**3. Results and Discussion**

X-ray diffraction patterns (XRD) of BiOCl, NaNbO3, and a series of BiOCl/NaNbO3 piezoelectric composites are shown in Figure 1a. The distinct diffraction peaks of pure BiOCl can be related to tetragonal BiOCl (PDF card no. 82–0485, space group: P4/nmm), and the diffraction peaks of pure NaNbO3 can be indexed to orthorhombic NaNbO3 (PDF card no. 77–0873, space group: P21ma). As for BiOCl/NaNbO3 piezoelectric composites (BN-1, BN-2, BN-3, BN-4), there are both BiOCl and NaNbO3 peaks can be observed. In addition, the crystallite sizes were calculated by Scherrer formula: *<sup>D</sup>* <sup>=</sup> *<sup>K</sup><sup>λ</sup> <sup>β</sup>cos<sup>θ</sup>* , where *<sup>D</sup>* is crystallite size (nm), *K* is 0.9 (Scherrer constant), *λ* is 0.15406 nm (wavelength of the X-ray sources). The average crystallite sizes of BiOCl, NaNbO3, and BN-3 are 57, 22, and 56 nm. With the increase of NaNbO3 content, the diffraction peaks increased. The UV–vis diffuse reflectance spectra (DRS) of BiOCl, NaNbO3, and BN-3 are exhibited in Figure 1b, which indicate the absorbance threshold of NaNbO3, BiOCl, and BN-3 are the same. The estimated band gaps (*Eg*) of BiOCl and NaNbO3 are computed in Figure 1c by (*Ahv*) 2/*<sup>n</sup>* <sup>∼</sup> *hv* <sup>−</sup> *Eg*, where *A* is for absorbance, *hv* is for irradiation energy [20], and the obtained values are 3.44 and 3.52 eV, respectively. The valance band X-ray photoelectron spectroscopy (VB XPS) spectra in Figure 1d show that the valance band values of BiOCl and NaNbO3 are 2.57 and 2.50 eV. Together with the band gaps, the conductive band (CB) position can be calculated by *EVB* = *ECB* − *Eg*, which are −0.87 (BiOCl) and −1.02 eV (NaNbO3).

The morphology and microstructure of the BN-3 powder were investigated by scanning electron microscopy (SEM), element mapping and transmission electron microscope (TEM), and the results were shown in Figure 2. From Figure 2a, the irregular particles can be observed and the distribution of the corresponding main elements are shown in Figure 2b–f. The different colored areas suggest that Nb-, Na-, O-, Bi-, and Cl-enriched areas of the BN-3 composite, respectively. The TEM image of the BN-3 powder is displayed in Figure 2g. The lattice spacing of 0.343 and 0.273 nm in Figure 2h–i are corresponding to the (101) of BiOCl and (121) plane of NaNbO3, respectively. The result agrees well with that in the XRD patterns as shown in Figure 1a.

**Figure 1.** (**a**) XRD patterns of BiOCl, NaNbO3, and BiOCl/NaNbO3 piezoelectric composites; (**b**) UV– vis absorption spectra; (**c**) the estimated band gaps of BiOCl and NaNbO3; (**d**) VB XPS spectra of BiOCl and NaNbO3.

X-ray photoelectron spectroscopy (XPS) spectra of the BiOCl, NaNbO3, and BN-3 piezoelectric composite are shown in Figure 3. From Figure 3a, the peaks of Bi 4f of BiOCl (BN-3) located at 159.60 (159.24 eV) and 164.90 eV (164.52 eV) can be assigned to Bi 4f7/2 and Bi 4f5/2, respectively, suggesting the Bi3+ exists in the BiOCl (BN-3). In Figure 3b, the Cl 2p peaks at 198.29 (197.90 eV) and 199.93 eV (199.54 eV) can be attributed to Cl 2p3/2 and Cl 2p1/2, respectively, which indicate the Cl<sup>−</sup> in BiOCl (BN-3) [21]. The peak at 1070.62 eV (1071.42 eV) in Figure 3c is ascribed to Na 1s in NaNbO3 (BN-3). In Figure 3d, it is clearly seen that the binding energies located at 206.67 (207.05 eV) and 209.40 eV (209.78 eV) belong to Nb 3d5/2 and Nb 3d3/2, respectively, reflecting that Nb is in the Nb (+5) chemical state [22]. As shown in Figure 3e, the peaks located at 530.39 (BiOCl), 529.60 (NaNbO3), and 529.98 eV (BN-3) correspond to O 1s. Compared with BiOCl, the blue shift of all peaks for BN-3 can be observed, while the red shift compared with NaNbO3. The XPS survey spectra also indicate that BiOCl is composed of Bi, O, and Cl elements, and NaNbO3 is mainly composed of Na, O, and Nb elements, while BN-3 contains all elements above, as shown in Figure 3f. In short, the XPS results demonstrate that the BN-3 piezoelectric composite is composed of BiOCl and NaNbO3.

