Tailoring of PVDF for retrieval of piezoelectric powders to optimize piezo-catalytic water treatment†

Kaiyu Feng ^a, Yan Zhang *^a, Xuefan Zhou ^a, Yan Zhao ^a, Hanyu Gong ^a, Xiang Zhou ^a, Hang Luo ^a, Dou Zhang ^a and Chris Bowen ^b
aState Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China. E-mail: yanzhangcsu@csu.edu.cn
^bDepartment of Mechanical Engineering, University of Bath, United Kingdom, Bath BA2 7AY, UK

First published on 25th July 2024

---

---

Introduction

With the rapid development of industry and growth in the world population, the increase in industrial wastewater and domestic sewage has led to increasing pollution of water resources, thus aggravating the availability of clean water resources and leading to potential harm to human health and safety.^1,2 As a result, effective routes to treat water have been explored to address the above problems; these include adsorption,³ photocatalysis,⁴ and electrocatalysis,⁵ where their catalytic performance and complex catalytic reaction conditions are being explored.⁶ The mechanism of piezoelectric catalysis is an emerging process that is attracting interest since it is not reliant on a light source or an external source of electricity^7–9 and it can be triggered by a mechanical stress generated from water flow,¹⁰ wind,¹¹ or ultrasound.^12,13 The application of a stress to a piezoelectric generates charges or a built-in electric field within the material, thereby producing hydroxyl radicals (˙OH) or superoxide radicals (˙O₂⁻)⁷ that degrade pollutants.^14,15

Piezoelectric catalysts can be divided into both powder-based catalysts^14,16–19 and bulk catalysts.^20,21 Powder-based catalysts are able to achieve a high catalytic activity due to their large specific surface area and high level of dispersion in a solution. Yu et al.²² synthesized ferroelectric Ba_xSr_1−xTiO₃ particles as a piezoelectric catalyst to degrade carbamazepine (CBZ), where the removal rate of CBZ by Ba_0.5Sr_0.5TiO₃ reached 94.5% within 30 minutes during the application of ultrasonic vibrations. Su et al.²³ prepared a novel piezoelectric catalyst based on Bi₂Fe₄O₉ nanosheets to improve the activation efficiency of peroxydisulfate (PDS), where the degradation rate of bisphenol A (BPA) reached 98.3% within 30 minutes. Wang et al.²⁴ used polarized BaTiO₃ nanoparticles for the degradation of indigo carmine (IC) and, on the application of ultrasonic vibration, more than 90% of the dye was degraded within 35 minutes. In addition, MoS₂ nanoflowers synthesized by Wu et al.²⁵via a hydrothermal method exhibited an extremely high piezoelectric catalytic activity, which was able to achieve a 93% degradation of rhodamine B (RhB) within 60 seconds. When used in powder form, piezoelectric catalysts are able to degrade pollutants efficiently; however, since the powders cannot be easily retrieved from water after use, especially when employed at the nano-scale, this approach has potential to lead to secondary pollution due to the use of a powder catalyst.

As a result, there has been interest in the development of piezoelectric catalysts that can be used in bulk form, which can be readily retrieved and reused.²⁶ In this regard, Liu et al.²⁰ prepared a BaTiO₃ scaffold using a direct ink-writing method and decorated the surface of the scaffold with TiO₂ nanowires. When subjected to ultrasound, the scaffold was able to degrade 15.7% of indigo carmine (IC) in 40 minutes, which was significantly lower than that of powder-based catalysts. Combining ceramic powder catalysts with other materials to form bulk composite materials is also an effective solution to address the poor catalytic performance of individual bulk piezoelectric catalysts. Cheng et al.¹² combined piezoelectric particles of (K_0.5Na_0.5)_0.94Li_0.06NbO₃ with a polydimethylsiloxane (PDMS) matrix to produce a flexible and retrievable porous piezoelectric composite that can achieve 91% degradation of RhB in 180 minutes. While combining ceramic catalyst fillers with PDMS can effectively address the issue of secondary pollution, PDMS, as the primary component of the composite material, does not contribute to the catalytic process. As a result, it does not significantly enhance the piezoelectric catalytic performance of the composite material. As a piezoelectric polymer, polyvinylidene fluoride (PVDF) is widely used due to its high stability, mechanical flexibility and ability to be retrieved after use.^27–30 Shi et al.³¹ embedded BaTiO₃ nanoparticles into a porous PVDF scaffold, thereby forming a PVDF–BaTiO₃ composite foam that was capable of degrading 87% of RhB within 80 minutes. The piezoelectric catalytic activity of this composite material is relatively low compared to conventional piezoelectric ceramic powders; for example 0.82Ba(Ti_0.89Sn_0.11)O₃–0.18(Ba_0.7Ca_0.3)TiO₃ is capable of degrading more than 90% RhB within 40 minutes.³² The piezoelectric properties of PVDF are lower than those of piezoelectric ceramics and piezoelectric semiconductors. In addition, the low catalytic activity was attributed to the discontinuous interfacial bonding between the ceramic filler and PVDF, resulting in a disruption in the arrangement of dipole moments within PVDF.³³ Furthermore, the degree of interaction between the ceramic fillers in the PVDF matrix and the reacting solution is restricted. Balancing the need to enable retrieval of a catalyst to avoid secondary pollution, whilst also being able to provide a simple and cost-effective method of maintaining a high catalytic activity, therefore poses a significant challenge.

