Biomaterial (Garlic and Chitosan)-Doped WO3-TiO2 Hybrid Nanocomposites: Their Solar Light Photocatalytic and Antibacterial Activities.

In this work, WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 nanocomposites were synthesized by the sol-gel and precipitation technique. The synthesized nanocomposites were characterized by XRD, FE-SEM, HR-TEM, EDX, UV-DRS, FT-IR, and TG-DTA analysis. The photocatalytic efficiency of the three synthesized nanocomposites on the degradation of dyes such as rhodamine B (Rh-B), methylene blue (MB), and methyl orange (MO) as organic pollutants was evaluated under solar light irradiation. The results show that garlic-loaded WO3-TiO2 nanocomposites act as an excellent photocatalyst than chitosan-blended WO3-TiO2 and WO3-TiO2 nanocomposites. Further, the antimicrobial activity of the synthesized nanocomposites was examined against Gram-negative bacteria (Escherichia coli) by the well diffusion method. Garlic-loaded WO3-doped TiO2 nanocomposites have demonstrated good antibacterial activity over chitosan-blended WO3-TiO2 nanocomposites and WO3-TiO2 nanocomposites. The possible reason may be the presence of organic sulfur compounds in garlic.

Introduction

Over the past decades, the catalytic degradation method is one of the best ways to prevent water pollution from dyes. The catalytic degradation technique contains a photocatalytic material that acts as a catalyst by absorbing light (ultraviolet or visible) and enhances the photochemical reaction. TiO2, ZnO, CuO, ZrO2, etc., are some of the metal oxides used as photocatalysts. Out of these metal oxides, TiO2[1] and ZnO[2] have been used extensively as photocatalysts. The photocatalyst generates electrons and holes when it absorbs light energy. The electrons of the valence band (VB) get excited and move to the conduction band (CB) through the band gap, creating e– (negative) and h+ (positive) pairs, and the phenomenon is named as the “photoexcitation” state. The positive holes of the photocatalyst cleave the H2O molecules to produce H2 gas and hydroxyl radicals. The electrons react with the O2 molecule to create a superoxide anion and this cyclic process continues until light is available. In most cases, the electrons and positive holes recombine and decrease the photocatalytic efficiency. To overcome these shortcomings, certain metal oxides are added as dopants.[3,4] To increase the photocatalytic efficiency further, hybrid photocatalysts are prepared to employ biomaterials. This hybrid photocatalyst not only increases photocatalytic activity but also acts as good antimicrobial agents.[5] The hybrid materials are potentially applied in many biomedical fields and they could also enhance the mechanical properties of the materials.[6−8]

The WO3-TiO2 nanocomposite has been used as an energy storage photocatalyst, which could store the electrons produced under the illumination of light and release these electrons in the absence of light, which takes place in electron-mediated reactions.[9−12] The band gap of WO3 is 2.8 eV (wavelength of ∼440 nm), which is lower than that of anatase (3.2 eV) so that WO3 shows significant photocatalytic activity due to the enhancement of the ability of visible-light photons.[13] Sun et al. designed spindle-like WO3-TiO2, which proves better photocatalytic activity compared to the single TiO2 and WO3. Crystallinity played a significant role in the photocatalyst activity.[14]

Currently, chitosan-based hybrid materials are applied for various applications. Chitosan is a biomaterial, which is basically a polysaccharide prepared from the deacetylation of chitin. It is a biodegradable, biocompatible, nontoxic, and pH-sensitive polymer offering a wealth of advantages. It is the second-most abundant biopolymer widely present in crustaceans and insects.[15,16] The chitosan structure is similar to that of cellulose. It has an excellent adsorbing capacity, which is around 1100 g kg–1, and is more superior to activated carbon, which is usually used as an adsorbent.[17] The adsorption capacity of chitosan with the contaminants may be due to the presence of amino and hydroxyl groups on the surface. The hybrid photocatalyst prepared by using chitosan and TiO2 has a potential significance in catalytic activity and antimicrobial property. The chitosan adsorbed the organic substrates as a result of electrostatic attraction between the −NH2 groups and solutes. The presence of chelating groups (−NH2 and −OH groups) on the chitosan also increases the chitosan binding ability for metal ions.[18]

