Thermodynamic, Physical, and Structural Characteristics in Layered Hybrid Type (C2H5NH3)2MCl4 (M = 59Co, 63Cu, 65Zn, and 113Cd) Crystals.

The thermal, physical, and molecular dynamics of layered hybrid type (C2H5NH3)2MCl4 (M = 59Co, 63Cu, 65Zn, and 113Cd) crystals were investigated by thermogravimetric analysis (TGA) and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The temperatures of the onset of partial thermal decomposition were found to depend on the identity of M. In addition, the Bloembergen-Purcell-Pound curves for the 1H spin-lattice relaxation time T1ρ in the rotating frames of CH3CH2 and NH3, and for the 13C T1ρ of CH3 and CH2 were shown to exhibit minima as a function of the inverse temperature. These results confirmed the rotational motion of 1H and 13C in the C2H5NH3 cation. Finally, the T1ρ values and activation energies Ea obtained from the 1H measurements for the H‒Cl···M (M = Zn and Cd) bond in the absence of paramagnetic ions were larger than those obtained for the H‒Cl···M (M = Co and Cu) bond in the presence of paramagnetic ions. Moreover, the Ea value for 13C, which is distant from the M ions, was found to decrease upon increasing the mass of the M ion, unlike in the case of the Ea values for 1H.

1. Introduction

Layered hybrid compounds have drawn great attention as a new generation of high performance materials due to their interesting physical and chemical properties obtained through the combination of organic and inorganic materials at the molecular level [1,2,3]. They consist of a wide range of inorganic anion chains, alternating with a large variety of organic cations as building blocks. The organic component of the hybrid complex provides several useful properties, such as structural flexibility and optical properties, while the inorganic part is responsible for the mechanical and thermal stabilities, in addition to interesting magnetic and dielectric transitions [4,5]. The diversity of such hybrid materials is therefore large, and so offers a wide range of structures, properties, and potential applications [6,7,8,9,10,11]. More specifically, hybrid layered compounds based on the perovskite structure are interesting materials due to their potential application in solar cells [2,3]. However, the toxicity and chemical instability of halide perovskites limit their use. As a result, the replacement of the lead in present in the perovskite structure with alternative cost-effective materials that are environmentally friendly, less-toxic, and more readily available (e.g., transition metals) is necessary for the extended application of perovskites in solar cells [3]. The structure of (CH2NH3)2MCl4 compounds, where n = 1, 2, 3… and M represents a divalent metal (M = Co2+, Cu2+, Zn2+, and Cd2+), has been described as a sequence of alternating organic-inorganic layers [2,3,12]. The structures of (C2H5NH3)2MCl4 crystals with n = 2 are similar within each group but dissimilar between groups due to differences between either the inorganic or organic components. For example, the inorganic frames where M = Cu2+ and Cd2+ are corner-sharing MCl6 octahedra, while those of M = Co2+ and Zn2+ are simple MCl4 tetrahedra [13]. In addition, the organic chains are joined by weak hydrogen bonds between the NH3 groups and the Cl ions. Indeed, the structural geometries and molecular dynamics of the organic molecules within the layered hybrid structures are important for determining the influence of temperature on the structural phase transitions.

As an example, (C2H5NH3)2CoCl4 crystallizes as an orthorhombic structure, which undergoes a reversible phase transition at 235 K [14]. In addition, (C2H5NH3)2CuCl4 undergoes phase transitions at 236, 330, 357, and 371 K [7,15,16,17,18,19], its crystal structure at room temperature is orthorhombic [20]. In contrast, (C2H5NH3)2ZnCl4 undergoes five phase transitions at 231, 234, 237, 247, and 312 K [21], crystallizing as an orthorhombic system at room temperature [22]. Finally, (C2H5NH3)2CdCl4 undergoes structural phase transitions at 114, 216, 358, and 470 K [9,23,24], whereby the room temperature orthorhombic phase has the Abma space group [23]. The structure of the organic component consists of a double layer of alkylammonium ions with the charged nitrogen atoms oriented to the nearest MCl4 tetrahedra or MCl6 octahedra. The phase transition temperatures, lattice constants, structures, and space groups for the four crystals are summarized in Table 1.

