Biosynthesis of zero-dimensional TiO₂ nanostructures for potential photocatalytic applications

Initially, garlic cloves were gathered, peeled, and thoroughly cleaned with deionized and tap water to remove surface contaminants. The cloves were then finely chopped. The pieces were then combined with 100 mL of deionized water to create the extract that was going to be utilized as a stabilizing agent. In order to facilitate the effective extraction of bioactive components from the pieces, this mixture was heated for 25 minutes at 100 °C. Subsequently, the extracts were cooled to ambient temperature before being filtered through Whatman No. 1 qualitative filter paper to get rid of any remaining pieces of solid contaminants. The resulting filtrate, containing the garlic clove extract, was refrigerated at 4 °C for future use.

For the production of TiO₂ nanoparticles, a solution of titanium isopropoxide (TTIP) at a concentration of 0.1 M was made. Then, 80 mL of deionized water was employed to dissolve this solution. Then, 40 mL of extract from garlic clove was added to the mixture. After that, the mixture was magnetically agitated for 3 h at 600 rpm while a thermocouple was used to keep the temperature constant at 60 °C. A small amount of NH₃ was supplemented to regulate the pH of the mixture. The produced solution was then aged for 24 h and maintained for gelation. After that, ethanol and deionized water were used to centrifuge and clean the reaction mixture. The TiO₂ NPs were then dried at 80 °C in a hot air oven for an entire night in order to remove any remaining moisture. After succeeding, the calcination was performed for 2 h at 450 °C. The NPs were then preserved for additional characterization.

To find out the characteristics of the synthesized TiO₂ nanostructures, several experiments were conducted. The characterization involved the use of a SHIMADZU LabX XRD-6100 instrument with Cu Kα radiation at 1.5418 Å for powder XRD, with the diffraction angle range (2θ) adjusted between 20° and 80°. The XRD data was subjected to Rietveld refinement using the FullProf software. Using a JEOL JSM-7600F model, Field Emission Scanning Electron Microscopy was utilized to examine the particles' morphology. After that, the particle size distribution from the FESEM image was estimated using ImageJ software. With ethanol, the TiO₂ nanoparticles' optical properties were determined by a Shimadzu UV-2600i UV-vis spectrophotometer. The bandgap of the NPs was evaluated from the experimental curve and the absorption spectra is obtained between 300 and 800 nm in wavelength.

Under UV light at a wavelength of 450–800 nm and an intensity of 120 μW/cm², the degradation of MB was carried out to examine the photocatalytic activity of biosynthesized TiO₂ NPs. Initially, 5 gm of TiO₂ and weighed MB dye were combined. After being installed in complete darkness for 2h, the mixture was exposed to UV light. Next, the mixture's light absorption at various periods was measured. The following equation (Eqn. 1) was used to quantify the photo deterioration proficiency [6]:

where, C₀ and C₁ correspond to the absorption of MB solution before and after the adding of TiO₂ nanoparticles as a function of time t (0, 30, 60 and 90 minutes).

Figure 1a displays the XRD pattern of TiO₂ nanostructures that were synthesized using biosynthesis method. The Bragg’s reflection planes of (101), (004), (200), (105), (204), (116), and (215) are corresponding to the diffraction angle (2θ) at 25.35°, 37.90°, 48.01°, 54.65°, 62.69°, 68.86°, and 75.25°, respectively. The prominent (101) peak at 25.35° indicated the high crystallinity of the synthesized particles. The overall XRD pattern matched the tetragonal crystal structure of TiO₂ as referenced in JCPDS card number 89-4920. To further elucidate the structural parameters, Rietveld refinement was performed using FullProf software and a Pseudo-Voigt profile function (Figure 1b). The refined lattice parameters (a, b, c) and unit cell volume were determined. The goodness-of-fit indicators, including the weighted profile R-factor (Rwp), expected profile R-factor (Rexp), profile R-factor (Rp), and goodness-of-fit (GOF) values, were all below 10%, confirming the reliability of the Rietveld refinement results (Table 1). Additionally, based on the refined lattice parameters, the crystallographic structure was visualized using the VESTA software (inset of Figure 1b).

