КОЛЛОИДНЫЙ ЖУРНАЛ, 2013, том 75, № 1, с. 118-122
УДК 541.18
A NEW AND SIMPLE METHOD FOR SULFUR NANOPARTICLES SYNTHESIS
© 2013 г. M. Soleimani1, F. Aflatouni, A. Khani
Department of Chemistry, Imam Khomeini International University (IKIU)
Qazvin, IRAN Поступила в редакцию 20.01.2012 г.
Sulfur nanoparticles were successfully synthesized via novel water-in-oil microemulsion system. The microemulsion system contained cyclohexane as an oil phase, Triton X-100 as a surfactant, butanol as a co-surfactant and sodium polysulfide solution or hydrochloric acid solution as aqueous phase, respectively. The sulfur nanoparticles were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy. The results showed that the as-prepared monoclinic sulfur nanoparticles exhibited high purity and spherical shape with an average size of about 22 nm.
DOI: 10.7868/S0023291212060146
1. INTRODUCTION
In the last years synthesis of nanoparticles has been extensively investigated. Nanosized materials show novel catalytic [1], magnetic [2], optical [3], mechanical and electrical [4, 5] properties, which are significantly different from those of bulk materials.
Sulfur is a chemically and biologically active element and is widely used in many fields, such as the production of sulfuric acid, chemical fibers, plastics, chemical fertilizers, antimicrobial agents, insecticides, fumigants, vulcanization of rubber, gun-powder pharmaceutical industry and manufacture of dyes, etc [6—8]. Sulfur nanoparticles are used for modification of carbon nanotubes [9], synthesis of sulfur nanocom-posites for lithium batteries [10, 11], synthesis of sulfur nanowires with carbon to form hybrid materials with useful properties for gas sensor and catalytic applications [12].
Many methods have been exploited for the synthesis of nanoparticles in the recent years. Among those, chemical reactions in microemulsion system have attracted considerable interest for synthesis of monodisperse nanoparticles. Microemulsion consists of a ternary mixture of oil, surfactant and water or a quaternary mixture of oil, surfactant, co-surfactant and water. Depending on the proportion of various components of mixture and hydrophilic-lipophilic balance of the surfactant, there are two kinds of micro-emulsions (Fig. 1). In the case of water-in-oil (W/O) or reverse microemulsions, the hydrophilic parts of the surfactant molecules are adsorbed at the surface of water microdroplets, whereas the hydrophobic parts are oriented toward the oil. Oil-in-water (O/W) micro-emulsions are the systems, in which lipophilic parts of the surfactant are adsorbed at the surface of oil micro-
1 Corresponding author; E-mail: m-soleimani@hotmail.com
droplets and hydrophilic parts are oriented toward the water [13, 14]. Water-in-oil microemulsion technique is one of the most widely applied methods due to several advantages: i) it is a simple technique, which does not require extreme temperatures or pressure conditions, ii) it can be used to perform several chemical reactions and does not require any special equipment, and iii) it can be used to prepare uniform and size-controllable nanoparticles [15, 16].
W/O microemulsions are optically transparent and isotropic liquid media with nanosize water droplets that form thermodynamically stable dispersion in continuous oil phase; they are stabilized by surfactant molecules at the water/oil interface. Microemulsion droplet nanoreactors therefore offer a unique microenvironment for the formation of small crystallites with a narrow size distribution [17, 18]. Once two mi-croemulsions, one of which containing the precursor and the other harboring the precipitating agent, are uniformly mixed, the desired reaction takes place in a controlled manner in the interior of micelles, which are suitable environment for the synthesis of nanopar-ticles [19].
-# Water •
W/O microemulsion
Fig. 1. Schematic illustration of two types of microemulsions [13].
A few methods have been reported for synthesis of sulfur nanoparticles by different investigators [6, 7, 20]. Deshpand et al. [6] have synthesized rhombic sulfur or a-sulfur nanoparticles by catalytic oxidation of hazardous H2S gas using biodegradable iron chelates in W/O microemulsion system. They obtained a-sulfur particles with average size of 10 nm. Guo et al. [7] have reported the synthesis of monoclinic or P-sulfur nanoparticles in a reverse microemulsion system; but the materials they used to prepare microemulsions are not easily available. They have also investigated the effect of inorganic reactant. They obtained sulfur nanoparticles with an average particle size of about 20 nm or 35 nm by using two different kinds of inorganic re-actants. Chaudhuri et al. [20] have reported synthesis of a-sulfur nanoparticles in aqueous surfactant/thio-sulphate solution. They have studied the effect of different surfactants on particle size and the lowest particle size which they could obtain was 30 nm.
