Synthesis of ZnPc/BiVO₄ Z-Scheme Heterojunction for Enhanced Photocatalytic Degradation of Tetracycline Under Visible Light Irradiation

Abstract

The construction of semiconductor heterojunctions is an effective strategy to improve the photocatalytic degradation efficiency of organic pollutants. Herein, ZnPc/BiVO₄ Z-scheme heterojunction was synthesized via a physical mixing method and was used for the photocatalytic degradation of tetracycline (TC) under visible light irradiation. Compared with BiVO₄ and ZnPc, the 15ZnPc/BiVO₄ sample exhibited improved light absorption capacity, and the electron-hole separation efficiency and redox capacity were enhanced due to the formation of the Z-scheme heterojunction. The 15ZnPc/BiVO₄ composite exhibited an optimal TC degradation rate of 83.1% within 120 min. Additionally, 15ZnPc/BiVO₄ exhibited excellent stability in cycling experiments, which maintained a high TC degradation rate of 79.5% after four cycles. Free radical trapping experiments indicated that superoxide radicals (·O₂⁻) were the main active substances in the photocatalytic degradation of TC.

1. Introduction

Over the past few decades, antibiotics have been extensively utilized for the treatment and prevention of bacterial infections. However, the widespread presence of antibiotic residues in aquatic environments has become a significant concern, posing threats to ecosystems. Tetracycline (TC), a broad-spectrum antibiotic, is commonly employed in both veterinary and human medical practices [1]. Substantial quantities of TC are consequently released into the environment. Due to their complex chemical structure and low volatility, TC is particularly resistant to biodegradation, making its removal challenging [1]. Therefore, there is an urgent need to identify cost-effective and sustainable methods to mitigate the environmental impact of TC. Fortunately, photocatalysis has emerged as a promising, eco-friendly, and efficient technology for the degradation of TC.

Among various developed photocatalysts, BiVO₄ is a popular visible photocatalyst for the photodegradation of various organic pollutants because of its excellent photoelectric properties and narrow band gap (~2.4 eV) [2]. However, the photocatalytic efficiency of pure BiVO₄ is still unsatisfactory due to the high recombination efficiency of photogenerated carriers and the low utilization efficiency of visible light, which severely limits the practical application of photodegradation. Furthermore, the valence band (VB) potential and conduction band (CB) potential of BiVO₄ with a narrow band gap are not sufficiently positive or negative. The photogenerated electrons and holes lack redox activity and are unable to drive specific photocatalytic reactions. Specifically, it is difficult to produce free radicals, including hydroxyl radicals (·OH) or superoxide radicals (·O₂⁻), which are crucial for photocatalytic pollutant removal [3]. Therefore, a novel strategy to enhance the photogenerated charge separation and redox activity of BiVO₄ during the photocatalytic reaction is highly desirable.

In recent years, numerous efforts have been devoted to enhancing the photocatalytic performance of BiVO₄. The construction of heterojunctions, especially Z-scheme heterojunctions, is considered a promising strategy to overcome the problems of severe photogenerated electron–hole recombination and poor redox capacity for pure BiVO₄. Z-scheme heterojunctions facilitate the separation of photogenerated charge carriers. Specifically, the photogenerated electrons on the CB of the oxidation photocatalyst combine with holes in the VB of the reduction photocatalyst, while the electrons and holes with stronger reduction and oxidation abilities are retained to participate in the photocatalytic redox reaction [4,5].

Besides the construction of heterojunctions, the sensitization of BiVO₄ with dye is another promising approach to promote charge separation efficiency, which can facilitate photocatalytic reactions by harvesting light photons of longer wavelengths. As effective photosensitizers, metal phthalocyanines (MPcs) have been employed to enhance photoactivity due to their excellent redox activity, high thermal stability, and nontoxicity [6]. Zinc phthalocyanine (ZnPc) is an effective photosensitizer with a wide visible light response from 600 to 800 nm [7]. ZnPc is an ideal reduction photocatalyst for the construction of Z-scheme heterojunctions, which can effectively separate photogenerated carriers and generate activated species such as ·O₂⁻ for the degradation of organic pollutants. Thus, considering the suitable match of the band structures ZnPc and BVO₄, the construction of a wide-visible-light-responsive artificial ZnPc/BiVO₄ Z-scheme heterojunction is a reasonable strategy. Furthermore, ZnPc is analogous to light-harvesting chlorophyll with a porphyrin ring. The conjugated macrocycle structure is conducive to forming a dimensionally matched interface with BiVO₄, further enhancing the photocatalytic performance.