Table 1

The phase transition temperatures, TC, lattice constants, structures, and space groups of (C2H5NH3)2MCl4 (M = Co, Cu, Zn, and Cd) crystals at room temperature.

M	TC (K)	Lattice Constant (Å)			Structure	Space Group
Co	235	a = 10.025	b = 7.403	c = 17.603	orthorhombic	Pnma
Cu	236, 330, 357, 371	a = 7.47	b = 7.35	c = 21.18	orthorhombic	Pbca
Zn	231, 234, 237, 247, 312	a = 10.043	b = 7.397	c = 17.594	orthorhombic	Pna21
Cd	114, 216, 358, 470	a = 7.354	b = 7.478	c = 22.11	orthorhombic	Abma

Based on our previously reported nuclear magnetic resonance (NMR) results, the molecular dynamics of the cation present in (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) crystals were discussed in terms of temperature-dependent chemical shifts and spin-lattice relaxation times T1ρ in the rotating frames for the 1H and 13C nuclei [25,26,27].

Thus, to better elucidate the thermal stability in (C2H5NH3)2CoCl4 single crystals grown by the slow evaporation method, we herein describe the use of thermogravimetric analysis (TGA), in addition to structural analysis by variable-temperature 1H magic angle spinning (MAS) NMR spectroscopy and 13C cross-polarization (CP/MAS) NMR spectroscopy. Furthermore, the spin-lattice relaxation times T1ρ in the rotating frames are measured for the 1H and 13C nuclei to better understand the physical and structural properties of (C2H5NH3)2CoCl4. The obtained results are compared with those of the previously reported (C2H5NH3)2CuCl4, (C2H5NH3)2ZnCl4, and (C2H5NH3)2CdCl4, and the properties dependent on the characteristics of the metal anion and the organic cation are identified.

2. Results and Discussion

2.1. Thermal Stability

The thermal stabilities of the various (C2H5NH3)2MCl4 were examined by TGA, and the results are presented in Figure 1. Upon comparison of the TGA results with the possible chemical reactions taking place, the solid residues formed for (C2H5NH3)2MCl4 were calculated based on Equations (1)–(4) [28]: (C (C (C (C

Figure 1

Thermogravimetric analysis (TGA) curve for crystals of (C2H5NH3)2MCl4 (M = Co, Cu, Zn, and Cd).

For the M = Co, Cu, Zn, and Cd species, the first mass losses were observed at approximately 378, 430, 460, and 550 K, respectively, which represent the onset of partial thermal decomposition, Td. From the results calculated using the molecular weights, mass losses of 25.97, 24.51, 24.36, and 21.05% for the different M ions were attributed to decomposition of the 2HCl moieties. These results are consistent with the TGA experiment results shown by dotted lines in Figure 1. Moreover, the final decomposition product is MCl2, which corresponds to mass losses of 53.76, 54.81, 54.47, and 47.08%. These results indicate some differences between the calculated and experimental values. The difference between the calculation and experimental value of the final decomposition product is presumably dependent on the heating rate in the TGA experiment. Another difference is thought to be due to experimental conditions in air or N2 atmosphere. The decomposition temperature, Td, and mass loss of 2HCl, and final decomposition product for four crystals are summarized in Table 2.

Table 2

The decomposition temperature, Td, mass loss of 2HCl, and final decomposition product MCl2 for four crystals.

