Green and Phytogenic Fabrication of Co-Doped SnO2 Using Aqueous Leaf Extract of Tradescantia spathacea for Photoantioxidant and Photocatalytic Studies

The influence of cobalt doping in SnO2 crystal lattice tailored the optical, structural, and surface properties of SnO2. Co2+ was successfully doped in SnO2 (Co-SnO2) via green synthesis using Tradescantia spathacea aqueous leaf extract. Powder X-ray diffraction patterns of the synthesized nanoparticles showed a rutile structure with no impurities. As Co-doping was increased, the average crystallite size increased from 13.25 nm to 32.32 nm and BET results showed reduced surface area. The presence of organic compounds of the aqueous leaf extract was confirmed by Fourier-transform infrared spectroscopy. UV-visible diffuse reflectance spectroscopy showed a red shift suggesting a band gap reduction with Co-doping. The photoluminescence study showed a peak quenching with the increase in Co-doping. Spherical and smaller particles were observed by scanning electron microscopy. The density of states was proposed using X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy data. A novel antioxidant study of SnO2 and Co-SnO2 nanoparticles was done under visible light irradiation using 2,2-diphenyl-1-picrylhydrazyl free radicals and was compared to conventional antioxidant method in the dark. Photocatalytic 4-nitrophenol conversion was also conducted in the dark and under visible light irradiation. The enhancement in the photoantioxidant activities and photocatalytic conversion of 4-nitrophenol to 4-nitrophenolate using SnO2 and Co-SnO2 was observed under visible light irradiation.


Introduction
Nowadays, the growth of numerous hazardous human illnesses is overwhelming such as cancer, liver damage, and gastritis. These diseases can be possibly avoided by the use of antioxidants which protect the human body from active oxygen species and free radicals. Since antioxidants are able to slow down and hinder the development of these dangerous diseases, there is an urgent need to develop antioxidants to strengthen human health [1,2].
On the other hand, p-nitrophenol also known as 4hydroxynitrobenzene and 4-nitrophenol (4-NP) is well recognized to be anthropogenic, noxious, and inhibitory in nature as at high concentration of 4-NP, it would not be mineralized or degraded [3]. This leads to some severe symptoms to both human beings and animals such as vomiting, headaches, and impairment to the liver, kidney, and central nervous system. Besides, 4-NP is stable and soluble in water which might lead to ecological stress. Henceforth, researchers have developed several methodologies to remove or minimize it in which one of the methods is to convert 4-NP to 4-nitrophenolate ions [1].
It is known that a smaller particle size can improve the capability of the nanoparticles in fields such as photocatalysis and antioxidant [16,17]. Hence, it is desirable to synthesize SnO 2 with smaller size and spherical particles. Moreover, with the aid of dopants, it can enhance the activity of SnO 2 . A wide variety of metal ions have been used as dopants to increase the performance of SnO 2 such as Ni [18], Cr [19], Zn [20], Co [21], and Mn [22]. In a previous study, nickel (Ni) was used as the dopant to see the effect of doping [23]. In the present study, cobalt (Co) was chosen as it plays a vital role in the optical properties of SnO 2 . Furthermore, Co is a high corrosion-resistant material, is ferromagnetic, and is a conductor of heat and electricity [24]. The ionic radius of Co 2+ is 74.5 pm and 65.0 pm in the high spin and low spin states, respectively. Therefore, the substitution of Sn 4+ with Co 2+ would result in surface morphology alteration, structural property distortion, and impact on the optical properties of the Co-SnO 2 NPs.
There are few studies on the synthesis of Co-SnO 2 which have been reported. Bouaine et al. reported the synthesis of Co-SnO 2 using co-precipitation method producing nanoparticles with average particle size between 21.9 and 85.6 nm [25]. Patil et al. synthesized SnO 2 with an average particle size of 200 nm using spray pyrolysis technique and investigated acetone vapor sensing activity using the synthesized Co-SnO 2 [4]. Bagheri-Mohagheghi et al. has also utilized spray pyrolysis technique to synthesize Co-SnO 2 and managed to achieve smaller particle size of 19 to 82 nm [21]. Dalui et al. [26] used pulsed laser deposition method and succeeded in obtaining a particle size of 23.7 to 29.2 nm. Co-SnO 2 nanoparticles were also synthesized using chemical precipitation method. Saravanakumar et al. reported spherical and smaller particles size around 5 to 25 nm, while Mani et al. obtained small particle size of about 32 to 42 nm using the same method [24,27].
Meanwhile, others have reported different synthesis methods to synthesize SnO 2 such as solvothermal [28], microwave [29], hydrothermal [30], and sol-gel [31]. However, these methods usually use harsh chemicals and high instrumentation cost. Present research work has been focused on the progress of green synthesis method that does not require harsh chemicals which offer a better solution to prevent from using harsh chemicals that may risk the environment and human health [32].
Plant extract-mediated synthesis method is one of the green methods which attracts researchers [33]. There are numerous studies on the green synthesis of metal/metal oxides; for example, AgNPs were synthesized using Berberis vulgaris extract according to Behravan et al. [34]. AgNPs were also prepared by Khan et al. using the leaf extract of Trigonella foenum-graecum [35]. Other than that, Javaid et al. prepared a review article on the fabrication of AgNPs using different bacteria [36]. Other than that, ZnO NPs were fabricated using the bulb extract of Costus woodsonii as reported by Khan et al. [5]. Various plant extracts used in the synthesis of SnO 2 were reported in a review article by Matussin et al. [32]. For example, orange peel was used in the synthesis of SnO 2 by Luque et al. [37].
However, according to the authors' knowledge, the synthesis of metal oxides specifically Co-SnO 2 using Tradescantia spathacea has not been reported. Thus, in this current work, aqueous leaf extracts of Tradescantia spathacea were used to synthesize SnO 2 and (1at%, 5at% and 10at%) Co-SnO 2 NPs. As reported by Kadam et al., phytochemicals present in Tradescantia spathacea extract are alkaloids, saponins, flavonoids, terpenoids, and coumarins [38]. These phytochemicals might be responsible in the synthesis of SnO 2 [39]. Moreover, Tradescantia spathacea possess antibacterial and antioxidant properties as reported by Tan et al. [40]. A novel study on photoantioxidant activity of S-SnO 2 and Co-SnO 2 NPs was also conducted using 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radicals both in the dark and under illumination of visible light for 30 min. The photocatalytic conversion of 4nitrophenol to 4-nitrophenolate was also conducted using S-SnO 2 and Co-SnO 2 NPs in the dark and under visible light irradiation for 180 min.