In this work, a ZnPc/BiVO₄ composite photocatalyst was prepared by a facile assembly strategy involving ultrasound-assisted mechanical stirring, which was applied for the photocatalytic degradation of TC under visible light irradiation. The ZnPc/BiVO₄ organic/inorganic heterojunction effectively promotes the separation of photogenerated electrons and holes, thus enhancing the photocatalytic TC degradation performance. In addition, the photocatalytic reaction mechanism of TC degradation over ZnPc/BiVO₄ was revealed.

2. Results and Discussion

The XRD patterns of BiVO₄, ZnPc, and ZnPc/BiVO₄ composites are shown in Figure 1a, which determines the crystal structure. The diffraction peaks at 18.68°, 18.98°, 28.95°, 30.55°, 34.49°, 35.22°, 42.46°, 53.31°, 58.53°, and 59.26° correspond to the (110), (011), (121), (040), (200), (002), (051), (161), (321), and (123) facets of monoclinic BiVO₄ (JCPDS No. 14-0688), respectively [8,9]. The diffraction peaks at 6.86°, 9.14°, 10.57°, 12.70°, 23.83°, and 26.21° are attributed to the (001), (201), (200), (202), (211), and (311) facets of β-ZnPc (JCPDS No. 39-1882), respectively [10,11,12]. The peaks of BiVO₄ are mainly observed in the XRD patterns of ZnPc/BiVO₄ composites, which have no shift compared to those in the XRD pattern of BiVO₄, indicating that the added ZnPc is dispersed on the surface of BiVO₄ rather than entering the lattice of BiVO₄ [13]. With the increase in ZnPc amount, the peaks of the (001) and (201) facets of β-ZnPc gradually strengthen in the XRD patterns of the ZnPc/BiVO₄ composite. The above results prove the successful preparation of the ZnPc/BiVO₄ composite photocatalyst. Furthermore, compared with that of BiVO₄, the intensity of the diffraction peak at 30.55° of the ZnPc/BiVO₄ composites was significantly greater, which may be due to the overlap of the same peak positions [14].

The FT-IR spectra of BiVO₄, ZnPc, and ZnPc/BiVO₄ are shown in Figure 1b. The peaks at 837 and 739 cm⁻¹ are attributed to the symmetric stretching vibrations and asymmetric stretching vibrations of the V–O bonds in BiVO₄, respectively [15]. The sharp IR peaks between 600 and 1500 cm⁻¹ are ascribed to the phthalocyanine skeletal vibrations of ZnPc [16]. Among them, the peaks at 777 and 1060 cm⁻¹ are attributed to C–H out-of-plane bending vibrations and C–H in-plane bending vibrations, respectively. The peak at 1282 cm⁻¹ corresponds to the telescopic vibration peak of C–N in the phthalocyanine structure. The peak at 1411 cm⁻¹ is assigned to the aromatic phenyl ring [17]. With the increase in ZnPc amount, the characteristic peaks of ZnPc are gradually enhanced in the FT-IR spectra of ZnPc/BiVO₄ composites, indicating the successful compounds of ZnPc and BiVO₄. Furthermore, the peak observed at 3411 cm⁻¹ is attributed to the stretching vibrations of O–H groups on the surface. However, with the addition of ZnPc, the intensity of the O–H bond decreases, indicating the interaction between ZnPc and BiVO₄. As shown in Figure S1, the N₂ adsorption–desorption isotherms and the BJH pore size distribution plots were obtained to determine the textural properties of the catalysts, and the surface area, pore volume, and average pore diameter of BiVO₄, ZnPc, and 15ZnPc/BiVO₄ composites are shown in Table S1. As shown in Table S1, after the addition of ZnPc, the specific surface area of the ZnPc/BiVO₄ composite (1.00 m² g⁻¹) greatly increased compared to that of BiVO₄ (0.16 m² g⁻¹), which is beneficial to enhance the photocatalytic reaction.

