Experimental and theoretical study of catalytic dye degradation and bactericidal potential of multiple phase Bi and MoS₂ doped SnO₂ quantum dots

Abstract

In the present study, different concentrations (1 and 3%) of Bi were incorporated into a fixed amount of molybdenum disulfide (MoS₂) and SnO₂ quantum dots (QDs) by co-precipitation technique. This research aimed to increase the efficacy of dye degradation and bactericidal behavior of SnO₂. The high recombination rate of SnO₂ can be decreased upon doping with two-dimensional materials (MoS₂ nanosheets) and Bi metal. These binary dopants-based SnO₂ showed a significant role in methylene blue (MB) dye degradation in various pH media and antimicrobial potential as more active sites are provided by nanostructured MoS₂ and Bi³⁺ is responsible for producing a variety of different oxygen vacancies within SnO₂. The prepared QDs were described via morphology, optical characteristics, elemental composition, functional group, phase formation, crystallinity, and d-spacing. In contrast, antimicrobial activity was checked at high and low dosages against Escherichia coli (E. coli) and the inhibition zone was calculated utilizing a Vernier caliper. Furthermore, prepared samples have expressed substantial antimicrobial effects against E. coli. To further explore the interactions between the MB and Bi/MoS₂–SnO₂ composite, we modeled and calculated the MB adsorption using density functional theory and the Heyd–Scuseria–Ernzerhof hybrid (HSE06) approach. There is a relatively strong interaction between the MB molecule and Bi/MoS₂–SnO₂ composite.

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1 Introduction

The most fundamental requirement for all living things is clean water.¹ An essential component of life is organic compounds but they also have a significant function in the degradation of the natural environment and have essential practical benefits in businesses such as the cosmetics, painting, and dying industries. Dyes are organic compounds, categorized into two types, cationic and anionic. Heavy metal ions (lead, chromium, arsenic), azo dyes such as methylene blue (MB) and rhodamine B (RhB), and phenol present in wastewater find their way to the aqueous environment and pollute the water. This results in harm to all marine life, land animals, and plant life^2–7 and has adverse effects on human health, such as anemia, bladder irritation, gastrointestinal issues, and so on. Furthermore, waterborne microbes namely Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) make a route of nosocomial disease in human beings.⁸

Catalysis is a significant process that focuses on nanomaterial semiconductors owing to chemical stability, their low toxicity, and environmental friendliness.⁹ The admiration of semiconductor nanoparticles has increased attributed to catalysis capabilities and photo reactivity. Numerous transition-metal oxides, ZnO, SnO₂, Fe₂O₃, TiO₂, and Bi₂O₃ have been employed in photocatalysis to convert multiple organic contaminants into harmless CO₂ and H₂O. Among them, SnO₂ has now been suggested as a potentially useful photocatalyst due to its low toxicity, efficiency, relatively high chemical stability and cost-effective.¹⁰

Additionally, SnO₂ is an n-type semiconductor that has a wide band gap energy (E_g) of 3.6 eV, less resistivity, high electron mobility (200 cm² V⁻¹ s⁻¹), and photoexcited e⁻/h⁺ by reason of these characteristics, it is suitable for various applications.^11–17 The quick e⁻/h⁺ recombination of SnO₂ limits its applications to reduce dye effluents.^18,19 Abdel-Messih et al. SnO₂ prepared through sol–gel methodology, exhibited 10% decolorization of RhB within 120 min under UV-light. Sayfa Bano et al. Synthesized SnO₂ via chemical oxidation polymerization, co-precipitation method, and RhB removal using UV light to achieve 10% methanol efficacy within 20 min.^20,21 To deal with the aforesaid flaw, several approaches such as oxygen vacancy formation and doping with non-metals, carbon materials, and metals heterostructure development were applied for narrowing the E_g and expanding the absorption spectral response towards visible area.²² Vadivel and Rajarajan²³ and Rawal et al.²⁴ demonstrated that tungsten (W) doping improved the dye degradation capabilities of SnO₂ against different dye contaminants. Sharf Illahi Siddiqui et al. Prepared monohybrids Fe₂O₃/SnO₂ and showed 95% degradation of MB.²⁵

