Surfactant effects on the synthesis of durable tin-oxide nanoparticles and its exploitation as a recyclable catalyst for the elimination of toxic dye: a green and efficient approach for wastewater treatment

Abstract

A green synthesis of SnO₂ nanoparticles was successfully developed using urea by a microwave heating method. This method resulted in the formation of spherical, microcrystalline SnO₂ nanoparticles with an average size of ∼4.0 nm. The role of cationic and non-ionic surfactants, namely cetyl pyridinium chloride (CPC) and triton X-100, in the synthesis of SnO₂ nanoparticles are investigated. In this reaction, surfactants act as capping agents. The addition of a surfactant along with urea leads to the formation of spherical and microcrystalline SnO₂ nanoparticles. The average particle size of the CPC assisted SnO₂ nanoparticles is ∼4.5 nm, while that of triton X-100 assisted SnO₂ nanoparticles is ∼5.8 nm. An increase in band gap energy is observed with a decrease in particle size because of three dimensional quantum confinement effect shown by synthesized SnO₂ nanoparticles in their electronic spectra. The band gap energy of SnO₂ nanoparticles synthesized using urea is ∼4.30 eV, whereas that of CPC assisted SnO₂ nanoparticles and triton X-100 assisted SnO₂ nanoparticles are ∼4.25 and ∼4.15 eV, respectively. The synthesized SnO₂ nanoparticles were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Fourier transformed infrared spectroscopy (FT-IR). The optical properties were investigated using UV-visible spectroscopy. The synthesized SnO₂ nanoparticles act to be an efficient photocatalyst in the degradation of rhodamine B and methyl violet 6B dye under direct sunlight. For the first time, methyl violet 6B and rhodamine B dye were degraded by solar irradiation using SnO₂ nanoparticles as catalyst.

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1. Introduction

The pollution of air and water caused by various organic contaminants leads to serious environmental problems. Dyes constitute a major class of organic compounds having huge applications in our daily life. Most of the dyes are toxic but are used in textile industries, dyeing, printing, and cosmetics. The effluents coming out from these industries contaminate water systems, thereby causing water pollution. This poses a threat to water bodies and our ecosystem. Therefore, the complete removal of dye from industrial waste water is vital for reducing water pollution. In this study, rhodamine B (RhB) and methyl violet 6B (MV6B) dyes are selected. Rhodamine B (RhB) is a water soluble dye, used as a laser dye, colorant in textile industries and food stuffs, and fluorescent water tracer. It is suspected to be carcinogenic, and it also causes irritation to eyes, skin and respiratory tract if swallowed by human beings and animals. Methyl violet 6B is a water soluble dye, used in textile industries, paper dyeing, paints, and printing ink. In the biomedical field, MV6B is the active ingredient in Gram's stain for bacterial classification. MV6B is also used as a disinfectant, and it is found to be very poisonous for animals. MV6B is carcinogenic in nature. Therefore, both RhB and MV6B dyes cause adverse health effects, and they are real threat to human, animal and aquatic life. Numerous efforts have been devoted for the removal of dyes from industrial waste water. The search for the effective means of reducing water pollution is a big challenge to many researchers. The conventional method, such as adsorption of dye on activated carbon, is inadequate for the treatment of dye waste water. Adsorption is a non-destructive method which transfers dye from one substance to another and gives rise to a new kind of pollution, which requires further treatment.¹ In contrast, the photocatalytic treatment of dye using solar or UV irradiation in presence of a suitable photocatalyst is a green technique and proves to be an effective method for the degradation of dye. This method converts dye molecule into non-toxic compounds. In this respect, nanostructural semiconductor metal oxides, being inexpensive and stable, act as an excellent photocatalyst for the degradation of dye, and this prove to be an efficient method for the reduction of water pollution.² Several metal oxide semiconductors, such as SnO₂, TiO₂, ZnO, NiO, and V₂O₅, have been used as photocatalysts for the degradation of organic pollutants in water.^2–7 Among them, SnO₂ is known to be one of the most effective photocatalysts because of its high surface reactivity, large number of active sites and high absorption power of light radiation.²

Tin oxide (SnO₂) is an n-type semiconductor with a wide band gap of 3.6 eV.⁸ SnO₂, with a rutile crystal structure, is the most intensively explored metal oxide because of its potential applications in catalysis, gas sensors, dye-based solar cells, transparent conducting electrodes and rechargeable lithium batteries.^9–16 Numerous synthetic methods have been developed for the synthesis of SnO₂ nanostructures, such as sol-gel, homogeneous precipitation, microwave heating method and hydrothermal method.^17–20 The cost for the preparation of SnO₂ nanoparticles on an industrial scale is a challenging job in the production of materials. Therefore, it is very important to design a synthetic method using cheap and non-toxic reagents. The size, morphology, stability and properties of synthesized SnO₂ nanoparticles are of great importance and should be taken into consideration.

