Introduction

TMDs are layered materials, similar to graphene, in which the transition metal atomic layer is sandwiched between two layers of the chalcogenide, bonded together by weak Van der Wall forces, which are strong in-plane bonding and weak out-of-plane bonding1.VS2 and MoS2 are two different kinds of TMDs; V is the group 5th element of the periodic table, and Mo is the group 6th element. Their electronic configuration is [Ar] 3d3 4s2 and [Kr] 4d5 5s1, respectively, and for sulfur, it is [Ne] 3s2 3p4. TMDs have different stacking sequences, namely 1T, 2H, and 3R for trigonal, hexagonal, and rhombohedral, respectively. Among them, the 1T and 2H polytypes of VS2 and MoS2 are thermodynamically stable phases. The distinct electronic properties of metallic and semiconducting MoS2 arise from their different electron configurations, with the metallic phase exhibiting partially filled valence bands, while the semiconducting phase has fully filled valence bands. Although the 1T and 2H phases share a similar crystal structure, these differences in electronic arrangements may influence their relative stability due to variation in electronic energy. For instance, the 1T phases of MoS2 have formation energies that are 0.84 eV, and 0.55 eV per formula unit higher than those of the 2H phase. Due to the lowest free energy, the phases are thermodynamically stable2,3.The 1T poly-type of VS2 is known for its different optical and electronic properties. The hexagonal structure is the most common form of VS2 4.Due to the variable bandgap properties, TMDs are very useful in optoelectronic applications5,6,7,8,9,10,11.Apart from other applications, these TMDs also show good antimicrobial activity, which leads to biomedical applications12.The high surface-to-volume ratio of 2D material is beneficial for different biomedical applications, including biosensing13,drug delivery14,15, photothermal therapy16,bioimaging17,18, and antibacterial applications. According to a previous report, the antibacterial activity of chemically exfoliated MoS₂ sheets was evaluated using the colony counting method. When E. coli DH5a was incubated for two hours with 80 mg mL− 1, only 40% of bacteria were killed by raw MoS2, and 90% were killed by chemically exfoliated MoS219.The incorporation of MoSe₂ with H₂O₂ enhances the antibacterial activity compared to H₂O₂ alone. This enhanced effect is particularly evident against Escherichia coli (E. coli), and Bacillus subtilis (B. subtilis). The MoSe₂-based films exhibit strong resistance to bacterial infection, which can be attributed to their intrinsic peroxidase-like activity20.The antibacterial activity of MoS2, WS2, and MoSe2 with their IT phases was tested against gram-negative E. coli bacteria. TMDs were Prepared for this activity by chemical exfoliation and the antibacterial activity by colony counting method21. Doping is also a good approach to improve antibacterial activity because it enhances the active surface area. Nitrogen-doped MoS2 and WS2, which is sowing improved peroxidase-like catalytic activity by producing more toxic OH by catalyzing H2O2. It is able to fight more efficiently against E. coli and B. subtilis bacteria than non-doping MoS2 and WS2. Here, the material was prepared using the hydrothermal method followed by ultrasonic exfoliation22. WX2(X = S, Se) is exfoliated using single-standard DNA and shows antibacterial activity against Escherichia coli K-12 MG1655 in phosphate-buffered saline23. Bismuth-doped MoS2 prepared by hydrothermal method shows increased antimicrobial response against S. aureus and E. coli bacteria24. Fe3O4@MoS2-Ag enzyme with a rough defect-rich surface was created using in-situ photo deposition of Ag nanoparticles and a straightforward hydrothermal process. By releasing Ag + and producing reactive oxygen species, the enzyme demonstrated strong antibacterial activity against E. coli (~ 69.4%). Additionally, the nano enzyme could attain exceptional synergistic disinfection (~ 100%) when combined with the near-infrared photothermal characteristic of Fe3O4@MoS2-Ag25. Zr-doped MoS2 nano catalyst synthesized by the hydrothermal method enhances the antibacterial activity against both bacteria26. Hydrothermally synthesized Ce-MoS2/WO3 shows exceptional antibacterial activity in visible light. The antibacterial rate against E. coli increased to 97.8% when compared with MoS2/WO3(62.8%) and pure MoS2. (44.1%). Therefore, for quick sterilization, the composite’s enzyme-like catalytic activity and photodynamic effect are enhanced by the synergetic reaction of Ce-doped WO3 and MoS227. TMDs like VS2 and MoS2 can generate reactive oxygen species upon light exposure or in the presence of certain catalysts. Previously, the antibacterial activity of VS2/MoS2 was also studied, and the antibacterial efficacy against both gram- and gram-positive bacteria was also studied28. CoS2/MoS2 shows remarkable antibacterial performance with good enzymatic and photocatalytic properties. CoS2/MoS2 nanosheets exhibit remarkable enzyme-mimicking capabilities in neutral media, addressing the significant limitation that nanozymes typically function only in acidic environments29. Several other features of the VS2 and MoS2 nanocomposites make them suitable for the treatment of cancerous cells. The sharp edges30 of VS2 and MoS2 nanosheets can physically puncture and disrupt microbial cell membranes, resulting in eventual cell death by leakage of cellular contents. These materials can release metal ions (V5+ from VS2 and Mo6+ from MoS2) that have toxic effects on microorganisms. These ions can interfere with essential cellular processes and enzymes, leading to microbial death. MoS2 is an exceptional member of transition metal dichalcogenides (TMDs) due to its expansive surface area, strong near-infrared (NIR) absorption, high biocompatibility, and low toxicity toward cells. MoS2-based nanomaterials have displayed efficacy in inhibiting bacterial growth across various systems31. Premeditated the antimicrobial efficiency of CAU-17/MoS2 against both gram-negative (E. coli) and gram-positive (S. aureus) as well as Methicillin Resistant S. aureus (MRSA) and established a result which shows it has an impressive antibacterial efficacy against the test micro-organisms due to its photocatalytic properties. VS4/Ag2WO4 nanocomposites were synthesized using straightforward co-precipitation and ultrasonic-assisted techniques. These nanocomposites demonstrated significant antimicrobial response against both gram-negative E-coli and gram-positive Bacillussubtilis32. The nanocomposite of MoS2 and chitosan was evaluated and found to be successfully constraining the growth of both E. coli and S. aureus along with breast cancer cells, i.e., MCF733. These findings underscore the nanocomposites’ substantial potential as highly effective photo-catalysts for water pollutant removal and as potent antimicrobial agents. The newly synthesized nanocomposite can also be used in the biomedical field as a novel antibacterial agent against different bacterial diseases.

