Synthesis of TiO2 nanoparticles by chemical and green synthesis methods and their multifaceted properties

In this present work, Titanium dioxide nanoparticles (TiO2 NPs) successfully synthesized using the chemical as well as the green synthesis routine. The ethanol provoked the chemical reduction of ions. In the green synthesis, jasmine flower extract was used as a reducing and stabilizing agent because it contains alkaloids, coumarins, flavonoids. The Rutile phase of TiO2 NPs with an average crystalline size of 31–42 nm was revealed from the XRD pattern. From the UV–Visible spectroscopy, the optically active region of TiO2 NPs at 385 nm represents the visible region spectrum. The Ti–O–Ti and Ti–O vibration bond formation confirms the formation of TiO2 NPs. The SEM image of TiO2 NPs reveals that the spherical shaped NPs with randomly arranged manner. The obtained results have revealed that the property of TiO2 nanoparticles was similar in both processes. The Photodegradation of methylene blue dye was investigated and resulted in the maximum degradation efficiency of 92% is achieved at 120 min of irradiation. The Photodegradation study shows the biosynthesized TiO2 NPs exhibits a higher degradation efficiency compared to chemically synthesized TiO2 NPs. The antibacterial activity of prepared TiO2 NP’s was studied using grams-positive and gram-negative strains. The biological activities of green synthesized TiO2 NPs are enhanced compared to the chemically synthesized TiO2 NPs. Hence the degradation efficiency and zone inhibition layer indicate that the prepared TiO2 NPs are the potential candidate for environmental and biomedical applications.

This present study is to investigate the chemical and bio-synthesis of the TiO 2 nanoparticles. The phytochemicals present in jasmine flower extracts are alkaloids, coumarins, flavonoids, tannins, terpenoids, glycosides, embodies, steroids, essential oil, and saponins [35]. These phytochemicals are responsible for the reduction of Titanium tetra Isopropoxide to titanium dioxide nanoparticles. The structural, morphological, vibrational, and optical properties of the TiO 2 NPs were analyzed. The photodegradation of methylene blue dye were visualized uisng UV-Visible irradiation technique. As well as the antibacterial activity were tested against both gram-positive and gramnegative strains . The different processes of TiO 2 nanoparticles synthesis were studied in detail.

Materials
Titanium Tetra Isopropoxide (TTIP, C 12 H 28 O 4 Ti, 97%), Ethanol (C 2 H 5 OH, 96%), Methylene blue (C 16 H 18 CIN 3 S), and distilled water was purchased from Merck India. Jasmine flowers were collected from the local market. All chemicals and reagents are of analytic grade and used without further purification. Bacterial pathogens, such as Staphylococcus aureus (gram-positive bacteria), Klebsiella pneumonia and E-coli (gram-negative bacteria) were used to study biological activities.

Synthesis of TiO 2 by hydrothermal method
The slight modifications were made on the synthesis of TiO 2 NPs from the previously reported literature [36]. Initially, 0.1 N of titanium tetra isopropoxide is dissolved in 20 ml of ethanol solution under continuous stirring for 30 min. After that, add a few drops of distilled water to form the dispersion medium. The product was placed on the ultrasonic bath for 20 min. After sonication, the solution was transferred into an autoclave at 150 °C for 3 h. Then the solution was cool to room temperature, and it was washed and centrifuged with deionized water to remove the impurities. Then it is filtered with Whatman No. 1 Filter paper. The filtered sample was dried oven at 110 °C for 5 h, and it is further annealed at 500 °C for 2 h. The resultant TiO 2 NPs was collected and processed with further characterization.

Characterization of TiO 2 nanoparticles
X-Ray Diffraction pattern of investigated titanium dioxide nanoparticles was recorded by using PANanalytical XPERT PRO Diffractometer. FT-IR spectrum was recorded by using the Perkin Elmer spectrophotometer recorded from 400 to 4000 cm −1 . The Surface morphology of TiO 2 nanoparticles was visualized using SEM. EDS spectrum is used to determine its homogeneity and its elemental distribution of elements in the investigated compound. SEM with EDS spectrum was recorded with the help of Quanta FEG-250. UV-Visible Diffuse Reflectance Spectrophotometer (DRS) spectrum was recorded using a Shimadzu 2700 spectrophotometer. The reflectance spectrum was recorded in the range of 200-800 nm. The antibacterial activity of TiO 2 nanoparticles was studied for both gram-positive and gram-negative bacteria by the disk diffusion method.

