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Iron oxide nanoparticles for magnetically assisted patterned coatings
Journal of Magnetism and Magnetic Materials 388 (2015) 49–58
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Iron oxide nanoparticles for magnetically assisted patterned coatings Gianina Dodi, Doina Hritcu n, Dan Draganescu, Marcel I. Popa Faculty of Chemical Engineering and Environmental Protection, “Gheorghe Asachi” Technical University of Iasi, 73, Prof. Dr. Docent Dimitrie Mangeron Road, 700050 Iasi, Romania
art ic l e i nf o
a b s t r a c t
Article history: Received 6 October 2014 Received in revised form 6 April 2015 Accepted 6 April 2015 Available online 8 April 2015
Iron oxide nanoparticles able to magnetically assemble during the curing stage of a polymeric support to create micro-scale surface protuberances in a controlled manner were prepared and characterized. The bare Fe3O4 particles were obtained by two methods: co-precipitation from an aqueous solution containing Fe3 þ /Fe2 þ ions with a molar ratio of 2:1 and partial oxidation of ferrous ions in alkaline conditions. The products were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and magnetization measurement. They were subsequently functionalized using oleic acid, sodium oleate, or non-ionic surfactant mixtures with various hydrophilic to lipophilic balance (HLB) values. Composite nanoparticle-polymer films prepared by spraying were deposited and cured by drying on glass slides under a static magnetic field in the range of 1.5–5.5 mT. Magnetic field generated surface roughness was evidenced by optical and scanning electron microscopy. The optimum hierarchical patterning was obtained with the nanoparticles produced by partial oxidation and functionalized with hydrophobic surfactants. Possible applications may include ice-phobic composite coatings. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetite nanoparticles Surfactant functionalization Composite patterned coatings
1. Introduction The interest for research directed toward achieving a better control over the mechanisms governing the interaction between solid surfaces and water drops is steadily growing. Synthetic surfaces with wetting behavior that is tailored according to the desired application are relevant to a wide array of modern technological fields, such as self-cleaning coatings for satellite dishes and automobile windshields, non-fouling surfaces in biomedical devices, drag-force reducing materials for micro-fluidic or aquatic sport applications, anti-corrosion or ice-accretion reducing treatments in power-lines, aviation or building industry. The strategies for engineering synthetic coatings with water/ice repellent properties have been inspired by the features of naturally occurring superhydrophobic materials, namely their resistance to water penetration and their low adhesion to droplets [1,2]. Examples of such natural surfaces that display a contact angle higher than 150° with the water drops, include the lotus and other plant leaves, bird and butterfly wings or the water strider's leg. Recent studies conclude that an optimum combination of low surface tension and hierarchical roughness is able to produce the superhydrophobic effect [3]. In natural surfaces, the morphology of the surface is the determining factor for its wetting properties: for example, the n
Corresponding author. Fax: þ 40 232 271311. E-mail addresses: [email protected], [email protected] (D. Hritcu).
http://dx.doi.org/10.1016/j.jmmm.2015.04.011 0304-8853/& 2015 Elsevier B.V. All rights reserved.
lotus leaf is superhydrophobic, while the rose petal presents high adhesion to droplets although both surfaces have low surface tension. The difference is in the size of the protuberances (nano versus micro features). The presence of solid/liquid interface roughness in the nano-size range favors air pocket entrapment underneath the water drop, thus minimizing the contact area with it (Cassie–Baxter wetting regime) [4]. Various synthetic methods to produce patterned surface morphologies have been proposed, such as templating, lithography, etching, vapor-deposition, plasma treatment, sol–gel processes, layer-by-layer deposition [5,6]. While precise surface patterning and the high cost associated with multistep processing may be justified for special applications (transparent coatings for electronics, surface treatments for biomedical devices or aircraft parts), accessible, less expensive technologies are also needed for high volume applications such as anti-icing coatings for automotive and construction parts, power lines or pipes. Current development in nanotechnology brought a possible solution. Among the various approaches for creating surfaces with hierarchical roughness, nanoparticle assembly is known to be facile and cost effective [7], but relatively difficult to control due to the nature of the particle interactions (mainly van der Waals forces) [8]. In order to improve the process control level, we propose the use of magnetic field assisted nanoparticle aggregation. Magnetic nanoparticles, especially iron oxides, are currently intensely studied due to their unique properties and wide array of applications in biomedicine, separation and purification processes,
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catalysis and electronic data storage [9,10]. A variety of synthetic procedures for their preparation has been reported, such as coprecipitation, the reverse micelle method, micro/nano emulsion technology, laser pyrolysis, sol–gel techniques, pulsed wire evaporation, thermal decomposition, hydrothermal methods, freeze drying, ultrasound irradiation, microwave plasma synthesis and flame spray synthesis [11,12], oxidation [13] or by using bio-inspired routes [14]. The properties may be tuned according to specific applications by adding surface functional layers (surfactants, polymers, metals or metal oxide, silica, carbon) [12,15]. Despite the fact that magnetic iron oxide nanoparticles present multiple advantages such as easy manipulation in magnetic field, non-toxicity, cost effective preparation and functionalization, their use in water/ice repellent composite coatings has not been investigated yet. This study aims to develop a method to prepare functional iron oxide nanoparticles that are capable to interact magnetically and assemble during the curing stage of a polymeric support to create micro/nano-scale surface protuberances in a controlled manner.
