Chiral 1D Hybrid Metal Halides with Piezoelectric Energy Harvesting and Sensing Properties

Chiral hybrid metal halides have been widely studied in the field of photondetectors, chiral optics, and spintronics due to their great structural flexibility and suitable bandgaps. Inspired by such great progress made in the abovementioned fields, continuous efforts need to be paid to seek other interesting applications for chiral hybrid metal halides. Herein, the synthesis of a pair of one‐dimensional enantiomorphic hybrid metal halide piezoelectrics, R/SMPCdCl4(R/SMP = R/S‐2‐methylpiperazine), and their applications in piezoelectric energy harvesting and sensing are reported. Density functional theory calculations show that these chiral piezoeletrics possess low elastic properties and high piezoelectric constants (16.71, 8.39, and 7.35 pC N−1). In addition, devices made of RMPCdCl4/PDMS (PDMS = polydimethylsiloxane) composite films are fabricated for piezoelectric energy harvesting and sensing. The piezoelectric energy harvesters with optimized content of 15 wt% RMPCdCl4/PDMS show not only excellent performance with an open‐circuit voltage of 2.57 V, short‐circuit current of 0.37 μA, and power density of 0.55 μW cm−2 but also outstanding stability of more than 3500 cycles. Meanwhile, these piezoelectric energy harvesters exhibit prominent sensing properties for detecting tapping, finger bending, walking, and running. It is demonstrated that chiral hybrid metal halides hold great potential in intelligent wearable and sensing devices.

In this work, we report the structures, elastic properties, and piezoelectricity of chiral hybrid metal halides, R/SMPCdCl 4 (R/SMP = R-/S-2-methylpiperazine), via the experimental methods and first-principles calculations.Meanwhile, R/SMPCdCl 4 / PDMS composite films with different contents were fabricated and explored their applications in piezoelectric energy harvesting and human motion sensing.[35] The different surface colors of red, white, and blue represent the different van der Waals distances which are shorter, equal to, and longer, respectively.As shown in the insert of Figure 1b,d, the Hirshfeld surfaces of R/SMP 2þ both showed bright red areas in the -NH 2 groups suggesting the strong intermolecular interactions between the organic cations and inorganic chains.As for the 2D fingerprints, the d i and d e represent the distances between the organic cation and the surrounding atoms inside and outside of the Hirshfeld surface, respectively.One striking feature for both crystals is that the 2D fingerprints show a sharp peak (d i , d e ) in the bottom left with (0.76, 1.35) and (0.77, 1.36) for R/SMPCdCl 4 , respectively.From these peaks of the 2D fingerprints (d i < d e ), it can be found that organic cations play the role of the strong hydrogen bond donor.Further, the sum (d i þ d e ) is relatively small suggesting the strong hydrogen bond between the chains and the organic cations.Powder X-ray diffractions (PXRD) of RMPCdCl 4 were conducted as shown in Figure S3a, Supporting Information, and the experimental pattern is well matched with the simulated one to verify the crystal powder purity.Further thermogravimetric (TG) (Figure S3b, Supporting Information) experiments of RMPCdCl 4 show that the compound does not decompose until 245 °C.Both Raman and infra-red spectroscopy (Figure S4, Supporting Information) are identical, which indicates their enantiomeric crystal structures.

Results and Discussion
Further, UV-visible absorptions of two crystals were measured at room temperature.The UV-visible absorptions of crystal powder were measured at room temperature to investigate the optical bandgaps.As shown in Figure 1e, both crystals show a clear absorption edge starting at around 250 nm.Then the optical bandgaps of crystals are determined as 4.96 eV.To verify the chirality of these enantiomers, circular dichroism (CD) spectroscopy was recorded using drop-casted thin films in the range of 200-400 nm as shown in Figure 1f.Due to the Cotton effect, R/SMPCdCl 4 films show typical chiral nature in which the opposite CD signals are observed at the same wavelength (typically at 215 nm).This result indicates that the chirality of the organic cations is transferred to the coordination chains which is widely reported in previous chiral hybrids.

