- Email: [email protected]
Preparation of Perovskite-Derived One Dimensional Single Crystals based on Edge-Shared Octahedrons with Pyridine Derivatives
Journal Pre-proofs Preparation of Perovskite-Derived One Dimensional Single Crystals based on Edge-Shared Octahedrons with Pyridine Derivatives Thi-Mai Huong Duong, Shunpei Nobusue, Hirokazu Tada PII: DOI: Reference:
S0022-0248(20)30100-7 https://doi.org/10.1016/j.jcrysgro.2020.125577 CRYS 125577
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
4 December 2019 20 February 2020 25 February 2020
Please cite this article as: T.H. Duong, S. Nobusue, H. Tada, Preparation of Perovskite-Derived One Dimensional Single Crystals based on Edge-Shared Octahedrons with Pyridine Derivatives, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro.2020.125577
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.
© 2020 Published by Elsevier B.V.
Preparation of Perovskite-Derived One Dimensional Single Crystals based on Edge-Shared Octahedrons with Pyridine Derivatives Thi-Mai Huong Duong, Shunpei Nobusue, and Hirokazu Tada* Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Email: [email protected]
ABSTRACT
Single crystals of hybrid materials based on 1D lead iodide perovskite networks with two pyridine (Py) derivatives, namely 4-ethyl (4Et)Py and 4-aminomethyl (4AM)Py, have been prepared. The compounds assembled into edge-shared lead iodide chains that formed needle-shaped crystals with lengths greater than 1 cm. The crystal containing (4Et)Py formed in the triclinic system with P1 symmetry. The molecules were stacked along with the novel triple chain of 1D perovskite-derived networks. The crystal containing 4AMPy formed in the monoclinic system with P21/a symmetry. The amino group formed hydrogen bonds to iodide anions. The optical band gaps for the 4EtPy-based and 4AMPy-based crystals were estimated to be 2.73 eV and 2.64 eV, respectively. The slight difference between the functional groups of the two molecules governed their crystal structures and hence optical properties.
Keywords: B1. Perovskite-derived, A1. Single crystal, B1. Pyridine derivatives, A1. Onedimensional, A2. Growth from high temperature solutions, B2. Optical properties
INTRODUCTION
Organic–inorganic hybrid perovskites have received considerable attention because of their superior optical and electronic properties, including broad absorption [1], considerable exciton diffusion lengths [2,3], and high charge carrier mobility levels [4,5]. Threedimensional (3D) hybrid perovskites, abbreviated as ABX3, are made up each of an organic cation such as CH3NH3+ at the A position, a metal cation (usually Pb2+ or Sn2+) at the B 1
position, and a halogen anion (Br–, I–, or Cl–) at the X position. In these structures, a framework of corner-sharing metal halide octahedra extends in all three dimensions, where small organic cations fit into the voids between the octahedra. According to Goldschmidt’s rule [6], the diameter of the organic cation must be smaller than that of the void between the octahedra (∼2.6 Å for CH3NH3PbI3) to keep the 3D structure. When larger organic cations are inserted, the Goldschmidt’s rule is broken. The connectivity of the inorganic network reduced to two-dimensional sheets (2D) [7–9], one-dimensional chains (1D) [10–12], and zero-dimensional clusters (0D) [13,14], of which the formula is not ABX3. In related publications, some derived names were used, such as “perovskite-like” [15-19], “perovskitoid” [20], or “perovskite network” [21] to define the new materials. In this paper, we use "perovskite-derived" crystals. Three types of octahedral connectivity are considered to define 1D perovskite-derived, namely conner-, edge- (side-), and face-shared 1D chain.”
The choice of the organic cation, when under the same reaction stoichiometry, has been pointed out to be the most influential parameter on the conformation of the resultant inorganic framework [6]. One of the best ways to clarify the relationship between the type of organic cation inserted and the crystal structure is to examine the effects on the crystal structure of two different molecules having almost the same size and structure. In this work, we used two organic molecules, 4-aminomethyl pyridine (4AMPy) and 4-ethyl pyridine (4EtPy), whose sizes and structures are almost the same, and set out to test the effect of the functional group on the crystal structure. Most of the reported hybrid perovskite-derived compounds investigations have used polycrystalline films whose properties are strongly affected by the presence of recombination sites of carriers and excitons. For example, the carrier diffusion lengths in lead halide perovskite thin films have been reported to be 100 times less than those in single crystals [2,3]. It is thus important to synthesize large single crystals of such compounds based perovskites to effectively characterize their intrinsic properties. Here, we prepared 1D single crystals based on lead iodide perovskites-derived with lengths greater than 1 cm, achieved by carefully controlling the temperature during the crystal growth.
