Synthesis, Characterization, Photocatalysis, and Antibacterial Study of WO3, MXene and WO3/MXene Nanocomposite

Tungsten oxide (WO3), MXene, and an WO3/MXene nanocomposite were synthesized to study their photocatalytic and biological applications. Tungsten oxide was synthesized by an easy and cost-effective hydrothermal method, and its composite with MXene was prepared through the sonication method. The synthesized tungsten oxide, MXene, and its composite were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR), energy-dispersive X-ray analysis (EDX), and Brunauer–Emmett–Teller (BET) for their structural, morphological, spectral, elemental and surface area analysis, respectively. The crystallite size of WO3 calculated from XRD was ~10 nm, the particle size of WO3 was 130 nm, and the average thickness of MXene layers was 175 nm, which was calculated from FESEM. The photocatalytic activity of as-synthesized samples was carried out for the degradation of methylene blue under solar radiation, MXene, the WO3/MXene composite, and WO3 exhibited 54%, 89%, and 99% photocatalytic degradation, respectively. WO3 showed maximal degradation ability; by adding WO3 to MXene, the degradation ability of MXene was enhanced. Studies on antibacterial activity demonstrated that these samples are good antibacterial agents against positive strains, and their antibacterial activity against negative strains depends upon their concentration. Against positive strains, the WO3/MXene composite’s inhibition zone was at 7 mm, while it became 9 mm upon increasing the concentration. This study proves that WO3, MXene, and the WO3/MXene nanocomposite could be used in biological and environmental applications.


Introduction
In the last few decades, environmental remediation technologies have been the most challenging for effective and efficient water cleaning, primarily through the photocatalytic method [1][2][3][4][5][6]. Such catalysts are cost-effective and have a suitable energy and electronic structure. Minimal amounts of contaminants such as phenols, textile dyes, and poly chlorinated biphenyls (PCBs) not only pollute the water, but also reduce the growth of aqueous organisms [7]. For the removal of such pollutants from water, various physical and chemical methods were reported [8,9]. However, these methods are either expensive or suitable for large amounts of contaminants [10]. So, the photocatalytic degradation of organic contaminants has gained much attention due to its efficiency and cost-effectiveness [7,11].

Synthesis of MXene
MAX powder (Ti 3 AlC 2 ) was used to prepare the MXene with the Ti 3 C 2 T x formula in a 50 mL Teflon vessel. For this purpose, Al was etched by using an HF solution. For the preparation of MXene, 10 mL HF was poured inside the Teflon vessel and then placed in a fume hood. Then, 0.5 g of MAX powder was slowly added into the HF solution pinch by pinch. Then, the whole mixture was stirred magnetically at room temperature for about 24 h for maximal etching. DI water was added to the resultant product for dilution, and multilayered MXene was obtained by centrifugation at 5000 rpm. The washing of these precipitates was repeated continuously until its pH became 6. The vacuum filtration of the aqueous dispersion was carried out by using a PTFE membrane. The filtrate containing Nanomaterials 2022, 12, 713 3 of 19 Ti 3 C 2 T x was then freeze-dried for 24 h. Schematic illustration for preparation of MXene is shown in Figure 1.  24 h for maximal etching. DI water was added to the resultant product for dilution, and multilayered MXene was obtained by centrifugation at 5000 rpm. The washing of these precipitates was repeated continuously until its pH became 6. The vacuum filtration of the aqueous dispersion was carried out by using a PTFE membrane. The filtrate containing Ti3C2Tx was then freeze-dried for 24 h. Schematic illustration for preparation of MXene is shown in Figure 1.

Synthesis of Tungsten Oxide (WO3)
2.5 g of sodium tungstate and 3.0 g of sodium sulfate were dissolved in 80 mL of distilled water. A 3M HCl solution was added dropwise to the clear solution under continuous stirring, and the pH of the solution was set to 1.5. After 10 min of stirring, the mixture was transferred into a Teflon-lined stainless-steel autoclave and was kept at 180 °C for 48 h. After that, the product was collected by centrifugation at 4500 rpm, and washed with distilled water and ethanol to obtain neutral solution; then, the product was obtained by drying at 60 °C in air.