In this paper, we report a green synthesis of SnO₂ nanoparticles by microwave heating method. Among various methods, microwave heating method is selected because of its numerous advantages over others, such as short reaction time, good control over particle size and uniform nucleation of powders in suspension. Herein, we develop a microwave heating method using urea, which is a versatile reagent. The use of urea may lead to the enhancement of properties and morphology of SnO₂ nanoparticles.

This communication also illustrates a new surfactant-mediated method to prepare nanocrystalline SnO₂ powders. Surfactants are widely used, and they have a wide range of applications because of their remarkable ability to influence the properties of surfaces and interfaces. The most accepted classification of surfactants is based on their dissociation in water: anionic surfactants, cationic surfactants and nonionic surfactants. Surfactants have a major role in shape-controlled synthesis of nanoparticles. Both hydrophobic and hydrophilic groups are present in surfactants. These groups effectively prevent the agglomeration of the particles and control their morphology. Surfactant molecules have also been used as capping agents.²¹ The surfactant molecules anchored on the surface of nanoparticles act to be the hybrid building blocks for conversion into higher-order structures.²² The surfactant not only provides favorable site for growth of the particulate assemblies, but it also influences the formation process, including nucleation, growth, coagulation and flocculation. The surfactant mediated method applied herein provides a promising preparative approach for tin oxide nanoparticles. In this case, we introduce a cationic (cetyl pyridinium chloride, CPC) and a non-ionic surfactant (triton X-100) along with urea to investigate their roles in the synthesis of SnO₂ nanoparticles. The effect of surfactants on size and morphology of SnO₂ nanoparticles are also studied. To the best knowledge of the authors, the synthesis of SnO₂ nanoparticles using SnCl₂·2H₂O and urea along with CPC and triton X-100 has not been reported in the literature.

In this paper, we report the photocatalytic activity of synthesized SnO₂ nanoparticles in the photodegradation of rhodamine B and methyl violet 6B dyes under direct sunlight. In this case, for the first time, direct sunlight is used for the photodegradation of methyl violet 6B and rhodamine B dye in presence of SnO₂ nanocatalyst. The photodegradation of dye by solar irradiation is a comparatively greener approach than that with UV-light. To the best knowledge of the authors, methyl violet 6B dye is degraded for the first time using the SnO₂ photocatalyst.

2. Experimental

2.1. Materials

The reagents, stannous chloride dihydrate (SnCl₂·2H₂O), urea, cetyl pyridinium chloride, triton X-100, methyl violet 6B and rhodamine B were of analytical grade (AR) and purchased from Sigma-Aldrich. These reagents were used without further purification. Double distilled water is used for the synthesis of SnO₂ nanoparticles. All the reactions were carried out in a domestic microwave oven of 300 W.

2.2. Synthesis of SnO₂ nanoparticles

2.2.1. Synthesis of SnO₂ nanoparticles using urea (S1). For the synthesis of SnO₂ nanoparticles, 0.01 M SnCl₂·2H₂O was treated with 100 ml aqueous solution of 0.01 M urea. The mixture was then kept in a microwave oven and irradiated with thirty 10 s 300 W shots. A white precipitate was formed. The obtained precipitate was centrifuged and washed several times with double distilled water. The final white product was dried at 70 °C and collected for characterization. The sample was marked as S1.

2.2.2. Synthesis of SnO₂ nanoparticles using urea and cetyl pyridinium chloride (S2). The synthesis of SnO₂ nanoparticles were carried out by treating 0.01 M SnCl₂·2H₂O with 100 ml aqueous solution of 0.01 M urea. A 10 ml aqueous solution of 60 mmol cetyl pyridinium chloride (cationic surfactant) was added dropwise with constant stirring. The mixture was then kept in a microwave oven and irradiated with thirty 10 s 300 W shots. A white precipitate was formed. The obtained white precipitate was centrifuged and washed several times with double distilled water. The final white product was dried at 70 °C and collected for characterization. The sample was marked as S2.

2.2.3. Synthesis of SnO₂ nanoparticles using urea and triton X-100 (S3). Likewise, SnO₂ nanoparticles were also synthesized by treating 0.01 M SnCl₂·2H₂O with 100 ml aqueous solution of 0.01 M urea, and then 10 ml of 10% triton X-100 (non-ionic surfactant) solution was added dropwise with constant stirring. The mixture was then kept in a microwave oven and irradiated with thirty 10 s 300 W shots. A white precipitate was obtained, which was marked as S3. The obtained white precipitate was centrifuged and washed several times with double distilled water. The final white product was dried at 70 °C and collected for characterization.

2.3. Characterization of SnO₂ nanoparticles

SnO₂ nanoparticles were characterized by powder X-ray diffraction (XRD) method using a Phillips X'Pert PRO diffractometer with CuKα radiation of wavelength 1.5418 Å. The size, morphology and diffraction ring pattern of SnO₂ nanoparticles were determined by a JEM-2100 transmission electron microscope. Infrared spectra were recorded in the wave number range from 400 to 4000 cm⁻¹ using a Bruker Hyperion 3000 FTIR spectrometer. The UV-visible absorption spectra of the synthesized SnO₂ nanoparticles were recorded on Cary 100 BIO UV-visible spectrophotometer equipped with 1 cm quartz cell.

