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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.

The antiferroelectric (Pb0.985Sm0.01) (Zr1-xTix)O3 (Ti-PSZO) thin films were synthesized on Pt(111)/Ti/SiO2/Si substrates using a chemical solution deposition method. The films were crystallized in the perovskite phase with a preferential orientation along (111) direction. With Ti doping in PSZO, a gradual transformation from antiferroelectric to ferroelectric phase transition was noticed at room temperature owing to the Ti doping induced lattice distortion. The phase transition has been confirmed through the P - E hysteresis loops, X-ray diffraction (peak shifting), capacitance-voltage measurements, and Raman scattering analysis. The thin film with Ti = 0.15 doping displayed a ferroelectric behavior with high dielectric constant and large dielectric tunability of about 62%. Also, Ti doping altered the Curie temperature (Tc) and enhanced the order of dielectric diffuseness. It is believed that Ti-doping in PSZO is an effective way to induce an antiferroelectric - ferroelectric phase transition and to tailor the electrical characteristics of PSZO thin films.

Developing new techniques that can synthesize one-dimensional piezoelectric materials on a large scale is of great significance for boosting piezocatalytic applications. In this work, we proposed a high-efficiency template hydrothermal method for large-scale synthesis of piezoelectric Na0.5Bi0.5TiO3 (NBT) nanowires. By ion-exchange with Bi3+, Na2Ti3O7 template nanowires can be easily and entirely transformed to NBT. The piezocatalytic activity of the NBT nanowires was thoroughly investigated with respect to their capability to degrade typical organic pollutants, including Rhodamine B, methylene blue, methyl orange, tetracycline hydrochloride, phenol, and bisphenol A. The NBT nanowires exhibited the highest efficiency in piezocatalytic degradation of Rhodamine B, which was completely decomposed within 80 min (rate constant ∼0.0575 min−1). The electron spin resonance spin-trapping technique and active species capture experiments were employed to characterize free radicals. The present work is advantageous for the high yield of NBT nanowires and the excellent piezocatalytic performance. The reported template hydrothermal method can potentially be extended to the synthesis of other perovskite nanowires.

Excellent dielectric frequency, bias, and temperature stability of bismuth silicate (Bi2SiO5, BSO) thin films with a low dielectric loss has been obtained in this study. The thin films were prepared on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method at a relatively low annealing temperature of 500 °C. The BSO films have a preferred growth along (200) orientation with dense fine-grained surface morphology. The dielectric constant and dielectric loss of the thin film annealed at 500 °C are 57 and 0.01, respectively, at 100 kHz, with little change between 1 kHz and 100 kHz and in the bias electric field range between −250 kV/cm and 250 kV/cm, indicating that the thin film exhibits a low dielectric loss as well as excellent frequency and bias field stability. The dielectric-temperature measurements confirmed that the BSO thin film annealed at 500 °C also has good temperature stability between 150 K and 450 K. Our results suggest that the BSO thin films have potential applications in the next-generation integrated capacitors.

The piezocatalytic performance of Sm-doped Pb(Mg1/3Nb2/3)O3-xPbTiO3 solid solutions, which exhibit ultrahigh piezoelectric coefficient with a dependence on PbTiO3 content, was thoroughly investigated. We focused mainly on the influence of piezoelectric property modulation on the piezocatalytic performance of the materials by composition arrangement. The maximal piezocatalytic activity was achieved as the composition approached the morphotropic phase boundary (MPB) at x = 0.29, in which the piezoelectric coefficient also reached the maximum. The linear relationship between the two properties proves that the piezoelectric effect plays a key role in the piezocatalytic performance. As a direct evidence of catalytic reactions, free radicals were detected by electron spin resonance (ESR) technique. Revolution of intermediate products observed by liquid chromatography-mass spectrometry (LC–MS) further confirms the catalytic reactions in the piezoelectricity induced dye degradation process. This work is of great significance for understanding the mechanism of piezocatalytic effect.

Antiferroelectric Sm-doped PbZrO3 (PSZO) thin films were prepared on Pt(111)/Ti/SiO2/Si substrates using a chemical solution deposition method. The effects of Sm doping concentration on structural, morphology, dielectric, antiferroelectric, and energy storage characteristics of PSZO thin films were investigated. With Sm doping, all the thin films were crystallized in perovskite phase along with a strong (111)- preferential orientation. It is found that Sm doping significantly altered the Curie temperature (Tc) and electric field-induced phase switching of the films. The PSZO film with Sm = 0.01 exhibited a maximum polarization of 53.7 μC/cm2, an energy storage density of 17.1 J/cm3 at ~444 kV/cm, and a dielectric constant of 175 at 1 kHz. The improved energy storage performance was attributed to high (111) degree of preferred orientation and site occupation of Sm ions.

