A Study of the Optical and Structural Properties of SnO 2 Nanoparticles Synthesized with Tilia cordata Applied in Methylene Blue Degradation

: Water contamination is one of the most worrisome problems in the world. Industrial dyes are discharged without previous treatment, promoting water pollution and affecting the environ-ment. In this paper, semiconductor SnO 2 nanoparticles (NPs) were synthesized using Tilia cordata extract, as a reducing agent, at different concentrations, 1%, 2%, and 4% (weight/volume; w / v ). These NPs were used as photocatalysts characterize an alternative for degrading wastewater compounds. Nanoparticle symmetry is an important factor for understanding the properties that provide tools for further treatments. Additionally, the structural, morphological, and optical properties of the green-synthesized SnO 2 NPs were studied. Fourier transform infrared spectroscopy (FTIR) showed the characteristic absorption band of Sn–O centered at 609 cm −1 . Meanwhile, X-ray diffraction (XRD) confirmed a tetragonal rutile-type crystalline phase without impurities whose crystallite size in-creased from 15.96 nm and 16.38 nm to 21.51 nm for SnO 2 -1%, SnO 2 -2%, and SnO 2 -4%, respectively, as extract concentration was increased. NPs with a quasi-spherical morphology with agglomera-tions were observed through scanning electron microscopy (SEM). On the other hand, the bandgap remained at ~3.6 eV throughout all samples, even at variable extract concentrations. The NPs yielded great photocatalytic activity capable of degrading methylene blue (MB) dye under ultravi-olet radiation and solar radiation, achieving degradation percentages of 90% and 83% of MB under UV and solar radiation at 90 and 180 min, respectively.


Introduction
Water pollution is one of the major problems worldwide, caused by industrial waste discharge such as organic dyes, which are considered pollutants in concentrations from 1 ppm [1]. Due to their chemical structure, dyes are resistant to solar radiation and to the action microorganisms [2], which makes them slow-degrading and nonbiodegradable contaminants. In humans, they are considered mutagenic and can cause malnutrition as a result of bioaccumulation. In addition, they cause skin and eye irritation, kidney, and respiratory problems [3][4][5][6]. Furthermore, dyes prevent reoxygenation and sunlight passage through bodies of water, causing adverse effects on aquatic life [7]. Throughout history, different physical or chemical methods have been developed to remove dyes, for instance, adsorption [8], coagulation-flocculation [9], ozonation [10], and biological methods [11], among others. These procedures are considered highly efficient; however, they require additional treatment since they generate byproducts, and even worse, some use toxic chemical agents. Alternatively, photocatalysis, reported in 1987 by Fujishima and Honda [12], has positioned itself as one of the most promising advanced oxidation chemical methods today [13] due to its ability to degrade organic dyes until their mineralization and to its working conditions, atmospheric temperature, and pressure and under solar or UV radiation [14]. SnO2 being photoexcited by irradiating it with an energy equal to its bandgap or greater, generating electron-hole pairs that interact with H2O and O2 molecules in the solution, giving rise to reactive species that act on the dye degrading it [15]. Semiconductor oxide nanoparticles, such as TiO2 [16], ZnO [17], Co/Co3O4 [18], Nb2O5 [19], BiVO4 [20], DyBa2Fe3O7.988/DyFeO3 [21] and SnO2 [22], whose high surface area allows greater interaction with dyes, have been widely used in photocatalysis. SnO2 has stood out for its high stability, high oxidation potential, corrosion resistance, and nontoxicity, as well as for its excellent photocatalytic activity [23]. Within the NPs, SnO2 semiconductor synthesis methods excel on the basis of using natural precursors like extracts from plants [24][25][26], algae [27], fungi [28], and bacteria [29] that contain primary metabolites and secondary polyphenols such as flavonoids and terpenes, which can reduce metal salts to metal ion NPs. In addition, they can stabilize NPs by avoiding contact between nuclei, preventing fusion and hence controlling their size. Using natural extracts decreases the inherent environmental impacts of chemical processes, making them safe, biocompatible, and eco-friendly [30]. Diverse research on SnO2 NP green synthesis using plant extracts has been reported in the literature. Suresh et al. [31] used Delonix elata for synthesizing NPs between 13 and 19 nm whose photocatalytic activity in the rhodamine B degradation was evaluated, rendering a maximum degradation of 92.8% after 150 min under UV radiation. Karthik et al. [32] obtained SnO2 NPs through microwave-assisted green synthesis using Andrographis paniculata extracts; the NPs were evaluated for their optical and photocatalytic properties in Congo red dye degradation. Rathinabala et al. [33] used Canna indica leaf extracts obtaining SnO2 NPs that were analyzed for rhodamine B degradation under UV and sun radiation; cytotoxicity was also evaluated. To the best of our knowledge, there are no reports within the literature on SnO2 NPs biosynthesized utilizing Tilia cordata extracts. Tilia cordata, also known as lime or linden, is a straight-stemmed tree with a smooth bark that can reach 18 m in height. Its leaves are heart-shaped, dark green on the upper side, and bluish green on the underside. Its flowers are yellowish in color, grouped in clusters of one to six flowers that mature into globose-looking fruits [34]. At least 24 water-soluble phenolic compounds have been detected in its extract with a flavonoids and antioxidants high content, in a 58.86 ± 21.51 y 82.99 ± 13.13 mg/g proportion, respectively [35]. These molecules can be used in NP semiconductor green synthesis. In the present work, semiconductor SnO2 NPs were synthesized through biosynthesis utilizing Tilia cordata extracts at different concentrations to evaluate their photocatalytic activity in MB degradation.

