Construction of Zn_0.5Cd_0.5S/Bi₄O₅Br₂ Heterojunction for Enhanced Photocatalytic Degradation of Tetracycline Hydrochloride

Abstract

The development of efficient catalysts with visible light response for the removal of pollutants in an aqueous environment has been a hotspot in the field of photocatalysis research. A Zn_0.5Cd_0.5S (ZCS) nanoparticle/Bi₄O₅Br₂ ultra-thin nanosheet heterojunction was constructed by ultrasound-assisted solvothermal method. The morphology, structure, and optoelectronic properties of the composite were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectra. Under simulated visible light illumination, the photocatalytic performance was evaluated through degradation of tetracycline hydrochloride. Results show that the degradation effect by the optimum ZCS/Bi₄O₅Br₂ catalyst is superior to pure materials with the kinetic constant that is 1.7 and 9.6 times higher than those of Bi₄O₅Br₂ and ZCS, and also has better stability and reusability. Trapping experiments and electron paramagnetic resonance tests find that free radicals in the photocatalytic system are superoxide radicals and holes. This work provides a referable idea for the development of more efficient and recyclable photocatalysts.

1. Introduction

Tetracycline hydrochloride (TCH) is widely used in medicine and agriculture as a broad-spectrum antibiotic. It is stable and difficult to degrade naturally, and its overuse poses a serious threat to the environment and human beings [1,2]. The increasing amounts of various types of organic dyes discharged from industrial wastewater also pose a great challenge for water purification [3]. Conventional treatment technologies have difficulty removing pollutants in an efficient and environmentally friendly manner. In recent years, photocatalytic technology has emerged as a potential strategy for degrading hazardous pollutants [4,5]. Semiconductor photocatalytic materials have a strong redox capacity and are capable of converting most organic pollutants into H₂O, CO₂, and other small inorganic molecules for purifying harmful substances. Therefore, developing a new efficient photocatalyst is particularly important.

Bismuth oxyhalide BiOX (X = Cl, Br, I) materials have become a research hotspot in the field of photocatalysis due to their unique layered structure and superior photocatalytic performance [6]. However, their positive conduction band position (CB) and poor carrier separation ability limit the practical applications [7]. Density-functional-transfer (DFT) calculations of BiOBr show that its valence band is hybridized by O 2p and Br 4p orbitals, and the conduction band consists of Bi 6p orbitals; thus, its energy band position and band gap width can be changed by adjusting the ratio of Bi, O, and Br [8]. Bi₄O₅Br₂ constructed by this bismuth-rich strategy retains the 2D layered structure of BiOBr but has a narrower band gap, stronger photogenerated carrier separation efficiency, and better photocatalytic performance [9,10]. However, the photocatalytic role of pure Bi₄O₅Br₂ is limited, with low visible light utilization efficiency and high complexation rate of a photogenerated electron–hole pair [11,12]. Therefore, modifications are needed to improve the catalytic activity, including changing the morphology [13], constructing heterojunctions [14,15,16], doping [17,18], and introducing surface defects [19,20]. Selecting ideal semiconductors with suitable energy band structure to construct heterojunctions with excellent photocatalytic performance is of great significance. For example, Zhang et al. constructed In₂O₃/Bi₄O₅Br₂ S-scheme heterojunctions with interfacial oxygen vacancies using a hydrothermal and chemical precipitation method. The obtained catalyst was capable of degrading tetracycline efficiently [21]. Qian et al. synthesized a 2D/2D Bi₄O₅Br₂/Bi₂WO₆ Z-scheme heterojunction by chemical co-precipitation and hydrothermal method, which was effective for the stable removal of ciprofloxacin [22]. Therefore, rationally designing heterojunction materials is an effective strategy for improving the photocatalytic activity of Bi₄O₅Br₂ while preserving the redox capacity.

In recent years, Zn_xCd_1−xS solid solution with excellent photocatalytic properties has been recognized as a promising semiconductor material. One of the highlights is that its forbidden bandwidth and optical response range can be flexibly tuned by varying the molar ratio of Zn²⁺ and Cd²⁺ [23]. Among the series of zinc cadmium sulfide solid solutions, ZCS has been the most extensively researched and widely used for photocatalytic hydrogen precipitation [24,25], degradation of pollutants [26,27], and carbon dioxide reduction [28]. However, pure ZCS has some limitations, such as low efficiency of photogenerated electron–hole pair separation and photocorrosion [29]. These defects can be compensated by constructing heterojunctions with other semiconductors to improve the photocatalytic activity.

