Synthesizing BaTiO3 Nanostructures to Explore Morphological Influence, Kinetics, and Mechanism of Piezocatalytic Dye Degradation

The role of BaTiO3 nanoparticles as photocatalysts in the synthesis and characterization of novel fruit dyes is investigated

In the present work, the photocatalytic activity against the natural dye extracted from the novel fruits has been studied by the BaTiO3 nanoparticles (NPs) under a ultra-violet (UV) light source. The large concentrations of an essential phenolic agent present in this phytochemical extract superimposed with cloths fibers make strong stain and degrade into another form of toxic, which is excluded from the many textiles industries as the colorful waste waters without recycling and removal of that dye pigments have been investigated using both photodegradation and photoluminescence techniques. The entitled nanoparticles (NPs) were prepared using the soft chemical root-modified solvothermal synthesis combo method and exposure to heat treatment such that the annealing process has been done for different temperatures ranging from 100°C to 250°C. As for as concern the characterization, as a start, structural and morphology studies have been reported here that highly crystalline oriented peaks data using powder x-ray diffraction techniques (PXRD) as well as the surface morphology including the size, shape, and mass distribution using the field emission scanning electron microscopy (FESEM) techniques, which purely belong to rutile tetragonal structure of the crystal system and circular and noncircular flakes like rough surface morphology materials respectively. The lattice dissociation constant 'ε' value of the BaTiO3 NPs has determined to be ~2.71 × 10-3 using the Williamson-Hall (W-H plot) analysis of crystallographic data. In the UV visible spectroscopy findings, since the extreme quantum confinement of BaTiO3 nanoflakes/nanodisc, the optical energy bandgap has been estimated to be a range of 1.98 to 2.67 eV (~2.48 eV) found from the Tauc plot analysis, which contributes to the significantly owing to the enhanced photocatalytic efficiency with excellent performance along exciton formation, superoxide ions, and hydroxyl free radicals generations under UV-vis light irradiation resulting in efficient degradation of typical novel fruit organic dye. Photoluminescence spectra observed at room temperature and low temperature have been observed for the BaTiO3 nanoflakes, which exhibit the blue emission due to the crystalline defects such as the appearance of Ba vacancies leads to the conceivable beginning of p-type conductivity and the origination of free exciton emission reveals the direct bandgap transition nature of nanoflakes. RESEARCH HIGHLIGHTS: According to our findings, 89.71% of the natural syzygium cumin is degraded by photocatalysis reaction. As a plausible mechanism for the destruction of natural dyes under solar light, photocatalytic destruction has been proposed. The reaction between these reactive free radical species leads to high efficiency photodegradation with a short decay time. In addition to water treatment and environmental cleaning applications, the excellent performance of this photocatalyst makes it a promising candidate for other applications. Hence, the synthesized BaTiO3 nanoflakes showcase a highly significant advancement towards the development of a textiles dye recycling method.

Degradation of dye through mechano-catalysis using BaBi4Ti4O15 catalyst

Ferroelectric BaBi4Ti4O15 was prepared using solid-state calcination at 950 °C for four hours. X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy were utilized to understand its microstructure and other structural aspects. Particle size was around < 1.5 µm. This oxide is able to demonstrate piezocatalysis and tribocatalysis as reflected in its dye degradation performance. This oxide showed piezocatalytic activity around 40% in 2 h and tribocatalytic activity around 90% in 12 h. The rate constant for the piezocatalytic reaction is 0.003 min-1 and for tribocatalytic reaction is 0.169 h-1. The rotation speed also affected the tribocatalytic activity of the oxide. Oxide showed 25%, 90%, and 94% tribocatalytic activity at 300, 500, and 700 rpm respectively. This material has demonstrated notable performance of catalysis under different types of mechanical energy sources and under different mechanisms.

Aeration-driven piezoelectric activation of peroxymonosulfate achieves effective mitigation of antibiotic resistance dissemination

The antibiotic resistance dissemination in water has become a globally concerned issue, and the wastewater discharge, especially medical wastewater, is considered as one of the most important sources for antibiotic resistance genes (ARGs). However, the effectiveness of current disinfection techniques in the ARGs reduction still remains controversial. In this study, a novel aeration-driven piezoelectric peroxymonosulfate (PMS) activation system using oxygen-vacancy engineered BaTiO3 (BTO) was developed to effectively eliminate antibiotic resistant bacteria (ARB) and ARGs from water. The ARB can be completely inactivated and ∼3.0 logs of ARGs can be removed by the PMS/BTO/aeration system within 1 h, and the spent BTO nanoparticles can be facilely reused after simple rinsing. The aeration can not only provide the driving force for the piezocatalytic process but also more dissolved oxygen in water that played an important role in the generation of free radicals. The radical quenching experiments and electron spin-resonance (ESR) confirmed that all the free radicals, including singlet oxygen (1O2), hydroxyl radical (OH•), sulfate radical (SO4•-) and superoxide radical (•O2-), contributed to the ARGs reduction and 1O2 radicals were identified as the dominant active species. This work provides a high-efficiency and energy saving approach for the mitigation of ARGs from water as the universal use of aeration in water treatment processes and the good reusability of BTO nanoparticles.

