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Synthesis of sulfur nanoparticles in aqueous surfactant solutions
Journal of Colloid and Interface Science 343 (2010) 439–446
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of sulfur nanoparticles in aqueous surfactant solutions Rajib Ghosh Chaudhuri, Santanu Paria * Department of Chemical Engineering, National Institute of Technology, Rourkela 769 008, Orissa, India
a r t i c l e
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Article history: Received 30 September 2009 Accepted 2 December 2009 Available online 6 December 2009 Keywords: Sulfur nanoparticle Aqueous surfactant solution Interparticle exchange Film flexibility Nucleation
a b s t r a c t Sulfur is a widely used element in different applications such as fertilizers, pharmaceuticals, rubber, fiber industries, bioleaching processes, anti microbial agents, insecticides, and fumigants, etc. Nanosize sulfur particles are useful for pharmaceuticals, modification of carbon nano tubes, and synthesis of nano composites for lithium batteries. In this study we report a surfactant assisted route for the synthesis of sulfur nanoparticles by an acid catalyzed precipitation of sodium thiosulphate. We use both the inorganic and organic acids, and find that organic acid gives lower size sulfur particles. The size of the particles also depends on the reactant concentration and acid to reactant ratio. The effect of different surfactants (TX-100, CTAB, SDBS, and SDS) on particle size shows that the surfactant can significantly reduce the particle size without changing the shape. The size reducing ability is not same for all the surfactants, depending on the type of surfactant. The anionic surfactant SDBS is more effective for obtaining a uniform size in both the acid media. Whereas, the lowest size (30 nm) particles were obtained in a certain reactant concentration range using CTAB surfactant. The objective of this study is to synthesize sulfur nanoparticles in aqueous media and also study the effect of different surfactants on particle size. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Elemental sulfur in nano, micro or bulk state is widely used for different industrial applications such as production of sulfuric acid, nitrogenous fertilizers, phosphatic fertilizers, plastics, enamels, antimicrobial agents, gun powders, petroleum refining, other petrochemicals, ore leaching processes, pulp and paper industries, and in different other agrochemical industries [1]. Nanosize sulfur particles also have many important applications like in pharmaceuticals, synthesis of nano composites for lithium batteries [2–5], modification of carbon nano tubes [6], synthesis of sulfur nano wires with carbon to form hybrid materials with useful properties for gas sensor and catalytic applications [7]. In agricultural field, sulfur is used as a fungicide against the apple scab disease under colder conditions [8], in conventional culture of grapes, strawberry, many vegetables, and several other crops. Sulfur is one of the oldest pesticides used in agriculture and it has a good efficiency against a wide range of powdery mildew diseases as well as black spot. Different methods were used for nanosize particle synthesis; among those, microemulsion method is one of the very important methods to control the particle size. But microemulsion itself is a very complicated system, composing of oil, surfactant, co-surfactant and aqueous phases with the specific compositions. The main disadvantages of the microemulsion method are the difficulties in * Corresponding author. Fax: +91 661 246 2999. E-mail addresses: [email protected], [email protected] (S. Paria). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.12.004
process scale up, separation and purification of the particles from the microemulsion, and finally this method is consumed huge amounts of surfactants. Despite many exciting applications, there are only a few recent literatures available on synthesis of sulfur nanoparticles by different investigators [9–12] in both aqueous and microemulsion phase by different routes. Deshpande et al. [9] have synthesized sulfur nanoparticles from H2S gas by using biodegradable iron chelate catalyst in reverse microemulsion technique. They found a-sulfur or rhombic sulfur of average particle size 10 nm with a particle size range of 5–15 nm. They have also studied the antimicrobial activity of sulfur nanoparticles and shows it is very much effective, especially when the particle size is low. Guo et al. [10] have prepared sulfur nanoparticles from sodium polysulfide by acid catalysis in reverse microemulsion technique. They found monoclinic or b-sulfur with an average particle size of around 20 nm. Xie et al. [12] have prepared nanosized sulfur particles from sublimed sulfur. They added aqueous cystine solution drop wise on a saturated alcoholic sulfur solution with constant ultrasonic treatment and cystine–nano-sulfur sol was obtained. Now, realizing the importance of sulfur nanoparticles in different applications, the development of an easy synthesis method is highly essential for nanosize sulfur particles. In the method of Deshpande et al. [9] H2S gas was used as a reactant, therefore the arrangement of the heterogeneous phase reaction (gas–liquid) between gaseous H2S and iron chelate solution is more complicated apart from the complexity of microemulsion. The disadvantage of the method proposed by Guo et al. [10,11] was the use of
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polysulfide as a source of sulfur, where, micro sized sulfur particles were used as a raw material for the synthesis of polysulfide. The method proposed by Xie et al. [12] was also required small sized sulfur particle as a raw material for the synthesis of nanoparticles. In the present method, sulfur can be synthesized in aqueous media from thiosulphate solution. Large amount of sulfur particles can be synthesized easily by using a cheap reactant for agricultural and other applications where the consumption is more. This reaction was studied by LaMer and coworkers [13–15] and proposed mechanism and kinetics of particle formation in aqueous acidic media. This study is mainly concentrated on effect of surfactants on particle size in aqueous media, which has not been studied before. Moreover, from the basic understanding point of view it is also very important for apply this route in microemulsion based synthesis. In this study, orthorhombic or a-sulfur with S8 structure have been synthesized in an aqueous micellar solution. An attempted has been made to understand the basic mechanism of particle formation in the presence of different inorganic and organic acids, reactant to acid ratio, effect of reactant concentration, and the effect of different micellar solutions (TX-100, SDBS, SDS, and CTAB). The studies on kinetics of particle formation in different surfactant assisted media are also in progress and will be communicated soon.
2. Materials and method 2.1. Materials The chemicals used for this study were taken from the following companies: sodium thiosulphate (Na2S2O35H2O), oxalic acid (H2C2O42H2O) from Rankem (India), Triton X-100 (TX-100) with 98% purity, sodium dodecyl sulphate (SDS) with 99.5% purity, cetyl trimethylammoniumbromide (CTAB) with 99% purity from Loba Chemie Pvt. Ltd., (India), sodium dodecyl benzene sulphonate from Sigma Aldrich (Germany) (technical grade, Cat No. 28995-7), and hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3) Merck (India). All the chemicals were used as those were received without any further purification. Ultrapure water of 18.2 MX cm resistivity and pH 6.4–6.5 (Sartorius, Germany) was double distilled again and used for all the experiments. 2.2. Particle synthesis Stock sodium thiosulphate was prepared by dissolving solid thiosulphate in double distilled water and different acid solutions were prepared from the pure stock. Both the reagents were filtered with 0.2 l nylon 6, 6 membrane filter paper from Pall Life science, USA. In an acidic solution, sodium thiosulphate undergoes through a disproportionation reaction to sulfur and sulfonic acid according to
Na2 S2 O3 þ 2HCl ! 2NaCl þ SO2 þ S # þ H2 O SO2 þ H2 O ! H2 SO3 After mixing the reactants, 30 and 40 min equilibrium time was given for the completion of reaction in the case of inorganic and organic acids, respectively. After equilibration, the sample was sonicated in a bath for 2 min and particle size was measured by DLS method immediately. CMC values of CTAB (0.93 mM), SDBS (1.2 mM), and TX-100 (0.15 mM) were taken from our previous study [16], and that of SDS (9 mM) was measured by Wilhelmy plate technique with a surface tensiometer (DCAT-11EC, Data Physics, Germany). A constant temperature 28 ± 0.5 °C was maintained throughout the experiments.
