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Article

Fabrication of Effective Co-SnO2/SGCN Photocatalysts for the Removal of Organic Pollutants and Pathogen Inactivation

by
Mohsin Javed
1,
Sana Iqbal
1,
Muhammad Azam Qamar
1,*,
Mohammad Shariq
2,*,
Inas A. Ahmed
3,
Amal BaQais
4,
Hanan Alzahrani
2,
Syed Kashif Ali
5,
N. A. Masmali
2,
Talal M. Althagafi
6 and
Mohd. Shakir Khan
7
1
Department of Chemistry, School of Science, University of Management and Technology, Lahore 54770, Pakistan
2
Department of Physics, College of Science, Jazan University, Jazan 45142, Saudi Arabia
3
Department of Chemistry, Faculty of Science, King Khalid University, Abha 62224, Saudi Arabia
4
Department of Chemistry, College of Science, Princess Nourah Bint Abdulrahman University, Riyadh 11671, Saudi Arabia
5
Department of Chemistry, College of Science, Jazan University, Jazan 45142, Saudi Arabia
6
Department of Physics, College of Science, Taif University, Taif 21944, Saudi Arabia
7
Department of Physics, College of Science, Al-Zulfi, Majmaah University, Al-Majmaah 11952, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(2), 163; https://doi.org/10.3390/cryst13020163
Submission received: 20 December 2022 / Revised: 6 January 2023 / Accepted: 12 January 2023 / Published: 17 January 2023
(This article belongs to the Special Issue Synthesis, Characterization and Application of Novel Nanoparticles)
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Versions Notes

Abstract

:
Substantial improvement is needed in efficient and affordable decolorization and disinfection methods to solve the issues caused by dyes and harmful bacteria in water and wastewater. This work involves the photocatalytic degradation of methylene blue (MB) as well as gram-negative and gram-positive bacteria by cobalt-doped tin oxide (Co-SnO2) nanoparticles (NPs) and Co-SnO2/SGCN (sulfur-doped graphitic carbon nitride) nanocomposites (NCs) under sunlight. The coprecipitation approach was used to synthesize the photocatalysts. Maximum methylene blue (MB) photocatalytic degradation was seen with the 7% Co-SnO2 NPs compared to other (1, 3, 5, and 9 wt.%) Co-SnO2 NPs. The 7% Co-SnO2 NPs were then homogenized with different amounts (10, 30, 50, and 70 weight %) of sulfur-doped graphitic carbon nitride (SGCN) to develop Co-SnO2/SGCN heterostructures with the most significant degree of MB degradation. The synthesized samples were identified by modern characterization methods such as FT-IR, SEM, EDX, UV-visible, and XRD spectroscopies. The Co-SnO2/50% SGCN composites showed a significant increase in MB degradation and degraded 96% of MB after 150 min of sunlight irradiation. Both gram-negative (E. coli) and gram-positive (B. subtiles) bacterial strains were subjected to antibacterial activity. All samples were shown to have vigorous antibacterial activity against gram-positive and gram-negative bacteria, but the Co-SnO2/50% SGCN composites exhibited the maximum bactericidal action. Thus, the proposed NC is an efficient organic/inorganic photocatalyst that is recyclable and stable without lowering efficiency. Hence, Co-SnO2/50% SGCNNC has the potential to be employed in water treatment as a dual-functional material that simultaneously removes organic pollutants and eradicates bacteria.

