BiVO4-Based Photocatalysts for the Degradation of Antibiotics in Wastewater: Calcination Role after Solvothermal Synthesis

Citation: Aguirre-Cortes, J.M.; Moral-Rodríguez, A.I.; Bailón-García, E.; Carrasco-Marín, F.; Pérez-Cadenas, A.F. BiVO4-Based Photocatalysts for the Degradation of Antibiotics in Wastewater: Calcination Role after Solvothermal Synthesis. Catalysts 2024,14, 474. https://doi.org/ 10.3390/catal14080474 Academic Editor: Carlo Santoro Received: 14 July 2024 Revised: 19 July 2024 Accepted: 22 July 2024 Published: 25 July 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article BiVO4-Based Photocatalysts for the Degradation of Antibiotics in Wastewater: Calcination Role after Solvothermal Synthesis Jhon Mauricio Aguirre-Cortes *, Adriana Isabel Moral-Rodríguez, Esther Bailón-García, Francisco Carrasco-Marín and Agustín Francisco Pérez-Cadenas * UGR-Carbon, Materiales Polifuncionales Basados en Carbono, Departamento Química Inorgánica, Unidad de Excelencia Química Aplicada a Biomedicina y Medioambiente, Universidad de Granada, ES18071 Granada, Spain; [email protected] (A.I.M.-R.); [email protected] (E.B.-G.); [email protected] (F.C.-M.) *Correspondence: [email protected].es (J.M.A.-C.); [email protected] (A.F.P.-C.) Abstract: BiVO 4 is an important n-type semiconductor used in photocatalysis due to its high capacity to absorb solar light in the 400–700 nm range, abundance, high chemical stability, non-toxicity, and low cost. However, research on physicochemical modifications to increase its catalytic activity via simple procedures is limited. In this work, the influence of different synthesis parameters, such as calcination temperatures or silver doping, on the structural and physicochemical characteristic of the BiVO 4 -based photocatalysts and their photocatalytic performance in degrading sulfamethoxazole from aqueous solution under blue-LED irradiation was evaluated. BiVO 4 -based photocatalysts were synthesized using a solvothermal method. The monoclinic phase (m-s) was successfully kept stable even after the thermal treatments at 300, 450, and 600 ◦ C and the corresponding silver doping. The low bandgap of 2.40 eV and the average particle size of 18 nm of the BiVO 4 catalyst treated at 300 ◦ C seems to be the key. Afte doping, Ag/BiVO 4 photocatalyst treated at the optimal found calcination temperature (300 ◦C) showed the best photocatalytic behavior. Keywords: BiVO4; solar light; SMX; photocatalysts 1. Introduction Wastewater treatment plants (WWTPs) have received significant attention in recent decades for their role in preventing micropollutants from entering the environment [ 1 ]. Among these micropollutants, antibiotics stand out as a vital class of medicaments that have revolutionized modern medicine in the 21st century, playing a crucial role in treating infectious diseases and enabling vital therapies [2]. However, the disproportionate use of antibiotics can contribute to both genotoxic effects in humans and aquatic species and the emergence of antibiotic-resistant bacteria (ARB) [ 3 – 6 ]. Because of their chemical characteristics and antibacterial nature, antibiotics cannot be completely eliminated or degraded through conventional Urban Wastewater Treatment Plants (UWTPs) that employ conventional mechanical–biological technology, such as UASB (Upflow Anaerobic Sludge Blanket) reactors or active sludges [7]. This limitation in technical capabilities motivates them to be considered as emergent contaminants [ 6 , 8 , 9 ], of special interest in modern society, and a global concern. This category includes highly pharmaceutical products, personal care products, and microplastics [ 10 ], which are spreading globally among people, food, animals, and plants via soil, water, and the air depending on factors such as geographic locations, sources of contamination, or environmental conditions [ 11 ], as recognized by the World Health Organization (WHO) [3,12]. Therefore, accurate and timely detection of emerging contaminants among waterborne pathogens is essential for safeguarding water quality. However, challenges related to cost, complexity, limited sensitivity, and selectivity must be addressed to provide a more accurate guide on water treatment [13,14]. Catalysts 2024,14, 474. https://doi.org/10.3390/catal14080474 https://www.mdpi.com/journal/catalysts Catalysts 2024,14, 474 2 of 21 Among the most commonly used antibiotics in both human and veterinary therapy are aminoglycoside [ 15 ], trimethoprim (TMP) [ 16 ], β -lactams [ 17 ], chloramphenicols [ 18 ], macrolides [ 19 ], nitrofurans [ 20 ], sulfonamides [ 12 ], tetracyclines [ 21 ], and sulfamethoxazole (SMX) [ 22 ]. SMX, an antibiotic belonging to the group of synthetic antimicrobial agents, is widely used in human and animal therapy to treat various bacterial and protozoal infections. Sulfanilamide is a structural analog of para-aminobenzoic acid (PABA) [ 23 ] and is frequently used in human medicine to treat bronchitis and urinary tract infections [ 24 – 26 ]. This antibiotic ranks among the top ten formulated in Europe and North America and is considered an essential medicine by the WHO. For effective action, it is regularly dosed in combination with trimethoprim as trimethoprim–sulfamethoxazole (TMP–SMX), with recommended doses between 10 and 20 mg kg−1day−1[27]. However, unfortunately, approximately 15% of this medication is excreted from the human body unchanged, leading to concentrations ranging from 0.01 to 2.0 µg L−1in wastewater. As an alternative to mitigating the constraints in tertiary treatment, conventional process UWTPs for degrading or eliminating micropollutants such as drugs, advanced processes such as Advanced Oxidation Processes (AOPs) and Advanced Reduction Processes (ARPs) have been demonstrated to be effective in removing drugs and pathogens from wastewater due to their low energy consumption, non-toxicity, and cost-effectiveness [ 28 ]. Among AOPs, photocatalysis has been used to eliminate antibiotics such as trimethoprim [29], sulfonamides (up to 69%) [30], and nitrofurans (up to 93%) [31]. Photocatalysis, which involves a reaction facilitated by both a catalyst and light, is a promising technology that can be applied at specific wavelengths derived from clean energy and renewable sources, representing one of the simplest and most effective approaches [ 32 ]. Photocatalysis utilizes solar energy, a unique natural resource abundantly available at no cost, which is non-polluting and adaptable for various applications. In this direction toward environmental remediation, as renewable source of clean energy, sunlight can be employed to stimulate heterogeneous photocatalysts such as metal oxides [ 33 ], chalcogenides [ 34 ], and semiconductor materials [35,36]. Currently, one of the most used semiconductor materials is TiO 2 ; however, its bandgap energy is relatively high (3.2 eV), limiting its use to wavelengths in the UV region of the electromagnetic spectrum, which are not abundant in