In this work, we provide a new route to combine the high performance of powder-based catalysts with the ease of retrieval of bulk catalysts. Porous PVDF was combined with piezocatalytic BaTiO₃ particles, where polydopamine was coated on the particle surface to facilitate the anchoring of BaTiO₃ onto PVDF. Notably, this new approach to piezoelectric catalysis provides a route to achieve a high piezoelectric catalytic activity that is comparable to that of BaTiO₃ powder, whilst enabling retrieval after use to limit secondary pollution. Detailed characterisation and measurement of the catalytic activity of PVDF and powder, along with their ability to be retrieved, were conducted. Furthermore, the universality of the applicability of this new retrieval strategy was validated by assessing a range of piezoelectric powders and potential dyes. This approach presents an effective solution to the challenge of retrieving catalytic powders after use and holds promise for further advancing the practical application of piezoelectric catalysis for other particle-based catalysts.

Results and discussion

Synthesis and phase analysis

Fig. 1a and b show schematics of the retrieval strategy for powder based piezo-catalysts. The BaTiO₃ (BT) particles were encapsulated with polydopamine (PDA) in a Tris–HCl buffer solution to produce BT@PDA.^34,35 Simultaneously, PVDF was dissolved in N,N-dimethylformamide (DMF) followed by uniformly spraying with water mist to achieve rapid solvent exchange.²¹ After drying, porous PVDF with high surface roughness and a thickness of ∼2 mm was obtained, as shown by the optical image in Fig. 1c. When subjected to ultrasonic vibrations, the BT@PDA powder, which was dispersed in a solution containing pollutants, adhered to the surfaces of porous PVDF films to form a BT@PDA/PVDF composite whilst leading to piezo-catalytic degradation of the pollutants (Fig. 1b).³⁶ As shown in Fig. 1b, the porous PVDF films turned from a light color to a darker color with an increase in ultrasound time, indicating that BT@PDA powders are gradually anchored onto the PVDF surface in the presence of ultrasound and that the BT@PDA powders do not detach from PVDF after undergoing ultrasonic cycles in the pollutant degradation experiments.

Fig. 1d shows the X-ray diffraction (XRD) patterns of BT, BT@PDA, PVDF and BT@PDA/PVDF. The diffraction peaks attributed to the tetragonal phase were observed in BT and BT@PDA.¹⁴ Compared to the XRD pattern of pure PVDF, diffraction peaks corresponding to BaTiO₃ with a tetragonal phase were observed in BT@PDA/PVDF, indicating the BT@PDA powders were anchored onto PVDF successfully.

Morphology and structure analysis

Fig. 2 shows the microstructure of the prepared samples. As shown in Fig. 2a and b, the BT powders and BT@PDA powders exhibited a spherical microstructure. The diameter of BT@PDA powders and thickness of the PDA shell was ∼50 nm and 4 nm, as seen in the transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images in Fig. 2c and d, respectively. The crystalline interplanar spacing of the BaTiO₃ powders was 0.405 nm. Furthermore, the polycrystalline nature of BT@PDA was confirmed by a selected-area electron diffraction (SAED) technique, see Fig. S3a.†Fig. 2e shows the porous microstructure of PVDF with a pore size of approximately 0.5–1 μm. As shown in Fig. 2f and S3b,† a clear interface between BT@PDA powders and PVDF was obtained, and the differences of distribution in Ti, Ba and F were observed at the two sides of the PVDF/BT@PDA interface, as shown in Fig. 2g–i.