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 Bi

Orthorhombic Bi2SiO5 thin films were fabricated on Pt/Ti/SiO2/Si substrates by incorporating a NiO thin layer between Bi2SiO5 and bottom electrode. Compared with those bare Pt/Bi2SiO5/Pt devices, a remarkably improved uniformity of resistive switching parameters such as electroforming voltages, reset voltages, and a resistance ratio of low/high states was demonstrated in the Bi2SiO5 devices with an embedded NiO layer. This improvement was attributed to the formation of the partial conductive filaments resulted from sufficient oxygen vacancies at the interface. Our results provide a method for the optimization of the operation voltage control toward forefront applications in nonvolatile memory.

We report the coexistence of resistive switching and magnetism modulation in the Pt/Co3O4/Pt devices, where the effects of thermal annealing and film thickness on the resistive and magnetization switching were investigated. The sol-gel derived nanocrystalline Co3O4 thin films obtained crack-free surface and crystallized cubic spinel structure. The 110 nm Co3O4 film based device annealed at 600 °C exhibited optimum resistive switching parameters. From I–V curves fitting and temperature dependent resistance, the conduction mechanism in the high-voltage region of high resistance state was dominated by Schottky emission. Magnetization-magnetic field loops demonstrated the ferromagnetic behaviors of the Co3O4 thin films. Multilevel saturation magnetization of the Co3O4 thin films can be easily realized by tuning the resistance states. Physical resistive switching mechanism can be attributed to the rejuvenation and annihilation of conductive filament consisting of oxygen vacancies. Results suggest that Pt/Co3O4/Pt device shows promising applications in the multifunctional electromagnetic integrated devices.

Orthorhombic Bi2SiO5 thin films with dense surface were synthesized by using a chemical solution deposition method. The crystallized films were first utilized to implement resistive memory cells with Pt/Bi2SiO5/Pt sandwich architecture. It exhibited outstanding switching parameters including concentrated distributions of low and high resistance states, uniform switching voltages, cycling endurance, and long retention. Furthermore, the model of formation and rupture of conductive filaments consisted of oxygen vacancies was used to well explain resistive switching behavior. The results revealed that the solution-processed Bi2SiO5 thin film devices have great potential for forefront application in nonvolatile memory.

Structure, dielectric, ferroelectric, fatigue, and photoluminescent properties of Bi3.86−kSm0.14LakTi3O12 (BSLT) thin films synthesized by sol–gel method were investigated. The XRD analysis indicated that the BSLT thin films formed single phase orthorhombic structure, and orthorhombic distortion was found to increase with increasing lanthanum doping content. Raman spectra revealed that BSLT thin films have significantly structure distorted because of La3+ doping, which is consistent with XRD results. Increasing La substitution results in improvement in dielectric and ferroelectric properties of BSLT thin films due to distorted structure and defects reduction. The films show almost fatigue-free ferroelectric polarization behavior at room temperature. Besides, the BSLT thin films show orange-red color emission at 604 nm and long florescence lifetime (about 0.75 ms). This study indicates that the thin films with an appropriate doping concentration of Sm3+, La3+ ions may have potential applications in multifunctional optoelectronic devices.

Ni-Cu ferrite (Ni1-xCuxFe2O4, x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) thin films were synthesized on Pt/Ti/SiO2/Si substrates via sol-gel method. The influence of Cu2+ ions substitution on the resistive switching and magnetism of nickel ferrite thin films were investigated. The appropriate 0.2 Cu2+ ions substituted Pt/NFO/Pt device exhibited improved resistive switching parameters such as lower electroforming voltages, narrow distribution of SET/RESET voltages, good endurance (103 cycles), and long data retention upto 105 s at 25 °C and 85 °C, respectively. With increasing Cu2+ ions substitution, the saturation magnetization of Ni-Cu ferrite thin films decreased. The 0.2 Cu2+ ions substituted Pt/NFO/Pt device illustrated the controlling magnetism by accompanying with high saturation magnetization to switch into low saturation magnetization during the resistive switching process. The modulation of Cu2+ ions substitution on the magnetic properties was linked with magnetic moments and oxygen vacancies concentrations. Current transport conduction mechanisms were Ohmic behavior in low resistance state and Schottky emission in high field region of high resistance state. Moreover, temperature dependence of resistance revealed that origin of switching mechanism was related with the hybrid Cu mental and oxygen vacancies conducting filaments. Our results suggest that controlling the paths of hybrid conductive filaments is an effective strategy to modulate resistive switching performance and magnetism in ferrite based devices.

