Skip to main content
NCBI home page
Ultrasonics Sonochemistry logoLink to Ultrasonics Sonochemistry
. 2021 Sep 11;78:105754. doi: 10.1016/j.ultsonch.2021.105754

KNbO3/ZnO heterojunction harvesting ultrasonic mechanical energy and solar energy to efficiently degrade methyl orange

Yi Li a, Huafeng Chen a, Linkun Wang a, Tiantian Wu a, Ying Wu b,⁎, Yiming He a,b,⁎
PMCID: PMC8445898  PMID: 34530388

Graphical abstract

graphic file with name ga1.jpg

Open in a new tab

Keywords: KNbO3/ZnO, Piezocatalytic, Photocatalytic, Methyl orange

Highlights

  • •

    KNbO3/ZnO composite was prepared via a two-step hydrothermal method.

  • •

    KNbO3/ZnO can utilize solar and vibration energy to catalyze the degradation of MO.

  • •

    KNbO3/ZnO presented much better performance in piezo/photocatalytic MO degradation than ZnO or KNbO3.

  • •

    The enhanced performance was mainly ascribed to the improved charge separation.

Abstract

In this paper, KNbO3/ZnO nanocomposite was synthesized and used in piezo/photocatalytic degradation of methyl orange (MO) under simulated sunlight and ultrasonic vibration. Under simulated solar light, the optimal KNbO3/ZnO sample presented a MO degradation rate of 0.047 min−1, which is 2.47 times higher than that of ZnO. The promotion effect of KNbO3 on ZnO was also observed in the piezoelectric catalytic reaction. In addition, the co-utilization of solar and mechanical energy can further increase the MO degradation rate. Piezoelectric property and photoresponse capability are the origins of the piezo/photo catalytic behavior of the KNbO3/ZnO composite. Owing to the different band potentials of KNbO3 and ZnO, the electric potential field at their interface can drive the second distribution of the photo/piezoinduced charge carriers and hence promote the photo/piezocatalytic activity. This phenomenon was verified by the analysis on transient photocurrent and piezocurrent response. Trapping experiments on reactive species were also conducted. Superoxide radicals, holes, and hydroxyl radicals were found to be the main reactive species during the photo/piezocatalytic reaction. Recycling test showed that the KNbO3/ZnO composite exhibited good catalytic stability during six consecutive uses. Given its advantages of good catalytic activity and stability, the synthesized KNbO3/ZnO nanocomposite material has great potential in the further use of solar and mechanical energy to develop new water purification technologies.

1. Introduction

With the rapid development of the economy, environmental pollution has become a huge problem for humans. Photocatalysis can decompose organic compounds in wastewater into non-toxic inorganic substances by using solar energy, thus providing a green approach for wastewater treatment [1]. Piezocatalysis is another new green technology that uses mechanical vibration to enrich heat-excited free electrons and holes on the surface of piezocatalyst particles, thus generating active free radicals with strong oxidation ability to decompose organic pollutants [2]. The abundant low-frequency mechanical energy source in nature guarantees the wide application prospective of this technique. Given that piezocatalysis can realize the effective degradation of dye molecules in the absence of light or weak light, this method can effectively compensate for the shortage of photocatalysis.

ZnO nanostructures have been widely used in organic pollutant degradation due to their high photocatalytic efficiency, low cost, and environmental friendliness [3], [4], [5], [6], [7]. When the light energy is equal to or exceeds the band gap energy of zinc oxide, the electrons are excited from the valence band (VB) to the conduction band (CB) and leave the same number of holes. The photogenerated charge carriers then migrate to the surface and induces photocatalysis. In addition, ZnO has good piezoelectric properties and can trigger the piezocatalytic reaction [8]. Nevertheless, the piezocatalytic activity of pure ZnO is still limited. Various methods, such as morphology modification, oxygen vacancy introduction, noble metal loading, and semiconductor coupling, have been used to improve the piezocatalytic performance of ZnO. Ning et. al synthesized ZnO nanorods (NRs) and nanoparticles (NPs) via a solid-state chemical method [9]. ZnO NRs exhibit 5–18 times higher piezoelectric potential and thus have higher piezocatalytic activity than ZnO NPs in MB degradation. Bai et al. introduced oxygen vacancy into ZnO via heating the ZnO powders in the H2-Ar atmosphere [10]. The introduced oxygen vacancies increase the content of free charge carriers and hence enhance the catalytic activity of ZnO in piezocatalytic RhB degradation. Xiang et al. reported that the loading of Ag nanoparticles improves the separation of charge carriers via trapping the heat-excited free electrons, thereby enhancing the piezo/photo catalytic activity of ZnO [11]. Zhang et al. reported that decorating BiOI on ZnO nanorod arrays greatly improves the activity in the piezo/photocatalytic degradation of organic pollutants [12] because of the enhanced charge separation due to the coupling effect of ZnO nanorod and BiOI. Ren et al. found that ZnS coupling presents a similar promotion effect on ZnO during the piezocatalytic degradation of MB [13]. Among the aforementioned examples, heterostructure construction is the most convenient and efficient [14], [15], [16]. Therefore, the piezocatalytic activity of ZnO can be enhanced by decorating it with a semiconductor exhibiting suitable band potential. KNbO3 is a typical perovskite composite material. The built-in electric field generated by its spontaneous polarization can effectively prevent the recombination of electron-hole pairs, which endows the good activity of KNbO3 in photocatalytic H2 evolution, organic pollutant degradation, and N2 fixation [17], [18], [19], [20]. In addition, KNbO3 exhibits good piezoelectric property and has been applied in the piezocatalytic degradation of MO [21], [22]. In general, KNbO3 is used as a key component of the catalyst in photocatalysis and piezoelectric catalysis. However, in some cases, KNbO3 can also be used as a good co-catalyst [23], [24], [25]. The CB and VB potentials of KNbO3 are approximately −1.51 and 1.62 eV [26], respectively, which are suitable with the band structure of ZnO (ECB = 0.30 eV, EVB = 2.8 eV) [27]. The decoration of KNbO3 on ZnO can fabricate a type-II heterojunction and effectively hinder the recombination of electron-hole pairs, thus facilitating the photo/piezocatalytic reaction. However, no research has been conducted on the photocatalytic and piezoelectric degradation of organic pollutants by KNbO3/ZnO composite materials.

