Piezocatalytic Effect Induced Hydrogen Production from Water over Non-noble Metal Ni Deposited Ultralong GaN Nanowires

Asymmetric Single-Unit-Cell Layer Enriching Polar Inherent Hydroxyls Eliminates Interlayer Electric Field Shielding Effect and In Situ Self-Polarize for Piezocatalytic Water Splitting

Piezocatalytic two-electron water splitting into spontaneously isolated H2 and H2O2 shows huge prospects in meeting industrial requirements. Herein, asymmetric single-unit-cell Bi2O2(OH)(NO3) monolayer (BON-M) with superb force-sensitivity are developed for pure water and seawater dissociation. The formation of a monolayer structure allows sufficient exposure of polar inherent hydroxyls and eliminates the interlayer electric field screening induced by hydrogen bonding between [Bi2O2OH] slices and [NO3] layers, resulting in larger piezoelectricity and strengthened internal electric field. It also benefits surface charge carrier decoupling and renders more favorable H2O molecules adsorption and H* desorption. Particularly, the mechanical strain can induce the in situ self-polarization of BON-M, which further enhances electric field intensity and reduces energy barriers of H* desorption and key intermediate *OH formation, facilitating water splitting to H2 and H2O2 kinetically and thermodynamically. An exceptional piezocatalytic H2 and H2O2 production rate up to 2071.05 and 970.27 µmol g-1 h-1 is delivered by BON-M from pure water. It also accumulates H2 output of 12 429.68 µmol g-1 within 8 h from seawater splitting, along with mechanical-to-hydrogen efficiency of 0.15%. This work develops an effective strategy for exploiting high-performance piezocatalyst by building ultrafine nanostructure enriched with inherent polar groups on the surface.

Enhancing Piezo-Catalytic Hydrogen Evolution on BiOCl through UV Irradiation

Hydrogen production from water using piezo-catalysis is increasingly popular, but it typically requires expensive noble metal cocatalysts to speed up the reactions. In this study, we found that a UV irradiation treatment on the piezoelectric material BiOCl enhances its H2 evolution by 6.8 folds, from 0.41 to 2.81 mmol/g/h, even outperforming those loaded with noble metal cocatalysts. Our experiments and simulations revealed that UV irradiation prompts the in situ reduction of lattice Bi3+ in BiOCl, creating Bi metal nanoparticles on the surface. These nanoparticles serve as cocatalysts, enhancing hydrogen production by (1) capturing piezo-generated electrons from BiOCl with Bi's higher work function; (2) facilitating charge transfer between Bi and BiOCl due to their intimate contact; and (3) improving H2 evolution kinetics due to Bi's more suitable H* adsorption strength. This UV-enhancement technique can be applied to various Bi-based materials (e.g., BiOBr, BiOI, and Bi2WO6, etc.) as well as piezo-catalytic dye degradation reactions, demonstrating its versatility and potential for broader applications. This research provides a straightforward and cost-effective approach to improve the piezo-catalytic performance of Bi-based piezoelectric materials without the need for costly noble metal cocatalysts.

Polarization: A Universal Driving Force for Energy, Environment, and Electronics

The sustainable future relies on the synergistic development of energy, environmental, and electronic systems, founded on the development of functional materials by exploring their quantum mechanisms. Effective control over the distribution and behavior of charges within these materials, a basic quantum attribute, is crucial in dictating their physical, chemical, and electronic properties. At the core of charge manipulation lies "polarization"-a ubiquitous phenomenon marked by separating positive and negative charges. This review thoroughly examines polarization techniques, spotlighting their transformative role in catalysis, energy storage, solar cells, and electronics. Starting with the foundational mechanisms underlying various forms of polarization, including piezoelectric, ferroelectric, and pyroelectric effects, the perspective is expanded to cover any asymmetric phenomena that generate internal fields, such as heterostructures and doping. Afterward, the critical role of polarization across various applications, including charge separation, surface chemistry modification, and energy band alignment, is highlighted. Special emphasis is placed on the synergy between polarization and material properties, demonstrating how this interplay is pivotal in overcoming existing technological limitations and unlocking new functionalities. Through a comprehensive analysis, a holistic roadmap is offered for harnessing polarization across the broad spectrum of applications, thus finding sustainable solutions for future energy, environment, and electronics.