Sol-gel method was used to prepare the Pr3+ ions-doped (1-x)Na0.5Bi0.5TiO3-xCaTiO3 (Pr-NBT-xCTO) (x = 0, 0.04, 0.06, 0.08, 0.1, 0.12, and 0.16) thin films on Pt/Ti/SiO2/Si and fused silicon substrates. The structure phase of thin films was evolving from rhombohedral (R3c) to orthorhombic (Pnma) with increasing CTO content. Owing to the morphotropic phase boundary (MPB), the improved ferroelectric and dielectric properties were obtained at x = 0.06–0.1. The MPB was formed from the concomitant phase of rhombohedral (R3c) and orthorhombic (Pnma). The Pr-NBT-0.08CTO thin film showed the best ferroelectric and dielectric properties, as well as strong relaxor behavior (the diffusion factor is γ = 1.79). In addition, all the films exhibited strong red emission as excited by UV light, and wide optical band-gap (3.44–3.47 eV), which might be influenced by grain size and structural variation. Our results indicate that Pr-NBT-xCTO thin films may have potential applications in ferroelectric-luminescence multifunctional optoelectronic devices.

Spinel Co3O4 thin films were synthesized using a sol–gel technique to study the annealing atmosphere effect on resistive switching (RS) and magnetic modulation properties. Compared with oxygen and air annealed Pt/Co3O4/Pt stacks, the nitrogen annealed Pt/Co3O4/Pt stack shows optimal switching parameters such as a lower forming voltage, uniform distribution of switching voltages, excellent cycle-to-cycle endurance (>800 cycles), and good data retention. Improvement in switching parameters is ascribed to the formation of confined conducting filaments (CFs) which are composed of oxygen vacancies. From the analysis of current–voltage characteristics and their temperature dependence, the carrier transport mechanism in the high-field region of the high resistance state was dominated by Schottky emission. Besides, temperature dependent resistance and magnetization variations revealed that the physical mechanism of RS can be explained based on the formation and rupture of oxygen vacancy based CFs. In addition, multilevel saturation magnetization under different resistance states is attributed to the variation of oxygen vacancy concentration accompanied with the changes in the valence state of cations. Results suggested that using a nitrogen annealing atmosphere to anneal the thin films is a feasible approach to improve RS parameters and enhance the magnetic properties of Co3O4 thin film, which shows promising applications to design multifunctional electro-magnetic coupling nonvolatile memory devices.

A variety of nanostructured Bi4Ti3O12 materials with diverse morphologies were synthesized by a novel hydrothermal method using layered titanate Na2Ti3O7 as a synthetic precursor. Among these materials, decussated nanoplates exhibit superior piezocatalytic activity compared with other piezocatalysts of the perovskite family. The enhanced piezocatalytic activity is attributed to the large piezoelectric potential difference and the short distance between polar surfaces, which may help enhance the driving force of charge transport. The finite element method (FEM) simulation of piezoelectric response in different Bi4Ti3O12 nanostructures was performed to illustrate the influence of morphological features on the piezocatalytic performance. The catalytic mechanism of Bi4Ti3O12 was investigated by the detection and characterization of free radicals and intermediate products with electron spin resonance (ESR) spin-trapping technique and liquid chromatography-mass spectrometry (LC-MS). This work may push forward the development of piezocatalytic materials, and provide insights into piezocatalysis for environmental applications.

