1 Introduction

Tin oxide is an n-type semiconductor with a wide band gap of 3.6 eV and has been used in a large range of technological applications due to its tailor-made properties. It is particularly interesting because of its unique optical, electrical, and catalytic properties and has a wide range of applications mainly gas sensors, optoelectronic devices, lithium-ion batteries, photocatalysis, and dye-sensitized solar cells [1–7]. Recently, SnO2 has become of much interest in the area of photocatalysis due to its high photochemical stability, strong oxidizing power, low-cost, and non-toxic nature. Casados et al. [4] show that pure tin oxide has photocatalytic properties similar to that of commercial titanium oxide (Degussa P25) under UV light irradiation in the degradation of methylene blue (MB) for reaction times in the range 0–1 h. However, due to the large band gap observed for SnO2, it can be photoactivated only by UV irradiation which constitutes only 4–5 % of the entire solar energy, leaving most of the visible portion of solar radiation. This limitation can be overcome by doping SnO2 with transition metal ions such as Co, Ni, and Mn that extends the absorption spectrum from UV to visible regions, and therefore, the solar energy could be used effectively [8, 9]. In addition to photocatalytic activity, inorganic metal oxides such as TiO2, ZnO, and SnO2 doped with transition metal ions have received increasing attention in antimicrobial applications because such materials can achieve effective disinfection without the formation of any harmful by-products [10–14].

There are several methods for the synthesis of SnO2 nanoparticles such as sol–gel method, solvothermal method, thermal decomposition method, and hydrothermal route. [15–19]. Among them, sol–gel method offers several advantages over other methods such as high purity, better homogeneity, phase-pure powders at a lower temperature, ability to control the grain size, and capability in controlling the textural and surface properties of metal oxides. In this article, pure and Co-doped SnO2 nanoparticles have been prepared through sol–gel method, and their application toward the photocatalytic degradation of MB under daylight illumination was investigated. The antimicrobial effect of the prepared samples on standard bacterial and fungal strains was also investigated. To the best of our knowledge, very less work has been reported in the antimicrobial properties and photocatalytic activity (under sunlight) of Co-doped SnO2 nanoparticles.

2 Experimental

2.1 Materials

The chemicals used in this study were SnCl4·5H2O (Sigma-Aldrich), NH4OH (25 %, Merck), Co(Cl)2·6H2O (Merck), methylene blue (Merck). All reagents were used as received without any further purification, and double distilled water was used throughout the experiments.

2.2 Synthesis of pure and Co-doped SnO2 nanoparticles

Pure and Co-doped tin oxide (SnO2) nanoparticles have been prepared by sol–gel method with different concentrations of cobalt (0.75, 3, and 4 at%). In a typical synthesis, 5.8 g of tin (IV) chloride was dissolved in 100 ml ultrapure water and stirred for half an hour. To this solution, 3.2 M ammonia solution was added drop wise under a controlled feed rate of 0.5 ml per min with constant stirring until pH 2 was reached. The sol was then kept at room temperature for 1 day to complete the aging process. The resultant gel was washed several times with ethanol and double distilled water and dried in an oven at 80 °C for 24 h. After grinding, the synthesized powder was calcined at 400 °C for 2 h. Co-doped SnO2 nanopowders were prepared in a similar manner, by the addition of calculated amount of Co(Cl)2·6H2O into SnCl4·5H2O solution.

2.3 Photocatalytic measurements

The photocatalytic activity of the samples was determined by the photodecomposition of MB under sunlight. The whole experiments were carried out on a hot sunny day. The decolorization of MB is taken as a model reaction for evaluating the photocatalytic activity of all the samples. In a typical experiment, 50 mg of photocatalyst is added to 100 ml of 10 μmol MB solution, and the solution is sonicated for 10 min by ultrasonic vibration. Prior to irradiation, the suspension is kept in the dark for 30 min under continuous stirring to establish an adsorption/desorption equilibrium between the photocatalyst and the dye. The solution is then kept under sunlight with constant stirring, and sampling was done at regular time intervals (1 h), centrifuged at 5000 rpm to remove any catalyst particles. The change in MB concentration with irradiation time was determined from their characteristic absorption (λ max) at 665 nm with a UV–visible spectrophotometer. The efficiency of the photocatalysts in the photodegradation of MB under sunlight exposure was calculated using the formula:

$$\% D = {\raise0.7ex\hbox{${\left( {C - C_{0} } \right)}$} \!\mathord{\left/ {\vphantom {{\left( {C - C_{0} } \right)} {C_{0} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${C_{0} }$}} \times 100$$

where C 0 is the initial concentration of MB solution and C its concentration at different intervals of irradiation time.

