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## *Article* **Fabrication of Novel g-C3N4@Bi/Bi2O2CO3 Z-Scheme Heterojunction with Meliorated Light Absorption and Efficient Charge Separation for Superior Photocatalytic Performance**

**Hongxia Fan, Xiaohui Ma, Xinyang Li, Li Yang, Yongzhong Bian \* and Wenjun Li \***

Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, University of Science and Technology Beijing, Beijing 100083, China

**\*** Correspondence<EMAIL_ADDRESS>(Y.B<EMAIL_ADDRESS>(W.L.)

**Abstract:** Herein, a novel g-C3N4@Bi/Bi2O2CO3 Z-scheme heterojunction was synthesized via simple methods. UV/Vis diffuse reflectance spectroscopy (DRS) revealed that the visible light absorption range of heterojunction composites was broadened from 400 nm to 500 nm compared to bare Bi2O2CO3 . The XRD, XPS and TEM results demonstrated that metal Bi was introduced into g-C3N4@Bi/Bi2O2CO3 composites, and Bi may act as an electronic bridge in the heterojunction. Metal Bi elevated the separation efficiency of carriers, which was demonstrated by photocurrent and photoluminescence. The performance of samples was assessed via the degradation of Rhodamine B (RhB), and the results exhibited that g-C3N4@Bi/Bi2O2CO3 possessed notably boosted photocatalytic activity compared with g-C3N4 , Bi2O2CO3 and other binary composites. The heterojunction photocatalysts possessed good photostability and recyclability in triplicate cycling tests. Radical trapping studies identified that h+ and •O2 − were two primary active species during the degradation reaction. Based on the energy band position and trapping radical experiments, the possible reaction mechanism of the indirect Z-scheme heterojunction was also proposed. This work could provide an effective reference to design and establish a heterojunction for improving the photocatalytic activity of Bi2O2CO3 .

**Keywords:** photocatalytic; Bi2O2CO3 ; g-C3N4 ; Z-scheme heterojunction

### **1. Introduction**

With the increasingly serious problem of environmental pollution, people have been looking for available approaches to solve pollution challenges [1,2]. Advanced oxidation processes and photocatalysis in water treatment receive more and more attention because they could utilize solar energy to degrade organic pollutants [3,4]. Photocatalytic technology uses photocatalysts to produce active species (e.g., h+ , •O2 −, •OH) to oxidize organic pollutants in the photocatalysis process. Therefore, the development of efficient visible light photocatalysts is decisive for photocatalytic technology [5,6]. Recently, bismuth-based photocatalysts have attracted extensive attention, such as BiVO4 [7,8], Bi2O2CO3 [9,10], BiOX [11,12], and Bi2O3 [13,14]. Among them, Bi2O2CO3 (BOC) is deemed to be an attractive semiconductor for degrading organic dyes [15,16]. Deplorably, the poor activity of unmodified Bi2O2CO3 limits its environmental application because of the wide band gap. At present, many efforts have been attempted to improve the photocatalytic activity of Bi2O2CO3, such as by controlling morphology [17,18], constructing heterojunctions [19,20], and loading precious metals [21]. Constructing a heterojunction is considered as an ideal method because it can expand light absorption and improve carrier separation efficiency at the same time. Gao et al. [19] reported that the p-n BiOI/Bi2O2CO3 heterojunction presented splendid photocatalytic performance for MB and RhB. Huang et al. [22] synthesized a Bi2O2CO3/Bi2WO6 heterostructure, which possessed outstanding photocatalytic

**Citation:** Fan, H.; Ma, X.; Li, X.; Yang, L.; Bian, Y.; Li, W. Fabrication of Novel g-C3N4@Bi/Bi2O2CO3 Z-Scheme Heterojunction with Meliorated Light Absorption and Efficient Charge Separation for Superior Photocatalytic Performance. *Molecules* **2022**, *27*, 8336. https:// doi.org/10.3390/molecules27238336

Academic Editor: Yucheng Lan

Received: 8 November 2022 Accepted: 25 November 2022 Published: 29 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

degradation activity for RhB. Even so, due to unsatisfactory activity, it is still necessary to explore other Bi2O2CO3-based photocatalysts for organic degradation.

More recently, g-C3N4 (CN) was applied for photocatalytic degradation because of its stability, excellent absorption properties, easy fabrication, and suitable band gap [23–25]. Based on the matched energy band levels of BOC and CN, they could form a direct Z-scheme heterojunction for elevated photodegradation activity. The direct Z-scheme heterojunctions are not sufficiently efficacious to boost carrier separation, resulting from the unsatisfactory e− transport ability of BOC. It has been reported that the use of metal (Au and Ag, etc.) as Z-scheme bridges to form indirect Z-scheme heterojunctions could further accelerate the separation efficiency of photocarriers and thus promote the photocatalytic activity of a direct Z-scheme heterojunction system [26,27]. Owing to its opportune work function and outstanding electrical conductivity, low-cost metal bismuth (Bi) is expected to become a substitute for precious metals as an electronic bridge and gradually come into people's view [28,29]. Additionally, a Bi bridge is acquired via the in situ reduction of BOC, which is awfully conducing to the constitution of Bi/BOC compact heterostructure. Thus, Bi as the bridge in the heterostructure g-C3N4@Bi/Bi2O2CO3 heterojunction is anticipated to remarkably promote charge separation for high activity. The photocatalytic degradation of the g-C3N4@Bi/Bi2O2CO3 heterojunction system has been not reported.

In this work, a novel g-C3N4@Bi/Bi2O2CO3 Z-scheme heterojunction was successfully constructed. Benefiting from the formed heterojunction, g-C3N4@Bi/Bi2O2CO3 composites possessed notably enhanced photocatalytic degradation activity of RhB due to promoted carriers separation efficiency and expanded photo-response range. We studied the g-C3N4@Bi/Bi2O2CO3 heterojunction system with an electron-conduction bridge under a photodegradation experiment.

### **2. Results and Discussion**

The microscopic structure and morphology of BOC, CN and BOC-CN-Bi photocatalysts are analyzed via SEM and EDS. Pure BOC is an irregular agglomerated grain structure (Figure 1A). As shown in Figure 1B, g-C3N4 possesses a sheet structure that is tens of nanometers thick. The microstructure of BOC-CN-Bi heterojunction composites is shown in Figure 1C. Figure 1E–H displays SEM images and elemental mapping images of BOC-CN-Bi composites, which manifests an even distribution of Bi, C, O and N elements. Table S1 (Supplementary Materials) shows that the Bi element is excessive, indicating the presence of metallic Bi in BOC-CN-Bi composites. The SEM results reveal that BOC-CN-Bi Z-scheme heterojunction composites are successfully prepared. Figure 2A–F shows TEM and HRTEM images of CN, BOC, and BOC-CN-Bi samples. The morphology shown by TEM images was in good agreement with the SEM results. In Figure 2D, CN possesses an amorphous structure as reported in the literature. Furthermore, pure BOC has good crystallinity and its (161) crystal plane spacing is 0.294 nm (Figure 2E). Interestingly, for BOC-CN-Bi (Figure 2F), both CN and BOC could be clearly observed. Meanwhile, metal Bi can also be found by reduction with EG. These results definitely illustrate the successful preparation of BOC-CN-Bi ternary heterostructure composites.

The electrochemical impedance spectroscopy (EIS) and photocurrent response were recorded via an electrochemical workstation (CHI660E). The detailed condition of the elec-

The degradation of RhB dyes was utilized to determine the photocatalytic degradation performance of all samples via a 400 W Xe lamp (≥420 nm). Before each experiment, 30 mg samples were adequately dispersed in a 30 mL RhB solution. In the dark, the obtained suspension solution was consumingly stirred for 1 h on behalf of an adsorptiondesorption equilibrium. During the photoreaction process, 3 mL solution was collected every 30 minutes and centrifuged. A UV-vis spectrophotometer was used to test the RhB

The microscopic structure and morphology of BOC, CN and BOC-CN-Bi photocatalysts are analyzed via SEM and EDS. Pure BOC is an irregular agglomerated grain structure (Figure 1A). As shown in Figure 1B, g-C3N4 possesses a sheet structure that is tens of nanometers thick. The microstructure of BOC-CN-Bi heterojunction composites is shown in Figure 1C. Figure 1E–H displays SEM images and elemental mapping images of BOC-CN-Bi composites, which manifests an even distribution of Bi, C, O and N elements. Table S1 (Supplementary Materials) shows that the Bi element is excessive, indicating the presence of metallic Bi in BOC-CN-Bi composites. The SEM results reveal that BOC-CN-Bi Zscheme heterojunction composites are successfully prepared. Figure 2A–F shows TEM and HRTEM images of CN, BOC, and BOC-CN-Bi samples. The morphology shown by TEM images was in good agreement with the SEM results. In Figure 2D, CN possesses an amorphous structure as reported in the literature. Furthermore, pure BOC has good crystallinity and its (161) crystal plane spacing is 0.294 nm (Figure 2E). Interestingly, for BOC-CN-Bi (Figure 2F), both CN and BOC could be clearly observed. Meanwhile, metal Bi can also be found by reduction with EG. These results definitely illustrate the successful prep-

trochemical tests is the same as we reported before.

aration of BOC-CN-Bi ternary heterostructure composites.

*2.3. Photocatalytic Experiments*

**3. Results and Discussion**

concentration in the culture at 553 nm.

**Figure 1.** The SEM of BOC (**A**), CN (**B**) and BOC-CN-Bi (**C**,**D**), the mapping image of BOC-CN-Bi in (**E**) (Bi), (**F**) (O), (**G**) (C) and (**H**) (N). **Figure 1.** The SEM of BOC (**A**), CN (**B**) and BOC-CN-Bi (**C**,**D**), the mapping image of BOC-CN-Bi in (**E**) (Bi), (**F**) (O), (**G**) (C) and (**H**) (N). *Molecules* **2022**, *27*, x FOR PEER REVIEW 4 of 13

**Figure 2.** The TEM and HRTEM of CN (**A**,**D**), BOC (**B**,**E**) and BOC-CN-Bi (**C**,**F**). **Figure 2.** The TEM and HRTEM of CN (**A**,**D**), BOC (**B**,**E**) and BOC-CN-Bi (**C**,**F**).

Figure 3 displays the XRD patterns of as-prepared BOC, CN, BOC-Bi and BOC-CN-Bi photocatalysts. In the XRD patterns of BOC, BOC-Bi and BOC-CN-Bi composites, the peaks of BOC match with the orthorhombic phase of BOC (JCPDS card No. 84-1752). Eleven major peaks at 12.91°, 23.88°, 26.07°, 30.27°, 32.64°, 39.49°, 42.23°, 46.89°, 52.28°, 56.84° and 68.62° are indexed to the planes of (0 4 0), (1 2 1), (0 8 0), (1 6 1), (0 0 2), (0 12 2), (2 8 0), (2 0 2), (1 11 2), (1 6 3) and (4 0 0), respectively. Simultaneously, compared to BOC, there is a new weak peak at 27.16° in the patterns of BOC-Bi and BOC-CN-Bi composites, which can be indexed to metallic Bi [30]. The peaks ((0 0 2) and (1 0 0)) of CN are consistent Figure 3 displays the XRD patterns of as-prepared BOC, CN, BOC-Bi and BOC-CN-Bi photocatalysts. In the XRD patterns of BOC, BOC-Bi and BOC-CN-Bi composites, the peaks of BOC match with the orthorhombic phase of BOC (JCPDS card No. 84-1752). Eleven major peaks at 12.91◦ , 23.88◦ , 26.07◦ , 30.27◦ , 32.64◦ , 39.49◦ , 42.23◦ , 46.89◦ , 52.28◦ , 56.84◦ and 68.62◦ are indexed to the planes of (0 4 0), (1 2 1), (0 8 0), (1 6 1), (0 0 2), (0 12 2), (2 8 0), (2 0 2), (1 11 2), (1 6 3) and (4 0 0), respectively. Simultaneously, compared to BOC, there is a new weak peak at 27.16◦ in the patterns of BOC-Bi and BOC-CN-Bi composites, which can be indexed to metallic Bi [30]. The peaks ((0 0 2) and (1 0 0)) of CN are consistent

with that reported in the literature [31]. The peaks of single BOC, CN and Bi could be observed in the patterns of BOC-CN-Bi composites. Additionally, no impurity peaks are

port SEM and EDS results and will be further determined by XPS analysis.

with that reported in the literature [31]. The peaks of single BOC, CN and Bi could be observed in the patterns of BOC-CN-Bi composites. Additionally, no impurity peaks are detected in the XRD patterns. The aforementioned results indicate that BOC-CN-Bi Z-scheme heterojunction composites have been successfully prepared. The XRD results support SEM and EDS results and will be further determined by XPS analysis.

The surface chemical compositions and the oxidation states of BOC-CN-Bi composites are analyzed via XPS, further confirming the coexistence of BOC, CN and Bi in heterojunction composites. The peaks at 164.3 and 158.9 eV correspond to Bi 4f7/2 and Bi 4f5/2, respectively (Figure 4A). The bismuth is a Bi3+ cation in the BOC-CN-Bi composite [32]. The peak located at 157.7 eV is attributed to metallic Bi in the BOC-CN-Bi composites. Figure 4B shows two typical peaks of C 1s situate at 288.1 and 284.8 eV, which associates with N-C=N and C-C, respectively. The N-C=N is caused by the sp2 -bonded carbon in the nitrogenous aromatic ring. The C-C bond is caused by amorphous or graphitic carbons. In Figure 4C, the N 1s peak of composites is fitted as three peaks at 398.3, 399.6 and 400.9 eV, corresponding to sp2-hybridized nitrogen involved in triazine rings (C=N-C), tertiary N bonded in N-(C)3 groups, and N-H groups, respectively [33,34]. As shown in Figure 4D, the peaks at 529.7, 530.7, and 532.8 eV are assigned to Bi-O binding, carbonate ions, and absorbed H2O on the surface [35], respectively. Obviously, the aforementioned results confirm the formation of BOC-CN-Bi Z-scheme heterojunction. *Molecules* **2022**, *27*, x FOR PEER REVIEW 5 of 13

**Figure 3.** XRD patterns of BOC, CN, BOC-Bi and BOC-CN-Bi photocatalysts (\* indicates peaks of metallic Bi). **Figure 3.** XRD patterns of BOC, CN, BOC-Bi and BOC-CN-Bi photocatalysts (\* indicates peaks of metallic Bi).

