Crystallographic Investigations into Properties of Acentric Hybrid

Article pubs.acs.org/cm

Crystallographic Investigations into Properties of Acentric Hybrid Perovskite Single Crystals NH(CH3)3SnX3 (X = Cl, Br) Yangyang Dang,† Cheng Zhong,‡ Guodong Zhang,† Dianxing Ju,† Lei Wang,† Shengqing Xia,† Haibing Xia,† and Xutang Tao*,† †

State Key Laboratory of Crystal Materials & Institute of Crystal Materials, Shandong University, No. 27 Shanda South Road, Jinan 250100, P. R. China ‡ Solar and Photovoltaic Engineering Research Center (SPERC), King Abdullah University of Science and Technology (KAUST), 23955-6900 Thuwal, Saudi Arabia S Supporting Information *

ABSTRACT: The hybrid perovskites with special optoelectronic properties have attracted more attention to the scientific and industrial applications. However, because of the toxicity and instability of lead complexes, there is interest in finding a nontoxic substitute for the lead in the halides perovskites and solving the ambiguous crystal structures and phase transition of NH(CH3)3SnX3 (X = Cl, Br). Here, we report the bulk crystal growths and different crystal morphologies of orthorhombic hybrid perovskites NH(CH3)3SnX3 (X = Cl, Br) in an ambient atmosphere by bottom-seeded solution growth (BSSG) method. More importantly, detailed structural determination and refinements, phase transition, band gap, band structure calculations, nonlinear optical (NLO) properties, XPS, thermal properties, and stability of NH(CH3)3SnX3 (X = Cl, Br) single crystals are demonstrated. NH(CH3)3SnCl3 single crystal undergoes reversible structural transformation from orthorhombic space group Cmc21 (no. 36) to monoclinic space group Cc (no. 9) and NH(CH3)3SnBr3 belongs to the orthorhombic space group Pna21 (no. 33) by DSC, single-crystal X-ray diffraction and temperature-dependent SHG measurements, which clarify the former results. These results should pave the way for further studies of these materials in optoelectronics.

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INTRODUCTION Hybrid perovskite materials ABX3 (A = organic cation; B = Sn2+, Pb2+ or Ge2+; X = Cl, Br, or I) with special optoelectronic properties1−4 have attracted more attention to the scientific and industrial applications. There have been many reports mostly focused on lead (Pb) hybrid perovskite materials.5,6 On the basis of the toxicity and instability of lead (Pb) element under ambient moist air, these issues have urged to investigate the exploration interest of new lead-free hybrid perovskite materials. Recently, syntheses and fundamental measurements of lead free perovskite materials, such as Cs3Sb2I9,7 Cs2AgBiX6 (X = Br, Cl),8 (CH3NH3)3Bi2I9,9 have been reported by some groups. Moreover, the bulk growths and stability of ASnI3 (A = CH3NH3, CH(NH2)2) single crystals have been investigated by our research group.10 Single crystals are best candidates for revealing the intrinsic properties of these materials. However, the former reports on NH(CH3)3SnX3 (X = Cl, Br) materials mainly focused on their crystal structure, molecular motion and phase transition,11−13 and the synthesis of NH(CH3)3SnX3 (X = Cl, Br) materials mainly by the solid state reaction method11 or gel-growth method12−14 under a vacuum atmosphere. Nowadays, none have carried out the growth and properties of bulk NH(CH3)3SnX3 (X = Cl, Br) single crystals in an ambient atmosphere. Here, we © 2016 American Chemical Society