1. Introduction

Human activities such as industrialization and urbanization are increasing day by day with the increase of urbanization [1]. Organic pollutants such as industrial dyes and agricultural wastes are the major causes of aquatic pollution, due to which human health and its ecosystem are affected badly [2,3,4,5]. Pharmaceutical, chemical, and textile industries dispose of organic pollutants and dyes directly into the environment and produce colored wastewater [4,5,6,7]. Textile industries are releasing about 10–50 mg/L of dye during the dying process which is a high enough concentration to pollute water along with colors [8]. In developing countries, 1.6 million deaths per year occur just because of drinking contaminated, untreated water [9,10]. Methylene blue is the most commonly used synthetic cationic industrial dye and is used for silk, wood, and cotton dyeing [11,12,13,14]. Long-term exposure to methylene blue causes serious health risks in humans, including increased heart rate, eye injury, shocks, vomiting, and necrosis [14,15]. Due to their complex structure, organic pollutants are resistant to degradation by simple treatment methods [16,17,18].
Advanced oxidation processes (AOPs) have huge potential for treating a wide range of contaminants along with improving wastewater biodegradability [15,16,17]. For industrial wastewater treatment, photodegradation by photocatalysis is regarded as an advantageous technique due to its low-cost photons, lower toxicity, and lack of secondary pollution [18,19,20,21,22,23]. Heterogenous catalysis is involved in photocatalytic degradation, where sunlight is absorbed by semiconductor photocatalysts. As a result, electron-hole pairs are generated, which then interact with contaminants and give carbon dioxide and water as the final products [20,24,25,26].
For the degradation of organic pollutants, many types of photocatalysts are used, such as metal oxides, metal sulfides semiconductors, and nanomaterials. Among these materials, oxides (for example SnO2, ZnO, TiO2, CuO, and Fe2O3) are extremely important and are widely used in the degradation of pollutants and dyes [27,28,29,30,31]. SnO2 have great importance and have an energy band gap of 3.5 to 3.8 eV, which falls under the UV region [32]. Tin oxide, such as titanium oxide, exhibits high photocatalytic activity due to its rutile structure [33]. It is widely used in chemical sensors, transparent electrodes, solar cells, and gas sensors because of its excellent properties [34,35,36]. Tin oxide (SnO2) can be synthesized by various methods such as Hydrothermal, Coprecipitation, Sol-gel, Solvothermal, and Thermal decomposition [37,38,39,40]. Coprecipitation is a wet chemical method that is widely used for the synthesis of nanomaterials because it is an energy-efficient, simple, low-temperature technique that also provides homogeneity [41,42,43,44,45]. Kim et al. adopted the coprecipitation method in order to prepare tin oxide particles in the nanosize range. Synthesized SnO2 nanoparticles were used for the photodegradation of methylene blue [46]. The co-precipitation approach for the manufacture of SnO2 nanoparticles was demonstrated by Tazikeh et al. [47].
However, the tin oxide can be photoactivated only by UV irradiation due to its large band gap. It accounts for only 4–5% of solar radiation, leaving a significant portion of solar energy unaccounted for [48,49]. By doping tin oxide with suitable metal such as cobalt, the band gap can be reduced, which lies in the visible region [50,51]. Furthermore, n-type semiconductors used in the fabrication of S-scheme photocatalysts with an appropriate band gap can improve the optical and photocatalytic properties of the material [52,53]. Cobalt-doped tin oxide nanoparticles and sulphur-doped graphitic carbon nitride have been recommended for S-scheme Co-SnO2/SGCN heterojunction. In previous projects, the removal of pollutants with Co-SnO2 [42,48], SnO2/r GO [54,55], TiO2/SnO2 [56], g-C3N4/rGO/SnO2 [57] has been reported.
In this project, the synthesis of tin oxide, cobalt-doped tin oxide with various percentages of cobalt (1, 3, 5, 7, and 9), and 7% Cobalt doped tin oxide composites with various percentages of sulphur doped graphitic carbon nitride (10, 30, 50, 70) were prepared by the simple and facile coprecipitation method. Photodegradation of methylene blue dye was carried out with these synthesized photocatalysts under sunlight. Both gram-negative (E. coli) and gram-positive (B. subtiles) bacterial strains were subjected to antibacterial activity.

2. Experimental

2.1. Materials

Distilled water, Cobalt nitrate hexahydrate (Co(NO3)2.6H2O) (purity 98%, sigma-Aldrich), Tin chloride dihydrate (SnCl2.2H2O) (98%, sigma-Aldrich), Sodium hydroxide (NaOH) (purity 98%, sigma-Aldrich), Thiourea CH4 N2S, (AppliChem, 98% purity) Nutrient agar, Nutrient broth Microgen and MB (C16H18ClN3S) Simpsons.

2.2. Synthesis of SnO2 Nanoparticles

Chemical precipitation was used to produce tin oxide nanoparticles. SnCl2.2H2O was used as a source of tin. In this method, 5 g of tin chloride dihydrate was weighed and dissolved into 100 mL of distilled water by continuous vigorous stirring on a magnetic stirrer at room temperature for 20 to 30 min, obtaining a clear and homogeneous solution. Then a 1 M solution of NaOH was added dropwise into the solution on constant stirring until pH reached 8. On observing white precipitates, the solution was filtered with filter paper (Grade 42) and washed again and again in order to remove chloride ions until the pH was maintained at seven, which was continually checked with pH meter. After washing, the precipitates were dried in an oven at 70 °C for 24 h. After that, the precipitates were annealed in a muffle furnace at 600 °C for 3 h. The obtained tin oxide was ground with a mortar and pestle to obtain powdered tin oxide.

2.3. Synthesis of Co-SnO2

A chemical precipitation method was used to synthesize a series of Co-doped SnO2 by varying the cobalt percentage (1, 3, 5, 7, and 9 wt.%). For the synthesis of 1% Co-doped SnO2, 0.063 g of cobalt nitrate hexahydrate and 4.129 g of tin chloride dihydrate were dissolved in 100 mL of distilled water on vigorous and constant stirring with the magnetic stirrer. Stirring continued until a clear and homogenous solution was obtained. The pH was then kept at seven by adding drops of a 1 M solution of NaOH to the stirring solution, resulting in the observation of white precipitates. The solution was filtered and washed again and again to remove chloride ions. The precipitates were then dried in an oven at 70 °C for 24 h. Dried precipitates were annealed for 3 h in a muffle furnace in an air atmosphere at 600 °C. The obtained material was ground in a mortar and pestle to obtain powdered cobalt-doped tin oxide nanoparticles. The same procedure was used to synthesize 3, 5, 7, and 9 wt.% cobalt-doped tin oxide nanoparticles. The amount of cobalt salt and tin salts for the composition of cobalt-doped tin oxide is mentioned in Table 1.