The surface chemical compositions and the oxidation states of BOC-CN-Bi composites are analyzed via XPS, further confirming the coexistence of BOC, CN and Bi in heterojunction composites. The peaks at 164.3 and 158.9 eV correspond to Bi 4f7/2 and Bi 4f5/2, respectively (Figure 4A). The bismuth is a Bi3+ cation in the BOC-CN-Bi composite [32]. The peak located at 157.7 eV is attributed to metallic Bi in the BOC-CN-Bi composites. Figure 4B shows two typical peaks of C 1s situate at 288.1 and 284.8 eV, which associates with N-C=N and C-C, respectively. The N-C=N is caused by the sp2-bonded carbon in the nitrogenous aromatic ring. The C-C bond is caused by amorphous or graphitic carbons. In Figure 4C, the N 1s peak of composites is fitted as three peaks at 398.3, 399.6 and 400.9 eV, corresponding to sp2-hybridized nitrogen involved in triazine rings (C=N-C), tertiary N bonded in N-(C)3 groups, and N-H groups, respectively [33,34]. As shown in Figure 4D, the peaks at 529.7, 530.7, and 532.8 eV are assigned to Bi-O binding, carbonate ions, and absorbed H2O on the surface [35], respectively. Obviously, the aforementioned results

confirm the formation of BOC-CN-Bi Z-scheme heterojunction.

**Figure 4.** XPS spectra: (**A**) Bi 4f, (**B**) C 1s, (**C**) N 1s and (**D**) O 1s of BOC-CN-Bi composites. **Figure 4.** XPS spectra: (**A**) Bi 4f, (**B**) C 1s, (**C**) N 1s and (**D**) O 1s of BOC-CN-Bi composites.

The UV-vis diffuse reflectance spectra are acquired to assess the optical response range of all samples. Pristine BOC and CN reveal severally conspicuous absorption edges at approximately 400 and 500 nm (Figure 5A). The absorption edges for BOC-Bi and CN-Bi possess a slight redshift compared to pristine BOC and CN, which suggests that metal Bi could slightly ameliorate the visible light absorption capacity of photocatalysts [36,37]. Intriguingly, BOC-CN-Bi ternary photocatalysts have a wider visible light absorption range than BOC and BOC-Bi, insinuating the likelihood of better photocatalytic performance for BOC-CN-Bi ternary photocatalysts. The wider visible light absorption range assures that BOC-CN-Bi could provoke ample photoinduced e−-h+ pairs. As presented in Figure 5B, the band gap of pristine BOC and CN are estimated as 3.17 and 2.38 eV via the Kubelka-Munk formula. Furthermore, the band gap of all composites is shown in Figure S1. The band gap for BOC-CN-Bi is 2.6 eV. Compared with BOC, the BOC-CN-Bi composite has a narrower band gap and higher carrier separation for efficient photocatalytic hy-The UV-vis diffuse reflectance spectra are acquired to assess the optical response range of all samples. Pristine BOC and CN reveal severally conspicuous absorption edges at approximately 400 and 500 nm (Figure 5A). The absorption edges for BOC-Bi and CN-Bi possess a slight redshift compared to pristine BOC and CN, which suggests that metal Bi could slightly ameliorate the visible light absorption capacity of photocatalysts [36,37]. Intriguingly, BOC-CN-Bi ternary photocatalysts have a wider visible light absorption range than BOC and BOC-Bi, insinuating the likelihood of better photocatalytic performance for BOC-CN-Bi ternary photocatalysts. The wider visible light absorption range assures that BOC-CN-Bi could provoke ample photoinduced e−-h+ pairs. As presented in Figure 5B, the band gap of pristine BOC and CN are estimated as 3.17 and 2.38 eV via the Kubelka-Munk formula. Furthermore, the band gap of all composites is shown in Figure S1. The band gap for BOC-CN-Bi is 2.6 eV. Compared with BOC, the BOC-CN-Bi composite has a narrower band gap and higher carrier separation for efficient photocatalytic hydrogen production. *Molecules* **2022**, *27*, x FOR PEER REVIEW 7 of 13

In Figure 6A, the photocatalytic performance of all photocatalysts is evaluated via the photodegradation of the rhodamine B (RhB) under visible light (≥420 nm). Before the

tion equilibrium. The value of RhB adsorption for all samples is shown in Table S2 (Supplementary Materials). Pristine BOC reveals an inconsequential photodegradation rate of 30% after 120 min because of its wide band gap. As for CN, only 50% of RhB is disintegrated after 120 min. However, after the introduction of metal Bi, the binary BOC-Bi and CN-Bi exhibit better degradation rates (40% and 60%, respectively). Simultaneously, the photodegradation rate of the RhB for the BOC-CN compound is 45%. Interestingly, the ternary composites (BOC-CN-Bi) reveal the best photocatalytic activity compared to other samples, which is approximately 93% after 120 min. Additionally, the ternary BOC-CN-Bi heterojunction composites still possess high photocatalytic performance for RhB degradation compared to the reported BOC-CN photocatalysts. Therefore, we can reach the conclusion that metal Bi has a remarkable effect on enhancing photocatalytic performance. To explore the kinetic reaction, the kinetics of RhB degradation for all samples are analyzed. The degradation data of all catalysts are linearly fitted according to the equation:

> Ln ቀ େ େబ

where C, C0 and k are the concentration of RhB under the different times, the concentration of RhB after the adsorption-desorption equilibrium, and the apparent reaction rate constant, respectively. As shown in Figure 6B, the relationship between Ln(C/C0) and t of all samples conforms to the first-order kinetic model. The k of every sample is estimated and shown (Figure 6B). Obviously, BOC-CN-Bi possesses the fastest photodegradation rate, which is 6.2 and 3.5 times than that of BOC and CN. The aforementioned results forcefully verified that BOC-CN-Bi ternary composites possess a superior photocatalytic activity. We also explored the effects of pH, dye concentration and catalyst amount on photocatalytic degradation (Figure S2). Obviously, the more photocatalytic content, the better the degradation effect. However, the degradation efficiency and the catalyst increase disproportionately. Moreover, the concentration of organic pollutants increased, and the degradation efficiency decreased slightly. When pH increased, the degradation efficiency decreased significantly. When pH decreased, the catalyst completely adsorbed organic pollutants after darkness, and the degradation efficiency could not be measured (Figure S3, Supplementary Materials). It is well known that the stability of photocatalysts is crucial in practical applications. Therefore, the cycling experiments of BOC-CN-Bi composites were carried out. As shown in Figure 6C, the photocatalytic performance is not significantly reduced in three cycles, which confirms the outstanding stability of the com-

ቁ ൌ kt a

**Figure 5.** The DRS (**A**) of the as-obtained samples; the band gaps (**B**) of BOC and CN. posites. **Figure 5.** The DRS (**A**) of the as-obtained samples; the band gaps (**B**) of BOC and CN.

In Figure 6A, the photocatalytic performance of all photocatalysts is evaluated via the photodegradation of the rhodamine B (RhB) under visible light (≥420 nm). Before the photoreaction process, all samples were kept in the dark to realize the adsorptiondesorption equilibrium. The value of RhB adsorption for all samples is shown in Table S2 (Supplementary Materials). Pristine BOC reveals an inconsequential photodegradation rate of 30% after 120 min because of its wide band gap. As for CN, only 50% of RhB is disintegrated after 120 min. However, after the introduction of metal Bi, the binary BOC-Bi and CN-Bi exhibit better degradation rates (40% and 60%, respectively). Simultaneously, the photodegradation rate of the RhB for the BOC-CN compound is 45%. Interestingly, the ternary composites (BOC-CN-Bi) reveal the best photocatalytic activity compared to other samples, which is approximately 93% after 120 min. Additionally, the ternary BOC-CN-Bi heterojunction composites still possess high photocatalytic performance for RhB degradation compared to the reported BOC-CN photocatalysts. Therefore, we can reach the conclusion that metal Bi has a remarkable effect on enhancing photocatalytic performance. To explore the kinetic reaction, the kinetics of RhB degradation for all samples are analyzed. The degradation data of all catalysts are linearly fitted according to the equation:

$$\text{Ln}\left(\frac{\text{C}}{\text{C}\_0}\right) = \text{kt} + \text{a}$$

where C, C0 and k are the concentration of RhB under the different times, the concentration of RhB after the adsorption-desorption equilibrium, and the apparent reaction rate constant, respectively. As shown in Figure 6B, the relationship between Ln(C/C0) and t of all samples conforms to the first-order kinetic model. The k of every sample is estimated and shown (Figure 6B). Obviously, BOC-CN-Bi possesses the fastest photodegradation rate, which is 6.2 and 3.5 times than that of BOC and CN. The aforementioned results forcefully verified that BOC-CN-Bi ternary composites possess a superior photocatalytic activity. We also explored the effects of pH, dye concentration and catalyst amount on photocatalytic degradation (Figure S2). Obviously, the more photocatalytic content, the better the degradation effect. However, the degradation efficiency and the catalyst increase disproportionately. Moreover, the concentration of organic pollutants increased, and the degradation efficiency decreased slightly. When pH increased, the degradation efficiency decreased significantly. When pH decreased, the catalyst completely adsorbed organic pollutants after darkness, and the degradation efficiency could not be measured (Figure S3, Supplementary Materials). It is well known that the stability of photocatalysts is crucial in practical applications. Therefore, the cycling experiments of BOC-CN-Bi composites were carried out. As shown in Figure 6C, the photocatalytic performance is not significantly reduced in three cycles, which confirms the outstanding stability of the composites.

active species, and h+ acts in a dominant role for RhB degradation.

Under photocatalytic degradation, organic pollutants are decomposed by active species, e.g., active holes (h+), hydroxyl radicals (•OH) and superoxide radicals (•O2−). To verify the dominant active species of BOC-CN-Bi ternary composites under photocatalytic degradation, trapping experiments are performed by adding different scavengers (Figure 6D). In the experiment, benzoquinone (BQ), sodium oxalate (Na2C2O4) and isopropanol (IPA) are used as a quencher of •O2−, h+ and •OH. After adding IPA, the degradation efficiency decreases slightly, implying that •OH is not decisive active species in the photodegradation experiment. Evidently, when BQ and Na2C2O4 are added, the degradation efficiency of pollutants is remarkably suppressed, which could declare that •O2− and h+ species have a pivotal role in degrading pollutants. More importantly, the degradation rate of pollutants for Na2C2O4 is obviously lower than that of BQ, suggesting that h+ is a more vivacious species. The results evidently demonstrate that •O2− and h+ are the main

**Figure 6.** The photodegradation of different organic pollutants (**A**) and kinetics of the RhB decomposition (**B**) over as-prepared samples, the cycling experiments (**C**) and the trapping experiment (**D**) the result of BOC-CN-Bi ternary composites by degrading RhB. **Figure 6.** The photodegradation of different organic pollutants (**A**) and kinetics of the RhB decomposition (**B**) over as-prepared samples, the cycling experiments (**C**) and the trapping experiment (**D**) the result of BOC-CN-Bi ternary composites by degrading RhB.

Photoluminescence (PL) is used to study the electron-hole separation efficiency. The PL spectra of all samples are shown in Figure 7A,B. Commonly, the lower PL intensity signifies the lower recombination of electron-hole pairs [38,39]. CN-Bi has lower PL intensity compared with pure CN (Figure 7A). As shown in Figure 7B, the PL spectra of BCO-CN and BCO-CN-Bi obviously reduce, suggesting that CN and Bi can restrain the carrier recombination of BCO. Among all samples, BCO-CN-Bi has the lowest PL spectra, indicating that BCO-CN-Bi has the highest carrier separation efficiency. To further study the behaviors of charge transfer for as-prepared samples, the EIS and transient photocurrent responses are carried out. It is well known that the smaller the radius of the arc, the smaller the interfacial resistance [40,41]. As disclosed in Figure 7C, BOC-CN-Bi photocatalysts possess the smallest arc compared to pure BOC, CN and other binary samples, which implies the lowest charge transfer resistance. We also execute photocurrent measurement to Under photocatalytic degradation, organic pollutants are decomposed by active species, e.g., active holes (h+ ), hydroxyl radicals (•OH) and superoxide radicals (•O2 −). To verify the dominant active species of BOC-CN-Bi ternary composites under photocatalytic degradation, trapping experiments are performed by adding different scavengers (Figure 6D). In the experiment, benzoquinone (BQ), sodium oxalate (Na2C2O4) and isopropanol (IPA) are used as a quencher of •O2 −, h+ and •OH. After adding IPA, the degradation efficiency decreases slightly, implying that •OH is not decisive active species in the photodegradation experiment. Evidently, when BQ and Na2C2O4 are added, the degradation efficiency of pollutants is remarkably suppressed, which could declare that •O2 − and h+ species have a pivotal role in degrading pollutants. More importantly, the degradation rate of pollutants for Na2C2O4 is obviously lower than that of BQ, suggesting that h+ is a more vivacious species. The results evidently demonstrate that •O2 − and h+ are the main active species, and h+ acts in a dominant role for RhB degradation.

Photoluminescence (PL) is used to study the electron-hole separation efficiency. The PL spectra of all samples are shown in Figure 7A,B. Commonly, the lower PL intensity signifies the lower recombination of electron-hole pairs [38,39]. CN-Bi has lower PL intensity compared with pure CN (Figure 7A). As shown in Figure 7B, the PL spectra of BCO-CN and BCO-CN-Bi obviously reduce, suggesting that CN and Bi can restrain the carrier recombination of BCO. Among all samples, BCO-CN-Bi has the lowest PL spectra, indicating that BCO-CN-Bi has the highest carrier separation efficiency. To further study the behaviors of charge transfer for as-prepared samples, the EIS and transient photocurrent responses are carried out. It is well known that the smaller the radius of the arc, the smaller the interfacial resistance [40,41]. As disclosed in Figure 7C, BOC-CN-Bi photocatalysts possess the smallest arc compared to pure BOC, CN and other binary samples, which implies the lowest charge transfer resistance. We also execute photocurrent measurement to further investigate the separation rate of e−-h+ pairs for samples. The photocurrent intensity of BOC-CN-Bi photocatalysts was much stronger than BOC and CN alone (Figure 7D),

explaining that the designed ternary Z-scheme heterojunction among BOC, Bi and CN boosts the separation efficiency of photoinduced carriers [42]. 7D), explaining that the designed ternary Z-scheme heterojunction among BOC, Bi and CN boosts the separation efficiency of photoinduced carriers [42].

further investigate the separation rate of e−-h+ pairs for samples. The photocurrent intensity of BOC-CN-Bi photocatalysts was much stronger than BOC and CN alone (Figure

*Molecules* **2022**, *27*, x FOR PEER REVIEW 9 of 13

**Figure 7.** The PL (**A**,**B**), Nyquist (**C**) and photocurrent spectra (**D**) of over as-prepared samples. **Figure 7.** The PL (**A**,**B**), Nyquist (**C**) and photocurrent spectra (**D**) of over as-prepared samples.