report the NH(CH3)3SnX3 (X = Cl, Br) orthorhombic hybrid perovskite single crystals grown in an ambient atmosphere by bottom-seeded solution growth (BSSG) method, similar to previous reports.10,15,16 Structural redetermination and refinements, phase transition, band gap, nonlinear optical (NLO) properties, XPS, band structure calculations and thermal properties of NH(CH3)3SnX3 (X = Cl, Br) single crystals have been also implemented in detail. The crystal structures of NH(CH3)3SnX3(X = Cl, Br) are both redetermined and refined with the orthorhombic system with polar point groups mm2. NH(CH3)3SnCl3 exhibits the layered perovskite structure with the space group Cmc21 (no. 36) at room temperature, while NH(CH3)3SnBr3 belongs to the space group Pna21 (no. 33). Band structure calculations demonstrate that NH(CH3)3SnX3 (X = Cl, Br) single crystals are both direct band gap semiconductors, the band gaps of which are approximately 3.59 and 2.76 eV, respectively, by UV−vis spectra measurements. These crystals show relatively good stability when exposed to air. These behaviors of NH(CH3)3SnX3 (X = Cl, Br) single crystals provide guidance for further studies of these materials. Received: June 30, 2016 Revised: September 7, 2016 Published: September 8, 2016 6968

DOI: 10.1021/acs.chemmater.6b02653 Chem. Mater. 2016, 28, 6968−6974

Article

Chemistry of Materials

Figure 1. NH(CH3)3SnX3 (X = Cl, Br) single crystals and their growth morphologies. These graphics depict that the NH(CH3)3SnX3 (X = Cl, Br) single crystals were obtained by BSSG crystal growth method, and their growth morphologies were deduced by Bravais−Friedel−Donnay−Harker (BFDH) method.17

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EXPERIMENTAL SECTION

Reagents. Analytical grade reactants of SnO, SnCl2·2H2O, NH(CH3)3Cl, H3PO2 solution, and HX (X = Cl, Br) solution were purchased and utilized without further purification. Synthesis of NH(CH3)3SnCl3 Single Crystals. SnCl2·2H2O (22.565 g, 0.10 mol) and NH(CH3)3Cl (9.600 g, 0.10 mol) were dissolved in 150 mL of HCl and 100 mL of H3PO2 mixed solution under constant stirring, forming a colorless solution. Solutions were saturated at 54 °C. Colorless single crystals of NH(CH3)3SnCl3 have been grown by bottomseeded solution growth (BSSG) method at water bath in an ambient atmosphere for about one month, as shown in Figure 1a and Figure S1. Synthesis of NH(CH3)3SnBr3 Single Crystals. SnO (6.755 g, 0.05 mol) and NH(CH3)3Cl (4.800 g, 0.05 mol) were dissolved in 200 mL of HBr and 50 mL of H3PO2 mixed solution under constant stirring, forming a colorless solution. Solutions were saturated at 56 °C. Colorless single crystals of NH(CH3)3SnBr3 have been grown by bottom-seeded solution growth (BSSG) method in an ambient atmosphere for about one month, as shown Figure 1b and in Figure S1. Characterizations. Measurements for X-ray diffractions, UV−vis diffuse spectra, SHG effect, XPS, thermal properties, and band gap calculations were described in detail in the Supporting Information [CCDC 1475929 for NH(CH3)3SnBr3 at 296 K; 1475930, 1475931, and 1482638 for NH(CH3)3SnCl3 at 296, 420, and 260 K, respectively, contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.]

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RESULTS AND DISCUSSION Crystal Growth. Bulk NH(CH3)3SnX3 (X = Cl, Br) single crystals with the dimensions of 13 mm × 8 mm × 6 mm and 8 mm × 6 mm × 4 mm, were both grown in ambient atmosphere in about one month. The grown crystal and calculated morphology are shown in Figure 1. The exhibited facets of NH(CH3)3SnCl3 were determined to be (200), (110), (111), and (002), whereas the exhibited facets of NH(CH3)3SnBr3

Figure 2. Solubility curves of NH(CH3)3SnX3 (X = Cl, Br) in HX (X = Cl, Br) and H3PO2 mixed solution, respectively. This graphics depict that the solubility of NH(CH3)3SnX3 (X = Cl, Br) was measured at the different temperature, respectively (a, b). 6969