2.4. Synthesis of Sulfur Doped Graphitic Carbon Nitride

Sulfur-doped graphitic carbon nitride was synthesized by the thermal polycondensation method from thiourea. For this purpose, the calculated amount of thiourea is weighed, placed in a crucible, and kept in a muffle furnace under an air atmosphere at a temperature starting from 25 °C. Heat at this temperature for half an hour at a rate of 5 °C per minute until the temperature reaches 550 °C. The yellow-colored material was obtained, which was ground into fine powder.

2.5. Synthesis of Co-SnO2/SGCN Nanocomposites

Chemical precipitation was used to create a series of cobalt-doped tin oxide composites with sulfur-doped graphitic carbon nitride by altering the percentage of SGCN (10, 30, 50, and 70 wt.%). For this purpose, 0.27 g of sulphur-doped graphitic carbon nitride was dissolved in 100 mL of solution and sonicated for 15 min. After 15 min, this solution was put on the stirrer to add 4.600 g of SnCl2.2H2O to the solution of SGCN in 100 mL of water. Then, as a cobalt source, 0.447 g of cobalt nitrate hexahydrate was added to the vigorously stirring solution. It is stirred continually until a clear and homogeneous solution is observed. To achieve pH 8, a 1 M solution of NaOH was added dropwise. When white precipitates were observed, the solution was filtered with filter paper and washed with distilled water to remove chloride ions until the pH maintained at 7. After washing, the precipitates were dried in an oven at 60 °C for 24 h. This dried material was then placed into the muffle furnace in an air atmosphere at 600 °C for 3 h. Cobalt-doped tin oxide composites with SGCN were ground to a fine powder in a mortar and pestle. The remaining percentage of composites was also prepared in the same way, and the amount of salt for each series is given in Table 2.

2.6. Material Characterization

Various synthesized nanoparticles and nanocomposites were characterized by using various techniques. The crystalline structure was determined by a powder X-ray diffractometer with a scan rate of 0.2° s−1 ranging from 20 to 80°. Cu Kα radiation (k = 1.54056 Å) was applied at 40 kV and 30 mA. The surface morphology of synthesized materials was examined by transmission electron microscope (TEM, JEOL-JEM-1230). FTIR analysis to obtain the functional group study was carried out on FTIR spectrometer in the range of 4000–400 cm−1. The photocatalytic degradation efficacy of samples was measured by a UV-visible spectrophotometer (Shimadzu 1700) with a wavelength range of 200–800 nm. The percentage of elemental composition was obtained by using energy-dispersive spectroscopy (EDS).

2.7. Photocatalytic Activity

To check the photocatalytic activity of prepared materials, methylene blue was used as a standard pollutant. The photocatalytic efficiency of pure tin oxide, doped with cobalt and composite with SCN against methylene blue was monitored via UV-Visible spectroscopy. For this purpose, 0.2 g of prepared photocatalysts was dissolved in 100 mL of methylene blue solution (0.12 g/L in distilled water) in petri dishes. After sonicating the solution for 15 min, it was placed in darkness for 30 min to reach absorption-desorption equilibrium. After that, solutions were placed under the irradiation of sunlight (68–73 klux) until they were completely degraded. Five mL of aliquots were taken from each sample after 30 min interval, and changes in the absorption band (664 nm) were recorded using the UV-Vis spectrum. This was done to look into photocatalytic activity.

2.8. Antibacterial Activity

The antibacterial activity of tin oxide, cobalt-doped tin oxide, and composites with Sulphur doped graphitic carbon nitride were studied by the agar-well diffusion method. For this purpose, strains of gram-positive (B. subtiles) and gram-negative (E. coli) were examined.

2.8.1. Preparation of Inoculum

Inoculum for both positive and negative strains was prepared separately by dissolving 0.32 g nutrient broth in 25 mL of distilled water and then stirring for 15 min. After 15 min, flasks were taken out of the autoclave and cooled to room temperature. In this solution, 2–3 drops of bacterial strains were added. Both the flasks were then placed in the shaker for 24 h.

