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Study the effect of HLB of surfactant on particle size distribution of hematite nanoparticles prepared via the reverse microemulsion
Solid State Sciences 14 (2012) 622e625
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Study the effect of HLB of surfactant on particle size distribution of hematite nanoparticles prepared via the reverse microemulsion Mohammad Reza Housaindokht*, Ali Nakhaei Pour Bimolecular Research Center, Department of Chemistry, Ferdowsi University of Mashhad, P.O. Box: 91775-1436, Mashhad, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2011 Received in revised form 20 December 2011 Accepted 23 January 2012 Available online 30 January 2012
Hematite nanoparticles have been synthesized via reverse microemulsion route at room temperature. The microemulsion system, contained water, chloroform, 1-butanol, and surfactant, was combined with iron nitrate solution to result iron oxide nanoparticles precipitation. Three technical surfactants, with different structures and HLB (hydrophileelipophile balance) values were employed and the effects of the HLB values on the hematite particle size were investigated. The prepared particles were evaluated by BET, XRD and TEM techniques. These results showed that the iron oxide particle size and particle size distribution increased with increasing surfactant HLB values. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Iron-nanoparticles Microemulsion Hematite Particle size distribution HLB
1. Introduction Nanostructured inorganic particles are promising systems as oxidation catalysts due to the high surface-to-volume ratio. Especially, nanoscale iron dioxide particles are a very attractive catalytic systems for FischereTropsch synthesis (FTS),and Wateregas-shift (WGS) reaction [1]. Different approaches have been proposed and investigated for preparing and producing of nanoparticles, e.g. physical and chemical vapor deposition, a well-controlled mixing in bulk precipitation, and microemulsions [2e7]. A microemulsion media was formed on addition of an aliphatic alcohol (co-surfactant) to an ordinary emulsion [6e8]. There are three types of microemulsions: water-in-oil (w/o), oil-in-water (o/w), and bicontinuous microemulsions. The surfactant molecule, which has polar heads and nonpolar organic tails, stabilizes the water droplets. The organic (hydrophobic) portion faces toward the oil phase, and the polar (hydrophilic) group toward water. The relationship (or balance) between the hydrophilic to the lipophilic portion of the nonionic surfactants is called HydrophileeLipophile Balance (HLB) [6e10]. In diluted water (or oil) solutions, the emulsifier dissolves and exists as a monomer, but when its concentration exceeds a certain limit,
* Corresponding author. Tel.: þ+98 5118797022; fax: þ98 5118796416. E-mail addresses: [email protected], [email protected] (M.R. Housaindokht). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.01.016
which is called the critical micelle concentration (CMC), the molecules of emulsifier associate spontaneously to form aggregates called micelles. In the hydrophilic interior of these droplets, a certain amount of water-soluble material can be dissolved; for example, transition metal salts that in the next step serve as precursor(s) for the final metal particles. The aim of the present work is to synthesize and control the particle size of the hematite with various surfactants, which have different HLB values. In this work the effect of HLB values on the particle size and particle size distribution of prepared hematite nanoparticles is discussed. 2. Experimental 2.1. Preparation of iron oxides nanoparticles Hematite nanoparticles were prepared by coprecipitation in a water-in-oil microemulsion [11e15]. The precipitation was performed in the single-phase microemulsion operating region. For this purpose, a water solution of metal precursor, Fe(NO3)3-9H2O (Fluka, 98e100%) was added to a mixture of an oil phase (Chloroform, Aldrich, >99%) and the commercial available surfactants. Besides, 1-butanol as a co-surfactant was added to system in order to obtain a proper microemulsion. For a quaternary system of surfactant (S, g)/oil (O, g)/alcohol (A, g)/water (W, g), the following symbols were defined, a represents the mass fraction of oil-in-
M.R. Housaindokht, A. Nakhaei Pour / Solid State Sciences 14 (2012) 622e625
623
Table 1 Characterization of surfactants. Surfactant
Chemical name
Chemical formula
HLB
Type
SDS SDBS Triton X-100
Sodium dodecyl sulfate Sodium dodecyl benzene sulfonate Polyoxethylene-10-octylphenylether
C12H25SO4Na C12H25(C6H4)SO3Na C8H17eC6H4e(OCH2-CH2)10OH
40 10.6 13.4