A TiO2-garlic hybrid material was used as a photocatalyst, which revealed significant improvement in the photocatalytic activity in the visible region compared to commercially available catalysts like Degussa P25.[19] Garlic (Allium sativum) is a herb containing 33 sulfur compounds, 17 amino acids, glycosides, aginine, selenium, and certain enzymes like allinase, peroxidases, myrosinase, etc.[20] It is used as a flavoring agent for cooking foods and also serves as a medicine to cure certain diseases. When garlic is cut, it is influenced by the presence of an alliinase enzyme, a cysteine sulfoxidelyase, and converts into allicin. Allicin is responsible for the strong smell of garlic, which provides good antioxidant and antibacterial properties.[21] Allicin is the most essential biochemically active component of freshly crushed garlic, which may easily undergo decomposition under the influence of heat and time into stable compounds.[22]

To the best of our knowledge, binding of chitosan with WO3-doped TiO2 and the effect of organic sulfur compounds from garlic such as ajoene, allicin, phenolic, and diallyl sulfide compounds in composition with WO3-doped TiO2 to enhance its activity toward the visible light range have not been researched previously.

Results and Discussion

XRD Analysis

The X-ray diffraction (XRD) patterns of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 hybrid nanocomposites are presented in Figure a–c, respectively, which show the TiO2 anatase phase and WO3 tetragonal phase. For the TiO2 anatase phase, the 2θ values are 25.356°, 37.847°, 48.145°, 53.974°, 55.186°, 62.812°, 68.879°, and 70.812°, which correspond to the (101), (004), (200), (105), (211), (204), (116), and (220) planes, respectively, which are in good agreement with the standard JCPDS pattern (89-4921). The presence of peaks at 2θ = 24.06, 28.12, 33.61, 34.13, 44.73, and 50.15° corresponding to the (001), (111), (201), (220), (221), and (112) planes, respectively, indicates the tetragonal WO3 (89-1287) and their crystal parameters are tetragonal, primitive, a = 7.39 Å, b = 7.39 Å, c = 3.88 Å, α = 90°, β = 90°, and γ = 90°.

Figure 1

XRD patterns of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

The average crystallite sizes of the nanocomposites have been deduced from the full width at half maximum (FWHM) of the 101 anatase peak of TiO2 using the Scherrer equation (eq )where t is the crystallite size, K is the shape factor of a value of 0.9, λ is the wavelength of the X-ray used, θ is Bragg’s diffraction angle, and β is the corrected line broadening, β = βb – βs, where βb is the broadened profile width of the experimental sample and βs is the standard profile width of the reference (high-purity silica) sample. According to eq , the average crystallite sizes of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 are 13, 11, and 9 nm, respectively. It is essential to indicate that the crystallite size decreased gradually by the influence of biomaterials in the synthesized nanocomposites. The transformation of phases (from more stable anatase to rutile) only after growing to a crystallite size above 14 nm, but below 14 nm, of the anatase phase is the most stable one.[25]

Electron Microscopic Studies

Figure a–c shows the field emission scanning electron microscopy (FE-SEM) images of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 hybrid nanocomposites, respectively. The SEM micrographs of the synthesized nanocomposites indicate that the particles have a nonuniform size with a high degree of agglomeration. The agglomerated spherical size of the particle falls between 20 and 40 nm. It was observed that the morphology of the formed nanocomposites was strongly influenced by the hydrolysis agent. The moist air, both at 50 and 100% relative humidity levels, used as the hydrolysis agent, allowed the occurrence of the precursor’s thermal decomposition, resulting in the formation of micrometer-size agglomerates.

Figure 2

FE-SEM images of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

Figure a–c illustrates the high-resolution transmission electron microscopy (HR-TEM) images of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 hybrid nanocomposites, respectively. The HR-TEM images of the nanocomposites reveal a composite network with sizes in the range of 10–50 nm having a mean average particle distribution of 35 nm. ImageJ software is used to measure the average particle size distribution diameter. TEM investigations show that nanocomposites are randomly distributed with a nonuniform particle size.

Figure 3

HR-TEM images of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

EDX Analysis

Energy-dispersive X-ray (EDX) spectroscopy images of the WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 hybrid nanocomposites are shown in Figure a–c, respectively. EDX analysis of the nanocomposites in a TiO2-based matrix reveals peaks corresponding to carbon, oxygen, titanium, and doped metals. The elements present in the nanocomposites are confirmed due to the presence of corresponding peaks and the atomic and weight percentages are tabulated.

