Xiaoli Xu

The flexoelectric effect, an electromechanical coupling phenomenon ubiquitous in all materials (including centrosymmetric crystals) under non-uniform strain gradients, has recently given rise to the emerging field of flexocatalysis. By eliminating the need for external light or electrical biases and circumventing the symmetry constraints of piezoelectric materials, it represents a catalytic paradigm for green chemistry, with applications spanning water splitting for hydrogen production, hydrogen peroxide generation, pollutant degradation, and biomedical therapies (e.g., tumor treatment). This review systematically outlines the fundamentals of the flexoelectric effect and the strain gradient-driven charge separation mechanisms. Recent advances in representative applications across energy, environmental, and biomedical fields are comprehensively summarized. Additionally, preliminary strategies to enhance catalytic efficiency are categorized into two main approaches: (1) amplifying the flexoelectric response, primarily interface engineering, the synergistic coupling of piezoelectric and flexoelectric effects, and doping; and (2) optimizing charge separation and transfer, mainly via introduction of vacancies, photo-flexoelectric synergistic coupling, and single-atom modification. Finally, the current challenges and future opportunities of flexocatalysis are highlighted, providing a foundational roadmap that bridges solid-state physics and interfacial catalysis.

Piezoelectric materials with high piezoelectric coefficients often exhibit low carrier concentrations, which hinders the generation of free charges for piezocatalytic processes. To address this, defect engineering has emerged as a promising approach to adjust the electronic structure and enhance the carrier concentration of piezocatalytic materials. However, the introduction of defects can negatively affect the piezoelectric performance by increasing charge screening and reducing domain wall mobility. In this work, we demonstrate that a high carrier concentration can be achieved without compromising the piezoelectric properties, by applying an external electric field to enhance polarization and rearrange oxygen defect dipoles. This poling process significantly improves both piezoelectric and piezocatalytic properties. Specifically, the effective piezoelectric coefficient of a polarised, oxygen-deficient Bi₂WO_6-x material increased from d^*₃₃ ~ 4.79 pm V^‒1 to d^*₃₃ ~ 39.89 pm V^‒1. Moreover, its carrier concentration remained high, approaching N_d ~ 7.39×10²⁰cm^‒3, when subjected to ultrasonic vibrations. As a result of the high piezoelectric activity and carrier concentration, the piezocatalytic hydrogen production efficiency (per unit power) attained ~3.49 μmol g⁻¹ h⁻¹ W⁻¹. This work enables the design of efficient and energy-saving piezocatalytic materials, providing a significant step forward for hydrogen production via piezocatalytic water splitting.

Piezocatalysis is an innovative, clean catalytic technology. The efficiency requires enhancement due to the low level of piezoelectricity, low carrier concentration, and rapid recombination rates of the materials. Composition modulation is a critical strategy for enhancing the piezoelectric activity of ferroelectric materials. In this work, the influence of composition changes on the piezocatalytic performance of barium strontium titanate (Ba _{1‐ x} Sr _x TiO ₃ (0 ≤ x ≤ 0.3)) with varying Ba/Sr molar ratios is investigated. As the Sr content ( x ) increases from x = 0 to x = 0.3, the effective piezoelectric coefficient exhibits a significant enhancement from d _eff = 46.8 pm·V ⁻¹ to d _eff = 205.6 pm·V ⁻¹ , leading to an increase in the carrier concentration increase ( ΔN _d ), due to the application of vibrations from ΔN _d = 0.11 × 10 ²⁰ cm ⁻³ (for x = 0) to ΔN _d = 1.69 × 10 ²⁰ cm ⁻³ (for x = 0.3). The optimal hydrogen production for each composition occurs near, but not exceeding, its Curie point ( T _C） . The optimized piezocatalytic hydrogen production (per unit power) of Ba _0.7 Sr _0.3 TiO ₃ at room temperature is 53.4 µmol·g ⁻¹ ·h ⁻¹ ·W ⁻¹ , which is one of the highest reported values to date. This work investigates the fundamental reasons behind composition modulation of piezocatalysis, providing constructive guidance for designing optimized piezocatalysts.

Pure and Eu ²⁺ ‐activated fluoroborate BaGaBO ₃ F ₂ was prepared using high‐temperature solid‐state reaction. BaGaBO ₃ F ₂ is a wide band semiconductor with the indirect transition characteristic. The excitation and luminescence spectra of the phosphor were measured, and it was found that Eu ²⁺ ‐activated BaGaBO ₃ F ₂ exhibits a bright blue color under ultraviolet (UV) light. The narrow emission band peaked at 425 nm is attributed to the transitions of 4f ⁶ 5d→4f ⁷ ( ⁸ S _7/2 ), and the Stokes shift estimated for this phosphor sample is 3140 cm ⁻¹ . The lifetime of the luminescence is also reported. The absolute quantum efficiency (QE) of the phosphor was evaluated, and it was found that the absolute QE decreases with increasing Eu ²⁺ concentration. The phosphor shows an excellent quantum efficiency of 72.5% and a high thermal activation energy of 0.342 eV. The study concludes that Eu ²⁺ ‐doped BaGaBO ₃ F ₂ phosphor has promising luminescence application abilities and can be used as a blue‐emitting phosphor in a variety of applications.