Advanced oxidation processes including piezocatalysis and photocatalysis are considered as promising strategies for environmental remediation. More efforts have been made to clarify the factors that can improve the catalytic efficiency in piezocatalysis. Diphasic Ba1−xCaxTiO3 piezoelectric materials near a solubility limit have been found to exhibit potential for piezoelectric performance far beyond their corresponding single-phasic materials. It is interesting to explore how phase structures (single phase and diphase) will influence the catalytic performance of nanoscale piezoelectric materials. In this study, Ba1−xCaxTiO3 nanowires were used as representative materials to explore the influence of phase structures on catalytic performance. It is found that when x = 0.2 (near the solubility limit), the Ba1−xCaxTiO3 nanowires exhibit better piezocatalytic and photocatalytic performance. This can be ascribed to higher spontaneous polarization induced by the interaction of a ferroelectric tetragonal Ba-rich phase and a normal dielectric orthorhombic Ca-rich phase and band alignment of Ba-rich and Ca-rich phases, which can both effectively enhance charge carrier separation rates. In addition, with the simultaneous assistance of ultrasonic vibration and ultra-violet light, there is a coupling catalytic effect of Ba1−xCaxTiO3 nanowires, with a rate constant kobs of 71% higher than the simple sum-up of the rate constants of piezocatalytic and photocatalytic reactions. The increasing piezo-/photocatalytic degradation efficiency is mainly attributed to increased free charge carrier concentrations (thermally-excited and photo-induced charge carriers) and a strong piezoelectric field induced by the piezoelectric effect, which is beneficial for the separation of free charge carriers. We believe that this work provides mechanistic insights into how phase structures affect catalytic performance and a novel route to furnish environmentally-friendly catalysts that can efficiently utilize multiple energy sources.

Piezocatalysis has emerged as a novel technique to make use of mechanical energy in dealing with organic pollutants in wastewater. In this work, ferroelectric Bi2WO6 (BWO) nanosheets with a characteristic layered crystal structure and in-plane polarization were investigated, with special interest in the influence of their crystal symmetry and oxygen vacancies on the piezocatalytic performance. The nanosheets were synthesized by a hydrothermal process along with thermal annealing at different temperatures and atmospheres. The crystal symmetry, grain size and oxygen vacancies of the crystals were characterized by XRD, SEM, XPS and Raman spectroscopy. The piezocatalytic properties were investigated by degrading Rhodamine B (RhB) solution under ultrasonic vibration. The free radicals generated by ultrasonic treatment were detected and characterized using the electron spin resonance (ESR) method. The essential role of hydroxyl radicals in the dye degradation process was demonstrated by controlled experiments with different scavenger additives. The results show that the crystalline size and oxygen vacancies affect the degree of orthorhombic distortion and formation of defect energy levels, respectively, which will further affect the distribution of piezoelectric potential and piezocatalytic activity. This work will enable us to understand the influence of oxygen vacancies and crystal symmetry on piezocatalysis more intuitively and design more efficient piezocatalysts via a heat treatment control strategy.

In this Letter, spinel ferrite NiFe2O4 films prepared through pulsed laser deposition are used to fabricate Pt/NiFe2O4/Pt devices with the coexistence of unipolar resistive switching (URS) and bipolar resistive switching (BRS). The device demonstrated nonvolatile and stable resistive switching (RS) properties under URS and BRS modes. The current–voltage (I–V) fitting analysis and temperature dependence measurements suggested that one Pt/NiFe2O4/Pt RRAM cell follows two significantly different RS and carrier transport mechanisms under URS and BRS modes. Under URS mode, the carrier mechanism alternates between the high resistance state (HRS) with Schottky emission and low resistance state (LRS) with the Ohmic mechanism. In contrast, under BRS mode, carrier transport at HRS and LRS is dominated by space-charge-limited current and nearest-neighboring hopping, respectively. Finally, a physical model, combining oxygen ion (O2−) migration and electric field distribution simulated by COMSOL Multiphysics, is proposed to further clarify the coexistence of two distinct RS and carrier transport mechanisms in the single Pt/NiFe2O4/Pt RRAM device.

Powder-form piezocatalysts suffer from poor recyclability and pose a potential threat of creating serious secondary pollution, which restrict their practical applications. Thin-film piezocatalysts, which not only exhibit good recyclability but also fully contact with solution, are believed to be one of the solutions to address these problems. In this work, the nanostructured BaTiO

Switching instability significantly affects the development of resistive random access memories. An enhanced resistive switching uniformity was developed by embedding well-ordered Pt/Au nanostructures (NSs) in Pt/NiFe2O4/Pt devices. The well-ordered Pt/Au NSs arrays were developed on Pt/Ti/SiO2/Si substrates using 2D colloidal crystals as the deposition templates. The NiFe2O4 (NFO) thin films were then deposited on the Pt/Au NSs array via a spin coating technique, followed by annealing treatment. The memory devices were fabricated via Pt top electrode sputtering on the NFO film surface. The embedded device showed more uniform SET and RESET voltages and lower forming voltage than the non-embedded device. Distortion of electric field and temperature gradient around the Pt/Au NSs suppresses the random development of conductive filaments (CFs). Furthermore, the temperature gradient near the Au NSs could be relatively high during the forming process than in Pt NSs since Au has high thermal conductivity, promoting oxygen vacancy migration. Therefore, CFs easily nucleate in HRS and develop near Au NSs under the same bias voltage, leading to better resistive switching properties in the Au NSs array device than in the Pt NSs array device. This study provides an effective path for enhancing the resistive switching performances of devices.

