Enabling PIEZOpotential in PIEZOelectric Semiconductors for Enhanced Catalytic Activities

Ultrathin Ba0.75Sr0.25TiO3 nanosheets with highly exposed {001} polar facets for high-performance piezocatalytic application

The development of piezoelectrics with high catalytic activity to address environmental pollution and energy shortage has long been pursued. In this work, for the first time, a "three-birds-with-one-stone" strategy is proposed to design high-activity piezocatalysts. Interestingly, we achieved ultrathin, highly exposed polar facets and ferroelectric-paraelectric phase transitions in Ba1-xSrxTiO3 nanosheets simultaneously. As expected, Ba0.75Sr0.25TiO3 shows superior piezocatalytic performance for organic pollutant degradation due to its excellent flexibility, highly exposed polar area, and short carrier migration distance. Then, the piezoelectric potential distribution and electron transport ability on the interface of Ba0.75Sr0.25TiO3 were investigated through finite element method (FEM) simulation and density-functional theory (DFT) calculations, which provided a deep insight into the enhanced mechanism. This work thus presents a novel strategy for designing high-performance piezocatalysts and provides new insights for the optimization of the piezocatalytic activity by combining multiple advantages.

Dual-Vacancy-Engineered ZnIn2S4 Nanosheets for Harnessing Low-Frequency Vibration Induced Piezoelectric Polarization Coupled with Static Dipole Field to Enhance Photocatalytic H2 Evolution

This study investigates the impact of In- and S-vacancy concentrations on the photocatalytic activity of non-centrosymmetric zinc indium sulfide (ZIS) nanosheets for the hydrogen evolution reaction (HER). A positive correlation between the concentrations of dual In and S vacancies and the photocatalytic HER rate over ZIS nanosheets is observed. The piezoelectric polarization, stimulated by low-frequency vortex vibration to ensure the well-dispersion of ZIS nanosheets in solution, plays a crucial role in enhancing photocatalytic HER over the dual-vacancy engineered ZIS nanosheets. The piezoelectric characteristic of the defective ZIS nanosheets is confirmed through the piezopotential response measured using piezoelectric force microscopy. Piezophotocatalytic H2 evolution over the ZIS nanosheets is boosted under accelerated vortex vibrations. The research explores how vacancies alter ZIS's dipole moment and piezoelectric properties, thereby increasing electric potential gradient and improving charge-separation efficiency, through multi-scale simulations, including Density Functional Theory and Finite Element Analysis, and a machine-learning interatomic potential for defect identification. Increased In and S vacancies lead to higher electric potential gradients in ZIS along [100] and [010] directions, attributing to dipole moment and the piezoelectric effect. This research provides a comprehensive exploration of vacancy engineering in ZIS nanosheets, leveraging the piezopotential and dipole field to enhance photocatalytic performances.

Piezoelectric Hydrogels: Hybrid Material Design, Properties, and Biomedical Applications

Hydrogels show great potential in biomedical applications due to their inherent biocompatibility, high water content, and resemblance to the extracellular matrix. However, they lack self-powering capabilities and often necessitate external stimulation to initiate cell regenerative processes. In contrast, piezoelectric materials offer self-powering potential but tend to compromise flexibility. To address this, creating a novel hybrid biomaterial of piezoelectric hydrogels (PHs), which combines the advantageous properties of both materials, offers a systematic solution to the challenges faced by these materials when employed separately. Such innovative material system is expected to broaden the horizons of biomedical applications, such as piezocatalytic medicinal and health monitoring applications, showcasing its adaptability by endowing hydrogels with piezoelectric properties. Unique functionalities, like enabling self-powered capabilities and inducing electrical stimulation that mimics endogenous bioelectricity, can be achieved while retaining hydrogel matrix advantages. Given the limited reported literature on PHs, here recent strategies concerning material design and fabrication, essential properties, and distinctive applications are systematically discussed. The review is concluded by providing perspectives on the remaining challenges and the future outlook for PHs in the biomedical field. As PHs emerge as a rising star, a comprehensive exploration of their potential offers insights into the new hybrid biomaterials.

Mechanochemical reduction of alkyl and aryl halides using mesoporous zinc oxide

In this study, we propose a mechanochemical approach that combines mesoporous ZnO (m-ZnO) as a mechanoredox catalyst and silane-mediated atom transfer chemistry to achieve efficient hydrodehalogenation of organic halides. The reaction can be conducted under mild conditions without the use of a large amount of organic solvent. Substrates ranging from activated alkyl halides to unactivated aryl halides were converted to the corresponding debrominated hydrogenation products in moderate to excellent isolated yields (50-95%). In addition, m-ZnO can be recycled and reused without appreciable loss of catalytic activity.

Calcium Single Atom Confined in Nitrogen-Doped Carbon-Coupled Polyvinylidene Fluoride Membrane for High-Performance Piezocatalysis

A piezoelectric polymer membrane based on single metal atoms was demonstrated to be effective by anchoring isolated calcium (Ca) atoms on a composite of nitrogen-doped carbon and polyvinylidene fluoride (PVDF). The addition of Ca-atom-anchored carbon nanoparticles not only promotes the formation of the β phase (from 29.8 to 56.3%), the most piezoelectrically active phase, in PVDF, but also introduces much higher porosity and hydrophilicity. Under ultrasonic excitation, the fabricated catalyst membrane demonstrates a record-high and stable dye decomposing rate of 0.11 min-1 and antibacterial efficiencies of 99.8%. Density functional theory calculations reveal that the primary contribution to catalytic activity arises from single-atom Ca doping and that a possible synergistic effect between PVDF and Ca atoms can improve the catalytic performance. It is shown that O2 molecules can be easily hydrogenated to produce ·OH on Ca-PVDF, and the local electric field provided by the β-phase-PVDF might enhance the production of ·O2-. The proposed polymer membrane is expected to inspire the rational design of piezocatalysts and pave the way for the application of piezocatalysis technology for practical environmental remediation.