2.4 Antibacterial activity

The antibacterial activity of pure and Co-doped tin oxide nanoparticles was determined using agar-well diffusion method and screened against different Gram-positive bacteria (Enterococcus faecalis, Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli, Enterobacter spp., Pseudomonas aeruginosa). Mueller-Hinton agar (MHA) plates were swabbed with inoculums of test organisms grown to turbidity equal to 0.5 McFarland scale. After swabbing, agar wells of approximately 10 mm was bored using a well cutter, and the wells in each plate were loaded with different concentrations (25, 50, 100 µg) of pure and Co-doped tin oxide nanoparticles. The inoculated plates were then incubated at 37 °C for 24 h. The antibacterial activity was assayed by measuring the diameter of the inhibition zone (mm) formed around the well. The larger the zone of inhibition (ZOI) around the well, the greater the antimicrobial activity of the sample will be. Gentamicin was used as the positive control against all the bacteria to compare the efficacy of the test samples.

2.5 Antifungal activity

Aspergillus niger and Candida albicans were employed for testing the antifungal activity of pure and Co-doped tin oxide nanoparticles using agar-well diffusion method. Potato dextrose agar (PDA) plates were prepared, and fungal spores/cells were swabbed. Wells (10 mm) were bored, and samples of different concentrations (25, 50, 100 μg/ml) were added. The ZOI was measured after 3 days and 24-h incubation for Aspergillus niger and Candida albicans, respectively, and compared with the standard antifungal drug, Clotrimazole.

3 Characterization and measurements

The X-ray diffraction patterns were recorded with Bruker Advance diffractometer using CuKα (λ = 1.5406 Å) radiation in 2θ ranging from 20° to 80° at a scanning rate of 0.02 s−1. The morphology, particle size, and structure of the synthesized samples were observed under a transmission electron microscope using JEOL-JEM2100. The compositional analysis of the doped samples was determined from energy-dispersive X-ray spectroscopy (EDAX) spectra using Horiba EMAX. The specific surface area of pure and doped samples was determined from the adsorption isotherm of nitrogen at 77 K on the basis of the Brunauer–Emmett–Teller (BET) method using a micrometrics Gemini 2375, after degassing the samples at 200 °C for 2 h. The UV–visible diffuse reflectance measurements were recorded at room temperature using PerkinElmer Lambda 35 spectrophotometer over a range of 200–800 nm.

4 Results and discussion

4.1 Characterization of the photocatalysts

Figure 1 shows the XRD patterns of pure (CT0), 0.75 at% Co-doped (CT1), 3 at% Co-doped (CT2), and 4 at% Co-doped (CT3) tin oxide nanoparticles calcined at 400 °C synthesized by the sol–gel method. All the diffraction peaks can be indexed to the tetragonal rutile-type SnO2 phase, and the results are in good agreement with the standard JCPDS (Card No. 41-1445) data. Further, no characteristic peaks of impurities such as cobalt oxides or other tin oxides were observed in the XRD pattern showing the single phase sample formation. The average crystallite size (d) of the sample was estimated using Scherrer’s equation [20]:

$$d = 0.9\lambda /\beta \cos \theta$$

where λ is the wavelength of the X-ray used, θ is the Bragg’s angle, and β is the full width at half maximum (FWHM) of the peak. The average crystallite size of the samples and the deviation, which is the standard deviation between the sizes estimated along the three different crystallographic directions, are given in Table 1. As compared to pure SnO2, broadening of diffraction peaks and degradation of crystallinity can be observed with increase in cobalt content, which implies the reduction in crystallite size [10, 21–24]. Table 1 shows the lattice parameters of pure and Co-doped SnO2 nanoparticles. The increase in lattice parameters with increase in cobalt concentration shows that Co ions systematically substituted Sn ions in the samples as the ionic radius of Co2+ (r = 0.75 Å) is larger than that of Sn4+ (r = 0.69 Å) [8, 25].