M	Td (K)	Weight Loss of 2HCl (%)(cal. Value)	Weight Loss of 2HCl (%)(exp. Value)	Final Decomposition Product (%)(cal. Value)
Co	378	25.97	26.09	53.76
Cu	430	24.51	24.84	54.81
Zn	460	24.36	23.17	54.47
Cd	550	21.05	21.00	47.08

Optical polarizing microscopy was used in order to determine whether these transformations are structural phase transitions or chemical reactions, as presented in Figure 2. In the case of (C2H5NH3)2CoCl4, the crystals are blue at room temperature, and no change in the crystal state was observed upon increasing temperature to 360 or 460 K, although melting was observed to commence at 465 K. In contrast, the (C2H5NH3)2CuCl4 crystals are dark yellow at room temperature, although they present a slightly inhomogeneous hue due to surface roughness. Upon increasing the temperature, the crystal color changed from dark yellow (300 K), to brown (380 K), to dark brown (450 and 500 K), and start melting was observed at 530 K. Interestingly, the crystals of (C2H5NH3)2ZnCl4 remained colorless and transparent (300, 450, and 460 K), and melting was observed between 470 and 475 K. Similarly, in the case of (C2H5NH3)2CdCl4, the crystals remained colorless and transparent between 300 and 480 K, although they became slightly opaque at approximately 540 K, prior to becoming fully opaque close to 570 K. Here, the sample temperatures shown in Figure 2 were kept constant during 2 min each temperature. For all four crystals, it was apparent that the phenomenon above Td was not related to any structural phase transitions, but rather to a thermal decomposition, suggested by Lee [29].

Figure 2

The states of single crystals according to the temperature (a) (C2H5NH3)2CoCl4, (b) (C2H5NH3)2CuCl4, (c) (C2H5NH3)2ZnCl4, (d) (C2H5NH3)2CdCl4.

2.2. Investigation of the Structural Properties and Molecular Dynamics by 1H MAS NMR

The 1H MAS NMR spectra of (C2H5NH3)2CoCl4 were recorded at a range of temperatures as shown in Figure 3. More specifically, at 300 and 370 K, the 1H signals for C2H5 and NH3 could not be distinguished, and the superimposed peak was rather broad; at 300 and 370 K, single peaks were observed at δ = 1.68 and δ = 0.02 ppm, respectively. In Figure 3, the spinning sidebands for the protons of C2H5NH3 are marked with asterisks. At 420 and 430 K, signals with chemical shifts of δ = 1.76 and 4.36 ppm, and δ = 1.79 and 4.37 ppm, were observed, respectively, which represent the protons of the C2H5 and NH3 ions. In addition, at these higher temperatures, the obtained signals became more intense, and the full-width at half-maximum (FWHM) values narrowed significantly, which were attributed to a high internal mobility.

Figure 3

1H magic angle spinning (1H MAS) NMR spectra of (C2H5NH3)2CoCl4 at 300 K, 370 K, 420 K, and 430 K. The spinning sidebands for central peak are marked with asterisk.

The magnetization recovery traces for both the C2H5 and NH3 protons in (C2H5NH3)2CoCl4 can be described by a single exponential function [30,31] P( where P(t) is the magnetization as a function of the spin-locking pulse duration t, and P0 is the total nuclear magnetization of the proton at thermal equilibrium. The recovery traces of the 1H nuclei for delay times ranging from 1 μs to 50 ms at 300 K are presented in the inset of Figure 4. Here, the asterisks represent spinning sidebands for the center peak. The T1ρ values were obtained from the slopes of the delay time vs. the signal intensity, and were plotted as a function of the inverse temperature in Figure 4. As shown, the T1ρ values sharply decrease close to 430 K, while near the phase-transition temperature TC, no changes are evident. At higher temperatures, the T1ρ values for the C2H5 and NH3 protons were comparable within the range of error, and from the slope of T1ρ vs. the inverse temperature, the activation energy Ea for the rotational motion below 400 K was determined to be Ea = 3.11 ± 0.15 kJ/mol.