MoS2@MWCNTs with Rich Vacancy Defects for Effective Piezocatalytic Degradation of Norfloxacin via Innergenerated-H2O2: Enhanced Nonradical Pathway and Synergistic Mechanism with Radical Pathway

Molybdenum disulfide (MoS2)-based materials for piezocatalysis are unsatisfactory due to their low actual piezoelectric coefficient and poor electrical conductivity. Herein, 1T/3R phase MoS2 grown in situ on multiwalled carbon nanotubes (MWCNTs) was proposed. MoS2@MWCNTs exhibited the interwoven morphology of thin nanoflowers and tubes, and the piezoelectric response of MoS2@MWCNTs was 4.07 times higher than that of MoS2 via piezoresponse force microscopy (PFM) characterization. MoS2@MWCNTs exhibited superior activity with a 91% degradation rate of norfloxacin (NOR) after actually working 24 min (as for rhodamine B, reached 100% within 18 min) by pulse-mode ultrasonic vibration-triggered piezocatalysis. It was found that piezocatalysis for removing pollutants was attributed to the synergistic effect of free radicals (•OH and O2•-) and nonfree radical (1O2, key role) pathways, together with the innergenerated-H2O2 promoting the degradation rate. 1O2 can be generated by electron transfer and energy transfer pathways. The presence of oxygen vacancies (OVs) induced the transformation of O2 to 1O2 by triplet energy transfer. The fast charge transfer in MoS2@MWCNTs heterostructure and the coexistence of sulfur vacancies and OVs enhanced charge carrier separation resulting in a prominent piezoelectric effect. This work opens up new avenues for the development of efficient piezocatalysts that can be utilized for environmental purification.

Piezocatalysis for Chemical-Mechanical Polishing of SiC: Dual Roles of t-BaTiO3 as a Piezocatalyst and an Abrasive

Chemical mechanical polishing (CMP) offers a promising pathway to smooth third-generation semiconductors. However, it is still a challenge to reduce the use of additional oxidants or/and energy in current CMP processes. Here, a new and green atomically smoothing method: Piezocatalytic-CMP (Piezo-CMP) is reported. Investigation shows that the Piezo-CMP based on tetragonal BaTiO3 (t-BT) can polish the rough surface of a reaction sintering SiC (RS-SiC) to the ultra-smooth surface with an average surface roughness (Ra) of 0.45 nm and the rough surface of a single-crystal 4H-SiC to the atomic planarization Si and C surfaces with Ra of 0.120 and 0.157 nm, respectively. In these processes, t-BT plays a dual role of piezocatalyst and abrasive. That is, it piezo-catalytically generates in-situ active oxygen species to selectively oxidize protruding sites of SiC surface, yielding soft SiO2, and subsequently, it acts as a usual abrasive to mechanically remove these SiO2. This mechanism is further confirmed by density functional theory (DFT) calculation and molecular simulation. In this process, piezocatalytic oxidation is driven only by the original pressure and friction force of a conventional polishing process, thus, the piezo-CMP process do not require any additional oxidant and energy, being a green and effective polishing method.

In Situ Piezoelectric-Catalytic Anti-Inflammation Promotes the Rehabilitation of Acute Spinal Cord Injury in Synergy

Relieving inflammation via scavenging toxic reactive oxygen species (ROS) during the acute phase of spinal cord injury (SCI) proves to be an effective strategy to mitigate secondary spinal cord injury and improve recovery of motor function. However, commonly used corticosteroid anti-inflammatory drugs show adverse side effects which may induce increased risk of wound infection. Fortunately, hydrogen (H2), featuring selective antioxidant performance, easy penetrability, and excellent biosafety, is being extensively investigated as a potential anti-inflammatory therapeutic gas for the treatment of SCI. In this work, by a facile in situ growth approach of gold nanoparticles (AuNPs) on the piezoelectric BaTiO3, a particulate nanocomposite with Schottky heterojunction (Au@BT) is synthesized, which can generate H2 continuously by catalyzing H+ reduction through piezoelectric catalysis. Further, theoretical calculations are employed to reveal the piezoelectric catalytic mechanism of Au@BT. Transcriptomics analysis and nontargeted large-scale metabolomic analysis reveal the deeper mechanism of the neuroprotective effect of H2 therapy. The as-prepared Au@BT nanoparticle is first explored as a flexible hydrogen gas generator for efficient SCI therapy. This study highlights a promising prospect of nanocatalytic medicine for disease treatments by catalyzing H2 generation; thus, offering a significant alternative to conventional approaches against refractory spinal cord injury.

Boosting Efficiency in Piezo-Photocatalysis Process Using Poled Ba0.7Sr0.3TiO3 Nanorod Arrays for Pollutant Degradation and Hydrogen Production

Recently, the combination of the piezoelectric effect in the photocatalytic process, referred to as piezo-photocatalysis, has gained considerable attention as a promising approach for enhancing the degradation of organic pollutants. In this investigation, we studied the piezo-photocatalysis by fabricating arrays of barium strontium titanate (Ba0.7Sr0.3TiO3) nanorods (BST NRs) on a glass substrate as recoverable catalysts. We found that the degradation rate constant k of the rhodamine B solution achieved 0.0447 min-1 using poled BST NRs in the piezo-photocatalytic process, indicating a 2-fold increase in efficiency compared to the photocatalytic process (0.00183 min-1) utilizing the same material. This is mainly ascribed to the generation of the piezopotential in the poled BST NRs under ultrasonic vibration. Moreover, the BST NR array demonstrated a hydrogen (H2) production rate of 411.5 μmol g-1 h-1. In the photoelectrochemical process, the photocurrent density of poled BST NRs achieved 1.97 mA cm-2 at an applied potential of 1.23 V (ERHE (reversible hydrogen electrode)) under ultrasonic vibrations, representing a 1.7-fold increase compared with the poled BST NRs without ultrasonic vibrations. The measurement results from the liquid chromatograph mass spectrometer (LC-MS) demonstrated the formulation of a degradation pathway for rhodamine B molecules. Moreover, ab initio molecular dynamics (AIMD) simulation results demonstrate the dominance of hydroxyl radicals (•OH) rather than superoxide radicals (•O2-) in the degradation process. This study not only benefits the understanding of the principle of the piezo-photocatalytic process but also provides a new perspective for improving the catalytic efficiency for organic pollutants degradation.