2.4. Photocatalytic activity of synthesized SnO₂ nanoparticles

The photocatalytic activity of SnO₂ nanoparticles (S1) were evaluated by the degradation of methyl violet 6B (MV6B) and rhodamine B (RhB) under direct sunlight. To evaluate the photocatalytic activity, 10 mg of SnO₂ photocatalyst (S1) was dispersed in 200 ml of 10⁻⁴ M aqueous solution of two different dyes by sonication. These dyes were then exposed to sunlight irradiation. The experiments were carried out on a sunny day (23^rd May 2014) at Silchar city between 10 a.m.–3 p.m. (atmospheric temperature 35–40 °C). At regular intervals of time, 4 ml of the two different suspensions were withdrawn and immediately centrifuged. The progress of the reaction was monitored by taking UV-visible spectroscopy at regular intervals of time.

3. Results and discussion

3.1. FT-IR studies

Fig. 1(a–c) represents the FT-IR spectra of the synthesized S1, S2 and S3 nanoparticles, respectively. The assignment of FT-IR bands of the synthesized SnO₂ nanoparticles (S1, S2 and S3) are summarized in Table 1. The band observed around 3439–3442 cm⁻¹ is due to the –OH vibration of water adsorbed on the surface of SnO₂ nanoparticles. The presence of water is also confirmed by a sharp peak at 1629 cm⁻¹ which is due to H₂O deformation. The peak around 610–600 cm⁻¹ is assigned to the Sn–O–Sn stretching mode of surface bridging oxide formed by the condensation of adjacent surface hydroxyl groups.^4,9,17,23

Fig. 1 FT-IR spectra of the synthesized SnO₂ nanoparticles (a) S1 (b) S2 and (c) S3.

Table 1 Assignment of FT-IR bands of the synthesized tin oxide nanoparticles

SnO2 nanoparticles	FT-IR bands (cm^−1)
	νO–H	νN–H	νC–H	νH2O (def.)	νC–O	νSn–O–Sn
S1	3439	—	—	1629	—	610
S2	3442	3062	2919 (asym.), 2850 (sym.)	1629	—	600
S3	3439	—	2942 (asym.), 2876 (sym.)	1619	1248 (asym.), 1114 (sym.)	610

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FT-IR spectra is recorded not only to detect the formation of SnO₂ nanoparticles but also to perceive the existence of capping agents (CPC and triton X-100) adsorbed on the surface of SnO₂ nanoparticles. In the synthesis of S2 nanoparticles, CPC is used as a capping agent. FT-IR spectra of S2 nanoparticles show bands in the region 3000–3100 cm⁻¹ due to N–H stretching, which indicates that some molecules of CPC are adsorbed on the surface of SnO₂ nanoparticles along with traces of urea. The presence of alkyl chain is indicated by the peak around 2919 cm⁻¹ and 2850 cm⁻¹, which are due to asymmetric and symmetric C–H stretching, respectively. This further confirms the presence of CPC on the surface of SnO₂ nanoparticles as a capping agent.

The FT-IR spectra of S3 nanoparticle is represented in Fig. 1c. In the synthesis of S3 nanoparticles, the surfactant triton X-100 is used as a surface capping agent. The band observed around 3439 cm⁻¹ is assigned to the –OH vibration of triton X-100 along with water adsorbed on the surface of SnO₂ nanoparticles. The peak around 1248 cm⁻¹ and 1114 cm⁻¹ is due to asymmetric and symmetric C–O stretching of triton X-100, respectively. The bands around 2942 cm⁻¹ and 2876 cm⁻¹ indicates the presence of alkyl chain. This confirms that some of the molecules of triton X-100 are adsorbed on the surface of SnO₂ nanoparticles. Therefore, from the FT-IR spectra, it is evident that surfactant CPC and triton X-100 are adsorbed on the surface of SnO₂ nanoparticles and thus act as a good capping agent.

3.2. XRD studies

Fig. 2 represents the XRD patterns of the synthesized SnO₂ nanoparticles (S1, S2, and S3). The XRD pattern of the synthesized S1 nanoparticles (Fig. 2a) show peaks at 2θ values of 26.8°, 34.06°, 38.02°, 52.01°, 54.9°, 58.1°, 62.02°, 64.9°, 66.03°, 71.6° and 79.1°, which correspond to (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (1 1 2), (3 0 1), (2 0 2) and (3 2 1) planes, respectively. All the peaks are well indexed to the tetragonal rutile structure of SnO₂ nanoparticles (JCPDS 41-1445).^24,25

Fig. 2 XRD pattern of the synthesized SnO₂ nanoparticles (a) S1 (b) S2 and (c) S3.

