Bandgap-Tunable Zn_xCd_1−xS Solid Solutions for Effective Photocatalytic Degradation of Norfloxacin Under Visible Light and Natural Sunlight

Abstract

Due to its broad-spectrum antibacterial activity, norfloxacin (NOR) has been widely used over the past few decades. However, the residual NOR in aquatic ecosystems could pose risks to human health from bacteria with resistance genes that potentially cause serious infectious diseases. Herein, a series of bandgap-tunable Zn_xCd_1−xS (x = 0~1) solid solutions were hydrothermally synthesized and used for NOR photodegradation under visible light and natural sunlight. Benefitting from the suitable bandgap, band structure, and unique tetrapod shape nanostructure, the Zn_0.1Cd_0.9S solid solution exhibited the best photocatalytic activity, with high degradation efficiencies of 83.23% and 86.28% under visible light and natural sunlight, respectively, within 60 min, which is remarkable among reported Zn_xCd_1−xS-based photocatalysts and other materials. The in situ reactive-species trapping experiment revealed that holes (h⁺) were the primary species, and a possible photodegradation mechanism was thus suggested. Moreover, Zn_0.1Cd_0.9S also exhibited decent reusability and stability after five cycles of experiments. This work provides a comprehensive exploration of the application of bandgap-tunable Zn_xCd_1−xS solid solutions for NOR photodegradation under visible light and natural sunlight, demonstrating the promising application of as-synthesized Zn_0.1Cd_0.9S in the photocatalytic degradation of antibiotics.

1. Introduction

As one of the greatest achievements in the field of medicine in the 20th century, the discovery and application of antibiotics have effectively prevented viral infections, reduced mortality rates, and promoted medical development [1,2]. Antibiotics are not only extensively utilized in the human medical field, but also in animal husbandry and aquaculture [1,2]. Meanwhile, ecological safety concerns caused by their excessive and indiscriminate use, as well as illegal discharge, have become one of the most prominent challenges in the field of contemporary environmental science. When antibiotics enter water bodies, they not only directly inhibit the physiological functions of aquatic organisms but may also accumulate in sediment through adsorption, forming long-term pollution sources [1,2,3]. Due to the difficulty of completely degrading antibiotics in the environment, incompletely degraded antibiotics have been observed and can induce microorganisms to produce resistance genes, leading to the emergence of multidrug-resistant strains and increasing the difficulty of clinical treatment caused by bacterial infections [1,2]. Furthermore, antibiotics remaining in the environment can also be transmitted to the human body through various pathways, such as the food chain, posing potential risks to human health [1,2]. As a second-generation quinolone antibiotic, norfloxacin (NOR) is extensively used in the treatment of enteritis, diarrhea, and urinary tract infections due to its broad-spectrum antibacterial activity [4,5]. However, most of NOR cannot be metabolized by humans and animals and is therefore discharged into the environment and has been detected in many different surface waters, leading to environmental residues and ecologically toxic compounds, as well as the emergence of resistant bacteria [4,5]. Thus, developing economical and efficient methods to eliminate antibiotic pollution is a critical public health issue [4,5].

Over the past few decades, a number of treatment methods have been developed, such as flocculation, photocatalysis, electrolysis, adsorption, ozonation, membrane filtration, microbial degradation, and reverse osmosis [4]. Among these different technologies, semiconductor-based photocatalysis, a promising method, has become a research focus due to its high efficiency and environmental friendliness [3,5,6]. As it can directly convert solar energy into chemical energy under moderate reaction conditions, and organic compounds can be thoroughly degraded to avoid secondary pollution, researchers are delving deeply into this technology, and remarkable strides are being made in this field [3,5,6].