Aqueous Leaf Extract Preparation
The preparation of aqueous leaf extract was carried out according to the study reported by Matussin et al. [39]. The leaves were washed with distilled water and were airdried prior to use. Subsequently, exactly 2.0 g of leaf was crushed using pestle and mortar and blended in 50 mL distilled water. Prior to filtration, it was stirred consistently for 1 h at room temperature. The fresh filtrate was used directly to synthesize SnO 2 and Co-SnO 2 NPs.

Synthesis of SnO 2 Nanoparticles
SnO 2 NPs was prepared following the procedures stated in the previous work by Matussin et al. [39]. In short, exactly 1:1 volume ratio of the leaf extract and 0.1 M SnCl 4 solutions, respectively, were mixed together. In this paper, 20 mL of both leaf extract and SnCl 4 solution were used. The mixture was consistently stirred and heated at 80°C until it became a paste. Subsequently, it was calcined at 800°C (lower temperature would result in less crystalline SnO 2 ) for 2 h and the product was ground to yield very fine powder.

Synthesis of Co-Doped SnO 2 Nanoparticles
Co-SnO 2 NPs was synthesized following the synthesis protocol reported by Matussin et al. [23]. Briefly, 20 mL of (0.1 M) SnCl 4 solution was prepared. After few seconds, specific amounts of CoCl 2 were mixed with the SnCl 4 solution to p r e pa r e 1a t % , 5 a t % , a nd 1 0a t % C o-Sn O 2 N P s . Subsequently, 20 mL of Tradescantia spathacea aqueous leaf extract was taken and mixed with the CoCl 2 -SnCl 4 solution. The mixture was heated at 80°C until it became a paste. The paste was then calcined at 800°C in a furnace for 2 h to produce powders. Fine powder of Co-doped SnO 2 NPs was obtained after it was ground. The schematic synthesis procedures can be found in Fig. 1.