Herein, a series of ZCS/Bi₄O₅Br₂ heterojunctions were constructed by using a facile ultrasound-assisted solvothermal method. The prepared composite catalysts exhibit the following properties: (1) The ZCS/Bi₄O₅Br₂ heterojunction forms a built-in electric field and accelerates the transport of photogenerated carriers at the interface. (2) Bi₄O₅Br₂ serves as an oxidation semiconductor to capture the holes of ZCS and strengthen the photocatalytic stability, thereby slightly mitigating the photocorrosion of ZCS nanoparticles. The photocatalytic performance of the material was optimized by appropriately adjusting the ratio of ZCS and Bi₄O₅Br₂, which showed an obvious removal effect for several pollutants (TCH, OTC, RhB, and MB). The stability and reusability were demonstrated by cycling tests, and possible TCH degradation pathways and mechanistic predictions were suggested.

2. Results and Discussion

2.1. Material Characterization

The samples were characterized by X-ray diffraction (XRD), as displayed in Figure 1. The diffraction peaks at 24.23°, 29.58°, 31.82°, and 45.65° are observed in pure Bi₄O₅Br₂, corresponding to the (112), (11-3), (020), and (422) crystal planes of the monoclinic phase (JCPDS No. 37-0699), respectively [30]. The peaks between cubic CdS and ZnS appearing at 27.44°, 45.49°, and 53.67° belong to the (111), (220), and (311) crystallographic planes of ZCS, respectively, which indicates the successful synthesis of ZCS [31,32]. The main diffraction peaks of as-prepared ZCS/Bi₄O₅Br₂ composites clearly correspond to Bi₄O₅Br₂ and ZCS without the appearance of other impurity peaks. With the gradual introduction of ZCS, the peak intensity of ZCS in the composite catalysts was strengthened, while the crystal surface strength of Bi₄O₅Br₂ was weakened and then increased. This represented the gradual loading of ZCS to Bi₄O₅Br₂, and the materials were successfully composited.

The microscopic morphology and structure of ZCS, Bi₄O₅Br₂, and ZCS/Bi₄O₅Br₂ were observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). Figure 2a shows the morphology of ZCS, which consists of small, aggregated nano-microspheres. The pure Bi₄O₅Br₂ exhibits irregular stacked nanosheets with relatively smooth surfaces, as shown in Figure 2b. A scanning image of 1.5-ZCS/Bi₄O₅Br₂ composite (Figure 2c) shows that ZCS nanoparticles are uniformly loaded on Bi₄O₅Br₂ sheet structures, and the surfaces become rough. EDS and elemental mapping images (Figure 3) confirm the uniformly distributed elements of Zn, Cd, S, Bi, Br, and O in 1.5-ZCS/Bi₄O₅Br₂.

The morphology of nanoparticles and ultra-thin nanosheets was similarly detected in TEM images (Figure 4a), with ZCS nanoparticles dispersed on the surface of Bi₄O₅Br₂ nanosheets. The material was further analyzed by HRTEM (Figure 4b,c), the lattice fringes reveal that the lattice spacing of 0.302 nm corresponds to the (11-3) crystal plane of Bi₄O₅Br₂, and 0.335 nm matches with the (111) crystal plane of ZCS from the boxed section. The result is also matched with XRD, further demonstrating the successful synthesis of the ZCS/Bi₄O₅Br₂ heterojunction interface.

