Cost-effective synthesis method of facile environment friendly SnO₂ nanoparticle for efficient photocatalytic degradation of water contaminating compound

Pharmaceutical, textile and chemical industries being the cradle of all scientific and technical progress imparts majorly for the advancement of human society. However, these chemicals, when they find their way to the aqueous environment, results in severe environmental pollution that negatively effects the entire ecosystem. To get rid of these toxic effluents, biodegradation was used traditionally. The cost-effectiveness and low efficacy are the major disadvantages of the biodegradation process (Li et al. 2018). In the field of wastewater treatment, employment of nanostructured materials has fascinated the researchers due to its outstanding physico-chemical properties (An et al. 2005; Ali et al. 2010; Al-Hamdi et al. 2015) which are dissimilar from the bulk state (Bagheri-Mohagheghi et al. 2008).

The most widely used metal oxides such as zinc oxide (ZnO) and titanium dioxide (TiO₂) with wide band gap semiconductors are used currently in the decontamination of wastewater. Quite a number of previously published works were centered on TiO₂ and ZnO for degradation of organics. Many researchers reported that TiO₂, ZnO and tin dioxide (SnO₂) are the most active catalysts for the degradation of dyes, phenols and pesticides (Al-Hamdi et al. 2015).

In this regard, there are few literature reports showing the study of tin oxide (SnO₂) of rutile-type crystal structure making it a topic of immense interest for the researchers. SnO₂ is a typical n-type semiconductor with a wide band gap of about 3.6 eV (Viet et al. 2016). This low cost material shows high electron mobility excellent optical, gas-sensing properties and chemical stability (Hu et al. 2017) which was widely used in sensors (Tripathy et al. 2013), solar cells (Hara et al. 2011; Chen et al. 2012) and lithium ion batteries (Lin et al. 2012). However, for the degradation of organic pollutants, water splitting and hydrogen production, SnO₂ has proved to be an excellent photocatalyst (Prakasha et al. 2016). SnO₂ is poorly absorbed by the human body when injected or inhaled (Kim et al. 2016). So it has no adverse health impacts on the human body. Thus, it can be stated that SnO₂ is an ideal photocatalyst to work with. Few literature reports the use of pure SnO₂ nanoparticle in the photocatalysis (Singh & Nakat 2013).

Various synthetic methods are there for preparing SnO₂ nanoparticles which includes sono-chemical method (Yu et al. 2011), sol-gel (Lin et al. 2008), two-step solid state reaction (Li et al. 2002) and spray pyrolysis (Begum et al. 2016b). Precipitation method is simple, inexpensive and does not require high temperature and pressure. A huge amount of photocatalyst could be synthesized without using many reagents. Impurities in the precipitate can easily be eliminated by filtration and repeated washing (Patil et al. 2012; Pan et al. 2014; Nadaf & Venkatesh 2016).

Different research groups have synthesized SnO₂ nanoparticles and studied their applicative perspectives towards reduction and photo degradation of aromatic compounds (Begum et al. 2016a), photo degradation of methyl violet 6B dye and reduction of p-nitrophenol to p-aminophenol (Bhattacharjee et al. 2015), photocatalytic activity in methylene blue degradation (Kim et al. 2016), photocatalytic removal of NO gas (Huy et al. 2018) and degradation of carbamazepine from aqueous phase (Begum & Ahmaruzzaman 2018), etc.

4-Aminopyridine (4-AP) showed agricultural use as Avitrol and is used for repelling and killing bird pests (Takenaka et al. 2013). 4-AP is used by people with multiple sclerosis (Kenneth et al. 2000). In the laboratory of physics and biophysics, it is used in the pharmacology of various potassium conductances. But overdoses with 4-AP can lead to parenthesis, seizure and some common side effects like kidney or bladder infections, headache, nausea, weakness and back pain. The discharge of this toxic chemical as industrial effluent, agricultural, laboratory and hospital wastes contaminates the water sources. Although low in concentration, a continuous release of 4-AP, a highly stable chemical, to the aquatic environment can lead to undesirable effects causing harm to aquatic life and human health (Ljubas et al. 2018).

