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Self-cleaning superhydrophobic epoxy coating based on fibrous silica-coated iron oxide magnetic nanoparticles
Journal of Colloid and Interface Science 513 (2018) 349–356
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Regular Article
Self-cleaning superhydrophobic epoxy coating based on fibrous silica-coated iron oxide magnetic nanoparticles Haleema Alamri, Abdullah Al-Shahrani, Enrico Bovero, Turki Khaldi, Gasan Alabedi, Waleed Obaid, Ihsan Al-Taie, Aziz Fihri ⇑ Oil and Gas Network Integrity Division, Research & Development Center, Saudi Aramco, Dhahran 31311, Saudi Arabia
g r a p h i c a l a b s t r a c t
a r t i c l e
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Article history: Received 4 October 2017 Revised 13 November 2017 Accepted 14 November 2017 Available online 15 November 2017 Keywords: Magnetic nanoparticles Fiber core-shell Superhydrophobicity Wettability
a b s t r a c t Inspired by the self-cleaning lotus leaf, a facile method of fabricating superhydrophobic silica coated magnetite nanoparticles using a cost-effective process is presented in this work. The structural characterizations and magnetic properties of the obtained core-shell magnetic nanoparticles were characterized by means of X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). TEM analysis revealed that the particles present flower‐like dendrimeric fibers morphology. The particles were uniformly dispersed on the surface of an epoxy resin coating with the purpose to increase the roughness and reduce the surface energy of the surface. The resulting superhydrophobic surface provides robust water-repellent surface under harsh conditions, thanks to its self-cleaning characteristic. The superhydrophobicity of this surface was confirmed based on the measurements of a water contact angle around 175°, which surpasses the theoretical limit of the superhydrophobicity. The simplicity and the costeffectiveness of the process developed in this study appears to be a promising route for the preparation of other magnetic superhydrophobic organic-inorganic hybrid materials that would be beneficial in a wide variety of applications. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (A. Fihri). https://doi.org/10.1016/j.jcis.2017.11.042 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
Ideal prototypes for superhydrophobic surfaces can be found in nature, in entities such as lotus leaves and the unique body
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structures of a number of insects, all of which are characterized by low surface energy and have contact angles greater than 150° [1,2]. Such natural structures have inspired the design of artificial superhydrophobic surfaces, an area that has attracted extensive attention over the last two decades in both academic and industrial fields due to their potential benefits in a variety of applications, such as anticorrosion [3,4], self-cleaning [5,6], anti-icing [7], oilwater separation [8]. Today, the mechanism behind these superhydrophobic natural structures is no longer a mystery, and several studies have been conducted involving the use of Scanning electron microscopy analysis for unveiling the full mechanism underlying this phenomenon [9]. These studies provide evidence of the coexistence of a low surface energy material with a high degree of roughness in hierarchical micro/nanostructure. Both of these characteristics are, in fact, necessary for the preparation of artificial superhydrophobic surfaces [10]. These findings have inspired scientists to explore a range of pathways for the fabrication of a variety of superhydrophobic structures that have varied functional systems, with the goal of developing the next generation of hybrid/functional materials [11,12]. Combining the synthesis of a superhydrophobic structure with materials that are responsive to different external stimuli such as mechanical force, temperature, or magnetic and electric fields, is currently of major interest in the materials science research community [13,14]. In the context of responsive materials, magnetic nanoparticles (MNPs) have become a topic of tremendously expanded study owing to their outstanding magnetic properties, which makes them highly useful in many diverse areas, including magnetic separation, water purification, and biomedical applications such as the hyperthermic treatment of cancerous tumors [15–17]. Of these magnetic materials, magnetite (Fe3O4) nanoparticles have been increasingly under investigation due to their unique physical and chemical properties [18]. It has been reported that adding an inert layer on the surfaces of the magnetite particles helps preventing their aggregation in a dispersant, thus enhancing their chemical reactivity and stability [19]. The recent synthesis of magnetic core-shell materials has attracted considerable scrutiny and the synthesis of various kinds of Fe3O4 core-shell composite materials has been reported, including Fe3O4/polymers [20], Fe3O4/carbon [21], and Fe3O4/MOF [22]. However, the majority of studies have been focused on the synthesis of a silica-coated magnetite particle core-shell. This emphasis is attributable to the high level of stability of magnetic silicates and the possibility of modifying their surface using a variety