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. 2022 Dec 9;7(50):46428–46439. doi: 10.1021/acsomega.2c05133

Comparative Study of Sonophotocatalytic, Photocatalytic, and Catalytic Activities of Magnesium and Chitosan-Doped Tin Oxide Quantum Dots

Sawaira Moeen †, Muhammad Ikram †,*, Ali Haider ‡, Junaid Haider §, Anwar Ul-Hamid ∥, Walid Nabgan ⊥,*, Tahira Shujah #, Misbah Naz ∇, Iram Shahzadi ○
PMCID: PMC9773341  PMID: 36570226

Abstract

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The present study demonstrates the hydrothermal synthesis of SnO2 quantum dots (QDs) doped with different concentrations (2, 4 wt %) of magnesium (Mg) and a fixed amount of chitosan (CS). The obtained samples were investigated through a number of characterizations for optical analysis, elemental composition, crystal structure, functional group presence, interlayer spacing, and surface morphology. The XRD spectrum revealed the tetragonal structure of SnO2 with no significant variations occurring upon the addition of CS and Mg. The crystallite size of QDs was reduced by incorporation of dopants. The optical absorption spectra revealed a red shift, assigned to the reduction of the band gap energy upon doping. TEM analysis proved that the few nanorod-like structures of CS overlapped with SnO2 QDs, and agglomeration was observed upon Mg doping. The incorporation of dopants little enhanced the d-spacing of SnO2 QDs. Moreover, the synthesized nanocatalyst was utilized to calculate the degradation percentage of methylene blue (MB) dye. Afterward, a comparative analysis of catalytic activity, photocatalytic activity, and sonophotocatalytic activity was carried out. Notably, 4% Mg/CS-doped QDs showed maximum sonophotocatalytic degradation of MB in basic medium compared to other activities. Lastly, the prepared nanocatalyst was found to be efficient for dye degradation in any environment and inexpensive.

1. Introduction

At an international level, environmental pollution is creating difficulties, including insufficient availability of distilled water for a large number of people. An estimated 750 million people are facing the deficiency of unharmed and unsoiled drinking water because of the continuously increasing global population. About 97.5% of global water is saline and 2.5% is conserved for humankind′s nourishment.1 Organic dyes used in different industries such as cosmetic, paper, textile, and chemical manufacturing are sources of contaminated unsoiled water, causing a threat to the environment. Specifically, cationic dye, MB, possesses a stable aromatic structure that is nonbiodegradable, posing a significant threat to aquatic life.2 Many traditional treatments, including ion interchange, carbon filter, biological, and reverse osmosis, are used to lessen these global problems. These approaches have several restrictions, are costly and disadvantageous involving complex methods, and utilize a huge amount of energy.3−5 Thus, logical technologies with the above-mentioned assets need to be developed; sonophotocatalytic activity (SPCA), photocatalytic activity (PCA), and catalytic activity (CA) are environmentally friendly, easy to handle, and cost-effective.6 PCA is considered a highly optimistic approach to reduce the MB dye due to its large efficacy and ambient working conditions. However, this method is not suitable at a large scale because of the slow degradation response time and several photocatalyst disadvantages including large band gap energy (Eg) and increased electron–hole (e–-h+) recombination.6 To overcome these features, more modern technologies and better catalysts are required. Ultrasonic irradiation has also been extensively researched as a primary or secondary technique for the reduction of the dye. Cavitation bubbles are produced in the liquid due to ultrasonic waves. The quick collapse of the bubbles creates localized severe conditions including large pressure and large temperature, and the critical conditions cause dioxygen and water molecules to cleave, producing OH, O2–, and other radical species. These species have the ability to oxidize the organic contaminants in water. However, various disadvantages of the ultrasonic irradiation technique, including time-consuming reactions and utilization of a large amount of energy, have limited its applicability in dye reduction, and it has been observed that ultrasonic irradiation alone is not very efficient for degradation of pollutants. To resolve this issue, several efforts have been devoted to combine the ultrasonic irradiation process with an advanced oxidation process (AOP) like PCA. The degradation of dyes is increased in ultrasonication coupled with photocatalysis (sonophotocatalysis) due to the production of a large number of active species radicals.7−10