In the past few decades, advanced oxidation processes (AOPs) including semiconductor powders as photocatalysts have been engaged to treat a variety of industrial wastewaters carrying a number of products such as dyes, plastics, resin, pharmaceuticals, pesticides, etc. (ChangSong et al. 2000; Taib & Sorrell 2007; Saleh & Gupta 2012). Recently, researchers have focused their attention to apply semiconductor nanomaterials for the degradation of these water contaminating industrial wastes under the irradiation of UV light, visible light or even under solar light (Regmi et al. 2018).

Previous study reports that Fenton and photo-Fenton degradation process has been applied for removal of 3-aminopyridine (Karale et al. 2014). There is only a report demonstrating degradation of 4-AP using bulk ZnO photocatalyst (Chakraborty et al. 2017).

To the best of our knowledge, there are no previous reports focused on the photocatalytic application of synthesized SnO₂ nanoparticle towards the degradation of 4-AP under UV irradiation and solar light irradiation separately.

Analytical reagent (AR) grade chemicals such as tin dichloride dihydrate (SnCl_2.2H₂O) and ammonia (Merck, India), were used for synthesis of SnO₂ nanoparticles without any further purification. 4-AP of 99.9% purity was purchased from Sigma Aldrich, India. The double distilled water was used as a solvent during the reaction process.

In the typical synthesis process, at first 2 g (0.1 M) stannous chloride dihydrate (SnCl₂.2H₂O) was dissolved in 100 mL water, when the pH of the solution was found to be acidic (pH = 3). Complete dissolution was followed by addition of 4 mL ammonia solution to the above aqueous solution with magnetic stirring. Nonstop stirring was continued for 20 min. Immediately, white gel precipitate was formed. It was allowed to settle for 12 h. Then it was filtered and washed with water 2–3 times by using deionized water. The obtained precipitate was mixed with 0.27 g carbon black powder (charcoal activated). The obtained mixer was kept in vacuum oven at 70 °C for 24 h so that the mixer turns completely in to dried powder. Then this dry product was crushed into a fine powder by grinder. Now the obtained product of fine nano-powder of SnO₂ was calcinated at 700 °C up to 6 h in the auto controlled muffle furnace (Gayatri Scientific, Mumbai, India) so that the impurities from the product were completely removed.

Optical absorption spectrum was recorded in a double beam spectrophotometer. To study the surface morphology of the prepared SnO₂ nanoparticle, field emission scanning electron microscopy (FESEM) was carried out. The crystalline phase was analyzed by X-ray powder diffraction (XRD) using Pan Analytical Expert Pro Diffractometer with copper k a radiation and a graphite monochromator to produce X-rays of wavelength 1.54 A. Diffractogram of powders were recorded in 2Ө scan configuration, in the 20–80 °C 2Ө range. Infrared spectrum was recorded in the wave number range from 500 to 4,000 cm⁻¹ by Bruker Hyperion 3,000 Fourier transform infrared (FTIR) spectrometer.

The photocatalysis of the synthesized SnO₂ nanoparticle observed under UV and direct sunlight was investigated by the degradation of 4-AP (5 ppm) compound analyzed spectrophotometrically at room temperature. The degradation was carried out for 2 h. For the UV irradiation, the above method was followed but a photo-reactor was designed for the UV photocatalytic activity. The reactions were conducted in a batch process in a cylindrical photo reactor with a total volume of 1.0 L (diameter 12 cm and height 13.3 cm). The catalyst (SnO₂ nanoparticle) was added to the reactor containing the sample. To house UV light (λ = 240 nm) source, the unit was set up with an annular cylindrical glass reactor consisting of a quartz tube at the center and a jacket was provided for cooling purpose. A continuous circulation of cooling media (water) was provided. The reactor was provided with inlets for intake of reactants, and outlets for collecting samples. The reactor was open to air with a magnetic stirring bar placed in the bottom for homogenization. The reactor was covered with a wooden box to prevent the exposure of UV light. Samples were periodically withdrawn for recording the absorbance of the selected compound (4-AP).