of functional groups, based on the presence of reactive silanol groups that enable control of the surface properties and porous structure of the materials [23–26]. An additional factor is that Fe3O4 particles possess a large number of magnetic dipoles that could absorb a significant amount of incident electromagnetic radiation. When these later propagate throughout the materials, the Fe3O4 magnetic dipoles and the electromagnetic radiation collide, thus inducing an energy reduction [27]. Fe3O4 can therefore play a role as an electromagnetic interference shield against radiation penetration. However, the use of Fe3O4 as an electromagnetic interference shielding material is limited because of its susceptibility to contamination, especially under harsh environmental conditions such as rain. This challenge can be overcome by making use of the selfcleaning and anti-contamination properties of a superhydrophobic shell around the Fe3O4. The use of silica as a shell for preventing Fe3O4 corrosion seems an appealing solution, and a large number of superhydrophobic surfaces have recently been prepared by processing a variety of functional silica nanoparticles [28–30]. The importance of magnetic silica nanoparticles in this context is also related to their remarkable magnetic properties, which open up new avenues for future functional materials research. More recently, the development of magnetic superhydrophobic coatings on wood composites [31] and on a wide range of substrates such as
glass [11] copper, and aluminum [32] has gained prominence due to their wide-ranging potential applications [14]. However, little research has been devoted to the design of magnetic superhydrophobic coatings with improved stability and durability, and only a few published reports have described the functionalization of MNPs using fluoride moieties to lower the surface energy of coated MNPs [33,34]. The reason of this scarceness is that these fluorinated moieties are typically expensive, and a more costeffective approach is usually preferable. In this work, we report the development of an alternative, simpler approach that employs inexpensive reagents for the synthesis of a superhydrophobic silica shell grafted around core MNP-based materials (Fe3O4@H-SiO2/ KCC-1). The fabricated magnetic-core/silica-shell-grafted nanoparticles exhibited superior and durable superhydrophobic performance under a variety of corrosive conditions. These superhydrophobic MNPs are easily separable by an external magnetic field, which makes them good candidates for a variety of crucial applications such as anti-icing, oil-water separation, and anticorrosion. Moreover, when these superhydrophobic nanoparticles were deposited on a crosslinked epoxy matrix, they also exhibited an excellent self-cleaning ability with good acid-base resistance. 2. Experimental 2.1. Materials High purity iron (II) chloride (FeCl24H2O, 99%), ammonia (28%), tetraethoxysilane (TEOS, 98%), trimethoxy(octyl)silane (OTS, 97.5%), cetyltrimethylammonium bromide (CTAB, 99%), urea (99%), toluene (99.7%) and ethanol (99.8%) were purchased from the Sigma-Aldrich chemical company and were used without further purification. 2.2. Synthesis of the Fe3O4 magnetic nanoparticles Magnetite nanoparticles were prepared according to the procedure reported in the literature but with a minor modification [35]. Starting with a typical synthesis, 2 g of FeCl24H2O were dissolved in 100 mL of water and stirred at room temperature for 30 min. Then 15 mL of ammonium hydroxide were added, and the mixture was stirred continuously for 60 min, thus permitting the iron to be oxidized. The resulting solution was stirred for 60 min at room temperature and then transferred into a Teflon hydrothermal reactor to be heated at 120 °C for 5 h in an oven. The black powder obtained was collected and purified by repeated centrifugation, washed several times with deionized water, and then dried at 120 °C for 5 h. 2.3. Preparation of the Fe3O4@SiO2/KCC-1 The Fe3O4 nanoparticles were covered with silica particles to permit further growth of fibrous silica and were prepared as described in an earlier work with minor modifications [36,37]. Firstly, 2 g of Fe3O4 NPs were dispersed in a mixture composed of 200 mL of ethanol, 50 mL of deionized water, and 10 mL of ammonia solution and the mixture was stirred for 60 min following which, 4 mL of TEOS were added dropwise. The mixture was then stirred at room temperature for 6 h, following which, the solid obtained was collected by filtration, washed thoroughly with deionized water and ethanol, and then dried. To make the Fe3O4@SiO2/KCC-1, 2 g of the Fe3O4@SiO2 powder obtained above were dispersed in 120 mL of an aqueous solution containing 2.4 g of urea and 4 g of CTAB. The suspension was then added to a mixture composed of 6 mL of 1-pentanol, 120 mL of cyclohexane, and 10 g of TEOS. The resultant mixture was stirred at room temperature for
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1 h and then transferred into a sealed stainless steel autoclave. Heat was applied at 120 °C for 5 h in an oven, after which the autoclave was cooled to room temperature and the Fe3O4@SiO2/KCC-1 obtained was isolated by filtration, washed several times with deionized water and ethanol, dried overnight in a vacuum drying oven at 150 °C.
