Two bilayer organic-inorganic hybrid perovskite compounds exhibiting reversible phase transition and dielectric anomalies

Journal Pre-proof Two bilayer organic-inorganic hybrid perovskite compounds exhibiting reversible phase transition and dielectric anomalies Jiang ZhenTao, Hao Yanhuan, Wang Hui, Zhang Xiuxiu, Yao Jiaojiao, Li Mingli, Wei Zhenhong PII:

S0022-4596(19)30609-7

DOI:

https://doi.org/10.1016/j.jssc.2019.121104

Reference:

YJSSC 121104

To appear in:

Journal of Solid State Chemistry

Received Date: 13 October 2019 Revised Date:

30 November 2019

Accepted Date: 30 November 2019

Please cite this article as: J. ZhenTao, H. Yanhuan, W. Hui, Z. Xiuxiu, Y. Jiaojiao, L. Mingli, W. Zhenhong, Two bilayer organic-inorganic hybrid perovskite compounds exhibiting reversible phase transition and dielectric anomalies, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/ j.jssc.2019.121104. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.

Author contributions

Zhenhong Wei designed research;

Zhentao Jiang, Yanhuan Hao, Hui Wang, Xiuxiu Zhang, Jiaojiao Yao, Mingli Li. performed research;

Zhenhong Wei and Yanhuan Hao wrote the paper.

Graphic abstract

Two bilayer Ruddlesden-Popper 2D organic-inorganic hybrid perovskite compounds showed reversible phase transitions and dielectric anomalies above the room temperature.

Two bilayer organic-inorganic hybrid perovskite compounds exhibiting reversible phase transition and dielectric anomalies ZhenTao Jiang, Yanhuan Hao, Hui Wang, Xiuxiu Zhang, Jiaojiao Yao, Mingli Li, Zhenhong Wei* College of chemistry, Nanchang University, Nanchang city, 330031, China

Abstract

Ruddlesden-Popper two-dimensional (2D) organic-inorganic hybrid perovskites have been explored for multiple applications, benefitting from their structural tunability, superior stability and congenital quantum-well effects. Herein, two bilayer hybrid perovskite-type compounds with the general formula [A2Bn-1PbnI3n+1] (n is the number of layer) [(C5H8NS)2(MA)Pb2I7] (1) and [(C5H8NS)2(FA)Pb2I7] (2) (C5H8NS: 2-thiophenemethylamine, MA: methylamine, FA: formamidine) are reported. The differential scanning calorimetry (DSC) measurements revealed both compounds 1 and 2 exhibit reversible phase transitions above the room temperature, accompanied by dielectric anomalies near the Curie temperature (Tc). The variable-temperature X-ray single-crystal diffraction disclosed the phase transitions derive from the order-disorder transformation of organic cations 2-thiophenemethylamine, methylamine and formamidine. Furthermore, compounds 1 and 2 exhibited strong emission peaks at about 600 nm when under an excitation wavelength of 475 nm.

Introduction

Organic-inorganic hybrid perovskites with multilayer structures are rapidly evolving as high performance semiconductors[1-4], particularly for three-dimensional (3D) perovskite materials that exhibit superior performance in photovoltaic and photovoltaic device applications[5-7]. In addition to 3D hybrid perovskites, 2D organic-inorganic hybrid perovskites with structural diversity and tunability have received blooming interests[8-14]. They have been regarded as promising materials due to their diverse and intriguing physical properties, such as semiconducting, phase transitions, and optical properties[15-18]. From the structural viewpoint, 2D perovskites tend to have superior stability than 3D perovskites due to the use of longer-chain hydrophobic organic cations in hybrid perovskite[19-22]. The 2D perovskites can be achieved by introducing organic molecules containing amine group during the synthesis of the 3D perovskites[16, 23-25], in which organic molecules can act both as spacers between the inorganic layers and as charge balancing cations of the structure[9, 26-28], Scheme 1. To the best of our knowledge, the layer 2D materials can adjust the thickness of the perovskite layer by changing functional organic cations, which has a greater performance than 3D perovskites. In addition, these mixed perovskites have unique properties of organic and inorganic components, this is very important for creating new materials with attractive features and functions. One of the most pregnant characteristics is that the dynamic motions of organic cation residing between two

adjacent inorganic perovskite layers can be subtly controlled by applying external stimulation.