2.8.2. Preparation of Petri Plates

First of all, Petri dishes were sterilized in an autoclave and divided into four sections with permanent markers. These sections included one blank hole and three holes for sample solutions with varying concentrations. Next, 4 g of nutrient agar and 2 g of nutrient broth were added to a flask containing 100 mL of distilled water. This mixture was sterilized in an autoclave and then boiled for 5–10 min. The mixture was poured into petri dishes, which were already prepared and kept in a laminar flow cabinet. When the mixture was cooled down, the inoculum was spread on the surface of the gel. When it was completely dry, small wells were bored with a borer in each section. Standard solutions of varying concentrations, i.e., 100 ppm, 150 ppm, 200 ppm, and 250 ppm solutions, were poured into the wells with the help of a micropipette. All the petri dishes were placed in the incubator for 24 h. Results were obtained by measuring the diameter of the zone of inhibition (ZOI), which was compared with the blank hole.

3. Results and Discussion

3.1. XRD Analysis

The crystallinity and crystalline structure of all the samples were analyzed by X-ray diffractometer at 10–80°. The diffraction patterns are given in Figure 1. The obtained pattern for SnO2 revealed that no additional impurity peaks appeared in the pattern. It is revealed that all the diffraction peaks are according to the tetragonal rutile structure of SnO2. The obtained pattern shows 11 peaks with 2ϴ, which are due to (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), (321) facets of crystals. All these diffraction peaks are correctly matched with the reported values given in ICDD 01-088-0287 and JCPDS 00-072-1147 [32]. This result shows that the pattern of 5% Co-SnO2 exactly resembles that of pure tin oxide peaks in an XRD pattern with a decrease in peak intensity and a slight shift of the (101) peak. However, by doping cobalt and changing the lattice parameters, the average grain size is decreased. So, crystallinity increases by doping tin oxide with cobalt. This is because Co2+ has a smaller ionic radius (0.060 nm) than Sn4+ (0.069 nm) and has lower valence states than the host Sn4+ cations [32,58]. Figure 1 shows the XRD record for sulphur-doped graphitic carbon nitride. Two unique peaks may be detected in this pattern, at 13.02° and 27.3°, which correspond to the crystal’s 100 and 002 facets, respectively. The effective synthesis of sulphur-doped graphitic carbon nitride is documented in JCPDS number 00-087-1526. Figure 1, shows the X-ray diffraction pattern of a nanocomposite of 50 percent sulphur doped graphitic carbon nitride and 7 percent cobalt-doped tin oxide (7% Co-SnO2/50% SGCN). Various peaks can be observed in this diffractogram. The peaks at 27.4° and a minor peak at 14° are due to the presence of sulphur-doped graphitic carbon nitride in the 002 facet. The presence of Co-doped SnO2 is confirmed by the other peaks located at (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), (321). The intensity of the peaks obtained from the XRD pattern of 7% Co-SnO2/50% SGCN was lower than that of pure SnO2 nanoparticles and 7% cobalt-doped tin oxide nanoparticles. Moreover, peak broadening with a slight shift towards lower theta at 27.08° to 26.86° was observed which confirms the successful synthesis of the nanocomposite, which contains cobalt-doped tin oxide and 50% sulphur-doped graphitic carbon nitride. These results indicate that by inserting SGCN in cobalt-doped tin oxides nanoparticle, the growth of grain size is reduced, which provides high porosity and a large surface area. The average crystallite size calculated for SnO2 and Co-SnO2 using the Scherrer equation from XRD data was found to be 15 nm and 13 nm respectively.

3.2. Morphology and Elemental Composition Analysis

EDX spectrum of SnO2, Co-SnO2, and Co-SnO2 composite with SGCN are shown in Figure 2a–c. The spectrum of SnO2 (Figure 2a) and Co-SnO2 (Figure 2b) revealed the existence of tin (Sn), oxygen (O), and cobalt (Co) elements with slightly higher atomic and weight percentages of tin. Similarly, the EDX spectrum of Co-SnO2 composite with SGCN (Figure 2c), shows the existence of carbon ©, cobalt (Co), nitrogen (N), oxygen (O), sulfur (S), and tin (Sn) elements. The percentage composition of these components is also given along with the elemental spectrum. Detected elements in the EDX spectrum, confirm the successful synthesis of the Co-SnO2/SGCN composite.
Figure 3a–c shows SEM images used to describe the morphological structure of SnO2, Co-SnO2 NPs, and Co-SnO2/SGCNNCs. The SEM images of SnO2 (Figure 3a) and Co-SnO2 NPs (Figure 3b) exhibit spherical morphology with enough agglomeration. The images in Figure 3c reveal the morphology of SGCN/Co-SnO2 NCs. These images also show a high degree of agglomeration, and the distinction between NPs and SGCN is not possible.