The elemental compositions and valence states were analyzed using X-ray photoelectron spectroscopy (XPS). The full spectrum of 1.5-ZCS/Bi₄O₅Br₂ shown in Figure 5a reveals the presence of Bi, O, Br, Zn, Cd, and S. Figure 5b displays that the two characteristic peaks Zn 2p_1/2 and Zn 2p_3/2 of the Zn 2p spectrum in 1.5-ZCS/Bi₄O₅Br₂ are attributed to 1044.64 and 1021.61 eV, respectively, proving the presence of Zn²⁺ [33]. The Cd 3d peaks in 1.5-ZCS/Bi₄O₅Br₂ at 411.38 and 404.37 eV, belonging to Cd 3d_3/2 and Cd 3d_5/2 of Cd²⁺ (Figure 5c), respectively [34]. In the Bi 4f spectrum shown in Figure 5d, Bi 4f_5/2 and Bi 4f_7/2 are attributed to 163.78 and 158.48 eV, respectively, indicating that Bi element exists in a +3 valence state [11]. Meanwhile, the S 2p peaks of S²⁻ are observed from the spectrum with binding energies at 161.82 and 160.98 eV belonging to S 2p_1/2 and S 2p_3/2, respectively [35]. The result is consistent with the S 2p peaks in pure ZCS. The peaks appearing at 68.80 and 67.80 eV in Figure 5e correspond to Br 3d_3/2 and Br 3d_5/2, respectively, confirming the existence of Br⁻ [36]. Additionally, the O 1s peaks (Figure 5f) can be fitted to three peaks at 532.25, 530.76, and 529.26 eV related to Bi-O bond and surface-adsorbed oxygen [37]. Compared with pure Bi₄O₅Br₂ or ZCS, the binding energies of Bi, Br, and O from 1.5-ZCS/Bi₄O₅Br₂ are slightly negatively shifted possibly due to the escape of electrons, and the elements involving Zn, Cd, and S are positively shifted due to the increase of the surrounding electron density, indicating the electronic interaction between Bi₄O₅Br₂ and ZCS [38].

The N₂ adsorption–desorption isotherms and pore size distributions of the corresponding samples are presented in Figure 6. The curves of Bi₄O₅Br₂ and 1.5-ZCS/Bi₄O₅Br₂ are clearly consistent with type IV isotherm, indicating the presence of mesoporous structure of the materials. Evidently, 1.5-ZCS/Bi₄O₅Br₂ (68.7 m²/g, 0.232 cm³/g) has a higher specific surface area and pore volume than Bi₄O₅Br₂ (24.6 m²/g, 0.167 cm³/g). The result demonstrates that the introduction of ZCS can significantly increase the specific surface area of Bi₄O₅Br₂, thus providing more catalytically active sites with enhanced adsorption capacity and photocatalytic performance.

2.2. Optoelectronic Properties

The light absorption capacities and energy band structures of ZCS, Bi₄O₅Br₂, and ZCS/Bi₄O₅Br₂ were revealed by UV–vis diffuse reflectance spectroscopy (DRS). Figure 7a shows the diffuse reflection spectra of the obtained samples in the region of 200–800 nm. The absorption edges of ZCS and Bi₄O₅Br₂ are located around 519 and 538 nm, respectively. Compared with pure Bi₄O₅Br₂, the composite photocatalysts exhibit enhanced light absorption intensity and a significantly red-shifted characteristic of the absorption edges, indicating that the combination of ZCS and Bi₄O₅Br₂ can expand the absorption range of sunlight [39]. Results demonstrates that the effective complexation of ZCS with Bi₄O₅Br₂ can improve the absorption capacity and utilization of visible light. The energy bandgap (E_g) values of Bi₄O₅Br₂ (2.65 eV) and ZCS (2.43 eV) are calculated in Figure 7b according to the Kubelka–Munk formula [40]:

$$(\alpha \mathit{hv})=\mathrm{A}{(\mathit{hv}-E_g)}^{\mathrm{n}/2}$$

(1)

The bandgap value of 1.5-ZCS/Bi₄O₅Br₂ is slightly reduced, which is roughly at 2.38 eV. The narrow band gap made it easier for electrons to jump, and the photocatalytic activity was enhanced. The flat band potential positions −0.49 V (vs. Ag/AgCl) of Bi₄O₅Br₂ and −0.88 V (vs. Ag/AgCl) of ZCS are obtained from the tangent intercepts of Mott–Schottky curves, respectively (Figure 7c,d). Converted to standard hydrogen electrode potentials according to the following formula [26]:

$${\mathrm{E}}_{\mathrm{NHE}}={ \mathrm{E}}_{\mathrm{Ag}/\mathrm{AgCl}}+0.198$$

(2)

The flat band potentials for Bi₄O₅Br₂ and ZCS are −0.29 and −0.68 V (vs. NHE), respectively. The tangent slopes of M–S curves are both positive, indicating that ZCS and Bi₄O₅Br₂ are typical n-type semiconductors [41]. For n-type semiconductors, the flat band is generally 0.1 or 0.2 V higher than the conduction band (CB) potential, so the CB positions of Bi₄O₅Br₂ and ZCS are −0.49 and −0.88 V (vs. NHE), respectively. Finally, applying the following equation [42]:

$${\mathrm{E}}_{\mathrm{VB}}{=\mathrm{E}}_{\mathrm{CB}}+{\mathrm{E}}_{\mathrm{g}}$$

(3)

The valence band (VB) potentials of Bi₄O₅Br₂ and ZCS are calculated to be 2.16 and 1.55 V, respectively.