2.3. Particle characterization The structure of sulfur particles was characterized by X-ray diffraction (XRD) using Philips (PW 1830 HT) X-ray diffractometer with scanning rate of 0.01°/s in the 2h range from 20° to 40°. Particle size measurement was carried-out by dynamic light scattering (DLS) using Malvern zeta size analyzer, UK (Nano ZS) with the help of cumulants fitting model and intensity distribution. The size and shape of particles were observed under a scanning electron microscope (JEOL JSM-6480LV) and transmission electron microscope (Tecnai S-twin). 3. Results and discussion 3.1. Effect of stoicheometry ratio on particle size According to the stoicheometry of the reaction one mole of thiosulphate reacts with two moles of mono-basic acid to precipitate 1 mol of sulfur. Firstly, we have studied the effect of stoicheometry ratio of acid (H+) to thiosulphate concentration on particle size for a particular reactant concentration (5 mM) and later we used the same ratio for that particular acid. Fig. 1 shows that with increasing acid (H+) to thiosulphate molar ratio the particle size increases, but the size is almost constant above the ratio of 2:1 for inorganic mono-basic acid, 4:1 for inorganic di-basic acid, and 6:1 for organic di-basic acid. Hence, all the experiments with inorganic mono-basic (HCl, HNO3) and di-basic (H2SO4) acids were carried-out with the acid (H+) to thiosulphate ratio of 2:1 and 4:1, respectively. However, in the case of organic di-basic acid (oxalic acid) we used that ratio of 6:1 for the all experiments. As a general rule, larger the ionization constant values (Ka) stronger the acid, to get an idea about the ionization constants of different acids we have presented the values in Table 1. The acid requirement of H2SO4 is about twice than that of HCl, probably due to first ionization constant value of H2SO4 is high but the second is low and the requirement of oxalic acid is further more due to low values of both the ionization constants. 3.2. Effects of acid type and reactant concentration on particle size Fig. 2 shows the effects of acid types and thiosulphate concentration on particle size. Let us first consider the effect of reactant concentration on particle size for a particular acid; it is very clear
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Fig. 1. The effect of acid to thiosulphate ratio on the size of sulfur particles. Thiosulphate concentration 5 mM.
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Ionization constant (Ka)
HCl [17] HNO3 [18] H2SO4 [19] (H SO ) K 2 41 K2 HSO1 4 H2C2O4 [20] (H C O ) K 2 2 4 1 K2 HC2 O1 4
pKa
6
1.74 10 40
6.24 1.60
2.4 106 1.02 102
6.38 2.00
5.6 102 5.1 105
2.75 5.71
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Particle Size (nm)
1200 1000 800 600 HCl H SO
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2
4
(COOH) , 2H O 2
200
2
HNO
3
0
0
2 4 6 8 Reactant Concentration (mM)
10
Fig. 2. The effect of reactant concentration on the particle size in different acidic aqueous media.
from figure that the particle size increases with the increase in thiosulphate concentration. The particle size is mainly influenced by two factors: (i) nucleation and followed by (ii) particle growth and agglomeration. For this reaction nucleation is an instantaneous process and completed within 2 min after mixing the reactant with acid [21]. A nucleus is essentially a small group of atoms with a critical size that have taken up arrangement in space, is stable and capable of further growth. Sub-critical particles are called embryos, and of larger size are called nuclei. As well as, when the particles are achieved a critical radius (nuclei), then the growth process predominates over the nucleation. The rate of reaction also increases with the reactant concentration. According to LaMer and Zaiser [21] the rate of reaction depends on both the concentration of thiosulphate and acid types. They proposed the rate of reaction = k[T]1.5[A]0.5, where, k is the reaction rate constant, [T] and [A] are the thiosulphate and acid concentrations, respectively. Therefore, in higher reactant concentration media the density of nuclei will be more, as a result, after the growth process the particle density will also be high. When the particle density is more, the collision between the particles lead to more agglomeration, that will increase the ultimate equilibrium particle size [22]. FurediMilhofer et al. [23] have mentioned that for nucleation from a super saturated solution growth process will start when the nuclei density is <107 cm3 and >1012 cm3 for heterogeneous and homogeneous nucleation respectively. Furthermore, when collision between those new born particles increases because of higher particle density larger particles are formed and they are stabilized by minimizing the overall energy of the system, this is termed as Ostwald ripening or coarsening [24]. In addition, comparing the results in the presence of different acids it is clear that the particle size also depends on the acid types. There is a distinct size difference between the organic and inor-
ganic acids at a higher reactant concentration; the organic acid shows smaller size particles. The system with lower reactant concentration in the presence of HCl shows lowest particle size among all the acids but the differences are less. Among the inorganic acids particle size in H2SO4 is always higher than HNO3 but HCl is showing a different behavior. At high reactant concentration HCl is showing the highest particle size but at lower reactant concentration it is just reverse, shows lowest size. In addition, we can clearly see from Fig. 2 that at higher reactant concentration (10 mM) the increasing order of particle size in the presence of different acids are: C2H2O4 < HNO3 < H2SO4 < HCl which is exactly the same as the order of acid ionization constants, even though, the differences between the inorganic acids are less. Because of all the inorganic acids are strong and ionization is very fast, the reaction rate is also expected to be fast in compare to organic acid. Apparently there is a large difference in Ka value of HNO3 with HCl and H2SO4. Generally, acids with a pKa value of less than about 2 are said to be strong acids and completely dissociated in aqueous solution. From Table 1 we can see the pKa value of HNO3 is close to 2 and the literature also shows that it almost completely (93% at 0.1 mol/L) ionizes in aqueous solution. In the presence of inorganic acids as the reaction is expected to be very fast, formation of the nuclei also will be very fast with more nuclei density, which leads to higher equilibrium particle size. Whereas, oxalic acid catalyzed reaction may have slower rate of nucleation as well as lower the particle density in compare to that of inorganic acids, which may leads to formation of smaller particle size. In the case of oxalic acid, another reason of lower particle size is probably because of the adsorption of oxalate ion ðC2 O2 4 Þ on the particle surface, which is clear from zeta potential value. Zeta potential of the particles in the presence of oxalic acid is more negative (8.05 mV) than in hydrochloric acid (2.99 mV) at the same pH 2.8. The zeta potential was measured by DLS Malvern zeta size analyzer using Smoluchowski model. The zeta potentials in the presence of all other inorganic acids are very close. Higher zeta potential in the presence of oxalic acid may reduce the agglomeration tendency of the particles and finally the size becomes small and the particle size distribution is also sharp. The values of zeta potentials in the presence of different acids are given in Table 2. From Table 2, it is clear that the agglomeration tendency of the particles in the presence of inorganic acids is high because zeta potential values are low. Apart from the average particle size, the size distribution is also very important parameter in particle synthesis. Fig. 3 shows the comparisons of particle size distribution among four acids used in our study. Sulfuric and nitric acids have similar size distribution, hydrochloric acid is having little sharp distribution than the other two acids but the change is not very significant. The difference was found for the oxalic acid where the distribution is significantly narrow. 3.3. Particle size in the presence of surfactant solution The effect of surfactants on sulfur particle size was tested in the presence of HCl and oxalic acids catalyzed reactions with different Table 2 Zeta potential of sulfur nanoparticles in different media. Thiosulphate concentration 5 mM. Medium
Acid
Zeta potential (mV)
Water Water Water Water TX-100 SDS SDBS CTAB
HCl H2SO4 HNO3 (COOH)2 HCl HCl HCl HCl
2.99 1.85 2.17 8.05 0.557 76.3 85.0 23.8
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35 HCl H SO
30
2
4
Intensity (%)
HNO
3
25
(COOH) , 2H O 2
2
20 15 10 5 0
400
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2400
Fig. 3. Particle size distribution in different acid media at 10 mM thiosulphate concentration.