Piezoelectric response and retrievability of the catalysts

Fig. 3a–e show the piezoelectric properties of BT@PDA powders measured by piezoelectric force microscopy (PFM). Fig. 3a shows the PFM morphology of the BT@PDA powders, and the strong contrast in Fig. 3b and c after applying a DC bias indicates the existence of multi-domain structures and 180° domain walls. Fig. 3d and e show a butterfly-like amplitude–voltage loop and a 180° phase loop, respectively, confirming the local piezoelectric response of BT@PDA. Fig. 3f–i show the Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) results of the BaTiO₃ powders, porous PVDF, BT@PDA and BT@PDA/PVDF composite. As shown in Fig. 3f and g, the peaks at 1358 cm⁻¹ and 1568 cm⁻¹ from BT@PDA and BT@PDA/PVDF were attributed to PDA.³⁵Fig. 3h shows the FT-IR spectroscopy spectra of BT and BT@PDA, where BT@PDA exhibited new peaks at 1284 cm⁻¹, 1342 cm⁻¹, 1511 cm⁻¹, and 1606 cm⁻¹ corresponding to the bending or stretching vibrations of C–O, C–N, C–C, and N–H, respectively, indicating the presence of amide and aromatic groups.^37,38 In addition, a broad peak ranging from 3100 to 3600 cm⁻¹ corresponds to the stretching vibrations of O–H or N–H of PDA.³⁸ Due to the formation of hydrogen bonding between the –NH₂ groups from dopamine and the C–F groups from PVDF,³⁵ a peak shift is observed as shown in Fig. 3i. X-ray photoelectron spectroscopy (XPS) was used to further characterize the polydopamine coating on the surface of BaTiO₃. As shown in Fig. 4a, a peak at ∼400 eV with a high intensity was observed in the BT@PDA spectrum compared to that of BaTiO₃, which was attributed to the nitrogen-containing groups from dopamine and indicated the covalent binding between BaTiO₃ and PDA.^38,39 The O 1s spectrum is shown in Fig. S4b,† and the peaks observed at 530.9 eV and 532.9 eV in BT@PDA correspond to the C–O/Ba–O bond and the O–H bond, respectively;^38,40 this suggests the formation of a stable covalent bond linkage between polydopamine and BaTiO₃. The XPS results indicate that dopamine, after in situ polymerization, forms a covalent bond with BaTiO₃, thereby tightly binding them together. The nature of the covalent bond enhances the interface interaction between BaTiO₃ and polydopamine, ensuring strong adsorption of polydopamine on the surface of BaTiO₃ nanoparticles.

Fig. 4b and c show the thermogravimetric analysis (TGA) of the samples. For BT@PDA, a rapid decrease in weight begins at approximately 250 °C; see Fig. S5a.† At approximately 800 °C, the weight changes of both BT and BT@PDA become stable. At this point, the weight of BT@PDA is 87.23% of the initial weight, as shown in Fig. 4b, exhibiting a larger weight change compared to BT alone (97.89%). This difference in weight is attributed to the degradation of polydopamine, indicating that the weight fraction of polydopamine in BT@PDA is approximately 10%. The TGA and derivative thermogravimetry (DTG) curves of BT@PDA/PVDF and PVDF have a similar shape, indicating that the loading of BT@PDA does not affect the thermal stability of PVDF; see Fig. 4c and S5b.† On reaching the pyrolysis temperature of PVDF, approximately 460–480 °C, both PVDF and BT@PDA/PVDF exhibit a rapid weight decrease. After thermal decomposition at 800 °C, the weight difference is approximately 16.08%, suggesting that the weight fraction of BT@PDA in BT@PDA/PVDF is 16.08%. From the results of TGA, PVDF can efficiently retrieve a high weight fraction of BT@PDA powders.

To quantify the weight fraction of powders that could be retrieved and the resulting retrieval performance, the binding efficiency was determined using eqn (1):

	(1)

where ε represents the binding efficiency, m₀ is the initial mass of powders, and m is the mass of the remaining powder after the retrieval process and filtration. Fig. 4d shows the binding efficiency between PVDF and BT@PDA powders with an ultrasound time of up to 60 minutes and an interval of 10 minutes. When the ultrasound time increased from 10 to 60 minutes, the binding efficiency (ε) increased from 32 to 94%, indicating a large fraction of the BT@PDA particles anchored onto the porous PVDF. Fig. 4e shows the relationship between the binding efficiency and the mass of PVDF added, where the ultrasonication time was fixed at 40 minutes; it can be seen that ε increased with an increase in the mass addition of PVDF. When the addition of PVDF was 7 g, ε reached 93%. This retrieval strategy can be applied in various dye solutions to retrieve BaTiO₃ powders and purify water, as shown in Fig. S6.† As shown in Fig. 4f, S7 and Table S1,† molecular dynamics (MD) simulations were conducted to explore and understand the underlying mechanism of the interaction between BT@PDA and PVDF. The radial distribution functions (RDFs) of the F atom with respect to the –NH₂ group and the –OH group were determined, and the RDFs for the two functional groups were similar at distances of 2–3 Å. However, compared to –OH groups, the –NH₂ groups from dopamine demonstrated a stronger binding tendency with the –CF₂ groups of PVDF at distances of 3.1–4.3 Å. An independent gradient model (IGM) method was employed to identify the interaction between PVDF and dopamine. Fig. 4g and h indicate the formation of strong hydrogen bonds between PVDF and PDA.⁴¹ Hence, the BaTiO₃ powders with a PDA coating were firmly attached to PVDF due to the strong interactions between the –CF₂ and –NH₂ groups under the application of ultrasound.