During the last few decades, in the area of environmental growth, semiconductor nanomaterials have initiated an excellent curiosity in researchers on account of the exceptional physicochemical characteristics, including nontoxicity and environmental sustainability.11 Various approaches to synthesize metal oxides such as TiO2 (sol–gel technique, nanoparticles with a spherical morphology),12 CeO2 (coprecipitation technique, QDs),13 ZnO (chemical precipitation method, nanorods),14 SnO2 (hydrothermal method, spherical-shaped nanoparticles),15 etc. are used for reduction of the MB dye.16 Among these, hydrothermally synthesized SnO2 QDs have attracted a significant deal of research interest in view of the exceptional optical and electrical characteristics such as low resistive power, large specific capacity, and appropriate Eg (3.6 eV). Moreover, SnO2 QDs have large electron flexibility (∼100 to 200 cm2 V–1s–1), revealing a rapid transfer of photoexcited electrons.15,17 As a result, SnO2 is considered to be an excellent photocatalyst. Pure SnO2 is rarely used in PCA, perhaps because of the difficulty of simultaneous synthesis of Sn2+ and even Sn0, which implies that mixed phases of SnO2 and SnO or Sn are present in the catalyst.7 However, SnO2 has a limitation in degrading dyes due to the large Eg and the inability to utilize visible sunlight.18 Several efforts have been undertaken to overcome these problems by enhancing the photocatalytic degradation rate of SnO2 by doping polymer composites such as chitosan (CS), cellulose, and starch. Among these, CS is found to be a plenteous natural biopolymer extensively used in water remediation possessing exceptional properties, including inexpensiveness, nontoxicity, and bio- and eco-friendliness [15]. Hydroxyl (−OH) and amino groups (−NH2) in CS impart various antimicrobial activities, gas sensing, and wastewater treatment capability. SnO2 QDs can utilize visible light for dye degradation by doping CS, as the Eg is reduced.19,20 The SPCA and PCA of SnO2 QDs can be further increased by metal doping, which modifies the SnO2 photocatalyst to operate efficiently in visible light. The behavior of dopant ions, their quantity, the technique used for doping, and the mechanism of lowering Eg depend upon the PCA of SnO2 by metal doping.21−23 By doping alkaline metals including Ca, Mg, Ca, and Ra, the Eg is reduced, enhancing the dye degradation activity of SnO2. The alkaline metal-doped SnO2 generates lattice defects such as oxygen vacancies and facilitates the production of reactive oxygen species (ROS) that improve the PCA of SnO2 QDs. Hence, we choose alkaline earth metals as dopants.24

This work presents the novel synthesis of Mg/CS-doped SnO2 QDs by utilizing a hydrothermal technique to compare SPCA, PCA, and CA for the degradation of the MB dye.

2. Experimental Part

2.1. Materials

SnCl2·2H2O, magnesium chloride hexahydrate (H12Cl2MgO6 > 99%), NaOH > 98%, and chitosan (C56H103N9O39 > 99%) were obtained from Sigma-Aldrich (Germany).

2.2. Synthesis of Mg/CS-doped SnO2

To synthesize SnO2 QDS by a hydrothermal method, SnCl2·2H2O (0.5 M) was prepared under vigorous stirring at 90 °C for 30 min. Then, an appropriate amount of NaOH solution was added to sustain pH ∼ 9 of the above-mentioned solution under stirring for 1 h. Subsequently, several concentrations of Mg were added with a fixed amount of (0.05%) CS to SnO2 QDs in an airtight 100 mL Teflon-lined autoclave at 150 °C for 12 h. Then, the autoclaved samples were cooled at room temperature and centrifuged at 7500 rpm for 6 min. The collected sample was dried at 150 °C for 12 h and ground with a mortar and pestle to attain fine powder, as depicted in Figure 1.

Figure 1.

Figure 1

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Schematic representation for the synthesis of Mg/CS-doped SnO2.

