Photocatalytic Degradation of Sulfamethoxazole by Cd/Er-Doped Bi₂MoO₆

Abstract

Bi₂MoO₆ (BMO) is a typical bismuth-based semiconductor material, and its unique Aurivillius structure provides a broad space for electron delocalization. In this study, a new type of bismuth molybdate Cd/Er-BMO photocatalytic material was prepared by co-doping Er³⁺ and Cd²⁺, and the performance of the photocatalytic degradation of sulfamethoxazole (SMZ) was systematically studied. The research results showed that the efficiency of SMZ degradation by Cd/Er-BMO was significantly improved after doping Er³⁺ and Cd²⁺ ions, reflecting the synergistic catalytic effect of Cd²⁺ and Er³⁺ co-doping. Cd/Er-BMO doped with 6% Cd had the highest degradation efficiency (93.89%) of SMZ under visible light irradiation. The material revealed excellent stability and reusability in repeated degradation experiments. In addition, 6% Cd/Er-BMO had a smaller particle size and a larger specific surface area, which is conducive to improving the generation efficiency of its photogenerated electron-hole pairs and reducing the recombination rate, significantly enhancing the photocatalysis of the material. This study not only provides an effective photocatalyst for degrading environmental pollutants such as SMZ, but also provides an important scientific basis and new ideas for the future development of efficient and stable photocatalytic materials.

1. Introduction

With the development of global industrialization, the human living environment has gradually deteriorated, and a large amount of waste has been discharged. Water pollution is particularly prominent among many pollution problems as it contains a large amount of organic pollutants, many of which are difficult to degrade, such as phenols, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons. These organic pollutants possess a relatively high biological toxicity, which seriously threatens human health and life. According to the survey, a large number of additives that pollute the environment and are harmful to the human body are used in the process of textile printing and dyeing. Most of these additives are discharged into the water environment, resulting in water pollution. In addition, upon the increase in the use of antibiotics in human society, antibiotics inevitably enter the water environment. Among them, sulfamethoxazole (SMZ) is an organic compound that is mainly used to treat diseases such as urinary tract infections and respiratory system infections that are caused by sensitive bacteria. It has shown strong antibacterial properties in clinical practise. However, studies have shown that 45%–90% will be directly excreted in the form of metabolites through urine and feces and enter the environment [1]. Conventional methods, such as physical adsorption and Fenton methods, are difficult to effectively degrade these compounds, leading to the long-term deterioration of water quality, which, in turn, affects the virtuous cycle of the ecosystem. Moreover, sulfonamides are degraded slowly after entering the environment and are likely to remain for a long time, which will eventually result in an adverse impact on human health. As such, the degradation treatment of this type of wastewater is of great importance [2].

In order to remove harmful pollutants from the water environment, photocatalytic degradation was adopted in this work. Photocatalysis is a pollutant degradation technology that makes use of radiation, usually in the UV and visible range, in the presence of an adequate catalyst to produce highly reactive oxidizing species. It is a promising environmental technology due to its high efficiency, environmental friendliness, and energy saving characteristics. It can remove reluctant-to-degrade organic pollutants in wastewater at a low cost [3]. This method can decompose organic substances that are harmful to the human body and the environment without causing a waste of resources or producing secondary pollutants. A large number of studies have shown that organic pollutants can be effectively photocatalytically degraded, decolorized, and mineralized into small inorganic molecules, thereby eliminating pollution to the environment. Photocatalysts have also attracted much attention given their strong redox ability, good stability, low pollution, and easy recycling properties. UV-excited TiO₂-based photocatalysts have been widely studied [4]. The principle of this technology is to use TiO₂ to generate active free radicals, thereby achieving the photocatalytic degradation of organic pollutants and generating CO₂, H₂O, and simple inorganic substances. However, as a result of the wide band gap of TiO₂ (3.2 eV), it can only absorb ultraviolet light and does not have good catalytic activity in the visible light range, which greatly limits the practical application of such catalysts. Studies have shown that the properties of titanium dioxide can be changed by making composite materials [5,6,7]. In addition, in-depth research on semiconductor materials such as CdS [8,9], ZrO₂ [10,11], and SnO₂ [12,13] has been widely conducted, and has achieved remarkable results. However, the photogenerated electrons and holes in CdS are easy to recombine, which reduces its photocatalytic efficiency. The wide band gap energy of ZrO₂ results in less light absorption and its photocatalytic activity is limited by the high charge carrier recombination efficiency. The inherent defects of SnO₂ in photocatalysis, such as structural design and morphology control, have been widely documented, which can affect its photocatalytic performance.

