MAPbI3 perovskite structure to form 1-dimensional double-chain or 0D MAPbI3 perovskite variants, thus resulting in irreversible degradation of MAPbI3 perovskite.18 Lead-free all inorganic perovskite compositions, e.g., Cs2SnI6, have also been explored as an alternative for potential applications as hole transport materials or light absorber materials for solar cell applications21-24. Due to its high oxidation state of Sn4+, Cs2SnI6 is stable in ambient environment and can maintain its efficiency over 30 days in air,21,25 despite a relatively-low efficiency (~1%). However, Cs2SnI6 is still not very environmental stable. Previously, we reported a rapid dissolution and phase decomposition of Cs2SnI6 in water solution.26 Different in the phase behavior of MAPbI3 in water vapor, no hydrated phases have been observed when exposing Cs2SnI6 in a high humidity environment. Interestingly, a partially-reversible process was observed, leading to the formation of octahedral-shaped Cs2SnI6 precipitates from solutions upon dehydration, indicating a strong self-healing ability. To further improve the environmental stability, the mixed halide perovskites has been explored by varying different halogen anions due 3 ACS Paragon Plus Environment


to the significant differences in size and electronegativity among the halide anions. MAPbI3-xClx exhibits better film morphologies, improved device performance and enhanced moisture stability in comparison with the pure end member of MAPbI3.3,27-29 Through Cl incorporation, the environmental stability of Cs2SnIxCl6-x can further be enhanced and the high Cl compositions can be stable in ambient air for over 2 month.24 However, the effects of halogen anions and different halide compositions on the environmental stability are not fully-understood yet, and limited knowledge are available on the interaction and degradation mechanisms of the hybrid halide lead-free all inorganic perovskite (Cs2SnIxCl6-x) with water molecules. In this work, the water interaction and the environmental stability of Cl-enriched perovskites in the binary system of Cs2SnIxCl6-x were systematically studied by in-situ synchrotron X-ray powder diffraction and in-situ micro-Raman spectroscopy. The phase behaviors and degradation mechanisms, particularly the dissolution-precipitation of the binary (Cs2SnIxCl6-x) system, were investigated upon water exposure. Cs2SnCl6 displays an enhanced water stability as compared with the end member of Cs2SnI6. Partial dissolution Cs2SnCl6 occurs, leading to the formation of Sn(OH)4 amorphous alteration phase as a result of hydrolysis process. 2D Cs2SnCl6 flakes could be precipitated out during dehydration process at well-controlled solvent conditions. The solid solution Cs2SnI0.9Cl5.1 with small amount of iodine incorporation shows a rapid iodine dissolution, transforming to the Cl-enriched composition and displaying a similar degradation behavior as the pure end member Cs2SnCl6. The enhanced environmental stability of the Cs2SnCl6 and Cl-enriched compositions can be attributed to greater ionic potentials of chloride as compared with iodide due to its smaller ionic radius and thus stronger atomic bonds.

EXPERIMENTAL SECTION Materials 4 ACS Paragon Plus Environment

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Unless stated otherwise, all materials and solvents used were as-purchased without further purification. Cesium iodide (CsI, 99.9%), n-Butyl acetate (99% min), and hydroiodic acid (HI, 55-58%) were purchased from Alfa Aesar. Tin powders (Sn, ≥99%), iodine (I2, ≥99.99%), and tin (IV) chloride pentahydrate (SnCl4·5H2O, 98%) were from Sigma Aldrich. Cs2SnIxCl6-x preparation The method for preparing the end members and Cl-enriched perovskite compositions was described in details in a previous report24. Typically, for Cs2SnI6, CsI (1 mmol) was dissolved in 10 mL methanol and heated to ~60 oC in a water bath. In a separated 25 mL beaker, SnI4 (0.5 mmol) was dissolved in 4 mL n-Butyl acetate with addition of 2 mL hydriodic acid. SnI4 was prepared following the method described in the previous report.24 Addition of the acid SnI4 solution to the warm CsI solution under vigorous stirring led to spontaneous precipitation of fine black powders. The mixture was stirred for 30 mins to complete reaction in the 60 oC water bath. Then the solid was washed by n-Butyl acetate twice via centrifuging and dried in an oven at 80 oC

overnight. For Cs2SnCl6, following a similar procedure, CsCl (1 mmol) was dissolved in 2