reactant concentration. The surfactant concentration was kept three fold CMC of respective surfactants. Fig. 4 shows the effect of surfactants on particle size by HCl catalyzed reaction. In aqueous solution the particle size is continuously increased with the reactant concentration, and after 10 mM reactant concentration the particles are settled down from the liquid phase because of larger size. However, from Fig. 4, it is clear that in the presence of surfactants there is a significant change in particle size than that in the absence of surfactant at 10 mM thiosulphate concentration. Furthermore, it is worthy to note that the effect is not same for all the surfactants. In addition, lower thiosulphate concentration (0.5 mM) where the particle size is small in aqueous media, the effect of surfactant is not very significant. At that reactant concentration, SDBS is showing almost no changes in particle size but other surfactants are showing little higher particle size. It is observed from figure that the plot of particle size vs. reactant concentration in the presence of anionic and nonionic surfactants pass through a maximum. The maximum particle size was observed at the same reactant concentration (5 mM) in the presence of both anionic sur-
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Particle Size (nm)
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Water CTAB TX-100 SDS SDBS
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5 mM thio 4 mM thio
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Fig. 4. Variation of particle size with the thiosulphate concentration in the absence and presence of surfactants by hydrochloric acid catalyzed reaction. Inset shows the particle size distribution at 4 mM and 5 mM thiosulphate concentration in the presence of CTAB.
factants (SDBS and SDS) and at higher concentration (10 mM) in the presence of nonionic surfactant (TX-100). Comparisons of maximum particle size obtained in the presence of surfactants show the following order: SDBS < TX-100 < SDS. However, the behavior in the presence of CTAB is little bit uncommon and interesting. From the experimental result it was found that at 5 mM reactant concentration there is a sharp decrease in particle size (55 nm) then again the particle size increases up to 10 mM and after that the trend is as usual to that of other surfactants. Due to this sudden change between 5 mM and 10 mM reactant concentration we have repeated the experiments several times and got repeatable results. The zeta potential values of the particles in this concentration range are also not changed significantly (+20.1 to +31.1 mV). We are still unable to completely explain this behavior at present and it needs further investigation. The above observations can be explained as follows. As the sulfur particles are hydrophobic in nature, they are having low zeta potential as well as low surface energy. Low surface energy particles are unstable in high surface tension polar liquid such as water, especially when the particle size is very small. Ultimately, in pure aqueous media the growth process is favorable to minimize the contact area between the particle and water, as a result, there is a constant increase in particle size with the increase in reactant concentration. However, in aqueous surfactant media the mechanism of particle formation is little bit different than that from the pure aqueous system. In the presence of SDBS and TX-100 we can observe initially the particle size increases with increasing the reactant concentration and after a critical concentration again there is a decrease in particle size. The researchers have also reported both the increasing [23,25] and decreasing [26] trends in particle size with reactant concentration for the separate systems. In those literatures the researchers have studied a narrow reactant concentration range to show the increasing or decreasing trends in particle size. A wide range of reactant concentration is studied here and both the trends are observed in the same system. In the first regime as soon as the nuclei are formed, surfactants are adsorbed through the tailgroup and a micelle like structure is formed with the nuclei inside the core. Thus the hydrophobic sulfur nuclei are also stabilized in the aqueous media. Then, similar to intermicellar exchange in reverse microemulsion [25], due to the collision between the surfactant coated nuclei, the growth process started and the particle size increases. Therefore, at high reactant concentration when the number of nuclei is more, the probability of inter-particle collision is also more and that may be the reason of increase in particle size. In the decreasing regime, above a limiting reactant concentration, the nucleation rate is very fast, as a result the size of the nuclei also increases rapidly. In this case inter-particle exchange is less important for the particle growth similar to that in microemulsion [26,27]. Since the nuclei concentration is very high at that regime immediately after the formation, the nuclei reached a certain size as well as stabilized by the adsorbed layer of surfactant molecules. There may be only diffusion of small nuclei through the micelles like structure of adsorbed surfactant layer is occur instead of exchange by collision to get final lower particle size. Inset of Fig. 4 clearly shows the size distribution in 5 mM reactant concentration is very narrow whereas at 4 mM it is significantly wide with a large particle size. We believe that the type of surfactant plays an important role for this type of behavior. The reason of minimum particle size obtained in the presence of CTAB is not completely understood but it may be due the change in mechanism depending on the reactant concentration. This can be attributed as in the increasing particle size regime, the inter-particle exchange may be the main mechanism, but after reaching a critical size (nuclei) at a particular reactant concentration film stability of the adsorbed surfactant layer is more important. As a result, lower particle size is obtained when inter-particle exchange
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1000
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0
0
2
4 6 8 10 Reactant Concentration (mM)
Fig. 6. Variation of particle size with the thiosulphate concentration in the absence and presence of surfactants by oxalic acid catalyzed reaction.
of surfactants the particle size is not continuously increased with the reactant concentration. In the presence of higher surfactant concentration (thrice of CMC), the monomer surfactant molecules are adsorbed on the sulfur particle surface through its tailgroup. As a result, after the adsorption of the ionic surfactant molecules, a charge is developed on the particle surface. Therefore, even at high reactant concentration agglomeration tendency of the particles are reduced due to the electrostatic repulsion between the particles adsorbed by ionic surfactants. For TX-100 solution, due to hydration of the headgroups there is also minimum tendency towards agglomeration. This explanation can be supported by zeta potential values of sulfur particles in aqueous and different surfactant solutions. Zeta potentials values of sulfur particles in different media are given in Table 2. From table, it is clear that sulfur is having low negative potential at aqueous media and closed to zero in the presence of TX-100. Among the ionic surfactants, anionic surfactants show higher zeta potential value than cationic, and between two anionic surfactants (SDS and SDBS), SDBS shows more negative potential. So, lower particle size and sharp distribution in the presence of SDBS can be explained in terms higher negative zeta
35 SDBS Water CTAB TX-100
30 25 Intensity (%)
Intensity (%)
25
600
200
30 SDBS CTAB Water TX-100 SDS
Water CTAB TX-100 SDBS
800 Particle Size (nm)
rate is low due to higher adsorbed surfactant film stability. Indeed, this may be the similar phenomenon that the effect of surfactant film flexibility in intermicellar exchange in reverse microemulsion system reported by Tojo and his coworkers [28–30]. The other portion of the curve in CTAB can be explained similarly that of SDBS and TX-100. For 5 mM thiosulphate concentration, we also studied the effect of surfactant concentration on particle size. It shows with the increasing surfactant concentration the particle size decreases and shows a minimum of 30 nm size particle at 1 mM (close to CMC) surfactant concentration but after that again size of the particle increases and becomes constant at 50–60 nm size (Fig. 8). Fig. 5 shows the particle size distribution at 10 mM thiosulphate solution in different surfactant media. From figure it is clear that the particle size distribution in the presence of SDBS is sharper than the other surfactant solutions at a constant reactant concentration (10 mM). In addition to narrow size distribution at a constant reactant concentration, for the total range of the reactant concentration studied here the change in size with thiosulphate concentration is less as the height of maxima is less in the presence of SDBS. Therefore, SDBS can be use as better size controlling agent. Fig. 6 shows the change in particle size by oxalic acid catalyzed reaction in the presence of different surfactants. The main difference found in the presence of organic acid is the absence of maximum in particle size with the thiosulphate concentration in SDBS solution. Furthermore, the change in particle size with the increase in reactant concentration is also less for SDBS similar to that in HCl catalyzed reaction. TX-100 shows higher particle size than other two ionic surfactants similar to that is observed in the presence of HCl. The change in particle size with reactant concentration is also less in the presence of TX-100 in contrast to that for HCl. By comparing the results in the presence of SDBS and TX-100 we can conclude that the change in particle size with the increase in reactant concentration is less, as well as overall lower particle size is obtained by oxalic acid catalyzed reaction than HCl. In the presence of CTAB similar to HCl catalyzed reaction minimum particle size was found between 4 mM and 6 mM thiosulphate concentration range. Minimum particle size was obtained 35 nm at 4 mM thiosulphate concentration. The particle size distribution at 10 mM thiosulphate concentration (see Fig. 7) also shows the distribution is narrow in oxalic acid and SDBS combination. Unlike the aqueous media, in the presence
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Fig. 5. Plot of particle size distribution in different media at 10 mM thiosulphate solution catalyzed by HCl.