Anionic Anionic Non-anionic
water plus oil, a ¼ O/(W þ O), b the mass ratio of surfactant in the whole system, b ¼ S/(W þ O þ S þ A), and 3 , the mass fraction of alcohol in the whole system, 3 ¼ A/(W þ O þ S þ A) [6e10]. In the prepared microemulsion system, the parameters are, a ¼ 0.62, b ¼ 0.03, 3 ¼ 0.28. After stirring well and stabilizing for 24 h, a transparent mixture was obtained. Then, hydrazine was added to reduce metal precursor. The several steps of the complete reduction process were followed through color-changes that occurred in the solution (final color is black), which suggests a change in the oxidation state of the iron or copper, and formation of a colloidal suspension of metallic particles. The resulting mixture was decanted overnight. The solid was recovered by centrifuging and then washing thoroughly with distilled water and ethanol for several time and then dried at 393 K and calcined at 673 K for 4 h. There different surfactants as listed in Table 1 were applied to evaluate the surfactant effects on particle size, and other characteristics of iron oxides. In this work the amount of water, oil, alcohol and surfactant is kept at fixed values, but the kind of surfactant was changed. Therefore, the middle-phase behavior and the solubilization power of the microemulsion formed by sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and Triton X-100, were studied. 2.2. Characterization The surface area was calculated from the Brunauer-EmmettTeller (BET) equation, pore volume, average pore diameter, and pore size distribution of the iron nanoparticles were determined by N2 physisorption using a Micromeritics ASAP 2010 automated system. A 0.5 g catalyst sample was degassed in the Micromeritics ASAP 2010 at 373 K for1 h and then at 573 K for 2 h prior to analysis. The analysis was done using N2 adsorption at 77 K. Both the pore volume and the average pore diameter were calculated by Barret-Joyner-Hallender (BJH) method from the adsorption isotherm. Assuming that the Fe2O3 particles are spherically, the corresponding particle size (dBET) can be estimated as [14,16]:
dBET ¼
6
(1)
rA
where r is the true density of Fe2O3, ie 5.24 g/cm3 and A is the specific surface area of samples. X-ray diffraction (XRD) technique was used to determine the phase composition of iron nanoparticles after calcination. The XRD spectrum of the samples were collected using an X-ray diffractometer, Philips PW1840 X-ray diffractometer, using monochromatized Cu/Ka radiation (40 kV, 40 mA) and a step scan mode at a scan rate of 0.02 (2q) per second from 10e80 . XRD peak identification was performed by comparison with the JCPDS database software. The average crystallite size of samples, dxRD, can be estimated from XRD patterns by applying full-with half-maximum (FWHM) of characteristic peak (104) Fe2O3 located at 2q ¼ 33.3 peak to Scherrer equation. The morphology of prepared iron nanoparticles after calcinations was observed with a transmission electron microscope (TEM, LEO 912 AB, Germany). An appropriate amount of Fe2O3 suspension that was directly taken from the reaction solution was dropped onto the carbon-coated copper grids for TEM observation. The average particle size (dTEM) and particle size distribution were also determined by TEM images by counting more than 100 particles. 3. Results 3.1. Crystalline structure of prepared nanoparticles Fig. 1 shows the XRD patterns of iron oxides prepared by the microemulsion and bulk methods. From this Fig., the characteristic peaks corresponding to (012), (104), (110), (113), (024), (116), (018), (214), (300) planes are located at 2q ¼ 24.3, 33.3, 35.8, 40.8, 49.6, 54.1, 57.6, 64.1 and 65.6o, respectively [11,15]. They represent very close to the ones with cubic hematite structured Fe2O3 crystal in JCPDS database. Diffraction data indicate that the crystalline phases of all samples demonstrate the cubic hematite structured Fe2O3 crystal, independent to existence of surfactant in microemulsion system. This strongly infers that microemulsion system can only modules physical properties of reaction medium without any changes in the reaction paths and arrangements of crystal structure. The characteristic peak at 2q ¼ 33.3 corresponds to the hematite 104 plane and this value is used to calculate the average metal particle size by the Scherrer equation, which is listed in Table 2. 3.2. Textural properties of the prepared nanoparticles Textural properties and pore size distributions of the iron oxides are shown in Table 3 and Figs. 2 and 3, respectively. N2 Table 2 Average particle size determined by TEM, BET and XRD techniques.
Fig. 1. X-ray diffraction patterns of the fresh samples after calcination.
