Polyvinylidene difluoride, or PVDF, has long been a favorite of flexible electronics engineers because it combines the mechanical resilience of a plastic with genuine piezoelectric behavior. The catch is that piezoelectricity in PVDF depends entirely on which of its several crystal phases dominates. The nonpolar alpha phase, which forms under ordinary processing conditions, contributes essentially nothing to energy conversion. The polar beta phase, in which the fluorine and hydrogen atoms on the polymer chain are arranged so that each unit cell carries a permanent dipole moment, is the phase that actually generates charge when the material is stretched or compressed. In untreated electrospun PVDF fibers, the beta-phase fraction typically hovers around two-thirds of the crystalline content, leaving considerable room for improvement.

The Pakistani-led team attacked this problem by dispersing BST particles throughout the polymer solution before electrospinning it into fibers. Electrospinning itself already helps: the intense electric field applied as the polymer jet is drawn toward the collector stretches the chains and encourages them to fold into the polar beta configuration. The researchers found that the ceramic particles act as nucleation sites that amplify this effect. Their negatively charged surfaces attract the positively polarized CH2 groups of the PVDF chains, locking the polymer into the all-trans beta conformation as the fiber solidifies. The result, confirmed by both X-ray diffraction and Fourier transform infrared spectroscopy, was an increase in beta-phase content from roughly 65.7 percent in pristine PVDF fibers to about 87.4 percent at the optimal filler loading.

That optimum turned out to be 30 weight percent BST. The team systematically tested loadings of 10, 20, 30, 40, and 50 weight percent, and the structural measurements showed the polar fraction rising steadily up to the 30 percent mark before declining again at higher concentrations. The decline at high loadings is a familiar pattern in composite piezoelectrics: once too much ceramic is packed into the matrix, the particles begin to agglomerate, disrupting the polymer chains instead of aligning them and simultaneously making the fibers stiffer and more brittle. The sweet spot balances nucleation benefits against these aggregation penalties, and the spectroscopic data pinpointed it precisely.

The electrical payoff was substantial. Piezoelectric nanogenerators built from the optimized 30 percent BST/PVDF composite fibers delivered an open-circuit voltage of about 23.3 volts and a short-circuit current density of roughly 0.194 milliamperes per square meter, with a maximum power density of approximately 4.52 milliwatts per square meter measured at a load resistance of 60 megaohms. By comparison, devices made from the pristine PVDF fibers produced an open-circuit voltage of only about 11.8 volts, a current density of about 0.118 milliamperes per square meter, and a peak power density near 1.39 milliwatts per square meter at 50 megaohms. In other words, the ceramic additive roughly doubled the voltage and more than tripled the power output, a leap that stems directly from the higher density of aligned dipoles available to respond to mechanical strain.

The mechanism behind this enhancement involves more than simple nucleation. Barium titanate is itself a classic ferroelectric perovskite, and substituting a small fraction of strontium for barium modifies its lattice in ways that previous studies have linked to improved piezoelectric and dielectric response. When the BST particles are embedded in the polar polymer matrix, local electric fields around the ceramic inclusions interact with the dipoles of the surrounding PVDF chains, and the electrospinning process ensures that both components are oriented along the fiber axis. The composite therefore benefits from the flexibility and processability of the polymer alongside the strong polarization of the ceramic, with the interface between the two phases acting as the critical region where charge transfer and dipole coupling take place.

To demonstrate that the devices could do real work, the researchers connected their nanogenerators to practical loads. The BST/PVDF fiber devices were able to illuminate multiple light-emitting diodes and to power a digital multimeter and a hygrometer, the kind of low-power electronics that define the growing internet of things. This demonstration matters because many piezoelectric energy harvesters reported in the literature produce impressive voltage numbers on an oscilloscope but deliver too little current or power to drive anything tangible. Showing that the composite fibers can sustain multiple LEDs and run measurement instruments places the material firmly in the category of functional, application-ready energy harvesters rather than laboratory curiosities.

The broader context is the surging demand for sustainable power sources in flexible and wearable devices. Sensors for health monitoring, motion tracking, and environmental measurement are proliferating, but batteries impose weight, rigidity, charging requirements, and eventual disposal problems. Piezoelectric nanogenerators offer an alternative: they scavenge energy from the very motions that the devices experience, whether that is a footstep, a heartbeat, a breeze, or the flexing of a fabric. PVDF-based systems are particularly attractive for these roles because they are lead-free, unlike the widely used lead zirconate titanate ceramics, and because electrospinning can produce them as soft fiber mats that integrate naturally into textiles and conformal coatings. The challenge has always been output, and the BST strategy reported here represents a straightforward, scalable route to closing that gap.

