Flexible Composites for Piezocatalysis

Despite piezoelectric materials have a long history of application, piezoelectric catalysis has continued to be a hot topic in recent years. Flexible piezoelectric materials have just emerged in recent years due to their versatility and designability. In this paper, we review the recent advances in flexible piezoelectric materials for catalysis, discuss the fundamentals of the catalytic properties of composite materials, and detail the typical structures of these materials. We pay special attention to the types of filler in flexible piezoelectric composites, their role and the interaction between the particles and the flexible substrate. Notable examples of flexible piezoelectric materials for organic pollutants degradation, enhanced piezo-photocatalysis and antibacterial applications are also presented. Finally, we present key issues and future prospects for the development of flexible piezoelectric catalysts.

Hydrogen Bond-Induced Activation of Photocatalytic and Piezophotocatalytic Properties in Calcium Nitrate Doped Electrospun PVDF Fibers

In this study, polyvinylidene fluoride (PVDF) fibers doped with hydrated calcium nitrate were prepared using electrospinning. The samples were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), optical spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman, and photoluminescence (PL) spectroscopy. The results are complementary and confirm the presence of chemical hydrogen bonding between the polymer and the dopant. Additionally, there was a significant increase in the proportion of the electroactive polar beta phase from 72 to 86%. It was shown that hydrogen bonds acted as a transport pathway for electron capture by the conjugated salt, leading to more than a three-fold quenching of photoluminescence. Furthermore, the optical bandgap of the composite material narrowed to the range of visible light energies. For the first time, it the addition of the salt reduced the energy of the PVDF exciton by a factor of 17.3, initiating photocatalytic activity. The calcium nitrate-doped PVDF exhibited high photocatalytic activity in the degradation of methylene blue (MB) under both UV and visible light (89 and 44%, respectively). The reaction rate increased by a factor of 2.4 under UV and 3.3 under visible light during piezophotocatalysis. The catalysis experiments proved the efficiency of the membrane design and mechanisms of catalysis are suggested. This study offers insight into the nature of chemical bonds in piezopolymer composites and potential opportunities for their use.

Recent Advances in Efficient Photocatalysis via Modulation of Electric and Magnetic Fields and Reactive Phase Control

The past several years has witnessed significant progress in enhancing photocatalytic performance via robust electric and magnetic fields' modulation to promote the separation and transfer of photoexcited carriers, and phase control at reactive interface to lower photocatalytic reaction energy barrier and facilitate mass transfer. These three research directions have received soaring attention in photocatalytic field. Herein, recent advances in photocatalysis modulated by electric field (i.e., piezoelectric, pyroelectric, and triboelectric fields, as well as their coupling) with specific examples and mechanisms discussion are first examined. Subsequently, the strategy via magnetic field manipulation for enhancing photocatalytic performance is scrutinized, including the spin polarization, Lorentz force, and magnetoresistance effect. Afterward, materials with tailored structure and composition design enabled by reactive phase control and their applications in photocatalytic hydrogen evolution and carbon dioxide reduction are reviewed. Finally, the challenges and potential opportunities to further boost photocatalytic efficiency are presented, aiming at providing crucial theoretical and experimental guidance for those working in photocatalysis, ferroelectrics, triboelectrics, piezo-/pyro-/tribo-phototronics, and electromagnetics, among other related areas.

Synergistically active piezoelectrical H2O2 production composite film achieved from catalytically inert PVDF-HFP matrix and SiO2 fillers

Local and decentralized H 2 O 2 production via piezoelectrical process promise smart biological utilization as well as environmental benefits. However, stable, bio/environmental- safe, and easily applied H 2 O 2 generation materials are still lacking. Here we report a novel flexible H 2 O 2 generation polymeric film composed of catalytically inert PVDF-HFP (Poly(vinylidene fluoride-co-hexafluoropropylene)) matrix and SiO 2 nanoparticle fillers. The film is bio-/environmentally benign at resting states, but effectively produces H 2 O 2 upon ultrasonic motivation at a production rate of 492 μmol [[EQUATION]] in one hour. Experimental and simulation methods in combination indicate that the effective H 2 O 2 generation capabilities stem from the synergistic existence of piezoelectrical fields and the air-liquid-solid three-phase regions around the porous film. The chemical conversions are motivated by the adsorbed charges. The silicon hydroxyl groups properly stabilize the *OOH intermediate and facilitate the chemical conversions of 2e - ORR of ambient O 2 . We expect the report to inspire H 2 O 2 piezoelectrical generation materials and promote the novel production strategies of H 2 O 2 as well as piezoelectrical functional materials.

