Unveiling Solvent-Related Effect on Phase Transformations in CsBr

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Unveiling Solvent-related effect on Phase Transformations in CsBr-PbBr2 System: Coordination and Ratio of Precursors Mei Liu, Jiangtao Zhao, Zhenlin Luo, Zhihu Sun, Nan Pan, Huaiyi Ding, and Xiaoping Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00537 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Chemistry of Materials

Unveiling Solvent-related effect on Phase Transformations in CsBr-PbBr2 System: Coordination and Ratio of Precursors Mei Liu1, Jiangtao Zhao2, Zhenlin Luo2, Zhihu Sun2, Nan Pan1, Huaiyi Ding1*, Xiaoping Wang1* 1

Hefei National Laboratory for Physical Sciences at the Microscale, University of

Science and Technology of China, Hefei, Anhui 230026, P. R. China 2

National Synchrotron Radiation Laboratory and CAS Key Laboratory of Materials

for Energy Conversion, University of Science and Technology of China, Hefei, Anhui, 230026, China. *Correspondence and requests for materials should be addressed to Huaiyi Ding, Xiaoping Wang

(email: [email protected], [email protected],)

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Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract All-inorganic cesium lead-halide perovskite nanocrystals have emerged as attractive optoelectronic nanomaterials owing to their stabilities and highly efficient photoluminescence. However, the inorganic perovskites of CsPbBr3 synthesized by the solution method are often suffered from by-products such as Cs4PbBr6 and CsPb2Br5. Herein, we have investigated thoroughly the solvent-related effect on the phase formation in CsBr-PbBr2 system through single crystal X-ray diffraction measurement. It is found that the prepared product is dominantly determined by the coordination number (CN) of Pb (II) and the ratio of precursors. Using dimethylsulfoxide (DMSO) or dimethylformamide (DMF) as the solvent, Pb2+ is found to be surrounded by six-coordination sites, and the products can be tuned from CsPbBr3 to Cs4PbBr6 by increasing the precursor ratio of CsBr to PbBr2. On the contrary, in the solvent of water, only Pb2+ eight-coordinated crystal of CsPb2Br5can be produced, regardless of the ratio of CsBr to PbBr2. More importantly, with the investigation on the extended X-ray absorption fine structure (EXAFS) for Pb L-II edge in precursor solutions, we identify that the CN of Pb (II) of resultants are the same as those of the corresponding plumbite oligomers in precursor solutions. In addition, the phase transitions from Cs4PbBr6 to CsPbBr3, amorphous state and CsPb2Br5triggered by water vapor have also been observed clearly. This work not only enriches our understanding of the phase formation of CsBr-PbBr2 system but also provides the knowledge of the degradation of halide perovskites in the environment of humidity.


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The CsBr-PbBr2 system possesses a complicated phase diagram containing three compounds, CsPbBr3, CsBr-rich Cs4PbBr6, and PbBr2-rich CsPb2Br5 (CPBs).1 Recently, the narrower band-gap semiconductor material CsPbBr3 with perovskite structure attracts a great deal of research attention in the field of optoelectronic devices

2-5

by the virtue of high photoluminescence (PL) quantum yield, large

absorption cross-sections and facile emission color tunability. 6 Whereas the studies of CsPb2Br5 and Cs4PbBr6 are very rare, resulting in greatly divergences on their photophysical behavior. Actually, both of them were regarded as materials with strong green-emission and green-lasing properties in some reports initially,

7-9

but the

follow-up studies suggested that they are wide-gap semiconductors with band-gap 2.979 eV and 3.95 eV respectively and have no visible PL emission.

10-12

These

conflicting results may stem from a few CsPbBr3 nanocrystals embeddedinCs4PbBr6 or CsPb2Br5, owing to the fact that CPBs are easily interconvertible to form mixed phasese.

11, 13-14

On the other hand, the composites of the CsPbBr3 containing

Cs4PbBr6 or CsPb2Br5 might have a better performance in optoelectronic devices than intrinsic CsPbBr3.