In this work, KNbO3/ZnO composites were prepared by the hydrothermal method. The synthesized KNbO3/ZnO was used for the photocatalytic and piezocatalytic degradation of methyl orange (MO) under simulated solar light and ultrasonic vibration. Results showed that KNbO3 introduction elevated the photocatalytic and piezocatalytic performance of ZnO. The stability of the KNbO3/ZnO catalyst and the reactive species during the photo/piezo catalytic reaction were also investigated.

2. Experimental

2.1. Preparation of catalyst

All chemicals used in the manuscript are analytical pure, and were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China).

KNbO3 nanorods were prepared using a hydrothermal method. In brief, 1.222 g (4.6 mmol) of Nb2O5 was added to 70 mL of KOH solution (10 mol/L) and stirred for 1 h. The obtained mixture was then hydrothermally treated at 160 °C for 18 h to obtain white KNbO3 powders.

KNbO3/ZnO catalyst was also synthesized by the hydrothermal method. ZnCl2 solution was prepared via dissolving 0.02 mol (2.7260 g) ZnCl2 in 70 mL of deionized water. A certain amount (0.5%, 1%, and 1.5%) of KNbO3 nanorods was uniformly dispersed in the solution by ultrasonic treatment for 0.2 h. The NaOH solution (5 mol/L, 8 mL) was then added under stirring. After the mixture was treated hydrothermally (100 mL kettle) at 160 °C for 18 h, the KNbO3/ZnO sample was obtained via centrifugation, washing with deionized water and ethanol, and drying at 80 °C for 12 h. In the absence of KNbO3 powders, pristine ZnO was prepared by a similar method.

2.2. Piezocatalytic and photocatalytic experiments

The piezoelectric catalytic activity of different KNbO3/ZnO catalysts was examined by measuring the degradation rate of MO dye under ultrasonic vibration at room temperature. An ultrasonic generator (Jiemeng JP030, 40 kHz, 120 W) was used as the vibration source. In brief, 100 mL of MO solution (10 ppm) was used as the reaction solution, and the catalyst dosage is 0.1 g during each test. Prior to the test, an adsorption–desorption equilibrium between the catalyst and MO solution was established via continuous stirring for 1 h. During the reaction, 3 mL of the solution was collected at every 15 min interval, and the MO concentration was measured with a UV–vis spectrophotometer (Beijing Purkinje, TU-1950). Piezocatalytic experiments with different scavengers were also implemented in the same way, and the scavenger concentration is 0.001 mol/L.

The photocatalytic activity of the KNbO3/ZnO composites was investigated by conducting the photocatalytic degradation of MO under simulated sunlight. The catalytic test is the same as that of the piezocatalytic procedure, except for the vibration source that was replaced by a Xe lamp (Perfect Light, PLS-SXE300, 300 W, 300 nm < λ < 1000 nm).

Information on catalyst characterization is provided in the supplementary materials.

3. Result and discussion

3.1. Characterization of the KNbO3/ZnO composite

The structure of the KNbO3/ZnO catalysts was investigated by XRD. The XRD patterns of ZnO, KNbO3, and the KNbO3/ZnO samples are presented in Fig. 1. Pristine ZnO shows several strong characteristic peaks at 2θ = 31.8°, 34.6°, 36.2°, 47.5°, 56.5°, 62.9°, which correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), and (1 0 3) planes of hexagonal ZnO (PDF# 36–1451), respectively. For KNbO3, the characteristic peaks appear at 22.1°, 31.6°, 45.2°, 50.9°, and 56.1°, corresponding to the (1 1 0), (1 1 1), (0 0 2), (2 2 1), and (3 1 1) planes of orthorhombic structure of KNbO3 (PDF#32–0822), respectively. The XRD patterns of the KNbO3/ZnO catalyst only show the characteristic peaks of ZnO and KNbO3. No other phases are observed, suggesting the successful formation of the KNbO3/ZnO composite. When the KNbO3 content increases, the KNbO3 peaks of the composite catalyst are gradually enhanced. By contrast, the ZnO peaks only slightly change due to the low KNbO3 concentration. (Fig. S1). The Raman spectra of the ZnO, KNbO3, and KNbO3/ZnO composites are presented in Fig. 1b. Neat KNbO3 shows several strong Raman signals at 188, 277, 528, 590, and 830 cm−1, which can be assigned to the vibration of Nb–O bonds [28]. For ZnO, the characteristic peaks appear at 330 and 436 cm−1, which originates from the E2H-2L and E2H modes of ZnO, respectively [29]. KNbO3 and ZnO phases are detected in the Raman spectra of the KNbO3/ZnO composite. The Raman peak of KNbO3 gradually reduces with the decrease in KNbO3 concentration. This result is consistent with the XRD result. However, different from the XRD patterns, the KNbO3 Raman signal is strong, though its content in the composite catalyst is low. This phenomenon may be ascribed to the strong Raman response of KNbO3 under Ar ion laser excitation. Another possible reason is that the KNbO3 may exist on the ZnO surface in the detected area, resulting in its abnormally strong Raman signal.

Fig. 1.

Fig. 1

Open in a new tab

XRD patterns (a) and Raman spectra (b) of KNbO3/ZnO composite.

