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Abstract
The coordination of piezoelectric and plasmonic effects to regulate the separation and migration of photo-generated carriers is still a significant method to improve the performance of visible-light photoresponse. Herein, we propose the PVDF@Ag-ZnO/Au composite nanofiber membranes utilizing the piezoelectric and plasmonic effects to promote the photocatalytic degradation of organic dyes. Here, ZnO nanorods can generate a built-in electric field under vibration to separate electron-hole pairs. The Schottky junction formed by noble metal/semiconductor can not only inhibit the recombination of photo-generated carriers and accelerate the migration of carriers, but also enhance the utilization of visible light. In addition, the structure has excellent flexibility and easy recycling characteristics. We demonstrate that the plasmonic effect of noble metal can enhance the light response of membranes and broaden light absorption from ultraviolet to visible light region. With the help of the surface-enhanced Raman scattering (SERS), modulation effects of the piezoelectric effect on light response is proved. For catalytic processes, rhodamine B (98.8%) can be almost completely degraded using PVDF@Ag-ZnO/Au within 120 minutes in the piezoelectric photocatalysis process, which is 2.2 and 2.8 times higher than photocatalysis and piezoelectric catalysis, respectively. This work provides a promising strategy for harnessing solar and mechanical energy.
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1. Introduction
As an efficient, safe and environmentally friendly purification technology, photocatalysis can convert light energy into chemical energy, which has considerable potential in solving energy and environmental problems [1–3]. To solve these problems, efficient photocatalysts should have strong light absorption and high separation rate of electron-hole generated by light [4,5]. Among photocatalysts, oxide and sulfide semiconductors, such as TiO2, ZnO, Fe2O3, CuO, CdS, etc, have become the main body of photocatalytic materials owing to their low cost and high stability [5–9]. But the further development has been greatly limited, due to the low light energy utilization and the high recombination rate of the photo-generated carriers. In order to solve these issues, some researches have proposed to generate a built-in electric field using piezoelectric effect in the photocatalyst, which can efficaciously promote the photocatalytic performance [10]. Under the action of external pressure, the piezoelectric potential will be generated due to the polarization inside the non-central symmetric crystal [11–13]. The directional movement of carriers can reduce the recombination rate because of the restriction of internal piezoelectric potential. Therefore, the synergistic effect of piezoelectric and photoelectric effect in semiconductor materials has attracted much attention in the field of pollutant degradation and hydrogen evolution [14].
Hexagonal wurtzite zinc oxide (ZnO) has been universally utilized in photocatalysis owing to its low cost, high biocompatibility and photoelectronic effects [15]. Currently, ZnO-based photocatalysts such as nanospheres, nanoflowers and nanorods have been studied and implemented to degrade organic dyes [5,15–18]. However, ZnO nanoparticles tend to agglomerate, which may decrease the photocatalytic activity. What’s more, nanoparticle-based catalysts are mostly in the form of powder in the application, which are difficult to separate and reuse from the solution and are easy to cause secondary pollution, heavily restricting the practical application [19,20]. ZnO nanorods (ZnO NRs) with high photocatalytic efficiency and immobilization can overcome these problems. Compared with the spherical structure and other structures, the spontaneous polarization of one-dimensional ZnO NRs under applied stress is stronger and regular, which will form along the longitudinal direction (c-axis). And, opposite piezoelectric potential will be generated on both sides when affected by transverse force, which substantially reduce the recombination rate of photo-generated carriers [21–23]. Chen et al. grown ZnO NRs vertically on 3D Ni foam, and achieved the degradation of pollutants in water [24]. Ma et al. reported that the bi-harvesting of light and vibration energy was realized in the hydrothermal synthesized ZnO NRs, and the degradation rate of Acid Orange 7 reached up to 80.8% within 100 min [5]. Despite these fruitful advances reported so far, the design of ZnO-based photocatalysts has still met with restricted success. The main reason is that ZnO, as an n-type semiconductor, has a wide bandgap (∼3.3ev) which is higher than the photon energy of visible light. Therefore, it is an imperative matter to further improve the utilization of light [25,26].