3.3. FTIR Analysis

To reveal the functional group analysis, FTIR is an important technique [59,60]. Samples of SnO2, 7% Co-SnO2, and 7% Co-SnO2/50% SGCN were examined using an FTIR spectrometer in the range of 4000–400 cm−1. Figure 4 represents the FTIR spectra of the above-mentioned nanomaterial.
The peak at 3300–3400 cm−1 is due to the O–H stretching vibration, which is most likely related to the fact that the spectrum was not captured in situ and some water adsorption from the surrounding environment occurred. Another band is observed at 1600–1700 cm−1, which represents the vibrational peak of C=O. A peak at 1087 cm−1 has also appeared, which represents C-O bond stretching. This may occur due to the release of CO2 during annealing in the furnace A minor peak of Sn-OH is also observed at 1250 cm−1. The peak of the O-Sn-O functional group of tin oxide is observed at about 638, 632, and 620 cm−1 for SnO2, 7% Co-SnO2, and 7% Co-SnO2/50% SGCN respectively, which ensures the presence of crystalline phase of tin oxide. The positions, widths, and forms of IR peaks change dramatically after Co doping, showing that the Co ion has been integrated into the SnO2 host. Furthermore, due to the modest quantity of cobalt concentration utilized, the peak position and intensity for Co-doped tin oxide barely changed. Shifting of metal-oxygen peak from 638 to 620 cm−1, as well as other peaks, also shifted towards higher wavelengths and lower wavenumber. The shifting of peaks towards the lower region shows a red shift or bathochromic effect on absorbance in the visible region. These results confirm that Co ions have been substituted into the regular lattice of the SnO2 site. Furthermore, due to the modest quantity of SGCN, the peak position and intensity for Co-doped tin oxide nanoparticles and composites barely changed. When cobalt-doped tin oxide is attached to SGCN, it clearly indicates a red shift, which causes absorption in the visible region.

3.4. Optical Properties

All the synthesized nanomaterials were examined using UV-visible spectroscopy, and Figure 5a shows the respective absorption spectra. While the band gap calculation of prepared samples was done by following equations:
E = hv = 1240 λ
and
α = 4 π ( A b s o r b a n c e ) λ
The relation among E and α can be given as:
(αhv) = B(hv − Hg)1/2
Equations (1)–(3) were used to draw Tauc’s plot for pure tin oxide nanoparticles. From Tauc’s plot, the observed band gaps of pure tin oxide nanoparticles and SGCN were found at 3.42 eV and 2.78 eV, respectively. These values were very close to the reported values [60,61]. The results showed a remarkable reduction in the band gap of Co-SnO2, which was obtained at 3.21 eV. Similarly, the estimated band gap for 7% Co-SnO2/50% SGCN was noted at 2.59 eV. These results showed a remarkable reduction in the band gap of nanocomposites and doped nanoparticles as compared to pure tin oxide, which played a vital role in the photocatalytic degradation of methylene blue. Figure 5b shows band gaps for SnO2, 7% Co-SnO2, SGCN, and 7% Co-SnO2/50% SGCN.

