Ethylene glycol mediated synthesis of Ag₈SnS₆ nanoparticles and their exploitation in the degradation of eosin yellow and brilliant green

Abstract

Ag₈SnS₆ (silver tin sulfide) nanoparticles (17–18 nm) were synthesized using a chemical coprecipitation method using ethylene glycol as a solvent and capping agent and thiourea as a sulfur source at a temperature of 160 °C, 6 h. The as synthesized nanoparticles were characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM). The optical properties were investigated by using UV-visible NIR spectroscopy. The XRD pattern shows the formation of single phase Ag₈SnS₆ with a rhombohedral structure. Ethylene glycol mediated synthesis resulted in the formation of spherical Ag₈SnS₆ nanoparticles as seen from TEM images. Diffuse reflectance spectra show that Ag₈SnS₆ possesses an absorption profile across the whole visible-light and near-infrared region. Ag₈SnS₆ exhibits a band gap energy 1.12 eV. Ag₈SnS₆ shows powerful visible light photodegradation activity towards the degradation of eosin yellow and brilliant green dyes. The enhanced photocatalytic activity of Ag₈SnS₆ particles is attributed to its improved visible light absorption and efficient separation of photogenerated charge carriers. Studies with different quenchers suggest that superoxide radicals play a major role in the photodegradation process.

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1. Introduction

Since the discovery of photocatalytic water splitting by K. Honda and A. Fujishima in 1972, photocatalysis has received great interest. An immense research effort is continuing in the field of photocatalysis for various applications such as solar water splitting, solar cells, photofuel cells, and remediation of contaminants.¹ In order to utilize visible light energy efficiently fundamental progress has been made in developing visible light responsive catalysts for the degradation of organic pollutants. Although many photocatalysts have been reported for this purpose, their practical application is limited by their low efficiency under visible light irradiation. For example, TiO₂, and ZnO are promising photocatalysts but require UV activation due to their wide band gap. Therefore, alternative photocatalysts having visible light activity need to be explored.

The semiconducting metal sulfides are of interest because of their light-absorbing ability in the visible and short-wavelength near-infrared regions, which enable them to work as a promising visible light-driven photocatalyst because of its valence band contains S 3p orbitals that is lower in energy than O 2p orbitals. Different metal sulfides such as Cu₂S,² In₂S₃,³ Ag₂S–ZnS,⁴ and I–III/IV–VI semiconductor compounds like AgInS₂,⁵ AgGaS₂ nanoplates,⁶ ZnS–AgInS₂,⁷ ZnIn₂S₄,⁸ Cu–Zn–In–S⁹ etc. have been extensively studied due to their photovoltaic and photocatalytic application. Among them, Ag₈SnS₆ (silver tin sulfide) is semiconductor compound which responds in the whole solar energy spectrum and possesses a narrow band gap (1.3–1.5 eV). It is inexpensive, environmentally benign and has good chemical stability. Ag₈SnS₆ exhibit absorption coefficient¹⁰ at about 1 × 10⁴–1.5 × 10⁴ cm⁻¹ in the visible light. It has been used as catalyst in the degradation of dyes and a light absorber in photovoltaics.

Various methods have been reported to prepare silver tin sulfide, Ag₈SnS₆. Hu et al.¹¹ have prepared orthorhombic Ag₈SnS₆ submicropyramids (400–600 nm) with surface energy by solvent less synthetic method using carbon disulfide as a sulfur source and studied their photodegradation activity using methylene blue under tungsten-halogen lamp. Tang et al.¹² prepared polygonal flakes (200–300 nm) of Ag₈SnS₆ at 180 °C, 14–20 h by hydrothermal method using thiourea as a source of sulfur. Li et al.¹³ prepared Ag₈SnS₆ (60 nm) in one-step using solvothermal process in the temperature range of 180 °C for 14–18 h. Li et al.¹⁴ prepared Ag₈SnS₆ (20 nm) by a solvothermal process using ethylenediamine as solvent and elemental sulfur as a source of sulfur at 100 °C for 10 h. The above methods require much time to prepare Ag₈SnS₆ particles. Moreover, a very few reports are available in literature where Ag₈SnS₆ are used in photocatalysis.