0
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400 600 800 1000 Particle Size (nm)
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Fig. 7. Plot of particle size distribution in different media for 10 mM thiosulphate solution catalyzed by oxalic acid.
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Size at 10 mM thio soln Size at 5 mM thio soln Zeta Potential (mV) at 10 mM thio soln
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Zeta Potential (mV)
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concentration of thiosulphate is more than 50 mM particles are separate very fast from the aqueous media except the presence of anionic surfactants.
25
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200 0
0
0.5
1 1.5 2 2.5 3 3.5 Surfactant Concentration (mM)
4
5
Fig. 8. Variation of particle size and zeta potential with CTAB concentration in the presence of 5 mM and 10 mM reactant concentrations by HCl catalyzed reaction.
potential due to adsorption of surfactant, which prevent growth as well as the agglomeration of particles. Moreover, we have observed that the dispersing ability of the particles in the presence of both the anionic surfactants is more than TX-100 or CTAB. When the
3.4. Effect of surfactant concentration on particle size From the previous discussion it is clear that using different surfactant we can control the size of the sulfur particles. In all the above studies we have used the concentration of surfactants thrice of their respective CMCs. Here, we have studied the effect of surfactant concentration (CTAB) on particle size for a fixed reactant concentration (10 mM) and the results are plotted in Fig. 8. Fig. 8 shows there is a sharp decrease in particle size till little below the CMC, above CMC the size becomes almost constant. Zeta potentials were also measured for different CTAB concentration for the same reactant concentration. Figure clearly indicates that the zeta potential values are increasing with the increase in surfactant concentration and ultimately become constant close to the CMC of the surfactant where particle size is also becomes constant. The increase in zeta potential is due to the adsorption of surfactant molecules on sulfur surface by its tailgroup and forming a complete saturated monolayer near to CMC, also zeta potential values indirectly proofs there is no bi-layer (hemimicelle) formation in the adsorption. The particle size in the presence of surfactant
Fig. 9. XRD pattern of sulfur nanoparticles: (a) in different acid media and (b) in the presence of different surfactants by HCl catalyzed reaction.
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is minimum when surface charge is maximum and surface is saturated by the adsorbed monomer surfactant molecules.
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The XRD samples are prepared by successive washing by fresh water, so we got only pure sulfur peak. The sample was prepared without any washing shows sulfur peak mainly as well as it also shows surfactant and Na2SO4 peaks (pattern is not shown here).
3.5. X-ray diffraction (XRD) analysis Figs. 9a and b show the XRD pattern of sulfur particles in different acidic solution and in the presence of surfactants and HCl media, respectively. The positions and intensities of the diffraction peaks are in good agreement with the literature values for orthorhombic or a-phase sulfur with S8 structure (74-1465 from JCPDS PDF Number). The phase obtained is also independent of acid media. Fig. 9b also shows the similar structure is formed, with the difference of sharp peak indicates highly crystalline nature in the presence of surfactant system than aqueous media except SDBS.
3.6. Analysis by electron microscope (SEM and TEM) Figs. 10A–D show the SEM images of sulfur particle synthesize by HCl (Fig. 10A) and oxalic acid (Fig. 10B) catalyzed media without any surfactant, by HCl catalyzed in the presence of CTAB (Fig. 10C), and SDBS (Fig. 10D) surfactants for thiosulphate concentration of 5 mM. From the figures it is clear that the sulfur particles are almost spherical shape and uniform size. Fig. 11 shows the TEM image of sulfur nanoparticle formed at 5 mM thiosulphate
Fig. 10. SEM micrographs of sulfur nanoparticles from 5 mM thiosulphate concentration: (A) HCl catalyzed, (B) oxalic acid catalyzed, (C) CTAB and HCl (D) SDBS and HCl.
Fig. 11. TEM micrographs of sulfur nanoparticle formed at 5 mM thiosulphate concentration by inorganic acid catalyzed reaction in CTAB surfactant media.
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concentration by inorganic acid catalyzed reaction in CTAB media and the size of the particles obtained is close to 50–55 nm. This again confirms the formation of lower size particle in CTAB media. We have also done SEM (figure not shown) of samples by HCl media in the presence of CTAB surfactant for 4, 9 mM thiosulphate concentration solution and found the particles sizes are higher than that of 5 mM. Form both the SEM and TEM figures we can say sulfur has a tendency towards agglomeration, so we are not getting separate discrete sulfur particle image. 4. Conclusion Mostly spherical shape orthorhombic or a-sulfur with S8 structure is formed by acid catalyze thiosulphate reaction. Sulfur particle size is depends on the ionization constant value of the acids as well as acid (H+) to thiosulphate molar ratio. Inorganic acids produce larger size particles than the organic acid. The particle size distribution is also more uniform by organic acid catalyzed reaction, as zeta potential of the particles is high in organic acid media to prevent agglomeration tendency. All the surfactants, anionic, cationic, and nonionic surfactants play a major role in the lowering the particle size. Compared to nonionic and cationic surfactants, in anionic surfactant media the particle size is low and even in our working concentration range the change in particle size also less, and among two anionic surfactants studied here SDBS is more effective for controlling the size of the particles. Surfactant concentration is also one important factor for the particle sizes and it seems that above CMC is required. In the presence of CTAB within a certain reactant concentration range lower particle sizes are obtained in contrast to the other surfactants if all other conditions are remain same. Sulfur nanoparticles of approximately 30 nm particle size can be synthesized by organic acid catalyzed precipitation of thiosulphate in the presence of aqueous CTAB media. Acknowledgments The financial support from Department of Science and Technology (DST) under Nanomission, New Delhi, India, Grant No. SR/S5/
NM-04/2007, for this project is gratefully acknowledged. We also acknowledge Saha Institute of Nuclear Physics (SINP), Kolkata for giving the opportunity to access their TEM facility. References [1] J.A. Ober, Materials Flow of Sulfur: US Geological Survey Open File Report 02298, 2003.