Fig. 1
figure 1

XRD patterns of pure and Co-doped SnO2 nanoparticles

Full size image
Table 1 Comparison of particle size, lattice parameters, band gap energy, rate constant, and photocatalytic degradation efficiency of pure and Co-doped SnO2 nanoparticles
Full size table

Figure 2a provides the TEM image of sample CT3 and the particle size distribution with log-normal profile and selected area diffraction (SAED) patterns in its inset, and Fig. 2b shows the corresponding high-resolution transmission electron microscopy (HRTEM) image.TEM image shows nearly spherical-shaped particles, and average particle size obtained from TEM analysis for all the samples is depicted in Table 1. From the TEM image, the average particle size and the size distribution were estimated by log-normal curve fitting to the observed histograms. It can be observed that the size of nanoparticles obtained from the TEM images is in close agreement with those obtained from XRD patterns. The SAED patterns exhibited the polycrystalline rings, which could be well indexed to diffractions from the (110), (101), and (211) planes which well characterize the tetragonal SnO2 structure. HRTEM image clearly reveals the interplanar spacing of 0.34 nm, which corresponds to the (110) crystallographic planes of SnO2. Hence, the interplanar distances measured from HRTEM image agree well with the data obtained from XRD pattern.

Fig. 2
figure 2

a TEM image of sample CT3, inset showing the particle size distribution with log-normal profile and SAED patterns, b HRTEM image of sample CT3

Full size image

Figure 3 shows the EDAX spectrum of sample CT3 which confirms the existence of Co2+ ions in doped samples. Thus, the successful doping of Co2+ ions in SnO2 is confirmed from EDAX and XRD analyses.

Fig. 3
figure 3

EDAX spectrum of sample CT3

Full size image

Figure 4 shows the N2 adsorption/desorption isotherms of pure and Co-doped SnO2 nanoparticles. They exhibit type IV isotherms with a H2-type hysteresis loop observed in the relative pressure (p/p 0) range of 0.45–0.95, revealing the characteristic of mesoporous materials. The specific surface areas of the prepared samples were calculated using the BET method and are given in Table 1, and it was found that the specific surface area increases with increase in Co content.

Fig. 4
figure 4

Nitrogen adsorption/desorption isotherms of pure and Co-doped SnO2 nanoparticles

Full size image

Figure 5 shows the UV–visible DRS of pure and Co-doped samples. The optical absorption was recorded as the Kubelka–Munk function F(R) = (1 − R)2/2R, where R is the reflectance and F(R) is the absorbance [26]. It can be clearly observed that as the doping percentage of cobalt increases, the absorption edge positions shift toward longer wavelength or lower energies. As compared to pure SnO2, the doped samples exhibit an extra peak between 375 and 505 nm. Biswajit et al. [27] reported similar peak in Co-doped TiO2 nanoparticles, and they related this region to the crystal field splitted d-electronic transition of Co2+ in octahedral or pseudo-octahedral coordination and ascribed to 4T1g–4T1g(P) transition.

Fig. 5
figure 5

UV–visible spectra of pure and Co-doped SnO2 nanoparticles

Full size image

Figure 6 shows the optical band gap (E g) of the samples determined by extrapolating the linear portion of [F(R)hν]2 versus the photon energy (hν) curve (Tauc plot), to zero absorption. The band gaps determined are summarized in Table 1. It was found that the band gap of doped samples decreases with increasing dopant content. The redshift of band gap in doped samples can be attributed to the sp-d exchange interactions between the band electrons and the localized d electrons of the Co2+ ions substituting Sn4+ ions [8, 28, 29]. Fang et al. [30] in their work reported that redshift in the band gap of Fe3+-doped SnO2 nanoparticles is due to the charge–transfer transition between the Fe3+ d-electrons and the SnO2 conduction or valence band. Similar shift in the band gap has been reported in cobalt- and other transition metal ion-doped semiconductor materials [28, 31–34]. The lowering of band gap clearly indicates the incorporation of cobalt ions in SnO2 crystal lattice. This would have played an important role for the enhancement of photocatalytic activity in doped samples as the defect levels formed act as charge carrier trapping site and minimize the electron–hole recombination.

Fig. 6
figure 6

Band gap energy of pure and Co-doped SnO2 nanoparticles

Full size image

4.2 Photocatalytic studies

Figure 7 shows the photocatalytic activity of sample CT3 evaluated by the degradation of MB at different time intervals of sunlight exposure. The decrease in the intensity of absorption peak at 665 nm with time of exposure in sunlight clearly indicates the degradation of dye.