The synthesized SnO₂ nanoparticles (S2) (Fig. 2b) show diffraction peaks at 2θ = 26.8°, 34.1°, 38.2°, 52.01°, 55.05°, 58.2°, 62.1°, 64.9°, 66.3°, 71.6° and 78.9°, which correspond to (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (1 1 2), (3 0 1), (2 0 2) and (3 2 1) planes, respectively. The obtained peak positions are in excellent agreement with the tetragonal rutile structure of SnO₂ nanoparticles (JCPDS 41-1445).^24,25

The XRD pattern of SnO₂ nanoparticles (S3) (Fig. 2c) show diffraction peaks at 2θ = 26.9°, 34.3°, 38.1°, 52.03°, 55.07°, 58.4°, 62.3°, 65.09°, 66.09°, 71.5° and 78.8° which correspond to the lattice planes (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (1 1 2), (3 0 1), (2 0 2) and (3 2 1), respectively. All the peaks are well indexed to the tetragonal rutile structure of SnO₂ nanoparticles (JCPDS 41-1445).^24,25

Therefore, the XRD data (Fig. 2) confirms the formation of SnO₂ nanoparticles possessing a tetragonal rutile crystal structure. The average crystallite size of synthesized SnO₂ nanoparticles can be calculated from the XRD data using the Debye–Scherrer equation:^24,25

where D is the crystallite size, λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the diffraction peak, θ is the Bragg diffraction angle and k is the so-called shape factor, whose value is about 0.9. The average crystallite size of S1, S2 and S3 nanoparticles calculated using eqn (1) are 4.2, 4.8 and 6.0 nm respectively.

Therefore, it is evident that the grain size of SnO₂ nanoparticles increased when surfactants are introduced as capping agents. The particle size of SnO₂ nanoparticles formed using the non-ionic surfactant (triton X-100) is greater than that formed using the cationic surfactant (CPC). Hence, from the above studies it is evident that the size of SnO₂ nanoparticles can be tuned by introducing surfactants as capping agent.

3.3. TEM and SAED studies

The morphology and size distribution of synthesized SnO₂ nanoparticles (S1) can be depicted from the TEM images (Fig. 3a) and SAED pattern (Fig. 3d). The microstructure of the nanoparticles is examined by high resolution transmission electron microscopy (HRTEM). Fig. 3c shows the HRTEM image of the SnO₂ nanoparticles (S1). From the TEM image (Fig. 3a), it is evident that spherical SnO₂ nanoparticles are formed with an average particle size of ∼4.0 nm. Interestingly, a clock-like structure is observed in the domain of spherical SnO₂ nanoparticles (Fig. 3b). The lattice spacing is calculated from the HRTEM image (Fig. 3c) and found to be 0.23 nm, which corresponds to (200) lattice plane. The SAED pattern (Fig. 3d) indicates the micro-crystalline structure of SnO₂ nanoparticles. The lattice spacings are calculated from Fig. 3d and found to be 0.32 nm, 0.25 nm and 0.17 nm, which correspond to the lattice planes (110), (101) and (211), respectively. The observed lattice planes are in excellent agreement with the reported tetragonal rutile structure of SnO₂ nanoparticles.^24,25

Fig. 3 (a) TEM microphotograph of the synthesized SnO₂ nanoparticles (S1), (b) a clock-like structure observed in the domain of synthesized spherical SnO₂ nanoparticles (S1), (c) HRTEM image of synthesized SnO₂ nanoparticles (S1), (d) SAED pattern of the synthesized SnO₂ nanoparticles (S1).

Fig. 4(a and c) show the TEM image and SAED pattern of SnO₂ nanoparticles formed using urea and cetyl pyridinium chloride (CPC). The microstructure of the nanoparticles is investigated by the HRTEM images (Fig. 4b). Fig. 4a shows the formation of spherical SnO₂ nanoparticles with an average particle size of ∼4.5 nm. The lattice spacing calculated from the HRTEM image (Fig. 4b) is found to be 0.17 nm, which corresponds to the (211) lattice plane. The SAED pattern (Fig. 4c) shows diffused concentric rings and reveals the microcrystalline nature of the SnO₂ nanoparticles. The diffusion is preferably due to the attachment of the surfactant at the surface of the nanoparticles. The lattice spacings are found to be 0.34 nm, 0.26 nm, 0.22 nm and 0.17 nm, which correspond to the lattice planes (110), (101), (200) and (211), respectively. The lattice planes obtained from the SAED pattern are in good agreement with the reported tetragonal rutile structure of SnO₂ nanoparticles.^24,25

Fig. 4 (a) TEM microphotograph of the synthesized SnO₂ nanoparticles (S2), (b) HRTEM image of the synthesized SnO₂ nanoparticles (S2), (c) SAED pattern of the synthesized SnO₂ nanoparticles (S2).