Mott-Schottky was used to study the conduction band of CN. The conduction band of CN is −1.1 V. According to our previous studies, BOC has a conduction band of 0.33 V [43]. The valence band of BOC and CN is severally 3.5 and 1.28 V according to Eg = EVB − ECB. In consideration of the dominant active species, the elevated carrier separation efficiency and the relative band positions of BOC and CN, we designed and discussed a novel indirect heterojunction system to expatiate the feasible reaction mechanism of BOC-CN-Bi photocatalysts for degrading pollutants (Figure 8). When illuminated by visible light, both BOC and CN could produce photoinduced electrons in the conduction band. Simultaneously, the valence band of BOC and CN left a lot of holes. Metal Bi with excellent conductivity could readily capture and transmit electrons. Therefore, the electrons of BOC tended to transfer to metal Bi and consumed the holes of CN because of their different band level, greatly improving the separation efficiency of e−–h+ pairs. The bi bridge was responsible for the elevated separation efficiency of carriers between BOC and CN, which was proved by the mentioned electrochemical and PL results. As a result, the electrons of BOC combined with the holes of CN resulted in the agglomeration of h+ in the VB of BOC (3.5 V) and e− in the CB of CN (−1.1 V) for efficient carrier separation. The reductive electrons in the CB of CN could reduce the absorbed O2 to O2− due to E(O2/O2−) = −0.33 V [44,45]. Ultimately, the holes of BOC together with the produced ·O2− were able to degrade the adsorbed organic pollutants. The remarkably better photocatalytic degradation performance for the ternary BOC-CN-Bi composites was beneficial from the intensive sepa-Mott-Schottky was used to study the conduction band of CN. The conduction band of CN is −1.1 V. According to our previous studies, BOC has a conduction band of 0.33 V [43]. The valence band of BOC and CN is severally 3.5 and 1.28 V according to Eg = EVB − ECB. In consideration of the dominant active species, the elevated carrier separation efficiency and the relative band positions of BOC and CN, we designed and discussed a novel indirect heterojunction system to expatiate the feasible reaction mechanism of BOC-CN-Bi photocatalysts for degrading pollutants (Figure 8). When illuminated by visible light, both BOC and CN could produce photoinduced electrons in the conduction band. Simultaneously, the valence band of BOC and CN left a lot of holes. Metal Bi with excellent conductivity could readily capture and transmit electrons. Therefore, the electrons of BOC tended to transfer to metal Bi and consumed the holes of CN because of their different band level, greatly improving the separation efficiency of e−–h+ pairs. The bi bridge was responsible for the elevated separation efficiency of carriers between BOC and CN, which was proved by the mentioned electrochemical and PL results. As a result, the electrons of BOC combined with the holes of CN resulted in the agglomeration of h+ in the VB of BOC (3.5 V) and e− in the CB of CN (−1.1 V) for efficient carrier separation. The reductive electrons in the CB of CN could reduce the absorbed O2 to O2 − due to E(O2/O2 −) = −0.33 V [44,45]. Ultimately, the holes of BOC together with the produced ·O2 − were able to degrade the adsorbed organic pollutants. The remarkably better photocatalytic degradation performance for the ternary BOC-CN-Bi composites was beneficial from the intensive separation of carriers and the extended visible-light absorption range.

ration of carriers and the extended visible-light absorption range.

**Figure 8.** The schematic of BOC-CN-Bi Z-scheme heterojunction by degrading RhB. **Figure 8.** The schematic of BOC-CN-Bi Z-scheme heterojunction by degrading RhB.

### **3. Materials and Methods**

#### **4. Conclusions**  *3.1. Preparation of Photocatalyst*

To sum up, we have successfully fabricated a novel BOC-CN-Bi Z-scheme heterojunction. As shown in UV-vis DRS, the BOC-CN-Bi composites have a wider visible-light absorption range than pure BOC. Bi metal as an electron bridge significantly improves the separation efficiency of photogenerated carriers in a heterojunction composite, which is The preparation of g-C3N4: firstly, the melamine was thoroughly ground. The obtained powders were then calcined at 550 ◦C (heating rate: 5 ◦C min−1 ) for 3 h in a muffle furnace. Until they reached room temperature, the products were ground for further use and labeled as CN.

demonstrated by the photocurrent and PL. The BOC-CN-Bi composites possess significantly superior photocatalytic degradation activity compared to single BOC and CN. Radical trapping studies identify that h+ and •O2− are two primary active species during the degradation reaction. Cycling tests manifest the remarkable stability of the BOC-CN-Bi heterojunction composites. This work implies that BOC-CN-Bi photocatalysts might be efficient and promising novel materials for removing organic pollutants from wastewater. The preparation of Bi2O2CO3 and g-C3N4-Bi2O2CO3: 0.238 g g-C3N4, 0.5 g cetyl trimethyl ammonium bromide and 2.99 g Na2CO3 were added into 40 mL deionized water under continuous stirring, and this was labeled as solution A. Meanwhile, 1.71 g Bi(NO3)3·5H2O and 10 mL HNO3 (1 mol L−1 ) were added into 40 mL deionized water under continuous stirring to form solution B. We then mixed solution A and B and continued stirring for 3 hours. After that, the products were obtained via centrifugation, washed and dried, labeling them as BOC-CN. Without adding CN, Bi2O2CO3 (BOC) was synthesized.

**Supplementary Materials:** The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: The band gaps of BOC-Bi, CN-Bi, BOC-CN, and BOC-CN-Bi.; Figure S2: The photodegradation different organic pollutant of BOC-CN-Bi under different conditions.; Figure S3: The color of the solution for BOC-CN-Bi after the dark at pH = 4; Figure S4: Mott-Schottky plots of CN; Table S1: Element content of BOC-CN-Bi composites by EDS. **Author Contributions:** Conceptualization, H.F.; Methodology, H.F. and Y.B.; Formal analysis, X.L., L.Y. and Y.B.; Investigation, X.M.; Data curation, X.M.; Writing—original draft, H.F.; Writing—review & editing, H.F.; Project administration, W.L.; Funding acquisition, W.L. All authors have read The preparation of g-C3N4-Bi, Bi/Bi2O2CO3 and g-C3N4@Bi/Bi2O2CO3: 1.71 g Bi(NO3)3·5H2O was added into ethylene glycol (EG). After stirring for 2 h, 0.238 g g-C3N4 was added to the above solution and was stirred for 3 h. Finally, the mixed solution was heated at 180 ◦C for 10 h in a reaction kettle. The products were obtained via centrifugation, washed and dried, and labeled as CN-Bi. Similarly, Bi/Bi2O2CO3 and g-C3N4@Bi/Bi2O2CO3 were prepared by replacing 0.238 g g-C3N4 with 0.238 g Bi2O2CO3 or g-C3N4-Bi2O2CO3 during synthesis, and labeled as BCO-Bi and BOC-CN-Bi, respectively.

#### and agreed to the published version of the manuscript. *3.2. Materials Characterization*

**Funding:** We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 21271022). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest. The elementary composition, phase structure and morphology of samples were measured by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The UV-vis diffuse reflectance spectra (DRS) and photoluminescence (PL) of samples were obtained by a Lambda 950 Spectrophotometer and Fluorescence Spectrophotometer. The electrochemical impedance spectroscopy (EIS) and photocurrent response were recorded via an electrochemical workstation (CHI660E). The detailed condition of the electrochemical tests is the same as we reported before.

### *3.3. Photocatalytic Experiments*

The degradation of RhB dyes was utilized to determine the photocatalytic degradation performance of all samples via a 400 W Xe lamp (≥420 nm). Before each experiment, 30 mg samples were adequately dispersed in a 30 mL RhB solution. In the dark, the obtained suspension solution was consumingly stirred for 1 h on behalf of an adsorptiondesorption equilibrium. During the photoreaction process, 3 mL solution was collected every 30 minutes and centrifuged. A UV-vis spectrophotometer was used to test the RhB concentration in the culture at 553 nm.

### **4. Conclusions**

To sum up, we have successfully fabricated a novel BOC-CN-Bi Z-scheme heterojunction. As shown in UV-vis DRS, the BOC-CN-Bi composites have a wider visible-light absorption range than pure BOC. Bi metal as an electron bridge significantly improves the separation efficiency of photogenerated carriers in a heterojunction composite, which is demonstrated by the photocurrent and PL. The BOC-CN-Bi composites possess significantly superior photocatalytic degradation activity compared to single BOC and CN. Radical trapping studies identify that h+ and •O2 − are two primary active species during the degradation reaction. Cycling tests manifest the remarkable stability of the BOC-CN-Bi heterojunction composites. This work implies that BOC-CN-Bi photocatalysts might be efficient and promising novel materials for removing organic pollutants from wastewater.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27238336/s1, Figure S1: The band gaps of BOC-Bi, CN-Bi, BOC-CN, and BOC-CN-Bi; Figure S2: The photodegradation different organic pollutant of BOC-CN-Bi under different conditions; Figure S3: The color of the solution for BOC-CN-Bi after the dark at pH = 4; Figure S4: Mott-Schottky plots of CN; Table S1: Element content of BOC-CN-Bi composites by EDS.

**Author Contributions:** Conceptualization, H.F.; Methodology, H.F. and Y.B.; Formal analysis, X.L., L.Y. and Y.B.; Investigation, X.M.; Data curation, X.M.; Writing—original draft, H.F.; Writing—review & editing, H.F.; Project administration, W.L.; Funding acquisition, W.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 21271022).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

## *Article* **Enhance ZnO Photocatalytic Performance via Radiation Modified g-C3N4**

**Yayang Wang 1,2,†, Xiaojie Yang 2,†, Jiahui Lou 2 , Yaqiong Huang 2 , Jian Peng 3,\* , Yuesheng Li 2,\* and Yi Liu 1,4,\***

> 1 School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430074, China

**Abstract:** Environmental pollution, especially water pollution, is becoming increasingly serious. Organic dyes are one type of the harmful pollutants that pollute groundwater and destroy ecosystems. In this work, a series of graphitic carbon nitride (g-C3N4 )/ZnO photocatalysts were facilely synthesized through a grinding method using ZnO nanoparticles and g-C3N4 as the starting materials. According to the results, the photocatalytic performance of 10 wt.% CN-200/Z-500 (CN-200, which g-C3N4 was 200 kGy, referred to the irradiation metering. Z-500, which ZnO was 500 ◦C, referred to the calcination temperature) with the CN-200 exposed to electron beam radiation was better than those of either Z-500 or CN-200 alone. This material displayed a 98.9% degradation rate of MB (20 mg/L) in 120 min. The improvement of the photocatalytic performance of the 10 wt.% CN-200/Z-500 composite material was caused by the improvement of the separation efficiency of photoinduced electron–hole pairs, which was, in turn, due to the formation of heterojunctions between CN-200 and Z-500 interfaces. Thus, this study proposes the application of electron-beam irradiation technology for the modification of photocatalytic materials and the improvement of photocatalytic performance.

**Keywords:** g-C3N4 ; g-C3N4/ZnO; electron beam irradiation; photocatalytic performance

### **1. Introduction**

With increasing industrialization, environmental pollution is becoming more and more serious, especially water pollution [1–3]. Therefore, the treatment of industrial wastewater from various sources has attracted extensive attention from researchers. It is an important challenge to remove pollutants from bodies of water. Among the various types, organic dyes are some of the most harmful pollutants that pollute groundwater and destroy ecosystems. Photocatalysis can effectively degrade organic dyes and is widely used in the degradation of these types of pollutants [4,5].

Photocatalytic technology, due to the excellent stability of the catalyst, its low cost, and its environmental friendliness, has been widely studied by researchers [6–8]. ZnO, with good chemical stability, has low toxicity, low cost, and many other advantages, thus, it has been extensively studied in the field of photocatalytic degradation [9–12]. The narrow spectral absorption (Eg = 3.37 eV) and low degree of separation of photogenerated electron– hole pairs of ZnO, however, limit its applications in photocatalysis [13]. Improving its light absorption capacity and promoting the separation of photogenerated electron–hole pairs can effectively improve the photocatalytic performance of ZnO [14–17]. Graphitic carbon nitride (g-C3N4) possesses many good characteristics, such as a low band gap (Eg = 2.7 eV), stable physical and chemical properties, good absorption capacity of visible

**Citation:** Wang, Y.; Yang, X.; Lou, J.; Huang, Y.; Peng, J.; Li, Y.; Liu, Y. Enhance ZnO Photocatalytic Performance via Radiation Modified g-C3N4. *Molecules* **2022**, *27*, 8476. https://doi.org/10.3390/ molecules27238476

Academic Editors: Hongda Li, Mohammed Baalousha and Victor A. Nadtochenko

Received: 3 November 2022 Accepted: 28 November 2022 Published: 2 December 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

light, simple synthetic methods, thermal stability, and low cost [18–22]. To further improve the performance of g-C3N4, electron beam radiation technology is used [23]. Enhancement of the photocatalytic activity of ZnO can be achieved by compounding g-C3N4 with it [24–28]. Ding et al. [29] reported a composite catalyst, AgCl/ZnO/g-C3N4, which was prepared via calcination, hydrothermal reaction, and in-situ deposition processes. The catalyst was assessed for its photocatalytic efficiency in eliminating tetracycline hydrochloride from pharmaceutical wastewater under visible light. Ganesh et al. [30] prepared ZnO, Sb-doped ZnO, and ZnO: Sb/g-C3N4 nanocomposite using a simple chemical route. ZnO: Sb and g-C3N4, which reduced luminous intensity due to increased charge transfer, can effectively improve photocatalytic performance. The degradation efficiency of the ZnO: Sb/g-C3N4 sample was 86% in 60 min. The efficiency of the degradation of methylene blue (MB) (1 × 10−5 M) by ZnO: Sb/g-C3N4 was 86%. Based on these studies, we modified the materials by electron beam radiation to study their degradation efficiency against dyes in high concentrations.

In this study, ZnO was prepared at different annealing temperatures and its photocatalytic properties were examined. The carbon nitride photocatalysts, g-C3N4 (CN-X (where X is the radiation absorption dose and X = 100, 200, 300, and 400 kGy)), were prepared by direct calcination of urea and CN-200 was prepared by the electron beam radiation technique. The presence of the CN-200/Z-500 heterostructures produced a large number of catalytic active sites, enhancing the catalytic performance. It was found that 10 wt.% CN-200/Z-500 particles degraded 98.9% of MB (20 mg/L) in 120 min. The improvement of the photocatalytic performance of 10 wt.% CN-200/Z-500 nanocomposite resulted from the improvement of the separation efficiency of photoinduced electron–hole pairs, which was due to the formation of heterojunctions between CN-200 and ZnO interfaces.