Synthesis of WO3/MXene Composites
The composite of WO3/MXene (1:1) was fabricated by sonication method. Then, 2 g of MXene was added in 50 mL of water and sonicated for 3 h. Afterwards, 2 g of tungsten oxide was added to it, again sonicated for 2 h, and then dried in an oven. Synthesis of WO3 and WO3/MXene composites is shown in Figure 2.

Synthesis of Tungsten Oxide (WO 3 )
2.5 g of sodium tungstate and 3.0 g of sodium sulfate were dissolved in 80 mL of distilled water. A 3M HCl solution was added dropwise to the clear solution under continuous stirring, and the pH of the solution was set to 1.5. After 10 min of stirring, the mixture was transferred into a Teflon-lined stainless-steel autoclave and was kept at 180 • C for 48 h. After that, the product was collected by centrifugation at 4500 rpm, and washed with distilled water and ethanol to obtain neutral solution; then, the product was obtained by drying at 60 • C in air.

Synthesis of WO 3 /MXene Composites
The composite of WO 3 /MXene (1:1) was fabricated by sonication method. Then, 2 g of MXene was added in 50 mL of water and sonicated for 3 h. Afterwards, 2 g of tungsten oxide was added to it, again sonicated for 2 h, and then dried in an oven. Synthesis of WO 3 and WO 3 /MXene composites is shown in Figure 2.

Characterization
An XRD diffractometer using Cu Kα radiation (λ = 1.54 Å) as a light source, at a scan rate of 30 min by applying a voltage of 40 kV, was used for the structural and phase analysis of the as-synthesized samples. ZEISS LEO SUPRA 55 field emission scanning electron microscope and JEOL JCM-6000Plus SEM were used for morphological characterization and elemental analysis, respectively. Functional group analysis and the surface properties of the as-synthesized samples were measured by Fourier transform infrared spectroscopy (FTIR). For the measurement of the BET surface areas, nitrogen adsorption-desorption was conducted by flowing liquid nitrogen at 77 K (−196 • C) by using the Micromeritics ASAP 2020 Physisorption analyzer.

Characterization
An XRD diffractometer using Cu Kα radiation (λ = 1.54 Å) as a light source, at a scan rate of 30 min by applying a voltage of 40 kV, was used for the structural and phase analysis of the as-synthesized samples. ZEISS LEO SUPRA 55 field emission scanning electron microscope and JEOL JCM-6000Plus SEM were used for morphological characterization and elemental analysis, respectively. Functional group analysis and the surface properties of the as-synthesized samples were measured by Fourier transform infrared spectroscopy (FTIR). For the measurement of the BET surface areas, nitrogen adsorption-desorption was conducted by flowing liquid nitrogen at 77 K (−196 °C) by using the Micromeritics ASAP 2020 Physisorption analyzer.

Photocatalytic Degradation
WO3, MXene, and the WO3/MXene nanocomposite were used as a photocatalyst to measure the photocatalytic degradation of methylene blue in the presence of solar radiation for 80 min. For these measurements, 100 mL of 5 ppm methylene blue solution was poured in a beaker, and 5 mg of photocatalyst was added into the solution. It was then stirred continuously for 60 min in the dark. Adsorption-desorption equilibrium could thus be achieved between methylene blue and photocatalyst. The solution was then placed in solar light with constant stirring. In order to measure the degradation percentage of methylene blue, 5 mL of a solution containing both dye and sample was taken after every 10 min, and a UV-vis spectrophotometer was used to measure the degradation efficiency of the samples [5,48].
The degradation percentage of the as-synthesized samples was measured by using following equation: where, Ct is the concentration of the solution at time t, and Co is the concentration of the solution at time zero.

Antibacterial Activity
The disc diffusion method was utilized to study the antibacterial activity of WO3, MXene, and WO3/MXene nanocomposite. Staphylococcus aureus (S. aureus) was used as a

Photocatalytic Degradation
WO 3 , MXene, and the WO 3 /MXene nanocomposite were used as a photocatalyst to measure the photocatalytic degradation of methylene blue in the presence of solar radiation for 80 min. For these measurements, 100 mL of 5 ppm methylene blue solution was poured in a beaker, and 5 mg of photocatalyst was added into the solution. It was then stirred continuously for 60 min in the dark. Adsorption-desorption equilibrium could thus be achieved between methylene blue and photocatalyst. The solution was then placed in solar light with constant stirring. In order to measure the degradation percentage of methylene blue, 5 mL of a solution containing both dye and sample was taken after every 10 min, and a UV-vis spectrophotometer was used to measure the degradation efficiency of the samples [5,48].
The degradation percentage of the as-synthesized samples was measured by using following equation: where, C t is the concentration of the solution at time t, and C 0 is the concentration of the solution at time zero.

