Self-standing films (45-μm thick) of native cellulose nanofibrils (CNF) were synthesized and characterized for their piezoelectric response. The surface and the microstructure of the films were evaluated with image-based analysis and scanning electron microscopy (SEM). The measured dielectric properties of the films at 1 kHz and 9.97 GHz indicated a relative permittivity of 3.47 and 3.38 and loss tangent tan  of 0.011 and 0.071, respectively. The films were used as functional sensing layers in piezoelectric sensors with corresponding sensitivities of 4.7 to 6.4 pC/N in ambient conditions. This piezoelectric response is expected to increase remarkably upon film polarization resulting from the alignment of the cellulose crystalline regions in the film. The CNF sensor characteristics were compared with those of polyvinylidene fluoride (PVDF) as reference piezoelectric polymer. Overall, the results suggest that CNF is a suitable precursor material for disposable piezoelectric sensors, actuator or energy generators with potential applications in the fields of electronics, sensors and biomedical diagnostics.

Energy harvesting technology may be considered an ultimate solution to replace batteries and provide a long-term power supply for wireless sensor networks. Looking back into its research history, individual energy harvesters for the conversion of single energy sources into electricity are developed first, followed by hybrid counterparts designed for use with multiple energy sources. Very recently, the concept of a truly multisource energy harvester built from only a single piece of material as the energy conversion component is proposed. This review, from the aspect of materials and device configurations, explains in detail a wide scope to give an overview of energy harvesting research. It covers single-source devices including solar, thermal, kinetic and other types of energy harvesters, hybrid energy harvesting configurations for both single and multiple energy sources and single material, and multisource energy harvesters. It also includes the energy conversion principles of photovoltaic, electromagnetic, piezoelectric, triboelectric, electrostatic, electrostrictive, thermoelectric, pyroelectric, magnetostrictive, and dielectric devices. This is one of the most comprehensive reviews conducted to date, focusing on the entire energy harvesting research scene and providing a guide to seeking deeper and more specific research references and resources from every corner of the scientific community.

An ABO3-type perovskite solid-solution, (K0.5Na0.5)NbO3 (KNN) doped with 2 mol.% Ba(Ni0.5Nb0.5)O3-δ (BNNO) is reported in this communication. Such a composition yields a much narrower bandgap (~1.6 eV) compared to the parental composition -pure KNN -and other widely used piezoelectric and pyroelectric materials (e.g. Pb(Zr,Ti)O3, BaTiO3).Meanwhile, it exhibits the same large piezoelectric coefficient as that of KNN (~100 pC N -1 ) and a much larger pyroelectric coefficient (~130 µC m -2 K -1 ) compared to the previously reported narrow bandgap material (KNbO3)1-x-BNNOx. The unique combination of these excellent ferroelectric and optical properties opens the door to the development of multi-source energy harvesting or multi-functional sensing devices for the simultaneous and efficient conversion of solar, thermal and kinetic energies into electricity simultaneously and efficiently in a single material. Individual and comprehensive characterizations of the optical, ferroelectric, piezoelectric, pyroelectric and photovoltaic properties are investigated with single and co-existing energy sources. No degrading interaction between ferroelectric and photovoltaic behaviors was observed. This composition may fundamentally change the working principles of state of the art hybrid energy harvesters and sensors, and thus significantly increase the unit 2 volume energy conversion efficiency and reliability of energy harvesters in ambient environments.Various energy harvesting (EH) techniques have been investigated in recent decades in order to overcome the shortcomings of batteries in terms of lifespan, overall cost-effectiveness and chemical safety. [1] However, the power level and stability provided by a single-source energy harvester are often insufficient for practical applications. In order to address this issue, various hybrid energy harvesters have been developed and investigated. [2][3][4] However, as such hybrid energy harvesters have mostly been simple physical combinations of individual harvesters made from different materials/structures, the effective size of the entire system can become much larger than its individual counterparts. [2,3] In such a case one has to compromise either on the number of simultaneously harvested energy sources or on the space taken by different energy harvesting components. [5] This compromise usually leads to the loss of the advantage of energy harvesters over batteries. A similar situation may occur in hybrid sensors.One method to solve the problem is to design or discover a single composition/material which enables the simultaneous harvesting/detection of multiple energy sources. At the same time the individual conversion efficiency of the material for each energy source should be neither reduced nor interrupted in this multi-task performance. This requires different energy conversion effects exhibited by the same material to be independent of each other, or coupled but working in the same direction, and to be functional simultaneously. This communication reports a perovski...

Lithium molybdate disks were fabricated by moistening watersoluble Li 2 MoO 4 powder with deionized water and compressing it under a pressure of 130 MPa. Disks were postprocessed at room temperature, at 120°C, and at 540°C, which is a common sintering temperature for Li 2 MoO 4 . Regardless of the postprocessing temperature, densities as high as 87%-93% of the theoretical value were achieved. The X-ray diffraction patterns of processed disks were all the same with no signs of hydrates or constitutional water. The samples also exhibited very similar microstructures and microwave dielectric properties with a relative permittivity of 4.6-5.2 and a Q 3 f value of 10 200-18 500 at 9.6 GHz, depending on the postprocessing temperature.