The morphology and the size distribution of SnO₂ nanoparticles formed using urea and triton X-100 can be depicted from TEM images (Fig. 5a) and SAED pattern (Fig. 5c). Fig. 5b shows the HRTEM images of the SnO₂ nanoparticles. The TEM image (Fig. 5a) depicts the formation of spherical SnO₂ nanoparticles with an average particle size of about ∼5.8 nm. The lattice spacings calculated from the HRTEM image (Fig. 5b) are found to be 0.21 nm and 0.17 nm, which correspond to (200) and (211) lattice planes. The SAED pattern (Fig. 5c) shows concentric diffraction rings, which reveal the microcrystalline nature of the SnO₂ nanoparticles. The lattice spacings are observed to be 0.33 nm, 0.25 nm, 0.23 nm and 0.17 nm, which correspond to the lattice planes (110), (101), (200) and (211), respectively. The obtained lattice plane depicts the tetragonal rutile structure of SnO₂ nanoparticles.^24,25

Fig. 5 (a) TEM microphotograph of the synthesized SnO₂ nanoparticles (S3), (b) HRTEM image of the synthesized SnO₂ nanoparticles (S3), (c) SAED pattern of the synthesized SnO₂ nanoparticles (S3).

Therefore, from the TEM images and SAED pattern, it is confirmed that SnO₂ nanoparticles are formed using urea with various surfactants. The results obtained from the TEM images and SAED pattern are in excellent agreement with the XRD data. The surface modification of SnO₂ nanoparticles is achieved by the introduction of cationic and non-ionic surfactants, namely CPC and triton X-100, respectively. The synthesized SnO₂ nanoparticles are spherical and microcrystalline in nature. The average particle size of SnO₂ nanoparticles formed using urea is about 4.0 nm, whereas the average particle size of CPC assisted SnO₂ nanoparticles is about 4.5 nm and that of triton X-100 assisted SnO₂ nanoparticles is about 5.8 nm. Therefore, it is evident that the grain size of SnO₂ nanoparticles increased when surfactants are introduced as capping agents. The particle size of SnO₂ nanoparticles formed using non-ionic surfactants (triton X-100) is greater than that of SnO₂ nanoparticles formed using cationic surfactants (CPC). This may be attributed to the fact that the surfactants used in the synthesis formed micellar encapsulation of different shapes and sizes. Accordingly, the size and shape of nanoparticles were influenced by that of the micelle formed.

3.4. Optical properties

Semiconductor nanoparticles showed a three dimensional quantum size effect in their electronic spectra. It was observed that the band gap energy increases with a decrease in particle size, and the absorption edge shows a blue shift. Fig. 6a shows the absorption spectra of the synthesized SnO₂ nanoparticles (S1). From the UV-vis spectra, it is apparent that the absorption onset is about 285 nm. The absorption spectra of CPC assisted SnO₂ nanoparticles (S2) is represented in Fig. 7a, which shows that the absorption onset is about 290 nm. Fig. 8a represents the UV-visible spectra of triton X-100 assisted SnO₂ nanoparticles (S3); the absorption onset is about 296 nm. Therefore, the absorption edge shows a red shift with an increase in the particle size. This indicates that the particles are in quantum regime.

Fig. 6 (a) Absorption spectra of the synthesized SnO₂ nanoparticles (S1), (b) plot of (αhν)² versus incident photon energy (hν) for the synthesized SnO₂ nanoparticles (S1).

Fig. 7 (a) Absorption spectra of the synthesized SnO₂ nanoparticles (S2), (b) plot of (αhν)² versus incident photon energy (hν) for the synthesized SnO₂ nanoparticles (S2).

Fig. 8 (a) Absorption spectra of the synthesized SnO₂ nanoparticles (S3), (b) plot of (αhν)² versus incident photon energy (hν) for the synthesized SnO₂ nanoparticles (S3).

The band gap energy (E_g) of the SnO₂ nanoparticles can be calculated from the absorption spectra using Tauc plot. For semiconductor nanoparticles, the following equation has been used to relate the absorption coefficient with incident photon energy:

α(ν)hν = K(hν − E_g)ⁿ (2)

where E_g is the band gap energy, hν is the incident photon energy, K is a constant, α(ν) is the absorption coefficient, which can be defined by the Beer–Lambert's law as follows:

α(ν) = 2.303Aρ/cl, where A is the absorbance, ρ is the density of the SnO₂ nanoparticles, c is the concentration, and l is the path length. The exponent ‘n’ in the eqn (2) depends on the type of transition, and n may have values 1/2, 2, 3/2, and 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. In the case of SnO₂ nanoparticles, the value of n is 1/2, for allowed direct transition. Therefore, by plotting (αhν)² versus hν, and by extrapolating the curve to the zero absorption coefficient, band gap (E_g) can be determined using eqn (2).²⁶

Fig. 6b, 7b, and 8b represent the plot of (αhν) ² versus photon energy (hν) for S1, S2 and S3 nanoparticles, respectively. By extrapolating the curve to the zero absorption co-efficient, band gap energy can be calculated using eqn (2). The intercept of the tangent to the plot provides a good estimation of the band gap energy. The band gap energy of SnO₂ nanoparticles synthesized using urea (S1) is 4.30 eV. The band gap energy of CPC assisted SnO₂ nanoparticles (S2) and triton X-100 assisted SnO₂ nanoparticles (S3) are found to be 4.25 eV and 4.15 eV, respectively.