3.5. Photocatalytic Study

The photocatalytic proficiency of the photocatalysts (SnO2 and Co-SnO2 NPs, and Co-SnO2/50% SGCN NCs) was assessed against a model dye (MB) by checking their dye degradation capability under sunlight. employing a UV-Vis spectrophotometer with a 200–700 nm wavelength range. The UV-visible spectra exhibited that the absorbance intensity of MB for all the doped nanoparticles decreased with increasing time, indicating an increase in the degradation of MB. At the same time, the rate of degradation of methylene blue with various concentrations of synthesized nanomaterials was determined by the following equation.
Rate = C/Co
Figure 6b shows the photocatalytic efficiency of SnO2 and (1, 3, 5, 7, and 9 wt.%) Co-SnO2 NPs against MB. From Figure 6a,b, it can be seen that 7% Co-SnO2 NPs showed the maximum degradation of MB (83% in 150 min) as compared to other fabricated samples. From Figure 6b, the plots of C/Co versus time for the degradation of MB in the presence of photocatalysts show that the pure SnO2 degraded less dye than the nanoparticles of Co-SnO2. The reason for the highest degradation by 7% Co-SnO2 is that Co ions and oxygen states that act as electron and hole pair trapping centers introduce defect levels between the conduction and valance bands. These trapping centers reduce the recombination of electron-hole pairs and increase photocatalytic activity. However, photocatalytic activity was increased at the optimal level of cobalt doping (7%); after that, the photocatalytic activity began to decrease. When there is a high level of doping, it provides recombination centers for electron-hole pair recombination and decreases photocatalytic activity [58,62,63]. The bar graph in Figure 6c shows the final % degradation of MB dye by various Co-SnO2 NPs after 150 min.
To make Co-SnO2 work better as a photocatalyst, SGCN was used as a substrate to make nanocomposites of it. The catalytic performance of the produced NCs was tested under sunlight every 15 min, using the same procedure as in Section 2.7. Following the method described by Qamar et al. the photocatalysts were left in the dark until an adsorption-desorption equilibrium was reached between the dye and the NCs [64]. Nanocomposites exhibited better absorption than pure tin oxide and cobalt-doped tin oxide, which led to a higher degradation rate. From Figure 7a,b, it can be seen that the Co-SnO2/50% SGCN nanocomposites showed the highest degradation among all the samples, and it degraded 98% of the MB dye in 120 min. The good photocatalytic activity of 7% Co-SnO2/50% SGCN is ascribed to the formation of good heterojunction between Co-SnO2 and SGCN, which prevents electron-hole pair recombination. The good synergetic impact between Co-SnO2 and SGCN provides more defects in the heterostructure, which provides more sites for the dye to attach. Then the attached dye is more susceptible to degradation.
Figure 7c presents the bar graph of the % degradation of MB by NCs. It is evident from the graph that the lowest degradation rate is shown by the methylene blue solution, which is kept without a photocatalyst. The methylene blue degradation rate with 7% Co-SnO2/50% SGCN nanocomposite is faster than all other composites, as shown in Figure 8b,c. A further increase in SGCN concentrations > 50% decreases the catalytic efficiency and suppresses MB’s degradation. It means that the optimum concentration of SGCN to enhance photocatalytic activity is 50%. The photocatalytic effectiveness of the 7% Co-SnO2/50% SGCN NC is higher than that of the previously reported composites because Co-SnO2 and SGCN successfully formed good heterojunctions [60,61,62,63,64,65,66,67,68]. The transport and separation of e− and h+ in the composite may also be facilitated by cobalt atoms.
Superoxide radicals(·O2−), hydroxyl radicals(·OH), and photogenerated holes (h+) are often the main active species for the degradation of organic contaminants in water. The importance of active species (·OH, h+, and ·O2−) in the photocatalytic reaction was investigated in order to explore the photodegradation mechanism of MB by the Co-SnO2/50% SGCN catalyst (Figure 8). Particularly, benzoquinone (BQ) was used to trap ·O2−, isopropanol (IPA) was used to trap ·OH, and EDTA-2Na was used to capture holes (h+). The efficiency of MB deterioration was reduced by 88% after adding benzoquinone. However, the degradation rates of MB were only significantly reduced by 65% and 31%, respectively, by IPA and EDTA-2Na. The scavenging effect of the free radicals is given by the bar graph as shown in Figure 8. The results showed that, instead of holes (h+), the ·OH and ·O2− are the major reactive species involved in photocatalytic dye degradation.
The MB photodegradation recycling experiment evaluated the stability of Co-SnO2/50% SGCN NCs. The photocatalytic efficiency curve of Co-SnO2/50% SGCN NCs against MB for five cycles under sunlight is shown in Figure 9a. In the sixth cycle, 90% of the MB is degraded, compared to 98% in the first cycle. This suggests that photocatalysis slows down less and is more stable. After five degradation experiments, the XRD showed only a slight decrease in peak intensities (Figure 9b). This experiment validates the stability of the fabricated composite. Homogeneous hybridization of SGCN layers and Co-SnO2 NPs enhances the stability of Co-SnO2/50% SGCN NCs. Table 3 compares the photocatalytic effectiveness of Co-SnO2/SGCN NCs to earlier works.
Antibacterial activity was performed against gram-positive (B. subtilis) and gram-negative bacteria (E. coli). The observed ZOI without light irradiation during the antibacterial activity experiment was negligible, indicating that the substances themselves were not toxic to E. coli and B. subtilis. Table 4 shows the antibacterial activity of pure tin oxide, cobalt-doped tin oxide, and its composite with sulphur-doped graphitic carbon nitride at various concentrations i.e., at 100 ppm, 150 ppm, 200 ppm, and 250 ppm under light (LED lamp).
It has been observed from the photocatalytic activity that all the prepared samples act as photocatalysts under sunlight irradiation, which degrades the dye, i.e., methylene blue, by the formation of reactive oxygen species (ROS). The amount of ROS formed is light-dependent; as the production of ROS increases under the sun, the antibacterial activity of nanoparticles and nanocomposites also increases significantly [70,71]. Another factor that affects the antimicrobial activity of tin oxide and its composites is the energy band gap. The lower band energy and the higher surface area provide good antibacterial results. By evaluating the results and comparing zones of inhibition, it was found that tin oxide showed no antibacterial activity at 100 ppm concentration, while at higher concentrations it showed activity against both gram-positive and gram-negative bacteria. At all concentrations, the nanocomposite demonstrated good antibacterial activity against gram-negative (E. coli) (Figure 10a) and gram-positive (B. subtiles) (Figure 10b) bacterial strains. The lowest antibacterial activity was shown by tin oxide, cobalt-doped tin oxide also showed good results.