Scheme 1 The 2D multilayer inorganic perovskite layers [(4-FBA)2(MA)n-1PbnI3n + 1] (4-FBA = 4-FC6H4CH2NH3, M A= CH3NH3) were obtained by cutting the interface of 3D perovskite structure[29].

Taking these considerations into account, we report two bilayer compounds: [(C5H8NS)2(MA)Pb2I7] (1) [30] and [(C5H8NS)2(FA)Pb2I7] (2) which can be also regarded as being obtained by slicing the 3D perovskites [(MA)PbI3] and [(FA)PbI3] (FA = formamidine) along the specific crystallographic planes with 2-thiophenemethylamine. The crystal structures of two compounds at room temperature and high temperature were studied by the X-ray single crystal diffraction. It is interesting that both 1 and 2 show reversible phase transitions and temperature-dependent dielectric anomalies. This work provides an avenue to construct novel 2D functional materials with tunable properties.

Experimental

Synthesis All reagents were purchased from Sigma-Aldrich or Titan and used without further purification. Compound 1 was a known product and was synthesized according the reported literature[30].

The synthesis of compound [(C5H8NS)2(FA)Pb2I7] (2): PbI2 (0.9220 g, 2 mmol) was dissolved in mixture of 47% w/w HI aqueous solution (5.0 mL) and 50% H3PO2 (2 mL) aqueous solution in a sample beaker. The mixture was heated to boiling under constant magnetic stirring, and the formamidine salt CH5N2I (0.2222 g, 1.3 mmol) solid was added. In a separate beaker, 2-thiophenemethylamine (0.0717 g, 0.63 mmol) was neutralized with HI 47% w/w (4 mL) and added into the above yellow solution. The mixed solution was kept stirring for fifty minutes under heating. After the reaction was cooled to room temperature, red plate crystals 2 were formed, which were isolated by filtration. Yield: 0.9701 g, 61% (based on PbI2).

Physical Measurement PXRD data were recorded in a D8 ADVANCE diffractometer with Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV/15 mA with a Kβ foil filter. Differential scanning calorimetry (DSC) was carried out on a Perkin–Elmer Diamond DSC instrument by heating and cooling the samples in the temperature range 320-450 K with a heating rate of 10 K/min under nitrogen at atmospheric pressure in aluminum crucibles. The pressed-powder pellet sandwiched between two parallel copper electrodes was used

for dielectric constant measurements. The dielectric permittivity was measured on a Tonghui TH2828A instrument in the temperature range between 280-450 K and over the frequency range of 1 KHz to 1 MHz with an applied electric field of 1 V. Optical diffuse reflectance measurements were performed using a UV–Vis Curry 60 spectrophotometer operating in the 250–800 nm region at room temperature and BaSO4 as the 100% reflectance reference. Fluorescent spectra were carried on a Hitachi F-4600 spectrophotometer.

Single-crystal X-ray crystallography Variable-temperature X-ray single-crystal diffraction data were collected on a Bruker SMART CCD diffractometer with MoKα radiation (λ = 0.71073 Å) at 293 K, 368 K for 1 and 293 K, 420 K for 2. The structures were solved by direct methods and refined using the full-matrix method based on F2 by means of the SHELXLTL software package[31]. Non-H atoms were refined anisotropically using all reflections with I>2σ(I). All H atoms were generated geometrically and refined using a riding model with Uiso = 1.2Ueq (C and N). The packing views were drawn with DIAMOND. Crystallographic data and structure refinements are listed in Table 1.