Figure 4

1H spin-lattice relaxation times T1ρ in the rotating frame in C2H5NH3 cation of (C2H5NH3)2CoCl4 as a function of inverse temperature. The black square and red triangle at 410 K and 420 K is for 1H T1ρ in the C2H5 and NH3 group, respectively (inset: the 1H recovery traces according to the delay times at 300 K).

The previously reported 1H T1ρ values for C2H5 and NH3 of (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) are shown in Figure 5 as a function of the inverse temperature. More specifically, the 1H T1ρ values in the presence of the paramagnetic Co2+ and Cu2+ ions are particularly short, i.e., 0.01–20 ms, while those of the non-paramagnetic Zn2+ and Cd2+ ions are longer, i.e., 2–200 ms. In addition, the 1H T1ρ values for C2H5 are longer than those for NH3. In contrast, the relaxation times for the 1H nuclei in the presence of M = Cu, Zn, and Cd reach minimum values, unlike in the case of Co2+. For (C2H5NH3)2CuCl4, the T1ρ for the 1H nucleus reaches its minimum values at 190 and 200 K for C2H5 and NH3, respectively, while for (C2H5NH3)2ZnCl4, the minimum values of 2.17 and 2.48 ms were reached at 260 and 330 K, respectively. Moreover, in case of (C2H5NH3)2CdCl4, the T1ρ shows a minimum value at 270 K. It is therefore apparent that the 1H T1ρ values for (M = Cu, Zn, and Cd) vary due to molecular motion according to the Bloembergen–Purcell–Pound (BPP) theory [30], while no such molecular motion is observed for the (M = Co) species. Indeed, the T1ρ values are related to the corresponding values of the rotational correlation time, τC, which is a direct measure of the rate of molecular motion. The experimental value of T1ρ can therefore be expressed in terms of τC for the molecular motion as suggested by the BPP theory [26,29,31,32,33]. T where the quantities f(ω) are spectral density functions, i.e., Fourier transforms of the time correlation functions. ωH and ωC are the Larmor frequencies of proton and carbon, respectively, and ω1 is the frequency of the spin-locking field. The parameter τC is a characteristic correlation time, that is, the time scale of the motion of the C2H5 and NH3 ions. F is defined as a relaxation constant: F = (N/20)(γ where γH and γC are the proton and carbon gyromagnetic ratios, respectively, N is the number of directly bound protons, rH–C is the H–C internuclear distance, and ħ is the reduced Planck constant. The obtained data were analyzed assuming that T1ρ has a minimum at ω1τC = 1, and the BPP relationship was applied between T1ρ and the characteristic frequency ω1. The value of the relaxation constant F was therefore obtained using Equation (7). From these results, the temperature dependences of the τC values for the rotational motions of C2H5 and NH3 were calculated from the F values. The temperature dependence of τC follows the simple Arrhenius equation: τ where Ea is the activation energy, τ0 is the high temperature limit of the correlation time, T is the temperature, and R is the gas constant. The slope of the linear portion of the semi-log plot represents the Ea, and the Ea for the rotational motion can be obtained from the log τC vs. 1000/T curve. Thus, the calculated Ea values for the four compounds are summarized in Table 3; the activation energies for molecular motion in the presence of paramagnetic Co2+ and Cu2+ ions were smaller than those for the species containing Zn2+ and Cd2+.

Figure 5

1H spin-lattice relaxation times T1ρ in the rotating frame in (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) as a function of inverse temperature.

Table 3

The spin-lattice relaxation times, T1ρ, and activation energies, Ea, for 1H and 13C of (C2H5NH3)2MCl4 (M = Co, Cu, Zn, and Cd) crystals.

