Ethylene glycol mediated synthesis of SnS quantum dots and their application towards degradation of eosin yellow and brilliant green dyes under solar irradiation

Abstract

SnS (tin sulfide) quantum dots (QDs) were synthesized by 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, 4 h. The as synthesized SnS QDs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis-NIR spectroscopy and FT-Raman spectroscopy. XRD patterns show the formation of single phase SnS QDs with rhombohedral structure. Ethylene glycol mediated synthesis resulted 2.5–3 nm SnS QDs. The UV-Vis-NIR optical absorption spectra of the SnS QDs displayed that the SnS QDs possess an absorption profile across the whole visible-light and near-infrared region. The direct band gap and indirect band gap energy for SnS QDs are found to be 1.17 eV and 1.11 eV, respectively. FT-Raman spectra of SnS QDs demonstrate vibrational modes at 73, 97, 162, 188, 222 cm⁻¹. The Brunauer–Emmett–Teller (BET) surface area of SnS QDs was found to be 5.63 m² g⁻¹. SnS QDs showed powerful photodegradation activity towards degradation of eosin yellow and brilliant green dyes under sunlight. The enhanced photocatalytic activity of SnS QDs is attributed to improved visible light absorption and efficient separation of photogenerated charge carriers. In addition, the quenching effects of different quenchers suggest that superoxide radicals play a major role in the photodegradation process.

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1. Introduction

Pollution of the environment by textiles, cosmetics and food industries is increasing. Industrial waste water mostly contains dyes like eosin yellow, brilliant green etc., which are used in textile industries. These are very hazardous for environment due to their toxicity to living organisms and carcinogenetic effects. Therefore, these dyes have to be removed in order to reuse the water. Different materials like mesoporous TiO₂,¹ carbon functionalized TiO₂ nanofibers,² Ag nanoparticles-coated TiO₂ nanofiber composites,³ TiO₂–NiO mixed oxide nanocrystals,⁴ nanofibrous TiO₂-core/conjugated polymer composite⁵ etc. have been employed for removal of dyes from waste water. The degradation of organic dyes is achieved under UV-light. Photocatalytic degradation of organic pollutants from wastewater using solar radiation is a potential approach to control the environmental pollution as solar energy is renewable. Hence, it is essential to develop effective, visible light responsive catalytic materials to solve the issues related to energy and environment. For harnessing the solar radiation effectively as solar radiation contains around 40% visible light, researchers are developing the suitable semiconductor photocatalysts based on metal oxides, sulfides, oxynitrides and graphitic carbon nitride^6–9 to make the process efficient. An improved visible light absorption and enhanced photodegradation has been informed in number of cases.

The IV–VI series of semiconducting materials like CdS, CdSe, PbS, SnS, PbSe and SnTe etc. are of interest primarily because of their narrow band gap and potential applications in solar cells, detectors as a optically active components in the near-infrared and infrared spectral region.¹⁰ These materials show efficient multiple exciton generation which is the ability to generate more than one electron–hole pair per high energy photon absorbed and provide a way to overcome the Shockley–Queisser efficiency.¹¹

SnS (tin sulfide) is an important binary chalcogenide. It is a layered semiconductor and possess an orthorhombic crystal structure. It is an inexpensive, environmentally benign and has good chemical stability. Tin sulfide exist in different phases^12,13 such as SnS, SnS₂, Sn₂S₃, Sn₃S₄ and Sn₄S₅. Among these, SnS is p-type and SnS₂ is n-type semiconductor.¹⁴ SnS has both a direct optical gap located at 1.3 eV and indirect optical band gap located at 1.09 eV. The absorption coefficient of SnS is 10⁴ cm⁻¹ and possess high conductivity.¹⁵ SnS have been used as a light absorber in photovoltaics, anode material in lithium ion batteries, sensors, capacitors, near-infrared detector and holographic recording and a visible light driven photocatalyst.^16,17