Enhanced Generation of Reactive Oxygen Species via Piezoelectrics based on p-n Heterojunctions with Built-In Electric Field

Tuning the charge transfer processes through a built-in electric field is an effective way to accelerate the dynamics of electro- and photocatalytic reactions. However, the coupling of the built-in electric field of p-n heterojunctions and the microstrain-induced polarization on the impact of piezocatalysis has not been fully explored. Herein, we demonstrate the role of the built-in electric field of p-type BiOI/n-type BiVO4 heterojunctions in enhancing their piezocatalytic behaviors. The highly crystalline p-n heterojunction is synthesized by using a coprecipitation method under ambient aqueous conditions. Under ultrasonic irradiation in water exposed to air, the p-n heterojunctions exhibit significantly higher production rates of reactive species (·OH, ·O2-, and 1O2) as compared to isolated BiVO4 and BiOI. Also, the piezocatalytic rate of H2O2 production with the BiOI/BiVO4 heterojunction reaches 480 μmol g-1 h-1, which is 1.6- and 12-fold higher than those of BiVO4 and BiOI, respectively. Furthermore, the p-n heterojunction maintains a highly stable H2O2 production rate under ultrasonic irradiation for up to 5 h. The results from the experiments and equation-driven simulations of the strain and piezoelectric potential distributions indicate that the piezocatalytic reactivity of the p-n heterojunction resulted from the polarization intensity induced by periodic ultrasound, which is enhanced by the built-in electric field of the p-n heterojunctions. This study provides new insights into the design of piezocatalysts and opens up new prospects for applications in medicine, environmental remediation, and sonochemical sensors.

Piezoelectricity activates persulfate for water treatment: A perspective

Advanced oxidation processes (AOPs) utilizing persulfate (PS) offer great potential for wastewater treatment. Yet, the dependency on energy and chemical-intensive activation techniques, such as ultraviolet radiation and transition metal ions, constrains their widespread adoption. Recognizing this limitation, researchers are turning towards the piezoelectric effect-a novel, energy-efficient method for PS activation that capitalizes on the innate piezoelectric characteristics of materials. Intriguingly, this method taps into weak renewable mechanical forces omnipresent in nature, ranging from wind, tides, water flow, sound, and atmospheric forces. In this perspective, we delve into the burgeoning realm of piezoelectric/PS-AOPs, elucidating its fundamental principles, the refinement of piezoelectric materials, potential mechanical force sources, and pertinent application contexts. This emerging technology harbors significant potential as a pivotal element in wastewater pretreatment and may spearhead innovations in future water pollution control engineering.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Boosting Piezocatalytic Performance of BaTiO3 by Tuning Defects at Room Temperature

Defect engineering constitutes a widely-employed method of adjusting the electronic structure and properties of oxide materials. However, controlling defects at room temperature remains a significant challenge due to the considerable thermal stability of oxide materials. In this work, a facile room-temperature lithium reduction strategy is utilized to implant oxide defects into perovskite BaTiO3 (BTO) nanoparticles to enhance piezocatalytic properties. As a potential application, the piezocatalytic performance of defective BTO is examined. The reaction rate constant increases up to 0.1721 min-1, representing an approximate fourfold enhancement over pristine BTO. The effect of oxygen vacancies on piezocatalytic performance is discussed in detail. This work gives us a deeper understanding of vibration catalysis and provides a promising strategy for designing efficient multi-field catalytic systems in the future.

Macroscopic Spontaneous Piezopolarization and Oxygen-Vacancy Coupled Robust NaNbO3/FeOOH Heterojunction for Pharmaceutical Drug Degradation and O2 Evolution: Combined Experimental and Theoretical Study

Prompt recombination of photoproduced charges in bulk and surface of a photocatalyst significantly impedes catalytic efficiency. To address these challenges, FeOOH nanorods (NRs) anchored NaNbO3 (NNO) piezoelectric microcubes (MCs) have been fabricated for ciprofloxacin (CIP) degradation and oxygen evolution through water splitting by coupling macroscopic spontaneous piezoelectric polarization and a built-in electric field. The local electric field induced by surface oxygen vacancies (Ovs) and orientation of FeOOH NRs over NNO MCs afford the polarization electric field a significant boost, driving the quick separation/migration of charge carriers from bulk to the surface. The polarized NNO/FeOOH composite with ample Ovs demonstrates an outstanding piezophotocatalytic CIP degradation of 93% in 1 h, higher than pristine materials (NNO and FeOOH), and a high O2 evolution rate of 1155 μmol h-1. The effect of piezoelectric polarization on the catalytic activity is supplemented by theoretical simulations. This work offers an avenue for selective pollutant remediation and water splitting through the rational design of piezoelectric polarization-mediated heterostructure systems with surface Ovs.

Single-Crystal Ferroelectric-Based (K,Na)NbO3 Microcuboid/CuO Nanodot Heterostructures with Enhanced Photo-Piezocatalytic Activity

Developing single-crystal-based heterostructured ferroelectrics with high-performance photo-piezocatalytic activity is highly desirable to utilize large piezopotentials and more reactive charges that can trigger the desired redox reactions. To that end, a single-crystal-based (K,Na)NbO3 (KNN) microcuboid/CuO nanodot heterostructure with enhanced photo-piezocataytic activity, prepared using a facile strategy that leveraged the synergy between heterojunction formation and an intense single-crystal-based piezoelectric effect, is reported herein. The catalytic rhodamine B degrading activity of KNN/CuO is investigated under light irradiation, ultrasonication, or co-excitation with both stimulations. Compared to polycrystalline KNN powders and bare KNN single-crystals, single-crystal-based KNN/CuO exhibits a higher piezocurrent density and an optimal energy band structure, resulting in 5.23 and 2.37 times higher piezocatalytic degradation activities, respectively. Furthermore, the maximum photo-piezocatalytic rate constant (≈0.093 min-1 ) of KNN/CuO under 25 min ultrasonication and light irradiation is superior to that of other KNN-based catalysts, and 1.6 and 48.6 times higher than individual piezocatalytic and photocatalytic reaction rate constants, respectively. The excellent photo-piezocatalytic activity is attributed to the enhanced charge-carrier separation and proper alignment of band structure to the required redox levels by the appropriate p-n heterojunction and high piezoelectric potential. This report provides useful insight into the relationships between heterojunctions, piezoelectric responses, and catalytic mechanisms for single-crystal-based heterostructured catalysts.