In the present report, the VS2/MoS2 nanocomposites are prepared through the one-pot hydrothermal method by varying Mo concentration. Structural investigation by XRD confirms the presence of both VS2 and MoS2 phases. The UV-Vis analysis infers the absorption shift resulting in the bandgap change with Mo content. The morphological images were taken using FESEM, which shows the nanoflower-like structure. The compositional study reveals the presence of constituent elements in the samples. XPS study of the material presents the details regarding the oxidation states of the different elements. The antimicrobial activity was performed on gram-positive bacteria (ATCC 25923, Staphylococcus aureus and gram-negative (ATCC 25922, Escherichia coli). It was observed that there was a very good response of anti-microbial activity against gram-negative bacteria.

Experimental procedures

Materials

Ammonium molybdate tetrahydrate (NH4)6Mo7O24. 4H2O,99.3%, Loba Chemie), Ammonium metavanadate (NH4VO3, 99%, GLR Innovations), Thioacetamide (C2H5NS, Thiourea (CH4N2S,99%, Loba Chemie), Spectrochem), Oxalic acid dehydrate((COOH)2.2H2O,99.5%, SRL Chemicals), Polyvinyl pyrrolidone K30 L R((C6H9NO) n, SDFCL) (PVP), Ammonia soln. Abt. 30%. (NH3, SDFCL), all the chemicals were used directly without any modification. The solvent used is deionized water; deionized (DI) water and ethanol were used for the removal of unreacted precursors and impurities to extract the required nanoparticle.

Methods

VS2 nanosheet preparation

A facile hydrothermal synthesis method was used to prepare VS2 nanosheets. In a beaker containing 75 mL DI water and 5 mL ammonium hydroxide solution,2.5 g PVP, 0.585 g NH4VO3, and 3.755 g C2H5NS were added one after another, respectively. The solution is stirred for 2 h at 900 rpm at ambient temperature to make it a homogeneous solution. The solution color gradually changes from light yellow to deep brown and finally black. Then, the prepared solution was put inside a 100 mL stainless-steel autoclave (Teflon-lined). The heating was done for 24 h at 200 °C. After that, the autoclave was allowed to cool naturally. The powder was obtained after centrifugation several times by DI water and ethanol. Then, it was vacuum dried for 24 h at 60 °C. The dried sample was finely grinded with mortar pastel to obtain the VS2 nanosheets. Scheme 1 represents the schematic of the preparation of the VS2 nanosheet.

Scheme 1
figure 1

Preparation method of VS2 nanomaterial.

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VS2/MoS2 nanocomposite preparation

First, 0.2 g of prepared VS2 powder was taken and mixed with 40 mL of DI water in a beaker. Then, the solution was ultrasonicated in ice water for 30 min to prevent oxidation34. At the same time, another beaker was taken, added with 30 mL of DI water and 0.3 g of (NH4)6Mo7O24. 4H2O, and 0.8 g of CH4N2S. Finally, 0.4 g of oxalic acid was used for regulating pH. Then stirring of the mixture was done for 30 min at 900 rpm at normal temperature. Next, the mixing of both solutions was carried out with a stirring for 2 h to make it a homogeneous one. Then, the solution was put inside a 100 mL stainless steel autoclave (Teflon–lined). The heating was done for 24 h at 200 °C in a hot air oven. It was allowed to cool automatically. By the same cleaning process as like VS2 nanosheet, the VS2/MoS2-1 nanocomposite powder was obtained. A schematic diagram is shown in Scheme 2. By varying the concentration of molybdenum precursor (0.5 g (5.7797 mmol) and 0.7 g (8.0915 mmol), another two composites are prepared, namely VS2/MoS2-2 and VS2/MoS2-3, respectively. These names are denoted throughout the paper.