The Schottky junction, formed between semiconductor and noble metal nanoparticles (NPs) (Au, Ag, Pt, etc.) with local surface plasmon resonance (LSPR), is regarded as an efficacious solution to improve the efficiency of photo-electrochemical energy conversion [9,27–29]. The photon-excited electrons can transfer from the noble metal to the conduction band (CB) of the adjacent semiconductor to achieve directional migration owing to the Schottky junction, thus markedly reducing the recombination rate of the electron-hole pairs [30]. Especially for one-dimensional metal/semiconductor structures, they can greatly engender a strong local plasmonic electric field, which would optimize the directional separation efficiency of carriers. In addition, the LSPR wavelength of Au NPs is located in the visible light range, and a large number of hot electrons generated on the surface of the plasmonic nanoparticles under the light excitation of a specific wavelength which can directly participate in the redox reaction [12,24,31–33]. Therefore, the ZnO/Au heterostructure can further enhance the separation and migration of photo-generated carriers, and on the other hand, expand the light response of pure ZnO from the ultraviolet region to the ultraviolet visible region, enhancing the utilization of light [30,34,35].
Polyvinylidene fluoride (PVDF), as an organic piezoelectric material, not only has a high piezoelectric coefficient, but also has the characteristics of light weight, strong processability and considerable flexibility compared with inorganic materials, endowing it as an ideal candidate for photocatalysis [36,37]. Electrospinning of PVDF with high voltage is a widely used method to prepare PVDF thin nanofiber membranes at present. In this process, PVDF membranes with high flexibility and β phase content can be produced due to the existence of high voltage, which can be directly used as flexible piezoelectric materials without another follow-up treatment [38,39]. However, the advancement of β phase is limited only by electrospinning. The incorporation of Au, Ag or other noble metal nanoparticles can cause strong electrostatic interaction between metal nanoparticles and fluorine atoms in the polymer, which plays a positive role in the arrangement of polymer chain and further promotes the formation of β phase [40]. On the one hand, the mechanical vibration energy in the degradation environment can be fully utilized by using PVDF electrospinning membrane doped with Ag particles as the substrate. On the other hand, it can satisfy different application environments by virtue of its flexible characteristics.
In this paper, we designed a PVDF@Ag-ZnO/Au nanofiber membranes that can effectively utilize mechanical vibration energy and solar energy to improve the photocatalytic efficiency. Herein, besides serving as a photoactive semiconductor for catalysis, ZnO NRs can also inhibit the recombination of photo-generated carriers by the virtue of the piezoelectric effects, which can generate a built-in piezoelectric field. Au NPs can not only improve the separation of electron-hole pairs, but also further enhance the light response of the composite structure by LSPR effect. With the addition of noble metals Ag NPs, the formation of β phase in PVDF can be promoted, which will improve the piezoelectric properties of the electrospun flexible PVDF substrate. In this study, we demonstrated that the plasmonic effect of noble metals can effectively enhance the light response and broaden light absorption from ultraviolet to visible light region. In addition, the modulation effect of piezoelectric effect on light response is proved by the surface-enhanced Raman scattering (SERS). Using the COMSOL Multiphysics, the electrical enhancement of PVDF and ZnO loaded noble metal particles under stress was simulated. For the degradation of organic dyes, a synergistic effect of plasmonic and piezoelectric effect was achieved to attain efficient degradation of rhodamine B (RhB) dye (98.8%), which is 280% and 220% higher than that of photocatalysis and piezocatalysis, respectively. Furthermore, considering that high-frequency ultrasound is difficult to obtain in daily life, a high efficiency (61%) was implemented by simulating the degradation of RhB dye by river flow and illumination. The functional mechanism in dyes on the composites under ultrasound and light was also discussed. Moreover, the flexible PVDF@Ag-ZnO/Au nanofiber membranes can be readily recycled without complex centrifugation or precipitation treatments. It is anticipated that the development of the flexible composite nanofiber membranes will provide an efficient strategy for enhancing the utilization of solar and mechanical energy.