Metal dichalcogenides have recently been discovered to be remarkable in the realm of dye degradation,²² in this work fixed concentration of MoS₂ doped into SnO₂. MoS₂ acts as one of the valuable materials for catalysis. It demonstrates the particular optical features, chemical inertness, and high conductivity that show it as cardinal for numerous applications (phototransistors, sensing, catalysis).^26,27 Nanostructured MoS₂ gives more active sites other than bulk MoS₂ due to the reason it has a direct band gap.^28,29 Non-metal based doping has significant promise for biosensing, contaminants removal, and bioimaging purposes,³⁰ herein bismuth has been chosen as a doping agent. Two different (dichalcogenides and post-transition) metals doped into SnO₂ and a relative discussion of their applications as well as characteristics, which to the extent feasible knowledge has not yet been published. Distinctive modification of SnO₂ has been an expected promising approach to get better efficacy of CA at small time intervals and microbial activity.

In the current work, the co-precipitation technique was used to introduce MoS₂ and Bi into SnO₂ to synthesize Bi-doped MoS₂–SnO₂ QDs. The prepared QDs investigated the antibacterial potential for the inhibition of E. coli and improved degradation of MB. Additionally, detailed analyses of synthesized QDs were examined by numerous techniques.

2 Experimental section

2.1 Material

Tin chloride dihydrated (SnCl₂·H₂O, 98%), bulk molybdenum disulfide (∼6 μm, 99%), sodium hydroxide (NaOH, 98%), was acquired from Sigma-Aldrich (Germany). Bismuth nitrate (Bi (NO₃)₃·5H₂O) was obtained from England. HCl (37% dilute) and sodium nitrate (NaNO₃) were received from ‘Analar’ and ‘Merck’, respectively. All materials had been consumed exactly as received.

2.2 Liquid-phase exfoliation of MoS₂

The technique applied to prepare liquid phase exfoliated (LPE) MoS₂ nanosheets (NSs) is depicted in Fig. 1a. Using a beaker, NaNO₃ (6.0 g) was dissolved in 16.0 mL of 37% diluted HCl. After that, bulk MoS₂ (1.2 g) was incorporated, and the reaction was stopped by the addition of an appropriate quantity of H₂O. The suspension was then ultrasonically exfoliated for 5 h at 30 °C in a setup designed to capture hazardous gas. Finally, to extract the nanosheet, the resulting supernatant fraction was centrifuged for 30 min at 6000 rpm. Precipitated formed grayish black MoS₂ nanosheets were obtained.³¹

Fig. 1 (a) Schematic diagram of MoS₂ and, (b) synthesis of Bi/MoS₂-doped SnO₂.

2.3 Synthesis of Bi-doped MoS₂–SnO₂

The chemical co-precipitation approach was utilized for the preparation of bare SnO₂ and Bi/MoS₂-doped SnO₂ (Fig. 1b). A solution of SnCl₂ with a concentration of 0.5 M was prepared and subjected to heating at 80 °C while constant stirring was maintained. Hereafter, a fixed amount (11.28 mg) of MoS₂ had been integrated into the SnO₂ solution. After that various concentrations (5.64 and 16.92 mg) of Bi were incorporated into the solution mixture at continuous stirring. After half an h, 1 M NaOH was incorporated into the aforesaid colloidal solution in a drop-by-drop manner to sustain the pH ∼ 10. After that, the produced precipitates have been washed many times by centrifugation at 7000 rpm for 7 min to achieve a product that was free from contaminants, and therefore they were dried at 90 °C for 12 h. The final product was crushed into fine powder.

2.4 Catalysis

In the presence of NaBH₄, the catalytic activity of SnO₂ and Bi/MoS₂-doped SnO₂ QDs was evaluated by the decolorization of MB into leucomethylene blue (LMB). Positively charged MB dye is utilized as an oxidizing agent, colorless when reduced and blue color when oxidized³² and NaBH₄ is act as a reducing agent. MB (3 mL) was incorporated in 0.1 M NaBH₄ solution (400 μL) in a quartz cell. Moreover, the solution of MB was dissolved in 400 μL prepared QDs. The absorption reaction process had been examined spectrophotometrically at regular intervals. The percentage of dye degradation was determined as:
where C₀ and C_t are referred to as the initial and final MB concentrations, respectively.