X-ray photoelectron spectroscopy (XPS) was conducted to investigate the valence state of the elements in the KNbO3/ZnO samples. Fig. 2a displays the Zn 2p XPS spectra of ZnO and 1%KNbO3/ZnO catalysts. The Zn 2p3/2 and Zn 2p1/2 peaks of ZnO are located at 1021.6 and 1044.8 eV, respectively, indicating that the zinc element has a valence state of +2 [30]. Compared those of with pristine ZnO, the Zn 2p peaks of KNbO3/ZnO move 0.2 eV to the lower binding energy (BE) direction. This phenomenon can be ascribed to the different Femi levels of KNbO3 and ZnO. Given that both samples are n-type semiconductors and the CB of KNbO3 is higher than that of ZnO [26], [27], KNbO3 has a higher Femi level than ZnO. When the two semiconductors are in close contact to form a heterojunction structure, electron transfer from KNbO3 to ZnO may occur. This process increases the electron density around Zn and subsequently reduces the BE of Zn. For the KNbO3 phase in the KNbO3/ZnO composite, the outflow of electrons decreases the density of the electron cloud and increases the BE of the related elements. As shown in Fig. 2b, pure KNbO3 presents two K2p peaks at 290.7 and 293.5 eV, thus confirming the +1 valence state of the K element [31]. For the 1% KNbO3/ZnO sample, an obvious blue shift of the two K2p peaks can be observed in addition to the greatly reduced peak intensity due to the low KNbO3 content. A similar result is also noted for the Nb 3d XPS spectra. The Nb3d BE values of 1% KNbO3/ZnO are 206.4 and 209.1 eV, which are 0.4 eV higher than that of KNbO3. This finding confirms that a strong interaction exists between ZnO and KNbO3 and the synthesized KNbO3/ZnO is a composite rather than a physical mixture [32], [33], [34]. Fig. 2d shows the O1s spectra of ZnO, KNbO3, and 1%KNbO3/ZnO samples. The lattice oxygen signal of KNbO3 and ZnO appears at 529.2 and 530.0 eV, respectively [35]. For the 1%KNbO3/ZnO sample, the O1s spectrum is similar to that of ZnO due to the low KNbO3 concentration. The atomic contents of all elements in ZnO, KNbO3, and 1%KNbO3/ZnO were calculated. The result in Table S1 indicates that the K/Nb ratio in the KNbO3 and 1%KNbO3/ZnO catalysts is close to 1.0, which is consistent with the theoretical value for KNbO3. Meanwhile, the K/Zn ratio of the 1%KNbO3/ZnO is 0.6%, which is lower than the theoretical value of 1.0%. Given that the larger particle size of KNbO3 than ZnO limits the uniform dispersion of the former in the composite piezocatalyst (Fig. 3), the variation in the K/Zn ratio is acceptable.

Fig. 2.

Fig. 2

Open in a new tab

XPS spectra of ZnO, KNbO3, and 1%KNbO3/ZnO composite (a) Zn2p; (b) O1s; (c) K 2p; (d) Nb 3d.

Fig. 3.

Fig. 3

Open in a new tab

SEM images of KNbO3 (a), ZnO (b), and KNbO3/ZnO (c); TEM images of 1% KNbO3/ZnO sample (d, e, f).

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were implemented to reveal the morphology of the KNbO3/ZnO composite. Fig. 3 displays the SEM and TEM pictures of KNbO3, ZnO, and 1%KNbO3/ZnO. Pure KNbO3 appears as nanocubes (Fig. 3a), while ZnO can be seen as nanosheets attached with some nanoparticles (Fig. 3b). The SEM image of the 1%KNbO3/ZnO sample only shows the ZnO sheets. KNbO3 nanocubes are not observed mainly due to the low content of KNbO3 and the serious aggregation of catalyst particles. However, in the TEM image, the KNbO3 nanocubes can be easily distinguished due to their lesser electron penetrating ability than the ZnO nanosheets. As shown in Fig. 3d and e, ZnO nanosheets and nanoparticles adhere to the surface of KNbO3 nanocubes. The HR-TEM image displays that the nanocube and the nanosheets show clear lattice fringes of 0.2813 and 0.2098 nm, which correspond to the (1 0 0) and (1 1 0) planes of KNbO3 and ZnO, respectively. Combined with the XRD and XPS finding, this result verifies that the KNbO3 nanocubes have closely adhered onto the ZnO surface.

The locale piezoelectric response of the KNbO3/ZnO composite was investigated by using the piezoelectric force microscope (PFM) technique. Fig. 4a, b, and c display the atomic force microscope (AFM), PFM amplitude, and PFM phase images of the 1%KNbO3/ZnO sample, respectively. The images do not provide the morphological information of the composite material as clear as SEM, and only several nanoparticles can be observed. However, the locale hysteresis loop (Fig. 4d) indicates that these particles (1% KNbO3/ZnO sample) present a phase angle change of approximately 180° under the reversal of 10 V DC bias field, indicating that the 1%KNbO3/ZnO composite has piezoelectric property and can drive the piezoelectric catalytic reaction.

Fig. 4.

Fig. 4

Open in a new tab

The AFM picture (a), PFM amplitude image (b), PFM phase image (b), and hysteresis loop (d) of the prepared 1.0% KNbO3/ZnO composite.

Fig. 5 shows the DRS spectra of KNbO3/ZnO composite with different KNbO3 contents. ZnO and KNbO3 present strong absorption peaks at the UV region. The light absorption edges are at approximately 400 nm. According to the Kubelka–Munk plot, the band gaps of ZnO and KNbO3 are 3.07 and 3.03 eV, respectively. Therefore, the DRS spectra of KNbO3/ZnO composites are similar to those of ZnO and KNbO3. The consequence in Fig. 5 shows that the synthesized KNbO3/ZnO can only work under UV light.

Fig. 5.

Fig. 5

Open in a new tab

DRS spectra of KNbO3/ZnO composite (a) and the corresponding band gaps (b) of ZnO and KNbO3.