2. Experimental
2.1 Synthesis of PVDF@Ag compositional nanofibers and ZnO nanorods/Au heterostructures
Figure 1 schematically shows the fabrication process of PVDF@Ag-ZnO/Au. PVDF nanofibers were fabricated by the method of electrospinning with 20 wt% PVDF (Mw∼275000, Sigma-Aldrich) solution in N, N-dimethylformamide (DMF, AR, ≥99.5%, Aladdin) and aceton (AR, ≥99.5%, Sinopharm Chemical Reagent Co. Ltd.) mixture in volume ratio of 7:3 under a constant voltage of 15 kV. The PVDF solution was electrospun by electrospinning equipment at a feeding rate of 1 ml/h on a collector covered by aluminum foil for 30 min. The distance between the needle tip and the collector was 12 cm. The silver particles prepared using a modified method reported by Xia et al were added into the PVDF solution and the electrospinning process was carried out with the same conditions of pure PVDF solution [41,42]. The prepared PVDF@Ag nanofiber membranes were then immersed in 70 mM zinc acetate ((CH3COO)2Zn, AR, ≥99.0%, Sinopharm Chemical Reagent Co. Ltd.) ethanol solution and 0.1 M sodium hydroxide (NaOH, AR, ≥96.0%, Sinopharm Chemical Reagent Co. Ltd.) ethanol solution successively, and fully dried at 125 °C to form the ZnO seed layer. In order to realize a comprehensive coverage of the seed layer, the above steps should be repeated more than three times, and the nanofiber membranes were lightly washed with deionized water and dried again at 125 °C. The PVDF@Ag with seed ZnO layer was placed in a mixture of 50 mM Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, AR, ≥99.0%, Tianjin Damao Chemical Reagent Factory) and 50 mM hexamethylenetetramine (HMTA, AR, ≥99.0%, Sinopharm Chemical Reagent Co. Ltd.) deionized water. Following that, the mixture was transferred into a high-pressure reactor and placed in a 90 °C incubator for 12 h. After the whole reactor was lowered to room temperature, the membranes were removed and cleaned with deionized water. After that, ZnO NRs were grown successfully on the surface of PVDF@Ag nanofibers (PVDF@Ag-ZnO) [43]. Whereafter, the AuNPs were adorned on the ZnO NRs by in-situ reduction reaction of HAuCl4 [44]. During this process, the nanofiber membranes were immersed in 0.1 mM HAuCl4 solution to obtain AuNPs on ZnO NRs surface. After the sample was removed, it was washed successively with ethanol and deionized water and dried in an oven at 60 °C to finally form PVDF@Ag-ZnO/Au.
Fig. 1. Illustration for the synthesis procedure of the PVDF@Ag-ZnO/Au composite nanofiber membrane.
2.2 Apparatus and characterization
The morphologies and elemental distribution of prepared samples were observed by using a scanning electron microscopy (SEM, ZEISS Sigma500) with acceleration voltage of 3kV and energy dispersive spectrometer (EDS). X-ray diffraction (XRD, SmartLab9) analysis and Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet IS50) were utilized to study the crystalline structure. Photoluminescence (PL) spectra were used to analyze the recombination rate of the photogenerated electron-hole pairs, where the PL spectroscopy (F-4500, Hitachi, Tokyo, Japan) was operated at room temperature at the excitation wavelength of 325 nm. The photocurrent density was collected using electrochemical working station (CHI 760E) with a standard three-electrode system, where the composite nanofiber membranes were used as the working electrode, and Pt net and Ag/AgCl electrode (KCl saturated) were served as the counter and reference electrode, respectively. Piezoelectric potential output signals were detected using an electrochemical workstation (CHI 760E) via two copper plates connected to the upper and lower surfaces of the sample for measurement. Raman spectra were probed by a Raman spectrometer (Horiba HR Evolution) with an excitation laser of 532 nm, laser energy of 0.48 mW. The UV-vis absorption spectra of the samples were measured by UV-VIS-NIR spectrophotometer (SolidSpec-3700i DUV, Shimadzu). The absorbance for the catalytic test was evaluated using a UV–vis spectrophotometer (UV-1800 PC, Mapada).
2.3 Piezo-photocatalytic activity measurement
The catalytic performance of the samples was evaluated by the degradation of rhodamine B (RhB) and malachite green (MG) under ultrasonic stimulation at (200 W; 40 kHz), visible light irradiation (Xe lamp, 300 W; without Cut-off filter) or under their combined conditions. For the selection of external stimuli, magnetic stirring and ultrasonic action can achieve the same purpose of degradation [23,24]. In a routine experiment, the prepared samples were kept in 50 ml RhB and MG solution (10mg/L) for 30 min in darkness to achieve an adsorption-desorption equilibrium between the catalyst and dye molecule. To prevent the influence of a thermal effect, the device was cooled using circulating water flow in order to maintain the temperature of the device at room temperature throughout the process. Under visible light illumination, 2 ml RhB or MG solution was extracted every 20 min, and the supernatant was collected after centrifugation. The concentration of RhB or MG was analyzed by recording the absorption band in the UV-Vis spectra. Photocatalytic measurements were repeated five times under the same catalytic conditions, and the average was calculated to avoid chance.