The Piezocatalytic Degradation of Sulfadiazine by Lanthanum-Doped Barium Titanate

Piezocatalysis, a heterogeneous catalytic technique, leverages the periodic electric field changes generated by piezoelectric materials under external forces to drive carriers for the advanced oxidation of organic pollutants. Antibiotics, as emerging trace organic pollutants in water sources, pose a potential threat to animals and drinking water safety. Thus, piezoelectric catalysis can be used to degrade trace organic pollutants in water. In this work, BaTiO3 and La-doped BaTiO3 were synthesized using an improved sol-gel-hydrothermal method and used as piezocatalytic materials to degrade sulfadiazine (SDZ) with ultrasound activation. High-crystallinity products with nano cubic and spherical morphologies were successfully synthesized. An initial concentration of SDZ ranging from 1 to 10 mg/L, a catalysis dosage range from 1 to 2.5 mg/mL, pH, and the background ions in the water were considered as influencing factors and tested. The reaction rate constant was 0.0378 min-1 under the optimum working conditions, and the degradation efficiency achieved was 89.06% in 60 min. La-doped BaTiO3 had a better degradation efficiency, at 14.98% on average, compared to undoped BaTiO3. Further investigations into scavengers revealed a partially piezocatalytic process for the degradation of SDZ. In summary, our work provides an idea for green environmental protection in dealing with new types of environmental pollution.

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.

Facile synthesis of a GO-g-C3N4/BaTiO3 ternary nanocomposites for visible-light-driven photocatalytic degradation of rhodamine B

Photogenerated charge carriers can undergo rapid recombination in conventional photocatalyst systems, reducing their photocatalytic efficiency. To address this bottleneck, a g-C3N4/BaTiO3 (CNB) heterojunction composite was decorated with different mass ratios of graphene oxide (GO) to form a novel visible-light responsive ternary GO-g-C3N4/BaTiO3 (GOCNB) nanocomposite using a facile fabrication method. The GOCNB photocatalyst exhibited significantly higher light absorption and greater charge transfer than CNB, g-C3N4, or BaTiO3. The photodegradation performance of GOCNB was optimized with a 2% mass loading of GO, and it achieved a degradation rate constant of 14.9 × 10-3 min-1 for rhodamine B with an efficiency of 94% within 180 min. The rate constant was 8-fold and 6-fold higher than that of bare BaTiO3 and CNB, respectively. The stronger photocatalytic activity was attributed to the synergistic effect of GO, g-C3N4, and BaTiO3, with g-C3N4 and BaTiO3 promoting charge transfer within a wider visible light range and GO promoting electron mobility and the photocatalyst's adsorption capacity. In particular, the proposed system maintained the spatial separation of photogenerated electron-hole pairs, which is vital for high photocatalytic activity. This study provides new insights into semiconductor-based photocatalytic systems and suggests a route for more environmentally sustainable technologies.

Synergistic sono-adsorption and adsorption-enhanced sonochemical degradation of dyes in water by additive manufactured PVDF-based materials

The present study proposes the first mechanistic model accounting for the most meaningful physico-chemical phenomena taking place in liquid phase adsorption processes under ultrasound. Initially, this study was aimed at developing an easy-to-make and easy-to-recover piezocatalyst for the degradation of RhB in water by combining the high piezocatalytical performance of BaTiO3 with a compatible piezoelectric support such as PVDF, manufactured by a customised additive manufacturing - direct ink writing system with in-situ poling. However, initial results showed that the resulting PVDF-BaTiO3 composite slabs performed worse than BaTiO3 piezocatalysts on their own, and that poling did not have any effect on their performance (82% RhB removal after 2 h when using either poled or unpoled PVDF-BaTiO3 composite slabs compared to 92% RhB removal after 2 h in presence of BaTiO3 piezocatalysts). Further investigation with pure PVDF materials demonstrated that, instead of piezocatalysis, synergistic ultrasound-assisted adsorption and sonochemical degradation were taking place, enabling the removal of >95% of the dye within 40 min of ultrasound treatment in the presence of 4 g L-1 of additive manufactured PVDF slabs. The results of this study and their evaluation with the mechanistic model proposed for liquid phase adsorption under ultrasound suggest that the adsorption of RhB on additive manufactured PVDF slabs was enhanced by the structure, higher specific surface ratio and higher volume of mesopores achieved through the 3D-printing process, as well as the minimisation of film resistance to mass transport due to ultrasound. Moreover, adsorption on additive manufactured PVDF enhanced the sonochemical degradation of the dye due to its high concentration in the adsorbed phase. This study demonstrates that adsorption processes, especially in the presence of PVDF materials, may be significantly more important in piezocatalysis than what has been reported to date, to the point that the synergistic combination of sono-adsorption and sonochemical degradation in presence of additive-manufactured PVDF slabs may be enough to achieve high removal rates of dyes in water.

An Emerging Family of Piezocatalysts: 2D Piezoelectric Materials

Piezocatalysis is an emerging technique that holds great promise for the conversion of ubiquitous mechanical energy into electrochemical energy through piezoelectric effect. However, mechanical energies in natural environment (such as wind energy, water flow energy, and noise) are typically tiny, scattered, and featured with low frequency and low power. Therefore, a high response to these tiny mechanical energies is critical to achieving high piezocatalytic performance. In comparison to nanoparticles or 1D piezoelectric materials, 2D piezoelectric materials possess characteristics such as high flexibility, easy deformation, large surface area, and rich active sites, showing more promise in future for practical applications. In this review, state-of-the-art research progresses on 2D piezoelectric materials and their applications in piezocatalysis are provided. First, a detailed description of 2D piezoelectric materials are offered. Then a comprehensive summary of the piezocatalysis technique is presented and examines the piezocatalysis applications of 2D piezoelectric materials in various fields, including environmental remediation, small-molecule catalysis, and biomedicine. Finally, the main challenges and prospects of 2D piezoelectric materials and their applications in piezocatalysis are discussed. It is expected that this review can fuel the practical application of 2D piezoelectric materials in piezocatalysis.

