3.1 Morphology and crystal structure
Figure 1(a1)-(b1) and Fig. 1(a2)-(b2) were the digital photographs and SEM images of the (C6H5CH2NH3)2BiCl5 and (C6H5CH2NH3)BiI4 single crystals, respectively. The (C6H5CH2NH3)2BiCl5 possesses granular shape, while (C6H5CH2NH3)BiI4 showed simple prismatic shape. In order to analyze the crystal structure of the single crystal, single crystal diffraction and powder X-ray diffraction (PXRD) were carried out at room temperature. The crystal structures of single crystals were shown in Fig. 1(a3)-(b3). At the same time, the single crystal X-ray diffraction data of (C6H5CH2NH3)2BiCl5 and (C6H5CH2NH3)BiI4 were summarized in Table 1. Owing to different coordinate axes chosen, the (C6H5CH2NH3)2BiCl5 crystals belongs to the monoclinic system and P21/c space group, while the (C6H5CH2NH3)BiI4 crystals the monoclinic system and P21/n space group. The crystal structures of the anisotropic displacement parameters and other structural information have been deposited with the Cambridge Crystallographic Data Centre under the registration number CCDC 1886951, 1883198, respectively.
Table 1
Crystal data and structure refinement for (C6H5CH2NH3)aBibXc
Empirical formula
|
(C6H5CH2NH3)2BiCl5
|
(C6H5CH2NH3)BiI4
|
Formula weight (g mol− 1)
|
602.55
|
1649.48
|
Crystal system
|
monoclinic
|
monoclinic
|
Space group
|
P21/c
|
P21/n
|
α(°)
|
90
|
90
|
β(°)
|
97.195(3)
|
90.455(4)
|
γ(°)
|
90
|
90
|
a(Å)
|
13.5528(4)
|
12.4166(6)
|
b(Å)
|
12.8311(3)
|
7.8330(4)
|
c(Å)
|
12.1296(3)
|
15.9264(7)
|
Volume(Å3)
|
2092.68(9)
|
1548.95(12)
|
Z
|
4
|
2
|
Density (g cm− 3)
|
1.912
|
3.537
|
Diffractometer/scan
|
Agilent G8910A CCD
|
Agilent G8910A CCD
|
Radiation, λ (Å),
|
0.71073
|
0.71073
|
Monochromator θ range (°)
|
6.772 to 58.546
|
6.564 to 58.13
|
Unique reflections
|
15133
|
11205
|
F(0 0 0)
|
1144.0
|
1416.0
|
Index ranges
|
-18 ≤ h ≤ 17, -17 ≤ k ≤ 15, -16 ≤ l ≤ 14
|
-15 ≤ h ≤ 16, -10 ≤ k ≤ 10, -15 ≤ l ≤ 20
|
GOF on F2
|
1.036
|
1.138
|
Absorption correction
|
Spherical harmonics
|
Spherical harmonics
|
Data/restraints/parameters
|
5052/0/201
|
3672/0/120
|
Final R indexes [I > 2σ (I)]
|
R1 = 0.0341, wR2 = 0.0664
|
R1 = 0.0379, wR2 = 0.0790
|
Final R indexes (all data)
|
R1 = 0.0591, wR2 = 0.0777
|
R1 = 0.0487, wR2 = 0.0849
|
Min/max (q/e Å−3)
|
1.44/-1.34
|
1.89/-1.07
|
(C6H5CH2NH3)2BiCl5 presented typical two-dimensional (2D) structure. The basic structural unit of (C6H5CH2NH3)2BiCl5 compound consisted of benzylammonium cation and corner-sharing [BiCl6] octahedra. Each Bi3+ in the structure was surrounded by six halogen anions Cl− to form an inorganic octahedral structure. The octahedrons were connected by sharing two halogen anions Cl−. [C6H5CH2NH3]+ cations occupyed the gaps of the octahedral, and each inorganic layer [BiCl6]− connected two adjacent organic layers [C6H5CH2NH3]+ to form an alternating organic-inorganic hybrid structure. The organic group and the inorganic octahedron were connected by unequal N-H⋯Cl hydrogen bonds, combined with weak van der Waals interactions to form a two-dimensional perovskite structure.