### **2. Results and Discussion**

### *2.1. XRD and FT-IR Analysis*

The crystal phases of the samples were examined by XRD. Figure 1a–d shows the XRD patterns of g-C3N4, ZnO, and CN-200/Z-500. g-C3N4 has its strongest peak at 2θ = 27.33◦ corresponding to the (002) planes (Figure 1a) due to the typical interplanar stacking structures of graphitic materials. The g-C3N4, which was irradiated by an electron beam, shows a red shift for the reflection of the (002) crystal planes. Electron beam irradiation was beneficial in increasing the spacing between crystal planes. To investigate the influence of different annealing temperatures on the crystalline properties of ZnO, the prepared materials were investigated by XRD, as presented in Figure 1b. Obviously, the peak position did not change with annealing temperature, suggesting that the crystal form did not change after calcination due to the material temperature. The crystal forms of CN-200, Z-500, and 10 wt.% CN-200/Z-500 are shown in Figure 1c. The diffraction peak of CN-200 at 27.65◦ corresponds to the (002) planes (JCPDS NO. 87-1526). The high-intensity diffraction peak of CN-200 at 27.65◦ corresponds to the packing of the conjugated aromatic system. Z-500 exhibits prominent characteristic peaks corresponding to the hexagonal wurtzite phase at 31.77◦ , 34.42◦ , 36.25◦ , 47.54◦ , 56.60◦ , 62.86◦ , 66.38◦ , 67.96◦ , 69.10◦ , 72.56◦ , and 76.95◦ , corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes (JCPDS NO. 36–1451), respectively. The diffraction peaks at different positions are well-matched with the wurtzite diffraction peaks. This showed that the prepared hexagonal wurtzite zinc oxide samples have high crystallinity. Figure 1d presents the FT-IR spectra of CN, CN-200, Z-500, and 10 wt.% CN-200/Z-500. The peak at 815 cm−1 in the spectrum of CN corresponds to the s-triazine ring system. The peaks at 1242.35 cm−1 and 1627.17 cm−1 correspond to the C–N and C=N stretching vibrations, respectively. Comparing the infrared spectra of CN and CN-200, it can be seen that the electron beam irradiation did not change the structure of graphitic carbon nitride. In the case of Z-500, the band at 438.32 cm−1 due to the Zn–O stretching mode indicates the formation of ZnO crystal. The FT-IR spectrum of 10 wt.% CN-200/Z-500 was similar to the characteristic spectra of both CN-200 and

Z-500, suggesting the retention of the typical graphitic structure of CN-200 after it was compounded with Z-500. spectra of both CN-200 and Z-500, suggesting the retention of the typical graphitic structure of CN-200 after it was compounded with Z-500.

the band at 438.32 cm−1 due to the Zn–O stretching mode indicates the formation of ZnO crystal. The FT-IR spectrum of 10 wt.% CN-200/Z-500 was similar to the characteristic

*Molecules* **2022**, *27*, 8476 3 of 12

**Figure 1.** (**a**) XRD patterns of CN prepared under different absorbed radiation doses; (**b**) XRD patterns of ZnO prepared at different calcination temperatures; (**c**) XRD patterns of CN-200, Z-500, and 10 wt.% CN-200/Z-500; (**d**) infrared characterization of CN, CN-200, Z-500, and 10 wt.% CN-200/Z-**Figure 1.** (**a**) XRD patterns of CN prepared under different absorbed radiation doses; (**b**) XRD patterns of ZnO prepared at different calcination temperatures; (**c**) XRD patterns of CN-200, Z-500, and 10 wt.% CN-200/Z-500; (**d**) infrared characterization of CN, CN-200, Z-500, and 10 wt.% CN-200/Z-500.

### 500. *2.2. XPS Analysis*

*2.2. XPS Analysis* X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical elements on the surface of the composite material. The XPS spectra of C 1s, N 1s, O 1s, and Zn 2p are shown in Figure 2. Three peaks were observed, which were at 288.54 eV, 287.13 eV, and 285.15 eV, respectively, corresponding to the C=N, C–N, and C–C bonds in Figure 2a. The peak at 287.13 eV belongs to the N–C=N bonds with sp2 hybridization. The peak at 288.54 eV was assigned to sp2-bonded C–NH2. As shown in Figure 2b, the N 1s spectrum of 10 wt.% CN-200/Z-500 has three characteristic peaks with binding energies of 401.19, 399.09, and 398.87 eV. The peak at 401.19 eV was attributed to C–N–H bonds, while the other peaks located at 399.09 and 398.87 eV were ascribed to N–(C)3 and C=N–H, respectively. The XPS spectrum of Zn 2p exhibits two peaks at 1045.17 and 1022.05 eV (Figure 2c), which correspond to Zn 2p1/2 and Zn 2p3/2, respectively. The Zn 2p peaks of 10 wt.% CN-200/Z-500 showed a negative shift in comparison with the pristine ZnO, which was reported by the authors of [31,32]. As shown in Figure 2d, three peaks were resolved from the O 1s spectrum of 10 wt.% CN-200/Z-500. The peak with binding energy of 530.50 eV X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical elements on the surface of the composite material. The XPS spectra of C 1s, N 1s, O 1s, and Zn 2p are shown in Figure 2. Three peaks were observed, which were at 288.54 eV, 287.13 eV, and 285.15 eV, respectively, corresponding to the C=N, C–N, and C–C bonds in Figure 2a. The peak at 287.13 eV belongs to the N–C=N bonds with sp2 hybridization. The peak at 288.54 eV was assigned to sp2-bonded C–NH2. As shown in Figure 2b, the N 1s spectrum of 10 wt.% CN-200/Z-500 has three characteristic peaks with binding energies of 401.19, 399.09, and 398.87 eV. The peak at 401.19 eV was attributed to C–N–H bonds, while the other peaks located at 399.09 and 398.87 eV were ascribed to N–(C)3 and C=N–H, respectively. The XPS spectrum of Zn 2p exhibits two peaks at 1045.17 and 1022.05 eV (Figure 2c), which correspond to Zn 2p1/2 and Zn 2p3/2, respectively. The Zn 2p peaks of 10 wt.% CN-200/Z-500 showed a negative shift in comparison with the pristine ZnO, which was reported by the authors of [31,32]. As shown in Figure 2d, three peaks were resolved from the O 1s spectrum of 10 wt.% CN-200/Z-500. The peak with binding energy of 530.50 eV was determined to belong to the lattice oxygen of Zn–O, which further verified the successful combination of Z-500 and CN-200. The other two peaks, located at 532.55 and 532.45 eV, were assigned to water molecules and surface hydroxyl, respectively.

was determined to belong to the lattice oxygen of Zn–O, which further verified the successful combination of Z-500 and CN-200. The other two peaks, located at 532.55 and

532.45 eV, were assigned to water molecules and surface hydroxyl, respectively.

**Figure 2.** XPS spectra of 10 wt.% CN-200/Z-500: (**a**) C 1s; (**b**) N 1s; (**c**) Zn 2p; (**d**) O 1s. **Figure 2.** XPS spectra of 10 wt.% CN-200/Z-500: (**a**) C 1s; (**b**) N 1s; (**c**) Zn 2p; (**d**) O 1s.

#### *2.3. FE-SEM and HR-TEM Analysis 2.3. FE-SEM and HR-TEM Analysis*

The FE-SEM images of the CN, CN-200, Z-500, and CN-200/Z-500 samples are shown in Figure 3. A typical multilayer stacking structure of the CN is shown in Figure 3a,b. Compared with CN, CN-200 shows better dispersion, as shown in Figure 3c,d. Figure 3e,f indicates that Z-500 was mainly composed of non-uniform spherical particles. It is clear from Figure 3g,h that the composite material CN-200/Z-500 was formed from nanosheets of CN-200 and nanoparticles of Z-500. Figure 3g,h presents an SEM image of 10 wt.% CN-200/Z-500 photocatalyst. As seen above, the Z-500 photocatalyst was composed of irregular nanoparticles, which contributed to its low photocatalytic activity under visible light. The SEM image of the 10 wt.% CN-200/Z-500 shows that the Z-500 was evenly distributed on the surface of the CN-200, which favored the formation of heterojunctions and resulted The FE-SEM images of the CN, CN-200, Z-500, and CN-200/Z-500 samples are shown in Figure 3. A typical multilayer stacking structure of the CN is shown in Figure 3a,b. Compared with CN, CN-200 shows better dispersion, as shown in Figure 3c,d. Figure 3e,f indicates that Z-500 was mainly composed of non-uniform spherical particles. It is clear from Figure 3g,h that the composite material CN-200/Z-500 was formed from nanosheets of CN-200 and nanoparticles of Z-500. Figure 3g,h presents an SEM image of 10 wt.% CN-200/Z-500 photocatalyst. As seen above, the Z-500 photocatalyst was composed of irregular nanoparticles, which contributed to its low photocatalytic activity under visible light. The SEM image of the 10 wt.% CN-200/Z-500 shows that the Z-500 was evenly distributed on the surface of the CN-200, which favored the formation of heterojunctions and resulted in enhanced photocatalytic activity.

in enhanced photocatalytic activity. The 10 wt.% CN-200/Z-500 with the best photocatalytic activity was further analyzed by TEM and high-resolution TEM (HR-TEM). Figure 4a,b revealed typical TEM images of the composite photocatalyst 10 wt.% CN-200/Z-500.  This observation is consistent with the XRD and SEM results.

### *2.4. Photocatalytic Activity*

The photoluminescence (PL) emission spectra was conducted to determine the recombination rate of photoinduced charge carriers. The properties of photogenerated charge carriers between CN-200, Z-500, and 10 wt.% CN-200/Z-500 were studied using 365 nm as the excitation wavelength. As shown in Figure 5a, the PL strength of 10 wt.% CN-200/Z-500 was much weaker than that of pure CN-200, indicating that the photoinduced charge carrier recombination rate of 10 wt.% CN-200/Z-500 composite is greatly reduced. Figure 5b showed UV–visible spectra of 10 wt.% CN-200/Z-500 composites, CN-200 and Z-500. The corresponding band gaps of ZnO and g-C3N4 were acquired from the indirect

estimation of Tauc plot (Ahv)n vs. The estimated Eg values for Z-500, CN-200, and 10 wt.% CN-200/Z-500 composites were 3.24 eV, 2.89 eV, and 3.22 eV, respectively. Z-500 had a narrower band gap after doping with CN-200, which indicated that it can absorb more visible light and show a better light response under visible light irradiation.   This observation is consistent with the XRD and SEM results.

 **Figure 4.** (**a**,**b**) TEM images of CN-200/Z-500 photocatalyst; (**c**,**d**) HR-TEM images of CN-200/Z-500 photocatalyst.

photocatalyst.

more visible light and show a better light response under visible light irradiation.

The photoluminescence (PL) emission spectra was conducted to determine the recombination rate of photoinduced charge carriers. The properties of photogenerated charge carriers between CN-200, Z-500, and 10 wt.% CN-200/Z-500 were studied using 365 nm as the excitation wavelength. As shown in Figure 5a, the PL strength of 10 wt.% CN-200/Z-500 was much weaker than that of pure CN-200, indicating that the photoinduced charge carrier recombination rate of 10 wt.% CN-200/Z-500 composite is greatly reduced. Figure 5b showed UV–visible spectra of 10 wt.% CN-200/Z-500 composites, CN-200 and Z-500. The corresponding band gaps of ZnO and g-C3N4 were acquired from the indirect estimation of Tauc plot (Ahv)n vs. The estimated Eg values for Z-500, CN-200, and 10 wt.% CN-200/Z-500 composites were 3.24 eV, 2.89 eV, and 3.22 eV, respectively. Z-500 had a narrower band gap after doping with CN-200, which indicated that it can absorb

**Figure 5.** (**a**) Photoluminescence (PL) emission spectra of 10 wt.% CN-200/Z-500 composites, CN-200 and Z-500; (**b**) UV-Visible spectra of 10 wt.% CN-200/Z-500 composites, CN-200 and Z-500. **Figure 5.** (**a**) Photoluminescence (PL) emission spectra of 10 wt.% CN-200/Z-500 composites, CN-200 and Z-500; (**b**) UV-Visible spectra of 10 wt.% CN-200/Z-500 composites, CN-200 and Z-500.

#### *2.5. Photocatalytic Activity 2.5. Photocatalytic Activity*

*2.4. Photocatalytic Activity*

Figure 6a,b shows the photocatalytic activities of different catalysts for MB (20 mg/L) degradation. The catalytic performance of ZnO prepared at different calcination temperatures while keeping the amount of CN-200 doping constant is shown in Figure 6a. The results showed that 10 wt.% CN-200/Z-500 exhibited the best catalytic activity. A very rapid degradation rate of MB was observed initially and then it slowed down until no more degradation occurred. As can be seen from Figure 6a, Z-500 showed better MB degradation than the other Z-T samples (T = 400, 600, and 700 °C). It is noteworthy that Z-500 and CN-200 played a crucial role in the construction of effective heterojunctions between the two components. As expected, the photocatalytic activity of CN-200/Z-500 composite was enhanced as compared to Z-500. Under visible light irradiation, the 10 wt.% CN-200/Z-500 decomposed MB solution (20 mg/L) with a high degradation rate of 98.9% within 120 min. This showed that CN-200 was well incorporated into Z-500, effectively enhancing the photocatalytic performance of the composite under visible light irradiation. Figure 6b shows the influence of different composite proportions on photocatalytic performance. It can be seen that, when the doping ratio was 10 wt.%, the photocatalytic per-Figure 6a,b shows the photocatalytic activities of different catalysts for MB (20 mg/L) degradation. The catalytic performance of ZnO prepared at different calcination temperatures while keeping the amount of CN-200 doping constant is shown in Figure 6a. The results showed that 10 wt.% CN-200/Z-500 exhibited the best catalytic activity. A very rapid degradation rate of MB was observed initially and then it slowed down until no more degradation occurred. As can be seen from Figure 6a, Z-500 showed better MB degradation than the other Z-T samples (T = 400, 600, and 700 ◦C). It is noteworthy that Z-500 and CN-200 played a crucial role in the construction of effective heterojunctions between the two components. As expected, the photocatalytic activity of CN-200/Z-500 composite was enhanced as compared to Z-500. Under visible light irradiation, the 10 wt.% CN-200/Z-500 decomposed MB solution (20 mg/L) with a high degradation rate of 98.9% within 120 min. This showed that CN-200 was well incorporated into Z-500, effectively enhancing the photocatalytic performance of the composite under visible light irradiation. Figure 6b shows the influence of different composite proportions on photocatalytic performance. It can be seen that, when the doping ratio was 10 wt.%, the photocatalytic performance was the best. *Molecules* **2022**, *27*, 8476 7 of 12

**Figure 6.** (**a**,**b**) Photocatalytic activity of MB degradation photocatalyst under visible light irradiation; (**c**,**d**) first-order kinetic curve of MB degradation photocatalyst under visible light irradiation. **Figure 6.** (**a**,**b**) Photocatalytic activity of MB degradation photocatalyst under visible light irradiation; (**c**,**d**) first-order kinetic curve of MB degradation photocatalyst under visible light irradiation.

can be expressed quantitatively with the use of the pseudo-first-order model: ln(C0/Ct) = kKt = K′t, where C0 refers to the original dye concentration; Ct refers to the MB concentration; K′ represents the pseudo-first-order rate constant; and t represents the irradiation time. In accordance with this equation, K′ can be acquired from a linear plot of ln(C0/Ct) against t, which represents the degradation rate and is proportional to the photocatalytic

The kinetics of photo-oxidation/photoreduction of MB over the different samples are presented in Figure 6c,d. The MB photo-oxidation/photoreduction kinetics of different samples are shown in Figure 6c,d and K′ is obtained after data fitting. Specific data are shown in Tables 1 and 2. Combined with the data analysis, the photocatalytic degradation of MB conforms to the first-order kinetic model, R2 > 0.92, indicating a good degree of fit. Based on Figure 6d, the photocatalytic degradation efficiency is 98.86% for the 10 wt.% CN-200/Z-500, 80.97% for the Z-500, and 89.64% for the 5 wt.% CN-200/Z-500 after 120

**Table 1.** Degradation rate of MB after compound of zinc oxide and CN prepared at different anneal-

10% CN-200/Z-400 Y = 0.02194X + 0.07045 0.96537 94.6% 10% CN-200/Z-500 Y = 0.02796X + 0.27089 0.97759 97.376% 10% CN-200/Z-600 Y = 0.01819X + 0.07533 0.99565 89.529% 10% CN-200/Z-700 Y = 0.0247X + 0.15485 0.97284 96.195%

**y = ln(C0/Ct) R2 Degradation Rate (%)**

MB dye was used to simulate pollutants and the photocatalytic activities of the catalysts were assessed by studying the MB degradation efficiency under visible light. The

degradation rate.

ing temperatures.

min.