As the core of photocatalysis, efficient photocatalytic materials are critical to advancing the development and application of photocatalytic technology [3,5,6,7,8,9]. Among the numerous photocatalytic materials reported, metal sulfides such as CdS and ZnS have attracted more and more research interest because of their appropriate bandgap and band structure, which is favorable for the photodegradation of organic contaminants, photocatalytic water splitting, and so on [10,11,12,13,14]. In particular, combining the advantages of visible light-responsive CdS with a relatively narrow bandgap and photostable ZnS with a wide bandgap, solid ternary Zn_xCd_1−xS solutions have a tunable bandgap and an adjustable visible-light absorption range through varying the Zn/Cd ratio, as well as a relatively negative conduction band potential, showing promise towards the efficient separation of photoinduced charge carriers. This ability is as a result of its nano twinned structure with a twin-boundary-dependent potential and an uneven distribution of charges that induce spontaneous polarization and form an internal electric field in solid solution [15,16,17,18]. Therefore, Zn_xCd_1−xS series materials have garnered considerable interest and have been widely used in visible light-driven photocatalytic hydrogen production and CO₂ reduction [15,16,17,18]. However, their photocatalytic activity toward the degradation of NOR under visible light, especially under natural sunlight, has not been fully investigated [19,20,21].

Herein, we synthesized a series of bandgap-tunable Zn_xCd_1−xS (x = 0~1) solid solutions for the efficient photocatalytic degradation of NOR under visible light and natural sunlight. Through adjusting the amount of Zn²⁺, the optimum photocatalytic performance could be obtained by Zn_0.1Cd_0.9S (x = 0.1), which could be attributed to the appropriate bandgap and band alignment, as well as the distinctive tetrapod nanostructure of Zn_0.1Cd_0.9S. Moreover, the degradation efficiency of NOR by Zn_0.1Cd_0.9S could reach as high as 83.23% and 86.28% under visible light and natural sunlight within 60 min. Mechanistic studies suggested that h⁺ plays a prominent role in the photocatalytic degradation of NOR. Furthermore, the as-prepared Zn_0.1Cd_0.9S solid solution also displayed robust stability and good reusability, showing promising application in environmental remediation.

2. Results

2.1. Structural Characterization

The structure and phase of as-synthesized Zn_xCd_1−xS solid solutions were first explored by XRD characterization. As shown in Figure 1a, the diffraction peaks of CdS and ZnS samples all are consistent with the standard cards (PDF#41-1049 and PDF#89-2191), confirming that both as-synthesized CdS and ZnS are pure-phase and have a hexagonal crystal structure. The XRD pattern of Zn_0.1Cd_0.9S resembles that of CdS, with small differences in the intensity and slight migration of diffraction peaks, suggesting that Zn_0.1Cd_0.9S still maintains a hexagonal structure similar to CdS though a small amount of Cd atoms is substituted by Zn atoms [22,23]. Meanwhile, as shown in Figure 1b, Zn_xCd_1−xS shows multiphase characteristics, and its diffraction peaks shifted to a higher diffraction angle with an increase in x. This is because the radius of the Zn²⁺ ion (0.74 Å) is smaller than that of the Cd²⁺ ion (0.97 Å), and as Zn²⁺ ions are incorporated in the lattice of CdS, the lattice of CdS will shrink, leading to a certain decrease in the lattice plane distance and a shift in diffraction peaks to larger diffraction angles according to Bragg’s equation [22,23]. Additionally, the continuous shifts in diffraction peaks of Zn_xCd_1−xS solid solutions with the increase in Zn content also indicate that as-synthesized Zn_xCd_1−xS is a solid solution rather than simply a mixture of CdS and ZnS [22,23].

