The piezo‐assisted photocatalysis system, which can utilize solar energy and mechanical energy simulteneously, is promising but still challenging in the environmental remediation field. In this work, a novel metal–semiconductor BaTiO3@ReS2 Schottky heterostructure is designed and it shows high‐efficiency on piezo‐assisted photocatalytic molecular oxygen activation. By combining experiment and calculation results, the distorted metal‐phase ReS2 nanosheets are found to be closely anchored on the surface of the BaTiO3 nanorods, through interfacial ReO covalent bonds. The Schottky heterostructure not only forms electron‐transfer channels but also exhibits enhanced oxygen activation capacity, which are helpful to produce more superoxide radicals. The polarization field induced by the piezoelectric BaTiO3 can lower the Schottky barrier and thus reduce the transfer resistance of photogenerated electrons directing to the ReS2. As a result of the synergy effect between the two components, the BaTiO3@ReS2 exhibits untrahigh activity for degradation of pollutants with an apparent rate constant of 0.133 min−1 for piezo‐assisted photocatalysis, which is 16.6 and 2.44 times as that of piezocatalysis and photocatalysis, respectively. This performance is higher than most reported BaTiO3‐based piezo‐assisted photocatalysis systems. This work paves the way for the design of high‐efficiency piezo‐assisted photocatalytic materials for environmental remediation through using green energies in nature.

Directing Charge Transfer in a Chemical‐Bonded BaTiO3@ReS2 Schottky Heterojunction for Piezoelectric Enhanced Photocatalysis

In this work, the centrosymmetry of Bi 2 S 3 nanorods is broken due to the introduction of large quantities of S i defects in the crystal lattice, which then endows the Bi 2 S 3 nanorods with the ability to efficiently reduce Cr(VI) by piezocatalysis. Most importantly, the piezocatalytic reduction could be significantly boosted by applying an alternating magnetic field (AMF). The AMF coupled piezocatalysis is achieved by H 2 O 2 and·O 2 - . AMF is found to facilitate the separation of electrons and holes due to the magnetoresistance effect of Bi 2 S 3 nanorods, thus remarkably enhancing the piezocatalysis by largely increasing the generation of H 2 O 2 . This work paves a new way for inducing the piezoelectricity of centrosymmetric materials by introducing defects and improving their catalytic activity through AMF, which has a great application potential in energy and environment fields. • Bi 2 S 3 nanorods were endowed with piezocatalytic performance due to the symmetry breaking effect. • Cr(VI) is highly efficient reduced to Cr(III) of the piezocatalysis. • The piezocatalysis is greatly boosted by applying an alternating magnetic field. • H 2 O 2 and·O 2 - were found to be generated and reduce Cr(VI) while pieozcatalysis. 

Symmetry-Breaking Induced Piezocatalysis of Bi2S3 Nanorods and Boosted by Alternating Magnetic Field

The application of promising persulfate-based advanced oxidation processes (AOPs) for environment remediation is limited by the poor charge separation and the sluggish kinetic for active centers regeneration of the catalysts. Piezocatalytic activation has been proposed to solve the above concern recently, but little is known on its mechanism. Herein, novel piezocatalyst Bi2Fe4O9 nanosheets (BFO NSs) are demonstrated to significantly improve peroxydisulfate (PDS) activation efficiency via piezocatalysis, resulting SO4•− and •OH as the dominant active species for organic pollutants degradation. Further DFT calculations and experimental investigations confirm that the Fe2+ in BFO NSs as active sites donate electrons for PDS to generate SO4•−, and simultaneously that the piezo-electrons can accelerate the regeneration of Fe2+, thus leading to the superior catalytic activity. This work provides atomic-scale insights of catalytic sites and mechanisms of piezocatalytic PDS activation and may inspire the development of more efficient piezocatalysts and AOPs. 

Piezo-promoted regeneration of Fe2+ boosts peroxydisulfate activation by Bi2Fe4O9 nanosheets

Since the third scientific and technological revolution, electronic information technology has developed rapidly, and piezoelectric materials that can convert mechanical energy into electrical energy have become a research hotspot. Among them, piezoelectric polymers are widely used in various fields such as water treatment, biomedicine, and flexible sensors due to their good flexibility and weak toxicity. However, compared with ceramic piezoelectric materials, the piezoelectric properties of polymers are poor, so it is very important to improve the piezoelectric properties of polymers. Electrospinning technology can improve the piezoelectric properties of piezoelectric polymers by adjusting electrospinning parameters to control the piezoelectrically active phase transition of polymers. In addition, the prepared nanofibrous membrane is also a good substrate for supporting piezoelectric functional particles, which can also effectively improve the piezoelectric properties of polymers by doping particles. This paper reviews the piezoelectric properties of various electrospun piezoelectric polymer membranes, especially polyvinylidene fluoride (PVDF)-based electrospun nanofibrous membranes (NFs). Additionally, this paper introduces the various methods for increasing piezoelectric properties from the perspective of structure and species. Finally, the applications of NFs in the fields of biology, energy, and photocatalysis are discussed, and the future research directions and development are prospected.