As shown in the SEM images of BiVO₄ (Figure 2a,b) and ZnPc (Figure 2c), the prepared BiVO₄ presents a regular decahedral morphology with a relatively smooth surface and sharp edges, while the prepared ZnPc exhibits an irregular block morphology [18]. From the SEM images of the ZnPc/BiVO₄ composites, ZnPc nanoparticles with different sizes are evenly distributed on the surface of BiVO₄. The tight contact between ZnPc and BiVO₄ indicates the successful compounds of ZnPc and BiVO₄, which is advantageous for the transfer of photogenerated carriers. However, when the amount of ZnPc added is 25 wt% in ZnPc/BiVO₄ composites, a large amount of ZnPc stacked on the surface of BiVO₄ will cover the active sites of BiVO₄, which may restrict the photocatalytic degradation performance. The EDX spectrum of the 15ZnPc/BiVO₄ sample (Figure S2) confirms the presence of Zn, C, N, Bi, O, and V, further verifying the successful construction of the 15ZnPc/BiVO₄ composite. Moreover, from the EDS mapping of 15ZnPc/BiVO₄ composites (Figure 2g), the elements of Bi, V, O, Zn, C, and N are uniformly distributed on the surface, further verifying the successful construction of the ZnPc/BiVO₄ composite.

The TEM and HRTEM images of the 15ZnPc/BiVO₄ composite were further observed in the interface between ZnPc and BiVO₄. As shown in Figure 3, BiVO₄ is evenly covered with ZnPc nanoparticles. The lattice fringes with a spacing of 0.307 nm correspond to the (121) planes of BiVO₄ [19], which is consistent with the results of the XRD pattern. The amorphous regions without clear lattice fringes are ascribed to ZnPc. Meanwhile, the TEM and HRTEM images of the 15ZnPc/BiVO₄ composite determine the close contact interface between ZnPc and BiVO₄, facilitating the separation of photogenerated carriers.

The XPS spectra of BiVO₄ and 15ZnPc/BiVO₄ are performed to determine the elemental composition and chemical states (Figure 4, the colored solid line represents the XPS fitting results and the dashed line represents the peak location). As shown in Figure 4a, the elements of Bi, V, O, Zn, C, and N are found in the XPS survey spectra of 15ZnPc/BiVO₄ composites, indicating the successful compounds of ZnPc and BiVO₄. From the XPS spectra of Bi 4f (Figure 4b), the peaks at 158.9 and 164.3 eV are attributed to Bi 4f_7/2 and Bi 4f_5/2, respectively, indicating the existence of Bi³⁺ in BiVO₄ [19]. Moreover, the peaks at 516.6 and 524.1 eV are ascribed to V 2p_3/2 and V 2p_1/2, respectively, illustrating the form of V⁵⁺ in BiVO₄ (Figure 4c) [20]. From Figure 4d, the peaks at 529.7, 531.5, and 533.0 eV correspond to lattice oxygen (Bi–O), surface hydroxyl (OH), and adsorbed water on the surface, respectively [21,22,23]. As shown in Figure 4e, the peaks at 1022.3 and 1045.3 eV are attributed to Zn 2p_3/2 and Zn 2p_1/2, respectively, indicating the form of Zn²⁺ in ZnPc [24,25]. From Figure 4f, the peaks at 398.9 and 400.6 eV are ascribed to the C–N and C=N bonds of ZnPc, respectively [26,27]. Additionally, the peaks of Bi 4f, V 2p, and O 1s in 15ZnPc/BiVO₄ shift in the direction of increasing binding energy in comparison to those in BiVO₄, suggesting an interaction between ZnPc and BiVO₄ [14]. This shift proves the formation of the ZnPc/BiVO₄ organic/inorganic heterojunction, which accelerates the transfer of photogenerated charges at the contact interface between ZnPc and BiVO₄.

The optical absorption properties are an important index to evaluate photocatalysts, which are investigated by UV-vis DRS spectra. This is due to the intrinsic band gap absorption BiVO₄ presented the visible light response below 550 nm (Figure 5a) [28]. As shown in Figure 5b, ZnPc demonstrated excellent light capture ability in the range of 230–850 nm [29]. The light absorption ranges of the ZnPc/BiVO₄ composites are redshifted compared with those of BiVO₄. Additionally, with the increasing ZnPc amount, the light absorption properties of the ZnPc/BiVO₄ composites gradually increased in the visible light range. The ZnPc nanoparticles distributed on the surface of BiVO₄ could improve the absorption of visible light, which is consistent with the SEM results of ZnPc/BiVO₄ composites. Therefore, the successful introduction of ZnPc extended the absorbance of BiVO₄ to the far-visible region. The relationships between (F(R_∞)hν)^1/2 and the photon energies of BiVO₄ and ZnPc are shown in Figure 5c,d [30]. According to the Kubelka–Munk function, through the (F(R_∞)hν)² against hν plots, the energy band gap (E_g) of BiVO₄ and ZnPc was determined by extending the straight part of the curves to abscissa. As indicated by the dashed lines and arrows, the band gaps (E_g) of BiVO₄ and ZnPc were calculated to be 2.33 eV and 1.93 eV, respectively.