The polarization electric field hysteresis loop (P-E) and electric-field-induced strain (S-E) curves of BN-3 composite are displayed in Figure 4. From Figure 4a, a saturated and nearly squared P-E loop can be observed, and the remnant polarization (Pr) is 35.13 μC/cm<sup>2</sup> and the coercive field (Ec) is 8.72 kV/mm. The result shows that BN-3 composite has well ferroelectric properties, favoring the spatial separation and transportation of photo-induced carriers [23]. The S-E curve in Figure 4b exhibits an asymmetric butterfly shape, confirming the piezoelectricity of the BN-3 composite [24,25].

**Figure 2.** (**a**) SEM image; (**b**–**f**) EDS element mappings; (**g**) TEM image; (**h**,**i**) lattice fringes images of BN-3.

Consequently, the piezo/photocatalytic activities of BiOCl, NaNbO3, and BiOCl/ NaNbO3 piezoelectric composites were evaluated by the degradation of Rhodamine B (RhB) under the condition of light irradiation and ultrasound. From Figure 5a, BN-3 exhibits better piezo/photocatalytic performance than that of BiOCl, NaNbO3, and other content BiOCl/NaNbO3 composites. The rate constant *k* values are obtained from Figure 5a via the pseudo-first-order equation [26]: ln(*C*0/*Ct*) = −*kt*, where *C*<sup>0</sup> is RhB concentration for initial and *Ct* is for after irradiation time *t*. And the decomposition ratio is calculated via the formula: *η* = <sup>1</sup> <sup>−</sup> *Ct C*0 × 100%. As shown in Figure 5b, the apparent reaction rate constant *k* for BiOCl, BN-1, BN-2, BN-3, BN-4, and NaNbO3 is 0.0089, 0.0112, 0.0134, 0.0192, 0.0168, and 0.0037 min−1, respectively. The piezo/photodegradation rate of RhB for BN-3 is 2.2 and 5.2 times higher than that of BiOCl and NaNbO3, the histogram in Figure 5c reflects this directly. The degradation percentages of BiOCl, NaNbO3 and BN-3 are 29.8%, 61.9% and 87.4%, respectively. In addition, the BET surface areas of BiOCl, NaNbO3, and BN-3 are 0.24, 2.00, and 1.32 m2/g, and the pore volumes are 0.0008, 0.0088, and 0.0037 cm3/g, respectively. The BET surface areas of the samples are in the same order of magnitude, which means the BET surface areas not can decisive the catalytic activity. This result indicates the heterojunction in BN-3 exerts a tremendous advantage on the piezo/photocatalytic process. To investigate the most reactive species during the process of RhB decomposition, the radical trapping experiments were carried out in the presence of BN-3 as a catalyst. From Figure 5d, the piezo-photodegradation efficiency of RhB is remarkably inhibited while adding the triethanolamine (TEOA, 50 μL) scavenger for

trapping hole (h+) to the mixed solution, demonstrating an important role of h+ in the piezophotocatalytic process. While L-ascorbic acid (VC, 40 mg) for superoxide radical (·O2 −) was added, the degradation efficiency also decreased rapidly. The RhB degradation efficiency is decreased slightly by adding the isopropanol (IPA, 50 μL), reflecting the hydroxyl radical (·OH) plays a secondary role in this process. These results indicate that the effect in this piezo-photocatalytic process is: h<sup>+</sup> <sup>&</sup>gt;·O2 <sup>−</sup> >·OH.

**Figure 3.** XPS survey spectra of BiOCl, NaNbO3, and BN-3: (**a**) Bi 4f; (**b**) Cl 2p; (**c**) Na 1s; (**d**) Nb 3d; (**e**) O 1s; (**f**) Survey.

**Figure 4.** (**a**) The ferroelectric P-E loop and (**b**) electric field-induced S-E curve of BN-3 composite.

**Figure 5.** (**a**) The kinetic curves of piezo-photodegradation RhB performance for BiOCl, NaNbO3, and BiOCl/NaNbO3 piezoelectric composites; (**b**) the dynamics of degradation reaction [(−ln(*Ct*/*C*0)]; (**c**) the histogram of corresponding reaction rate constant; (**d**) piezo-photodegradation curves with disparate scavengers of BN-3 composite.