Scheme 2
figure 2

Preparation method of VS2 nanomaterial.

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Impact of PVP on nanoparticle synthesis

The role of PVP in nanoparticle synthesis varies with the specific material system and reaction conditions, where it can act as a reducing agent, growth regulator, surface stabilizer, and dispersant. By coordinating with small ions, PVP restricts nanoparticle growth, enabling the formation of ultra-small particle sizes35. Widely employed in nanoparticle synthesis, PVP is a nonionic, biocompatible polymer composed of C = O, C–N, and CH₂ functional groups. Its molecular structure contains both a hydrophilic pyrrolidone moiety and a hydrophobic alkyl group. Due to the highly polar amide group in the pyrrolidone ring, along with methylene and methine groups in the ring and backbone, PVP is soluble in water as well as in many nonaqueous solvents. The steric hindrance effect, arising from hydrophobic carbon chains extending into the solvent and repelling each other, makes PVP an effective stabilizer that prevents nanoparticle agglomeration36,37,38. In PVP-mediated synthesis, residual PVP may remain after the reaction. Typically, purification involves centrifugation and ethanol washing to remove excess reactants and byproducts. Nevertheless, some PVP molecules often remain bound to the nanoparticle surface, functioning as ligands and stabilizing agents. Previous studies further indicate that oxidized PVP, such as poly(vinyl(pyrrolidone)x-(succinimide)y), exhibits stronger binding affinity, which may account for its persistent association with nanoparticles39.

Characterization techniques

The powder XRD data for the samples were measured by using CuKα radiation (λ = 1.5418 Å) source (XRD Instrument-Bruker D8 ADVANCE). The data were recorded at 30 kV and 40 mA. The data scan was done with step size 0.02° in 2θ range of 10° − 80° at a grazing angle of 1°. Raman data was recorded from the HORIBA 1024*256-OE spectrometer and a charge-coupled device (CCD) detector. The spectra were taken from 50 to 700 cm− 1 by using a 633 nm helium-neon laser with a resolution of 0.5 nm. The morphology of the samples was assessed using FESEM (JEOL, JMS-7610 F plus) operated around 20 to 200 kV. The surface morphological view of the samples was imaged by the FESEM unit at various portions of the film with different scales. The high-resolution surface images were collected via TEM supplied by JEOL, TEM-2100 PLUS instrument. The planes present in the sample were verified from the selected area electron diffraction (SAED) pattern. The element confirmation of the samples was determined by an energy-dispersive X-ray (EDX) spectrometer. The elemental mapping for the samples showing the distribution of elements was taken through EDX. The XPS instrument (Axis Ultra Kratos Analytical, UK) was employed to study modifications in the chemical structure and compositions. Core level XPS spectra were obtained using Al Kα X-rays (1486.6 eV) under a vacuum of 2 × 10− 9 Torr. The XPS data consisted of survey scans of the entire binding energy (BE) and selected scans of the core level peaks of interest. An energy increment of 1 eV was used for recording the survey spectra and 0.05 eV for the case of core level spectra. The core level peaks were recorded by sweeping the retarding field and using the constant pass energy of 30 eV. Data were averaged over three scans. The reproducibility of the measurements was checked on different regions of the investigated surfaces. For this study, adventitious carbon was used as a reference, and the BE of the reference C 1s line was set as 284.6 eV. The reflectance data was captured by JASCO V-770 UV-visible spectrophotometer between 200 and 1200 nm at 0.5 nm spectral resolution. The data were obtained at an interval of 1 nm.

Minimum inhibitory concentration (MIC) by broth Dilution method

In this procedure, the MIC was done to check the minimum concentration of the materials required to kill or inhibit the growth of bacteria. Here, both gram-negative (ATCC 25922, Escherichia coli) and gram-positive bacteria (ATCC 25923, Staphylococcus aureus) strains were used to perform MIC. First of all, solutions of material were prepared at a concentration of 25 mg/mL, 50 mg/mL, and 100 mg/mL by adding the appropriate amount of material in 1 mL of Dimethylformamide (DMF). MIC was performed by adding 10 µl of prepared solution of materials from each concertation to the Nutrient Broth (NB), and in that fresh culture of the standard strains of both gram-positive (E. coli) and gram-negative (S. aureus) (0.5 McFarland) was inoculated, and at last, PBS was added to maintain the volume. Then, the mixture was incubated at 37 °C for 36 h. Turbidity was observed afterward.