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Synthesis of sulfur nanoparticles in aqueous surfactant solutions Rajib Ghosh Chaudhuri, Santanu Paria * Department of Chemical Engineering, National Institute of Technology, Rourkela 769 008, Orissa, India
a r t i c l e
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Article history: Received 30 September 2009 Accepted 2 December 2009 Available online 6 December 2009 Keywords: Sulfur nanoparticle Aqueous surfactant solution Interparticle exchange Film flexibility Nucleation
a b s t r a c t Sulfur is a widely used element in different applications such as fertilizers, pharmaceuticals, rubber, fiber industries, bioleaching processes, anti microbial agents, insecticides, and fumigants, etc. Nanosize sulfur particles are useful for pharmaceuticals, modification of carbon nano tubes, and synthesis of nano composites for lithium batteries. In this study we report a surfactant assisted route for the synthesis of sulfur nanoparticles by an acid catalyzed precipitation of sodium thiosulphate. We use both the inorganic and organic acids, and find that organic acid gives lower size sulfur particles. The size of the particles also depends on the reactant concentration and acid to reactant ratio. The effect of different surfactants (TX-100, CTAB, SDBS, and SDS) on particle size shows that the surfactant can significantly reduce the particle size without changing the shape. The size reducing ability is not same for all the surfactants, depending on the type of surfactant. The anionic surfactant SDBS is more effective for obtaining a uniform size in both the acid media. Whereas, the lowest size (30 nm) particles were obtained in a certain reactant concentration range using CTAB surfactant. The objective of this study is to synthesize sulfur nanoparticles in aqueous media and also study the effect of different surfactants on particle size. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Elemental sulfur in nano, micro or bulk state is widely used for different industrial applications such as production of sulfuric acid, nitrogenous fertilizers, phosphatic fertilizers, plastics, enamels, antimicrobial agents, gun powders, petroleum refining, other petrochemicals, ore leaching processes, pulp and paper industries, and in different other agrochemical industries [1]. Nanosize sulfur particles also have many important applications like in pharmaceuticals, synthesis of nano composites for lithium batteries [2–5], modification of carbon nano tubes [6], synthesis of sulfur nano wires with carbon to form hybrid materials with useful properties for gas sensor and catalytic applications [7]. In agricultural field, sulfur is used as a fungicide against the apple scab disease under colder conditions [8], in conventional culture of grapes, strawberry, many vegetables, and several other crops. Sulfur is one of the oldest pesticides used in agriculture and it has a good efficiency against a wide range of powdery mildew diseases as well as black spot. Different methods were used for nanosize particle synthesis; among those, microemulsion method is one of the very important methods to control the particle size. But microemulsion itself is a very complicated system, composing of oil, surfactant, co-surfactant and aqueous phases with the specific compositions. The main disadvantages of the microemulsion method are the difficulties in * Corresponding author. Fax: +91 661 246 2999. E-mail addresses: [email protected], [email protected] (S. Paria). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.12.004
process scale up, separation and purification of the particles from the microemulsion, and finally this method is consumed huge amounts of surfactants. Despite many exciting applications, there are only a few recent literatures available on synthesis of sulfur nanoparticles by different investigators [9–12] in both aqueous and microemulsion phase by different routes. Deshpande et al. [9] have synthesized sulfur nanoparticles from H2S gas by using biodegradable iron chelate catalyst in reverse microemulsion technique. They found a-sulfur or rhombic sulfur of average particle size 10 nm with a particle size range of 5–15 nm. They have also studied the antimicrobial activity of sulfur nanoparticles and shows it is very much effective, especially when the particle size is low. Guo et al. [10] have prepared sulfur nanoparticles from sodium polysulfide by acid catalysis in reverse microemulsion technique. They found monoclinic or b-sulfur with an average particle size of around 20 nm. Xie et al. [12] have prepared nanosized sulfur particles from sublimed sulfur. They added aqueous cystine solution drop wise on a saturated alcoholic sulfur solution with constant ultrasonic treatment and cystine–nano-sulfur sol was obtained. Now, realizing the importance of sulfur nanoparticles in different applications, the development of an easy synthesis method is highly essential for nanosize sulfur particles. In the method of Deshpande et al. [9] H2S gas was used as a reactant, therefore the arrangement of the heterogeneous phase reaction (gas–liquid) between gaseous H2S and iron chelate solution is more complicated apart from the complexity of microemulsion. The disadvantage of the method proposed by Guo et al. [10,11] was the use of
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polysulfide as a source of sulfur, where, micro sized sulfur particles were used as a raw material for the synthesis of polysulfide. The method proposed by Xie et al. [12] was also required small sized sulfur particle as a raw material for the synthesis of nanoparticles. In the present method, sulfur can be synthesized in aqueous media from thiosulphate solution. Large amount of sulfur particles can be synthesized easily by using a cheap reactant for agricultural and other applications where the consumption is more. This reaction was studied by LaMer and coworkers [13–15] and proposed mechanism and kinetics of particle formation in aqueous acidic media. This study is mainly concentrated on effect of surfactants on particle size in aqueous media, which has not been studied before. Moreover, from the basic understanding point of view it is also very important for apply this route in microemulsion based synthesis. In this study, orthorhombic or a-sulfur with S8 structure have been synthesized in an aqueous micellar solution. An attempted has been made to understand the basic mechanism of particle formation in the presence of different inorganic and organic acids, reactant to acid ratio, effect of reactant concentration, and the effect of different micellar solutions (TX-100, SDBS, SDS, and CTAB). The studies on kinetics of particle formation in different surfactant assisted media are also in progress and will be communicated soon.
2. Materials and method 2.1. Materials The chemicals used for this study were taken from the following companies: sodium thiosulphate (Na2S2O35H2O), oxalic acid (H2C2O42H2O) from Rankem (India), Triton X-100 (TX-100) with 98% purity, sodium dodecyl sulphate (SDS) with 99.5% purity, cetyl trimethylammoniumbromide (CTAB) with 99% purity from Loba Chemie Pvt. Ltd., (India), sodium dodecyl benzene sulphonate from Sigma Aldrich (Germany) (technical grade, Cat No. 28995-7), and hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3) Merck (India). All the chemicals were used as those were received without any further purification. Ultrapure water of 18.2 MX cm resistivity and pH 6.4–6.5 (Sartorius, Germany) was double distilled again and used for all the experiments. 2.2. Particle synthesis Stock sodium thiosulphate was prepared by dissolving solid thiosulphate in double distilled water and different acid solutions were prepared from the pure stock. Both the reagents were filtered with 0.2 l nylon 6, 6 membrane filter paper from Pall Life science, USA. In an acidic solution, sodium thiosulphate undergoes through a disproportionation reaction to sulfur and sulfonic acid according to
Na2 S2 O3 þ 2HCl ! 2NaCl þ SO2 þ S # þ H2 O SO2 þ H2 O ! H2 SO3 After mixing the reactants, 30 and 40 min equilibrium time was given for the completion of reaction in the case of inorganic and organic acids, respectively. After equilibration, the sample was sonicated in a bath for 2 min and particle size was measured by DLS method immediately. CMC values of CTAB (0.93 mM), SDBS (1.2 mM), and TX-100 (0.15 mM) were taken from our previous study [16], and that of SDS (9 mM) was measured by Wilhelmy plate technique with a surface tensiometer (DCAT-11EC, Data Physics, Germany). A constant temperature 28 ± 0.5 °C was maintained throughout the experiments.