We report on the structure, dielectric, ferroelectric, and photoluminescent properties of Sm3+-doped Bi4Ti3O12 thin films which were prepared on fused silica and Pt/Ti/SiO2/Si substrates by sol-gel method. The X-ray diffraction analysis confirmed that the Bi4-xSmxTi3O12 (BSmT) thin films were well crystallized in layered perovskite structure without any secondary phase. Raman spectra indicated that the structure of BSmT thin films was significantly distorted because of the Sm3+ doping. An appropriate doping amount of Sm3+ ions leads to obvious enhancement in ferroelectric and dielectric properties of BSmT thin films due to structure distortion and reduction in defects. In addition, the BSmT thin films also show orange-red color emission at 601 nm and long florescence lifetime (> 0.6 ms). This study indicated that lead-free BSmT thin films, which are featuring good electrical and photoluminescent properties, may have potential applications in integrated optoelectronic devices.

In this work, we have fabricated the MgZnO thin films directly on low resistive silicon substrates by using a chemical solution deposition route. Besides the investigation of the structures, morphologies, chemical states, and photoluminescence properties of MgZnO thin films, the bipolar resistive switching behaviours have been evaluated in the simple Ag/MgZnO/Si structures. The self-compliance resistive switching characteristics simplify the device configuration and circuit design for practical applications. In addition, the multilevel storage capabilities have also been investigated by regulating different operation voltages or compliance currents, respectively. The four level storage capabilities show long retention time and good cycling performance. The resistive switching mechanism has been explicated by the formation and rupture of Ag filament based on the electrochemical metallization model. This work suggests that the Ag/MgZnO/Si structures have potential non-volatile multilevel memory applications.

This work examined carefully the electrical performance of Pt/InGaZnO/W memory device and found the co-existence of volatile and non-volatile resistive switching effects dependent on the electroforming process. The Pt/InGaZnO/W device exhibited stable and volatile switches without electroforming process, which were originated from electron trapping and detrapping in intrinsic defects of InGaZnO thin films. However, after electroforming process, the device showed non-volatile bipolar resistive switching properties with high reliability including long retention, good endurance and concentrated switching voltages. On the basis of the resistance temperature dependence and the change of chemical states, the non-volatile resistive switching effects were explained by the formation and rupture of oxygen vacancies related filament. This study provides new insights into the effects of electroforming process and defect types on resistive switching behaviours.

A novel catalytic effect of pyroelectric materials induced by a change in temperature, namely pyroelectric catalysis, was found to be attractive due to its ability to utilize waste heat in pollution treatment. In this work, the pyroelectric catalytic properties of BaTiO3 (BTO) nanowires synthesized by a template hydrothermal method have been thoroughly investigated. The nanowires with an elongated polar axis show a superior pyroelectric catalytic performance in comparison with the equiaxial nanoparticles. Our numerical simulation results with a finite element method indicate that the enhanced catalytic efficiency of BTO nanowires can be attributed to the higher pyroelectric potential. On the basis of the pyroelectric effect and our experimental results, a pyroelectric catalytic degradation mechanism has been proposed by taking into account the migration of charge carriers and the formation of reaction radicals. This study for enhancing the pyroelectric catalytic activity by using BTO nanowires may provide a facile, promising, and new reusable strategy for the catalytic degradation of organic dye pollutant by means of temperature variation. It is hoped that the present work gives a clear understanding of the mechanism of pyroelectric catalysis.

