Field-Induced Antiferroelectric-Ferroelectric Transformation in Organometallic Perovskite Displaying Giant Negative Electrocaloric Effect.

Antiferroelectric materials with an electrocaloric effect (ECE) have been developed as promising candidates for solid-state refrigeration. Despite the great advances in positive ECE, reports on negative ECE remain quite scarce because of its elusive physical mechanism. Here, a giant negative ECE (maximum ΔS ∼ -33.3 J kg-1 K-1 with ΔT ∼ -11.7 K) is demonstrated near room temperature in organometallic perovskite, iBA2EA2Pb3I10 (1, where iBA = isobutylammonium and EA = ethylammonium), which is comparable to the greatest ECE effects reported so far. Moreover, the ECE efficiency ΔS/ΔE (∼1.85 J cm kg-1 K-1 kV-1) and ΔT/ΔE (∼0.65 K cm kV-1) are almost 2 orders of magnitude higher than those of classical inorganic ceramic ferroelectrics and organic polymers, such as BaTiO3, SrBi2Ta2O9, Hf1/2Zr1/2O2, and P(VDF-TrFE). As far as we know, this is the first report on negative ECE in organometallic hybrid perovskite ferroelectric. Our experimental measurement combined with the first-principles calculations reveals that electric field-induced antipolar to polar structural transformation results in a large change in dipolar ordering (from 6.5 to 45 µC/cm2 under the ΔE of 18 kV/cm) that is closely related to the entropy change, which plays a key role in generating such giant negative ECE. This discovery of field-induced negative ECE is unprecedented in organometallic perovskite, which sheds light on the exploration of next-generation refrigeration devices with high cooling efficiency.

Acquiring High-TC Layered Metal Halide Ferroelectrics via Cage-Confined Ethylamine Rotators.

Two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) ferroelectrics have attracted widespread interest in the field of optoelectronics due to the combination of excellent semiconducting and ferroelectric properties. The Curie temperature (TC ), below which ferroelectricity exists, is a crucial parameter for ferroelectrics. However, the lack of research on TC tuning of 2D OIHP ferroelectrics hinders their further progress. Here, through incorporating ethylammonium (EA) as cage-confined rotators, we obtained two 2D OIHP ferroelectrics, (IBA)2 (EA)Pb2 Br7 (2L; IBA=isobutylammonium), and (IBA)2 (EA)2 Pb3 Br10 (3L). Intriguingly, TC is successfully tuned from 326 K (2L) to 370 K (3L) with increasing layer thickness. Structural and computational analyses suggest that the improvement of TC is due to the higher phase-transition energy barrier triggered by the cage-confined EA rotators with increased layer thickness. This work suggests that EA is an effective "cage-confined rotator" to rationally design high-TC 2D OIHP ferroelectrics.

High-density electron transfer in Ni-metal-organic framework@FeNi-layered double hydroxide for efficient electrocatalytic oxygen evolution.

The electrochemical oxygen evolution reaction is a bottleneck reaction in hydrolysis and electrolysis because the four-step electron transfer leads to slow reaction kinetics and large overpotentials. This situation can be improved by fast charge transfer by optimizing the interfacial electronic structure and enhancing polarization. Herein, a unique metal (Ni) organic (diphenylalanine, DPA) framework Ni(DPA)2 (Ni-MOF) with tunable polarization is designed to bond with FeNi-LDH (layered double hydroxides) nanoflakes. The Ni-MOF@FeNi-LDH heterostructure delivers excellent oxygen evolution performance exemplified by an ultralow overpotential of 198 mV at 100 mA cm-2 compared to other (FeNi-LDH)-based catalysts. Experiments and theoretical calculations show that FeNi-LDH exists in an electron-rich state in Ni-MOF@FeNi-LDH due to polarization enhancement caused by interfacial bonding with Ni-MOF. This effectively changes the local electronic structure of the metal Fe/Ni active sites and optimizes adsorption of the oxygen-containing intermediates. Polarization and electron transfer of Ni-MOF are further enhanced by magnetoelectric coupling consequently giving rise to better electrocatalytic properties as a result of high-density electron transfer to active sites. These findings reveal a promising interface and polarization modulation strategy to improve electrocatalysis.

Accelerated Discovery of Targeted Environmentally Friendly A(II)B(I)X3-Type Three-Dimensional Hybrid Organic-Inorganic Perovskites for Potential Light Harvesting via Machine Learning.

The engineered hybrid organic-inorganic perovskites (HOIPs) with outstanding multifunctionalities have realized overarching targeted-driven applications and thus aroused intense research interest. The emergence of three-dimensional (3D) A(II)B(I)X3-type HOIPs in 2018 brought a breakthrough to extend the 3D perovskite family and successfully realized prominent ferroelectricity at the same time. Here, we focus on these new-type HOIPs to perform machine-learning (ML)-based molecular design to screen promising candidates for versatile light harvesting, involving photovoltaics (77 ones), water splitting (216 ones), and photodetection (178 ones), out of 3180 A(II)B(I)X3 perovskites in total. These candidates await future experimental synthesis and characterization. Our high-throughput ML-based screening of 3D A(II)B(I)X3 HOIPs would enrich the material inventory by successfully introducing a class of new 3D HOIPs to realize property-oriented light harvesting and additional versatile energy harvesting due to their potential multifunctionalities such as ferroelectricity and electrocaloricity.