2.3. Sonophotocatalytic and Photocatalytic Activity

The PCA of the pristine and doped SnO2 was characterized in a photoreactor, and SPCA was characterized in a photoreactor containing an ultrasonic bath. The mercury lamp utilized as as visible light source in the photoreactor was 400 W with a wavelength and frequency of 400–700 nm and 430–790 THz, respectively.25 The stock solution of MB (1 g/1 mL) with the suspension of 10 mg of photocatalyst under vigorous stirring was prepared and placed in the dark for 30 min to enable substantial absorbance. After vigorous stirring, 30 mL of each sample solution was reassigned to an ultrasonic bath in a photoreactor. The 3 mL suspension was used to determine dye reduction capability using an UV–Vis spectrophotometer. At 665 nm, the maximum absorption of MB was achieved for all samples. The efficiency of photocatalysts was calculated using the following equation

2.3. 1

where C0 is the preliminary concentration of dye in the absence of light and Ct is the degradation concentration at constant intervals in the presence of light.

2.3.1. Reaction Mechanism and Kinetics

The reaction mechanism of PCA or SPCA for dye degradation is demonstrated in Figure 2. The SPCA and PCA start with photoexcitation, which is promoted by the photon possessing energy equal or greater than Eg. These high-energy photons shift the valence band (VB) electron to the conduction band (CB). e––h+ pairs are generated in the VB as a result of the excitation process described in the equation below

2.3.1. 2
Figure 2.

Figure 2

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Sonophotoctalysis and photocatalysis mechanism of Mg/CS-doped SnO2 QDs.

The photogenerated electron reacts with O2 to produce the superoxide radical O2•–.

2.3.1. 3

Hydroperoxyl (HO2•) and hydroxyl radicals (OH•) are generated after the reaction of O2•– with water (eq 4), which play a key role in the degradation of the dye.

2.3.1. 4

Photogenerated h+ reacts with water to produce hydroxyl radicals (OH•).

2.3.1. 5
2.3.1. 6

Under ultrasound, acoustic cavitation pyrolyzes the H2O molecules into H• and OH• and H2O2 is formed by combination of OH•. Finally, H2O2 decomposes into OH• radicals as given below.26

2.3.1. 7
2.3.1. 8
2.3.1. 9

Ultimately, OH• radicals produced by both the processes oxidize the organic molecules into CO2 and H2O.

2.3.1. 10

Moreover, even small recombination of e––h+ pairs, which may be destroyed finally, will disturb the PCA of materials.

2.3.1. 11

2.4. Catalytic Activity

CA of the dopant-free and Mg/CS-doped SnO2 QDs was analyzed in the existence of NaBH4 to decolorize the dye. Initially, the desired amount of NaBH4 solution was incorporated in a 3 mL MB solution, followed by introducing 400 μL of pure and (2 and 4%) Mg/CS-doped SnO2. The dye degradation was conducted by conversion of MB to leucomethylene blue (LMB) in the presence of a reducing agent (NaBH4). The reduction of MB was noticed at constant intervals with the help of an UV–Vis spectrophotometer.

2.4.1. Catalysis Mechanism

The incorporation of the synthesized catalysts and NaBH4 into MB plays a key role in the catalysis mechanism, as shown in Figure 3. MB acts like an oxidizing agent, while NaBH4 works as a reducing agent. First, NaBH4 split into ions and shifting an electron from the reducer to MB stimulated the redox reaction. As a result, dye degradation occurs that manifested in electron absorption in MB. Additionally, the reduction of MB in the presence of the reducing agent is slow and time-consuming.27 The additions of the synthesized catalysts into the redox reaction act as an electron relay that permits electron shifting from a donor (BH4–) to MB and reduces the MB to LMB. Adsorption of MB and BH4– was enhanced by adding the prepared dopant-free and Mg/CS-doped SnO2. The size of the nanocatalyst also affects the CA, as small particles possess a large surface-to-volume ratio, resulting in efficient dye reduction.27 The two main reactions to MB degradation are shown in eqs 12 and 13.

2.4.1. 12
2.4.1. 13
Figure 3.