Eosin yellow is an anionic halogen containing dye. It is highly water soluble and belongs to the fluorescein class of dye. The toxicological information of eosin yellow reveals that the dye may cause severe skin and eye irritation. It also damages DNA in gastrointestinal organs of living beings resulting thereby in several types of diseases in the human body. Inhalation of the dye reduces the pulmonary gas exchange capacity of the lungs. Its metabolites are highly toxic and carcinogenic¹⁵ in nature. Brilliant green is a triphenylmethane organic dye finds wide applications in biological staining, modern textile and leather industries, etc. These dyes have been reported to cause hypersensitivity, carcinogenicity and toxicity to living organisms. Brilliant green induces vomiting when swallowed and is toxic when ingested. It can cause eye damage and ophthalmic chemical burns.¹⁶ The photocatalytic degradation is a significant approach to destruct hazardous chemical wastes, from wastewater polluted with organic compounds. Keeping in view of toxicity of the eosin yellow and brilliant green, it was considered for its removal from wastewater using Ag₈SnS₆.

In the present study, Ag₈SnS₆ have been prepared in one step by chemical coprecipitation method using ethylene glycol as a solvent and capping agent and thiourea as a sulfur source at 160 °C, 6 h. This method is a simple, economical, green route and requires low reaction temperature and short reaction time to synthesize Ag₈SnS₆ nanoparticles. By using this route spherical Ag₈SnS₆ nanoparticles were obtained. For the first time, we evaluated the photodegradation activity of the synthesized Ag₈SnS₆ in the degradation of eosin yellow under solar irradiation, and brilliant green under tungsten lamp irradiation and solar irradiation.

2. Experimental section

2.1. Materials

Silver nitrate, (AgNO₃, 99%), ethylene glycol (99%) were purchased from Himedia Ltd. SnCl₂·2H₂O (99%, Merck), thiourea, (Merck, 99%) were of analytical-reagent grade and was used as received. Double distilled water was used for preparing solutions.

2.2. Synthesis of Ag₈SnS₆

Ag₈SnS₆ nanoparticles was prepared by chemical coprecipitation method¹⁷ using ethylene glycol solvent. 0.8 M AgNO₃, 0.1 M SnCl_2, 10 gm thiourea (SCN₂H₄) and 100 ml ethylene glycol was taken in three necked flask fitted with air condenser and then heated up to 160 °C, 6 h. The black precipitate was obtained. It was then centrifuged and washed with methanol and dried at 80 °C.

Ag⁺ and Sn²⁺ ions form a complex with EG, resulting in particle capping upon nucleation. Upon addition of thiourea into preformed Ag–EG, Sn–EG complex a competition between the thiourea and EG is introduced. The strong complexation¹⁸ between Ag⁺, Sn²⁺ ions and thiourea leads to the formation of Ag–thiourea and Sn–thiourea complexes in the precipitation method which prevent the production of a large number of free S²⁻ in the solution. Metal salts and thiourea both easily dissolves in ethylene glycol solvent which shows the formation of M–thiourea complexes. On heating at 160 °C the Ag–thiourea and Sn–thiourea complexes undergo thermal decomposition to produce EG capped silver tin sulfide, Ag₈SnS₆.

In the synthesis, ethylene glycol (EG) works not only solvent but also plays the role of a complexing and capping agent which restrict the growth during the synthesis of nanoparticles. Secondly, it has lower viscosity and high boiling point (197 °C) than other polyol provides reaction conditions adequate to greatly enhance solubility, diffusion and crystallizations that is to say, the nucleation and growth of Ag₈SnS₆ nanoparticles.

8Ag⁺ + 4(SCN₂H₄) → Ag₈(SCN₂H₄)₄

Sn²⁺ + 2(SCN₂H₄) → Sn(SCN₂H₄)₂

2.3. Characterization

Crystal structure of Ag₈SnS₆ particles were determined by using X-ray diffraction (XRD) analysis conducted on ‘X'Pert PRO, PANalytical X-ray diffractometer using CuKα radiation (λ = 1.5406 A). Measurements were performed in the 2θ range from 10° to 90°. High resolution transmission electron microscope imaging was done by using JEOL Model JEM-2100. UV–visible diffuse reflectance spectra (DRS) were recorded on Varian, Cary 5000 UV-Vis-NIR spectrophotometer. Photocatalytic activity of Ag₈SnS₆ particles was evaluated by using dye solutions: eosin yellow (20 mg L⁻¹) under solar radiation and brilliant green (10 mg L⁻¹) under solar radiation and in presence of a tungsten lamp (200 watt, Phillips). The absorbance spectra of the dye solutions before and after photo degradation were recorded on UV-VIS spectrophotometer (Cary 100 Bio).