2.3. Particle characterization The structure of sulfur particles was characterized by X-ray diffraction (XRD) using Philips (PW 1830 HT) X-ray diffractometer with scanning rate of 0.01°/s in the 2h range from 20° to 40°. Particle size measurement was carried-out by dynamic light scattering (DLS) using Malvern zeta size analyzer, UK (Nano ZS) with the help of cumulants fitting model and intensity distribution. The size and shape of particles were observed under a scanning electron microscope (JEOL JSM-6480LV) and transmission electron microscope (Tecnai S-twin). 3. Results and discussion 3.1. Effect of stoicheometry ratio on particle size According to the stoicheometry of the reaction one mole of thiosulphate reacts with two moles of mono-basic acid to precipitate 1 mol of sulfur. Firstly, we have studied the effect of stoicheometry ratio of acid (H+) to thiosulphate concentration on particle size for a particular reactant concentration (5 mM) and later we used the same ratio for that particular acid. Fig. 1 shows that with increasing acid (H+) to thiosulphate molar ratio the particle size increases, but the size is almost constant above the ratio of 2:1 for inorganic mono-basic acid, 4:1 for inorganic di-basic acid, and 6:1 for organic di-basic acid. Hence, all the experiments with inorganic mono-basic (HCl, HNO3) and di-basic (H2SO4) acids were carried-out with the acid (H+) to thiosulphate ratio of 2:1 and 4:1, respectively. However, in the case of organic di-basic acid (oxalic acid) we used that ratio of 6:1 for the all experiments. As a general rule, larger the ionization constant values (Ka) stronger the acid, to get an idea about the ionization constants of different acids we have presented the values in Table 1. The acid requirement of H2SO4 is about twice than that of HCl, probably due to first ionization constant value of H2SO4 is high but the second is low and the requirement of oxalic acid is further more due to low values of both the ionization constants. 3.2. Effects of acid type and reactant concentration on particle size Fig. 2 shows the effects of acid types and thiosulphate concentration on particle size. Let us first consider the effect of reactant concentration on particle size for a particular acid; it is very clear
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Fig. 1. The effect of acid to thiosulphate ratio on the size of sulfur particles. Thiosulphate concentration 5 mM.
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Ionization constant (Ka)
HCl [17] HNO3 [18] H2SO4 [19] (H SO ) K 2 41 K2 HSO1 4 H2C2O4 [20] (H C O ) K 2 2 4 1 K2 HC2 O1 4
pKa
6
1.74 10 40
6.24 1.60
2.4 106 1.02 102
6.38 2.00
5.6 102 5.1 105
2.75 5.71
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Particle Size (nm)
1200 1000 800 600 HCl H SO
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2
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(COOH) , 2H O 2
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2
HNO
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0
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2 4 6 8 Reactant Concentration (mM)
10
Fig. 2. The effect of reactant concentration on the particle size in different acidic aqueous media.
from figure that the particle size increases with the increase in thiosulphate concentration. The particle size is mainly influenced by two factors: (i) nucleation and followed by (ii) particle growth and agglomeration. For this reaction nucleation is an instantaneous process and completed within 2 min after mixing the reactant with acid [21]. A nucleus is essentially a small group of atoms with a critical size that have taken up arrangement in space, is stable and capable of further growth. Sub-critical particles are called embryos, and of larger size are called nuclei. As well as, when the particles are achieved a critical radius (nuclei), then the growth process predominates over the nucleation. The rate of reaction also increases with the reactant concentration. According to LaMer and Zaiser [21] the rate of reaction depends on both the concentration of thiosulphate and acid types. They proposed the rate of reaction = k[T]1.5[A]0.5, where, k is the reaction rate constant, [T] and [A] are the thiosulphate and acid concentrations, respectively. Therefore, in higher reactant concentration media the density of nuclei will be more, as a result, after the growth process the particle density will also be high. When the particle density is more, the collision between the particles lead to more agglomeration, that will increase the ultimate equilibrium particle size [22]. FurediMilhofer et al. [23] have mentioned that for nucleation from a super saturated solution growth process will start when the nuclei density is <107 cm3 and >1012 cm3 for heterogeneous and homogeneous nucleation respectively. Furthermore, when collision between those new born particles increases because of higher particle density larger particles are formed and they are stabilized by minimizing the overall energy of the system, this is termed as Ostwald ripening or coarsening [24]. In addition, comparing the results in the presence of different acids it is clear that the particle size also depends on the acid types. There is a distinct size difference between the organic and inor-
ganic acids at a higher reactant concentration; the organic acid shows smaller size particles. The system with lower reactant concentration in the presence of HCl shows lowest particle size among all the acids but the differences are less. Among the inorganic acids particle size in H2SO4 is always higher than HNO3 but HCl is showing a different behavior. At high reactant concentration HCl is showing the highest particle size but at lower reactant concentration it is just reverse, shows lowest size. In addition, we can clearly see from Fig. 2 that at higher reactant concentration (10 mM) the increasing order of particle size in the presence of different acids are: C2H2O4 < HNO3 < H2SO4 < HCl which is exactly the same as the order of acid ionization constants, even though, the differences between the inorganic acids are less. Because of all the inorganic acids are strong and ionization is very fast, the reaction rate is also expected to be fast in compare to organic acid. Apparently there is a large difference in Ka value of HNO3 with HCl and H2SO4. Generally, acids with a pKa value of less than about 2 are said to be strong acids and completely dissociated in aqueous solution. From Table 1 we can see the pKa value of HNO3 is close to 2 and the literature also shows that it almost completely (93% at 0.1 mol/L) ionizes in aqueous solution. In the presence of inorganic acids as the reaction is expected to be very fast, formation of the nuclei also will be very fast with more nuclei density, which leads to higher equilibrium particle size. Whereas, oxalic acid catalyzed reaction may have slower rate of nucleation as well as lower the particle density in compare to that of inorganic acids, which may leads to formation of smaller particle size. In the case of oxalic acid, another reason of lower particle size is probably because of the adsorption of oxalate ion ðC2 O2 4 Þ on the particle surface, which is clear from zeta potential value. Zeta potential of the particles in the presence of oxalic acid is more negative (8.05 mV) than in hydrochloric acid (2.99 mV) at the same pH 2.8. The zeta potential was measured by DLS Malvern zeta size analyzer using Smoluchowski model. The zeta potentials in the presence of all other inorganic acids are very close. Higher zeta potential in the presence of oxalic acid may reduce the agglomeration tendency of the particles and finally the size becomes small and the particle size distribution is also sharp. The values of zeta potentials in the presence of different acids are given in Table 2. From Table 2, it is clear that the agglomeration tendency of the particles in the presence of inorganic acids is high because zeta potential values are low. Apart from the average particle size, the size distribution is also very important parameter in particle synthesis. Fig. 3 shows the comparisons of particle size distribution among four acids used in our study. Sulfuric and nitric acids have similar size distribution, hydrochloric acid is having little sharp distribution than the other two acids but the change is not very significant. The difference was found for the oxalic acid where the distribution is significantly narrow. 3.3. Particle size in the presence of surfactant solution The effect of surfactants on sulfur particle size was tested in the presence of HCl and oxalic acids catalyzed reactions with different Table 2 Zeta potential of sulfur nanoparticles in different media. Thiosulphate concentration 5 mM. Medium
Acid
Zeta potential (mV)
Water Water Water Water TX-100 SDS SDBS CTAB
HCl H2SO4 HNO3 (COOH)2 HCl HCl HCl HCl
2.99 1.85 2.17 8.05 0.557 76.3 85.0 23.8
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35 HCl H SO
30
2
4
Intensity (%)
HNO
3
25
(COOH) , 2H O 2
2
20 15 10 5 0
400
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Fig. 3. Particle size distribution in different acid media at 10 mM thiosulphate concentration.