EXPERIMENTAL
Materials. The acid solution used for our syntheses was prepared by mixing aqueous 2
hydroiodic acid (HI) solution (57% w/w, Tokyo Chemical Industry) and aqueous H3PO2 solution (50% w/w, Wako Pure Chemical Industries) in a 9:1 v/v ratio.
Syntheses. Compound 1. A mass of 560 mg (2.50 mmol) of PbO powder was mixed with 10 mL of the acid solution and the resulting mixture was heated until the powder dissolved. In a separate flask, a mass of 268 mg (2.50 mmol) of 4-ethyl pyridine (C7H9N, Tokyo Chemical Industry,) was neutralized with 4 mL of the acid solution in an ice bath. The solution of 4-ethyl pyridine was slowly added to the solution of PbI2, and the resulting mixture was heated until all of the components dissolved. Then the temperature of the solution was decreased very slowly to room temperature. Crystals formed during the slow cooling. The crystals were collected using filtration, and a mass of 0.895 g of pale-yellow needle-shaped crystals (58% yield based on total Pb content) was in this way obtained. Anal. Calcd. for compound 1 (C14H20N2Pb3I8): C, 9.07; H, 1.09; N, 1.51%. Found: C, 8.60; H, 0.71; N, 1.43%.
Compound 2. A mass of 560 mg (2.50 mmol) of PbO powder was mixed with 10 mL of the acid solution and the resulting mixture was heated until the powder dissolved. In a separate flask, a mass of 270 mg (2.50 mmol) of 4-aminomethyl pyridine (C6H8N2, Wako Pure Chemical Industries) was neutralized with 4 mL of the acid solution in an ice bath. The solution of 4-aminomethyl pyridine was slowly added into the solution of PbI2, and the resulting mixture was heated until all of the components dissolved. The temperature of the solution was then decreased gradually to room temperature. Yellow needle-shaped crystals formed during slow cooling. The crystals were collected using filtration, and a mass of 0.903 g of the yellow needle-shaped crystals (43% yield based on total Pb content) was in this way obtained. Anal. Calcd. for compound 2 (C6H10N2PbI4): C, 8.75; H, 0.8; N, 3.25%. Found: C, 8.54; H, 1.1; N, 3.4%.
Characterization. X-ray crystallographic studies of the single crystals of compounds 1 and 2 were performed at room temperature using a Rigaku XtaLAB P100 diffractometer with Mo-Kα radiation (λ = 0.71075 Å), operated at 50 kV and 24 mA. X-ray diffraction data were collected using an imaging plate diffractometer with graphite-monochromated MoKα radiation. The positional and thermal parameters of non-hydrogen atoms were refined anisotropically on F2 by using the full-matrix least-squares method with SHELXL 2013. 3
Hydrogen atoms were placed at calculated positions and refined while “riding” on their corresponding carbon atoms. In the subsequent refinement, the function Σw(|Fo| - |Fc|)2 was minimized, with |Fo| and |Fc| denoting the observed and calculated structure factor amplitudes, respectively. The agreement indices were defined as R1 = Σ (||Fo|-|Fc||)/Σ|Fo| and wR2 = [Σw (|Fo|-|Fc|)2/Σ(wFo2)2]1/2. Absorption spectra of the single crystals of compounds 1 and 2 were acquired at room temperature
with
a
Shimadzu
UV
3600
double-beam,
double-monochromator
spectrophotometer. From these absorption spectra, the band gaps of the materials were calculated. Specifically, (ahν)^(1/n) was plotted as a function of hν, where α denotes the optical absorption coefficient and was calculated from the absorbance (A) and thickness of the sample (t) using the formula α = 2.303A/t. Then, by extrapolating the straight portion of the graph to the hν axis, i.e. for a = 0, the band gaps of the compounds 1 and 2 were obtained.