mL of water. In a separate 50 mL beaker, SnCl4·5H2O (0.5 mmol) was dissolved in 4 mL of warm ethanol (~60 oC) to obtain a clear transparent solution. Addition of the alcoholic SnCl4 solution to the aqueous CsCl solution to precipitate white Cs2SnCl6 powders. 2D Cs2SnCl6 flakes were synthesized by dispersing Cs2SnCl6 powder (~5 mg) in 5 mL water, followed by addition of 2 droplets of the dispersion on a TEM copper grid and dried at room temperature. To obtain the mixed halide perovskite (Cs2SnI0.9Cl5.1), CsCl (1 mmol) was dissolved in 10 mL of methanol and kept at ~60 oC. SnCl4·5H2O (0.5 mmol) was dissolved in a mixture of 5 mL n-Butyl acetate and 2 mL of hydriodic acid. Then, the mixture of both solutions led to the precipitation of the desired mixed halide perovskite.



Water interaction and in-situ characterization In-situ synchrotron X-ray powder diffraction (XRD) was performed at the Advanced Photon Source on the beamline 17-BM-B at Argonne National Laboratory using a Si (111) monochromator (λ = 0.39433 Å). The experiment was carried out by loading approximately 5 mg of powder sample in a Kapton tube of 1.1 mm diameter with addition of 250 µL water. The diffraction pattern was recorded by an amorphous silicon-based area detector from PerkinElmer (2048×2048, 200 µm pixels). All data was collected at room temperature. To monitor the reaction at real time, the diffraction patterns were continuously collected with 60 seconds per frame. The obtained patterns were analyzed using the Rietveld refinement functions implemented in GSAS II.30,31 In-situ micro-Raman spectra were measured on a Renishaw micro-Raman spectrometer equipped with an Ar+ ion laser (λ = 514.5 nm) at room temperature. Two drops of water were added on top of the powder and the spectra were collected at every 5 min interval to monitor the interaction between the perovskites and water. A typical spectrum was acquired with an exposure time of 30 seconds and 2 accumulations under an operational power of 20 mW. To investigate the perovskite morphological changes before/after the water reaction, scanning electron microscopy (SEM) observation was carried out using a Carl Zeiss Supra 55 field emission scanning electron microscope with an Oxford energy-dispersive X-ray spectrometer (EDX). Transmission electron microscopy (TEM) analysis was conducted on a TEM microscope (JEOL 2011) at an operation voltage of 200 kV.

RESULTS AND DISCUSSION The water interaction with lead free all inorganic perovskite (Cs2SnIxCl6-x) was monitored by in situ synchrotron X-ray powder diffraction (XRD). Based on the previous reports21-25,32, Cs2SnI6 6 ACS Paragon Plus Environment

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is stable in the moisture air and does not show obvious degradation even under a high humidity environment (~80%) over 7 hours. Thus, to accelerate the degradation process and observe the water interaction with Cs2SnI6, we loaded Cs2SnI6 powders in a tube with addition of water droplets directly on the powders (see the experimental section for more details). Figures 1a-d are a sequence of typical diffraction patterns selected from a large set of data collected by in-situ synchrotron X-ray diffraction experiments at different durations of water interaction. The full set of diffraction data is shown in Figure S1. Figure 1a shows the original Cs2SnI6 powder diffraction pattern without water addition. Once water added on the sample, the diffraction intensities of Cs2SnI6 perovskite decrease rapidly due to the degradation. Within 10 mins, the powder was fully dissolved, and no obvious crystallinity was observed (Fig. 1b). As water evaporated, new diffraction patterns started to be observed, indicating that crystals were precipitated from the solution and the diffraction belongs to Cs2SnI6 (Figure 1c). Figure 1d shows the final compositions obtained as water fully evaporated, mainly Cs2SnI6 and SnI4. It indicates the reaction between Cs2SnI6 and water is not completely reversible. The integrated curves are plotted in Figure 1e, demonstrating the rapid dissolution process of Cs2SnI6 in water. During the dehydration process, Cs2SnI6 reprecipitates with decomposition product SnI4, as shown in Figure 1e. It is worthy to note that the recovery process appears to take only several minutes, as shown in Figure S2. The degradation mechanisms and the dissolution-precipitation of Cs2SnI6 were discussed in details in our previous publication26. Further Rietveld refinement of the synchrotron X-ray diffraction pattern after dehydration determines that the phase fractions of Cs2SnI6 and SnI4 are 40.4 and 59.6 wt%, respectively (Figure 1f). No XRD peaks from CsI are observed, suggesting that CsI as the other decomposition phase may distribute outside of the X-ray beam spot (beam size of 300 µm × 300 7 ACS Paragon Plus Environment


µm). To confirm it, X-ray scans were taken at different regions and CsI was identified as shown in Figure S3. A new broad peak (Q = ~1.35 Å-1) appeared during the dissolution process and then disappeared in the final product, which can be attributed to Sn(OH)4 due to the hydrolysis of the decomposition product SnI4 as reported previously26.