Table 1 Crystal data and structure refinements for 1 and 2 Compounds

1

Formula

C11H22N3S2Pb2I7

Temperature

293 K

2

C11H21N4S2Pb2I7 368 K

293 K

Formula Mass

1563.16

1563.16

1576.16

Crystal system

Orthorhombic

Orthorhombic

Orthorhombic

Space group

Aba2

Ccca

Cmc2(1)

a (Å)

8.8398(3)

8.8318(3)

41.233(5)

b (Å)

41.5238

41.681(3)

8.9166(10)

c (Å)

8.7740(4)

8.8318(3)

8.8147(10)

β (°)

90

90

90

V (Å)3

3220.6(2)

3251.1(3)

3240.8(6)

Z

24

4

4

Dcalc (g·cm−3)

3.224

3.181

3.230

F (000)

2704

2680

2728

θmax

24.99

23.50

27.57

Total no. of reflns.

8451

6608

9165

No. of unique reflns.

2848[R(int)= 0.0962]

1203[R(int)= 0.0977

3802[R(int)=0.0557

No. of variables

84

87

94

R1, wR2 (obsd data)

0.0669, 0.1880

0.0666, 0.1913

0.0655, 0.1989

R1, wR2 (all data)

0.0687, 0.1898

0.0705, 0.1965

0.0819, 0.2145

GOF

1.088

1.073

1.044

Results and discussion Synthesis and characterization Compounds 1 and 2 were prepared as red flake crystals by heating the stoichiometric amounts of 2-thiophenemethylamine, methylamine and PbI2 for 1, 2-thiophenemethylamine, formamidine and PbI2 for 2 in concentrated HI solution.

The structures of 1 and 2 were confirmed by X-ray single crystal diffraction at room temperature. Their phase purities of the bulk were verified using the powder X-ray diffraction (PXRD) patterns which matched very well with the simulated one in terms of the single-crystal X-ray data as shown in Figure 1.

Figure 1 The powder X-ray diffraction patterns of compounds 1 and 2 with the simulated one in red and the experimental one in black.

Phase transitions Differential scanning calorimetry (DSC) is often used to test the thermal properties of

compounds. By testing the change of heat flux during the change of compound 1 and 2 with temperature, the phase transition behavior of compounds can be judged and identified. DSC analysis of the compound 1 was showed in Figure 2-1. During the temperature rise, the thermal anomalous absorption peak appeared at the temperature of 355 K, and at the temperature of 342 K, a thermal anomaly appeared during the cooling process. There are reversible thermal anomalies in the process of heating and cooling. Therefore, it is concluded that the compound 1 has a reversible phase transition, and the first-order phase transition is judged based on the large thermal hysteresis during the heating and cooling processes. As for compound 2, it can be seen from Figure 2-2, during the heating process, the compound shows a little thermal anomaly at 406 K, but during the cooling process, when the temperature at 391 K, the thermal anomaly occurs, so the compound 2 also shows a reversible phase transition.

Figure 2 DSC curves of 1 and 2 in the heating and cooling runs.

Switchable dielectric properties For structural phase change crystal materials, due to the rearrangement of molecules in the unit cell, the dielectric constant of the compound near the phase transition temperature is abnormal, that is, dielectric anomaly. Its dielectric constant also has significant temperature dependence and frequency dependence. The dielectric permittivity of compounds 1 and 2 were investigated over the temperature range of 280 K to 420 K with the frequencies of 5 kHz, 10 kHz, 100 kHz and 1 MHz using the powder tablet of the crystal. Figure 3a shows the dielectric constant of compound 1 with temperature at a frequency of 1 MHz. When the temperature was 365 K during the heating process and the temperature was 360 K during the cooling process, the dielectric anomaly occurred. Figure 3b shows the variation of dielectric constant with temperature at the different frequencies. It can be obtained from the graph, at temperature around 365 K, there are dielectric anomalies, which is near the phase

transition temperature of 350 K. The dielectric anomalies may be caused by the disordered rotation of the organic portion in the structure 1.