Elastic Properties of R/SMPCdCl 4
The elastic properties are of great significance for chiral piezoelectrics since they can not only reflect the mechanical properties when the devices are working but also determine the intrinsic piezoelectric performance.The elastic stiffness constants (C ij ) are first obtained through first-principles calculations [36] and listed in Table S2 and S3 S4 and S5, Supporting Information.Taking an example of G of RMPCdCl 4 , the threedimensional representation shows that the G max is 11.81 GPa along (001) <010> direction and the G min is 12.20 GPa along (À110) <110> direction.It could be easily rationalized from the crystal packing in Figure S6, Supporting Information.In the (001) <010> direction, the inorganic chains are linked via two line-sharing connections resulting in a more stable structure and fewer shear strains along this direction in Figure S6a, Supporting Information.On the contrary, the inorganic chains are more flexible and thus resulting in greater shear strains along (À110) <110> direction in Figure S6b, Supporting Information.Meanwhile, the anisotropy of G (G max /G min ) is 2.40, which is much smaller than those of many reported hybrid materials. [22,37,38]Young's moduli also show smaller values and anisotropies compared with the typical perovskite oxides.
Based on the elastic constant, we further obtained the piezoelectric constants of R/SMPCdCl 4 .Like the typical piezoelectric crystal, Rochelle salt, [39,40] R/SMPCdCl 4 belongs to the  S6, Supporting Information.As for RMPCdCl 4 , values of d 14 , d 25 , and d 36 are 16.71, 8.39, and 7.35 pC N À1 , respectively.Such values are comparable with those of crystal materials. [18,41,42]From the above-mentioned equation of piezoelectric constants, it can be concluded that to improve the natural piezoelectricity is to increase the polarization and decrease the crystal stiffness.So, these high shear piezoelectric constants might be attributed to the small elastic stiffness constants and the overall low Young's and shear moduli.

Characterizations of RMPCdCl 4 /PDMS Composite Films
Considering the soft nature and the good piezoelectric activity, R/SMPCdCl 4 might be a potential candidate for the flexible wearing self-power device.To investigate the piezoelectric energy harvesting and sensing properties and optimize the components, a series of composite films of RMPCdCl 4 /PDMS with a weight ratio (wt%) of crystal powder from 5, 10, 15, to 20 wt% were prepared.As shown in Figure 3a, the homogeneous white composite films of 15 wt% with 2 Â 2 cm 2 can be easily folded with tweezers and display good flexibility for practical applications.Moreover, PXRD patterns of different content films (Figure 3b and S8, Supporting Information) show that the structure of the crystal is well preserved with the remaining peaks at 8.82°/(011) face, 20.26°/(112) face, and 21.10°/(121) face.As the content increase, the other diffraction peaks are also shown indicating the gradually improved crystallinity.In detail for 15 wt% content, the PXRD pattern is well matched with the simulated pattern indicating that all the crystal particles are randomly and evenly distributed in the composite film.Furthermore, the SEM image of 15 wt% content film (Figure 3c) shows the uniform morphology of the cross-section with a thickness of approximately 200 μm.The EDS mapping of Si, Cd, and Cl, clearly shows that the crystal particles are distributed evenly in the PDMS matrix.The uniform distribution of crystal particles is also found in the polarizing photos in Figure S9, Supporting Information.The lamellar crystals of RMPCdCl 4 are uniformly dispersed in the PDMS substrate.As the content of crystal particles increases, the size of crystal particles is also getting bigger as expected.The statistical particle dimeters are varied from 9.63 to 12.72 μm as the content of RMPCdCl 4 is increasing from 5 to 20 wt%.Such particle aggregation effect is also seen in the  The R/SMPCdCl 4 /PDMS composite films are further utilized to fabricate the piezoelectric energy harvester with a sandwich structure as shown in Figure 3a.As for the practical piezoelectric applications, the dielectric property is one of the key factors.The relative permittivity (ϵ r ) and quality factor (Q ) were collected to investigate the impendence properties of 15 wt% RMPCdCl 4 piezoelectric energy harvester (Figure 3d).Based on the frequencydependent capacitance (Figure S12, Supporting Information), the relative permittivity is recorded and shows a slight fall as the frequency increases in the range of 1 k-1.2MHz with a range from 1.62 to 1.71.Such lower relative permittivity of RMPCdCl 4 piezoelectric energy harvester compared with the pure compound (Figure S13, Supporting Information) and the traditional perovskite ceramics would promote the piezoelectric response according to the relation of g = d/ϵ r , where g is the piezoelectric voltage constant, ϵ r is the relative permittivity, and d is the piezoelectric constant.Further, RMPCdCl 4 piezoelectric energy harvester shows a relatively small Q factor of 74.4, which might indicate the potential applications in sensors and detectors.The relative permittivity (ϵ r ) and quality factor (Q ) of 15 wt% SMPCdCl 4 PEG are also obtained in Figure S14, Supporting Information.Furthermore, the impedances of RMPCdCl 4 crystals and composite films were compared in Figure S15, Supporting Information.The composite films show a smaller impedance than RMPCdCl 4 , which suggests that the piezoelectric devices would produce a greater power density that RMPCdCl 4 pellets.