3.2. Photo/piezocatalytic activity

The photocatalytic performances of the synthesized KNbO3/ZnO composites were examined by degrading the MO solution under simulated sunlight. Owing to the wide band gap of the KNbO3/ZnO catalyst, only the UV light is harvested and can stimulate the photocatalytic reaction. As shown in Fig. 6a, no MO is degraded in the absence of a photocatalyst. Pure KNbO3 shows negligible photocatalytic activity, whereas ZnO exhibits good photocatalytic performance with a MO degradation rate of 0.019 min−1 (Fig. 6b). The KNbO3/ZnO composite shows higher photocatalytic activity than ZnO, suggesting that the added KNbO3 hastens the photocatalytic procedure of ZnO. The KNbO3 concentration in the KNbO3/ZnO composite is closely related to its photocatalytic activity. When the KNbO3 content increases, the photoactivity of the KNbO3/ZnO composite enhances first and then reduces. The 1% KNbO3/ZnO composite demonstrates the maximum MO degradation rate of 0.047 min−1, which is 2.47 times higher than that of ZnO. In addition to the photocatalytic activity, the photocatalytic stability of KNbO3/ZnO catalyst was evaluated by the cyclic test. The consequence in Fig. 6c indicates that the KNbO3/ZnO composite still displays high photoactivity after five cyclic tests, implying its good stability. Fig. 6d shows the effect of different capture agents on the photoactivity of the 1% KNbO3/ZnO sample. Isopropyl alcohol (IPA), potassium iodide (KI), and p-benzoquinone (BQ) were added to the reaction solution as the capture agents of •OH, h+, and •O2–, respectively [36], [37]. Fig. 6d shows that all scavengers can retard the photocatalytic reaction, indicating that the aforementioned reactive species contribute to the photocatalytic reaction. The order of the inhibitory effect is KI > BQ > IPA, which represents that the importance of the reactive species in the photocatalytic reaction is h+ > •O2– > •OH.

Fig. 6.

Fig. 6

Open in a new tab

Photocatalytic activity of ZnO, KNbO3, KNbO3/ZnO composites (a) and the corresponding degradation rate (b), the cycling test (c) and the reactive species trapping experiments (d) of 1% KNbO3/ZnO composite.

In addition to its high photocatalytic performance, the KNbO3/ZnO composite presents good piezoelectric properties, and thus has potential in the piezoelectric catalytic reaction. Thus, the piezocatalytic activity of the synthesized KNbO3/ZnO was evaluated via the piezocatalytic degradation of MO under ultrasonic vibration, and the result is presented in Fig. 7. The MO degraded under ultrasonic vibration can be ignored. In the presence of ZnO or KNbO3, the MO content decreases when the vibration time increases. Consistent with literature [8], [9], [21], this finding indicated that the two semiconductors present good piezoelectric properties and can induce the piezocatalytic reaction. The degradation rates of ZnO and KNbO3 are 0.0055 and 0.0047 min−1, respectively (Fig. 7b). For the ZnO sample, this value in piezocatalysis is lower than that in photocatalytic reaction. However, for the KNbO3 sample, its piezocatalytic degradation rate is higher than its photocatalytic efficiency. The KNbO3/ZnO composite has higher piezocatalytic activity than pure ZnO or KNbO3, indicating the synergistic effect of ZnO and KNbO3. The 1% KNbO3/ZnO is still the optimal sample with a MO degradation rate of 0.008 min−1, which is approximately 1.5- and 1.7-folds faster than those of ZnO and KNbO3, respectively. Therefore, the coupling of KNbO3 and ZnO fabricates an efficient piezocatalyst for MO degradation. Cyclic test and reactive trapping experiment were also performed for the 1%KNbO3/ZnO composite. The result indicates that the KNbO3/ZnO has high piezocatalytic stability (Fig. 7c), and the main reactive species are still h+, •O2–, and •OH species (Fig. S2). Photo/piezoelectric synergistic catalytic reaction shows that the KNbO3/ZnO sample has a higher MO degradation efficiency under the combined action of simulated sunlight and ultrasonic vibration (Fig. 7d). This result indicates that the catalyst can be applied to treat wastewater under different conditions and can achieve a high degradation efficiency through the synergistic effect of light and ultrasonic vibration. The KNbO3/ZnO catalyst may have broad application prospects in pollutant degradation.

Fig. 7.

Fig. 7

Open in a new tab

Piezocatalytic activity of ZnO, KNbO3, KNbO3/ZnO composites (a) and the corresponding degradation rate (b), the cycling test of 1% KNbO3/ZnO composite under ultrasonic vibration (c), and the catalytic activity of 1% KNbO3/ZnO under simulated sunlight and ultrasonic vibration (d).

3.3. Discussion

Photocatalytic and piezocatalytic reaction test shows the synergetic effect of KNbO3 and ZnO in the photocatalytic and piezocatalytic degradation of MO, indicating that coupling KNbO3 with ZnO generates an efficient photo/piezocatalyst. Herein, a detailed discussion was conducted to clarify the origin of the increased catalytic activity based on the characterization results.

ZnO and KNbO3 are well-known semiconductor photocatalysts. Various ZnO or KNbO3 based composite photocatalysts have been reported, such as ZnO/KNb3O8 [38], NiO/KNbO3 [26], MoS2/ZnO [39], and In2S3/KNbO3 [40]. Almost all the composite catalysts have achieved an increase in photocatalytic performance due to the increased efficiency in charge separation. Therefore, the improved photocatalytic activity of KNbO3/ZnO has originated from the synergistic effect of KNbO3 and ZnO in hindering the recombination of photoexcited electrons and holes. VB XPS analysis indicates that the VB top of KNbO3 and ZnO are 2.13 and 2.54 eV, respectively (Figure S3). On the basis of DRS result and Eg = EVB − ECB, the conduction bottoms of KNbO3 and ZnO are determined to be −0.90 and −0.53 eV, respectively. The energy band positions of KNO3 and ZnO can also be calculated based on the empirical formula of EVB = X − Ee + 0.5Eg (Table S2) [41], [42]. Owing to the use of different testing methods, the CB and VB positions of KNbO3 and ZnO are slightly different [26], [27]. However, their relative position is not changed. KNbO3 exhibits higher CB and VB position than ZnO. The matched band potential reveals that KNbO3 and ZnO can fabricate a type-II heterojunction structure, which leads to the directional migration of free electrons and the decreased recombination possibility of charge carriers (Fig. 8). Hence, the photoactivity of the KNbO3/ZnO composite is increased.

Fig. 8.

Fig. 8

Open in a new tab

Possible mechanism of KNbO3/ZnO in photocatalysis and piezocatalysis.