Catalyst
dBET (nm)
dXRD (nm)
dTEM (nm)
FeeSDS FeeTriton FeeSDBS
25.3 22.2 15.4
23.8 20.4 13.2
22 19 12
624
M.R. Housaindokht, A. Nakhaei Pour / Solid State Sciences 14 (2012) 622e625
Table 3 Textural properties of prepared samples. Catalyst
BET surface area (m2/g)
Average pore size (nm)a
Pore volume (cm3/g)a
FeeSDS FeeTriton FeeSDBS
45.2 51.5 76.5
29.1 17.2 7.1
0.17 0.21 0.19
a
These values were calculated by BJH method from desorption isotherm.
P0 ¼ 0.8e0.95, which is characteristic of hexagonal mesoporous materials with cylindrical channels [16]. Also, SDBS prepared catalyst displays type IV isotherm pattern with H4-type hysteresis loop at the relative pressure of P/P0 ¼ 0.5e0.95 which is characteristic of vesicle-like structure channels [16]. The pore size distributions of samples calculated by BarettJoyner-Halenda (BJH) method for adsorption step are presented in Fig. 3 and Table 3. They indicate that surfactant play an important role in the structure of iron nanoparticles in the microemulsion system. Furthermore, the mean pore sizes and pore size distribution decreased while the HLB value increases. In addition, Table 3 shows that BET surface area of the samples monotonously declines with increase of the surfactant HLB values. 3.3. Particle size distribution of iron particles The TEM images and particle size distributions for the samples prepared are demonstrated in Figs. 4. and 5 and Table 2. The average particle size (dTEM) and particle size distribution were determined by TEM images considering more than 100 particles. Itindicates that the average particle size of resultant Fe2O3 particles is increased from 12e22 from SDS to water prepared particles. As shown in these Figs., when surface HLB value increases in
Fig. 2. N2 adsorptionedesorption isotherms with hysteresis loop of prepared samples with deferent surfactant. The effect of HLB of the surfactant on the isotherms was shown.
adsorptionedesorption isotherms of samples are presented in Fig. 2. As shown in this Fig., prepared iron oxides with SDS and Triton surfactants show type IV of Brunauer’s classification isotherms with H1-type hysteresis loop at the relative pressure of P/
Fig. 3. Pore size distribution of prepared samples with deferent surfactant based on adsorption BJH graph.
Fig. 4. TEM images of prepared samples. The effect of HLB of the surfactant on the morphology of the samples was shown.
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the droplet is very fast, so the rate-determining step will be the initial communication step of the microdroplets with different droplets. The presence of the surfactant strictly prevents the nuclei to grow fast. Consequently the particles will grow at the same rate, favoring the formation of particles of more homogeneous size distribution. The result is a suspension of small particles stabilized by the surfactant molecules that prohibit coalescence in which without it the system would lead to further agglomeration. The size of the droplets will influence the size of the nuclei but the size of the final particle will be controlled by the surrounding surfactant molecules. As shown in Table 2, the particle size of final iron oxide increases when surfactant HLB value augments. Produced solid particle size via microemulsion system may be attained from lower nucleation rate and/or higher rate of nuclei growth. The surfactant with lower HLB value decreases the surface energy more and thus increases the nucleation rate. The higher surfactant HLB number indicates more hydrophilic properties and the lower number shows more lipophilic characteristic. In a w/o microemulsion system, the more lipophilic surfactant leads to lower droplet size, which decreases the mass transfer amounts inside droplets. Therefore, lower HLB value for surfactant causes a lower growth of nucleus and diminishes the final particle size. As shown in present results, SDBS with lower HLB value Produces smaller particle size via the same microemulsion system than SDS or Triton X-100 surfactant. 5. Conclusion Nanosized iron oxide particles were synthesized by reverse microemulsion method with different HLB values. The iron oxide particle size and particle size distribution increased while surfactant HLB values augmented. The more lipophilic affinity produces lower droplet size and decreases the mass transfer inside the droplets. Thus lower HLB value for surfactant proceeds a lower growth of nucleus and therefore decreases the particle size. Acknowledgments Financial support of the Ferdowsi University of Mashhad, Iran (P/15369/1-89/8/5) is gratefully acknowledged. References [1] [2] [3] [4] Fig. 5. Particle size distributions (PSD) of prepared samples based on PSD ¼ ni
P
ni .