2. Materials and methods 2.1. Materials Iron (III) chloride hexahydrate p.a. (FeCl3 6H2O), iron (II) chloride tetrahydrate p.a. (FeCl2 4H2O), oleic acid, Tween 80, Span 80 for synthesis and potassium nitrate (KNO3) were purchased from Merck, Germany. Sodium oleate powder purum grade, sodium hydroxide p.a. grade, low molecular weight chitosan (CS_LMW) (Mw ¼50–190 kDa; degree of deacetylation 84.5%), acetic acid (99.8–100.5% by weight) and dodecane p.a. grade were obtained from Sigma-Aldrich, Germany; aqueous ammonia solution (25% by weight), ethanol, acetone and isopropanol (IPA) were procured from the Chemical Company (Iasi, Romania). Analytical grade chemicals were used as received, without further purification. All solutions were prepared with ultrapure water. 2.2. Magnetite nanoparticles preparation 2.2.1. Co-precipitation method Magnetite nanoparticles (the lot obtained by this method designated as batch Mag 20) were prepared by co-precipitation from an aqueous solution containing Fe3 þ /Fe2 þ ions with a molar ratio of 2:1, upon addition of aqueous sodium hydroxide solution. In a typical synthesis, iron (III) chloride hexahydrate (0.0551 mol) dissolved in 120 mL water and iron (II) chloride tetrahydrate (0.0275 mol) dissolved in 120 mL water were mixed (500 rpm) under mild nitrogen blanket in a 500 mL three-necked flask equipped with mechanical overhead stirring placed in a temperature controlled water bath. Aqueous sodium hydroxide solution (12.8 g in 120 mL water) was then added using a peristaltic
pump with flow rate of 10 mL/min while stirring at 65 °C. The reaction was continued for 1 h under the same conditions. 2.2.2. Oxidation method The Fe3O4 nanoparticles (the lot obtained by this method identified as batch Mag 24) were obtained by partial oxidation of ferrous ions, using nitrate ion as a mild oxidizing agent, in alkaline solution. Briefly, iron (II) chloride tetrahydrate (3.78 mmole) dissolved in 150 mL water was mixed with 22.5 mL 1 M acetic acid solution and homogenized by ultrasonication under reduced pressure for 15 min. The mixture was transferred to a 500 mL three-necked round bottom flask previously filled with nitrogen, equipped with mechanical overhead stirrer and placed in a temperature controlled water bath. The reaction pH was gradually increased by the addition of 0.5 M aqueous ammonia solution with a peristaltic pump (10 mL/min flow rate), under stirring and gentle nitrogen blanket. The temperature of the bath was increased to 70 °C after adding 60 mL ammonia solution. A second portion of ammonia solution (67.5 mL) was subsequently added at the same flow rate and the resulting dark-green complex was maintained for 2 h at 70 °C, with stirring and nitrogen blanket. The mixture was then oxidized using 30 mL aqueous 10% KNO3 solution and aged under stirring for another hour at the same temperature. The black particles produced by either co-precipitation or oxidation were collected by magnetic sedimentation and washed repeatedly with water until neutral pH. Each batch was then dispersed in 200 mL water and stored at 4 °C for further study. The solid content (w/w) was determined by drying an aliquot at 105 °C in a Mettler-Toledo HG63 moisture analyzer. 2.3. Particle functionalization The magnetic materials obtained as previously described were subsequently functionalized in aqueous suspension using sodium oleate, oleic acid or mixtures of non-ionic surfactants (Span 80 and Tween 80) using the following procedure: a 10 mL aliquot of the respective 1% aqueous magnetite suspension (batch Mag 20 or Mag 24) was mixed with a certain amount of surfactant solution, as shown in Table 1, and placed in a thermostatic laboratory shaker at 70 °C, for 1 h, then left to cool and mature overnight at room temperature. The resulting product was washed several times with ethanol using magnetic field separation. The functionalized particles were re-dispersed in isopropyl alcohol (approximately 1% solid content by weight) and stored at 4 °C for further use. 2.4. Two-phase partition In order to check the functionalization uniformity, the nanoparticles were tested by addition to a two-phase water-dodecane system. A sample of the 1% functionalized magnetite suspension in IPA, weighing 0.6 g, was added to a vial containing 10 mL water and 5 mL dodecane; the mixture was gently shaken, then allowed
Table 1 Functionalization study. Batch Mag Mag Mag Mag
20/Mag 20/Mag 20/Mag 20/Mag
24_AcOl 24_NaOl 24_Span 24_S/T 75/25
Magnetite suspension, mL Surfactant
Surfactant solution amount, mL Surfactant HLB value
10 10 10 10
2.0 2.0 0.4 0.4
– – 4.3 7.0
0.4
9.6
0.4
15.0
Mag 20/Mag 24_S/T 50/50 10 Mag 20/Mag 24_Tween
10
Oleic acid 1% in ethanol Sodium oleate 1% in water Span 80 1% in ethanol Span 80 1% (75%) þ Tween 80 1% (25%) in water/ ethanol Span80 1% (50%) þTween 80 1% (50%) in water/ ethanol Tween 80 1% in water
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to separate. The distribution of the nanoparticles in the two-phase water/dodecane system was visually examined and a photograph was taken with a Nikon Coolpix camera. 2.5. Composite film preparation The ability of the functionalized particles to generate composite films with patterned roughness was tested by using chitosan as a model film forming polymeric support, according to the following procedure: 0.3 g functionalized magnetite suspension (1% by weight in IPA) was added to 5 g chitosan solution (1% by weight in 1 M aqueous acetic acid). The mixtures were homogenized by sonication, deposited by spraying on glass slides and subsequently cured by drying (20 min at 75 °C) under a static magnetic field (strength range 1.5–5.5 mT) generated with the aid of an electromagnet, to yield composite films. The magnetic field strength was measured using a FH 51 GaussTesla meter. 2.6. Characterization The morphology of the bare magnetite nanoparticles was investigated by transmission electron microscopy (TEM) on a dry sample. Magnetization curves were generated using a VSM 7410 vibrating sample magnetometer. X-ray powder diffraction (XRD) patterns were recorded on a Bruker AXS D8 diffractometer using monochromatic CuKα radiation (λ ¼ 0.154 nm) and operating at 40 kV and 50 mA over a 2θ range from 4° to 70°. The phases were identified by comparison with the Joint Committee Powder Diffraction Standards (JCPDS) (Patent no. 82-1533). Qualitative chemical composition assessment of functionalized nanoparticles was performed by FTIR analysis (Bomem MB 104 spectrometer). The composite film morphology was observed by optical upright microscopy (Leica DM 2500) in bright field and by Scanning Electron microscopy (SEM) with field emission operating at 30 kV.
3. Results and discussions 3.1. Magnetite nanoparticles preparation and characterization The magnetite nanoparticles were prepared by two wet methods, in order to compare their functionalization and assembly ability within the polymeric matrix under the influence of the magnetic field. A first batch, denoted Mag 20, was produced by co-precipitation, the most widely used method for obtaining synthetic magnetite, described by the chemical equation:
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Fe2 + + 2Fe 3 + + 8OH− = Fe 3 O4 + 4H2 O For comparison, a second batch (Mag 24) was produced by partial oxidation of ferrous ions in alkaline solution, as shown below:
12Fe(OH)2 + NO−3 = 4Fe3 O4 + NH3 + 10H2 O+ OH−
3.1.1. TEM analysis Comparative TEM images and particle size distribution histograms are presented in Figs. 1 and 2. The results show that both methods produced nanoparticles that are relatively uniform in size and shape. The histograms were plotted by measuring 250 particles on 5 different micrographs for each batch using ImageJ v.1.48 image analysis software. The number weighted average particle size was 13 nm (S.D. ¼3 nm) in the co-precipitation method and respectively 14 nm (S.D. ¼4 nm) in the oxidation method. 3.1.2. X-ray diffraction The crystalline structure of the magnetite nanoparticles obtained by both methods was evaluated by powder X-ray diffraction (XRD) and comparative patterns are shown in Fig. 3. All the evidenced peaks were analyzed and indexed using Joint Committee on Powder Diffraction Standards (JCPDS) database. Both curves have diffraction peaks at 2θ ¼18.6°, 30.4°, 35.7°, 43.2°, 53.7°, 57.3°, and 62.8°, that can be indexed respectively to the (111), (220), (311), (400), (422), (511), and (440) planes, characteristic to the face centered cubic structure of Fe3O4. The results confirm that crystalline magnetite was produced by both methods. As noticed in Fig. 3, batch Mag 24 displays stronger intensities for all peaks with minimal background noise, indicating a better phase purity than batch Mag 20. This result is consistent with the higher saturation magnetization (discussed in the next paragraph) and also with the product appearance. The darker color of the nanoparticles obtained by oxidation suggests that it contains mostly magnetite, while the slightly brownish color of the coprecipitation product might be due to the presence of some maghemite. Both phases display the same spinel structure with similar XRD patterns [16]. The crystallite size calculated with Scherrer's formula [17] using the width of the (311) reflection peak was 10 nm for the nanoparticles produced by co-precipitation and respectively 25 nm for those prepared by the oxidation method. While this result is in good agreement with the estimated particle size reported from TEM micrographs for batch Mag 20, it is apparently significantly different for batch Mag 24, due to the extremely sharp peaks in its XRD pattern. Considering the fact that the
Fig. 1. TEM picture (A) and size distribution histogram (B) of the nanoparticles produced by co-precipitation (batch Mag 20).
