Direct and converse piezoelectric responses at the nanoscale from epitaxial BiFeO3 thin films grown by polymer assisted deposition

Ferrite bismuth-based nanomaterials: From ferroelectric and piezoelectric properties to nanomedicine applications

Bismuth ferrite (BiFeO3), a perovskite-type oxide, possesses unique morphology and multiferroicity, rendering it highly versatile for various applications. Recent investigations have demonstrated that BiFeO3 exhibits enhanced Fenton-like and photocatalytic behaviors, coupled with its piezoelectric/ferroelectric properties. BiFeO3 can catalytically generate highly oxidative reactive oxygen species (ROS) when exposed to hydrogen peroxide or light irradiation. Consequently, bismuth ferrite-based nanomaterials have emerged as promising candidates for various biomedical applications. However, the precise fabrication of BiFeO3-based materials with controllable features and applications in diverse biomedical scenarios remains a formidable challenge. In this review, we initially summarize the Fenton reaction property, ferroelectric, and piezoelectric properties of BiFeO3. We further survey the current methodologies for synthesizing BiFeO3 nanomaterials with diverse morphologies. Subsequently, we explore the effects of element doping and heterojunction formation on enhancing the photocatalytic activity of BiFeO3, focusing on microstructural, electronic band structure, and modification approaches. Additionally, we provide an overview of the recent advancements of BiFeO3-based nanomaterials in biomedicine. Finally, we discuss the prevailing obstacles and prospects of BiFeO3 for biomedical applications, offering valuable insights and recommendations for forthcoming research endeavors.

Effect of Different Ca2+ and Zr4+ Contents on Microstructure and Electrical Properties of (Ba,Ca)(Zr,Ti)O3 Lead-Free Piezoelectric Ceramics

In the preparation of (Ba,Ca)(Zr,Ti)O3 lead-free piezoelectric ceramics, different Ca2+ and Zr4+ contents will greatly affect the phase structure, microstructure, and electrical properties of the ceramics. XRD shows that all samples have pure perovskite phase structure, and the (Ba0.85Ca0.15)(ZryTi1−y)O3 ceramics morphotropic phase boundary region from tetragonal phase to rhombohedral phase near 0.08 ≤ y ≤ 0.1. From the dielectric temperature curve, the phase transition temperature (TO-T) was found near room temperature at 0.12 ≤ x ≤ 0.18 for the (Ba1−xCax)(Zr0.1Ti0.9)O3 ceramics. Both Ca2+ and Zr4+ increase have a significant decrease on the Curie temperature Tc. All samples were revealed as relaxers with diffusivities in the range 1.29 ≤ γ ≤ 1.82. Different from the undoped ceramics, ceramics doped with Ca and Zr ions exhibit saturated P–E hysteresis loops, and their ferroelectric properties are significantly optimized. In particular, the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic demonstrated optimal properties, namely d33 = 330 pC/N, kp = 0.41, εr = 4069, Pr = 4.8 μC/cm2, and Ec = 3.1 kV/cm, indicating that it is a viable lead-free piezoelectric contender. Variations in Ca and Zr content have a significant effect on the crystal grain sizes and densities of ceramics, which is strongly associated with their piezoelectricity.

Recent Advances in Perovskite Catalysts for Efficient Overall Water Splitting

Hydrogen is considered a promising clean energy vector with the features of high energy capacity and zero-carbon emission. Water splitting is an environment-friendly and effective route for producing high-purity hydrogen, which contains two important half-cell reactions, namely, the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). At the heart of water splitting is high-performance electrocatalysts that efficiently improve the rate and selectivity of key chemical reactions. Recently, perovskite oxides have emerged as promising candidates for efficient water splitting electrocatalysts owing to their low cost, high electrochemical stability, and compositional and structural flexibility allowing for the achievement of high intrinsic electrocatalytic activity. In this review, we summarize the present research progress in the design, development, and application of perovskite oxides for electrocatalytic water splitting. The emphasis is on the innovative synthesis strategies and a deeper understanding of structure–activity relationships through a combination of systematic characterization and theoretical research. Finally, the main challenges and prospects for the further development of more efficient electrocatalysts based on perovskite oxides are proposed. It is expected to give guidance for the development of novel non-noble metal catalysts in electrochemical water splitting.

Interplay of piezoresponse and magnetic behavior in Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramics

In the pristine as well as the doped BiFeO3 samples, ferroelectric domains show switching behavior. The regions marked by yellow color loops show either 71° or 109°-domains, whereas those marked by white loops are 180°-domains.

A Fully Biodegradable Ferroelectric Skin Sensor from Edible Porcine Skin Gelatine

High-performance biodegradable electronic devices are being investigated to address the global electronic waste problem. In this work, a fully biodegradable ferroelectric nanogenerator-driven skin sensor with ultrasensitive bimodal sensing capability based on edible porcine skin gelatine is demonstrated. The microstructure and molecular engineering of gelatine induces polarization confinement that gives rise the ferroelectric properties, resulting in a piezoelectric coefficient (d33) of ≈24 pC N-1 and pyroelectric coefficient of ≈13 µC m-2K-1, which are 6 and 11.8 times higher, respectively, than those of the conventional planar gelatine. The ferroelectric gelatine skin sensor has exceptionally high pressure sensitivity (≈41 mV Pa-1) and the lowest detection limit of pressure (≈0.005 Pa) and temperature (≈0.04 K) ever reported for ferroelectric sensors. In proof-of-concept tests, this device is able to sense the spatially resolved pressure, temperature, and surface texture of an unknown object, demonstrating potential for robotic skins and wearable electronics with zero waste footprint.

