Piezoelectric actuators are indispensable over a wide range of industries for their fast response and precise displacement. Most commercial piezoelectric actuators contain lead, posing environmental challenges. We show that a giant strain (1.05%) and a large-signal piezoelectric strain coefficient (2100 picometer/volt) are achieved in strontium (Sr)–doped (K,Na)NbO 3 lead-free piezoceramics, being synthesized by the conventional solid-state reaction method without any post treatment. The underlying mechanism responsible for the ultrahigh electrostrain is the interaction between defect dipoles and domain switching. The fatigue resistance, thermal stability, and strain value (0.25%) at 20 kilovolt/centimeter are comparable with or better than those of commercial Pb(Zr,Ti)O 3 -based ceramics, showing great potential for practical applications. This material may provide a lead-free alternative with a simple composition for piezoelectric actuators and a paradigm for the design of high-performance piezoelectrics.

For traditional piezoelectric sensors based on poled ceramics, a low curie temperature (Tc) is a fatal flaw due to the depolarization phenomenon. However, in this study, we find the low Tc would be a benefit for flexible piezoelectric sensors because small alterations of force trigger large changes in polarization. BaTi0.88Sn0.12O3 (BTS) with high piezoelectric coefficient and low Tc close to human body temperature is taken as an example for materials of this kind. Continuous piezoelectric BTS films were deposited on the flexible glass fiber fabrics (GFF), self-powered sensors based on the ultra-thin, superflexible, and polarization-free BTS-GFF/PVDF composite piezoelectric films are used for human motion sensing. In the low force region (1–9 N), the sensors have the outstanding performance with voltage sensitivity of 1.23 V N−1 and current sensitivity of 41.0 nA N−1. The BTS-GFF/PVDF sensors can be used to detect the tiny forces of falling water drops, finger joint motion, tiny surface deformation, and fatigue driving with high sensitivity. This work provides a new paradigm for the preparation of superflexible, highly sensitive and wearable self-powered piezoelectric sensors, and this kind of sensors will have a broad application prospect in the fields of medical rehabilitation, human motion monitoring, and intelligent robot.

Transparent ferroelectrics, with promising prospects in transparent optoelectronic devices, have unique advantages in self-powered photodetection. The self-powered photodetectors based on the photovoltaic effect have quicker responses and higher stability compared with those based on the pyroelectric effect. However, the ferroelectric ceramics previously applied are always opaque and have no infrared light-stimulated photovoltaic effect. Thus, it would be very meaningful to design photodetectors based on infrared light-stimulated photovoltaic effect and/or transparent ferroelectric ceramics. In this work, highly optical transparent pristine lead lanthanum zirconate titanate (PLZT) and band gap-engineered Ni-doped PLZT ceramics with excellent piezoelectric/ferroelectric properties were prepared by hot-pressing sintering. Stable and excellent photovoltaic performance was obtained for pristine PLZT and band gap-engineered PLZT. The value of short-circuit current density is at least 2 orders of magnitude larger than those in PLZT reported in previous works. The transparent PLZT and Ni-doped PLZT ferroelectric ceramics are applied as self-powered photodetectors for the first time for 405 nm and near-infrared light, respectively. The devices based on PLZT under 405 nm light exhibit high detectivity (7.15 × 10 7 Jones) and quick response (9.5 ms for rise and 11.5 ms for decay), and those devices based on Ni-doped PLZT, under near-infrared light filtered from AM 1.5 G simulated sunlight, also exhibit high detectivity (6.86 × 10 7 Jones) and short response time (8.5 ms), both presenting great potential for future transparent photodetectors.

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Bandgap engineered ferroelectrics have raised the utmost interest in achieving visible/near-infrared light utilization. The gap states in ferroelectrics can not only realize the absorption of visible/near-infrared light but also keep their excellent piezoelectricity. However, the impurity energy levels provide limited photogenerated carrier, and the doping ions may act as trapping sites, which make it difficult to convert enough sunlight to chemical or electrical energy. In this work, we report a novel strategy to achieve the cooperative interaction of codopants and significantly enhance the photocatalytic activity of the host ferroelectrics. The (N3–, Ni2+)-codoped (Na0.5Bi0.5)­TiO3–BaTiO3 composition was taken as the concrete example, which has a lower bandgap of 2.06 eV without gap state. Largely enhanced photocatalytic activity including organic pollutant degradation and photocatalytic oxidative desulfurization were realized, in which the photodegradation efficiency of RhB was increased by 250% and dibenzothiophene could be degraded to lower than 10 ppm within 150 min. Moreover, a long-standing confusion about the appearance/disappearance of bandgap state in transition-metal-doped ferroelectrics has been properly clarified based on theoretical calculation result. Our work could provide a new insight for advancing the bandgap engineering and improving the energy conversion efficiency.

In the past century, ferroelectrics are well known in electroceramics and microelectronics for their unique ferroelectric, piezoelectric, pyroelectric, and photovoltaic effects. Nowadays, the advances in understanding and tuning of these properties have greatly promoted a broader application potential especially in energy and environmental fields, by harvesting solar, mechanical, and heat energies. For example, high piezoelectricity and high pyroelectricity can be designed by defect or microstructure engineering for piezo‐ and pyro‐catalyst, respectively. Moreover, highly piezoelectric and broadband (UV–Vis–NIR) light‐responsive ferroelectrics can be designed via defect engineering, giving rise to a new concept of photoferroelectrics for efficient photocatalysis, piezocatalysis, pyrocatalysis, and related cocatalysis. This article first summarizes the recent developments in ferroelectrics in terms of piezoelectricity, pyroelectricity, and photovoltaic effects based on defect and microstructure engineering. Then, the potential applications in energy generation (i.e., photovoltaic effect, H2 generation, and self‐powered multisource energy harvesting and signal sensing) and environmental protection (i.e., photo‐piezo‐pyro‐ cocatalytic dye degradation and CO2 reduction) are reviewed. Finally, the outlook and challenges are discussed. This article not only covers an overview of the state‐of‐art advances of ferroelectrics, but also prospects their applications in coping with energy crisis and environmental pollution.

Large-strain (K,Na)NbO 3 (KNN) based piezoceramics are attractive for nextgeneration actuators because of growing environmental concerns. However, inferior performance with poor temperature stability greatly hinders their industrialized procedure. Herein, a feasible strategy is proposed by introducing. defect dipoles and constructing grain orientation to enhance the strain performance and temperature stability of KNN-based piezoceramics. This textured ceramics with 90.3% texture degree exhibit a giant strain (1.35%) and a large converse piezoelectric coefficient d 33 * (2700 pm V −1 ), outperforming most lead-free piezoceramics and even some single crystals. Meanwhile, the strain deviation at high temperature of 100 °C-200 °C is obviously alleviated from 61% to 35% through texture engineering. From the perspective of practical applications, piezo-actuators are commonly utilized in the form of multilayer. In order to illustrate the applicability on multilayer actuators, a stack-type actuator consisted of 5 layers of 0.4 mm thick ceramics is fabricated. It can generate large field-induced displacement (11.6 µm), and the promising potential in precise positioning and optical modulation are further demonstrated. This work provides a textured KNN-based piezoceramic with temperature-stable giant strain properties, and facilitates the lead-free piezoceramic materials in actuator applications.

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