3.6. Photocatalytic Degradation Mechanism

As anticipated by a schematic diagram, the creation of e−/h+ pairs in the photocatalysts may be responsible for the rapid degradation of methylene blue and the antibacterial action of photocatalysts (Figure 11). When Co-SnO2/S-g-C3N4 is exposed to solar radiation, both Co-SnO2 and S-g-C3N4 are energized and e−/h+ pairs are photogenerated on their conduction band (CB) and valence band (VB), respectively [71,72]. Since the conduction band (CB) of Co-SnO2 had a lower potential than the CB of S-g-C3N4 (−1.12 eV) based on the energy level of CB/VB, the photo-induced electrons moved between the two bands with ease. Additionally, Co-SnO2 may absorb the holes that were made in the VB of S-g-C3N4 [61]. The presence of Co atoms in the hybrid composite not only reduced the value of Eg but also facilitated the transfer of electrons from S-g-C3N4 to Co-SnO2. Therefore, doping might considerably reduce the likelihood of charge recombination by raising the separation of photoexcited e/h+ pairs. Reactive oxygen species (ROS) are created when the photogenerated e/h+ interact with oxygen and water molecules that are on the photocatalyst’s surface. The produced radicals then degrade MB and kill bacteria in an oxidative manner to degrade MB [72].

4. Conclusions

Doping cobalt into SnO2 nanostructures and then integrating it into sulfur-doped graphitic carbon nitride has been found to be an excellent way to increase catalytic efficiency under solar light. The dye degradation results of a series of Co-SnO2 samples were compared via UV-visible spectroscopy, and it was found that by increasing the concentration of cobalt in SnO2 nanoparticles, the rate of degradation was enhanced regularly. The band gap of SnO2 narrowed from 3.42 eV to 3.21 eV by Co-doping of SnO2 nanoparticles (7 wt.%) In order to achieve the highest degradation in a short time period, the 7% Co-SnO2 NPs with the best catalytic efficiency were integrated into different concentrations of S-g-C3N4. The NCs containing the 50%SGCN showed higher photocatalytic activity. The nanomaterials were tested to see how efficiently they killed both gram-positive and gram-negative bacteria. The 7% Co-SnO2/50% SGCN NCs demonstrated higher photocatalytic and antibacterial activity. Ultimately, the findings of this research give us a new understanding of how to create efficient photocatalysts for bactericidal and organic pollutant degradation applications.

Author Contributions

Conceptualization, M.J.; methodology, S.I.; software, S.K.A.; validation, I.A.A., M.S.K. and N.A.M.; formal analysis, M.S.; investigation, M.A.Q. and S.I.; resources, H.A.; data curation, M.J.; writing—original draft preparation, S.I. and M.A.Q.; writing—review and editing, M.S. and N.A.M.; visualization, T.M.A.; supervision, M.J.; project administration, I.A.A.; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript.

Funding

The current work was assisted financially by the Dean of Science and Research at King Khalid University via the General Research Project: Grant no. (R.G.P.1/320/43). This research work was supported by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R230), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Data Availability Statement

Not Applicable.