Piezoelectricity, Pyroelectricity, and Ferroelectricity in Biomaterials and Biomedical Applications

Piezoelectric, pyroelectric, and ferroelectric materials are considered unique biomedical materials due to their dielectric crystals and asymmetric centers that allow them to directly convert various primary forms of energy in the environment, such as sunlight, mechanical energy, and thermal energy, into secondary energy, such as electricity and chemical energy. These materials possess exceptional energy conversion ability and excellent catalytic properties, which have led to their widespread usage within biomedical fields. Numerous biomedical applications have demonstrated great potential with these materials, including disease treatment, biosensors, and tissue engineering. For example, piezoelectric materials are used to stimulate cell growth in bone regeneration, while pyroelectric materials are applied in skin cancer detection and imaging. Ferroelectric materials have even found use in neural implants that record and stimulate electrical activity in the brain. This paper reviews the relationship between ferroelectric, piezoelectric, and pyroelectric effects and the fundamental principles of different catalytic reactions. It also highlights the preparation methods of these three materials and the significant progress made in their biomedical applications. The review concludes by presenting key challenges and future prospects for efficient catalysts based on piezoelectric, pyroelectric, and ferroelectric nanomaterials for biomedical applications.

Multimodal energy harvesting and catalysis of piezoelectric nanosheets for efficient and round-the-clock wastewater treatment

Solar-driven pollutants degradation is an important way for green wastewater treatment, but it is still limited by the intermittent solar flux. Here, we have prepared piezoelectric Bi4Ti3O12 (BTO) nanosheets with abundant physical properties, which can convert extensive solar energy, mechanical energy and temperature variation energy into electrical and chemical energy. It can be used for round-the-clock wastewater treatment by harvesting multi-modal energy. More importantly, the degradation rate of piezoelectric nanosheets can reach 153.4 × 10-3 min-1, and nanosheets can degrade many organic pollutants. In addition, we fabricate porous foam catalysts based on BTO-polydimethylsiloxane (PDMS) composite to prevent secondary contamination. Our results suggest that BTO nanosheets with photoelectric, piezoelectric and pyroelectric catalysis offer a potential approach for round-the-clock wastewater degradation by harvesting solar energy, ambient mechanical energy, and cyclic thermal energy.

Construction of [NbO]6-x -xS Structure to Change Charge Density and Regulate Spontaneous Polarization to Achieve Efficient Pyro-Photo-Electric Water Splitting System of NaNbO3

Pyroelectric materials in the field of photoelectrochemical (PEC) water splitting still face the problems of difficult low spontaneous polarization intensity and excessive carrier recombination. Based on the above problems, we altered the interaction between S-Nb-S in the [NbO]6-x -xS structure, and the constructed [NbO]6-x -xS structure achieved the regulation of charge density change and spontaneous polarization. The results show that under the stimulation of light and temperature fluctuations, the current density of the NS-4 photoanode is as high as 0.574 mA/cm2 at 1.23 VRHE , which is about 1.59 times higher than the pure NaNbO3 current density value, and the NS -4 photoanode achieves IPCE value of 16.08 %. The first-principles density-functional theory calculations (DFT) reveal the principle of the [NbO]6-x -xS structure for the suppression function of the carrier recombination and the improvement function of the pyroelectric effect. The analysis shows that the S-doping leads to the weakening of S-Nb-S interactions in the [NbO]6-x -xS structure, which improves the pyroelectric effect and suppresses the photo/pyro-generated carrier recombination, and effectively enhances the performance of the pyro-photo-electric synergistic water splitting system. This work promotes the development of pyroelectric materials in the field of photoelectrochemical water splitting.

Reactive oxygen species for therapeutic application: Role of piezoelectric materials

This perspective article emphasizes the significant role of reactive oxygen species (ROS) in in vivo remedial therapy of various diseases and complications, capitalizing on their potential reactivity. Among the various influencers, herein, piezoelectric materials driven ROS generation activity is primarily considered. Intrinsic non-centrosymmetry of piezoelectric materials makes them suitable for distinct dipole formation in the presence of external mechanical stimuli. Such characteristics prompt the positioning of opposite charged carriers to execute associated redox transformations that effectively participate to generate ROS in the aqueous media of the cell cytoplasm, organelles and nucleus. The immense reactivity of piezoelectric material driven ROS is fostered to terminate cellular toxicity or curtail tumor cell growth, due to their higher specificity. This perspective considers the conjugated performance of piezoelectric materials and ultrasound which can remotely generate electrical charges that promote ROS production for therapeutic application. In particular, a substantial synopsis is provided for the remedial activity of numerous piezocatalytic materials in tumor cell apoptosis, antibacterial treatment, dental care and neurological disorders. Subsequently, the report precisely demonstrates the methods involving various spectrophotometric approaches for the analysis of the ROS. Finally, the key challenges of piezoelectric material-based therapy are discussed and systematic future progress is outlined.

Engineering Sub-Nanometer Hafnia-Based Ferroelectrics to Break the Scaling Relation for High-Efficiency Piezocatalytic Water Splitting

Reversible control of ferroelectric polarization is essential to overcome the heterocatalytic kinetic limitation. This can be achieved by creating a surface with switchable electron density; however, owing to the rigidity of traditional ferroelectric oxides, achieving polarization reversal in piezocatalytic processes remains challenging. Herein, sub-nanometer-sized Hf0.5 Zr0.5 O2 (HZO) nanowires with a polymer-like flexibility are synthesized. Oxygen K-edge X-ray absorption spectroscopy and negative spherical aberration-corrected transmission electron microscopy reveal an orthorhombic (Pca21 ) ferroelectric phase of the HZO sub-nanometer wires (SNWs). The ferroelectric polarization of the flexible HZO SNWs can be easily switched by slight external vibration, resulting in dynamic modulation of the binding energy of adsorbates and thus breaking the "scaling relationship" during piezocatalysis. Consequently, the as-synthesized ultrathin HZO nanowires display superb water-splitting activity, with H2 production rate of 25687 µmol g-1  h-1 under 40 kHz ultrasonic vibration, which is 235 and 41 times higher than those of non-ferroelectric hafnium oxides and rigid BaTiO3 nanoparticles, respectively. More strikingly, the hydrogen production rates can reach 5.2 µmol g-1  h-1 by addition of stirring exclusively.