the solar spectrum [ 35 ]. To address these challenges, bismuth vanadate (BiVO 4 ) is an alternative to TiO 2 for the effective removal of emergent contaminants. This semiconductor can be readily stimulated by visible light because of its low band gap (2.4 eV in the monoclinic scheelite structure), high crystallinity (main characteristic peaks at 2 θ : 18.7 ◦ , 28.8 ◦ , 30.5 ◦ , 35.2 ◦ , and 53.3 ◦ ), and simple synthesis method [37,38]. Recently, some research groups focusing on photocatalyst applications have studied different alternatives. For example, Xu et al. [ 39 ] used BiVO 4 –graphene nanocomposites for the photodegradation of rhodamine B (RhB), driven by visible light from a 450 W high-pressure mercury lamp as the irradiation source. In this work, with BiVO 4 containing 0.25% graphene, the photodegradation reached 87% after 20 h of irradiation. Ni et al. [ 40 ] designed and integrated an excellent 2D morphology structure with a smart BiOI/BiVO 4 2D heteronanostructure under hydrothermal conditions. This layered lattice structure improved photocatalytic performance compared to individual BiVO 4 and BiOI under visible-light irradiation from a 300 W Xe arc lamp equipped with a UV cutoff filter. When applied to the photodegradation of RhB and salicylic acid (SA), the degradation rates were 29.8-times (RhB) and 15.7-times (SA) higher than those of individual BiVO 4 , and 6.4-times (RhB) and 10-times (SA) higher than those of BiOI, respectively. Pham et al. [ 41 ] conducted a study on the photodecomposition of methylene blue (MB) dye using LED light as the irradiation source, consisting of six daylight LEDs with a power of 10 W and a maximum light output of 1052 lm. To control the crystal morphology and crystal faces, BiVO 4 samples obtained via the solvothermal method were calcined at 400 ◦ C. The experimental process employed 50 mg of catalyst dispersed in 100 mL of MB solution (15 ppm), and the sample prepared at 140 ◦C and pH 7 (BV-140-7) exhibited 82.30% MB removal in 180 min. Catalysts 2024,14, 474 3 of 21 Silver as the active center for antibiotic photocatalytic degradation was investigated by Liu et al. [ 42 ], with an Ag(I)-based coordination polymer [Ag 2 (dib)(H 2 L) 2 ] used for the photocatalytic degradation of nitrofurantoin (NFT), achieving 97.95% degradation under ultraviolet irradiation in 60 min when the concentration of NFT was 30 ppm. As an alternative to heterogeneous tests, Tsoumachidou et al. [ 43 ] demonstrated the homogeneous photocatalytic treatment of PABA using photo-Fenton and ferrioxalate processes. They used a thermostated Pyrex cell with artificial UV-A or visible light (9 W) of identical dimensions and geometry. From aqueous stock solutions at 0.2 g L −1 , which is substantially higher than that typically found in environmental samples (ranging from nanograms per liter to micrograms per liter), they showed that elevating the reaction temperature increased the apparent reaction rate kfor both UV-A and visible light. For example, at 323 K after 30 min of treatment with visible irradiation, PABA removal was almost 86%. Herein, due to easy preparation and thermal treatment modification, bismuth vanadate salt was synthesized using the solvothermal method to investigate the effect of calcination temperature (BiV/T, where T represents the thermal treatment temperature) on the formation of the monoclinic scheelite phase (m-s), the most active phase for the photocatalytic degradation of organic molecules. Due to its facile implementation, photocatalysis was selected as the AOP to test the removal efficiency of the samples. Beyond the design and experimental results, the theoretical studies focused on the Langmuir–Hinshelwood– Hougen–Watson (LHHW) model, which required minimization of the objective function using an Excel sheet for different pseudo-reaction orders. Inspired by these concepts, we aim in this work to enhance our understanding about some variables which affect the development of BiVO 4 -based photocatalysts, such as the calcination temperature and the doping with silver, to promote the degradation of emergent pollutants. 2. Results and Discussion Figure 1shows powder samples after the synthesis protocol. The physical characteristics change when the powders are processed at different temperatures. Different yellow color intensities for fine powders with similar grain sizes are relatively easy to observe as the selected temperature (80, 300, 450, or 600 ◦C) increases during thermal treatment. Catalysts 2024, 14, x FOR PEER REVIEW 3 of 20 samples obtained via the solvothermal method were calcined at 400 °C. The experimental process employed 50 mg of catalyst dispersed in 100 mL of MB solution (15 ppm), and the sample prepared at 140 °C and pH 7 (BV-140-7) exhibited 82.30% MB removal in 180 min. Silver as the active center for antibiotic photocatalytic degradation was investigated by Liu et al. [42], with an Ag(I)-based coordination polymer [Ag 2 (dib)(H 2 L) 2 ] used for the photocatalytic degradation of nitrofurantoin (NFT), achieving 97.95% degradation under ultraviolet irradiation in 60 min when the concentration of NFT was 30 ppm. As an alternative to heterogeneous tests, Tsoumachidou et al. [43] demonstrated the homogeneous photocatalytic treatment of PABA using photo-Fenton and ferrioxalate processes. They used a thermostated Pyrex cell with artificial UV-A or visible light (9 W) of identical dimensions and geometry. From aqueous stock solutions at 0.2 g L −1 , which is substantially higher than that typically found in environmental samples (ranging from nanograms per liter to micrograms per liter), they showed that elevating the reaction temperature increased the apparent reaction rate 𝑘 for both UV-A and visible light. For example, at 323 K after 30 min of treatment with visible irradiation, PABA removal was almost 86%. Herein, due to easy preparation and thermal treatment modification, bismuth vanadate salt was synthesized using the solvothermal method to investigate the effect of calcination temperature (BiV/T, where T represents the thermal treatment temperature) on the formation of the monoclinic scheelite phase (m-s), the most active phase for the photocatalytic degradation of organic molecules. Due to its facile implementation, photocatalysis was selected as the AOP to test the removal efficiency of the samples. Beyond the design and experimental results, the theoretical studies focused on the Langmuir–Hinshelwood– Hougen–Watson (LHHW) model, which required minimization of the objective function using an Excel sheet for different pseudo-reaction orders. Inspired by these concepts, we aim in this work to enhance our understanding about some variables which affect the development of BiVO 4 -based photocatalysts, such as the calcination temperature and the doping with silver, to promote the degradation of emergent pollutants. 