Currently, polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET) are among the most frequently detected types of microplastics in the natural environment^[37]. Because the different chemical structures of common microplastics determine their distinct preferred reaction sites and oxidation pathways, there is no single reaction pathway for microplastic degradation that applies to all polymers. The rate at which they age in the environment and the difficulty of their degradation are primarily determined by the chemical structure of their polymer main chains. Specifically, PE, PP, PS, and PVC are all typical carbon-chain polymers, and their backbones are composed of relatively stable C-C bonds. Therefore, their catalytic activity typically depends on the oxidation state of the C-H/C-C bonds on their surfaces. In contrast, because PET contains ester bonds in its structure, it is easier for hydrolysis reactions to cause chain breakage and degradation in an aqueous environment^[38]. The characteristics of these common microplastics are summarized in Table 1.

Polyethylene (PE) and polypropylene (PP) are typical polyolefin microplastics; their backbones consist primarily of stable C-C and C-H bonds, and their structures lack functional groups. Therefore, before catalyzing the degradation of such microplastics, chemical treatment can be used to increase the number of oxygen-containing functional groups on their surfaces, thereby improving their surface wettability in aqueous solutions. Furthermore, because polyethylene and polypropylene are typically highly hydrophobic, they cannot come into extensive contact with the aqueous solution or the catalyst, which significantly reduces the degradation efficiency. Therefore, the strategy in degrading PP and PE mainly relies on oxidation-induced chain scission^[39,48,49]. For PE, C-H abstraction typically involves oxidation to form carbon-centered radicals, followed by reaction with O₂ to form peroxyl radicals and hydroperoxides, which ultimately lead to the formation of carbonyl-containing products and the cleavage of C-C bonds^[50,51]. Unlike polyethylene, PP contains tertiary C-H sites with the methyl-substituted backbone in its structure. These sites are generally more prone to hydrogen abstraction. The resulting carbon-centered radicals can subsequently undergo peroxide formation and alkoxy radical formation, which may ultimately lead to the formation of ketones and alcohols^[41,52,53]. However, the extent of degradation is still influenced by the stability of the crystalline regions and the number of surface reactive sites. Since PS contains aromatic phenyl side chains, its catalytic or oxidative transformation often results in the formation of certain aromatic oxygen-containing compounds, such as benzaldehyde and benzoic acid^[42,43]. Therefore, the degradation of PS cannot be assessed solely based on mass loss or surface oxidation; it also requires analysis of the intermediates formed during the catalytic process. The PET main chain contains both hydrolyzable ester bonds and aromatic structures. The ester bonds can be hydrolyzed through simple chemical treatment or enzyme-catalyzed hydrolysis; however, the highly ordered crystalline regions inhibit the penetration of water and catalytic sites into the polymer interior to facilitate the reaction, thereby reducing the efficiency of microplastic degradation^[44]. In the case of PVC, the presence of C-Cl bonds makes its degradation process more complex; therefore, when evaluating the degradation of PVC, attention must be paid to its dechlorination efficiency and the toxicity of its byproducts^[46,47].

Because of their properties, microplastics are typically insoluble or poorly dispersed in water. Therefore, different chemical pretreatment methods may be used to modify microplastics^[54]. Among the polymers, polyethylene and polypropylene both have poor wettability, which reduces the contact area between the polymer surface and catalyst. As a result, the oxidation reaction occurs primarily on the exposed surface and has difficulty propagating into the polymer. Previous studies on the photocatalytic degradation of polyethylene-based microplastics have shown that degradation efficiency is closely related to the particle size and morphology of the microplastics, as well as the degree of contact between the photocatalyst and the polymer particles^[55]. Crystallinity is also one of the key factors. Amorphous regions typically exhibit higher chain mobility and better reactant accessibility, which make these regions more sensitive to the reactive species. This means that even polymers of the same type may exhibit significantly different degradation kinetics depending on their crystallinity^[56].

At this stage, chemical pretreatment becomes particularly important. It is primarily used to effectively separate microplastics from complex matrices prior to the analysis of environmental samples, or to improve the surface wettability of microplastics and enhance their reactivity prior to catalytic reforming. Although these two applications serve different purposes, they may both alter the initial physical and chemical properties of the polymer.