In view of the above shortcomings of photocatalytic materials, more scholars have begun to study new ones in recent years. Bismuth-based semiconductor materials have attracted extensive attention worldwide as a result of their easy regulatory morphology, good photochemical stability, and unique electronic band structure [14]. Bismuth molybdate (Bi₂MoO₆) is a compound with remarkable photocatalytic properties and is one of the most important members of the Aurivillius oxide materials family [15], with its structure providing sufficient space for electron delocalization, thereby facilitating catalytic reactions [16]. However, the practical application of the Bi₂MoO₆ monomer is severely limited given its poor photocarrier separation, high recombination rate, and narrow photoresponse range [17]. To overcome these limitations, a variety of modifications have been made in recent years. Current modification methods mainly focus on doping with non-metallic ions, transition metals, and rare earth metals [18]. Wang et al. [19] synthesized Cu-doped Bi₂MoO₆ using a simple solvothermal method. Cu doping reduces the work function and improves the charge separation efficiency, which is considered to be the main reason for the enhanced photoactivity. In addition, Cu doping has little effect on the morphology of Bi₂MoO₆, though it has a greater effect on the energy band structure, which makes the reducibility of Cu-doped Bi₂MoO₆ stronger. As such, Cu-Bi₂MoO₆ exhibits a higher photocatalytic efficiency than pure Bi₂MoO₆. Meng et al. [20] prepared an efficient Bi₂MoO₆ nitrogen-fixing photocatalyst using iron as the medium. The Fe-induced reduction in the surface work function facilitates charge transport to the catalyst surface. In addition, Fe doping can also improve charge collection through the Fe³⁺/Fe²⁺ redox pathway and can become an active site for nitrogen reduction. Given the above advantages, Fe-mediated Bi₂MoO₆ significantly enhances the photocatalytic activity of nitrogen fixation driven by visible light. Li et al. [21] prepared Eu³⁺-doped bismuth molybdate phosphor using the sol–gel method, studied the effect of different calcination temperatures on BMO: Eu³⁺, and characterized its structure and properties. The crystal structure and related parameters were obtained using the Rietveld method. The relationship between crystal structure and luminescence properties was analyzed in detail, and BMO: Eu³⁺ phosphor was applied to the preparation of luminescent films and LED devices. As such, it is very important to select a suitable doping atom to improve the photocatalytic degradation efficiency.

Previous studies have shown that sulfamethoxazole and rhodamine B can be photocatalytically degraded under visible light by doping γ-Bi₂MoO₆ nanomaterials with heterovalent cadmium. Researchers have successfully synthesized a Cd-BMO photocatalyst with an excellent pollutant degradation efficiency through a simple hydrothermal method [18,22]. Compared with BMO, Cd-BMO has a smaller particle size, a larger specific surface area, a greater charge separation efficiency, and a greater electron excitation capability, thereby achieving a higher degradability of pollutants. The latest research indicates that the co-doping of multiple elements can lead to more effective photocatalysts through the synergistic interaction between multiple ions. For example, Li et al. [23] used a hydrothermal method to synthesize a new type of Gd/Er/Lu-doped Bi₂MoO₆ photocatalyst for the degradation of rhodamine B (RhB) and tetrachlorophenol, significantly enhancing the photocatalytic degradation performance. Specifically, after doping with Gd³⁺, hydroxyl radicals were generated to improve the oxidation efficiency of Bi₂MoO₆. The introduction of Er³⁺ provided an energy upconversion centre, thereby improving light absorption. After doping with Lu³⁺ ions, abundant oxygen vacancies were generated in the Bi₂MoO₆ crystal, thereby promoting carrier separation. Wang et al. [24] used the citric acid complexation method to prepare Eu³⁺ and Fe³⁺ co-doped bismuth molybdate to improve its photocatalytic performance. The results indicated that Eu³⁺ and Fe³⁺ co-doped bismuth molybdate achieved the highest photocatalytic activity. The synergistic effect of the two ions resulted in a 94.1% degradation of the sample within 50 min, and the photocatalytic activity of Eu/Fe-BMO was not simply the sum of the photocatalytic properties of the Eu-BMO and Fe-BMO catalysts. Inspired by the synergistic effect of co-doping, this study used a hydrothermal method to prepare Er³⁺ and Cd²⁺ co-doped photocatalytic materials. Through material characterization and photocatalytic degradation experiments, the synergistic effect of the two doping ions was studied to further modify the intrinsic material and improve its photocatalytic performance.

2. Materials and Methods

2.1. The Preparation of the Catalyst

Er-BMO and Cd/Er-BMO materials were prepared using the hydrothermal method. The chemical reagents used in the preparation process were not purified. The detailed preparation process is as follows: Bi(NO₃)₃·5H₂O (4 mmol) was dissolved in 10 mL HNO₃ (2 mol·L⁻¹) to obtain solution A, and (NH₄)₆Mo₇O₂₄·4H₂O (0.2857 mmol) was dissolved in 10 mL NaOH (1 mol·L⁻¹) to obtain solution B. Solution B was slowly added to solution A and was stirred continuously for 20 min to obtain solution C, before a certain amount of Er(NO₃)₃·5H₂O (0.12 mmol) was added. Then, solid Cd(NO₃)₂·4H₂O (0.08, 0.16, 0.24, 0.32, and 0.4 mmol) was weighed and added while continuously stirring until uniform. The pH of the solution was adjusted to 8 with ammonia solution. Finally, the precursor solution was transferred to a 100 mL polytetrafluoroethylene liner, kept at 180 °C for 12 h, and was cooled naturally to room temperature. The samples were washed with deionized water and anhydrous ethanol, respectively, and were dried at 80 °C for 12 h. The samples were labelled as X Cd/Er-BMO (X = 2%, 4%, 6%, 8%, and 10%). In addition, the γ-Bi₂MoO₆ sample containing only Er³⁺ was prepared using the same method and was labelled as Er-BMO.