2.4 COMSOL modeling
The piezoelectric effects of PVDF and ZnO NRs under ultrasound were simulated by COMSOL Multiphysics. The geometrical parameters for the modeling were defined as follows: the diameter and length of the PVDF nanofiber is 800 nm and 2 µm; the width of ZnO NR is 100 nm and the length is 1 µm; the radius of Ag NP and Au NP is 50 nm. The pressure applied to PVDF was considered in the case of the radial direction. The bottom surface on the left side of the model was fixed and grounded, and the pressure was applied normally to the 1/4 area of the side. In addition, the Ag NPs were fixed in the interior 100 nm from the edge of the PVDF. Similarly, the pressure on ZnO NR was considered in the radial case. The bottom surface of the model was fixed and grounded, and different radial forces were applied to its side. Meanwhile, Au NP was established at different positions on the outside of ZnO NR. In this case, the results of the two models under external pressure are calculated separately.
3. Result and discussion
3.1 Characterization of PVDF@Ag-ZnO/Au composites
The morphology of the samples was characterized with SEM. Figure 2(a) shows the image of Ag NPs, where we can be seen that the size of Ag NPs is 93.47+/-10.4224 nm as shown in Supplement 1, Fig. S1a. The optical extinction peak of the prepared Ag NPs is around 440 nm, which can be attributed to the plasmonic effect and is beneficial for improving the light response (Supplement 1, Fig. S1b) [45]. The image of the prepared PVDF@Ag is shown in Fig. 2(b), where the composite nanofibers present a three-dimensional multi-layer structure without visible particles on its surface. The reason for this phenomenon is that, on the one hand, the size of Ag NPs prepared are much smaller than nanofibers, and on the other hand, Ag NPs are wrapped in the nanofibers due to the strong interaction between Ag NPs and fluorine atoms. In addition, the average diameter of PVDF@Ag and PVDF nanofibers was 0.9 µm and 0.85 µm (Supplement 1, Fig. S1c,d), respectively, which demonstrated the successful incorporation of Ag NPs. To make better use of the solar and mechanical energy, ZnO NRs were introduced on the surface of PVDF@Ag. As shown in Fig. 2(c), PVDF@Ag is completely covered by ZnO NRs and presents a three-dimensional hierarchical structure. As can be observed from the large magnification illustration (Fig. 2(c)), ZnO NRs exhibit a hexagonal cross section which reveals that the crystal phase of ZnO NRs grow well. In order to further enhance the utilization of visible light, ZnO/Au heterojunction was fabricated on the surface of ZnO NRs as shown in Fig. 2(d), where the ZnO NRs are still fixed on the surface of PVDF@Ag, and the surface becomes rough and the tip turned sharp due to the reduction of Au NPs on the sample surface. The EDS maps further disclose the uniform distribution of the detected elements (Fig. 2(e)), confirming the successful growth of ZnO NRs and Au NPs on the surface of the nanofibers. The reason why there is no silver element is that the Ag NPs are embedded in the innermost part of the sample and the intensity of the electron beam cannot reach the innermost part.
Fig. 2. SEM images of (a) Ag NPs, (b) PVDF@Ag, (c) PVDF@Ag-ZnO and (d) PVDF@Ag-ZnO/Au nanofiber membranes at 3 kV acceleration voltage, respectively. The insets in (c) and (d) show the corresponding enlarged views. (e) EDS analysis was performed for F (e1), Zn (e2) and Au (e3) elements.
In order to further determine the successful combination of each structure and explore the optimal conditions of silver adding, XRD and FTIR were used to detect the crystalline structure and elemental compositions. The XRD patterns of PVDF@Ag-ZnO/Au, PVDF@Ag-ZnO and PVDF@Ag crystal structures indicate that the diffraction peaks at 18.3° and 20.4° correspond to the α and β phases of PVDF (Fig.3a). In addition, diffraction peaks are observed at 36.7°, 34.4°, 36.2°, 47.5°, 56.6° and 62.8°, corresponding to the (100), (002), (101), (102), (110) and (103) planes of hexagonal wurtzite structured ZnO (JCPDS card no.35-1451), respectively [5,23]. Strong ZnO diffraction peak can be obviously observed, which can be proved the possession of great crystal quality and it is the hexagonal noncentrosymmetric structure that makes ZnO NRs have great piezoelectric properties. Diffraction peaks at 38.1° and 44.3° corresponding to the Ag face-centered cubic structure are also detected in the XRD diffraction pattern of PVDF@Ag, [46] which indicates that Ag NPs was successfully embedded into PVDF. In addition, in order to further figure out the optimal conditions of silver embedding, FTIR data measured for pure PVDF and PVDF@Ag nanofibers are shown in Fig. 3(b). The characteristic bands of 763 cm-1,795 cm-1 and 976 cm-1 correspond to the TGTG conformation of α crystal phase, while the TTTT conformation of the polar β crystal phase is 840 cm-1,1276 cm-1 and 1431 cm-1. Both samples show non-polar α phase and polar β phase [47]. For quantitatively characterize the content of polar β phase, the relative percentage of the sample was calculated based on absorbance measurements at 763 cm-1 and 840 cm-1. These could be calculated by Eq. (1), supposing that FTIR absorption obeys Lambert-Beer law [48].