Doping Engineering to Modulate Lattice and Electronic Structure for Enhanced Piezocatalytic Therapy and Ferroptosis

Piezocatalytic therapy, which generates reactive oxygen species (ROS) under mechanical force, has garnered extensive attention for its use in cancer therapy owing to its deep tissue penetration depth and less O2 -dependence. However, the piezocatalytic therapeutic efficiency is limited owing to the poor piezoresponse, low separation of electron-hole pairs, and complicated tumor microenvironment (TME). Herein, a biodegradable, porous Mn-doped ZnO (Mn-ZnO) nanocluster with enhanced piezoelectric effect is constructed via doping engineering. Mn-doping not only induces lattice distortion to increase polarization but also creates rich oxygen vacancies (OV ) for suppressing the recombination of electron-hole pairs, leading to high-efficiency generation of ROS under ultrasound irradiation. Moreover, Mn-doped ZnO shows TME-responsive multienzyme-mimicking activity and glutathione (GSH) depletion ability owing to the mixed valence of Mn (II/III), further aggravating oxidative stress. Density functional theory calculations show that Mn-doping can improve the piezocatalytic performance and enzyme activity of Mn-ZnO due to the presence of OV . Benefiting from the boosting of ROS generation and GSH depletion ability, Mn-ZnO can significantly accelerate the accumulation of lipid peroxide and inactivate glutathione peroxidase 4 (GPX4) to induce ferroptosis. The work may provide new guidance for exploring novel piezoelectric sonosensitizers for tumor therapy.

Acoustic-Activated Se Crystalline Nanodomains at Atomically-Thin Liquid-Metal Piezoelectric Heterointerfaces for Synergistic CO2 Conversion

Acoustic-activated polarization at two-dimensional (2D) domains provide supplementary mechanisms for adjustment of empty and occupied orbitals at material heterointerfaces, activating a wide range of physicochemical applications. The piezoelectric nanodomains grown at 2D liquid-metal heterointerfaces represent a new class of polarization-dependent hybrid nanostructures with a highly challenging fabrication process. Here, the controlled growth of selenium-rich piezoelectric nanodomains on the nonpolar 2D surface of liquid Ga-based nanoparticles (NPs) enabled highly efficient and sustainable CO2 conversion. The Ga-based NPs were engulfed in carbon nanotube (CNT) frameworks. The initial hindrance effects of CNT frameworks suppressed the undesirable Ga-Se amalgamation to guarantee the suitable functions of piezocatalyst. Simultaneously, the CNT-Se mesoporous network enhances the transport and interaction of ionic species at heterointerfaces, providing unique selectivity features for CO2 conversion. Driven by acoustic energy, the multiple contributions of Ga-Se polarized heterointerfaces facilitated the piezoelectric switching and therefore increased the CO2 conversion efficiency to the value of 95.8%. The inherent compositional and functional tunability of the Ga-Se nanojunction reveal superior control over the catalyst heterointerfaces and thereby show promising potential for nanoscale applications.

Experimental Investigations upon Ultrasound Influence on Calefaction of AdBlue in Selective Catalytic Reduction Systems (SCR)

The present paper intends to provide an analysis of how the process of calefaction occurs in a selective catalytic reduction (SCR) system and the mechanisms by which the deposition of AdBlue crystals on a hot surface evolve. Experimentally, two aluminium samples heated to 200 °C were used, over which AdBlue droplets with different atomisation rates were dropped, maintaining the same dynamic flow parameters, in order to observe the influence of temperature effects on the degree of deposition of crystallised sediment on the surface. The authors proposed the use of calefaction in an ultrasonic environment to prevent deposition and to increase droplet fragmentation by a break-up process. To prove the performance of this method one sample was subjected to a normal flow regime while a second sample was exposed to ultrasound. Both samples were assembled on a magneto-strictive concentrator operating at a frequency of 20 kHz. The obtained results indicated that the sample exposed to ultrasound demonstrated lower urea crystallisation compared to the sample that was not exposed to this treatment. Thus, it can be seen that the proposed method of injecting AdBlue into an ultrasonic zone gives the desired results.

Intrinsically morphological effect of perovskite BaTiO3 boosting piezocatalytic uranium extraction efficiency and mechanism investigation

Developing a convenient, efficient and eco-friendly approach for the recovery of U(VI) ion is a key measure to solve the environmental problems arising from the utilization of nuclear energy. Herein, the high efficiency of uranium extraction is realized by the piezo property of perovskite BaTiO₃, revealing the intrinsically morphological engineering effect on the piezocatalytic performance. Especially, BaTiO₃ nanowires (BTO NWs) exhibit not only an excellent piezocatalytic activity with U(VI) extraction rate of 96.8% in a UO₂(NO₃)₂ aqueous solution compared to 71.3% of BaTiO₃ nanoparticles (BTO NPs), but also a promising piezocatalyst for U extraction in a real U-mining wastewater with various pH ranges. Piezo response force microscopy and finite elemental simulation show that the piezo response of BTO NWs is much higher than BTO NPs. Additionally, some factors (pH, various ions, different powers) are explored on piezocatalytic efficiency for U(VI) extraction. The results from electron spin resonance and the charge/radical capture experiments confirm that the active species (e^-, •O^2-, •OH) stemmed from the piezo induction of BTO NWs and BTO NPs in the piezocatalytic U(VI) reduction process. The present work reveals the structure-performance correlation during piezocatalysis and highlights the crucial role of piezocatalysis in dealing with environmental problems.

Ultra-fast Piezocatalysts Enabled By Interfacial Interaction of Reduced Graphene Oxide/MoS2 Heterostructures

The catalytic activity has been investigated in 2D materials, and the unique structural and electronic properties contribute to their success in conventional heterogeneous catalysis. Heterojunction-based piezocatalysis has attracted increasing attention due to the band-structure engineering and the enhanced charge carrier separation by prominent piezoelectric effect. However, the piezocatalytic behavior of van der Waals (vdW) heterostructures is still unknown, and the finite active sites, catalyst poisoning, and poor conductivity are challenges for developing good piezocatalysts. Herein, a reduced graphene oxide (rGO)-MoS₂ heterostructure is rationally designed to tackle these challenges. The heterostructure shows a record-high piezocatalytic degradation rate of 1.40 × 10² L mol^-1 s^-1 , which is 7.86 times higher than MoS₂ nanosheets. Piezoresponse force microscope measurements and density functional theory calculation reveal that the coupling between semiconductive and piezoelectric properties in the vdW heterojunction is vital to break the metallic state screening effect at the MoS₂ edge for keeping the piezoelectric potential. The dynamic charges generated by MoS₂ and the fast charge transfer in rGO activate and maintain catalytically active sites for pollutant degradation with an ultra-fast rate and good stability. The working mechanism opens new avenues for developing efficient catalysts significant to wastewater treatments and other applications.

