Ferroelectric Organic–Inorganic Hybrid Ammonium Halogenobismuthate(III) for Piezoelectric Energy Harvesting

Halogenobismuthate(III) compounds are of recent interest because of their low toxicity and distinct electrical properties. The utility of these materials as ferroelectrics for piezoelectric energy harvesters is still in its early stages. Herein, we report a hybrid ammonium halogenobismuthate(III) [BPBrDMA]2·[BiBr5], crystallizing in a ferroelectrically active polar noncentrosymmetric Pna21 space group. Its noncentrosymmetric structure was confirmed by the detection of the second harmonic generation response. The ferroelectric P–E hysteresis loop measurements on the thin film sample of [BPBrDMA]2·[BiBr5] gave a saturation polarization (Ps) of 5.72 μC cm–2. The piezoresponse force microscopy analysis confirmed its ferroelectric and piezoelectric nature, showing characteristic domain structures and signature hysteresis and butterfly loops. The piezoelectric energy harvesting attributes of [BPBrDMA]2·[BiBr5] were further probed on its polylactic acid (PLA) composites. The 15 wt % [BPBrDMA]2·[BiBr5]-PLA polymer composite resulted in a high output voltage of 26.2 V and power density of 15.47 μW cm–2. The energy harvested from this device was further utilized for charging a 10 μF capacitor within 3 min.

4-Bromo-N,N-dimethylaniline was purchased from Merck, benzyl bromide was procured from Lobachemie, India and bismuth(II) oxide and hydrobromic acid were acquired from Avra Chemicals, India and directly employed without further purification.Thermogravimetric analyses were performed on the PerkinElmer STA-6000 analyzer, with a heating rate of 10 °C/min, and the differential scanning calorimetry (DSC) measurements using a TA Q20 differential scanning calorimeter, with heating and cooling rates of 10 °C/min.Both these measurements were performed under a dry nitrogen atmosphere.Melting points without correction were recorded on a Buchi M-560 melting point apparatus.The FT-IR spectra were obtained using the Bruker (Invenio R), covering the range of 400-4000 cm -1 .Powder X-ray diffraction (PXRD) data, in the 2θ range of 5 to 50º, were collected using the Bruker-D8 Advance X-ray diffractometer.The field-emission scanning electron microscopy (FE-SEM) images of all composite films (with different weight percentages of [BPBrDMA]2•[BiBr5]) were recorded using the Zeiss ultra plus FE-SEM instrument with a minimum spatial resolution of 1 μm.

Single Crystal X-ray Diffraction Analysis:
The single-crystal X-ray diffraction data for compounds BPBrDMA•Br and [BPBrDMA]2•[BiBr5] were collected on a Bruker Smart Apex Duo diffractometer with Mo Kα radiation (λ=0.71073Å).The crystal structures were solved through the direct method and refined using full-matrix least-squares against F 2 , employing the SHELXL-2014/7 program integrated into the Apex 3 software. 1Anisotropic refinement was applied to all nonhydrogen atoms, while hydrogen atoms were constrained in geometric positions relative to their parent atoms. 2The data shows some amount of residual electron densities that could be attributed to the presence of solvated molecules.Owing to the diffuse nature of solvated atoms (in the vicinity of bulky Bi and Br atoms), they could not be modeled appropriately even at 120 K. Hence, these were treated as diffuse contributions to the overall scattering and removed by the SQUEEZE/PLATON method for better refinement data.The volumes squeezed in the structure were found to be 540 Å 3 , which approximately corresponds to 7 methanol molecules and 3 water molecules in the unit cell of [BPBrDMA]2•[BiBr5] as determined from thermogravimetric analysis (TGA).Structural illustrations were generated using the DIAMOND-3.1 software.

Hirshfeld Surface Analysis:
The Crystal Explorer 3.

Nonlinear Optical Measurements:
The Kurtz-Perry powder tests were conducted using a Coherent Astrella Ti: Sapphire regenerative amplifier (RA) to generate femtosecond laser pulses with a repetition rate of 1 kHz.These laser pulses were directed through a wavelength-tuneable Topaz Prime Vis-NIR optical parametric amplifier (OPA) to achieve the desired wavelength of 1300 nm.The laser beams, with a fluence of 0.20 mJ cm -2 at 1300 nm, were unfocused.The SHG relative efficiency of [BPBrDMA]2•[BiBr5] was determined through the Kurtz-Perry powder method, utilizing potassium dihydrogen phosphate (KDP) as the SHG reference.Microcrystals of [BPBrDMA]2•[BiBr5] and KDP were independently ground and sieved using an Aldrich mini-sieve set to obtain a microcrystal size fraction of 250−177 μm.The laser beam was directed at the samples at an angle of 45° degrees, and the diffused SHG spectra were recorded by an Ocean Optics Flame T spectrograph after suppressing the scattered pumping radiation with a 750 nm short-pass dielectric filter.