XRD analysis was performed to determine the size and structural properties of the synthesized SnO₂ nanoparticle. Structural identification of SnO₂ nanoparticle was carried out with X-ray diffraction in the range of angle 2 theta between 20 °C and 80 °C. Figure 1 shows the XRD pattern of synthesized SnO₂ nanoparticle formed at 700 °C which was crystalline in nature. XRD pattern showed peak at 2Ө values of 26.8 °C, 34.1 °C, 39.2 °C, 52.05 °C, 54.9 °C, 58.2 °C, 62.04 °C, 65 °C and 66.16 °C which corresponded to the (110), (101), (200), (211), (220), (002), (310), (112) and (301) planes respectively. The obtained peaks coincided well with previous similar observations of tetragonal rutile structure of SnO₂ nanoparticle (JCPDS 41-1445) [35].

The synthesized SnO₂ nanoparticle showing UV-Vis spectrum has been depicted in Figure 2. The figure shows that the absorption onset of SnO₂ nanoparticle was about 290 nm at 700 °C calcination temperature.

The morphology and particle size of the prepared SnO₂ sample was determined by FESEM analysis as shown in Figure 3(a). The figure shows the FESEM picture of SnO₂ nanoparticles synthesized by using aqueous-phase method via conventional precipitation. It was observed from micrographs, particles were found to be spherical in shape and the maximum particle size distribution found in the range about 25–50 nm from Figure 3(b). The average particle size observed in both FESEM and XRD measurement was found to be nearly equal. The composition of the powders was confirmed by EDAX analysis from Figure 3(c).

The idea of the chemical composition can be obtained from the FTIR spectra. Figure 4 has given the FTIR spectrum of SnO₂ nanoparticle. The bands acquired at around 554 cm⁻¹ and 618 cm⁻¹ were recognized as the Sn-O stretching modes of Sn-OH and Sn-O-Sn, respectively. The bands at around 1,630 cm⁻¹ and 3,410 cm⁻¹ were due to the bending vibrations of absorbed water molecule on the SnO₂ nanoparticle and the stretching vibrations of –OH, respectively. These bands were generally due to the moisture acquired during sample preparation.

The two forms of ammonia present in water are ammonium hydroxide (NH₃) or as the ammonium ion (NH₄⁺). Ammonia is present as ammonium ion when pH of the water is less than 7.

It should be noted that when SnCl₂.5H₂O was dissolved in water, the pH of the medium became acidic (pH = 3). On addition of ammonia to this solution, a milky white precipitate was formed. Ammonia played a pH adjusting role due to the formation of hydroxyl ion. The formed hydroxyl ion led to the formation of Sn(OH)₃⁻ complex followed by the precipitation of Sn(OH)₂. This precipitate on annealing at 700 °C decomposed to produce SnO₂ nanoparticles.

The degradation of toxic 4-AP water solution (5 ppm) using synthesized SnO₂ nanoparticle was studied. The catalyst dosage and the pH were fixed at 1.5 gm/L and 6.5, respectively. In this case, UV light and direct sunlight was used as irradiation sources, separately as shown in Figure 5(i). As the illumination time increased, the degradation of 4-AP has also increased under UV light and solar irradiation both. The figure depicts the degradation rate under UV radiation was 33%, 76%, 92%, 97% and 3%, 5%, 8%, and 11% under direct sunlight.

In the solar spectrum, only 4% of UV light is present. Thus, it can be said that as the amount of UV light is very less, few upon it was absorbed for degradation. For this reason, the degradation was much lower under solar irradiation than that of UV irradiation. So, experiment under UV irradiation has been continued further but neglected under solar light.

The possible mechanism for photodegradation of 4-AP can be explained from Figure 5(ii). SnO₂ photocatalyst was excited when light with greater energy than the band gap energy was irradiated resulted in the formation of hole-electron pair in the valence band (VB) and conduction band (CB) of SnO₂ nanoparticle. Consequently, hole (h⁺) may react with H₂O adsorbed on the surface of SnO₂ to form hydroxyl radicals (°OH). In the VB, photo-oxidation occurred where the holes are trapped by water molecules to form °OH. At the same time, photo-reduction also occurred where the electrons in the CB of the SnO₂ semiconductor react with dissolved molecular oxygen present in water to produce superoxide radical anion O₂°⁻. This O₂°⁻ reacted with H₂O to furnish °OH and HOO° having powerful oxidizing ability. The °OH is the strong oxidizing agent which helped in the degradation of the organic compound, 4-AP (Begum et al. 2016a).

The physico-chemical factors (like pH of the medium and catalyst dosage) that influences photocatalytic degradation has been discussed below.