Photo‐ferroelectric single crystals and highly oriented thin‐films have been extensively researched recently, with increasing photovoltaic energy conversion efficiency (from 0.5% up to 8.1%) achieved. Rare attention has been paid to polycrystalline ceramics, potentially due to their negligible efficiency. However, ceramics offer simple and cost‐effective fabrication routes and stable performance compared to single crystals and thin‐films. Therefore, a significantly increased efficiency of photo‐ferroelectric ceramics contributes toward widened application areas for photo‐ferroelectrics, e.g., multisource energy harvesting. Here, all‐optical domain control under illumination, visible‐range light‐tunable photodiode/transistor phenomena and optoelectrically tunable photovoltaic properties are demonstrated, using a recently discovered photo‐ferroelectric ceramic (K0.49Na0.49Ba0.02)(Nb0.99Ni0.01)O2.995. For this monolithic material, tuning of the electric conductivity independent of the ferroelectricity is achieved, which previously could only be achieved in organic phase‐separate blends. Guided by these discoveries, a boost of five orders of magnitude in the photovoltaic output power and energy conversion efficiency is achieved via optical and electrical control of ferroelectric domains in an energy‐harvesting circuit. These results provide a potentially supplementary approach and knowledge for other photo‐ferroelectrics to further boost their efficiency for energy‐efficient circuitry designs and enable the development of a wide range of optoelectronic devices.

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has been investigated for the optical reading of ferroelectric random-access memories, [14,15] photodiodes, [16] and photovoltaic [4,[17][18][19][20][21][22][23] applications. Current research is focused on developing band-gap tuned ferroelectric materials as their performance is restricted by low mobility of charge carriers and short diffusion lengths. [24][25][26] Band-gap tuned ferroelectric materials are capable of hosting multiple functionalities (due to ferroelectricity and semiconducting features), which suggest the possibilities of multiple energy conversions (piezoelectric, pyroelectric, and photovoltaic) simultaneously. In this context, Bai et al. synthesized a novel band-gap (1.6 eV down from >4 eV) engineered lead-free ferroelectric composition, i.e. (K 0.5 Na 0.5 )NbO 3 -2 mol% Ba(Ni 0.5 Nb 0.5 )O 3−δ (KNBNNO or KNN-BNNO), and demonstrated multiple functionalities (piezoelectric, pyroelectric, and photovoltaic effects) and their cumulative effect using this single material. [27][28][29] (K 0.5 Na 0.5 )NbO 3 supports off-center distortion and is responsible for ferroelectric nature of KNN-BNNO while Ba(Ni 0.5 Nb 0.5 ) O 3−δ controls the electronic states in the gap of parent (K 0.5 Na 0.5 ) NbO 3 using oxygen vacancies and Ni +2 ions. [24] Simultaneous presence of ferroelectric and semiconducting features makes this composition alluring for photovoltaic applications. In addition, it also provides an opportunity to understand how ferroelectric properties in semiconductors could help in improving the photovoltaic response and vice versa. In this context, the present study aims to provide an insight into light-induced changes in the ferroelectric behavior of KNN-BNNO. Recent demonstration of light-driven switching of nanodomains in (K 0.5 Na 0.5 )NbO 3 also makes it interesting to further explore KNN-BNNO. [30] Light-induced changes in ferroelectrics could persist due to a number of reasons such as (i) localized heating assisted diffusion of alkali element (as in LiNbO 3 ) and oxygen vacancies, [31] (ii) change in oxidation state of the central atom (e.g., in BaTiO 3 ), [32] and (iii) Jahn-Teller distortions due to light-induced pyroelectric current or charge injection in the conduction band (as in SbSI). [7,13] Warren et al. suggested that similar to electrical and thermal fluctuations, an exposure to light involves "locking domains by electronic charge trapping at domain boundaries." [1] Any change in the macroscopic behavior could be attributed to nanoscale changes in the ferroelectric domains. Moreover, Domain wall nanoelectronics constitutes a potential paradigm shift for nextgeneration energy conversion and von-Neumann devices. In this context, attempts have been made to achieve energy-efficient control over ferromagnetic, ferroelectric, and ferroelastic domain walls through electric and magnetic fields or applied stress. However, optical control of ferroic domains offers an additional degree of freedom and significant advantages of reduced hysteresis and Joule heating losses, creating novel opport...

Niobate perovskites like the (K,Na)NbO 3 (KNN) family are among the most important lead-free ferroelectrics. Ba and Ni have been co-doped into KNN to induce Ni 2+ -oxygen vacancy defect dipoles to significantly reduce the band gap while maintaining the ferroelectricity. This opens doors to novel optoferroelectric applications such as multisensors and photocatalysts. However, obtaining a single phase of the above co-doped KNN is difficult due to the sensitive stoichiometry on phase formation and arduous nickel diffusion into the KNN unit cells during synthesis. This paper reports an alternative approach to simultaneously tune the band gap and ferroelectricity. A-site vacancies are intentionally introduced into the mixtures of starting reactants. The homogeneously distributed vacancies trigger self-assembly of a niobate perovskite and niobate tungsten bronze phase and thus form a composite. The interface between the two phases, which mimics a heterojunction, rather than any individual phase, is proven to be responsible for the resultant narrow band gap and strong ferroelectricity. Hypotheses are proposed based on the results of ferroelectric, photoconductivity, and density functional theory-based studies to explain the mechanism. This paper offers an additional option to engineer polarizations and band structures in complex photoferroelectric oxides, especially alkaline niobates.