Disk diffusion method

Several methods have been used to evaluate antimicrobial properties40,41. Among all of these, the disk diffusion system is one of the most used techniques42,43,44,45. In order to evaluate diffusion and the antimicrobial efficacy of these newly designed materials, the disk diffusion method was used. The antibacterial response of the synthesized material (VS2/MoS2-3) was evaluated by using both gram-negative (ATCC 25922, Escherichia coli) and gram-positive bacteria (ATCC 25923, Staphylococcus aureus). The pure colonies were isolated from ATCC samples by quadrant streaking. Then, fresh culture was prepared using a Nutrient Broth (NB) medium, and the bacterial concentration of 0.5 McFarland constant (1.5 × 108) was prepared. Afterward, 100 µl of fresh culture was swabbed all over the surface of the Mueller Hinton Agar (MHA) plate using a sterile cotton swab and left for drying for about 20 min. In the meantime, newly synthesized material was dissolved in Dimethylformamide (DMF) at 100 mg/mL concenmlmltration, and 6 mm sterile disks were soaked in a solution of material. Some disks were soaked in DMF (for control) for 6 h as well. Afterward, 2 disks were placed on the swabbed plate, one with only DMF as positive control and another impregnated with a material solution. Then, incubation of these plates was done at 37 °C for 24 h. The material’s antimicrobial activity was observed using macroscopic images and their respective inhibition zones.

Result and discussions

XRD study

VS2 and VS2/MoS2 nanocomposite XRD pattern is illustrated in Fig. 1. The prepared VS2 nanomaterial shows a hexagonal phase (JCPDS:36-1139)34. Two prominent peaks of VS2 at 15.70o and 36.50o were directed along the (001) and (101) crystallographic plane. The fingerprint of pure hexagonal MoS2 (ICSD:01-075-1539) is also shown in Fig. 1. Three prominent peaks of MoS2/VS2 nanocomposite are observed at 13.93o, 33.33o, and 59.25o, which correspond to (002), (100), and (110) planes, respectively. The broader peak observed is due to the merging of pure VS2 and MoS2 peaks. The first (13.93o), second (33.33o), and third (59.25o) peaks of the composite are due to the (001) plane of pure VS2, (002) plane of pure MoS2, (100) plane of pure VS2, (101) plane of pure MoS2 and (110) plane of pure VS2, (111) plane of pure MoS2 respectively34,46,47. MoS2/VS2 nanocomposite diffraction peak located at 13.93o and corresponding to the MoS2 (002) plane is less than that of pure MoS2(2θ = 14.4o), indicating the (002) plane’s larger interlayer distance48. The reduction of grain size is due to the broader and weaker peak of the nanocomposite.

Fig. 1
figure 1

XRD pattern of VS2 and VS2/MoS2 nanocomposite.

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The X-ray diffraction data gives us valuable information such as crystalline phase, crystal structure, crystallographic orientation, crystallite size, strain and phase purity, etc48. Broader peaks are observed that lead to smaller crystallite sizes. Debye-Scherrer equation is used to determine the crystallite size (D) by 49,

$$\:\text{D}\:=\:\frac{K}{\beta\:\text{c}\text{o}\text{s}\left(\theta\:\right)}$$
(1)

with K = 0.9 as the shape factor. λ = 1.54 Å as the CuKα wavelength of. β as the full width at half maximum (FWHM). The calculated values are listed in Table 1 .The average crystallite size decreased with the increase in the molybdenum concentration; values are 4.6 nm,4.39 nm, and 4.29 nm, respectively. The D value of VS2 is 13.81 nm. All calculated values are presented with their corresponding standard deviations, and the detailed calculations are provided in Table S1. The Dislocation density is the number of dislocation lines per unit volume of a crystal as calculated49.

$$\:\text{D}\text{i}\text{s}\text{l}\text{o}\text{c}\text{a}\text{t}\text{i}\text{o}\text{n}\:\text{d}\text{e}\text{n}\text{s}\text{i}\text{t}\text{y}\:\left({\updelta\:}\right)\:=\:\frac{\:1}{{D}^{2}}$$
(2)

In nano-crystalline materials, lattice distortion, characterized by micro-strain and lattice parameter change, is commonly encountered. The fundamental explanation for the lattice distortion is thought to be the internal stress caused by the extra volume at grain boundaries31. The micro-strain produced inside the material is given by,49

$$\:\text{M}\text{i}\text{c}\text{r}\text{o}-\text{s}\text{t}\text{r}\text{a}\text{i}\text{n}\:\left({\upepsilon\:}\right)\:=\:\frac{\beta\:}{4\text{t}\text{a}\text{n}\left(\theta\:\right)}$$
(3)
Table 1 Structural parameter of VS2 and VS2/MoS2 nanocomposite.
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Fig. 2
figure 2

Raman spectra of VS2 and VS2/MoS2 nanocomposites.