2. Results and Discussion Figure 1 shows powder samples after the synthesis protocol. The physical characteristics change when the powders are processed at different temperatures. Different yellow color intensities for fine powders with similar grain sizes are relatively easy to observe as the selected temperature (80, 300, 450, or 600 °C) increases during thermal treatment. Figure 1. Photographs of undoped BiV/T (T = 80, 300, 450, and 600 °C) photocatalyst. For all samples, the physicochemical properties of the particles induce significant changes in the color of the bulk powder. Physical changes can be observed, such as an opaquer color in samples treated at lower temperatures (80 and 300 °C). In contrast, those treated at higher temperatures exhibited a brighter appearance (450 and 600 °C). 2.1. X-ray Diffraction (XRD) Thermal treatments can significantly affect the crystalline structure of solids [44,45]. At low temperatures (below 500 °C), thermal expansion of crystals commonly occurs, altering their dimensions and leading to modifications in their physicochemical properties. Figure 1. Photographs of undoped BiV/T (T = 80, 300, 450, and 600 ◦C) photocatalyst. For all samples, the physicochemical properties of the particles induce significant changes in the color of the bulk powder. Physical changes can be observed, such as an opaquer color in samples treated at lower temperatures (80 and 300 ◦ C). In contrast, those treated at higher temperatures exhibited a brighter appearance (450 and 600 ◦C). 2.1. X-ray Diffraction (XRD) Thermal treatments can significantly affect the crystalline structure of solids [ 44 , 45 ]. At low temperatures (below 500 ◦ C), thermal expansion of crystals commonly occurs, altering their dimensions and leading to modifications in their physicochemical properties. At intermediate temperatures (between 500 ◦ C and 1000 ◦ C), both modifications in the physicochemical properties and partial recrystallization of the sample can occur [41]. For BiVO 4 -based photocatalysts, the effect on the crystalline structure depends on the treatment temperature, heating time, and rate [ 46 ]. This solid can crystallize into three different structures: tetragonal zircon (t-z), monoclinic scheelite (m-s), and tetragonal Catalysts 2024,14, 474 4 of 21 scheelite (t-s). Several studies have shown that the monoclinic form is much more active in the visible spectrum region than the other crystalline forms, which is generally attributed to its small band-gap magnitude [47]. To initiate our studies and identify the photocatalyst with the best performance as a function of calcination temperature, the undoped samples were tested in the photodegradation of SMX as a probe molecule. The relationship between photodegradation capacity and temperature indicates the temperature that maximizes photodegradation efficiency. As observed in Figure 2, The optimal temperature is 335 ◦ C, achieving a significant photodegradation of approximately 50%. Catalysts 2024, 14, x FOR PEER REVIEW 4 of 20 At intermediate temperatures (between 500 °C and 1000 °C), both modifications in the physicochemical properties and partial recrystallization of the sample can occur [41]. For BiVO4-based photocatalysts, the effect on the crystalline structure depends on the treatment temperature, heating time, and rate [46]. This solid can crystallize into three different structures: tetragonal zircon (t-z), monoclinic scheelite (m-s), and tetragonal scheelite (t-s). Several studies have shown that the monoclinic form is much more active in the visible spectrum region than the other crystalline forms, which is generally attributed to its small band-gap magnitude [47]. To initiate our studies and identify the photocatalyst with the best performance as a function of calcination temperature, the undoped samples were tested in the photodegradation of SMX as a probe molecule. The relationship between photodegradation capacity and temperature indicates the temperature that maximizes photodegradation efficiency. As observed in Figure 2, The optimal temperature is 335 °C, achieving a significant photodegradation of approximately 50%. Figure 2. Photodegradation results for SMX degradation after 540 min of irradiation using undoped BiV/T (T = 80, 300, 450, and 600 °C) photocatalysts as a function of calcination temperature. The optimal temperature seems to be 335 °C. Among the prepared photocatalysts, the sample treated at 300 °C, named BiV/300, was selected as the closest to the optimal temperature due to its proximity to the theoretical value of 335 °C, and for this reason, the silver-doped samples prepared were those named as Ag/BiV/80 and Ag/BiV/300 for the corresponding photodegradation study. Figure 3a shows the X-ray diffractogram of the BiV/T and Ag/BiV/T photocatalyst system (T corresponds to the calcination temperature in °C) and the Miller indices attributed to the main peaks of the monoclinic scheelite phase (m-s) assigned to JCPDS No. 00-075-1866, detected in all photocatalysts [48]. Figure 3b presents an enlargement of the diffraction patterns in the 28–31.5° range. The intensities at 28.5° and 30.5° correspond to the crystalline planes (121) and (040), respectively. In Figure 3b, an increase in the intensity of the diffraction peak of the (040) crystalline plane concerning (121) was also observed as the temperature was increased from 80 to 300 °C, during the synthesis of the photocatalysts. The increase in the intensity of the (040) plane peak indicates higher crystallinity, which can significantly enhance the photocatalytic activity of the materials to degrade organic pollutants [49,50]. Regarding the photocatalysts containing Ag (Ag/BiV/80 and Ag/BiV/300), the main diffraction peaks attributed to metallic Ag (2θ = 38° and 44°) (JCPDS No. 65-2871) were observed in both samples, but at low intensity, due to the low weight percentage of Ag added (3.5 wt.% Ag) [51]. Figure 4a,b show the correlations of the intensity ratio I(121)/I(110) and I(040)/I(110) concerning the synthesis temperature of the photocatalysts for the BiV/T (T = 80, 300, 450, and 600 °C) and Ag/BiV/T (T = 80 and 300 °C) samples. Figure 2. Photodegradation results for SMX degradation after 540 min of irradiation using undoped BiV/T (T = 80, 300, 450, and 600 ◦ C) photocatalysts as a function of calcination temperature. The optimal temperature seems to be 335 ◦C. Among the prepared photocatalysts, the sample treated at 300 ◦ C, named BiV/300, was selected as the closest to the optimal temperature due to its proximity to the theoretical value of 335 ◦ C, and for this reason, the silver-doped samples prepared were those named as Ag/BiV/80 and Ag/BiV/300 for the corresponding photodegradation study. Figure 3a shows the X-ray diffractogram of the BiV/T and Ag/BiV/T photocatalyst system (T corresponds to the calcination temperature in ◦ C) and the Miller indices attributed to the main peaks of the monoclinic scheelite phase (m-s) assigned to JCPDS No. 00-075-1866, detected in all photocatalysts [48]. Figure 3b presents an enlargement of the diffraction patterns in the 28–31.5 ◦ range. The intensities at 28.5 ◦ and 30.5 ◦ correspond to the crystalline planes (121) and (040), respectively. In Figure 3b, an increase in the intensity of the diffraction peak of the (040) crystalline plane concerning (121) was also observed as the temperature was increased from 80 to 300 ◦ C, during the synthesis of the photocatalysts. The increase in the intensity of the (040) plane peak indicates higher crystallinity, which can significantly enhance the photocatalytic activity of the materials to degrade organic pollutants [ 49 , 50 ]. Regarding the photocatalysts containing Ag (Ag/BiV/80 and Ag/BiV/300), the main diffraction peaks attributed to metallic Ag (2 θ = 38 ◦ and 44 ◦ ) (JCPDS No. 65-2871) were observed in both samples, but at low intensity, due to the low weight percentage of Ag added (3.5 wt.