Fig. 7
figure 7

UV–visible absorption spectra showing the degradation of MB at different time intervals of sunlight exposure using sample CT3

Full size image

A basic mechanism of photocatalytic reaction can be explained as follows. When a photocatalyst is irradiated with light having energy equal to or greater than its band gap energy \((h\nu \ge E_{gb} )\), then electrons migrate from valence band (VB) to conduction band (CB), and holes are formed in VB. The photogenerated holes and electrons then undergo oxidation and reduction process with water and dissolved oxygen, which results in the generation of reactive oxygen species such as hydroxyl radicals (\(\cdot {\text{OH}}\)) and superoxide radical anions (\(\cdot {\text{O}}_{2}^{ - }\)). The superoxide radical anions thus obtained reacted with \({\text{H}}^{ + }\) producing (\({\text{HO}}_{2}^{ \cdot }\)) radicals, which upon subsequent collision with electrons produce hydrogen peroxide anions (\({\text{HO}}_{2}^{ - }\)). They then react with hydrogen ions to produce molecules of hydrogen per oxide (\({\text{H}}_{2} {\text{O}}_{2}\)). The overall mechanism of photocatalytic degradation of dye can be represented by the following equations:

$${\text{SnO}}_{2} + h\nu \to h^{ + }_{{ ( {\text{VB)}}}} + \bar{e}_{{ ( {\text{CB)}}}}$$
$$h^{ + }_{{ ( {\text{VB)}}}} + {\text{H}}_{2} {\text{O}} \to \cdot {\text{OH}} + {\text{H}}^{ + }$$
$$\bar{e}_{{ ( {\text{CB)}}}} + {\text{O}}_{2} \to \cdot {\text{O}}_{2}^{ - }$$
$$\cdot {\text{O}}_{2}^{ - } + {\text{H}}^{ + } \to {\text{HO}}_{2}^{ \cdot }$$
$${\text{HO}}_{2}^{ \cdot } + {\text{H}}^{ + } \to {\text{H}}_{2} {\text{O}}_{2}$$
$${\text{Dye}} + \cdot {\text{OH}} \to {\text{degradation}}\;{\text{products}}$$
$${\text{Dye}} + \cdot {\text{O}}_{2}^{ - } \to {\text{degradation}}\;{\text{products}}$$
$${\text{Dye}} + {\text{H}}_{2} {\text{O}}_{2} \to {\text{degradation}}\;{\text{products}}$$

Several studies in the field of electrocatalysis of rutile-type oxides also report the role of similar hydroxo, oxo, or peroxo intermediate species in the enhancement of oxygen evolution reactions in those oxides [35–37]. Figure 8a presents the effect of Co-doping concentration on the degradation of MB. It can be found that the degradation of dye is faster in sample CT3, and about 97 % of the dye was degraded within 4 h under sunlight exposure. It is evident from UV spectra that the incorporation of Co2+ ions into SnO2 matrix introduces new defect levels in the band gap of SnO2, which can act as trap centers and thereby minimizes the recombination rate by the effective separation of electron–hole pairs. Moreover, the larger surface area of doped samples provides more photocatalytic reaction centers for adsorbing a large number of dye molecules, causing the degradation of dye effectively [38–40]. Wang et al. [38] and Stengl et al. [41] reported that large surface area and the presence of mesoporosity enhances the photocatalytic activity.

Fig. 8
figure 8

a Effect of Co-doping concentration on the degradation of MB solution under sunlight exposure, b Kinetic fit for the degradation of MB with pure and Co-doped SnO2 nanoparticles, c Stability test for sample CT3 for four runs

Full size image

Figure 8b shows the plot of \(\ln \left( {{\raise0.7ex\hbox{${C_{0} }$} \!\mathord{\left/ {\vphantom {{C_{0} } C}}\right.\kern-0pt} \!\lower0.7ex\hbox{$C$}}} \right)\) versus time. The linear relationship of the plot indicates that the photodegradation of the dye obeys pseudo-first-order kinetics, and the rate expression is given by the equation [42–44],

$$\ln \left( {{\raise0.7ex\hbox{${C_{0} }$} \!\mathord{\left/ {\vphantom {{C_{0} } C}}\right.\kern-0pt} \!\lower0.7ex\hbox{$C$}}} \right) = kt$$

where k is the pseudo-first-order rate constant, C 0 and C are the concentration at time t = 0 and t, respectively. The pseudo-first-order rate constants (k) of all the photocatalysts obtained from the plot of \(\ln \left( {{\raise0.7ex\hbox{${C_{0} }$} \!\mathord{\left/ {\vphantom {{C_{0} } C}}\right.\kern-0pt} \!\lower0.7ex\hbox{$C$}}} \right)\) with time are given in Table 1. The rate constants obtained clearly indicate that the photocatalytic activity of sample CT3 had a faster dye degradation of 8.4 times than the undoped sample.

The stability tests of the photocatalytic activity of sample CT3 for four runs are shown in Fig. 8c. The results clearly indicated that the photocatalyst was stable even after four runs without any significant loss of photocatalytic activity, showing its efficiency for the degradation of organic pollutants.