15-19

For instance, Wang et al. synthesized embedded ligands-free

CsPbBr3 nanocrystals in a wider band-gap Cs4PbBr6 matrix as temperature-insensitive frequency-upconverted optical gain. Lack of ligands in CsPbBr3 and protection from Cs4PbBr6 matrix ensure its thermal stability and high temperature operation. 19 The single crystals of CPBs can be readily synthesized in solution-process through inverse temperature and anti-solvent vapor-assisted crystallization method, which has greater yield and low-cost.

20-22

Generally, dimethyl sulfoxide (DMSO) or

dimethylformamide (DMF) and H2O are frequently used as effective solvents for dissolving lead halides and CsBr. Interestingly, the resultants are deeply affected by the solvents. CsPbBr3 and Cs4PbBr6 bulk crystals have been synthesized in DMSO, and they are often cocrystallized.

13-14, 23

Pure powders of CsPb2Br5 were obtained in

water by M. Rodová et al. 22 Meanwhile，it was reported that the lead halide in solvent forms a serious of PbXn2-n (n≥2, X=Cl, Br, I) complexes, which play a key role in building PbBrn polyhedron framework of CPBs crystals.

24-27

However, the evolution

process from PbXn2-n to the CPBs in different solvents is still ambiguous. ACS Paragon Plus Environment


In this paper we focused on the roles of the precursors ratio and solvent properties in the formation of CPBs. Based on the extended X-ray absorption fine structure (EXAFS) of Pb L-II edge in precursor solutions and single crystal X-ray diffraction (XRD) measurement on the resultants, we reveal that the the coordination number (CN) of the Pb (II) of resultants are controlled crucially by the intermediates mediated by the solvents in the precursor solutions, while the stoichiometry of CPBs can be regulated by the precursors ratio of CsBr to PbBr2. Furthermore, by utilizing the coordination engineering and solubility of precursors, we achieved the phase transitions from Cs4PbBr6 to CsPbBr3 and CsPb2Br5 in humid environment. The ratio of precursors. We began our investigation on solution synthesis with the solvent of DMSO. In a typical anti-solvent vapor-assisted crystallization, 21 10 mL of a DMSO solution containing CsBr (0.2 M) and PbBr2 with various concentrations (Pb:Cs) 9:1, 4:1, 1:1, 1:4 and 1:9 acted as the precursors, and dichloromethane (DCM) as anti-solvent. XRD patterns of the resultants are shown in Figure 1a. We found that only PbBr2•2DMSO is produced when the ratio of PbBr2 to CsBr is 9. However, a precursor with a ratio of 4 yields pure CsPbBr3 (orthorhombic, space group: Pbnm; a = 8.207 Å,

b = 8.255 Å, c = 11.759 Å, α = β = γ = 90o) 22 precipitate. As decreasing

the ratio of PbBr2 to CsBr to 1, Cs4PbBr6 (trigonal, space group: R-3c; a = b =13.722 Å, c = 17.299 Å; α = β = 90o, γ = 120o)

28

appears gradually and dominates the

products at the ratio down to 1/4. Further decreasing the PbBr2 proportion, CsPbBr3 disappears and Cs4PbBr6-CsBr is crystallized from the solution. The results suggest that the variation tendency of stoichiometry of the products is consistent well with the ingredients of the precursors, i.e., with PbBr2 ratio decreasing in the precursor solution, the products are gradually changed from PbBr2•2DMSO to CsPbBr3 and then to Cs4PbBr6 and finally to CsBr. Similarly, when using DMF as the solvent, PbBr2•DMF, CsPbBr3, Cs4PbBr6 and CsBr crystals are formed as the products, which results are shown in Figure S1 and Table S1. We next change the solvent to H2O with cooling crystallization method through cooling down the precursor solutions.