Quantum dots (QDs) have unique advantages^18,19 of having simple synthesis, tunable band gap energy by controlling the particle size, multiple exciton generation from single photon absorption and large intrinsic dipole moment. To our knowledge, there are a very few reports that present synthetic methodologies to forming SnS QDs and their application in the photocatalysis. Muthuvinayagam et al.¹⁷ prepared SnS QDs (2–5 nm) by one-pot hydrothermal approach using SnCl₂·2H₂O and thiourea at 200 °C, 8 h. Prastani et al.¹¹ prepared SnS QDs with a size of ∼4 nm using colloidal route. The synthesis was carried out using precursors such as SnCl₂·2H₂O, triethanolamine, sodium sulphide, ethylene glycol and trioctylphosphine oxide at room temperature under nitrogen atmosphere. Xu et al.²⁰ prepared SnS QDs using triethanolamine ligand, SnBr₂ and sodium sulfide. Deepa and Nagaraju²¹ prepared SnS QDs by SILAR method. Tang et al.²² synthesized SnS nanoparticles (5 nm) using starting materials like SnCl₂·2H₂O, octadecene, trioctylphosphine, oleic acid, oleyamine and thioacetamide at 135 °C and reported their photocatalytic activity towards rhodamine B under halogen lamp. Das and Dutta²³ prepared SnS nanorods by using mercaptoacetic acid as capping agent and proved to be an efficient photocatalyst for dye degradation of trypan blue dye under sunlight.

In the present study, we report facile synthesis of SnS QDs by chemical coprecipitation method using ethylene glycol as a solvent and complexing agent and thiourea as a sulfur source at 160 °C, 4 h. This method is a simple, economical route and requires low reaction temperature and short reaction time to synthesize SnS QDs. By using this route spherical SnS QDs were obtained. For the first time, we evaluated the photodegradation activity of as-synthesized SnS QDs in the degradation of eosin yellow and brilliant green under solar irradiation. Our results indicate that the formed SnS QDs showed the enhanced photodegradation for eosin yellow and brilliant green because of improved visible light absorption and efficient separation of photogenerated charge carriers.

2. Experimental section

2.1. Materials

Stannous chloride, SnCl₂·2H₂O (Merck, 99%), ethylene glycol (Himedia, 99%), thiourea (SCN₂H₄) (Merck, 99%), P25 TiO₂ (Sigma Aldrich, 99.5%), eosin yellow and brilliant green (Himedia) were of analytical-reagent grade and was used as received.

2.2. Synthesis of SnS QDs

SnS QDs were prepared by chemical coprecipitation method^24,25 by treating SnCl₂ with thiourea in ethylene glycol medium. Briefly, SnCl₂ (2.25 g, 0.1 M) was dissolved in 100 ml water and mixed this solution with 100 ml ethylene glycol (EG). The mixture was then transferred to three necked flask fitted with air condenser and 10 g thiourea was added into it and refluxed to 160 °C for 4 h. The black precipitate was obtained. It was then centrifuged and washed with methanol and dried at 80 °C, 1 h.

Sn²⁺ ions form a complex with EG. Upon addition of thiourea into preformed Sn–EG complex, a competition between the thiourea and EG is introduced. The strong complexation between Sn²⁺ ions and thiourea leads to the formation of Sn–thiourea complex in the precipitation method which prevent the production of a large number of free S²⁻ in the solution. SnCl₂ and thiourea both easily dissolves in ethylene glycol solvent which indicates the formation of [Sn(SCN₂H₄)_n]²⁺ complex. The produced complex serve as both the tin source and the sulfur source. On heating at 160 °C, Sn–thiourea complex undergo thermal decomposition to produce EG capped tin sulfide QDs due to the rupture of coordinate bonds between Sn²⁺ and thiourea. EG acts as the stabilizing ligand. Hydroxyl groups on EG control the size of SnS QDs. EG has two hydroxyl groups to bind more strongly to the QDs as they grow.

SnCl₂ + n(EG) → [Sn(EG)_n]²⁺

[Sn(EG)_n]²⁺ + SCN₂H₄ → [Sn(SCN₂H₄)_n]²⁺

2.3. Characterization

Crystal structure of SnS QDs were determined by using X-ray diffraction (XRD) analysis conducted on ‘X'Pert PRO, PANalytical X-ray diffractometer using Cu Kα radiation (λ = 1.5406 A). Measurements were performed in the 2θ range from 10° to 90°. High resolution transmission electron microscope (TEM) imaging was done by using JEOL Model JEM-2100. UV-visible diffuse reflectance spectra (DRS) of SnS QDs was recorded on Varian, Cary 5000 UV-Vis-NIR spectrophotometer. Fourier Transform (FT)-Raman spectra of SnS QDs was recorded on Bruker RFS 27 Stand-alone FT-Raman spectrometer with scan range 50–4000 cm⁻¹ and resolution 2 cm⁻¹. The laser source was Nd:YAG 1064 nm. The Brunauer–Emmett–Teller (BET) surface area of SnS QDs was determined by nitrogen adsorption–desorption isotherms using Micromeritics ASAP 2010 surface area analyzer by nitrogen adsorption at 77 K. Photocatalytic activity of SnS QDs was evaluated by using dye solutions such as eosin yellow (20 mg L⁻¹) and brilliant green (10 mg L⁻¹) under solar radiation. 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 SnS particles was dispersed in a beaker containing 100 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 SnS QDs 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 SnS QDs. The XRD pattern of SnS QDs matches with JCPDS file 01-073-1859 (SnS) and has rhombohedral structure. The set of lattice planes and their corresponding d-spacing are shown in Table 1. The lattice parameters of the synthesized SnS QDs were calculated by using cell refine software, and were found to be a = 11.98 Å, b = 3.98 Å, c = 4.32 Å. These values are consistent with JCPDS 01-073-1859. The crystallite size of SnS QDs 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 15 nm.