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Raman analysis

Raman spectra analysis was performed to verify the microstructure and insight into the details of the phonon active mode of VS2 and VS2/MoS2 nanocomposite. The Raman data was taken in the 50–700 cm− 1 range for all the prepared nanocomposites, as presented in Fig. 2. The characteristic pristine VS2 peaks are located approximately at 192, 219, 271,286.5 and 346 cm− 1, which are arises due to the different rocking and stretching vibration of V-S bond vibrational modes50,51.192 cm− 1 and 219 cm− 1 peaks are due to the V-S bond stretching. The peak formed at 271 cm− 1 and 286 cm− 1 is due to the E2g1 in-plane phonon mode arises due to the opposite vibration of two S atoms with respect to V atom52 .

The characteristic peak of VS₂ at 192 cm⁻¹ is observed in the composite sample, while the other peaks are not clearly visible. Both in-plane(E12g) and out-of-plane (A1g) vibrational mode peaks of MoS2 located at 378.9 and 406.9 cm− 1 are clearly observed in all three nanocomposite samples53. In the nanocomposite the peak appeared at 228 is due to the MoS254,55. The peak intensity gradually increased with higher molybdenum concentration. The presence of both VS2 and MoS2 vibrational mode peaks of Raman also confirms the formation of the composite.

FESEM and EDX analysis

The morphology of the nanomaterial is determined by FESEM images. Figure 3.a, b illustrates the FESEM pictures at different magnifications of VS2/MoS2-1. From the observation, it is found that the VS2/MoS2-1 sample morphology exhibits a flower-like structure. An essential method for examining the compositional variance in the material is elemental mapping. Figure 3. c-f shows the elemental mapping of VS2/MoS2-1. The material’s homogeneous distribution of V, S, and Mo components confirms the successful completion of a uniform VS2/MoS2 nanocomposite through the elemental mapping. EDX spectra of VS2/MoS2-1, from Fig. 3.g, confirm the elemental presence of the prepared sample. Figure 4. represents the FESEM pictures of two other compositions, such as VS2/MoS2-2 and VS2/MoS2-3, with two different magnifications.

Fig. 3
figure 3

(a, b) FESEM images at 100 nm and 1 μm scale; corresponding elemental maps of (c) V, (d) S, (e) Mo, (f) overall map, and (g) EDX spectra of VS2/MoS2-1 sample.

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Fig S1a, b represents the FESEM images of VS2 with different magnifications. Some other reports also show similar morphology56. An average thickness of 43 nm is found for the nanosheets. All other EDX spectra are provided in Figure S2a, b, c. Different peaks for the various elements present in the material can be seen in the EDX spectrum. 0.511 keV and 4.952 keV are the energies corresponding to the Lα and Kα transitions for the element Vanadium. 2.293 keV is the Lα transition for molybdenum. The Sulfur peak at 2.307 keV is due to the Kα transition.

Fig. 4
figure 4

FESEM images of (a, b) VS2/MoS2-2, (c, d) VS2/MoS2-3 at two different magnifications.

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TEM analysis

Figure 5 represents the TEM images of VS2/MoS2-3 with different magnifications, which clearly show the nanosheets stacked together, forming a flower-shaped structure. The other two TEM images are shown in Figure S3. Figure 5c-e shows the HRTEM images with different scales and distinct crystallographic planes. (002) and (103) are the planes corresponding to MoS2, with lattice spacing of d = 6.3 Å and d = 2.21 Å, respectively. (101) plane corresponds to VS2, which confirms the inter-planar spacing of 2.51 Å. It matches well with the XRD data, conforming to the hexagonal phase of both VS2 and MoS2. Figure 5f is the SAED pattern confirming the presence of both VS2 and MoS2. Each circular ring present demonstrates the (002), (004), (100), and (102) planes corresponding to MoS2 and VS2, respectively. It shows the polycrystalline nature of the prepared sample.

Fig. 5
figure 5

(a, b) TEM images at various magnifications; corresponding (c–e) HRTEM images of VS2/MoS2-3 (f) SAED pattern.

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XPS analysis

XPS is widely recognized as a powerful tool for surface analysis. It enables the identification of a material’s surface elemental composition, oxidation states, and bonding environments, offering precise insights into the outermost layers, generally within the top ~ 10 nm of the sample. This data further confirms the presence of S, V, and Mo in the prepared sample. The comparison between pure VS2 spectra and VS2/MoS2 spectra will give proof of nanocomposite formation. The XPS spectra of pristine VS2 and VS2/MoS2 nanocomposite are presented in Fig. 6.a. The characteristic core levels for VS2 are located at 524.7 eV and 517.4 eV. The V-2p1/2 and V- 2p3/2 atomic orbitals correspond to the V4+ state. V2+ oxidation state is also present, confirmed by the peak located at 514 eV (Fig.S4(a))46. Due to the overexposure of the sample to air, another characteristic peak located at 530.1 eV is present due to O1s atomic orbital. The 162.8 eV and 163. 9 eV peaks are for S-2p3/2 and S-2p1/2 atomic orbitals, respectively, with the state S2− Fig. S4(b)57.

Fig. 6
figure 6

(a) VS2 and VS2/MoS2-3 XPS survey spectra: (b) V-2p, (c) S-2p, and (d) Mo-3d core level peaks.