Figure 4

EDX spectra of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

UV-DRS Analysis

Diffuse reflectance spectroscopy (DRS) spectra of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 hybrid nanocomposites are displayed in Figure a–c, respectively, which indicates that the absorption appeared mostly in the visible region. DRS of the garlic-loaded WO3-TiO2 nanocomposite shows intense absorption in the visible region. Chitosan-blended WO3-TiO2 and WO3-TiO2 nanocomposites also absorb visible light significantly, which may be due to the implantation of nanocrystalline WO3 in TiO2 nanocomposites. As it can be seen, in the case of garlic-loaded TiO2 calcinated at 450 and 700 °C, the absorption edge was observed in the visible region of the solar spectrum, representing that the catalyst excitation efficiently exploits more photons. Such absorption mentioned the substitution of lattice titanium by S6+ and the formation of a newly isolated band above the TiO2 valence band and consequently narrowed a band gap.[26] Furthermore, the results suggest that loading greatly promotes the band gap to red-shift, which eases the electron excitation from the VB to the CB that results in higher photocatalytic activity.

Figure 5

DRS of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

FT-IR Analysis

Figure a–c displays the Fourier transform infrared (FT-IR) spectra of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 nanocomposites, respectively. Figure a shows that the strong band that appeared around 3400 cm–1 represents O–H. The band at 2300 cm–1 corresponding to vibrations of atmospheric CO2 and the band around 1630 cm–1 represent the deformation of water δH–OH. The different vibrational modes of TiO2 appeared between 650 and 800 cm–1. Chitosan-blended WO3-TiO2 hybrid nanocomposites (Figure b) show peaks around 3400 and 1630 cm–1, which indicate amine (−NH2) and hydroxyl (−OH) functional groups that act as coordination and reaction sites for the adsorption of the organic species. The existence of a peak around 700 cm–1 represents TiO2. The vibrations of atmospheric CO2 appeared around 2300 cm–1. Garlic-loaded WO3-TiO2 hybrid nanocomposites (Figure c) show a strong band around 3400 cm–1, indicating O–H group stretching vibrations.[27] The presence of a peak around 460 cm–1 is attributed to the S–S stretching of sulfur. The band at 1130 cm–1 may be corresponding to the C–O–H bending of the carboxylic group found in garlic extract compounds.[28] A weak band at 2300 cm–1 corresponds to atmospheric CO2. The presence of bands between 650 and 800 cm–1 are attributed to the different vibrational modes of TiO2.[29] The FT-IR results supported the formation of hybrid nanocomposites.

Figure 6

FT-IR spectra of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

TG-DTA Analysis

Thermal stability of the synthesized nanomaterials was analyzed by thermogravimetry–differential thermal analysis (TG-DTA) between 50 and 800 °C under a N2 atmosphere, as shown in Figure a–c. The TGA analysis of the WO3-TiO2 nanocomposite (Figure a) indicates two weight losses. The first major weight loss occurs at 283 °C and the second weight loss occurs at 523 °C. The first and second weight losses may be attributed to the decomposition of the residual −OH groups and the condensation of nonbonded oxygen and crystallization of TiO2, respectively. In DTA, a strong intense exothermic peak is obtained at 298 °C, which corresponds to the decomposition of the residual −OH groups.

Figure 7

TG-DTA of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

In the TGA analysis of the chitosan-blended WO3-TiO2 nanocomposite (Figure b), there was no change up to 341 °C. After 341 °C, a significant decrease was observed and then a steady state was reached. The weight loss between 300 and 400 °C was allocated to the complex dehydration of the saccharide rings, depolymerization, and decomposition of the acetylated and deacetylated units of the polymer.[30−32]

TGA analysis of the garlic-loaded WO3-TiO2 nanocomposite (Figure c) shows the observance of a sharp decrease from 250 to 280 °C and 420 to 580 °C. The first and second weight losses can be attributed to the decomposition of the residual −OH groups and the condensation of nonbonded oxygen and crystallization of TiO2, respectively. DTA analysis reveals two peaks that correspond to the condensation of nonbonded oxygen and crystallization of TiO2.

Photocatalytic Activities

Figure a–c shows the photocatalytic activities of TiO2, WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 nanocomposites with visible light, which have been evaluated using rhodamine B (Rh-B), methylene blue (MB), and methyl orange (MO). The degradation studies of Rh-B under visible light are found to be good for the three synthesized nanocomposites such as garlic-loaded WO3-TiO2, chitosan-blended WO3-TiO2, and WO3-TiO2 but feeble for TiO2. The orders of photocatalytic degradation of Rh-B, MB, and MO by the catalysts are as follows: garlic-loaded WO3-TiO2 > chitosan-blended WO3-TiO2 > WO3-TiO2 > TiO2.