Acknowledgments

The authors are grateful to the Dean of Science and Research at King Khalid University for making financial support available. This research work was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R230), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00163 g001 550
Figure 1. XRD pattern of SnO2, 7%Co-SnO2, SGCN, and 7%Co-SnO2/50% SGCN.
Figure 1. XRD pattern of SnO2, 7%Co-SnO2, SGCN, and 7%Co-SnO2/50% SGCN.
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Crystals 13 00163 g002 550
Figure 2. EDX images of SnO2 (a), Co-SnO2 NPs (b), and Co-SnO2/SGCN NCs (c).
Figure 2. EDX images of SnO2 (a), Co-SnO2 NPs (b), and Co-SnO2/SGCN NCs (c).
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Crystals 13 00163 g003 550
Figure 3. SEM images of SnO2 (a), Co-SnO2 NPs (b), and Co-SnO2/SGCN NCs (c).
Figure 3. SEM images of SnO2 (a), Co-SnO2 NPs (b), and Co-SnO2/SGCN NCs (c).
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Crystals 13 00163 g004 550
Figure 4. FTIR spectra of SnO2, 7% Co-SnO2 and 7% Co-SnO2/50% SGCN.
Figure 4. FTIR spectra of SnO2, 7% Co-SnO2 and 7% Co-SnO2/50% SGCN.
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Crystals 13 00163 g005 550
Figure 5. Absorption spectra (a) and bandgap of SnO2, 7% Co-SnO2, and 7% Co-SnO2/50% SGCN (b).
Figure 5. Absorption spectra (a) and bandgap of SnO2, 7% Co-SnO2, and 7% Co-SnO2/50% SGCN (b).
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Figure 6. Degradation contours (a) Degradation rate (b) % Degradation of MB (c) by SnO2 and Co-SnO2 NPs.
Figure 6. Degradation contours (a) Degradation rate (b) % Degradation of MB (c) by SnO2 and Co-SnO2 NPs.
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Crystals 13 00163 g007 550
Figure 7. Degradation contours (a) Degradation rate (b) % Degradation of MB (c) by Co-SnO2/SGCN.
Figure 7. Degradation contours (a) Degradation rate (b) % Degradation of MB (c) by Co-SnO2/SGCN.
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Crystals 13 00163 g008 550
Figure 8. Effect of radical scavengers in the catalytic degradation of MB with Co-SnO2/50% SGCN.
Figure 8. Effect of radical scavengers in the catalytic degradation of MB with Co-SnO2/50% SGCN.
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Crystals 13 00163 g009 550
Figure 9. The stability and recyclability of the photocatalyst (a). X-ray diffraction patterns of the Co-SnO2/50% SGCN NCs before the experiment and after the 6th cycle (b).
Figure 9. The stability and recyclability of the photocatalyst (a). X-ray diffraction patterns of the Co-SnO2/50% SGCN NCs before the experiment and after the 6th cycle (b).
Crystals 13 00163 g009
Crystals 13 00163 g010 550
Figure 10. ZOI diameter of SnO2, Co-SnO2 and 7% Co-SnO2/50% SGCN against E. coli. (a) and B. subtiles (b).
Figure 10. ZOI diameter of SnO2, Co-SnO2 and 7% Co-SnO2/50% SGCN against E. coli. (a) and B. subtiles (b).
Crystals 13 00163 g010
Crystals 13 00163 g011 550
Figure 11. An illustration of the catalytic degradation mechanism of MB under sunlight over the Co-SnO2/S-g-C3N4 NCs.
Figure 11. An illustration of the catalytic degradation mechanism of MB under sunlight over the Co-SnO2/S-g-C3N4 NCs.
Crystals 13 00163 g011
Table
Table 1. Composition of Co-SnO2 nanoparticles.
Table 1. Composition of Co-SnO2 nanoparticles.
Percentage SnCl2.2H2O (g)Co(NO3)2.6H2O (g)
1%4.1290.063
3%4.817 0.187
5%4.712 0.315
7%4.607 0.447
9%4.5150.576
Table
Table 2. Composition of Co-SnO2/SGCN NCs.
Table 2. Composition of Co-SnO2/SGCN NCs.
Sr. No % Age of SGCN in NCs SnCl2.2H2O (g) Co(NO3)2.6H2O (g) Sulfur Doped Graphitic Carbon Nitride
110%4.6070.4470.275 g
230%4.6070.4470.827 g
350%4.6070.4471.379 g
470%4.6070.4471.931 g
Table
Table 3. Efficiency comparisons for photocatalysis of the Co-SnO2/SGCN NCs with some previous works.
Table 3. Efficiency comparisons for photocatalysis of the Co-SnO2/SGCN NCs with some previous works.
Sr. NoPhotocatalystContaminantLight SourceRadiation Time (min.)Degradation %Ref.
1SnO2MBSolar180100[65]
2SnO2-TiO2RhBVisible12090[66]
3Co-SnO2RhBVisible25060[32]
4ZnO-SnO2MBXe lamp18095[67]
5Zn2SnO4-SnO2MBUV-Visible36099.4[68]
6Fe2O3/SnO2MBUV9070[69]
7TiO2/Ag/SnO2MBVisible14090[70]
8Co-SnO2/SGCNMBSolar12098Present Work
Table
Table 4. Antibacterial activity against E. coli and B. subtilis.
Table 4. Antibacterial activity against E. coli and B. subtilis.
Diameter of ZOI (mm) for Different Concentrations (ppm)
SamplesE. coliB. subtilis
100150200250100150200250
Tin oxide-121517--1114
7% Co-SnO21922253016192123
7% Co-SnO2/SGCN 3134394220242931
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Javed, M.; Iqbal, S.; Qamar, M.A.; Shariq, M.; Ahmed, I.A.; BaQais, A.; Alzahrani, H.; Ali, S.K.; Masmali, N.A.; Althagafi, T.M.; et al. Fabrication of Effective Co-SnO2/SGCN Photocatalysts for the Removal of Organic Pollutants and Pathogen Inactivation. Crystals 2023, 13, 163. https://doi.org/10.3390/cryst13020163

AMA Style

Javed M, Iqbal S, Qamar MA, Shariq M, Ahmed IA, BaQais A, Alzahrani H, Ali SK, Masmali NA, Althagafi TM, et al. Fabrication of Effective Co-SnO2/SGCN Photocatalysts for the Removal of Organic Pollutants and Pathogen Inactivation. Crystals. 2023; 13(2):163. https://doi.org/10.3390/cryst13020163

Chicago/Turabian Style

Javed, Mohsin, Sana Iqbal, Muhammad Azam Qamar, Mohammad Shariq, Inas A. Ahmed, Amal BaQais, Hanan Alzahrani, Syed Kashif Ali, N. A. Masmali, Talal M. Althagafi, and et al. 2023. "Fabrication of Effective Co-SnO2/SGCN Photocatalysts for the Removal of Organic Pollutants and Pathogen Inactivation" Crystals 13, no. 2: 163. https://doi.org/10.3390/cryst13020163

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