In the case of semiconductors, band gap energy depends on the particle size. From Fig. 6b, 7b and 8b, it is evident that the band gap energy decreases with an increase in the grain size of SnO₂ nanoparticles, and the absorption edge shows a red shift. This is due to three dimensional quantum confinement effects that occur in semiconductor nanoparticles. This indicates that the synthesized SnO₂ nanoparticles are in quantum regime. The band gap energy of synthesized SnO₂ nanoparticles are greater than that of the bulk SnO₂ (3.6 eV). The selective growth of crystal and distinct morphology (clock-like structure) may influence a more significant blue shift of the band gap.

Table 2 summarizes the result obtained from FT-IR, XRD, TEM and UV-visible spectra for the synthesized SnO₂ nanoparticles (S1, S2 and S3). The bands in the FT-IR spectra confirm that the surfactant molecules act as capping agents in the formation of SnO₂ nanoparticles. Interestingly, the average particle size of the synthesized SnO₂ nanoparticles is found to increase by the introduction of cationic and non-ionic surfactant molecules, which act as capping agents. The grain size of SnO₂ nanoparticles synthesized using triton X-100 (non-ionic surfactant) is greater than that obtained using CPC (cationic surfactant). This may be due to the fact that the surfactants used in the synthesis form micellar encapsulation of various sizes and shapes. Accordingly, the size and shape of the nanoparticles will be influenced by that of the micellar structure formed. From the TEM images, it is evident that the synthesized SnO₂ nanoparticles (S1, S2 and S3) are spherical in nature. The band gap energy of the synthesized SnO₂ nanoparticles showed a clear blue shift from 4.15 to 4.30 eV with a decrease in particle size from 5.8 to 4.0 nm. This is due to the three dimensional quantum confinement effect shown by the synthesized SnO₂ nanoparticles.

Table 2 Summary of results obtained from FT-IR, XRD, TEM and UV-visible spectroscopy for synthesized SnO₂ nanoparticles (S1, S2 and S3)

SnO2 NPs	Precursor	Surfactant	FT-IR bands (cm^−1)	XRD (average crystallite size, nm)	TEM (average particle size, nm)	Morphology	Band gap energy (eV)
S1	SnCl2 + urea	—	3439 cm^−1 (νO–H str.), 1629 cm^−1 (νH2O def.), 610 cm^−1 (νSn–O–Sn str.)	4.2	4.0	Spherical (a clock-like structure obtained in the domain of spherical SnO2 nanoparticles)	4.30
S2	SnCl2 + urea	CPC (cationic surfactant)	3442 cm^−1 (νO–H str.), 3062 cm^−1 (νN–H str.), 2919 cm^−1 (νC–H asym. str.), 2850 cm^−1 (νC–H sym. str.), 1629 cm^−1 (νH2O def.), 600 cm^−1 (νSn–O–Sn str.)	4.8	4.5	Spherical	4.25
S3	SnCl2 + urea	Triton X-100 (non-ionic surfactant)	3439 cm^−1 (νO–H str.), 2942 cm^−1 (νC–H asym. str.), 2876 cm^−1 (νC–H sym. str.), 1619 cm^−1 (νH2O def.), 1248 (νO–H asym. str.), 1114 (νO–H sym. str.), 610 cm^−1 (νSn–O–Sn str.)	6.0	5.8	Spherical	4.15

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3.5. Reaction mechanism for the synthesis of SnO₂ nanoparticles

The most plausible mechanism for the synthesis of SnO₂ nanoparticles can be visualized as follows:

On microwave heating, urea decomposes to give ammonium hydroxide and carbon dioxide. The produced ammonium hydroxide then reacts with the precursor molecule, SnCl₂·2H₂O to form a white precipitate of tin hydroxide, Sn(OH)₂. This on further microwave heating decomposes to give tin oxide (SnO₂) nanoparticles.

Step I:

Step II:

SnCl₂·2H₂O + NH₄OH → Sn(OH)₂

Step III:

Sn(OH)₂ → SnO₂

3.6. Role of cationic and non-ionic surfactant in the synthesis of SnO₂ nanoparticles

The formation of a particle is a very complex process. It involves nucleation, growth, coagulation and flocculation, which are considerably influenced by the surfactant assemblies.²⁷ The addition of surfactants, viz. CPC and triton X-100, can affect the nucleation during the crystallization process of oxides. After nucleation, the surfactants can influence particle growth, coagulation and flocculation. Therefore, surfactants play an important role in the preparation of metal oxide nanoparticles.