reactant concentration. The surfactant concentration was kept three fold CMC of respective surfactants. Fig. 4 shows the effect of surfactants on particle size by HCl catalyzed reaction. In aqueous solution the particle size is continuously increased with the reactant concentration, and after 10 mM reactant concentration the particles are settled down from the liquid phase because of larger size. However, from Fig. 4, it is clear that in the presence of surfactants there is a significant change in particle size than that in the absence of surfactant at 10 mM thiosulphate concentration. Furthermore, it is worthy to note that the effect is not same for all the surfactants. In addition, lower thiosulphate concentration (0.5 mM) where the particle size is small in aqueous media, the effect of surfactant is not very significant. At that reactant concentration, SDBS is showing almost no changes in particle size but other surfactants are showing little higher particle size. It is observed from figure that the plot of particle size vs. reactant concentration in the presence of anionic and nonionic surfactants pass through a maximum. The maximum particle size was observed at the same reactant concentration (5 mM) in the presence of both anionic sur-
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5 mM thio 4 mM thio
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Fig. 4. Variation of particle size with the thiosulphate concentration in the absence and presence of surfactants by hydrochloric acid catalyzed reaction. Inset shows the particle size distribution at 4 mM and 5 mM thiosulphate concentration in the presence of CTAB.
factants (SDBS and SDS) and at higher concentration (10 mM) in the presence of nonionic surfactant (TX-100). Comparisons of maximum particle size obtained in the presence of surfactants show the following order: SDBS < TX-100 < SDS. However, the behavior in the presence of CTAB is little bit uncommon and interesting. From the experimental result it was found that at 5 mM reactant concentration there is a sharp decrease in particle size (55 nm) then again the particle size increases up to 10 mM and after that the trend is as usual to that of other surfactants. Due to this sudden change between 5 mM and 10 mM reactant concentration we have repeated the experiments several times and got repeatable results. The zeta potential values of the particles in this concentration range are also not changed significantly (+20.1 to +31.1 mV). We are still unable to completely explain this behavior at present and it needs further investigation. The above observations can be explained as follows. As the sulfur particles are hydrophobic in nature, they are having low zeta potential as well as low surface energy. Low surface energy particles are unstable in high surface tension polar liquid such as water, especially when the particle size is very small. Ultimately, in pure aqueous media the growth process is favorable to minimize the contact area between the particle and water, as a result, there is a constant increase in particle size with the increase in reactant concentration. However, in aqueous surfactant media the mechanism of particle formation is little bit different than that from the pure aqueous system. In the presence of SDBS and TX-100 we can observe initially the particle size increases with increasing the reactant concentration and after a critical concentration again there is a decrease in particle size. The researchers have also reported both the increasing [23,25] and decreasing [26] trends in particle size with reactant concentration for the separate systems. In those literatures the researchers have studied a narrow reactant concentration range to show the increasing or decreasing trends in particle size. A wide range of reactant concentration is studied here and both the trends are observed in the same system. In the first regime as soon as the nuclei are formed, surfactants are adsorbed through the tailgroup and a micelle like structure is formed with the nuclei inside the core. Thus the hydrophobic sulfur nuclei are also stabilized in the aqueous media. Then, similar to intermicellar exchange in reverse microemulsion [25], due to the collision between the surfactant coated nuclei, the growth process started and the particle size increases. Therefore, at high reactant concentration when the number of nuclei is more, the probability of inter-particle collision is also more and that may be the reason of increase in particle size. In the decreasing regime, above a limiting reactant concentration, the nucleation rate is very fast, as a result the size of the nuclei also increases rapidly. In this case inter-particle exchange is less important for the particle growth similar to that in microemulsion [26,27]. Since the nuclei concentration is very high at that regime immediately after the formation, the nuclei reached a certain size as well as stabilized by the adsorbed layer of surfactant molecules. There may be only diffusion of small nuclei through the micelles like structure of adsorbed surfactant layer is occur instead of exchange by collision to get final lower particle size. Inset of Fig. 4 clearly shows the size distribution in 5 mM reactant concentration is very narrow whereas at 4 mM it is significantly wide with a large particle size. We believe that the type of surfactant plays an important role for this type of behavior. The reason of minimum particle size obtained in the presence of CTAB is not completely understood but it may be due the change in mechanism depending on the reactant concentration. This can be attributed as in the increasing particle size regime, the inter-particle exchange may be the main mechanism, but after reaching a critical size (nuclei) at a particular reactant concentration film stability of the adsorbed surfactant layer is more important. As a result, lower particle size is obtained when inter-particle exchange
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1000
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0
0
2
4 6 8 10 Reactant Concentration (mM)
Fig. 6. Variation of particle size with the thiosulphate concentration in the absence and presence of surfactants by oxalic acid catalyzed reaction.
of surfactants the particle size is not continuously increased with the reactant concentration. In the presence of higher surfactant concentration (thrice of CMC), the monomer surfactant molecules are adsorbed on the sulfur particle surface through its tailgroup. As a result, after the adsorption of the ionic surfactant molecules, a charge is developed on the particle surface. Therefore, even at high reactant concentration agglomeration tendency of the particles are reduced due to the electrostatic repulsion between the particles adsorbed by ionic surfactants. For TX-100 solution, due to hydration of the headgroups there is also minimum tendency towards agglomeration. This explanation can be supported by zeta potential values of sulfur particles in aqueous and different surfactant solutions. Zeta potentials values of sulfur particles in different media are given in Table 2. From table, it is clear that sulfur is having low negative potential at aqueous media and closed to zero in the presence of TX-100. Among the ionic surfactants, anionic surfactants show higher zeta potential value than cationic, and between two anionic surfactants (SDS and SDBS), SDBS shows more negative potential. So, lower particle size and sharp distribution in the presence of SDBS can be explained in terms higher negative zeta
35 SDBS Water CTAB TX-100
30 25 Intensity (%)
Intensity (%)
25
600
200
30 SDBS CTAB Water TX-100 SDS
Water CTAB TX-100 SDBS
800 Particle Size (nm)
rate is low due to higher adsorbed surfactant film stability. Indeed, this may be the similar phenomenon that the effect of surfactant film flexibility in intermicellar exchange in reverse microemulsion system reported by Tojo and his coworkers [28–30]. The other portion of the curve in CTAB can be explained similarly that of SDBS and TX-100. For 5 mM thiosulphate concentration, we also studied the effect of surfactant concentration on particle size. It shows with the increasing surfactant concentration the particle size decreases and shows a minimum of 30 nm size particle at 1 mM (close to CMC) surfactant concentration but after that again size of the particle increases and becomes constant at 50–60 nm size (Fig. 8). Fig. 5 shows the particle size distribution at 10 mM thiosulphate solution in different surfactant media. From figure it is clear that the particle size distribution in the presence of SDBS is sharper than the other surfactant solutions at a constant reactant concentration (10 mM). In addition to narrow size distribution at a constant reactant concentration, for the total range of the reactant concentration studied here the change in size with thiosulphate concentration is less as the height of maxima is less in the presence of SDBS. Therefore, SDBS can be use as better size controlling agent. Fig. 6 shows the change in particle size by oxalic acid catalyzed reaction in the presence of different surfactants. The main difference found in the presence of organic acid is the absence of maximum in particle size with the thiosulphate concentration in SDBS solution. Furthermore, the change in particle size with the increase in reactant concentration is also less for SDBS similar to that in HCl catalyzed reaction. TX-100 shows higher particle size than other two ionic surfactants similar to that is observed in the presence of HCl. The change in particle size with reactant concentration is also less in the presence of TX-100 in contrast to that for HCl. By comparing the results in the presence of SDBS and TX-100 we can conclude that the change in particle size with the increase in reactant concentration is less, as well as overall lower particle size is obtained by oxalic acid catalyzed reaction than HCl. In the presence of CTAB similar to HCl catalyzed reaction minimum particle size was found between 4 mM and 6 mM thiosulphate concentration range. Minimum particle size was obtained 35 nm at 4 mM thiosulphate concentration. The particle size distribution at 10 mM thiosulphate concentration (see Fig. 7) also shows the distribution is narrow in oxalic acid and SDBS combination. Unlike the aqueous media, in the presence
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0 0
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1000 1500 Particle Size (nm)
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Fig. 5. Plot of particle size distribution in different media at 10 mM thiosulphate solution catalyzed by HCl.