The separation efficiency of photogenerated carriers in semiconductors was examined by photoluminescence (PL) spectroscopy. PL spectra of Bi₄O₅Br₂, ZCS, and 1.5-ZCS/Bi₄O₅Br₂ heterojunction measured at the excitation light wavelength of 325 nm are shown in Figure 8a. The fluorescence intensity of 1.5-ZCS/Bi₄O₅Br₂ is weaker than that of uncompounded photocatalysts, illustrating that the possibility of its photogenerated carrier recombination becomes significantly lower [43]. The transfer and separation efficiency of light-induced carriers were further evaluated by impedance tests. Figure 8b displays the electrical impedance spectra (EIS) of Bi₄O₅Br₂, ZCS, and 1.5-ZCS/Bi₄O₅Br₂ heterojunction. The impedance arc radius of 1.5-ZCS/Bi₄O₅Br₂ is obviously smaller than that of ZCS and Bi₄O₅Br₂, which suggests that the introduction of ZCS can reduce the resistance of the material. Evidently, ZCS/Bi₄O₅Br₂ composite catalysts showed low charge migration resistance and fast electron transport.

2.3. Photocatalytic Performance

The photocatalytic performance of ZCS, Bi₄O₅Br₂, and ZCS/Bi₄O₅Br₂ was evaluated by degrading TCH. The adsorption–desorption equilibrium experiment was first conducted, as shown in Figure S1. After stirring in the dark for 30 min, the concentration of TCH remains essentially constant, indicating that equilibrium had been reached. In Figure 9a, there is almost no degradation that occurred without the presence of the photocatalyst, proving that TCH is very stable in the natural environment. The degradation rate by pure Bi₄O₅Br₂ is 65.8% within 60 min of irradiation, while the degradation rate of ZCS is 14.4%. However, the photocatalytic activity of ZCS/Bi₄O₅Br₂ heterojunctions improved significantly. Among them, 1.5-ZCS/Bi₄O₅Br₂ shows the highest degradation rate of 84.5%, which is 1.3 and 5.9 times that of Bi₄O₅Br₂ and ZCS, respectively. In Figure 9b,c, the removal process of TCH is further analyzed applying the pseudo-first-order kinetic model. It is calculated that 1.5-ZCS/Bi₄O₅Br₂ (0.02189 min⁻¹) has the highest apparent rate constant, which is about 1.7 and 9.6 times higher than that of pure Bi₄O₅Br₂ (0.01297 min⁻¹) and ZCS (0.00229 min⁻¹), respectively. The above findings corroborate that the effective combination of ZCS and Bi₄O₅Br₂ can improve the photocatalytic degradation performance. From the UV–vis absorption spectra (Figure 9d), it can be observed that the intensity of the main characteristic peak of TCH diminishes with time, proving the occurrence of degradation. The reported photocatalytic materials for degrading TCH are displayed in

Table 1. Comparison of degradation rates of TCH by different catalysts.

Catalyst	Dosage (g/L)	Concentration (mg/L)	Time (min)	Degradation Rate (%)	Reference
PANI/Bi4O5Br2	0.4	20	240	85.7	[44]
BiOBr@ZnFe-MOF	0.15	5	90	79.2	[45]
Bi4O5I2/Bi4O5Br2	0.4	20	120	90.2	[19]
FeVO4/Bi4O5Br2	0.5	20	150	88	[46]
BiOCl0.9I0.1/β-Bi2O3	0.4	20	120	82.4	[47]
Zn0.5Cd0.5S/Bi4O5Br2	0.4	20	60	84.5	This work

, and it can be concluded from the comparison that ZCS/Bi₄O₅Br₂ catalyst has advantages in degrading TCH. Further, the suitability of 1.5-ZCS/Bi₄O₅Br₂ for different pollutants was further investigated. Figure 9e demonstrates that the degradation efficiency of OTC, RhB, and MB can reach 83.3%, 97.7%, and 97.3%, respectively, implying its potential for the removal of contaminants.