In fact, the actual effectiveness of chemical pretreatment mainly depends on the type of polymer and the treatment conditions. Previous comparative studies on PE, PS, PET, and PP have shown that H₂SO₄ causes dramatic changes in the surface morphology and functional groups of these materials. In contrast, HCl, H₂O₂, KOH, and NaOH only have relatively limited effects on most plastics^[54]. However, high temperature and pressure can also cause chemical changes in polymers. Some researchers have found that oxygen-containing functional groups appear on the surface of PP after hydrothermal treatment^[57]. Others have also used ozone to subject microplastics to intense oxidation. The results show that this process successfully introduces a great variety of oxygen-containing functional groups, and leads to physical cracking on the plastic surface^[58]. However, since some chemical pretreatment methods themselves can cause oxidation and partial degradation of the plastic, researchers must distinguish the chemical changes resulting from pretreatment separately or from the photocatalytic process. Therefore, a blank control experiment that includes only the pretreatment steps is necessary.

In addition, while pristine microplastics are commonly used in laboratory studies, microplastics in real-world environments have typically undergone varying degrees of photoaging, thermal oxidation, mechanical abrasion, and biological colonization. Environmental aging alters the oxygen-containing functional groups, roughness, surface charge, wettability, and crystallinity of polymer surfaces. Oxidation and surface cracking may enhance the contact between active species and microplastics in water; therefore, the catalytic behavior of pristine and aged microplastics cannot be simply equated, and the aging state of the microplastics should be clearly reported when comparing degradation performance across different studies^[59-61]. In summary, the ultimate goal of pretreatment should be to lay the groundwork for improving subsequent catalytic efficiency; therefore, when designing catalysts and studying surface reaction mechanisms, it is important to compare the catalysts’ degradation performance on microplastics that have undergone pretreatment under different conditions, as well as the selectivity of the products.

The photocatalytic conversion of microplastics relies on the absorption of incident light by semiconductors. When light strikes the surface of the catalyst, electrons in the valence band are excited to the conduction band, thereby forming electron-hole pairs. These conduction-band electrons can participate in reduction reactions, while the holes can participate in the oxidation process of microplastics^{[17,19,62,63]}. However, because photocatalysts are prone to electron-hole recombination, and microplastics possess the characteristics described in Section STRUCTURAL CHARACTERISTICS AND DEGRADATION CHALLENGES OF MICROPLASTICS, this process affects the conversion of charge carriers into active species. Therefore, simply modulating the electronic structure will not result in higher conversion efficiency for microplastics. Therefore, the piezoelectric effect is introduced to fix the direction of electron migration in the photocatalytic process. A piezocatalyst undergoes lattice deformation when stimulated by mechanical stresses such as ultrasonic waves. Piezo-photocatalysis, however, does not involve the superposition of the two catalytic processes; rather, it regulates carrier migration through piezoelectric polarization and piezoelectric potential generated by mechanical deformation. The principle is illustrated in Figure 2^[21,64]. When compared with other advanced oxidation processes, piezo-photocatalysis can utilize mechanically induced polarization to regulate the separation of photogenerated charges, thereby enabling interfacial redox reactions. However, other AOPs, such as ozone oxidation, photocatalytic Fenton oxidation, and electrochemical oxidation, require the generation of reactive species through chemical oxidants, ozone, electrochemical pathways, or the Fenton reaction, whereas piezoelectric photocatalysis operates without the need for additional oxidants^[65,66]. However, the external mechanical energy required for piezoelectric catalysis may partially offset the energy efficiency. Therefore, when reporting a comparison between piezo-photocatalysis and other AOP technologies, chemical inputs and total energy consumption should also be considered^[67]. Therefore, the advantage of piezo-photocatalysis over conventional photocatalysis should not be limited to the additional utilization of mechanical energy, but rather to the dynamic regulation of redox chemistry and interfacial charge transfer at the reaction surface. Furthermore, to demonstrate that piezo-photocatalysis does indeed enhance the degradation efficiency of microplastics, it is not sufficient to merely compare the reaction rates under the three conditions of photocatalysis, piezocatalysis, and piezo-photocatalysis. To further compare the conventional heterogeneous photocatalysis, piezocatalysis and piezo-photocatalysis, we established Table 2 to clarify the advantages and limitations. The next section will further discuss how piezoelectric polarization modulates local potentials and the migration of photo-generated charge carriers, as well as how these effects should be verified at the mechanistic level.