Figure 8

Solar light photodegradation profiles of dyes (a) Rh-B, (b) MB, and (c) MO.

The higher photocatalytic efficacy of the garlic-loaded WO3-TiO2 nanocomposite may be due to the presence of organic sulfur, which does have an excellent degradation property.

The higher rate of photodegradation of the three dyes by the chitosan-blended WO3-TiO2 photocatalyst may be due to the fact that the surface of the positively charged chitosan matrix increased the anionic dye adsorption. The presence of amine groups in chitosan composites can undergo protonation (forming protonated amine) that adsorbs the dye molecules/metallic ions by using different types of interaction mechanisms like electrostatic attraction, chelation, etc. They can be high-capacity adsorbents for the efficient removal of contaminants from wastewater. The presence of amine groups in chitosan also creates active sites for the formation of complexes with attracted molecules, thereby enhancing the solar light photocatalytic activity.[33]

The WO3-doped TiO2 nanocomposite also shows good photocatalytic activity but not like the chitosan-blended and garlic-loaded WO3-TiO2 nanocomposites. The good photocatalytic activity of the WO3-TiO2 nanocomposite is due to the dye-sensitized photocatalytic degradation.

The photocatalytic degradation of the three dyes by TiO2 is minimal and there is no significant change in the degradation process. TiO2 is very much inferior to the other three photocatalysts.

The reason for photocatalytic activity enhancement of the hybrid nanocomposites may be (i) the high degree of crystallization of loaded/doped anatase and stability that ease the electron transfer and subsequently decrease recombination within the photogenerated holes and/or (ii) oxygen vacancies that have increased as a result of doping or deformation in the defects of the lattice, which trap the photoinduced electrons, suppressing the recombination of holes and electrons.[34−36] It is usually agreed that doping materials may distort the TiO2 lattice and the substitution of either Ti4+ or O2– takes place. The crystallite size, crystallinity, the morphology of particles, and phase/chemical composition of the catalysts affect the photocatalytic degradation process. Thus, the holes present in the VB could be trapped by OH– or the radicals have been produced by the adsorption of H2O species on the surface of the catalyst, whereas the reduction of adsorbed oxygen into ·O2– has been caused by the photogenerated electrons present in the CB, which contribute to the enhancement of catalytic activity.[37] Additionally, the effective oxidation of the target pollutants adsorbed on the catalyst surface was achieved by the hole itself.[38]

The degradation of all the three dyes (Rh-B, MB, and MO) progressed over the catalyst surface[39] by the synergistic effect of produced radicals and holes and was not processed in the bulk of the solution because the lifetime of the photogenerated radicals was short and leaned to the recombination.[40] In addition, the enhancement of degradation of all the dyes in the liquid phase has occurred by the introduction of dopants of the slightly distorted lattice and the occurrence of the anatase phase with a high degree of crystallinity.[41,42]

Antimicrobial Activities

The antimicrobial activities of WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 were determined by using a well diffusion method employing Gram-negative bacteria (Escherichia coli) with four different concentrations (250, 500, 750, and 1000 μg). The inhibition zones of all the nanocomposites are shown in Figure with the corresponding bar diagram (Figure ).

Figure 9

Well diffusion assay of (a) WO3-TiO2, (b) chitosan-blended WO3-TiO2, and (c) garlic-loaded WO3-TiO2 nanocomposites.

Figure 10

Comparable inhibition zone (mm) for WO3-TiO2, chitosan-blended WO3-TiO2, and garlic-loaded WO3-TiO2 nanocomposites in an E. coli bacterial strain.

The WO3-TiO2 nanocomposite is inert to the antibacterial activity. Chitosan-blended WO3-TiO2 has no activity for 250 and 500 μg but shows significantly less activity for 750 and 1000 μg. Garlic-loaded WO3-TiO2 exhibited good activity for all the four concentrations. In garlic-loaded, metal oxide-doped TiO2 nanocomposites, the inhibitory effect of garlic on the growth of E. coli is due to the presence of the allicin compound in the extract of garlic. It is a volatile compound that decomposes into other sulfurous compounds such as diallyl disulfide and ajoene. Sulfurous compounds interact with the cell wall of bacteria by rupturing their layer and change their total metabolic activity, thereby inhibiting the activity of bacteria.