% Ag) [ 51 ]. Figure 4a,b show the correlations of the intensity ratio I(121)/I(110) and I(040)/I(110) concerning the synthesis temperature of the photocatalysts for the BiV/T (T = 80, 300, 450, and 600 ◦C) and Ag/BiV/T (T = 80 and 300 ◦C) samples. Catalysts 2024,14, 474 5 of 21 Catalysts 2024, 14, x FOR PEER REVIEW 5 of 20 (a) (b) Figure 3. XRD pattern (a) of undoped BiV/T (T = 80, 300, 450, and 600 °C) and doped Ag/BiV/T (T = 80 and 300 °C) photocatalysts. (b) Enlargement of the 2θ degree area representing the (121) and (040) crystalline planes of (m-s) BiVO4, 7.13 and 9.37. Figure 4. Correlation of the intensity ratio of I(121)/I(010) and I(040)/I(110) in the diffraction patterns of (a) undoped BiV/T (T = 80, 300, 450, and 600 °C) and (b) doped Ag/BiV/T (T = 80 and 300 °C) photocatalyst with synthesis temperature. In Figure 4a, an increase in the intensity ratio I(121)/I(110) was observed with increasing temperature from 80 to 300 °C. From 300 to 600 °C, the intensity ratio I(121)/I(110) decreased slightly as the synthesis temperature increased. On the other hand, the increase in temperature during the synthesis of the photocatalysts showed a greater effect on the intensity ratio of I(040)/I(110) for all the samples. For example, the intensity ratio I(040)/I(110) increased from 4.6 to 8.5 (1.84 times) as the temperature increased from 80 to 300 °C. These results indicated that temperature can influence the orientation of the crystal growth during the synthesis of BiVO4. In Figure 4b, it can be seen that the intensity ratios I(121)/I(110) and I(040)/I(110) increased slightly as the synthesis temperature was raised from 80 to 300 °C. It can also be observed that the intensities I(121)/I(110) are slightly higher, compared to the photocatalysts not containing Ag (See Figure 4a). The average crystal size (dp) of the photocatalysts was determined using the DebyeScherrer equation [52], the results are shown in Table 1. The dp varied from 16 to 20 nm as the temperature increased during the synthesis of the photocatalysts. These results can be attributed to the formation of crystal defects due to oxygen evolution and decomposition 10 15 20 25 30 35 40 45 50 55 60 Normalized Intensity (u.a.) 2 θ (Degree) Ag/BiV/300 BiV/600 Ag/BiV/80 BiV/450 BiV/300 BiV/80 (m-s) BiVO 4 (Ag) (002) (110) (011) (121) (040) (200) (220) (301) (105) (015) (123) (204) (024) (125) (116) (411) (Ag) (111) (200) Figure 3. XRD pattern (a) of undoped BiV/T (T = 80, 300, 450, and 600 ◦ C) and doped Ag/BiV/T (T = 80 and 300 ◦ C) photocatalysts. (b) Enlargement of the 2 θ degree area representing the (121) and (040) crystalline planes of (m-s) BiVO4, 7.13 and 9.37. Catalysts 2024, 14, x FOR PEER REVIEW 5 of 20 (a) (b) Figure 3. XRD pattern (a) of undoped BiV/T (T = 80, 300, 450, and 600 °C) and doped Ag/BiV/T (T = 80 and 300 °C) photocatalysts. (b) Enlargement of the 2θ degree area representing the (121) and (040) crystalline planes of (m-s) BiVO4, 7.13 and 9.37. Figure 4. Correlation of the intensity ratio of I(121)/I(010) and I(040)/I(110) in the diffraction patterns of (a) undoped BiV/T (T = 80, 300, 450, and 600 °C) and (b) doped Ag/BiV/T (T = 80 and 300 °C) photocatalyst with synthesis temperature. In Figure 4a, an increase in the intensity ratio I(121)/I(110) was observed with increasing temperature from 80 to 300 °C. From 300 to 600 °C, the intensity ratio I(121)/I(110) decreased slightly as the synthesis temperature increased. On the other hand, the increase in temperature during the synthesis of the photocatalysts showed a greater effect on the intensity ratio of I(040)/I(110) for all the samples. For example, the intensity ratio I(040)/I(110) increased from 4.6 to 8.5 (1.84 times) as the temperature increased from 80 to 300 °C. These results indicated that temperature can influence the orientation of the crystal growth during the synthesis of BiVO4. In Figure 4b, it can be seen that the intensity ratios I(121)/I(110) and I(040)/I(110) increased slightly as the synthesis temperature was raised from 80 to 300 °C. It can also be observed that the intensities I(121)/I(110) are slightly higher, compared to the photocatalysts not containing Ag (See Figure 4a). The average crystal size (dp) of the photocatalysts was determined using the DebyeScherrer equation [52], the results are shown in Table 1. The dp varied from 16 to 20 nm as the temperature increased during the synthesis of the photocatalysts. These results can be attributed to the formation of crystal defects due to oxygen evolution and decomposition 10 15 20 25 30 35 40 45 50 55 60 Normalized Intensity (u.a.) 2 θ (Degree) Ag/BiV/300 BiV/600 Ag/BiV/80 BiV/450 BiV/300 BiV/80 (m-s) BiVO 4 (Ag) (002) (110) (011) (121) (040) (200) (220) (301) (105) (015) (123) (204) (024) (125) (116) (411) (Ag) (111) (200) Figure 4. Correlation of the intensity ratio of I(121)/I(010) and I(040)/I(110) in the diffraction patterns of (a) undoped BiV/T (T = 80, 300, 450, and 600 ◦ C) and (b) doped Ag/BiV/T (T = 80 and 300 ◦ C) photocatalyst with synthesis temperature. In Figure 4a, an increase in the intensity ratio I(121)/I(110) was observed with increasing temperature from 80 to 300 ◦ C. From 300 to 600 ◦ C, the intensity ratio I(121)/I(110) decreased slightly as the synthesis temperature increased. On the other hand, the increase in temperature during the synthesis of the photocatalysts showed a greater effect on the intensity ratio of I(040)/I(110) for all the samples. For example, the intensity ratio I(040)/I(110) increased from 4.6 to 8.5 (1.84 times) as the temperature increased from 80 to 300 ◦ C. These results indicated that temperature can influence the orientation of the crystal growth during the synthesis of BiVO4. In Figure 4b, it can be seen that the intensity ratios I(121)/I(110) and I(040)/I(110) increased slightly as the synthesis temperature was raised from 80 to 300 ◦ C. It can also be observed that the intensities I(121)/I(110) are slightly higher, compared to the photocatalysts not containing Ag (See Figure 4a). Catalysts 2024,14, 474 6 of 21 The average crystal size (dp) of the photocatalysts was determined using the DebyeScherrer equation [ 52 ], the results