25 mg and 100 mg of Ag₈SnS₆ particles was dispersed in a beaker containing 80 ml of eosin yellow and brilliant green solution, respectively (pH = 7.5). The mixture was continuously stirred using a magnetic stirrer at ambient temperature in the dark for 15 min in order to attain adsorption–desorption equilibrium prior to light irradiation. 5 ml of the aliquot was withdrawn at regular time intervals. The aliquot was centrifuged and quantitative determination of eosin yellow was performed by measuring its absorbance at λ = 509 nm and for brilliant green at λ = 624 nm.

The photodegradation efficiency of Ag₈SnS₆ particles was determined by equation, % degradation = [(C₀ − C_t)/C₀] × 100. Where C₀ is the initial concentration of dye and C_t is the concentration of dye after “t” minutes visible light irradiation. The degradation profiles¹⁹ were fitted in first order equation ln(C₀/C) = kt, where C is the concentration after degradation and C₀ is the concentration of the dye after dark adsorption, respectively, k is rate constant and t is time for light irradiation. The rate constant (k) was calculated from slope of straight line obtained by plotting ln(C₀/C) versus time (t).

3. Results and discussion

3.1. Structural, morphological and optical studies

Fig. 1 shows the XRD pattern of Ag₈SnS₆. The XRD pattern of Ag₈SnS₆ matches with JCPDS file 00-038-0434 (Ag₈SnS₆ Canfieldite, syn) and has single phase crystalline structure. The strong peaks at 2θ° = 27.54, 28.86, 31.60, 32.87, 35.90, 36.53, 43.78, and 47.94 are corresponding to the (4 1 1), (0 2 2), (5 1 0), (3 1 3), (5 1 2), (4 1 3), (6 0 3) and (4 2 4) planes of rhombohedral Ag₈SnS₆, respectively. The lattice parameters of the synthesized Ag₈SnS₆ were calculated by using cell refine software, and were found to be a = 15.23 Å, b = 7.54 Å, c = 10.68 Å. These values are consistent with JCPDS 00-038-0434. The crystallite size of Ag₈SnS₆ particles was calculated by using Debye Scherer equation, D = 0.99λ/β cos θ where, β full width at half maximum of the strongest peak, λ is the X-ray wavelength and θ is the angle of diffraction and was found to be 20 nm.

Fig. 1 (a) XRD pattern of silver tin sulfide, Ag₈SnS₆.

Fig. 2(a) shows TEM image of Ag₈SnS₆. TEM images showed the formation of spherical Ag₈SnS₆ nanoparticles with an average diameter of 17–18 nm. From the high resolution TEM image (Fig. 2(b)), the lattice spacing is obtained (0.302 nm) which corresponds to (1 2 2) lattice plane of Ag₈SnS₆. The selected area electron diffraction pattern of Ag₈SnS₆ (Fig. 2(c)) obtained from spherical particles indicated its crystalline nature.

Fig. 2 (a) TEM image of Ag₈SnS₆ particles, (b) high resolution TEM image and (c) electron diffraction pattern of Ag₈SnS₆ nanoparticles.

The use of ethylene glycol as a solvent, operating reaction temperature and time influences the crystallite size and morphology of Ag₈SnS₆.

The absorption thresholds of the prepared Ag₈SnS₆ particles was obtained from the UV-vis DRS curve (Fig. 3) using the tangent line extrapolation technique on the curve. The band gap energy (E_bg) was calculated using equation: E_bg = 1240/λ, where λ = 1100 nm. The band gap energy is found to be 1.12 eV, which is smaller than that of the bulk (1.39 eV).¹¹ A clear blue shift is observed in the band gap energy of synthesized Ag₈SnS₆ nanoparticles. This is because of enhancement of quantum confinement effect resulting from the decrease in the size of nanoparticles. The band gap allows Ag₈SnS₆ particles to respond the visible and short-wavelength near-infrared spectrum to drive photocatalytic degradation of organic pollutants.

Fig. 3 UV-vis DRS spectrum of prepared Ag₈SnS₆ nanoparticles.