2.2. Characterization

The crystal structure of the samples was analyzed using X-ray diffraction (XRD), in the range of 5–90°, at a scanning rate of 2°/min and a step size of 0.02°. The samples were studied using scanning electron microscopy (SEM) at a working voltage of 15 kV to reveal the morphology of the samples. The microstructure of the samples was analyzed using transmission electron microscopy (TEM) at 250 kV. The specific surface area of the samples was determined using the Bruauer-Emmett-Teller (BET) method, and the average pore size of the samples was determined using nitrogen adsorption at 77 K. The samples were degassed under vacuum conditions at 150 °C for 6 h. The instrument used in the measurement process was Quanta 4000 (Quanta Computer, Taoyuan, China). The elemental valence state of the samples was analyzed using X-ray photoelectron spectroscopy (XPS) using Moser K-ALPHA (Thermo Fisher Scientific, Waltham, MA, USA). The prepared samples were measured using ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) using a UV/VIS/NIR Lambda 1050 (PerkinElmer, Waltham, MA, USA). The optical response and electron transfer characteristics of the sample were analyzed by using the transient photocurrent response. Among them, the suspension prepared using Na₂SO₄ was used for photocurrent tests, and the samples were tested under a 300 W xenon light source and electrochemical workstation. Photoluminescence (PL) was used to analyze the recombination rate of photogenerated electron-hole pairs, and the time-resolved fluorescence data of the samples were measured using an Hitachi F-7100 (Hitachi High-Technologies, Tokyo, Japan). The photoresponse ability of the prepared samples was analyzed using the transient photocurrent response.

2.3. Photocatalytic Experiment

The prepared sample was placed under a long arc xenon lamp to study its photocatalytic degradation performance of the SMZ solution. A total of 50 mL of SMZ pollutant solution (5 mg·L⁻¹) and 50 mg of catalyst powder were placed in a quartz tube and were irradiated with a 400 W xenon lamp. The solution was stirred continuously for 30 min without visible light irradiation, and part of the suspension was taken after reaching the adsorption-desorption equilibrium. The xenon lamp was turned on and a certain amount of suspension (SMZ solution) was withdrawn every 45 min. It is important to use a cooling jacket to control the temperature of the photocatalytic reaction at 15 °C. Finally, the pollutant concentration was determined spectrophotometrically in the range of 190–338 nm using a UV1901PC instrument (Unico (Shanghai) Instruments Co., Ltd., Shanghai, China).

3. Results and Discussion

3.1. Characterization of Photocatalysts

X-ray diffraction (XRD) was used to analyze and confirm the crystal structure and phase composition of the existing catalyst (Figure 1a). As clearly shown in the figure, the XRD pattern of the Cd/Er-BMO sample is very similar to the reference pattern JCPDS No. 21-0102 of the original BMO; the diffraction peak is sharp, indicating that the crystallinity of the prepared sample is good [25]. In the Cd/Er-BMO sample, the 2θ diffraction angles are 11.0°, 27.2°, 28.3°, 32.5°, 33.2°, 36.1°, 46.7°, 47.1°, 55.6°, and 56.4°, respectively, corresponding to the PDF card of γ-Bi₂MoO₆ (JCPDS No. 21-0102) of (020), (140), (131), (200), (060), (151), (202), (260), (133), and (082). Among them, the most significant diffraction peak of the sample was observed at the (131) crystal plane. Upon the addition of different concentrations of Cd²⁺, the peak at (131) continued to change but no other diffraction peaks were generated, further illustrating the successful doping of Cd²⁺ into Er-BMO without producing other substances [20]. Meanwhile, the resolution of the diffraction peak of the sample was high, indicating that the crystallinity was good. In particular, note that under different doping ratios, almost no additional diffraction peaks were revealed in the XRD images of Cd/Er-BMO, indicating that the crystal structure of Er-BMO was not changed after the introduction of Cd²⁺. The results showed that compared with Er-BMO, the (131) diffraction peak of Cd/Er-BMO gradually increased with the increase in Cd²⁺ doping concentration, and the highest peak appeared in the Cd/Er-BMO sample when Cd was 6%. The peak intensity increased and the shape of the diffraction peak was clear, indicating that the formation of nanocrystals was good. However, the subsequent peak drop can be explained by the excessive Cd²⁺ doping concentration inhibiting the growth of crystal size, resulting in a smaller crystal size [26]. Figure 1b shows that the Cd/Er-BMO (131) diffraction peak shifts to larger diffraction angles with increasing Cd²⁺ doping concentration and reaches a maximum of 28.42° at 6% Cd/Er-BMO. The shift of the diffraction peak reflects the contraction of the crystal and the reduction in the crystal plane spacing [27]. This is because the small ionic radius element replaces the large ionic radius element, because the Bi³⁺ ionic radius (1.08 Å) is larger than the Cd²⁺ ionic radius (0.97 Å). The subsequent shift to the left is due to lattice distortion caused by the high Cd²⁺ doping concentration [18].