Figure 1. In situ synchrotron diffraction patterns of Cs2SnI6 powders upon addition of water droplets at different durations: (a) 0, (b) 8, (c) 70, and (d) 90 minutes; (e) The typical integrated curves of the X-ray patterns obtained by GSAS-II program; and (f) Rietveld refinement of the Xray diffraction pattern of the dehydrated sample. The phase fractions of Cs2SnI6 and SnI4 are 40.4 and 59.6 wt%, respectively (Black squares: the experimental data; the red line: the fittings; the blue line: the difference between experimental data and fitting results; purple sticks: the corresponding Bragg diffractions). Following the similar experimental procedure for Cs2SnI6, the phase behavior of Cs2SnCl6 in water was monitored by in-situ synchrotron X-ray diffraction (see Figure 2 and Figure S4). 8 ACS Paragon Plus Environment

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Figure 2a displays the synchrotron XRD pattern of original Cs2SnCl6 powders showing welldefined polycrystalline diffraction rings. Upon water added on the powders, Cs2SnCl6 reveals enhanced stability and maintains well crystalline features over 70 mins (Figures 2b and 2c). An amorphous ring is evident in Figure 2d with a broad peak at ~2 Å-1 as contributed to the effect of water. As water evaporated at room temperature, the broad peak intensity decreases and another broad peak appears at ~1.35 Å-1 (Figures 2c and 2d), which might be the precipitation of the hydrolysis product of Cs2SnCl6. Figure 2e shows the Rietveld refinement of the synchrotron XRD pattern acquired from the final sample after water evaporation. Only polycrystalline Cs2SnCl6 with a small amount of amorphous phase is present in the dehydrated sample, indicating a much lower dissolution rate as compared with the pure end member of Cs2SnI6. Part of Cs2SnCl6 are hydrolyzed, leading to the formation of amorphous precipitates. To investigate the final products, X-ray diffractions scanning was performed alone the tube at every 1 mm interval (see Figure S5). The results manifest that a small amount of CsCl could be identified at the part of the sample, indicating the redistribution of the hydrolysis products during the drying process.



Figure 2. (a-c) 2D X-ray diffraction patterns of Cs2SnCl6 interacted with water droplets captured at different durations up to 70 minutes; (d) The typical integrated curves at different time; (e) The observed and Rietveld refined profiles of the final dehydrated sample showing only small amount of the amorphous alteration phase. Black squares represent the observed data, the red lines represent the fit, and the blue lines are difference. The positions of the Bragg diffractions are labelled by the vertical purple sticks. In-situ micro-Raman spectra were measured at various time to monitor the interaction of water with Cs2SnCl6 (Figure 3a). The as-prepared Cs2SnCl6 exhibits three Raman peaks at 168 cm-1, 231 cm-1, and 309 cm-1, which can be attributed to the asymmetric bending X-Sn-X deformation, the asymmetric Sn-X stretching vibration, and the Sn-X symmetric stretching mode, 10 ACS Paragon Plus Environment

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respectively33. Among them, the strongest Raman peak is the Sn-X symmetric stretching at 309 cm-1. Upon water addition, the Raman peak intensities decreased significantly, which could arise from Cs2SnCl6 submerged into water and part of Cs2SnCl6 was dissolved. Upon water evaporation at room temperature, the three peaks attributed to Cs2SnCl6 could be easily identified, and a new broad peak appeared around 560 cm-1 which could be assigned to Sn(OH)4, the hydrolysis product of Cs2SnCl6. To monitor the morphological change, SEM measurements were performed, as shown in Figures 3b-e. Figure 3b shows the original Cs2SnCl6 powders with a small crystal size around 50 nm. As water evaporated, Cs2SnCl6 reprecipitates from the solution and grows to much larger crystals with octahedral shapes, as shown in Figure 3c. Meanwhile, a layered structure is evidenced in the recrystallized Cs2SnCl6 crystals, suggesting the layer-bylayer growth nature (as shown in Figures 3d and e).