The discovery of piezocatalysts opens up a new route to wastewater decontamination. The piezoelectricity of barium titanate nanocrystals induced by ultrasonic vibration was found effective to degrade organic pollutants. In the present work, the products of (Ba,Sr)TiO3 (BST) nanocrystals with different morphologies including nanowires (NWs), nanoparticles (NPs) and their composite were synthesized by a template hydrothermal method. And, the piezocatalytic behavior of them was thoroughly investigated with respect to a possible influence of lowered phase transition temperature on the piezocatalytic behavior. It was found that the elongated BST nanowires were more effective in degrading dye pollutants in comparison with the pure BaTiO3 nanowires and BST nanoparticles, which makes the BST nanowires a better candidate as piezocatalyst. The catalytic nature of the stress induced dye degradation was confirmed by the detection of free radicals and intermediate products during the degradation process. The improveme...

Effect of oxygen annealing on bipolar resistive switching (BRS) properties of TiN/ZnO/CeO2-x/Pt devices was investigated. Bilayer ZnO/CeO2-x thin films were fabricated by rf-magnetron sputtering. It was observed that the improvement in cycle-to-cycle endurance degradation and uniformity of the bilayer ZnO/CeO2-x thin film is optimum at 400 °C annealing temperature due to decrease in oxygen vacancies during annealing, as confirmed by x-ray photoelectron spectroscopy. The BRS could be caused by the formation of interfacial TiON layer, which is most likely to be accountable for creating an adequate quantity of oxygen vacancies necessary for the formation and rupture of conductive filaments. Smaller Gibbs free energy of the formation of interfacial TiON ( − 611 k J m o l − 1 ) layer as compared to bilayer film ZnO ( − 650 k J m o l − 1 ) and CeO2 ( − 1024 k J m o l − 1 ) results in an easier re-oxidation of the filaments through the oxygen exchange with TiN top electrode. The analysis of current–voltage characteristics shows that the charge transport mechanism is Schottky emission. Moreover, the temperature dependence of high resistance state (HRS) and low resistance state (LRS) revealed the physical origin of the RS mechanism, which entails the oxygen vacancies for the formation and rupture of conducting paths.

Nano-crystalline zinc ferrite (ZnFe2O4) magnetic thin film was prepared on Pt/Ti/SiO2/Si substrate by chemical solution deposition method though spin coating technique. Reproducible nonpolar resistive switching (RS) characteristics in Pt/ZnFe2O4/Pt devices are reported. By changing the magnitude and polarity of the applied electric bias, reversible transitions among all the four nonpolar RS modes can be realized. For each switching mode, I-V characteristics were measured in the temperature range of 200 K–340 K. Temperature dependent I-V characteristics of high resistance state (HRS) have been found to support the conduction mechanism across the metal/insulator/metal (MIM) systems to be Schottky emission. Associated parameters such as activation energy and effective height of Schottky barrier at zero biasing (ϕo) have been evaluated at different temperature and electric field. In addition, the increase in resistance of low resistance state (LRS) with temperature revealed that the conduction through ZnFe2O4 in LRS is metallic. Based on the temperature-dependent characteristics, the reversible switching among the four modes of nonpolar RS via formation and rupture of the conducting filaments has been attributed to the joint effect of field-induced migration of oxygen vacancies and metallic Zn atoms.

Piezoelectric effect, commonly known as a change in electric polarization in piezoelectric/ferroelectric materials under mechanical stress, is extensively employed as a driving force for the catalytic degradation of organic pollutants. However, the relationship between electric polarization and piezocatalytic activity is still unclear. In this work, we investigated the role of ferroelectric polarization in the piezocatalytic activity of BaTiO3 nanoparticles through annealing BaTiO3 at different temperatures or poling BaTiO3 at different electric fields. The BaTiO3 nanoparticles annealed at 800 °C exhibit effectively enhanced piezocatalytic activity compared with those annealed at other temperatures. The polycrystalline particles annealed at higher temperatures exhibit a greatly reduced catalytic activity. After poling, the piezocatalytic activity of the polycrystalline BaTiO3 particles was obviously improved. In addition, we identified the free radical species and the intermediate products of the catalytic reaction. We also well-explained the dependence of electric polarization in the BaTiO3 piezocatalyst on annealing temperature and ultrasonic vibration theoretically. Our study indicates that increasing ferroelectric polarization (but not crystallite size) can effectively enhance the piezocatalytic activity. We believe that the present work provides a clear understanding of the role of ferroelectric polarization in piezocatalysis.

Lead-free ferroelectric Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO2/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700 °C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr3+ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.