The morphology and particle size of the synthesized samples were observed and studied using scanning electron microscopy (SEM). No particles, cracks, or surface roughness changes were observed in Er-BMO (Figure 1c) and 6% Cd/Er-BMO (Figure 1d). Compared with Er-BMO materials, the size of the 6% Cd/Er-BMO nanosheets was significantly reduced, changing from the original sheet shape to the needle shape. The emergence of the needle-like structure indicated that the addition of Cd²⁺ resulted in a significant increase in the specific surface area of the intrinsic material and an increase in the contact area for the reaction. The appearance of the needle-like structure may be attributable to the doping of Cd²⁺ ions, which affected the growth of Er-BMO [28].

The microstructure and lattice planes of Er-BMO and 6% Cd/Er-BMO were measured using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (Figure 1e,f) to investigate the microstructure and properties of the materials. Both the Er-BMO and 6% Cd/Er-BMO samples have complete morphology, changing from an irregular flake structure to a needle-like structure. It is worth noting that the size of 6% Cd/Er-BMO was smaller than that of Er-BMO, which is consistent with the SEM analysis. HRTEM images showed the lattice features of Er-BMO (Figure 1e) and 6% Cd/Er-BMO (Figure 1f). The lattice fringes of Er-BMO and Cd/Er-BMO were clear, corresponding to the (131) and (200) planes of the orthorhombic crystal of γ-Bi₂MoO₆, respectively, indicating that there was no change in the orthorhombic phase before and after doping. The TEM and HRTEM images confirmed that the doping of Cd²⁺ had no significant effect on the crystal lattice and did not produce any other crystalline phase, which is consistent with the XRD results mentioned above [23].

The specific surface area is an important parameter in the photocatalytic degradation process that affects the performance and efficiency of the catalyst. A larger specific surface area can increase the active sites, improve the adsorption capacity, and increase light absorption. The specific surface area (BET) of Er-BMO and 6% Cd/Er-BMO was determined using N₂ adsorption/desorption isotherms. The pore size distribution was determined using the Barrett-Joyner-Halenda method (BJH). As shown in Figure 2a,b, in the relative pressure range of 0.8 to 1.0, the isotherms of the two samples showed typical type IV characteristics, namely, the adsorption amount was caused by capillary condensation [29] and formed a hysteresis loop in the medium and high relative pressure region. The figure shows that the 6% Cd/Er-BMO crystals had a stronger adsorption/desorption curve. Figure 2b reveals that the pore size distribution of the sample was between 2 and 50 nm. This pore size is between micropores and macropores, which belong to a mesoporous structure. Mesoporous materials usually have a high specific surface area and good fluid transport properties. The specific surface area and average pore size of the tested samples are shown in the insert table in Figure 2. When doping with Cd²⁺, the specific surface area of Er-BMO increased from 8.414 to 12.638 m²·g⁻¹, indicating that the doping of Cd²⁺ ions resulted in a larger specific surface area of Er-BMO, as well as more active sites. Meanwhile, the increase in active sites equates to more catalytic active centres, which can effectively improve the rate and efficiency of the catalytic reaction. This is consistent with the above SEM analysis results [30]. In addition, the study also showed that the average pore size of Cd/Er-BMO was better than that of Er-BMO, which resulted in a larger number of active surface sites [31].

An energy spectrum (EDS) element diagram of 6% Cd/Er-BMO (Figure 1g–k) shows that the Cd, Er, Bi, Mo, and O elements are uniformly distributed, which means that Cd²⁺ ions are successfully doped into Er-BMO.