2.5 Isolation and identification of MDR E. coli

2.5.1 Sample collection. From determined breastfeeding cows sold at several markets, farms, and veterinary clinics in Punjab Pakistan, raw milk samples were obtained by direct milking to decontaminating glassware. After being collected at 4 °C, raw milk was promptly deposited in the laboratory. Coliforms count in raw milk was carried out on MacConkey agar. All plates were incubated at 37 °C for 48 h.

2.5.2 Identification and characterization of bacterial isolates. The initial identification of E. coli was based on a variety of biochemical tests and Gram stain colonial morphology, under Bergey's Manual of Determinative Bacteriology.³³

2.5.3 Antibiotic susceptibility. The disk diffusion method of Bauer' et al.³⁴ Mueller Hinton agar (MHA) was used for the antibiotic susceptibility test. This test was performed to demonstrate the resistance of E. coli to various kinds of antibiotics; imipenem (Imi) 10 μg (carbapenem), ciprofloxacin (Cip) 5 μg (quinolones), azithromycin (Azm) 15 μg (macrolides), tetracycline (Te) 30 μg (tetracyclines), ceftriaxone (Cro) 30 μg (cephalosporins), amoxycillin (A) 30 μg (penicillins) and gentamicin (Gm) 10 μg (aminoglycosides).³⁵ Uncontaminating cultures of E. coli were grown and engaged to 0.5 MacFarland turbidity. On Muller Hinton Agar (MHA) (Oxoid Limited, Basingstoke, UK), it was then expanded plate and the prevention of overlapping of inhibition zones. The antibiotic discs were spaced apart on the surface of the inoculated plate. Plates were incubated for 24 h at 37 °C and according to the clinical and laboratory standard institute, the consequences were elucidated.³⁶ A bacterium having at least three antibiotic resistances was announced as MRD.³⁷

2.5.4 Antimicrobial activity. The in vitro antibacterial action potential of NSs on 10 representative isolates of MRD E. coli assembled from mastitis milk was estimated using an agar well diffusion approach. On MacConkey agar, Petri dishes were swabbed with 1.5 × 10⁸ CFU mL⁻¹ (0.5 MacFarland standard) MRD E. coli. An Uninfected cork borer was utilized to form wells of 6 mm in diameter. Various concentrations of Bi/MoS₂ doped SnO₂ were applied as (0.5 mg/50 μL) and (1.0 mg/50 μL). DI water was utilized as the negative control (50 μL) and ciprofloxacin (0.005 mg/50 μL) as the positive control.³⁸

2.5.5 Statistical analysis. The antibacterial effectiveness had been estimated in respect of inhibition zone size (mm), and inhibition zone diameters were statistically investigated using one-way analysis of variance (ANOVA) in SPSS 20.³⁹

2.5.6 Characterizations. The crystallographic behavior of SnO₂ and Bi/MoS₂-doped SnO₂ QDs were evaluated using a PANalytical Xpert PRO X-ray diffraction (XRD) instrument in the 20–70° range of 2θ using Cu Kα radiation (λ ∼ 0.154 nm). A PerkinElmer 3100 FTIR spectrometer was used with 32 scans in the 4000–400 cm⁻¹ range to detect the presence of functional groups in the synthesized catalyst. A UV-Vis Genesys 10S spectrophotometer with a wavelength range of 210–600 nm was employed to analyze the optical properties. The morphological characteristics of QDs were studied using the JSM-6460LV FE-SEM scheme in combined with an EDS spectrometer.

3 Results and discussion

MoS₂ nanosheets were synthesized by liquid-phase exfoliation, illustrated in Fig. 1a. Bi-doped MoS₂–SnO₂ QDs were synthesized using a cost-effective co-precipitation method with different concentrations (1 and 3%) of Bi and fix concentration of MoS₂ dopant as shown in schematic diagram Fig. 1b.