Piezoelectric materials and techniques for environmental pollution remediation

With the rapid development of industrialization and agriculture, a series of critical imminent environmental problems and water pollution have caught wide attention from the public and society. Piezoelectric catalysis technology with piezoelectric materials is a green and environmental method that can efficiently improve the separation of electron-hole pairs, then generating the active substances such as OH, H₂O₂ and O₂^-, which can degrade water pollutants. Therefore, we firstly surveyed the piezoelectric catalysis in piezoelectric materials and systematically concluded and emphasized the relationship between piezoelectric materials and the piezoelectric catalytic mechanism, the goal to elucidate the effect of polarization on piezoelectric catalytic performance and enhance piezoelectric catalytic performance. Subsequently, the applications of piezoelectric materials in water treatment and environmental pollutant remediation were discussed including degradation of organic pollutants, removal of heavy mental ions, radionuclides, bacteria disinfection and water splitting for H₂ generation. Finally, the development prospects and future outlooks of piezoelectric catalysis were presented in detail.

The fundamentals and applications of piezoelectric materials for tumor therapy: recent advances and outlook

Malignant tumors are one of the main diseases leading to death, and the vigorous development of nanotechnology has opened up new frontiers for antitumor therapy. Currently, researchers are focused on solving the biomedical challenges associated with traditional anti-tumor medical methods, promoting the research and development of nano-drug carriers and new nano-drugs, which brings great hope for improving the curative effect and reducing toxic and side effects. Among the new systems being investigated, piezoelectric nano biomaterials, including ferroelectrics, piezoelectric and pyroelectric materials, have recently received extensive attention for antitumor applications. By coupling force, light, magnetism or heat and electricity, polarized charges are generated in these materials microscopically, forming a piezo-potential and establishing a built-in electric field. Polarized charges can directly act on the materials in the tumor micro-environment and also assist in the separation of carriers and inhibit recombination based on piezoelectric theory and piezoelectric optoelectronic theory. Based on this, piezoelectric materials convert various forms of primary energy (such as light energy, mechanical energy, thermal energy and magnetic energy) from the surrounding environment into secondary energy (such as electrical energy and chemical energy). Herein, we review the basic theory and principles of piezoelectric materials, pyroelectric materials and ferroelectric materials as nanomedicine. Then, we summarize the types of piezoelectric materials reported to date and their wide applications in treatment, imaging, device construction and probe detection in various tumor treatment fields. Based on this, we discuss the relevant characteristics and post-processing strategies of nano piezoelectric biomaterials to obtain the maximum piezoelectric response. Finally, we present the key challenges and future prospects for the development of ferroelectric, piezoelectric and pyroelectric nanomaterial-based nanoagents for efficient energy harvesting and conversion for desirable therapeutic outcomes.

Recent Advances in Efficient Photocatalysis via Modulation of Electric and Magnetic Fields and Reactive Phase Control

The past several years has witnessed significant progress in enhancing photocatalytic performance via robust electric and magnetic fields' modulation to promote the separation and transfer of photoexcited carriers, and phase control at reactive interface to lower photocatalytic reaction energy barrier and facilitate mass transfer. These three research directions have received soaring attention in photocatalytic field. Herein, recent advances in photocatalysis modulated by electric field (i.e., piezoelectric, pyroelectric, and triboelectric fields, as well as their coupling) with specific examples and mechanisms discussion are first examined. Subsequently, the strategy via magnetic field manipulation for enhancing photocatalytic performance is scrutinized, including the spin polarization, Lorentz force, and magnetoresistance effect. Afterward, materials with tailored structure and composition design enabled by reactive phase control and their applications in photocatalytic hydrogen evolution and carbon dioxide reduction are reviewed. Finally, the challenges and potential opportunities to further boost photocatalytic efficiency are presented, aiming at providing crucial theoretical and experimental guidance for those working in photocatalysis, ferroelectrics, triboelectrics, piezo-/pyro-/tribo-phototronics, and electromagnetics, among other related areas.

Pyroelectric Janus nanomotors for synergistic electrodynamic-photothermal-antibiotic therapies of bacterial infections

Antibacterial electrotherapy is currently activated by external electric field or self-powered generators, but usually needs complicated power management circuits. Herein, near-infrared illumination (NIR) of pyroelectric nanoparticles (NPs) produces a built-in electric field to address the effectiveness and safety concerns in the antibacterial treatment. Janus tBT@PDA NPs were obtained by capping polydopamine (PDA) on tetragonal BaTiO₃ (tBT) NPs through defining the polymerization time, followed by ciprofloxacin (CIP) loading on the PDA caps to fabricate Janus tBT@PDA-Cip NPs. NIR illumination of PDA caps creates temperature variations on tBT NPs to generate photothermal and pyroelectric effects. Finite element simulation reveals a pyroelectric potential of over 1 V and sufficient reactive oxygen species (ROS) are produced to exhibit pyroelectric dynamic therapy (PEDT). The elevated temperature on one side of the Janus NPs produces thermophoretic force to drive NP motion, which enhances interactions with bacteria and overcomes limitations in the short action distance and lifespan of ROS. The pyroelectric field accelerates CIP release through weakening the π-π stacking and electrostatic interaction with PDA and also interrupts membrane potentials of bacteria to enhance CIP invasion into bacteria. The synergistic antibacterial effect of pyroelectric tBT@PDA-Cip NPs causes the fully recovery of S. aureus-infected skin wounds and regeneration of intact epidermis, blood vessels and hair follicles, while no obvious pathological change or inflammatory lesion is detected in the major organs. Thus, the pyroelectric Janus nanomotors demonstrate synergistic PEDT/photothermal/antibiotic effects to enhance antibacterial efficacy while avoiding the necessity of excessive heat, ROS and antibiotic doses. STATEMENT OF SIGNIFICANCE: Antibacterial treatment is challenged by antibiotics-derived side effects and the evolution of resistant strains. Phototherapy is commonly associated with excessive heat and oxidative stress, and their combinations with other agents are especially encouraged to strengthen antibacterial efficacy while alleviating the associated side effects. Electric field is another activator to generate antibacterial abilities, but usually requires complicated power management and bulk electrodes, making it inconvenient in a biological setup. To address these challenges, we propose a strategy to generate microelectric field on nanoparticles themselves and achieve synergistic electrodynamic-photothermal-antibiotic therapies. The pyroelectric effect weakens interactions between nanoparticles and antibiotics to accelerate drug release, and the built-in pyroelectric field increases membrane fluidity to enhance bacterial uptake of antibiotics.