3.2. Evaluation of photodegradation activity

Fig. 4(a) shows the absorbance spectra of eosin yellow dye solution in presence of Ag₈SnS₆ particles under solar irradiation. It has been observed that the maximum absorbance at 509 nm for eosin yellow disappears completely after solar irradiation indicating the complete destruction of the chromophoric structure of the dye. The photodegradation efficiency of eosin yellow using of Ag₈SnS₆ particles is represented by Fig. 4(b). It is observed that 92.23% of eosin yellow degraded photochemically within 60 min using Ag₈SnS₆. The photodegradation of eosin yellow reaction follows first order kinetics. The rate constant was calculated from Fig. 4(d) and was found to be 0.0545 min⁻¹ (Table 1).

Fig. 4 (a) UV-visible absorbance spectra of eosin yellow (20 mg L⁻¹) before and after solar light irradiation. (b) Percentage efficiency of photodegradation of eosin yellow with time. (c) Effect of different quenchers on photodegradation of eosin yellow using Ag₈SnS₆ particles in 45 minutes. (d) Plot of ln(C₀/C) versus irradiation time, t, for photodegradation of eosin yellow dye using as synthesized Ag₈SnS₆ nanoparticles.

Table 1 % Degradation, rate of photogdegradation for eosin yellow (20 mg L⁻¹) under solar light, and brilliant green (10 mg L⁻¹) under solar light and tungsten lamp irradiation

Catalyst	Eosin yellow		Brilliant green
	Solar irradiation		Tungsten lamp		Solar irradiation
	Rate (min^−1)	% Degradation	Rate (min^−1)	% Degradation	Rate (min^−1)	% Degradation
Ag8SnS6	0.0545	92.23	0.0264	92.31	0.0036	73.43

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Fig. 5(a) and (b) shows the absorbance spectra of brilliant green dye solution in presence of Ag₈SnS₆ particles under tungsten lamp and solar irradiation, respectively. The maximum absorbance at 624 nm for brilliant green disappears completely in presence of tungsten lamp irradiation.

Fig. 5 UV-visible absorbance spectra of brilliant green (10 mg L⁻¹) under (a) tungsten lamp irradiation and (b) solar light irradiation. (c) Percentage efficiency of photodegradation of brilliant green with time under tungsten lamp irradiation. (d) Effect of different quenchers on photodegradation of brilliant green using Ag₈SnS₆ particles in 90 minutes. (e) Plot of ln(C₀/C) versus irradiation time, t, for photodegradation of brilliant green dye using synthesized Ag₈SnS₆ nanoparticles.

The photodegradation efficiency of brilliant green using of Ag₈SnS₆ particles is represented by Fig. 5(c). From the graph, it is evident that 92.31% of brilliant green dye degraded within 90 min using Ag₈SnS₆ as a photocatalyst under tungsten lamp whereas only 73.43% degradation of brilliant green solution has been achieved after 5 h solar irradiation (Fig. 5(b)). The photodegradation of brilliant green follows first order kinetics. The rate constant was calculated from Fig. 5(e) and was found to be 0.0264 min⁻¹ (Table 1). The enhanced photo degradation of eosin yellow and brilliant green is attributed to small size and spherical morphology of Ag₈SnS₆ and more visible light absorption ability of Ag₈SnS₆.

The catalytic activity of Ag₈SnS₆ nanoparticles was compared using statistical method. Ag₈SnS₆ submicropyramids¹¹ decomposes 90% of MB molecules after 120 min of visible irradiation, and P25 TiO₂ decomposes only 25% under tungsten halogen lamp. Whereas, as-synthesized Ag₈SnS₆ spherical particles shows 92.23% of eosin yellow degradation within 60 min under sunlight irradiation and 92.31% of brilliant green dye degraded within 90 min under tungsten lamp irradiation. Interestingly, Ag₈SnS₆ spherical nanoparticles exhibit superior photocatalytic performance than Ag₈SnS₆ submicropyramids and commercial P25 TiO₂ under visible light, which may be due to small size and spherical morphology of Ag₈SnS₆.

For the regeneration of catalyst, the used catalysts were collected by centrifugation and washed with methanol and dried in air oven.

