Fabrication of different SnO2 nanorods for enhanced photocatalytic degradation and antibacterial activity

The acid-mediated (oxalic acid [OXA], cinnamic acid [CA], and itaconic acid [IA]) SnO2 nanorods were synthesized by the hydrothermal method. The synthesized SnO2 nanorods, in turn, were analyzed with various physico-chemical techniques such as the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and Raman spectroscopy. Furthermore, the photocatalytic activity of the different SnO2 nanorods was investigated with the malachite green (MG) dye under visible light illumination. The OXA-SnO2 nanorods displayed an excellent degradation performance with observed value at 91% and it was compared to CA and IA-SnO2 nanomaterials. This tetragonal phase was identified and confirmed by XRD studies. In this regards, obtained band gap energy is low then optimally performed to the photocatalytic evolution. The OXA-SnO2 materials were tested for antibacterial and antifungal studies; this was as shown in good biological activities with admire to the different bacterial strains. The Candida albicans (antifungal) and Enterococcus faecalis (Gram-positive) bacteria were not affected in the microbial studies.


Introduction
A novel nanomaterial has most attractive and considered due to the photocatalytic applications. Recently, food and pharmaceuticals, textile, leather, and paper industries reported that materials containing several organic aromatic groups and molecules produced toxic gas and other moieties, which are highly damaging in an environment (Bouras and Slaoui 2019;Yadav et al. 2017). However, these dyes are highly toxic and affect human, animal, and bird health disorders and damaging ecosystems (Gruzeł 2019;Prasad et al. 2021;Gupta et al. 2021). Nowadays, pollution can be controlled with soluble dyes used in various fields in the emerging world. For instance, Sirajul (2019) reported that the Cd ions incorporated into the SnO 2 nanoparticles are favored for the absorption process and thermodynamic studies. Meanwhile, the researchers developed SnO 2 -based nanomaterials for several applications such as in electrochemical sensors (Dilip and Jayaprakash 2018), solar cells , battery studies (Mao and Tian 2019), fuel cells (Dou et al. 2013), supercapacitors (Saravanakumar et al. 2019), biological studies (Srivastav et al. 2018;Singh et al. 2019;Sharma et al. 2015), and photocatalytic activities (Honarmand 2019). In case some people used different toxic chemicals and synthesis methods, it has been explained by the morphology and band gap energy. Therefore, the photocatalytic activity of different acid-mediated SnO 2 nanorods was tested.
Moreover, several metal oxides are currently reported to be used in the field of photocatalysis, such as ZnS-based materials (Tie et al. 2019), Sn(IV)/TiO 2 /AC (Sun et al. 2006), Ag/ZnO (Hosseini et al. 2019;Zhang et al. 2017), PVDF/TiO 2 nanofibers (Dong et al. 2017), 3D g-C 3 N 4 (Heo and Shukla 2019), and β-Bi 2 O 3 @Bi 2 S 3 (Yu et al. 2018). However, these types of materials, created for the first time, displayed a better efficiency in photocatalytic performances with a well-known photo-generated recombination used in different light sources with respect to the various organic pollutants and dyes. These semiconductor materials possess more favorable photocatalytic studies boosting the degradation efficiency as well as exhibiting the synergistic effects.
This study aims to introduce the SnO 2 nanorods which have a tetragonal crystal system with the materials having been characterized by the surface morphology and various physico-chemical methods. The SnO 2 nanorod has a few advantages including being facile, eco-friendly, easily available, having mild reactions and reduced toxicity, and lower costs of the synthesized materials. The synthesized OXA, CA, and IA-SnO 2 nanomaterials were used for the photocatalytic studies under the visible light source with the malachite green dye. However, the OXA-SnO 2 nanomaterials gained a higher efficiency when compared with the CA and IA-SnO 2 nanomaterials.

Materials
The number of standard chemicals required for the material synthesis was the recommended analytical grade chemicals, such as stannous chloride (SnCl 2 ·2H 2 O), oxalic acid (C 2 H 2 O 4 ), cinnamic acid (C 9 H 8 O 2 ), and itaconic acid (C 5 H 6 O 4 ), with NaOH materials with methanol and ethanol used as solvents.
Synthesis of SnO 2 nanorods (OXA, CA, and IA-SnO 2 ) The SnO 2 nanorods were first synthesized by the hydrothermal method. In a typical procedure, 1 mm of the SnCl 2 ·2H 2 O was dissolved in methanol, making three sets with each set of the solution added drop wise in different acids such as oxalic acid, cinnamic acid, and itaconic acid. These precursors were stirring for 1 h, then, the liquid NaOH (0.1M) was added and the solution stirred overnight for 12 h. The obtained homogeneous solution was then transferred to the stainless steel autoclave, maintaining the temperature at 180°C. The obtained product was washed with ethanol and water several times, followed by drying at a vacuum oven at 25°C overnight.