22

XRD results of the products with various


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concentration ratios (Pb: Cs) of 9:1, 4:1, 1:1, 1:4 and 1:9 are shown in Figure 1b. As seen, with the ratio decreasing, the products are gradually changed from 3PbBr2•2H2O to CsPb2Br5 (tetragonal, space group: I4/mcm; a = b = 8.45 Å and c = 15.07 Å, α = β = γ = 90o).29 However, further decreasing the ratio of PbBr2: CsBr to as low as 1:9, only CsPb2Br5 is obtained, no CsPbBr3 and Cs4PbBr6 signal can be detected in XRD patterns. Therefore, we suggest that, in addition to the ratio of precursors, the solvent also plays an important role in the phases of final products.

Figure 1. XRD patterns of the resultants produced by varying the precursor ratios of PbBr2 to CsBr in different solvents. (a) DMSO and (b) H2O. It should be mentioned that, besides the solvent, both of the anti-solvent and the crystallization technique also play roles in the final phase formation. This is because that the anti-solvent with higher dielectric constant can screen the interaction between the solvated salts and the solvent, and hence govern the final products, as reported by Rakita et al.

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At the same time, in the solution preparation of crystal, the route of

crystallization is dominantly controlled by either thermodynamic or kinetic process, which distinctly depends on the crystallization technique. In this work, we adopted cooling crystallization method and DCM as anti-solvent, which has less impact on the final products due to its small relatively dielectric constant (ε≈9.1) as compared to the solvents of DMSO (ε≈42) and DMF (ε≈38) solvent. The coordination of Pb (II). To get more insight into the solvent effect on CPBs formation, we focus on the CN of Pb (II) of products. It is well-known that the



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structures of CPBs can be divided into two categories by their building blocks of CPBs: one is CsPbBr3 or Cs4PbBr6 with [PbBr6]4− octahedral units (six-coordinate sites), and the other is CsPb2Br5 with [PbBr8]6− hendecahedral units (eight-coordinate sites), whereas there are rarely reports on the products of PbBr2•2DMSO, PbBr2•DMF and 3PbBr2•2H2O. To this end, the single crystals of PbBr2•2DMSO, PbBr2•DMF and 3PbBr2•2H2O are firstly separated respectively from PbBr2 solutions in DMSO, DMF and H2O solvents to determine their corresponding Pb (II) CN. Figure 2 and Figure S2 show their crystal structures deduced from single crystal X-ray crystallographic analysis. As seen, each lead ion is coordinated by four bromide ions and two oxygen atoms of DMSO ligands for PbBr2•2DMSO (Figure 2a),

30

but by five bromide ions

and one oxygen atom of DMF ligands for PbBr2•DMF (FigureS2). In other words, Pb (II) ions in both of these crystals have six-CN. However, as shown in Figure 2b, each Pb (II) ion has eight-CN in 3PbBr2•2H2O crystal, where Pb (II) binds either eight bromide ions or seven bromides ions and one oxygen atom of H2O ligands. The results imply that the coordination number of Pb (II) resultants is six in the crystals produced from the solvent of DMSO or DMF, but becomes eight for H2O solvent.

Figure 2. The crystal structures of (a) PbBr2•2DMSO30 and (b) 3PbBr2•2H2O characterized and deduced by single crystal X-ray crystallographic analysis. The whole synthesis process of lead perovskite consists of the dissolution of reagent salts in solvent and the crystallization from the supersaturation solution. During the process, the solvent molecules or Br– was coordinated to Pb (II) to form PbBrn2-n anion, such as PbBr2, PbBr3− and PbBr42− species when the lead halide precursor dissolved in aqueous, DMF or other polar solutions. 31-32 In this regards, it is