In practice application, the situation was complex and variable. Thus, the effect of different conditions on the degradation of ZCS/Bi₄O₅Br₂ heterojunction was explored, including the initial pH and concentration of TCH solution, as well as the catalyst dosage. As shown in Figure S2a, 1.5-ZCS/Bi₄O₅Br₂ catalyst has the best photodegradation ability at a neutral initial pH value, and also maintains favorable TCH removal under different pH gradients, indicating a wider pH application range. Figure S2b shows the removal rate of TCH with different catalyst dosages. The dosage increases from 0.2 to 0.5 g/L, and the degradation rate increases from 78.0% to 86.0%. More pollutants can be adsorbed to the catalyst surface to participate in the photoreaction, and more reactive sites can be provided. The differences in TCH removal effects were not significant when 1.5-ZCS/Bi₄O₅Br₂ dosage was 0.4 and 0.5 g/L, respectively. Therefore, 0.4 g/L was the optimal catalyst dosage. Figure S2c shows the degradation efficiency at different TCH concentrations. The concentrations range from 10 to 40 mg/L, and the degradation rates are 87.3%, 84.5%, 80.2%, and 75.5%, respectively. As the pollutant concentration increased, the removal rate decreased, which was due to the accumulation of substrates and intermediates masking some active sites on the surface of catalyst. And the active radicals produced by a certain amount of catalyst were also limited. Considering the practical application and economic value, 20 mg/L of TCH solution was selected for the following experiments.

The stability and reusability of the photocatalyst are also an important evaluation index. The solution after each round of degradation was filtered off the supernatant, and the catalyst was collected by centrifugation, washed, and dried for the next round of photodegradation experiments. As illustrated in Figure 10a, the photocatalytic activity of 1.5-ZCS/Bi₄O₅Br₂ decreases slightly after five cycles of degradation experiments but is still superior to that of the pure ZCS and Bi₄O₅Br₂. The result indicates that the catalyst has a long service life. Peak structures of XRD pattern (Figure 10b) before and after cycling remain basically the same, which also demonstrates the stability of its crystalline phase structure.

2.4. Analysis of Possible Degradation Pathways

The intermediate products in the degradation of TCH were identified using LC-MS. On the basis of the mass spectrometry m/z results (Figure S3), the corresponding intermediates and two possible degradation pathways are deduced. The strong peak at m/z = 445 corresponds to the mass-to-charge ratio of TCH. In pathway I, the introduction of a hydroxyl group produces the intermediate P1 (m/z = 459), followed by N-demethylation and hydroxylation occurring with the detection of P2 (m/z = 417). P2 dissociates to P3 (m/z = 274) via ring-opening and reduction reaction. Furthermore, P2 is dehydrated to produce P4 (m/z = 399) and then degrades to P5 (m/z = 338) by ring-opening, demethylation, and hydroxylation reactions. In pathway II, TCH is degraded to the intermediate P6 (m/z = 391) via deamination reaction and hydroxylation, and then by a ring-opening reaction to produce P7 (m/z = 234), which undergoes a dehydration process and other reactions to produce P8 (m/z = 217). These intermediate substances may be further degraded to obtain smaller masses of organic molecules (P9–P11, m/z = 194, 171, and 114), finally mineralizing to CO₂ and H₂O. The postulated degradation pathways are shown in Figure 11.

2.5. Photocatalytic Mechanism

During the photocatalysis process, organic pollutants were degraded through the generation of active species including ·O₂⁻, h⁺, and ·OH by the catalysts. Corresponding scavengers such as para-benzoquinone (BQ, ·O₂⁻ scavenger), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, h⁺ scavenger), and isopropanol (IPA, ·OH scavenger) were added during the degradation of TCH to identify the main active substances by trapping experiments. As depicted in Figure 12a, the degradation rates decrease from 84.5% to 44.5% (·O₂⁻), 61.6% (h⁺), and 83.5% (·OH), indicating that ·O₂⁻ and h⁺ are the most important active radicals. The EPR tests further confirmed the production of ·O₂⁻ and h⁺ in the photocatalytic system. In Figure 12b, the signal of DMPO-·O₂⁻ could not be recognized under dark condition for 1.5-ZCS/Bi₄O₅Br₂. The EPR peak intensity increases after 15 min of light, and the signal is obviously enhanced. The signal generated under dark condition belongs to the capture agent TEMPO, and the intensity of the EPR peak decreases after providing light instead, precisely because the generation of h⁺ neutralizes the signal from TEMPO, making the signal weaker (Figure 12c).