Beyond Conventional Catalysts: Monoelemental Tellurium as a Game Changer for Piezo-Driven Hydrogen Evolution

The increasing demand for clean hydrogen production over fossil fuels necessitates the development of sustainable piezoelectrochemical methods that can overcome the limitations of conventional electrocatalytic and photocatalytic approaches. In this regard, existing piezocatalysts face challenges related to their low piezoelectricity or active site coverage for hydrogen evolution reaction (HER). Driven by global environmental concerns, there is a compelling push to engineer practical materials for highly efficient HER. Herein, monoelemental 2D tellurium (Te) is presented as a class of layered chalcogenide with a non-centrosymmetric crystal structure (P3121 space group). The refined Te nanosheets demonstrate an unprecedented highly efficient H2 production rate ≈9000 µmol g-1 h-1 under ultrasonic mechanical vibration due to built-in piezo-potential in the system. The remarkable piezocatalytic performance of Te nanosheets arises from a synergistic interplay between their semi-metallic nature, favorable free energy landscape, enhanced electrical conductivity and outstanding piezoelectricity. As a proof of concept, the theoretical approach based on Density Functional Theory (DFT) validates the findings due to the gradual exposure of active sites on the Te nanosheets leading to a self-optimized catalytic performance for hydrogen generation. Therefore, mechanically driven Te emerges as a promising piezocatalyst with the potential to revolutionize highly efficient and sustainable technology for futuristic applications.

Tracking Charge Carrier Paths in Freestanding GaN/AlN Nanowires on Si(111)

Functional and abundant substrate materials are relevant for applying all sophisticated semiconductor-based device components such as nanowire arrays. In the case of GaN nanowires grown by metalorganic vapor phase epitaxy, Si(111) substrates are widely used, together with an AlN interlayer to suppress the well-known Ga-based melt-back-etching. However, the AlN interlayer can degrade the interfacial conductivity of the Si(111) substrate. To reveal the possible impact of this interlayer on the overall electrical performance, an advanced analysis of the electrical behavior with suitable spatial resolution is essential. For the electrical investigation of the nanowire-to-substrate junction, we used a four-point probe measurement setup with sufficiently high spatial resolution. The charge separation behavior of the junction is also demonstrated by an electron beam-induced current mode, while the n-GaN nanowire (NW) core exhibits good electrical conductivity. The charge carrier-selective transport at the NW-to-substrate junction can be attributed to different, local material compositions by two main effects: the reduction of Ga adatoms by shadowing of the lower part of the NW structure by the top part during growth, i.e. the protection of the pedestal footprint from Ga adsorption. Our combination of investigation methods provides direct insight into the nanowire-to-substrate junction and leads to a model of the conductivity channels at the nanowire base. This knowledge is crucial for all future GaN bottom-up grown nanowire structure devices on conductive Si(111) substrates.

Electrochemical Reduction of CO2 into Syngas by N-Modified NiSb Nanowires

Carbon dioxide reduction reaction (CO2RR) provides a promising method for syngas synthesis. However, it is challenging to balance the CO2RR activity and hydrogen (H2)/carbon monoxide (CO) ratios due to the limited mass transport and inefficient catalytic interface. Herein, we adopt a nitrogen (N)-modification method to synthesize N-modified nickel antimony nanowires (N-NiSb NWs/C), which are efficient for producing syngas with controllable H2/CO ratios. Significantly, the optimized N-NiSb NWs/C, with boosted electrochemical CO2RR activity, have the flexibility to control H2/CO ratios in syngas from nearly 1 to 4 in a wide potential range. The mechanistic discussion shows that the electronic structure of NiSb NWs/C can be optimized by using the synergistic effect between Ni and Sb, as well as the reasonable surface modification, so that a controllable syngas can be obtained. Our design provides an ideal platform for generating syngas with widely controllable H2/CO ratios.