Recyclability and Reusability

The recyclability experiments have been investigated and are displayed in Figure . After every experiment, the synthesized nanocatalysts were washed two to three times with absolute alcohol, filtered, dried at 70 °C, and reused. Garlic-loaded WO3-TiO2 and chitosan-blended WO3-TiO2 nanocomposites exhibit outstanding recyclability with no significant loss in the activity of the catalyst even after three times of recycling. After each cycle, some amount of loss in the catalytic activity was observed. The decrease in the rate of degradation (recyclability) might be weakening the ability of the absorbance or some amount of photocatalyst lost at the time of the collection of the catalysts.

Figure 11

Degradation percentage of the prepared nanocomposites after recyclability.

Conclusions

Novel garlic-loaded WO3-TiO2, chitosan-blended WO3-TiO2, and WO3-TiO2 nanocomposites were synthesized by a simple sol–gel and precipitation method. The FT-IR results supported the formation of the hybrid nanocomposites. The thermal behavior and crystallinity of the nanocomposites were analyzed by TG-DTA and XRD. The surface morphology and the evidence for the immobilization of the samples were examined by FE-SEM and HR-TEM techniques. The garlic-loaded WO3-TiO2 catalyst showed higher photocatalytic activity for the photodegradation of the Rh-B, MO, and MB dyes under the illumination of solar light for 120 min. The result displayed that recycling the use of nanocomposites for three times did not eminently affect their photocatalytic activity. The antibacterial effect of the nanomaterials against E. coli was determined by CFU. It was evidenced that garlic-loaded WO3-TiO2 nanocomposites exhibited superior antibacterial activity than chitosan-blended WO3-TiO2 and WO3-TiO2 nanocomposites. Novel hybrid nanocomposites could be used as an eco-friendly, economical, and considerable material for removal of dyes with excellent photocatalytic and antimicrobial activities.

Experimental Section

Reagents

Titanium tetraisopropoxide (TTIP) was purchased from Sigma Aldrich Chemicals. Tween-80 (templating agent) was purchased from Loba Chemie Pvt. Ltd., India. Tungstic oxide (WO3), isopropyl alcohol, glacial acetic acid, methyl orange powder, methylene blue powder, and rhodamine B powder were obtained from SD Fine Chemicals, India. Hydrochloric acid and sodium hydroxide were purchased from SRL Chemicals, India. Crab shells were obtained from a seafood market (Chennai). During the synthesis, Milli-Q water was used.

Synthesis of Undoped TiO2 Nanocomposite

The sol–gel method was used to synthesize the TiO2 nanocomposite using titanium tetraisopropoxide (TTIP) as the precursor and Tween-80 as the templating agent. TTIP was mixed with isopropyl alcohol and then blended with Tween-80 and stirred for 30 min. The gel formed was subjected to aging for 24 h undisturbed. The gel was then separated and calcinated at 3 h for 500 °C to get TiO2 nanocomposites.

Synthesis of WO3-Doped TiO2 Nanocomposite

In the synthesis of the WO3-doped TiO2 nanocomposite, 0.070 g of WO3 was added in distilled ethanol (20 mL) for attaining homogeneous dispersion, which was allowed to stir for 30 min. Tween-80 (3.0 mL) was added to this homogeneous dispersion and stirring was continued for further 30 min. To this suspension, a mixture of TTIP (3.0 mL) in isopropyl alcohol (10 mL) was added dropwise and stirred continuously for 2 h to get a gel. The resulting gel was isolated and thoroughly washed with aqueous ethanol, filtered, and kept in an oven at 6 h for 120 °C. The obtained solid material was calcined at 3 h for 500 °C using an electrical muffle furnace.

Synthesis of Chitosan

The synthetic procedure for the preparation of chitosan from a crab shell was followed from the previously reported literature.[23]

Synthesis of Chitosan-Blended WO3-TiO2 Nanocomposite

WO3 (0.070 g) was added in distilled ethanol (20 mL) for attaining homogeneous dispersion, which was allowed to stir for 30 min. Tween-80 (3.0 mL) was added to this homogeneous dispersion and stirring was continued for further 30 min. To this suspension, a mixture of TTIP (3.0 mL) in isopropyl alcohol (10 mL) was added dropwise and stirred continuously for 2 h to get a gel. Chitosan solution (1 g of chitosan in 100 mL of 1% (v/v) acetic acid) was added to this gel and stirred for 1 h. The obtained mixture was filtered and thoroughly washed with aqueous ethanol, filtered, and kept in the oven for 6 h at 120 °C. The solid samples were calcined at 500 °C for 3 h using an electrical muffle furnace.