Composition-driven morphological evolution of BaTiO3 nanowires for efficient piezocatalytic hydrogen production

Hydrogen production from water by piezocatalysis is very attractive owing to its high energy efficiency and novelty. BaTiO3, a highly piezoelectric material, is particularly suitable for this application due to its high piezoelectric potential, non-toxic nature, and physicochemical stability. Owing to the critical role of morphology on properties, one-dimensional (1D) materials are expected to exhibit superior water-splitting performance and thus there is a need to optimise the processing conditions to develop outstanding piezocatalysts. In the present work, piezoelectric BaTiO3 nanowires (NWs) were hydrothermally synthesised with precursor Ba:Ti molar ratios of 1:1, 2:1, and 4:1. The morphology, defect chemistry, and hydrogen evolution reaction (HER) efficiency of the as-synthesised BaTiO3 NWs were systematically investigated. The results showed that the morphological features, aspect ratio, structural stability and defect contents of the 1D morphologies collectively have a significant impact on the HER efficiency. The morphological evolution mechanism of the 1D structures were described in terms of ion exchange and dissolution-growth processes of template-grown BaTiO3 NWs for different Ba:Ti molar ratios. Notably, the BaTiO3 NWs synthesised with Ba:Ti molar ratio of 2:1 displayed high crystallinity, good defect concentrations, and good structural integrity under ultrasonication, resulting in an outstanding HER efficiency of 149.24 μmol h-1g-1 which is the highest obtained for nanowire morphologies. These results highlight the importance of synthesis conditions for BaTiO3 NWs for generating excellent piezocatalytic water splitting performance. Additionally, post-ultrasonication tested BaTiO3 NWs demonstrated unexpected photocatalytic activity, with the BTO-1 sample (1:1 Ba:Ti) exhibiting 56% photodegradation of RhB in 2 h of UV irradiation.

Universal Piezo-Photocatalytic Wastewater Treatment on Realistic Pollutant Feedstocks by Bi4TaO8Cl: Origin of High Efficiency and Adjustable Synergy

Clean water is a fundamental human right but millions struggle for it daily. Herein, we demonstrate a new piezo-photocatalyst with immense structural diversity for universal wastewater decontamination. Single-crystalline Bi₄TaO₈Cl nanoplates with exposed piezoelectric facets exhibit visible-light response, piezoelectric behavior with coercive voltages of ±5 V yielding 0.35% crystal deformation, and pressure-induced band-bending of >2.5 eV. Using five common contaminants of textile and pharmaceutical industries, we show that the nanoplates can mineralize them in all piezocatalytic, photocatalytic, and piezo-photocatalytic approaches with efficiencies higher than most catalysts developed for just one contaminant. Their efficiencies for feedstocks differing over 2 orders of magnitude in concentrations, the highest to date, are also demonstrated to simulate real-life situations. These extensive studies established that combining piezocatalytic and photocatalytic approaches can lead to a tremendous synergy exceeding >45%. The origin of synergy has been illustrated for the first time using band-bending models and improved charge transfer from valence and conduction band electronic surfaces. We further quantified synergy across reactants, concentrations, and ultrasonic frequency and power to demonstrate their versatility and unpredictability. Finally, seven parameters that contribute to synergy but create unpredictability have been identified for the rational design of piezo-photocatalysts for wastewater treatment.

Insight into the synergistic mechanism of sonolysis and sono-induced BiFeO3 nanorods piezocatalysis in atenolol degradation: Ultrasonic parameters, ROS and degradation pathways

Herein, BiFeO₃ nanorods (BFO NRs) was synthesized as the piezoelectric catalyst. The synergistic mechanism of sonolysis and sono-induced BFO-piezocatalysis in atenolol degradation was revealed and the effect of ultrasonic parameters on it was investigated for the first time. The results indicated that 100 kHz was the optimal frequency for the sonolytic and sono-piezocatalytic degradation of atenolol in ultrasound/BFO nanorods (US/BFO NRs) system, with the highest synergistic coefficient of 3.43. The piezoelectric potential differences of BFO NRs by COMSOL Multiphysics simulations further distinguishing that the impact of cavitation shock wave and ultrasonic vibration from sonochemistry reaction (i.e., 2.48, -2.48 and 6.60 V versus 0.008, -0.008 and 0.02 V under tensile, compressive and shear stress at 100 kHz). The latter piezoelectric potentials were insufficient for reactive-oxygen-species (ROS) generation, while the former contributed to 53.93% •OH yield in US/BFO NRs system. Sono-piezocatalysis was found more sensitive to ultrasonic power density than sonolysis. The quenching experiments and ESR tests indicated that the ROS contribution in atenolol degradation followed the order of •OH > ¹O₂ > h⁺ > O₂^•- in US/BFO NRs system and ¹O₂ generation is exclusively dissolved-oxygen dependent. Four degradation pathways for atenolol in US/BFO NRs system were proposed via products identification and DFT calculation. Toxicity assessment by ECOSAR suggested the toxicity of the degradation products could be controlled.

