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The low photocatalytic conversion efficiency, poor light absorption and high charge recombination rate of traditional semiconductor photocatalysts continues to be a significant research challenge. In this paper, by combining detailed experimental and modeling techniques, we report on the unique potential of CaBi2Nb2O9(CBN) platelets that can couple both piezo- and photo- multi-field effects to overcome these issues and realize high-efficiency hydrogen production and dye degradation. The surface adsorption of OH−and dye molecules is improved as a result of the built-in electric field, thereby demonstrating an enhanced piezo- and photo-catalytic H2production activity, with a high rate of 96.83 μmol g−1h−1. The piezo-photocatalytic decomposition ratio for 100 mL RhB dye of 10 mg/L can reach up to 98.7 % in 32 min using only 0.05 mg of CBN platelets (k = 0.131 min−1). It is shown that the careful introduction of regularly arranged layers of (Bi2O2)2+into the CBN platelet structure provides a high transport of photoelectrons via a pathway of (Bi2O2)2+→ (CaNb2O7)2−→ CBN surface. The electron density distribution of Bi atoms is also found to be enriched on the facets of (020) and (200) crystal planes in the CBN platelets, which is beneficial to the oxidation reduction reaction. Furthermore, the large deformation of CBN platelet during the application of ultrasound leads to an increase of the piezo-induced built-in electric field to improve charge separation and migration. This work therefore provides a new perspective in the design and manufacture of advanced materials with enhanced piezo- and photo-catalytic performance by exploiting multi-field coupling effects.

Persistent organic pollution (POPS) with the feature of difficult to be degraded, highly toxic and global spread has become a challenge for the whole human beings. Recently, piezo-photoelectric catalysis is emerging as a promising way to degrade organic pollutions. In order to avoid the secondary pollution caused by the difficult recycling of powder catalyst, a three-dimensional barium titanate (BaTiO3, BT) scaffold is prepared by direct writing technique in this study, of which the surface is decorated by titanium oxide (TiO2, TO) nanowire arrays. Piezoelectric potential can be generated in the BT scaffold, especially in the direction parallel to the direct writing printing, and the piezoelectric electric field can effectively promote the separation of photo-generated carriers. This design of the structure can effectively produce a larger deformation compared with the bulk structure in the catalytic process, and it can also increase the contact area between the catalyzer and solution with enhanced reactive site. Due to the piezo-phototronic coupling effect, BT piezoelectric ceramic scaffold with TiO2array exhibits an excellent dye degradation efficiency, of which the rate constant reaches approximately 0.109 min−1for 100 mL of 10 mg/L of indigo carmine (IC) degradation. This work demonstrated that 3D piezoelectric ceramics by direct ink writing method shows a great potential in promoting the practical application of catalysis.

The induced built-in electric field by piezoelectric materials has proven to be one of the most effective strategies for modulating the charge-transfer pathway and inhibiting carrier recombination. In this work, a series of core-shell structured BaTiO3@TiO2nanowires (BT@TiO2NWs) heterojunctions were synthesized and the significant coupling effects between BaTiO3(BT) and TiO2resulted in surperior piezo-photocatalytic performance, which was demonstrated by three typical types of dyes with high concentrations. The degradation efficiency of 30 mg/L Rhodamine B (RhB), Methylene blue (MB) and Indigo Carmine (IC) solutions by 0.5 g/L BT@TiO2NWs reached 99.5% in 75 min, 99.8% in 105 min and 99.7% in 45 min, respectively, which are much higher than piezo-photocatalysis systems reported before. To reveal the coupling mechanisms, photoelectrochemical measurements and band diagram analysis were carried out. The carrier concentration was increased from 2.28 × 1017cm−3to 4.91 × 1018cm−3and the lifetime of charges was improved from 50.37 ms to 60.98 ms due to the construction of a heterojunction between TiO2and BT. It was proposed that the tilting and bending of the energy band caused by the introduction of a piezoelectric polarization can facilitate carrier separation both in the bulk phase and at the surfaces of semiconductors, resulting in outstanding piezo-photocatalytic properties for highly concentrated dye degradation. This work provides a universal catalyzer for highly concentrated dye degradation.

The harvesting of mechanical energy from our living environment via piezoelectric energy harvesters to provide power for next generation wearable electronic devices and sensors has attracted significant interest in recent years. Among the range of available piezoelectric materials, porous piezoelectric ceramics exhibit potential for both sensing and energy harvesting applications due to their reduced relative permittivity and enhanced piezoelectric sensing and energy harvesting figures of merit. Despite these developments, the low output power density and the lack of optimized structural design continues to restrict their application. Here, to overcome these challenges, a lead-free multi-layer porous piezoelectric composite energy harvester with a highly aligned pore structure and three-dimensional intercalation electrodes is proposed, fabricated and characterized. The effect of material structure and multi-layer configuration of the porous piezoelectric ceramic on the dielectric properties, piezoelectric response and energy harvesting performance was investigated in detail. Since the relative permittivity is significantly reduced due to the introduction of aligned porosity within the multi-layer structure, the piezoelectric voltage coefficient, energy harvesting figure of merit and output power are greatly enhanced. The multi-layer porous piezoelectric composite energy harvester is shown to generate a maximum output current of 80 μA, with a peak power density of 209 μW cm-2, which is significantly higher than other porous piezoelectric materials reported to date. Moreover, the generated power can charge a 10 μF capacitor from 0V to 4.0V in 150s. This work therefore provides a new strategy for the design and manufacture of porous piezoelectric materials for piezoelectric sensing and energy harvesting applications.

The coupling between piezoelectricity and photoexcitation has recently been emerged as an effective strategy to improve the catalytic performance of semiconductors. Lead-free Bi0.5Na0.5TiO3(BNT) is a promising system for effective piezo-photocatalysis owing to its favorable piezoelectricity, photoexcited response, and electrical conductivity. In this work, one-dimensional BNT nanorods prepared by a high-temperature molten-salt method were developed for piezo-photocatalysis applications for the first time, which exhibited excellent catalytic activity. The degradation efficiency of RhB solution (initial concentration C0= 5mg/L) in the presence of BNT nanorods reached 95% in 30min, leading to a high first-order rate constant kof 0.094min-1. For reveal the underlying mechanisms, the piezoresponse characterization, piezoelectric potential simulation, band diagram analysis, and photoelectrochemical measurements were carried out, and the BNT nanospheres synthesized by hydrothermal method were simultaneously studied for comparison. A higher piezoelectric coefficient d33⁎of 200pm/V and a larger potential output under pressure were achieved in the BNT nanorods. The BNT nanorods also displayed the narrower band gap, stronger photocurrent response, and lower charge transfer resistance. These merits enabled the BNT nanorods to exhibit strong piezo-photocatalytic activity. Moreover, it was found that the BNT nanorods showed better piezo-photocatalytic activity for cationic dyes (RhB and MB) than anionic dyes (MO). For RhB, MB, and MO solutions with C0of 10mg/L, kvalues of 0.041, 0.075, and 0.013min-1were achieved, respectively.