Piezoelectric Energy Harvesting and Human Motion Sensing
To investigate the piezoelectric performance of R/SMPCdCl 4 , the as-prepared piezoelectric energy harvester is tested in a homemade piezoelectric test system with the periodic mechanical tapping of 10 Hz.Upon applying the pressing force of 2 N to the piezoelectric energy harvester, an electric dipole could be generated, resulting in piezoelectric charges flowing to the electrodes, and the open-circuit piezoelectric voltages (V oc ) of different content are shown in Figure 4a.As the content increases from 5 to 15 wt%, the values of V oc are also increasing from 0.97, 1.17, to 2.57 V.The output performance of V oc is more outstanding than that of chiral molecular crystals [41] and chiral lead halide perovskites.It suggests that the piezoelectric outperformance of the composite films is positively dependent on the active piezoelectric component content among the relatively low content and the optimized content for the RMPCdCl 4 composite films is 15 wt%.Within the content range, the piezoelectric active components play a dominant role in improving the piezoelectric energy harvester performance.However, with the further increase of the content up to 20 wt%, the performance of V oc conversely decreases to 0.91 V.Such deteriorated performance might arise from the excessive aggregation of active particles as shown in the SEM and polarizing images.The significant increase of the particle size in the 20 wt% RMPCdCl 4 /PDMS films inevitably results in enhanced dielectric constants and a Maxwell-Wagner-Sillars polarization so that the high content of the piezoelectric particle would deteriorate rather than improve the piezoelectric response.The same trend is observed in the short-circuit current (I sc ) in Figure 4d that the piezoelectric energy harvester of optimized 15 wt% content shows a better performance of 0.37 μA.Based on the output performance of different contents, piezoelectric energy harvester with 15 wt% RMPCdCl 4 films is selected to conduct further piezoelectric tests.
To confirm the piezoelectric effect, the outputs of V oc (Figure 4b) and I sc (Figure 4e) were recorded when the electrodes were connected forward and reversed.Taking an example of Figure 4b, V oc shows typically positive values when the electrodes were connected forward while it shows almost identical but negative values when the electrode was connected reversed.The flipping of outputs verifies these signals truly come from the piezoelectric effect rather than the triboelectric effect. [26,30]urther the output signals were collected via applying different external forces from 1 to 5 N as shown in Figure 4c,f.With the increasing loading forces, the V oc values reached from 2.23 to 3.55 V, and the I sc values reached from 0.20 to 0.38 μA showing a positive proportional trend.As for SMPCdCl 4 piezoelectric energy harvester, it also shows the same optimized content of 15 wt% with the output performance of 1.81 V in Figure S16a, Supporting Information.The electrode flipping test shows good reversibility for 5 wt% under the compressing force of 2 N in Figure S16b, Supporting Information.The output performance is also increasing up to 1.55 V with the increasing forces for 5 wt% piezoelectric energy harvesters