Enhanced piezocatalytic hydrogen evolution performance of bismuth vanadate by the synergistic effect of facet engineering and cocatalyst engineering

Developing piezocatalysts with excellent piezocatalytic hydrogen evolution reaction (HER) performance is highly desired but also challenging. Here, facet engineering and cocatalyst engineering are employed to synergistically improve the piezocatalytic HER efficiency of BiVO₄ (BVO). Monoclinic BVO catalysts with distinct exposed facets are synthesized by adjusting pH of hydrothermal reaction. The BVO with highly exposed {110} facet exhibits a superior piezocatalytic HER performance (617.9 μmol g^-1h^-1) compared with that with {010} facet, owing to the strong piezoelectric property, high charge transfer efficiency, and excellent hydrogen adsorption/desorption capacity. The HER efficiency is enhanced by 44.7% by selectively depositing cocatalyst of Ag nanoparticles specifically on the reductive {010} facet of BVO, where the Ag-BVO interface provides the directional electron transport for high-efficiency charge separation. Under the collaboration between cocatalyst of CoO_x on {110} facet and the hole sacrificial agent of methanol, the piezocatalytic HER efficiency is evidently enhanced by 2 times because CoO_x and methanol can impede the water oxidation and improve the charge separation. This easy and simple strategy provides an alternative perspective on designing high-performance piezocatalysts.

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.

Hyaluronic acid-covered piezoelectric nanocomposites as tumor microenvironment modulators for piezoelectric catalytic therapy of melanoma

Increasing the formation of reactive oxygen species (ROS) and reducing the elimination of ROS are the two main objectives in the development of novel inorganic sonosensitizers for use in sonodynamic therapy (SDT). Therefore, BTO-Pd-MnO₂-HA nanocomplexes with targeted tumor cells and degradable oxygen-producing shells were designed as piezoelectric sonosensitizers for enhancing SDT. The deposition of palladium particles (Pd NPs) leads to the formation of Schottky junctions, promoting the separation of electron-hole pairs and thereby increasing the efficiency of toxic ROS generation in SDT. The tumor microenvironment (TME) triggers the degradation of MnO₂, and the released Mn²⁺ ions catalyze the generation of hydroxyl radicals (•OH) from H₂O₂ through a Fenton-like reaction. BTO-Pd-MnO₂-HA can continuously consume glutathione (GSH) and generate O₂, thereby improving the efficiency of SDT and chemodynamic therapy (CDT). A multistep enhanced SDT process mediated by the piezoelectric sonosensitizers BTO-Pd-MnO₂-HA was designed, targeted by hyaluronic acid (HA), activated by decomposition in TME, and amplified by deposition of Pd. This procedure not only presents a new alternative for the improvement of sonosensitizers but also widens the application of piezoelectric nanomaterials in biomedicine.

Hierarchically Designed Biodegradable Polylactide Particles with Unprecedented Piezocatalytic Activity and Biosafety for Tooth Whitening

At-home tooth whitening solutions with good efficacy and biosafety are highly desirable to meet the ever-growing demand for aesthetic dentistry. As a promising alternative to the classic peroxide bleaching that may damage tooth enamel and gums, piezocatalysis has been recently proposed to realize non-destructive whitening by toothbrushing with piezoelectrical particles. However, traditional particles either pose potential threats to human health or exhibit low piezoresponse to weak mechanical stimuli in the toothbrushing. Here, biocompatible and biodegradable polylactide particles constructed from interlocking crystalline lamellae have been hierarchically designed as next-generation whitening materials with ultra-high piezocatalytic activity and biosafety. By simultaneously controlling the chain conformation within lamellae and the porosity of such unique lamellae network at the nano- and microscales, the particles possessing unprecedented piezoelectricity have been successfully prepared due to the markedly increased dipole alignment, mechanical deformability, and specific surface area. The piezoelectric output can reach as high as 18.8 V, nearly 50 times higher than that of common solid polylactide particles. Consequently, their piezocatalytic effect can be readily activated by a toothbrush to rapidly clean the teeth stained with black tea and coffee, without causing detectable enamel damage. Furthermore, these particles have no cytotoxicity. This work presents a paradigm for achieving high piezoelectric activity in polylactide, which enables its practical application in tooth whitening.

Contact-Piezoelectric Bi-Catalysis of an Electrospun ZnO@PVDF Composite Membrane for Dye Decomposition

The treatment of organic pollutants in wastewater is becoming a great challenge for social development. Herein, a novel contact-piezoelectric bi-catalysis of a ZnO@ PVDF composite membrane was prepared by electrospinning technology. The obtained ZnO@PVDF composite membranes is superior to the pure PVDF membrane in decomposing methyl orange (MO) under ultrasonication at room temperature, which is mainly attributed to the synergy effect of the contact-electro-catalysis of dielectric PVDF, as well as the piezoelectric catalysis of tetrapodal ZnO and the β-phase of PVDF. The heterostructure of the piezoelectric-ZnO@dielectric-PVDF composite is beneficial in reducing the electron/hole pair recombination. As compared to the pure PVDF membrane, the catalytic degradation efficiency of the ZnO@PVDF composite membrane was improved by 444.23% under ultrasonication. Moreover, the reusability and stability of the composite membrane are comparable to those of the traditional powdered catalyst. This work offers a promising strategy for improving the pollutant degradation by combining contact-electro-catalysis with piezoelectric catalysis.