Fig. 3. Crystalline structure and elemental compositions of the composite nanofiber membranes. (a) XRD spectra of PVDF@Ag, PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au nanofiber membranes. (b) FTIR spectra of pure PVDF and PVDF@Ag with different amounts of Ag NPs.
3.2 Optical response of PVDF@Ag-ZnO/Au composites
To reveal the optical properties of membranes prepared, the obtained samples were further characterized. The PL spectra was used to evaluate the migration and separation ability of the photo-generated electron-hole pairs in the samples. According to Fig. 4(a), PVDF@Ag-ZnO/Au shows the lowest PL intensity at the peak of 560 nm under a light excitation (λEX = 325 nm) compared with the sample without Au, which indicates that the ZnO/Au heterojunction is successfully formed and the migration of carriers in PVDF@Ag-ZnO/Au can be effectively enhanced [49,50]. It is worth noting that, the PL intensity decreases with the increase of Au loading, and further elevates with the extension of Au reduction time, where the intensity was the lowest when the deposition time was 4 mins. This phenomenon may be caused by the formation of composite centers when excessive Au NPs are loaded onto ZnO NRs. Since AuNPs with a reduction time of 4 mins have the lowest PL intensity and the strongest separation rate of photo-generated carriers, this condition is adopted in this report. To investigate the response range of samples to light, Uv-vis absorption spectra tests were performed (Fig. 4(b)). The results reveal that bare PVDF has no obvious absorption peaks in the range of 325-800 nm, indicating that this sample have little photocatalytic activity in this spectral range. It should be noted that with the addition of Ag NPs, the absorption peak for PVDF@Ag appears around 440 nm, indicating that the plasmonic effect of Ag NPs can enhance the utilization of light. The spectra of PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au have strong absorption in the UV light region owing to the large bandgap of ZnO (∼3.3 eV) [25,51]. In comparison, the absorption intensity in the visible light range increases significantly and a shoulder peak appears near 530 nm after loading Au NPs, for both PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au. This phenomenon can be attributed to the LSPR effect of Au NPs, which can broaden the light absorption from UV to visible light region and utilize light more efficiently [9]. The instantaneous photocurrent response is assessed to characterize the generation and separation efficiency of photo-generated electron-hole pairs, which is the key factors affecting photocatalytic activity. Generally speaking, higher photocurrent response means greater carrier density, so as to separate charges more effectively, which is conducive to the improvement of photocatalytic performance. Under light irradiation (Xe lamp, 300 W; without Cut-off filter), the time-dependent photocurrent curves of the four samples were recorded (Fig. 4(c)). The current densities of PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au increase rapidly and stabilize at about 1.4 µA·cm-2 and 1.0 µA·cm-2, respectively, and reduce rapidly with the extinction of the lamp. What’s more, the intensity of the photocurrent was consistent with the results of PL spectra. Among them, PVDF@Ag-ZnO/Au exhibits the largest photocurrent, reflecting excellent separation and transmission efficiency of carriers. Furthermore, under the on-off irradiation period (with an interval of 20s), the instantaneous photocurrent of PVDF@Ag-ZnO/Au recovers high reversibility and repeatability, which demonstrates the well electrochemical stability. It's worth noting that the photocurrent of the sample growing ZnO NRs showed a sharp rising peak and a sharp falling peak when the light turned on or off. It is attributed to when the sudden light irradiation, a large number of photo-generated electron-hole pairs are separated, and, a small number of charge carriers are recombined while the light tends to be stable, resulting in the photocurrent finally becoming stable. To verify the promoting effect of light response of samples, we studied the SERS intensity of the samples by Raman detection (Fig. 4(d)). It is obvious that the SERS spectral intensity is significantly enhanced after the addition of Ag NPs contrasted with bare PVDF. In the same case, the intensity of the peak at 1617 cm-1 on the PVDF@Ag-ZnO/Au increased by 2.3 times than that of the PVDF@Ag-ZnO. The reason for these phenomena is that LSPR effect can enhance SERS intensity with the addition of noble metal [52–56]. It should be noted that the SERS spectral intensity decreases remarkably after the growth of ZnO NRs, which can be attributed to the change of the hydrophobic performance, resulting in the weakened accumulation of molecules (Supplement 1, Fig. S2). Compared with PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au, the hydrophobicity is slightly improved after Au loading, which may be because the surface of ZnO NRs becomes rough and sharp [57]. In addition, PVDF@Ag-ZnO/Au is more hydrophilic than PVDF and PVDF@Ag membrane, which is conducive to the generation of oxygen radical species for the decomposition of organic dyes.