Figure 3

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Schematic diagram of the catalysis mechanism of Mg/CS-doped SnO2 QDs.

2.5. Radical Scavenging Assay (DPPH)

The free radical scavenging activity of the fabricated nanostructures was analyzed using a modified version of DPPH assay. Mg/CS-doped SnO2 nanoparticles (50–500 μg/mL) were mixed with an equal volume of (0.1 mM) DPPH solution. This mixture was vortexed and incubated for 30 min at room temperature in the dark. A standard solution of ascorbic acid was employed as a reference sample. The degradation of the DPPH solution (λ = 517 nm) was employed to calculate the scavenging rate (%) of each sample by eq 14

2.5. 14

Here, A0 and A1 = control absorbance and standard absorbance, respectively.

2.6. Characterizations

The crystal structures of SnO2 and Mg/CS-doped QDs were investigated through a PANalytical Xpert PRO X-ray diffraction (XRD) instrument in the 2θ range of 20–70° utilizing Cu Kα radiation (λ ∼ 0.154 nm). A FTIR PerkinElmer 3100 spectrometer was employed in the range of 4000–500 c/m with 32 scans to detect the existence of functional groups in the synthesized photocatalyst. To analyze the optical characteristics, a UV–Vis Genesys 10S spectrophotometer with the range of 210–600 nm was utilized. The morphological characteristics of QDs were determined using the JSM-6460LV FE-SEM scheme combined with an EDX spectrometer. PL spectroscopy of QDs was carried out using a JASCO FP-8300 system. XPS Peak 41 and Origin 9 software tools were used for binding energy calibration and peak fittings.

3. Results and Discussion

XRD spectra of SnO2 and Mg/CS-doped SnO2 in the 2θ range from 20 to 70° are depicted in Figure 4a. Diffraction peaks were located at ∼26, 33.71, 37.06, 51.34, and 65°, assigned to the (110), (101), (200), (211), and (301) facets respectively. These planes correlate to the tetragonal configuration of SnO2 linked with the standard spectrum (JCPDS card no. 411445) along the P42/mm space group. Other reflection peaks were detected at ∼31.55 and 45.40° attributed to the (02̅1) and (131) planes, respectively, and well-indexed with the anorthic structure of Sn2O3 (JCPDS no. 0251259). Upon CS doping, neither a distinct peak of the dopant nor any variations in the tetragonal configuration of SnO2 were observed. The broadening of peaks and a significant decrease in intensity have been observed upon the introduction of CS, attributed to the intermolecular interaction and synergistic effect between CS and SnO2 QDs.28Figure 4a clearly demonstrates that with Mg doping, there is no change in the phase structure; only slight shift alternation in the diffraction peak of SnO2 can be seen. It is noted that the intensity of the diffraction peaks reduced with the increasing concentration of Mg assigned to the inclusion of Mg2+ into the Sn4+ ions, as there is a clear difference between the ionic radii of Mg2+ (0.072 nm) and Sn4+ (0.071 nm).29 The average crystallite size of SnO2 was calculated by the Debye–Scherrer formula, which was decreased by the incorporation of CS and Mg, as depicted in Table 1. Broad peaks upon doping of CS and Mg were observed, attributed to the decrease in the crystallite size.30 This indicated that the existence of Mg inhibits the development of SnO2. Other studies have found similar outcomes with different dopants in the past.22,23 As the crystallite size reduced, the surface area of the photocatalyst becomes large, improving the adsorption of the reactant and absorption of light over the photocatalyst. Consequently, PCA was increased by doping CS and Mg in SnO2 because the crystallite size was reduced, as elaborated in Table 1.31 The CA of SnO2 was also increased as the crystallite size decreased, attributed to the large surface area of the catalyst.31 Moreover, SAED analysis of primeval and doped samples revealed discrete and bright rings corresponding to different facets (110), (211), (101), and (02̅1) of the XRD pattern, as elaborated in Figure 4c–f. These circular rings containing bright spots exhibited the polycrystalline nature of SnO2, CS/SnO2, and (2, 4 wt %) Mg/CS SnO2. The crystallinity of the samples was reduced by the addition of CS and Mg, as divulged by the XRD data. The FTIR technique was used to investigate the functional groups in the pristine and Mg/CS-doped SnO2 QDs, as demonstrated in Figure 4b. Pure SnO2 QDs confirm the Sn–O–Sn bending and stretching and C–O stretching vibrations at 530, 640, and 2301 cm–1 bands, respectively.32 The transmission bands at 3356 and 1633 cm–1 can be allocated to the O–H vibration.33 Upon doping of CS, bands move toward the lower wavenumber, representing the presence of the −OH or NH2 functional group in CS.27 The transmittance band in the region of 3291–361 cm–1 corresponds to the N–H and O–H stretching that identifies the presence of CS.34