MB dye was used to simulate pollutants and the photocatalytic activities of the catalysts were assessed by studying the MB degradation efficiency under visible light. The degradation of MB was monitored by observing the decrease in the intensity of the characteristic absorption peak at 662 nm with time. The degradation reaction kinetics of MB can be expressed quantitatively with the use of the pseudo-first-order model: ln(C0/Ct) = kKt = K0 t , where C0 refers to the original dye concentration; Ct refers to the MB concentration; K0 represents the pseudo-first-order rate constant; and t represents the irradiation time. In accordance with this equation, K0 can be acquired from a linear plot of ln(C0/Ct) against t, which represents the degradation rate and is proportional to the photocatalytic degradation rate.

The kinetics of photo-oxidation/photoreduction of MB over the different samples are presented in Figure 6c,d. The MB photo-oxidation/photoreduction kinetics of different samples are shown in Figure 6c,d and K0 is obtained after data fitting. Specific data are shown in Tables 1 and 2. Combined with the data analysis, the photocatalytic degradation of MB conforms to the first-order kinetic model, R2 > 0.92, indicating a good degree of fit. Based on Figure 6d, the photocatalytic degradation efficiency is 98.86% for the 10 wt.% CN-200/Z-500, 80.97% for the Z-500, and 89.64% for the 5 wt.% CN-200/Z-500 after 120 min.

**Table 1.** Degradation rate of MB after compound of zinc oxide and CN prepared at different annealing temperatures.

**Table 2.** Degradation rates of MB with different catalysts.

The cyclic stability of 10 wt.% CN-200/Z-500 composite catalyst was evaluated by cyclic experiments of photocatalytic degradation of MB. The used catalyst would be centrifuged at high speed after each cycle test, washed, and vacuum-dried overnight at 60 ◦C for collection before use. Figure 7a showed the photocatalytic efficiency curves of 10 wt.% CN-200/Z-500 in three cycles. As shown in Figure 7a, after three cycles, the degradation rate of MB was 91.7%, while the degradation rate of the first cycle was 98.4%, indicating that the degradation efficiency of the catalyst did not decrease significantly with the progress of the photocatalytic process. The results indicated that the prepared 10 wt.% CN-200/Z-500 catalyst was stable and reusable in the photocatalytic degradation process. The trapping experiments were carried out to explore the photocatalytic mechanism. To test the role of these reactive species, Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), isopropyl alcohol (IPA), and β-benzoquinone (BQ) were employed as scavengers for h+ , •OH, and •O2 −, respectively. As can be seen in Figure 7b, the degradation ratios for MB were all reduced after the addition of the three trapping reagents, indicating that the h+ , •OH, and •O2 − are all responsible for the degradation process. When the EDTA-2Na was added into the reaction system, the degradation efficiency is remarkably decreased, suggesting that h + takes a crucial part in the degradation of organic pollutants.

for h+, •OH, and •O2

the h+, •OH, and •O2

−

**Table 2.** Degradation rates of MB with different catalysts.

5% CN-200/Z-500 Y = 0.01705X + 0.33072 0.92233 89.637% 10% CN-200/Z-500 Y = 0.03473X + 0.21363 0.98704 98.857% 15% CN-200/Z-500 Y = 0.02599X + 0.23163 0.98333 96.249% 20% CN-200/Z-500 Y = 0.02417X + 0.35889 0.95197 96.133%

Z-500 Y = 0.01223X + 0.14589 0.95744 80.971% CN-200 Y = 0.02809X + 0.03753 0.99826 96.776%

cyclic experiments of photocatalytic degradation of MB. The used catalyst would be centrifuged at high speed after each cycle test, washed, and vacuum-dried overnight at 60°C for collection before use. Figure 7a showed the photocatalytic efficiency curves of 10 wt.% CN-200/Z-500 in three cycles. As shown in Figure 7a, after three cycles, the degradation rate of MB was 91.7%, while the degradation rate of the first cycle was 98.4%, indicating that the degradation efficiency of the catalyst did not decrease significantly with the progress of the photocatalytic process. The results indicated that the prepared 10 wt.% CN-200/Z-500 catalyst was stable and reusable in the photocatalytic degradation process. The trapping experiments were carried out to explore the photocatalytic mechanism. To test the role of these reactive species, Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), isopropyl alcohol (IPA), and β-benzoquinone (BQ) were employed as scavengers

for MB were all reduced after the addition of the three trapping reagents, indicating that

creased, suggesting that h+ takes a crucial part in the degradation of organic pollutants.

The cyclic stability of 10 wt.% CN-200/Z-500 composite catalyst was evaluated by

**Figure 7.** (**a**) Cycle experiments for degradation of MB over 10 wt.% CN-200/Z-500; (**b**) Influence of different scavengers on the degradation of MB over the 10 wt.% CN-200/Z-500. **Figure 7.** (**a**) Cycle experiments for degradation of MB over 10 wt.% CN-200/Z-500; (**b**) Influence of different scavengers on the degradation of MB over the 10 wt.% CN-200/Z-500.

**y = ln(C0/Ct) R2 Degradation Rate (%)**

, respectively. As can be seen in Figure 7b, the degradation ratios

− are all responsible for the degradation process. When the EDTA-

On the basis of the results of the photodegradation and photogenerated carrier trapping test, a possible photodegradation mechanism for 10 wt.% CN-200/Z-500 was explained to clarify the enhanced photocatalytic activity of it for the degradation of MB. The degradation mechanism was shown in Figure 8. CN-200 absorbs visible light to induce Π–Π\* transition, transporting the excited-state electrons from the HOMO to the lowest unoccupied molecular orbital (LUMO). The LUMO potential of CN-200 (−1.12 eV) is more negative than the conduction band (CB) edge of Z-500 (−0.5 eV), so the excited electron on CN-200 could directly inject into the CB of Z-500. Meanwhile, the holes on the VB of Z-500 transfer to the CN-200 because the VB edge potential of Z-500 was more positive than the CN-200. This can effectively restrain the recombination of photoinduced electron–hole pairs for Z-500 and CN-200. The O2 molecules trapped photoelectron on the surface of 10 On the basis of the results of the photodegradation and photogenerated carrier trapping test, a possible photodegradation mechanism for 10 wt.% CN-200/Z-500 was explained to clarify the enhanced photocatalytic activity of it for the degradation of MB. The degradation mechanism was shown in Figure 8. CN-200 absorbs visible light to induce Π–Π\* transition, transporting the excited-state electrons from the HOMO to the lowest unoccupied molecular orbital (LUMO). The LUMO potential of CN-200 (−1.12 eV) is more negative than the conduction band (CB) edge of Z-500 (−0.5 eV), so the excited electron on CN-200 could directly inject into the CB of Z-500. Meanwhile, the holes on the VB of Z-500 transfer to the CN-200 because the VB edge potential of Z-500 was more positive than the CN-200. This can effectively restrain the recombination of photoinduced electron–hole pairs for Z-500 and CN-200. The O2 molecules trapped photoelectron on the surface of 10 wt.% CN-200/Z-500 and generate the superoxide anion radical (•O2 −). Due to its high activity and disability of penetrating in bulk of the sample, •O2 − mostly degrades the adsorbed MB molecules on the surface of the catalyst. Simultaneously, h+ reacted with water molecules to produce hydroxyl radicals. The resulting •OH and h+ oxidize the organic pollutants into small molecules, such as CO2 and H2O [33]. The photocatalytic reaction process is described below:

$$\text{CN-200} + \text{hv} \rightarrow \text{h}^+ + \text{e}^-$$

$$\text{Z-500} + \text{hv} \rightarrow \text{h}^+ + \text{e}^-$$

$$\text{CN-200 (e}^- \text{CB} \rightarrow \text{Z-500 (e}^- \text{CB})$$

$$\text{Z-500 (h}^+ \text{vB} \rightarrow \text{CN-200 (h}^+ \text{vB})$$

$$\text{Z-500 (e}^- \text{)} + \text{O}\_2 \rightarrow \bullet \text{O}\_2^-$$

$$\bullet \text{O}\_2^- + 2 \text{H}^+ + \text{e}^- \rightarrow \text{H}\_2\text{O}\_2$$

$$\text{H}\_2\text{O}\_2 + \text{e}^- \rightarrow \bullet \text{OH} + \text{OH}^-$$

h + + •OH + •O2 − + pollutants → CO2 + H2O + degradation intermediate

**Figure 8.** Schematic diagram for the proposed photocatalytic reaction mechanism of 10 wt.% CN-200/Z-500. **Figure 8.** Schematic diagram for the proposed photocatalytic reaction mechanism of 10 wt.% CN-200/Z-500.

#### **3. Experimental 3. Experimental**

#### *3.1. Preparation of CN-X by Irradiation (X = 100, 200, 300, and 400 kGy) 3.1. Preparation of CN-X by Irradiation (X = 100, 200, 300, and 400 kGy)*

wt.% CN-200/Z-500 and generate the superoxide anion radical (•O2

sorbed MB molecules on the surface of the catalyst. Simultaneously, h+ reacted with water molecules to produce hydroxyl radicals. The resulting •OH and h+ oxidize the organic pollutants into small molecules, such as CO2 and H2O [33]. The photocatalytic reaction

> CN-200 + hv → h+ + e-Z-500 + hv → h+ + e− CN-200 (e−CB) → Z-500 (e−CB) Z-500 (h+VB) → CN-200 (h+VB)

> > ) + O2 → •O2

− + 2H+ + e− → H2O2

H2O2 + e− → •OH + OH-

−

− + pollutants → CO2 + H2O + degradation intermediate

Z-500 (e−

•O2

tivity and disability of penetrating in bulk of the sample, •O2

process is described below:

h+ + •OH + •O2

−

). Due to its high ac-

− mostly degrades the ad-

g-C3N4 preparation: 20 g urea (CH4N2O) was calcined at 550°C with a heating rate of 5°C·min−1 for 180 min in an alumina crucible in a muffle furnace. The sample was crushed into powder after calcination. g-C3N4 preparation: 20 g urea (CH4N2O) was calcined at 550 ◦C with a heating rate of 5 ◦C·min−1 for 180 min in an alumina crucible in a muffle furnace. The sample was crushed into powder after calcination.

Sample preparation for irradiation: 0.12 g g-C3N4, 12 mL isopropanol, 120 μL water, and 240 μL ammonia were mixed in a reagent bottle. The mixture was ultrasonically stirred for 10 min, which was repeated three times. The sample was transferred to a polyethylene bag and sealed under a vacuum. The sample was irradiated by a 1 MeV electron accelerator (1 MeV; Wasik Associates, Dracut, MA, USA) with a dose rate of 20 kGy/pass. The sample was repeatedly washed with ethanol and water, subsequently dried in an Sample preparation for irradiation: 0.12 g g-C3N4, 12 mL isopropanol, 120 µL water, and 240 µL ammonia were mixed in a reagent bottle. The mixture was ultrasonically stirred for 10 min, which was repeated three times. The sample was transferred to a polyethylene bag and sealed under a vacuum. The sample was irradiated by a 1 MeV electron accelerator (1 MeV; Wasik Associates, Dracut, MA, USA) with a dose rate of 20 kGy/pass. The sample was repeatedly washed with ethanol and water, subsequently dried in an oven at 60 ◦C, and ground for later use. The samples with different irradiation doses of CN were denoted as CN-X, where X is the radiation absorption dose and X = 100, 200, 300, and 400 kGy, respectively.

### *3.2. Preparation of ZnO*

ZnO was prepared using the co-precipitation method. Solution A was prepared by dissolving ZnSO4 (100 mmol) and cetyltrimethylammonium bromide (CTAB, 4.3 mmol) in 125 mL of distilled water. Solution B consisted of ammonium bicarbonate (50 mmol) dissolved in 125 mL of distilled water. Solution B was then quickly poured into solution A, sonicated, and stirred. A gray precipitate resulted, which was then stirred for 60 min. The sample was washed several times with ethanol and water and then dried at 80 ◦C. Finally, it was calcined for 3 h at the rate of 2 ◦C·min−1 at 400, 500, 600, and 700 ◦C, respectively. The samples were labeled as Z-T (T = 400, 500, 600, and 700 ◦C).

### *3.3. Synthesis of CN-200/Z-500*

During the earlier studies, it was found that the best photocatalytic efficiency for C3N4/ZnO was obtained when the annealing temperature was 500 ◦C and the radiation absorption dose was 200 kGy. Therefore, CN-200 and Z-500 were compounded together by the grinding method. CN-200 (50 mg) was added to the desired mass of Z-500. During the grinding process, 5 mL of absolute ethanol was added every 5 min, and after grinding for 30 min, the mixture was transferred to an oven at 60 ◦C and dried for 24 h.

### *3.4. Characterization*

The crystalline samples were characterized by an X-ray diffractometer (XRD, DMAX-D8X; Rigaku, Tokyo, Japan). The dye degradation was studied by using an ultravioletvisible (UV-Vis) (TU-1950; Persee, suzhou, China). The morphologies of the materials were analyzed by field-emission scanning electron microscopy (FE-SEM, SU8220; Tokyo, Japan) and field emission transmission electron microscopy (FE-TEM, FEI Tecnai G2 F30; Hillsboro, OR, USA). KBr pellets of the samples were made to analyze the samples using Fouriertransform infrared spectroscopy (FT-IR) (Thermo Ferret iS10 infrared spectrometer, Thermo Fisher Scientific, Waltham, MA, USA). Photoluminescence spectra (PL) of samples was recorded on an Edinburgh FLS1000 fluorescent spectrophotometer (UK). Ultraviolet–visible diffuse reflectance spectra (UV-Vis DRS; Japan) was performed on a Shimadzu, UV-3600i Plus UV-Vis spectrometer with a scan range of 200–800 nm.