In addition to its good performance in photocatalytic reaction, KNbO3/ZnO presents an impressive performance in the piezocatalytic degradation of MO under ultrasonic vibration. The good piezoelectric property of KNbO3 and ZnO is undoubtedly the key factor. Semiconductors has low contents of the heat-excited free charges under normal circumstances [43]. No redox reaction can be observed. Under ultrasonic vibration, however, the mechanical force generated by the cavitation phenomenon strikes the surface of ZnO or KNbO3 and causes them to form local strain. A piezoelectric potential is thus produced because of the collectively induced polarization of charges in whole unit cells in the crystal. This potential attracts the heat-excited free charges in the piezoelectric material towards the opposite directions of the crystal surfaces. These separated free charges accumulate at the interface and react with hydroxyl or dissolved oxygen to form reactive species, which then induces the surface electrochemical reactions to degrade dye molecules [43], [44]. For the ZnO/KNbO3 heterojunction, the different band potentials of the two semiconductors induce an electric potential field at the interface, which drives the second distribution of the piezoinduced charge carriers. The electrons prefer to accumulate on the ZnO side, and the positive charges are enriched on the KNbO3 particles (Fig. 8). This process improves the separation and utilization of free charge carriers and thereby increases the piezocatalytic activity in MO degradation. When the light and ultrasonic vibration simultaneously act on the catalyst, the bending of the energy band induced by the piezoelectric effect speed up the migration of photoexcited charge carriers [45], [46], [47]. Therefore, the separation efficiency of electron-hole pairs is further improved, resulting in good photo/piezocatalytic activity.

For confirmation, a series of electrochemical experiments was implemented to examine the separation of electron-hole pairs in the KNbO3/ZnO composite. Fig. 9a shows the transient photocurrent (PC) response of ZnO, KNbO3, and KNbO3/ZnO composites under simulated sunlight irradiation. ZnO electrode displays higher photocurrent response than KNbO3, indicating its better capability in separating electron-hole pairs and higher photocatalytic activity [48], [49], [50]. This finding is consistent with Fig. 7. KNbO3/ZnO composites possess higher photocurrent than ZnO, implying the synergistic effect of KNbO3 and ZnO in retarding the recombination of charge carriers and confirming the suggested mechanism in Fig. 8. The highest photocurrent is observed in the 1% KNbO3/ZnO sample, indicating its best capability in charge separation. Considering that the heterojunction structure of KNbO3 and ZnO is the main reason for the improved charge separation, the 1% KNbO3/ZnO may have a suitable KNbO3 concentration. Therefore, this sample has the most heterojunction structures in the composite and efficiently accelerates the charge separation, thereby leading to its highest photo/piezocatalytic activity. In addition to the photocurrent experiment, electrochemical impedance spectroscopy (EIS) was conducted to confirm the promotion effect of KNbO3/ZnO heterojunction in charge separation. The EIS curves of ZnO, KNbO3, and KNbO3/ZnO composites with different KNbO3 contents are shown in Fig. 9b. In general, a small arc size of the Nyquist circles represents a low electron transfer interface resistance [51], [52], [53]. The smallest arc radius is observed in the Nyquist circle of the 1% KNbO3/ZnO composite, suggesting its lower interfacial resistance than other catalysts. Therefore, the introduced KNbO3 increases the transfer and separation of carriers of ZnO by forming a heterojunction structure. This finding is in agreement with the PC results.

Fig. 9.

Fig. 9

Open in a new tab

The transient photocurrent response (a) and EIS (b) of ZnO, KNbO3, and KNbO3/ZnO composites with different KNbO3 contents.

Fig. 10 shows the piezoelectric current response of KNbO3, ZnO, and 1%KNbO3/ZnO samples. The current response is still observed under periodic ultrasonic vibration, thus confirming the piezoelectric property of the three samples. The piezoelectric current is only approximately 0.1% of the photocurrent, which can be ascribed to the low content of heat-excited free electrons in the samples. The 1%KNbO3/ZnO electrode produces the highest piezoelectric current, thus proving that the heterostructure of KNbO3 and ZnO improves the separation of piezoelectric-induced charge carriers [54], [55], [56]. Therefore, the KNbO3/ZnO composite displays an enhanced piezocatalytic performance.

Fig. 10.

Fig. 10

Open in a new tab

The generated current response of ZnO, KNbO3, and 1% KNbO3/ZnO under ultrasonic vibration.

4. Conclusion

A novel piezoelectric catalyst KNbO3/ZnO composite was successfully synthesized by a two-step hydrothermal method and applied in the piezo/photocatalytic degradation of MO. KNbO3 and ZnO showed a strong interaction that triggered the formation of a type-II heterojunction and enhanced the separation efficiency of electron-hole pairs. Hence, the KNbO3/ZnO nanocomposite shows higher efficiency than pure ZnO or KNbO3 in harvesting vibration energy or solar energy for MO degradation. In addition, the KNbO3/ZnO nanocomposite also maintains stable photo/piezocatalytic efficiency in cyclic test, indicating its potential application in water purification.

CRediT authorship contribution statement

Yi Li: Writing – original draft, Investigation, Formal analysis, Data curation. Huafeng Chen: Investigation, Validation. Linkun Wang: Investigation. Tiantian Wu: Investigation. Ying Wu: Conceptualization, Resources, Methodology, Writing – review & editing. Yiming He: Conceptualization, Supervision, Project administration, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was financially supported by National Natural Science Foundation of China (No. 22172144), Nature Science Foundation of Zhejiang Province (No. LY20B030004) and the National Undergraduate Training Program for Innovation and Entrepreneur-ship (No. 202010345055).

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2021.105754.

Contributor Information

Ying Wu, Email: yingwu@zjnu.cn.