Piezoelectric materials can convert mechanical energy into electrical energy, and further piezocatalysis can be designed based on the product between piezoelectric effect and electrochemical redox effect, which is hopeful for the actual dye decomposition. Tourmaline is a natural mineral with spontaneous polarization, which is an ideal piezocatalytic material and is rarely reported in dye decomposition. In this work, after 100 mg tourmaline powder is vibrated for 120 min, the piezocatalytic dye decomposition ratios of rhodamine B (RhB), methylene blue, and methyl orange are ∼95%, ∼46%, and ∼23%, respectively. The corresponding first‐order reaction kinetic values for these three dyes are 0.02681, 0.00639, and 0.00246 min ⁻¹ , respectively. By adjusting the pH value of the initial RhB solution to 5, the decomposition ratio can reach ∼99% after only 60 min of vibration. In the process of dye decomposition, it is determined that the main active substances are hydroxyl radicals and superoxide radicals via electron spin–resonance spectra test. The spontaneous polarization performance of tourmaline can effectively adsorb dye molecules to promote the catalytic reaction. Natural tourmaline minerals are abundant, low cost, and environmentally friendly making them suitable for large‐scale piezocatalysis of mineral materials.

Pyroelectric materials have the ability to convert the environmental cold–hot thermal energy such as day–night temperature alternation into electrical energy. The novel pyro-catalysis technology can be designed and realized on the basis of the product coupling between pyroelectric and electrochemical redox effects, which is helpful for the actual dye decomposition. The organic two-dimensional (2D) graphic carbon nitride (g-C₃N₄), as an analogue of graphite, has attracted considerable interest in the field of material science; however, its pyroelectric effect has rarely been reported. In this work, the remarkable pyro-catalytic performance was achieved in the 2D organic g-C₃N₄ nanosheet catalyst materials under the continuous room-temperature cold–hot thermal cycling excitation from 25 °C to 60 °C. The pyro-catalytic RhB dye decoloration efficiency of the 2D organic g-C₃N₄ can reach ~92.6%. Active species such as the superoxide radicals and hydroxyl radicals are observed as the intermediate products in the pyro-catalysis process of the 2D organic g-C₃N₄ nanosheets. The pyro-catalysis of the 2D organic g-C₃N₄ nanosheets provides efficient technology for wastewater treatment applications, utilizing the ambient cold–hot alternation temperature variations in future.

In this work, explicitly pyro-catalytic performance is observed in sol-gel-synthesized ferroelectric Bi_0.5Na_0.5TiO₃ lead-free nanomaterials, and its application for dye wastewater purification is also actualized under temperature fluctuations varying from 23 °C to 63 °C. The decomposition ratios of the pyro-catalytic Bi_0.5Na_0.5TiO₃ nanomaterials on Rhodamine B, methyl blue and methyl orange can reach 96.75%, 98.35% and 19.97%, respectively. In the pyro-catalytic process, the probed active species such as hydroxyl radicals, superoxide radicals and holes play an extremely important role in decomposing dye molecules. The ferroelectric Bi_0.5Na_0.5TiO₃ lead-free nanomaterials will have an excellent prospect for dye wastewater purification due to its explicit pyro-catalysis.

All-inorganic perovskite solar cells suffer from low performance due to unsatisfactory carrier transport and light harvesting efficiency. Semiconductor nanopillar arrays can reduce light reflection loss and suppress exciton recombination dynamics in optoelectronic devices. In all-inorganic perovskite solar cells, few studies employing TiO₂ nanopillar arrays (TiO₂ NaPAs) have been reported to improve the device performance. Herein, well-arranged TiO₂ NaPAs are chosen to enhance the interfacial contact between perovskite and electron transporting layers for improving the carrier transport. Notably, TiO₂ NaPAs can be directly fabricated on rigid/flexible substrates at roughly room temperature by unique glancing angle deposition, which is more available than high-temperature hydrothermal/solvothermal methods. By embedding TiO₂ NaPAs into chemical processable CsPbI₂Br layers, continuous and intimate films are readily formed, guaranteeing large physical contact for facilitating more effective electron injection and charge separation. The vertically grown TiO₂ NaPAs also provide a straightforward electron transporting path to electrodes. In addition, TiO₂ NaPAs can guide the incident light and enhance the light-harvesting ability of CsPbI₂Br films. As a result, the solar cell with TiO₂ NaPAs displays a power conversion efficiency of 11.35% higher than planar control of 10.04%, and exhibits better long-term thermal stability. This strategy provides an opportunity by constructing direct interfacial regulation towards the performance improvement of inorganic perovskite solar cells.