microemulsion system, the average iron oxide particle size enlarges and particle size distribution broads. 4. Discussion The formation of particles proceeds in two steps: first, the nucleation process inside the droplet, and then the aggregation process to form the final particle. The nucleation rate defined as the number of nuclei produced per unit volume per unit time. The rate of particle growth is controlled by mass transfer inside the droplets. The reduction, nucleation, and growth occur inside the droplets, which controls the final particle size. The chemical reaction within
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
G.P. van der Laan, A.A.C.M. Beenackers, Catal. Rev. Sci. Eng. 41 (1999) 255. A.T. Bell, Science 299 (2003) 1688. J.Y. Ying, Chem. Eng. Sci. 61 (2006) 1540. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Zarkesh, M.R. Alaei, J. Mol. Catal. A 330 (2010) 112. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Zarkesh, J. Nat. Gas. Chem. 19 (2010) 284. X. Zhou, W. Xu, G. Liu, D. Panda, P. Chen, J. Am. Chem. Soc. 132 (2010) 138. P. Kumar, K.L. Mittal, Handbook of Microemulsions: Science and Technology, Marcel Dekker, New York, 1999. S. Eriksson, U. Nylén, S. Rojas, M. Boutonnet, Appl. Catal. A 265 (2004) 207. I. Capek, Adv. Coll. Inte. Sci. 110 (2004) 49. Z. Tian, W. Huang, Y. Liang, Cer. Inter. 35 (2009) 661. T. Herranz, S. Rojas, F.J. Pérez-Alonso, M. Ojeda, P. Terreros, J.L.G. Fierro, Appl. Catal. A 311 (2006) 66. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Zarkesh, J. Nat. Gas. Chem. 19 (2010) 107. A. Nakhaei Pour, S. Taghipoor, M. Shekarriz, S.M.K. Shahri, Y. Zamani, J. Nanosci. Nanotech. 9 (2009) 4425. A. Nakhaei Pour, M.R. Housaindokht, E.G. Babakhani, M. Irani, S.M.K. Shahri , Journal of Industrial and Engineering Chemistry 17 (2011) 596e602. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Ind. Eng. Chem. 16 (2010) 1025. H.I. Chen, H.Y. Chang, Coll. Surf. A: Phy. Eng. Asp. 242 (2004) 61.
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Study the effect of HLB of surfactant on particle size distribution of hematite nanoparticles prepared via the reverse microemulsion Mohammad Reza Housaindokht*, Ali Nakhaei Pour Bimolecular Research Center, Department of Chemistry, Ferdowsi University of Mashhad, P.O. Box: 91775-1436, Mashhad, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2011 Received in revised form 20 December 2011 Accepted 23 January 2012 Available online 30 January 2012
Hematite nanoparticles have been synthesized via reverse microemulsion route at room temperature. The microemulsion system, contained water, chloroform, 1-butanol, and surfactant, was combined with iron nitrate solution to result iron oxide nanoparticles precipitation. Three technical surfactants, with different structures and HLB (hydrophileelipophile balance) values were employed and the effects of the HLB values on the hematite particle size were investigated. The prepared particles were evaluated by BET, XRD and TEM techniques. These results showed that the iron oxide particle size and particle size distribution increased with increasing surfactant HLB values. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Iron-nanoparticles Microemulsion Hematite Particle size distribution HLB
1. Introduction Nanostructured inorganic particles are promising systems as oxidation catalysts due to the high surface-to-volume ratio. Especially, nanoscale iron dioxide particles are a very attractive catalytic systems for FischereTropsch synthesis (FTS),and Wateregas-shift (WGS) reaction [1]. Different approaches have been proposed and investigated for preparing and producing of nanoparticles, e.g. physical and chemical vapor deposition, a well-controlled mixing in bulk precipitation, and microemulsions [2e7]. A microemulsion media was formed on addition of an aliphatic alcohol (co-surfactant) to an ordinary emulsion [6e8]. There are three types of microemulsions: water-in-oil (w/o), oil-in-water (o/w), and bicontinuous microemulsions. The surfactant molecule, which has polar heads and nonpolar organic tails, stabilizes the water droplets. The organic (hydrophobic) portion faces toward the oil phase, and the polar (hydrophilic) group toward water. The relationship (or balance) between the hydrophilic to the lipophilic portion of the nonionic surfactants is called HydrophileeLipophile Balance (HLB) [6e10]. In diluted water (or oil) solutions, the emulsifier dissolves and exists as a monomer, but when its concentration exceeds a certain limit,
