**1. Introduction**

There is increasing social concern over the current energy and environmental issues, especially wastewater pollution (originating from rapid industrial development), which poses a direct threat to human health [1,2]. Researchers are seeking effective, cost-efficient, stable, and safe ways to degrade and remove hazardous compounds in water. Semiconductor photocatalysis is one of the prominent strategies, not only converting sustainable solar energy into hydrogen energy but also utilizing visible light to degrade organic pollutants. A large number of semiconductor photocatalysts and their composites have been reported to improve the water splitting or pollutant degradation performance, such as ZnO and WO3 [3–5]. Among the various semiconductors, bismuth oxychloride (BiOCl) as a p-type bismuth oxyhalides semiconductor has an open layered structure composing of [Bi2O2] 2+ layers sandwiched between [Cl2] <sup>2</sup><sup>−</sup> plates, which can facilitate the separation of photo-produced electrons and holes due to the more space to polarize the atoms and orbitals involved [6]. However, BiOCl is limited by its wide band gap (~3.4 eV), representing favorable photocatalytic performance only under ultraviolet light irradiation. Therefore, heterojunction–construction engineering is one of the most powerful strategies to achieve a broader photoresponse and improved photocatalytic activity by constructing heterostructured BiOCl photocatalysts [7–9].

In addition to photocatalysis, piezoelectric catalysis is also an efficient and environmentally friendly dye degradation method [10–12]. Vibration is one of the most common

**Citation:** Li, L.; Cao, W.; Yao, J.; Liu, W.; Li, F.; Wang, C. Synergistic Piezo-Photocatalysis of BiOCl/ NaNbO3 Heterojunction Piezoelectric Composite for High-Efficient Organic Pollutant Degradation. *Nanomaterials* **2022**, *12*, 353. https://doi.org/ 10.3390/nano12030353

Academic Editor: Dong-Joo Kim

Received: 30 December 2021 Accepted: 20 January 2022 Published: 22 January 2022

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sources of energy in our environment. In piezoelectric catalysis, mechanical vibration generates an electric charge on the surface of the piezoelectric catalyst through the piezoelectric effect, which in turn reacts with the dye molecules, leading to the decomposition of the dye [13,14]. Combining with photocatalysis, piezoelectric materials possess the ability to reduce the carrier recombination rate due to the existence of an internal piezoelectric field in piezoelectric materials. In past decades, perovskite niobates, including NaNbO3, have attracted intensive attention because of their nonlinear optics, ionic conductive, piezoelectric, and photocatalytic properties [15–17]. In addition, the piezo-photocatalysis and pyroelectric catalysis of NaNbO3 have been thoroughly studied in recent years. For example, Jia's group has found that NaNbO3 nanofibers possess a highly efficient piezoelectrically and pyroelectrically bi-catalysis for decomposition of organic dye [18]. It is worth noting that BiOCl is a piezoelectric material with piezocatalytic activity in response to ultrasound. Jia's group have also reported that BiOCl shows highly efficient dye wastewater decomposition under the condition of light (300 W Xenon lamp) and ultrasound (120 W, 40 kHz) together, which is much greater than that of only with light or only with ultrasound, respectively [19]. However, there are not any reports for piezo-photocatalysis of BiOCl/NaNbO3 heterojunction piezoelectric composite. Therefore, it is necessary to explore the mechanism of BiOCl/NaNbO3 piezoelectric composite for enhancing degradation efficiency by using the synergistic effect of piezoelectric catalysis and photocatalysis.

In this work, the BiOCl/NaNbO3 piezoelectric composites were prepared by the hydrothermal method. The polarization electric field hysteresis loop (P-E) and electric-fieldinduced strain (S-E) curves confirm the BiOCl/NaNbO3 composite has good ferroelectric and piezoelectric properties. The catalytic performance of BiOCl/NaNbO3 piezoelectric composite was remarkably enhanced by the heterojunction construction and the synergy effect of piezocatalysis and photocatalysis, which greatly promote the separation of electronhole pairs under electric field.