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plate morphology, which was in good agreement with layered crystal structure of NH(CH3)3SnCl3 along the [001] direction in Figure 4, the difference of which between the calculated and experimental morphology for NH(CH3)3SnCl3 single crystals was attributed to the influence of the crystal growth conditions, such as the seed orientation, temperature control, and rotation speed. The crystal growth conditions are selected with proper temperature control. The solubility curves of NH(CH3)3SnX3 (X = Cl, Br) in HX (X = Cl, Br) and H3PO2 mixed solution are shown in Figure 2; these solubility data are guaranteed to conduct crystal growth successfully. When the starting crystal growth temperature is above 60 °C, opacity of NH(CH3)3SnX3 (X = Cl, Br) single crystals will appear due to the volatilization of HX (X = Cl, Br) solvents, as shown in Figure S2. Consequently, the growth of NH(CH3)3SnCl3 single crystal was executed via the reaction of NH(CH3)3Cl and SnCl2·2H2O in an HCl-H3PO2 mixed solution at the 54 °C temperature in ambient atmosphere, whereas the growth of NH(CH3)3SnBr3 single crystal was performed by the reaction of NH(CH3)3Cl and SnO in an HBr-H3PO2 mixed solution at the 56 °C temperature in ambient atmosphere. The H3PO2 solution was utilized as a reducing regent to prevent the Sn2+ ion from being oxidized into Sn4+ ion during the synthesis and crystal growth of NH(CH3)3SnX3 (X = Cl, Br).10 Besides, phase purities of obtained NH(CH3)3SnX3 (X = Cl, Br) samples were checked by powder X-ray diffraction, PXRD patterns of which agreed well with the calculated XRD diffraction patterns of these single crystals, as shown in Figure 3.

Figure 3. Calculated and experimental X-ray diffraction patterns for NH(CH3)3SnX3 (X = Cl, Br).

were determined to be (200), (011), (110), (111), (201), and (002) using X-ray diffraction. The theoretical morphologies of the orthorhombic NH(CH3)3SnX3 (X = Cl, Br) at room temperature were also calculated according to the Bravais− Friedel−Donnay−Harker (BFDH) method17 using the Mercury program.18 All the predicted facets were almost observed on the grown crystal. But in Figure 1a, NH(CH3)3SnCl3 also exhibited

Table 1. Crystal Data and Structure Refinement for NH(CH3)3SnX3 (X = Cl, Br) Single Crystals empirical formula formula weight/g·mol−1 temperature/K wavelength/Å crystal color crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 crystal size (mm3) Z density/g·cm−3 μ(mm−1) F (000) GOF on F2 absolute flack factor absorption correction extinction coefficient max and min transmission refinement method data/restraints/parameters R1, wR1 [I > 2σ (I)] R2, wR2 (all data) min/max Δρ /eÅ−3 CCDC

NH(CH3)3SnCl3 260 Ka

Monoclinic Cc (no. 9) 9.370(7) 8.267(7) 12.355(10) 90.00 92.172(9) 90.00 956.4(13) 0.13 × 0.11 × 0.10 4 1.980 3.431 544 1.065 0.21(14) 0.011(2) 0.7254 and 0.6640 1445/2/83 0.0805, 0.2178 0.0815, 0.2186 −0.948/2.380 1482638

285.16 296 Kb 0.71073 Colorless Orthorhombic Cmc21 (no.36) 9.476(5) 8.290(5) 12.340(5) 90.00 90.00 90.00 969.4(9) 0.17 × 0.15 × 0.13 4 1.954 3.385 544 1.098 0.02(3) semiempirical from equivalents 0.0065(4) 0.6673and 0.4299 full-matrix least-squares on F2 1169/1/46 0.0171, 0.0432 0.0177, 0.0435 −0.450/0.403 1475930

NH(CH3)3SnBr3 420 Kc

418.54 296 Kd

Orthorhombic Cmc21 (no.36) 9.598(5) 8.368(5) 12.416(5) 90.00 90.00 90.00 997.2(9) 0.30 × 0.20 × 0.10 4 1.899 3.291 544 1.043 0.06(5)