Materials
Stems, leaves, and flowers of Tilia cordata were obtained at a local market. Tin chloride (SnCl2•2H2O, 98%), used as tin precursor salt, and Methylene blue, as model dye, were purchased at Sigma-Aldrich. Deionized water was used as environmentally friendly solvent. All reagents were used as received.

Tilia Cordata Extract Preparation
Tilia cordata extract was prepared using a thermal method in an aqueous solution. Briefly, to prepare Tilia cordata extract, the stems, leaves, and flowers were pulverized. Subsequently, 1%, 2%, and 4% (w/v) extract solutions were prepared in 50 mL deionized water. The solutions remained under magnetic stirring for 2 h. They were consecutively placed in a water bath at 60 °C for 1 h. Afterward, the extracts were vacuum filtered to separate them from the organic matter. The extracts were stored for further use.

Green Synthesis of SnO2 NPs
In a typical procedure: 2 g of SnCl2•2H2O were dissolved in 42 mL of the Tilia cordata extract, 1%, 2%, and 4% (w/v), labeled SnO2-1%, SnO2-2%, and SnO2-4%, respectively. The solutions were mechanically stirred for 1 h until the Sn precursor's complete dissolution, then placed in a thermoregulated water bath at 60 °C for 12 h to complete the reaction until a pasty consistency was obtained. Finally, they were submitted to heat treatment at 400 °C for 1 h. The grayish-white products were ground and stored for later characterization and photocatalytic activity evaluation.

Characterization Techniques of the NPs
The SnO2-1%, SnO2-2%, and SnO2-4% NPs were characterized by different analytical techniques. The functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR) with attenuated total reflectance (ATR), using a Perkin Elmer Spectrum Two Spectrophotometer; the spectra were taken in a 4500 cm −1 to 300 cm −1 range. The XRD diffraction patterns were obtained in a Buker X-ray D2 phase diffractometer with a 1.5406 Å (Cu Kα) irradiation wavelength. The scanning angle was measured in a 10° to 80° range at a 0.02° step. Morphology was determined utilizing a JEOL JSM-6310LV scanning electron microscope (SEM) operating at 200 kV. Sample purity was probed by elemental composition analysis using energy-dispersive X-ray spectroscopy (EDX). The optical properties analyses were carried out by visible-ultraviolet light (UV-Vis) and photoluminescence (PL) spectroscopies. The UV-Vis analysis was performed in a Perkin Elmer Lambda 365 equipment at a 600 nm/min scanning speed, in the 200 nm to 800 nm wavelength range. The PL analysis was performed on a Horiba Nanolog PL spectrophotometer over a 200 nm to 600 nm wavelength range and 350 nm excitation wavelength.