RESULTS AND DISCUSSION
Figures 1 (a) and 1(b) show the crystal structures of compounds 1 and 2, respectively, which were determined by the X-ray analysis described in Experimental. Selected information of the crystallographic data parameters and statistics are outlined in Table I. Figures 2 (a) and 2 (b) show the morphologiesy of compounds 1 and 2, respectively, deduced using from the Bravais-Friedel Donnay-Harker (BFDH) method [22]. The compounds formed needleshaped crystals adopt needle shapes with the lengths of hundreds of micrometers to onecentimeter as shown in the insets of figure 2. Essentially one-dimensional lead iodide chains were found to extend along the longitudinal directions of the crystals. Regarding compound 1, the single-crystal X-ray diffraction data showed that it crystallized in the triclinic system with the P1 space group. The chemical formula of compound 1 was determined from the X-ray analysis to be in the form A Pb32 I 8 (A=HNC5H4-C2H5). One proton was donated from an HI acid to the nitrogen of the Py ring during the acid-base reaction in the synthesis procedure. Three octahedrons were found to be connected via an edge-shared system to form the unit of 1D triple-edge-shared chains extending along the aaxis, as can be seen in figure 1(a). The organic component of 4EtPyH+ adopted columnarlike stacking structures between the inorganic lead idodie chains. There have been two reports on double chain structures so far [23,24]: one being a corner-shared double-chain
4
structure [23] and the other an edge-shared double-chain one [24]. An edge-shared triplechain structure like that found in the present study has not previously, to the best of our knowledge, been reported for any hybrid perovskite. Figure 3 (a) shows an enlarged representation of the structure of compound 1, as well as some atomic distances. In this figure, N1 denotes the nitrogen of the pyridine ring. The distance between N1 (H1) and the nearest I (I1) was estimated to be 3.66 Å (2.89 Å) as shown in figure S1 (a). The N1–I1 distance was close to the sum of the Van der Waals radii of the nitrogen (1.55 Å) and iodide (1.98 Å) atoms. And it was almost the same as the distances between N and I in N–H–I hydrogen bonds reported for 2D and 1D hybrid compounds based on lead iodide octahedral [24,25].
5
Table 1. Crystal data and structure refinement statistic for compounds 1 and 2.
Compound
1
2
Organic component
4-Ethylpyridine
4-Aminomethylpyridine
1D structural type
Edge-shared triple-
Edge-shared single-chain
chain Formula
C14H20N2Pb3I8
C6H10N2PbI4
Formula weight
1857.16
824.98
Crystal system
Triclinic
Monoclinic
Space group
P1
P21/a
Crystal color
yellow
yellow
a (Å)
4.6002(6)
13.020(6)
b (Å)
12.794(5)
14.413(8)
c (Å)
14.693(5)
8.389(4)
α (°)
80.937(18)
90
β (°)
82.20(2)
105.766
γ (°)
80.03(2)
90
Volume (Å3)
835.8(5)
1514.9(13)
Z
2
4
Density (calcd) (g/cm3)
7.363
3.617
Absorption coefficient (mm−1)
22.50
19.26
F(000)
1576.00
1416.00
54.9
54.9
6345/3/244
3452/0/119
R1 (I > 2σ(I))
0.028
0.052
wR2 (all data)
0.071
0.127
GOF
0.944
0.918
CCDC number
1968131
1968137
2
max
(°)
Independent reflections/restraints/parameters
6
Figure 1. Schematic representations of the crystal structures of (a) compound 1 and (b) compound 2. Hydrogen atoms are omitted for clarity.
( (a)
( (b)
Figure 2. (Upper) crystal photograph (mm scale bars) of compounds 1 (a) and 2 (b), (lower) crystal morphologies deduced from the Bravais-Friedel Donnay-Harker (BFDH) method of compounds 1 (a) and 2 (b). Hydrogen atoms are omitted for clarity. 7
Figure 3. Enlarged representations of the crystal structures of compounds 1 (a) and 2 (b). Some atomic distances are given.