2.2. Surface Morphology Characterization

The surface morphologies of as-prepared Zn_xCd_1−xS solid solutions were characterized by SEM. As shown in Figure 2, pure ZnS is formed as serious agglomerated irregular small nanoparticles of ca. 20 nm. With a decrease in x, though there are still random aggregations between small nanoparticles due to the large surface energy, the size of Zn_xCd_1−xS solid solutions gradually becomes larger, the morphology is transformed from spherical to a nanorod-like structure, and particles adhere to each other, which is in line with reports in the literature. In particular, Zn_0.1Cd_0.9S displays a tetrapodal rod-like nanostructure with a length of 100~200 nm and a diameter of 30~80 nm. This special tetrapod shape of Zn_0.1Cd_0.9S is consistent with that observed in the previous literature and favors charge separation and transportation in photocatalysis due to the internal electronic field formed at the interface of the nanorods and nuclei, as well as the nanorod structure of the arms [24]. Furthermore, as shown in Figure 2i and Figure S1, the energy dispersive spectroscopy (EDS) results revealed the elemental composition of different samples, and the measured ratios of Zn to Cd for different Zn_xCd_1−xS solid solutions are close to the theoretical values, which also confirms the successful preparation of Zn_xCd_1−xS (x = 0.1, 0.3, 0.5, 0.7, 0.9) solid solutions.

2.3. Optical Absorption

To determine the optical properties and band structure of as-synthesized samples, solid-state UV-vis diffuse reflectance spectra (DRS) were measured. As depicted in Figure 3a, the absorption edges of the samples red-shifted with the decrease in x of Zn_xCd_1−xS solid solutions. Moreover, the absorption edges for Zn_xCd_1-xS solid solutions were distributed between those of CdS and ZnS, which lay just within the visible light region (approximately 450~600 nm), indicating that the Zn_xCd_1−xS solid solutions possess an appropriate visible light-harvesting ability to play a role in visible light catalysis. This could also be inferred from the gradual change in color of series Zn_xCd_1−xS solid solutions from orange to white gray with the increase in x (Figure S2).

The direct bandgaps (E_g) of Zn_xCd_1−xS solid solutions could be estimated from the Tauc plots shown in Figure 3b. The bandgaps of pure CdS and ZnS were found to be ca. 2.16 eV and 3.61 eV, respectively, while the bandgaps of the Zn_xCd_1−xS solid solutions increased from 2.38 eV to 3.19 eV with the increase in x, confirming the declined visible light absorption ability of Zn_xCd_1−xS solid solutions with high x values. Additionally, the E_g of ZnS was close to the value of ZnS nanoparticles in the previous literature [25], indicating the presence of point defects (including S or Zn vacancies) that could increase photoabsorption capability and enhance the photocatalytic activity and stability of the photocatalyst.

2.4. Photocatalytic Degradation Performance

To investigate the photocatalytic behavior of as-synthesized Zn_xCd_1−xS solid solutions, a series of photocatalytic experiments for NOR photodegradation were carried out and the stability of the NOR solution without a photocatalyst under irradiation with different light sources was first determined. As shown in Figure S3, the blank experiments under full-spectrum solar light irradiation with/without a 420 nm filter show that NOR remains stable, with a negligible 1.7% photo-induced self-decomposition within 60 min under visible light irradiation, suggesting a good photostability of NOR under visible light. On the other hand, under simulated sunlight, within the same period, 50% of NOR self-decomposed. Therefore, in order to avoid the self-decomposition of NOR, we chose visible light as the light source for subsequent experiments. Then, the time for the catalyst to achieve an adsorption–desorption equilibrium in the dark was explored. As shown in Figure S4, the maximum absorption peak of NOR gradually decreased after adding a typical Zn_0.5Cd_0.5S catalyst under dark conditions as time increased, and the absorbance of NOR remained stable after 60 min, indicating that a adsorption–desorption equilibrium between the NOR solution and the Zn_xCd_1−xS photocatalyst was achieved within 1 h under dark conditions. Therefore, a dark adsorption time of 60 min was adopted for the subsequent photocatalytic experiments. Additionally, to improve the efficiency of the experiment, the illumination time was controlled within 1 h.