To demonstrate the piezocatalysis, photocatalysis, and the synergy effect of piezocatalysis and photocatalysis of BN-3 piezoelectric composite, the RhB degradation capability within 100 min was measured under the condition of ultrasound only, light only, and ultrasound + light together. UV–vis absorption spectra of RhB for BN-3 under different conditions are shown in Figure 6a,c,e, which correspond to light, ultrasound, and both light and ultrasound, respectively. From Figure 6b,d,f, the degradation rate is the lowest under the condition of only ultrasound, the rate constant *k* (0.0005 min−1) and decomposition ratio (9.9%) are well below those of the condition of only light (0.0044 min−<sup>1</sup> and 40.4%) and light + ultrasound together (0.0192 min−<sup>1</sup> and 87.4%). The decomposition ratio of RhB under synergy of piezocatalysis and photocatalysis is 8.8 and 2.2 times higher than that of piezocatalysis and photocatalysis, respectively. The rate constant *k* under synergy of

piezocatalysis and photocatalysis is 38.4 and 4.36 times higher than that of only ultrasound and only light. In addition, based on the same condition, compared to other piezoelectric materials past reported, the k value of BN-3 is higher than that of NaNbO3/CuBi2O4 nanocomposites (0.0112 min<sup>−</sup>1) [27], and closing to that of BaTiO3/KNbO3 heterostructure (0.01492 min−1) [28]. The result confirms that the synergy effect of piezocatalysis and photocatalysis of BiOCl/NaNbO3 piezoelectric composite plays an important role in the highly efficient degradation of RhB. One of the key parameters in the piezo-photocatalyst is reproducibility, and Figure 7 shows the cycling performance of the piezo-photocatalytic activity of BN-3 for degrading RhB. After three cycles, the degradation efficiency is just reduced a little. This result evidences that BN-3 possesses a high reproducibility.

**Figure 6.** UV–vis spectral absorption of RhB for BN-3 under the condition of (**a**) only light, (**c**) only ultrasound and (**e**) light + ultrasound; (**b**) the kinetic curves of RhB degradation for BN-3 under these three control conditions; (**d**) the dynamics of degradation reaction [(−ln(Ct/C0)]; (**f**) the histogram of corresponding reaction rate constant and decomposition ratio.

On the basis of the above analysis, the possible mechanism for piezocatalytic, photocatalytic, and their synergetic catalytic process of BiOCl/NaNbO3 piezoelectric composites are shown in Figure 8. According to our experiment, the valance band of NaNbO3 is 2.50 eV, while BiOCl is 2.57 eV; the conductive band of NaNbO3 is −1.02 eV, while BiOCl is −0.87 eV. In the condition of only light, the photoelectrons are excited from the valance band to the conductive band, and the electrons will transfer from the conductive band of NaNbO3 to the conductive band of BiOCl, and thus build an inner electric field. The built-in electric field can promote the separation of electrons and holes. However, there is still a combination of electrons and holes in the inner of BiOCl/NaNbO3 piezoelectric composites because the built-in electric field is easily prone to be screened by electrostatic compensated free space charges [29]. This reduces the degradation efficiency of RhB. In

the condition of only ultrasound, the cavitation bubbles will form, expand, and burst, an amount of electric charge can be generated [30,31]. These positive and negative charges will transfer to the opposite directions under the influence of the alternating built-in electric field. In the condition of both light and ultrasound, the electrons and holes located at the conductive band and valance band will transfer to the opposite directions under the internal piezoelectric potential, causing electrons to accumulate in the conductive band of BiOCl and holes accumulate in the valance band of NaNbO3 [32–34]. Subsequently, the electrons on the CB of BiOCl combined with the absorbed O2 to produce ·O2 −. Meanwhile, part holes on the VB of NaNbO3 will oxidize hydroxyl to form ·OH. Finally, the reactive species ·OH, h+, and ·O2 − will participate in the oxidative degradation of RhB. The combination rate of electrons and holes will be reduced significantly under the built-in electric field, thus the decomposition ratio of BiOCl/NaNbO3 piezoelectric composite increased remarkably.

**Figure 7.** The cycling performance of the piezo-photocatalytic activity of BN-3 for degrading RhB solution.

**Figure 8.** Possible piezocatalytic, photocatalytic, and piezo-photocatalytic mechanism of BiOCl/ NaNbO3 piezoelectric composites.