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Fig. 2. TEM picture (A) and size distribution histogram (B) of the nanoparticles produced by oxidation (batch Mag 24).
crystallite diameter estimated from XRD is statistically related to the volume weighted average particle size, as opposed to the number weighted value resulted from TEM micrographs, we conclude that the particles obtained by oxidation had a wider volume distribution, skewed towards larger diameter. The contribution of the over-sized particles is statistically more important in the volume weighted average calculation, shifting the result to higher value. 3.1.3. Magnetization measurements Comparative magnetization curves recorded at room temperature for both lots are presented in Fig. 4. The saturation magnetization was 66.0 emu/g for the co-precipitation product and 88.3 emu/g respectively for the oxidation one. The higher value obtained for batch Mag 24 is consistent with its superior crystallinity, phase purity and also with the slightly larger particle size noticed in the XRD analysis. Based on previously published studies, it is generally demonstrated that the saturation magnetization decreases with decreasing particle size due to the contribution of a superficial spin-canted layer that becomes more important with increasing surface area [18]. The saturation value obtained for the Mag 24 batch is very close to the theoretical magnetization for bulk magnetite (92 emu/g) and, to our knowledge, one of the highest reported in the literature for synthetic
magnetite nanoparticles obtained by wet aqueous methods [19,20]. In terms of the magnetization curve shape, batch Mag 20 shows zero coercivity, while a narrow hysteresis loop is obtained for batch Mag 24. This may be due to the difference in particle size, since the superparamagnetic behavior (zero coercivity) occurs under a certain critical value of the nanoparticle diameter [21]. The presence of a remanent magnetization is in fact advantageous for our intended application because it creates the premises to obtain more stable nanoparticle aggregates within the composite coating. 3.2. Functionalization study The aim of the functionalization step was to produce particles with various degrees of hydrophobicity in order to subsequently study their aggregation behavior within the polymeric matrix under the influence of the magnetic field. Oleic acid derivatives add a hydrophobic shell to the nanoparticles, while the non-ionic surfactants bring the same hydrophobic part (the oleic acid functionality), but modulated by a hydrophilic tail, namely a sorbitan residue in the Span 80 and respectively a polyoxyethylene sorbitan in the Tween 80. The hydrophilic to lipophilic balance values for the prepared non-ionic surfactant mixtures are shown in the last column of Table 1. Oleic acid and its salts are known to have high
Fig. 3. X-ray diffraction patterns for nanoparticles prepared by co-precipitation (batch Mag 20) and by oxidation (batch Mag 24).
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Fig. 4. Magnetization measurements for nanoparticles prepared by co-precipitation (batch Mag 20) and by oxidation (batch Mag 24).
Fig. 5. Two-phase partition of functionalized nanoparticles.
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affinity for magnetic nanoparticles, both through chemical and physical adsorption mechanisms [22]. The nanoparticles obtained by both methods were successfully covered with all the surfactants shown in Table 1, producing colloidally stable suspensions upon functionalization and cleaning, with only one exception: batch Mag 24 that agglomerated at the end of oleic acid functionalization procedure. This is probably due to the effect of a sudden surface charge inversion upon oleic acid addition. The reason for which batch Mag 20 behaved differently is explained below. The bare magnetite nanoparticles are stabilized against agglomeration only by the surface hydroxyl groups. The difference between the two batches of magnetite prepared consists in the nature of the adsorbed surface counter ions: sodium ions in coprecipitation product (where sodium hydroxide was used to increase the pH during synthesis) versus ammonium ions in the oxidation method. The presence of sodium ions most probably led to the formation of sodium oleate on the surface of Mag 20 and it contributed to create a buffered protective layer [23], thus avoiding agglomeration. 3.2.1. Two-phase partition In order to check the functionalization uniformity, the nanoparticles were tested by addition to a two-phase water-dodecane system. After mild mixing, most of them partitioned totally or partially in the organic phase as shown in Fig. 5 and described below. The magnetite particles obtained by co-precipitation (batch Mag 20) and coated with sodium oleate partitioned partially in dodecane, most likely due to non-uniform coating. Mag 20 nanoparticles functionalized uniformly with Span 80 and its mixtures with Tween 80 partitioned in an emulsion located at the interface. The batch covered with Tween partitioned in the aqueous phase,
probably due to insufficient surfactant incorporation. The magnetite particles prepared by oxidation (batch Mag 24) and coated with sodium oleate, Span 80 and the mixtures between Span 80 and Tween 80 (S/T 75/25 and S/T 50/50) were uniformly covered with enough surfactant to partition and disperse well in dodecane. The batch functionalized with Tween 80 produced an emulsion at the organic/aqueous interface, evidencing the fact that the surface had the most hydrophilic character. The results show that batch Mag 20 most likely adsorbed less surfactant than batch Mag 24 in all cases. The coating behavior difference between the two types of synthesized magnetite is probably due to an increased density of hydroxyl groups on the nanoparticle surfaces obtained by co-precipitation [21]. Their superficial layer is prone to incorporating less surfactant due to steric shielding. 3.2.2. FTIR analysis The magnetic nanoparticles functionalized with ionic surfactants and their bare precursors were characterized by FTIR spectroscopy in the wavelength range of 4000–400 cm 1. Fig. 6 shows the comparative spectra of bare magnetite prepared by co-precipitation (batch Mag 20) and oxidation (batch Mag 24)) and respectively of oleic acid coated magnetite (Mag 20_AcOl) and of sodium oleate functionalized magnetite (batch Mag 24_NaOl). The peaks appearing on the FTIR spectra of both bare magnetite lots at 584 cm 1 and respectively 582 cm 1 are assigned to the Fe–O bond in Fe3O4, that is located close to this region according to the literature [24]. The peaks at 3319 cm 1 and respectively 3454 cm 1 are attributed to the stretching vibrations due to the superficial OH groups. In order to understand the adsorption mechanism of the oleic acid and sodium oleate, respectively, on the surface of Fe3O4
Fig. 6. Comparative FTIR spectra of the bare magnetite produced by co-precipitation (batch Mag 20) and oxidation (batch Mag 24)), oleic acid coated magnetite (Mag 20_AcOl) and sodium oleate coated magnetite (Mag 24_NaOl).
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Fig. 7. Comparative optical micrographs of composite films prepared with functionalized nanoparticles in magnetic field of 5.5 mT. Functionalized particle lots designated as explained in Table 1.
nanoparticles, the spectra obtained for the functionalized particle samples were compared with the pure oleic acid and sodium oleate spectra presented in the literature (data not shown) [25,26]. The peaks found at 3435 cm 1 and 3460 cm 1 are attributed to the stretching vibrations of OH , also assigned to OH absorbed by the nanoparticles. On the oleic acid coated magnetite FTIR spectrum (Mag 20_AcOl), the sharp bands at 2923 cm 1 and 2852 cm 1 are attributed to asymmetric and symmetric CH vibrations of the methylene groups, slightly shifted from those characteristic to pure
oleic acid spectrum. The C ¼O stretch band of the carboxyl group present at 1710 cm 1 in the spectrum of the pure oleic acid [23] was absent in the spectrum of the coated nanoparticles. Therefore, the newly appeared bands at 1604 cm 1 and 1406 cm 1 were ascribed to the symmetric and asymmetric COO stretches. A medium adsorption peak located at 1043 cm 1 is due to C–O single bond stretching. These results suggest that the oleic acid functionality was partially bound covalently to the Fe3O4 nanoparticles as an ester. In the case of sodium oleate (Mag 24_NaOl) coated magnetite,
Fig. 8. Comparative SEM micrographs of composite films prepared with functionalized nanoparticles in magnetic field of 5.5 mT.