As shown in Figure S5, the absorbance of NOR gradually decreased with time under visible light irradiation when different Zn_xCd_1−xS (x = 0~1) solid solutions were used as photocatalysts, which is in contrast with the above blank experiment. As displayed in Figure 4a,b, Zn_0.1Cd_0.9S exhibits the best photocatalytic degradation among all catalysts, with a NOR degradation rate of 83.23% over 60 min of visible light irradiation, whereas only 8.6% and 67.64% NOR were degraded with pure ZnS and CdS. Moreover, the degradation rate of NOR gradually decreased with the increase in x. Even after the NOR adsorption rate of different Zn_xCd_1−xS (x = 0~1) solid solutions (Figure S6a) was taken into account, Zn_0.1Cd_0.9S still exhibited the highest NOR removal rate among all catalysts (Figure S6b).

Figure 4c illustrates the linear fitting curves of ln(C₀/C_t) vs. time for the various catalysts; the corresponding apparent rate constants (K_app) were calculated form the pseudo-first-order reaction equation ln(C₀/C_t) = K_app × t according to previous studies [26,27,28]. Moreover, as shown in Figure 4d, the K_app of Zn_0.1Cd_0.9S is 0.0343 min⁻¹, which is the largest value among all catalysts, ca. 21 and 1.6 times faster than those of pure ZnS (K_app = 0.0016 min⁻¹) and CdS (K_app = 0.0220 min⁻¹). Moreover, as shown in Table S1, Zn_0.1Cd_0.9S exhibited the best performance of photocatalytic degradation of NOR among all as-synthesized Zn_xCd_1−xS solid solutions in terms of highest removal rate (84.20%), degradation rate (83.23%), K_app (0.0343 min⁻¹), and shortest half-life time (t_1/2) (20.21 min). This prominent photocatalytic activity of as-synthesized Zn_0.1Cd_0.9S under visible light positions it at the forefront among reported Zn_xCd_1−xS-based photocatalysts and other materials, performing even better than some heterostructures under simulated solar light (Table S2). Such a competitive photocatalytic activity can be attributed to the suitable bandgap and unique tetrapod-shaped nanostructure of Zn_0.1Cd_0.9S, which is conducive to the efficient absorption and utilization of visible light. These characteristics also lead to the high efficiency of separation and directional transportation of charge along the longitude of the nanorod, which occurs via the internal electronic field formed in the tetrapod nanostructure of this photocatalyst according to reports in the literature [22,23,24,29,30].

The effect of initial NOR concentration on degradation efficiency with a fixed quantity of photocatalysts was also assessed. As shown in Figure 5a, degradation efficiency shows a negative relationship with the initial concentration of NOR solution, and this is also reflected in the decrease in corresponding K_app with the elevated initial concentration of the NOR solution (Figure 5b). This decline in efficiency with rising NOR concentration is probably due to the following two reasons [31]. On one hand, as the quantity of photocatalysts is fixed, when the initial concentration of NOR is increased, the surface of the photocatalyst will be occupied by the adsorbed NOR molecules, while the limited active sites will not be capable of generating enough reactive species for the photodegradation. On the other hand, a higher concentration of NOR will decrease the number of photons reaching the catalyst surface, which deceases the light absorption ability of the photocatalyst, causing the decline in degradation efficiency [31].

To investigate the photocatalytic degradation mechanism of NOR, free radical capture experiments were performed to determine the active species generated in the process of photodegradation. In total, 1 mM of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), L-histidine, 2, 2, 6, 6-tetramethyl-1, 4-piperidinediol (TEMPO), and isopropanol (IPA) were added into the NOR solution to selectively quench holes (h⁺), singlet oxygen (¹O₂), superoxide radicals (·O₂⁻), and hydroxyl radicals (·OH), respectively. As shown in Figure 6, compared with the control sample without scavengers, all scavengers had a certain inhibitory effect on degradation efficiency, and with the addition of EDTA-2Na, the degradation rate significantly decreased from 83.23% to 9.73%, implying that the photo-generated h⁺ is the most active species responsible for the photocatalytic degradation of NOR by Zn_0.1Cd_0.9S. On the contrary, the degradation rate decreased to 64.03%, 68.68%, and 70.1% when L-histidine, TEMPO and IPA, respectively, were used as scavengers, suggesting that ¹O₂, ·O₂⁻, and ·OH take part in the photocatalytic degradation of NOR yet have a negligible impact. The results of the active species capture experiment demonstrate that photo-generated holes play a predominant role in the photocatalytic degradation of NOR.