Using the Scherrer equation, the avg. crystallite size of the resulting TiO₂ nanostructures is calculated to be 6.45 nm. An extended analysis utilizing the Williamson-Hall plot (Figure 2), taking into account all of the diffraction peak, showed that the avg. crystallite size was a bit smaller at 5.57 nm. The measured crystallite size is smaller compared to values reported in previous studies [9], [10], which can be referred to the relatively low calcination temperature employed in this present work. Additionally, the positive slope of the W-H plot indicated the presence of tensile strain within the material. Lattice strain values, along with other crystallographic parameters, are summarized in Table 1. Using the X-ray diffraction data, the regions corresponding to the crystalline peaks and the amorphous region have been quantified to determine the degree of crystallinity and calculated to be 74.91%.

The shape, morphology, and average particle size of the synthesized NPs were investigated using the FESEM. Using ImageJ software, the particles size distribution of TiO₂ NPs were evaluated. The FESEM micrograph (Figure 3) showing that the biosynthesized NPs are spherical in shape and uniformly dispersed. The size distribution histogram of TiO₂ NPs is shown in the inset of the same diagram, indicating that the majority of particles fall within the 30-60 nm range. The average particles size was found to be 45.08 nm. This nanoscale dimension is expected to enhance photocatalytic activity due to an increased surface-to-volume ratio [11].

Investigating optical characteristics is essential for photocatalytic study because it provides information on the number of incident photons absorbed throughout the process [12]. Figure 4 shows the optical absorption spectra of TiO₂ NPs measured by a UV-vis spectrophotometer. The wavelength range where the intense absorptions were found was roughly 300–370 nm. TiO₂ nanoparticles light absorption properties were evaluated using UV-visible light having wavelengths between 300 and 800 nm. The observation revealed that ~361 nm was the absorption maxima of the TiO₂ nanostructures. The bandgap energy (E_g) of the TiO₂ nanostructures was calculated employing the Tauc plot, using the Eqn. 2 [13].

where, α is absorbance coefficient, hυ is photon energy, E_g is the bandgap energy, and A is the equation constant. The calculated direct bandgap energy of the TiO₂ NPs was found to be 3.28 eV (inset of Fig. 4) and this closely aligns with the reported band gap value of 3.27 eV for bulk TiO₂ [14].

TiO₂ nanoparticles are valued for photocatalytic activity due to their potent oxidizing ability, long-term stability, and lack of toxicity [15]. The photocatalytic performance of TiO₂ nanostructures strongly depends on particle size, surface area, crystallinity, and band gap [16]. Biosynthesis can enhance these by introducing unique surface modifications and morphology, improving light absorption and charge separation. In this study, using an organic dye (Methylene Blue) and solar light, the photocatalytic activity of biosynthesized TiO₂ NPs was demonstrated. The initial indication of dye degradation was observed through a color change. The dye displayed a characteristic visible light absorption spectrum, with its highest absorbance at 664 nm (Figure 5). This peak served as a reference for measuring and monitoring the dye concentration in the solution. From the absorbance spectra associated with dye degradation, it was clear that TiO₂ nanoparticles facilitated a steady rise in the percentage of dye degradation over time, demonstrated by the declining absorbance levels. Degradation rates for 30 and 60 min were found to be 34.59% and 56.02%, respectively. This finding signified that the percent deterioration increased with longer irradiation times and peaked at 86.23% after 90 minutes of solar irradiation. The amount of light energy impinging on the catalyst surface was absorbed as the irradiation time increased. Due to this, there was an increase in the generation of photo-excited species, which raised photocatalytic activity [17]. This investigation showed that increasing the exposure period improved the photocatalytic dye degradation process (Figure 5).