The CB and VB positions of ZCS are −0.88 and 1.55 V, while Bi₄O₅Br₂ are −0.49 and 2.16 V, respectively. Both semiconductors have more negative potentials and can produce ·O₂⁻ (O₂/·O₂⁻, −0.33 V vs. NHE) [48]. The redox capacity of Bi₄O₅Br₂ enables H₂O molecules to produce ·OH (H₂O/·OH, 1.99 V vs. NHE) [49], but ·OH (OH⁻/·OH, 2.27 V vs. NHE) derived from OH⁻ cannot be generated [50]. Thus, ·OH had minimal participation in the degradation of TCH, which matched the trapping experimental results. Under visible light excitation, photogenerated electrons and holes were produced in the VB and CB of ZCS and Bi₄O₅Br₂, respectively. If the photocatalytic mechanism followed the traditional type II heterojunction transfer theory, ·O₂⁻ would not be generated due to the more positive CB of ZCS than ·O₂⁻ (O₂/·O₂⁻), and the VB of Bi₄O₅Br₂ would be more negative than ·OH (·OH/H₂O) or ·OH (·OH/OH⁻) and unable to produce ·OH, which was contrary to the trapping experiments. Combining the above analyses, the transfer mechanism could be hypothesized in Figure 13. When the two semiconductors contacted closely, the electrons of ZCS spontaneously flowed to Bi₄O₅Br₂ until the Fermi energy levels reached equilibrium. Similarly, electron consumption accelerates the transfer of photogenerated holes in the VB of Bi₄O₅Br₂ to that of ZCS with a higher potential. At the heterojunction interface, Bi₄O₅Br₂ was negatively charged and ZCS was positively charged, thus forming an internal electric field. The recombination of electrons in the CB of Bi₄O₅Br₂ and holes in the VB of ZCS under Coulomb force and photoexcitation promoted photogenerated charge separation. Therefore, the soluble oxygen could be reduced to ·O₂⁻ with the presence of photogenerated electrons in the CB of ZCS, and the remaining highly oxidized h⁺ in the VB of Bi₄O₅Br₂ was directly involved in the degradation of pollutants. The whole process is summarized as follows:

$$\mathrm{ZCS}/{\mathrm{Bi}}_4{\mathrm{O}}_5{\mathrm{Br}}_2+\mathrm{h}\mathrm{v} \to \mathrm{ZCS}/{\mathrm{Bi}}_4{\mathrm{O}}_5{\mathrm{Br}}_2({\mathrm{e}}^{-},{\mathrm{h}}^{+})$$

(4)

$${\mathrm{e}}^{-}({\mathrm{Bi}}_4{\mathrm{O}}_5{\mathrm{Br}}_2)+{\mathrm{h}}^{+}\left(\mathrm{ZCS}\right)\to \mathrm{Recombination}$$

(5)

$${\mathrm{e}}^{-}(\mathrm{ZCS})+{\mathrm{O}}_2\to ·{\mathrm{O}}_2^{-}$$

(6)

$${\mathrm{h}}^{+}({\mathrm{Bi}}_4{\mathrm{O}}_5{\mathrm{Br}}_2)+{\mathrm{H}}_2\mathrm{O} \to ·\mathrm{OH}+{\mathrm{h}}^{+}$$

(7)

$$·{\mathrm{O}}_2^{-}+{\mathrm{h}}^{+}+·\mathrm{OH}+\mathrm{TCH}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{Small} \mathrm{molecules}$$

(8)

3. Experimental Section

3.1. Preparation of Zn_0.5Cd_0.5S

Zn_0.5Cd_0.5S was easily prepared by using the solvothermal method. Quantities of 0.3296 g of Zn(CH₃COO)₂·2H₂O and 0.3998 g of Cd(CH₃COO)₂·2H₂O were completely dissolved by ultrasonication with 40 mL of ethylene glycol solution, then Na₂S solution was added dropwise according to the molar ratio of Zn: Cd: S = 1: 1: 3 and stirred for 4 h. Next, the mixed solution was poured into a 100 mL Teflon-lined autoclave at 160 °C for 4 h. The sample was washed with ethanol and deionized water repeatedly by centrifugation after natural cooling and dried under vacuum at 60 °C overnight to obtain ZCS nanoparticles.