The Compound 2 crystallized in the monoclinic system with the space group P21/a. The chemical formula of compound 2 was determined from the X-ray analysis to be in the form A2+Pb2+I–4 (A= HNC5H4-CH2NH3). It is considered that one proton is bound with nitrogen in the Py ring and the other is bound with that of the amino group. To the best of our knowledge, the APbI4 formula was previously found only for 2D hybrid perovskites-like structure [7], with the current work being the first example of this formula for a 1D crystal. Inspection of the vertical and horizontal views of the representation of the structure of compound 2 shown in Figure 1b indicated the presence of edge-shared structures in which the lead iodide octahedrons were connected via two iodine atoms to form the 1D chain. Figure 3 (b) shows an enlarged representation of the structure of compound 2, with N1 and N2 denoting the nitrogen atoms in the Py ring and amino group, respectively. The shortest N1–I (I2) distance here was measured to be 3.82 Å, longer than that of compound 1. The 8
distances between N2 and the nearest I anions (I1-I4) were in the range 3.56 to 3.7 Å. We estimated the I-H distances for various conformations of the amino group. While the distances between I4 and the H atoms of the amino group (labeled as H1-H3) were greater than 3.2 Å, those between I and H for I1–H1, I2–H2 and I3–H3 were approximately 2.7-2.8 Å (figure S1 (b)), indicating the formation of N–H–I hydrogen bonds. This formation was associated with a shift of 4 AMPy toward the void, which resulted in the greater N1–I2 distance in compound 2 than in compound 1. Hence, the slight difference between the end groups of the two inserted molecules resulted in very different crystal structures.
Figure 4. Absorption spectra of the crystals of compounds 1 (a) and 2 (b) at room temperature. Figure 4 shows the absorption spectra of the single crystals of compounds 1 and 2 at room temperature. For each spectrum, extrapolating the linear part of the curve at the absorption edge to the horizontal axis yielded an estimation of the band gap. In this way, the band gaps of compounds 1 and 2 were estimated to be 2.73 and 2.64 eV, respectively. This difference between the band gaps may have been due to the difference between the Pb–I–Pb bond angles in the inorganic chains. Theoretical studies for a layered perovskite-like structure containing an SnI4 framework have suggested that the in-plane Sn–I–Sn bond angle has the greatest influence on the band gap [26]. In these studies, larger Sn-I-Sn bond angles in the hybrid perovskites-like yielded narrower band gaps. Such was also the case for the 1D crystal studied in the current work. Pb–I–Pb bond angles of 91.82 and 88.04 degrees were measured for compound 1 and 93.92 and 91.83 degrees for compound 2, as shown in figure S2. Thus the broader band gap of compound 1, compared to that of compound 2, may have been due to its smaller Pb–I–Pb bond angles.
9
CONCLUSIONS
We demonstrated the preparation of single crystals of two different one-dimensional hybrid compounds based on lead iodide perovskites-derived containing pyridine derivatives. The sizes of the single crystals can be tuned from several micrometers to on the order of a centimeter by controlling the cooling rate in the crystal growth procedure. We found that the slight difference between the end functional groups of the organic components of compounds 1 and 2 gave rise, despite the similar shapes and sizes of their organic molecules, to quite different crystal structures: compound 1 crystallized in the monoclinic system and formed a novel edge-shared triple-chain perovskite-derived; while compound 2 crystallized in the triclinic system and formed an edge-shared single-chain perovskite-derived. The amino group in compound 2 was found to interact with the lead iodide chain through N2–H– I hydrogen bonds. Both compounds showed relatively broad band gaps, with the specific values of their band gaps apparently affected by the angle of the Pb–I–Pb bond. SUPPLEMENTARY MATERIAL See the supplementary material for I–H distances, Pb–I– Pb bond angles, and powder XRD results and discussion.
ACKNOWLEDGEMENTS This work was supported by JSPS KAKENHI Grant Number JP18H03899, and JSPS DC2.
REFERENCES
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Highlights 1, The sizes of the single crystals can be tuned from several micrometers to centimeter order by controlling crystal growth procedure. 2, The slight difference between the functional groups of the molecules governed their crystal structures and hence optical properties. 3, The band gaps of the compound affected by the angle of the Pb–I–Pb bond.