Characterization of SnO 2 and Co-Doped SnO 2 Nanoparticles
The X-ray diffraction (XRD) patterns of the S-SnO 2 and Co-SnO 2 NPs powders were recorded using a powder Xray diffractometer (Advanced Powder X-ray diffractometer, Bruker, Germany, model D8) with a diffraction angle between 20 and 80°at room temperature. To investigate the different functional groups related with the extract and NP formation, Fourier-transform infrared spectroscopy (FTIR) of the pre-dried samples with finely ground KBr was carried out using FT-IR (Alpha T Bruker) in the range of 400-4000 cm −1 via KBr method at room temperature. The optical properties and band gap of the materials were examined using ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy (DRS, VARIAN, Cary 5000, USA) and photoluminescence study (PL, HORIBA scientific) was carried out with excitation wavelength of 325 nm at ambient conditions. Determination of the chemical states and compositions of S-SnO 2 and Co-SnO 2 was performed using X-ray photoelectron spectroscopy (XPS, Kratos Analytical, Axis Nova). Surface morphology study of the NPs was done using field emission scanning electron microscopy (FESEM, JSM-7610F), and the materials were coated with carbon using a JEC-560 auto carbon coater. Brunauer-Emmett-Teller (BET, Micromeritics 3 Flex surface characterization analyzer) specific surface area analysis was measured using N 2 adsorption at 77 K. In the case of photoantioxidant and photoconversion of 4-NP activities, the activities were carried out using a photochemical reactor Toption (TOPT-V) and the absorbance Fig. 1 The schematic diagram of Co-SnO 2 NPs synthesis using Tradescantia spathacea aqueous leaf extract of DPPH and 4-NP was measured using UV-visible spectrophotometer (Shimadzu UV-1900, Japan).

Photoantioxidant Activities of S-SnO 2 and Co-SnO 2 NPs
The study of photoantioxidant activities of S-SnO 2 and Co-SnO 2 NPs were evaluated using 2,2-diphenyl-1-picrylhydazyl (DPPH) free radicals. The concentrations of S-SnO 2 and Co-SnO 2 NPs were varied, where at 0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL in methanol were added separately to 2 mL of DPPH (0.1 mM, dissolved in methanol). The reactions were carried out at room temperature for 30 min in two different conditions: in the dark and under visible light. The absorbance was recorded at OD 515nm .
The antioxidant capacity (radical scavenging activity) of S-SnO 2 and Co-SnO 2 using DPPH free radicals was calculated using Eq. (1) [41]: where A blank is the absorbance of DPPH without catalyst and A sample is the absorbance of DPPH with the catalyst.

Photocatalytic Conversion of 4-Nitrophenol to 4-Nitrophenolate Ions Using S-SnO 2 and Co-SnO 2 NPs
Photocatalytic conversion of 4-nitrophenol to 4nitrophenolate ions tests were performed using S-SnO 2 and Co-SnO 2 NPs as the photocatalysts. Each sample of 5.0 mg was transferred into a boiling tube containing 50 mL of 10 ppm 4-nitrophenol. The reaction was completed in the dark and under visible light irradiation for 180 min. The reduction in the absorption peak at λ = 317 nm and the increase in the absorption peak at λ = 400 nm was used to monitor the conversion of 4-nitrophenol to 4-nitrophenolate ions.

Results and Discussion
S-SnO 2 and (1at%, 5at%, and 10at%) Co-SnO 2 NPs were successfully synthesized through a facile and green method using aqueous leaf extract of Tradescantia spathacea. The fine powders were obtained after calcination and grinding. The color of the samples changed with the increase in Co content from light yellow to dark gray. The samples were abbreviated as S-SnO 2 and 1at% Co-SnO 2 , 5at% Co-SnO 2 , and 10at% Co-SnO 2 for synthesized SnO 2 and Co-doped SnO 2 NPs (with different concentrations) respectively.