Figure 3. (a) Micro-Raman spectra of Cs2SnCl6 obtained at various time upon water addition; (b) A SEM image of the original Cs2SnCl6 powder; (c) The precipitated Cs2SnCl6 crystals; (c) and (d) A close-up view of the Cs2SnCl6 crystals showing the layered growth steps as observed by SEM at high magnifications.



As inspired by the layered feature of the reprecipitated Cs2SnCl6 crystals, 2D Cs2SnCl6 crystals may be precipitated out from water solution under well controlled conditions. Figure 4a shows the original Cs2SnCl6 powders with the size in the nano-metered regime. To obtain Cs2SnCl6 crystals in a 2D geometry, a small amount of Cs2SnCl6 powders were dispersed in water, and then 2 droplets of the dilute solution were added on a TEM copper grid and dried at room temperature. Cs2SnCl6 flakes in a 2D geometry deposited on the grid, as shown in the SEM images (Figures 4b and 4c). The 2D flakes have an average lateral size around 3 µm, much larger than original powders. Figure 4d displays a bright field transmission electron microscopy (TEM) image of an individual 2D flake with a flat surface and hexagonal shape, and the inset in Figure 4d is the corresponding selected area electron diffraction (SAED) pattern. The sharp diffraction patterns from the same flake also supports the high-quality single-crystalline structure of these Cs2SnCl6 flakes. The diffraction pattern can be well indexed to the cubic structure of Cs2SnCl6, and the exposed facets of the crystal are parallel to the ( crystal grows along

) plane. These results suggest that the

] direction vertically; and horizontally, the 2D crystals grow along [111]

and [002] directions. Due to its feasible processability, this 2D featured Cs2SnCl6 crystals may have a great interest in the wide bandgap optoelectronics. Thus, further investigation on this material is still ongoing.


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Figure 4. (a) A SEM image of the nanosized Cs2SnCl6 powder; (b) 2D Cs2SnCl6 flakes on a TEM grid precipitated from water solution and dried in air; (c) A high magnification view of the 2D crystals; and (d) A bright field TEM image of a Cs2SnCl6 flake with hexagonal shape showing well defined crystal facets and growth directions. Inset is the indexed SAED pattern of the 2D Cs2SnCl6 flake. The enhanced water stability of Cs2SnCl6 can be attributed to greater ionic potential of chloride than iodide. Ionic potential is defined as the ionic charge divided by its radius, and is a good indicator of the materials’ thermal stability and chemical durability.34 Due to the significantly larger ionic radius for negatively-charged iodide (2.20 Å) than chloride (1.81 Å)35, chloride has a higher ionic potential than iodide. The higher ionic potential may form a shorter bond length and stronger bond strength, responsible for the enhanced environmental stability for Cs2SnCl6. The larger ionic potential is also consistent with thermal stability in which Cs2SnCl6 displays greater phase decomposition temperature than Cs2SnI624. Cs2SnI6 and Cs2SnCl6 also 13 ACS Paragon Plus Environment


exhibit different dissolution behaviors in water. Cs2SnI6 directly decomposes into CsI and SnI4 in water and the hydrolysis of SnI4 leads to Sn(OH)4 precipitation. Instead of direct dissolution, Cs2SnCl6 is much stable in water at the same exposure condition and its crystalline nature can be well maintained upon water interaction except the formation of limited amorphous alteration phase during hydration process. Therefore, it is of interests to study the water interaction with the mixed halide lead free perovskites (Cs2SnIxCl6-x) and how different halogen anions affecting their environmental stability. Figure 5a show the sequence of the selected typical synchrotron X-ray diffraction patterns of the mixed halide perovskite interaction with water at various time. Upon water interaction, the synchrotron x-ray diffraction peaks of the mixed perovskite shift to higher Q, indicating a lattice contraction as a result of the fast dissolution of iodide, as shown in Figure 5b. The chemical composition of the original mixed halide perovskite powder was determined by the Rietveld refinement (Figure 5c). The refined profile displays the original perovskite possesses the cubic structure with a lattice parameter of 10.6740(2) Å, larger than that of Cs2SnCl624. From the refinement of site occupancies, the composition could be determined as Cs2SnI0.9Cl5.1. After 5 mins in water, the lattice parameter of Cs2SnI0.9Cl5.1 decreases to 10.4413(3) Å and its composition changes to Cs2SnI0.12Cl5.88 (Figure S6). After that, the chloride-enriched perovskite undergoes much slower iodide dissolution and remains its crystalline with amorphous alteration phases occurred during dehydration process, similar to Cs2SnCl6 (Figures 5a and 5b). The lattice parameter of the final water treated sample is determined to be 10.4248(2) Å and the composition becomes Cs2SnI0.09Cl5.91 as determined from the refinement results (Figure 5d). These results suggest that Cl-enriched perovskite composition with the incorporation of a small amount of iodide can still maintain good water stability. 14 ACS Paragon Plus Environment

