Layered double hydroxides (LDHs) with positively charged brucite-like layers and negatively charged interlayer anions are considered to be one of the critical nanoscale building blocks, showing promise for their potential applications in catalysts, biological sensors, and optical, electrical, and magnetic devices. However, one key component in modern devices we use daily that has received relatively little attention yet to date, is an energy conversion device, even though LDHs are theoretically predicted to be centrosymmetric structure. Here we report the first theoretical study, experimental observation of the flexoelectric nature in LDH nanosheets, and a high energy conversion.

Ferroelectrics strongly couple changes in polarization of a material with its deformation, which induces the useful electromechanical phenomena such as flexoelectric effect. Flexoelectricity is the generation of an electric field that is generated by a strain gradient.1-5It is just recently that the strain gradients couple to polarization in all dielectrics through flexoelectricity.6-8 Coupling between polarization and strain gradients was used to tune the symmetry and properties of the ferroelectric thin films.9-10Recently, there has been interest in flexoelectricity including epitaxial ferroelectric thin films because these material systems have a large strain gradients.11-14Flexoelectricity is large at the nanoscale because the size of the strain gradients is inversely proportional to the relaxation length such as the sample size.12
Zinc aluminum layered double hydroxides (LDHs) have received considerable attention as an emerging class of ionic lamellar structured materials possessing various compelling advantages such as their tunable composition, anion exchange ability, excellent adsorption capacity, good biocompatibility and stability for both fundamental research and potential technological applications.15-20Zn-Al:LDHs are structurally described as the stacking of positively charged layers intercalated with hydrated anions, which showed similar structure to the brucite one.21-23Not only hydrogen bonding but also electrostatic attraction between the positively charged layers ((Zn,Al)(OH)2+) and the interlayer anions ((CO3)2-) hold the layers together in LDHs. Based on the structural characteristics (Centrosymmetric structure, space group of Rm ) of the LDHs, if the growth characteristics of the LDHs can control the degree of freedom of (CO3)2- anions with respect to the positively charged ions, strain gradients by an applied force can induce the polarization within the LDHs, which resulted in the flexoelectricity. In the present study, output power signals by an applied force and bending were, for the first time, demonstrated via LDH nanosheets grown with perpendicular orientation to the AZO/Au/AZO/PET substrate. In order to confirm the energy conversion via authentic flexoelectricity of the LDH nanosheets, that via LDH thin films (with parallel orientation to the substrate) deposited via spin-coating was compared in the present study.