Optical Studies Using UV-Vis DRS and Photoluminescence of S-SnO 2 and Co-SnO 2 NPs
UV-Vis DRS analysis was used to determine the band gap energies of the powdered or opaque samples. The influence of doping on the optical band gap of the SnO 2 and Co-SnO 2 NPs was investigated. The Kubelka-Munk function plots are shown in Fig. 2(a). The band gap energies of all these samples were estimated by the Kubelka-Munk Eq. (2) [42]: where R is the measured absolute reflectance of the samples. The band gap can be obtained from the plots of [F(R)hv] 1/2 versus hv, as the intercept of the extrapolated linear part of the plot at [F(R)hv] 1/2 = 0, assuming that the absorption coefficient (α) is proportional to the Kubelka-Munk function F(R).
The band gap energy of commercial SnO 2 , syntheszed SnO 2 , 1at% Co-SnO 2 , 5at% Co-SnO 2 , and 10at% Co-SnO 2 NPs was reduced from 3.33 to 2.18 eV ( Table 1). The S-SnO 2 successfully obtained band gap energy lower than C-SnO 2 . In addition, cobalt doping also help to reduce the energy difference between the conduction band and valence band of SnO 2 , which enhances the electronic conductivity of SnO 2 .
The UV-Vis DRS spectrum of S-SnO 2 showed high absorbance with Co-doping while absorption edge of Co-SnO 2 (1at%, 5at% and 10at% Co-SnO 2 ) greatly shift into the visible region (inset in Fig. 2(a)). Based on that, the higher absorbance might be related to the number of defect states [43]. Therefore, 10at% Co-SnO 2 has more defect states. Moreover, the increase in absorbance with increasing cobalt content might correspond to the intercalation of Co into the lattice of SnO 2 thus narrowing the band gap energy [44].
Photoluminescence (PL) is a tool for analytically searching electron-hole surface processes of semiconducting materials and the determination of surface defects in the samples. Figure 2(b) exhibits the PL emission spectra of C-SnO 2 , S-SnO 2 and Co-SnO 2 NPs at excitation wavelength of 325 nm measured from 350 nm to 800 nm. All samples possessed two similar peak patterns. The first PL peak was probably because of the radiative recombination of electrons and holes in the conduction and valence bands which might be attributed to the near band edge emission (NBE). At approximately 590 nm, it was showing a strong PL emission peak which is mainly due to several surface defects and states in the S-SnO 2 and Co-SnO 2 NPs [45,46].
The PL peak intensity is obviously decreasing with the increase in the amount or % of Co-doping. This might be due to the non-radiative process or charge transfer process on the integration of Co into SnO 2 revealing that fluorescence quenching occurred [47]. In addition, with the increase in Co content, defects such as the distortion centers in the lattice and surface defects also increases as the photo-excited electrons can be transferred from valence band to the new energy levels below conduction band by cobalt doping [28]. Similar behavior was observed by Tianping et al. where the addition of Codoping reduced the PL intensity [43].

Morphological Studies of S-SnO 2 and Co-SnO 2 NPs Using Scanning Electron Microscopy
Scanning electron microscopy (SEM) was utilized to study the surface morphology of the S-SnO 2 and Co-SnO 2 NPs. Figure 3 exhibits SEM images of (a) S-SnO 2 , (b) 1at% Co-SnO 2 , (c) 5at% Co-SnO 2 , and (d) 10at% Co-SnO 2 NPs. The addition of Co 2+ into SnO 2 lattice showed a slight decrease in particle size. As observed in Fig. 3(b) and (c), the observed particle size was smaller than S-SnO 2 ( Fig. 3(a)) which indicate that Co 2+ aids in inhibiting the particle size growth. However, the 10at% Co shown in Fig. 3(d) had the biggest particle size among the Co-doped SnO 2 samples. Nonetheless, its average particle size is still smaller than the S-SnO 2 .
The calculated average particle size was determined using imageJ software and presented in Table 2. The average particle size attained for S-SnO 2 is 59.40 nm. With doping of Co content from 1at% Co to 5at% Co, the average particle size was seen to reduce from 53.18 to 51.81 nm. However, this trend was not acquired for 10at% Co content. As can be observed, the average particle size for 10at% Co-SnO 2 increased to 54.54 nm. The particles were spherical and less agglomerated in S-SnO 2 compared to 1at% Co-SnO 2 . However, the particles became less agglomerated with increasing Co (1at% and 5at%) content.