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Fig. 9. Optical micrographs at different magnetic field strengths: A. 0 mT; B. 1.5 mT; C. 2.8 mT; D. 5.5 mT; and SEM images at E. 2.8 mT and F. 5.5 mT magnetic strength of composite films prepared with functionalized nanoparticles (batch Mag 24_NaOl).
the peaks at 2923 cm 1 and 2852 cm 1 are also attributed to the stretching vibrations of –CH2 and –CH3 from sodium oleate. The characteristic carbonyl stretching vibration characteristic to sodium oleate at 1710 cm 1 (pure oleate spectrum from the literature) is missing from the functionalized particle spectrum entirely, and the asymmetric and symmetric COO vibrations appear instead at 1610 cm 1 and 1562 cm 1. The peaks at 1396 cm 1 and 1365 cm 1 are attributed to the vibration of –CH, and the single C–O bond stretching at 1099 cm 1. These all suggest that sodium oleate is also partially bound chemically to the Fe3O4 nanoparticles. The higher intensity of these peaks on the spectrum of functionalized Mag 24 particles compared to the corresponding peaks on the functionalized Mag 20 spectrum qualitatively confirm higher surfactant coverage on the former, as noticed in the two-phase partition experiment. The peak assigned to the Fe–O bond vibration was slightly shifted to a higher frequency region in both functionalized particle spectra.
3.3. Composite film characterization Composite nanoparticle-chitosan films prepared by spraying and cured by drying under a 5.5 mT magnetic field were analyzed by optical microscopy and comparative micrographs are shown in Fig. 7. Chitosan was chosen as a polymeric model support because the film is easy to prepare from aqueous solution and the resulting coating is transparent. The magnetite nanoparticle content in all the model films was relatively low (6% based on solid weight) in order to keep them transparent and be able to observe their morphology under the optical microscope. For real life applications in which superhydrophobic behavior is desired, the nanoparticle content should be higher, in the range of 30–50%, according to the literature [27–29], but the actual value will be determined according to the intended application. All model films were prepared using the same procedure. Since particle movement during the film curing stage is a result of the equilibrium between the aligning effect of the magnetic field and the drag force, the film morphology depends on the surfactant nature and its concentration. It is expected that faster moving particles will align more with the magnetic field than
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slower moving ones. The results show that the particles functionalized with oleic acid or sodium oleate are the fastest moving ones. This is due to the fact that in this case the drag force is minimal (the friction between the hydrophilic matrix and the hydrophobic nanoparticle surface is at the lowest point). The composite films prepared with Mag 20_AcOl and respectively Mag 24_NaOl show therefore the highest degree of alignment, with striped patterns comprising of micro-rods. As the surfactant becomes more hydrophilic, the drag force increases and the degree of particle alignment with the field is less evidenced. In the Mag 24 series, the film morphology changes to narrower stripes (comprising of shorter micro-rods) in the Span functionalized batch, respectively to a mix of branches and micro-rods in the mixed Span/Tween batches and finally to branches only, in the film prepared with the Tween functionalized lot. The nanoparticles produced by oxidation are therefore able to produce a variety of patterns, depending on the surfactant nature. By comparison, the particles produced by co-precipitation produced patterns only with the most hydrophobic surfactants (oleic acid and Span, respectively). They align less than their respective counterparts produced by oxidation due to slower movement caused both by their lower magnetization and insufficient surfactant coverage. SEM micrographs shown in Fig. 8 confirm that the coatings produced with Mag 24 batch present hierarchical protuberances alternating with cavities (micro-rods show nano-sized features) when functionalized with both the most hydrophobic and the most hydrophilic surfactant from the tested series. By comparison, batch Mag 20 produced a film with patterned roughness only with the most hydrophobic surfactant, but even in this case the protuberances are only at micro-level. The oxidation method is the optimum one for the envisioned application since functionalized Mag 24 particles assembled in chain-like structures, thus forming ordered protuberances within the polymeric film and hierarchical morphology that may create premises to lock in air pockets upon wetting. In a second series of experiments aimed to observe the pattern dependence on the magnetic field strength, while the surfactant nature was kept constant, the Mag 24 particles functionalized with sodium oleate were used. Optical microscope images for composite films prepared in magnetic field varying in the range of 0– 5.5 mT are shown in Fig. 9. They evidence that the nanoparticles remain uniformly dispersed within the matrix when no magnetic field is applied (0 mT – Fig. 9A). When the lowest magnetic field is applied, the morphology of the film shows chain-like aggregates (micro-rods with nanostructured features) aligned to the field lines (1.5 mT – Fig. 9B). The micro-rods become longer as the magnetic field strength increases (2.8 mT – Fig. 9C and 5.5 mT – Fig. 9D). The last two films, prepared at magnetic field of 2.8 and respectively 5.5 mT, are also shown here in SEM micrographs. The images (Figs. 9E and 9F) evidenced a morphology with cavities and protuberances that are better structured when the magnetic field is stronger. Nanoparticles move faster under the stronger field and, given the same amount of time, produce patterning in wider scale.
4. Conclusions Magnetite nanoparticles bearing variable hydrophobic functionality were prepared by co-precipitation and the partial oxidation method, followed by surfactant coating. While both synthesis methods yielded well crystallized, uniformly sized magnetite nanoparticles that were able to generate patterned composite films under the influence of magnetic field, the product obtained by partial oxidation proved more suitable for the envisioned application due to
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1. higher phase purity and saturation magnetization; 2. better surfactant coverage; 3. ability to produce films in which the degree of aggregation and patterning may be controlled by the surfactant nature or by the magnetic field strength; 4. coatings with hierarchical roughness, consisting in micro-rods with nano-scale structuring that may facilitate air pocket entrapment beneath water drops. The results show that textured surface composite coatings may be prepared in a relatively simple manner with the aid of a magnetic field. Moreover, the versatility of the method is demonstrated by the possibility to choose the surfactant combination with the appropriate HLB value, depending on the support properties, to encourage the desired surface patterning. Future work will be carried out for synthesizing a polymeric matrix modified with a low surface energy reagent in order to produce composite films with ice-phobic properties.
Acknowledgments This work was supported by a grant of the Ministry of National Education, CNCS-UEFISCDI, Project no. PN-II-ID-PCE-2012-4-0433.
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[20] K. Nishio, M. Ikeda, N. Gokon, S. Tsubouchi, H. Narimatsu, Y. Mochizuki, S. Sakamoto, A. Sandhu, M. Abe, H. Handa, Preparation of size-controlled (30– 100 nm) magnetite nanoparticles for biomedical applications, J. Magn. Magn. Mater. 310 (2007) 2408–2410. [21] G.F. Goya, T.S. Berquo, F.C. Fonseca, M.P. Morales, Static and dynamic magnetic properties of spherical magnetite nanoparticles, J. Appl. Phys. 94 (2003) 3520–3528. [22] X. Liu, M.D. Kaminski, Y. Guan, H. Chen, H. Liu, A.J. Rosengart, Preparation and characterization of hydrophobic superparamagnetic magnetite gel, J. Magn. Magn. Mater. 306 (2006) 248–253. [23] W. Jiang, Y. Wu, B. He, X. Zeng, K. Lai, Z. Gu, Effect of sodium oleate as a buffer on the synthesis of superparamagnetic magnetite colloids, J. Colloid Interface Sci. 347 (2010) 1–7. [24] R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses, second ed., Wiley- VCH Verlag GmbH&Co., KGaA, Weinheim, 2003.