Yiming He, Email: hym@zjnu.cn.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Supplementary data 1
mmc1.docx (222.4KB, docx)

References:

  • 1.Pasini S.M., Valério A., Yin G.L., Wang J.F., Souza S.M.A.G.U., Hotza D., Souza A.A.U. An overview on nanostructured TiO2–containing fibers for photocatalytic degradation of organic pollutants in wastewater treatment. J. Water Pro. Eng. 2021;40 [Google Scholar]
  • 2.Tu S.C., Guo Y.X., Zhang Y.H., Hu C., Zhang T.R., Ma T.Y., Huang H.W. Piezocatalysis and piezo-photocatalysis: Catalysts classification and modification strategy, reaction mechanism, and practical application. Adv. Funct. Mater. 2020;30:2005158. [Google Scholar]
  • 3.Sheydaei M., Fattahi M., Ghalamchi L., Vatanpour V. Systematic comparison of sono-synthesized Ce-, La- and Ho-doped ZnO nanoparticles and using the optimum catalyst in a visible light assisted continuous sono-photocatalytic membrane reactor. Ultrason. Sonochem. 2019;56:361–371. doi: 10.1016/j.ultsonch.2019.04.031. [DOI] [PubMed] [Google Scholar]
  • 4.Krishnakumar B., Alsalme A., Alharthi F.A., Mani D., Anandan K., Amutha P., Sobral A.J.F.N. Synthesis, characterization of gelatin assisted ZnO and its effective utilization of toxic azo dye degradation under direct sunlight. Opt. Mater. 2021;113 [Google Scholar]
  • 5.Krishnakumar B., Balakrishna A., Nawabjan S.A., Pandiyan V., Aguiar A., Sobral A.J.F.N. Solar and visible active amino porphyrin/SiO2–ZnO for the degradation of naphthol blue black. J. Phys. Chem. Solids. 2017;111:364–371. [Google Scholar]
  • 6.Krishnakumar B., Ima T. Chemically modified novel PAMAM-ZnO nanocomposite: synthesis, characterization and photocatalytic activity. Appl. Catal. A: Gen. 2014;486:170–175. [Google Scholar]
  • 7.Krishnakumar B., Imae T., Miras J., Esquena J. Synthesis and azo dye photodegradation activity of ZrS2–ZnO nano-composites. Sep. Purif. Technol. 2014;132:281–288. [Google Scholar]
  • 8.Xu X., Jia Y., Xiao L., Wu Z. Strong vibration-catalysis of ZnO nanorods for dye wastewater decolorization via piezo-electro-chemical coupling. Chemosphere. 2018;193:1143–1148. doi: 10.1016/j.chemosphere.2017.11.116. [DOI] [PubMed] [Google Scholar]
  • 9.Ning X., Hao A., Cao Y., Hu J., Xie J., Jia D. Effective promoting piezocatalytic property of zinc oxide for degradation of organic pollutants and insight into piezocatalytic mechanism. J. Colloid Interf. Sci. 2020;577:290–299. doi: 10.1016/j.jcis.2020.05.082. [DOI] [PubMed] [Google Scholar]
  • 10.Bai Y., Zhao J., Lv Z., Lu K. Enhanced piezocatalytic performance of ZnO nanosheet microspheres by enriching the surface oxygen vacancies. J. Mater. Sci. 2020;55(29):14112–14124. [Google Scholar]
  • 11.Zhang L., Zhu D., He H., Wang Q., Xing L., Xue X. Enhanced piezo/solar-photocatalytic activity of Ag/ZnO nanotetrapods arising from the coupling of surface plasmon resonance and piezophototronic effect. J. Phys. Chem. Solids. 2017;102:27–33. [Google Scholar]
  • 12.Zhang C., Fei W., Wang H. P-n Heterojunction of BiOI/ZnO nanorod arrays for piezo-photocatalytic degradation of Bisphenol A in water. J. Hazard. Mater. 2020;399 doi: 10.1016/j.jhazmat.2020.123109. [DOI] [PubMed] [Google Scholar]
  • 13.Ren Z., Li X., Guo L., Wu J.Z., Li Y.Q., Liu W.L., Li P., Fu Y.M., Ma J. Facile synthesis of ZnO/ZnS heterojunction nanoarrays for enhanced piezo-photocatalytic performance. Mater. Lett. 2021;292 [Google Scholar]
  • 14.Zhou X.F., Yan F., Wu S.H., Shen B., Zeng H.R., Zhai J.W. Remarkable piezophoto coupling catalysis behavior of BiOX/BaTiO3 (X = Cl, Br, Cl0.166Br 0.834) piezoelectric composites. Small. 2020;16:2001573. doi: 10.1002/smll.202001573. [DOI] [PubMed] [Google Scholar]
  • 15.Chen L., Dai X.Q., Li X.J., Wang J.F., Chen H.F., Hu X., Lin H.J., He Y.M., Wu Y., Fan M.H. Novel Bi2S3/KTa0.75Nb0.25O3 nanocomposite with high efficiency for photocatalytic and piezocatalytic N2 fixation. J. Mater. Chem. A. 2021;9:13344–13354. [Google Scholar]
  • 16.Hong D., Zang D.W., Guo X. High Piezo-Photocatalytic Efficiency of CuS/ZnO nanowires co-using solar and mechanical energy for degrading organic dye. ACS Appl. Mater. Interfaces. 2016;8:21302–21314. doi: 10.1021/acsami.6b05252. [DOI] [PubMed] [Google Scholar]
  • 17.Yu J., Chen Z., Chen Q., Wang Y., Lin H., Hu X., Zhao L., He Y. Giant enhancement of photocatalytic H2 production over KNbO3 photocatalyst obtained via carbon doping and MoS2 decoration. Int. J. Hydrogen Energy. 2018;43(9):4347–4354. [Google Scholar]
  • 18.Wermuth T.B., Arcaro S., Venturini J., Ribeiro T.M.H., Rodriguez A.A.L., Machado E.L., Oliveira T.F., Oliveira S.E.F., Baibich M.N., Bergmann C.P. Microwave-synthesized KNbO3 perovskites: photocatalytic pathway on the degradation of rhodamine B. Ceram. Int. 2019;45:24137–24145. [Google Scholar]