Current research has shown that piezoelectric enhancement works by modulating the charge-transfer barrier and the built-in electric field, which can alter the charge migration direction and utilization efficiency of photogenerated carriers. In the chemically bonded BaTiO₃@ReS₂ Schottky heterojunction system developed by Liu et al., the Schottky junction between ReS₂ and BaTiO₃ provides an interfacial channel for electron transfer, while the mechanically induced BaTiO₃ polarization field further lowers the Schottky barrier^[69]. The electrons can then transfer more easily toward ReS₂, thereby enhancing O₂ activation and ·O₂^- formation. The piezo-photocatalytic apparent rate constant reported in this research reached 0.133 min^-1, which is 16.6 and 2.44 times that of piezocatalysis and photocatalysis^[69]. Li et al. found that the surface potential of a Bi₂O₂(OH)NO₃/BiOI type-II heterojunction is approximately 1.6 times that of Bi₂O₂(OH)NO₃ alone^[70]. They demonstrated that piezoelectric polarization and the heterojunction-induced electric field can synergistically enhance charge separation efficiency. In the most recent research work, Niu et al. used piezoresponse force microscopy (PFM), Kelvin probe force microscopy (KPFM), and density functional theory (DFT) to further elaborate on this mechanism in a ZnO/N-CuO S-scheme heterojunction^[71]. Under simultaneous light irradiation and ultrasound, this system achieved 98.3% cefotaxime removal. However, compared to the ultimate degradation efficiency, the experiments indicate that the key to heterojunction piezo-photocatalysis does not lie simply in the simultaneous presence of a built-in field. They mainly consider whether mechanically induced polarization can enhance the favorable interfacial potential gradient under actual operating conditions and then ultimately drive the directional migration of charge carriers^[71]. Jing et al. recognized that it is still insufficient to infer the piezoelectric contribution based solely on steady-state photoluminescence (PL) quenching or photocurrent enhancement; therefore, in addition to the BiVO₄/BiFeO₃ n-p heterojunction, they also analyzed the electron diffusion coefficient, diffusion length, and electron lifetime^[72]. The coupled reaction rate of this system reached 6.12 and 4.36 times that of photocatalysis and piezocatalysis^[72].

On the other hand, some researchers have also found that the piezoelectric response does not simply have a linear effect on catalytic efficiency. BaTiO₃ samples of different sizes exhibit different properties. A higher piezoelectric amplitude did not result in a higher reactivity. Furthermore, the authors found that the photocurrent response of small BaTiO₃ platelets was 2.89 times that of larger samples, a phenomenon primarily attributed to switchable surface screening charges^[73]. This result indicates that the key factor determining the actual catalytic enhancement is the catalyst’s ability to maintain effective surface charges for interfacial exchange under dynamic mechanical stimulation. This work also explains why the mechanism of piezocatalysis remains controversial. In short, Wang et al. summarized the main explanations as energy-band theory and the screening-charge effect^[22]. The former emphasizes the role of the piezopotential in modulating semiconductor band bending and carrier migration, while the latter mainly discusses the role of polarization-induced surface charges and screening charges in interfacial redox processes. More recent articles have further pointed out that, in addition to the two models mentioned above, the displacement-current mechanism based on the nanogenerator principle has also been used to explain some piezocatalytic phenomena; therefore, studies of the mechanisms underlying these reactions should integrate evidence from pressure-dependent spectroscopy, KPFM, chronoamperometry, and in situ electron paramagnetic resonance (EPR)/Fourier-transform infrared spectroscopy (FTIR)^[74].