Surface Modification and Enhancement of Ferromagnetism in BiFeO3 Nanofilms Deposited on HOPG

BiFeO3 (BFO) films on highly oriented pyrolytic graphite (HOPG) substrate were obtained by the atomic layer deposition (ALD) method. The oxidation of HOPG leads to the formation of bubble regions creating defective regions with active centers. Chemisorption occurs at these active sites in ALD. Additionally, carbon interacts with ozone and releases carbon oxides (CO, CO2). Further annealing during the in situ XPS process up to a temperature of 923 K showed a redox reaction and the formation of oxygen vacancies (Vo) in the BFO crystal lattice. Bubble delamination creates flakes of BiFeO3-x/rGO heterostructures. Magnetic measurements (M-H) showed ferromagnetism (FM) at room temperature Ms ~ 120 emu/cm3. The contribution to magnetization is influenced by the factor of charge redistribution on Vo causing the distortion of the lattice as well as by the superstructure formed at the boundary of two phases, which causes strong hybridization due to the superexchange interaction of the BFO film with the FM sublattice of the interface region. The development of a method for obtaining multiferroic structures with high FM values (at room temperature) is promising for magnetically controlled applications.

Template-free fabrication of BiFeO3 nanorod arrays: multiferroic and photo-electrochemical performances

BiFeO₃ (BFO) has been widely investigated in many forms and morphologies because of its combined multiferroic and photovoltaic properties. However, direct growth of vertically aligned BFO nanorods on an underlying substrate has remained a challenge. In this work, we report template free growth of BiFeO₃ nanorod arrays on fluorine doped tin oxide coated glass substrate. This has been achieved by a two-step process, in which FeOOH nanorods are grown by chemical bath deposition and converted into BFO using bismuth (Bi) coating by physical vapour deposition (PVD). Both DC sputtering and thermal evaporation are attempted under PVD and the results suggest that Bi deposited by DC sputtering leads to well-defined BFO nanorods, which show superior performance in both multiferroic and photoelectrochemical studies. Piezoelectric force microscopy data shows the signature butterfly loop that confirms piezoelectric behaviour with a d ₃₃ value of 8 pmV^-1 in the BFO nanorods grown by DC sputtering. Further, the M-H hysteresis curve for the same samples reveals a remanent magnetization (M_r) value of 0.54 emu cc^-1 and antiferromagnetic nature at room temperature. Finally, a stable photocurrent density of 0.05 mA cm^-2 is achieved at 0.8 V vs Ag/AgCl under 1 Sun illumination. This work opens up new avenues for BFO in applications involving 1D nanostructures.

Ferroelectricity in Si-Doped Hafnia: Probing Challenges in Absence of Screening Charges

The ability to develop ferroelectric materials using binary oxides is critical to enable novel low-power, high-density non-volatile memory and fast switching logic. The discovery of ferroelectricity in hafnia-based thin films, has focused the hopes of the community on this class of materials to overcome the existing problems of perovskite-based integrated ferroelectrics. However, both the control of ferroelectricity in doped-HfO₂ and the direct characterization at the nanoscale of ferroelectric phenomena, are increasingly difficult to achieve. The main limitations are imposed by the inherent intertwining of ferroelectric and dielectric properties, the role of strain, interfaces and electric field-mediated phase, and polarization changes. In this work, using Si-doped HfO₂ as a material system, we performed a correlative study with four scanning probe techniques for the local sensing of intrinsic ferroelectricity on the oxide surface. Putting each technique in perspective, we demonstrated that different origins of spatially resolved contrast can be obtained, thus highlighting possible crosstalk not originated by a genuine ferroelectric response. By leveraging the strength of each method, we showed how intrinsic processes in ultrathin dielectrics, i.e., electronic leakage, existence and generation of energy states, charge trapping (de-trapping) phenomena, and electrochemical effects, can influence the sensed response. We then proceeded to initiate hysteresis loops by means of tip-induced spectroscopic cycling (i.e., “wake-up”), thus observing the onset of oxide degradation processes associated with this step. Finally, direct piezoelectric effects were studied using the high pressure resulting from the probe’s confinement, noticing the absence of a net time-invariant piezo-generated charge. Our results are critical in providing a general framework of interpretation for multiple nanoscale processes impacting ferroelectricity in doped-hafnia and strategies for sensing it.

Piezoelectric materials for ultrasound-driven dissociation of Alzheimer's β-amyloid aggregate structure

Piezoelectric materials can evoke electrochemical reactions by transferring charge carriers to reactants upon receiving mechanical stimuli. We report a newly discovered function of piezoelectric bismuth oxychloride (BiOCl) nanosheets for dissociating Alzheimer's β-amyloid (Aβ) aggregates through ultrasound-induced redox reactions. The accumulation of Aβ aggregates (e.g., Aβ fibrils, plaques) in the central nervous system is a major pathological hallmark of Alzheimer's disease (AD). Thus, clearing Aβ aggregates is considered a key for treating AD, but the dissociation of Aβ aggregates is challenging due to their extremely robust structure consisting of β-sheets. BiOCl nanosheets are a biocompatible piezoelectric material with piezocatalytic activity in response to ultrasound. Our analyses using multiple spectroscopic and microscopic tools have revealed that BiOCl nanosheets effectively disassemble Aβ fibrils under ultrasound stimulation. Sono-activated BiOCl nanosheets produce piezo-induced oxidative stress, which effectively destabilizes the β-sheets in Aβ fibrils. In vitro evolution has also shown that sono-activated BiOCl nanosheets can effectively alleviate the neuro-toxicity of Aβ fibrils. Furthermore, ex vivo evolution demonstrated that amount of Aβ plaques in AD mouse's brain slices was drastically reduced by treatment with sono-activated BiOCl nanosheets.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.