  • 19.Qu Z., Wang J., Tang J., Shu X., Liu X., Zhang Z., Wang J. Carbon quantum dots/KNbO3 hybrid composites with enhanced visible-light driven photocatalytic activity toward dye waste-water degradation and hydrogen production. Mol. Catal. 2018;445:1–11. [Google Scholar]
  • 20.Zhang W., Xing P., Zhang J., Chen L., Yang J., Hu X., Zhao L., Wu Y., He Y. Facile preparation of novel nickel sulfide modified KNbO3 heterojunction composite and its enhanced performance in photocatalytic nitrogen fixation. J. Colloid Interf. Sci. 2021;590:548–560. doi: 10.1016/j.jcis.2021.01.086. [DOI] [PubMed] [Google Scholar]
  • 21.Jia S.F., Su Y.P., Zhang B.P., Zhao Z.C., Li S., Zhang Y.F., Li P.C., Xu M.Y., Ren R. Few-layer MoS2 nanosheet-coated KNbO3 nanowire heterostructures: piezo-photocatalytic effect enhanced hydrogen production and organic pollutant degradation. Nanoscale. 2019;11:7690–7700. doi: 10.1039/c9nr00246d. [DOI] [PubMed] [Google Scholar]
  • 22.Zhang Y.H., Shen G.G., Sheng C.H., Zhang F., Fan W. The effect of piezo-photocatalysis on enhancing the charge carrier separation in BaTiO3/KNbO3 heterostructure photocatalyst. Appl. Surf. Sci. 2021;562 [Google Scholar]
  • 23.Ma Y.Y., Shen Y.Q., Gao X., Wu S.Y., Zhou Z.X. First-principles investigation on hydrogen evolution reaction in KNbO3(100)/g-C3N4 heterojunction. Appl. Catal. A: Gen. 2019;58 [Google Scholar]
  • 24.Ye X., Zhao S., Meng S., Fu X., Bai L., Guo Y., Wang X., Chen S. Remarkable enhancement of photocatalytic performance via constructing a novel Z-scheme KNbO3/Bi2O3 hybrid material. Mater. Res. Bull. 2017;94:352–360. [Google Scholar]
  • 25.Shi H.F., Zhang C.L., Zhou C.P., Chen G.Q. Conversion of CO2 into renewable fuel over Pt–g-C3N4/KNbO3 composite photocatalyst. RSC Adv. 2015;5:93615–93622. [Google Scholar]
  • 26.Xing P.X., Zhang W.Q., Chen L., Dai X.Q., Zhang J.L., Zhao L.H., He Y.M. Preparation of NiO/KNbO3 nanocomposite via photodeposition method and its superior performance in photocatalytic N2 fixation. Sustain. Energy Fuels. 2020;4:1112–1117. [Google Scholar]
  • 27.Shi Z.T., Qi X.Q., Zhang Z., Zhang J., Guo C., Liu K., Wei X., Jun X., Zhu Z. Facile synthesis of ZnO/PdSe2 core-shell heterojunction for efficient photodetector application. Chem. Eng. J. 2021;413 [Google Scholar]
  • 28.Chen Z., Chen P., Xing P., Hu X., Lin H., Wu Y., Zhao L., He Y. Novel carbon modified KTa0.75Nb0.25O3 nanocubes with excellent efficiency in photocatalytic H2 evolution. Fuel. 2018;233:486–496. [Google Scholar]
  • 29.Hansen M., Truong J., Xie T.N., Hahm J.I. Spatially distinct Raman scattering characteristics of individual ZnO nanorods under controlled polarization: intense end scattering from forbidden modes. Nanoscale. 2017;9:8470–8480. doi: 10.1039/c7nr02672b. [DOI] [PubMed] [Google Scholar]
  • 30.Kang Q.M., Ren T.J., Zhao W.Q., Wang H., Zhang A.X., Xu Y. Preparation of Zn2GeO4 nanosheets with MIL-125(Ti) hybrid photocatalyst for improved photodegradation of organic pollutants. Mater. Res. Bull. 2021;133 [Google Scholar]
  • 31.Dai X., Chen L., Li Z., Li X., Wang J., Hu X., Zhao L., Jia Y., Sun S.-X., Wu Y., He Y. CuS/KTa0.75Nb0.25O3 nanocomposite utilizing solar and mechanical energy for catalytic N2 fixation. J. Colloid Interf. Sci. 2021;603:220–232. doi: 10.1016/j.jcis.2021.06.107. [DOI] [PubMed] [Google Scholar]
  • 32.You Z., Wu C., Shen Q., Yu Y., Chen H., Su Y., Wang H., Wu C., Zhang F., Yang H. A novel efficient g-C3N4@ BiOI p–n heterojunction photocatalyst constructed through the assembly of g-C3N4 nanoparticles. Dalton Trans. 2018;47(21):7353–7361. doi: 10.1039/c8dt01322e. [DOI] [PubMed] [Google Scholar]
  • 33.Cao J., Xu B., Lin H., Chen S. Highly improved visible light photocatalytic activity of BiPO4 through fabricating a novel p–n heterojunction BiOI/BiPO4 nanocomposite. Chem. Eng. J. 2013;228:482–488. [Google Scholar]
  • 34.Chen Y., Jiang Z., Xu L., Liu C., Cheng Y., Zou Y., Zhang Q. Composite magnetic photocatalyst Bi24O31Br 10/NiFe2O4: hydrothermal preparation, characterization and photocatalytic mechanism. Mater. Sci. Semicon. Proc. 2021;126:105669. [Google Scholar]
  • 35.Yu J., Chen Z., Zeng L., Ma Y., Feng Z., Wu Y., Lin H., Zhao L., He Y. Synthesis of carbon-doped KNbO3 photocatalyst with excellent performance for photocatalytic hydrogen production. Sol. Energy Mat. Sol. C. 2018;179:45–56. [Google Scholar]
  • 36.Ma Y., Chen Y., Feng Z., Zeng L., Chen Q., Jin R., Lu Y., Huang Y., Wu Y., He Y. Preparation, characterization of Bi3O4Cl/g-C3N4 composite and its photocatalytic activity in dye degradation. J. Water Process Eng. 2017;18:65–72. [Google Scholar]
  • 37.Ismail M., Wu Z., Zhang L., Ma J., Jia Y., Hu Y., Wang Y. High-efficient synergy of piezocatalysis and photocatalysis in bismuth oxychloride nanomaterial for dye decomposition. Chemosphere. 2019;228:212–218. doi: 10.1016/j.chemosphere.2019.04.121. [DOI] [PubMed] [Google Scholar]