### *3.5. Photocatalytic Experiments*

The photocatalytic activities of the samples were evaluated by using the dye MB as the model. All experiments were performed in an open system at room temperature. The photocatalyst (50 mg) was dispersed in 50 mL of MB solution (20 mg/L) and stirred for 30 min in dark circumstances to achieve adsorption–desorption equilibrium between the photocatalyst and the MB dye in the photocatalysis study.

The catalysis reaction was performed under visible light by taking out 2 mL of solution from each sample suspension every 20 min. High-speed centrifugation was employed to remove photocatalysts. The concentrations of MB at various reaction times were measured with a UV-Vis spectrophotometer at λmax = 662 nm, and the degradation rate (%) was calculated using the following equation:

$$\text{Depradation Rate} \left( \% \right) = \frac{\mathcal{C}\_0 - \mathcal{C}\_t}{\mathcal{C}\_t} \times 100\% = \frac{A\_0 - A\_t}{A\_0} \times 100\% \tag{1}$$

Here, *Ct* and *C*0 represent pollutant concentrations at times *t* and *t*0, respectively. *A*0 and *At* are the absorbances of the MB solution before irradiation and after irradiation, respectively.

### **4. Conclusions**

In this study, ZnO was prepared at different annealing temperatures and its photocatalytic properties were examined. The best photocatalytic degradation of 20 mg/L MB dye was achieved by ZnO prepared at 500 ◦C under visible light radiation. g-C3N4 photocatalysts were prepared by direct calcination of urea and CN-200 was prepared by irradiation with electron beams. The radiation dose of CN-200 was 200 kGy. A series of CN-200 doped ZnO hetero-structured photocatalysts were successfully prepared using the grinding method. The samples were characterized using XRD, FT-IR, FE-SEM, and HR-TEM. MB was chosen as the photodegradation target, and the photocatalytic activity of the composites was evaluated. Interestingly, 10 wt.% CN-200/Z-500 particles degraded 98.9% of the MB in 120 min, owing to the presence of a large number of catalytic active sites in CN-200/Z-500 heterostructures that enhanced its catalytic performance. Thus, the 10 wt.% CN-200/Z-500 has great potential to degrade pollutants. Thus, it can be concluded that electron beam irradiation technology can be effectively used for the modification of photocatalytic materials and the improvement of their photocatalytic performance.

**Author Contributions:** Conceptualization and methodology, Y.W. and X.Y.; validation, Y.W. and J.L.; formal analysis and investigation, Y.H.; writing—original draft preparation, X.Y.; writing—review and editing, J.P. and Y.L. (Yi Liu); provided technical and financial support, Y.L. (Yuesheng Li); proposed the guiding support and the final review of the paper, Y.L. (Yi Liu). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Xianning City Program of Science & Technology (No. 2022ZRKX051), the Hubei University of Science and Technology Doctoral Research Initiation Project (No. BK202217), the Science Development Foundation of Hubei University of Science & Technology (No. 2021F005, No. 2021ZX14, Nos. 2020TD01, 2021ZX01, 2022FH09), the Hubei Provincial Colleges and Universities Outstanding Young and Middle-aged Technological Innovation Team Project (No. T2020022), and Xianning City Key Program of Science & Technology (No. 2021GXYF021).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.

### **References**

## *Article* **Z-Scheme CuOx/Ag/TiO2 Heterojunction as Promising Photoinduced Anticorrosion and Antifouling Integrated Coating in Seawater**

**Xiaomin Guo 1 , Guotao Pan 1 , Lining Fang 2 , Yan Liu 1,\* and Zebao Rui 1,\***

**Abstract:** In the marine environment, steel materials usually encounter serious problems with chemical or electrochemical corrosion and fouling by proteins, bacteria, and other marine organisms. In this work, a green bifunctional Z-scheme CuOx/Ag/P25 heterostructure coating material was designed to achieve the coordination of corrosion prevention and antifouling by matching the redox potential of the reactive oxygen species and the corrosion potential of 304SS. When CuOx/Ag/P25 heterostructure was coupled with the protected metal, the open circuit potential under illumination negatively shifted about 240 mV (vs. Ag/AgCl) and the photoinduced current density reached 16.6 µA cm−2 . At the same time, more reactive oxygen species were produced by the Z-shape structure, and then the photocatalytic sterilization effect was stronger. Combined with the chemical sterilization of Ag and the oxide of Cu, the bacterial survival rate of CuOx/Ag/P25 was low (0.006%) compared with the blank sample. This design provides a strategy for developing green dual-functional coating materials with photoelectrochemical anticorrosion and antifouling properties.

**Keywords:** antibacterial effect; antifouling effect; CuOx/Ag/P25; photoelectrochemical cathodic protection; bifunctional coatings

### **1. Introduction**

Metal corrosion and fouling in the marine environment are very serious because of the presence of oxygen, Cl− ions, sunlight, and micro-organisms [1–3], causing huge economic losses and significant harm to the human living environment. Corrosion generally results from chemical or electrochemical reactions between the metal and oxygen, while the metal surface submerged in seawater or in the humid marine atmosphere is often fouled by marine organisms such as proteins, bacteria, algae, and mollusks, which usually increase the metal weight and accelerate material surface damage [4,5].

Various methods, such as coatings [6], corrosion inhibitors [7], and cathodic protection [8], have been proposed to help reduce metal corrosion in the marine environment. However, these traditional anticorrosion techniques usually suffer from the high cost of resources or additional energy, together with the problem of environmental pollution. Photoelectrochemical cathodic protection (PECCP) technology, which couples a semiconductor material with the protected metal, is a new and green anticorrosion technology [1,3,9]. During the PECCP process, under illumination the semiconductor material generates photoelectrons, which are transferred to the protected metal surface to make the metal potential more negative than its corrosion potential. The typical semiconductor materials, TiO2 [2,10,11], ZnO [12], and SrTiO3 [9,13], have been evaluated for the PECCP process. Among them, TiO2 is one of the most widely used photoelectric anode materials, and

**Citation:** Guo, X.; Pan, G.; Fang, L.; Liu, Y.; Rui, Z. Z-Scheme CuOx/Ag/TiO2 Heterojunction as Promising Photoinduced Anticorrosion and Antifouling Integrated Coating in Seawater. *Molecules* **2023**, *28*, 456. https:// doi.org/10.3390/molecules28010456

Academic Editors: Hongda Li, Mohammed Baalousha and Victor A. Nadtochenko

Received: 1 December 2022 Revised: 26 December 2022 Accepted: 1 January 2023 Published: 3 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

it has the advantages of high photoelectrochemical activity, low cost, high stability, and nontoxicity [14,15]. However, TiO2 has a low utilization rate of sunlight (<5%), with a relatively wide band gap (~3.2 eV) [16] and a fast recombination rate of photogenerated carriers, which seriously limits its application in the PECCP field. Currently, forming a heterojunction by coupling narrow-band-gap metal oxides with TiO2 is an effective way to improve the absorption of visible light and promote the separation of electron/hole pairs [17,18]. Tian et al. [3] deposited Cu2O nanoparticles on the surface of TiO2 nanotubes to prepare the Cu2O/TiO2 p-n heterojunction composite photoelectrodes to accelerate the photogenerated carriers' separation and improve PECCP performance.

Regarding the protection against microbial corrosion, the addition of antibacterial components to the protective paints or coatings has been commonly used to achieve antifouling and antibacterial effects [19]. Although many heavy metals and rare earth elements, such as Cu, Ag, Ce, La, etc., have a strong bactericidal effect [20–22], their adverse effects on the environment need to be considered [23]. It is reported that some semiconductors can be also used for photocatalytic sterilization and antifouling, which is correlated with the generation of reactive oxygen species (ROS, such as superoxide radicals •O2 − and hydroxyl radicals •OH) under irradiation and their inhibition to the growth of bacteria [24]. Wang et al. [24] reported that AgVO3/BiO2−x inactivated bacteria in full spectrum, which is due to a large amount of ROS caused by internal structural defects and the formation of heterostructures. Moreover, the integration of chemical sterilization with photocatalytic sterilization has become a key research goal in the antimicrobial field [25–27]. Yang et al. [25] designed Cu2O/Ag composites with strong and long-term antibacterial activities.

Apparently, both the PECCP performance and photocatalytic antibacterial properties of semiconductors are related to the light absorption, photoinduced charges separation, and the conduction/valence-band positions. In this sense, we herein design a bifunctional CuOx/Ag/TiO2 Z-type heterojunction, which is anticipated as coating material for metal anticorrosion/antifouling in the marine environment on the basis of the following considerations. (i) Both CuOx and TiO2 hold higher conduction-band positions against the potential of stainless steel (e.g., 304SS) and make the transfer of photoinduced electrons to the metal for PECCP possible. (ii) The combination of Ag nanoparticles (NP) with a local surface plasmon resonance (LSPR) effect and small-band-gap CuOx can improve the visible light absorption of TiO2 [28,29]. (iii) The unique Z-type CuOx/Ag/TiO2 heterojunction provides an effective charge transfer path for realizing the efficient separation of electron-hole pairs without sacrificing redox ability [30]. (iv) The bacteria, viruses, and other micro-organisms can be inhibited or chemically killed by Ag and CuOx and by active ROS groups produced by photoinduced electrons and holes from the heterojunction [25–27,31,32]. As expected, the as-designed CuOx/Ag/TiO2 not only well demonstrates photocathodic protection performance for 304SS in the simulated seawater but also possesses chemical and photocatalytic synergistic bactericidal activities. Such a green bifunctional coating material shows great promise in metal anticorrosion and antifouling applications in the marine environment.

### **2. Results and Discussion**

### *2.1. Structures, Compositions, and Morphologies*

XRD measurements were employed to study the crystalline structures of samples. As shown in Figure 1a, the diffraction peaks for P25 could be assigned to anatase and rutile TiO2 (JCPDS 84-1285 and JCPDS 86-0148). The diffraction peak intensity at 2θ = 25.3 ◦ was strong, and the peak shape was symmetrical, indicating that the crystallinity of the material is very good. The reference material CuOx presented characteristic diffraction peaks at 2θ = 36.4 and 42.3 ◦ , corresponding to the (111) and (200) crystal planes of the Cu2O phase of cubic hematite (JCPDS 05-0667). Compared with P25 and CuOx, the diffraction peaks for CuOx/Ag/P25 remained unchanged, indicating that CuOx and Ag did not affect the crystal structure of P25, which may have been due to the small load, the small particle size, and the low crystallinity of CuOx and Ag [3,33]. The morphologies and nanostructures

of CuOx/Ag/P25 were analyzed by TEM and HRTEM. As shown in Figure 1b, the lattice spacings of 2.5 and 3.6 Å corresponded to the (101) crystal plane of anatase TiO2 and the (101) crystal plane of rutile TiO2, respectively. The lattice spacings of 2.1 and 2.4 Å were ascribed to the (100) and (111) crystal planes of Cu2O [3,16,34]. The (200) crystal plane of Ag with a lattice spacing of about 2.08 Å was found at the interface between the CuOx phase and the P25 phase [35]. Figure 1c shows that CuOx/Ag/P25 was mainly composed of irregular particles with a diameter of 20~25 nm. The EDS element distribution mapping in Figure 1d exhibited that the Ti, O, Ag, and Cu elements were evenly distributed in CuOx/Ag/P25. The corresponding element content is listed in Figure 1i.

**Figure 1.** (**a**) XRD patterns of P25, CuOx, CuOx/P25, Ag/P25, and CuOx/Ag/P25; (**b**) HRTEM and (**c**) TEM images of CuOx/Ag/P25; (**d**–**h**) EDS element mapping images of CuOx/Ag/P25; (**i**) list of element content of CuOx/Ag/P25.

The atomic valence states and energy-band structures of CuOx/P25, Ag/P25, and CuOx/Ag/P25 were further studied by XPS. The Ti 2p spectra for P25 in Figure 2a showed a pair of spin-orbital doublets at ~464.2 and ~458.5 eV, corresponding to Ti 2p1/2 and Ti 2p3/2 of Ti4+, respectively [3,28,36]. Compared with P25, the peaks for CuOx/Ag/P25 showed a shift to a higher binding energy, which was possibly due to the strong interaction between CuOx, Ag, and P25 in the heterojunction structure of CuOx/Ag/P25 [37]. The O1s spectra of the samples shown in Figure 2b can be divided into four types of peaks in total. Peak A, around 529.8 eV, belonged to the oxide peak, which was related to the lattice oxygen in TiO2. The slightly shifted peak B, at ~530.9 eV, also belonged to the metal (Ag and Cu) oxide peak, which was attributed to the O atom near the oxygen vacancy [38]. Peak C, around ~532 eV, belonged to the -OH group, chemically adsorbed on the material surface [39], and peak D, around ~533 eV, belonged to adsorbed H2O molecules on the surface [40]. Figure 2c shows that the Ag 3d spectra of Ag/P25 and CuOx/Ag/P25 can be deconvoluted into two peaks, Ag 3d3/2 (373.5 eV) and Ag 3d5/2 (367.5 eV), indicating the existence of Ag metal in the material [30]. In the Cu 2p spectra for CuOx (Figure 2d), the peaks with binding energies of 934 and 932.6 eV can belong to the Cu2+ [41] and Cu+ [42] species, respectively. In addition, the satellite peaks corresponding to Cu2+ species can be seen between 940 and 945 eV. This indicates that Cu+ and Cu2+ coexisted in CuOx. For CuOx/P25 and CuOx/Ag/P25, only peaks at 932.6 eV corresponding to the binding state of Cu+ species were observed, which demonstrates that the strong interaction between the carrier (P25) and the load (CuOx and Ag) made the Cu species stable.