Surface Area Measurements Using BET
BET analysis determined the nitrogen adsorption and desorption isotherms, pore area, and pore volume of S-SnO 2 and Co-SnO 2 NPs. Figure 4(a) shows the nitrogen adsorption and desorption isotherms of C-SnO 2 , S-SnO 2 , and Co-SnO 2 which all of them show a type IV isotherm with a type H1 hysteresis which can be attributed to mesoporous materials [48]. The H1 hysteresis loop indicates homogenous distribution of enhanced pore size materials as well as showing the enhanced pore connectivity of materials [49]. Figure 4(b) and (c) displays the pore area and pore volume distributions of S-SnO 2 and Co-SnO 2 NPs, respectively. A unimodal distribution was observed in the case of S-SnO 2 and Co-SnO 2 (1at%, 5at%, and 10at% Co) NPs in both pore area and pore volume. Among them, S-SnO 2 showed broader pore area and volume distribution. Table 3 presented the BET surface area, pore volume, and pore size of the samples. S-SnO 2 showed higher surface area than others and followed by 1at% Co-SnO 2 . This could be due to the smaller particle size observed. Higher BET surface area corresponds to smaller particle size. However, S-SnO 2 NPs showed higher particle size calculated in SEM. Nonetheless, according to the XRD, the obtained crystallite size of S-SnO 2 is lower than the Co-SnO 2 . As observed from SEM, as the cobalt content increases (1at% to 5at% Co), the average particle size also decreases and the particle size was increased with 10at% Co. This is in contrary to the BET surface area reduction phenomena for Co-SnO 2 . However, the average crystallite size of Co-SnO 2 from XRD was increased with doping which supports the reduction Fig. 2 (a) Kubelka-Munk function extrapolation (inset: UV-Vis diffuse reflectance spectra) and (b) photoluminescence of C-SnO 2 , SSnO 2 , 1at%Co-SnO 2 , 5at% Co-SnO 2 , and 10at% Co-SnO 2 NPs in BET surface area. Moreover, there was no significant change in the pore volume for the S-SnO 2 and Co-SnO 2 NPs. Nevertheless, the pore size was increased from S-SnO 2 to 10at% Co-SnO 2 NPs.

X-Ray Diffraction Analysis of S-SnO 2 and Co-SnO 2 NPs
X-ray diffraction can provide broad information related to the crystallographic nature and chemical structure of synthetic as well as natural materials. The XRD patterns of the S-SnO 2 and Co-SnO 2 NPs are shown in Fig. 5(a) and (b) with diffraction peaks in the range of 20 to 80°. The peak positions of each sample refer to the rutile-type tetragonal structure of SnO 2 which were confirmed from the JCPDS no 00-041-1445 [50]. No impurity peaks corresponding to adjacent crystal phases were detected for Co-SnO 2 . It shows that the substitution of Co at Sn sites did not impact the phase and structure of SnO 2 confirming the tetragonal crystal lattice remained intact. This is due to the similar ionic radii of both cations. It is obvious that the lattice parameter constants of the samples were almost consistent in the rutile structure before and after Co-doping. Moreover, the XRD patterns of Co-SnO 2 NPs demonstrated sharp and intense peaks which proved they possess crystalline nature. This suggests that as the amount of Co increases, the intensity ascends and it displays the promotion of crystallinity.
The average crystallite sizes were calculated using Debye Scherrer's formula [51]: where β is the full width at the half maximum (FWHM) in radian of the peak with given (hkl) value, λ = 1.5406 Å of the CuKα radiation and θ is the diffracting angle. The average crystallite sizes were found to be 13.25 nm, 14.45 nm, 24.74 nm, and 32.32 nm for S-SnO 2 , 1at%, 5at%, and 10at% Co-SnO 2 NPs, respectively ( Table 4). The crystallite size of 1at% Co-SnO 2 was the smallest, while 10at% Co-SnO 2 was the biggest. The increase in the crystallite size as the Co content was increased is probably due to cobalt ionic radius which is bigger than Sn. The degree of crystallite size increment has been reported to be influenced by the level of the Fig. 3 SEM images of (a) S-SnO 2 , (b) 1at% Co-SnO 2 , (c) 5at% Co-SnO 2 , and (d) 10at% Co-SnO 2 NPs doping material on the surface of NPs [52]. Moreover, the increase in Co might lead to atomic diffusion which results in the enlargement in crystallite size [53]. In addition, the cell volume was also slightly increased with the increase of Co. The Sn 4+ and Co 2+ have slight difference in ionic radius which are 0.71 Å and 0.745 Å, respectively. The Co atoms can be located in Sn positions in the lattice which causes crystal imperfection and distortion in the lattice system hence a strain in the system [54]. The average lattice strain (ε) in these nanoparticles was calculated using the following relation (4) [55]: The average lattice strain was reduced with Co-doping. This shows an agreement with the crystalline nature of SnO 2 with doping [56].
Furthermore, it is worth noting that, the first two XRD peaks showed similar intensity as can be observed in Fig. 5(a), except for the two XRD peaks of S-SnO 2 from JCPDS No. 00-041-1445 which showed a significant difference in intensity. This is due to the change in growth direction. The preferential growth of preferred planes (hkl) can be determined by estimating the texture coefficient using Eq. (5) [57]: where I (hkl) is the measured relative intensity of the sample preferred planes (hkl) hence in this case, (101) was used; N is the maximum number of reflections observed in XRD; and I 0(hkl) is the standard relative intensity. The raise in diffraction Fig. 4 a The N 2 adsorption and desorption isotherms plot, b pore area, and c pore volume of C-SnO 2 , S-SnO 2 , and Co-SnO 2 (1at%, 5at% and 10at%) NPs intensity is due to the methodical arrangement of atoms on crystal surface and grain interfaces which is in good agreement with the lattice strain calculation. To summarize, the lattice strain was reduced, while the crystallite size of S-SnO 2 and Co-SnO 2 NPs was increased with the increase in the amount of Co-doping. Despite the integration of Co into SnO 2 , the lattice parameter showed comparable values. The texture coefficient on the other hand showed T c value larger than 1 which indicates the preferable particle growth in the direction of (101).