Antibacterial Activity
The disc diffusion method was utilized to study the antibacterial activity of WO 3 , MXene, and WO 3 /MXene nanocomposite. Staphylococcus aureus (S. aureus) was used as a positive strain, and Escherichia coli (E. coli), Klebsiella pneumonia (K. pneumonia) and Proteus vulgaris (P. vulgaris) were used as negative strains. For standard/positive control, an antimicrobial agent (ciprofloxacin) was used. First, the aqueous solution of the as-prepared samples was prepared by sonicating the samples with distilled water. Then, they were placed on the corners of a nutrient agar plate with the use of forceps. After incubating the samples for 24 h at 37 • C, the zone of inhibition could be seen on the edges of the agar plate. The formation of these zones of inhibition confirmed the antibacterial activity, while the lack of these zones of inhibition showed no antibacterial activity. The mm units were used for the measurement of these inhibition zones.
By using the Debye-Scherer equation, the crystallite size of the as-fabricated tungsten oxide was calculated [58].
where D is the crystalline size; K is the Scherer constant; λ is the X-ray wavelength of the copper source used in XRD, which was equal to 1.5406 Å; Bragg's angle was given by θ; and β represents full width at half maximum (FWHM) [59]. The crystalline size of WO3 nanoparticles, determined by XRD, was 6.19 nm. The measurement of the crystalline size  [52,53]. Due to the presence of the Al, the pure MAX powder showed a characteristic peak at 2θ = 38.61 • , which corresponds to (104). Al was completely etched by using HF in order to fabricate good-quality MXene [54,55]. During the first 2 h of the reaction, the peak intensity at 38.61 • increased [56]. After 24 h of the reaction, the characteristic peak of MAX at (104) vanished, as shown in Figure 3a. A peak shift was also observed in the peak at 9.11 • [57].
By using the Debye-Scherer equation, the crystallite size of the as-fabricated tungsten oxide was calculated [58].
where D is the crystalline size; K is the Scherer constant; λ is the X-ray wavelength of the copper source used in XRD, which was equal to 1.5406 Å; Bragg's angle was given by θ; and β represents full width at half maximum (FWHM) [59]. The crystalline size of WO 3 nanoparticles, determined by XRD, was 6.19 nm. The measurement of the crystalline size of MXene was not possible by using the Debye-Scherer formula because MXene is a 2D layered material.

FESEM and EDX Analysis
For FESEM analysis, the samples were gold-sputtered for 120 s at 15 mA before imaging. Figure 4a,b show the morphology of WO 3 and WO 3 /MXene nanocomposite, respectively. Figure 4a demonstrates the block-/rodlike morphology of WO 3 . Figure 4b clearly shows that MXene was impregnated on the nanorods of WO 3 . The nanosheet-like structure in Figure 4c represents the formation of MXene. The particle size of WO 3 was 130 nm, which was calculated from the FESEM image. The average layer thickness of MXene calculated from micrograph was~175 nm.

FESEM and EDX Analysis
For FESEM analysis, the samples were gold-sputtered for 120 s at 15 mA before imaging. Figure 4a,b show the morphology of WO3 and WO3/MXene nanocomposite, respectively. Figure 4a demonstrates the block-/rodlike morphology of WO3. Figure 4b clearly shows that MXene was impregnated on the nanorods of WO3. The nanosheet-like structure in Figure 4c represents the formation of MXene. The particle size of WO3 was ~130 nm, which was calculated from the FESEM image. The average layer thickness of MXene calculated from micrograph was ~175 nm. Energy-dispersive X-ray analysis (EDX) was used for the elemental analysis of the synthesized material. Figure 5a,b show the elemental composition of WO3 and WO3/MXene composites, respectively, which confirmed the purity of the as-synthesized samples. Energy-dispersive X-ray analysis (EDX) was used for the elemental analysis of the synthesized material. Figure 5a,b show the elemental composition of WO 3 and WO 3 /MXene composites, respectively, which confirmed the purity of the as-synthesized samples.

