Magnetoelectricity-Mediated Tunable Absorption and Release of Peroxide Dianions.

Peroxide dianion (O22-) has strong oxidizing activity and ease of proton abstraction and is extremely unstable. Direct and controllable adsorption and release of O22- has large application implication and is a large challenge so far. Here, we use a unique metal (Ni)-organic (diphenylalanine, DPA) framework (MOF), Ni(DPA)2, as adsorbents for absorption and release of O22-. This MOF structure has room-temperature magnetoelectricity via distortion of the Ni-centered octahedron {NiN2O4} and thus possesses a tunable ferroelectric polarization under applied electric/magnetic fields. Controllable adsorption and release of O22- are realized in such a MOF system via electrochemical redox measurements. Structural/spectroscopic characterization and calculations reveal that a number of NH active sites in the nanopores of MOF can effectively adsorb O22- by hydrogen bonds and then tunable ferroelectric polarization induces controllable release of O22- under applied magnetic fields. This work presents a constructive way for controllable adsorption and release of reactive oxygen species.

Light-Induced Tunable Ferroelectric Polarization in Dipole-Embedded Metal-Organic Framework.

Reversible regulation of ferroelectric polarization possesses great potentials recently in bionic neural networks. Photoinduced cis-trans isomers have changeable dipole moments, but they cannot be directed to some specific orientation. Here, we construct a host-guest composite structure which consists of a porous ferroelectric metal (Ni)-organic framework [Ni(DPA)2] as host and photoisomer, azobenzene (AZB), as guest molecules. When AZB molecules are embedded in the nanopores of Ni(DPA)2 in the form of a single molecule, polarization strength tunable regulation is realized after ultraviolet irradiation of 365 and 405 nm via cis-trans isomerism transformation of AZB. An intrinsic built-in field originating from the distorted {NiN2O4} octahedra in Ni(DPA)2 directs the dipole moments of AZB to the applied electric field. As a result, the overlapped ferroelectric polarization strength changes with content of cis-AZB after ultraviolet and visible irradiation. Such a connection of ferroelectric Ni(DPA)2 structure with cis-trans isomers provides an important strategy for regulating the ferroelectric polarization strength.

Polarization-induced efficient charge separation in an electromagnetic coupling MOF for enhancing CO2 photocatalytic reduction.

Rapid recombination of photogenerated carriers severely impairs the performance of photocatalysts, while polarization is an effective driving force for increasing the charge separation and hence improving the photocatalytic activity. In this work, a series of magnetoelectric-coupled layered metal-organic framework (MOF) catalysts with different Co-doped contents (denoted as Ni-MOF and CoxNi1-x-MOF) are fabricated with different polarities and employed as novel photocatalysts for CO2 photocatalytic reduction reaction. Our experiments show that the highest charge separation efficiency occurs in the Co0.1Ni0.9-MOF sample which has a maximal polarization. This Co0.1Ni0.9-MOF material has a best CO2 reduction performance of 38.74 µmol g-1h-1 which is at a high level in the currently reported layered materials. Meanwhile, it is found that a series of CoxNi1-x-MOF samples all display selectivity close to 100% for CO2 reduction to CO, which is desirable for industrial applications. Theoretical analysis shows that Co doping alters the degree of distortion of the asymmetrical Ni-centered octahedron {NiN2O4} in Ni-MOF by replacing Ni due to the magnetoelectric coupling effect and Jahn-Teller effect, which results in adjustable polarity of CoxNi1-x-MOF. This work provides new insights on how to improve photogenerated charge separation in MOF by enhancing polarization.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity.

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN2O4} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field µ0H from 0 to 2 T decreases linearly from 0.041 to 0.011 µC/cm2 at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Selective and high-sensitive label-free detection of ascorbic acid by carbon nitride quantum dots with intense fluorescence from lone pair states.

Label-free detection of ascorbic acid (AA) with high sensitivity and specificity based on the effects of AA on fluorescence quenching of carbon nitride quantum dots (CNQDs) is described. The CNQDs with a high fluorescence quantum yield of 21% and good dispersibility in water are prepared by reacting g-C3N4 sheets with ethylenediamine (EN). Fluorescence at 510 nm from the CNQDs is quenched gradually by addition of AA. As the AA concentration increases, the activity of the lone pair (LP) state of the CNQDs diminishes resulting in reduced fluorescence from the CNQDs and the mechanism of the static quenching effect is discussed. Since the CNQDs have a large specific surface area and abundant amino groups and AA exists in the anionic form at the physiological pH, the electrostatic interaction between CNQDs and AA inhibits excitation and emission of the LP states in the CNQDs. Owing to steric effects and hydrogen bonding, the CNQDs constitute a sensitive and selective detection platform for AA in a wide range from 0.5 to 200 µM with a detection limit of 150 nM (signal-to-noise ratio of 3). More importantly, the strategy can successfully be applied to the detection of AA concentrations in serum samples. This simple method which provides quantitative AA determination has large potential in clinical and health-related applications and the mechanism provides insights into intracellular AA monitoring.