Functional Groups Determination Using Fourier-Transform Infrared Spectroscopy
FTIR spectroscopic studies were carried out for S-SnO 2 and Co-SnO 2 NPs within the range of 450-4500 cm −1 at room temperature (Fig. 5(c)). The possible functional groups  The band at 1630 cm −1 is attributed for aromatic compounds which confirmed the involvement of these compounds in the production process of NPs [52]. The peak at around 1133 cm −1 is attributed to secondary amine which may be involved in the capping as well as stabilizing the NPs synthesis [52]. The gradual decrease in peak intensity was observed with increase in cobalt doping (5at % to 10at %).

X-Ray Photoelectron Spectroscopy
In order to investigate the chemical state and the electronic structure of the elements in S-SnO 2 and Co-SnO 2 NPs, X-ray photoelectron spectroscopy (XPS) was performed at room temperature and represented in Fig. 6. Figure 6(a) displays the total survey scan of all the samples showing the presence of Sn 3d, O 1s, and Co 2p. The Sn 3d XPS of S-SnO 2 , 1at% Co, and 10at% Co-SnO 2 samples can be observed in Fig. 6(b).
Owing to the strong spin orbit coupling, the XPS analysis scan of Sn 3d showed two separate peaks centered at 488.0 eV and 496.4 eV which is responsible for Sn 3d 5/2 and Sn 3d 3/2 sub-bands indicating that Sn 4+ valence state existed in the form of SnO 2 [58]. In case of Co-SnO 2 , the peaks were shifted to lower energy as the percentage of Co content increases. However, the Sn 3d XPS peak of 10at% Co-SnO 2 show doublet peaks which might indicate the presence of Sn 4+ and Sn 2+ species. It suggests that Sn 4+ might have been reduced to Sn 2+ in the synthesis process [59]. These results however were not detected by XRD which suggests that very small amount of Sn 2+ ions are attached on the surface of 10at% Co-SnO 2 and this is confirmed by XPS measurement. The XPS spectra of O 1s (Fig. 6(c)) shows the binding energy at 531.9 eV. The lower binding energy indicates the oxygen present in the forms of O 2− ions in the tetragonal structure of Sn 2+ ion array.
With the doping, the O 1s peak shifted to lower energy level which might be due to the oxide (O 2− ) component bound on the SnO 2 . The higher binding energy is attributed to the adsorbed water on the surface or structural water molecules [59]. From Fig. 6(d), Co 2p 3/2 and Co 2p 1/2 are located at binding energy of 780.9 eV and 796.9 eV, respectively, for 1at% Co-SnO 2 and 781.9 eV and 796.8 eV for 10at% Co-SnO 2 , respectively. There are two peaks which correspond to the main and minor doublet pairs. Co 2p 3/2 :Co 2p 1/2 at 780.9:796.9 eV and 781.9:796.8 eV can be assigned to Co 3+ (high spin) and Co 2+ (low spin) states, respectively [58]. The elemental compositions of Sn 3d, O 1s, and Co 2p of S-SnO 2 and Co-SnO 2 NPs estimated using XPS can be found in Table 5. The Co is estimated to be 1.40 at% which is higher Fig. 6 XPS spectra of (a) survey scan spectra, (b) Sn 3d, (c) O 1s, and (d) Co 2p of S-SnO 2 , 1at%Co-SnO 2 , and 10at% Co-SnO 2 NPs than the expected value for 1at% (which is 0.33% theoretically), while for 10at% Co-SnO 2 , the estimated elemental percentage of Co 2p is 5.02 at% which is also higher than the expected value which is about 3.33%. It is worth to note that in XPS the atomic percentage depends largely on the aggregation state [60].
The valence band XPS (VB-XPS) spectra of S-SnO 2 and Co-SnO 2 NPs were carried out and shown in Fig. 7(a). The phenomenon of band gap reduction was observed using the zoomed in area of each sample which is separately shown in Fig. 7(b (i)-(iii)). The VB maximum of S-SnO 2 was at 3.30 eV with a band tailing at 1.89 eV. The 1at% Co-SnO 2 shows the VB maximum of 2.40 eV without a band tailing seen. Meanwhile, 10at% Co-SnO 2 exhibits VB maxima at 2.63 eV followed by a band tail at 0.89 eV. The optical band gap energy of S-SnO 2 , 1at% Co-SnO 2 , and 10at% Co-SnO 2 are 2.81 eV, 2.48 eV, and 2.18 eV, respectively, which were obtained from UV-Vis DRS. Therefore, the proposed DOS scheme is shown in Fig. 7(c) which is based on the UVvisible diffuse absorption and VB-XPS results. The conduction band minimum would occur at 0.49 eV, − 0.08 eV, and 0.45 eV for S-SnO 2 , 1at% Co-SnO 2 , and 10at% Co-SnO 2 NPs, respectively.