are shown in Table 1. The dp varied from 16 to 20 nm as the temperature increased during the synthesis of the photocatalysts. These results can be attributed to the formation of crystal defects due to oxygen evolution and decomposition or destruction of the BiVO 4 crystal at high temperatures [ 53 , 54 ]. The photocatalysts containing Ag showed a larger dp size compared to those without Ag. The dp was 23 and 22 nm for Ag/BiV/80 and Ag/BiV/300, respectively. Table 1. Average crystal size (from XRD) and band-gap values of undoped BiV/T (T = 80, 300, 450, and 600 ◦C) and doped Ag/BiV/T (T = 80 and 300 ◦C) photocatalysts. Photocatalyst dp Band Gap nm eV BiV/80 16 2.37 BiV/300 18 2.40 BiV/450 19 2.49 BiV/600 20 2.45 Ag/BiV/80 23 2.48 Ag/BiV/300 22 2.47 Since the solvothermal synthesis temperature of all photocatalysts was 80 ◦ C and the calcination temperature of 300 ◦ C was the closest to optimal, the undoped materials treated at these temperatures were selected to be studied via nitrogen adsorption–desorption at 77 K to analyze the textural changes between the freshly prepared precursor and the one that would subsequently be doped with silver. 2.2. Nitrogen Adsorption–Desorption Isotherms at 77 K The textural properties of the BiVO 4 samples at lower temperatures (80 and 300 ◦ C) are shown in Figure 5and Table 2. The Figure 5shows the nitrogen adsorption–desorption isotherms at 77 K for the photocatalysts. In both cases, the isotherms share similar characteristics and belong to type II according to the IUPAC classification which is a characteristic feature of non-porous samples [55,56]. Catalysts 2024, 14, x FOR PEER REVIEW 6 of 20 or destruction of the BiVO4 crystal at high temperatures [53,54]. The photocatalysts containing Ag showed a larger dp size compared to those without Ag. The dp was 23 and 22 nm for Ag/BiV/80 and Ag/BiV/300, respectively. Since the solvothermal synthesis temperature of all photocatalysts was 80 °C and the calcination temperature of 300 °C was the closest to optimal, the undoped materials treated at these temperatures were selected to be studied via nitrogen adsorption–desorption at 77 K to analyze the textural changes between the freshly prepared precursor and the one that would subsequently be doped with silver. Table 1. Average crystal size (from XRD) and band-gap values of undoped BiV/T (T = 80, 300, 450, and 600 °C) and doped Ag/BiV/T (T = 80 and 300 °C) photocatalysts. Photocatalyst dp Band Gap nm eV BiV/80 16 2.37 BiV/300 18 2.40 BiV/450 19 2.49 BiV/600 20 2.45 Ag/BiV/80 23 2.48 Ag/BiV/300 22 2.47 2.2. Nitrogen Adsorption–Desorption Isotherms at 77 K The textural properties of the BiVO4 samples at lower temperatures (80 and 300 °C) are shown in Figure 5 and Table 2. The Figure 5 shows the nitrogen adsorption–desorption isotherms at 77 K for the photocatalysts. In both cases, the isotherms share similar characteristics and belong to type II according to the IUPAC classification which is a characteristic feature of non-porous samples [55,56]. Figure 5. N2 adsorption–desorption isotherms of undoped BiV/T (T = 80 and 300 °C) photocatalysts. Table 2. Textural properties of doped and undoped BiV/T (T = 80 and 300 °C) photocatalysts. Photocatalyst SBET SDR Smic Wo (N2) W0.95 Vmeso Lo (N2) m2 g−1 m 2 g−1 m 2 g−1 cm3 g−1 cm3 g−1 cm3 g−1 nm BiV/80 5.1 5.8 0.0 0.002 0.005 0.003 3.3 BiV/300 2.2 2.9 0.2 0.001 0.002 0.001 1.9 BiV/80/Ag 5.3 5.5 0.0 0.002 0.005 0.003 3.1 BiV/300/Ag 2.0 2.8 0.0 0.001 0.002 0.001 2.0 0.00 0.01 0.02 0.03 0.04 0.0 0.2 0.4 0.6 0.8 1.0 Volume adsobed Vliq(cm 3 ·g -1 ) (P/P 0 ) BiV/80 BiV/300 Figure 5. N 2 adsorption–desorption isotherms of undoped BiV/T (T = 80 and 300 ◦ C) photocatalysts. Catalysts 2024,14, 474 7 of 21 Table 2. Textural properties of doped and undoped BiV/T (T = 80 and 300 ◦C) photocatalysts. Photocatalyst SBET SDR Smic Wo(N2) W0.95 Vmeso Lo(N2) m2g−1m2g−1m2g−1cm3g−1cm3g−1cm3g−1nm BiV/80 5.1 5.8 0.0 0.002 0.005 0.003 3.3 BiV/300 2.2 2.9 0.2 0.001 0.002 0.001 1.9 BiV/80/Ag 5.3 5.5 0.0 0.002 0.005 0.003 3.1 BiV/300/Ag 2.0 2.8 0.0 0.001 0.002 0.001 2.0 Table 2shows that the specific surface areas calculated from the S BET equation were 5.1 and 2.2 m 2 g −1 for BiV/80 and BiV/300, respectively. Based on the type and shape of the adsorption isotherm (see Figure 5), as well as the surface area BET calculation, it can be concluded that the samples are non-porous due to the negligible value of specific surface areas [55]. 2.3. UV-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) Figure 6shows the UV-Vis absorption spectra of the photocatalysts. The absorption spectra of the photocatalyst system showed strong absorption in the UV-Vis region, confirming the optical response of these materials. Catalysts 2024, 14, x FOR PEER REVIEW 7 of 20 Table 2 shows that the specific surface areas calculated from the SBET equation were 5.1 and 2.2 m2 g−1 for BiV/80 and BiV/300, respectively. Based on the type and shape of the adsorption isotherm (see Figure 5), as well as the surface area BET calculation, it can be concluded that the samples are non-porous due to the negligible value of specific surface areas [55]. 2.3. UV-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) Figure 6 shows the UV-Vis absorption spectra of the photocatalysts. The absorption spectra of the photocatalyst system showed strong absorption in the UV-Vis region, confirming the optical response of these materials. These values were obtained using the Kubelka–Munk method [F(R)-E]1/2 [57]. The photocatalysts showed Eg values of 2.37 eV (BiV/80), 2.40 eV (BiV/300), 2.49 eV (BiV/450), 2.45 eV (BiV/600), 2.48 eV (Ag/BiV/80), and 2.47 eV (Ag/BiV/300) (see Table 1). These values slightly increase with the rise in synthesis temperature and correspond to the Eg values of the (m-s) BiVO4 phase [49]. The Eg values for Ag/BiV/80 and Ag/BiV/300 were similar to those without Ag. This result might be due to the low weight percentage of Ag added. However, its presence, even in low weight percentages, can significantly enhance the photocatalytic activity [58]. (a) (b) Figure 6. (a) UV-Vis absorbance spectra, (b) Eg calculations of undoped BiV/T (T = 80, 300, 450 and 600 °C) and doped Ag/BiV/T (T = 80 and 300 °C) photocatalysts. 