Analyzing the chemical state of the doped sample benefits the study of the bonding of elements. As such, XPS analysis was used to monitor the valence state and elemental composition of Er-BMO and Cd/Er-BMO. Prior to the analysis of the data, C 1s was used for calibration at 284.8 eV (Figure 2c) given its stability, universality, clear peaks, and simple electronic structure. It can be observed that the characteristic peaks of the C, Cd, Er, Bi, Mo, and O atoms appeared in the XPS spectrum of 6% Cd/Er-BMO. Among them, the presence of the C element is attributed to the surface contamination of carbon materials (Figure 2d) [32], and the peak at 288.4 eV may be attributable to C-N or C-(N)₃ in the aromatic lattice [33]. As shown in Figure 2e, Cd 3d is located at 405.0 and 411.6 eV, corresponding to Cd 3d_5/2 and Cd 3d_3/2 of Cd²⁺, respectively. Two strong peaks at 158.9 and 164.3 eV were observed in the Bi 4f XPS spectrum of 6% Cd/Er-BMO, which corresponds to Bi 4f_7/2 and Bi 4f_5/2, respectively (Figure 2f), indicating that the valence state of Bi in 6% Cd/Er-BMO is +3 [18]. Similarly, Mo 3d corresponded to the two peaks of Mo 3d_5/2 and Mo 3d_3/2 that were located at 232.2 and 235.3 eV, respectively (Figure 2g), indicating that Mo exists only in the Mo⁶⁺ oxidation state in 6% Cd/Er-BMO [34]. Two strong peaks at 170.2 and 169.0 eV were shown in the spectrum of Er 4d, corresponding to Er 4d_3/2 and Er 4d_5/2, respectively (Figure 2h). The O 1s peaks of Er-BMO and 6% Cd/Er-BMO are shown in Figure 2i. The O 1s peak was fitted using three Gaussian superposition peaks, corresponding to 530.0, 531.2, and 532.7 eV, respectively. The peak at 530 corresponds to lattice oxide species, while the peak at 531.2 corresponds to surface hydroxyl groups and the peak at 532.6 corresponds to weakly adsorbed water [35]. The comparison result regarding the peaks of Er-BMO and Cd/Er-BMO indicated that doping Cd²⁺ has no effect on the valence state of Er-BMO. Cd/Er-BMO has similar peaks to Er-BMO, indicating that their crystal structures are similar. The reduction in electron density weakens the electron shielding effect, thereby affecting the binding energy and bonding environment. After doping, the Gaussian peak of the Er-BMO sample shifts toward the high bond energy direction, indicating that doping with Cd²⁺ will change the chemical environment [36], replacing Bi³⁺ or Er³⁺ to form ionic bonds with oxygen atoms, or Cd²⁺ is inserted into the Er-BMO lattice, resulting in a decrease in the electron density of Bi³⁺, Mo⁶⁺, and O²⁻ [37]. This also proves that Cd is successfully incorporated into the Er-BMO lattice [38].

The photocatalytic performance of materials is closely related to the recombination of electrons and holes as the photoexcitation, generation, separation, and recombination of electron-hole pairs have a significant impact on the efficiency of the photocatalytic reaction. As such, the electron-hole-related properties are analyzed by measuring the UV-visible diffuse reflectance spectrum, transient photocurrent response, and photoluminescence spectrum to understand the optical properties of the material in one step. As shown in Figure 3a, the UV spectrum of Er-BMO has a band gap absorption edge near 471.23 nm. Compared with Er-BMO, upon the increase in the Cd²⁺ doping ratio, the light absorption range of Cd/Er-BMO in the visible light region gradually undergoes a slight red shift, indicating that its light absorption capacity continues to increase and its photocatalytic efficiency continues to improve. The red shift phenomenon stops at 6% Cd/Er-BMO (493.30–513.19 nm). Upon the continued increase in the Cd²⁺ concentration, a slight blue shift (513.19–484.11 nm) gradually occurs. The red shift indicates that Cd²⁺ doping can broaden the light absorption range of Er-BMO and enhance visible light absorption, which leads to the generation of more electrons and holes and, eventually, may improve the photocatalytic reaction efficiency [39]. The subsequent blue shift can be explained by the increased Cd²⁺ doping concentration. An excessive exposure resulted in a change in the Er-BMO structure. In addition, new absorption peaks were found at 481.8 and 614.8 nm, which can be attributed to the transition of erbium ions 4F_{3/2, 5/2} and 4F_9/2 from the 4I_15/2 ground state to the excited state, further confirming that Er³⁺ can provide the energy upconversion centre and promote the transfer of electrons in the system [38]. By plotting the relationship between (αhν)² and photon energy (hν), the band gap energy (Eg) of the Cd²⁺-doped Er-BMO photocatalyst can be calculated; this is shown in Figure 3b. In the figure, α, h, and ν are the absorption coefficient, Planck constant, and optical frequency, respectively. As can be seen from Figure 3b, the band gaps of Er-BMO, 2% Cd/Er-BMO, 4% Cd/Er-BMO, 6% Cd/Er-BMO, 8% Cd/Er-BMO, and 10% Cd/Er-BMO are 2.81, 2.72, 2.63, 2.60, 2.74, and 2.78 eV, respectively. The energy band of the Cd²⁺-doped Er-BMO photocatalyst is reduced in line with the trend in adsorption edge, and the minimum energy band is displayed at 6% Cd/Er-BMO. The smaller the band, the narrower the band width. The narrow band width can improve the photocatalytic efficiency. Therefore, 6% Cd/Er-BMO showed the largest photocatalytic activity [26].