To elucidate the active species responsible for the visible light photocatalytic degradation, various quenchers were added to aqueous solution containing dyes: eosin yellow (20 mg L⁻¹) and 25 mg Ag₈SnS₆, brilliant green (10 mg L⁻¹) and 100 mg Ag₈SnS₆ and the photodegradation activity were studied. Quenchers are certain chemicals which hinder the action of certain specific species for the degradation reaction by trapping them during the course of photocatalytic experiment. The quenchers²⁰ employed were 5 ml isopropanol (Pr) for hydroxyl radicals, 0.1 g ammonium oxalate (AO) to trap photogenerated holes (h⁺), 10⁻³ M 1,4-benzoquinone (BQ) for superoxide anion radical. Fig. 4(c) and 5(d) shows the effect of different quenchers on the photocatalytic activity of Ag₈SnS_6. On adding isopropanol no distinct changes were observed in the performance of Ag₈SnS₆ indicating that bulk hydroxyl radicals do not take part in the degradation process. When ammonium oxalate is added to the solution the decolorization rate of eosin yellow is also increased. That means that holes (h⁺) do not take part in the degradation process. When benzoquinone is added into the reaction solution, photocatalytic efficiency Ag₈SnS₆ was decreased indicating O₂˙⁻ radicals play a major role in the degradation process of eosin yellow under solar light irradiation. Similar kind of reaction was also observed with brilliant green dye degradation when benzoquinone was added in the solution in presence of tungsten lamp (Fig. 4(d)). It is obvious that addition of BQ shows a major effect on the photodegradation process, manifesting that superoxide radical (O₂˙⁻) played a significant role in photodegradation of dyes: eosin yellow and brilliant green. On addition of BQ, there has been 23.29% and 26.31% decrease in degradation efficiency of eosin yellow and brilliant green occurred, respectively. On the basis of above results, the possible reactions occurring during the photocatalytic degradation process of eosin yellow and brilliant green over Ag₈SnS₆ particles are given below.

Ag₈SnS₆ + hν → Ag₈SnS₆ (h_VB⁺ + e_CB⁻)

e_CB⁻ + h_VB⁺ → energy

e_CB⁻ + O₂ → O₂˙⁻

O₂˙⁻ + H⁺ → ˙OOH

˙OOH + ˙OOH → H₂O₂ + O₂

O₂˙⁻ + eosin yellow, brilliant green → intermediates → CO₂ + H₂O

˙OOH + eosin yellow, brilliant green → CO₂ + H₂O

In the presence of visible light, electrons are promoted from the valence band (VB) to the conduction band (CB). As a result of this, negative-electron (e⁻) and positive-hole (h⁺) pair are generated. Both these entities can migrate to the catalyst surface, where they can enter in a redox reaction with other species present on the surface. e_CB⁻ can react with O₂ to produce superoxide radical anion of oxygen which have a powerful oxidation ability to degrade eosin yellow and brilliant green dyes.

4. Conclusions

In this paper, we reported a cost-effective, green method for synthesizing Ag₈SnS₆ nanoparticles by coprecipitation method using ethylene glycol solvent and thiourea as a sulfur source where ethylene glycol acts as a good capping agent. The formation of extremely small sized Ag₈SnS₆ nanoparticles took place within 6 h at 160 °C. X-ray powder diffraction pattern and transmission electron microscope images showed that the product is the Ag₈SnS₆ which is well crystallized. TEM images showed the formation of spherical Ag₈SnS₆ nanoparticles with an average diameter of 17–18 nm. Hence, the ethylene glycol mediated coprecipitation method proved to be an efficient and environmental friendly method for the synthesizing Ag₈SnS₆ nanoparticles. XRD and SAED pattern confirms the rhombohedral crystalline structure of Ag₈SnS₆ nanoparticles. The optical properties were studied using UV-visible NIR spectroscopy. A clear blue shift was observed in the band gap energy of synthesized Ag₈SnS₆ nanoparticles (1.12 eV) from the bulk Ag₈SnS₆ (1.39 eV) because of quantum confinement effect. The synthesized Ag₈SnS₆ nanoparticles act as an efficient catalyst in the degradation of eosin yellow and brilliant green dyes. The complete degradation of eosin yellow took within 45 minutes under solar irradiation and brilliant green dye degraded within 90 min under tungsten lamp irradiation. Superoxide anions are the main active species involved in photodegradation of eosin yellow and brilliant green dyes. Ag₈SnS₆ particles are found to be photo-stable and can be recycled. Ag₈SnS₆ provides the basis for a very promising and practical environmental technology for the efficient treatment of dye effluent.

Acknowledgements

The author thanks to SAIF Cochin, SAIF Chandigarh for characterization assistance.

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