Antibacterial studies
The antibacterial activity of the SnO 2 nanorods was evaluated for the different pathogens by using the agar well diffusion method (Gnanamoorthy et al. 2020). This bacterial analysis was performed by the antibiotic condition. The synthesized SnO 2 nanorod was tested to the Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative), Pseudomonas aeruginosa (Gram-negative), Candida albicans (antifungal), and Enterococcus faecalis bacteria (Grampositive) all cultured from the Mueller-Hinton agar with incubated temperatures at 32-35°C for 48 h. The 0.9% saline solution was used for washing with observed bacterial strain intensity OD values for 0.5 at 571 nm, and then different higher concentrations (100 μg/mL, 200 μg/mL, and 500 μg/ mL) were added in the well and different positive controls (20 μg/mL) (S. aureus, E. faecalis-amoxicillin, E. coli, P. aeruginosa-levofloxacin, C. albicans-fluconazole) were used, after being measured for the zone of inhibition.

Characterization
The three different synthesized SnO 2 nanomaterials were characterized and confirmed by XRD (Rigaku, Dmax-2500) with the surface morphology images captured by SEM (Hitachi, S-4800). The phase orientation confirmation was recorded by FT-IR and Raman spectroscopy (WQF-410 and LABRAM-HR system with laser excitation of 514.5 nm). The dye degradation studies were carried out by UV-visible spectroscopy (Hitachi U-3010). The photocatalytic measurement was recorded by the photocatalytic reactor (Techinstro).

Structural analysis
The three different SnO 2 nanorods were synthesized by the hydrothermal method.  (321) planes. The synthesized SnO 2 nanorods were identified the tetragonal system in all three samples. The CA and IA-SnO 2 samples obtained for the lower intensity had some differences when compared to the OXA-SnO 2 nanorods. These three acid-mediated SnO 2 nanorods showed diffraction peaks without shifting 2θ values than the lattice parameter which also increased. The OXA, CA, and IA-SnO 2 nanorod crystallite sizes were calculated by the Debye-Scherrer formula (Eq. 1).
Here, K is the shape factor, the wavelength, and the diffraction angle. The evaluated crystallite size is in decreasing order of the materials, OXA-SnO 2 ˃ CA-SnO 2 ˃ IA-SnO 2 nanomaterials, 38 nm, 37 nm, and 32 nm, respectively.

FT-IR spectroscopy
The chemical composition of the three different synthesized SnO 2 nanorods was characterized by the FT-IR spectroscopy method. The SnO 2 nanorods' FT-IR comparison spectra are shown in Fig. 2a-c and evaluated by the several vibration peaks. The vibration peak at 400-640 cm −1 corresponds to the O-Sn-O and Sn-O stretching vibrations which is similar to the previously reported work (Chen et al. 2015;. The peak 1608 cm −1 is attributed to the bending vibration modes of the N-H group and 3340 cm −1 can be ascribed to the O-H stretching or N-H stretching vibrations of absorbed water molecules (Gnanamoorthy et al. 2019;Tavker et al. 2021). The CA-SnO 2 nanoparticles' vibration peak transmittance has been decreased depending on the formation of molecules. Hence, synthesized SnO 2 functional groups were confirmed and the results coincide with the Raman spectroscopy. Therefore, the materials' functional groups were confirmed, which have been used for the subsequent application process. nanorods were synthesized at a temperature of 180°C and the optical band gap energy was evaluated using the Kubelka-Munk equation (given below Eq. 2).

Raman spectroscopy
where α is the proportionality constant, A the absorption coefficient, hν the Planck constant, and Eg the band gap respectively. The obtained DRS-UV results show that the OXA, CA, and IA-SnO 2 nanorods have identified red shift regions of the transition. The OXA, CA, and IA-SnO 2 nanoparticles' band gap energies (in-direct) were at 2.5 eV, 2.8 eV, and 3.0 eV, while preparing the low band gap energy compared to the reported band gap values (Yadav et al. 2020a;Kar et al. 2019). The OXA-SnO 2 nanoparticles exhibited lower band gap energies (in-direct) when compared to the CA and IA-SnO 2 materials. As a result, all these SnO 2 nanomaterials were used to enhance the photocatalytic performances.