The transient photocurrent method is a widely used characterization technique to detect charge carrier photogeneration and extraction kinetics in optoelectronic devices. In order to illustrate the carrier separation efficiency, the response speed [40] and the calculated transient photocurrent response is shown in Figure 3c. As shown in Figure 3c, the magnitude of the photoresponse intensity was 6% Cd/Er-BMO > 2% Cd/Er-BMO > 10% Cd/Er-BMO > 8% Cd/Er-BMO > 4% Cd/Er-BMO > Er-BMO, among which the photoresponse intensity of 6% Cd/Er-BMO was the largest. This proves that the content of electron-hole pairs generated by its photoexcitation is the highest. Compared with that of Er-BMO, the photoresponse intensity of Cd/Er-BMO increased to varying degrees, indicating that Er-BMO crystals can generate more electron-hole pairs under photoexcitation when doped with Cd²⁺, thereby further promoting the photocatalytic reaction. PL is a non-destructive and non-contact optical method for probing the electronic structure of materials. When light hits the sample, it begins a process called photoexcitation, in which the light is absorbed and excess energy is given to the material [41]. In addition, photoluminescence (PL) spectroscopy can be used to further analyze the separation and transport of photogenerated charges in materials and to further study the recombination efficiency of photocarriers in photocatalysis [42]. This is the spectrum of the intensity or energy distribution of light with different wavelengths formed by the recombination of electrons and holes in the quasi-equilibrium state of the material. The higher the peak value, the higher the electron-hole recombination efficiency of the material and the worse the photocatalytic performance [43]. The PL spectra of Er-BMO and Cd/Er-BMO are shown in Figure 3d. The typical response peaks are identified in the emission spectra of Er-BMO and Cd/Er-BMO. Compared with the strong response peak of Er-BMO, Cd-BMO shows a relatively weak peak (the intensity of 10% Cd/Er-BMO is the highest), which may be attributable to an excessive doping concentration leading to the photoinduced recombination between electrons and holes and the subsequent increase in the PL intensity [44]. This indicated that the appropriate amount of Cd²⁺ doping can promote the transfer of electrons and holes. In summary, Cd/Er-BMO can effectively promote the generation of photogenerated carriers, which inhibit and improve the recombination and separation of electron-hole pairs by doping Cd ions in Er-BMO. In particular, 6% Cd/Er-BMO showed a significantly improved electron excitation and transfer ability and revealed an excellent photocatalytic degradation performance.

3.2. Photocatalytic Degradation

In order to explore the photocatalytic pollutant degradation ability of Er-BMO and Cd/Er-BMO with different doping ratios, 5 mg·L⁻¹ SMZ was photocatalytically degraded under simulated visible light radiation under a 400 W long arc xenon lamp. Meanwhile, recycling degradation experiments and free radical capture experiments were conducted to study the stability of bismuth molybdate materials and the free radical degradation mechanism. For the repeatability of the experiment, we tested the irradiation intensity of the catalyst in the experiment, which is 4.72 × 10⁴ Lux (Err: 0.13 × 10⁴ Lux).

Firstly, the degradation efficiency diagram of the Er-BMO and Cd/Er-BMO crystals for the SMZ solution under simulated visible light irradiation to study the photocatalytic degradation performance is shown in Figure 4a. The relative concentration (C₀ − C/C₀) of the SMZ solution (each point separated by 45 min) was plotted as a function of reaction time. The concentration of pollutant solution was measured using a UV-visible spectrophotometer, and the degradation efficiency under the visible illumination time can be calculated according to the following formula:

$$\mathsf{\eta }={\displaystyle \frac{{\mathrm{C}}_0 -\mathrm{C}}{{\mathrm{C}}_0}}\times 100\%$$

As shown in the figure, the adsorption degree of SMZ at 6% Cd/Er-MBO (more than 20%) is large, while the adsorption on Er-MBO can be ignored, which proves that the existence of Cd²⁺ is related to the adsorption capacity. This may be related to surface acidity, as the addition of Cd²⁺ leads to an increase in the number of oxygen vacancies. The degradation efficiency of Er-BMO for SMZ was only 3.11% within the first 180 min. Other than that, the degradation efficiency of SMZ by BMO was only 4.1% in 210 min, which proved that pure BMO was not active in the degradation process [18]. After adding Cd²⁺, the degradation efficiency was significantly improved with 6% Cd/Er-BMO revealing the most obvious increase, which reached 93.89% degradation efficiency in 180 min. For different contents of Cd/Er-BMO, the order of the degradation efficiencies is 6% Cd/Er-BMO > 8% Cd/Er-BMO > 10% Cd/Er-BMO > 4% Cd/Er-BMO > 2% Cd/Er-BMO. The results fully reflect the good photocatalytic ability of Cd/Er-BMO.