M	1H T1ρ (ms)	Ea (kJ/mol)	13C T1ρ (ms)	Ea (kJ/mol)
Co	0.01–2	3.11 (for C2H5NH3)	0.1–10	45.98 (for CH3)
Cu	7–20	12.19 (for C2H5 below 240 K)	1–100	21.35 (for CH3)
		8.33 (for NH3 below 240 K)		19.72 (for CH2)
Zn	2–200	39.41 (for C2H5NH3 above 290 K)	6–100	21.13 (for C2H5)
		57.59 (for C2H5NH3 below 290 K)
Cd	2–100	22.63 (for C2H5NH3)	5–100	18.05 (for CH3)

2.3. Investigation of the Structural Properties and Molecular Dynamics by 13C CP/MAS NMR

The structural analysis of (C2H5NH3)2CoCl4 was also performed using 13C CP/MAS NMR over a range of increasing temperatures. Thus, the two peaks corresponding to the CH3 and CH2 species at 360 K were observed at chemical shifts of δ = 49.65 and 176.55 ppm, respectively, as shown in the inset of Figure 6. The CH3 and CH2 results obtained by 13C MAS NMR were distinguished in that the signals corresponding to CH2 could not be observed at low temperatures. In these experiments, the chemical shift of CH3 remained relatively constant, while that of CH2 decreased with increasing temperature, and a sharp decrease was observed close to 420 K.

Figure 6

13C chemical shift in CH3 and CH2 groups in (C2H5NH3)2CoCl4 as a function of temperature (inset: 13C MAS NMR spectrum at 360 K).

To obtain the corresponding 13C T1ρ values, the nuclear magnetization recovery traces were measured as a function of the delay time. The signal intensities of the magnetization recovery curves for 13C were analyzed by a single exponential function of Equation (5) at all temperatures, and the 13C T1ρ values for CH3 and CH2 in (C2H5NH3)2CoCl4 were plotted as a function of inverse temperature (see Figure 7). Indeed, the 13C T1ρ curve for CH3 at low temperatures can be reproduced by the BPP theory [32], and the BPP curve shows a minimum of 0.57 ms at 260 K. This characteristic of T1ρ means that distinct molecular motions existed. The correlation time was then obtained using Equation (6), and the activation energy was obtained from these results. More specifically, the Ea for the rotational motion was determined to be 45.98 ± 1.78 kJ/mol from the log τC vs. 1000/T curve shown in Figure 7.

Figure 7

13C spin-lattice relaxation times in the rotating frame for CH3 and CH2 groups in (C2H5NH3)2CoCl4 as a function of inverse temperature (inset: Arrhenius plots of the natural logarithm of the correlation time for CH3 as a function of inverse temperature).

The T1ρ values of the previously reported (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) (see Figure 8) were compared with those of (C2H5NH3)2CoCl4 determined herein. In addition, the molecular motions influenced by 13C T1ρ in (C2H5NH3)2CoCl4 were found to exhibit BPP trends, unlike in the case of the 1H T1ρ results. Furthermore, for (C2H5NH3)2CuCl4, the temperature dependences of the 13C T1ρ values for CH2 and CH3 appeared similar, and the BPP curves for CH3 and CH2 showed minima at 190 K. The T1ρ curve for (C2H5NH3)2ZnCl4 can be also represented by the BPP theory, with a minimum being observed at 260 K in the curve. Finally, in case of (C2H5NH3)2CdCl4, the T1ρ curves show minima at 260 and 250 K for CH3 and CH2, respectively. The 13C T1ρ and Ea values obtained from the 13C results for the four compounds are summarized in Table 2, whereby it is apparent that the 13C T1ρ values for compounds containing paramagnetic ions are shorter than those without paramagnetic ions, since the relaxation time should be inversely proportional to the square of the magnetic moment of the paramagnetic ions. Therefore, the T1ρ values of (C2H5NH3)2MCl4 (M = Co and Cu) were driven by fluctuations of the magnetic dipoles of the paramagnetic Co2+ and Cu2+ species, and the Ea values for 13C decreased upon increasing the mass of the M2+ ion, unlike in the case of the 1H Ea values. These differences are due to variations in the electronic structures of the M2+ ions, and in particular, the d electrons, which screen the nuclear charge from the motion of the outer electrons.