Figure 3 Temperature dependence of the dielectric constant of compound 1 at 1 MHz (a) and at various frequencies (b).

For compound 2, Figure 4a shows the dielectric anomaly measured at different 1 MHz frequency. During the temperature rise, the dielectric constant increases from 25 to 58 at temperature of 418 K at 1 MHz. The dielectric constant in the high dielectric state

is about 2.3 times than that in the low dielectric state, indicating a remarkable peak-like dielectric anomaly at 418 K. Similar dielectric anomalies were obtained at other different frequencies, Figure 4b. Combined with the structure analyses of 1, the change of dielectric constant of compound 2 may be also attributed to the order-disorder phase transition.

Figure 4 Temperature dependence of the dielectric constant of compound 2 at 1 MHz (a) and at various frequencies (b).

Single crystal structures To further explore the mechanism of the structural phase transitions, the crystal structures of compounds 1 and 2 were characterized at the selected temperatures. At the temperature of 293 K, the crystal 1 crystallizes in the orthorhombic crystal system, the space group is non-centered Aba2, the unit cell parameters are a = 8.8398(3) Å, b = 41.5238 Å, c = 8.7740(4) Å, β = 90°, the unit cell parameters are in accordance with the two-dimensional layered organic-inorganic hybrid perovskite. The a-axis and c-axis are the same in the unit cell parameters, indicating that the two-dimensional inorganic layer is on the ac plane, and the inorganic layer and the organic layer are stacked along the b-axis. The asymmetric unit of 1 contains one C4H3SCH2NH3+, one CH3NH3+, and one discrete [PbI4]2- anion. The inorganic part of lead iodide is deposited into two layers in the b-axis direction by apex sharing, the methylamine CH3NH3+ cation is located in the cavity of the octahedron, and the C4H3SCH2NH3+ cation is arranged in the middle of the octahedron in the b-axis direction, Figure 5. The inorganic portion and the organic portion are connected by N−H···I hydrogen bonding to form a typical two-dimensional layered organic-inorganic hybrid perovskite structure. The Pb−I bond length and cis-Pb−I−Pb bond angle range: 3.1368(14)~3.2654(6) Å and 88.21(5)~95.70(5)°, the horizontal Pb−I−Pb bond angle is around 154.79(8)~177.78(14)°, the amino moiety of 2-thiophenemethylamine with I atom in the inorganic octahedron forms N−H··· I hydrogen bond with the average N−I distance is at 3.68(6) Å. When the temperature increased to 368 K, the crystal 1 is also crystallized in the

orthorhombic crystal system, but the space group is Ccca, the unit cell parameters are a = 8.8318(3) Å, b = 41.681(3) Å, c = 8.8318(3) Å, β = 90°. The unit cell parameters are similar to those at 293 K. The octahedral Pb−I bond length and Pb−I−Pb bond angle range: 3.108(2)~3.2691(8) Å and 88.81(3)~91.19(3)°, the horizontal direction Pb−I−Pb bond angles are 157.32(6)°~177.62(5)°. Similarly, the inorganic and organic moieties are linked by hydrogen bonds, and the average N−I distance is at 3.703(5) Å. Compared to the low temperature phase, the configuration of the inorganic layer anion at the high temperature has no significant change, but the organic part of C5H8NS+ and CH3NH3+ cations move violently, resulting in a very pronounced disorder of the cations. As shown in Figure 5, the 2-thiopheneamine and the methylamine exhibit severe disorder with the C, N and S atoms apparently existing in two possible occupied positions, which lead to a related disordered state and create crystallographic mirror planes.

Figure 5 The packing diagrams of 1 at 293 K and 368 K viewed from c-axis.