0
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400 600 800 1000 Particle Size (nm)
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Fig. 7. Plot of particle size distribution in different media for 10 mM thiosulphate solution catalyzed by oxalic acid.
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Size at 10 mM thio soln Size at 5 mM thio soln Zeta Potential (mV) at 10 mM thio soln
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Zeta Potential (mV)
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concentration of thiosulphate is more than 50 mM particles are separate very fast from the aqueous media except the presence of anionic surfactants.
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200 0
0
0.5
1 1.5 2 2.5 3 3.5 Surfactant Concentration (mM)
4
5
Fig. 8. Variation of particle size and zeta potential with CTAB concentration in the presence of 5 mM and 10 mM reactant concentrations by HCl catalyzed reaction.
potential due to adsorption of surfactant, which prevent growth as well as the agglomeration of particles. Moreover, we have observed that the dispersing ability of the particles in the presence of both the anionic surfactants is more than TX-100 or CTAB. When the
3.4. Effect of surfactant concentration on particle size From the previous discussion it is clear that using different surfactant we can control the size of the sulfur particles. In all the above studies we have used the concentration of surfactants thrice of their respective CMCs. Here, we have studied the effect of surfactant concentration (CTAB) on particle size for a fixed reactant concentration (10 mM) and the results are plotted in Fig. 8. Fig. 8 shows there is a sharp decrease in particle size till little below the CMC, above CMC the size becomes almost constant. Zeta potentials were also measured for different CTAB concentration for the same reactant concentration. Figure clearly indicates that the zeta potential values are increasing with the increase in surfactant concentration and ultimately become constant close to the CMC of the surfactant where particle size is also becomes constant. The increase in zeta potential is due to the adsorption of surfactant molecules on sulfur surface by its tailgroup and forming a complete saturated monolayer near to CMC, also zeta potential values indirectly proofs there is no bi-layer (hemimicelle) formation in the adsorption. The particle size in the presence of surfactant
Fig. 9. XRD pattern of sulfur nanoparticles: (a) in different acid media and (b) in the presence of different surfactants by HCl catalyzed reaction.
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is minimum when surface charge is maximum and surface is saturated by the adsorbed monomer surfactant molecules.
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The XRD samples are prepared by successive washing by fresh water, so we got only pure sulfur peak. The sample was prepared without any washing shows sulfur peak mainly as well as it also shows surfactant and Na2SO4 peaks (pattern is not shown here).
3.5. X-ray diffraction (XRD) analysis Figs. 9a and b show the XRD pattern of sulfur particles in different acidic solution and in the presence of surfactants and HCl media, respectively. The positions and intensities of the diffraction peaks are in good agreement with the literature values for orthorhombic or a-phase sulfur with S8 structure (74-1465 from JCPDS PDF Number). The phase obtained is also independent of acid media. Fig. 9b also shows the similar structure is formed, with the difference of sharp peak indicates highly crystalline nature in the presence of surfactant system than aqueous media except SDBS.
3.6. Analysis by electron microscope (SEM and TEM) Figs. 10A–D show the SEM images of sulfur particle synthesize by HCl (Fig. 10A) and oxalic acid (Fig. 10B) catalyzed media without any surfactant, by HCl catalyzed in the presence of CTAB (Fig. 10C), and SDBS (Fig. 10D) surfactants for thiosulphate concentration of 5 mM. From the figures it is clear that the sulfur particles are almost spherical shape and uniform size. Fig. 11 shows the TEM image of sulfur nanoparticle formed at 5 mM thiosulphate
Fig. 10. SEM micrographs of sulfur nanoparticles from 5 mM thiosulphate concentration: (A) HCl catalyzed, (B) oxalic acid catalyzed, (C) CTAB and HCl (D) SDBS and HCl.
Fig. 11. TEM micrographs of sulfur nanoparticle formed at 5 mM thiosulphate concentration by inorganic acid catalyzed reaction in CTAB surfactant media.
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concentration by inorganic acid catalyzed reaction in CTAB media and the size of the particles obtained is close to 50–55 nm. This again confirms the formation of lower size particle in CTAB media. We have also done SEM (figure not shown) of samples by HCl media in the presence of CTAB surfactant for 4, 9 mM thiosulphate concentration solution and found the particles sizes are higher than that of 5 mM. Form both the SEM and TEM figures we can say sulfur has a tendency towards agglomeration, so we are not getting separate discrete sulfur particle image. 4. Conclusion Mostly spherical shape orthorhombic or a-sulfur with S8 structure is formed by acid catalyze thiosulphate reaction. Sulfur particle size is depends on the ionization constant value of the acids as well as acid (H+) to thiosulphate molar ratio. Inorganic acids produce larger size particles than the organic acid. The particle size distribution is also more uniform by organic acid catalyzed reaction, as zeta potential of the particles is high in organic acid media to prevent agglomeration tendency. All the surfactants, anionic, cationic, and nonionic surfactants play a major role in the lowering the particle size. Compared to nonionic and cationic surfactants, in anionic surfactant media the particle size is low and even in our working concentration range the change in particle size also less, and among two anionic surfactants studied here SDBS is more effective for controlling the size of the particles. Surfactant concentration is also one important factor for the particle sizes and it seems that above CMC is required. In the presence of CTAB within a certain reactant concentration range lower particle sizes are obtained in contrast to the other surfactants if all other conditions are remain same. Sulfur nanoparticles of approximately 30 nm particle size can be synthesized by organic acid catalyzed precipitation of thiosulphate in the presence of aqueous CTAB media. Acknowledgments The financial support from Department of Science and Technology (DST) under Nanomission, New Delhi, India, Grant No. SR/S5/
NM-04/2007, for this project is gratefully acknowledged. We also acknowledge Saha Institute of Nuclear Physics (SINP), Kolkata for giving the opportunity to access their TEM facility. References [1] J.A. Ober, Materials Flow of Sulfur: US Geological Survey Open File Report 02298, 2003.