Construction of a novel S-scheme heterojunction piezoelectric photocatalyst V-BiOIO3/FTCN and immobilization with floatability for tetracycline degradation

A high-performance piezoelectric photocatalyst (V-BiOIO₃/FTCN) was constructed to improve removal efficiency of tetracycline hydrochloride (TCH). The role of V-BiOIO₃ in the composite was to introduce piezoelectric effect and construct S-scheme heterojunction photocatalyst with fish scale tubular carbon nitride (FTCN). The morphology, structure, chemical composition and optoelectronic characteristics of the as-prepared photocatalysts were studied by SEM, TEM, XRD, XPS and UV-Vis DRS. Combined with UV-Vis DRS, XPS valence band, Mott-schottky curve and theoretical calculations, the mechanism of TCH degradation was deeply analyzed. A series of degradation experiments showed that the V-BiOIO₃/FTCN could effectively degrade TCH, and the removal efficiency was further improved under the action of ultrasound. Importantly, the further immobilized V-BiOIO₃/FTCN/MS could float on the water surface to degrade TCH without additional stirring, which facilitated the recovery of photocatalysts and showed excellent practical application value. This work provided a reference for the design and immobilization of carbon nitride-based piezoelectric photocatalysts.

Piezocatalytic Techniques in Environmental Remediation

As a consequence of rapid industrialization throughout the world, various environmental pollutants have begun to accumulate in water, air, and soil. This endangers the ecological environment of the earth, and environmental remediation has become an immediate priority. Among various environmental remediation techniques, piezocatalytic techniques, which uniquely take advantage of the piezoelectric effect, have attracted much attention. Piezoelectric effects allow pollutant degradation directly, while also enhancing photocatalysis by reducing the recombination of photogenerated carriers. In this Review, we provide a comprehensive summary of recent developments in piezocatalytic techniques for environmental remediation. The origin of the piezoelectric effect as well as classification of piezoelectric materials and their application in environmental remediation are systematically summarized. We also analyze the potential underlying mechanisms. Finally, urgent problems and the future development of piezocatalytic techniques are discussed.

Mechanically Driven Atom Transfer Radical Polymerization by Piezoelectricity

Targeting sustainable and eco-friendly polymer synthesis, we demonstrate here a synergistically catalyzed atom transfer radical polymerization (ATRP) induced and controlled by interplay between ball milling (BM) and piezoelectric nanoparticles (piezoNPs). BM-induced electron transfer can be achieved through piezoNPs deformation under impact force, serving as an external stimulus to mediate polymerization. The ppm level of copper loading is sufficient in fabrication of a polymer with well-defined molecular weight and low polydispersity. High-molecular-weight polymers ranging from 33 to 74 kDa were prepared successfully through DMSO-assisted grinding. Besides, its good performance on availability of water as liquid-assisted grinding additive, the recyclability of piezoNPs, and the formation of cross-linker-free composite resin make our ATRP approach a green and practical option alongside the existent heat-, electro-, and photo-induced methods.

Pyro-photocatalytic Coupled Effect in Ferroelectric Bi0.5Na0.5TiO3 Nanoparticles for Enhanced Dye Degradation

Advanced oxidation processes (AOPs), achieved through the continuous attack of reactive oxygen species (ROS), are considered the most efficient way to mineralize organic pollutants. Among them, photocatalysis is the most environmentally friendly strategy for pollution mitigation but is hampered by low conversion efficiency. By exploiting the coupling effect without changing the properties of the semiconductor, the application of pyroelectric fields can significantly improve the catalytic performance. The degradation rate of rhodamine B by Bi_0.5Na_0.5TiO₃ (BNT) nanoparticles under temperature fluctuations and visible light irradiation was up to 98%. The performance was enhanced by 216.54% and 31.48% compared to the pyroelectric catalysis and photocatalysis alone, respectively. The improved performance is due to the introduced pyroelectric potential with the imposition of temperature fluctuations, which can make the domains enhance the polarization of ferroelectrics, thus promoting the charge separation. This method can significantly advance the coupled pyro-photocatalytic reaction of ferroelectric semiconductors and also can enable the synergistic utilization of multiple energy sources such as solar and thermal energy, which is a promising strategy for environmental remediation.

Enhanced Piezocatalytic Reactive Oxygen Species Production Activity and Recyclability of the Dual Piezoelectric Cu3B2O6/PVDF Composite Membrane

Piezocatalysts have attracted considerable attention due to their ability to convert natural mechanical energy into chemical energy. However, the inefficient chemical reactions of the free charges and the poor mechanical endurance of the powder piezoelectric materials have largely restricted their wide application. Here, by combining piezocatalyst Cu₃B₂O₆ (CBO) and polyvinylidene fluoride (PVDF), a composite membrane CBO/PVDF with superior stability and excellent piezo-performance is prepared for the first time. This composite membrane shows a high efficiency for the degradation of antibiotics and organic dyes under ultrasonication; particularly, the removal efficiency is 33.9 times higher than that of a pure PVDF membrane for amoxicillin degradation, and it maintains a high efficiency after 16 cycling tests. The polarization electric field in the dual piezoelectric composite membrane significantly enhances the redox reaction of the intrinsic free carrier with dissolved oxygen and water molecules to generate reactive oxygen species. The results provide a strategy for combining the borate with the polymer membrane to lead piezocatalysis to real future applications.