Surprising Effects of Ti and Al2O3 Coatings on Tribocatalytic Degradation of Organic Dyes by GaN Nanoparticles

GaN is more stable than most metal oxide semiconductors for the photocatalytic degradation of organic pollutants in harsh conditions, while its catalytic efficiency has been difficult to be substantially improved. In this study, the tribocatalytic degradation of organic dyes by GaN nanoparticles has been investigated. Stimulated through magnetic stirring using homemade Teflon magnetic rotary disks in glass beakers, the GaN nanoparticles were found to induce negligible degradation in rhodamine B (RhB) and methyl orange (MO) solutions. Surprisingly, the degradation was greatly enhanced in beakers with Ti and Al2O3 coatings on their bottoms: 99.2% and 99.8% of the 20 mg/L RhB solutions were degraded in 3 h for the Ti and Al2O3 coatings, respectively, and 56% and 60.2% of the 20 mg/L MO solutions were degraded in 24 h for the Ti and Al2O3 coatings, respectively. Moreover, the MO molecules were only broken into smaller organic molecules for the Ti coating, while they were completely degraded for the Al2O3 coating. These findings are important for the catalytic degradation of organic pollutants by GaN in harsh environments and for achieving a better understanding of tribocatalysis as well.

Mechanically Enhanced Detoxification of Chemical Warfare Agent Simulants by a Two-Dimensional Piezoresponsive Metal-Organic Framework

Chemical warfare agents (CWAs) refer to toxic chemical substances used in warfare. Recently, CWAs have been a critical threat for public safety due to their high toxicity. Metal-organic frameworks have exhibited great potential in protecting against CWAs due to their high crystallinity, stable structure, large specific surface area, high porosity, and adjustable structure. However, the metal clusters of most reported MOFs might be highly consumed when applied in CWA hydrolysis. Herein, we fabricated a two-dimensional piezoresponsive UiO-66-F4 and subjected it to CWA simulant dimethyl-4-nitrophenyl phosphate (DMNP) detoxification under sonic conditions. The results show that sonication can effectively enhance the removal performance under optimal conditions; the reaction rate constant k was upgraded 45% by sonication. Moreover, the first-principle calculation revealed that the band gap could be further widened with the application of mechanical stress, which was beneficial for the generation of 1O2, thus further upgrading the detoxification performance toward DMNP. This work demonstrated that mechanical vibration could be introduced to CWA protection, but promising applications are rarely reported.

Significant hydrogen generation via photo-mechanical coupling in flexible methylammonium lead iodide nanowires

The flexoelectric effect, which refers to the mechanical-electric coupling between strain gradient and charge polarization, should be considered for use in charge production for catalytically driving chemical reactions. We have previously revealed that halide perovskites can generate orders of higher magnitude flexoelectricity under the illumination of light than in the dark. In this study, we report the catalytic hydrogen production by photo-mechanical coupling involving the photoflexoelectric effect of flexible methylammonium lead iodide (MAPbI3) nanowires (NWs) in hydrogen iodide solution. Upon concurrent light illumination and mechanical vibration, large strain gradients were introduced in flexible MAPbI3 NWs, which subsequently induced significant hydrogen generation (at a rate of 756.5 μmol g-1 h-1, surpassing those values from either photo- or piezocatalysis of MAPbI3 nanoparticles). This photo-mechanical coupling strategy of mechanocatalysis, which enables the simultaneous utilization of multiple energy sources, provides a potentially new mechanism in mechanochemistry for highly efficient hydrogen production.

Enhancing piezocatalytic H2O2 production through morphology control of graphitic carbon nitride

Piezocatalytic H₂O₂ production has attracted significant attention as a green alternative to traditional anthraquinone methods with heavy environmental pollution and high energy consumption. However, since the efficiency of piezocatalyst in producing H₂O₂ is poor, searching for a suitable method to improve the yield of H₂O₂ is of great interest. Herein, a series of graphitic carbon nitride (g-C₃N₄) with different morphologies (hollow nanotube, nanosheet and hollow nanosphere) are applied to enhance the piezocatalytic performance in yielding H₂O₂. The hollow nanotube g-C₃N₄ exhibited an outstanding H₂O₂ generation rate of 262 umol·g^-1·h^-1 without any co-catalyst, which is 1.5 and 6.2 times higher than nanosheets and hollow nanospheres, respectively. Piezoelectric response force microscopy, piezoelectrochemical tests, and Finite Element Simulation results revealed that the excellent piezocatalytic property of hollow nanotube g-C₃N₄ is mainly attributed to its larger piezoelectric coefficient, higher intrinsic carrier density, and stronger external stress absorption conversion. Furthermore, mechanism analysis indicated that piezocatalytic H₂O₂ production follows a two-step single-electro pathway, and the discovery of ¹O₂ furnishes a new insight into explore this mechanism. This study offers a new strategy for the eco-friendly manufacturing of H₂O₂ and a valuable guide for future research on morphological modulation in piezocatalysis.