Piezo-catalytic performance and the piezo-catalytic mechanism

Fig. 5a and b show the degradation ratio of indigo carmine (IC) and the first-order kinetic rate constant for the materials examined. According to the Beer–Lambert law, the concentration of the solution is directly proportional to the absorbance (A) of the dye; this relationship can be represented by eqn (2):⁴²

A = KbC	(2)

where K corresponds to the molar absorption coefficient and b represents the effective thickness of the absorbing layer. The first-order kinetic rate constant (k) can be calculated using eqn (3):⁴³

ln(C/C0) = −kt	(3)

where C₀ is the initial concentration of the dye solution, C is the concentration of the extracted dye at an interval of 10 min, and t is time. The degradation ratio (D) can be represented by eqn (4):

D = (1 − C/C0) × 100%	(4)

After stirring in the dark for 30 min to eliminate the effects of adsorption, the degradation ratio of the IC solution was 94.8% and 95.3% when using BT@PDA and BT powders as the catalysts respectively, as shown in Fig. 5a. The result indicates that polydopamine does not affect the piezocatalytic performance of BaTiO₃. When BT@PDA and PVDF were added simultaneously, a 94.3% degradation ratio of IC was achieved within 40 minutes, which is comparable to that when using the BT powders alone, further validating the feasibility of the strategy for combined efficient dye degradation and powder retrieval. Furthermore, BT@PDA/PVDF exhibited a higher degradation ratio compared to pure PVDF, indicating the potential of the BT@PDA/PVDF combination for dye degradation. Among the prepared samples, the lowest degradation ratio of 0.5% within 40 min was observed when using pure PVDF as the catalyst without an applied ultrasound, see Fig. 5b. Our study of prolonged stirring also eliminates any adsorption effects of BT@PDA on the dye; see Fig. S9.† Similar experiments were conducted using rhodamine B (RhB) and methyl orange (MO), see Fig. S10,† where 89.4% of MO was degraded within 70 minutes when BT@PDA powders and PVDF were used together to form a BT@PDA/PVDF composite, with a degradation ratio that was similar to that of BT powders (90.1% within 70 minutes). Similarly, the BT@PDA and PVDF combination was able to degrade 90.7% of RhB within 40 minutes, which is approximately the same degradation ratio as that of BT powders (93.6% within 40 minutes). The catalytic performances of recent typical piezoelectric catalysts including both retrievable and irretrievable catalysts (Table S2†) have been compared. The retrieval strategy shows a significant advantage among retrievable catalysts.

To further investigate the reaction mechanism during the piezocatalytic process, the electron spin resonance (ESR) spin trapping technique with DMPO (DMPO = 5,5-dimethyl-1-pyrroline N-oxide) as the spin trapper was used to confirm the presence of radical species ˙O₂⁻ (Fig. 5c) and ˙OH (Fig. 5d). When applying ultrasound and adding BT@PDA and PVDF into the aqueous solution, the characteristic peaks of spin adducts of DMPO-˙O₂⁻⁴⁴ adducts and DMPO-˙OH⁴⁵ adducts were observed. However, the characteristic peaks of DMPO-˙O₂⁻ and DMPO-˙OH disappeared without the addition of BT@PDA and PVDF, which confirms the generation of radical species ˙O₂⁻ and ˙OH during the piezo-catalytic degradation of dyes. A mechanism based on the screening charge effect of piezoelectric materials is proposed, as shown in Fig. 5e–h. Initially, equivalent positive and negative charges will be absorbed on the surfaces perpendicular to the direction of spontaneous polarization to maintain the electrical neutrality of the piezoelectric material. When nearby cavitation bubbles are generated and collapse, which is followed by the application of a compressive stress on the piezoelectric, the level of polarization decreases and screening charges are released, as shown in Fig. 5f and g. Subsequently, when the compressive stress applied to piezoelectric is removed, the subsequent increase in spontaneous polarization leads to the formation of more bound charges and the capture of charges from the solution, thereby promoting further oxidation–reduction reactions.

By applying a periodic stress during the continuous application of ultrasound and associated cavitation events, radical species of ˙O₂⁻ and ˙OH can be continuously generated and dye degradation can be triggered, as illustrated in eqn (5)–(9). Fig. 5i shows a schematic of hydrogen bonding between PVDF and dopamine, which facilitated the retrieval of powders and avoided secondary pollution.⁴⁶

BT@PDA + ultrasound → BT@PDA (q+ + q−)	(5)

PVDF + ultrasound → PVDF (q+ + q−)	(6)

O2 + q− → ·O2−	(7)

H2O + q+ → ·OH	(8)

·O−2 + ·OH + Dye → Oxidation products	(9)