Figure 8

13C spin-lattice relaxation times T1ρ in the rotating frame in (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) as a function of inverse temperature.

3. Materials and Methods

Single crystals of (C2H5NH3)2MCl4 (M = Co, Cu, Zn, and Cd) were grown from CH3CH2NH2∙HCl (ethylamine hydrochloride, Aldrich 98%), and CoCl2 (cobalt chloride, Aldrich 97%), CuCl2 (copper chloride, Aldrich 97%), ZnCl2 (zinc chloride, Aldrich 98%), and CdCl2 (cadmium chloride, Aldrich 99.99%), respectively, which were weighed in stoichiometric proportions at 300 K. These crystals were obtained by slow evaporating aqueous solutions containing of CH3CH2NH2 HCl and MCl2 in the molar ratio of 2:1.

The thermodynamic properties were measured by TGA (TA, Q600) and optical polarizing microscopy. The differential scanning calorimetry (DSC) and TGA data were recorded between 300 and 770 K under a N2 atmosphere using a heating rate of 10 °C/min.

The 1H MAS NMR and 13C CP/MAS NMR spectra for the rotating frame of (C2H5NH3)2MCl4 were measured at the Larmor frequencies of 400.13 and 100.61 MHz, respectively, using a Bruker 400 MHz Avance II+ NMR spectrometer (BRUKER, Germany) at the Korea Basic Science Institute, Western Seoul Center. The powder samples were placed in a 4 mm MAS probe, and the MAS rate was set at 10 kHz for the 1H MAS and 13C CP MAS measurements to minimize any overlap of the spinning sidebands with respect to the central peak. The chemical shifts are listed using tetramethylsilane (TMS) as an internal reference. The spin-lattice relaxation times T1ρ for the rotating frame of (C2H5NH3)2MCl4 were determined using a π/2−t sequence by variation of the spin-locking pulses. The NMR spectra and T1ρ values were recorded between 180 and 430 K.

4. Conclusions

We herein discussed the thermodynamic, physical, and structural properties of (C2H5NH3)2MCl4 (M = Co, Cu, Zn, and Cd) layered hybrid materials, where we replaced Pb with nontoxic M metals for the production of lead-free perovskite solar cells, and investigated their potential toward solar cell applications based on NMR studies.

The temperature of Td and the degree of mass loss for the decomposition of the 2HCl moieties were both found to depend on the M ion present in the structure. Furthermore, the cation dynamics in layered (C2H5NH3)2MCl4 single crystals were investigated as a function of temperature by 1H MAS NMR and 13C CP/MAS NMR experiments. To obtain detailed information regarding the cation dynamics of these crystals, the T1ρ values for both 1H and 13C were obtained, revealing that these atoms undergo rotational motion.

The reason why 1H T1ρ of C2H5 is longer than 1H T1ρ of NH3 is as follows; the rotational motion for C2H5 is activated, and that for NH3 at the end of the organic cation is less strongly activated. In addition, the reason why 13C T1ρ of CH2 is longer than 13C T1ρ of CH3 is as follows; the amplitude of the cation motion is enhanced at its CH3 end, and the central CH2 moiety is fixed to the NH3 group in the organic cation.

Overall, it was found that all components of this series exhibit an orthorhombic structure at room temperature. However, the lattice constants of the crystals containing Co2+ and Zn2+ ions differed from those of the crystals containing Cu2+ and Cd2+ ions. It was also found that the inorganic frames of the M = Cu2+ and Cd2+ species are corner-sharing MCl6 octahedra, while those of M = Co2+ and Zn2+ are simple MCl4 tetrahedra. Finally, it was concluded that the physical properties of these species depend on the characteristics of the organic cation and the inorganic metal ion, but are independent of the arrangements of the MCl4 tetrahedra and the MCl6 octahedra. The presence of different paramagnetic ions and different lattice constants may also account for these differences.