A Novel Strategy for Excellent Piezocatalytic Activity in Lead-Free BaTiO3-Based Materials via Manipulating the Multiphase Coexistence

Piezocatalysis is regarded as a fascinating technology for water remediation and possible disease treatment. A high piezoelectric coefficient (d₃₃) is one of the most important parameters to determine piezocatalytic performance, which can be manipulated via phase boundary design. Herein, a novel strategy for excellent piezocatalytic activity in lead-free BaTiO₃-based materials via manipulating the multiphase coexistence is proposed. The piezocatalyst of 0.82Ba(Ti_0.89Sn_0.11)O₃-0.18(Ba_0.7Ca_0.3)TiO₃ (0.82BTS-0.18BCT) with multiphase coexistence is prepared, and a large d₃₃ can be obtained. As a result, 0.82BTS-0.18BCT exhibits excellent piezocatalytic performance for the degradation of Rhodamine B (RhB). Furthermore, the removal rate of RhB could reach more than 90% after vibration for 30 min, and the reaction rate constant (k) could reach 0.0706 min^-1, which is much superior to that of most other representative perovskite-structured piezoelectric materials. Excellent piezocatalytic performance can be attributed to the strong local ferro-/piezoelectric response induced by the multiphase coexistence, as confirmed by the in situ piezoresponse force microscopy (PFM). Finally, the piezocatalytic degradation mechanism is analyzed systemically and proposed. This work not only provides a high-efficiency piezocatalyst but also sheds light on developing efficient BT-based piezocatalysts by manipulating the multiphase coexistence.

Enhanced piezotronics by single-crystalline ferroelectrics for uniformly strengthening the piezo-photocatalysis of electrospun BaTiO3@TiO2 nanofibers

Turning the built-in electric field by modulating the morphology and microstructure of ferroelectric materials is considered a viable approach to enhancing the piezo-photocatalytic activity of the ferroelectric/oxide semiconductor heterojunctions. Here, hydrothermally synthesized single-crystalline BaTiO₃ nanoparticles are employed to construct BaTiO₃@TiO₂ hybrid nanofibers by sol-gel assisted electrospinning of TiO₂ nanofibers and annealing. Because of the obvious enhancement of the synergetic piezo-photocatalytic effect under both ultrasonic and ultraviolet (UV) light irradiation, the piezo-photocatalytic degradation rate constant (k) of BaTiO₃@TiO₂ hybrid nanofibers on methyl orange (MO) reaches 14.84 × 10^-2 min^-1, which is approximately seven fold that for piezocatalysis and six fold that for photocatalysis. Moreover, BaTiO₃@TiO₂ core-shell nanoparticles are also synthesized for comparison purposes to assess the influence of microstructure on the piezo-photocatalysis by a wet-chemical coating of TiO₂ on BaTiO₃ nanoparticles. Such a high piezo-photocatalytic activity is attributed to the enhancement of the piezotronic effect by the single-crystalline ferroelectric nanoparticles and the nanoconfinement effect caused by the one-dimensional boundary of nanofibers with high specific surface areas. The mechanically induced uniform local built-in electric fields originated from the single-crystalline ferroelectric nanoparticles can enhance the separation of photogenerated electron and hole pairs and promote the formation of free hydroxyl radicals, resulting in a strong piezotronic effect boosted photochemical degradation of organic dye. This work introduces the single-crystalline ferroelectrics to construct ferroelectric/oxide semiconductor heterojunctions, and the enhanced local piezotronic effect uniformly strengthens the photochemical reactivity, which offers a new option to design high-efficiency piezo-photocatalysts for pollutant treatment.

Piezo-promoted persulfate activation by SrBi2B2O7 for efficient sulfadiazine degradation from water

Combining piezoelectric effect and persulfate (PS) activation is a newly developed strategy for refractory emerging contaminants removal. In this work, borate SrBi₂B₂O₇ (SBBO) is firstly developed as a piezoelectric material to piezo-assisted activation of PS for the removal of sulfadiazine (SDZ) under ultrasonic irradiation (US). SDZ could be efficiently degraded by 85.61 % in the system of PS/SBBO/US with a pseudo-first-order rate constant of 0.0520 min^-1, which is faster than that in the systems of PS/SBBO (0.0210 min^-1), SBBO/US (0.0041 min^-1), PS/US (0.0074 min^-1), and PS/BaTiO₃/US (0.0120 min^-1). The excellent degradation performance of the PS/SBBO/US system is mainly attributed to the piezoelectric effect of the SBBO which plays an important role in PS activation and accelerating reaction. Two oxidation processes, radical process (•O₂^- and •SO₄^-) and non-radical process (¹O₂ and electron transfer), exist during the SDZ degradation. The system of PS/SBBO/US also attains excellent removal efficiency in different SDZ contained water bodies. The possible degradation pathways mainly include cleavage of bonds, ring-opening, and hydroxylation process, and the toxicity of intermediates was predicted by T.E.S.T. software. This study provides new insight into piezoelectric catalysis associated with PS activation for SDZ removal.

Enhanced Piezocatalytic Activity in Bi1/2Na1/2TiO3 for Water Splitting by Oxygen Vacancy Engineering

Piezoelectric materials have demonstrated applicability in clean energy production and environmental wastewater remediation through their ability to initiate a number of catalytic reactions. In this study, we used a conventional sol-gel method to synthesize lead-free rhombohedral R3c bismuth sodium titanate (BNT) particles of various sizes. When used as a piezocatalyst to generate H₂ through water splitting, the BNT samples provided high production rates (up to 506.70 μmol g^-1 h^-1). These piezocatalysts also degraded the organic pollutant methylene blue (MB, 20 mg L^-1) with high efficiency (up to k = 0.039 min^-1), suggesting their potential to treat polluted water. Finally, we found that the piezopotential caused band tilting in the semiconductor and aided charge transfer such that recombination was suppressed and the rate of H₂ production increased. The mechanism of piezoelectric catalysis involved oxygen vacancies, the size of the catalyst, and the internal electric field playing important roles to enhance electron-hole separation, which further enhanced the catalysis reactions.