Application of the retrievable strategy in various powder-based piezo-ceramics

To fully validate the universality of the retrievable strategy without compromising the piezoelectric catalytic performance, a range of ceramic powders including Bi_0.5Na_0.5TiO₃@PDA (BNT@PDA), BiFeO₃@PDA (BFO@PDA), Ba_0.75Sr_0.25TiO₃@PDA (BST@PDA), Bi₄Ti₃O₁₂@PDA (BIT@PDA) and Al₂O₃@PDA were prepared and examined. Al₂O₃ served as a control group based on a non-piezoelectric material. All phases were confirmed by XRD, as shown in Fig. S11.†Fig. 6a–e demonstrate the degradation capabilities of these four powders when combined with PVDF in the presence of an indigo carmine (IC) solution. The degradation ratio within 40 minutes reached 96.9% (BNT@PDA), 97.3% (BFO@PDA), 90.4% (BST@PDA), 96.1% (BIT@PDA) and 4.8% (Al₂O₃@PDA), respectively. In addition, the powder retrieval of PVDF for the five types of powders is similar to that of previously studied BaTiO₃ powders, as observed in the optical images where PVDF is loaded with significant amounts of the powder catalyst; see Fig. 6f–j. Fig. 6k–o and S12† show schematics and SEM images of the microstructures of the powders. The prepared BNT@PDA, BFO@PDA, BST@PDA, and BIT@PDA exhibit a range of morphologies, which include fiber, polyhedron, spherical, and sheet forms, respectively. It was observed that all the powder-based catalysts with different morphologies could be coated with polydopamine and loaded on the surface of the porous PVDF substrate through hydrogen bonding. Furthermore, the possibility of achieving the same effect under stirring conditions was further investigated and retrieval of the BT@PDA powders during magnetic stirring (300 rpm) was also observed. As shown in Fig. S13,† the experimental results demonstrated that on increasing the stirring time period, more powders could be loaded onto PVDF. However, compared to the use of ultrasound, there remained a decrease in catalytic efficiency, which indicates that stirring can also achieve powder retrieval, but with a lower efficiency. This difference in results from large forces is induced by high intensity cavitation effects during the ultrasound process,⁴⁷ which propel the powders onto the surface of PVDF and facilitate the formation of hydrogen bonding. In addition, ultrasound creates a negative pressure within the porous PVDF,⁴⁸ making it easier for the powders to adhere. Nevertheless, its feasibility under stirring conditions can further expand the application prospects of this strategy.

Experimental

Fabrication of BT@PDA powder

A mass of 0.2 g of BaTiO₃ (99.9%, Shanghai Aladdin Biochemical Technology Co., Ltd) and 0.189 g of dopamine hydrochloride (99%, Alfa Aesar Chemical Co. Ltd) were dissolved in a 0.01 M Tris–HCl buffer solution with a pH of 8.5. The solution was then stirred at 60 °C for 12 hours. Afterward, the solution was subjected to multiple filtration processes and washing. The resulting material was then dried in a vacuum drying oven at 50 °C for 5 hours to obtain the BT@PDA powders.

Fabrication of porous PVDF with a rough surface by solvent exchange

A mass of 2 g PVDF (6020, Solvay, Shanghai, China) particles was added into a 20 ml N,N-dimethylformamide (DMF, 99.5%, Sinopharm, Shanghai, China) solution and stirred at 60 °C for 4 hours to obtain a mixture of PVDF and DMF. Then, the mixture was poured into a Petri dish and evenly sprayed with pure water over the mixture in the Petri dish, via a spray bottle, to facilitate a rapid solvent exchange. After five minutes, PVDF was turned over and pure water was sprayed onto the other side to ensure that PVDF would be uniformly in contact with water. PVDF was dried after completion of the solvent exchange process in an oven to obtain porous PVDF.

Retrieval performance to assess PVDF and BT@PDA powder

The specific procedure to assess ease of retrieval involved mixing 50 mg of BT@PDA powders with a specific weight of PVDF (1, 2, 3, 4, 5, 6 and 7 g) in a 50 ml of the 10 mg per l IC dye solution. After a specific period of ultrasonication (10, 20, 30, 40, 50 and 60 min), BT@PDA/PVDF was removed, and the remaining mixture was filtered to obtain the residual powders, which were then dried at 60 °C for 12 h. The weight of the dried powders was measured and compared to that of the initial powder weight to calculate the binding efficiency of PVDF to BT@PDA powders, which represents the retrieval efficiency for the powders.

Piezocatalytic degradation of dyes

A mass of 50 mg BT@PDA powder and 5 g of prepared PVDF were separately weighed and placed in a beaker containing 50 ml of 10 mg per l IC dye under dark conditions. The mixture was stirred for 30 minutes in the dark. Dye degradation was carried out with the application of ultrasound, and 5 ml of the mixed solution was collected every 10 minutes. The upper clear liquid obtained after centrifugation was used to measure the concentration of the IC dye using a UV-Vis spectrophotometer. In addition, 5 mg per l solutions of RhB and MO dyes were prepared, and their degradation by BT@PDA powders and PVDF was measured in the same manner.

Calculation details of the independent gradient model (IGM)

The IGM method was conducted using the Gaussian 09 program package (Gaussian 09 (Revision D.01), Gaussian Inc., Wallingford CT. 2009). Dispersion corrections were computed using Grimme's D3(BJ) method during the optimization process. All structures were calculated at the B3LYP-D3(BJ)/6-311G(d,p) level of theory to obtain the corresponding energies and the 6-311G(d,p) basis set for C, N, O and H atoms. Revealing noncovalent interactions through methods such as RDG (reduced density gradient) not only identifies the locations of weak interactions, but also provides a visualization of their strength and type. Detailed information regarding molecular dynamics is provided in the ESI.†