3.2. Preparation of Bi₄O₅Br₂

Bi₄O₅Br₂ nanosheets were prepared according to a previous report with minor modifications [51]. Specifically, 0.7277 g of Bi(NO₃)₃·5H₂O was added into 15 mL of ethylene glycol solution, and 0.1786 g of KBr was added into 10 mL of deionized water, dissolved by sonication for 0.5 h. Thereafter, the KBr solution was slowly dripped into Bi(NO₃)₃ solution, and the white colloid appeared gradually. The pH value was adjusted to about 9 by using dilute ammonia and continued stirring. Finally, the homogeneously mixed solution of Bi₄O₅Br₂ precursor was kept at 140 °C for 12 h. The reaction product was washed and dried to obtain milky white Bi₄O₅Br₂ powder.

3.3. Preparation of Zn_0.5Cd_0.5S/Bi₄O₅Br₂

A simple ultrasound-assisted solvothermal method was used to synthesize Zn_0.5Cd_0.5S/Bi₄O₅Br₂ composites. A total of 0.1000 g of Bi₄O₅Br₂ was dispersed ultrasonically in 20 mL of ethanol to form a homogeneous solution. Next, ZCS was added at the mass ratio X:1 wt.% (X = 0.25, 0.50, 1.0, 1.5, 2.0, which was the mass ratio of ZCS and Bi₄O₅Br₂) and sonicated for 1 h, followed by stirring for 2 h. The solutions were transferred into a 50 mL Teflon-lined autoclave, and 100 °C heat was employed for 10 h. After cooling, the composites were washed with ethanol and deionized water to obtain the final substance named X-ZCS/Bi₄O₅Br₂ (X = 0.25, 0.50, 1.0, 1.5, 2.0). The whole preparation process is displayed in Scheme 1.

3.4. Characterization

Specific characterizations are detailed in the Supplementary Materials.

3.5. Photocatalytic Performance Measurements

The photocatalytic performance was evaluated by using TCH (20 mg/L), OTC, RhB, and MB (10 mg/L) as target pollutants, pH = 7 for all contaminant solutions, and a 300 W xenon lamp as a simulated light source. About 20 mg of catalyst was weighed and dispersed homogeneously into 50 mL of contaminant solution and stirred under dark condition for 0.5 h to reach adsorption–desorption equilibrium. Then, 4 mL of suspension was extracted at 15 min intervals after irradiation and centrifuged to test the maximum absorbance at 357 nm of the obtained supernatant with a Shimadzu UV-1900 spectrophotometer. The degradation rate is calculated via the following equation:

$$\mathrm{Removal} \mathrm{rate}\%=\left(1 -{ \mathrm{C}}_{\mathrm{t}}/{\mathrm{C}}_0\right) \times 100\%$$

(9)

where C₀ is the initial concentration of the pollutant solution, and C_t is the concentration at a certain degradation time t (min).

4. Conclusions

In this work, a ZCS/Bi₄O₅Br₂ heterojunction was successfully prepared. The better degradation properties of optimized ZCS/Bi₄O₅Br₂ were attributed to the formation of an electric field inside the heterojunction, realizing the fast separation of carriers at the interface between ZCS and Bi₄O₅Br₂ while also preserving their redox capabilities. Compared with ZCS (14.3%) and Bi₄O₅Br₂ (65.8%), 1.5-ZCS/Bi₄O₅Br₂ could degrade TCH up to 84.5% and exhibited excellent stability, as indicated by recycling experiments. Here, ·O₂⁻ and h⁺ played a dominant role during the elimination process. The suitability for other pollutants’ degradation was validated, and the results showed that it also had a better removal effect on OTC (83.3%), RhB (97.7%), and MB (97.3%). Therefore, this study provides a possible strategy for the modification of bismuth-rich materials and the preparation of efficient photocatalysts.