XRD analysis has been utilized to examine the crystallographic structure, and phase purity of a material, 2θ ranging from 20° to 70° of SnO₂ and Bi (1 and 3%) MoS₂-doped SnO₂ (Fig. 2a). Diffracted peaks emerged at 26.26° (110), 33.87° (101), 37.86° (200), 51.40° (211), 54.79° (220), 61.76° (310) and 65.75° (301) revealed the tetragonal phase (t-SnO₂) authenticated by (JCPDF 01-077-0452/00-046-1088) and 31.48° (020) to the orthorhombic phase (o-SnO₂). Furthermore, peaks at 29.88° (101) 45.32° (024), and 56.49° (3-1-1) owing to SnO and Sn₂O₃ respectively confirmed by (JCPDF 00-006-0395/01-077-2296/00-025-1259). Different phases of tin oxide were observed. The peak intensity of MoS₂-doped SnO₂ was sharp and less broadened, advocating the crystallinity is enhanced and crystal size increased.⁴⁰ Upon doping of different concentrations of Bi, the peak intensity decreased and broadened as compared to the pure sample. Suppression in intensity ascribed to Bi has an inhibitory impact on crystallinity and the larger ionic radii of Bi corresponding to Sn and O, which minimize the ability of its substitution in SnO₂.⁴¹ Upon incorporation of Bi, few peaks were diminished, suggesting that Bi has an inhibitory effect on crystallinity and particle growth because Bi ions at the particle's surface compete for rearrangement and diffusion in SnO₂; hence crystal growth vanished.⁴² The crystallite size calculated using the Debye–Scherrer formula from the most intense peak of prepared samples was 9.21 nm, 9.59 nm, 3.74 nm, and 3.94 nm for SnO₂, MoS₂-doped SnO₂ and, Bi (1 and 3 wt%)/MoS₂-doped SnO₂ QDs, respectively.

Fig. 2 (a) XRD patterns, (b) FTIR spectra, and (c–f) SAED images of SnO_2, MoS₂-doped SnO_2, and Bi (1 and 3%)/MoS₂-doped SnO₂ QDs.

FTIR analysis was investigated to determine the presence of the functional group, modes of vibration for chemical bonds, and surface chemistry of synthesized samples in the 4000–400 cm⁻¹ wavenumber range (Fig. 2b). The bending vibration of water and stretching vibration of absorbed hydroxyl function group were associated with the band appeared at 1640 cm⁻¹ and 3300–3500 cm⁻¹ respectively.^43,44 The transmission band at 581 cm⁻¹ was manifested to O–Sn–O bending and stretching modes of SnO₂ (ref. 45) and a possible O–Sn–O mode of vibration has been assigned to the band at 682 cm⁻¹.⁴⁶ No extra peaks were generated after doping which might be due to the incorporation of less concentration of dopants.

Bright dots in SAED pattern (Fig. 2c–f) depict the crystalline nature of the as-synthesized specimen. The indexing of SAED patterns revealed the rings were found to correlate to the planes (110), (211), (101), (020), (200), (024), (301), and (220). The planes and the observed peaks are well agreed with XRD and substantiate one another result.

The optical properties were estimated by electronic spectroscopy in the range of 200 to 800 nm. In UV-vis spectra, the 3-D quantum confinement effect was observed by semiconductor NSs. The absorption peak at 285 nm for synthesized SnO₂ was revealed by electronic spectra (Fig. 3a). Similarly, the absorption peaks of MoS₂-doped SnO₂, and Bi (1 and 3 wt%)/MoS₂-doped SnO₂ were observed to be increased and the transfer of electrons was π–π*.⁴⁷ Hence, the absorption ability of light increased with MoS₂ and the concentration of Bi. Absorption spectra shifted towards a longer wavelength and the absorption edge indicated the redshift.^48–50 Tauc plot was used to measure the band gap energy (E_g) of bare and doped QDs. SnO₂ QDs have direct E_g ∼ 4.30 eV while E_g of doped samples was decreased as concentration increased (4.20–3.59 eV), mentioned in Fig. 3b.