Sample characterization

The crystal structures of the BT powder, BT@PDA powders, PVDF, and the BT@PDA/PVDF composite were measured using an X-ray diffractometer (XRD, Smartlab SE, Japan). The microscale morphology and structure of the materials were characterized using scanning electron microscopy (SEM, TESCAN MIRA4, Czech Republic) and transmission electron microscopy (TEM, JEOL JEM-F200, Japan). The elemental distribution within the materials was determined using Energy Dispersive X-ray Spectroscopy (EDS, Ultim Max 80). The Raman spectra and Fourier-transform infrared spectra of the BT@PDA powders and BT@PDA/PVDF composite were measured using a Raman spectrometer (Renishaw inVia) and Fourier-transform infrared spectrometer (FT-IR, Thermofisher, Is-50), respectively. The elemental composition and chemical states of BT@PDA powders were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA). A mechanical force for testing the piezoelectric catalytic performance was provided using an ultrasound source (200 W, 45 kHz, KQ-200VDE, China). The piezoelectric properties of BT@PDA powders were examined using a piezo-response force microscope (PFM, NanoMan VS, USA). Prior to measurement, the BT@PDA powders were dispersed in ethanol and then dropped onto platinum-coated silicon wafers. Ethanol was then evaporated on a hot plate and the BT@PDA powders were fixed onto the silicon wafer.

Conclusion

In this paper, we have proposed a new strategy for the retrieval of powders used for catalytic dye degradation and water treatment. This new strategy harnesses the advantages of high-efficiency piezocatalytic powders and the ease of retrieval of bulk catalysis, allowing for the degradation of a variety of dyes and simultaneous in situ retrieval of catalyst particles to limit secondary pollution. By coating catalytic BaTiO₃ nanoparticles with polydopamine (PDA), we synthesized BT@PDA, which was combined with a porous PVDF material with high surface roughness to allow retrieval of catalytic particles. During the application of ultrasound and the dye degradation process, the BT@PDA powder catalysts are gradually anchored onto the surface of porous PVDF via hydrogen bonding; this is aided by cavitation events and the negative pressure generated within the porous PVDF. The catalytic process and anchoring mechanisms were elucidated by a combination of detailed characterisation and molecular dynamics modeling. This new strategy yields a high piezocatalytic activity comparable to that of using only powder catalysts and allows the porous PVDF substrate to be used to retrieve the catalyst from the solution. Furthermore, we examined a range of alternative catalytic particles, with different morphologies, and potential dyes to confirm the universality of this strategy. In conclusion, this new strategy is able to provide a high piezocatalytic activity while simultaneously avoiding secondary pollution, thereby effectively addressing the challenge of powder catalyst retrieval. The universality of the approach also holds promise for applications in a variety of other fields where a catalyst is used in powder form, such as photo-catalysis.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Author contributions

Kaiyu Feng: data curation, formal analysis, methodology, project administration, resources, software, validation, writing – original draft. Yan Zhang: conceptualization, data curation, funding acquisition, supervision, visualization, writing – review & editing. Xuefan Zhou: investigation, methodology, project administration. Yan Zhao: data curation, formal analysis, investigation. Hanyu Gong: conceptualization, data curation, investigation, resource. Xiang Zhou: software, validation. Hang Luo: investigation, resources. Dou Zhang: formal analysis, funding acquisition, supervision. Chris Bowen: software, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program (2022YFB3807404) and the National Natural Science Foundation of China (No. 52302158), the Overseas Talent Introduction Project of China, the Hundred Youth Talents Program of Hunan, the Xiaomi Young Talents Program and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. Informed written consent from all human participants was obtained prior to the research.