Photoantioxidant Activities of the S-SnO 2 and Co-SnO 2 NPs Using DPPH Radicals
The capacity of DPPH free radical scavenging using S-SnO 2 and Co-SnO 2 NPs was evaluated and depicted in bar graph plot as observed in Fig. 8. The DPPH radical scavenging was done in conventional method (dark) (Fig. 8(a)) and under visible light irradiation ( Fig. 8(b)). The positive control used in the experiment was ascorbic acid (AA). Under dark conditions, AA showed an increase in antioxidant activity as the AA concentration was increased. S-SnO 2 and Co-SnO 2 NPs showed almost consistent value with increase in the sample dosage. This indicates that the increase in sample dosage has no significant effect on the activity of DPPH radical scavenging. Nonetheless, 10at% Co-SnO 2 showed better scavenging capacity than others. The incorporation of cobalt has thus enhanced the DPPH free radicals scavenging activity. 1at% Co-SnO 2 and 5at% Co-SnO 2 have slightly lowered radical scavenging activity. However, they showed better response than S-SnO 2 . Reported antioxidant activities of SnO 2 synthesized using various plant parts of different plants are found in Table 6. S-SnO 2 and Co-SnO 2 in this study showed antioxidant activity of less than 40% in comparison to SnO 2 synthesized using the same leaf extracts as reported by Matussin et al. [39]. However, with doping, the antioxidant activity under visible light showed enhanced activity up to almost 60%.
On the other hand, the samples showed quite enhanced DPPH free radical scavenging activities under visible light irradiation. AA showed lower activity as opposed to S-SnO 2, whereas the samples' ability to scavenge the DPPH radicals was enhanced. S-SnO 2 showed the highest activity which increased from 29% (under dark) to about 60% (under visible light). This improvement was probably due to the visible light irradiation of visible light active materials, i.e., S-SnO 2 and Co-SnO 2 NPs. Co-SnO 2 NPs showed slightly lowered activity than S-SnO 2 although the enhancement was seen from under dark to under visible light conditions. This could be owing to the smaller crystallite size of S-SnO 2 than Co-SnO 2 . Nevertheless, the proposed mechanisms of DPPH radical scavenging activity are described in Fig. 9.
In general, the S-SnO 2 donates hydrogen atom (H atom) to the free radical DPPH which finally reduced to DPPH 2 [63]. When this occurs, decolorization is observed from violet to pale yellow (or yellowish brown). In the dark, it is predicted that the H atom was contributed from the organic functional groups coated on the green synthesized SnO 2 ( Fig. 9(a)) [64]. FTIR spectra confirm the presence of organic functional groups adhered on the surface of the SnO 2 NPs. Other than that, it is also evident that Tradescantia spathacea leaf extract exhibits antioxidant properties [65].
In the experiment under visible light irradiation, the excitation of electrons was involved. The generation of electron-hole pairs was resulted from the excited electrons on the surface of SnO 2 NPs. The excited electrons can pair with DPPH radicals as shown in Fig. 9(b) [64]. Moreover, the H atom from the coated biomolecules aided in the scavenging of DPPH radicals. In the case of Co-SnO 2 , the introduction of Co into SnO 2 lattice creates mid-gap states which reduced the band gap energy [45]. Hence, an enhancement under visible light was observed. Furthermore, as reported by Chaudari et al., the produced superoxide radical anions and hydroxyl radicals might react with other compounds; thus, they might produce photoproducts [66]. To conclude, band gap energy plays a major role in the activity under visible light irradiation. Nevertheless, smaller particle size results in higher catalytic activity as well as due Fig. 7 (a) Valence band XPS spectra of S-SnO 2 , 1at% Co-SnO 2 , and 10at% Co-SnO 2 ; (b) zoomed valence band spectra of (i) S-SnO 2 , (ii) 1at% Co-SnO 2 , and (iii) 10at% Co-SnO 2 to determine the band gap; and (c) proposed density of electronic states (DOS) for (i) S-SnO 2 , (ii) 1at% Co-SnO 2 , and (iii) 10at% Co-SnO 2 NPs to high surface area to volume ratio, i.e., more active sites.