It is believed that pH plays a key role in the photocatalytic degradation of 4-AP using SnO₂ photocatalyst. The effect of variable pH has been shown in Figure 5(iii). The study was performed using SnO₂ dosage of 1.5 gm/L under UV light irradiation. The figure depicts catalytic degradation of 4-AP by SnO₂ at pH 3 (acidic), 6.5 (neutral) and 10 (basic) medium by adjusting the pH with the help of 0.1 M HCl or 0.1 M NaOH. The removal efficiency of 4-AP by SnO₂ was 99% at basic, 98% at neutral and 81% at acidic medium. The above results revealed that the degradation of SnO₂ in acidic pH was appreciably lower than that in the neutral as well as in the alkaline medium, which showed almost similar removal efficiency. So the degradation study had been done at neutral pH.

The influence of pH on degradation rates of 4-AP can be explained by the electrical double-layer of a solid electrolyte interface (Soltani et al. 2012). Under acidic condition the SnO₂ nanoparticle surface as well as 4-AP molecules were protonated (Uddin et al. 2016). Hence protonated SnO₂ surface hinders the adsorption of cationic 4-AP, thus explaining the lower removal efficiency in acidic pH.

To study the effect of catalyst on degradation efficiency, the SnO₂ nanoparticle with different dosage ranges from (0.5–2) gm/L was added to the 5 ppm 4-AP solution and kept under ultra-violet radiation. Figure 5(iv) shows the photocatalytic activity of SnO₂ under UV light illumination condition. The performance of the photocatalytic degradation depended immensely on the variation of catalyst loading. The degradation efficiency under catalyst dosage 0.5 gm/L, 1 gm/L, 1.5 gm/L and 2 gm/L were found to be 88, 95, 97 and 99, respectively. Up to 60 min, degradation had shown a sharp increase but after 60 min of degradation, the efficiency found to become stable. The optimum catalyst dosage was 1.0 gm/L.

This phenomenon could be explained as the low dosage of catalyst leads to the less absorption of photons by SnO₂ thereby little utilization for the photocatalytic activity. When the catalyst loading increases, the photon absorbed also increases followed by the increase in active centers on the surface of the catalyst (Hao & Jiaqiang 2010). Thus, the degradation efficiency also increased with catalyst loading.

A comparison between the present study and the previous study has been shown in Table 1. The SnO₂ nanoparticle had shown its high degradation efficiency under the UV radiation.

Recycling and stability performance had been studied for SnO₂ photocatalyst. The recycling performance of catalytic efficiency was checked for three cycles (Figure 6(i)). The experimental conditions, i.e. initial concentration of pollutant solution was 5 ppm, catalyst dosage, 1.0 g/L and pH was neutral. The synthesized SnO₂ nanoparticle showed almost same degradation performance in three consecutive runs.

To check the stability of the synthesized SnO₂ photocatalyst after degradation of 4-AP in three consecutive cycles, the photocatalyst was characterized by XRD (Figure 6(ii)), FTIR (Figure 6(iii)) and FESEM (Figure 6(iv)). The figure demonstrats that there was no such change as compared to synthesized fresh SnO₂ nanoparticles.

In this paper, we developed a simple chemical precipitation method for synthesizing SnO₂ nanoparticles. The XRD pattern proved the tetragonal rutile structure of SnO₂ nanoparticles. The formation of spherical shape was evident from the FESEM images and XRD pattern. The maximum particle size distribution found in the range about 25–50 nm. The band gap energy of SnO₂ nanoparticles was obtained at 3.8 eV. The degradation efficiency of water contaminant 4-AP under UV light and solar light irradiation for 120 min were found to be 97% and 11%, respectively. These SnO₂ nanoparticles were also found to be stable photocatalysts after photocatalytic degradation. The reusability of the catalyst was checked and has been found stable after three photocatalytic runs.

We, the authors, express our heartfelt thanks and gratitude to the Director, NIT Durgapur for providing laboratory facilities with reagents and instrumentation. Our special thanks to the department of MME for providing FESEM, XRD data and the Department of Chemistry for FTIR, spectrophotometric data. The financial support from the Department of Science and Technology under the SERB (N-PDF) sponsored project (No. PDF/2017/000390), Government of India, is gratefully acknowledged.

The authors declare that they have no conflict of interest.