References

1. C. Yu, X. Huang, H. Chen, H. C. J. Godfray, J. S. Wright, J. W. Hall, P. Gong, S. Ni, S. Qiao, G. Huang, Y. Xiao, J. Zhang, Z. Feng, X. Ju, P. Ciais, N. C. Stenseth, D. O. Hessen, Z. Sun, L. Yu, W. Cai, H. Fu, X. Huang, C. Zhang, H. Liu and J. Taylor, Nature, 2019, 567, 516–520 CrossRef CAS PubMed .
2. W. Liu, P. F. Wang, Y. H. Ao, J. Chen, X. Gao, B. H. Jia and T. Y. Ma, Adv. Mater., 2022, 34, 2202508 CrossRef CAS PubMed .
3. G. Crini, E. Lichtfouse, L. D. Wilson and N. Morin-Crini, Environ. Chem. Lett., 2019, 17, 195–213 CrossRef CAS .
4. K. Y. Su, H. F. Liu, Z. Y. Gao, P. Fornasiero and F. Wang, Adv. Sci., 2021, 8, 2003156 CrossRef CAS PubMed .
5. C. Du, P. Li, Z. H. Zhuang, Z. Y. Fang, S. J. He, L. G. Feng and W. Chen, Coord. Chem. Rev., 2022, 466, 214604 CrossRef CAS .
6. W. Qian, K. Zhao, D. Zhang, C. R. Bowen, Y. Wang and Y. Yang, ACS Appl. Mater. Interfaces, 2019, 11, 27862–27869 CrossRef CAS .
7. E. Z. Lin, Z. H. Kang, J. Wu, R. Huang, N. Qin and D. H. Bao, Appl. Catal. B, 2021, 285, 119823 CrossRef CAS .
8. J. Ma, L. Xia, D. Wu, Y. Feng, C. Ban, L. Ruan, J. Guan, M. Zhang, R. Zhou, J. Y. Dai, L. Y. Gan and X. Zhou, Nano Energy, 2024, 121, 109258 CrossRef CAS .
9. J. Ma, D. Wu, Y. Feng, C. Ban, L. Xia, L. Ruan, J. Guan, Y. Wang, J. Meng, J. Y. Dai, L. Y. Gan and X. Zhou, Nano Energy, 2023, 115, 108719 CrossRef CAS .
10. C. Yu, J. He, S. Lan, W. Guo and M. Zhu, Environ. Sci. Ecotechnology, 2022, 10, 100165 CrossRef CAS .
11. Y. C. Lai, Y. C. Hsiao, H. M. Wu and Z. L. Wang, Adv. Sci., 2019, 6, 1801883 CrossRef PubMed .
12. X. Cheng, Z. Liu, Q. Jing, P. Mao, K. Guo, J. Lu, B. Xie and H. Fan, J. Colloid Interface Sci., 2023, 629, 11–21 CrossRef CAS PubMed .
13. Y. Zhao, Y. Zhang, Q. Q. Xu, H. Y. Gong, M. Y. Yan, K. Y. Feng, X. Zhou, X. F. Zhou and D. Zhang, J. Mater. Chem. A, 2024, 12, 1753–1763 RSC .
14. P. L. Wang, X. Y. Li, S. Y. Fan, X. Chen, M. C. Qin, D. Long, M. O. Tad and S. M. Liu, Appl. Catal. B, 2020, 279, 119340 CrossRef CAS .
15. P. F. Shen, P. Yin, Y. T. Zou, M. Li, N. Q. Zhang, D. Tan, H. Y. Zhao, Q. J. Li, R. S. Yang, B. Zou and B. B. Liu, Adv. Mater., 2023, 35, 2212172 CrossRef CAS PubMed .
16. J. Zhang, Q. Bai, X. Bi, C. Zhang, M. Shi, W. W. Yu, F. Du, L. Wang, Z. Wang, Z. Zhu and N. Sui, Nano Today, 2022, 43, 101429 CrossRef CAS .
17. H. L. You, Z. Wu, L. H. Zhang, Y. R. Ying, Y. Liu, L. F. Fei, X. X. Chen, Y. M. Jia, Y. J. Wang, F. F. Wang, S. Ju, J. L. Qiao, C. H. Lam and H. T. Huang, Angew. Chem., Int. Ed., 2019, 58, 11779–11784 CrossRef CAS PubMed .
18. J. Ma, S. Jing, Y. Wang, X. Liu, L. Y. Gan, C. Wang, J. Y. Dai, X. Han and X. Zhou, Adv. Energy Mater., 2022, 12, 2200253 CrossRef CAS .
19. J. Ma, X. Xiong, D. Wu, Y. Wang, C. Ban, Y. Feng, J. Meng, X. Gao, J. Y. Dai, G. Han, L. Y. Gan and X. Zhou, Adv. Mater., 2023, 35, 2300027 CrossRef CAS PubMed .
20. Q. Liu, Z. Li, J. Li, F. Zhan, D. Zhai, Q. Sun, Z. Xiao, H. Luo and D. Zhang, Nano Energy, 2022, 98, 107267 CrossRef CAS .
21. M. N. Chai, W. S. Tong, Z. H. Wang, Z. S. Chen, Y. C. An and Y. H. Zhang, J. Hazard. Mater., 2022, 430, 128446 CrossRef CAS PubMed .
22. C. Yu, S. Y. Lan, S. T. Cheng, L. X. Zeng and M. S. Zhu, J. Hazard. Mater., 2022, 424, 127440 CrossRef CAS PubMed .
23. C. Su, R. Li, C. Li and W. Wang, Appl. Catal. B, 2022, 310, 121330 CrossRef CAS .
24. Y. Wang, X. Wen, Y. Jia, M. Huang, F. Wang, X. Zhang, Y. Bai, G. Yuan and Y. Wang, Nat. Commun., 2020, 11, 1328 CrossRef CAS PubMed .