Photocatalytic Activity
The SnO2 NPs photocatalytic activity was evaluated for MB dye degradation at a 15 ppm concentration. Degradation studies were performed using UV and solar radiation. In the experiment under UV radiation, 10 W Polaris UV-1C lamps with an energy of 18 mJ/cm 2 were used. In a typical experiment, 50 mg of SnO2-1%, SnO2-2%, and SnO2-4% NPs were added to 50 mL of MB dye solution. The obtained suspension was then mechanically shaken in the dark for 30 min to obtain the adsorption-desorption equilibrium. Subsequently, the suspensions were placed under UV radiation. During the experiment, aliquots were taken at progressive times for up to 180 min. Analogously, the experiment carried out under solar radiation followed a procedure similar to that described above. MB photodegradation was studied from 10:00 am to 1:00 pm on sunny days during October in Ensenada, Baja California, Mexico. The degraded samples' absorbance measurements were provided by UV-Vis spectroscopy. Degradation calculations were performed according to Equation (1): where η is degradation efficiency, Ct is dye concentration at any given time, and C0 is the initial dye concentration. Figure 1 shows the FTIR spectra in ATR mode for the Tilia cordata extract and the SnO2-1%, SnO2-2%, and SnO2-4% NPs. The Tilia cordata spectrum in Figure 1a shows an absorption band centered at 3323 cm −1 that is attributed to the vibrations of the O-H bond; the signals located at 2925 and 2851 cm −1 can be attributed to the C-H bond vibrations of methyls and methylenes, respectively, whereas the absorption bands centered at 1731-1600 cm −1 are attributed to the C=O bond vibrational mode [36]. The SnO2 NPs with 1%, 2%, and 4% extract (Figure 1a,b) presented 2 characteristic signals centered at 605 and 480 cm −1 , which are attributed to the asymmetric and symmetric Sn-O and Sn-O-Sn stretching, respectively [37,38]. Additionally, it is possible to observe the absorption bands corresponding to Tilia cordata located at 3400 cm −1 , 2925 cm −1 , 2851 cm −1 , and 1636 cm −1 , indicating the presence of extract molecules in the SnO2 synthesized products [39,40].
The SnO2 NPs crystallite size was determined using the Debye-Scherrer model, Equation (2).
where τ is the average crystallite size, K is the dimensionless shape factor at ~0.9, λ is the X-ray wavelength, β is the full width at half maximum intensity (FWHM), and θ is the Bragg angle. According to Equation (2) Figure 3 illustrates the UV-Vis absorption spectra of the SnO2-1%, SnO2-2%, and SnO2-4% NPs. In all the syntheses, an absorption edge centered ~300 nm was detected, which is characteristic of SnO2 semiconductors according to reports in the literature [44,45]. Bandgap calculation was performed using the Tauc model, Equation (3): [46] α(v)hv = K(hv − Eg) n (3) where Eg corresponds to bandgap energy, hv is photon energy, K is a constant, α is the absorption coefficient, and 'n' is an index that89 can take on different values depending on the interband transition mechanism [47].

Photoluminescence
Photoluminescence studies are important for investigating structural defects in NPs such as oxygen vacancies. Figure 4 shows the PL spectra for the SnO2 NPs synthesized with different concentrations of Tilia cordata extract. The spectra were taken using a 350 nm excitation wavelength. SnO2-1%, SnO2-2%, and SnO2-4% NPs present different emission bands centered at 360 nm, 379 nm for UV region and bands located at 448 nm, 466 nm, 480 nm, 490 nm, 509 nm, 525 nm and 559 nm, which correspond to the visible region. UV bands are related to the direct combination of electrons (e − ) from the conduction band (CB) to holes (h + ) in the valence band (VB) [50]. On the other hand, the green emission centered at 509 nm is attributed to the oxygen vacancies that help reduce the recombination rate of the electron-hole pairs, increasing their lifetime [51].

Surface Morphology
The SEM micrographs in Figure 5 show the surface morphology changes of the synthesized SnO2 NPs with different Tilia cordata extract concentrations. Figure 5a depicts the SnO2-1% NPs, which present a quasi-spherical morphology with agglomerations. Figure  5b illustrates a similar SnO2-2% NP morphology to SnO2-1%, but scattered and uniform in size with a few larger particles. Moreover, the SnO2-4% NPs shown in Figure 5c present bumps supported on cluster-like particles and scattered, quasi-spherical particles; this could be due to the low nanoparticles nucleation caused by the high organic matter content [52,53], similar to those reported in the literature for SnO2 NPs biosynthesis using natural extracts [30]. The EDX analyses (Table 1) elucidated three main elements, C, O, and Sn. Carbon can be associated with organic matter from Tilia cordata extracts, as shown by FTIR, because the amount of carbon increases as the concentration of the extract increases [54]. The occurrence of Sn and O elements confirms that the NPs obtained are SnO2 [55].

