The Effect of Multi-Fields Synergy from Electric/Light/Thermal/Force Technologies on Photovoltaic Performance of Ba0.06 Bi0.47 Na0.47 TiO3 Ferroelectric Ceramics via the Mg/Co Substitution at A/B Sites.

Currently, it is widely reported that the photovoltaic effect in ferroelectric materials can be promoted by the application of a piezoelectric force, an external electric field, and intense light illumination. Here, a semiconducting ferroelectric composition is introduced, (1-x) Ba0.06 Bi0.47 Na0.47 TiO3 -xMgCoO3 (abbreviated as xMgCo, where x = 0.02-0.08), synthesized through Mg/Co ions codoping. This process effectively narrows the optical bandgaps to a spectrum of 1.38-3.06 eV. Notably, the system exhibits a substantial increase in short-circuit photocurrent density (Jsc ), by the synergy of the electric, light, and thermal fields. The Jsc can still be further enhanced by the extra introduction of a force field. Additionally, the Jsc also shows an obvious increase after the high field pre-poling. The generation of a considerable number of oxygen vacancies due to the Co2+ /Co3+ mixed valence state (in a 1:3 ratio) contributes to the reduced optimal bandgap. The integration of Mg2+ ion at the A-site restrains the loss and sustains robust ferroelectricity (Pr  = 24.1 µC cm-2 ), high polarizability under an electric field, and a significant piezoelectric coefficient (d33  = 102 pC N-1 ). This study provides a novel perspective on the physical phenomena arising from the synergy of multiple fields in ferroelectric photovoltaic materials.

Realizing an Ultrahigh Depolarization Temperature near the Curie Point and Large d33 with Superior Temperature Stability in Lead-Free Ceramics via a Sandwich Structure Design.

An imminent challenge of lead-free Bi0.5Na0.5TiO3-based (BNT) piezoceramics is that the giant piezoelectric constant (d33) caused by the morphotropic phase boundary is incompatible with a high depolarization temperature (Td) and ultralow temperature coefficient (Ttc) of the real-time d33, which severely hinders their industrial application in the field of elevated temperatures. Herein, a sandwich-structured 0.94Bi0.5Na0.5TiO3-0.06BaTiO3/0.89Bi0.5Na0.5TiO3-0.11BaTiO3/0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (SWS-6/11/6BT-y, where y refers to the weight fraction of the BNT-11BT solid solution) ceramic composite is engineered for mitigating the conflict between d33, Td and Ttc. Following this strategy, ultrahigh Td near the Curie point (225 °C, close to that of the BNT-11BT layer) and relatively large d33 (130 pC/N, close to that of the BNT-6BT layer) are simultaneously realized in a SWS-6/11/6BT-40%-Q ceramic composite. More importantly, the ultralow Ttc (0.07%) of real-time d33 is also achieved in this work. The structural heterogeneity yields the high piezoresponse, and the built-in field resulting from layer-type ceramic composites provides the driving force to promote the diffused ferroelectric-relaxor phase transition and the resultant ferroelectric order with high Td. The above synergistic contributions realize the remission of the d33-Td-Ttc conflict in a sandwich-structural SWS-6/11/6BT-40% ceramic composite. Thus, our work provided a path for designing the BNT-based piezoceramics with potential for industrial applications.

Enhanced Visible Photocatalytic Hydrogen Evolution of KN-Based Semiconducting Ferroelectrics via Band-Gap Engineering and High-Field Poling.

In various ferroelectric-based photovoltaic materials after low-band-gap engineering, the process by which high-field polarization induces the depolarizing electric field (Edp) to accelerate the electron-hole pair separation in the visible light photocatalytic process is still a great challenge. Herein, a series of semiconducting KN-based ferroelectric catalytic materials with narrow multi-band gaps and high-field polarization capabilities are obtained through the Ba, Ni, and Bi co-doping strategy. Stable Edp caused by high-field poling enhanced the visible photocatalytic hydrogen evolution in a 0.99KN-0.01BNB sample with a narrow band gap and optimal ferroelectricity, which can be 5.4 times higher than that of the unpoled sample. The enhanced photocatalytic hydrogen evolution rate can be attributed to the synergistic effect of the significant reduction of the band gap and the high-field-polarization-induced Edp. The change in the band position in the poled sample further reveals that high-field poling may accelerate the migration of carriers through band bending. Insights into the mechanism by which catalytic activity is enhanced through high-field-polarization-induced Edp may pave the way for further development of ferroelectric-based catalytic materials in the photocatalytic field.

Ultrahigh Energy Storage Density and Efficiency in Bi0.5Na0.5TiO3-Based Ceramics via the Domain and Bandgap Engineering.

Environmentally friendly lead-free dielectric ceramics have attracted wide attention because of their outstanding power density, rapid charge/dischargerate, and superior stability. Nevertheless, as a hot material in dielectric ceramic capacitors, the energy storage performance of Na0.5Bi0.5TiO3-based ceramics has been not satisfactory because of their higher remnant polarization value and low dielectric breakdown strength, which is a problem that must be urgently overcome. In this work, the (1 - x) (0.6Na0.5Bi0.5TiO3 - 0.4Sr0.7Bi0.2TiO3) - xBa(Mg1/3Ta2/3)O3 (BNST-xBMT) systems were designed based on a dual optimization strategy of domain and bandgap to solve the above problems. As a result, a record-breaking ultrahigh energy density and excellent efficiency (Wrec = 8.58 J/cm3, η = 93.5%) were obtained simultaneously under 565 kV/cm for the BNST-0.08BMT ceramic. The introduction of Sr0.7Bi0.2TiO3 induces the formation of nanodomains in BNT-based ceramics, leading to slim P-E curves, and the further modification of Mg/Ta reduces the grain sizes and increases the bandgap width, resulting in significant enhancement of the dielectric breakdown strength. Moreover, excellent stability and superior discharge performance (Wd = 4.7 J/cm3, E = 320 kV/cm) in the BNST-0.08BMT ceramic were also achieved. The results suggest that the BNST-0.08BMT ceramic shows potential applicability for dielectric energy storage ceramics. Simultaneously, the composition-design concept in the system provides a good reference for the further development of ceramic dielectric capacitors.

Improvement of dielectric properties and energy storage performance in sandwich-structured P(VDF-CTFE) composites with low content of GO nanosheets.

Polymer-based dielectric capacitors play a notable part in the practical application of energy storage devices. Graphene oxide (GO) nanosheets can improve the dielectric properties of polymer-based composites. However, the breakdown strength will greatly reduce with the increase of GO content. Hence, the construction of sandwich structure can enhance the breakdown strength without reducing the dielectric constant. Herein, single-layered and sandwich-structured poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) nanocomposites with low content of GO nanosheets (<1.0 wt%) are prepared via employing a straightforward casting method. Compared with the single-layered composites and pure P(VDF-CTFE), the sandwich-structured composites exhibit comprehensively better performance compared. The sandwich-structured composite with 0.4 wt% GO nanosheets show an excellent dielectric constant of 13.6 (at 1 kHz) and an outstanding discharged energy density of 8.25 J cm-3at 3400 kV cm-1. These results demonstrate that the growth of the dielectric properties is owing to 2D GO nanosheets and the enhancement of breakdown strength due to the sandwich structure. The results from finite element simulation provide theoretical support for the design of high energy density composites.