Surfactant Effects on the Particle Size and ... - ACS Publications

Chapter 10

Surfactant Effects on the Particle Size and Formation of Iron Oxides via a Sol-Gel Process 1

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Erin Camponeschi , Jeremy Walker , Hamid Garmestani , and Rina Tannenbaum Downloaded by UNIV OF ARIZONA on June 10, 2014 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch010

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School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 Department of Chemical Engineering, Technion, Haifa 32000, Israel

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This work illustrates the effects of adding a common surfactant, sodium dodecylbenzene sulfonate (NaDDBS), to the reaction mixture used in the formation of iron (III) oxide nanoparticles via a modified sol-gel process. In order to elucidate the role of the surfactant on the control of the resulting iron oxide particle size, experiments were conducted with two different metal salt precursors: Fe(NO ) · 9H O and FeCl · 6H O. The average particle size of the dried iron oxide gels, in the absence of the surfactant, was 4.5 nm and 3.6 nm for Fe(NO ) · 9H O and FeCl · 6H O as precursors, respectively. The addition of a surfactant inhibited gel formation in the Fe(NO ) · 9H O system, while in the FeCl · 6H O system the gelation process was delayed. The resultant particle sizes were 3.2 nm and 4.9 nm, respectively. It appears that even though the Fe(NO ) · 9H O system was unable to gel the surfactant was able to stabilize the nanoparticles to form even smaller particles than the gel counterpart. 3

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© 2008 American Chemical Society

In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Downloaded by UNIV OF ARIZONA on June 10, 2014 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch010

Introduction The study of nanomaterials is a very active research area that is constantly being fuelled by new ideas, new strategies, new insight and new methods for their design and applications (7-77). Nanomaterials represent a novel class of materials that possess distinctive physical and chemical properties that are very different than their bulk counterparts (2,4-8,10-17). For example, nanometer sized iron oxide exhibits exceptionally strong magnetic properties and as such, may have a wide number of applications in pigments, magnetic materials, catalysts, sensors, data storage, and medical devices. This is one of many systems that have recently become highly attractive and extensively explored (2, 5-8,10-13,15-17). The facile and reproducible creation of nanoparticles is the ultimate goal for their incorporation and utilization in specific devices, thus promoting the development of new technologies (15,17-19). There are several avenues for the formation metal oxide nanoparticles, including iron oxide nanoparticles, which, as stated before, present a particular interest: sol gel processing, synthesis in microemulsions, hydrothermal synthesis, and high temperature solution processing (2,5,8,10-13,15,17). A l l these methods generate iron oxide nanoparticles, but require well-controlled environments that may prohibit the scale-up efforts of these processes from the lab bench to bulk production (2,17,20,21), with the exception of the sol gel processing method. Sol-gel synthesis provides an extremely easy method of creating a large variety of metal oxides from their metal salts at ambient conditions and at low temperatures. However, the drawback of the method is that it generates 3D oxide networks , and hence, it is limited in its efficiency regarding the formation of independent, disconnected nanosized particles. Therefore, the advantages of a relatively simple synthesis method for the creation of nanosized particles, in which the probability for the formation of a 3D network would be diminished, might prove useful for the functionalization of iron oxide particles designated to be incorporated in magnetic nanostructures and in drug targeting and delivery nanoplatforms (20-24). In this work we describe the development of a modified sol gel processing method that was used to create iron (III) oxide nanoparticles. We was found that with the addition of a common surfactant, e.g. sodium dodecyl benzene sulfonate (NaDDBS), iron oxide nanoparticles can easily be synthesized at room temperature in just a few minutes without the formation of 3D iron oxide network. Moreover, in order to comply with aqueous compatibility requirements imposed by the potential use of these materials in conjunction with medical application, the sol gel method was also performed in a water-based environment.