The separation of photogenerated electron–hole pairs and transfer of photoproduction charges is crucial for photocatalytic performance. The generation and transfer of photoexcited charges can be confirmed by the transient photocurrent response [31]. As shown in Figure 6a, 15ZnPc/BiVO₄ exhibited the significantly highest photocurrent density, indicating that the concentration of photogenerated carriers increases with the compounds ZnPc and BiVO₄. The above results indicate that the addition of ZnPc provides a significant positive effect on promoting the separation of photogenerated electrons and holes. Additionally, BiVO₄, ZnPc, and 15ZnPc/BiVO₄ exhibited the approximate photocurrent intensity during four switching cycles under visible light irradiation, verifying the stability of BiVO₄, ZnPc, and 15ZnPc/BiVO₄.

EIS Nyquist plots are used to analyze the charge transfer resistance of the photocatalysts. The arc radius usually reflects the interfacial charge transfer resistance. Briefly, a smaller arc radius corresponds to a higher charge migration capacity [32]. As shown in Figure 6b, the relative arc radius of the photocatalysts was given in the following order: 15ZnPc/BiVO₄ < ZnPc < BiVO₄, indicating that the charge transfer resistance of 15ZnPc/BiVO₄ is less than that of ZnPc and BiVO₄. The results of the photocurrent response and EIS Nyquist plots confirm that the 15ZnPc/BiVO₄ photocatalyst presents the best separation efficiency of the photogenerated electron–hole pairs and the fastest transfer rate of photo-induced charges during the photocatalytic reaction, which boosts photocatalytic performance.

The photocatalytic degradation performances over BiVO₄, ZnPc, and ZnPc/BiVO₄ composites are shown in Figure 7a. Before the photocatalytic reaction, the reaction mixture of the photocatalyst and TC solution was stirred in the dark for 30 min to ensure an adsorption–desorption equilibrium. Under visible light irradiation, the photocatalytic degradation rates of TC over BiVO₄, ZnPc, 5ZnPc/BiVO₄, 15ZnPc/BiVO₄, and 25ZnPc/BiVO₄ were 55.3%, 7.3%, 79.9%, 83.1%, and 78.8%, respectively, confirming that the addition of ZnPc can promote photocatalytic performance. With the increase in ZnPc amount, the degradation rate of TC tended to increase and then gradually decrease. The TC degradation rate of 15ZnPc/BiVO₄ was the highest and was 27.8% higher than that of BiVO₄ and 75.8% higher than that of ZnPc. When the amount of ZnPc was 25 wt% in ZnPc/BiVO₄ composite, the excess ZnPc covered the active site of BiVO₄, thus inhibiting the photocatalytic degradation performance [33]. Furthermore, the degradation rates decreased with reaction time during the photocatalytic degradation of TC, which can be attributed to the inhibiting effect of the accumulated byproducts and the decrease in TC concentration in the reaction system [34]. Notably, the controlled experiment of the photolysis of TC under visible light irradiation in the absence of a catalyst was carried out. As shown in Figure S3, the tetracycline only exhibited a degradation rate of 2.3% after photolysis for 120 min. Therefore, the photolysis of tetracycline under visible light irradiation in the absence of a catalyst is negligible, and the degradation of TC was mainly due to the photocatalytic reaction.

The photocatalytic degradation of TC was fitted into the pseudo-first-order kinetic model. As shown in Figure S4, the apparent kinetic rate constant (k) of 15ZnPc/BiVO₄ (0.01701 min⁻¹) was the highest among all the samples, which was 2.61 and 77.3 times greater than that of BiVO₄ and ZnPc, respectively. Meanwhile, according to the references [35], the products generated in photocatalytic degradation of TC might be DP 431, DP 417, DP 461, DP 509, or DP 525. DP 431 was the demethylation product of TC and was further transformed into DP 417 [36]. DP 431 was the demethylation product of TC and was further transformed into DP 417. DP 461 was the hydroxylation product of TC and was transformed into DP 509 after the consecutive cleavage and oxidation of the phenol ring of TC. Moreover, DP 509 was further transformed into DP 525 by the addition of OH to the double-bond ring [37].