drophobic material to the epoxy, the dispersal was then pressurized on the top of the surface. The epoxy coating was cured at 40 °C for 2 days, following which, the excess of Fe3O4@H-SiO2/ KCC-1 that was not attached to the surface was removed using flowing deionized water and ethanol, and the surface was then dried at 50 °C for 4 h.
2.4. Preparation of the Fe3O4@H-SiO2/KCC-1
3. Characterization and measurement
Prior to silanization, the Fe3O4@SiO2/KCC-1 NPs were dried for 2 h at 80 °C. According to the usual practice, 20 g of Fe3O4@SiO2/ KCC-1 were dispersed in 100 mL of dry toluene and stirred for 1 h at room temperature. The next step was to add 40 mL of trimethoxy(octyl)silane and reflux the resulting mixture for 2 days as depicted in Scheme 1. The solution was then left to be cooled at room temperature, and the powder obtained was isolated by centrifugation, thoroughly washed with toluene and ethanol, and then dried in an oven at 120 °C for 6 h so that it could be systematically characterized.
Thermogravimetric analysis- Derivative Thermogravimetry (TGA-DTG) was performed with a TA Instruments SDT Q600 apparatus under air at a heating rate of 10 °C/min. X-ray diffraction (XRD) of the samples was performed at room temperature using a Rigaku Ultima IV X-ray diffractometer equipped with Cu Ka radiation in Bragg-Brentano geometry (h–2h). To determine the surface wettability, the contact angles of water droplets (4 lL) on the surface coating were measured using an optical tensiometer (Attension Theta, Biolin Scientific, Germany) at room temperature and analysis was performed by fitting drops with a Young-Laplace formula using the Theta Software. Magnetic properties were measured at room temperature using a vibrating sample magnetometer (VSM) (Lake Shore Company). Scanning electron microscopy (SEM) analyses were recorded on a FEI Helios NanoLab microscope. SEM aluminum stub holders with conductive carbon tape were used for fixing the powdered materials. The SEM was operated in low vacuum mode with varied water vapor pressure in order to reduce charging on imaging quality during analysis. Transmission electron microscopy (TEM) analyses were recorded on a Tecnai Twin TEM microscope operated at a 120 kV accelerating voltage. The chemical composition was analyzed using an energy disper-
2.5. Preparation of the superhydrophobic magnetic epoxy coating A schematic illustration of the fabrication process is shown in Scheme 2. Briefly, 10 g of acetone were mixed with 4 g of diglycidyl ether of bisphenol A epoxy resin. Next, 2 g of isophoronediamine were added, and the resulting solution was thoroughly mixed. The epoxy pre-coating was applied on a pressure-cleaned carbon steel substrate by hand brushing, and Fe3O4@H-SiO2/KCC-1 powder was then dispersed onto the epoxy surface coating using a 450 lm sieve. To enhance the adhesion of the magnetic superhy-
Scheme 1. Schematic representation of the Fe3O4@H-SiO2/KCC-1 preparation process.