Figure 4.

Figure 4

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(a) XRD pattern, (b) FTIR spectra, and (c–f) SAED pattern of pristine and Mg/CS-doped SnO2.

Table 1. Band Gap Energy, Average Crystallite Size, Average Particle Size, SPCA, PCA, and CA of Pure and Mg/CS-Doped SnO2.

Mg:CS-SnO2 band gap energy (eV) average crystallite size (nm) average particle size (nm) sonophotocatalytic activity in basic medium (%) photocatalytic activity in basic medium (%) catalytic activity in acidic medium (%)
0:0-1 3.63 9.02 9.05 92.29 87.23 61.81
0:0.05-1 3.56 8.71 9.01 97.42 91.91 66.71
0.02:0.05-1 3.47 6.32 8.56 98.37 92.72 70.18
0.04:0.05-1 3.38 5.56 6.76 99.006 96.92 73.64
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Figure 5a reveals the UV–Vis absorption spectra of pristine and Mg/CS-doped SnO2 in the wavelength range from 250 to 600 nm. The acquired spectra delivered facts about Eg and optical characteristics of the synthesized semiconductors. For semiconductor nanostructures, particle size reduction causes the absorption band to move toward the larger energy, attributed to the quantum confinement effect. Consequently, the absorption band of SnO2 observed at 263 nm corresponds to the n−σ* electronic transition.35,36 Tauc′s relation has been used to calculate the Eg as 3.63, 3.56, 3.47, and 3.38 eV for SnO2 and (2%, 4%) Mg/CS-doped QDs, as elaborated in Figure 5b.33 Upon doping of Mg and CS, absorption enhanced toward a higher wavelength (red shift), representing a reduction in Eg because of a larger concentration of oxygen vacancies in SnO2. Both the crystallite size and Eg decreased with CS and Mg doping due to the substitution of Mg2+ ions into SnO2 QDs and matched well with those in the literature.37−39 The reduction of Eg might be associated with the existence of holes in Mg-doped QDs. Due to the lower valence compared to that of Sn4+, all substitutional Mg atoms will produce two holes in the O 2p state that may create an impurity band in the Eg region. First, electrons recombine with holes in the impurity band, and after that, confined electrons consecutively combined with holes in CB, which decreased the Eg.40 Moreover, the PCA of semiconductors depends upon the reduction of Eg because a photon with sufficient energy is required to stimulate the electron from the VB to the CB. The e––h+ pairs generated are utilized to lessen and oxidize chemicals, which results in enhancing the PCA.41 As Eg of SnO2 decreased by doping CS and Mg, its PCA was enhanced, as represented in Table 1. PL spectroscopy was used to elucidate the variation in the charge carrier efficiency of bare and Mg/CS-doped QDs, as depicted in Figure 5c. The photoluminescence is displayed when charge carrier recombination occurs at an excitation wavelength assigned to the coulombic interaction and increased emission.32 The Stokes shift is the wavelength difference between the electronic and PL spectra that occurred due to the loss of vibrational energy in emission spectra. As a result, the wavelength of emission spectra increased and well matched with that in the literature.42−44 SnO2 shows a single high-intensity emission band in the visible region at 470 nm, which is attributed to the fluorescence phenomenon.32 Upon addition of CS, the intensity was reduced, suggesting a low carrier recombination rate, and the peak intensity was further decreased by the incorporation of Mg, ascribed to the phosphorescence phenomena. Among all samples, 4% Mg/CS–SnO2 has the lowest PL intensity, which signifies the largest charge carrier efficacy and remarkable PCA.32

Figure 5.