**Figure 2.** XPS spectra of the samples: (**a**) Ti 2p, (**b**) O 1s, (**c**) Ag 3d, and (**d**) Cu 2p.

### *2.2. Photoelectric Characterization*

UV-visible diffuse reflectance spectra (or UV-vis DRS) were measured to investigate the light absorption of samples. Figure 3a showed a typical TiO2 light-absorption range. The light-absorption range of as-prepared CuOx/Ag/P25 was extended to the visible region with an edge of about 436 nm after loading CuOx and/or Ag. At the same time, it can be observed that Ag/P25 and CuOx/Ag/P25 had strong and wide absorption peaks near 550 nm, which were related to the LSPR effect of Ag NPs [43,44]. Figure 3a shows the band-gap results of the materials. CuOx/Ag/P25 holds the narrowest band gap (2.17 eV) compared with P25 (3.01 eV), CuOx/P25 (2.87 eV) and Ag/P25 (2.49 eV). The photoluminescence (PL) spectra of the catalysts at an excitation wavelength of 550 nm at room temperature were displayed in Figure 3b. The fluorescence peak intensity for CuOx/Ag/P25 was lower than that of other compound materials, indicating the lowest recombination rate of e−/h+ pairs. The fluorescence intensity order was CuOx/Ag/P25 < Ag/P25 < CuOx/P25 < P25, which suggested that having Ag NP as the electron bridge promoted the separation and emigration of photogenerated electron-hole pairs [30,45]. By plotting the Kubelka–Munk function against the photon energy, the band0gap (*E*g) values of P25 and CuOx were estimated to be 3.01 and 2.05 eV, respectively (Figure 3c,d). Moreover, the *E*g values of the composites, such as Ag/P25, CuOx/P25, and CuOx/Ag/P25, became

narrower after loading CuOx and Ag, which confirmed the broadened light-response range. According to the XPS valence-band (or XPS-VB) spectra in Figure 3e,f, the corresponding *E*VB,XPS of P25 and CuOx were 2.55 and 1.5 eV, respectively. The VB positions of P25 and CuOx were calculated as 2.31 eV and 1.26 eV, respectively, by using the following equation: *E*VB,NHE = ϕ + *E*VB,XPS − 4.44, where ϕ is the work function of the instrument (4.2 eV [46]).

**Figure 3.** (**a**) UV−vis DRS, dependence of (αhν) 2 on the photon energy (the inserts) and (**b**) photoluminescence spectra (λex = 550 nm) of P25, CuOx/P25, Ag/P25, and CuOx/Ag/P25 at room temperature. Dependence of (αhν) 2 on the photon energy for (**c**) P25 and (**d**) CuOx. XPS valenceband spectra of (**e**) P25 and (**f**) CuOx.

The electron paramagnetic resonance (EPR) spectra of free radical adducts trapped by DMPO in the dark and under irradiation at room temperature for the catalysts are shown in Figure 4. The EPR signals for DMPO−•OH and DMPO−•O2 − were almost negligible on P25, CuOx/P25, Ag/P25, and CuOx/Ag/P25 in darkness. Both DMPO−•OH and DMPO−•O2 − signals with four characteristic peaks with intensity ratios of 1:2:2:1 and 1:1:1:1 [47], respectively, can be observed in the suspension under simulated solar irradiation for 12 min. EPR signals of DMPO−•OH and DMPO−O2• − were different in intensity. The intensity order of DMPO−•O2 − signals under irradiation was CuOx/Ag/P25 > Ag/P25 > CuOx/P25 > P25; the intensity order of DMPO−•OH signals under irradiation was CuOx/Ag/P25 > Ag/P25 ≈ CuOx/P25 > P25. CuOx/Ag/P25 showed the highest

intensity in the characteristic peaks of DMPO−•OH and DMPO−•O2 − when compared with those materials.

**Figure 4.** EPR spin-trapping spectra of DMPO−•O2 − and DMPO−•OH adducts (**a**,**c**) in the dark and (**b**,**d**) under illumination, respectively, for P25, CuOx/P25, Ag/P25, and CuOx/Ag/P25. The light used was full-spectrum light.

### *2.3. Photoelectrochemical Cathodic Protection Performance Evaluation*

In general, photogenerated cathodic protection works by supplying electrons to the protected metal through an external photosensitive semiconductor, causing the metal potential to be more negative than the original corrosion potential under irradiation. Thus, the photoinduced OCP value is an important parameter to evaluate the PECCP performance of photoanode materials. Figure 5a shows the photogenerated OCP-time curves of samples. The photoelectrodes that were coupled with 304SS were used as working electrodes. P25 showed the minimum negative potential shift of 90 mV under light on/off cycles, which was a little lower than the self-corrosion potential of 304SS (−50 mV), indicating that P25 hardly achieved PECCP toward 304SS. The negative shift order of OCP values for samples was CuOx/Ag/P25 > CuOx/P25 > Ag/P25 > P25. Compared with CuOx/P25 and Ag/P25, CuOx/Ag/P25 displayed a significantly more negative potential shift of 240 mV under illumination, indicating that the introduction of CuOx and Ag improved the separation efficiency of electron-hole pairs, and more electrons were transferred to the coupled 304SS to provide photoelectrochemical cathodic protection. Figure 5b shows the photoelectric response of P25, CuOx/P25, Ag/P25, and CuOx/Ag/P25 coupled with 304SS in a 3.65 wt.% NaCl solution with an applied bias potential of 0 V (*Eref*). P25, CuOx/P25, Ag/P25, and CuOx/Ag/P25 showed a positive current density change when the light was turned on, showing the characteristics of an n-type semiconductor [48]. It indicates that photoinduced electrons could be transferred from the semiconductor materials to the coupled 304SS. Compared with P25, the photocurrent densities of CuOx/P25 and CuOx/Ag/P25 greatly increased at the first light on/off switching. At the same time, using Ag particles as a conducting medium accelerated the separation of electron-hole pairs [3,30]. Thus, CuOx/Ag/P25 had a photocurrent density of 16.6 µA cm2 , which was ~1.8 times greater than that of P25, indicating that it had the best PECCP performance. The migration

ability of photogenerated carriers could be characterized by electrochemical impedance spectroscopy (EIS).

**Figure 5.** (**a**) The photoinduced OCP−time curves and (**b**) i−t curves of P25, CuOx/P25, Ag/P25, and CuOx/Ag/P25 coupled with 304SS. Nyquist plots measured (**c**) in the dark and (**d**) under illumination. Insert of (**d**) is the corresponding equivalent circuit.

Figure 5c,d show EIS results measured in the dark and under illumination, respectively. In darkness, the impedance arc radius of CuOx/Ag/P25 was larger than that of the other materials, which indicates that the thin film electrode could be used as a coating material to protect 304SS from corrosion [18]. Under illumination, the smaller the impedance arc radius of the impedance spectrum is, the faster the photogenerated carriers migrate, resulting in better PECCP properties [49]. Figure 5d shows that CuOx/Ag/P25 had the smallest arc radius, demonstrating the best carriers transfer efficiency and thus the greatest photochemical cathodic protection effect, which was consistent with the results of OCP response and the photoinduced current. In the corresponding equivalent circuit model, *R*s, *R*d, and *R*c represented the solution resistance, the depletion layer resistance, and the charge transfer resistance [50], respectively. The fitting results in Table 1 showed that CuOx/Ag/P25 had a higher *R*d value than P25, which may have been due to the bending of the Fermi level caused by the formation of a Z-type heterostructure. Moreover, CuOx/Ag/P25 had an *R*c value of 1.23 Ω, much smaller than P25 (2.47 × 104 ), CuOx/P25 (1.22 × 104 ), and Ag/P25 (300.5), respectively. Therefore, it could be understandable that CuOx/Ag/P25 had the best PECCP performance for 304 SS in a 3.65 wt.% NaCl solution.

**Table 1.** EIS fitting results of photoanode films.

### *2.4. Antibacterial Performance Evaluation*

The antibacterial activities of the materials were evaluated by using the CFU method, i.e., an *E. coli* colony–counting method. The optical pictures of *E. coli* colonies on nutrient agar plates in Figure 6a exhibited that CuOx/Ag/P25 and Ag/P25 coatings had fewer *E. coli* colonies than the other samples did, indicating better antibacterial activities under 24 h of illumination. Taking the blank sample without active catalysts as a counterpart in Figure 6b, it can be calculated that the bacterial survival rate of P25 and CuOx/P25 in a 1 × 106–times diluted bacterial solution were about 81.1% and 75.4%, respectively. Moreover, as shown in Figure 6c,d and Figure 7, the bacterial survival rates of Ag/P25 and CuOx/Ag/P25 in a 100-times diluted solution were as low as 0.116% and 0.006%, respectively. It is concluded that CuOx/Ag/P25 can effectively inhibit the growth of *E. coli*.

**Figure 6.** Optical photographs of agar plates coated with different catalysts and *E. coli* incubated solution with various dilutions: (**a**) comparison of the samples at 1 × 106–times dilution, (**b**) blank sample at 1 × 106–times dilution, (**c**) CuOx/Ag/P25 at 100-times dilution, and (**d)** Ag/P25 at 100-times dilution.

**Figure 7.** Comparison diagram of the bacterial survival rate in the media with different catalysts.

### **3. Discussion**

The energy-band positions of CuOx (−0.79 eV/1.26 eV) and P25 (−0.7 eV/2.31 eV) determined in this work (Figure 3c–f) and the reported Fermi level of Ag NP (~−0.22 eV) [44] are listed in Figure 8, indicating that a Z-scheme or type-II heterojunction structure was formed in CuOx/Ag/P25 [28,30]. Moreover, because the VB potential of CuOx is not positive enough for the production of •OH from H2O oxidation, the stronger DMPO- •OH signal of CuOx/Ag/P25, compared with that of Ag/P25 (Figure 4d), indicates that CuOx/Ag/P25 is inclined to form a Z-scheme system in which the photoinduced electrons in Ag/P25 transfer and combine with the holes on the valance band (VB) of CuOx, leaving the photoinduced holes at the VB of P25 for the production of •OH and the photoinduced electrons at the conduction band (CB) of CuOx for the photocathode protection and production of superoxide ions (O2• −). Additionally, the incorporation of Ag NP with the LSPR effect and CuOx with a narrow band gap greatly improved the visible light absorption (Figure 3a). All these together promoted the production of photoinduced charges and the ROS of •O2 − and •OH radicals (Figure 4) under illumination. On one hand, the efficient production and transfer of the photoinduced charges in the unique Z-scheme heterojunction of CuOx/Ag/P25 makes effective the transfer of photogenerated electrons from the CB of CuOx to the coupled 304SS, achieving enough of a negative shift of the 304SS potential and photoelectrochemical cathodic polarization protection toward the metal (Figure 5a). On the other hand, their strong ROS (superoxide radicals •O2 − and hydroxyl radicals •OH) production ability upon irradiation and their ability to inhibit the growth of bacteria, viruses, and other micro-organisms provide the antifouling ability of the CuOx/Ag/P25 coating [26,27]. Moreover, both CuOx and Ag NPs, especially the Cu2O species stabilized by the photoinduced electrons through the unique Z-scheme transfer path (Figure 2d), effectively maintain chemical disinfection properties [25,31,32]. The superior antibacterial performance of CuOx/Ag/P25 and their application potential as an antifouling coating are demonstrated in Figures 6 and 7.

**Figure 8.** Schematic diagram of the antibacterial and anticorrosion dual mechanism for CuxO/Ag/P25.

### **4. Experimental Section**

### *4.1. Synthesis of CuOx/Ag/TiO2*

First, noble metal Ag was loaded on TiO2 (P25) by using the wet chemical reduction method. Typically, 0.5 g of P25 was ultrasonically dispersed in 30 mL of deionized water. Then 5 mL of AgNO3 aqueous solution with a concentration of 1 mg mL−1 was added to the above suspension under vigorous stirring. After fully mixing, an appropriate amount of NaBH4 aqueous solution was added dropwise to reduce Ag+ . The molar ratio of NaBH4 to AgNO3 was 4:1. Under continuously stirring for 4 h, the resulting powder was obtained by centrifugation and washing with deionized water and absolute ethanol, separately, several times, followed by drying in an oven at 80 ◦C to obtain the Ag/P25 sample with a theoretical Ag loading of 1.0 wt.%.

Then, CuOx was loaded on P25 or Ag/P25 by using the deposition-precipitation method. Next, 0.3 g of P25 or Ag/P25 and 0.0074 g of CuSO4·5H2O were added into 60 mL of 0.2 mol mL−1 NaOH aqueous solution under magnetic stirring for 30 min, followed by the addition of 9.0 mL of 0.1 mol L−1 ascorbic acid aqueous solution, drop by drop. After reaction for 1.5 h, the product was centrifuged and washed with deionized water and absolute ethanol, separately, until the solution was neutral. At last, 1.0 wt.% CuOx/P25 or 1.0 wt.% CuOx/Ag/P25 was obtained after drying the sample overnight in the flowing N2 at 80 ◦C.

### *4.2. Preparation of Coatings*

The as-prepared sample was mixed with nafion/ethanol solution under ultrasonic treatment for the preparation of the coating material [51]. Typically, 1 mL of absolute ethanol was mixed with 100 µL of 5% nafion to obtain the solvent. Next, 0.5 mg of CuOx/Ag/P25 (or other sample) was then ultrasonic dispersed in the above 200 µL solvent. The as-obtained ultrasonic slurry was coated on FTO glass substrates with a coating area of 1 cm × 1 cm. Finally, the photoanode coating was obtained after drying in an oven at 40 ◦C for 3 h.

### *4.3. Characterizations*

The crystal structures of the prepared powders were analyzed by an X-ray powder diffractometer (Rigaku Ultima IV) equipped with a Cu Kα radiation source. The diffraction patterns in the range of 10 to 80◦ were recorded at a scan speed of 10◦ min−1 . The morphologies and element distribution of the samples were observed by high-resolution thermal field emission scanning electron microscope (FESEM, Gemini500). The crystal

interface structures of the samples were analyzed by high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30) with an accelerating voltage of 300 kV. The element composition and valence state of the catalysts were analyzed by X-ray photoelectron spectroscopy/ESCA (XPS, Thermo Fisher Scientific, Nexsa). A monochromatic Al Kα X-ray source (h*υ* = 1486.6 eV) under a vacuum degree of ~2 × 10−9 mbar was used. The surface pollution C1s (284.8 eV) was used as the standard for energy correction. The UV-vis diffuse reflectance spectroscopy (UV-vis DRS) response of catalysts in the wavelength range from 200 to 800 nm was analyzed by integrating a sphere UV-vis spectrophotometer (UV2600) with BaSO4 as a reference. Photoluminescence (PL) spectra were characterized by using Edinburgh FS5 fluorescence spectrometer with an excitation wavelength of 550 nm. Electron paramagnetic resonance (EPR) spectrometer (JES X320) was used to obtain signals of photogenerated radicals spin trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), e.g., DMPO-•OH and DMPO-O2• −, in the dark and under illumination (190–900 nm) at room temperature. The free radical signals were collected after 12 min.

### *4.4. Photoelectrochemical Measurements*

The electrochemical measurements were performed at room temperature using Gamry electrochemical workstation (Gamry Interface1010E). All measurements were carried out after the open circuit potential (OCP) value was stable. A 300 W Xenon lamp (CEL-HXF300) was used as the light source with an optical power density of 200 mW cm−2 . In a threeelectrode system, Pt plate was used as a counter electrode, Ag/AgCl electrode (saturated KCl) was used as a reference electrode, and the photoanode (catalyst coatings on FTO glass substrates, i.e., P25, Ag/P25, CuOx/P25, or CuOx/Ag/P25) was used as a working electrode. In the PECCP performance measurements, such as those from the OCP and photocurrent density-time (i-t) tests, the photoanode coupled with 304SS was used as a working electrode. Additionally, a 3.65 wt.% NaCl solution was used as an electrolyte solution to simulate the marine environment. Mott–Schottky (M–S) curves were measured in the potential scope of −1.5 to 0.2 V at a frequency of 1000 Hz. Electrochemical impedance spectroscopy (EIS) was performed in the AC voltage of 10 mV and the frequency range from 105 to 10−2 Hz with or without light illumination.