The stability and reusability of photocatalysts is a vital index. To explore the stability and reusability of 15ZnPc/BiVO₄ in the photocatalytic degradation of TC, the photocatalytic TC degradation performance of the four cycles is shown in Figure 7b. The degradation rates of TC over 15ZnPc/BiVO₄ still remain stable after four cycles. After the fourth cycle, the degradation rate of TC still reaches 79.5% within 120 min, which decreases by 3.6% compared with that in the first cycle. The above results demonstrate the favorable stability of the 15ZnPc/BiVO₄ composite photocatalyst.

To further reveal stability, the XRD patterns and FT-IR spectra of 15ZnPc/BiVO₄ before and after the photocatalytic TC degradation reaction are displayed in Figure S5. It can be seen from the XRD patterns that the locations of the diffraction peaks of 15ZnPc/BiVO₄ before and after the reaction are consistent. Notably, the peak at 15.20° disappears, while the intensity of the peak at 30.55° decreases after the reaction, which may be attributed to the shedding of fractional ZnPc from the 15ZnPc/BiVO₄ surface. Before and after the photocatalytic TC degradation reaction, the IR peaks of 15ZnPc/BiVO₄ are essentially consistent, indicating that the molecular structure and the surface group of 15ZnPc/BiVO₄ do not change after four cycles of stability testing. Furthermore, no new IR peaks were found after the reaction, indicating that TC and its intermediates adsorbed on the surface of 15ZnPc/BiVO₄ photocatalyst during the reaction can be removed by washing with deionized water. The above results demonstrate the stability and reusability of the ZnPc/BiVO₄ composite photocatalyst.

As shown in Figure 8a, the degradation rate of TC increases from 71.5% to 86.7% as the additive amount of 15ZnPc/BiVO₄ increases from 10 mg to 60 mg. The number of active sites increases with the increasing additive amount, which can act as reaction centers to accelerate TC degradation [38]. When the amount of 15ZnPc/BiVO₄ was 30 mg, the degradation rate of TC reached 83.1%. However, when the amount of 15ZnPc/BiVO₄ increases to 60 mg, the degradation rate of TC increases by only 3.6%. The superfluous photocatalyst may have agglomerated, resulting in a decrease in the number of active sites [39]. In addition, excess photocatalyst will produce a shielding effect, which prevents the partial photocatalyst from absorbing visible light, thus reducing the degradation performance [40,41]. According to the degradation rates of TC and economy, 30 mg was selected as the optimal additive amount of photocatalyst for TC degradation.

The influence of the initial TC concentration on the photocatalytic TC degradation rate of 15ZnPc/BiVO₄ is exhibited in Figure 8b. The degradation rate of TC decreased from 86.5% to 75.8%, with the initial TC concentration increased from 10 mg/L to 40 mg/L. Apparently, a higher initial TC concentration causes a negative effect on the degradation reaction process due to the decreased light penetration and the accumulation of intermediates [41]. On the one hand, as the TC concentration increases, the penetration of light into the solution decreases, which affects the utilization of available light. On the other hand, the higher the initial TC concentration, the more byproducts that are produced, which competes with TC molecules for the active site, delaying the photocatalytic reaction process [42]. Therefore, 20 mg/L was selected as the optimal initial concentration of TC.

The pH of a reaction solution not only affects the type and amount of active substances but also affects the charges on the photocatalyst surface, thus affecting the degradation rate of pollutants [43]. To investigate the effect of pH on the photocatalytic degradation of TC over 15ZnPc/BiVO₄, the initial pH of the TC solution was adjusted from 4 to 8 by 0.5 mol/L HCl or 0.5 mol/L NaOH. After 120 min of illumination, the photocatalytic degradation rates of TC over 15ZnPc/BiVO₄ were 66.8%, 83.1%, 82.6%, and 77.7% with the initial TC solutions at pH = 4, 6, 7, and 8, respectively. The photocatalytic degradation rate of TC is clearly restricted under acidic conditions (pH = 4). TC is mainly converted to TC⁺ in the acidic solution (pH = 4). The competitive adsorption of H⁺ and TC⁺ on the surface of 15ZnPc/BiVO₄ inhibits the contact between TC⁺ and 15ZnPc/BiVO₄, thus reducing photocatalytic activity [44]. In alkaline solutions (pH = 8), TC is mainly converted to TC⁻. The electrostatic repulsion between TC⁻ and 15ZnPc/BiVO₄ causes a reduction in the degradation performance of TC [45]. In conclusion, the neutral solution (pH = 6) was selected for the photodegradation of TC. Furthermore, considering the solution pH, initial TC concentration, the additive amount of photocatalysts, and reaction time, 15ZnPc/BiVO₄ exhibited an excellent degradation rate compared with other reported BiVO₄-based photocatalysts, as shown in Table S2.