  • 38.Shao X., Xin W.Y., Yin X.H. Hydrothermal synthesis of ZnO quantum dot/KNb3O8 nanosheet photocatalysts for reducing carbon dioxide to methanol. Beilstein J. Nanotechnol. 2017;8:2264–2270. doi: 10.3762/bjnano.8.226. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Xing P., Chen P., Chen Z., Hu X., Lin H., Wu Y., Zhao L., He Y. Novel ternary MoS2/C-ZnO composite with efficient performance in photocatalytic NH3 synthesis under simulated sunlight. ACS Sustain. Chem. Eng. 2018;6(11):14866–14879. [Google Scholar]
  • 40.Xu J.J., Liu C., Niu J.F., Chen M.D. Preparation of In2S3 nanosheets decorated KNbO3 nanocubes composite photocatalysts with significantly enhanced activity under visible light irradiation. Sep. Purif. Technol. 2020;230 [Google Scholar]
  • 41.Xu H., Li H., Wu C., Chu J., Yan Y., Shu H., Gu Z. Preparation, characterization and photocatalytic properties of Cu-loaded BiVO4. J. Hazard. Mater. 2008;153(1-2):877–884. doi: 10.1016/j.jhazmat.2007.09.039. [DOI] [PubMed] [Google Scholar]
  • 42.Alizadeh M., Tong G.B., Qadir K.W., Mehmood M.S., Rasuli R. Cu2O/InGaN heterojunction thin films with enhanced photoelectrochemical activity for solar water splitting. Renew. Energ. 2020;156:602–609. [Google Scholar]
  • 43.Lei H., Zhang H.H., Zou Y., Dong X.P., Jia Y.M., Wang F.F. Synergetic photocatalysis/piezocatalysis of bismuth oxybromide for degradation of organic pollutants. J. Alloys Compd. 2019;809 [Google Scholar]
  • 44.Wu J., Qin N., Bao D. Effective enhancement of piezocatalytic activity of BaTiO3 nanowires under ultrasonic vibration. Nano Energy. 2018;45:44–51. [Google Scholar]
  • 45.Lan S.Y., Zeng X.K., Rathera R.A., Lo I.M.C. Enhanced trimethoxypyrimidine degradation by piezophotocatalysis of BaTiO3/Ag3PO4 using mechanical vibration and visible light simultaneously. Environ. Sci.: Nano. 2019;6:554–564. [Google Scholar]
  • 46.Biswas A., Saha S., Jana N.R. ZnSnO3–hBN nanocomposite-based piezocatalyst: ultrasound assisted reactive oxygen species generation for degradation of organic pollutants. New J. Chem. 2020;44:9278–9287. [Google Scholar]
  • 47.Huang X.Y., Lei R., Yuan J., Gao F., Jiang C.K., Feng W.H., Zhuang J.D., Liu P. Insight into the piezo-photo coupling effect of PbTiO3/CdS composites for piezo-photocatalytic hydrogen production. Appl. Catal. B: Environ. 2021;282 [Google Scholar]
  • 48.Vinesh V., Ashokkumar M., Neppolian B. rGO supported self-assembly of 2D nano sheet of (g-C3N4) into rod-like nano structure and its application in sonophotocatalytic degradation of an antibiotic. Ultrason. Sonochem. 2020;68 doi: 10.1016/j.ultsonch.2020.105218. [DOI] [PubMed] [Google Scholar]
  • 49.Ha L.P.P., Vinh T.H.T., Thuy N.T.B., Thi C.M., Viet P.V. Visible-light-driven photocatalysis of anisotropic silver nanoparticles decorated on ZnO nanorods: synthesis and characterizations. J. Environ. Chem. Eng. 2021;9:10510. [Google Scholar]
  • 50.Maimaitizi H., Abulizi A., Zhang T., Okitsu K., Zhu J.J. Facile photo-ultrasonic assisted synthesis of flower-like Pt/N-MoS2 microsphere as an efficient sonophotocatalyst for nitrogen fixation. Ultrason. Sonochem. 2020;63 doi: 10.1016/j.ultsonch.2019.104956. [DOI] [PubMed] [Google Scholar]
  • 51.Chen L., Zhang W.Q., Wang J.F., Li X.J., Li Y., Hu X., Zhao L.H., Wu Y., He Y.M. High piezo/photocatalytic efficiency of Ag/Bi5O7I nanocomposite using mechanical and solar energy for N2 fixation and methyl orange degradation. Green Energy Environ. 2021 doi: 10.1016/j.gee.2021.04.009. [DOI] [Google Scholar]
  • 52.Kuila A., Saravanan P., Bahnemann D., Wang C. Novel Ag decorated, BiOCl surface doped AgVO3 nanobelt ternary composite with Z-scheme homojunction-heterojunction interface for high prolific photo switching, quantum efficiency and hole mediated photocatalysis. Appl. Catal. B: Environ. 2021;293 [Google Scholar]
  • 53.El-Bery H.M., Abdelhamid H.N. Photocatalytic hydrogen generation via water splitting using ZIF-67 derived Co3O4@C/TiO2. J. Environ. Chem. Eng. 2021;9 [Google Scholar]
  • 54.Lin P., Pan C., Wang Z.L. Two-dimensional nanomaterials for novel piezotronics and piezophototronics. Mater. Today Nano. 2018;4:17–31. [Google Scholar]
  • 55.Li Z., Zhang Q., Wang L., Yang J., Wu Y., He Y. Novel application of Ag/PbBiO2I nanocomposite in piezocatalytic degradation of rhodamine B via harvesting ultrasonic vibration energy. Ultrason. Sonochem. 2021;78:105729. doi: 10.1016/j.ultsonch.2021.105729. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 56.Cheng L., Huang D., Zhang Y., Wu Y. Preparation and piezoelectric catalytic performance of HT-Bi2MoO6 microspheres for dye degradation. Adv. Powder Technol. 2021;32(9):3346–3354. doi: 10.1016/j.apt.2021.07.021. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary data 1
mmc1.docx (222.4KB, docx)

Articles from Ultrasonics Sonochemistry are provided here courtesy of Elsevier

RESOURCES