Photocatalytic Activiy
The photocatalytic activity of the SnO2 NPs was evaluated for the MB dye photodegradation under UV and solar radiation. The results of MB degradation under UV radiation are shown in Figure 6a. It was observed that during the first hour, 73%, 90%, and 88% of dye was degraded by the SnO2-1%, SnO2-2%, and SnO2-4% NPs, respectively, proving to have a significant effect on dye degradation. Maximum MB degradation percentages of 94%, 99%, and 99% were reached at 180 min by the SnO2-1%, SnO2-2%, and SnO2-4% samples, respectively. The difference in MB-degradation percentage can be attributed to the SnO2 NPs size since the nanometric scale allows better dye, surface-adsorption, and provides higher availability of active sites to initiate degradation [56]. Additionally, the extract can function as a photosensitizer, thereby increasing the amount of extract increases photocatalytic activity [57]. Figure 6b shows the changes in the UV-Vis absorption spectrum of MB dye in the presence of SnO2-2% NPs, where absorbance decreases over time, indicating concentration reduction. Additionally, the photograph in Figure 6b shows the photocatalysis process, which is visible as the color changes gradually. Correspondingly, Figure 6c shows the MB degradation percentages under solar radiation, where 48%, 68%, and 83.5% of MB were degraded after 180 min with SnO2-1%, SnO2-2%, and SnO2-4% NPs, respectively, implying that under these study conditions there is a correlation between the amount of extract used in the synthesis of the NPs and the percentage of dye degradation.
These results confirm the successful synthesis of SnO2 NPs using Tilia cordata extract and their excellent photocatalytic activity towards MB degradation.

Degradation Mechanism
The dye degradation mechanism through photocatalysis using SnO2 NPs is shown in Figure 7. The process is activated by the incidence of solar or UV radiation that promotes electrons from the valence band (VB) to the conduction band (CB), originating electronhole pairs (e − /h + ) in both [61]. The e − /h + pairs can react directly with H2O and O2, generating highly reactive species such as hydrogen peroxide, hydroxyl radicals, and superoxide radicals. The radical species O2• and OH• interact with dye molecules, degrading them to mineralization [66,67]. The MB degradation process (Figure 8) by photocatalysis with SnO2 NPs can be set off by photogenerated holes as well as by OH• radicals, which attack the C-S + =C functional groups of MB, generating sulfoxide groups (C-S(=O)-C). Then, the OH• radicals attack the sulfoxide group causing ring dissociation; the byproducts react to generate intermediate byproducts until the dye is degraded to mineralization, as reported by Bhattacharjee et al. [68].

Conclusions
In summary, SnO2 NPs were successfully synthesized by a green and simple method, using Tilia cordata extract. The effects of extract concentration were evaluated, and it was determined that it had a significant influence on the nanoparticles' optical and morphological properties, as well as crystallite size and photocatalytic activity. The morphology analyses revealed that all SnO2 NPs presented a quasi-spherical morphology and agglomerations, while the structural analyses of all synthesized materials revealed a rutile-type tetragonal crystalline structure, with crystallite sizes of 15.96 nm, 16.38 nm, and 21.51 nm for SnO2-1%, SnO2-2%, and SnO2-4%, respectively, whereas the bandgap values remained at ~3.6 eV. The SnO2 NPs presented excellent photocatalytic activity; UV radiation had a significant effect on the degradation of MB, resulting in 73%, 90%, and 88% degradation by SnO2-1%, SnO2-2%, and SnO2-4% during the first hour. Moreover, the degradation percentages reached under solar radiation after 180 min were 48%, 68%, and 83.5%, with SnO2-1%, SnO2-2%, and SnO2-4%, respectively. These studies presented similar and improved photocatalytic properties correspondingly to literature reports, demonstrating that Tilia cordata, used as a reducing agent, is a good candidate for the biosynthesis of semiconductor materials with application in the degradation of organic dyes by photocatalysis.