In the actual wastewater treatment process, the coexistence of various anions might affect the photocatalytic degradation performance of TC. The coexisting anions act as free radical scavengers, converting strongly oxidizing active species into less oxidizing active species. As shown in Figure 8d, the presence of Cl⁻, HCO₃⁻, and SO₄²⁻ had no significant effect on the degradation rate of TC over 15ZnPc/BiVO₄. Dissimilarly, H₂PO₄²⁻ exhibits a significant inhibitory effect on the photodegradation of TC over 15ZnPc/BiVO₄, and the degradation rate of TC was only 46.8% after 120 min, which is attributed to the competitive adsorption of H₂PO₄⁻ and TC at the active site on the surface of 15ZnPc/BiVO₄ [46]. H₂PO₄⁻ provides a highly polar environment in the vicinity of the surface of 15ZnPc/BiVO₄, preventing the adsorption of TC. In addition, H₂PO₄⁻ can also be used as an ∙OH trap, which reduces the concentration of ∙OH in the photocatalytic degradation reaction.

The band structures of BiVO₄ and ZnPc were determined by XPS-VB spectra and Mott–Schottky plots. As shown in Figure 9a,b, the positive slopes of the Mott–Schottky plots indicate that both BiVO₄ and ZnPc are n-type semiconductors [23]. The flat-band potentials of BiVO₄ and ZnPc were −0.50 and −0.44 V (vs. Ag/AgCl, pH = 7), respectively. Generally, the Fermi level (E_F) is close to flat band potential. According to Equations (1) and (2), the Fermi levels were calculated to be 0.11 and 0.17 V (vs. NHE, pH = 0) [47].

As shown in Figure 9c,d, the E_VB′ of BiVO₄ and ZnPc were 2.06 and 1.10 eV, respectively. Based on Equations (3) and (4), the E_VB of BiVO₄ and ZnPc were calculated to be 2.17 and 1.27 V (vs. NHE, pH = 0), respectively. Thus, the E_CB of BiVO₄ and ZnPc were calculated to be −0.16 and −0.66 V (vs. NHE, pH = 0), respectively [48].

$$E_{\mathrm{F}}=E_{\mathrm{f}\mathrm{b}}=E(\mathrm{NHE})=E^{\theta }(\mathrm{Ag}/\mathrm{AgCl})+E+0.059pH$$

(1)

$$E^{\theta }(\mathrm{Ag}/\mathrm{AgCl})=0.197 \mathrm{V}$$

(2)

$$E_{\mathrm{VB}}={E_{\mathrm{VB}}}^{\prime }+E_{\mathrm{f}}$$

(3)

$$E_{\mathrm{CB}}=E_{\mathrm{VB}}-E_{\mathrm{g}}$$

(4)

To further explore the reaction mechanism for the photocatalytic degradation of TC, the active free radical species were determined by free radical trapping experiments. Isopropanol (IPA), 1,4-benzoquinone (p-BQ), and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were employed to eliminate hydroxyl radicals (∙OH), superoxide radicals (∙O₂⁻), and holes (h⁺), respectively [49]. As shown in Figure 10, after adding IPA, p-BQ, and EDTA-2Na, the photocatalytic degradation rate of TC decreased to 76.4%, 53.0%, and 71.5%, respectively, indicating that ∙OH, ∙O₂⁻ and h⁺ all play crucial roles in the photodegradation of TC. Among them, O₂⁻ played a major role, and the contribution of free radical species was as follows: ∙O₂⁻ > h⁺ > ∙OH.