Fig. 4. Optical and photoelectric properties of PVDF, PVDF@Ag, PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au. (a) Photoluminescence (PL) spectra of different Au deposition time at room temperature under 325 nm excitation wavelength. (b) Uv-vis absorption spectra of samples and (c) time-dependent photocurrent density under full-spectral irradiation. The insets in (b) depict the partial enlargement in the 400-600 nm range. (d) SERS data of MG on the different samples.
3.3 Piezoelectric response
In order to research the piezoelectric properties of the samples, we carried out open-circuit voltage test under the action of external forces, and used COMSOL Multiphysics to simulate the piezoelectric properties of the samples. So as to demonstrate the piezoelectric properties, the measurement was executed by applying periodic pressing-releasing force. A copper electrode sheet was attached to the upper and lower surfaces of the composite nanofiber membranes for electric measurements. The piezoelectric output of different composite piezoelectric generators under repeated pressure and release at a frequency of 2Hz is depicted in Fig. 5(a). All the samples show typical piezoelectric responses under the action of periodic pressing-releasing force. The output voltage of PVDF@Ag-ZnO/Au reaches 2.27V, which is significantly higher than that of other samples, and is ∼14.8 times higher as compared to that of the pure PVDF. The piezoelectric properties of the structure derived from different aspects are responsible for this result. On the one hand, it depends on the polar β phase in PVDF. The polar β phase in PVDF is effectively enhanced with the addition of Ag NPs, which corresponds to the previous FTIR results. On the other hand, the piezoelectric effect of ZnO also plays an important role.
Fig. 5. (a,b) Piezoelectric output voltage-time profile of PVDF, PVDF@Ag, PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au substrates. (c) The polarization intensities of PVDF and PVDF@Ag nanofibers under radial pressure. (d) Calculated piezoelectric voltage and the stress distribution of ZnO NR under radial pressure. (e) Calculated maximum electric potential difference between ZnO NR and Au NP with the various position of Au NP on ZnO NR surface under the radial action of 5 MPa and 10 MPa.
As shown in Fig. 5(c), compressive stress of 10 MPa was applied in the radial direction of both PVDF and PVDF@Ag in the initial state. In comparison, the polarization intensity increased obviously, and its maximum value increased from 1.73 to 1.93 mC/m2 after embedding Ag NPs into PVDF, which suggests that mechanical energy can cause the PVDF@Ag nanofibers to deform and introduces the enhancement of the piezoelectric effect. Moreover, the piezoelectric potential of the ZnO and ZnO/Au was also considered. Under the radial pressure of 10MPa, the ZnO NR surface displayed a high electric potential difference, which increases with the enhance of external force (Fig. 5(d)). It should be noted that Au NP plays an important role in catalysis as a site of oxygen reduction. Consequently, it is necessary to understand the potential difference between ZnO NR and Au NP (Fig. 5(e)). In order to studied the potential difference, Au NP was placed at different positions. The calculation results of potential difference under radial pressure are shown in Fig. 5(e). It can be seen that the maximum potential difference between Au NP and ZnO NR varies with the relative position of Au NP. The maximum value of Au NP is at the position of maximum deformation, and the potential difference decreases when moving to both sides. In addition, a larger pressure corresponds to a higher piezoelectric voltage at the same position. At 10MPa, the maximum electric potential difference between them is as high as 1.54V.