* Corresponding author. Tel.: þ+98 5118797022; fax: þ98 5118796416. E-mail addresses: [email protected], [email protected] (M.R. Housaindokht). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.01.016
which is called the critical micelle concentration (CMC), the molecules of emulsifier associate spontaneously to form aggregates called micelles. In the hydrophilic interior of these droplets, a certain amount of water-soluble material can be dissolved; for example, transition metal salts that in the next step serve as precursor(s) for the final metal particles. The aim of the present work is to synthesize and control the particle size of the hematite with various surfactants, which have different HLB values. In this work the effect of HLB values on the particle size and particle size distribution of prepared hematite nanoparticles is discussed. 2. Experimental 2.1. Preparation of iron oxides nanoparticles Hematite nanoparticles were prepared by coprecipitation in a water-in-oil microemulsion [11e15]. The precipitation was performed in the single-phase microemulsion operating region. For this purpose, a water solution of metal precursor, Fe(NO3)3-9H2O (Fluka, 98e100%) was added to a mixture of an oil phase (Chloroform, Aldrich, >99%) and the commercial available surfactants. Besides, 1-butanol as a co-surfactant was added to system in order to obtain a proper microemulsion. For a quaternary system of surfactant (S, g)/oil (O, g)/alcohol (A, g)/water (W, g), the following symbols were defined, a represents the mass fraction of oil-in-
M.R. Housaindokht, A. Nakhaei Pour / Solid State Sciences 14 (2012) 622e625
623
Table 1 Characterization of surfactants. Surfactant
Chemical name
Chemical formula
HLB
Type
SDS SDBS Triton X-100
Sodium dodecyl sulfate Sodium dodecyl benzene sulfonate Polyoxethylene-10-octylphenylether
C12H25SO4Na C12H25(C6H4)SO3Na C8H17eC6H4e(OCH2-CH2)10OH
40 10.6 13.4
Anionic Anionic Non-anionic
water plus oil, a ¼ O/(W þ O), b the mass ratio of surfactant in the whole system, b ¼ S/(W þ O þ S þ A), and 3 , the mass fraction of alcohol in the whole system, 3 ¼ A/(W þ O þ S þ A) [6e10]. In the prepared microemulsion system, the parameters are, a ¼ 0.62, b ¼ 0.03, 3 ¼ 0.28. After stirring well and stabilizing for 24 h, a transparent mixture was obtained. Then, hydrazine was added to reduce metal precursor. The several steps of the complete reduction process were followed through color-changes that occurred in the solution (final color is black), which suggests a change in the oxidation state of the iron or copper, and formation of a colloidal suspension of metallic particles. The resulting mixture was decanted overnight. The solid was recovered by centrifuging and then washing thoroughly with distilled water and ethanol for several time and then dried at 393 K and calcined at 673 K for 4 h. There different surfactants as listed in Table 1 were applied to evaluate the surfactant effects on particle size, and other characteristics of iron oxides. In this work the amount of water, oil, alcohol and surfactant is kept at fixed values, but the kind of surfactant was changed. Therefore, the middle-phase behavior and the solubilization power of the microemulsion formed by sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and Triton X-100, were studied. 2.2. Characterization The surface area was calculated from the Brunauer-EmmettTeller (BET) equation, pore volume, average pore diameter, and pore size distribution of the iron nanoparticles were determined by N2 physisorption using a Micromeritics ASAP 2010 automated system. A 0.5 g catalyst sample was degassed in the Micromeritics ASAP 2010 at 373 K for1 h and then at 573 K for 2 h prior to analysis. The analysis was done using N2 adsorption at 77 K. Both the pore volume and the average pore diameter were calculated by Barret-Joyner-Hallender (BJH) method from the adsorption isotherm. Assuming that the Fe2O3 particles are spherically, the corresponding particle size (dBET) can be estimated as [14,16]:
dBET ¼
6
(1)
rA