Figure 5

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(a) UV–Vis spectra, (b) Tauc plot for band gap energy, (c) PL pattern of pristine and Mg/CS-doped SnO2 QDs.

EDS was analyzed to assess the chemical composition to confirm the purity of the synthesized photocatalysts. Figure 6a exhibits the EDS spectra of the pure sample and verifies the formation of Sn and O. Additional peaks of C and Mg, as depicted in Figure 6b–d, confirm the substitution of the dopant species. The Na peak is ascribed to utilizing NaOH to maintain the pH during the preparation of the samples. The Cl peak is assigned to the precursor for the synthesis of SnO2. The formation of SnO2 is further confirmed through FTIR analysis corresponding to the bending and stretching vibrations of Sn–O–Sn at 530 and 640 cm–1 bands, respectively, as represented in Figure 4b.

Figure 6.

Figure 6

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EDS analysis of (a) SnO2, (b) CS-doped SnO2, (c) 2% Mg/CS-doped SnO2, (d) 4% Mg/CS-doped SnO2 QDs.

FESEM images of pure and CS/Mg-doped SnO2 are given in Figure 7a–d. SnO2 revealed nonuniform chunks with randomly distributed small-sized particles, as revealed in Figure 7a. Upon doping of CS, small-sized chunks with an increased quantity of nanosized particles were observed, as shown in Figure 7b. With the incorporation of Mg (2%), agglomerated small-sized particles were overlapped with chunks, and this pattern was increased with the higher concentration of Mg (4%) as elaborated in Figure 7c,d.

Figure 7.

Figure 7

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FESEM micrographs of (a) SnO2, (b) CS-doped SnO2, (c) 2% Mg/CS-doped SnO2, and (d) 4% Mg/CS-doped SnO2 QDs.

TEM analysis was undertaken to investigate the morphological features of the prepared photocatalysts. In Figure 8a, HR-TEM micrographs revealed the formation of the QDs of SnO2. The addition of CS into QDs showed that few nanorod-like structures overlapped with QDs (Figure 8b), confirming the partial interaction between CS and QDs. Upon Mg (2%) doping (Figure 8c), agglomeration was observed, which increased with the higher concentration of Mg (4%) (Figure 8d). This led to Mg being appropriately dissolved in the binary system of CS and QDs, which is evident in the doped species. The average particle size of SnO2 was decreased by incorporating CS and Mg, which enhanced the PCA of samples assigned to the large surface area of the small-sized photocatalyst, as represented in Table 1.

Figure 8.

Figure 8

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(a–d) TEM micrographs of (a) SnO2, (b) CS-doped SnO2, (c) 2% Mg/CS-doped SnO2, and (d) 4% Mg/CS-doped SnO2 QDs.

At a higher resolution (∼10 nm), HR-TEM microscopy was employed to calibrate the interlayer spacing of bare and doped SnO2, as represented in Figure 9b–d. In Figure 9a, the measured d-spacing value of SnO2 was ∼0.23 nm, which matches with the (200) facet and corresponds well with XRD. With the incorporation of CS and Mg, the interplanar spacing of QDs was slightly increased and found to be ∼0.33, 0.35, and 0.37 nm, in accordance with peak alteration with XRD as shown in Figure 9b–d.

Figure 9.

Figure 9

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d-spacing from HR-TEM of (a) SnO2, (b) CS-doped SnO2, (c) 2% Mg/CS-doped SnO2, and (d) 4% Mg/CS-doped SnO2 QDs.