Synthesis of Garlic-Loaded WO3-TiO2 Nanocomposite

Fresh garlic cloves were bought from a grocery shop, and they were cut into small pieces, ground well with a small quantity of water, and then filtered to get the garlic extract.

WO3 (0.070 g) was added in absolute alcohol (20 mL) and stirred for 30 min. Tween-80 (3.0 mL) was added to this homogeneous dispersion and stirring continued for 30 min. To this suspension, a mixture of TTIP (3.0 mL) in isopropyl alcohol (10 mL) was added dropwise and stirred continuously for further 2 h. A freshly prepared garlic extract (10 mL) was added under constant stirring and the resulting mixture was kept for aging (24 h). It was then filtered, dried at 120 °C in a hot air oven for 6 h, and calcinated for 3 h at 500 °C to get the corresponding garlic-loaded WO3-doped TiO2 nanocomposites.

Characterization Techniques

XRD studies were investigated using a Bruker D2 Phaser Desktop X-ray diffractometer equipped with Ni-filtered Cu Kα radiation (λ = 1.542 Å) and operated at an accelerating voltage and emission current of 30 kV and 10 mA, respectively. A DXS-10 ACKT scanning electron microscope was used to analyze FE-SEM equipped with EDAX. A JEOL JEM-3010 microscope was used to take TEM images with magnifications of 600k and 800k times operated at 300 keV. The DRS/UV–visible spectra for all the samples in the reported work were recorded using a Shimadzu 2100 UV–visible spectrophotometer in the range of 200–800 nm equipped with an integrating sphere and BaSO4 was used as the reference. The FT-IR spectra were recorded using a Perkin Elmer RX1 instrument within the wavenumber range of 400–4000 cm–1 at 25 °C using solid KBr pellets. TG-DTA analysis of the synthesized nanocomposites was done by employing a WATERS SDT Q600 TA model instrument.

Solar Light Photocatalytic Activities

Dye Estimation

Solutions of Rh-B, MB, and MO of the required ppm were prepared in doubly distilled water and their UV–visible spectra were recorded. Rh-B, MB, and MO show absorption maxima in the visible region at 555, 663, and 464 nm, respectively. The calibration curve was constructed by measuring the absorbance at different ppm (Rh-B up to 5 ppm, MB up to 5 ppm, and MO up to 20 ppm). The concentrations of the dyes prior to and post illumination were determined from their measured absorbance.

Photocatalytic Studies

To 50 mL of freshly prepared dye solution, a calculated amount of the catalyst was added followed by passing of air through the solution at a constant rate. After irradiation, the catalyst was separated and the dye was estimated spectrophotometrically.

Solar Light Intensity Measurements

The intensity of solar light was determined every 30 min and the average intensity of each experiment over the period was calculated. The solar light intensity was determined using a New 200,000 Lux Digital Meter Light Luxmeter Meter Photometer with Footcandle FC. The intensity was 1200 × 100 ± 100 lux and it was nearly constant throughout the experiments.

Photocatalytic Degradation of the Dyes

All the photocatalytic studies were investigated on sunny days between 12 and 3 p.m. under identical conditions. For each experiment, 50 mL of the reaction mixture was irradiated under sunlight. A 50 mL open borosilicate glass beaker was used as the reaction vessel and the irradiation process was performed under open-air conditions. The synthesized catalyst with 50 mL of the dye solution was aired constantly through a pump for providing oxygen and also for getting a homogeneous solution. The volatility of the solvent was not observed at the time of illumination. The sample (2–3 mL) was withdrawn at specific time intervals (15 min) and its absorbance values at 555, 663, and 464 nm were measured immediately to monitor the degradation of dyes Rh-B, MB, and MO, respectively.

Antibacterial Assay

The synthesized nanocomposites were tested with Gram-negative bacteria (E. coli) by using the well diffusion method. The nutrient for the bacteria was prepared by mixing agar in distilled water, transferred into a sterile Petri dish, and allowed to solidify. Four wells were dug by employing a sterile cork borer to which 24 h-cultured Gram-negative bacteria (E. coli) were swabbed. Four different concentrations of the nanocomposites were incorporated into the four wells separately and one well was loaded with control. The plates were placed in an incubator for 24 h and maintained at 37 °C. After the incubation period, the plates were removed and the inhibition diameters of the four different concentrations of the nanocomposites were measured. Identical procedures were adopted for the other nanocomposites.[24] The inhibition diameter was measured after the incubation and the inhibition percentage was determined by using the following equation (eq ).