As for the mineralization of NOR, the TOC concentrations of the NOR solution before and after 60 min photodegradation by Zn_0.1Cd_0.9S were measured. As shown in Figure S7, the TOC removal efficiency (1 − TOC_t/TOC₀) was 50.86% at 60 min, indicating that more than half of NOR had been mineralized and degraded into CO₂ and H₂O, while the remaining 49.14% were intermediates and other small molecule substances.

In order to further explore the photocatalytic degradation mechanism of NOR by Zn_0.1Cd_0.9S, the band structure of the catalyst was calculated according to previous studies (see Supplementary Materials for details) [22,23]. The conduction band (CB) and valence band (VB) of Zn_0.1Cd_0.9S were positioned at −0.56 eV and 1.82 eV, respectively. Therefore, the conduction band and valence band potentials of Zn_0.1Cd_0.9S were transformed to be −0.97 V and 1.41 V (vs. NHE, pH = 7), respectively [32,33].

Based on the above results, a possible schematic diagram of the mechanism and process of the photocatalytic degradation of NOR is shown in Figure 7 and discussed below. Under visible light irradiation, electrons are excited from the VB to the CB in Zn_0.1Cd_0.9S, creating holes in the VB. Since the CB potential of Zn_0.1Cd_0.9S is more negative than $E_{{\mathrm{O}}_2/{\mathrm{o}}_2^{-}}$ (−0.33 V vs. NHE, pH = 7), the photo-excited electrons from the CB of Zn_0.1Cd_0.9S may be sufficient to transfer surface chemically adsorbed O₂ molecules into ·O⁻₂ to oxidize NOR to CO₂, H₂O, and other small molecules or to further participate in the generation of ·OH. On the other hand, although the VB potential of Zn_0.1Cd_0.9S is not positive enough compared to $E_{\bullet \mathrm{O}\mathrm{H}/{\mathrm{H}}_2\mathrm{O}}$ (2.27 V vs. NHE, pH = 7) to react with H₂O to produce strong oxidative ∙OH via photo-generated holes, ∙OH could also be obtained from the photo-generated electrons and ·O⁻₂, according to previous studies [23,31,34]. In addition, the holes remaining in the VB could directly degrade NOR to CO₂, H₂O, and other small molecules or further react with ·O⁻₂ to produce ¹O₂ to degrade NOR, according to the results of the free radical capture experiments. Unlike the more complicated degradation pathways of NOR by ¹O₂, ·O⁻₂, and ∙OH, as the process of degradation by h⁺ is simpler, more direct, and efficient, it could be inferred that h⁺ is the predominant active species responsible for the photodegradation of NOR by Zn_0.1Cd_0.9S while ¹O₂, ·O⁻₂, and ∙OH have a mild effect on this process, which is consistent with the observed results of the free radical capture experiments.

Furthermore, to assess the potential of practical applications using the Zn_0.1Cd_0.9S catalyst, five cyclic degradation experiments were carried out. As shown in Figure 8a, the photodegradation rate of NOR decreased from 83.23% to 68.78% and was maintained above 80% after five cycles, which is probably due to the loss of a small amount of catalyst during recovering experiments, such as centrifugation and washing. Although a decrement in NOR photodegradation by Zn_0.1Cd_0.9S is observed, the structure, morphology, and composition of the catalyst were well retained after five recycling experiments, as revealed by the XRD and SEM-EDS characterization of the used Zn_0.1Cd_0.9S catalyst, shown in Figure 8. The above results demonstrate the good stability and reusability of the Zn_0.1Cd_0.9S catalyst, which is promising for the practical application of abating pollution to a certain extent. Moreover, an ICP-OES test of the NOR solution after five photodegradation cycles was carried out to assess Zn/Cd leaching, and the result shows that the concentrations of Zn²⁺ and Cd²⁺ are 9.6 ppb and below the detection limit of the instrument, respectively, which are far below the drinking water standards set by the World Health Organization (WHO) (Zn²⁺: 3 ppm; Cd²⁺: 3 ppb).