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Figure 5. (a) X-ray diffraction patterns of the mixed halide lead free perovskite (Cs2SnI0.9Cl5.1) exposed to water at various time; (b) The strongest reflection, (220), at Q = 1.66 Å-1, shifts to higher Q with increasing time in water; Rietveld refined X-ray diffraction patterns of (c) original powders and (d) after water treated sample. Black squares are the observed data, the red lines are the fittings, and the blue lines represent difference, and the corresponding Bragg diffractions are labelled by the purple sticks. Figure 6a displays the measured Raman spectra of the mixed halide perovskite at various time after adding water droplets. For the original powder, two Raman bands at 165 cm-1 and 306 cm-1 could be identified, corresponding to the Cl-Sn-Cl asymmetric bending and the Sn-Cl symmetric stretching, respectively. The Raman band at 128 cm-1 is due to the Sn-I symmetric



stretching vibration. The peak at 145 cm-1 could be ascribed to the vibration mode caused by the mixed halide ions (I, Cl). Similar phenomena has also been observed in the mixed organicinorganic perovskite (MAPbIxBr3-x).36 As water added on the sample, the Sn-I symmetric stretch at 128 cm-1 disappears, indicating the loss of iodide from the structure. While two Sn-Cl related Raman bands still remain, suggesting better water stability for higher Cl concentration mixed halide perovskite, which is consistent with the in-situ XRD results as mentioned above. Moreover, as water evaporated, the Sn-Cl related Raman bands become stronger and sharper, indicating the Cl-enriched perovskite reprecipitate from the solution. SEM imaging was performed to monitor the morphological changes, as shown in Figures 6b-e. Well-defined octahedra shapes are observed for the as-prepared Cs2SnI0.9Cl5.1 powder (Figure 6b). After water interaction, pitting formation was observed, suggesting the dissolution process undergoes through etch pits formation, resulting in a hollow structure, as shown in Figure 6c. It is in accordance with our previous study on the dissolution behavior of Cs2SnI6.26 Meanwhile, four fold structured crystals and dendrites could also be observed based on different growth kinetic conditions. From the EDX measurement, the partial dissolved crystals and precipitates are Clenriched perovskite phase; while the dendrites are mainly CsI formed from the released iodide, as shown in Figure S7. Therefore, it is evident that tunable water stability of the lead free perovskite could be obtained by mixing halides, and their water interaction behavior depends on the iodide and chloride ratio in the compounds.


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Figure 6. (a) Micro-Raman spectra of Cs2SnI0.9Cl5.1 upon water addition. (b) A SEM image of the original mixed halide perovskite. SEM images obtained on the sample after water evaporated with different morphologies, (c) an octahedra crystal with pitting holes, and (d) precipitates with four-fold structures, and (e) dendrites.

CONCLUSIONS In summary, the environmental stability and the dissolution-precipitation process of the Clenriched Cs2SnIxCl6-x perovskites (Cs2SnCl6, and Cs2SnI0.9Cl5.1) upon direct exposure to water were investigated via in-situ synchrotron X-ray powder diffraction and micro-Raman spectroscopy, and compared with the isostructural Cs2SnI6. The isostructural Cs2SnI6 and Cs2SnCl6 perovskites display different dissolution behaviors and degradation mechanisms. Cs2SnI6 experiences direct dissolution and decomposition into CsI and SnI4 in water, resulting in the formation of intermediate hydrolysis product of Sn(OH)4. Cs2SnCl6 displays greater environmental stability upon exposure to water than the isostructural Cs2SnI6 at the same condition with the formation of a minor amorphous phase. Crystalline Cs2SnCl6 in a 2D geometry can be precipitated out under controlled solvent conditions without 3D growth. The mixed halide lead-free perovskite (Cs2SnI0.9Cl5.1) shows two different dissolution stages. A rapid 17 ACS Paragon Plus Environment