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Figure 6a. presents the comparison between the XPS survey spectra of VS₂ and the VS₂/MoS₂-3 nanocomposite, while Fig. 6b and c display the high-resolution spectra of the V 2p and S 2p orbitals, respectively. In nanocomposite, the peaks for V-2p are slightly shifted towards the lower binding energy value as 524.5 and 517.04 eV that correspond to V-2p1/2 and V-2p3/2 with the same oxidation state V4+57. The peak for state V2+ is not present here. The sulfur peak for 2p atomic orbital also shifted towards the lower binding energy value. S-2p1/2 and S-2p3/2 atomic orbitals correspond to binding energy values 163.22 and 162.08 eV, respectively, for S2− state. Figure 6. d confirms the presence of Mo with the characteristic peak at 232.4 eV and 229.29 eV with the emission of electrons from the 3d3/2 and 3d5/2 atomic orbitals, and the Oxidation state is Mo4+. A small peak at 226.45 eV is for the S-2s orbital33. The peak at 235.75 eV indicates the formation of the Mo6+ state58. The BE of nanocomposite is also less than that of pure MoS236. This peak shift is due to the transfer of electrons from the VS2 to the MoS2. The shift amount for both v-2p and S-2p is shown in Fig. 6b and c, respectively. This is the effect of the low work function of VS2, the observed peak shifts indicate electron transfer from VS₂ to MoS₂, in conjunction with the strong interfacial coupling between the two materials48. There is no need for carbon correction because the peak for C-C is exactly at 284.6 eV, fig. S5(C). Individual XPS spectra and high-resolution XPS spectra are shown in Fig. S4 and Fig. S5.

The atomic percentage can be calculated for the area under different oxidation states of the different elements present in a sample. The S-2p area is more in the VS2/MoS2-3 composite as compared to VS2. Same for the S-2p oxidation states. The atomic percentage for each oxidation state is provided in the supporting information Table S2 and Table S3 for VS2 and VS2/MoS2-3 composite. The oxidation states identified by XPS directly correlate with the antibacterial performance, indicating that the surface oxidation states govern the generation of reactive species responsible for bacterial inhibition. It is reported in a journal that Vanadium serves as the primary catalytic site for peroxymonosulfate (PMS) activation, where the generation of reactive oxygen species(ROS) originates from the redox transition among V5+, V4+, and V3+.XPS analysis confirmed the V4+/V5+ redox cycling, which plays a decisive role in radical generation and thereby enables efficient degradation of organic pollutants in the VS4/PMS system59.Nano-VOx films have been shown to display strong antibacterial activity, particularly against S.aureus, with the effect intensifying at higher vanadium oxidation states due to enhanced intracellular ROS generation60.Upon ultrasound stimulation, VS4 generated abundant electrons that were efficiently captured by Mxene, resulting in enhanced electron-hole separation. This process promoted the production of multiple ROS species and imparted strong antibacterial activity, particularly through the POD-like catalytic generation of -OH by VS461.Mo@ZIF-8 nanozymes, synthesized by refluxing Na₂MoO₄ with ZIF-8 and pyrolyzing at 600 °C, exhibited peroxidase-like •OH generation with broad antibacterial activity against E. coli and S. aureus, demonstrating strong potential as biocompatible antibacterial agents62. Thus, the oxidation states V4+ and Mo4+ identified by XPS play a crucial role in generating reactive species, which in turn enhance the inhibition of bacterial growth against E.coli, and S.aureus.

Optical study

A straightforward technique for determining the optical characteristics of the powdered crystalline materials is demonstrated using diffuse reflectance spectra (DRS). The energy gap between the valence band and the conduction band, where electrons can move, is referred to as the band gap63. Diffuse reflection is the phenomenon that arises from the reflection, refraction, diffraction, and absorption of particles oriented in all directions64. Kubelka-Munk theory is valid at the condition where the particle size is equivalent to or lower than the wavelength of the incident ray. Figure 7(a) represents the reflectance plot of the samples. The sample’s thickness does not affect reflectance when it falls within the appropriate range. The Kubelka-Munk is written for any wavelength as65.

$$\:\frac{K}{S}\:=\frac{{(1-{R}_{\infty\:})}^{2}}{2{R}_{\infty\:}}=\text{F}\left(\text{R}{\infty\:}\right)$$
(4)

Here, R∞ is the diffuse reflectance, and f(R∞) is the Kubelka-Munk function. K as the absorption coefficient and S as the scattering coefficient. Absorption coefficient (α) and band gap (Eg) are related by the famous Tauc relation63. given by

$$\alpha {\text{h}}\nu ={\text{C }}{({\text{h}}\nu - {{\text{E}}_{\text{g}}})^{\text{d}}}$$
(5)

Here, the value of C is significantly influenced by transition probability. At the same time, ‘ν’ is the frequency of the incident light, and ‘h’ represents the Planck constant. Here, ‘m’ is related to the transition mode power factor. The ‘d’ value is different for various types of electronic transition, like d= ½ or 2 for direct and indirect allowed transitions. The absorption coefficient becomes twice for the perfect diffuse scattering of incident radiation. At this condition, scattering coefficient “s” remains constant with wavelength. Applying Eq. (5) for direct allowed transition, it can be written as64,65

$${[{\text{F}}({{\text{R}}_\infty }){\text{h}}\nu ]^{\text{2}}}={\text{ A }}({\text{h}}\nu - {{\text{E}}_{\text{g}}})$$
(6)

Now Eg calculation was done by plotting [F(R∞)hν]2 vs. hν. The evaluated values are listed in Table 2.