FTIR
FTIR spectroscopy was used for the spectral analysis of the samples, which indicates the composition of synthesized products. Figure 6 shows the FTIR spectra of MXene, WO3 and WO3/MXene nanocomposite. In the case of MXene, the absorption band present at around 3545 cm −1 was attributed to the absorbed water, which was due to the hydrophilic nature of MXene [60]. The bands present in the range of 2000-2500 cm -1 showed a methyl/methylene group (-CH3, CH2). The signals at 603 and 1529 cm −1 were characteristic of Ti-O and C-F, respectively. The FTIR spectrum of WO3 featured characteristics bands of W-O-W and W-O at around 735 and 836 cm −1 [49]. The spectrum of the WO3/MXene nanocomposite showed the absorption bands of both MXene and WO3.

BET Measurements
Average particle size, BET surface area, total pore volume, and average pore width were determined from nitrogen adsorption-desorption curves ( Figure 7) and their values are given in Table 1. From the BET results, it was predicted that the formation of the

FTIR
FTIR spectroscopy was used for the spectral analysis of the samples, which indicates the composition of synthesized products. Figure 6 shows the FTIR spectra of MXene, WO 3 and WO 3 /MXene nanocomposite. In the case of MXene, the absorption band present at around 3545 cm −1 was attributed to the absorbed water, which was due to the hydrophilic nature of MXene [60]. The bands present in the range of 2000-2500 cm -1 showed a methyl/methylene group (-CH 3 , CH 2 ). The signals at 603 and 1529 cm −1 were characteristic of Ti-O and C-F, respectively. The FTIR spectrum of WO 3 featured characteristics bands of W-O-W and W-O at around 735 and 836 cm −1 [49]. The spectrum of the WO 3 /MXene nanocomposite showed the absorption bands of both MXene and WO 3 .

FTIR
FTIR spectroscopy was used for the spectral analysis of the samples, which indicates the composition of synthesized products. Figure 6 shows the FTIR spectra of MXene, WO3 and WO3/MXene nanocomposite. In the case of MXene, the absorption band present at around 3545 cm −1 was attributed to the absorbed water, which was due to the hydrophilic nature of MXene [60].

BET Measurements
Average particle size, BET surface area, total pore volume, and average pore width were determined from nitrogen adsorption-desorption curves (Figure 7) and their values are given in Table 1. From the BET results, it was predicted that the formation of the

BET Measurements
Average particle size, BET surface area, total pore volume, and average pore width were determined from nitrogen adsorption-desorption curves ( Figure 7) and their values are given in Table 1. From the BET results, it was predicted that the formation of the composite of WO 3 with MXene would result in increased surface area and enhanced average pore width, while average particle size was reduced. The reason behind this is the 2D layer structure of MXene, which offers a greater surface area. However, the photocatalytic activity of WO 3 was higher than that of the composite because MXene only enhanced the surface area, but this increased surface area had no effect on the degradation of dyes because the adsorption capacity and band gap of MXene were much less, due to which charge separation was not effective.
composite of WO3 with MXene would result in increased surface area and enhanced average pore width, while average particle size was reduced. The reason behind this is the 2D layer structure of MXene, which offers a greater surface area. However, the photocatalytic activity of WO3 was higher than that of the composite because MXene only enhanced the surface area, but this increased surface area had no effect on the degradation of dyes because the adsorption capacity and band gap of MXene were much less, due to which charge separation was not effective.

Photocatalysis
The photocatalytic activity of WO3, MXene, and the WO3/MXene nanocomposite was measured for the degradation of methylene blue under solar radiation for 80 min. The initial concentration of methylene blue was determined by measuring the blank absorption of the dye solution. For the achievement of adsorption-desorption equilibrium between photocatalyst and methylene blue, the solution was placed in the dark for 1 h with continuous stirring. The solution containing both methylene blue and sample was then kept under solar radiation. By taking 5 mL solution after regular intervals, the degradation of the dye was measured by using a UV-vis spectrophotometer [61].
The absorption spectra of methylene blue using WO3, MXene, and the WO3/MXene nanocomposite as photocatalyst are shown in Figure 8a-c). For the description of the experimental data given in Figure 9, a pseudo-first-order model was utilized, and the values of K measured by this model were 0.05682, −0.0084, and 0.0346 for WO3, MXene, and the WO3/MXene nanocomposite, respectively.