A BaTiO3/WS2 composite for piezo-photocatalytic persulfate activation and ofloxacin degradation

Piezoelectric fields can decrease the recombination rate of photogenerated electrons and holes in semiconductors and therewith increase their photocatalytic activities. Here, a BaTiO₃/WS₂ composite is synthesized and characterized, which combines piezoelectric BaTiO₃ nanofibers and WS₂ nanosheets. The piezo-photocatalytic effect of the composite on the persulfate activation is studied by monitoring Ofloxacin (OFL) degradation efficiency. Under mechanical forces, LED lamp irradiation, and the addition of 10 mM persulfate, the OFL degradation efficiency reaches ~90% within 75 min, which is higher than efficiencies obtained for individual BaTiO₃, WS₂, or TiO₃, widely used photocatalysts in the field of water treatment. The boosted degradation efficiency can be ascribed to the promotion of charge carrier separation, resulting from the synergetic effect of the heterostructure and the piezoelectric field induced by the vibration. Moreover, the prepared composite displays good stability over five successive cycles of the degradation process. GC-MS analysis is used to survey the degradation pathway of OFL during the degradation process. Our results offer insight into strategies for preparing highly effective piezo-photocatalysts in the field of water purification.

NaNbO3 Nanorods: Photopiezocatalysts for Elevated Bacterial Disinfection and Wastewater Treatment

In the present work, ferroelectric sodium niobate (NaNbO₃) nanorods are formulated to attain photopiezocatalysis for water pollutant degradation and bacterial disinfection. NaNbO₃ nanorods, integrating the advantages of photocatalysis (generation of free charge carriers) and piezocatalysis (separation of these charge carriers), possess synergistic effects, which results in a higher catalytic activity than photocatalysis and piezocatalysis alone. Active species that are involved in the catalytic process are found to be ^•O₂ ^- < OH^• < h⁺, indicating the significance of piezocatalysis and photocatalysis. The degradation efficiency of sodium niobate (NaNbO₃) nanorods for Rhodamine B in the presence of both sunlight and ultrasonic vibration is 98.9% within 60 min (k = 7.6 × 10^-2 min^-1). The piezo potential generated by NaNbO₃ nanorods was reported to be 16 V. The antibacterial activity of the produced sample was found to be effective against Escherichia coli. With inhibitory zones of 23 mm, sodium niobate has a greater antibacterial activity.

Piezoelectric polarization promoted spatial separation of photoexcited electrons and holes in two-dimensional g-C3N4 nanosheets for efficient elimination of chlorophenols

Graphitic carbon nitride (g-C₃N₄) has been proved to be a potential photocatalyst for environment purification, but the high recombination rate of photogenerated carriers leads to the low photocatalytic efficiency. Herein, we report the enhanced degradation of chlorophenols by 2D ultrathin g-C₃N₄ nanosheets with intrinsic piezoelectricity through photopiezocatalysis strategy. Under the simultaneous visible-light irradiation and ultrasonic vibration, the 2D g-C₃N₄ presented improved removal efficiency for elimination of 2,4-dichlorophenol (2,4-DCP) with an apparent rate constant of 6.65 × 10^-2 min^-1, which was 6.7 and 2.2 times of the photocatalysis and piezocatalysis, respectively. The improved removal efficiency was attributed to the sufficient separation of free charges driven by the ultrasound-induced piezoelectric field in the 2D g-C₃N₄, which was demonstrated by the enhanced current response under photopiezocatalysis mode. Additionally, the photopiezocatalysis of 2D g-C₃N₄ was proved to possess well universality for removing different chlorophenols, as well as high durability and dechlorination efficiency. Finally, a possible photopiezocatalytic mechanism for removal of 2,4-DCP was proposed based on the electron paramagnetic resonance (EPR) technique and the determination of intermediates through liquid chromatography-mass spectrometry (LC-MS) analysis. This work provides a promising strategy for the design of energy-conversion materials towards capturing solar and mechanical energy in ambient environment.

A review of advances in multifunctional XTiO3 perovskite-type oxides as piezo-photocatalysts for environmental remediation and energy production

Over more than three decades, the field of engineering of photocatalytic materials with unique properties and enhanced performance has received a huge attention. In this regard, different classes of materials were fabricated and used for different photocatalytic applications. Among these materials, recently multifunctional XTiO₃ perovskites have drawn outstanding interest towards environmental remediation and energy conversion thanks to their unique structural, optical, physiochemical, electrical and thermal characteristics. XTiO₃ perovskites are able to initiate different surface catalytic reactions. Under ultrasonic vibration or heating, XTiO₃ perovskites can induce piezo-catalytic reactions due to the titling of their conduction and valence bands, resulting in the formation of separated charge carriers in the medium. In addition, under light irradiation, XTiO₃ perovskites are considered as a new class of photocatalysts for environmental and energy related applications. Herein, we addressed the recent advances on variously synthesized, doped and formulated XTiO₃ perovskite-type oxides showing piezo- and/or photocatalytic exploitation in environmental remediation and energy conversion. The control of structural crystallite size and phase, conductivity, morphology, oxygen vacancy control, doping agents and ratio has a significant role on the photocatalytic and piezocatalytic activities. The different piezo or/and photocatalytic processes mechanistic pathways towards varying applications were discussed. The current challenges facing these materials and future trends were addressed at the end of the review.