[25] R.P. Araújo-Neto, E.L. Silva-Freitas, J.F. Carvalho, T.R.F. Pontes, K.L. Silva, I.H. M. Damasceno, Dantas A.L. Egito, M.A. Morales, A.S. Carriço, Monodisperse sodium oleate coated magnetite high susceptibility nanoparticles for hyperthermia applications, J. Magn. Magn. Mater. 364 (2014) 72–79. [26] L. Zhang, R. He, H.-C. Gu, Oleic acid coating on the monodisperse magnetite nanoparticles, Appl. Surf. Sci. 253 (2006) 2611–2617. [27] B.J. Basu, V. Hariprakash, S.T. Aruna, R.V. Lakshmi, J. Manasa, B.S. Shruthi, Effect of microstructure and surface roughness on the wettability of superhydrophobic sol–gel nanocomposite coatings, J. Sol-Gel Sci. Technol. 56 (2010) 278–286. [28] L. Cao, A.K. Jones, V.K. Sikka, J. Wu, D. Gao, Anti-icing superhydrophobic coating, Langmuir 25 (2009) 12444–12448. [29] L. Yan, K. Wang, L. Ye, Super hydrophobic property of PVDF/CaCO3 nanocomposite coatings, J. Mater. Sci. Lett. 22 (2003) 1713–1717.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Iron oxide nanoparticles for magnetically assisted patterned coatings Gianina Dodi, Doina Hritcu n, Dan Draganescu, Marcel I. Popa Faculty of Chemical Engineering and Environmental Protection, “Gheorghe Asachi” Technical University of Iasi, 73, Prof. Dr. Docent Dimitrie Mangeron Road, 700050 Iasi, Romania
art ic l e i nf o
a b s t r a c t
Article history: Received 6 October 2014 Received in revised form 6 April 2015 Accepted 6 April 2015 Available online 8 April 2015
Iron oxide nanoparticles able to magnetically assemble during the curing stage of a polymeric support to create micro-scale surface protuberances in a controlled manner were prepared and characterized. The bare Fe3O4 particles were obtained by two methods: co-precipitation from an aqueous solution containing Fe3 þ /Fe2 þ ions with a molar ratio of 2:1 and partial oxidation of ferrous ions in alkaline conditions. The products were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and magnetization measurement. They were subsequently functionalized using oleic acid, sodium oleate, or non-ionic surfactant mixtures with various hydrophilic to lipophilic balance (HLB) values. Composite nanoparticle-polymer films prepared by spraying were deposited and cured by drying on glass slides under a static magnetic field in the range of 1.5–5.5 mT. Magnetic field generated surface roughness was evidenced by optical and scanning electron microscopy. The optimum hierarchical patterning was obtained with the nanoparticles produced by partial oxidation and functionalized with hydrophobic surfactants. Possible applications may include ice-phobic composite coatings. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetite nanoparticles Surfactant functionalization Composite patterned coatings
1. Introduction The interest for research directed toward achieving a better control over the mechanisms governing the interaction between solid surfaces and water drops is steadily growing. Synthetic surfaces with wetting behavior that is tailored according to the desired application are relevant to a wide array of modern technological fields, such as self-cleaning coatings for satellite dishes and automobile windshields, non-fouling surfaces in biomedical devices, drag-force reducing materials for micro-fluidic or aquatic sport applications, anti-corrosion or ice-accretion reducing treatments in power-lines, aviation or building industry. The strategies for engineering synthetic coatings with water/ice repellent properties have been inspired by the features of naturally occurring superhydrophobic materials, namely their resistance to water penetration and their low adhesion to droplets [1,2]. Examples of such natural surfaces that display a contact angle higher than 150° with the water drops, include the lotus and other plant leaves, bird and butterfly wings or the water strider's leg. Recent studies conclude that an optimum combination of low surface tension and hierarchical roughness is able to produce the superhydrophobic effect [3]. In natural surfaces, the morphology of the surface is the determining factor for its wetting properties: for example, the n
Corresponding author. Fax: þ 40 232 271311. E-mail addresses: [email protected], [email protected] (D. Hritcu).
http://dx.doi.org/10.1016/j.jmmm.2015.04.011 0304-8853/& 2015 Elsevier B.V. All rights reserved.
lotus leaf is superhydrophobic, while the rose petal presents high adhesion to droplets although both surfaces have low surface tension. The difference is in the size of the protuberances (nano versus micro features). The presence of solid/liquid interface roughness in the nano-size range favors air pocket entrapment underneath the water drop, thus minimizing the contact area with it (Cassie–Baxter wetting regime) [4]. Various synthetic methods to produce patterned surface morphologies have been proposed, such as templating, lithography, etching, vapor-deposition, plasma treatment, sol–gel processes, layer-by-layer deposition [5,6]. While precise surface patterning and the high cost associated with multistep processing may be justified for special applications (transparent coatings for electronics, surface treatments for biomedical devices or aircraft parts), accessible, less expensive technologies are also needed for high volume applications such as anti-icing coatings for automotive and construction parts, power lines or pipes. Current development in nanotechnology brought a possible solution. Among the various approaches for creating surfaces with hierarchical roughness, nanoparticle assembly is known to be facile and cost effective [7], but relatively difficult to control due to the nature of the particle interactions (mainly van der Waals forces) [8]. In order to improve the process control level, we propose the use of magnetic field assisted nanoparticle aggregation. Magnetic nanoparticles, especially iron oxides, are currently intensely studied due to their unique properties and wide array of applications in biomedicine, separation and purification processes,
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catalysis and electronic data storage [9,10]. A variety of synthetic procedures for their preparation has been reported, such as coprecipitation, the reverse micelle method, micro/nano emulsion technology, laser pyrolysis, sol–gel techniques, pulsed wire evaporation, thermal decomposition, hydrothermal methods, freeze drying, ultrasound irradiation, microwave plasma synthesis and flame spray synthesis [11,12], oxidation [13] or by using bio-inspired routes [14]. The properties may be tuned according to specific applications by adding surface functional layers (surfactants, polymers, metals or metal oxide, silica, carbon) [12,15]. Despite the fact that magnetic iron oxide nanoparticles present multiple advantages such as easy manipulation in magnetic field, non-toxicity, cost effective preparation and functionalization, their use in water/ice repellent composite coatings has not been investigated yet. This study aims to develop a method to prepare functional iron oxide nanoparticles that are capable to interact magnetically and assemble during the curing stage of a polymeric support to create micro/nano-scale surface protuberances in a controlled manner.