Fig. 1 XRD pattern of SnS QDs.

Table 1 Lattice planes and their corresponding d-spacing of SnS QDs

2[theta] (°)	d-Spacing (Å)	h k l values
16.57	5.34	(2 0 0)
22.15	4.01	(1 0 1)
26.15	3.40	(2 0 1)
27.57	3.23	(2 1 0)
30.57	2.92	(0 1 1)
31.62	2.82	(1 1 1)
32.00	2.79	(3 0 1)
39.09	2.30	(3 1 1)
44.77	2.02	(4 1 1)
45.53	1.99	(0 2 0)
48.68	1.87	(3 0 2)
50.35	1.81	(2 1 2)
51.30	1.78	(5 1 1)
54.29	1.68	(6 1 0)
56.6	1.62	(4 2 0)
64.24	1.44	(5 1 2)

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Fig. 2(a) and (b) shows TEM image of SnS QDs. TEM images showed the formation of spherical SnS QDs with size of 2.5–3 nm. The selected area electron diffraction (SAED) pattern of SnS QDs (Fig. 2(c)) obtained from spherical particles revealed the formation of circular diffraction rings, which reflects the polycrystalline nature of SnS QDs. The size distribution of SnS QDs is uniform and average particle size is 5.1 nm (the histogram in Fig. 2(d)). Energy dispersive X-ray spectroscopy (EDX) measurement was done to know the chemical composition of SnS QDs. The EDX spectrum revealed the characteristic L X-ray peak of Sn and the K X-ray peak of S (Fig. 3). The atomic concentrations of Sn and S were measured as 54.69% and 45.31% respectively.

Fig. 2 TEM image of SnS QDs (a and b) and (c) SAED pattern of SnS QDs and (d) histogram of SnS QDs size distribution.

Fig. 3 EDX pattern of SnS QDs.

Fig. 4(a) and (b) shows the UV-Vis-NIR optical reflectance and absorption spectra of SnS QDs, respectively. The spectra displayed a continuous absorption from the UV-Vis region to near infra-red region. The broad spectrum allows SnS QDs to act as a good visible light-sensitive photocatalyst to drive photocatalytic degradation of organic pollutants. The optical band gap of the prepared SnS QDs was obtained from the UV-Vis-NIR optical absorption curve (Fig. 4(b)) using the tangent line extrapolation technique on the curve. The band gap energy (E_bg) was calculated by using simple equation: E_bg = 1240/λ (absorption edge), assuming that the prepared SnS QDs are direct crystalline semiconductor. The as-synthesized SnS QDs exhibits an absorption onset at 1070 nm, which corresponds to band gap energy of 1.15 eV. In order to get more precise values of the optical band gaps, the values of E_g were calculated with the help of the Tauc equation,²⁶ (αhν) = A(hν − E_g)ⁿ, where α is the absorption coefficient. hν and A are the photon energy and a constant relative to the material, respectively and n is either 2 for a direct allowed electronic transition (direct band gap) or 1/2 for an indirect allowed electronic transition (indirect band gap). For this purpose, (αhν)^1/2 was plotted as a function of photon energy hν (eV) for the indirect gap and (αhν)² against photon energy hν (eV) for the direct gap. Fig. 4(c) and (d) are presentation of these plots. The linear intercept at hν on x-axis gives the value of optical band gap. The direct band gap and indirect band gap energy for SnS QDs are found to be 1.17 eV and 1.11 eV, respectively. The direct band gap is very close to that of silicon (1.11 eV).²⁷ The band gap energy varies with synthetic temperature of reaction according to previous reported results.²⁸ The optical direct band gap and indirect band gap¹⁶ in SnS nanoparticles are 3.6 eV and 1.6 eV, respectively and in bulk SnS, the direct band gap and the indirect band gap are 1.3 eV and 1.09 eV respectively. Band gap energy values of as prepared SnS QDs are compared with nanoparticles, it is seen that direct and indirect transition of SnS QDs are shifted towards lower energy values. Compared to bulk SnS, blue shift of 0.13 eV is observed for direct transition. This is because of enhancement of quantum confinement effect resulting from the decrease in the size of nanoparticles.