Piezo-phototronic effect on photocatalysis, solar cells, photodetectors and light-emitting diodes

The piezo-phototronic effect (a coupling effect of piezoelectric, photoexcitation and semiconducting properties, coined in 2010) has been demonstrated to be an ingenious and robust strategy to manipulate optoelectronic processes by tuning the energy band structure and photoinduced carrier behavior. The piezo-phototronic effect exhibits great potential in improving the quantum yield efficiencies of optoelectronic materials and devices and thus could help increase the energy conversion efficiency, thus alleviating the energy shortage crisis. In this review, the fundamental principles and challenges of representative optoelectronic materials and devices are presented, including photocatalysts (converting solar energy into chemical energy), solar cells (generating electricity directly under light illumination), photodetectors (converting light into electrical signals) and light-emitting diodes (LEDs, converting electric current into emitted light signals). Importantly, the mechanisms of how the piezo-phototronic effect controls the optoelectronic processes and the recent progress and applications in the above-mentioned materials and devices are highlighted and summarized. Only photocatalysts, solar cells, photodetectors, and LEDs that display piezo-phototronic behavior are reviewed. Material and structural design, property characterization, theoretical simulation calculations, and mechanism analysis are then examined as strategies to further enhance the quantum yield efficiency of optoelectronic devices via the piezo-phototronic effect. This comprehensive overview will guide future fundamental and applied studies that capitalize on the piezo-phototronic effect for energy conversion and storage.

A significant enhancement of bulk charge separation in photoelectrocatalysis by ferroelectric polarization induced in CdS/BaTiO3 nanowires

Efficient charge separation, in particular bulk charge separation (BCS), is one of the most critical factors in determining the performance of photoelectrochemical (PEC) water-splitting. The BCS enhancement of CdS/BaTiO3 (CdS/BTO) nanowires (NWs) in photoelectrocatalysis has rarely been reported. This paper describes a remarkable PEC properties promotion of the CdS/BTO NWs, which is confirmed to be a result of the enhanced BCS efficiency induced by the ferroelectric polarization. The vertical arrays of BTO NWs endow fast transfer of carriers. Meanwhile, CdS is decorated uniformly on the surface of BTO NWs, which ensures a wide range of light absorption. After two negative polarizations, the CdS/BTO NWs have successfully obtained a remarkable photocurrent density, achieving 459.53 μA cm-2 at 1.2 V(vs.RHE), which is 2.86 times that of the unpolarized sample. However, after two positive polarizations, the photocurrent density dramatically decreases to 40.18 μA cm-2 at 1.2 V(vs.RHE), which is merely 0.25 times the original value. More importantly, the photocurrent density reaches up to a prominent value of -71.09 mA cm-2 at -0.8 V(vs.RHE) after two successive negative polarizations, which is a 40.87 mA cm-2 enhancement with respect to the sample without poling. Significantly, at -0.8 V(vs.RHE), the BCS efficiency of the CdS/BTO NWs is as high as 91.87% after two negative polarizations. The effects of ferroelectric polarization on the PEC performance of CdS/BTO NWs have been systematically studied. The results demonstrate that ferroelectric polarization, especially negative polarization, results in an internal electric field to tune band bending of CdS/BTO NWs, thus prominently enhancing the PEC performance.

Synergistic interaction of Z-scheme 2D/3D g-C3N4/BiOI heterojunction and porous PVDF membrane for greatly improving the photodegradation efficiency of tetracycline

Designing photocatalytic membranes with excellent photocatalytic and self-cleaning ability based on the synergistic effect between the crystal structure of membrane matrix and photocatalyst is highly desirable. Herein, Z-scheme 2D/3D g-C₃N₄/BiOI heterojunction blended in beta-phase polyvinylidene fluoride membrane (β-phase PVDF) was prepared via solvent crystallization and phase inversion technique. As expected, the designed g-C₃N₄/BiOI/β-phase PVDF photocatalytic membranes (CN/BI/β-phase PVDF PMs) achieved exceptional photocatalytic degradation efficiency for tetracycline (94.6%) as compared to the CN/BI heterojunction power (84.0%) and two other control membrane matrixes (CN/BI/PAN and CN/BI/CA PMs) within 120 min. Meanwhile, the dynamic cyclic degradation system of CN/BI/β-phase PVDF PMs was also investigated that reached to be 94.8% in 80 min. Besides, the CN/BI/β-phase PVDF PMs not only had outstanding self-cleaning activity and remarkable permeability (up to 30,688 L·m^-2·h^-1) but also had high stability and reusability even after five runs. Importantly, the hydroxyl radical detection and ESR analysis identified that the β-phase PVDF membrane could promote photoinduced carrier separation efficiency of 2D/3D g-C₃N₄/BiOI heterojunction. This work may open up a novel strategy for designing and constructing high-efficient photocatalytic membranes for water treatment.