While (C6H5CH2NH3)BiI4 displayed typical one-dimensional (1D) structure which consisted of [C6H5CH2NH3]+ cations and infinite linear anionic chains [BiI6]−. In the crystal structure, each Bi3+ is surrounded by six halogen anions I− to form an inorganic octahedral structure. And the adjacent octahedrons were connected by sharing two anion I− to form a one-dimensional chain. The organic group [C6H5CH2NH3]+ occupied the octahedral gap which was connected with the inorganic octahedral structure through the N-H⋯I hydrogen bond to form an organic-inorganic hybrid structure.
To investigate the purity of two types of single crystals, the PXRD were performed as shown in Fig. 2(a1) (b2). It could be seen from the figure that the experimental test pattern was roughly consistent with the theoretical simulation pattern which indicated the reliability of refined single crystal structures and purity phase of those crystals.
3.2. Hirshfeld surface analysis
To research the various interactions that lead to the crystal structure in detail, Hirshfeld surfaces[13] and fingerprint plots were generated for the (C6H5CH2NH3)aBibXc compound based on the crystallographic information file (CIF) using CrystalExplorer[14]. Hirshfeld surfaces enable the visualization of intermolecular interactions with different colours and colour intensity representing short or long contacts and indicating the relative strength of the interactions.
It was a significant advance in understanding the crystal packing behavior[15] which using the 2D-fingerprint plots and shape index to analysis the intermolecular interactions. The distance of the three-dimensional (3D) molecular Hirshfeld surfaces from surface to the nearest nucleus inside the surface was defined ‘di’, and the distance from surface to the nearest nucleus outside the surface was defined ‘de’[16]. The normalized contact distance (dnorm) is given by:
(1)
‘rivdw’ and ‘revdw’ were defined the van der Waals radii of the appropriate atoms, respectively. Graphical plots of the molecular Hirshfeld surfaces mapped with dnorm employ the red-white-blue colour scheme where red area represents the strongest contact, blue area indicates weaker contact.
The ‘di’, ‘de’, ‘dnorm’, shape index, curvedness curves mapping on Hirshfeld surface of (C6H5CH2NH3)2BiCl5 were shown in the Fig. 3. The fixed color scale in Fig. 3a1-a5 were 0.901 Å (red) ~ 2.879 Å (blue), 0.903 Å~2.972 Å, -0.416 Å~1.558 Å, -1.000 Å~1.000 Å, and − 4.000Å~0.400 Å respectively. The corresponding 2D fingerprint plot of (C6H5CH2NH3)2BiCl5 was as shown in Figure S1. Globally, the H⋯Cl/Cl⋯H intermolecular interactions had the largest contribution to the Hirshfeld surface, with percentage contributions of 46.2%(19.4%+26.8%). Owing to the molecular surface had large amount of hydrogen, the second most abundant interactions contribution was from H⋯H contacts with percentage of 34.8%. The remaining C⋯Cl/Cl⋯C, H⋯C/C⋯H, Cl⋯Cl, Bi⋯Cl/Cl⋯Bi, C⋯C intermolecular contacts of the Hirshfeld surface only contributes 0.2%, 12.9%, 1.6%, 3.4%, 0.8% respectively.
The ‘di’, ‘de’, ‘dnorm’, shape index and curvedness curves surfaces of compound (C6H5CH2NH3)BiI4 were shown in the Fig. 4, the fixed color scale of 3D mapped surfaces were between 1.000 Å (red) ~ 2.940 (blue), 0.946 Å~2.823 Å, -0.356 Å~1.016 Å, -1.000 Å~1.000 Å, and − 4.000Å~4.000 Å respectively. Figure S2 provide us information about intermolecular interactions of 2D-fingerprint plots. The decomposition of the fingerprint plots show that H⋯I/ I⋯H contacts comprise 64% of the total Hirshfeld surface area for the molecule of (C6H5CH2NH3)BiI4 which confirms significant of these interactions on structural stability. Furthermore, the proportion of H⋯H interactions to the tune of 12.2% in Hirshfeld surfaces of (C6H5CH2NH3)BiI4 indicated that these contacts are the second most significant interaction. Apart from these above, the C⋯H /H⋯C, C⋯C, I⋯C/C⋯I,, I⋯I, Bi⋯H, Bi⋯I/I⋯Bi of the Hirshfeld surface only contributes 6.5%, 2.7%, 0.6%, 4.7%, 0.1%, 9.2% respectively.
According to the parameter analysis of the ‘di’, ‘de’, ‘dnorm’, shape index and curvedness curves of (C6H5CH2NH3)2BiCl5 and (C6H5CH2NH3)BiI4, the ratios of the bonds and the interaction ratios between the elements are different because of the difference in halogens.