Fig. 4 UV-visible-NIR (a) reflectance and (b) absorbance spectrum of as-prepared SnS QDs. Tauc plots for direct band gap (c) and indirect band gap (d).

Raman spectroscopy (RS) is used to probe the detailed structure of materials. It is a scattering technique based on Raman effect, i.e., frequency of a small fraction of scattered radiation is different from frequency of monochromatic incident radiation. It is based on the inelastic scattering of incident radiation through its interaction with vibrating molecules. The 24 vibrational modes²⁹ for orthorhombic structure of SnS are represented as Γ = 4A_g + 2B_1g + 4B_2g + 2B_3g + 2A_u + 4B_1u + 2B_2u + 4B_3u.

Among them, SnS has 21 optical phonons, of which 12 are Raman active modes (4A_g + 2B_1g + 4B_2g and 2B_3g), seven infrared active modes (3B_1u + 1B_2u and 3B_3u) and two are inactive (2A_u). Fig. 5 shows the FT-Raman spectra of the as-synthesized SnS QDs. SnS QDs shows vibrational modes at 73, 97, 162, 188, 222 cm⁻¹. The peak at 73 cm⁻¹, 97 cm⁻¹ and 162 cm⁻¹ are corresponding to B_1g or B_2g mode, A_g mode and B_3g mode, respectively. The peaks at 188 cm⁻¹ and 222 cm⁻¹ can be assigned to the A_g mode. It has been reported that vibrational modes^14,30 for SnS nanoparticles was detected at 77, 95, 163, 191, and 220 cm⁻¹. Vibrational modes of as prepared SnS QDs shows a slight shift towards lower wave number in comparison to SnS nanoparticles. This is attributed to phonon confinement. FT-Raman results indicate that the formation of SnS QDs from the present process.

Fig. 5 Raman spectra of as-prepared SnS QDs.

3.2. BET surface area

Fig. 6 shows the nitrogen adsorption–desorption isotherms of as-prepared of SnS QDs. The SnS QDs shows type III isotherm. This indicates that there is a weak interaction between adsorbate and adsorbent i.e. between N₂ molecules and SnS QDs. BET surface area of SnS QDs is found to be 5.63 m² g⁻¹ and pore volume of 0.035 cm³ g⁻¹. Type III isotherm is mainly due to ink-bottle pores and the mesoporosity of pore size distribution is under pore diameter 25 nm. Results from desorption BJH pore volume distribution and pore area distribution confirmed mesoporous structure of prepared SnS QDs.

Fig. 6 N₂ adsorption–desorption isotherms of as-prepared SnS QDs.

3.3. Evaluation of photodegradation activity

Fig. 7(a) shows the absorbance spectra of eosin yellow dye solution in presence of SnS QDs under solar irradiation. The maximum absorbance for eosin yellow is observed at 509 nm that disappears completely after solar irradiation indicating the complete destruction of the chromophoric structure of the dye. The photodegradation efficiency of eosin yellow by SnS QDs is represented by Fig. 7(b). It is observed that 91.67% of eosin yellow degraded photochemically within 60 min by SnS QDs. The photodegradation rate constant of eosin yellow was calculated from Fig. 7(c) and was found to be 0.0235 min⁻¹.

Fig. 7 (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) Plot of ln(C₀/C) versus irradiation time for photodegradation of eosin yellow dye using as synthesized SnS QDs. (d) Effect of different quenchers on photodegradation of eosin yellow using SnS QDs in 60 minutes. (e) Recyclability.

Fig. 8(a) shows the absorbance spectra of brilliant green dye solution in presence of SnS QDs under sunlight irradiation. The maximum absorbance of brilliant green dye exhibits at 624 nm which disappears completely in presence of solar irradiation. The photodegradation efficiency of brilliant green using SnS QDs is represented by Fig. 8(b). From the graph, it is evident that 88.82% of brilliant green dye degraded using SnS QDs as a photocatalyst within 90 min of sunlight irradiation. The photodegradation rate constant of brilliant green was calculated from Fig. 8(c) and was found to be 0.0158 min⁻¹.