Piezocatalysis is a catalytic effect driven by the stress-induced electricity of piezoelectric crystallites. In this study, piezoelectric BaTiO3 nanowires and nanoparticles were synthesized and their piezocatalytic activity was investigated. The BaTiO3 nanowires exhibit effectively enhanced piezocatalytic activity under ultrasonic vibration compared with the BaTiO3 nanoparticles. On the basis of the piezoelectric potential analysis by finite element method simulation for BaTiO3 nanowires and nanoparticles under ultrasonic vibration, the enhanced piezocatalytic activity of the BaTiO3 nanowires can be attributed to the larger piezoelectric potential along the polar axis. In addition, we also demonstrate that the intrinsic charge carriers (not piezoelectric charges) in piezoelectric crystallites play the role of charge transfer in the catalysis process through regulating the concentration of charge carriers. Our work provides further understanding of piezocatalysis of piezoelectric nanomaterials as well as insights on the relationship between piezoelectric potential and piezocatalysis.

Influence of Gd ion doping on resistive switching (RS) and magnetic properties of NiFe2O4 (NiFe2−xGdxO4, NFG-x) thin films prepared by chemical solution deposition method was investigated. The 0.075 Gd-doped NiFe2O4 based devices exhibited much reliable repeatability, endurance (>500 switching cycles), and good data retention (105 s at 25 and 100 °C) without any significant degradation in their RS performance. Dominant conduction mechanisms in devices were Ohmic conduction at low resistance state and at lower voltage region of high resistance state, while Schottky emission dominated at higher voltage region in high resistance state. The physical mechanism of resistive switching was linked with the formation and rupture of conducting filaments, and the improved stability of the switching parameters for Pt/NFG/Pt devices was attributed to appropriate concentration of oxygen vacancies which can easily minimize randomness for formation/rupture of conductive filaments. Due to variation in concentration of oxygen vacancies, saturation magnetization of the RS device at high resistance state was higher than those at virgin state and low resistance state, and changes of saturation magnetization maintained a stable value of 40% between HRS and LRS of Pt/NFG-0.075/Pt device. Our study indicates that doping rare earth element into ferrite thin films is a viable approach for enhancing resistance switching and magnetic properties.

As a new catalytic effect, piezocatalysis has attracted much attention due to its ability to degrade organic pollutants under mechanical vibration. However, improving the efficiency of piezocatalysis still remains a challenge. Here, we report an efficient route to enhance the piezocatalytic performance of BaTiO3 by surface modification with Ag nanoparticles (NPs). The Ag NPs were deposited by a facile in situ photo-reduction method. The piezocatalytic efficiency of the as-prepared samples was investigated by monitoring the degradation of methyl orange (MO) under ultrasonic vibration in the dark. The results indicate that the Ag loaded BaTiO3 particles show greatly enhanced piezocatalytic activity for degradation of MO molecules. A suitable amount of Ag deposition gives the highest piezocatalytic activity, while a higher Ag loading leads to reduced piezocatalytic activity. The degradation of MO and the possible by-products as well as the active radical species generated in the piezocatalytic system were investigated systematically. Based on the experimental results, a mechanism was proposed to explain the enhancement effect of Ag NPs on the piezocatalytic performance of BaTiO3. The strategy presented here provides a promising route towards the development of high-efficiency piezocatalysts for actual applications.

Au-doped nickel ferrite (NiFe2O4-Au, NFO-Au) magnetic thin films were prepared on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method to explore the effect of Au doping on unipolar resistive switching (RS) behavior. Experimental results reveal that 1% Au doped NFO film demonstrated considerably optimum switching parameters such as lower electroforming voltage, narrow distribution of SET/RESET voltages, good cycle-to-cycle endurance (103 cycles) and long data retention (105 s at 125 °C) as compared to NFO and Au-(0.5% and 2%) doped NFO films. The improved performance is attributed to suppressing of randomness of oxygen vacancies based filament by Au doping. Current transport conduction mechanisms are found to be Ohmic in low resistance state and Schottky emission in high resistance state. Physical model concerns for the rupture and formation of Au atoms assisted conductive filament inside the Au-doped NFO films based on the thermal effect of oxygen vacancies. Magnetic properties correlated with RS were measured and analyzed. The variation of saturation magnetization with different resistance state may be attributed to Au-doping induced different oxygen vacancies concentrations and make the conversion of valence states of Fe3+ and Fe2+ ions in octahedral sites easier. Our results provide a pathway towards the applications in nonvolatile RS memory and magnetoelectronic coupling devices.