in Figure S16c, Supporting Information.The output performance of the piezoelectric energy harvester was tested in dependence on frequency.As shown in Figure S17, Supporting Information, the output voltages increase with increasing frequency because the piezoelectric energy harvester can produce more charge per unit of time with increasing frequency.The potential utility of 15 wt% RMPCdCl 4 /PDMS device was demonstrated by storing the harvested energy in 1 μF capacitors (Figure S18, Supporting Information).As shown in the charging curve, the charges increase gradually and reach 0.35 μC after 50 s.
To obtain the power density of RMPCdCl 4 piezoelectric energy harvester, the various voltages are collected with the different external load resistances in Figure 4g.With the external loading resistances increase, the output voltage is also gradually increased from 0.45 to 1.41 V.According to the power density (P) equation of P = U 2 /R, where U is the output voltage and R is the external loading resistance, the different power densities were calculated in Figure 4g.The value of the power density reaches a maximum of 0.55 μW cm À2 at an external loading resistance of 2 MΩ which is comparable with that of pure PVDF films (Table S7, Supporting Information).Stability is another key parameter for practical applications.To check the mechanical and time stabilities of the 15 wt% RMPCdCl 4 piezoelectric energy harvester, the long-term output performance test is conducted under the continuous 2 N force at 10 Hz in Figure 4h.As the figure shows, piezoelectric energy harvester does not show any performance degradations throughout the 350 s testing, more than 3500 cycles.It suggests that RMPCdCl 4 piezoelectric energy harvester can guarantee stable output performance in working conditions.Subsequently, 15 wt% RMPCdCl 4 piezoelectric energy harvester was utilized as the sensor to detect human motions (tapping, finger bending, walking, and running).As shown in Figure 5a, the slight tapping (approximately 0.1 N) can provoke the device to generate 0.5 V output with sharp peaks suggesting good sensitivity.Due to the excellent flexibility of RMPCdCl 4 piezoelectric energy harvester, the device was strapped to the knuckle to detect the finger bending and releasing as shown in Figure 5b.RMPCdCl 4 piezoelectric energy harvester produces a positive voltage (about 0.2 V) with the finger bending while when the bending is releasing, it generates an identical but negative voltage.Because of the relatively slow motion compared with the tapping, the peaks of the finger bending and releasing are gentle and symmetrical.Furthermore, our piezoelectric energy harvester is equipped on the shoes to detect walking and running as shown in Figure 5c.The harvester can produce a positive voltage when the footstep falls while a relatively small negative voltage when the footstep is up.Compared to the voltages of gentle walking (1.3 V), running can produce a greater voltage (2.4 V).From the cycle numbers of output voltages, we can obtain the stride frequency per second of walking (1.2) and running (2.6).The abovementioned results imply that RMPCdCl 4 piezoelectric energy harvester has great potential in precise human motion sensors and other intelligent wearable devices.