Apart from stability and reusability, the photocatalytic performance of photocatalysts under practical natural sunlight is worth investigating for their application in real environments. Thus, we carried out the photodegradation of NOR by Zn_0.1Cd_0.9S under natural sunlight from 11:30 a.m. to 12:30 a.m. on 19 April 2025, at latitude 29°33′46″ N and longitude 103°44′44″ E. The weather was cloudy, with cloud cover approximately 6~8, and the UV index was 7~9. In addition, the solar irradiance measured using a pyranometer was 70.9~77.7 mW⋅cm⁻². As shown in Figure 9, under natural sunlight irradiation, NOR undergoes self-degradation, and with the addition of Zn_0.1Cd_0.9S, the degradation efficiency significantly improves from 27.86% to 86.28%, demonstrating the superior photocatalytic activity of Zn_0.1Cd_0.9S under natural sunlight. Compared with the degradation rate of blank NOR solution under visible light (Figure S8a), more NOR would be self-degraded under natural sunlight, which is probably due to the ultraviolet light in the spectrum of natural sunlight. Therefore, with the addition of the Zn_0.1Cd_0.9S catalyst, the degradation rate of NOR under natural sunlight is slightly higher than that under visible light (Figure S8b).

For real-world applications, an efficient, environmentally friendly, and low-cost photocatalyst that can be synthesized in large scale is required. However, it is difficult for a photocatalyst to meet all the above requirements. As for the Zn_0.1Cd_0.9S photocatalyst used in this work, though it exhibited a considerable photocatalytic degradation performance for NOR under visible light and natural sunlight, and the concern about potential Zn/Cd leaching is low according to the aforementioned results of the ICP-OES test, there are still practical limitations for real-world application. For example, as the Zn_0.1Cd_0.9S solid solution is solvothermally synthesized from Cd(Ac)₂·2H₂O, Zn(Ac)₂·2H₂O, NaOH, thioacetamide, and ethylenediamine at 180 °C for 18 h, the relative high temperature, the uniformity of raw materials during the reaction, the corrosiveness of organic amines, and the relative tedious subsequent purification process would result in a relatively high cost, which prevents the Zn_0.1Cd_0.9S photocatalyst from being used in large-scale production.

3. Materials and Methods

3.1. Materials

Cd(Ac)₂·2H₂O (AR, ≥99%), Zn(Ac)₂·2H₂O (AR, ≥99%), thioacetamide (TAA, AR, ≥99%), NaOH (AR, ≥99%), norfloxacin (NOR, ≥99%), and L-histidine (AR, ≥99%) were purchased from Dibai Biotechnology Co., Ltd. (Shanghai, China). 2,2,6,6-tetramethyl-1,4-piperidinediol (TEMPO, ≥98%), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, AR, ≥99%), and anhydrous ethanol (AR, ≥99%) were purchased from Aladdin Ltd. (Shanghai, China). Isopropanol (IPA, AR, ≥99%) and ethylenediamine (EDA, AR, ≥99%) were purchased from Chengdu Jinshan Chemical Co., Ltd (Cheng, China). All the chemical reagents were used as received, and ultra-pure water (18.25 MΩ·cm⁻¹) was used in the experiments.