Colorless Orthorhombic Pna21 (no.33) 15.877(5) 8.568(5) 23.078(5) 90.00 90.00 90.00 3139(2) 0.20 × 0.16 × 0.15 12 2.657 13.826 2280 1.134 0.033(10)

0.0161(9) 0.6743 and 0.4385

0.00187(7) 0.2309and 0.1686

1053/1/46 0.0262, 0.0611 0.0357, 0.0672 −0.478/0.293 1475931

7190/1/227 0.0389, 0.0840 0.0735, 0.0929 −0.638/1.033 1475929

a c

w = 1/[s2(Fo2) + (0.0242P)2 + 30.6032P] where P = (Fo2 + 2Fc2)/3. bw = 1/[s2(Fo2) + (0.0195)2 + 0.5551P] where P = (Fo2 + 2Fc2)/3. w = 1/[s2(Fo2) + (0.0307P)2 + 0.4179P] where P = (Fo2 + 2Fc2)/3. dw = 1/[s2(Fo2) + (0.0242)2 + 0.0000P] where P = (Fo2 + 2Fc2)/3. 6970


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point group mm2 at room temperature by single-crystal X-ray diffraction. The related single crystal parameters for NH(CH3)3SnX3 (X = Cl, Br) at different temperature are presented in Table 1. It is the distorted {SnX3} (X = Cl, Br) pyramidal structure units that create the NH(CH3)3SnX3 (X = Cl, Br) asymmetry in Figure 4. The acentric crystal structures of NH(CH3)3SnX3 (X = Cl, Br) motivate us to investigate their nonlinear optical (NLO) properties. The powder second-harmonic generation (SHG) effect of NH(CH3)3SnX3 (X = Cl, Br) under 1064 nm laser radiation is about 1 × KH2PO4 (KDP) and 2.5 × KH2PO4 (KDP) under particle size range (75−100 μm) condition of all the samples, respectively, as shown in Figure S3a. Additional measurements (Figure S3b) demonstrate that the SHG intensity decreases with the increase of particle size, indicating that NH(CH3)3SnX3 (X = Cl, Br) are not type-1 Phase-Matchable.19−21 NH(CH3)3SnCl3 is stable at room temperature with space group Cmc21 (no. 36), which agrees well with those reported by Halfpenny et al.12 Along the [001] direction, NH(CH3)3SnCl3 exhibits apparent layered crystal structure as shown in Figure 1a and 4a. Previous report stated that NH(CH3)3SnCl3 showed three phase transitions at 490, 399, and 277 K.11 However, when the NH(CH3)3SnCl3 crystals were cooled down from room temperature to 200 K, and then returned to room temperature by DSC measurement, the compound exhibited only a very obvious endo/exo peak at about 277 K. Meanwhile, with the temperature increasing, SHG intensity exhibited a prompt decrease during the phase transition (Figure S4a), which ensured the crystal structure determination at 260 K. The change of SHG signal was reversible, showing almost overlapping curves in the heating and cooling runs.

Figure 4. Ball−stick diagrams of crystal structure and the {SnX3} pyramidal structure units in the NH(CH3)3SnX3 (X = Cl, Br) single crystals at room temperature. (a) Along the [010] direction, the crystal structure and structure unit of NH(CH3)3SnCl3; (b) along the [010] direction, the crystal structure and structure units of NH(CH3)3SnBr3.

Crystal Structures, Nonlinear Optical (NLO) Properties and Phase Transition. NH(CH3)3SnX3 (X = Cl, Br) both exhibit orthorhombic perovskite crystal structures with polar

Figure 5. DSC and TGA data of NH(CH3)3SnX3 (X = Cl, Br). The DSC curves of NH(CH3)3SnCl3 (a, b) and NH(CH3)3SnBr3 (c) at different temperature. Insert: The phase transition point at 277 K (a); (d) the DSC and TGA data of NH(CH3)3SnCl3 (black) and NH(CH3)3SnBr3 (red). 6971


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Chemistry of Materials The X-ray studies demonstrated that NH(CH3)3SnCl3 displayed the obvious difference on the crystal parameters at 296 and 260 K, resulting in the structural transformation from orthorhombic space group Cmc21 (no. 36) to monoclinic space group Cc (no. 9) in Figure 6 and Table 1, which verified the

Figure 7. UV−vis diffuse reflectance spectra and band gap for NH(CH3)3SnCl3 (black) and NH(CH3)3SnBr3 (red).