where r is the true density of Fe2O3, ie 5.24 g/cm3 and A is the specific surface area of samples. X-ray diffraction (XRD) technique was used to determine the phase composition of iron nanoparticles after calcination. The XRD spectrum of the samples were collected using an X-ray diffractometer, Philips PW1840 X-ray diffractometer, using monochromatized Cu/Ka radiation (40 kV, 40 mA) and a step scan mode at a scan rate of 0.02 (2q) per second from 10e80 . XRD peak identification was performed by comparison with the JCPDS database software. The average crystallite size of samples, dxRD, can be estimated from XRD patterns by applying full-with half-maximum (FWHM) of characteristic peak (104) Fe2O3 located at 2q ¼ 33.3 peak to Scherrer equation. The morphology of prepared iron nanoparticles after calcinations was observed with a transmission electron microscope (TEM, LEO 912 AB, Germany). An appropriate amount of Fe2O3 suspension that was directly taken from the reaction solution was dropped onto the carbon-coated copper grids for TEM observation. The average particle size (dTEM) and particle size distribution were also determined by TEM images by counting more than 100 particles. 3. Results 3.1. Crystalline structure of prepared nanoparticles Fig. 1 shows the XRD patterns of iron oxides prepared by the microemulsion and bulk methods. From this Fig., the characteristic peaks corresponding to (012), (104), (110), (113), (024), (116), (018), (214), (300) planes are located at 2q ¼ 24.3, 33.3, 35.8, 40.8, 49.6, 54.1, 57.6, 64.1 and 65.6o, respectively [11,15]. They represent very close to the ones with cubic hematite structured Fe2O3 crystal in JCPDS database. Diffraction data indicate that the crystalline phases of all samples demonstrate the cubic hematite structured Fe2O3 crystal, independent to existence of surfactant in microemulsion system. This strongly infers that microemulsion system can only modules physical properties of reaction medium without any changes in the reaction paths and arrangements of crystal structure. The characteristic peak at 2q ¼ 33.3 corresponds to the hematite 104 plane and this value is used to calculate the average metal particle size by the Scherrer equation, which is listed in Table 2. 3.2. Textural properties of the prepared nanoparticles Textural properties and pore size distributions of the iron oxides are shown in Table 3 and Figs. 2 and 3, respectively. N2 Table 2 Average particle size determined by TEM, BET and XRD techniques.
Fig. 1. X-ray diffraction patterns of the fresh samples after calcination.
Catalyst
dBET (nm)
dXRD (nm)
dTEM (nm)
FeeSDS FeeTriton FeeSDBS
25.3 22.2 15.4
23.8 20.4 13.2
22 19 12
624
M.R. Housaindokht, A. Nakhaei Pour / Solid State Sciences 14 (2012) 622e625
Table 3 Textural properties of prepared samples. Catalyst
BET surface area (m2/g)
Average pore size (nm)a
Pore volume (cm3/g)a
FeeSDS FeeTriton FeeSDBS
45.2 51.5 76.5
29.1 17.2 7.1
0.17 0.21 0.19
a
These values were calculated by BJH method from desorption isotherm.
P0 ¼ 0.8e0.95, which is characteristic of hexagonal mesoporous materials with cylindrical channels [16]. Also, SDBS prepared catalyst displays type IV isotherm pattern with H4-type hysteresis loop at the relative pressure of P/P0 ¼ 0.5e0.95 which is characteristic of vesicle-like structure channels [16]. The pore size distributions of samples calculated by BarettJoyner-Halenda (BJH) method for adsorption step are presented in Fig. 3 and Table 3. They indicate that surfactant play an important role in the structure of iron nanoparticles in the microemulsion system. Furthermore, the mean pore sizes and pore size distribution decreased while the HLB value increases. In addition, Table 3 shows that BET surface area of the samples monotonously declines with increase of the surfactant HLB values. 3.3. Particle size distribution of iron particles The TEM images and particle size distributions for the samples prepared are demonstrated in Figs. 4. and 5 and Table 2. The average particle size (dTEM) and particle size distribution were determined by TEM images considering more than 100 particles. Itindicates that the average particle size of resultant Fe2O3 particles is increased from 12e22 from SDS to water prepared particles. As shown in these Figs., when surface HLB value increases in
Fig. 2. N2 adsorptionedesorption isotherms with hysteresis loop of prepared samples with deferent surfactant. The effect of HLB of the surfactant on the isotherms was shown.
adsorptionedesorption isotherms of samples are presented in Fig. 2. As shown in this Fig., prepared iron oxides with SDS and Triton surfactants show type IV of Brunauer’s classification isotherms with H1-type hysteresis loop at the relative pressure of P/
Fig. 3. Pore size distribution of prepared samples with deferent surfactant based on adsorption BJH graph.
Fig. 4. TEM images of prepared samples. The effect of HLB of the surfactant on the morphology of the samples was shown.