3.2. Synthesis of Zn_xCd_1−xS (x = 0~1) Solid Solutions

The Zn_xCd_1−xS (x = 0~1) solid solutions were prepared according to the reported literature with minor modification [35]. In brief, 0.32 g NaOH was added in a 12 mL mixture of EDA and H₂O (V/V = 1:5) first. Then, 2 mmol Cd(Ac)₂·2H₂O (0.5333 g) or Zn(Ac)₂·2H₂O (0.4390 g) or the mixture of Cd(Ac)₂·2H₂O and Zn(Ac)₂·2H₂O with different proportions (e.g., 0.2 mmol (0.0533 g) Cd(Ac)₂·2H₂O and 1.8 mmol (0.3951 g) Zn(Ac)₂·2H₂O) was added into the above solution swiftly and vigorously stirred for 30 min until all solid particles were dissolved. Then, 2 mmol TAA was added into the solution and stirred for a few minutes. Subsequently, the mixtures were transferred into a 20 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 180 °C for 18 h. After the autoclave was taken out of the oven and cooled down to R.T., the products were obtained via centrifugation at 8000 rpm for 5 min, washed with ethanol and ultrapure water three times at least, and finally dried at 60 °C under vacuum overnight.

3.3. Physicochemical Characterization of Obtained Samples

Powder X-ray diffraction (XRD) patterns were performed on an X-ray diffractometer (XRD, DX-2700, Dandong Haoyuan Instrument Co., Ltd., Dandong, China) with Cu-K_α (λ = 1.54184 Å) radiation. Morphology observation and elemental composition of as-prepared samples were performed on a field emission scanning electron microscope (SEM 5000, CIQTEK, Hefei, China) equipped with an energy dispersive spectroscopy (EDS) detector under an accelerating voltage of 15 kV and 30 s accumulation time. The solid-state ultraviolet–visible diffuse reflectance spectra (UV-Vis DRS) were measured with a UV-Vis spectrophotometer (UV-3900, Hitachi, Tokyo, Japan) with BaSO₄ powder as the standard reflectance reference. The absorption spectra of NOR solution were measured with a UV-Vis spectrophotometer (TU-1950, PERSEE General Instrument Co., Ltd., Beijing, China). The inductively coupled plasma emission spectrometry (ICPE-9820, Shimadzu, Kyoto, Japan) was used to measure the concentration of metal ions in the NOR solution after photodegradation. Total organic carbon (TOC) was measured by the TOC analyzer (Shimadzu, TOC-L CPH, Japan). Detailed information on the methods used to investigate the photocatalytic activity of the as-synthesized samples is provided in the Supplementary Materials file.

4. Conclusions

In summary, a series of Zn_xCd_1−xS (x = 0~1) solid solutions with a tunable bandgap were successfully prepared and investigated for their applicability in the photocatalytic degradation of NOR. By finely tuning the ratio of Zn:Cd, the band structure of Zn_xCd_1−xS (x = 0~1) solid solutions was highly adjustable, and a tunable photocatalytic activity was achieved. When x = 0.1, the as-synthesized Zn_0.1Cd_0.9S displayed the most efficient photocatalytic performance with degradation efficiency up to 83.23% and 86.28% under visible light and natural sunlight, respectively, within 60 min, which could be ascribed to its suitable bandgap and band alignment as well as its distinctive tetrapod nanostructure that favors the separation of photo-generated carriers and facilitates the directional transportation of charge along the nanorod. Furthermore, the radical trapping test confirmed that h⁺ was the primary reactive species which accounted for the degradation of NOR; meanwhile, ¹O₂, •O₂⁻, and •OH also participated in the degradation process. A probable degradation mechanism of NOR by the Zn_0.1Cd_0.9S photocatalyst was also proposed. In addition, tetrapod-shaped Zn_0.1Cd_0.9S also possessed excellent stability and good recyclability. This study represents a significant step toward the application of bandgap-tunable Zn_xCd_1−xS for the effective removal of antibiotics under visible light and natural sunlight, which provides a great basis for the design and construction of more high-performance photocatalysts based on as-synthesized tetrapod-shaped Zn_0.1Cd_0.9S solid solutions for the efficient removal of other antibiotics from wastewater.