2. Materials and methods 2.1. Materials Iron (III) chloride hexahydrate p.a. (FeCl3 6H2O), iron (II) chloride tetrahydrate p.a. (FeCl2 4H2O), oleic acid, Tween 80, Span 80 for synthesis and potassium nitrate (KNO3) were purchased from Merck, Germany. Sodium oleate powder purum grade, sodium hydroxide p.a. grade, low molecular weight chitosan (CS_LMW) (Mw ¼50–190 kDa; degree of deacetylation 84.5%), acetic acid (99.8–100.5% by weight) and dodecane p.a. grade were obtained from Sigma-Aldrich, Germany; aqueous ammonia solution (25% by weight), ethanol, acetone and isopropanol (IPA) were procured from the Chemical Company (Iasi, Romania). Analytical grade chemicals were used as received, without further purification. All solutions were prepared with ultrapure water. 2.2. Magnetite nanoparticles preparation 2.2.1. Co-precipitation method Magnetite nanoparticles (the lot obtained by this method designated as batch Mag 20) were prepared by co-precipitation from an aqueous solution containing Fe3 þ /Fe2 þ ions with a molar ratio of 2:1, upon addition of aqueous sodium hydroxide solution. In a typical synthesis, iron (III) chloride hexahydrate (0.0551 mol) dissolved in 120 mL water and iron (II) chloride tetrahydrate (0.0275 mol) dissolved in 120 mL water were mixed (500 rpm) under mild nitrogen blanket in a 500 mL three-necked flask equipped with mechanical overhead stirring placed in a temperature controlled water bath. Aqueous sodium hydroxide solution (12.8 g in 120 mL water) was then added using a peristaltic
pump with flow rate of 10 mL/min while stirring at 65 °C. The reaction was continued for 1 h under the same conditions. 2.2.2. Oxidation method The Fe3O4 nanoparticles (the lot obtained by this method identified as batch Mag 24) were obtained by partial oxidation of ferrous ions, using nitrate ion as a mild oxidizing agent, in alkaline solution. Briefly, iron (II) chloride tetrahydrate (3.78 mmole) dissolved in 150 mL water was mixed with 22.5 mL 1 M acetic acid solution and homogenized by ultrasonication under reduced pressure for 15 min. The mixture was transferred to a 500 mL three-necked round bottom flask previously filled with nitrogen, equipped with mechanical overhead stirrer and placed in a temperature controlled water bath. The reaction pH was gradually increased by the addition of 0.5 M aqueous ammonia solution with a peristaltic pump (10 mL/min flow rate), under stirring and gentle nitrogen blanket. The temperature of the bath was increased to 70 °C after adding 60 mL ammonia solution. A second portion of ammonia solution (67.5 mL) was subsequently added at the same flow rate and the resulting dark-green complex was maintained for 2 h at 70 °C, with stirring and nitrogen blanket. The mixture was then oxidized using 30 mL aqueous 10% KNO3 solution and aged under stirring for another hour at the same temperature. The black particles produced by either co-precipitation or oxidation were collected by magnetic sedimentation and washed repeatedly with water until neutral pH. Each batch was then dispersed in 200 mL water and stored at 4 °C for further study. The solid content (w/w) was determined by drying an aliquot at 105 °C in a Mettler-Toledo HG63 moisture analyzer. 2.3. Particle functionalization The magnetic materials obtained as previously described were subsequently functionalized in aqueous suspension using sodium oleate, oleic acid or mixtures of non-ionic surfactants (Span 80 and Tween 80) using the following procedure: a 10 mL aliquot of the respective 1% aqueous magnetite suspension (batch Mag 20 or Mag 24) was mixed with a certain amount of surfactant solution, as shown in Table 1, and placed in a thermostatic laboratory shaker at 70 °C, for 1 h, then left to cool and mature overnight at room temperature. The resulting product was washed several times with ethanol using magnetic field separation. The functionalized particles were re-dispersed in isopropyl alcohol (approximately 1% solid content by weight) and stored at 4 °C for further use. 2.4. Two-phase partition In order to check the functionalization uniformity, the nanoparticles were tested by addition to a two-phase water-dodecane system. A sample of the 1% functionalized magnetite suspension in IPA, weighing 0.6 g, was added to a vial containing 10 mL water and 5 mL dodecane; the mixture was gently shaken, then allowed
Table 1 Functionalization study. Batch Mag Mag Mag Mag
20/Mag 20/Mag 20/Mag 20/Mag
24_AcOl 24_NaOl 24_Span 24_S/T 75/25
Magnetite suspension, mL Surfactant
Surfactant solution amount, mL Surfactant HLB value
10 10 10 10
2.0 2.0 0.4 0.4
– – 4.3 7.0
0.4
9.6
0.4
15.0
Mag 20/Mag 24_S/T 50/50 10 Mag 20/Mag 24_Tween
10
Oleic acid 1% in ethanol Sodium oleate 1% in water Span 80 1% in ethanol Span 80 1% (75%) þ Tween 80 1% (25%) in water/ ethanol Span80 1% (50%) þTween 80 1% (50%) in water/ ethanol Tween 80 1% in water
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to separate. The distribution of the nanoparticles in the two-phase water/dodecane system was visually examined and a photograph was taken with a Nikon Coolpix camera. 2.5. Composite film preparation The ability of the functionalized particles to generate composite films with patterned roughness was tested by using chitosan as a model film forming polymeric support, according to the following procedure: 0.3 g functionalized magnetite suspension (1% by weight in IPA) was added to 5 g chitosan solution (1% by weight in 1 M aqueous acetic acid). The mixtures were homogenized by sonication, deposited by spraying on glass slides and subsequently cured by drying (20 min at 75 °C) under a static magnetic field (strength range 1.5–5.5 mT) generated with the aid of an electromagnet, to yield composite films. The magnetic field strength was measured using a FH 51 GaussTesla meter. 2.6. Characterization The morphology of the bare magnetite nanoparticles was investigated by transmission electron microscopy (TEM) on a dry sample. Magnetization curves were generated using a VSM 7410 vibrating sample magnetometer. X-ray powder diffraction (XRD) patterns were recorded on a Bruker AXS D8 diffractometer using monochromatic CuKα radiation (λ ¼ 0.154 nm) and operating at 40 kV and 50 mA over a 2θ range from 4° to 70°. The phases were identified by comparison with the Joint Committee Powder Diffraction Standards (JCPDS) (Patent no. 82-1533). Qualitative chemical composition assessment of functionalized nanoparticles was performed by FTIR analysis (Bomem MB 104 spectrometer). The composite film morphology was observed by optical upright microscopy (Leica DM 2500) in bright field and by Scanning Electron microscopy (SEM) with field emission operating at 30 kV.
3. Results and discussions 3.1. Magnetite nanoparticles preparation and characterization The magnetite nanoparticles were prepared by two wet methods, in order to compare their functionalization and assembly ability within the polymeric matrix under the influence of the magnetic field. A first batch, denoted Mag 20, was produced by co-precipitation, the most widely used method for obtaining synthetic magnetite, described by the chemical equation:
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Fe2 + + 2Fe 3 + + 8OH− = Fe 3 O4 + 4H2 O For comparison, a second batch (Mag 24) was produced by partial oxidation of ferrous ions in alkaline solution, as shown below:
12Fe(OH)2 + NO−3 = 4Fe3 O4 + NH3 + 10H2 O+ OH−
3.1.1. TEM analysis Comparative TEM images and particle size distribution histograms are presented in Figs. 1 and 2. The results show that both methods produced nanoparticles that are relatively uniform in size and shape. The histograms were plotted by measuring 250 particles on 5 different micrographs for each batch using ImageJ v.1.48 image analysis software. The number weighted average particle size was 13 nm (S.D. ¼3 nm) in the co-precipitation method and respectively 14 nm (S.D. ¼4 nm) in the oxidation method. 3.1.2. X-ray diffraction The crystalline structure of the magnetite nanoparticles obtained by both methods was evaluated by powder X-ray diffraction (XRD) and comparative patterns are shown in Fig. 3. All the evidenced peaks were analyzed and indexed using Joint Committee on Powder Diffraction Standards (JCPDS) database. Both curves have diffraction peaks at 2θ ¼18.6°, 30.4°, 35.7°, 43.2°, 53.7°, 57.3°, and 62.8°, that can be indexed respectively to the (111), (220), (311), (400), (422), (511), and (440) planes, characteristic to the face centered cubic structure of Fe3O4. The results confirm that crystalline magnetite was produced by both methods. As noticed in Fig. 3, batch Mag 24 displays stronger intensities for all peaks with minimal background noise, indicating a better phase purity than batch Mag 20. This result is consistent with the higher saturation magnetization (discussed in the next paragraph) and also with the product appearance. The darker color of the nanoparticles obtained by oxidation suggests that it contains mostly magnetite, while the slightly brownish color of the coprecipitation product might be due to the presence of some maghemite. Both phases display the same spinel structure with similar XRD patterns [16]. The crystallite size calculated with Scherrer's formula [17] using the width of the (311) reflection peak was 10 nm for the nanoparticles produced by co-precipitation and respectively 25 nm for those prepared by the oxidation method. While this result is in good agreement with the estimated particle size reported from TEM micrographs for batch Mag 20, it is apparently significantly different for batch Mag 24, due to the extremely sharp peaks in its XRD pattern. Considering the fact that the
Fig. 1. TEM picture (A) and size distribution histogram (B) of the nanoparticles produced by co-precipitation (batch Mag 20).