Antibacterial activity
The antibacterial activity of titanium dioxide nanoparticles was tested by the agar diffusion method. First, the nutrient agar was uniformly spread in the Petri dish plate. Then fix the 6 mm diameter well, which is used to study the inhibition zone. Place 50 μl of TiO 2 NPs in 6 mm diameter well. The culture medium was incubated at 37 °C C for 24 h under aerobic conditions. The zone of inhibition layer was measured using the millimeter region. The Zone of inhibition results in the antibacterial activity of TiO 2 NPs.

Photodegradation of Methylene blue
Methylene blue dye is used as a model pollutant for photodegradation. Take 100 mg of TiO 2 NPs in 250 ml beaker with contains 100 ml methylene blue solution under ultrasonication for 20 min. Furthermore, the mixed solution was kept in a chamber at the dark condition to attain the absorption desorption equilibrium. The photodegradation of methylene blue dye was recorded with the help of UV-Visible irradiation at every 30 min regular interval from 0 to 120 min. The absorbance of methylene blue dye was recorded using 200 µl volume and 10 cm length quartz cuvette. Then the dye degradation efficiency was calculated.

X-Ray Diffraction
The X-ray diffraction technique analyzed the crystalline phase, crystal structure, purity, and average crystalline size of the TiO 2 NPs. Figure 1 displays the XRD pattern  where D is an average crystalline size, K is a dimensionless shape factor with a value close to unity, λ is the wavelength of the X-ray, β is the full width half the maximum intensity (FWHM) and θ is the Bragg angle [40,41]. The average crystalline size of TiO 2 NPs was found in the range of 31-42 nm. Observed average crystalline size values wellmatched with previous reports [42][43][44]. However, there is a small difference in peak strength, phase shift, and average crystalline size due to the synthesis process. The green TiO 2 nanoparticles were exhibited higher intensity TiO 2 peaks due to the presence of polyphenolic compounds in the plant extract. XRD data were tabulated in Table 1.

Fourier transform infrared spectroscopy
The functional group and chemical compound present in the prepared TiO 2 NPs were identified using the FT-IR spectrum. Figure 2 shows the FT-IR spectrum of TiO 2 NPs. The broadband at 3709-3712 cm −1 correlates to the O-H Stretching vibration [45,46]. The band around 1513-1516 cm −1 reflects the bending vibration of functional groups C-H [47]. The thin band at 1269-1278 cm −1 displays the alcohol functional groups D = k ∕ cos , [48]. The band assigned at 1057-1055 cm −1 corresponds to C-O groups of aromatic stretching vibration. The strong band at 460 cm −1 and 900 cm −1 reveals the formation of Ti-O and Ti-O-Ti bending vibrations, respectively [49]. Peaks observed at 460-1000 cm −1 may disappear/partially decrease in intensity by annealing temperature [50]. The metal oxide bonds like Ti-O-Ti and Ti-O confirms the existence of TiO 2 in the prepared TiO 2 NPs. The presence of the Ti-O-Ti bond is due to the strong interaction (capped) of biomolecules with TiO 2 NPs which results in the presence of alkaloids, coumarins, flavonoids, tannins, and terpenoids [51]. These phytochemicals are responsible for reducing the bulk of titanium dioxide to stable TiO 2 in green synthesis [39]. The hydroxyl groups present at 3709-3712 cm −1 in TiO 2 NPs, which enhances the photocatalytic performance. The IR frequency of green synthesized TiO 2 NPs are slightly changed compared to chemically prepared TiO 2 NPs. The Band assignment corresponds to tentative frequency was tabulated in Table 2.

UV-Visible spectroscopy
The optical behavior of the TiO 2 NPs was investigated using the DRS spectrum. The UV-Visible reflectance spectrum of TiO 2 NPs was shown in Fig. 3. The spectra of TiO 2 NPs at 385 nm indicate the charge coordinated electronic transition between the O 2p state and Ti at 3d state [52]. During the biosynthesis process, the colloidal solution turns from white to yellowish-grey, which indicates the formation of titanium dioxide nanoparticles. The white colour dispersion shows the formation of TiO 2 NPs during the chemical process. The sharp absorption peak corresponds to the change in the crystalline phase and the average crystalline size [53]. Hence the investigated nanomaterial is applicable for catalytic application [54,55]. The sharp absorbance peak around 385-400 nm region confirms the formation of TiO 2 NPs. The reflectance spectra of TiO 2 NPs were well matched with the previous reports [56]. NPs. The SEM image of bio-mediated TiO 2 nanoparticles is a spherical shaped structure and the chemical synthesis TiO 2 nanoparticles sphere-like surface morphology. The average particle size of a spherical shaped TiO 2 NPs was found in the range of 32-48 nm. The Particle size obtained from SEM results is well correlated with the average crystalline size from XRD. In general, the decrease in particle size is inversely proportional to the surface volume of the material. Therefore the lower particle size material quickly penetrates the toxic elements as well as the bacterial surface that led the process of decomposition [57,58].