As shown in Figure 11, BiVO₄ and ZnPc present a staggered band structure, which can form type II heterojunctions or Z-scheme heterojunctions [50,51]. The active radical species and photogenerated carrier transfer pathways of these two heterojunctions are different. According to the mechanism of the type II heterojunction, under visible light irradiation, the photogenerated electrons are transferred from the CB of ZnPc to the CB of BiVO₄, and the holes are transferred from the VB of BiVO₄ to the VB of ZnPc. The reason for this was that the conduction band position of BiVO₄ (−0.16 V) was significantly positive for the potential of O₂ to ⋅O₂⁻ (−0.33 V) as shown by the blue dashed line. Therefore, the electrons on the CB of BiVO₄ could not reduce the absorbed O₂ to ⋅O₂⁻, which obviously contrasted with the results obtained from free radical trapping experiments (Figure 10) [52]. According to the Z-scheme heterojunction mechanism, the photogenerated electrons in the CB of BiVO₄ recombine with the holes in the VB of ZnPc. Thus, the more negative reduction potential (−0.66 V) in the 15ZnPc/BiVO₄ Z-scheme heterojunction satisfies the reduction potential of ⋅O₂⁻. Under visible light irradiation, photogenerated holes and O₂⁻ free radicals promote photocatalytic TC degradation. In addition, the VB potential of BiVO₄ and ZnPc is higher than the oxidization potential of H₂O to ∙OH⁻ (2.40 V), as shown by the blue dashed line, which is consistent with the results of the free radical trapping experiment. In summary, the 15ZnPc/BiVO₄ Z-scheme heterojunction effectively promotes the separation of photogenerated electron–hole pairs and accelerates enhanced photocatalytic TC degradation.

3. Experimental Section

3.1. Materials

All the chemicals used were analytically pure and original. Bismuth nitrate pentahydrate (Bi(NO₃)₃⋅5H₂O, Guangdong Guanghua Sci-Tech, Co., Ltd., Shantou, China, >99%), ammonium meta-vanadate (NH₄VO₃, Guangdong Guanghua Sci-Tech, Co., Ltd., >99%), nitric acid (HNO₃, Guangdong Guanghua Sci-Tech, Co., Ltd., >99%), ammonium hydroxide solution (NH₃⋅H₂O, Xilong Scientific Co., Ltd., Shantou, China, 25~28%), zinc phthalocyanine (C₃₂H₁₆N₈Zn, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China, >95%), tetracycline (C₂₂H₂₄N₂O₈, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China, >99%), anhydrous ethanol (C₂H₅OH, Xilong Scientific Co., Ltd., >99%), and deionized water (DI Water) were used throughout the entire experiment.

3.2. Preparation of BiVO₄ and ZnPc/BiVO₄

BiVO₄ was prepared via a hydrothermal method [53]. Typically, 6 mmol Bi(NO₃)₃⋅5H₂O and 6 mmol NH₄VO₃ were added to a 50 mL HNO₃ (2 mol/L) solution and stirred for 20 min. The mixed solution was then brought to pH = 2 with NH₃⋅H₂O and stirred for 2 h at room temperature. The precursor solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and treated at 200 °C for 24 h. After cooling to room temperature, the precipitate was collected via centrifugation and washed with deionized water and anhydrous ethanol 5 times. It was then dried at 60 °C for 12 h to obtain the BiVO₄ sample.

ZnPc/BiVO₄ composites were prepared by a physical mixing method. Specifically, a certain amount of ZnPc was dispersed in 20 mL of anhydrous ethanol by ultrasound at room temperature for 1 h. A total of 0.18 g of BiVO₄ was added to the ZnPc solution, which was continuously stirred for 12 h. The precipitate obtained by filtrating was washed several times with deionized water and then dried at 60 °C in a vacuum oven overnight. After sufficient grinding, the ZnPc/BiVO₄ composites were obtained and recorded as x-ZnPc/BiVO₄, where x is the mass fraction of ZnPc in the composite photocatalyst (5%, 15%, or 25%).

3.3. Characterization of the Catalysts

The phase structure of the samples was measured via X-ray diffraction (XRD, BRUKER AXSGMBH, Karlsruhe, Germany). The functional groups of the samples were detected via Fourier transform–infrared (FT-IR) spectroscopy (TENSOR II, BRUKER, Karlsruhe, Germany). The morphological characteristics of the samples were observed by scanning electron microscopy (SEM, ZEISS Gemini 300, Oberkochen, Germany). The microscopic morphology and interface structure of the samples were observed via high-resolution transmission electron microscopy (HR-TEM, FEI Talos F200X, Thermo Fisher Scientific, Waltham, MA, USA). The chemical state of the surface elements was analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, USA), and all the binding energies were calibrated to a C1s (248.8 eV) standard peak. In addition, the valence band spectra were analyzed by an X-ray photoelectron spectrometer. The optical absorption properties of the samples were investigated via a UV-vis spectrophotometer (UV-vis DRS, TU-19, Beijing Puxi General Instrument Co., Ltd., Beijing, China).