### *4.5. Antibacterial Performance Evaluation*

The antibacterial activities of the materials were evaluated by using the CFU method (a plate-counting method) using *Escherichia coli* (*E. coli*) in a beef-extract-peptone (BEP) medium at 37 ◦C for 24 h. The concentration of the bacterial suspension was adjusted to 3 × 107~3 × 108 CFU mL−1 . The catalysts were dispersed by deionized water to a concentration of 2 mg mL−1 . The mixture of 1 mL of catalyst, 1 mL of BEF, and 100 µL of *E. coli* suspensions was cultured on a shaking table (ZQLY-180GN, 150 rpm) under simulated solar irradiation at 37 ◦C for 24 h (16000LX). At the end of the incubation period, the culture medium was sampled to determine the viable counts of planktonic bacteria. The viable bacteria in the sampled suspension were counted by using a 10-times gradient dilution method. Specifically, 100 µL of a diluted sample was transferred to an LB agar plate, which was cultured in a bacterial incubator (MJX-150) at 37 ◦C for 24 h. Finally, the colonies were counted, and the antibacterial activities of the catalysts were compared according to the bacterial survival rate α, as calculated by

α = Number of viable bacteria in the control sample × Dilution ratio Number of viable bacteria in the blank × Dilution ratio × 100%

### **5. Conclusions**

In summary, we have successfully prepared CuOx/Ag/P25 coatings on FTO glass substrates, which displayed excellent photocathodic protection performance and antifouling activities under simulated solar illumination in a 3.65 wt.% NaCl solution. The photoelectrochemical measurements and characterizations revealed that a Z-type CuOx/Ag/P25

heterostructure provided a photogenerated carrier transfer channel, which facilitated carrier separation and resulted in a more negative OCP shift of 240 mV. Meanwhile, the Z-type heterojunction of coatings had high-conduction-band and deep-valance-band potentials, which generated more reactive oxygen species of •O2 − and •OH radicals, thus effectively killing *E. coli* bacteria, with a low survival rate of 0.006%.

**Author Contributions:** Conceptualization, Z.R.; methodology, Y.L. and X.G.; validation, X.G., Y.L. and Z.R.; formal analysis, X.G. and Z.R.; investigation, X.G. and G.P.; resources, Y.L. and Z.R.; data curation, X.G. and Z.R.; writing—original draft preparation, X.G.; writing—review and editing, L.F., Y.L. and Z.R.; supervision, Z.R.; funding acquisition, Y.L. and Z.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by the Natural Science Funds of Guangdong for Distinguished Young Scholars (No. 2022B1515020098), National Natural Science Foundation of China (No. 22002192), and Science and Technology Program of Guangzhou (No.<PHONE_NUMBER>72).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.

### **References**

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## *Review* **Application of Photocatalysis and Sonocatalysis for Treatment of Organic Dye Wastewater and the Synergistic Effect of Ultrasound and Light**

**Guowei Wang and Hefa Cheng \***

MOE Key Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +86-10-6276-1070; Fax: +86-10-6276-7921

**Abstract:** Organic dyes play vital roles in the textile industry, while the discharge of organic dye wastewater in the production and utilization of dyes has caused significant damage to the aquatic ecosystem. This review aims to summarize the mechanisms of photocatalysis, sonocatalysis, and sonophotocatalysis in the treatment of organic dye wastewater and the recent advances in catalyst development, with a focus on the synergistic effect of ultrasound and light in the catalytic degradation of organic dyes. The performance of TiO2 -based catalysts for organic dye degradation in photocatalytic, sonocatalytic, and sonophotocatalytic systems is compared. With significant synergistic effect of ultrasound and light, sonophotocatalysis generally performs much better than sonocatalysis or photocatalysis alone in pollutant degradation, yet it has a much higher energy requirement. Future research directions are proposed to expand the fundamental knowledge on the sonophotocatalysis process and to enhance its practical application in degrading organic dyes in wastewater.

**Keywords:** sonocatalysis; sonoluminescence; photocatalysis; organic dye; sonophotocatalysis; synergistic mechanism

### **1. Introduction**

In recent years, the textile industry has played a vital role in the global economy, but it is also a major contributor to environmental pollution, particularly in terms of organic dye wastewater [1–5]. The discharge of organic dye wastewater from textile production and utilization can cause significant damage to the aquatic ecosystem, and therefore, it is imperative to develop effective treatment methods to degrade these pollutants [6–8]. Among the various technologies geared toward the treatment of organic dye wastewater, photocatalysis, sonocatalysis, and sonophotocatalysis have received increasing attention owing to their high efficiency and potential for large-scale industrial applications [9–13].

Photocatalysis is a well-established technology for the treatment of organic pollutants in wastewater [14–17]. Qutub et al. investigated CdS/TiO2 nanocomposites for photocatalytic degradation of organic pollutants in wastewater [18]. The results showed that CdS-TiO2 nanocomposites exhibited the highest photocatalytic activity in the degradation of AB-29 dye, with a degradation efficiency of 84%, compared to 68% and 9% achieved by CdS and TiO2 under comparable conditions, respectively. The enhanced photocatalytic performance of CdS-TiO2 was attributed to reduced charge carrier recombination, improved charge separation, and expansion of the response of TiO2 to visible light. In photocatalytic systems, semiconductor photocatalysts, such as TiO2, are irradiated with light, generating electron-hole pairs that react with water or oxygen to form reactive species, e.g., hydroxyl radicals. These reactive species can later degrade the organic pollutants into harmless products [19–22]. Sonocatalysis, on the other hand, utilizes ultrasonic wave to generate cavitation bubbles in the solution, which collapse and produce high-energy conditions that can promote chemical reactions. Wang et al. developed a recyclable WO3/NiFe2O4/BiOBr

**Citation:** Wang, G.; Cheng, H. Application of Photocatalysis and Sonocatalysis for Treatment of Organic Dye Wastewater and the Synergistic Effect of Ultrasound and Light. *Molecules* **2023**, *28*, 3706. https://doi.org/10.3390/ molecules28093706

Academic Editors: Hongda Li, Mohammed Baalousha and Victor A. Nadtochenko

Received: 23 March 2023 Revised: 12 April 2023 Accepted: 18 April 2023 Published: 25 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

(WNB) composite with dual Z-scheme heterojunction for the degradation of levofloxacin (LEV) in aqueous solution [23]. The WNB composite showed the highest removal efficiency (97.97%) for LEV within 75 min under ultrasonic irradiation. The ternary composite comprises three different semiconductors suitable for harvesting full-spectrum light. The combination of sonocatalysis and photocatalysis, known as sonophotocatalysis, can further improve the efficiency of both processes, as the cavitation bubbles can create local "hot spots" that increase the photocatalytic activity for the catalyst [24–28]. Despite the promising results of sonophotocatalysis in pollutant degradation, there are still significant challenges that need to be addressed in order to enhance its performance for the treatment of organic dye wastewater [29]. For example, the high energy requirement of sonophotocatalysis limits its practical application, as it has high electricity consumption for generating the ultrasonic wave and producing the light [30–36]. In addition, mechanistic understanding on the synergistic effect of ultrasound and light in sonophotocatalysis is still not lacking, and more studies are required to clarify the underlying mechanism for optimization of the process [37,38]. Wang et al. synthesized Fe3O4@SiO2/PAEDTC@MIL-101(Fe), a mesoporous composite with a core-shell structure, and evaluated its sonophotocatalytic performance in degrading acid red 14 (AR14) [39]. The results showed that Fe3O4@SiO2/PAEDTC@MIL-101 (Fe)/UV/US exhibited excellent activity in the removal of AR14 and total organic carbon.

In this review, we aim to present an overview on the mechanisms of photocatalysis, sonocatalysis, and sonophotocatalysis in the treatment of organic dye wastewater [40,41]. We compare the performance of TiO2-based catalysts in photocatalytic, sonocatalytic, and sonophotocatalytic systems, with a focus on the synergistic effect of ultrasound and light in the catalytic degradation of organic dyes [42]. We further discuss the recent advances in catalyst development for sonophotocatalysis and point out the potential risks associated with the sonophotocatalytic process, such as the generation of toxic byproducts and the potential releases of nanoparticles into the environment [43–48]. It is essential to monitor the reaction products and assess their toxicity to ensure that the sonophotocatalytic process is safe for both the environment and human health. Finally, we propose future research directions to expand the fundamental knowledge on the sonophotocatalysis process and enhance its practical application in degrading organic pollutants [49–53]. The information presented in this review can provide valuable insights into the mechanisms and performance of photocatalysis, sonocatalysis, and sonophotocatalysis, and contribute to the development of more efficient and cost-effective treatment methods for organic dye wastewater. This review will be of great interest to researchers and practitioners in the field of environmental science and engineering, especially those involved in the development of sustainable wastewater treatment technologies.

### **2. Sonocatalytic and Photocatalytic Mechanisms**

### *2.1. Sonocatalytic Mechanism*

The sonocatalytic process is believed to be predominantly based on the "hot spots" and "sonoluminescence" that originate from the ultrasonic cavitation phenomenon [54]. Ultrasonic wave of a specific frequency and intensity can produce numerous small bubbles in liquids [55]. These minute bubbles trigger various physical and chemical transformations during their formation, oscillation, expansion, contraction, and ultimate collapse [56–58]. Figure 1 scehematically depicts the phenomenon of ultrasonic cavitation. It promotes the production of light with a range of wavelengths, called "sonoluminescence," and a large number of localized "hot spots" with very high temperatures (up to ~5000 K) and pressures (up to ~1000 atm) [59,60]. These localized "hot spots" can cause pyrolysis of H2O molecules, producing hydroxyl radicals (•OH) [61], which can effectively oxidize organic pollutants and even mineralize them into CO2 and H2O [62–66].

**Figure 1.** Schematic illustration of the acoustic generation of a cavitation bubble in water (after [66]). **Figure 1.** Schematic illustration of the acoustic generation of a cavitation bubble in water (after [66]).

In general, the sonolytic removal of organic pollutants involves oxidation through both pyrolysis and free radical attack [67]. However, due to the significant energy loss that occurs during thermal dissipation (exceeding 50%), radpid degradation often cannot occur when relying solely on ultrasound [68]. In recent years, the use of ultrasound in the presence of appropriate catalysts, known as sonocatalysis, has been increasingly used to degrade organic pollutants due to its numerous benefits, including convenient handling and low cost, as well as environmental friendliness [69]. Sonocatalytic degradation involves the use of a sonocatalyst to create additional active sites for the cavitation effect, leading to the formation of greater numbers with highly reactive radicals [70]. In general, these radicals could recombine to form H2O, •OH, H2O2, and •O2– in water [71,72]: In general, the sonolytic removal of organic pollutants involves oxidation through both pyrolysis and free radical attack [67]. However, due to the significant energy loss that occurs during thermal dissipation (exceeding 50%), radpid degradation often cannot occur when relying solely on ultrasound [68]. In recent years, the use of ultrasound in the presence of appropriate catalysts, known as sonocatalysis, has been increasingly used to degrade organic pollutants due to its numerous benefits, including convenient handling and low cost, as well as environmental friendliness [69]. Sonocatalytic degradation involves the use of a sonocatalyst to create additional active sites for the cavitation effect, leading to the formation of greater numbers with highly reactive radicals [70]. In general, these radicals could recombine to form H2O, •OH, H2O2, and •O2 − in water [71,72]:

$$(\text{H}\_2\text{O} + \text{)}) \rightarrow \bullet\text{H} + \bullet\text{OH} \tag{1}$$

$$\bullet \text{H} + \bullet \text{OH} \to \text{H}\_2\text{O}\_2 \tag{2}$$

O2 + •H → •HO2 (3)

$$\bullet \text{O}\_2 + \bullet \text{H} \to \bullet \text{HO}\_2 \tag{3}$$

$$\text{2} \bullet \text{OH} \rightarrow \text{H}\_{2}\text{O}\_{2} \tag{4}$$

$$\text{2} \bullet \text{HO}\_2 \rightarrow \text{O}\_2 + \text{H}\_2\text{O}\_2 \tag{5}$$

$$\text{H}\_2\text{O} + \bullet\text{OH} \rightarrow \text{H}\_2\text{O}\_2 + \bullet\text{H} \tag{6}$$

cess, it is essential to understand the underlying mechanisms of sonocatalysis [74]. Figure 2 depicts the major processes involved in sonocatalysis elucidated by extensive studies conducted in the field [75,76]. Sonocatalysis is a crucial technology in the degradation of pollutants, and radicals play an essential role in this process [73]. These radicals can initiate chain reactions that lead to the degradation of pollutants. To enhance the efficiency of the sonocatalytic process, it is essential to understand the underlying mechanisms of sonocatalysis [74]. Figure 2 depicts the major processes involved in sonocatalysis elucidated by extensive studies conducted in the field [75,76].

Sonocatalysis is a crucial technology in the degradation of pollutants, and radicals

**Figure 2.** Schematic diagram of sonocatalytic mechanism (after [23]). **Figure 2.** Schematic diagram of sonocatalytic mechanism (after [23]).

#### 2.1.1. Heterogeneous Nucleation Mechanism 2.1.1. Heterogeneous Nucleation Mechanism

Semiconductor particles have been observed to induce preferential formation of nuclei at solid surfaces or phase boundaries, leading to increased formation of cavitation bubbles and free radicals, such as •OH [54]. The phenomenon of heterogeneous nucleation has been found to be more applicable than homogeneous cavitation in sonocatalysis [56]. This can be attributed to the fact that the thermodynamic nucleation barriers on interfaces are generally lower than their bulk counterparts, promoting surface nucleation [18]. The relationship between the maximum energy barriers of heterogeneous and homogeneous nucleation processes can be expressed as [57,60]: Semiconductor particles have been observed to induce preferential formation of nuclei at solid surfaces or phase boundaries, leading to increased formation of cavitation bubbles and free radicals, such as •OH [54]. The phenomenon of heterogeneous nucleation has been found to be more applicable than homogeneous cavitation in sonocatalysis [56]. This can be attributed to the fact that the thermodynamic nucleation barriers on interfaces are generally lower than their bulk counterparts, promoting surface nucleation [18]. The relationship between the maximum energy barriers of heterogeneous and homogeneous nucleation processes can be expressed as [57,60]:

$$
\Delta \mathbf{G}^\*\_{\text{het}} = \frac{16\pi \text{o}^3}{\text{3P}^2} \text{ f}(\theta) = \Delta \mathbf{G}^\*\_{\text{hom}} \text{ f}(\theta) \tag{7}
$$