iodide dissolution occurs first, transforming to the Cl-enriched composition displaying similar degradation as the pure end member Cs2SnCl6. The enhanced environmental stability of the Cs2SnCl6 and Cl-enriched compositions can be attributed to greater ionic potential of chloride as compared with iodide due to its smaller ionic radius and thus stronger atomic bonds. The fundamental understandings of the water interaction of the Pb-free all-inorganic halide perovskites and the effects of different halides are useful for optimizing material compositions and synthesis processes for the further development of alternative perovskites with enhanced environmental stability and performance.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Jie Lian: 0000-0002-9060-8831 Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. In-situ Synchrotron X-ray diffraction was performed using Beamline 17-BM of the Advanced Photon Source (APS), a US DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.


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ASSOCIATED CONTENT Supporting Information Supporting Information Available: a full set of in-situ synchrotron X-ray diffraction data of Cs2SnI6 interaction with water, in-situ XRD of Cs2SnI6 collected during dehydration process, XRD data of Cs2SnI6 decomposition products, a full set of in-situ XRD data of Cs2SnCl6 in water, XRD data of the dehydrated Cs2SnCl6 sample, Rietveld refined data of the product of Cs2SnI0.9Cl5.1 after 5 mins in water, and EDX data on the dehydrated mixed halide perovskite sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. In situ synchrotron diffraction patterns of Cs2SnI6 powders upon addition of water droplets at different durations: (a) 0, (b) 8, (c) 70, and (d) 90 minutes; (e) The typical integrated curves of the X-ray patterns obtained by GSAS-II program; and (f) Rietveld refinement of the X-ray diffraction pattern of the dehydrated sample. The phase fractions of Cs2SnI6 and SnI4 are 40.4 and 59.6 wt%, respectively (Black squares: the experimental data; the red line: the fittings; the blue line: the difference between experimental data and fitting results; purple sticks: the corresponding Bragg diffractions). 177x109mm (300 x 300 DPI)



Figure 2. (a-c) 2D X-ray diffraction patterns of Cs2SnCl6 interacted with water droplets captured at different durations up to 70 minutes; (d) The typical integrated curves at different time; (e) The observed and Rietveld refined profiles of the final dehydrated sample showing only small amounts of the amorphous alteration phase. Black squares represent the observed data, the red lines represent the fit, and the blue lines are difference. The positions of the Bragg diffractions are labelled by the vertical purple sticks. 177x131mm (300 x 300 DPI)


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Figure 3. (a) Micro-Raman spectra of Cs2SnCl6 obtained at various time upon water addition; (b) A SEM image of the original Cs2SnCl6 powder; (c) The precipitated Cs2SnCl6 crystals; (c) and (d) A close-up view of the Cs2SnCl6 crystals showing the layered growth steps as observed by SEM at high magnifications. 177x74mm (300 x 300 DPI)



Figure 4. (a) A SEM image of the nanosized Cs2SnCl6 powder; (b) Growth of 2D Cs2SnCl6 flakes on a TEM grid precipitated from water solution and dried in air; (c) A high magnification view of the 2D crystals; and (d) A bright field TEM image of a Cs2SnCl6 flake with hexagonal shape showing well defined crystal facets and growth directions. Inset is the indexed SAED pattern of the 2D Cs2SnCl6 flake. 177x127mm (300 x 300 DPI)


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Figure 5. (a) X-ray diffraction patterns of the mixed halide lead free perovskite (Cs2SnI0.9Cl5.1) exposed to water at various time; (b) The strongest reflection, (220), at Q =1.66 Å-1, shifts to higher Q with increasing time in water; Rietveld refined X-ray diffraction patterns of (c) original powder and (d) after water treated sample. Black squares are the observed data, the red lines are the fittings, and the blue lines represent difference, and the corresponding Bragg diffractions are labelled by the purple sticks. 177x149mm (300 x 300 DPI)



Figure 6. (a) Micro-Raman spectra of Cs2SnI0.9Cl5.1 upon water addition. (b) A SEM image of the original mixed halide perovskite. SEM images obtained on the sample after water evaporated with different morphologies, (c) an octahedra crystal with pitting holes, and (d) precipitates with four-fold structures, and (e) dendrites. 177x72mm (300 x 300 DPI)


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