Photocatalysis
The photocatalytic activity of WO 3 , MXene, and the WO 3 /MXene nanocomposite was measured for the degradation of methylene blue under solar radiation for 80 min. The initial concentration of methylene blue was determined by measuring the blank absorption of the dye solution. For the achievement of adsorption-desorption equilibrium between photocatalyst and methylene blue, the solution was placed in the dark for 1 h with continuous stirring. The solution containing both methylene blue and sample was then kept under solar radiation. By taking 5 mL solution after regular intervals, the degradation of the dye was measured by using a UV-vis spectrophotometer [61].
The absorption spectra of methylene blue using WO 3 , MXene, and the WO 3 /MXene nanocomposite as photocatalyst are shown in Figure 8a-c). For the description of the experimental data given in Figure 9, a pseudo-first-order model was utilized, and the values of K measured by this model were 0.05682, −0.0084, and 0.0346 for WO 3 , MXene, and the WO 3 /MXene nanocomposite, respectively. Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 20     Figure 10 demonstrates the removal efficiency of WO 3 , MXene, and the WO 3 /MXene nanocomposite. WO 3 showed higher degradation ability as compared to that of MXene and the WO 3 /MXene composite. The reason behind this high photocatalytic activity is the greater band gap of WO 3 , which allowed for them to absorb a wide-spectrum range of sunlight and degrade the dye solution with this solar energy. MXene exhibited very low removal efficiency, while the degradation ability of WO 3 /MXene composite was between those of WO 3 and MXene. MXene is a 2D material that acts as a supporting material. WO 3 is material that involves the generation of photo produced electrons and holes. MXene merely increases the surface area and reduces the chances of recombination of these photogenerated electrons and holes. Figure 11 shows the comparison of the degradation percentage of methylene blue by WO 3 , MXene, and the WO 3 /MXene nanocomposite. Nanomaterials 2022, 12, x FOR PEER REVIEW 11 of 20 Figure 10 demonstrates the removal efficiency of WO3, MXene, and the WO3/MXene nanocomposite. WO3 showed higher degradation ability as compared to that of MXene and the WO3/MXene composite. The reason behind this high photocatalytic activity is the greater band gap of WO3, which allowed for them to absorb a wide-spectrum range of sunlight and degrade the dye solution with this solar energy. MXene exhibited very low removal efficiency, while the degradation ability of WO3/MXene composite was between those of WO3 and MXene. MXene is a 2D material that acts as a supporting material. WO3 is material that involves the generation of photo produced electrons and holes. MXene merely increases the surface area and reduces the chances of recombination of these photogenerated electrons and holes. Figure 11 shows the comparison of the degradation percentage of methylene blue by WO3, MXene, and the WO3/MXene nanocomposite.

Mechanism
An emerging degradation technology that leads to the removal of most contaminants is heterogeneous photocatalysis [44]. The comparison of current reported catalysts with already reported similar materials is given in Table 2.  Figure 10 demonstrates the removal efficiency of WO3, MXene, and the WO3/MXene nanocomposite. WO3 showed higher degradation ability as compared to that of MXene and the WO3/MXene composite. The reason behind this high photocatalytic activity is the greater band gap of WO3, which allowed for them to absorb a wide-spectrum range of sunlight and degrade the dye solution with this solar energy. MXene exhibited very low removal efficiency, while the degradation ability of WO3/MXene composite was between those of WO3 and MXene. MXene is a 2D material that acts as a supporting material. WO3 is material that involves the generation of photo produced electrons and holes. MXene merely increases the surface area and reduces the chances of recombination of these photogenerated electrons and holes. Figure 11 shows the comparison of the degradation percentage of methylene blue by WO3, MXene, and the WO3/MXene nanocomposite.

Mechanism
An emerging degradation technology that leads to the removal of most contaminants is heterogeneous photocatalysis [44]. The comparison of current reported catalysts with already reported similar materials is given in Table 2.

Mechanism
An emerging degradation technology that leads to the removal of most contaminants is heterogeneous photocatalysis [44]. The comparison of current reported catalysts with already reported similar materials is given in Table 2. The proposed mechanism involved in photocatalytic degradation consists of the following steps [74] and also depicted in Figure 12: 1.
Efficient photons from sunlight are absorbed by WO 3 :

2.
Ion sorption of oxygen takes place (start of oxygen reduction where the oxidation state of oxygen changes from 0 to −1/2). 3.
Photogenerated holes neutralize the -OH group and produce OH • radicals.