25. J. M. Wu, W. E. Chang, Y. T. Chang and C.-K. Chang, Adv. Mater., 2016, 28, 3718–3725 CrossRef CAS PubMed .
26. H. Y. Gong, Y. Zhang, J. J. Ye, X. Zhou, X. F. Zhou, Y. Zhao, K. Y. Feng, H. Luo, D. Zhang and C. Bowen, Adv. Funct. Mater., 2024, 2311091 CrossRef CAS .
27. X. M. Liao, X. Q. Chen, Y. Tang, M. D. Zhu, H. Y. Xie, Y. Xin, Y. H. Lin and X. Y. Fan, ACS Appl. Mater. Interfaces, 2022, 15, 1286–1295 CrossRef PubMed .
28. J. X. Wang, Y. T. Liang, Z. C. Wang, B. J. Huo, C. Liu, X. X. Chen, H. W. Xu, D. M. Li, Z. Y. Zhu, Y. L. Wang and F. Q. Meng, Chem. Eng. J., 2023, 458, 141409 CrossRef CAS .
29. M. Sharma, G. Singh and R. Vaish, J. Appl. Phys., 2021, 130, 085107 CrossRef CAS .
30. G. Singh, M. Sharma and R. Vaish, ACS Appl. Mater. Interfaces, 2021, 13, 22914–22925 CrossRef CAS PubMed .
31. J. D. Shi, W. Zeng, Z. H. Dai, L. Wang, Q. Wang, S. P. Lin, Y. Xiong, S. Yang, S. M. Shang, W. Chen, L. Y. Zhao, X. J. Ding, X. M. Tao and Y. Chai, Small Sci., 2021, 1, 2000011 CrossRef CAS .
32. J. Li, X. Wei, X.-x. Sun, R. Li, C. Wu, J. Liao, T. Zheng and J. Wu, ACS Appl. Mater. Interfaces, 2022, 14, 46765–46774 CrossRef CAS PubMed .
33. T. Li, M. H. Qu, O. R. Carlos, L. Gu, F. Jin, T. Yuan, X. W. Wu, J. J. Xiao, T. Wang, W. Dong, X. D. Wang and Z. Q. Feng, Adv. Mater., 2021, 33, 2006093 CrossRef CAS PubMed .
34. Y. Xie, Y. Yu, Y. Feng, W. Jiang and Z. Zhang, ACS Appl. Mater. Interfaces, 2017, 9, 2995–3005 CrossRef CAS PubMed .
35. X. Y. Guan, B. G. Xu and J. L. Gong, Nano Energy, 2020, 70, 104516 CrossRef CAS .
36. Y. Su, W. Li, X. Cheng, Y. Zhou, S. Yang, X. Zhang, C. Chen, T. Yang, H. Pan, G. Xie, G. Chen, X. Zhao, X. Xiao, B. Li, H. Tai, Y. Jiang, L.-Q. Chen, F. Li and J. Chen, Nat. Commun., 2022, 13, 4867 CrossRef CAS PubMed .
37. A. R. Chowdhury, A. M. Abdullah, I. Hussain, J. Lopez, D. Cantu, S. K. Gupta, Y. Mao, S. Danti and M. J. Uddin, Nano Energy, 2019, 61, 327–336 CrossRef CAS .
38. Y. Song, Y. Shen, H. Y. Liu, Y. H. Lin, M. Li and C. W. Nan, J. Mater. Chem., 2012, 22, 8063–8068 RSC .
39. Z. Wang, T. Wang, C. Wang, Y. Xiao, P. Jing, Y. Cui and Y. Pu, ACS Appl. Mater. Interfaces, 2017, 9, 29130–29139 CrossRef CAS PubMed .
40. Z.-H. Dai, T. Li, Y. Gao, J. Xu, Y. Weng, J. He and B.-H. Guo, Colloids Surf., A, 2018, 548, 179–190 CrossRef CAS .
41. R. Wang, H. S. Xu, S. Cheng, J. J. Liang, B. Gou, J. A. Zhou, J. Fu, C. Z. Xie, J. L. He and Q. Li, Energy Storage Mater., 2022, 49, 339–347 CrossRef .
42. S. K. Kohl, J. D. Landmark and D. F. Stickle, J. Chem. Educ., 2006, 83, 644 CrossRef CAS .
43. X. F. Zhou, Q. W. Sun, Z. D. Xiao, H. Luo and D. Zhang, J. Environ. Chem. Eng., 2022, 10, 108399 CrossRef CAS .
44. H. Zheng, X. Li, K. Zhu, P. Liang, M. Wu, Y. Rao, R. Jian, F. Shi, J. Wang, K. Yan and J. Liu, Nano Energy, 2022, 93, 106831 CrossRef CAS .
45. Y. Zhao, Q. Xu, X. Zhou, M. Yan, H. Gong, X. Yuan, H. Luo, K. Zhou, D. Zhang, C. Bowen and Y. Zhang, Ceram. Int., 2023, 49, 8259–8270 CrossRef CAS .
46. L. Wan, W. Tian, N. Li, D. Chen, Q. Xu, H. Li, J. He and J. Lu, Nano Energy, 2022, 94, 106930 CrossRef CAS .
47. Y. Deng, W. Wang, S. Zhao, X. Yang, W. Xu, M. Guo, E. Xu, T. Ding, X. Ye and D. Liu, Trends Food Sci. Technol., 2022, 122, 83–96 CrossRef CAS .
48. G. Wang, Q. Cui, L. J. Yin, Y. Li, M. Z. Gao, Y. Meng, J. Li, S. D. Zhang and W. Wang, Sep. Purif. Technol., 2020, 244, 115805 CrossRef CAS .

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03219e

This journal is © The Royal Society of Chemistry 2024