The crystal 2 belongs to the orthorhombic crystal system, the space group is Cmc2(1),

the asymmetric unit contains one C5H8NS+ cation, one [PbI4]2- and one CH5N2+ cation, the unit cell parameters are a = 41.233(5) Å, b = 8.9166 (10) Å, c = 8.8147(10) Å, β = 90°. The average bond length of the Pb−I bond length in the octahedron is 3.20 Å, and the I−Pb−I bond angle range is 86.98(5)−97.16(5)°, The organic moiety C5H8NS+ cation is bonded to the inorganic octahedron by hydrogen bonding, and its structure is similar to that of the compound 1, as shown in Figure 6, except that the organic cation in the octahedral cavity is different. But the crystal of compound 2 was influenced by the high temperature, when the temperature increased to 443 K above Tc, the single-crystal structures are not able to be determined because of the weak diffraction.

Figure 6 The packing diagrams of 2 at 293 K viewed from b-axis.

Optical properties The optical properties of compounds 1 and 2 were investigated by the solid-state UV-vis diffuse reflectance spectrum at room temperature. As shown in Figure 7, compounds 1 and 2 display an intense absorption at the band edge onset 585 nm, which can be attributed to the charge transfer transitions within the inorganic layers. The bandgaps were calculated by using the Tauc-equation (hv·F(R)) = A(hv − Eg).

(Where h is the Planck's constant, v is the frequency of vibration, F(R∞) is the Kubelka -Munk function, Eg is the band gap and A is the proportional constant). The band gap Eg for compounds 1 and 2 as a direct band semiconductor was estimated to be about 2.0 eV, Figure 7-inset. Compared with the previously reported monolayer compound [(C5H8NS)2PbI4] at 515 nm [30], the absorptions of 1 and 2 have 70 nm red-shifted, which was conformed to the rule that the bandgaps of the multilayer compounds decreased with the increase of inorganic perovskite layers. The emission spectra of compounds 1 and 2 were measured in the solid state at room temperature as listed in Figure 7b. By resonantly exciting the sample at 475 nm, a strong emission at 570 nm in compound 1, and two emissions at 578 and 610 nm in compound 2 can be observed, which were originated from the octahedral inorganic layer. The observed strong photoluminescence of 1 and 2 indicates that these bilayer 2D compounds can be excellent candidates for potential application as photoactive materials.

Figure 7 Room-temperature absorption (a) and photoluminescent (b) spectra of compounds 1 and 2.

Conclusions In this paper, two bilayer perovskite-type compounds [(C5H8NS)2(MA)Pb2I7] (1) and [(C5H8NS)2(FA)Pb2I7] (2) (C5H7NS: 2-thiophenemethylamine, MA: methylamine, FA: formamidine) were obtained by reactions of PbI2 with two mixed organic amines. The different sizes of the selected two cationic moieties accounted for the formation of multilayered structures, in which the MA+ and FA+ cations are confined in the central cavities enclosed by [PbI6] corner-sharing octahedra and the larger C5H8NS+ cations are located on the periphery of [PbI6] octahedra. Compounds 1 and 2 show reversible phase transition which were proved by the variable temperature X-ray diffraction and DCS and the dielectric constant. The origin of the switchable phase transition was ascribed to the movements of the cations and anions from the equilibrium position. Optical investigations on compounds 1 and 2 indicated they are semiconductor

materials.

Acknowledgement The National Natural Science Foundation of China (no. 21571094, 21661021, 21865015), and the Graduate Student Creativity Funding of Nanchang University (no.CX2017043) were thanked for financial support.

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Highlight

• Two bilayered organic-inorganic-hybrid perovskite are designed, synthesized, and characterized.

• The relationship between phase transitions and structural changes is addressed.

• The optical properties of the bilayer perovskite compounds are studied.

Declaration of Interest Statement

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled.

Zhenhong Wei (on the behalf of all the authors)

Nanchang University