13
Credit author statement Thi-Mai Huong Duong: sample synthesis, measurement and analysis data, writing original draft preparation. Shunpei Nobusue: structure analysis supervision. Hirokazu Tada: supervision, review and editing.
14
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-0248(20)30100-7 https://doi.org/10.1016/j.jcrysgro.2020.125577 CRYS 125577
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
4 December 2019 20 February 2020 25 February 2020
Please cite this article as: T.H. Duong, S. Nobusue, H. Tada, Preparation of Perovskite-Derived One Dimensional Single Crystals based on Edge-Shared Octahedrons with Pyridine Derivatives, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro.2020.125577
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.
© 2020 Published by Elsevier B.V.
Preparation of Perovskite-Derived One Dimensional Single Crystals based on Edge-Shared Octahedrons with Pyridine Derivatives Thi-Mai Huong Duong, Shunpei Nobusue, and Hirokazu Tada* Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Email: [email protected]
ABSTRACT
Single crystals of hybrid materials based on 1D lead iodide perovskite networks with two pyridine (Py) derivatives, namely 4-ethyl (4Et)Py and 4-aminomethyl (4AM)Py, have been prepared. The compounds assembled into edge-shared lead iodide chains that formed needle-shaped crystals with lengths greater than 1 cm. The crystal containing (4Et)Py formed in the triclinic system with P1 symmetry. The molecules were stacked along with the novel triple chain of 1D perovskite-derived networks. The crystal containing 4AMPy formed in the monoclinic system with P21/a symmetry. The amino group formed hydrogen bonds to iodide anions. The optical band gaps for the 4EtPy-based and 4AMPy-based crystals were estimated to be 2.73 eV and 2.64 eV, respectively. The slight difference between the functional groups of the two molecules governed their crystal structures and hence optical properties.
Keywords: B1. Perovskite-derived, A1. Single crystal, B1. Pyridine derivatives, A1. Onedimensional, A2. Growth from high temperature solutions, B2. Optical properties
INTRODUCTION
Organic–inorganic hybrid perovskites have received considerable attention because of their superior optical and electronic properties, including broad absorption [1], considerable exciton diffusion lengths [2,3], and high charge carrier mobility levels [4,5]. Threedimensional (3D) hybrid perovskites, abbreviated as ABX3, are made up each of an organic cation such as CH3NH3+ at the A position, a metal cation (usually Pb2+ or Sn2+) at the B 1
position, and a halogen anion (Br–, I–, or Cl–) at the X position. In these structures, a framework of corner-sharing metal halide octahedra extends in all three dimensions, where small organic cations fit into the voids between the octahedra. According to Goldschmidt’s rule [6], the diameter of the organic cation must be smaller than that of the void between the octahedra (∼2.6 Å for CH3NH3PbI3) to keep the 3D structure. When larger organic cations are inserted, the Goldschmidt’s rule is broken. The connectivity of the inorganic network reduced to two-dimensional sheets (2D) [7–9], one-dimensional chains (1D) [10–12], and zero-dimensional clusters (0D) [13,14], of which the formula is not ABX3. In related publications, some derived names were used, such as “perovskite-like” [15-19], “perovskitoid” [20], or “perovskite network” [21] to define the new materials. In this paper, we use "perovskite-derived" crystals. Three types of octahedral connectivity are considered to define 1D perovskite-derived, namely conner-, edge- (side-), and face-shared 1D chain.”
The choice of the organic cation, when under the same reaction stoichiometry, has been pointed out to be the most influential parameter on the conformation of the resultant inorganic framework [6]. One of the best ways to clarify the relationship between the type of organic cation inserted and the crystal structure is to examine the effects on the crystal structure of two different molecules having almost the same size and structure. In this work, we used two organic molecules, 4-aminomethyl pyridine (4AMPy) and 4-ethyl pyridine (4EtPy), whose sizes and structures are almost the same, and set out to test the effect of the functional group on the crystal structure. Most of the reported hybrid perovskite-derived compounds investigations have used polycrystalline films whose properties are strongly affected by the presence of recombination sites of carriers and excitons. For example, the carrier diffusion lengths in lead halide perovskite thin films have been reported to be 100 times less than those in single crystals [2,3]. It is thus important to synthesize large single crystals of such compounds based perovskites to effectively characterize their intrinsic properties. Here, we prepared 1D single crystals based on lead iodide perovskites-derived with lengths greater than 1 cm, achieved by carefully controlling the temperature during the crystal growth.