The repeated degradation experiment of 6% Cd/Er-BMO is shown in Figure 4b. Four repeated photocatalytic experiments were carried out on 6% Cd/Er-BMO under the same reaction conditions to investigate the degradation of SMZ. That is, after the experiment was completed, the sample was recovered and the experiment was repeated for a total of four times. After one experiment, the sample is usually rinsed with water and dried before the next experiment. There will be some losses during the experiment. We will reduce the amount of pollutant solution according to the quality of the remaining samples in the previous proportion to compensate for the losses in repeated experiments. The results showed that the degradation efficiencies of the four experiments were 93.89%, 91.59%, 90.03%, and 89.40%, respectively, indicating its good sustainability, durability, and stability. In addition, the 6% Cd/Er-BMO photocatalyst after the degradation experiment was characterized using XRD and SEM (Figure 4c,d). A comparison before and after the experiment showed that the peaks shown in the XRD spectrum of 6% Cd/Er-BMO were still consistent. The SEM image suggested the 6% Cd/Er-BMO maintained its morphology, which confirmed its stability and great potential.

At the same time, ICP-MS was used to detect the concentration of ions in the solution after photocatalytic degradation, and the concentration of the specified element was obtained using the correlation function. The test results and related functions are shown in

Table 1. ICP-MS experiment.

Sample	Cd2+ Intensity	Cd2+ Concentration (mg·L−1)	Bi3+ Intensity	Bi3+ Concentration (mg·L−1)
1 mg·L−1 Cd2+ solution	20,414.82	1.0124	\	\
1 mg·L−1 Bi3+ solution	\	\	569.90	1.0357
6% Cd/Er-BMO degradation solution	1684.75	0.0835	266.90	0.4839

Cd²⁺ Intensity = 20,163.71 × Concentration + 1.081; Bi³⁺ Intensity = 549.02 × Concentration + 1.227.

. Cd²⁺ (0.0835 mg·L⁻¹) and Bi³⁺ (0.4839 mg·L⁻¹) showed that there was a small amount of ion overflow during photocatalytic degradation, but the photocatalytic efficiency was not affected. It should be noted that the Cd²⁺ concentration shown in the table is 0.0835 mL·L⁻¹, which is lower than the continuous emission concentration of the Chinese government (0.1 mg·L⁻¹) (ref: GB 8978-1996; China National Comprehensive sewage discharge standard). In summary, our experiments were carried out under prescribed and reasonable conditions, which also indicated that 6% Cd/Er-BMO is an excellent photocatalytic material. Nevertheless, the concentration of 0.0835 mg·L⁻¹ is higher than the limit for drinking water (0.005 mg·L⁻¹) and irrigation water (0.01 mg·L⁻¹), demonstrating that the solution still needs further treatment before it can be used for drinking and irrigation, and that the presence of Cd²⁺ in the photocatalyst seems to place some limits on the application of this catalyst.

A capture experiment is an experimental method to detect and study particles, molecules, ions, or reaction intermediates. Free radicals and holes play a key role in many biological and chemical processes. Therefore, studying free radicals and holes plays an important role in understanding these processes. As shown in Figure 4e, the formation and reaction mechanism of common functional groups and holes during the degradation of SMZ solution were studied through capture experiments. Since hydroxyl radicals (·OH), superoxide radicals (·O₂₋), and holes (h⁺) are the most active in photocatalysis, 1 mM isopropanol (IPA), 1 mM sodium bicarbonate (NaHCO₃), and nitrogen (N₂) are used as capture agents to capture ·OH, h⁺, and ·O₂₋ functional groups. In addition, nitrogen was continuously introduced through a tube during the experiment to capture superoxide radicals. When IPA, NaHCO₃, and N₂ were added to the experiment, the photocatalytic degradation performance decreased significantly. Compared with the reaction system without the addition of a capture agent, the degradation effect of 6% Cd/Er-BMO decreased from 93.89% to 6.64% after adding IPA as a capture agent during the degradation process, which seriously affected the photocatalytic reaction activity. Secondly, the degradation effect dropped from 93.89% to 26.79% after adding nitrogen. After adding NaHCO₃, the degradation effect dropped from 93.89% to 46.43%. The results indicated that after co-doping, the ·OH, h⁺, and ·O₂₋ radicals are still important active substances in the photocatalytic degradation of organic matter, and they play an important role in the degradation of SMZ. According to previous studies, the addition of IPA in the case of doping with only Cd²⁺ did not significantly change the photocatalytic degradation performance [18], which further confirmed that the doping of Er³⁺ benefits the formation of ·OH [23].

3.3. Photocatalytic Mechanism Analysis

The photocatalytic mechanism refers to the process and principle of photocatalysts inducing chemical reactions under light conditions, the study of which is conducive to understanding the nature of the photocatalytic process, optimizing the design of catalysts, and improving the efficiency of photocatalysis. Semiconductor photoexcitation as a photocatalytic mechanism refers to the band structure of semiconductor particles, which consists of a low-energy valence band (VB) filled with electrons and an empty high-energy conduction band (CB) with a forbidden band between them. When a light source of appropriate energy irradiates the semiconductor, the semiconductor will be activated as a result of the absorption of the energy of the photon, resulting in electron jumping from the valence band to the conduction band, generating electron-hole pairs. The electron-hole pairs separate and migrate to the surface of the semiconductor, and some of them recombine during the migration process, losing their activity. As such, to improve the quantum yield of semiconductor photocatalytic reactions, it is necessary to make the acceptor potential lower than the semiconductor conduction band potential, as well as making the donor potential higher than the semiconductor valence band potential, to effectively inhibit the direct recombination of photogenerated electrons and holes.