Taking advantage of multiple energy sources is an effective way to enhance the catalytic efficiency of a catalytic system. Here, through rational design, we fabricated CoFe2O4 modified BiOCl hierarchical microspheres (CFO-BiOCl), which not only can make full use of photo, photothermal and mechanical energy but also can be easily recycled from liquid solutions. Photocatalytic activity measurement showed that CFO-BiOCl samples possessed outstanding photocatalytic activity, evident with 99% of rhodamine B decomposed in 5 min and 90% of tetracycline hydrochloride degraded in 10 min under full-spectrum light irradiation. Furthermore, piezo-/photocatalytic degradation rates of tetracycline hydrochloride, bisphenol A and phenol were around 2, 3 and 8 times higher than photocatalytic degradation rates under low-intensity light irradiation. Recycling tests showed that the piezo-/photocatalytic activity of CFO-BiOCl hardly decreased after 5 cycles. The excellent photocatalytic performance of CFO-BiOCl catalysts can be attributed to suitable band alignment of CoFe2O4 and BiOCl and photothermal effect that elevates organic pollutant molecules into a more active state. The generation of piezoelectric field in BiOCl crystals, providing strong driven force for the separation of photo-induced electrons and holes, gives rise to the high piezo-/photocatalytic efficiency. This work may bring a design pattern to fabricate catalysts that can utilize multiple energy sources and give a thorough discussion in the catalytic mechanism.

Nanocrystalline Ba1-xSrxTiO3 (x = 0–0.3) powders were investigated with special interest in their piezocatalytic performance for degradation of organic pollutants. A great improvement of piezocatalytic activity was obtained for the compositions x = 0.2, in which the polarization shows a large change under external stress. Furthermore, with the increase of working environment temperature, the piezocatalytic performance of Ba0.8Sr0.2TiO3 increases firstly and then decreases, giving the best piezocatalytic performance near its Curie temperature . For a better understanding of the piezocatalytic mechanism, the dynamics of charge carriers transfer on solid/liquid interface was discussed with respect to activation energy and overpotential of catalyst. It was suggested that the enhanced piezocatalytic activity of Ba 1-xSrxTiO3 comes from two aspects: (1) the overpotential provided by the piezoelectric effect decreases with the increasing Sr content, (2) the activation energy for the piezocatalytic reaction is greatly reduced at ferro-paraelectric phase transition. Combination of these two aspects gives rise to the nonlinear dependence of piezocatalytic activity on Sr content.

Bilayer CeO2/TiO2 films with high-k dielectric property were prepared by rf magnetron sputtering technique at room temperature. Effect of annealing treatment on resistive switching (RS) properties of bilayer CeO2/TiO2 films in O2 ambient at different temperature in the range of 350–550 °C was investigated. Our results revealed that the bilayer films had good interfacial property at 500 °C and this annealing temperature is optimum for different RS characteristics. Results showed that bilayer CeO2/TiO2 film perform better uniformity and reliability in resistive switching at intermediate temperature (i.e. 450 °C and 500 °C) instead of low and high annealing temperature (i.e. 350 °C and 550 °C) at which it exhibits poor crystalline structure with more amorphous background. Less Gibbs free energy of TiO2 as compared to CeO2 results in an easier re-oxidation of the filament through the oxygen exchange with TaN electrode. However, the excellent endurance property (>2500 cycles), data retentions (105 s) and good cycle-to-cycle uniformity is observed only in 500 °C annealed devices. The plots of cumulative probability, essential memory parameter, show a good distribution of Set/Reset voltage.

Pr-doped ZnO hexagonal structure thin films with c-axis preferred orientation were deposited on Pt/Ti/SiO 2 /Si substrates by chemical solution deposition technique. The effects of Pr doping amount on the resistive switching behavior of Pt/Zn 1−x Pr x O/Pt (x = 0, 0.01, 0.03, and 0.05) memory cells were investigated. The results showed that Pr-doping lowered the c-axis orientation degree of the ZnO thin films, but improved the resistive switching properties of Pt/Zn 1−x Pr x O/Pt devices. The resistive switching devices exhibited good endurance, long retention, and uniform switching voltages. I-V characteristics and their temperature dependence analysis indicated that the conduction mechanism of LRS was Ohmic behavior, and that of HRS at relatively higher voltage is trap-controlled space charge limited current. The physical origin of the resistive switching can be referred to the formation and rupture of the oxygen vacancies related filaments.