126 Experimental Procedure Ferric nitrate nonahydrate, Fe(N0 ) • 9H 0, ferric chlorate hexahydrate, FeCl • 6H 0, propylene oxide, C H 0 , and stock absolute ethanol, were purchased from Fisher Scientific and used as received. Sodium dodecyl benzene sulfonate (NaDDBS, molecular weight of 348.48 g/mol, purchased from TCI) was used in the same concentrations and methods as described in Matarredona et al. (19). The syntheses were performed at room temperature in glass scintillation vials. Six separate solutions were made in order to test the effects of the surfactant on the final iron oxide particle size. Solution 6 was formed by completely dissolving 0.65 g Fe(N0 ) • 9H 0 in 3.5 mL of ethanol. Then, 1.2 mL of propylene oxide was added as the gelation agent. Solutions 2 and 4 followed the same procedures as solution 6, but 3.5 mL of a 1.2 mM solution of NaDDBS was added before the addition of the propylene oxide. Solution 5 was formed by completely dissolving 0.42 g FeCl • 6H 0 in 3.5 mL of ethanol. Then, 1.2 mL of propylene oxide was added. Solutions 1 and 3 followed the same procedures as solution 5, but 3.5 mL of a 1.2 mM solution of NaDDBS was added before the addition of the propylene oxide. Upon gelation, an additional amount of 1.2 mM NaDDBS was added to solutions 1 and 2. All solutions were then placed in a Fisher Scientific isotemp oven to dry for several days at 100°C. After this time, water was added to solutions 3, 4, 5, and 6 (see Table 1). All the solutions were placed in a dismembrator (Fisher Scientific, 20 kHz) at 35 % amplification for 30 minutes, followed by the removal of small samples for transmission electron microscopy (TEM) imaging and analysis in order to determine particle size. A droplet from each solution was placed on a grid (Ted Pella, Inc. carbon coated copper, PELCO® Center-Marked Grids, 400 mesh, 3.0 mm O.D.) and dried in air. These samples were then analyzed using a Hitachi HF2000, 200kV transmission electron microscope. X-ray diffraction (XRD) was performed on the samples in order to determine composition. The samples were prepared and tested in the same manner as previously reported (20). The dried samples were then placed on a zero background holder and analyzed using a Philips PW 1800 X-ray diffractometer. Patterns from 20° to 90° were examined with a step size of 0.02° using monochromatic Cu Ka X-rays with a wavelength of 1.54056 A. 3

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Results and Discussion The main insight gained from this work showed that the surfactant used, NaDDBS, has the potential of altering the particle size of the resulting iron oxide nanoparticles formed via the modified sol gel processing method. Propylene oxide was used as a gelation agent because it is known to promote the formation


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Table 1. Solution Parameters Solution #

Metal Salt

Gelation Agent

NaDDBS Addition

Water Addition

1

FeCl 6H 0

C H 0

Before gelation & before drying

None

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Fe(N0 ) 9H 0

CH0

Before gelation & before drying

none

3

FeCl 6H 0

CH0

Before gelation

After drying

4

Fe(N0 ) 9H 0

CH0

Before gelation

After drying

5

FeCl 6H 0

CH0

None

After drying

6

Fe(N0 ) 9H 0

CH0

None

After drying

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of a monolithic wet gel in a very short amount of time (a few minutes) (5,77). Moreover, the presence of water, which was also introduced in the modified solgel process, is known to change gelation time in some metal salt systems (5). The experiments conducted in this study involved two metal salts: Ferric nitrate nonahydrate, Fe(N0 ) • 9H 0 and ferric chlorate hexahydrate, FeCl • 6H 0. These were chosen based on their previous use as precursors in classical sol-gel processes (2,5,17). The presence of NaDDBS increased the gel time for the FeCl • 6H 0 system from minutes to hours, and prevented Fe(N0 ) • 9H 0 from forming a gel altogether. There may be several reasons for this behavior: (a) The relative concentrations of the propylene oxide, iron salt and coordinated water may play a pivotal role in determining the gelation time. Gash et al. (5) showed that there was a 2 minute gelation time for the FeCl • 6H 0 precursor in water, while the precursor with a higher water content required several hours to gel; (b) The presence of NaDDBS molecules in the solution has the additional effect of delaying the onset of gelation in the FeCl • 6H 0 system, and preventing it altogether in the Fe(N0 ) • 9H 0 system. These surfactant molecules tend to organize themselves in the vicinity of the water molecules, preferentially through their ionic groups, as shown in Figure 1, thus creating a stabilization layer that would either delay or completely inhibit the accessibility of the coordinated water molecules to the propylene oxide and hence, prevent the gelation process. Solutions that gelled follow the same general procedure of hydrolysis, condensation, and heating. This process allows for the formation of iron (III) oxide particles through a series of reactions where A is an intermediate reaction 3

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Figure 1. A schematic representation of the dipole-dipole attractions between the metal cations (Fe ) and the oxygen (O ') in the surrounding water molecules, and the subsequent attraction with the hydrophilic group in the surfactant NaDDBS. 3+

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Table 2. Particle Size Results Solution #