The cationic surfactant, CPC is used as a capping agent in the synthesis of SnO₂ nanoparticles (S2). This method is based on the chelation of cations (metal) by a surfactant in an aqueous solution. On microwave irradiation, urea is attacked by the strong nucleophilic nitrogen atoms of cetylpyridinium chloride (CPC) molecules, which leads to the weakening of C=O double bonds in the structure of urea, and it forms transitional product, urea–cetylpyridinium chlorine (UCPC). At an appropriate temperature, the C=O bond will break and O²⁻ anion will be gradually generated, which then reacts with Sn²⁺ to form SnO₂ nanoparticles. With the formation of SnO₂ nanoparticles, the ligand (UCPC) interacts with them and effectively caps most of the surface of SnO₂ nanoparticles.²⁸

The synthesis of S3 nanoparticles were carried out in the presence of a non-ionic surfactant, triton X-100. This method involves the selective adsorption of surfactant molecules on the surface of the SnO₂ nanoparticles. The hydrophobic interactions between the surfactant molecules on adjacent nanoparticles are responsible for bringing together the obtained inorganic–organic hybrid building blocks. The reaction was carried out in an aqueous solution. During the reaction, hydrophilic poly(ethylene oxide) (PEO) chains of triton X-100 were immersed into water cores with the hydrophobic heads left outside. This leads to the formation of SnO₂ nanocrystals. The repulsion of the outer hydrophobic heads protected the nanoparticles from further aggregation.²⁹

3.7. Evaluation of photocatalytic activity of synthesized SnO₂ nanoparticles

Two different dyes, namely rhodamine B and methyl violet 6B, were chosen for evaluating the photocatalytic activity of SnO₂ nanoparticles. These dyes have different chromophoric groups and are selected from two different categories. Rhodamine B is a heteropolyatomic dye, whereas methyl violet 6B is a triphenyl methane dye.

The photocatalytic activity of SnO₂ nanoparticles (S1) were examined by adding 10 mg of the photocatalyst to 200 ml of 10⁻⁴ M aqueous solution of each dye. The degradation of the dye does not take place immediately when irradiated with sunlight. The degradation process involves photochemical reactions on the surface of the SnO₂ nanoparticles. Therefore, an increase in the surface area of the photocatalyst leads to the greater degradation of the dye. The size and the dispersion of the photocatalyst in the solution play an important role in the degradation of dye.

The photocatalytic activity of SnO₂ nanoparticles were examined by monitoring the changes in the absorption spectra of RhB and MV6B dye solution during their photodegradation process. Fig. 9a shows the absorption spectra of photocatalytic degradation of RhB dye using SnO₂ nanoparticles under direct sunlight. The UV-visible spectra of the dye (RhB) shows a strong absorption band around 553 nm, and the addition of SnO₂ nanoparticles leads to a decrease in the absorption band with time. It is observed that the intensity of the peaks gradually decreases with an increase in irradiation time. After completion of 3 h, the solution becomes colorless. The absorption band at 553 nm disappears indicating the complete decomposition of the dye. Fig. 10a represents the UV-visible spectra for the photocatalytic degradation of MV6B dye by sunlight irradiation. From the spectra, it is evident that MV6B shows a strong absorption band around 580 nm, which gradually decreases with irradiation time after the addition of SnO₂ nanoparticles. After 3 h, the solution becomes colorless, and the absorption band at 580 nm disappears, indicating complete destruction of chromophoric structure of the dye. The degradation of dye is a pseudo-first order reaction, and its kinetics may be expressed as follows:³⁰

ln(C₀/C) = kt (3)

where k is the rate constant, and C₀ and C are the absorbance or concentration before and after the degradation of dye, respectively.

Fig. 9 (a) Photocatalytic degradation of rhodamine B (RhB) dye by solar irradiation using synthesized SnO₂ nanoparticles (S1) as catalysts, (b) plot of ln(C₀/C) versus irradiation time for photodegradation of rhodamine B (RhB) dye using synthesized SnO₂ nanoparticles, (c) percentage efficiency of photodegradation of rhodamine B (RhB) dye with time.

Fig. 10 (a) Photocatalytic degradation of methyl violet 6B (MV6B) dye by solar irradiation using synthesized SnO₂ nanoparticles (S1) as catalysts, (b) plot of ln(C₀/C) versus irradiation time for photodegradation of methyl violet 6B (MV6B) dye using synthesized SnO₂ nanoparticles, (c) percentage efficiency of photodegradation of methyl violet 6B (MV6B) dye with time.

The rate constants for photodegradation of RhB and MV6B dyes can be calculated using eqn (3). Fig. 9(b) and 10(b) represent the plot of ln(C₀/C) versus irradiation time (t) for RhB and MV6B dyes. The plot gives a linear relationship. Therefore, slope of the line represents the rate constant (k) for photodegradation of RhB and MV6B dyes. The value of k is found to be 1.76 × 10⁻² min⁻¹ and 1.72 × 10⁻² min⁻¹ for RhB and MV6B dyes, respectively.

The percentage efficiency of photodegradation of dye was determined using the following equation:³¹

X = [(C₀ − C)/C] × 100 (4)

Fig. 9c graphically shows the percentage efficiency of photocatalytic degradation of RhB dye with time. It was observed that 99.01% of the dye photochemically degraded within 240 min under direct sunlight using SnO₂ nanoparticles. Fig. 10c represents the percentage efficiency of photocatalytic degradation of MV6B dye with time. It was evident that 98.3% of MV6B dye photochemically degraded within 240 min by solar irradiation using SnO₂ nanoparticles.