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Fig. 2. TEM picture (A) and size distribution histogram (B) of the nanoparticles produced by oxidation (batch Mag 24).
crystallite diameter estimated from XRD is statistically related to the volume weighted average particle size, as opposed to the number weighted value resulted from TEM micrographs, we conclude that the particles obtained by oxidation had a wider volume distribution, skewed towards larger diameter. The contribution of the over-sized particles is statistically more important in the volume weighted average calculation, shifting the result to higher value. 3.1.3. Magnetization measurements Comparative magnetization curves recorded at room temperature for both lots are presented in Fig. 4. The saturation magnetization was 66.0 emu/g for the co-precipitation product and 88.3 emu/g respectively for the oxidation one. The higher value obtained for batch Mag 24 is consistent with its superior crystallinity, phase purity and also with the slightly larger particle size noticed in the XRD analysis. Based on previously published studies, it is generally demonstrated that the saturation magnetization decreases with decreasing particle size due to the contribution of a superficial spin-canted layer that becomes more important with increasing surface area [18]. The saturation value obtained for the Mag 24 batch is very close to the theoretical magnetization for bulk magnetite (92 emu/g) and, to our knowledge, one of the highest reported in the literature for synthetic
magnetite nanoparticles obtained by wet aqueous methods [19,20]. In terms of the magnetization curve shape, batch Mag 20 shows zero coercivity, while a narrow hysteresis loop is obtained for batch Mag 24. This may be due to the difference in particle size, since the superparamagnetic behavior (zero coercivity) occurs under a certain critical value of the nanoparticle diameter [21]. The presence of a remanent magnetization is in fact advantageous for our intended application because it creates the premises to obtain more stable nanoparticle aggregates within the composite coating. 3.2. Functionalization study The aim of the functionalization step was to produce particles with various degrees of hydrophobicity in order to subsequently study their aggregation behavior within the polymeric matrix under the influence of the magnetic field. Oleic acid derivatives add a hydrophobic shell to the nanoparticles, while the non-ionic surfactants bring the same hydrophobic part (the oleic acid functionality), but modulated by a hydrophilic tail, namely a sorbitan residue in the Span 80 and respectively a polyoxyethylene sorbitan in the Tween 80. The hydrophilic to lipophilic balance values for the prepared non-ionic surfactant mixtures are shown in the last column of Table 1. Oleic acid and its salts are known to have high
Fig. 3. X-ray diffraction patterns for nanoparticles prepared by co-precipitation (batch Mag 20) and by oxidation (batch Mag 24).
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Fig. 4. Magnetization measurements for nanoparticles prepared by co-precipitation (batch Mag 20) and by oxidation (batch Mag 24).
Fig. 5. Two-phase partition of functionalized nanoparticles.
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affinity for magnetic nanoparticles, both through chemical and physical adsorption mechanisms [22]. The nanoparticles obtained by both methods were successfully covered with all the surfactants shown in Table 1, producing colloidally stable suspensions upon functionalization and cleaning, with only one exception: batch Mag 24 that agglomerated at the end of oleic acid functionalization procedure. This is probably due to the effect of a sudden surface charge inversion upon oleic acid addition. The reason for which batch Mag 20 behaved differently is explained below. The bare magnetite nanoparticles are stabilized against agglomeration only by the surface hydroxyl groups. The difference between the two batches of magnetite prepared consists in the nature of the adsorbed surface counter ions: sodium ions in coprecipitation product (where sodium hydroxide was used to increase the pH during synthesis) versus ammonium ions in the oxidation method. The presence of sodium ions most probably led to the formation of sodium oleate on the surface of Mag 20 and it contributed to create a buffered protective layer [23], thus avoiding agglomeration. 3.2.1. Two-phase partition In order to check the functionalization uniformity, the nanoparticles were tested by addition to a two-phase water-dodecane system. After mild mixing, most of them partitioned totally or partially in the organic phase as shown in Fig. 5 and described below. The magnetite particles obtained by co-precipitation (batch Mag 20) and coated with sodium oleate partitioned partially in dodecane, most likely due to non-uniform coating. Mag 20 nanoparticles functionalized uniformly with Span 80 and its mixtures with Tween 80 partitioned in an emulsion located at the interface. The batch covered with Tween partitioned in the aqueous phase,
probably due to insufficient surfactant incorporation. The magnetite particles prepared by oxidation (batch Mag 24) and coated with sodium oleate, Span 80 and the mixtures between Span 80 and Tween 80 (S/T 75/25 and S/T 50/50) were uniformly covered with enough surfactant to partition and disperse well in dodecane. The batch functionalized with Tween 80 produced an emulsion at the organic/aqueous interface, evidencing the fact that the surface had the most hydrophilic character. The results show that batch Mag 20 most likely adsorbed less surfactant than batch Mag 24 in all cases. The coating behavior difference between the two types of synthesized magnetite is probably due to an increased density of hydroxyl groups on the nanoparticle surfaces obtained by co-precipitation [21]. Their superficial layer is prone to incorporating less surfactant due to steric shielding. 3.2.2. FTIR analysis The magnetic nanoparticles functionalized with ionic surfactants and their bare precursors were characterized by FTIR spectroscopy in the wavelength range of 4000–400 cm 1. Fig. 6 shows the comparative spectra of bare magnetite prepared by co-precipitation (batch Mag 20) and oxidation (batch Mag 24)) and respectively of oleic acid coated magnetite (Mag 20_AcOl) and of sodium oleate functionalized magnetite (batch Mag 24_NaOl). The peaks appearing on the FTIR spectra of both bare magnetite lots at 584 cm 1 and respectively 582 cm 1 are assigned to the Fe–O bond in Fe3O4, that is located close to this region according to the literature [24]. The peaks at 3319 cm 1 and respectively 3454 cm 1 are attributed to the stretching vibrations due to the superficial OH groups. In order to understand the adsorption mechanism of the oleic acid and sodium oleate, respectively, on the surface of Fe3O4
Fig. 6. Comparative FTIR spectra of the bare magnetite produced by co-precipitation (batch Mag 20) and oxidation (batch Mag 24)), oleic acid coated magnetite (Mag 20_AcOl) and sodium oleate coated magnetite (Mag 24_NaOl).