To verify the piezoelectric contribution to catalysis, researchers should establish a chain of evidence that can demonstrate the material’s piezoelectric response contributes to the final chemical transformation. PFM and piezoelectric coefficients can confirm whether a material exhibits a measurable electromechanical response. However, those results cannot prove that piezoelectric polarization plays a role in the catalysis. Further stress-dependent KPFM is needed to determine whether mechanical deformation produces exploitable local potential changes. Based on this, photocurrent, electrochemical impedance spectroscopy, PL/ time-resolved photoluminescence (TRPL), and transient spectroscopic techniques can be used to determine whether mechanical stimulation alters the lifetime of photo-generated carriers and the charge separation process. However, further mechanistic validation should be extended to interfacial redox chemistry on the surface between the catalyst and plastics. Then, EPR/ electron spin resonance (ESR), reactive oxygen species (ROS)-selective probes, and radical scavenging experiments can be used to determine whether piezoelectric polarization further alters ROS generation. Therefore, a comprehensive degradation mechanism should establish a continuous relationship among the piezoelectric response, surface-potential modulation, photocarrier regulation, reactive-species evolution, and substrate conversion. At the same time, control experiments are also essential for identifying the contribution of piezoelectricity. At least three experimental conditions should be established, including light irradiation alone, mechanical stimulation alone, and the simultaneous application of light irradiation and mechanical stimulation. If ultrasound is used, a catalyst-free control under simultaneous light irradiation and ultrasound should also be included to evaluate the direct contribution of these external stimuli. Wu et al. compared non-piezoelectric Bi₂Fe₄O₉ with piezoelectric BiFeO₃ and Bi₁₂(Bi_0.5Fe_0.5)O_19.5 in a PET-MP piezocatalytic/Fenton-like system and evaluated the entire degradation process by analyzing the piezoelectric contribution^[75].

The SnO₂/g-C₃N₄/ poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) system was reported by Fazli et al. In this study, the researchers compared the effects of photocatalysis, piezocatalysis, and combined piezo-photocatalysis on PS nanoplastics and demonstrated the existence of a coupling effect^[68]. Furthermore, this system immobilized SnO₂/g-C₃N₄ on a piezoelectric PVDF-HFP nanofiber mat, thereby reducing excessive dispersion of the catalyst in water under mechanical agitation^[68]. We summarize the significance of piezoelectric enhancement for understanding the mechanisms of microplastic transformation in existing studies in Table 3.

The mechanistic evidence listed in Table 3 summarizes two types of evidence. Mechanistic analog studies elucidate the mechanism and viability of piezo-photocatalysis in pollutant degradation, whereas a direct plastic evidence section includes three studies directly involving plastic substrates. Since the literature on the piezo-photocatalytic degradation of microplastics remains very limited, these studies are included to provide mechanistic context for understanding the detailed mechanism in polymer conversion. Lu et al. developed a Fe_1-xS/FeMoO₄/MoS₂ hetero-interpenetrated catalyst and established a mechanistic framework connecting interfacial charge regulation, oxidant generation, ROS chemistry, and polymer conversion products, thereby providing relatively comprehensive mechanistic evidence specific to microplastics^[36]. Recent studies have also found that piezoelectric enhancement may alter the selectivity of ROS. Li et al. used Fe-doped carbon nanotubes to modulate the ZnO/carbon nitride heterojunction, making ¹O₂ the dominant ROS during the peroxymonosulfate (PMS) activation process, with a yield of 67.71 μmol L^-1[76]. Approximately 700 mg of high-density polyethylene (HDPE) microplastics (MPs) were used, resulting in a weight loss of approximately 39.2 wt.% and the formation of intermediates such as carboxylic acids; however, this process did not achieve complete mineralization^[76]. Therefore, establishing a chain of evidence for degradation is crucial. It can provide a foundation for the subsequent development of reliable microplastic degradation criteria and mechanism-guided catalyst design.

From the perspective of materials design, the current optimization of piezo-photocatalysts primarily focuses on defect engineering, heterojunction construction, and morphology/interface regulation. However, when it comes to microplastic conversion, simply increasing the bulk charge-separation efficiency is not enough. Material design for microplastics must address two issues simultaneously: first, how to maintain high piezo-photocatalytic activity through structural design; and second, how to concentrate active species at the interface between the catalyst and the microplastic. Defect engineering does not simply increase the number of surface active sites. For example, Yuan et al. successfully controlled oxygen vacancies (O_vs) in BaTiO₃^[77]. They found that an appropriate amount of O_vs can enhance local polarization while forming Ti³⁺-related sites^[77]. However, performance did not improve as the number of defects was further increased. To achieve the best performance, researchers must find the balance between the number of defects and the degree of polarization. Defective BaTiO_3-x in seawater also proves that defect-induced structural distortion can stabilize piezoelectrically active tetragonal domains. However, the activity in natural seawater is significantly lower than that in deionized water, indicating that the actual performance of defect-regulated materials is still influenced by the electrolyte environment. Therefore, when using microplastics, care should be taken to avoid excessive defects that could lead to carrier recombination. Point defects play a multifaceted regulatory role in heterojunction band alignment, interfacial charge transfer, and adsorption^[78].

The advantage of the heterojunction structure is its ability to simultaneously control both the migration of charge carriers and the generation of surface-active species. Recent advances in conventional photocatalysis have further demonstrated that S-scheme heterojunction construction can effectively regulate directional charge transfer while preserving strong redox capability^[79-81]. Future heterojunction designs should focus on how heterojunctions contribute to microplastic oxidation. It is important to ensure that electrons and holes are transported separately to surface sites, which can actually activate O₂ or oxidize polymers, rather than simply pursuing lower PL intensity. Zhao et al. constructed a hierarchical porous TiO₂ layer and adjusted the interface from plastophobic to plastophilic^[82]. The results showed that optimal performance was achieved under semi-wet conditions at the interface between the plastic and the catalyst. This system achieved a mineralization rate of 5.63 mmol g^-1 h^-1 for LDPE and a conversion of 26.3% after 20 h^[82]. Strong hydrophobic adsorption capacity does not equate to the interfacial affinity for plastics. If the catalyst surface is excessively hydrophobic toward microplastics, it may limit the transport of H₂O and O₂. Therefore, the equilibrium at the interface between the polymer and the oxide needs to be optimized. This interface requirement implies that future material designs should incorporate greater polymer specificity. Due to the varying characteristics of different microplastics, designs should be based on the surface chemistry of the target microplastics, and further investigation should be conducted to determine whether this approach truly enhances the efficiency of the oxidation process. Based on this understanding, two-dimensional materials - which have large dimensions, relatively short electron transport paths, and tunable surface chemistry - can theoretically serve as both a contact platform for microplastics and an electron transfer layer. This allows for the dispersion of additional piezoelectric domains or active sites on the surface of 2D materials. Nowadays, carbon-based materials have been developed that incorporate a hierarchical carbon nitride-supported single-atom Fe catalyst capable of stabilizing Fe-N₄ sites, enabling the conversion of various practical plastic substrates and stable operation over six consecutive cycles^[83]. However, there is currently insufficient evidence to prove that 2D-0D morphology is inherently superior to other common, high-performance material designs; therefore, this paper presents it as a structure worthy of systematic comparison. In direct water treatment, if large amounts of nanoscale catalysts are freely dispersed in water, this is likely to cause secondary pollution, thereby negating the environmental benefits of the process. Fazli et al. used a SnO₂/g-C₃N₄/PVDF-HFP nanofiber mat to immobilize the photocatalytic components on a flexible piezoelectric substrate to prevent catalyst dispersion^[68]. Confining both the capture of microplastics and their subsequent degradation to a single interface can significantly reduce the use of nanoparticle catalysts, which is proved by the application of immobilized photocatalytic membrane reactors in conventional MP treatment. This approach will provide a basis for the direct application in rivers or seawater^[84]. Therefore, rather than simply studying catalysts with smaller particle sizes, it may be more practical in the future to focus on fibrous mats, flexible films, and supported porous layers - structures that are not only easier to recover but also more readily stimulated by mechanical forces.