Conclusion
In summary, two homochiral crystals, R/SMPCdCl 4, and the corresponding racemic crystal have been successfully synthesized by introducing the homochiral and racemic organic cations.Density functional theory (DFT) calculations show that R/SMPCdCl 4 would have low elastic moduli and attractive piezoelectricity which makes it suitable for piezoelectric applications.Furthermore, the optimized content, 15 wt% RMPCdCl 4 / PDMS films are utilized in piezoelectric energy harvester and show attractive energy harvesting performance with maximum V oc of 2.57 V, I sc of 0.37 μA, and power density of 0.55 μW cm À2 under an external force of 2 N.Meanwhile, the homochiral composite piezoelectric energy harvester exhibits great sensing property for detecting human sensing (tapping, finger bending, walking, and running) so it holds promise in intelligent wear devices.Our work develops the homochiral hybrid crystals and implies great potential applications in piezoelectric applications.

Experimental Section
Crystal Synthesis: All the chemicals and solvents used in this work were purchased from commercial vendors and used directly without further purification.Single crystals were synthesized via a similar method by mixing the stoichiometric amounts of CdCl 2 and the corresponding R-/S-/Rac-2-methylpiperazine in HCl/H 2 O solution (1 mL:1 mL) in a bottle.All the precipitates were dissolved by an ultrasonic oscillator to obtain a clear solution.After the solvent evaporation for two weeks, colorless transparent rod-shaped crystals were obtained in the bottom of a bottle.Single Crystal Data Collection and Refinement: Single-crystal X-ray diffraction of R/SMPCdCl 4 was conducted on a Rigaku XtaLAB MM007 CCD diffractometer (Cu Ka λ = 1.5418Å).Structural solving using the ShelXT program and data refinement using the ShelXL Least Squares program were performed on the Olex2 software packages. [43,44]All nonhydrogen atoms were refined anisotropically, whereas the hydrogen atoms were located and refined geometrically.The X-ray crystallographic structures of RMPCdCl 4 , SMPCdCl 4 , and RacMPCd 2 Cl 6 •H 2 O have been deposited at the Cambridge Crystallographic Data Centre with the CCDC numbers of 2 208 000, 2 211 550, and 2 211 519, respectively.The crystal data and structure refinement parameters are summarized in Table S1, Supporting Information.
General Measurements: The circular dichroism (CD) spectra were acquired using a CD spectrometer (BioLogic, MOS-450).The solid-state ultraviolent-visible (UV-Vis) diffuse reflectance spectra were obtained on Thermo Fisher, Evolution 220.The SEM and EDS mapping were investigated by a field-emission scanning electron microscope, JSM-7800F.The powders and composite films X-ray diffractions were conducted on a Rigaku MiniFlex 600 diffractometer.The TG/DTA curves were obtained on a Rigaku standard TG-DTA analyzer from room temperature to 800 °C under an air atmosphere with a heating rate of 10 °C min À1 using an empty and clean Al 2 O 3 crucible as a reference.The impedance and conductivity were measured with a precision impedance analyzer (Wayne Kerr, 6500B).The polarizing photos were recorded on a Nikon LV100NPOL microscope.
R/SMPCdCl 4 /PDMS Composite Film Preparations: 5, 10, 15, and 20 wt% (125, 250, 375, and 500 mg) well-grinded R/SMPCdCl 4 powders were added into 2.5 g PDMS precursor.Then 15 mL dichloromethane (CH 2 Cl 2 ) was added into the precursor, and the mixture was stirred for 4 h to obtain a homogeneous solution.When CH 2 Cl 2 was almost evaporating out, 10 wt% (250 mg) hardeners were added to the viscous liquid and stirred for pre-cure.The mixture was placed in a vacuum oven to remove the remaining CH 2 Cl 2 and the bubbles in the precursor.Finally, the precursor was spread on the substrate to get a uniform Piezoelectric Energy Harvester Fabrication and Test: The obtained films were cut into 2 cm Â 2 cm squares and acting as the piezoelectric layers.The copper tapes were acting a role of a collector on both sides of the composite films.Finally, the entire device was encapsulated with PET tape for piezoelectric testing.The homemade piezoelectric output testing system was described in our previous work. [45]he capacitances, the impedances, and Q factors of composite films and the pure compound are obtained from a precision impedance analyzer (Wayne Kerr, 6500B).The relative permittivity is further obtained from the capacitances according to the equation: ε r = Cd/ε 0 •S, where C is the capacitances, d is the thickness, and S is the area.
The Statement for Human Subjects: The author who participated in the human motion sensing tests, has given informed signed consent.
48] The energy cutoff was set to 500 eV, and the k-point mesh spanning scheme of Monkhorst-Pack was 4 Â 2 Â 1. Perdew-Burke-Ernzerh of generalized gradient approximation was used as the exchange-correlation energy function to optimize the atomic positions and lattice parameters until the total energy converged to within 10 À7 eV and the residual forces on each atom were less than 0.01 eV Å À1 . [49]The elastic stiffness constants C ij were obtained by the finite-strain method with 0.015 Å of the maximum strain amplitude and 6 steps for each strain. [50,51]54]