Figure 6. Ball−stick diagrams of crystal structure and the {SnCl3} pyramidal structure units in the NH(CH3)3SnCl3 single crystals at different temperature. Along the [010] direction, the crystal structure and structure unit of NH(CH3)3SnCl3 at 296 K (a) and 260 K (b).

obvious phase transition, as shown in Figure 5a. When heating to room temperature, the space group of NH(CH3)3SnCl3 shifted back to Cmc21 and there were no phase transition peaks at about 399 K, as shown in Figure 5b, which agreed with the powder and single-crystal X-ray patterns at higher temperature (420 K), as shown in Figure S5a. These above processes demonstrated reversible phase transitions. Moreover, Thiele et al. reported that NH(CH3)3SnBr3 exhibited the monoclinic crystal structure with space group P21 (no. 4) and was isotopic with that of RbGeBr3 analogue with the space group Pn21a (no. 33) at room temperature.13,22 In our results, with the temperature increasing, SHG intensity also shows a gradual decrease in Figure S4b. There exists obvious SHG signal, which ensures the asymmetric crystal structure of NH(CH3)3SnBr3. The X-ray diffraction shows that NH(CH3)3SnBr3 belongs to the orthorhombic space group Pna21 (no. 33) at room temperature, the powder XRD pattern is different from the calculated XRD pattern with space group P21, as shown in Figure S5b. There exists no phase transition phenomenon in the NH(CH3)3SnBr3 single crystals in the temperature range from 190 to 400 K, as shown in Figure 5c. Band gap, Band Structure Calculations, and Stability. The absorption spectra are calculated from the reflectance spectra by using the Kubelka−Munk function.23 The UV−vis absorption spectra of NH(CH3)3SnX3 (X = Cl, Br) indicate that the band gap of NH(CH3)3SnX3 (X = Cl, Br) is approximately 3.59 and 2.76 eV, respectively, as shown in Figure 7. Band structure calculations are performed to further investigate the electronic structure of the NH(CH3)3SnX3 (X = Cl, Br). Band structure calculations indicate that

Figure 8. XPS measurements of NH(CH3)3SnX3 (X = Cl, Br) single crystals. (a) NH(CH3)3SnCl3 single crystals measured immediately after crystal growth (black line) and after exposure to air for one month (red line), both exhibit the Sn2+state; (b) NH(CH3)3SnBr3 single crystals measured immediately after crystal growth (black line) and after exposure to an ambient atmosphere for one month (red line) both exhibit the Sn2+state. Insert: XPS of Sn 3d5 at binding energy from 490 to 484 eV.

NH(CH3)3SnX3 (X = Cl, Br) are both direct band gap semiconductors in Figure 9. PBE functional calculations26,27 usually 6972


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(TGA) in Figure 5d. The thermal stability of NH(CH3)3SnCl3 is higher than that of NH(CH3)3SnBr3. These behaviors show that NH(CH3)3SnX3 (X = Cl, Br) single crystals are both not hygroscopic and have relatively good physical and chemical stability when they are exposed to the ambient atmosphere.