3.3. FT-IR Spectra study
In order to understand information about the organic group and inorganic framework of the compound (C6H5CH2NH3)aBibXc in more detail, FT-IR spectroscopy were performed (Fig. 5). And the detailed assignments of vibrational models were listed in Table 2. Obviously, the peaks between 500-3500cm− 1 belong to the benzylamine ring, and those peaks below 500 cm− 1 belong to the peak of (Bi-X) (X = Cl, I)[17]. The asymmetric and symmetric stretching bands of NH3+ were usuallylocated at 3330 and 3080 cm− 1[18], respectively. In (C6H5CH2NH3)aBibXc, the NH3+ group interacts with the halide atoms through N-H⋯X hydrogen bonds. As seen from Table 2, the δasym(NH3) and δsym(NH3) modes are coupled with the C-C ring stretching mode. The CH2 asymmetric and symmetric stretching modes are observed at 2849 to 2856 cm− 1 and at 2764 cm− 1[19].
Table 2
Measured resonance frequencies of vibrational modes and peak assignments for the (C6H5CH2NH3)aBibXc
(C6H5CH2NH3)2BiCl5
IR Wavenumbers (cm− 1)
|
(C6H5CH2NH3)BiI4
IR Wavenumbers (cm− 1)
|
Peak assignment
|
3443
|
3443
|
υasym(NH3+)
|
3117
|
3146
|
υsym(NH3+)
|
2920
|
2920
|
υasym(CH3)
|
2849
|
2856
|
υasym(CH2)
|
2764
|
-
|
υsym(CH2)
|
1628
|
1628
|
δasym(NH3) + υ(C–C)ring
|
1551
|
1558
|
δsym(NH3) + υ(C–C)ring
|
1459
|
1445
|
δsym(CH3) + δsym(CH2)
|
1359
|
1374
|
υasym(C-N)
|
1078
|
1057
|
β(C–H)
|
-
|
-
|
β(C–H)
|
1014
|
923
|
ρ(NH3)
|
880
|
845
|
γ(C–H)
|
810
|
781
|
γ(C–H)
|
753
|
746
|
υ(C–C) + β(C–C–C)
|
696
|
690
|
γ(C–C–C)
|
569
|
563
|
β(C–C–C)
|
484
|
478
|
υasym(Bi–X)
|
ν: stretching; δ: scissoring or bending; t:twisting; ρ: rocking; t: torsion; as: asymmetric, s:symmetric |
3.4. Thermogravimetric analysis and stability
To investigate the mass loss behavior of each component of the (C6H5CH2NH3)aBibXc single crystal materials in detail, the thermogravimetric curve were obtained from room temperature to 800℃. As Fig. 6a shown, (C6H5CH2NH3)2BiCl5 exhibited one thermal events and was stable up to 260℃, all organic and inorganic compounds decompose completely at 350℃. And the (C6H5CH2NH3)BiI4 is stable up to 300℃, decomposed completely at 410℃. In short, different halogens have different stability between Bi-X bonds, resulting in different stability of compounds.
3.5. Optical property
To calculate the optical band gap of (C6H5CH2NH3)aBibXc, UV–Visible absorption spectra were carried out, as shown in Fig. 7. The absorbance as a function of reflectance explained by Kubelka-Munk Eq. [20], the formula is as follows:
F(R) = α = (1-R)2/(2R) (2)
where R represents the value of light reflectivity and α means the optical absorption coefficient.
According to the UV-vis spectrum, it could clearly observe that the absorption edge of (C6H5CH2NH3)2BiCl5, (C6H5CH2NH3)BiI4 was located at about 440 nm, 890 nm which indicating that the band gap was 3.21 eV, 1.67 eV respectively. As the ionic radius of I− is larger than that of Cl−, the ultraviolet-visible absorption peak of (C6H5CH2NH3)BiI4 is red-shifted compared to (C6H5CH2NH3)2BiCl5, resulting in an increase in the light absorption range[21–23]. Therefore, the band gaps of (C6H5CH2NH3)BiI4 is smaller than the (C6H5CH2NH3)2BiCl5, the light absorption range of (C6H5CH2NH3)BiI4 is wider than the (C6H5CH2NH3)2BiCl5. In summary, the (C6H5CH2NH3)BiI4 have more application potential in light-emitting, Diodes (LEDs), photodetectors, lasers and other fields, compared to the (C6H5CH2NH3)2BiCl5.