Breathing-Driven Self-Powered Pyroelectric ZnO Integrated Face Mask for Bioprotection

Rapid spread of infectious diseases is a global threat and has an adverse impact on human health, livelihood, and economic stability, as manifested in the ongoing coronavirus disease 2019 (COVID-19) pandemic. Even though people wear a face mask as protective equipment, direct disinfection of the pathogens is barely feasible, which thereby urges the development of biocidal agents. Meanwhile, repetitive respiration generates temperature variation wherein the heat is regrettably wasted. Herein, a biocidal ZnO nanorod-modified paper (ZNR-paper) composite that is 1) integrated on a face mask, 2) harvests waste breathing-driven thermal energy, 3) facilitates the pyrocatalytic production of reactive oxygen species (ROS), and ultimately 4) exhibits antibacterial and antiviral performance is proposed. Furthermore, in situ generated compressive/tensile strain of the composite by being attached to a curved mask is investigated for high pyroelectricity. The anisotropic ZNR distortion in the bent composite is verified with changes in ZnO bond lengths and OZnO bond angles in a ZnO₄ tetrahedron, resulting in an increased polarization state and possibly contributing to the following pyroelectricity. The enhanced pyroelectric behavior is demonstrated by efficient ROS production and notable bioprotection. This study exploring the pre-strain effect on the pyroelectricity of ZNR-paper might provide new insights into the piezo-/pyroelectric material-based applications.

Laser-Shock-Driven In Situ Evolution of Atomic Defect and Piezoelectricity in Graphitic Carbon Nitride for the Ionization in Mass Spectrometry

Nanostructures─coupled with mass spectrometry─have been intensively investigated to improve the detection sensitivity and reproducibility of small biomolecules in laser desorption/ionization mass spectrometry (LDI-MS). However, the impact of laser-induced shock wave on the ionization of the nanostructures has rarely been reported. Herein, we systematically elucidate the laser shock wave effect on the ionization in terms of the in situ development of atomic defects and piezoelectricity in two-dimensional graphitic carbon nitride nanosheets (g-C₃N₄ NS) by short laser pulses. The mass analysis results of immunosuppressive drugs verify the enhanced LDI-MS performance, structurally originating from anisotropic lattice distortions in g-C₃N₄ NS, i.e., in-plane extension (contraction) and out-of-plane contraction (extension) that modulate the charge carrier motion. Along with the experimental investigations, density functional theory calculations on Mulliken charges and dipole moments demonstrate the contribution of defect and piezoelectricity to the ionization. The results of this study provide a mechanistic understanding of the underlying ionization processes, which is crucial for revealing the full potential of laser shock waves in LDI-MS.

Peroxymonosulfate Activation by BaTiO3 Piezocatalyst

Peroxymonosulfate (PMS) plays an important role in the advanced oxidation process for environmental remediation. In this study, barium titanate (BTO) piezocatalyst was selected for the activation of PMS driven by ultrasonic power. The degradation of Rhodamine B (RhB) by BTO single component, PMS single component, and BTO/PMS double components were investigated. The results indicated that PMS can be efficiently activated by BTO under an ultrasound with an RhB degradation rate of 98% within 20 min. The ultrasound not only promoted the activation of the PMS itself, but the surface charge carriers of BTO induced by the ultrasound also contributed to the activation of PMS. ·O2−, ·OH, and ·SO4− radicals were found to be the main active species that participated in the reaction. In order to verify the reaction’s environmental applicability, amoxicillin (AMX) as a typical environmental pollutant was studied. BTO/PMS displayed 80% removal efficiency of AMX, and the products generated were less toxic as demonstrated by eco-toxicity comparison. This work provides a promising strategy to improve the utilization of ultrasonic energy and apply it to the field of environmental pollutants treatment.

Bandgap-engineered ferroelectric single-crystalline NBT-BT based nanocomposites with excellent visible light-ultrasound catalytic performance

Bandgap engineered ferroelectrics exhibit encouraging multi-energy catalytic performance by coupling the piezoelectricity and photoexcitation, which shows immense potential for environmental remediation and fuel production. However, it is challenging to prepare nano single-crystalline ferroelectric piezo-photoelectric with strong visible light absorption ability. Here, Ni mediated NBT-BT(NBT-BNT) single-crystalline nanocubes around 100 nm with considerable visible light absorption were synthesized by a high-temperature hydrothermal method. The mechanism of Ni2+ on the formation of NBT-BT nanocubes was proposed. The catalytic efficiency of NBT-BNT nanocubes is enhanced by decorating carbon quantum dots (CQDs). The RhB can be degraded within 8 min and the hydrogen production rate reaches up to ∼350 μmol g-1h-1 under visible light-ultrasonic condition. Moreover, under the simulated sunlight-ultrasound condition, RhB can be degraded within merely 3 min and a high H2 production rate of ∼747 μmol g-1h-1 is achieved. This work presents a paradigm for preparing ferroelectric single-crystalline nanocatalysts for multi-energy catalytic application.

Polymer and small molecule mechanochemistry: closer than ever

The formation and scission of chemical bonds facilitated by mechanical force (mechanochemistry) can be accomplished through various experimental strategies. Among them, ultrasonication of polymeric matrices and ball milling of reaction partners have become the two leading approaches to carry out polymer and small molecule mechanochemistry, respectively. Often, the methodological differences between these practical strategies seem to have created two seemingly distinct lines of thought within the field of mechanochemistry. However, in this Perspective article, the reader will encounter a series of studies in which some aspects believed to be inherently related to either polymer or small molecule mechanochemistry sometimes overlap, evidencing the connection between both approaches.

Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001)

Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO₃. This intrinsically polar material cleaves along (001) into KO- and TaO₂-terminated surface domains. At TaO₂ terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO₂ terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.

Competing electronic states emerging on polar surfaces

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

Harvesting mechanical energy for hydrogen generation by piezoelectric metal-organic frameworks

Piezocatalysis, the process of directly converting mechanical energy into chemical energy, has emerged as a promising alternative strategy for green H₂ production. Nevertheless, conventional inorganic piezoelectric materials suffer from limited structural tailorability and small surface area, which greatly impedes their mechanically driven catalytic efficiency. Herein, we design and fabricate a novel UiO-66(Zr)-F₄ metal-organic framework (MOF) nanosheet for piezocatalytic water splitting, with the highest H₂ evolution rate reaching 178.5 μmol g^-1 within 5 h under ultrasonic vibration excitation (110 W, 40 kHz), far exceeding that of the original UiO-66 host. A reduced bandgap from 2.78 to 2.43 eV is achieved after introducing a fluorinated ligand. Piezoresponse force microscopy measurements demonstrate a much stronger piezoelectric response for UiO-66(Zr)-F₄, which may result from the polarity of the introduced fluorinated ligand. This work highlights the potential of MOF-based porous piezoelectric nanomaterials in harvesting mechanical energy to drive chemical reactions such as water splitting.