EXPERIMENTAL
Materials. The acid solution used for our syntheses was prepared by mixing aqueous 2
hydroiodic acid (HI) solution (57% w/w, Tokyo Chemical Industry) and aqueous H3PO2 solution (50% w/w, Wako Pure Chemical Industries) in a 9:1 v/v ratio.
Syntheses. Compound 1. A mass of 560 mg (2.50 mmol) of PbO powder was mixed with 10 mL of the acid solution and the resulting mixture was heated until the powder dissolved. In a separate flask, a mass of 268 mg (2.50 mmol) of 4-ethyl pyridine (C7H9N, Tokyo Chemical Industry,) was neutralized with 4 mL of the acid solution in an ice bath. The solution of 4-ethyl pyridine was slowly added to the solution of PbI2, and the resulting mixture was heated until all of the components dissolved. Then the temperature of the solution was decreased very slowly to room temperature. Crystals formed during the slow cooling. The crystals were collected using filtration, and a mass of 0.895 g of pale-yellow needle-shaped crystals (58% yield based on total Pb content) was in this way obtained. Anal. Calcd. for compound 1 (C14H20N2Pb3I8): C, 9.07; H, 1.09; N, 1.51%. Found: C, 8.60; H, 0.71; N, 1.43%.
Compound 2. A mass of 560 mg (2.50 mmol) of PbO powder was mixed with 10 mL of the acid solution and the resulting mixture was heated until the powder dissolved. In a separate flask, a mass of 270 mg (2.50 mmol) of 4-aminomethyl pyridine (C6H8N2, Wako Pure Chemical Industries) was neutralized with 4 mL of the acid solution in an ice bath. The solution of 4-aminomethyl pyridine was slowly added into the solution of PbI2, and the resulting mixture was heated until all of the components dissolved. The temperature of the solution was then decreased gradually to room temperature. Yellow needle-shaped crystals formed during slow cooling. The crystals were collected using filtration, and a mass of 0.903 g of the yellow needle-shaped crystals (43% yield based on total Pb content) was in this way obtained. Anal. Calcd. for compound 2 (C6H10N2PbI4): C, 8.75; H, 0.8; N, 3.25%. Found: C, 8.54; H, 1.1; N, 3.4%.
Characterization. X-ray crystallographic studies of the single crystals of compounds 1 and 2 were performed at room temperature using a Rigaku XtaLAB P100 diffractometer with Mo-Kα radiation (λ = 0.71075 Å), operated at 50 kV and 24 mA. X-ray diffraction data were collected using an imaging plate diffractometer with graphite-monochromated MoKα radiation. The positional and thermal parameters of non-hydrogen atoms were refined anisotropically on F2 by using the full-matrix least-squares method with SHELXL 2013. 3
Hydrogen atoms were placed at calculated positions and refined while “riding” on their corresponding carbon atoms. In the subsequent refinement, the function Σw(|Fo| - |Fc|)2 was minimized, with |Fo| and |Fc| denoting the observed and calculated structure factor amplitudes, respectively. The agreement indices were defined as R1 = Σ (||Fo|-|Fc||)/Σ|Fo| and wR2 = [Σw (|Fo|-|Fc|)2/Σ(wFo2)2]1/2. Absorption spectra of the single crystals of compounds 1 and 2 were acquired at room temperature
with
a
Shimadzu
UV
3600
double-beam,
double-monochromator
spectrophotometer. From these absorption spectra, the band gaps of the materials were calculated. Specifically, (ahν)^(1/n) was plotted as a function of hν, where α denotes the optical absorption coefficient and was calculated from the absorbance (A) and thickness of the sample (t) using the formula α = 2.303A/t. Then, by extrapolating the straight portion of the graph to the hν axis, i.e. for a = 0, the band gaps of the compounds 1 and 2 were obtained.