In this paper, sulfur nanoparticles were prepared via a simple method in a novel microemulsion system formed with easily available materials. The chemical reaction occurred between sodium polysulfide and hydrochloric acid in a reverse microemulsion system, containing cyclohexane, polyethylene glycol oc-tylphenyl ether (Triton X-100) and butanol as oil phase, surfactant and co-surfactant, respectively.
2. EXPERIMENTAL 2.1. Materials
Sulfur (particle size <40 ^m and purity >99%), sodium sulfide, hydrochloric acid, methanol, acetone, butanol and cyclohexane were the highest purity available from Merck (Darmstadt, Germany) and polyethylene glycol octylphenyl ether (Triton X-100) was purchased from AkzoNobel (Germany). All of the chemicals were used without further purification. All solutions were prepared by distilled water.
2.2. Apparatus
The FTIR spectra were recorded by using Bruker Tensor 27 FTIR spectrometer (Germany) employing a potassium bromide pellet method. X-ray powder diffraction pattern was obtained on a Siemens D5000 X-ray diffractometer (Germany) with a scanning rate of 1.2 degree/min in a range from 4° to 80°. A Philips XL-30 scanning electron microscope (Eindhoven, Netherland) equipped with an energy dispersive analyzer was used to study the shape and surface morphology of sulfur nanoparticles and their purity. Suspension of sulfur nanoparticles in ethanol was prepared ultrasonically. All glassware containers were carefully treated with 2.0 M nitric acid (guaranteed reagent) and rinsed with distilled water.
2.3. Method
2.3.1. Preparation of sodium polysulfide solution.
Sulfur powder (particle size <40 ^m) was ground fully in a mortar. A portion of 12.8 g of grounded sulfur powder was added into a flask that has been filled with 100 mL of sodium sulfide solution (2 mol/L). The reaction occurred at room temperature for 60 min under stirring (Eq. (1)). The color of the solution changed slowly to orange with dissolving of sulfur and sodium polysulfide (Na2Sx) solution was prepared [6].
(x - 1)S + Na2S ^ Na2Sx. (1)
2.3.2. Synthesis of sulfur nanoparticles. Stable reverse microemulsions were obtained by mixing cyclohexane as oil phase, butanol as co-surfactant, the nonionic surfactant Triton X-100 and 2 mol/L sodium polysulfide solution (microemulsion I) or 4 mol/L hydrochloric acid solution (microemulsion II) as aqueous phases. The mole ratio of the surfactant to co-surfactant was 1 : 4. The volume ratio of cyclohexane, Triton X-100, butanol and aqueous phase was 6 mL/2 mL/1.3 mL/1 mL. Cyclohexane, Triton X-100 and butanol were mixed under constant stirring with the mentioned proportion until the mixture became transparent. Then appropriate amount of 2 mol/L sodium polysulfide solution (microemulsion I) or 4 mol/L hydrochloric acid solution (microemulsion II) was added dropwise under vigorous stirring until the mixture became transparent. Then the microemulsion II was added dropwise to the microemulsion I under stirring, at room temperature. After the reaction, acetone was added to the microemulsion to cause precipitation of the sulfur nanopar-ticles synthesized in the microemulsion system. Then the precipitate was separated by centrifugation at 4000 rpm for 40min and repeatedly washed with acetone, methanol and water to remove the remaining of organic materials and the produced salt (NaCl) from the product to make it pure; then the product was dried in an oven at 60°C for 6 h. Equation (2) represents the reaction occurred by mixing these two mi-croemulsions.
Na2Sx + 2HCl ^ 2NaCl + H2S + (x - 1)S. (2)
3. RESULTS AND DISCUSSION 3.1. Microemulsion System
Herein, we mixed two water-in-oil microemulsions with the same organic phase, though with different re-actants in the aqueous phase. In this paper, sulfur nanoparticles were prepared in Triton X-100/bu-tanol/cyclohexane/water W/O microemulsion system. Aqueous phases were sodium polysulfide solution and hydrochloric acid solution. When compared with ionic surfactants, the nonionic compounds have several advantages: a) they are typically more hydrophobic, b) they tends to produce stable emulsions, c) they exhibit less toxicity [21, 22], d) they show less sensitivity to pH of the media as well as to the presence of reac-
KOnnOH^HBIH XyPHAtf TOM 75 № 1 2013
SOLEIMANI и др. Microemulsion I Microemulsion II
Aqueous phase sodium polysulfide solution
âÉ.
Aqueous phase HCl solution
Oil phase
Mix microemulsions I and II
Oil phase
Collision and fusion Of droplets
W
bbfw
Chemical reaction occurs
a #h
Precipitate (sulfur nanoparticles)
Fig. 2. Schematic of formation process for sulfur nanoparticles [18].
tants in the water phase. However, the phase behaviour of nonionic surfactants is typically temperature sensitive [23, 24]. With this regard, all the experiments were done at room temperature. When the microemulsion containing sodium polysulfide (precursor) and
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