Enhanced piezocatalytic hydrogen evolution performance of bismuth vanadate by the synergistic effect of facet engineering and cocatalyst engineering

Developing piezocatalysts with excellent piezocatalytic hydrogen evolution reaction (HER) performance is highly desired but also challenging. Here, facet engineering and cocatalyst engineering are employed to synergistically improve the piezocatalytic HER efficiency of BiVO₄ (BVO). Monoclinic BVO catalysts with distinct exposed facets are synthesized by adjusting pH of hydrothermal reaction. The BVO with highly exposed {110} facet exhibits a superior piezocatalytic HER performance (617.9 μmol g^-1h^-1) compared with that with {010} facet, owing to the strong piezoelectric property, high charge transfer efficiency, and excellent hydrogen adsorption/desorption capacity. The HER efficiency is enhanced by 44.7% by selectively depositing cocatalyst of Ag nanoparticles specifically on the reductive {010} facet of BVO, where the Ag-BVO interface provides the directional electron transport for high-efficiency charge separation. Under the collaboration between cocatalyst of CoO_x on {110} facet and the hole sacrificial agent of methanol, the piezocatalytic HER efficiency is evidently enhanced by 2 times because CoO_x and methanol can impede the water oxidation and improve the charge separation. This easy and simple strategy provides an alternative perspective on designing high-performance piezocatalysts.

Performance and Mechanism of Hydrothermally Synthesized MoS2 on Copper Dissolution

The recovery of copper from circuit boards is currently a hot topic. However, recycling copper from circuit boards economically and environmentally is still a considerable challenge. In this study, a simple hydrothermal method was used to synthesize MoS2 with nano-flower-like morphology using sodium molybdate dihydrate and thiourea as molybdenum and sulfur sources. The metal copper in the chip was successfully dissolved under the action of free radicals produced by ultrasound. The results show that under the catalytic action of hydrothermal synthesis MoS2, the concentration of Cu2+ dissolved by ultrasonic treatment for 10 h is 39.46 mg/L. In contrast, the concentration of Cu2+ dissolved by commercial MoS2 is only 2.20 mg/L under the same condition. The MoS2 is polarized by external mechanical forces and reacts with water to produce H+ and free electrons e−, which can combine with O2 and OH− to produce ·OH and ·O2− free radicals. Elemental Cu is converted to Cu2+ by the attack of these two free radicals.

Bi4TaO8Cl as a New Class of Layered Perovskite Oxyhalide Materials for Piezopotential Driven Efficient Seawater Splitting

Piezocatalytic water splitting is an emerging approach to generate "green hydrogen" that can address several drawbacks of photocatalytic and electrocatalytic approaches. However, existing piezocatalysts are few and with minimal structural flexibility for engineering properties. Moreover, the scope of utilizing unprocessed water is yet unknown and may widely differ from competing techniques due to the constantly varying nature of surface potential. Herein, we present Bi₄TaO₈Cl as a representative of a class of layered perovskite oxyhalide piezocatalysts with high hydrogen production efficiency and exciting tailorable features including the layer number, multiple cation-anion combination options, etc. In the absence of any cocatalyst and scavenger, an ultrahigh production rate is achievable (1.5 mmol g^-1 h^-1), along with simultaneous generation of value-added H₂O₂. The production rate using seawater is somewhat less yet appreciably superior to photocatalytic H₂ production by most oxides as well as piezocatalysts and has been illustrated using a double-layer model for further development.