RESULTS AND DISCUSSION
Figures 1 (a) and 1(b) show the crystal structures of compounds 1 and 2, respectively, which were determined by the X-ray analysis described in Experimental. Selected information of the crystallographic data parameters and statistics are outlined in Table I. Figures 2 (a) and 2 (b) show the morphologiesy of compounds 1 and 2, respectively, deduced using from the Bravais-Friedel Donnay-Harker (BFDH) method [22]. The compounds formed needleshaped crystals adopt needle shapes with the lengths of hundreds of micrometers to onecentimeter as shown in the insets of figure 2. Essentially one-dimensional lead iodide chains were found to extend along the longitudinal directions of the crystals. Regarding compound 1, the single-crystal X-ray diffraction data showed that it crystallized in the triclinic system with the P1 space group. The chemical formula of compound 1 was determined from the X-ray analysis to be in the form A Pb32 I 8 (A=HNC5H4-C2H5). One proton was donated from an HI acid to the nitrogen of the Py ring during the acid-base reaction in the synthesis procedure. Three octahedrons were found to be connected via an edge-shared system to form the unit of 1D triple-edge-shared chains extending along the aaxis, as can be seen in figure 1(a). The organic component of 4EtPyH+ adopted columnarlike stacking structures between the inorganic lead idodie chains. There have been two reports on double chain structures so far [23,24]: one being a corner-shared double-chain
4
structure [23] and the other an edge-shared double-chain one [24]. An edge-shared triplechain structure like that found in the present study has not previously, to the best of our knowledge, been reported for any hybrid perovskite. Figure 3 (a) shows an enlarged representation of the structure of compound 1, as well as some atomic distances. In this figure, N1 denotes the nitrogen of the pyridine ring. The distance between N1 (H1) and the nearest I (I1) was estimated to be 3.66 Å (2.89 Å) as shown in figure S1 (a). The N1–I1 distance was close to the sum of the Van der Waals radii of the nitrogen (1.55 Å) and iodide (1.98 Å) atoms. And it was almost the same as the distances between N and I in N–H–I hydrogen bonds reported for 2D and 1D hybrid compounds based on lead iodide octahedral [24,25].
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Table 1. Crystal data and structure refinement statistic for compounds 1 and 2.
Compound
1
2
Organic component
4-Ethylpyridine
4-Aminomethylpyridine
1D structural type
Edge-shared triple-
Edge-shared single-chain
chain Formula
C14H20N2Pb3I8
C6H10N2PbI4
Formula weight
1857.16
824.98
Crystal system
Triclinic
Monoclinic
Space group
P1
P21/a
Crystal color
yellow
yellow
a (Å)
4.6002(6)
13.020(6)
b (Å)
12.794(5)
14.413(8)
c (Å)
14.693(5)
8.389(4)
α (°)
80.937(18)
90
β (°)
82.20(2)
105.766
γ (°)
80.03(2)
90
Volume (Å3)
835.8(5)
1514.9(13)
Z
2
4
Density (calcd) (g/cm3)
7.363
3.617
Absorption coefficient (mm−1)
22.50
19.26
F(000)
1576.00
1416.00
54.9
54.9
6345/3/244
3452/0/119
R1 (I > 2σ(I))
0.028
0.052
wR2 (all data)
0.071
0.127
GOF
0.944
0.918
CCDC number
1968131
1968137
2
max
(°)
Independent reflections/restraints/parameters
6
Figure 1. Schematic representations of the crystal structures of (a) compound 1 and (b) compound 2. Hydrogen atoms are omitted for clarity.
( (a)
( (b)
Figure 2. (Upper) crystal photograph (mm scale bars) of compounds 1 (a) and 2 (b), (lower) crystal morphologies deduced from the Bravais-Friedel Donnay-Harker (BFDH) method of compounds 1 (a) and 2 (b). Hydrogen atoms are omitted for clarity. 7
Figure 3. Enlarged representations of the crystal structures of compounds 1 (a) and 2 (b). Some atomic distances are given.