Piezo-responsive bismuth ferrite nanoparticle-mediated catalytic degradation of rhodamine B and pathogenic E. coli in aqueous medium and its extraction using external magnetic stimulation after successful treatment

Piezocatalytic bismuth ferrite nanoparticles (BFO) were used for the degradation of organic dye (RhB) and pathogenic bacteria (E. coli), then extracted using external magnetic stimulation after the successful operation.

Highly efficient piezoelectric field enhanced photocatalytic performance via in situ formation of BaTiO3 on Ti3C2Tx for phenolic compound degradation

Novel BaTiO3/Ti3C2Tx piezo-photocatalysts are fabricated via an in-situ solvothermal method. The synergistic effect of BaTiO3 and Ti3C2Tx increases piezo-photocatalytic activity.

General Strategy for ATiO3 (A = Ca, Sr, or Ba) Submicrospheres with Large Surface Area and its Photocatalytic Applications

Porous CaTiO3 solid submicrospheres, SrTiO3 and BaTiO3 hollow submicrospheres, which had larger surface area (40.21 to 228.18 m2·g-1) and uniform particle size, were synthesized by self-template assisted hydrothermal method with...

Recent advances in piezocatalytic polymer nanocomposites for wastewater remediation

Among several forms of water pollutants, common pesticides, herbicides, organic dyes and heavy metals present serious and persistent threats to human health due to their severe toxicity. Recently, piezocatalysis based removal of pollutants has become a promising field of research to combat such pollutions by virtue of the piezoelectric effect. In reality, piezoelectric materials can produce electron-hole separation upon external vibration, which greatly enhances the production of various reactive oxygen species (ROS) and further increases the pollutant degradation rate. Piezocatalysis does not alter the quality or composition of water, like several other conventional techniques (adsorption and photocatalysis), which makes this technique non-invasive. The simplicity and tremendously high efficacy of piezocatalysis have attracted researchers worldwide and thus various functional materials are employed for piezocatalytic wastewater remediation. In this frontier, we highlight and demonstrate recent developments on polymer based piezocatalytic nanocomposites to treat industrial wastewater in a facile manner that holds strong potential to be translated into a clean and green technology.

Piezoelectric polarization modulated novel Bi2WO6/g-C3N4/ZnO Z-scheme heterojunctions with g-C3N4 intermediate layer for efficient piezo-photocatalytic decomposition of harmful organic pollutants

It is of great significance to understand the role of carrier in piezocatalysis of composites by studying the separation mode of carriers under dynamic polarization field. Herein, the separation and migration pathways of carriers under piezoelectric field are investigated by synthesizing heterojunctions with Bi₂WO₆ (BWO) nanosheets grown vertically on g-C₃N₄ (CN) coated ZnO nanorods and directly on ZnO. Compared with the photocatalysis, the piezocatalytic efficiency of Rhodamine B (RhB) by BWO/ZnO is significantly increased to 0.121 min^-1, which indicated the polarization field promotes band tilt and Z-scheme formation. After introducing the CN interlayer, the piezocatalytic efficiency of BWO/CN/ZnO is further improved (0.217 min^-1), which can be attributed to the unique core-shell structure with Z-scheme heterojunctions. This unique structure provides more active sites and excited carrier concentration, the intermediate layer CN also reduces the direct contact and recombination of electrons and holes controlled by polarization potential at the interface between BWO and ZnO. This work deeply analyzes the influence of carrier concentration, separation efficiency and transport process on piezocatalysis, which provides a reference for the design of efficient catalysts.

Ultrahigh piezocatalytic capability in eco-friendly BaTiO3 nanosheets promoted by 2D morphology engineering

Piezocatalysis, converting mechanical vibration into chemical energy, is an emerging technology to address environmental issues. In this work, we propose an efficient method to significantly improve the piezocatalytic activity by morphology engineering rather than composition design. The catalytic property in BaTiO₃ nanocrystallites with diverse morphologies is investigated by dye degradation and hydrogen production under ultrasonic vibration. The BaTiO₃ nanosheets exhibit an excellent piezocatalytic activity with a degradation rate of 0.1279 min^-1 for Rhodamine B, far beyond those in previous piezocatalytic literature and even comparable to excellent photocatalysts, and also a high hydrogen production rate of 92 μmol g^-1 h^-1. Compared with nanowires and nanoparticles, the 2D morphology greatly enhances the piezocatalytic activity in nanosheets owing to much larger piezoelectric potential. This proves that the piezocatalytic property is dominated by the morphology-dependent piezoelectricity, rather than specific surface area as other catalysis. Dominated by bending vibrating mode, the piezocatalytic activity reaches a maximum at the piezoelectric resonating frequency, and it increases with the ultrasonic power. Moreover, it has good reusability and wide versatility for catalytic degradation. This work gives an in-depth understanding of piezocatalytic mechanism and provides a way to develop high performance and eco-friendly piezocatalysts.