Harvesting mechanical energy for hydrogen generation by piezoelectric metal-organic frameworks

Piezocatalysis, the process of directly converting mechanical energy into chemical energy, has emerged as a promising alternative strategy for green H₂ production. Nevertheless, conventional inorganic piezoelectric materials suffer from limited structural tailorability and small surface area, which greatly impedes their mechanically driven catalytic efficiency. Herein, we design and fabricate a novel UiO-66(Zr)-F₄ metal-organic framework (MOF) nanosheet for piezocatalytic water splitting, with the highest H₂ evolution rate reaching 178.5 μmol g^-1 within 5 h under ultrasonic vibration excitation (110 W, 40 kHz), far exceeding that of the original UiO-66 host. A reduced bandgap from 2.78 to 2.43 eV is achieved after introducing a fluorinated ligand. Piezoresponse force microscopy measurements demonstrate a much stronger piezoelectric response for UiO-66(Zr)-F₄, which may result from the polarity of the introduced fluorinated ligand. This work highlights the potential of MOF-based porous piezoelectric nanomaterials in harvesting mechanical energy to drive chemical reactions such as water splitting.

Bismuth Vacancy-Mediated Quantum Dot Precipitation to Trigger Efficient Piezocatalytic Activity of Bi2WO6 Nanosheets

Point defects in piezoelectric semiconductors play a significant role in regulating the piezocatalytic performance. However, the role of metal vacancies in piezocatalysis has been less explored than that of oxygen vacancies. Herein, Bi₂WO₆ (BWO) nanosheets with tunable Bi defects were synthesized using an ion exchange method. High-resolution transmission electron microscopy directly revealed the existence of Bi vacancies in the lattice of BWO nanosheets and the precipitation of Bi quasiparticles. The BWO nanosheets with the highest concentration of Bi vacancies exhibited an excellent decomposition efficiency (7.83 × 10^-2 min^-1) over rhodamine B under ultrasound. The phenomenon is mainly attributed to the increased charge carrier concentration as a consequence of defect energy levels. In addition, the significant enhancement of light absorption capacity caused by the surface plasmon resonance effect of quasiparticles indicates that Bi ions escape from the lattice and combine with free electrons around BWO to form Bi quantum dots, which function as electron traps to facilitate the separation of charge carriers during the piezocatalytic process. This work systematically reveals the essential affiliation of metal vacancies and surface metal clusters in piezocatalysts and verifies the significance of vacancy engineering in piezocatalytic application.

Synergistic Enhancement of Piezocatalytic Activity of BaTiO3 Convex Polyhedrons Nanocomposited with Ag NPs/Co3O4 QDs Cocatalysts

Piezocatalysis is one of the green and promising catalytic technologies for the degradation of organic pollutants. Surface modifications such as exposed facet engineering and surface decoration of nanoparticles (NPs) are simple but useful enhancement strategies for a catalytic system. However, the synergistic effect and mechanism of facet engineering and dual-cocatalyst decoration on piezocatalytic activity are still ambiguous and more investigations are expected. Herein, the piezocatalytic activities of BaTiO₃ (BTO) polyhedrons with anisotropic {001} and {110} facets and BTO cubes with isotropic {001} facets were compared. Furthermore, BaTiO₃ (BTO) convex polyhedrons with selectively deposited Ag NPs and uniformly loaded Co₃O₄ quantum dots (QDs) are rationally synthesized through photochemical deposition. The individual and synergistic effects of Ag NPs and Co₃O₄ QDs on the piezocatalytic activities are systematically studied. It was found that dual-cocatalyst-modified BTO possesses the highest piezocatalytic activity in methyl orange degradation, with a reaction constant k of 0.0539 min^-1, around 5, 2.2, and 1.3 times higher than that of nonmodified and Ag NP- and Co₃O₄ QD-modified BTO, respectively. Moreover, dual-cocatalyst-decorated BTO also exhibits excellent piezocatalytic performance in nondye pollutant degradation, with ∼100% tetracycline hydrochloride decomposed in 60 min. By analyzing the contribution, quantifying the amount of different free radicals, and comparing the chemical states of surface elements before and after piezocatalytic measurements, it was inferred that facet-dependent Ag NPs acted as efficient electron-transport sites, while uniformly loaded Co₃O₄ QDs served as hole-transfer sites to fully facilitate the migration of electrons and holes in a piezocatalytic reaction. This research presents a rational and effectual modification strategy to enhance the piezocatalytic activity of piezocatalysts and gives a thorough discussion of the enhanced mechanism.