Circular polarized light-dependent anomalous photovoltaic effect from achiral hybrid perovskites.

Circular polarized light-dependent anomalous bulk photovoltaic effect - a steady anomalous photovoltaic current can be manipulated by changing the light helicity, is an increasingly interesting topic in contexts ranging from physics to chemistry. Herein, circular polarized light-dependent anomalous bulk photovoltaic effect is presented in achiral hybrid perovskites, (4-AMP)BiI5 (ABI, 4-AMP is 4-(aminomethyl)piperidinium), breaking conventional realization that it can only happen in chiral species. Achiral hybrid perovskite ABI crystallizes in chiroptical-active asymmetric point group m (Cs), showing an anomalous bulk photovoltaic effect with giant photovoltage of 25 V, as well as strong circular polarized light - sensitive properties. Significantly, conspicuous circular polarized light-dependent anomalous bulk photovoltaic effect is reflected in the large degree of dependence of anomalous bulk photovoltaic effect on left-and right-CPL helicity, which is associated with left and right-handed screw optical axes of ABI. Such degree of dependence is demonstrated by a large asymmetry factor of 0.24, which almost falls around the highest value of hybrid perovskites. These unprecedented results may provide a perspective to develop opto-spintronic functionalities in hybrid perovskites.

Self-assembled rhomboidal ammonia monolayer confined in two vertically stacked graphene oxide/graphene nanosheets.

Confined water molecules have attracted widespread research interest due to their versatile phase behaviors. Ammonia (NH3, isoelectronic with water) molecules are also expected to realize the delicate self-assembled hydrogen-bonded network like water in confinement. Here, the structures and phase behavior of NH3 monolayers confined in two structurally symmetrical graphene oxide (GO) or graphene (G) nanosheets are investigated using first-principles calculations and ab initio molecular dynamics simulations. A highly ordered new rhomboidal phase with all NH3 molecules adopting a Y-shaped configuration, in which one N-H bond is parallel to the confining planes and two other N-H bonds point to the top/bottom GO/G layers, respectively, was discovered at low temperature, resulting from the symmetrical confinement and subtle interlayer/intermolecular interactions. Remarkably, this new phase is so stable that a quite large strain is needed to destroy it. At room temperature, these NH3 monolayers behave like a liquid. These rhomboidal NH3 monolayers confined in GO/G nanosheets not only offer diverse hydrogen-bonded networks but also possess potential piezoelectricity for future device applications.

Molecular Design of Three-Dimensional Metal-Free A(NH4)X3 Perovskites for Photovoltaic Applications.

The intense research activities on the hybrid organic-inorganic perovskites (HOIPs) have led to the greatly improved light absorbers for solar cells with high power conversion efficiency (PCE). However, it is still challenging to find an alternative lead-free perovskite to replace the organohalide lead perovskites to achieve high PCE. This is because both previous experimental and theoretical investigations have shown that the Pb2+ cations play a dominating role in contributing the desirable frontier electronic bands of the HOIPs for light absorbing. Recent advances in the chemical synthesis of three-dimensional (3D) metal-free perovskites, by replacing Pb2+ with NH4 +, have markedly enriched the family of multifunctionalized perovskites (Ye et al., Science2018, 361, 151-155). These metal-free perovskites possess the chemical formula of A(NH4)X3, where A is divalent organic cations and X denotes halogen atoms. Without involving transition-metal cations, the metal-free A(NH4)X3 perovskites can entail notably different frontier electronic band features from those of the organohalide lead perovskites. Indeed, the valence and conduction bands of A(NH4)X3 perovskites are mainly attributed by the halogen atoms and the divalent A2+ organic cations, respectively. Importantly, a linear relationship between the bandgaps of A(NH4)X3 perovskites and the lowest unoccupied molecular orbital energies of the A2+ cations is identified, suggesting that bandgaps can be tailored via molecular design, especially through a chemical modification of the A2+ cations. Our comprehensive computational study and molecular design predict a metal-free perovskite, namely, 6-ammonio-1-methyl-5-nitropyrimidin-1-ium-(NH4)I3, with a desirable bandgap of ∼1.74 eV and good optical absorption property, both being important requirements for photovoltaic applications. Moreover, the application of strain can further fine-tune the bandgap of this metal-free perovskite. Our proposed design principle not only offers chemical insights into the structure-property relationship of the multifunctional metal-free perovskites but also can facilitate the discovery of highly efficient alternative, lead-free perovskites for potential photovoltaic or optoelectronic applications.