The choice of strontium doping level also reflects a deliberate design decision grounded in the ceramics literature. Prior work on Ba1-xSrxTiO3 systems has shown that modest strontium substitution can tune the ferroelectric transition temperature and enhance piezoelectric coefficients in the resulting ceramics, while higher substitution levels shift the material toward different structural regimes. By fixing the composition at Ba0.96Sr0.04TiO3, the team selected a formulation with favorable polarization characteristics and then focused their optimization effort on the parameter that mattered most for the composite: the filler concentration. This two-stage design logic, tuning the ceramic first and the composite second, offers a template that other groups working on related systems, from KNN/PVDF to PZT/PVDF composites, can readily adapt.

Funded in part by the Higher Education Department of Khyber Pakhtunkhwa through its Higher Education Research Endowment Fund, and carried out in the Nano Physics Laboratory at Abdul Wali Khan University Mardan, the study adds to a rapidly expanding body of work on PVDF composite fibers for energy harvesting. The authors suggest that their BST/PVDF composite fibers could serve as high-efficiency piezoelectric materials for sustainable energy sources in flexible electronic devices, and the numbers support that claim. With beta-phase content near 87 percent, voltages above 23 volts, and power densities exceeding 4.5 milliwatts per square meter from a lead-free, textile-compatible platform, the material brings self-powered wearables a practical step closer to reality, and it does so using processing techniques that are already compatible with large-scale fiber manufacturing.

Subject of Research: Strontium-doped barium titanate enhancement of beta-phase PVDF composite fibers for flexible piezoelectric energy harvesting

Article Title: Barium strontium titanate induced β-phase enhancement in electrospun polyvinylidene difluoride composite fibers for high-efficiency flexible piezoelectric nanogenerators

Article References: Khan, S., Din, J. U., Shah, S. K., Ali, N., Saleem, S., Yousaf, J., Rauf, M., & Hayat, K. (2026). Barium strontium titanate induced β-phase enhancement in electrospun polyvinylidene difluoride composite fibers for high-efficiency flexible piezoelectric nanogenerators. Polymer Bulletin, 83(12), Article 655. https://doi.org/10.1007/s00289-026-06719-5

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06719-5

Keywords: PVDF, barium strontium titanate, BST, piezoelectric nanogenerator, electrospinning, beta-phase, energy harvesting, flexible electronics, composite fibers, wearable devices, lead-free piezoelectrics, power density

Barium strontium titanate, often abbreviated BST, belongs to the perovskite family of oxides with the general formula ABO3, in which barium and strontium share the A-site while titanium occupies the B-site. The compound sits at a compositional sweet spot: the 1:1 ratio of barium to strontium optimizes structural stability while preserving the ferroelectric behavior that makes these materials valuable for actuators, sensors, capacitors, memory devices, and non-volatile storage systems. Ferroelectrics respond to an applied electric field by shifting their internal polarization, and the interplay among dielectric permittivity, remanent polarization, leakage current, and tunability determines how well a given composition performs in any of these roles. By adjusting what sits on the A-site, researchers can, in effect, reprogram the material’s electrical personality.

The choice of lutetium as the dopant was deliberate and chemically nuanced. Lutetium is the smallest and heaviest of the rare earth elements, and it carries a compact, stable 4f electron shell that allows it to dissolve into the BST lattice rather than segregating into unwanted secondary phases. Because its ionic radius is smaller than those of the A-site cations it replaces, lutetium acts as a donor dopant, introducing extra positive charge and a controlled population of defects and vacancies into the crystal. The researchers argue that this combination of ionic size, valency, and structural stability is what gives lutetium its outsized influence: a few percent of a tiny rare earth ion reshapes strain, defect chemistry, and electronic structure across the entire perovskite framework.

To make the powders, the team used the sol-gel method, a wet-chemistry route in which precursor compounds are mixed at the molecular level in solution before being gelled and calcined into crystalline ceramic. Compared with conventional solid-state reactions that grind bulk oxides together at high temperatures, sol-gel synthesis offers precise stoichiometric mixing of the barium, strontium, titanium, and lutetium sources with minimal secondary phase formation, which is critical when the dopant is meant to occupy a specific crystallographic site. The uniformity achieved in the gel translates directly into compositional homogeneity in the final nanocrystals, ensuring that the measured properties reflect true bulk behavior rather than artifacts of uneven mixing.