Table 2. Observed Optical and Calculated Band Gaps compounds

experimental values (eV)

calculated values (eV)

NH(CH3)3SnCl3 NH(CH3)3SnBr3

3.59 2.76

3.29 2.75

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CONCLUSIONS In summary, based on the interest of investigating the airstable, nontoxic alternatives to the lead halide perovskites and the ambiguous crystal structures and phase transition of NH(CH3)3SnX3 (X = Cl, Br). NH(CH3)3SnX3 (X = Cl, Br) single crystals have been grown by bottom-seeded solution growth (BSSG) method in ambient atmosphere. The crystal structures of NH(CH3)3SnX3 (X = Cl, Br) have been redetermined and refined by X-ray diffraction, nonlinear optical (NLO) and DSC measurements. More importantly, NH(CH3)3SnCl3 single crystal undergoes the reversible structural transformation from orthorhombic space group Cmc21 to monoclinic space group Cc by DSC, single-crystal X-ray diffraction and temperature-dependent SHG measurement, while NH(CH3)3SnBr3 belongs to the orthorhombic space group Pna21, which clarifies the previous results. Besides, NH(CH3)3SnX3 (X = Cl, Br) single crystals both exhibit direct band gap semiconductors by band gap calculations and relatively good stability when exposure to air. These behaviors should provide better understandings of the further study of these materials. Further studies for other properties of these materials are under way.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02653. The growth equipment and crystal structure parameters of NH(CH3)3SnX3 (X = Cl, Br); the photos of opaque NH(CH3)3SnX3 (X = Cl, Br) single crystals; the calculated and experimental powder X-ray diffractions at different temperatures and when exposed to air for one month; temperature-dependent powder SHG and phasematchable measurements of NH(CH3)3SnX3 (X = Cl, Br). (PDF) (CIF) CIF for NH(CH3)3SnCl3-260K (CIF) CIF for NH(CH3)3SnCl3-296K (CIF) CIF for NH(CH3)3SnCl3-420K (CIF) CIF for NH(CH3)3SnBr3

Figure 9. Band structure diagrams for NH(CH3)3SnX3 (X = Cl, Br). The Fermi energy is set to E = 0 and denoted with a dashed line.

underestimate the band gap. As a result, the calculated band gaps are almost consistent with the experimental values in Table 2. To predict the carrier mobilities, carrier effective masses were determined from the curvature of the band extrema (assuming parabolic bands).8,25 Along the G to F direction for NH(CH3)3SnCl3 (NH(CH3)3SnBr3), the electron effective mass is 5.21me (2.84me) and the hole effective mass along the same direction is −1.23me (−1.36me).These predictions provide only a qualitative comparison since they neglect possible differences in carrier scattering rates. Besides, XPS measurements demonstrate the oxidation state of Sn element in the NH(CH3)3SnX3 (X = Cl, Br) single crystals immediately after crystal growth and after exposure to air for one month in Figure 8. The Sn 3d5 peak exhibits oxidation state of Sn element at binding energy from 490 to 484 eV. The main band of Sn element at around 487 eV can be attached to Sn2+.10,24 The oxidation state of Sn element in the NH(CH3)3SnX3 (X = Cl, Br) in air for one month stays the same, which is consistent with the powder X-ray diffraction results, as shown in Figure S6. The thermal properties of NH(CH3)3SnX3 (X = Cl, Br) are investigated by thermogravimetric analysis

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(X.T.) State Key Laboratory of Crystal Materials & Institute of Crystal Materials, Shandong University, No. 27 Shanda South Road, Jinan, 250100, P. R. China. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 51321091, 51227002, 6973


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51272129) and the Program of Introducing Talents of Disciplines to Universities in China (111 program no. b06015). The authors greatly thank Prof. Yi Zhang and Prof. Ren-Gen Xiong, Southeast University, P. R. China for their kind help in temperature-dependent-second harmonic generation (SHG) measurements, The authors also thank Shaojun Zhang, Xiufeng Cheng, Nannan Li, and Xin Ye for their help in powder second harmonic generation (SHG), thermal properties, XPS and DSC measurements, respectively.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on September 23, 2016, with errors in the Crystal Structures, Nonlinear Optical (NLO) Properties and Phase Transition Section and the Band gap, Band Structure Calculations, and Stability Section. The corrected version reposted September 27, 2016.

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Crystallographic Investigations into Properties of Acentric Hybrid

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