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reasonable to speculate that the structure of intermediate PbBrn2-n in solution plays a key role on the formation of CPBs resultants. We want to know whether the coordination number of intermediate is six in the solvent of DMSO or DMF, and become eight in H2O? To this end, EXAFS measurement at Pb L-II edge was carried out to probe the CN of the Pb (II) in precursor solutions with different solvents of DMSO, DMF and H2O. It is well known that EXAFS is resulted from the oscillatory region of an X-ray absorption spectrum above an absorption edge. The fine oscillatory behavior is attributed to the backscattering of photoelectrons from various shells of neighboring atoms, and therefore it can be used to identify the coordination environment of the atoms. The details of the experiment can be found in the supplementary information. Figure 3 shows the experimental results (open squares) of Fourier transform (FT) curves of the [k2χ(k)] in the R space for the intermediates in different solvents, where k is the wave vector of photoelectron and χ(k) is the oscillation function. The FT curves exhibit two coordination peaks, corresponding to Pb−O and Pb−Br bonds, respectively, while the oscillation positions and intensities vary with the solvents. Note that the chemical bands of Pb-N and Pb-O in DMF solvent can not be distinguished directly from the EXAFS. However, with the knowledge DMF coordinated with Pb2+ ions in solid state (PbBr2•DMF) from our single crystal X-ray crystallographic analysis, we speculate that Pb2+ ion tends to be bound to O not N of DMF.In order to know the exact CN of Pb (II), the experiment data are fitted by the ATHENA (version 0.8.056) and ARTEMIS (version 0.8.011) modules implemented in the IFEFFIT software packages.

33-34

The fitting results are shown in the Figure 3

with solid lines, and the structural parameters extracted from the fitting are summarized in Table 1. As seen, the total coordination number of Pb (II), is around six in DMSO or DMF, but becomes around eight in H2O. We also notice that, due to the different sizes of DMSO, DMF and H2O molecules, Pb-O and Pb-Br have different bond lengths and CN under different solvents (Table S2). However, the results of the single crystal X-ray crystallographic analysis imply that the difference does not change the configuration of Pb coordinations but only distorts the structure of octahedron (or hendecahedron). Consequently, we can conclude that the CN of Pb (II) of resultants are the same as those of the corresponding plumbite oligomers in the precursor solutions, i.e., the structure of intermediate in solution plays a decisive role on the formation of CPBs product, and Pb-O bond is replaced by Pb-Br gradually and ACS Paragon Plus Environment


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the CN is kept in the crystallization process.

Figure 3. Experimental (open squares) and fitting (solid lines) results of the Fourier transformed [k2χ(k)] in the R space for the intermediates in DMSO, DMF and H2O solvents.

Table 1. Fitting parameters for real space EXAFS. Sample DMF

H2O

DMSO

Bond

N

Total N

R (Å)

σ2 (10-3Å2)

∆E0 (eV)

Pb-O

2.0±0.3

6.1±0.9

2.51

6.3

-4.0

Pb-Br

4.1±0.6

2.72

11.6

-9.9

Pb-O

4.1±0.6

2.59

7.2

1.7

Pb-Br

3.7±0.6

2.69

13.8

-17.2

Pb-O

1.9±0.3

2.63

5.7

3.2

Pb-Br

3.6±0.5

2.76

12.0

-11.3

7.8±1.2

5.5±0.8

N, coordination number; R, bond distance; σ2, Debye-Waller factor; ∆E0, edge energy shift. EXAFS CN values have 10-15% fitting errors. The reason why the coordination number of lead halide can be affected by the solvent is that DMSO, DMF and H2O can be as the ligands to participate the Pb(II) coordination. Owing to the hindrance role of sterics, the larger ligands favor lower coordination number, while the small ones incline to form complicated high


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coordination number. In this regard, the ligands of DMSO or DMF can form lower coordination to Pb (II) as compared to that of H2O. Based on the experimental observation and the above discussion, here we propose a plausible mechanism of growing CPBs in different solvents as schematically shown in Figure 4. As illustration in the top of Figure 4a,the coordination number of Pb (II) changes firstly from seven in PbBr2 crystal to six when adding the solvent of DMSO. With increasing the ratio of CsBr to PbBr2, as shown in the bottom of Figure 4a, the intermediate can be tuned from [PbBr2(DMSO)4] to higher-order plumbate complexes,

24

such as [PbBr6]4-,

whereas the coordination number of Pb (II) maintains six. Addition of anti-solvent can further drive PbBrn–DMSOm ions assembled into bulk crystal. As a consequence, the resultants change from PbBr2•2DMSO to CsPbBr3 and finally to Cs4PbBr6 successively. Similarly, as shown in Figure 4b, the coordination number of Pb (II) changes to eight when dissolves into H2O, and the resultants can be tuned accordingly from 3PbBr2•2H2O to CsPb2Br5 with increasing the ratio of CsBr to PbBr2.