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Processes of Electrospun Polyvinylidene Fluoride-Based Nanofibers, Their Piezoelectric Properties, and Several Fantastic Applications

Solar-driven overall water splitting is an ideal way to generate renewable energy while still challenging. For the first time, this work combined covalent organic frameworks (COFs) and piezoelectric material by covalent linkages to form Z-scheme core@shell heterostructure for overall water splitting. Benefiting from the synergistic effect between the polarized electric field and photo-generated charges, as well as the precise adjustment of shell thickness, the carrier separation and utilization efficiency is greatly improved. The optimal BiFeO3 @TpPa-1-COF photocatalyst revealed hydrogen (H2 ) and oxygen (O2 ) production rates of 1416.4 and 708.2 μmol h-1 g-1 under the excitation of ultrasonication coupled with light irradiation, which is the best performance among various piezo- and COF-based photocatalysts. This provides a new sight for the practical application of highly efficient photocatalytic overall water splitting. 

Piezo‐Photocatalytic Synergy in BiFeO <sub>3</sub> @COF Z‐Scheme Heterostructures for High‐Efficiency Overall Water Splitting

The built-in electric field can be generated in the piezoelectric materials under mechanical stress. The resulting piezoelectric effect is beneficial to charge separation in photocatalysis. Meanwhile, the mechanical stress usually gives rise to accelerated mass transfer and enhanced catalytic activity. Unfortunately, it remains a challenge to differentiate the contribution of these two factors to catalytic performance. Herein, for the first time, isostructural metal-organic frameworks (MOFs), i.e., UiO-66-NH2 (Zr) and UiO-66-NH2 (Hf), are adopted for piezo-photocatalysis. Both MOFs, featuring the same structures except for diverse Zr/Hf-oxo clusters, possess distinctly different piezoelectric properties. Strikingly, UiO-66-NH2 (Hf) exhibits ≈2.2 times of activity compared with that of UiO-66-NH2 (Zr) under simultaneous light and ultrasonic irradiation, though both MOFs display similar activity in the photocatalytic H2 production without ultrasonic irradiation. Given their similar pore features and mass transfer behaviors, the activity difference is unambiguously assignable to the piezoelectric effect. As a result, the contributions of the piezoelectric effect to the piezo-photocatalysis can be clearly distinguished owing to the stronger piezoelectric property of UiO-66-NH2 (Hf).

Piezo-Photocatalysis over Metal–Organic Frameworks: Promoting Photocatalytic Activity by Piezoelectric Effect

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH2, is integrated with metal oxides (MoO3 and V2O5) that feature appropriate work function and energy level to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH2, boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activity, respectively, compared to the pristine MOF for photocatalytic H2 production. Upon depositing Pt onto the MOF, ~6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution.


 Proton exchange membrane (PEM) is a key component of proton exchange membrane fuel cells (PEMFCs). In recent years, metal organic framework (MOF) and its composite membranes have become the research hotspots. [Co(L-Glu)(H2O)•H2O]n (Co-MOF, L-Glu = L-glutamate) was synthesized by hydrothermal method. Co2+ ions are coordinated with L-Glu ligands and water molecules to form one-dimensional chains extending along the a-axis, which are further bridged by L-Glu ligands to form a three-dimensional network structure. AC impedance analysis shows that the proton conductivity of Co-MOF reaches 3.14×10-4 S•cm-1 under 98% relative humidity (RH) and 338 K. To improve proton conductivity, different contents of Co-MOF were added in chitosan (CS) to form composite membranes Co-MOF@CS-X (mass fraction X= 5%, 10%, 15% wt). The results show the proton conductivity of the Co-MOF@CS-10 composite membrane is 1.73×10-3 S•cm-1 at 358 K and 98% RH, which is more than 5 times that of Co-MOF. As far as we known, this is the first composite made of amino acid MOFs and CS as proton exchange membrane. Furthermore, Co-MOF has an obvious quenching effect on L-histidine in aqueous solution, which can detect the content of L-histidine in water with high sensitivity, and the detection limit is 1×10-7 M.

A Dual-Function Cobalt Metal-Organic Framework for High Proton Conduction and Selective Luminescence Sensing of Histidine

Organosulfates (OSs), also known as organic sulfate esters, are ubiquitous in atmospheric particles and used as secondary organic aerosol (SOA) markers. However, the chemical transformation mechanism of these OSs remains unclear. Therefore, we investigated the heterogeneous OH oxidation of 3-methyltetraol sulfate (3-MTS), which is one of the most abundant particulate organosulfates, by using quantum chemical and kinetic calculations. 3-MTS can easily undergo abstraction reaction with OH radicals, and the reaction rate constant is about 7.87 × 10−12 cm3 per molecule per s. The generated HCOOH, CH3COOH, HCHO, CH3CHO and 2-methyl-2,3-dihydroxypropionic acid are low-volatility species with increased water solubility, which are the main components of SOA. In addition, the OH radicals obtained from the reaction can continue to promote the oxidation reaction. The results of this study provide insights into the heterogeneous OH reactivity of other organosulfates in atmospheric aerosols, and it also provides a new understanding of the conversion of sulfur (S) between its organic and inorganic forms during the heterogeneous OH oxidation of organic sulfates.

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Chenxi Zhang

Papers

Piezo-Photocatalysis over Metal–Organic Frameworks: Promoting Photocatalytic Activity by Piezoelectric Effect

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution.

A Dual-Function Cobalt Metal-Organic Framework for High Proton Conduction and Selective Luminescence Sensing of Histidine

Study on heterogeneous OH oxidation of 3-methyltetraol sulfate in the atmosphere under high NO conditions

High-Efficiency Photocatalytic Hydrogen Production by Combining Hierarchical Fecds-Cds Nanorods and Ni-Salens