M.R. Housaindokht, A. Nakhaei Pour / Solid State Sciences 14 (2012) 622e625
625
the droplet is very fast, so the rate-determining step will be the initial communication step of the microdroplets with different droplets. The presence of the surfactant strictly prevents the nuclei to grow fast. Consequently the particles will grow at the same rate, favoring the formation of particles of more homogeneous size distribution. The result is a suspension of small particles stabilized by the surfactant molecules that prohibit coalescence in which without it the system would lead to further agglomeration. The size of the droplets will influence the size of the nuclei but the size of the final particle will be controlled by the surrounding surfactant molecules. As shown in Table 2, the particle size of final iron oxide increases when surfactant HLB value augments. Produced solid particle size via microemulsion system may be attained from lower nucleation rate and/or higher rate of nuclei growth. The surfactant with lower HLB value decreases the surface energy more and thus increases the nucleation rate. The higher surfactant HLB number indicates more hydrophilic properties and the lower number shows more lipophilic characteristic. In a w/o microemulsion system, the more lipophilic surfactant leads to lower droplet size, which decreases the mass transfer amounts inside droplets. Therefore, lower HLB value for surfactant causes a lower growth of nucleus and diminishes the final particle size. As shown in present results, SDBS with lower HLB value Produces smaller particle size via the same microemulsion system than SDS or Triton X-100 surfactant. 5. Conclusion Nanosized iron oxide particles were synthesized by reverse microemulsion method with different HLB values. The iron oxide particle size and particle size distribution increased while surfactant HLB values augmented. The more lipophilic affinity produces lower droplet size and decreases the mass transfer inside the droplets. Thus lower HLB value for surfactant proceeds a lower growth of nucleus and therefore decreases the particle size. Acknowledgments Financial support of the Ferdowsi University of Mashhad, Iran (P/15369/1-89/8/5) is gratefully acknowledged. References [1] [2] [3] [4] Fig. 5. Particle size distributions (PSD) of prepared samples based on PSD ¼ ni
P
ni .
microemulsion system, the average iron oxide particle size enlarges and particle size distribution broads. 4. Discussion The formation of particles proceeds in two steps: first, the nucleation process inside the droplet, and then the aggregation process to form the final particle. The nucleation rate defined as the number of nuclei produced per unit volume per unit time. The rate of particle growth is controlled by mass transfer inside the droplets. The reduction, nucleation, and growth occur inside the droplets, which controls the final particle size. The chemical reaction within
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
G.P. van der Laan, A.A.C.M. Beenackers, Catal. Rev. Sci. Eng. 41 (1999) 255. A.T. Bell, Science 299 (2003) 1688. J.Y. Ying, Chem. Eng. Sci. 61 (2006) 1540. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Zarkesh, M.R. Alaei, J. Mol. Catal. A 330 (2010) 112. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Zarkesh, J. Nat. Gas. Chem. 19 (2010) 284. X. Zhou, W. Xu, G. Liu, D. Panda, P. Chen, J. Am. Chem. Soc. 132 (2010) 138. P. Kumar, K.L. Mittal, Handbook of Microemulsions: Science and Technology, Marcel Dekker, New York, 1999. S. Eriksson, U. Nylén, S. Rojas, M. Boutonnet, Appl. Catal. A 265 (2004) 207. I. Capek, Adv. Coll. Inte. Sci. 110 (2004) 49. Z. Tian, W. Huang, Y. Liang, Cer. Inter. 35 (2009) 661. T. Herranz, S. Rojas, F.J. Pérez-Alonso, M. Ojeda, P. Terreros, J.L.G. Fierro, Appl. Catal. A 311 (2006) 66. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Zarkesh, J. Nat. Gas. Chem. 19 (2010) 107. A. Nakhaei Pour, S. Taghipoor, M. Shekarriz, S.M.K. Shahri, Y. Zamani, J. Nanosci. Nanotech. 9 (2009) 4425. A. Nakhaei Pour, M.R. Housaindokht, E.G. Babakhani, M. Irani, S.M.K. Shahri , Journal of Industrial and Engineering Chemistry 17 (2011) 596e602. A. Nakhaei Pour, M.R. Husiandoukht, S.F. Tayyari, J. Ind. Eng. Chem. 16 (2010) 1025. H.I. Chen, H.Y. Chang, Coll. Surf. A: Phy. Eng. Asp. 242 (2004) 61.