Figure 4. Schematic of the CPBs formation in the solvent of (a) DMSO and (b) H2O. PbBr2dissolves in solvents forming lead polyhalide framework with different CN, six in DMSO and eight in H2O. Increasing the ratio of CsBr to PbBr2 induces the intermediate to higher-order plumbate complexes and different resultants. In situ XRD observation of phase transformation. Preventing the CPBs from degradation in humid environment is critical for their application. More importantly,



the transformation process of CPBs in moisture can supply more insight on the role of ratio and coordination of precursors discussed above. In the following, in-situ synchrotron XRD experiment was used to investigate the humid effect of environment on the material stability and phase change. The schematic of the experiment setup is shown in Figure S3. Briefly, the Cs4PbBr6 powder was placed in a homemade sample cell at the diffractometer center. The relative humidity (RH) in the cell was controlled by tuning the flow rate of nitrogen gas which carried water vapor. The RH value was real-time monitored by a commercial hygrometer and regulated to be about 80% during the whole experiment. The cell was sealed with kapton film to confine the moisture but not to block the incident and diffractive x-ray beam passing through. 35 Figure 5a is in situ XRD result for the successive phase transformations from Cs4PbBr6 to CsPbBr3 and CsPb2Br5 with time, and Figure 5b shows the typical XRD patterns at 1 min, 25 min, 33 min, and 45 min in the phase transformation process, respectively. As shown in Figure 5a and b, XRD patterns confirm that the initial CPBs is Cs4PbBr6. With time increasing, the intensities of the diffraction peaks of Cs4PbBr6 gradually decrease due to its degradation. At the same time, the orthorhombic CsPbBr3 diffraction peaks emerge and the related intensities become enhanced gradually. As time going, all the diffraction peaks decrease and disappear abruptly, and only a weak and broad diffraction peak left as shown in Figure 5b with blue line, indicating the emergence of anamorphous phase of CPBs. This amorphous state is not stable and can be re-crystallized to the CsPb2Br5 in the end. More XRD patterns corresponding to the phase transformation process can be found in Figure S4.


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Figure 5(a) In situ XRD observation for the phase transformations from Cs4PbBr6 to CsPbBr3 and CsPb2Br5. (b) Representative XRD patterns at 1 min, 25 min, 33 min, and 45 min in the phase transformation process, respectively. (c) Schematic of the possible degradation mechanism for the Cs4PbBr6 crystal. On the basis of the results discussed above, we suggest that the transformation process is probably controlled by both of the solubility of CsBr and coordination number of Pb (II) in water. A plausible schematic of the crystal structure evolution is displayed in Figure 5c, and the related processes can be described as follows: When