To verify the modulation effect of the piezoelectric effect on the light response, SERS signal intensity of different samples was measured at 20 s after pressing as shown in Fig. 6(a)-(d). It can be observed that the SERS signal intensity of PVDF and PVDF@Ag change barely, while the SERS spectral intensity of PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au after pressing is significantly improved. In particular, the SERS intensity of PVDF@Ag-ZnO/Au was detected every 10 s after 20 s of pressure (Fig. 6(e)). In order to compare the variation of Raman intensity more visually, Supplement 1, Fig. S3 shows the distinction of peak intensity corresponding to 1617cm-1. After pressing, the intensity decreases gradually over time until it is almost the same as that without pressing. The reason for these phenomena may be that the decay time of the surface potential of each sample after pressing is different. As shown in Fig. 6(f), we measured the variation of open-circuit voltage generated by different samples after pressing. During the first 20 s, PVDF and PVDF@Ag decreased rapidly until disappeared completely, while PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au decreased 81% and 70%, respectively. Especially for PVDF@Ag-ZnO/Au, the open-circuit voltage remained above 0.5V for 20 s, which can provide enough time for SERS detection and be attributed to the augmentation of the separation and migration of photo-generated carriers when Au NPs were added. The results demonstrate that the electric field generated by bending can enhance the SERS signal, and confirms our view on the enhancement effect of piezoelectric effect on optical response. Figure 6(g) presents a schematic diagram of Raman detection. After the sample surface is subjected to external pressure, the electric potential will be generated inside the ZnO NRs and the LSPR effect of Au NPs effectively adjusts the charge distribution on the surface, thus enhancing the local electromagnetic field and resulting in the enhancement of Raman signal [58–60].
Fig. 6. (a-e) SERS spectra recorded before and during pressing for PVDF, PVDF@Ag, PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au, respectively. (f) Output voltage-time profile of different samples after pressing. (g) Schematic diagram of piezoelectric modulation of SERS substrate.
3.4 Piezo-photocatalytic performance
In order to assessed the synergistic effect of piezoelectric and plasmonic on the photocatalytic activity of the composite nanofiber membranes, the degradation tests of pollutant RhB within 120 min were reported under ultrasonic stimulation (piezocatalysis), full-spectrum irradiation (photocatalysis) and concurrent ultrasonic stimulation/full-spectrum irradiation (piezo-photocatalysis), respectively. The degradation ratio (D) is calculated from Eq. (2):
Fig. 7. Catalytic performance of samples for degradation of Rhodamine B (RhB) dye. (a) Piezoelectric catalytic degradation of RhB by PVDF, PVDF@Ag, PVDF@Ag-ZnO and PVDF@Ag-ZnO/Au. (b) Photocatalytic degradation under full spectral light. (c) Piezo- photocatalysis under the combined action of light irradiation and ultrasound. (d) The variations of ln(C/C0) versus time over the PVDF@Ag-ZnO/Au in 120min under different conditions. (e) RhB degradation curve of PVDF@Ag-ZnO/Au in 120min under the combined effect of light and ultrasound. (d) Summary histogram of RhB degradation under different conditions.
Considering that ultrasonic wave is not universal in nature, especially high-frequency ultrasonic stimulation used in the laboratory, piezoelectric potential generated by circulating water flow was used to replace ultrasonic for the study of piezoelectric photocatalysis as shown in Fig. 8(a). The degradation curves show that within 120min, the degradation ratio of RhB reached 61.4% under circulating water flow and Xe lamp irradiation, and the reaction rate constant k was ∼0.0085 min-1 (Fig. 8(b)-(d)). Consequently, the experiment of catalytic degradation of RhB by light and water flow induced charge demonstrates that PVDF@Ag-ZnO/Au has tremendous potential in real water purification.
Fig. 8. (a) Schematic representation of the experimental setup. (b) Uv–Vis absorption spectrum of RhB dye solution degraded by PVDF@Ag-ZnO/Au composite catalyst through piezo-photocatalysis. (c) The piezo-photocatalytic degradation curves of RhB, and (d) Variations of ln(C/C0) versus time of the PVDF@Ag-ZnO/Au under the combined action of light irradiation and ultrasound.