Sustainable Internal Electric Field for Enhanced Photocatalysis: From Material Design to Energy Utilization

The intrinsic internal electric field in a ferroelectric photocatalyst is beneficial for improving the photocatalytic properties because of its positive effect on the separation and migration of photogenerated carriers. However, this kind of internal electric field is static and easily saturated by inner and outer shielding effects, seriously restricting its potential in photocatalysis. To overcome this problem, a sustainable internal electric field was introduced into photocatalysis based on piezoelectric and pyroelectric effect, which exhibits good capability in consistently boosting photocatalytic activity, thus becoming a hot research topic. In this Perspective we summarize the recent significant progress in the construction of sustainable internal electric fields for facilitating photocatalysis from material design to energy utilization. Moreover, the fascinating influence of sustainable internal electric fields on carrier behavior is also discussed. Finally, a summary and outlook for building a sustainable internal electric field to further enhance photocatalytic performance are provided.

Solution Blow Spinning of Polyvinylidene Fluoride Based Fibers for Energy Harvesting Applications: A Review

Polyvinylidene fluoride (PVDF)-based piezoelectric materials (PEMs) have found extensive applications in energy harvesting which are being extended consistently to diverse fields requiring strenuous service conditions. Hence, there is a pressing need to mass produce PVDF-based PEMs with the highest possible energy harvesting ability under a given set of conditions. To achieve high yield and efficiency, solution blow spinning (SBS) technique is attracting a lot of interest due to its operational simplicity and high throughput. SBS is arguably still in its infancy when the objective is to mass produce high efficiency PVDF-based PEMs. Therefore, a deeper understanding of the critical parameters regarding design and processing of SBS is essential. The key objective of this review is to critically analyze the key aspects of SBS to produce high efficiency PVDF-based PEMs. As piezoelectric properties of neat PVDF are not intrinsically much significant, various additives are commonly incorporated to enhance its piezoelectricity. Therefore, PVDF-based copolymers and nanocomposites are also included in this review. We discuss both theoretical and experimental results regarding SBS process parameters such as solvents, dissolution methods, feed rate, viscosity, air pressure and velocity, and nozzle design. Morphological features and mechanical properties of PVDF-based nanofibers were also discussed and important applications have been presented. For completeness, key findings from electrospinning were also included. At the end, some insights are given to better direct the efforts in the field of PVDF-based PEMs using SBS technique.

Efficient Photocatalysis of Composite Films Based on Plasmon-Enhanced Triplet-Triplet Annihilation

To avoid secondary environmental pollution caused by photocatalysts in their applications, our work offers a new strategy for fabricating photocatalytic films based on plasmon-enhanced triplet-triplet annihilation upconversion (TTA-UC). Polydimethylsiloxane (PDMS) films containing platinum (II)-octaethylporphyrin and 9,10-diphenylanthracene (PtDPAP), and gold nanoparticles (AuNPs) were prepared. While graphene (G) was used as an adhesive and conductive layer, CdS nanoparticles were deposited onto the films (AuNPs-PtDPAP/G/CdS) by plasma glow discharge pretreatment. The AuNPs-PtDPAP film had an enhancement in the green-to-blue upconversion compared with the pristine PtDPAP film. CdS can utilize the AuNPs plasmon-enhanced TTA-UC photons to realize efficient photocatalytic reactions. The pseudo-first-order rate constant (k_pfo) of the optimized active and stable photocatalytic film, 0.3 AuNPs-PtDPAP/G/CdS, reached 0.294 h^-1 for tetracycline degradation under green light irradiation. Its k_pfo in decomposing tetracycline under visible light is 2.62 times higher than that of the PtDPAP/G/CdS. The reported composite films provide a strategy to improve the photocatalytic activity and promote the practical applications of nanosize photocatalysts.

Seaweed bio-inspired ZnO piezoelectric cilia array applied in microreactors for enhanced photocatalytic performance

A piezoelectric built-in electric field is believed to be an effective way to separate photoinduced carriers and enhance photocatalytic performance.