Ligand-Metal Charge Transfer Induced via Adjustment of Textural Properties Controls the Performance of Single-Atom Catalysts during Photocatalytic Degradation

Because of their peculiar nitrogen-rich structure, carbon nitrides are convenient polydentate ligands for designing single atom-dispersed photocatalysts. However, the relation between catalysts' textural properties and their photophysical-photocatalytic properties is rarely elaborated. Herein, we report the preparation and characterization of a series of single-atom heterogeneous catalysts featuring highly dispersed Ag and Cu species on mesoporous graphitic C3N4. We show that adjustment of materials textural properties and therefore metal single-atom coordination mode enables ligand-to-metal charge transfer (LMCT) or ligand-to-metal-to-ligand charge transfer (LMLCT), properties that were long speculated in single-atom catalysis but never observed. We employ the developed materials in the degradation of organic pollutants under irradiation with visible light. Kinetic investigations under flow conditions show that single atoms of Ag and Cu decrease the number of toxic organic fragmentation products while leading to a higher selectivity toward full degradation. The results correlate with the selected mode of charge transfer in the designed photocatalysts and provide a new understanding of how the local environment of a single-atom catalyst affects the surface structure and reactivity. The concepts can be exploited further to rationally design and optimize other single-atom materials.

Cocatalyst Engineering in Piezocatalysis: A Promising Strategy for Boosting Hydrogen Evolution

Piezoelectric semiconductor-based piezocatalysis has emerged as a promising approach for converting mechanical energy into chemical energy for renewable hydrogen generation and wastewater treatment under the action of mechanical vibration. Similar to photocatalysis, piezocatalysis is triggered by the separation, transfer, and consumption of piezo-generated electrons and holes. Inspired by this, herein, we report that the cocatalyst, which is widely used in photocatalysis, can also improve the semiconductor-based piezocatalytic properties. In the proof-of-concept design, well-defined Pd as a model cocatalyst has been deposited on the surface of piezoelectric BiFeO₃ nanosheets, which not only facilitates the separation of charge carriers by accepting the piezoelectrons from BiFeO₃ but also lowers the activation energy/overpotential through supplying highly active sites for the proton reduction reaction. Consequently, the as-obtained hybrid piezocatalyst delivers a high H₂ evolution rate of 11.4 μmol h^-1 (10 mg of catalyst), 19.0 times as high as that of bare BiFeO₃. The band tilting induced by the piezoelectric potential is proposed to lower or eliminate the Schottky barrier and smooth the electron transfer from BiFeO₃ to Pd, while the exposed facet, domain size, and loading amount of Pd cocatalyst are proved to be the key parameters determining the ultimate piezocatalytic activity. Our work provides some enlightenment on advancing the design and fabrication of more efficient piezocatalysts for H₂ evolution based on rational engineering on the cocatalyst.

Monoclinic dibismuth tetraoxide (m-Bi2O4) for piezocatalysis: new use for neglected materials

Piezocatalysis is a promising approach for environmental pollutant removal. Monoclinic dibismuth tetraoxide (m-Bi₂O₄) was first applied to piezocatalyze organics under ultrasonic vibration. The built-in electric field with ultrasonic stress drives the separation of holes and electrons in m-Bi₂O₄. Its excellent piezocatalytic activity, reusability and chemical stability make m-Bi₂O₄ a new candidate of piezocatalysis.

Enhanced Piezocatalytic Activity of Sr0.5Ba0.5Nb2O6 Nanostructures by Engineering Surface Oxygen Vacancies and Self-Generated Heterojunctions

Piezocatalysis provides a promising strategy for directly converting weak mechanical energy into chemical energy. In this work, we report a simple one-step hydrogen reduction route for the simultaneous generation of surface defects and heterojunctions in Sr_0.5Ba_0.5Nb₂O₆ nanorods fabricated by a molten salt synthesis method. The as-fabricated Sr_0.5Ba_0.5Nb₂O₆/Sr₂Nb₂O₇ nanocomposites with controllable oxygen vacancies exhibited excellent piezocatalytic activity under ultrasonic vibration, with an about 7 times enhancement of the rate constant (k = 0.0395 min^-1) for rhodamine B degradation and an about 10 times enhancement of the water-splitting efficiency for hydrogen generation (109.4 μmol g^-1 h^-1) for the optimized sample (H₂ annealed at 500 °C) compared to pristine Sr_0.5Ba_0.5Nb₂O₆ nanorods. This work demonstrates the essential role of a well-modulated oxygen vacancy concentration in the piezocatalytic activity and provides an inspiring guide for designing self-generated heterojunction piezocatalysts.

Rhodium doping augments photocatalytic activity of barium titanate: effect of electronic structure engineering

Environmentally friendly BaTiO3 is emerging as a potential photocatalyst due to its tunable electronic structure. Although originally believed to be a poor photocatalyst due to its wide band gap, several strategies have been implemented to reduce its band gap. One such approach is doping, but this often leads to the formation of mid gap recombination centers and diminishes the efficiency of the material. In the present work, we study for the first time the effect of site occupancy of Rh on the electronic structure of BaTiO3. As the theoretical results reveal that if Rh occupies both Ba and Ti sites simultaneously it leads to the formation of mid gap states, an experimental approach is implemented to reduce the band gap of BaTiO3 while simultaneously avoiding the formation of recombination centers. The facile one pot hydrothermal approach reported here directs the Rh towards Ba sites leading to a decrease in the band gap due to the appearance of donor Rh3+ states, suppressing the formation of Rh4+ states. A promising photocatalytic activity of 96% degradation of methylene blue dye in 120 minutes was observed for the 0.5 Rh sample indicating the high efficiency of the material.