2. 1 .
Crystal Structures of R/SMPCdCl 4 Crystals of R/SMPCdCl 4 both crystalize in an orthorhombic system, a chiral space group P2 1 2 1 2 1 .The cell parameters of R/SMPCdCl 4 are a = 5.83/5.85,b = 13.24/13.35,and c = 15.32/15.38Å, respectively.As shown in Figure 1a,c, both R/SMPCdCl 4 exhibit a typical one-dimensional packing structure in which [CdCl 4 ] n 2À chains are propagating along a crystal axis.Due to the induction of enantiomers, the crystal structures of R/SMPCdCl 4 show a mirror-imaged relationship.In detail, each Cd 2þ is coordinated with six Cl À anions to form a [CdCl 6 ] 4À octahedron and further these octahedrons are linked with each other via two line-sharing connections to construct a wavy one-dimensional chain, [CdCl 4 ] n 2À as shown in Figure S1, Supporting Information.These organic anions are located around the inorganic chains via electrostatic force and N-H•••Cl hydrogen bonds (2.18-2.26Å) to further connect these chains.As for RacMPCd 2 Cl 6 •H 2 O in Figure S2, Supporting Information, it crystallizes in a monoclinic system, a centrosymmetric space group P2 1 /n with the cell parameters a = 12.13, b = 6.77, c = 18.50 Å, β = 94.09°.The octahedrons are linked via three line-sharing connections to build the inorganic chains.

Figure 1 .
Figure 1.Crystal structures of a) RMPCdCl 4 and c) SMPCdCl 4 along the a-axis.Note that the extra hydrogen atoms except these of N-H•••Cl hydrogen bonds are omitted for clarity.The 2D fingerprints of b) RMPCdCl 4 and d) SMPCdCl 4 .The inserts are the Hirshfeld surfaces of the organic cations.e) UV-visible absorptions and f ) CD signals of R/SMPCdCl 4 .
, Supporting Information, and showing almost identical values for the R/SMPCdCl 4 .Based on C ij , shear modulus (G), and Young's modulus (E) are depicted in Figure 2 and S5, Supporting Information, and the maximum and minimum values of various moduli and the corresponding directions are extracted and listed in Tables

Figure 3 .
Figure 3.The phase characteristics of the as-prepared RMPCdCl 4 /PDMS films.a) The optical image of the original composite film and the folded films with tweezers (top).The schematic representation of the piezoelectric device with a sandwich structure and the real device for the later experiments (bottom).b) The PXRD patterns of the RMPCdCl 4 /PDMS films with different contents.c) The cross-sectional SEM and EDS mapping images of 15 wt% RMPCdCl 4 /PDMS composite films.Note that the insert bar in SEM represents 100 μm.d) The relative permittivity and quality factor of 15 wt% RMPCdCl 4 piezoelectric energy harvester.

SMPCdCl 4
films in FigureS9and S10, Supporting Information.As shown in FigureS11, Supporting Information, 20 wt% RMPCdCl 4 /PDMS films show larger grain sizes which could be attributed to the agglomeration of the piezoelectric particles.

Figure 4 .
Figure 4.The piezoelectric output performance of RMPCdCl 4 piezoelectric energy harvesters.The piezoelectric energy harvester output performance of a) V oc and d) I sc with different contents of RMPCdCl 4 .The output signals flipping test of b) V oc and e) I sc The applying force-dependent c) V oc and f ) I sc performances with 15 wt% RMPCdCl 4 .g) The resistance-dependent voltages and power densities.h) The long-term output performance with 350 s, 3500 cycles under 2 N force of 10 Hz.

Figure 5 .
Figure 5. Human motion sensing tests.a) Tapping sensing and b) finger bending sensing of RMPCdCl 4 piezoelectric energy harvester.c) Human motion, walking, and running detection experiments.