Fig. 3 (a) Absorbance spectra and, (b) calculated band gap energy of SnO₂, MoS₂-doped SnO₂, and Bi (1 and 3%)/MoS₂-doped SnO₂ QDs.

Various proportions of elements (Sn, O, Mo, S, Bi, Na, Cl) of Bi (1 and 3%)/MoS₂-doped SnO₂ QDs represented by EDS, elaborated in Fig. S1a–d.† Elemental constituents of as-prepared samples using specific colors described by mapping (Fig. S1e†). A pure sample is authenticated by the strong peaks of O and Sn whereas the peaks of Mo, S, and Bi represented the existence of MoS₂ and Bi in SnO_2. Furthermore, sodium (Na) peak was observed by NaOH used to maintain the pH of samples, and the chlorine (Cl) peak behave as an impurity attributed to the precursor used for synthesis. The holder and coating used for the EDS analysis resulted in a minor trace of Au peak. Electrostatic charging affected the samples throughout the SEM examination, to reduce charging damage, Au was used for SEM/EDS analysis.⁵¹

TEM was utilized to identify the morphology and structure of as synthesized Bi/MoS₂-doped SnO₂ as shown in Fig. 4a–d. In Fig. 4a, TEM image represents the morphology of QDs of the control sample SnO₂. Upon doping of MoS₂ into QDs (Fig. 4b), seems that nanosheets overlapped the SnO₂ quantum dots. In Fig. 4c, the addition of Bi (1%) into the binary system (MoS₂–SnO₂) revealed the agglomeration of QDs which can be attributed to hydrogen bonding as the solvent was DI water. A high degree of agglomeration was observed upon a higher concentration of Bi (3%) into MoS₂–SnO₂ (Fig. 4d). The average particle size of SnO₂ QDs (Fig. 4a) ranging from 4.55 to 7.95 nm was calculated using Image J. software.

Fig. 4 TEM images of (a) SnO₂ (b) MoS₂-doped SnO₂ and (c and d) Bi (1 and 3%)/MoS₂-doped SnO₂ QDs.

Interlayer d-spacing was calculated for all prepared QDs using high-resolution (10 nm) TEM images, as proclaimed in Fig. S2a–d.† The interlayer d-spacing of SnO₂ QDs was 0.23 nm (200). The d-spacing of doped samples were 0.23, 0.20, 0.19, and 0.17 nm along with the planes (112), (024), and (211) respectively, matched well with XRD result. The interlayer d-spacing was decreased because of the overlapping of MoS₂ on QDs which was well consistent with TEM analysis.

MB was used as an alternative contaminant to evaluate the catalytic application of synthesized samples. CA measurements were used to determine the degradation rate of synthetic dyes in the presence of NaBH₄ and the synthesized nanocatalyst (Fig. S3†). The general mechanism has an electron donor NaBH₄ which donates an electron to the proceeding chemical reaction to behave as a reductant while MB accepts an electron from the reducing agent to serve as an oxidant and oxidation–reduction reaction exhibits. Initially, NaBH₄ and dye adsorb on the surface of the catalyst and give root to the breakage of MB to LMB. Breakage of double bonds from aromatic rings and dye's N happens when MB accepts the electron and H atom from BH₄⁻. H atom attaches to double bonded N atom of dye molecules via double bond breakage while e⁻ is accepted by the positively charged N atom, as a result, π conjugation takes place. Although the reaction is favorable in thermodynamics in the absence of a catalyst but not valuable in kinetics. MB degradation was prolonged and time taking in the presence of NaBH₄. To overwhelm these difficulties, the nanocatalyst was dissolved in a redox reaction providing a path to electrons and permits to migrate from BH⁻₄ to MB. QDs increased the adsorption rate of dye and BH₄⁻ ions and several active sites enhanced their reactivity which increase the efficiency of dye degradation.^52–55 The catalytic activity was affected by the particle size as a surface-to-volume ratio is high in small-sized particles, consequently, degradation increased.³²