Characterization of the two samples was exhaustive. Rietveld refinement of X-ray diffraction data using the FullProf program, together with Raman spectroscopy, established the crystalline orientation and phase formation and revealed that the lutetium-doped material possessed higher tetragonality, expressed as an increased c/a ratio of the unit cell. Williamson-Hall analysis of the diffraction peak broadening showed that the doped crystals were smaller, with higher microstrain and greater dislocation density, a picture confirmed independently by transmission electron microscopy. Ultraviolet-visible and infrared spectra traced the electronic and vibrational modes of the lattice, while scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy captured surface morphology, grain size, density, porosity, and composition. X-ray photoelectron spectroscopy completed the picture by pinning down oxidation states, chemical composition, electronic structure, and surface properties.

The dielectric results are where the payoff becomes dramatic. Lutetium-doped BST exhibited higher dielectric permittivity and reduced dielectric loss compared with the undoped compound, and its dielectric tunability, the fractional change in permittivity achievable under an applied bias, reached 67.1 percent at a direct current field of 260 kV/cm, far exceeding the 41.5 percent of the pristine material. Tunability of this magnitude is the key figure of merit for voltage-controlled capacitors, phase shifters, and agile microwave devices, because it determines how much the capacitance of a component can be swept electrically without any moving parts. The authors attribute the enhancement to the shift in diffraction peak positions and the increase in density brought about by lutetium sitting on the A-site, which together reshape how the lattice responds to external fields.

Ferroelectric measurements told a similarly encouraging story. The doped sample produced a more symmetric hysteresis loop, with a remanent polarization of 10 microcoulombs per square centimeter and a coercive field of 8 kilovolts per centimeter, both higher than the values extracted from the asymmetric loop of the undoped material. According to the researchers, donor doping at the A-site can enhance polarization by aligning defect dipoles with the external electric field while simultaneously introducing local strain into the lattice. A symmetric, well-saturated loop with substantial remanent polarization is exactly what developers of non-volatile ferroelectric memories look for, since the two stable polarization states of the material encode digital information that survives power loss.

Equally important for practical devices is the behavior of leakage current, the parasitic current that bleeds through any real dielectric and drains stored charge. The undoped BST leaked at approximately 10^-2 microamperes per square centimeter, while the lutetium-doped material reduced that to roughly 10^-3 microamperes per square centimeter. That order-of-magnitude improvement means capacitors built from the doped ceramic can hold their stored energy far longer, which is why the authors highlight energy storage as a direct application. Low leakage combined with high tunability and strong polarization positions lutetium-doped BST as a candidate for tunable capacitors and energy-storing devices where efficiency and retention both matter.

The same defect chemistry that improves the electrical properties also transforms the optical ones. Because the introduced defect states and vacancies pull the band gap to lower values, the doped perovskite becomes responsive to solar irradiation as a photocatalyst. The team demonstrated this by degrading crystal violet, a persistent organic dye, under sunlight, confirming the enhanced photocatalytic performance of the lutetium-doped compound. This finding connects the work directly to wastewater remediation, and the authors note that the material aligns with United Nations Sustainable Development Goals 6 and 7, which concern clean water and affordable clean energy. A single ceramic powder that can both condition a polluted effluent and populate the dielectric layer of an efficient capacitor illustrates the kind of multifunctionality that materials chemists have long pursued.

The broader message of the study is one of control: by tuning donor doping precisely at the A-site of a lead-free perovskite, researchers can dial in the band gap, dielectric response, leakage behavior, and ferroelectric polarization of a material almost independently, choosing the operating point best suited to a given application. The work also underscores the enduring value of the sol-gel route, which delivered phase-pure, compositionally uniform nanoparticles without exotic equipment. As demand grows for lead-free, sustainable alternatives in electronics and environmental technology, results like these suggest that the smallest and heaviest of the rare earths may hold the key to making one of the oldest ferroelectric ceramics newly relevant, both in the treatment plants that clean the world’s water and in the capacitors that will store the energy of the future.

Subject of Research: Lutetium-doped barium strontium titanate perovskite ceramics for photocatalytic wastewater remediation and tunable energy storage capacitors

Article Title: Synthesis of undoped and A-site lutetium donor–doped barium strontium titanate by the sol-gel method for wastewater remediation and tunable energy storage capacitor applications

Article References: Sreelakshmi, K., Priya, S. L., Rajesh, K. M., Manoj, N., Augustine, P., & Parameswaran, M. (2026). Synthesis of undoped and A-site lutetium donor–doped barium strontium titanate by the sol-gel method for wastewater remediation and tunable energy storage capacitor applications. Journal of Nanoparticle Research, 28(10), Article 254. https://doi.org/10.1007/s11051-026-06766-z

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06766-z

Keywords: barium strontium titanate, lutetium doping, perovskites, sol-gel synthesis, ferroelectrics, dielectric tunability, energy storage capacitors, leakage current, photocatalysis, wastewater remediation, donor doping, nanoparticles