Cs4PbBr6 stored in humid environment, H2O molecular from the moisture can initially permeate into it and strip the CsBr component out from the Cs4PbBr6 crystal. This will induce the shrink of the crystal lattice and transform the Cs4PbBr6 phase into CsPbBr3, while the framework of PbBr6 octahedron is retained. With time increasing, more water vapor can interact with CsPbBr3 intensely, resulting in not only striping more CsBr but also changing the coordination number of Pb (II) from six to eight. During the process, the distinct change of Pb-Br coordination environment leads CsPbBr3 crystal to be decomposed to an amorphous state, which can transform to CsPb2Br5 crystal finally due to the instability of intermediate amorphous phase. Absorption and Raman properties of CPBs. The absorption spectra of as grown pure CPBs crystals are shown in Figure 6a. The bandgaps value (Eg) are obtained on the basis of the Tauc plot as the intercept value of the plot of (αhν)1/m against light energy hν, as shown in Figure S5, where α is the absorption coefficient. It is found that m is equal to 1/2 for both CsPbBr3 and Cs4PbBr6, indicating the feature of direct-band-gap semiconductor. On the contrary, m is equal to 2 for CsPb2Br5, suggesting its indirect-band-gap. The band gaps are estimated to be about 3.67 eV, 2.26 eV and 3.23 eV for Cs4PbBr6, CsPbBr3 and CsPb2Br5, respectively, which are consistent with previously reports.10-13 Note that, due to the encapsulated nanoscale CsPbBr3, Cs4PbBr6 has a little absorption in the region of 340-530 nm, which is a common behavior in the synthesis of Cs4PbBr6 crystals.11 This can also be verified by the weak peak appeared at 17.2° in the XRD pattern of Cs4PbBr6 in Figure 5b (labeled with red # mark). However this interpretation is doubted by some reports that the origin of this emission to the shallow and deep energy defects of Cs4PbBr623, 28, 36-38. Raman spectra of these crystals provide the vibrational modes of the metal-halide lattice (Figure 6b). When the perovskite crystal was excited by 633 nm laser at room temperature, sharp and well-resolved Raman signals were collected. Strong peak at 70 cm−1 and broad peak at 125 cm−1 of CsPbBr3 are assigned to the vibrational mode of [PbBr6]4− octahedron and the motion of Cs+ cations, respectively. For Cs4PbBr6 crystal, strong peaks at 81.8 cm−1 and 123 cm−1 and a shoulder peak at 67.3 cm−1 are


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consistent to previous reports for melt-grown and solution-grown Cs4PbBr6 crystals.14, 39

In the Raman spectrum of CsPb2Br5, the strong peaks at 75.8 cm−1 and 131.5 cm−1

are observed.

Figure 6.(a) Absorption and (b) Raman scattering spectra of as-prepared CPBs. Inserts in (a) are their photographs. Conclusions. In this work, CPBs bulk single crystals with different phases have been synthesized using room-temperature solution method by changing the solvents and ratio of precursors. Using DMSO or DMF as the solvent, various CPBs with Pb (II) six-CN can be produced. On the contrary, in the solvent of water, only Pb (II) eight-coordinated crystals, such as CsPb2Br5 and 3PbBr2•2H2O, can be formed. Based on the EXAFS of Pb L-II edge in precursor solutions and single crystal X-ray diffraction measurement on the resultants, we identify that the CN of a plumbite oligomers in precursor solution are the same as those of the corresponding resultants. The finding suggests that the coordination number of the product is controlled dominantly by the solvents, while the stoichiometry of the resultants can be regulated by the precursors ratio of CsBr to PbBr2. Additionally, the phase change process of CPBs under the humid environment is further verified by the in-situ synchrotron radiation XRD. Knowledge of how the solvent affects the phase formation by coordination geometry of Pb (II) is very helpful in synthesis of a pure phase in CsBr-PbBr2 system and is also beneficial to understand the degradation of halide perovskites in humid environment.



ASSOCIATED CONTENT Supporting Information. Details of all of the experimental methods and synthesis; absorption spectra , extended X-ray absorption fine structure and single crystal X-ray diffraction measurements (PDF) Crystallographic data for PbBr2•DMF in CIF format (CIF) Crystallographic data for 3PbBr2•2H2O in CIF format (CIF)

ACKNOWLEDGMENT

We acknowledge the financial supports from MOST of China (2016YFA0200602), National Natural Science Foundation of China (21421063, 11474260, 11374274, 11504364, 21633007, 11675179), the Chinese Academy of Sciences (XDB01020200), the Fundamental Research Funds for the Central Universities (WK2030020027, WK2060190027, WK3510000004), and Anhui Natural Science Foundation (1608085QA17).

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Unveiling Solvent-Related Effect on Phase Transformations in CsBr

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