The quantum dots were examined by X-ray photoelectron spectroscopy (XPS), as illustrated in Figure 10a–c, to identify their chemical structure and to validate Mg doping. The fitting findings of O1 enabled us to identify three components. The principal signal at 530.9 eV is close toward the binding energy of oxygen in SnO2,45,46 as presented in Figure 10a. Simultaneously, the remaining two low-intensity and high-energy components could be assigned to oxygen-containing adsorbates: OH groups (532.2 eV) and water molecules (533.2 eV) as shown in Figure 10a.45−47 The existence of SnO2 was established by the attribution of principal peaks at 487.4 and 495.8 eV corresponding to Sn 3d5/2 and Sn 3d3/2, separately, of Sn (IV), as illustrated in Figure 10b.48 The high-resolution spectra reveal that the peak at 49.4 eV corresponds to the Mg2+ ion, showing excellent agreement with the findings provided for Mg 2p3/2 as depicted in Figure 10c.48

Figure 10.

Figure 10

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XPS spectra of Mg:CS–SnO2, (a) O 1s of SnO2, (b) Sn 3d, and (c) Mg 2p.

A comparative reduction efficiency of sonophotocatalysis, photocatalysis, and catalysis of MB solution was examined on pure and doped SnO2 QDs, as demonstrated in Figure 11a–c. For SPCA, the largest competitive degradation of 99% occurs in basic medium (pH 12) within 120 min, whereas at the same time, degradation of 85.67 and 55.82% for the neutral (pH ∼ 7) and acidic medium (pH ∼ 4) was noted, respectively, as displayed in Figure 11a. For PCA, the optimum reduction of dye was 96% in the basic medium, whereas 35.6 and 50.35% for the neutral and acidic medium were noted, respectively, as elaborated in Figure 11b. Figure 11c demonstrates that the CA of pristine and CS/Mg-doped SnO2 QDs revealed the optimum reduction of 61.81–73.64% in acidic medium, 63.49–70.26% in basic medium, and 56.11–68.14% in neutral medium. The pH of the solution controlled the PCA. Owing to the cationic nature of MB, its degradation was smaller at low pH and maximum at higher pH. In an acidic dye solution, adsorption of cationic species is limited and attributed to a positively charged catalyst surface. The adsorption of MB dye enhanced in basic medium, manifesting the large electrostatic interaction between the MB dye and negatively charged catalyst that causes the surface charges to become negative.39 Moreover, the addition of CS enhanced the photocatalytic degradation of MB, attributed to the occurrence of amino and hydroxyl groups in CS.38 The incorporation of Mg also enhanced the PCA, which served as an electron acceptor and separated the e––h+ pair; consequently, the trapped electron shifted to absorb the oxygen molecule. Simultaneously, more dye molecules are absorbed on the surfaces of Mg/CS-doped SnO2 that enhance dye reduction.37 Moreover, the CA of SnO2 is enhanced by adding CS, which is attributed to possessing a large number of active sites.36 The result indicated that 4% Mg/CS-doped SnO2 shows optimum (99%) sonophotocatalytic degradation of MB in basic medium compared to photocatalytic and catalytic degradation. From these outcomes, we deduce that SnO2 QDs proved to be an excellent material for the degradation of the MB dye by the sonophotocatalysis process. Table 2 presents the comparison of the prepared sonophotocatalyst with other reported sonophotocatalysts in terms of percentage degradation, reaction time, and concentration of dyes.

Figure 11.

Figure 11

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(a) Sonophotocatalysis, (b) photocatalysis, and (c) catalysis of Mg:CS–SnO2 in acidic, basic, and neutral medium.

Table 2. Comparison of the Prepared Sonophotocatalysts with Other Reported Sonophotocatalysts.