Table 2 Estimated optical parameter of the VS2 and VS2/MoS2 composites.
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Fig. 7
figure 7

(a) Reflectance spectra, (b) [F(R∞)hν]2 vs. hν plot, (c) Variation of theoretically calculated n with Eg of VS2 and VS2/MoS2 nanocomposites.

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Figure 7.b confirms that the band gap of the prepared sample is increasing with an increased amount of molybdenum concentration. The energy gap and refractive index are two key parameters that determine semiconductors’ optical and electrical behavior. The photon absorption threshold of the energy gap is determined by a semiconductor, while the transparency of the incident photon is measured by the refractive index66. The refractive index of materials can be evaluated through different approaches, including spectroscopic ellipsometry67, prism coupling68, refractometry69, and UV-is spectroscopy. Among these, spectroscopic ellipsometry is generally regarded as the most precise and widely accepted method for thin films and smooth bulk substrates, as it yields both the real and imaginary components of the refractive index with high accuracy. Nevertheless, this technique cannot be applied to powdered samples, since their surface irregularities and intense diffuse scattering hinder reliable polarization-based measurements.

For the hydrothermally synthesized VS2/MoS2 nanocomposite obtained in powder form, UV-Vis diffuse reflectance spectroscopy(DRS) serves as the most appropriate technique. The reflectance spectra can be converted into the Kubelka-Munk function, and in combination with Tauc analysis, the optical bandgap can be estimated. Using this bandgap, the refractive index may be approximated through an empirical relation such as the Dimitrov-Sakka or Herve-Vandamme models. Although these estimates are indirect and less accurate compared to ellipsometry, they remain suitable for nanocomposite powders, where DRS provides the most dependable means of optical characterization. Therefore, while ellipsometry stands as the reference method for flat films and bulk samples, DRS-based evaluation offers the most credible pathway for refractive index determination in the present composite system. There are many theories for calculating the refractive index; some of the methods are listed below. According to the Moss relation70, the refractive index (n) and energy gap (Eg) are related by the Eq.

$$\begin{gathered} E_{g} \:n_{M} ^{4} = K \hfill \\ n_{M} = \sqrt[4]{{\frac{{95}}{{E_{g} }}}} \hfill \\ \end{gathered}$$
(7)

Here, the K value is 95 eV. The Moss relation is valid within the 0.5 to 3.67 eV energy range. Another well-known relation is Ravindra’s relation71

$$\:{n}_{R}\:=\:4.084\:\--\:\left[\:0.62\:\text{*}\:\text{E}\text{g}\right],$$
(8)

where the ‘n’ and ‘Eg’ are linearly connected. This model deals with the band gap energy 1.50 eV < Eg<3.50 eV. Using oscillator theory, Herve and Vandamme developed a relation between band gap and UV resonance energy, which gives the materials a low optical energy gap72. The model works between the energy range 2.00 eV < Eg<4.00 eV. It is the redefined Moss rule that is suitable for low optical energy gap materials. It is expressed as:

$$\:{\:\:\:\:\:\:\:n}^{2}=1+{\:\left(\frac{A}{{E}_{g\:\:}+B}\right)}^{2}$$
(9)

The value of A = 13.6 eV and B = 3.74 eV, with A representing the hydrogen ionization energy. This equation is expressed by:

$$\:{n}_{\left[H\nu\:\right]}=\sqrt{1+{\left(\frac{13.6}{{E}_{g}+3.47}\right)}^{2}}$$
(10)

Tripathy also put forth a different ‘n’ and ‘Eg’ relationship, where he showed that the ‘n’ and ‘Eg’ are exponentially related73

$$\:{n}_{T}=1.73\times\:[1+1.9017\:\times\:\:{e}^{-0.539\times\:{E}_{g}}]$$
(11)

Another way of calculating the band gap is the Dimitrov and Sakka relation74, given by the Eq.

$$\:\frac{{{n}_{DS}}^{2}-1}{{{n}_{DS}}^{2}+2}=1-\sqrt{\frac{{E}_{g}}{20}}$$
(12)

The calculated values of the refractive index are listed in Table 2. The refractive index value in the higher wavelength region is higher than in the lower wavelength region, as seen in Fig. 7 c72,75,76. The blue shift in the absorption spectra increases the band gap of the material from VS2 to higher concentrations of molybdenum gradually. This is due to the quantum confinement effect77. This is also confirmed from the XRD data that the crystallite size of the prepared material gradually decreases with a higher amount of molybdenum concentration. The prepared material can be used in solar cells, photodetectors, and waveguides, as it has a lower band gap calculated from DRS data due to the high refractive index78.