4.
Protons neutralize the O
Oxygen is reduced for the second time, and the decomposition of H 2 O 2 occurs:

7.
OH • radical attacks the organic pollutant (dye) and ultimately causes its oxidation: 8. Direct oxidation takes place when it reacts with holes: Nanomaterials 2022, 12, x FOR PEER REVIEW 13 of 20 7. OH o radical attacks the organic pollutant (dye) and ultimately causes its oxidation: 8. Direct oxidation takes place when it reacts with holes: Figure 12. Z-scheme mechanism for the photocatalytic activity of WO3.

Antibacterial Activity
Among metal oxide nanoparticles, ZnO is a competitive candidate for the study of antibacterial activity. Recent studies showed that ZnO nanoparticles could activate endoplasmic reticulum stress and ultimately kill mammalian cells [75]. Therefore, scientists have been striving to explore new nano-antibacterial agents with better compatibility. Recently, WO3-x was verified to exhibit good biocompatibility and antibacterial activity [76]. In the current study, WO3, MXene, and the WO3/MXene nanocomposite were used as antibacterial agents for the study of antibacterial activity (Figures 13-16). Table 3 shows the zones of inhibition of WO3, MXene, and the WO3/MXene nanocomposite. The disc diffusion method was utilized to measure the inhibition zones of the as-prepared samples, and various positive strains (S. aureus) and negative strains E. coli, K. pneumonia and P. vulgaris were used for antibacterial activity measurements. Due to the structural differences of cell membranes and cell walls, the as-synthesized samples exhibited different sensitivity levels towards the positive and negative strains [77,78]. Table 3 shows that, with the positive strain (S. aureus), all samples showed good antibacterial activity, which increased with the increase in concentration. In the case of negative strains, all samples were active against K. pneumoniae, and the WO3/MXene composite showed good activity at a low concentration. When the concentration of MXene and WO3 increased, activity also increased. The

Antibacterial Activity
Among metal oxide nanoparticles, ZnO is a competitive candidate for the study of antibacterial activity. Recent studies showed that ZnO nanoparticles could activate endoplasmic reticulum stress and ultimately kill mammalian cells [75]. Therefore, scientists have been striving to explore new nano-antibacterial agents with better compatibility. Recently, WO 3−x was verified to exhibit good biocompatibility and antibacterial activity [76]. In the current study, WO 3 , MXene, and the WO 3 /MXene nanocomposite were used as antibacterial agents for the study of antibacterial activity (Figures 13-16). Table 3 shows the zones of inhibition of WO 3 , MXene, and the WO 3 /MXene nanocomposite. The disc diffusion method was utilized to measure the inhibition zones of the as-prepared samples, and various positive strains (S. aureus) and negative strains E. coli, K. pneumonia and P. vulgaris were used for antibacterial activity measurements. Due to the structural differences of cell membranes and cell walls, the as-synthesized samples exhibited different sensitivity levels towards the positive and negative strains [77,78]. Table 3 shows that, with the positive strain (S. aureus), all samples showed good antibacterial activity, which increased with the increase in concentration. In the case of negative strains, all samples were active against K. pneumoniae, and the WO 3 /MXene composite showed good activity at a low concentration. When the concentration of MXene and WO 3 increased, activity also increased. The WO 3 /MXene nanocomposite showed no activity against E. coli and P. vulgaris, while WO 3 and MXene exhibited good antibacterial activity, which was enhanced on the increase in concentration. The reason behind the low or zero antibacterial activity of the WO 3 /MXene composite against negative strains was the presence of an extra outer membrane that increased the resistance of Gram-negative strains to WO 3 /MXene. The WO 3 /MXene nanocomposite showed a decrease in antibacterial activity on an increase in concentration due to certain factors such as size and agglomeration. Due to these factors, these nanocomposites were not able to penetrate the bacterial cell wall; hence, its toxicity decreased. On the other hand, the pristine WO 3 and MXene showed an increase in antibacterial activity on increasing concentration.
creased the resistance of Gram-negative strains to WO3/MXene. The WO3/MXene nanocomposite showed a decrease in antibacterial activity on an increase in concentration due to certain factors such as size and agglomeration. Due to these factors, these nanocomposites were not able to penetrate the bacterial cell wall; hence, its toxicity decreased. On the other hand, the pristine WO3 and MXene showed an increase in antibacterial activity on increasing concentration.