sonophotocatalyst degradation efficiency (%) reaction time dyes concentration of dye refs
ZnO 54 100 min methyl orange 50 mg/L (49)
NiO 90 30 min MB 200 mL of 20 mg/L (50)
MgTi2O5 96 75 min triphenylmethane dyes 100 mL of 5.32 mg/L (51)
polyacrylonitrile/g-C3N4/CdS heterojunction 92 15 min rhodamine B 100 mL of 10 mg/L (52)
Bi2O3 67 60 min basic brown 1 500 mL of 10 mg/L (53)
graphene nanoribbon-CeO2 heterojunction 91.2 120 min tetracycline hydrochloride 20 mg/L (54)
CeO2/TiO2/SiO2 90.8 1.3 h chlorpyrifos 1–6 mg/L (55)
Co doped Fe2O3 82 90 min eosin B 250 mL of 5.32 mg/L (56)
Mg:CS–SnO2 99 120 min MB 1 g/ mL present study
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A total organic carbon (TOC) assessment of treated water was conducted for the estimation of the dye (MB) degree of mineralization. The study was performed on Mg/CS-doped SnO2 using varying time intervals up to 180 min. This analysis (Figure 12a) demonstrated that TOC of the MB solution treated with the synthesized compounds under visible light irradiation reduced continuously with reaction time, and a significant amount of mineralization of the dye was found after 180 min in Mg:CS–SnO2 (0.04:0.05-1). To check the stability of the photocatalyst, the degraded solution was kept in the absence of light for three days to see whether the dye decolorization was stable or not in the presence of a catalyst. An UV–Vis spectrophotometer was utilized to monitor the dye reduction efficiency every 24 h, as shown in Figure 12b. The % reduction efficiency was calculated using eq 1.

Figure 12.

Figure 12

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(a) Mineralization (TOC) of MB solution. (b) Stability of pristine and Mg/CS-doped SnO2 in basic medium.

The prepared samples are stable up to three cycles as shown in Figure 12. Additionally, the stability of 4% Mg/CS-doped SnO2 was also examined through XRD before and after the sonophotocatalytic reaction as depicted in Figure 13. The crystallinity of QDs was decreased after the 3rd cycle because of photodissolution and photocorrosion of the photocatalyst.57

Figure 13.

Figure 13

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XRD of 4%Mg:CS–SnO2 before and after SPCA.

DPPH scavenging was used to examine and quantify the antioxidant effects of active radical species (Figure 14.). Antioxidant properties of compounds are interrelated with their potential to transfer hydrogen or electrons atoms to the DPPH free radical, resulting in stable diamagnetic compounds. The ability of free radical reduction of this DPPH can be evaluated spectrophotometrically by measuring the degradation in absorbance (517 nm). The antioxidant activity of all prepared samples exhibited a dose-dependent behavior. Mg:CS–SnO2 (0.04:0.05-1) displayed the highest scavenging performance up to 71.45% at 500 μg/mL concentration and scavenged DPPH radicals through the donation of hydrogen atoms. The formed highly reactive •OH and •O2 radical species can interact with DPPH free radicals and result in its degradation, which is highly correlated with the standard (ascorbic acid).58

Figure 14.

Figure 14

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Mg:CS–SnO2 DPPH scavenging activity.

4. Conclusions

In this study, SnO2 and Mg/CS-doped SnO2 were effectively synthesized by a hydrothermal method to compare the MB reduction efficiency by sonophotocatalytic, photocatalytic, and catalytic processes. Among the samples, 4% Mg/CS-doped SnO2 QDs show a maximum of 99% sonophotocatalytic degradation of MB in basic medium compared to other activities. The XRD pattern verified the tetragonal configurations of SnO2, and crystallite size decreased from 9.02 to 5.56 nm upon the incorporation of CS and Mg. Meanwhile, FTIR and SAED analysis endorsed Sn–O–Sn vibration at 530 and 640 cm–1 and crystalline behavior. The TEM image elucidates the formation of QDs of SnO2, and upon doping of CS, a rod-like structure formed, which partially interacts with QDs. The calculated d-spacing was ∼0.23, 0.33, 0.35, and 0.37 nm for both pristine and Mg/CS-doped SnO2. The Eg of SnO2 was observed to be 3.63 eV, and upon doping of Mg and CS, Eg reduced to 3.38 eV. In conclusion, Mg/CS-doped SnO2 QDs were found to be an effective material for the sonophotocatalytic degradation of the MB dye.

Acknowledgments

The authors are thankful to higher education commission (HEC), Pakistan, via the National Research Program for Universities (NRPU) Project-20-17615 (Dr. Muhammad Ikram).

The authors declare no competing financial interest.

References

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