Antimicrobial properties

The procured result showed that there is a sequential reduction in turbidity of both gram-negative and gram-positive bacterial strains with an increase in the concentration of the test material, which is shown in the Figs. 8 and 9 respectively. There is a visible turbidity bacterial culture incubated along with DMF, which suggests that DMF is not meddlesome in bacterial growth. This implies that the test material can significantly inhibit the growth of both gram-positive and gram-negative bacteria.

Fig. 8
figure 8

Evaluation of MIC of material against gram-negative bacterial strain (E. coli) (A) Negative control, (B) positive control, (C) bacterial culture along with DMF (D–F) bacterial strain along with the test material at different concentration i.e. 25 mg/mL, 50 mg/mL, 100 mg/mL respectively.

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Fig. 9
figure 9

Evaluation of MIC of material against gram-positive bacterial strain (S. aureus) (A) Negative control, (B) positive control, (C) bacterial culture along with DMF (D–F) bacterial strain along with the test material at different concentrations i.e. 25 mg/mL, 50 mg/mL, 100 mg/mL, respectively.

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The result obtained from the disk diffusion method showed that the material has a significant antibacterial property against both gram-negative (E. coli) and gram-positive (S. aureus) bacteria. The inhibition zone diameter obtained against E. coli and S. aureus is 12 mm and 11 mm, respectively. It shows that the inhibition zone against E. coli is more prominent than S. aureus, which implies that the material has better antibacterial efficacy against gram-negative bacteria than gram-positive bacteria. The disk diffusion method also shows that the material is easily diffused over the agar plate to inhibit the growth of bacteria. Figure 10. illustrates the evaluation of antimicrobial properties of the material in both gram-negative and gram-positive strains, the related inhibition zone against (a) S. aureus, (ATCC 25923), (b) E. coli, (ATCC 25922), whereas 1 & 3 are positive control impregnated disks with DMF, and 2, 4 are disks impregnated with material dissolved in DMF.

Fig. 10
figure 10

Evaluation of antimicrobial properties of the material in both gram-positive and gram-negative strains, the related inhibition zone against (a) S. aureus, (ATCC 25923), (b) E. coli, (ATCC 25922), whereas 1 & 3 are positive control impregnated disks with DMF, and 2 & 4 are disks impregnated with material dissolved in DMF.

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In this manuscript, the primary method selected was the broth dilution method to evaluate the antibacterial efficacy of the material against both gram-negative (ATCC-25922, E. coli) and gram-positive (ATCC-25923, S. aureus) bacteria. In this particular method, three different concentrations of material were taken to evaluate which concentration has better efficacy against both, where all the concentrations that were taken are effective, significantly inhibiting the growth. For further confirmation of the antibacterial activity of the synthesized MoS2/VS2 nanocomposite, the disk diffusion method was adopted. In this process, a sterile disk was impregnated with material and then applied to a bacterial culture to observe the sensitivity of the bacteria towards the nanocomposite.

There are several methods being used for the antibacterial assay, such as flow cytometry, ATP bioluminescence assay, and time kill assay, which is better for real-time monitoring, but here we selected the broth dilution and disk diffusion method for their broad acceptability. These are conventional antibacterial assays for initial screening of antibacterial efficacy in a microbiology laboratory. The diffusion capacity of MoS2/VS2-3 nanocomposite and its sensitivity towards bacteria were well observed in the disk diffusion method. In this study, MoS2/VS2-3 nanocomposite has been used as the sole material for antibacterial assay; therefore, the comparison focused on the effect of the material on gram-negative and gram-positive bacteria rather than different samples.

Conclusion

The experimental data revealed the formation of VS2/MoS2 nanocomposite by hydrothermal synthesis. The hexagonal phases of both VS2 and VS2/MoS2 were identified from the XRD analysis, with decreased crystallite size from 4.6 to 4.29 with the increased Mo concentration. The micro strain increased from 36.77 to 38.09 with an increase in Mo content for VS2/MoS2 nanocomposites. The surface morphology revealed the flower-like structure for VS2/MoS2 with a high surface area and a nanosheet-like structure for VS2. Raman spectroscopy identified the vibrational modes of V-S and Mo-S bonds. The bandgap value increased from 1.08 eV to 1.25 eV, and the refractive index decreased from 3.29 to 3.16 with an increase in Mo content as revealed in the optical study. The XPS peak shift is due to the transfer of electrons from VS2 to MoS2. The various oxidation states of the elements were identified from the XPS data. The antimicrobial activity analyzed through the disk diffusion method on gram-positive and gram-negative bacteria revealed the superior performance of gram-negative bacteria with better efficacy over gram-positive ones. Such antimicrobial findings are suitable for various biological applications as well.