Morphology studies
Figure 5a-d shows the scanning electron microscope images of OXA-SnO 2 and these images explained by the nanorod like structure with bundles of rods edge-to-edge carefully merging with each other and obtained with a diameter range of 3-1 μm. The CA-SnO 2 surface morphology is shown in Fig. 6a-d, the synthesized material has shown layers like structure with diameter ranges 1, 2, and 10 μm. The IA-SnO 2 surface morphology is shown in Fig. 7a-d and the morphology illustrated that the tube tablet-like structure with a diameter range of 300 to 500 nm. All the synthesized SnO 2 nanomaterials were confirmed by various techniques and have shown low band gap energy, therefore, the photocatalytic activity should be enhanced. and 10c illustrate the calculated degradation percentage, here the OXA-SnO 2 nanorods have been shown an excellent degradation compared to other mediated SnO 2 nanomaterials, the kinetic first order scan rate was calculated and the value found to be R 2 = 0.997, 0.997, and 0.988 and the slope value 0.0299, 0.0184, and 0.034 for OXA-SnO 2 , CA-SnO 2 , and IA-SnO 2 , respectively. However, the intensity of the adsorption peak decreased within 90, 60, and 50 min, the monitored degradation (Figs. 8d, 9d, and 10d) process percentages at 91, 78, and 66. Summarized, the synthesized different SnO 2 nanorods evaluated an excellent photocatalytic performance under the visible light source.
The obtained degradation efficiency of the SnO 2 nanomaterials such as OXA-SnO 2 and CA-SnO 2 nanoparticles are 78% and 66%, the results of which show low degradation efficiency with electron transfer when compared to the OXA-SnO 2 due to it having low bandgap energy. This band gap energy plays a key role in the formation of desirable defects of suitable photocatalytic behavior. In the presence of visible light, the acid-mediated SnO 2 nanomaterials produced charge recombination barriers in the valance and conduction bands. For the conduction band, the whole pair of H 2 O/OH − interacted with the hydroxyl (OH˙) in a radical formation, the conduction band of O 2 produced to the O 2˙− and HO 2˙w ere converted to H 2 O 2 and OH˙formation which strongly separates the radical formation with equation is shown below and detailed mechanism has been displayed in Fig. 11.
OH • þ Malachite Green→Degradation products Therefore, the above experimental results suggest that the synthesized SnO 2 nanomaterial was enhanced to the photocatalytic dye degradation and the OXA-SnO 2 nanorods have enhanced the photocatalytic activity when compared to other reported materials (Kaviyarasu et al. 2016 Meenu et al. 2020). This SnO 2 material was repeated for four times recycled and did not varying to degrading curves, which were shown in Fig. 12.

Antibacterial and antifungal activities
SnO 2 nanoparticles were analyzed for antibacterial and antifungal activities, which corresponds to the Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative), Pseudomonas aeruginosa (Gram-negative), Candida albicans (antifungal), and Enterococcus faecalis bacteria (Gram-positive) zones of inhibition shown in Fig. 13a-e. Here, the first three bacteria (Staphylococcus aureus (Grampositive), Escherichia coli (Gram-negative), Pseudomonas aeruginosa (Gram-negative)) have higher antibacterial activity due to the particle size and Sn 2+ ions (Al-Hada et al. 2018;Yadav et al. 2020b). The Candida albicans (antifungal) and Enterococcus faecalis bacteria (Gram-positive) are not inhibited by the activity (Table 1). Among them, the strains were tested only for higher concentrations, as the lower concentration did not support the activity. Therefore, this SnO 2 material shown higher inhibition as compared to the results reported by Phukan et al. (2017) and Arularasu et al. (2018).

Conclusion
All three different (OXA, IA, and CA-SnO 2 ) nanorods were synthesized by the hydrothermal method. The prepared nanomaterial structure, functional groups, and the surface morphology were investigated and confirmed by the XRD, FT-IR, SEM, and Raman analysis. The XRD results confirmed the tetragonal structure of the SnO 2 nanorods and the metal oxide functional groups were identified and confirmsed by the FT-IR analysis. The peak at 240 to 700 cm −1 is different modes of M-O and M-O-M orientation and these results were similar to the FT-IR spectroscopy. This synthesized material has well-known specifications like low cost, easily available, eco-friendly, reusable, etc., then we are encourage and designed for this new material. OXA, IA, and CA-SnO 2 nanorods were applied to the photocatalytic performances with commonly used for malachite green dye. The synthesized a c b d R 2 -0.997