3.8. Mechanism of photodegradation of RhB and MV6B using SnO₂ nanoparticles

When the surface of the SnO₂ nanocatalyst is irradiated with an energy greater than its band-gap energy, it leads to the formation of holes (h⁺) in the valence band and an electron (e⁻) in the conduction band of SnO₂ nanoparticles. The holes (h⁺) act as an oxidizing agent and directly oxidize the pollutant or react with water to form hydroxyl radicals. The electron (e⁻) in the conduction band acts as a reducing agent and reduces the oxygen adsorbed on SnO₂ nanocatalyst.

The plausible mechanism for the photocatalytic degradation of RhB can be schematically visualized as:⁹

SnO₂ + hν → e⁻ + h⁺

H₂O + h⁺ → OH⁻ + H⁺

OH⁻ + h⁺ → ˙OH

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + H⁺ → ˙OOH

RhB + hν → RhB*

RhB* + SnO₂ → RhB + SnO₂ (e⁻)

SnO₂ (e⁻) + O₂ → SnO₂ + O₂⁻

SnO₂ (e⁻) + ˙O₂⁻ + H⁺ → SnO₂ + H₂O₂

SnO₂ (e⁻) + H₂O₂ → SnO₂ + ˙OH + OH⁻

h⁺ + RhB → degradation products

RhB* + O₂ or ˙OH or ˙O₂⁻ → degradation products

The probable degradation mechanism of MV6B on the SnO₂ nanocrystal may be visualized as follows:

SnO₂ + hν → e⁻ + h⁺

H₂O + h⁺ → OH⁻ + H⁺

OH⁻ + h⁺→ ˙OH

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + H⁺→ ˙OOH

MV6B + hν → MV6B*

MV6B* + SnO₂ → MV6B + SnO₂ (e⁻)

SnO₂ (e⁻) + O₂ → SnO₂ + O₂⁻

SnO₂ (e⁻) + ˙O₂⁻ + H⁺ → SnO₂ + H₂O₂

SnO₂ (e⁻) + H₂O₂ → SnO₂ + ˙OH + OH⁻

h⁺ + MV6B → degradation products

MV6B* + O₂ or ˙OH or ˙O₂⁻ → degradation products

The excited dye injects an electron to the conduction band of SnO₂, from which it is scavenged by pre-adsorbed oxygen, O₂, to form active oxygen radicals. These active radicals drive the photodegradation process. SnO₂ nanoparticles play a significant role as electron carriers. Such assisted photo processes provide an attractive route to treat dye pollutants using sunlight.

The schematic representation of photodegradation of rhodamine B and methyl violet 6B dyes using SnO₂ NPs is depicted as follows (Scheme 1).

Scheme 1 Schematic representation of the photodegradation process of rhodamine B and methyl violet 6B dye using SnO₂ NPs.

4. Conclusion

In this paper, we report a facile and green approach towards the synthesis of SnO₂ nanoparticles. A microwave heating method was developed for the synthesis of SnO₂ nanoparticles using urea. The synthesized SnO₂ nanoparticles were characterized by FT-IR, XRD, TEM, SAED and UV-visible spectroscopy. From the TEM images and SAED pattern, it is evident that the synthesized SnO₂ nanoparticles are spherical and microcrystalline in nature with a tetragonal rutile crystalline structure. The average particle size is about 4.0 nm. The role of cationic and non-ionic surfactants, namely, CPC and triton X-100 in the synthesis of SnO₂ nanoparticles were also studied. It was evident that CPC and triton X-100 act as capping agents. The introduction of surfactants with urea leads to an increase in the particle size of SnO₂ nanoparticles. From the TEM images and SAED pattern, it is apparent that CPC and triton X-100 assisted SnO₂ nanoparticles are spherical, considerably monodispersed and microcrystalline in nature with an average particle size of ∼4.5 nm and ∼5.8 nm, respectively. The particle size obtained from the TEM images is also in good agreement with that obtained from the XRD pattern. Therefore, it is apparent that the particle size increases by the introduction of surfactants which act as capping agents. This may be attributed to the fact that the surfactants that are used in the synthesis form micellar encapsulation of different sizes. Accordingly, the size and shape of the nanoparticles are influenced by that of the micellar structure formed by the surfactant molecules. The synthesized SnO₂ nanoparticles showed a three dimensional quantum confinement effect. Therefore, the band gap energy of synthesized SnO₂ nanoparticles shows a clear blue shift from 4.15–4.30 eV with a decrease in particle size from 5.8 nm–4.0 nm. Hence, from the above studies it can be stated that the size of SnO₂ nanoparticles can be tuned by introducing surfactants as capping agents. The synthesized SnO₂ nanoparticles act as an efficient photocatalyst for the degradation rhodamine B and methyl violet 6B under direct sunlight.

Acknowledgements

We, the authors, express our heartfelt thanks and gratitude to the Director, NIT Silchar and TEQIP-II for providing lab facilities and scholarship. Our special thanks are extended to NEHU, IIT Bombay and Gauhati University for providing TEM, IR and XRD data.

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