3.4. Photoelectrochemical Measurements

The photoelectrochemical measurements were performed on a CHI760E electrochemical workstation (Chenhua, Shanghai, China) using a standard three-electrode cell. An Ag/AgCl electrode, Pt electrode, and photocatalyst electrode were used as the reference electrode, auxiliary electrode, and working electrode, respectively. An aqueous solution of 0.5 mol/L Na₂SO₄ was used as an electrolyte. The working electrodes were prepared as follows: 10 mg of photocatalyst was added to 200 μL of alcohol, and 20 μL of Nafion solution was added to form a slurry. After ultrasonication for 90 min, the suspension was coated on the surface of indium tin oxide (FTO) conductive glass with an area of 1 cm² (1 cm × 1 cm) and dried naturally to obtain a working electrode.

3.5. Photocatalytic Activity Measurements

The photocatalytic activities of the photocatalysts were evaluated for their ability to degrade TC under visible light irradiation. Initially, 30 mg of the photocatalyst was dispersed into 100 mL of the TC solution (20 mg/L). The suspension was stirred in the dark for 30 min to reach an adsorption–desorption equilibrium. The suspension was subsequently placed under a 300 W xenon lamp (CEL-PAEM-D6, Beijing China Education Au-light Co., Ltd., Beijing, China) with a 400 nm cut filter for photocatalytic degradation. The light intensity was measured with an optical power meter (CE-NP2000, Beijing China Education Au-light Co., Ltd.). The light intensity was measured to be 95 mW/cm². Every 30 min, 4 mL of the suspension was collected and filtered through a 0.22 μm membrane filter to separate the solid particles. The concentration of TC was subsequently measured with an ultraviolet–visible spectrophotometer (UV-vis) at 357 nm. The degradation rate (η) of TC was calculated via Equation (5). According to Equation (6), the first-order reaction kinetic fit for the experimental data in the photoreaction phase yielded an apparent reaction rate constant.

$$\eta =(1-C_{\mathrm{t}}/C_0)\times 100\%$$

(5)

$$-\ln (C_{\mathrm{t}}/C_0)=\mathrm{k}t$$

(6)

where η is the degradation rate of TC, %; C₀ is the initial concentration of the TC solution, mg/L; and C_t is the concentration of TC at different reaction times, mg/L. It is worth noting that C₀ in Equation (6) is the TC concentration at the end of the dark reaction

3.6. Stability Test

In order to evaluate the stability of the 15ZnPc/BiVO₄ composites, multiple cyclic degradation experiments were carried out. First, at the end of each photodegradation experiment, the catalyst was collected by filtration, washed several times with deionized water, and then dried in a vacuum drying oven at 60 °C. Notably, to make sure the same amount of catalyst was used in each cyclic experiment, enough photocatalysts were collected through multiple photocatalytic degradation reactions. The collected photocatalyst was used for the next degradation experiment, and the total cycle degradation was performed 4 times.

4. Conclusions

In this work, a ZnPc/BiVO₄ composite was prepared via a physical mixing method. The ZnPc/BiVO₄ Z-scheme heterojunction was constructed and used for the photocatalytic degradation of TC. Compared with BiVO₄ and ZnPc, the ZnPc/BiVO₄ composites exhibited better photocatalytic performance. After the photocatalytic degradation of TC for 120 min, the degradation rate of TC over 15ZnPc/BiVO₄ reached 83.1%. The tight organic/inorganic contact interface and the Z-scheme heterojunction could promote the separation of photogenerated electron–hole pairs and accelerate the transfer of photogenerated charges, thus boosting the photocatalytic degradation performance of TC. In addition, free radical trapping experiment revealed that ⋅O₂⁻ plays a major role in the degradation process of TC. The photogenerated electrons on the CB of BiVO₄ were recombined with the holes on the VB of ZnPc so that the photogenerated holes on the VB of BiVO₄ and the photogenerated electrons on the CB of ZnPc would provide a strong redox capacity, which is conducive to increasing the concentration of ⋅O₂⁻ free radicals, thus promoting the photocatalytic TC degradation reaction.