Electronic spectroscopy was used to observe the catalytic activity of pure and doped SnO₂ QDs for MB degradation. As mentioned in Fig. 5a–c, SnO₂, MoS₂-doped SnO₂, and Bi (1 and 3%)/MoS₂-doped SnO₂ QDs manifested the maximal degradation of 78.74, 99.9, 99.9, and 99.9% in acidic, 79.70, 89.85, 94.92 and 99.9% in basic, and 58.79, 63.36, 63.36 and 67.94%, in neutral medium respectively. The factors that affected the CA were crystallite size, medium pH, and surface area. The maximum degradation in MB was investigated in Bi (1%)/MoS₂-doped SnO₂ in acidic, basic, and neutral media. In an acidic medium, both dopants MoS₂ and Bi showed maximum degradation attributable to the utmost fabrication of H ions adsorbed on the QDs.⁵⁶ In a basic medium, the surface of the catalyst gravitates to gain a negative charge while the positively charged surfaces of the catalyst constrain the absorption of cationic adsorbate species in acidic environment.^57,58 MoS₂ enhanced the catalytic activity as it helped to transfer electrons (electron capture) as compared to the control sample.⁵⁹ The defects as well as disorders ascribed to the Bi³⁺, act as the trapping center for e⁻/h⁺ pairs, and effectively hinder the photo-generated charge carriers recombination.⁶⁰

Fig. 5 Catalysis of SnO₂, MoS₂-doped SnO₂ and Bi (1 and 3%)/MoS₂-doped SnO₂ QDs in (a) acidic, (b) basic, and (c) neutral media.

Antibacterial activity of undoped and MoS₂-doped SnO₂ and Bi (1 and 3%)/MoS₂-doped SnO₂ QDs is encapsulated in the Table 1. At low and high dosages, inhibition diameters were reported in E. coli from (0.34–2.90) to (1.75–3.15) respectively, and displayed in Table 1. Additionally, the inhibition zone for ciprofloxacin (positive control) was estimated to be 4.85 mm corresponding to zero mm DI water (negative control) against E. coli. Pristine QDs illustrated less activity against E. coli rather than Bi/MoS₂-doped SnO₂. Upon incorporation of MoS₂, antibacterial activity enhanced because MoS₂ is useful for increasing surface area and reducing particle size. The cell wall of bacteria is easily penetrated by small size particles.⁶¹ The incorporation of Bi prevents the formation of biofilm and disrupts bacterial membrane integrity, which could improve Bi delivery inside bacterial cells and, as a result, increase its antimicrobial activity.⁶²

Table 1 Antibacterial potential of SnO₂, MoS₂-doped SnO₂ and Bi (1 and 3%)/MoS₂-doped SnO₂ QDs

Samples	Inhibition areas (mm)
	0.5 mg/50 μL	1.0 mg/50 μL
SnO2	0.35	1.75
SnO2–MoS2	2.40	2.75
1% Bi	2.56	2.95
3% Bi	2.90	3.15
Ciprofloxacin	4.85	4.85
DI water	0	0

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The ROS generation (O₂⁻, HO₂, OH, and H₂O₂), free radical formation, and reduction in cell membrane integrity have been strongly linked with microbicidal action. The ability of metal oxides to donate electrons generates ROS. The surface of the microbial cell membrane is nanometer-sized porous; hence the sufficient charge and size of nanomaterials pierce the membrane. Cell functions are disrupted by affecting DNA and protein using these nanomaterials and cell performance destroy ultimately.⁶³ ROS and metal ions released from nanomaterials are intended to generate inhibitory zones (Fig. S4†).