Ferroelectric, Dielectric and Piezoelectric Measurements:
To investigate the ferroelectric properties of [BPBrDMA]2•[BiBr5], P-E hysteresis loop measurements were conducted on a thin film sample of approximately 2.21 μm thickness drop-casted on an Indium tin oxide (ITO)coated glass surface, and GaIlium Indium eutectic was used to the make top contact.These measurements were performed using the aixACCT TF-2000E model hysteresis loop analyzer.The experiments were conducted by applying the dynamic leakage current compensation (DLCC) mode to reduce the contributions from non-hysteretic components of the loop.
The dielectric permittivity of [BPBrDMA]2•[BiBr5] was measured on its powder-pressed pellet sample.The measurements were conducted using the Solartron Analytical 1260 model Impedance Analyzer combined with a Dielectric Interface model 1296A.The sample was placed in a Janis 129610A cryostat sample holder, and the temperature was controlled using a Lakeshore 336 model temperature controller.

Piezoresponse Force Microscopy (PFM) Characterizations:
The PFM measurements were conducted using the Asylum Research MFP-3D atomic force microscopy (AFM) system for a drop-casted thin film of [BPBrDMA]2•[BiBr5] on ITO-coated glass surface.A contact mode AFM experiment was carried out, utilizing RMN-12PT300B cantilever probes with a spring constant of 1.12 N m -1 and a tip diameter of less than 8 nm to measure the piezoresponse of the crystal films.Vertical-PFM experiments were employed to acquire PFM data, with the bottom electrode being grounded, while an AC voltage was applied to the conductive AFM tip.PFM images were collected at the resonance frequency of 300 +/-20 kHz with an applied bias of 60 and 80 V.The measurements were performed using the dual AC resonance tracking (DART) mode of the PFM.The switching ability of the domains of the single crystal located on the thin film was recorded by the application of external DC bias of ±120 using the PFM tip in contact mode.

General Procedure for the Preparation of Polymer Composite Films and Devices:
To fabricate composite films of [BPBrDMA]2•[BiBr5], different proportions (5, 10, 15, and 20 wt%) of ferroelectric crystallites were dispersed in a nonpiezoelectric, biodegradable polylactic acid polymer (PLA) dissolved in chloroform (CHCl3).The solutions underwent mechanical stirring at 50 ˚C for 15 minutes, followed by vortex mixing for 15 minutes to ensure homogeneity.Subsequently, these solutions were poured onto a glass slide and left undisturbed to air-dry at room temperature for 8 hours.The resulting composite films were carefully peeled off from the glass slide, and copper conductive adhesive tapes were affixed to both sides to establish electrical contacts.
To complete the architecture, the devices were covered with Kapton tapes.A pristine PLA polymer film encapsulated with Kapton tape was also prepared and subjected to similar experimental conditions for comparative analysis.

Piezoelectric Energy Harvesting and Storage Measurements:
An in-house-designed periodic impact set-up was employed to conduct mechanical energy harvesting experiments, applying an impact force of 21 N.The Tektronix 2024 Mixed Signal Oscilloscope, featuring an input impedance of 1 MΩ, was utilized for measuring output voltages and currents.The test devices, with a thickness of approximately 0.5 mm and an active area of 300 mm 2 , were subjected to examination.In order to assess the energy storage capabilities of the devices during impact measurements, a 10 µF capacitor was connected to the device using a full-wave bridge four-diode circuit.

D-H•••A d(H•••A) Å d(D-A) Å <(DHA) ̊ Symmetry transformations to generate equivalent atoms
1 program was employed to perform Hirshfeld surface analysis on [BPBrDMA]2•[BiBr5].For Hirshfeld analysis, the single-crystal X-ray crystallographic information file (CIF) was utilized and the diverse interactions present in [BPBrDMA]2•[BiBr5] were visualized.The resulting 3D color mapping images illustrate the surface characteristics of the compound [BPBrDMA]2•[BiBr5]; red: intense interactions, blue: medium interactions, and white: weak interactions.A 2D fingerprint plot, essentially a histogram, was generated by compiling distances between atoms closest to the interior (di) and exterior (de) of the Hirshfeld surface.The plot featured various contours represented by blue and grey colors, offering valuable insights into the various molecular interactions present in the molecule.
Scheme S1.Schematic showing the synthesis of BPBrDMA•Br.

Figure S20 .
Figure S20.The output peak-to-peak current (IPP) profile as calculated from the voltage drop obtained by attaching a 1 MΩ resistor across the circuit for the [BPBrDMA]2•[BiBr5]-PLA composite devices.

Figure S22 .
Figure S22.The comparative peak (a) voltage drop and (b) current data for all the [BPBrDMA]2•[BiBr5]-PLA composite devices under various load resistances.

Table S4 .
Percentage interactions present in [