2.4. Scanning Electron Microscopy (SEM) To investigate the morphological changes due to the effect of calcination temperature, the SEM images of all undoped photocatalysts are presented in Figure 7 [59]. The morphology of the samples is closely linked to the thermal treatment temperature. Figure 7a,b for the sample at 80 °C (BiV/80) show agglomerated shapes with dimensions between 1 and 3 μm, while individual particles, nearly spherical in shape, have sizes around 150 nm. At calcination temperature of 300 °C (BiV/300), Figure 7c,d show few differences compared with the precursor; agglomerated shapes remain due to the union of individual particles. An important morphological feature corresponds to its pine-like architecture, as observed by Le-Duy [60]. However, this sample exhibits the smallest sizes among all the samples; some individual particles have reduced their size to values below 100 nm. For the sample treated at 450 °C (BiV/450), the images in Figure 7e,f show a significant change in the aggregate size. Contrary to the phenomenon observed in BiV/80 and BiV/300, the aggregates are now much larger, with sizes above 150 nm. At 600 °C, the 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 [F(R)-E]1/2 (a.u.) Energy (eV) BiV/80 BiV/300 BiV/450 BiV/600 Ag/BiV/80 Ag/BiV/300 Figure 6. (a) UV-Vis absorbance spectra, (b) Egcalculations of undoped BiV/T (T = 80, 300, 450 and 600 ◦C) and doped Ag/BiV/T (T = 80 and 300 ◦C) photocatalysts. These values were obtained using the Kubelka–Munk method [F(R)-E]1/2 [ 57 ]. The photocatalysts showed Eg values of 2.37 eV (BiV/80), 2.40 eV (BiV/300), 2.49 eV (BiV/450), 2.45 eV (BiV/600), 2.48 eV (Ag/BiV/80), and 2.47 eV (Ag/BiV/300) (see Table 1). These values slightly increase with the rise in synthesis temperature and correspond to the Eg values of the (m-s) BiVO 4 phase [ 49 ]. The Eg values for Ag/BiV/80 and Ag/BiV/300 were similar to those without Ag. This result might be due to the low weight percentage of Ag added. However, its presence, even in low weight percentages, can significantly enhance the photocatalytic activity [58]. 2.4. Scanning Electron Microscopy (SEM) To investigate the morphological changes due to the effect of calcination temperature, the SEM images of all undoped photocatalysts are presented in Figure 7[ 59 ]. The morphology of the samples is closely linked to the thermal treatment temperature. Figure 7a,b for the sample at 80 ◦ C (BiV/80) show agglomerated shapes with dimensions between 1 and 3µm, while individual particles, nearly spherical in shape, have sizes around 150 nm. Catalysts 2024,14, 474 8 of 21 Catalysts 2024, 14, x FOR PEER REVIEW 8 of 20 effect of temperature is noticeable when observing the images in Figure 7g,h for the BiV/600 sample. There is a significant loss of porosity; now the aggregates are much larger and less porous, with sizes reaching several microns. For example, Figure 7g shows that, at this temperature, the aggregates scarcely exhibit shapes below 100 nm and are much larger. This difference in sizes can be explained by the crystallization processes to which the samples were subjected. The silver-doped samples showed very similar SEM images as the corresponding undoped samples. Figure 7. SEM images of undoped BiV/T (T = (a,b) 80, (c,d) 300, (e,f) 450, and (g,h) 600 °C). 2.5. High-Resolution Transmission Electron Microscopy (HRTEM) Due to its superior performance in the photodegradation of the probe molecule, the sample containing Ag calcined at 300 °C (Ag/BiV/300) was analyzed with HRTEM. Figure 8a shows a single particle of the sample, highlighting its irregular characteristics, formed by the aggregation of smaller plate-like particles with heterogeneous shapes. The micrographs in Figure 8b,c show the lattice fringes of BiVO 4 with an approximate dimension of 0.36 nm, corresponding to the crystallographic planes (101) [61]. Figure 8d, shows the symmetrical arrangement of the white diffraction points, confirming the polycrystalline nature of BiVO 4 . Figure 8. HRTEM analysis of sample Ag/BiV/300. (a) single particle at 36,000× magnification, (b,c) lattice fringes of BiVO 4 at 105,000× magnification, (d) diffraction points of BiVO 4 , (e–h) EDX-mapping. Figure 7. SEM images of undoped BiV/T (T = (a,b) 80, (c,d) 300, (e,f) 450, and (g,h) 600 ◦C). At calcination temperature of 300 ◦ C (BiV/300), Figure 7c,d show few differences compared with the precursor; agglomerated shapes remain due to the union of individual particles. An important morphological feature corresponds to its pine-like architecture, as observed by Le-Duy [ 60 ]. However, this sample exhibits the smallest sizes among all the samples; some individual particles have reduced their size to values below 100 nm. For the sample treated at 450 ◦ C (BiV/450), the images in Figure 7e,f show a significant change in the aggregate size. Contrary to the phenomenon observed in BiV/80 and BiV/300, the aggregates are now much larger, with sizes above 150 nm. At 600 ◦ C, the effect of temperature is noticeable when observing the images in Figure 7g,h for the BiV/600 sample. There is a significant loss of porosity; now the aggregates are much larger and less porous, with sizes reaching several microns. For example, Figure 7g shows that, at this temperature, the aggregates scarcely exhibit shapes below 100 nm and are much larger. This difference in sizes can be explained by the crystallization processes to which the samples were subjected. The silver-doped samples showed very similar SEM images as the corresponding undoped samples. 2.5. High-Resolution Transmission Electron Microscopy (HRTEM) Due to its superior performance in the photodegradation of the probe molecule, the sample containing Ag calcined at 300 ◦C (Ag/BiV/300) was analyzed with HRTEM. Figure 8a shows a single particle of the sample, highlighting its irregular characteristics, formed by the aggregation of smaller plate-like particles with heterogeneous shapes. The micrographs in Figure 8b,c show the lattice fringes of BiVO 4 with an approximate dimension of 0.36 nm, corresponding to the crystallographic planes (101) [ 61 ]. Figure 8d, shows the symmetrical arrangement of the white diffraction points, confirming the polycrystalline nature of BiVO4. To identify the elemental composition of the sample, elemental mapping was performed using the EDS technique. The images corresponding to the presence of bismuth (blue color), vanadium (yellow color), and silver (pink color) are presented in Figure 8e–h. This morphology confirms that, after calcination, the crystallinity of the silver-doped sample was preserved. This is evidenced by the formation of silver metallic nanoparticles, as confirmed with X-ray diffraction, where the characteristic crystallographic plane (111) of Ag 0 was observed (see Figure 3). These characteristics suggest that Ag +1 cations have been reduced to Ag 0 during the thermal treatment process and that silver nanoparticles were possibly coated on the BiVO4surface. Catalysts 2024,14, 474 9 of 21 Catalysts 2024, 14, x FOR PEER REVIEW 8 of 20 effect of temperature is noticeable when observing the images in Figure 7g,h for the BiV/600 sample. There is a significant loss of porosity; now the aggregates are much larger and less porous, with sizes reaching several microns. For example, Figure 7g shows that, at this temperature, the aggregates scarcely exhibit shapes below 100 nm and are much larger. This difference in sizes can be explained by the crystallization processes to which the samples were subjected. The silver-doped samples showed very similar SEM images as the corresponding undoped samples. Figure 7. SEM images of undoped BiV/T (T = (a,b) 80, (c,d) 300, (e,f) 450, and (g,h) 600 °C). 