All calculations were performed within the density functional theory using the generalized gradient approximation (GGA) formulated by Perdew–Burke–Ernzerhof⁶⁴ for exchange and correlation. We used the computational package QuantumATK,⁶⁵ in which employs the linear combination of atomic orbital (LCAO) basis set approach to solve the single particle Kohn–Sham equations. The norm-conserving PseudoDojo pseudopotentials⁶⁶ with a medium basis set and a kinetic-energy cutoff energy of 75 Ha were used. For the weak dispersion forces, the DFT-D2 function of Grimme⁶⁷ was applied. The Heyd–Scuseria–Ernzerhof hybrid (HSE06) approach was used to obtain the more accurate electronic structures of the materials in the investigation.^68,69 The structure was optimized when the atomic force reached 0.05 eV Å⁻¹ and the most significant energy difference between the two stages was less than 10⁵ eV. The Brillouin zone is sampled with a 4 × 4 × 1 Monkhorst–Pack grid for the calculations. The optimized lattice parameters of bulk SnO₂ are found to be a = 4.825 Å, c = 3.244 Å, which matches well with experimental values of a = 4.737 Å, c = 3.186 Å.⁷⁰ We employed a nanocluster derived from stoichiometric cuts from the bulk crystal structure with (SnO₂)₆, which retains its atomic structure reasonably well concerning the original bulk atomic ordering and positions after structural relaxation, as shown in Fig. 7. A 3 × 3 × 1 supercell based on a hexagonal unit cell was adopted to model MoS₂ monolayer and a vacuum space of 20 Å was employed in the z-direction to avoid artificial interaction between neighboring layers,⁷¹ as shown in Fig. 7. We calculated the total and partial DOS of bulk SnO₂ using HSE06 functional, as illustrated in Fig. 6. From the analysis of the results, the band gap of bulk SnO₂ is found to be 3.27 eV, which agrees well with the experimental values of 3.60 eV (ref. 72) and other theoretical values.⁷³ The PDOS demonstrates that the maximum of the valence band is mainly composed of O 2p states and that the minimum of the conduction band is dominated by strongly hybridized Sn 5 s and O 2p states. The calculated HOMO–LUMO gap (5.08 eV) of (SnO₂)₆ nanocluster is larger than the band gap for SnO₂ crystal, because of the quantum confinement. The obtained gap value is consistent with our experimental measurements (4.40 eV).

Fig. 6 Calculated total and partial DOS of pristine bulk SnO₂.

Fig. 7 Optimized adsorption geometries of the MB on (a) MoS₂–SnO₂ system (b) Bi/MoS₂-doped SnO₂ composite (red: O; blue gray: Sn; light purple: Bi; purple: Mo: brown: C; blue: N; yellow: S; gray: H).

To further explore the interactions between the MB and Bi/MoS₂–SnO₂ composite, we calculated the MB adsorption on the Bi/MoS₂–SnO₂ system. The adsorption energy was calculated using the following expression;^74,75 E_ads = E_MB+complex − E_complex − E_MB, where E_MB+complex, E_complex, and E_MB are the total energies of the Bi/MoS₂–SnO₂ composite absorbed MB, the Bi/MoS₂–SnO₂ composite, and the isolated MB molecule, respectively. The calculated E_ads with respect to MB on the MoS₂ doped SnO₂ nanocluster and Bi/MoS₂ doped SnO₂ nanocluster are found to be −4.26 eV, and −3.06 eV respectively. Negative adsorption energy corresponds to an exothermic reaction. The MB molecule is found to interact relatively stronger on Bi/MoS₂–SnO₂ composite.

4 Conclusion

In this research, Bi/MoS₂-doped SnO₂ QDs have been prepared using a cost-effective co-precipitation method with different concentrations (1 and 3%) of Bi dopant to enhance the dye degradation and antimicrobial activity efficiency. Decreased peak intensity and broadening of FWHM of prepared specimen affirmed by XRD. A significant decrease in E_g was noticed with the higher concentration of dopants. FTIR confirmed the functional groups in prepared QDs while EDS and mapping were used to determine the elemental composition of control and doped samples. QDs morphology was recorded via TEM and HR-TEM calculated interplanar d-spacing (0.24, 0.16, 0.17, and 0.14 nm) well matched with XRD. Bi/MoS₂-doped SnO₂ in contrast with doped free sample elucidated supreme catalytic activity up to 99.9%. The significant inhibition zone was determined as 3.15 mm after doping at a high concentration. DFT calculation has completed the understanding of how MB molecules interacted with the Bi/MoS₂ doped SnO₂ QDs by exploring the electronic structure and absorption energy.

Data availability

Data will be available on demand.

Conflicts of interest

Manuscript is free from conflict of interest.

Acknowledgements

The authors are thankful to Higher Education Commission (HEC), Pakistan through NRPU-20-17615 (Dr Muhammad Ikram).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra00698k

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