4.3 Antimicrobial activity

Figure 9a shows the results of antibacterial study of sample CT3 against 2 Gram-positive bacteria (E. faecalis, S. aureus) and 3 Gram-negative bacteria (E. coli, Enterobacter spp., P. aeruginosa) for three different concentrations, and Fig. 9b shows its antifungal results with Candida albicans and Aspergillus niger. The results of the antimicrobial studies of all the samples are summarized in Tables 2 and 3.

Fig. 9
figure 9

a Antibacterial activity of sample CT3 against (a) E. Coli (b) Enterobacter spp., (c) Pseudomonas aeruginosa, (d) Enterococcus faecalis, and (e) Staphylococcus aureus. b Antifungal activity of sample CT3 against (a) Candida Albicans and (b) Aspergillus niger

Full size image
Table 2 Antibacterial activity of pure and Co-doped SnO2 nanoparticles at different concentrations (25, 50, 100 μg/ml)
Full size table
Table 3 Antifungal activity of pure and Co-doped SnO2 nanoparticles at different concentrations (25, 50, 100 μg/ml)
Full size table

It was found that the diameter of the ZOI increased with increase in cobalt-doping percentage and also with the sample concentrations, and sample CT3 exhibits marked antimicrobial activity as compared to sample CT0. According to many published reports, several mechanisms have been proposed for the antimicrobial activity such as the generation of hydrogen per oxide H2O2 [reactive oxygen species (ROS)], large surface area/small particle size of the nanoparticles, and efflux mechanisms leading to the release of constituent ions [45–49]. From the photocatalytic studies, it was found that Co-doped SnO2 nanoparticles were photocatalytic under visible light irradiation and could degrade the dye very effectively by ROS as explained in the mechanism of photocatalysis. The higher antibacterial activity of smaller nanoparticles is due to the generation of H2O2 which strongly depends on the surface area of the nanoparticles; larger the surface area, the more antibacterial activity [50]. Jalal et al. [51] studied the concentration-dependent antibacterial activity of ZnO nanofluids and found that the antibacterial activity increased with increasing nanofluid concentration and time, which is due to the increase in H2O2 concentration generated from the surface of ZnO. Furthermore, the binding of nanoparticles to the microbe surface by direct or electrostatic forces can also be bactericidal [52, 53]. Many studies have showed that even under ordinary room light, ROS generation could be possible from metal oxide nanoparticle suspensions, and the amount of ROS can be increased significantly when irradiated with visible light or with UV light, which in turn enhances the antimicrobial activity of the nanoparticles [11, 54, 55]. The antimicrobial activity of SnO2 nanoparticles also depends on their band gap energy; lower band gap energy and higher surface area results in more bacteria adsorption and more facility of catalytic activity even under dark conditions [56]. In the present study, the antimicrobial tests were performed under laboratory light conditions and found remarkable enhancement in the antimicrobial activity of Co-doped samples which is due to the large surface area and small particle size, lower band gap energy of the nanoparticles, and the production of ROS, especially H2O2 on the surface of the nanoparticles that can penetrate the cell membrane and can cause damage to DNA and cellular proteins, leading to cell death.

5 Conclusions

Pure and Co-doped tin oxide nanoparticles were successfully synthesized by sol–gel method. The UV–Vis absorption spectra showed redshift in the absorption edge of doped samples, which enhances their absorption toward the visible light region. Compared to pure SnO2, the doped samples exhibit an extra peak between 375 and 505 nm which can be related to the d–d electronic transition of Co2+ in SnO2 lattice. The photocatalytic efficiency of pure and doped samples was determined by the degradation of MB solution under daylight illumination. It was found that the photocatalytic activity of SnO2 is significantly enhanced by Co-doping because the incorporation of Co2+ ions into SnO2 matrix introduces new defect levels in the band gap of SnO2, which can act as trap centers and thereby minimize the recombination rate by the effective separation of electron–hole pairs. The antimicrobial studies investigated against standard bacterial and fungal strains showed enhanced antimicrobial activity in doped samples, which is due to the large surface area and small particle size, lower band gap energy of the nanoparticles, and the production of ROS, especially H2O2 on the surface of the nanoparticles that can penetrate the cell membrane and can cause damage to DNA and cellular proteins, leading to cell death. Thus, the present study clearly indicates that Co-doped SnO2 nanoparticles can be used as an excellent photocatalyst for the treatment of industrial waste water containing dye effluents and also as antimicrobial agents in consumer and industrial products.