2.5. High-Resolution Transmission Electron Microscopy (HRTEM) Due to its superior performance in the photodegradation of the probe molecule, the sample containing Ag calcined at 300 °C (Ag/BiV/300) was analyzed with HRTEM. Figure 8a shows a single particle of the sample, highlighting its irregular characteristics, formed by the aggregation of smaller plate-like particles with heterogeneous shapes. The micrographs in Figure 8b,c show the lattice fringes of BiVO 4 with an approximate dimension of 0.36 nm, corresponding to the crystallographic planes (101) [61]. Figure 8d, shows the symmetrical arrangement of the white diffraction points, confirming the polycrystalline nature of BiVO 4 . Figure 8. HRTEM analysis of sample Ag/BiV/300. (a) single particle at 36,000× magnification, (b,c) lattice fringes of BiVO 4 at 105,000× magnification, (d) diffraction points of BiVO 4 , (e–h) EDX-mapping. Figure 8. HRTEM analysis of sample Ag/BiV/300. (a) single particle at 36,000 × magnification, (b,c) lattice fringes of BiVO 4 at 105,000 × magnification, (d) diffraction points of BiVO 4 , (e–h) EDX-mapping. 2.6. X-ray Photoelectron Spectroscopy (XPS) In order to study the chemical nature for all samples, the surface chemistry properties were analyzed with XPS. High-resolution spectra were obtained for the BiVO 4 photocatalyst treated at various temperatures and those doped with silver, initially focusing on the four elements: C, Bi, V, and O. These elements were studied with the C 1s , Bi 4f , V 2p , and O 1s spectrums, respectively, as shown in Figures 9and 10. Additionally, Figure 10e illustrates the Ag 3d5/2 band, which was used to determine the silver content in the doped samples. Table 3displays the surface element content, binding energy, and percentage of the components used to fit the different spectral regions of the BiVO4samples. Catalysts 2024, 14, x FOR PEER REVIEW 9 of 20 To identify the elemental composition of the sample, elemental mapping was performed using the EDS technique. The images corresponding to the presence of bismuth (blue color), vanadium (yellow color), and silver (pink color) are presented in Figure 8e– h. This morphology confirms that, after calcination, the crystallinity of the silver-doped sample was preserved. This is evidenced by the formation of silver metallic nanoparticles, as confirmed with X-ray diffraction, where the characteristic crystallographic plane (111) of Ag 0 was observed (see Figure 3). These characteristics suggest that Ag +1 cations have been reduced to Ag 0 during the thermal treatment process and that silver nanoparticles were possibly coated on the BiVO 4 surface. 2.6. X-ray Photoelectron Spectroscopy (XPS) In order to study the chemical nature for all samples, the surface chemistry properties were analyzed with XPS. High-resolution spectra were obtained for the BiVO 4 photocatalyst treated at various temperatures and those doped with silver, initially focusing on the four elements: C, Bi, V, and O. These elements were studied with the C 1s , Bi 4f , V 2p , and O 1s spectrums, respectively, as shown in Figures 9 and 10. Additionally, Figure 10e illustrates the Ag 3d5/2 band, which was used to determine the silver content in the doped samples. Table 3 displays the surface element content, binding energy, and percentage of the components used to fit the different spectral regions of the BiVO 4 samples. Figure 9. XPS spectra of undoped BiV/T (T = 80, 300, 450, and 600 °C) photocatalysts (a) C 1s , (b) Bi 4f , (c) V 2p , and (d) O 1s . Figure 10. XPS spectra of silver-doped Ag/BiV/T (T = 80 and 300 °C) photocatalyst (a) C 1s , (b) Bi 4f , (c) V 2p , (d) O 1s , and (e) Ag 3d5/2 . Figure 9. XPS spectra of undoped BiV/T (T = 80, 300, 450, and 600 ◦ C) photocatalysts (a) C 1s , (b) Bi 4f , (c) V2p, and (d) O1s. Catalysts 2024,14, 474 16 of 21 Catalysts 2024, 14, x FOR PEER REVIEW 15 of 20 3.2. Photodegradation Study The photoactive properties of the synthesized powered photocatalysts were assessed in the photodegradation of SMX as a probe molecule, under blue-LED light irradiation using two 50 W LED chips (465 nm main emission, total electric power~100 W and 4080 lm W−1). A borosilicate glass reactor (as a homemade slurry batch LED reactor) was placed between both LED chips (Figure 13). Figure 13. Schematic photodegradation experiment using the homemade slurry batch LED reactor. (a) Schematic representation of the reaction, (b) box batch reactor containing 5 × 10 blue-LED bulbs and fans to promote cooling, (c) 250 cm3 borosilicate glass reactor containing slurry photocatalyst, and (d) batch reactor dimensions. The pH of the experiments was adjusted to 7 with different solutions of NaCl and HCl (0.01 N), and during the photodegradation experiments, the temperature was maintained at 30 °C. Before exposure to light, the catalysts (100 mg) were immersed in a pollutant solution (100 mL) and allowed to reach adsorption–desorption equilibrium under stirring in the dark. The initial concentration of the pollutant in the solution was adjusted to achieve a final concentration of 5 mg L−1, a value determined on the basis of the SMX adsorption isotherms on the materials. To assure the adsorption–desorption equilibrium, the suspensions were stirred at 220 rpm for 12 h in the dark before exposure to irradiation. 3.3. Sulfamethoxazole Determination Before the photocatalytic experiment, 100 cm3 of SMX pollutant solution at a concentration of 5 mg L−1 was prepared, as determined from the SMX adsorption isotherms on the materials. The pH was adjusted to 7.0 using NaCl and HCl solutions (0.01 N). The photodegradation experiment in both homemade LED reactors was initiated by switching to LED-on mode and was continued for 540 min at 30 °C. Aliquots of 1.5 cm3 were extracted to control the reaction. These aliquots were then filtered using a 0.45 μm pore size filter. The SMX concentration was determined using a UV-Vis 6505 JENWAY spectrophotometer by measuring the maximum absorption of SMX at 264 nm. The % of SMX photodegradation (% SMX) was calculated according to Equation (6). Here, C0 is the concentration of SMX after irradiation, and C is the concentration at a specific time x. % 𝑜𝑓 𝑆𝑀𝑋 𝑝ℎ𝑜𝑡𝑜𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 =100 𝐶–𝐶 𝐶 (6) Figure 13. Schematic photodegradation experiment using the homemade slurry batch LED reactor. (a) Schematic representation of the reaction, (b) box batch reactor containing 5 × 10 blue-LED bulbs and fans to promote cooling, (c) 250 cm3 borosilicate glass reactor containing slurry photocatalyst, and (d) batch reactor dimensions. The pH of the experiments was adjusted to 7 with different solutions of NaCl and HCl (0.01 N), and during the photodegradation experiments, the temperature was maintained at 30 ◦C. Before exposure to light, the catalysts (100 mg) were immersed in a pollutant solution (100 mL) and allowed to reach adsorption–desorption equilibrium under stirring in the dark. The initial concentration of the pollutant in the solution was adjusted to achieve a final concentration of 5 mg L −1 , a value determined on the basis of the SMX adsorption isotherms on the materials. To assure the adsorption–desorption equilibrium, the suspensions were stirred at 220 rpm for 12 h in the dark before exposure to irradiation. 