Fig. 8 (a) UV-visible absorbance spectra of brilliant green (10 mg L⁻¹) under solar light irradiation. (b) Percentage efficiency of photodegradation of brilliant green with time. (c) Plot of ln(C₀/C) versus irradiation time for photodegradation of brilliant green dye using as synthesized SnS QDs. (d) Effect of different quenchers on photodegradation of brilliant green using SnS QDs in 90 minutes. (e) Recyclability.

The photodegradation activity of SnS QDs was compared with commercial P25 TiO₂. The rate of degradation of eosin yellow using P25 TiO₂ is found be same as that of SnS QDs, while P25 TiO₂ decomposes only 68.36% of brilliant green within 90 min under sunlight irradiation. The enhanced photocatalytic activity of SnS QDs is attributed to improved visible light absorption and efficient separation of photogenerated charge carriers.

The used catalysts was regenerated by centrifugation and washing with methanol and finally 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 SnS, brilliant green (10 mg L⁻¹) and 100 mg SnS 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 quenchers³¹ employed were 5 ml isopropanol (Pr) for hydroxyl radicals, 0.1 g ammonium oxalate (AO) for holes, 10⁻³ M 1,4-benzoquinone (BQ) for superoxide radical. Fig. 7(d) and 8(d) shows the effect of different quenchers on the photocatalytic activity of SnS. On adding isopropanol no distinct changes in the performance of SnS were observed. This indicates that bulk hydroxyl radicals do not take part in the degradation process. When AO 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 BQ was added into the reaction solution, photocatalytic efficiency SnS QDs was decreased indicating superoxide radicals (O₂˙⁻) are responsible for degrading eosin yellow under solar light irradiation. Similar kind of reaction was also observed with brilliant green dye degradation when BQ was added in the solution in presence of solar light irradiation (Fig. 8(d)). It is obvious that addition of BQ shows a major effect on the photodegradation process, manifesting that (O₂˙⁻) played a significant role in photodegradation of dyes: eosin yellow and brilliant green. On addition of BQ, there has been 29.17% and 43.73% decrease in degradation efficiency of eosin yellow and brilliant green occurred, respectively. On the basis of above results, a possible reaction mechanism occurring during the photocatalytic degradation process of eosin yellow and brilliant green over SnS QDs are given below.

SnS + hν → SnS(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 in the valence band (VB) of SnS QDs are promoted to the conduction band (CB). As a result of this, a high amount of negative-electron (e⁻) and positive-hole (h⁺) pairs are generated. Both e⁻ and h⁺ pairs can migrate to the catalyst surface, where they can enter in a redox reaction with dyes 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. The stability of SnS QDs photocatalyst was checked by running the degradation experiment for three repeated cycles using the same catalyst. From Fig. 7(e) and 8(e), it is clear that photocatalytic activity is same for three repeated cycles indicating that the catalyst is stable and can be reused.

4. Conclusions

In this paper, we reported a cost-effective process for synthesizing SnS QDs by coprecipitation method using ethylene glycol solvent and thiourea as a sulfur source where ethylene glycol acts as a good complexing and capping agent. The formation of SnS QDs took place within 4 h at 160 °C. X-ray powder diffraction pattern and transmission electron microscope images showed that the product is stannous sulphide, SnS which is well crystallized. TEM images showed the formation of spherical SnS QDs with an average diameter of 3 nm. XRD and SAED pattern confirms the rhombohedral crystalline structure of SnS QDs. EDX measurement revealed the presence of tin and sulfur. FT-Raman study also endorses the formation of SnS QDs by EG route. The direct optical band gap of synthesized SnS QDs (1.17 eV) calculated through Tauc plot shows a blue shift of optical band gap energy from the bulk SnS (1.3 eV) because of quantum confinement effect. The photodegradation studies reveal that as-synthesized SnS QDs act as an efficient catalyst in the removal of eosin yellow and brilliant green dyes from waste water. The complete decolorization of eosin yellow and brilliant green took within 60 minutes and 90 minutes of solar irradiation, respectively. Superoxide ions are the main active species involved in photodegradation of eosin yellow and brilliant green dyes. SnS QDs are found to be photo-stable and can be recycled. SnS QDs play very important role in the photocatalytic property which may help in solving issues related to environmental pollution by utilizing solar energy effectively.

Acknowledgements

The author thanks to following institutions for providing technical support: STIC Cochin, SAIF Chandigarh, VNIT Nagpur and SAIF Madras for XRD, TEM, EDX, UV-Vis-NIR and FT-Raman studies. One of the author Arpita Chowdhury is thankful to Director NIT Silchar for PhD fellowship.

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