Mechanism of Antibacterial Activity
The mechanism involved with the antibacterial activity of the as-synthesized nanoparticles was the cell damage by electrostatic interactions between the cell membrane and metal oxide nanoparticles. The main sites of attraction of metal cations are the chemical groups of polymers on membranes of bacteria that are electronegative in nature. The carboxylic groups present in the proteins are the main reason behind the negative charge on the surface of bacteria. Electrostatic attraction is created due to the charge difference between bacterial membrane and metal oxide nanoparticles; thus, these nanoparticles accumulated on the cell surface and ultimately entered the bacteria. This interaction between membrane polymer and cationic metal oxide nanoparticles resulted in the cytoxicity of microorganisms. The available surface area and ratio of particle size to surface area determine the efficiency of metal oxide nanoparticles in bacterial growth inhibition. The permeability and structure of the cell membrane are changed due to the attachment of metal oxide nanoparticles. The disorganization of cell wall was due to the strong bond between positively charged metal oxide nanoparticles and membrane. Apart from binding with the cell membrane, these metal oxide nanoparticles also bind with mesosomes, resulting in the alteration of cell division, DNA replication, and cellular respiration [79].

Conclusions
In the current work, we prepared WO3, MXene, and a WO3/MXene nanocomposite, which exhibited their potential applications in the biological and environmental remediation fields. WO3, MXene, and the WO3/MXene nanocomposite were synthesized by hydrothermal method, wet chemical etching, and sonication method, respectively. XRD, FTIR, EDX, and FESEM were used to characterize the as-synthesized samples for structural, spectral, elemental, and morphological analysis, respectively. BET analysis was conducted for surface area determination. The photocatalytic degradation of methylene blue using WO3, MXene, and the WO3/MXene nanocomposite was 99%, 54%, and 89%,

Mechanism of Antibacterial Activity
The mechanism involved with the antibacterial activity of the as-synthesized nanoparticles was the cell damage by electrostatic interactions between the cell membrane and metal oxide nanoparticles. The main sites of attraction of metal cations are the chemical groups of polymers on membranes of bacteria that are electronegative in nature. The carboxylic groups present in the proteins are the main reason behind the negative charge on the surface of bacteria. Electrostatic attraction is created due to the charge difference between bacterial membrane and metal oxide nanoparticles; thus, these nanoparticles accumulated on the cell surface and ultimately entered the bacteria. This interaction between membrane polymer and cationic metal oxide nanoparticles resulted in the cytoxicity of microorganisms. The available surface area and ratio of particle size to surface area determine the efficiency of metal oxide nanoparticles in bacterial growth inhibition. The permeability and structure of the cell membrane are changed due to the attachment of metal oxide nanoparticles. The disorganization of cell wall was due to the strong bond between positively charged metal oxide nanoparticles and membrane. Apart from binding with the cell membrane, these metal oxide nanoparticles also bind with mesosomes, resulting in the alteration of cell division, DNA replication, and cellular respiration [79].

Conclusions
In the current work, we prepared WO 3 , MXene, and a WO 3 /MXene nanocomposite, which exhibited their potential applications in the biological and environmental remediation fields. WO 3 , MXene, and the WO 3 /MXene nanocomposite were synthesized by hydrothermal method, wet chemical etching, and sonication method, respectively. XRD, FTIR, EDX, and FESEM were used to characterize the as-synthesized samples for structural, spectral, elemental, and morphological analysis, respectively. BET analysis was conducted for surface area determination. The photocatalytic degradation of methylene blue using WO 3 , MXene, and the WO 3 /MXene nanocomposite was 99%, 54%, and 89%, respectively. The photocatalytic activity of WO 3 was significant. MXene is a 2D material, its photocatalytic activity is very low, and it only acted as supporting material by enhancing the photocatalytic ability of its composite with WO 3 . The as-prepared samples also exhibited good antibacterial activity against positive strain bacteria; in the case of negative strains, WO 3 , MXene, and the WO 3 /MXene nanocomposite exhibited antibacterial activity at high concentrations.