Elemental dispersive spectrum
The elemental analysis of the chemical compounds was investigated through EDS spectra. Figure 5 shows the EDS spectra of Bio-mediated TiO 2 NPs. The elements present in the synthesized TiO 2 NPs are Titanium (Ti), and Oxygen (O) [59]. In bio-mediated TiO 2 NPs, the composition of the titanium element is high compared to oxygen content. The atomic and weight percentage of the TiO 2 NPs are tabulated in Table 3.

Anti-bacterial activity
The antibacterial study of TiO 2 nanoparticles was examined by gram-positive and gram-negative bacteria.  destruction of the cytoplasm membrane and the cell wall of bacteria [60]. In this present report, gram-negative bacteria are highly potent when compared with gram-positive bacteria. The difference in diameter of zone of inhibition is due to the difference in susceptibility of bacteria, the morphology of nanoparticles, phase formations, particle size, shape, and synthesis method. The effect of inhibition of growth on both positive and negative bacteria owing to its vigorous antibacterial activity [61,62]. The zone of inhibition (ZOI) of prepared TiO 2 NPs shows an excellent antibacterial activity. Thus, the prepared TiO 2 NPs are highly applicable to biomedical applications. The efficient antimicrobial agents must be poisonous to pathogens with the capability to be covered as antimicrobial coverings on medical appliances, purity testing devices, wound dressings, textiles, biomaterials, consumer products, food packaging [63].

Photocatalytic activity
The photodegradation of methylene blue dye was studied with the help of UV-Visible irradiation technique. Figure 7a, b shows the schematic representation of the photodegradation of methylene blue. The Photodegradation   where C t is the temporal concentration of MB at time t and C 0 is the initial concentration of MB [64]. Photocatalytic activity of chemically and bio-mediated TiO 2 NPs were examined by methylene blue. In this study, methylene blue dye is used as a pollutant because it is widely utilized in the textile industry for colouring purposes, and also it is more harmful to human beings. So, the removal of methylene blue from wastewater is a challenging problem [65]. The photodegradation efficiency and the absorption spectra of methylene blue dye with a regular interval of time, as shown in Fig. 7a, b. The UV absorption spectra of methylene blue at 665 nm corresponds to π-π* transition. Absorption peak intensity reduction results indicate the degradation of methylene blue. The Biologically synthesized TiO 2 NPs have higher degradation efficiency compared to chemically synthesized TiO 2 NPs. The degradation efficiency increases due to the presence of the hydroxyl group in jasmine flower extract. Bio mediated TiO 2 NPs results in the maximum degradation of 89% under 120 min of irradiation. When TiO 2 NPs undergo UV-Visible irradiation, the electron-hole pair is generated. The positive holes of TiO 2 NPs break water molecules to form hydrogen gas/free radical and negative electron react with oxygen molecules to form superoxide anions [66]. The electron-hole pair results in the formation of a hydroxyl group (OH · ) and superoxide's (O 2 ·− ). These superoxide's and hydroxyl groups are responsible for the degradation of methylene blue [67]. During the reduction process, methylene blue is converted to Leuco methylene blue (LMB) [68]. The degradation efficiency of bio-mediated TiO 2 and chemically investigated TiO 2 NPs are 89% and 82% respectively.

Conclusion
In this present work, TiO 2 NPs are successfully synthesized by green synthesis and hydrothermal method (chemical method). Colour changes confirmed the reduction of bulk Titanium to nanoparticles. The photodegradation of methylene blue under UV-Visible irradiation results in the degradation of methylene blue to leuco methylene blue.
Bio-mediated TiO 2 shows maximum degradation efficiency of 89% under 120 min of irradiation. SEM image reveals that a uniform spherical shape surface morphology. The antibacterial activity of TiO 2 NPs was visualized by the agar diffusion method. Antibacterial activity of TiO 2 NPs was tested against bacterial pathogens such as Staphylococcus aureus (gram-positive bacteria) Escherichia coli and Klebsiella pneumonia (gram-negative bacteria). The biomediated TiO 2 NPs exhibit a good potent on antibacterial activity. The suggested results have inferred the property of TiO 2 nanoparticles is suited for biomedical and wastewater treatment (dye degradation) applications.

Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
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