Photocatalytic Conversion of 4-Nitrophenol to 4-Nitrophenolate Ions
In the presence of S-SnO 2 and Co-SnO 2 NPs, the photocatalytic conversion of 4-NP to 4-nitrophenolate ions was monitored by UV-vis spectroscopy under ambient conditions. Figure 10 shows bar graph plots of the 4-NP conversion to 4-nitrophenolate ions in dark and under visible light at every 30-min interval for 180 min. As can be observed in Fig. 10(a), at 30 min, the 4-NP conversion increased from S-SnO 2 to 5at% Co-SnO 2 and the performance decreased for 10at% Co-SnO 2 for the first 30 min. With time, the catalytic conversion of 4-NP was decreased and the reaction was maintained for the last 60 min. Interestingly, under visible light, 10at% Co-SnO 2 showed improved photocatalytic conversion of 4-NP to 4-nitrophenolate ( Fig. 10(b)). However, the response was similar at every 30 min for the whole reaction time. Within 180 min, the photocatalytic conversion performance of all samples showed a similar behavior.

Conclusion
In summary, the synthesis of S-SnO 2 and Co-doped SnO 2 NPs using aqueous leaf extract of Tradescantia spathacea at room temperature without using any corrosive or harsh chemicals was established. The aqueous leaf extract acts as a capping agent and the existence of organic functional groups was confirmed by FTIR. Upon varying the Co content in SnO 2 , the particle size obtained from SEM showed decreasing trend with higher Codoping except for 10at%. The band gap energy was also reduced with Co-doping. The reduction of the band gap energy phenomenon was confirmed by VB-XPS analysis. The drop in photoluminescence intensity with Co confirmed the successful doping of Co. The BET surface area decreases with Co. The photoantioxidant of NPs were evaluated using DPPH which was conducted in the dark and under visible light irradiation. The Co- SnO 2 NPs showed a dose-independent antioxidant activity where the activity enhancement was observed under visible light irradiation. The photocatalytic conversion of 4-NP to 4-nitrophenolate was also carried out, and it was found that under visible light irradiation, 5at% Co-SnO 2 showed an improved performance. Nevertheless, S-SnO 2 and Co-SnO 2 NPs might show better response with extended reaction time.