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Fig. 7. Comparative optical micrographs of composite films prepared with functionalized nanoparticles in magnetic field of 5.5 mT. Functionalized particle lots designated as explained in Table 1.
nanoparticles, the spectra obtained for the functionalized particle samples were compared with the pure oleic acid and sodium oleate spectra presented in the literature (data not shown) [25,26]. The peaks found at 3435 cm 1 and 3460 cm 1 are attributed to the stretching vibrations of OH , also assigned to OH absorbed by the nanoparticles. On the oleic acid coated magnetite FTIR spectrum (Mag 20_AcOl), the sharp bands at 2923 cm 1 and 2852 cm 1 are attributed to asymmetric and symmetric CH vibrations of the methylene groups, slightly shifted from those characteristic to pure
oleic acid spectrum. The C ¼O stretch band of the carboxyl group present at 1710 cm 1 in the spectrum of the pure oleic acid [23] was absent in the spectrum of the coated nanoparticles. Therefore, the newly appeared bands at 1604 cm 1 and 1406 cm 1 were ascribed to the symmetric and asymmetric COO stretches. A medium adsorption peak located at 1043 cm 1 is due to C–O single bond stretching. These results suggest that the oleic acid functionality was partially bound covalently to the Fe3O4 nanoparticles as an ester. In the case of sodium oleate (Mag 24_NaOl) coated magnetite,
Fig. 8. Comparative SEM micrographs of composite films prepared with functionalized nanoparticles in magnetic field of 5.5 mT.
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Fig. 9. Optical micrographs at different magnetic field strengths: A. 0 mT; B. 1.5 mT; C. 2.8 mT; D. 5.5 mT; and SEM images at E. 2.8 mT and F. 5.5 mT magnetic strength of composite films prepared with functionalized nanoparticles (batch Mag 24_NaOl).
the peaks at 2923 cm 1 and 2852 cm 1 are also attributed to the stretching vibrations of –CH2 and –CH3 from sodium oleate. The characteristic carbonyl stretching vibration characteristic to sodium oleate at 1710 cm 1 (pure oleate spectrum from the literature) is missing from the functionalized particle spectrum entirely, and the asymmetric and symmetric COO vibrations appear instead at 1610 cm 1 and 1562 cm 1. The peaks at 1396 cm 1 and 1365 cm 1 are attributed to the vibration of –CH, and the single C–O bond stretching at 1099 cm 1. These all suggest that sodium oleate is also partially bound chemically to the Fe3O4 nanoparticles. The higher intensity of these peaks on the spectrum of functionalized Mag 24 particles compared to the corresponding peaks on the functionalized Mag 20 spectrum qualitatively confirm higher surfactant coverage on the former, as noticed in the two-phase partition experiment. The peak assigned to the Fe–O bond vibration was slightly shifted to a higher frequency region in both functionalized particle spectra.
3.3. Composite film characterization Composite nanoparticle-chitosan films prepared by spraying and cured by drying under a 5.5 mT magnetic field were analyzed by optical microscopy and comparative micrographs are shown in Fig. 7. Chitosan was chosen as a polymeric model support because the film is easy to prepare from aqueous solution and the resulting coating is transparent. The magnetite nanoparticle content in all the model films was relatively low (6% based on solid weight) in order to keep them transparent and be able to observe their morphology under the optical microscope. For real life applications in which superhydrophobic behavior is desired, the nanoparticle content should be higher, in the range of 30–50%, according to the literature [27–29], but the actual value will be determined according to the intended application. All model films were prepared using the same procedure. Since particle movement during the film curing stage is a result of the equilibrium between the aligning effect of the magnetic field and the drag force, the film morphology depends on the surfactant nature and its concentration. It is expected that faster moving particles will align more with the magnetic field than
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slower moving ones. The results show that the particles functionalized with oleic acid or sodium oleate are the fastest moving ones. This is due to the fact that in this case the drag force is minimal (the friction between the hydrophilic matrix and the hydrophobic nanoparticle surface is at the lowest point). The composite films prepared with Mag 20_AcOl and respectively Mag 24_NaOl show therefore the highest degree of alignment, with striped patterns comprising of micro-rods. As the surfactant becomes more hydrophilic, the drag force increases and the degree of particle alignment with the field is less evidenced. In the Mag 24 series, the film morphology changes to narrower stripes (comprising of shorter micro-rods) in the Span functionalized batch, respectively to a mix of branches and micro-rods in the mixed Span/Tween batches and finally to branches only, in the film prepared with the Tween functionalized lot. The nanoparticles produced by oxidation are therefore able to produce a variety of patterns, depending on the surfactant nature. By comparison, the particles produced by co-precipitation produced patterns only with the most hydrophobic surfactants (oleic acid and Span, respectively). They align less than their respective counterparts produced by oxidation due to slower movement caused both by their lower magnetization and insufficient surfactant coverage. SEM micrographs shown in Fig. 8 confirm that the coatings produced with Mag 24 batch present hierarchical protuberances alternating with cavities (micro-rods show nano-sized features) when functionalized with both the most hydrophobic and the most hydrophilic surfactant from the tested series. By comparison, batch Mag 20 produced a film with patterned roughness only with the most hydrophobic surfactant, but even in this case the protuberances are only at micro-level. The oxidation method is the optimum one for the envisioned application since functionalized Mag 24 particles assembled in chain-like structures, thus forming ordered protuberances within the polymeric film and hierarchical morphology that may create premises to lock in air pockets upon wetting. In a second series of experiments aimed to observe the pattern dependence on the magnetic field strength, while the surfactant nature was kept constant, the Mag 24 particles functionalized with sodium oleate were used. Optical microscope images for composite films prepared in magnetic field varying in the range of 0– 5.5 mT are shown in Fig. 9. They evidence that the nanoparticles remain uniformly dispersed within the matrix when no magnetic field is applied (0 mT – Fig. 9A). When the lowest magnetic field is applied, the morphology of the film shows chain-like aggregates (micro-rods with nanostructured features) aligned to the field lines (1.5 mT – Fig. 9B). The micro-rods become longer as the magnetic field strength increases (2.8 mT – Fig. 9C and 5.5 mT – Fig. 9D). The last two films, prepared at magnetic field of 2.8 and respectively 5.5 mT, are also shown here in SEM micrographs. The images (Figs. 9E and 9F) evidenced a morphology with cavities and protuberances that are better structured when the magnetic field is stronger. Nanoparticles move faster under the stronger field and, given the same amount of time, produce patterning in wider scale.
4. Conclusions Magnetite nanoparticles bearing variable hydrophobic functionality were prepared by co-precipitation and the partial oxidation method, followed by surfactant coating. While both synthesis methods yielded well crystallized, uniformly sized magnetite nanoparticles that were able to generate patterned composite films under the influence of magnetic field, the product obtained by partial oxidation proved more suitable for the envisioned application due to
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1. higher phase purity and saturation magnetization; 2. better surfactant coverage; 3. ability to produce films in which the degree of aggregation and patterning may be controlled by the surfactant nature or by the magnetic field strength; 4. coatings with hierarchical roughness, consisting in micro-rods with nano-scale structuring that may facilitate air pocket entrapment beneath water drops. The results show that textured surface composite coatings may be prepared in a relatively simple manner with the aid of a magnetic field. Moreover, the versatility of the method is demonstrated by the possibility to choose the surfactant combination with the appropriate HLB value, depending on the support properties, to encourage the desired surface patterning. Future work will be carried out for synthesizing a polymeric matrix modified with a low surface energy reagent in order to produce composite films with ice-phobic properties.
Acknowledgments This work was supported by a grant of the Ministry of National Education, CNCS-UEFISCDI, Project no. PN-II-ID-PCE-2012-4-0433.
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