For the co-doping system in this paper, when the sample was irradiated with light with energy equal to or greater than the band gap energy, e⁻ and h⁺ are formed and exist in the conduction band and valence band, respectively. As Cd²⁺ was doped in Er-BMO, especially 6% Cd/Er-BMO, the band gap was decreased, the generation of electron-hole pairs was promoted with the direct recombination of photogenerated electrons, and the holes were effectively suppressed. As such, a large number of e⁻ and h⁺ were separated to the appropriate position to contact the pollutants, improving the degradation efficiency. ·O₂₋ was developed when e⁻ contacts O₂, and then the h⁺ and active radical (·O₂₋ and ·OH) were employed for the SMZ degradation based on the free radical capture experiment results. Note that h⁺ is mainly used directly for the degradation experiments, as opposed to using ·OH formed by h⁺. Thus, the possible degradation process is as follows: SMZ is hydrolyzed during the reaction to form the product TP 1, then h⁺ and ·OH break the S-N bond to form the products TP 2 and TP 3. Finally, all intermediate products further react with h⁺ and ·O₂₋ to form smaller molecules or mineralize to CO₂ and H₂O [2]. In addition, in order to simplify the picture, we will replace the pollutant in the left half of Figure 5 with R-H, which will generate H₂O and simple organic matter (R·) during the reaction process. The detailed photodegradation process is as follows:

$$\mathrm{C}\mathrm{d}/\mathrm{E}\mathrm{r}-\mathrm{B}\mathrm{M}\mathrm{O}+\mathrm{h}\mathsf{\mu }\to \mathrm{C}\mathrm{d}/\mathrm{E}\mathrm{r}-\mathrm{B}\mathrm{M}\mathrm{O}({\mathrm{e}}^{-}\mathrm{c}\mathrm{b}+{\mathrm{h}}^{+}\mathrm{v}\mathrm{b})$$

(1)

$${\mathrm{e}}^{-}\mathrm{c}\mathrm{b}+{\mathrm{O}}_2\to ·{\mathrm{O}}_{2^{-}}$$

(2)

$$·{\mathrm{O}}_{2^{-}}+{\mathrm{H}}_2\mathrm{O}\to ·\mathrm{O}\mathrm{H};{\mathrm{h}}^{+}\mathrm{v}\mathrm{b}+{\mathrm{H}}_2\mathrm{O}\to ·\mathrm{O}\mathrm{H}$$

(3)

$$\begin{gathered} \mathrm{SMZ}+{\mathrm{h}}^{+}\mathrm{v}\mathrm{b}+·\mathrm{O}\mathrm{H}\to {\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{N}}_2{\mathrm{O}}_3\mathrm{S} \\ {\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{N}}_2{\mathrm{O}}_3\mathrm{S}+{\mathrm{h}}^{+}\mathrm{v}\mathrm{b}+·\mathrm{O}\mathrm{H}\to {\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{N}}_2\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_2\mathrm{S} \\ {\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{N}}_2\mathrm{O}+{\mathrm{h}}^{+}/·{\mathrm{O}}_{2^{-}}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2 \\ {\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_2\mathrm{S}+{\mathrm{h}}^{+}/·{\mathrm{O}}_{2^{-}}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2 \end{gathered}$$

(4)

4. Conclusions

In this paper, a simple hydrothermal method was used to successfully prepare the photocatalyst Cd/Er-BMO, which can be applied to the photocatalytic degradation of antibiotics and can improve the efficiency of pollutant degradation. Compared with Er-BMO, Cd/Er-BMO has a higher specific surface area, improved electronic properties, and a higher catalytic activity. In addition, Cd/Er-BMO did not produce any other crystal phases during the doping process. Through the analysis of the UV-visible diffuse reflectance spectrum, transient photocurrent response, and photoluminescence spectrum, it is confirmed that 6% Cd/Er-BMO can not only effectively promote the generation of photogenerated carriers, but also inhibit the recombination of electron-hole pairs and their separation, thereby further enhancing the photocatalytic degradation performance of Cd/Er-BMO. In repeated degradation experiments, 6% Cd/Er-BMO showed an excellent sustainability and stability, confirming that the material can maintain a good degradation ability during photocatalytic degradation. Free radical capture experiments revealed that h⁺, ·O_2⁻, and ·OH were the main active substances for the effective degradation of SMZ, further confirming the effectiveness and advantages of Cd/Er-BMO in practical applications. In summary, this study systematically investigated the degradation process of SMZ doped with Cd²⁺ Er-BMO, and found that 6% Cd/Er-BMO demonstrated an excellent catalytic performance. This work identified a new effective photocatalyst for real-world degradation and provided ideas and directions for the future research and development of more efficient photocatalysts.