The Compound 2 crystallized in the monoclinic system with the space group P21/a. The chemical formula of compound 2 was determined from the X-ray analysis to be in the form A2+Pb2+I–4 (A= HNC5H4-CH2NH3). It is considered that one proton is bound with nitrogen in the Py ring and the other is bound with that of the amino group. To the best of our knowledge, the APbI4 formula was previously found only for 2D hybrid perovskites-like structure [7], with the current work being the first example of this formula for a 1D crystal. Inspection of the vertical and horizontal views of the representation of the structure of compound 2 shown in Figure 1b indicated the presence of edge-shared structures in which the lead iodide octahedrons were connected via two iodine atoms to form the 1D chain. Figure 3 (b) shows an enlarged representation of the structure of compound 2, with N1 and N2 denoting the nitrogen atoms in the Py ring and amino group, respectively. The shortest N1–I (I2) distance here was measured to be 3.82 Å, longer than that of compound 1. The 8
distances between N2 and the nearest I anions (I1-I4) were in the range 3.56 to 3.7 Å. We estimated the I-H distances for various conformations of the amino group. While the distances between I4 and the H atoms of the amino group (labeled as H1-H3) were greater than 3.2 Å, those between I and H for I1–H1, I2–H2 and I3–H3 were approximately 2.7-2.8 Å (figure S1 (b)), indicating the formation of N–H–I hydrogen bonds. This formation was associated with a shift of 4 AMPy toward the void, which resulted in the greater N1–I2 distance in compound 2 than in compound 1. Hence, the slight difference between the end groups of the two inserted molecules resulted in very different crystal structures.
Figure 4. Absorption spectra of the crystals of compounds 1 (a) and 2 (b) at room temperature. Figure 4 shows the absorption spectra of the single crystals of compounds 1 and 2 at room temperature. For each spectrum, extrapolating the linear part of the curve at the absorption edge to the horizontal axis yielded an estimation of the band gap. In this way, the band gaps of compounds 1 and 2 were estimated to be 2.73 and 2.64 eV, respectively. This difference between the band gaps may have been due to the difference between the Pb–I–Pb bond angles in the inorganic chains. Theoretical studies for a layered perovskite-like structure containing an SnI4 framework have suggested that the in-plane Sn–I–Sn bond angle has the greatest influence on the band gap [26]. In these studies, larger Sn-I-Sn bond angles in the hybrid perovskites-like yielded narrower band gaps. Such was also the case for the 1D crystal studied in the current work. Pb–I–Pb bond angles of 91.82 and 88.04 degrees were measured for compound 1 and 93.92 and 91.83 degrees for compound 2, as shown in figure S2. Thus the broader band gap of compound 1, compared to that of compound 2, may have been due to its smaller Pb–I–Pb bond angles.
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CONCLUSIONS
We demonstrated the preparation of single crystals of two different one-dimensional hybrid compounds based on lead iodide perovskites-derived containing pyridine derivatives. The sizes of the single crystals can be tuned from several micrometers to on the order of a centimeter by controlling the cooling rate in the crystal growth procedure. We found that the slight difference between the end functional groups of the organic components of compounds 1 and 2 gave rise, despite the similar shapes and sizes of their organic molecules, to quite different crystal structures: compound 1 crystallized in the monoclinic system and formed a novel edge-shared triple-chain perovskite-derived; while compound 2 crystallized in the triclinic system and formed an edge-shared single-chain perovskite-derived. The amino group in compound 2 was found to interact with the lead iodide chain through N2–H– I hydrogen bonds. Both compounds showed relatively broad band gaps, with the specific values of their band gaps apparently affected by the angle of the Pb–I–Pb bond. SUPPLEMENTARY MATERIAL See the supplementary material for I–H distances, Pb–I– Pb bond angles, and powder XRD results and discussion.
ACKNOWLEDGEMENTS This work was supported by JSPS KAKENHI Grant Number JP18H03899, and JSPS DC2.
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Highlights 1, The sizes of the single crystals can be tuned from several micrometers to centimeter order by controlling crystal growth procedure. 2, The slight difference between the functional groups of the molecules governed their crystal structures and hence optical properties. 3, The band gaps of the compound affected by the angle of the Pb–I–Pb bond.
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Credit author statement Thi-Mai Huong Duong: sample synthesis, measurement and analysis data, writing original draft preparation. Shunpei Nobusue: structure analysis supervision. Hirokazu Tada: supervision, review and editing.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: