Synergetic Adsorption‐Catalysis in Potassium Oxide Modified High‐Entropy Relaxor Ferroelectrics for Efficient Dye Removal Strategy

Water scarcity caused by extreme weather events has become a global issue, leading to the necessity of wastewater recycling. Developing new materials with fast pollutant removal efficiency has become a research focus in related fields for producing clean water resources quickly and stably from wastewater. Herein, a relaxor ferroelectric type of piezoelectric high entropy perovskites, Pb(Mg,Nb,Hf,Zr,Ti)O3 (PMNHZT), is used and prepared for industrial dye removal. The synthesized PMNHZT exhibits high dark adsorption of the methylene blue dye (removal efficiency of ≈60% in 30 min) and further catalytic dye degradation (removal efficiency of ≈80–90% in 30 min) through light illumination, sonication, or a combination of both. In the dark adsorption process, K2O compounds from the synthesized environment attached to the surface of PMNHZT play a significant role in the remarkable dark adsorption by promoting a large specific surface area and more negative surface potential. Furthermore, a self‐decomposition of dye into smaller fragments by PMNHZT is also observed during dark adsorption. The piezocatalysis mechanism dominates the dye degradation process in catalytic experiments, where hydroxyl radicals are the main reactive species. Herein, a promising adsorbent and catalyst is disclosed using high‐entropy perovskites for efficient wastewater treatment.


DOI: 10.1002/aesr.202300100
Water scarcity caused by extreme weather events has become a global issue, leading to the necessity of wastewater recycling.Developing new materials with fast pollutant removal efficiency has become a research focus in related fields for producing clean water resources quickly and stably from wastewater.Herein, a relaxor ferroelectric type of piezoelectric high entropy perovskites, Pb(Mg,Nb,Hf, Zr,Ti)O 3 (PMNHZT), is used and prepared for industrial dye removal.The synthesized PMNHZT exhibits high dark adsorption of the methylene blue dye (removal efficiency of %60% in 30 min) and further catalytic dye degradation (removal efficiency of %80-90% in 30 min) through light illumination, sonication, or a combination of both.In the dark adsorption process, K 2 O compounds from the synthesized environment attached to the surface of PMNHZT play a significant role in the remarkable dark adsorption by promoting a large specific surface area and more negative surface potential.Furthermore, a selfdecomposition of dye into smaller fragments by PMNHZT is also observed during dark adsorption.The piezocatalysis mechanism dominates the dye degradation process in catalytic experiments, where hydroxyl radicals are the main reactive species.Herein, a promising adsorbent and catalyst is disclosed using high-entropy perovskites for efficient wastewater treatment.degradation, Fenton reaction, ozonation, and ultraviolet irradiation.However, these techniques are generally expensive and commercially unappealing due to their dependence on specialized equipment and high-energy consumption.They result in substantial chemical and reagent usage and generate hazardous secondary pollutants, leading to disposal challenges. [9]Recently, advanced oxidation processes, particularly photocatalysis or piezocatalysis using functional semiconductors, have shown significant promise in the degradation of dyes.By utilizing light or ultrasonic sources, these catalytic processes excite electrons and holes, triggering a series of reactions that generate reactive oxygen species like hydroxyl or superoxide radicals.These radicals possess potent oxidizing abilities, enabling the efficient oxidation of diverse organic pollutants. [9,13,14]As a result, there is a growing research interest in exploring novel catalyst materials that can expedite organic dye removal.
In numerous material systems, ceramic-based ones have emerged as a compelling avenue for exploring adsorption and degradation capabilities.For example, ZnO-based nanocomposites [15] and doped ZnO nanoparticles [16] have exhibited notable proficiency in adsorbing cationic dyes and antibiotics due to their extensive surface areas and high surface reactivity.[19] The unique piezoelectric property inherent in ZnO enables the harnessing of vibration energy to facilitate the piezocatalytic degradation of dyes. [20,21][24] In piezoelectric materials, the internal field induced by mechanical agitation can effectively suppress the recombination of photo-excited electron-hole pairs, directing them toward disparate regions, thereby facilitating dye degradation.Similar phenomena have been unveiled in other piezoelectric materials, such as ferroelectric NaNbO 3 , [25,26] LiNbO 3 , [27] and BaTiO 3 [14,28,29]   , exhibiting more tremendous advantages than conventional photocatalysts.
This study demonstrated a new piezocatalyst based on a famous relaxor ferroelectric, Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMNPT).Relaxor ferroelectric PMNPT exhibits a significant piezoelectric coefficient and a slim hysteresis loop, ascribed to multiple-doped-elements-induced local lattice distortion.[32][33] While more elements with different ionic sizes are involved in the lattice, severer lattice distortion is procured, which anticipatedly leads to a higher piezoelectric effect.Yan et al. showed that adding In and Zr ions into PMN-PT can create additional local random fields (RFs) and suppress the aggregation of PNRs, obtaining an outstanding piezoelectric coefficient of 1820 pm V À1 . [34]This phenomenon conforms to the concept of high-entropy materials, possessing four key features: 1) the high entropy effect in thermodynamics; 2) kinetics sluggish diffusion; 3) lattice distortion; and 4) the cocktail effect. [35]Particularly for the lattice distortion effect, it describes that lattice distortion caused by strain can substantially lead to the formation of noncentrosymmetric atomic arrangement in the crystal lattice and enhance the magnitude of polarization of piezoelectric.Both perspectives from relaxor ferroelectrics and high-entropy materials converge, suggesting that increasing the element species within a lattice can effectively introduce more localized lattice distortion, consequently enhancing the piezoelectric coefficient and polarization.Even though doped PMNPT has demonstrated the highest piezoelectric coefficient, limited attention has been given to its potential in the realm of piezocatalysis.Prior investigations have primarily focused on the piezoelectric effect-assisted photocatalysis for dye degradation, employing heterostructures such as PMNPT/ TiO 2 and PMNPT/SnO 2 . [36,37]Remarkably, no study has yet explored the impact of relaxor ferroelectric materials on piezocatalytic efficiency.
To address the viability of relaxor ferroelectrics in direct piezocatalysis for dye degradation, we developed and synthesized a high-entropy perovskite oxide with the composition of 0.25Pb(Mg 1/3 Nb 2/3 )O 3 -0.75Pb(Hf0.2 Zr 0.42 Ti 0.38 )O 3 (PMNHZT) by introducing Hf and Zr ions into PMNPT.In addition to the local lattice distortion-induced piezoelectric behavior in PMNHZT, the incorporation of multiple transition metal elements as active sites engenders a diverse range of electronic configurations, which could cause more unexpected interactions with organic dye molecules.Our results show that PMNHZT possesses good dye degradation behavior and presents an unexceptionally strong dye absorption capability before performing a dye degradation experiment.The synergy of dye absorption and degradation achieves a quick dye removal rate of 90% in a relatively short period of 60 min, providing a promising method for water remediation.

Results and Discussion
Figure 1a shows the X-ray diffraction (XRD) patterns of PMNHZT synthesized under different KOH concentrations, ranging from 1 to 12 M.In the case of the sample synthesized at 1 M KOH, the diffraction pattern displays a distinctive twophase characteristic, where the symbols of blue stars and green triangles correspond to the crystal structures of standard materials PbO (Fm3m, JCPDS No. 65-7466) and PbTiO 3 (P4/mmm, JCPDS No. 06-0452), respectively.The related SEM image in Figure 1b reveals the microstructure of this powder sample, exhibiting irregular and rod-like shapes.Upon increasing the KOH concentration to 4 M, a two-phase feature remains, but the diffraction pattern corresponding to PbTiO 3 turns into that of Pb(MgNb 2 ) 0.33 O 3 (Pm3m, JCPDS No. 89-3117).The related microstructure in Figure 1c depicts aggregated particles synthesized with 4 M KOH, displaying a cubic shape.As the KOH concentration increases from 5 to 7 M, a single-phase crystal structure is observed, which can be associated with the cubic lattice structure based on the standard material Pb(MgNb 2 ) 0.33 O 3 .Simultaneously, the microstructure in Figure 1d,e still preserves the cubic shape, but the aggregation size decreases with KOH concentration.Subsequently, the cubic configuration of the aggregations begins to undergo alterations as the KOH concentration raises to 9 M and beyond (Figure 1f,g).The diffraction patterns for samples synthesized at 9 and 12 M KOH exhibit multiple splitting around the peak positions of Pb(MgNb 2 ) 0.33 O 3 , which is challenging to identify their actual crystal structures.This suggests a further alternation in the lattice of PMNHZT, possibly changing the crystal structure from cubic to tetragonal or other lower symmetry forms.The microstructure of the sample synthesized at 12 M KOH becomes a plate-like shape (Figure 1g), suggesting a completely different crystal structure from previous cases.In general, the determination of lattice form for samples relies on the positions of the diffraction peaks.Meanwhile, the grain size estimation can be accomplished by analyzing the full-width at half-maximum (FWHM) of the diffraction peaks.The average crystalline size for all samples was assessed by applying the Scherrer equation to the FWHM of the diffraction peak (111), as illustrated in Figure S1, Supporting Information.The variation in grain size exhibited a distinct trend compared to the aggregation size, with the maximum grain size occurring in the sample synthesized at 7 M. Therefore, the sample synthesized with 7 M KOH demonstrated the best crystallinity and the smallest aggregation size.
Elemental analyses were conducted utilizing energydispersive X-ray spectra (EDS) to confirm the composition ratios of the synthesized high-entropy perovskites.The morphological characteristics and the corresponding elemental maps for three representative cases synthesized at 1, 7, and 12 M KOH are depicted in Figure S2, Supporting Information, where the distributions for all constituents (Pb, Mg, Nb, Ti, Zr, and Hf ) demonstrate coherence with the respective morphological features.In addition to these primary elements, we also observed the K signal in EDS spectra.After identifying the compositions from the intensity of all elements (Table S1, Supporting Information), the proportion of potassium (K) element increases in direct correlation with the increment of KOH concentration, which can be attributed to the inevitable extra doping in or adsorption on the particle surface from the synthesis environment.Furthermore, although the samples of 1, 7, and 12 M KOH show a deviation from the designed nominal composition, the atomic ratios of all elements remain within an acceptably broad range in alignment with the requisites for high-entropy materials.For subsequent dye removal experiments, our focus is directed exclusively toward the sample synthesized at 7 M KOH because this sample is a single-phase solid solution with the best crystallinity.In the subsequent sections, the abbreviation PMNHZT specifically denotes this sample for simplicity.
Figure 2 provides an in-depth insight into the microstructure of PMNHZT measured by high-resolution transmission electron microscopy (HRTEM).Low magnification of the HRTEM image in Figure 2a displays a representative PMNHZT particle with a square shape, of which the size is around 100 nm.A higher  magnified HRTEM image in Figure 2b, extracted from the lower right corner of this particle (highlighted by a yellow square in Figure 2a), reveals a distinct atomic image.In addition, the electron diffraction pattern taken from the whole particle in Figure 2c presents a typical diffraction spot feature with a square lattice projected along the [100] axis, substantiating the presence of cubic symmetry.The lattice constant of PMNHZT estimated from HRTEM images and the electron diffraction pattern is around 0.4 nm, consistent with the value calculated from XRD.These results confirm that PMNHZT is a single-phase solid solution with a single-crystal-like quality.To investigate the electrical properties of the synthesized PMNHZT powder, we conducted a polarization versus electric field (P-E) curve analysis, as depicted in Figure S3, Supporting Information.The results reveal that the sample exhibits a slim hysteresis loop with a significantly low coercive field and remnant polarization.Such a feature is indicative of the typical relaxor ferroelectrics behavior.Additionally, the curve even demonstrates an approximately linear relationship while still maintaining dielectric nonlinearity, suggesting a closer proximity to superparaelectric characteristics. [38,39]This finding further implies the presence of piezoelectric properties within the synthesized powder, which can potentially be harnessed for subsequent piezocatalytic dye degradation applications.
To verify the dye removal mechanism of this new high-entropy perovskite, an initial prolonged dark adsorption stage is necessary before performing experiments such as photocatalytic degradation, piezoelectric degradation, and photo-piezoelectric degradation.The adsorption process hinges on various attractive forces, including hydrogen bonding, material surface potential, and van der Waals interactions between adsorbents and dyes.Notably, these forces do not result in the decomposition of dyes.On the other hand, catalytic dye degradation typically involves chemical reactions that fragment large dye molecules into smaller constituents.While both adsorption and catalytic degradation can lead to the decolorization of dye solutions, their interaction mechanisms with dyes are distinct in nature.Interestingly, our synthesized PMNHZT exhibited a dual capability for dye removal, where it concurrently engaged in both adsorption and catalytic degradation processes.Figure 3a,c present the ultraviolet-visible (UV-Visible) spectra and dye removal efficiency (C/C 0 ) of the dark adsorption experiment for PMNHZT in the MB solution, respectively.The result demonstrated a significant adsorption behavior, of which a 59% decrease in MB concentration was observed when PMNHZT was placed in the MB solution for 15 min, and a saturated removal efficiency of 66% was achieved after 30 min.Since the dye adsorption onto PMNHZT exhibits no apparent alteration beyond 30 min, subsequent photocatalysis, piezocatalysis, and photo-piezocatalysis experiments were conducted utilizing powders undergoing the 30 min dark adsorption period.The respective UV-vis spectra of photocatalytic, piezocatalytic, and photo-piezo-catalytic degradations were unveiled in Figure S4a,b, Supporting Information, and 3b.The corresponding dye removal efficiencies were estimated in Figure 3d.All three methods could further decolorize the MB solution within 60 min.Among these, photocatalysis and photo-piezocatalysis achieved the least (%16% of initial MB concentration) and the highest (%26% of initial MB concentration) enhancement in dye removal efficiency, respectively.The least enhancement in dye removal efficiency for the photocatalytic degradation experiment can be attributed to the incomplete utilization of the visible spectrum due to the wide bandgap of PMNHZT (%3.34 eV), as shown in Figure S4c, Supporting Information.By contrast, piezoelectric catalysis and photo-piezoelectric catalysis showed comparable performances.The catalytic reaction rates for these three methods were evaluated by a pseudo-first-order equation ln(C 0 /C) = kt, where k signifies the rate constant extracted from the slope of kinetic curves in Figure S4d, Supporting Information.The calculated k values are 0.0119, 0.0171, and 0.01948 min À1 for photocatalysis, piezocatalysis, and photo-piezocatalysis, respectively.Hence, a more significant catalytic reaction rate in piezodegradation and photopiezodegradation processes can be inferred that piezoelectric catalysis is the primary mechanism of dye degradation for PMNHZT.
To investigate the interaction between the MB dye and the synthesized PMNHZT, an initial exploration of the surface characteristics of PMNHZT was conducted using X-ray photoelectron spectroscopy (XPS).Figure 4a,b depicts the C 1s spectra for PMNHZT after dark adsorption and photo-piezocatalytic degradation.These spectra exhibit analogous features while displaying distinct relative peak intensities.These spectra were comprehensively analyzed by assigning various principal bond types in both MB and powder composition.After dark adsorption (Figure 4a), the PMNHZT surface reveals a notable presence of C─C/C─H bonds (284.6 eV), C─O/C─N bonds (286.0 eV), and C═O/C═N bonds (288.5 eV), which are attributed to the benzene ring and carbonyl functional group inherent to MB. [40][41][42] In addition to C 1s signals, significant signals of the K element located at 292.5 eV (2p 3/2 ) and 295.3 eV (2p 1/2 ) is also presented, which are close to the binding energy of K þ . [43]The presence of K is likely a residual remnant originating from the synthesis environment.For PMNHZT undergoing both light illumination and sonication process (Figure 4b), we can observe a noticeable reduction in the signals corresponding to C─O/C─N and C═O/C═N bonds.This decrease indicates a further degradation of the benzene ring and carbonyl functional group of MB.A significant attenuation of the K signal is also observed, suggesting that the presence of the K element predominantly manifests as a compound like K 2 O on the surface of PMNHZT rather than as a dopant introduced during the synthesis procedure.This assertion is substantiated by XRD measurements conducted on samples subjected to different states.The relative intensities and positions of all peaks remain consistent following dark adsorption, photocatalytic, piezocatalytic, and photo-piezocatalytic processes (Figure S5, Supporting Information), illustrating the invariable structure and composition of the synthesized PMNHZT sample during the dye removal experiments.
Notably, the K 2 O compound seemingly dominates a robust dark adsorption phenomenon for this hydrothermally synthesized PMNHZT.46] Although the mechanism of adsorption triggered by K 2 O is still unclear, a comparative evaluation against undecorated PMNHZT samples of identical composition, synthesized through the solidstate method, confirms this augmented K 2 O-mediated adsorption phenomenon.As depicted in Figure S6, Supporting Information, undecorated PMNHZT exhibits markedly diminished dark adsorption capability in contrast to its hydrothermal-synthesized PMNHZT.Although the K 2 O compound is hardly observed in the XRD and TEM results due to its minute amount within the powder or amorphous structure, the surface area and zeta potential analyses can support such a strong adsorption phenomenon assisted by K 2 O. Figure S7, Supporting Information exhibits the isothermal curves of nitrogen adsorption/desorption in PMNHZT powders synthesized by hydrothermal and solid-state routes.Hydrothermal-synthesized PMNHZT demonstrates superior adsorption/desorption capacities across the entire pressure range compared to solid-statesynthesized PMNHZT.The specific surface area, as deduced from these curves employing the Brunauer-Emmett-Teller (BET) model, is provided in Table S2, Supporting Information.Hydrothermal-synthesized PMNHZT possesses a specific surface area five times larger than its solid-statesynthesized counterpart.Furthermore, the determination of the Zeta potential serves as a tool to assess the surface charge of a material, with corresponding values also detailed in Table S2, Supporting Information.Both hydrothermal-synthesized and solid-state-synthesized PMNHZT present negatively charged surfaces, facilitating the attraction of cationic methylene blue dyes.The zeta potential for hydrothermal-synthesized PMNHZT is -37.49mV, more negative than that of -24.61 mV for solid-state-synthesized PMNHZT.Such a vast difference in specific surface area and more negative zeta potential can be attributed to the smaller particle size and surface chemical compositional variation modified by K 2 O, resulting in strong dark adsorption for hydrothermal-synthesized PMNHZT.
It is recalled that K 2 O-assisted adsorption typically belongs to the realm of chemisorption.This phenomenon hinges on ionic or covalent bonding, suggesting the occurrence of a chemical reaction between the molecules of MB and PMNHZT.Figure 4c unveils the variation of the MB solution after reacting with PMNHZT under different states using Fourier-transform infrared spectroscopy (FT-IR).The FT-IR spectrum of the pure MB solution exhibited a similar pattern to those reported in the literature.This profile featured characteristic bands corresponding to C─H out-of-plane bending vibration (γ(C─H)), C─H wagging vibration (ω(C─H)), and C─H twisting vibration (τ(C─H)) at 812 cm À1 , C─C in-plane bending vibration (δ(C─C)) at 859 cm À1 , C─S─C asymmetric stretching vibration (ν as (C─S─C)) at 1030-1075 cm À1 , CH in-plane bending vibrations (δ(CH)) at 1183 and 1400 cm À1 , C = S þ or C─N stretching vibration (ν(C═S þ ) or ν(C─N)) at 1346 cm À1 , C═N or C═C stretching vibration (ν(C═N) or ν(C═C)) at 1600 cm À1 , CH 3 asymmetric stretching vibration (ν as (CH 3 )) at 2925 cm À1 , C─H stretching vibration (ν(C─H)) at 3055 cm À1 , and OH stretching vibration (ν(OH)) at 3415 cm À1 . [47,48]These characteristic bands become smearing and broadening after performing every experiment, implying the decomposition of MB into smaller molecules, such as benzene rings, CH 3 , C─N, and C─H.Such decomposition occurs not only during photocatalytic, piezocatalytic, and photo-piezo-catalytic processes but also during dark adsorption.In the photocatalytic, piezocatalytic, and photo-piezo-catalytic pathways, it is unsurprisingly that the excitons from light illumination, sonication, or both excitation sources in piezocatalysts can generate reactive radicals (•OH or •O À 2 ) to decompose dyes. [14]owever, the initiation of dye decomposition in the dark adsorption phase is a rarity due to the absence of an external stimulus to induce electron-hole pair excitation theoretically.
Electron paramagnetic resonance (EPR) spectroscopy was carried out to verify the occurrence of dye decomposition in every state.Redox reactions with hydroxyl (•OH) and superoxide (•O À 2 ) radicals have been considered the primary mechanism for degrading dyes.However, the lifespan of these radicals is short, making the direct measurement of their existence challenging.The spin trap 5,5-dimethylpyrroline-N-oxide (DMPO) is commonly adopted to trap-free radicals generated from the photocatalytic or piezocatalytic process.Figure 5 shows the EPR spectra for the MB solutions added with DMPO under different states.The spectrum of the pure MB solution (black line) exhibited solely a baseline signal, indicating that DMPO captures no free radicals.After the 30-minute dark adsorption process, several characteristic resonance signals emerged at 3452, 3468, 3475, 3483, 3491, and 3506 G (labeled by purple inverted triangles) in the EPR spectrum (red line) are similar to those in the simulated spectrum of DMPO capturing •CH 3 free radicals (purple dashed line).It confirms that during the dark adsorption process, the MB dye molecule has already undergone initial decomposition, producing small molecules such as •CH 3 -free radicals.Such self-decomposition observed during the dark adsorption stage primarily arises from the interaction with K 2 O, which can be supported by the absence of •CH 3 radicals in the solid-statesynthesized PMNHZT subsequent to the dark adsorption stage.[51] These additional peaks are in good agreement with the characteristic peaks of •OH radicals (earthy yellow dash line), [49,51,52] illustrating that a small number of excitons can still be created after the illumination.It indicates that the MB solution can be further decolorized by reacting with these •OH radicals after the photocatalytic process.For the solution after 30 min piezocatalytic (blue line) and 30 min photo-piezo-catalytic (orange line) processes, both EPR spectra exhibit significant characteristics akin to that of DMPO-OH adduct, illustrating that a large amount of •OH radicals are generated.This suggests that sonication-induced piezoelectric catalysis is the principal mechanism for dye degradation for PMNHZT.It is noteworthy that •O À 2 radicals can also serve as an active species in the degradation of dyes.[55] A suspension solution of PMNHZT in anhydrous methanol solvent was prepared to investigate the generation of O À 2 radicals.As depicted in Figure S8, Supporting Information, the observed EPR spectrum remained flat with background signals after a 30 min photo-piezo-catalytic process, indicating that an undetectable •O À 2 amount was generated in this case.In a typical concept of piezocatalytic dye degradation, the internal field-induced band tilt generally allows for the simultaneous gerenation of •OH and •O À 2 radicals.The absence of •O À 2 radicals in the EPR measurement may be attributed to two possible scenarios: 1).The conduction band edge of PMNHZT, even after band tilt, may still be lower than the reduction potential of O 2 /•O À 2 , hindering the formation of 2 radicals may readily convert into •OH radicals in contact with the ambient environment, which contains moisture, during the transfer process for EPR measurement.Although further comprehensive investigations are required to confirm the precise scenario behind the absence of •O À 2 radicals, this outcome illustrates that •OH radicals are likely the primary active species responsible for the degradation of MB dye, following a common reaction pathway as depicted below [56] • The generation of •OH radicals through the piezoelectric effect of PMNHZT was also proven using photoluminescence (PL) spectroscopy.In this measurement, the PMNHZT powders were placed in the water solution with 5 Â 10 À4 M terephthalic acid (TA) but devoid of MB.These solutions were then subjected to dark adsorption, photocatalytic, piezocatalytic, and photopiezo-catalytic processes.When TA reacts with •OH radicals, it transforms into 2-hydroxyterephthalic acid, characterized by a fluorescence peak approximately at 425 nm.As shown in Figure S9, Supporting Information, the solutions after dark adsorption and photocatalytic processes exhibited no discernible fluorescence signal corresponding to 2-hydroxyterephthalic acid.
In contrast, the solutions that underwent piezocatalytic and photo-piezo-catalytic processes showed significant fluorescence signals, indicating an escalation in •OH radical concentration over reaction time.The outcomes from both the EPR and PL spectra analyses align coherently with the observations derived from preceding dye removal experiments, thereby effectively elucidating the enhanced dye degradation phenomenon in piezocatalytic and photo-piezo-catalytic processes.
Based on the XPS observations mentioned earlier, we have discerned that K 2 O compounds present on the surface of PMNHZT might undergo depletion during the dark adsorption stage, implying that the robust adsorption phenomenon is expected to occur only once.To assess the reproducibility of PMNHZT as a reusable absorbent and catalyst, a series of dye removal profiles focusing on the piezodegradation test was conducted over 5 cycles, as depicted in Figure 6.In the initial cycle, the profile resembles that illustrated in Figure 3d, featuring pronounced dark adsorption coupled with piezoelectric catalysis.However, during the second cycle, the efficacy of dark adsorption markedly declined, with dye removal primarily reliant on piezodegradation.The profile is similar to that of solid-statesynthesized PMNHZT as depicted in Figure S6b, Supporting Information, reinforcing the role of K 2 O in governing the robust adsorption phenomenon.To restore the robust dark adsorption capacity of PMNHZT, a rapid pretreatment approach was also introduced.Before commencing the third cycle dye removal experiment, the used PMNHZT powders were immersed in the 1 M KOH for 4 h.Subsequently, the powders were retrieved from the solution through centrifugation and subjected to air drying at 50 °C.In the dye removal profile of the third cycle, robust adsorption reemerged.The following fourth and fifth cycles replicated the procedures of the second and third cycles, respectively, yielding analogous outcomes.This observation permits us to deduce that K 2 O predominantly governs the adsorption phenomenon, while its role in piezodegradation appears to be negligible.A comparative evaluation of dye removal efficiency between K 2 O-modified and unmodified PMNHZT is presented in Figure S10, Supporting Information.To attain an 85% dye removal target, the requisite time for a standard process encompassing both dark adsorption and catalysis stages amounted to 60 min for K 2 O-modified PMNHZT, whereas unmodified PMNHZT necessitated 120 min.This observation emphasizes the potential synergistic interplay between adsorption and catalysis, which can offer a more efficient pathway for dye removal.

Conclusion
In this study, we have synthesized a high-entropy perovskite, PMNHZT, using a simple hydrothermal method.The effect of different KOH concentrations on the crystal structure and microstructure of PMNHZT has been investigated, where a moderate synthesis concentration (7 M) can acquire the best crystallinity and smallest aggregation size, making it a suitable candidate for further investigation.The corresponding structural and elemental analyses confirm that hydrothermal-synthesized PMNHZT has a single-crystal-like quality with atomic ratios conforming with the broad definition for high-entropy materials.Moreover, it exhibited both significant adsorption and catalytic degradation of methylene blue compared to that synthesized via the traditional solid-state method in a short period.The robust dark adsorption phenomenon could be attributed to the presence of K 2 O compounds at the surface of PMNHZT, which promotes chemisorb strength between the MB dye and the material.The subsequent photocatalytic, piezocatalytic, or photopiezo-catalytic experiments can further degrade the MB dye through light illumination, sonication, or combined processes, respectively.Particularly in the piezocatalytic and photopiezocatalytic experiments, both have a resembling efficiency, indicating that the piezoelectric effect of PMNHZT dominates the degradation of the MB dye.From the FTIR and EPR results, we have even found that the decomposition of the MB dye begins during the dark adsorption process, where smaller fragments of CH 3 radicals can be observed.As a result, while employing the identical dye removal procedure, the process involving robust adsorption coupled with the initial self-decomposition of the dye demonstrates greater efficiency compared to a process relying solely on catalytic dye degradation.This study provides an approach to exploring and synthesizing a potential adsorbent and catalyst based on the relaxor ferroelectric type of highentropy perovskites for quick wastewater remediation.

Experimental Section
Material Synthesis: High-entropy piezoelectric perovskite catalysts were synthesized using both the hydrothermal and solid-state methods.For the hydrothermal-synthesized PMNHZT catalyst, the initial precursor solution was prepared by dissolving precise quantities of each compound in 50 mL of ultrapure water (18.2MΩ cm) under magnetic stirring at 200 rpm for 30 min.The precursor solution consisted of 0.06 M Pb(CH 3 COO) 2 •3H 2 O (Acros, 99% purity), 0.005 M [CH 3 COCHC(O) CH 3 ] 2 Mg • 2H 2 O (Sigma-Aldrich, 98% purity), 0.01 M NbCl 5 (Alfa Aesar, 98% purity), 0.009 M HfCl 4 (Sigma-Aldrich, 98% purity), 0.0189 M ZrCl 4 (Acros, 99% purity), and 0.0171 M TiO 2 (Degussa, comprising of 20% Rutile and 80% Anatase).Subsequently, the prepared precursor solution was carefully added to a 20 mL KOH (Emperor Chemical, 95% purity) solution with varying concentrations (1, 4, 5, 6, 7, 9, or 12 M) while continuously stirring at 400 rpm for another 30 min.The resulting mixtures were then transferred into a Teflon-lined reaction vessel and placed inside a stainless-steel container, which was subsequently positioned in an oven set at 200 °C for a duration of 24 h.Following cooling of the container to room temperature, the resulting products were thoroughly washed with ultrapure water, separated via centrifugation, and subsequently dried at 50 °C.
For the PMNHZT synthesized by a typical solid-state method, the starting materials, comprising PbO (Thermo Scientific, 99.9% purity), MgNb 2 O 6 (Thermo Scientific, 99.9% purity), TiO 2 (Degussa, comprising of 20% Rutile and 80% Anatase), ZrO 2 (Sigma Aldrich, 99% purity), and HfO 2 (Combi Blocks, 95% purity) powders, were combined in a stoichiometric ratio equivalent to that of the hydrothermal-synthesized PMNHZT powders.The resulting powder mixture was introduced into a cylindrical container along with 1 mm zirconia balls and 95% ethanol, and subsequently subjected to grinding using the conventional ball milling method at a rotational speed of 300 rpm for a duration of 24 h.Following this, the powder mixture was dried in ambient air at 150 °C for 30 min.Subsequently, %0.13 mL of a 5 wt% polyvinyl alcohol (PVA) binder solution was added to the dried mixture, which was then compacted using a hydraulic press within a 13mm pellet mold under a pressure of 5000 psi for 10 min.After demolding, the pellet was placed in a furnace and heated at a rate of 10°C min À1 until reaching a temperature of 1250 °C.Sintering was carried out for a total duration of 8 h.Finally, the resulting material was finely ground into powders using an agate mortar and pestle.
Material Characterization: The crystal structure of the synthesized PMNHZT samples were characterized by a typical θ-2θ method with a scan rate of 0.5 o s À1 using the X-ray diffractometer (Bruker, D8 advance eco system) with the Cu Kα radiation source.The microstructure and lattice structure of the samples were probed by a field-emission transmission electron microscope operated at 200 kV (JEOL, JEM-2010).The morphologies and elemental analyses were performed at the acceleration voltage of 20 kV by scanning electron microscope equipped with an energy dispersive spectroscope (JEOL, IT-100).The bandgap of PMNHZT was measured by an ultraviolet-visible spectrophotometer (Hitachi, UH5700) with a scanning rate of 600 nm min À1 and a scanning wavelength range from 250 to 1200 nm.The surface composition identification was conducted by the XPS equipped with monochromatic Al Kα radiation (ULVAC-PHI, PHI 5000 VersaProbe).The specific surface area of the powders was obtained by the gas adsorption-desorption isotherm curve measured from the specific surface area and porosimetry analyzer (Anton-Paar, NOVAtouch LX2).The surface electrostatic potential of the powders was measured using a zeta potential analyzer (Particulate Systems, Nanoplus-3).
Polarization Measurement: The synthesized PMNHZT powder was hydraulically pressed into a disc-shape pellet with 1 cm diameter and 0.2 mm thick.The disc was then sintered at 1000 °C for 8 hr for densification.Using a typical sputtering process, the Sn-doped In 2 O 3 (ITO) thin films were deposited on both sides of the densified disc as the electrodes for the subsequent electrical measurement.The corresponding polarization curve was measured using a 100 mV AC voltage at a frequency of 100 Hz, ranging from À20 to 20 V, using the commercially available TFAnalyzer3000 from aixACCT Systems.
Batch Adsorption and Catalytic Experiments: The investigated dye solution was prepared by dissolving 10 mg MB in 1000 mL ultrapure water (%18.2MΩ cm) with a concentration of 10 ppm.45 mg PMNHZT powders were dispersed in a 15 mL MB solution for the dark adsorption, photocatalytic, piezocatalytic, and photo-piezocatalyitc experiments.
The batch dark adsorption experiments were performed by stirring the mixed solution containing the powders and MB in the dark at room temperature.Sampling was taken every 15 min for a total 90 min process.The solution samples were taken using a centrifuge every 15 min for a total 90 min process.
The mixed solution undergoing 30 min dark adsorption, which reached adsorption and desorption equilibrium, was adopted to carry out the subsequent photocatalytic, piezocatalytic, and photo-piezocatalytic experiments under the illumination, ultrasonication, and the combination of illumination and ultrasonication environments, respectively.The light source for photocatalytic experiments used a 150 W xenon lamp.The piezocatalytic experiments were performed in an ultrasonic cleaner with 40 kHz frequency and 350 W power in the dark.The photo-piezocatalytic experiments were performed under illumination and ultrasonication simultaneously with the same instrumental parameters as the photocatalytic and piezocatalytic experiments.In all catalytic experiments, the solution samples were taken using a centrifuge every 10 min for a total of 60 min.In addition, all experiments were roughly maintained at room temperature (25-30 °C) to avoid the temperature effect on dye adsorption and degradation.
Analyses of the Dye Solution: The centrifugated dye solutions were first analyzed using an ultraviolet-visible spectrophotometer.The dye removal profiles (C/C 0 ) for dark adsorption and catalytic experiments were determined by adopting the absorbance peak intensity at around 664 nm, corresponding to the excitation transition of the thiazine conjugated system in MB.The organic functional groups in the solutions were identified by FT-IR (Thermo Fisher Scientific, Nicolet 6700 FT-IR) using infrared radiation in a wavenumber range of 4000-650 cm À1 with a resolution of 4 cm À1 .The free radicals for dye degradation were characterized by EPR (Bruker, EMX-Plus X-Band) with a center field of 3481.0G, a sweep width of 100.0 G, a modulation amplitude of 1.00 G, a microwave frequency of 9.8 GHz, and a microwave power of 20 mW at room temperature.DMPO (5,5-dimethyl-1-pyrroline N-oxide) was used as a trapping agent for free radicals such as •CH 3 and •OH in the centrifuged solution.In addition, the PL spectroscopy (Kimmon Koha, Horiba Instruments Incorp (HII)) using a helium-cadmium laser with a wavelength of 325 nm was carried out to confirm the free radical species.Terephthalic acid (TA) was a common scavenger to trap hydroxyl radicals, leading to a new compound of 2-hydroxyterephthalic acid.The amount of hydroxyl radicals can be determined by the PL peak intensity of 2-hydroxyterephthalic acid recorded at around 425 nm.

Figure 1 .
Figure 1.a) The XRD patterns of the samples were synthesized under different KOH concentrations, ranging from 1 to 12 M. b-g) are the corresponding SEM images of the samples under different KOH concentrations.

Figure 2 .
Figure 2. a,b) are HRTEM images of the PMNHZT nanoparticles at 25 000X and 800 000X magnifications, respectively.c) The selective area electron diffraction pattern of PMNHZT with the projection along the [100] axis.

Figure 3 .
Figure 3. a) The UV-vis spectra of dark adsorption experiments versus time for PMNHZT in the MB solution.b) The UV-vis spectra of photo-piezocatalytic degradation experiments versus time for PMNHZT in the MB solution.c) The MB dye removal profile of dark adsorption versus time extracted from (a). d) The MB dye removal profile of photo-piezo-catalytic degradation versus time extracted from Figure S3a,b, Supporting Information and (b).

Figure 4 .
Figure 4. a) The XPS C 1s spectrum and the corresponding components for the PMNHZT powder after the dark adsorption process.b) The XPS C 1s spectrum and corresponding components for the PMNHZT powder after the photo-piezocatalytic process.c) The FTIR spectra for the MB solutions under different states: the pure MB solution (dark purple), the solutions after the dark adsorption (blue), photocatalytic (green), piezocatalytic (yellow), photo-piezo-catalytic (red) processes.

Figure 5 .
Figure 5.The EPR spectra of the MB solutions with DMPO under different states: the pure MB solution (black), the solutions after the dark adsorption (red), photocatalytic (green), piezocatalytic (blue), photo-piezo-catalytic (orange) processes.Two simulated spectra and the corresponding structure for DMPO radical adducts: DPMO-OH (earthy yellow) and DPMO-CH 3 (purple) are compared.Schematics on the left side depict the formation of corresponding free radical adducts by DMPO.

Figure 6 .
Figure 6.Stability test of PMNHZT in MB dye removal over five reuse circles.