3.3. Sulfamethoxazole Determination Before the photocatalytic experiment, 100 cm3of SMX pollutant solution at a concentration of 5 mg L −1 was prepared, as determined from the SMX adsorption isotherms on the materials. The pH was adjusted to 7.0 using NaCl and HCl solutions (0.01 N). The photodegradation experiment in both homemade LED reactors was initiated by switching to LED-on mode and was continued for 540 min at 30 ◦ C. Aliquots of 1.5 cm 3 were extracted to control the reaction. These aliquots were then filtered using a 0.45 µ m pore size filter. The SMX concentration was determined using a UV-Vis 6505 JENWAY spectrophotometer by measuring the maximum absorption of SMX at 264 nm. The % of SMX photodegradation (% SMX) was calculated according to Equation (6). Here, C 0 is the concentration of SMX after irradiation, and Cis the concentration at a specific time x. %o f SMX photodegradation =100 Co–C Co(6) 3.4. Characterizations Techniques Various techniques were used to characterize the photocatalysts: the crystallinity study of the samples was obtained through X-ray diffraction analysis, performed using a Catalysts 2024,14, 474 17 of 21 BRUKER (Billerica, MA, USA) D8 ADVANCE diffractometer with Cu K α radiation. The X-ray diffraction patterns were recorded in a 2 θ range of 10 to 60 ◦ . The average crystal size (dp) was determined using the Debye–Scherrer equation; the textural properties of the samples were studied using nitrogen adsorption–desorption isotherms at 77 K with a Quantachrome (Boynton Beac, FL, USA) Autosorb-6B analyzer, applying the BET and Dubinin–Radushkevich equations, as well as the BJH method [ 73 ] for pore size distribution and mesopore volume; the optical absorption spectra of the photocatalysts were obtained using a VARIAN (Andheri, India) CARY 5E spectrophotometer equipped with a diffuse reflectance accessory (DRS), and the reflectance spectra were analyzed using the Kubelka–Munk method to determine the band gap (E g ) of the samples. The study of texture and morphology was conducted using a LIBRA (Greenwich, UK) 120 PLUS transmission electron microscope, Carl Zeiss, and a Thermo Fisher Scientific (Waltham, MA, USA) Talos F200X high-resolution microscope. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos (Dallastown, PA, USA) Axis Ultra-DLD spectrometer, equipped with a hemispherical electron analyzer connected to a delay-line detector (DLD), a dual-anode X-ray source (Mg/Al) with a power output of 450 W, and an Al-K α monochromator with a nominal power of 600 W, adjusting the spectra to Lorentzian and Gaussian curves; thermogravimetric analysis (TGA) was carried out using a METTLER-TOLEDO (Columbus, OH, USA) TGA/DSC1 thermogravimetric analyzer, while elemental analysis was conducted using an ICP-OES Optima (Schwaebisch Hall, Germany) 8300 system; finally, the concentration of SMX was determined using a UV-Vis 6505 JENWAY (Staffordshire, UK) spectrophotometer at a wavelength of 264 nm. 4. Conclusions Bismuth vanadate photocatalysts were synthesized using the hydrothermal method with modifications at different calcination temperatures and dopant introduction. Within the used experimental conditions these photocatalysts achieved 46% efficiency in the photodegradation of sulfamethoxazole solution under blue LEDs without dopant at the optimal calcination temperature (300 ◦ C). After doping with Ag, the efficiency increased to 69.7%, enhancing the degradation capacity by an additional 23.7%. Therefore, very low Ag superficial contents (determined with XPS) enhance the photocatalytic performance of the BiVO4-based catalysts. X-ray diffraction confirmed that, with the hydrothermal synthesis method, the monoclinic phase (m-s) was successfully crystallized keeping stable even after the thermal treatments at 300, 450, and 600 ◦ C; the monoclinic phase was also obtained with silverdoped samples. The low band gap of 2.40 eV and the average particle size of 18 nm of the catalyst treated at 300 ◦ C could justify its most efficient photocatalytic behavior among the undoped samples. The high photoactivity of the samples is likely due to the stabilization of the monoclinic phase during thermal treatment and the reduction in e − /h + recombination by stabilizing the active sites. Langmuir–Hinshelwood–Hougen–Watson model fits the experimental results well despite the low apparent surface areas of these materials. In all tests, the non-zero kinetic rate constant A=K A /K SR indicates that the mass transfer process from the aqueous solution to the catalyst surface directly influences the overall photodegradation of sulfamethoxazole. Due to their simple preparation and right stabilization of the monoclinic active phase, these BiVO 4 -based photocatalysts are a promising option for the photodegradation of emerging contaminants in water using solar light. Author Contributions: Conceptualization, E.B.-G., A.F.P.-C. and F.C.-M.; methodology, F.C.-M. and J.M.A.-C.; validation, F.C.-M. and A.I.M.-R.; formal analysis, A.I.M.-R. and E.B.-G.; investigation, J.M.A.-C.; data curation, A.I.M.-R. and J.M.A.-C.; writing—original draft preparation, J.M.A.-C.; writing—review and editing, A.F.P.-C., A.I.M.-R. and J.M.A.-C.; supervision, E.B.-G. and A.F.P.-C.; project administration, A.F.P.-C.; funding acquisition, F.C.-M., E.B.-G. and A.F.P.-C. All authors have read and agreed to the published version of the manuscript. Catalysts 2024,14, 474 18 of 21 Funding: This research received no external funding. Data Availability Statement: The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors. Acknowledgments: This research has been supported by the Project PID2021-127803OB-I00 funded by MCIN/AEI/10.13039/501100011033/and by “ERDF A way of making Europe. Jhon Mauricio Aguirre-Cortés is grateful to the Colombian Ministry of Science, Technology, and Innovation (MINCIENCIAS) for the financial support provided through call 860 (2019). E. Bailón-García is grateful to MICINN for her postdoctoral fellowship (RYC2020-029301-I). Conflicts of Interest: The authors declare no conflicts of interest. References 1. Ben, W.; Zhu, B.; Yuan, X.; Zhang, Y.; Yang, M.; Qiang, Z. Occurrence, Removal and Risk of Organic Micropollutants in Wastewater Treatment Plants across China: Comparison of Wastewater Treatment Processes. Water Res. 2018,130, 38–46. [CrossRef] [PubMed] 2. Cook, M.A.; Wright, G.D. The Past, Present, and Future of Antibiotics. Sci. Transl. Med. 2022,14, eabo7793. [CrossRef] [PubMed] 3. Tangcharoensathien, V.; Chanvatik, S.; Sommanustweechai, A. Complex Determinants of Inappropriate Use of Antibiotics. Bull. World Health Organ. 2018,96, 141–144. [CrossRef] [PubMed] 4. Rokesh, K.; Sakar, M. 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