Metal Salt

Gel Formation

Average Particle Size (nm)

Sample Color

1

FeCl • 6H 0

Yes

4.9

Brown

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Fe(N0 ) • 9H 0

No

3.2

Light brown

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FeCl • 6H 0

Yes

84.6

Orange

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Fe(N0 ) • 9H 0

No

5.1

Light brown

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FeCl • 6H 0

Yes

3.6

Brown

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Fe(N0 ) • 9H 0

Yes

4.5

Light Brown

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129 product with a metal hydroxide reactive group. In the presence of polypropylene oxide the reaction is as follows: 3 +

C H 0 + Fe(H 0) 3

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-> C H 0+ + Fe(OH)(H of

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-^Fe 0

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In the presence of water the reaction is as follows: 3 +

Fe(H 0)


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+H 0''

A

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Fe

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It is expected that the presence of NaDDBS was essential in the early stages of the process, impeding the reaction with the epoxide and hydroxo ligand, thus preventing the gelation process. In the last step of this process, the material is heated to 100°C in order to remove all excess water and drive the formation of the iron (III) oxide. The change in particle size and color of the solutions is illustrated in Table 2 and Figure 2. It appears that the color of the solution is directly related to the size of the particles formed in solution. To determine the average particle size, high-resolution TEM imaging was used. The TEM images and histograms are shown in Figures 3 - 8 . Due to the low molecular weight of the iron oxide particles, the surfactant and the epoxide molecules, which can cloud transmission electron images, obtaining clear TEM images was difficult. The NaDDBS addition prior to gelation to the FeCl • 6H 0 system did not appear to have a large effect on the particle sizes compared to the same system in the absence of NaDDBS. However, upon drying, the system having NaDDBS that was added in the final stages of the synthesis (solution 1) appears to generate significantly smaller particle size than that of the system without NaDDBS added at this stage (solution 3). This leads to the conclusion that the NaDDBS molecules inhibit the growth of the particle by replacing water molecules in the coordination sphere of the growing oxide particles, thus preventing their aggregation. As for the Fe(N0 ) • 9H 0 systems, the changes in particle size is negligible in most samples, however, the smallest particle sizes were obtained in the solutions containing NaDDBS. This supports the realization that the addition of NaDDBS inhibits the growth of the iron oxide nanoparticles, even though these system do not form gels. The coupling of a high number of coordinated water molecules with the presence of a surfactant, inhibits gel formation on one hand, and stabilizes small nanoparticles on the other hand. Hence, a distribution of particle sizes in the 3-5 nm range that is characteristic to the primary particles 3

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Figure 2. Color variations in solutions



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Figure 3. (a) High resolution TEM image of solution 1 (b) Particle size distribution for solution 1




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In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 9

Figure 9(i). X-ray diffraction patterns for (a) Sample 1 (b) Sample 2, (c) Sample 3, (d) Sample 4, (e) Sample 5, (f) Sample 6


136 formed by the sol-gel process, may be achieved also in a system in which surfactants are present but the gelation is inhibited. X-ray diffractions of the iron oxide nanoparticles are shown in Figure 9 a-f for the as-synthesized samples. The diffraction patterns for samples 1, 2, 4, 5 and 6 are largely amorphous in character, having large diffraction peaks resulting from the nanoscale features of the material. Figure 9 g-1 illustrates possible alternative phases that may have formed; iron (III) oxide, hydroxide and oxyhydroxide phases: a-Fe 0 (hematite), Fe(OH) , a-FeO(OH) (ferrihydrite), y-Fe 0 , y -FeO(OH); and • 35

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Figure 9(ii). X-ray diffraction patterns for various iron oxide/oxyhydroxide phases: (g) a-Fe203, (h) Fe(OH) , (i) a-FeO(OH) i.e. ferrihydrite, (j) y-Fe O (k) y-FeO(OH), and (I) d-FeO(OH). 3

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Acknowledgements This work was supported by the Air Force/Boiling AFB/DC MURI project on Energetic Structural Materials, Grant No. F49620-02-1-0382. Erin Camponeschi was supported by an IPST- Georgia Institute of Paper Science and Technology Graduate Fellowship, and Richard Vance was supported by the Summer Undergraduate Research Fellowship at Georgia Tech through the N S F REU program Grant No. DMR-0139081. We would like to thank Grady Nunnery for all his help with the background research and experiments.

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Surfactant Effects on the Particle Size and ... - ACS Publications

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