3.5 Mechanism analysis
As stated by the above data, the possible mechanism for improving the catalytic activity of PVDF@Ag-ZnO/Au is summarized as shown in Fig. 9. The PVDF molecular chain used in this study has a large dipole moment from fluorine atoms to hydrogen atoms due to the existence of large electronic affinity differences. Owing to the strong dipolar characteristic of PVDF molecules generated by electrospinning, part of the positive charge of hydrogen atoms in PVDF will interact with part of the negative charge on the surface of Ag NPs, which can further optimize the molecular chain arrangement of PVDF to promote the formation of β phase (Fig. 9(a)). ZnO NR produces photo-generated electron-hole pairs under full-spectrum irradiation. Furthermore, a large number of thermal electrons with higher energy than the Schottky barrier can be excited in AuNPs because of the LSPR effect. When Au NPs were attached to ZnO NR surface, ZnO/Au Schottky barrier was found on the surface of ZnO NRs, which can effectively inhibit the recombination of electron-hole pairs. These electrons can be transported from AuNPs to the conduction band (CB) of the semiconductor through the Schottky barrier, and the holes will remain on the surface of AuNPs (Fig. 9(b)). However, when only light is applied, the uncertain diffusion direction causes the carriers to flow randomly and easily recombine in the semiconductor, so only a small fraction of the hot carriers can reach the catalytic position (Fig. 9(b)). Interestingly, the piezoelectric potential generated in ZnO NR is complex and changeable in different directions under periodic ultrasonic stimulation. After the introduction of ultrasonic vibration, the built-in electric field is generated through the deformation of the composite material under ultrasonic stimulation, which further prevents the recombination of photo-generated carriers, thus improving the photocatalytic activity (Fig. 9(c)-(d)). As displayed in Fig. 9(c), the Schottky barrier at the ZnO/Au interface increases when the piezoelectric polarization of ZnO NR is pointed to the left, and the hot electrons generated by LSPR are injected from Au NPs into CB in ZnO NR. In contrast (Fig. 9(d)), when the piezoelectric polarization of ZnO is pointed to the right, the piezoelectricity potential reduces the Schottky barrier of ZnO/Au and drives the photo-generated electrons of CB from ZnO to Au NPs. The hot electrons and holes are transferred to the catalytic position respectively, and redox reactions will take place in different location. A series of photocatalytic chemical reactions can be expressed in the following Eqs. (4)–(8):
Fig. 9. Schematic illustration of the enhanced catalytic performance induced by piezoelectric effect and nanostructure under light irradiation and ultrasonic actuation. (ΦSB, Schottky Barrier; ECB and EVB, respective energy of the conduction and valence bands of ZnO; EF, energy of the Fermi level).
A large number of photo-generated carriers formed by photon excitation are effectively separated and migrated at ZnO/Au heterojunction sites. Furthermore, under the simultaneous action of ultrasound and light, the separation and directional migration of electron-hole pairs occurred in ZnO NRs. Driven by an internal electric field, electrons and holes migrate to different locations and react with O2 and OH- in the solution to form O2- and ·OH. When these highly oxidizing and reductive products come into contact with dye molecules in solution, the dye molecules can be decomposed.
In general, under the coordination of piezoelectric and plasmonic effect, the built-in electric field can further inhibit the recombination of electron-hole pairs by enhancing the directional transfer ability of charge. Eventually, the carrier redox reaction occurs on the surface of semiconductor or metal, so as to improve the photocatalytic performance.
4. Conclusions
In summary, we have successfully prepared the ZnO NRs arrays with Au NPs on flexible PVDF@Ag substrates (PVDF@Ag-ZnO/Au) using a typical electrospinning and hydrothermal method. The presented study has shown that ZnO NRs not only acts as a photocatalyst, but also provides a built-in electric field to further promote the separation of photo-generated charges. Moreover, with the addition of the noble metals, photo-generated hot electrons migrate and separate directionally by the LSPR effect of Au NPs on the surface. Furthermore, the embedding of Ag NPs also further enhances the piezoelectric effect of PVDF to strengthen the utilization of mechanical energy. With the reasonable design, PVDF@Ag-ZnO/Au exhibited a stronger light response and a wider light absorption range, which can be attributed to the plasmonic effect of Ag NPs and Au NPs. SERS was used to investigate the synergy between plasmonic and piezoelectric effect. We achieved a higher piezo-photocatalytic efficiency, up to 98.8% within 120 mins using PVDF@Ag-ZnO/Au, which was significantly higher than the photocatalysis or piezocatalysis for the degradation of RhB. The significant improvement of catalytic performance of PVDF@Ag-ZnO/Au could be attributed to the cooperated effect of piezoelectric and plasmonic effect. Apart from the experimental content, we also carried out practical application and theoretical simulation of the structure. This work presents a new insight to reliable solution to optimize photoelectrochemical energy conversion by harvesting mechanical energy and solar light.
Funding
National Natural Science Foundation of China (11974222, 12174229, 12004226, 11904214, 11774208); Taishan Scholar Project of Shandong Province (tsqn201812104); Science and Technology Support Plan for Youth Innovation of Colleges and Universities of Shandong Province of China (2021KJJ006, 2019KJJ014, 2019KJJ017).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time, but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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