Scheme 2. Schematic illustration of the preparation of the magnetic superhydrophobicity epoxy surface.
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sive X-ray spectrometer (EDS) embedded on the TEM microscope. The diameter of particles was statically analyzed using ImageJ free software and the distribution histogram is obtained by measuring at least 160 nanoparticles.
4. Results and discussion The entire process was conducted under mild conditions, and no complicated procedures were needed. For this proposed procedure, we took advantage of the nature of the Fe3O4 nanoparticles which have a high density of hydroxyl groups estimated to be in the range of 5.1–9.8 OH groups per nm2 [38]. The active hydroxyl groups on the surface of the Fe3O4 nanoparticles, were reacted with the hydrolyzed acidic moieties formed by the TEOS during the hydrolysis stage. At this stage the presence of ammonia increases the pH value, thus allowing the polymerization of the silica, which occurs through the condensation of the silanol groups (Si-OH) and begins the formation of the first coating layer of silica, based on the creation of covalent siloxane bonds (SiAOASi) [39]. After being washed and dried, the Fe3O4@SiO2 along with CTAB as a directing agent, was used to be covered with a mesoporous silica shell. Alkyl chains were then grafted onto the mesoporous shell to render it superhydrophobic, with superior properties and durability under a variety of corrosive conditions. Fig. 1(a) shows that the several diffraction peaks at 2h values of 18.278°, 30.066°, 35.413°, 37.044°, 43.038°, 53.391°, 56.914°, and 62.497° of the corresponding XRD pattern of the Fe3O4 NPs match the 111, 220, 311, 222, 400, 422, 511, and 440 planes, respectively, of the Fe3O4 cubic inverse spinel structure. The XRD pattern is in good agreement with the data for pure cubic Fe3O4 as reported in JCPDS number card: 01-076-1849 and are consistent with those reported in the literature [40,41]. The sharp diffraction peaks confirm the formation of a pure phase of Fe3O4 with excellent crystallinity and no characteristic impurity peaks were observed under the experimental conditions in place. The other XRD diffraction pattern, Fig. 1(b) indicates that the Fe3O4@SiO2/KCC-1 has diffraction peaks similar to those of the parent Fe3O4 NPs with an additional broad peak at 21.364° corresponding to the amorphous silica. These findings indicate that the silica was successfully coated onto the surface of the Fe3O4 nanoparticles and that the coating later retained its phase structures despite being calcined at 550 °C. Thermogravimetric analysis was employed in order to examine the thermal stability of Fe3O4 and Fe3O4@H-SiO2/KCC-1 samples for temperatures ranging from room temperature to 800 °C; the results are displayed in Fig. 2. For the uncoated Fe3O4, the TGA curve showed three weights losses of about 4% from room temper-
Fig. 1. XRD patterns of Fe3O4 (a) and Fe3O4@H-SiO2/KCC-1 core-shell nanoparticles (b).
atures up to 455 °C. The first weight loss at 25–200 °C can be attributed to the evaporation of surface bonded water. The second weight loss. The second weight loss occurred between 210 and 340 °C can be attributed to the conversion of the magnetite into hematite. It important to stress that it was reported that the magnetite converted into maghemite at around 270 °C which will be then transformed into hematite at around 320 °C [42]. This oxidation process is expected to increase the sample weight [43]. In addition, the small weight loss observed at 450 °C can be attributed to release of the pore bound water [44]. In contrast, two different weight loss regions can be identified for the Fe3O4@H-SiO2/ KCC-1. The first was characterized by rather slow weight loss up to 230 °C due to the evaporation of physisorbed water and pore bound water, whereas in the second region, the major weight loss of 5.2% observed between 230 °C and 600 °C can be attributed to the conversion of magnetite to hematite and to the loss of the covalently bonded octylsilane group [45]. This weight loss does not occur at a specific temperature since the organic silane groups decompose slowly from the silica surface. The magnetic hysteresis loops of the Fe3O4 and Fe3O4@H-SiO2/ KCC-1 are shown in Fig. 3. It is clearly evident that, for both samples, magnetization increased quickly with an increase in the magnetic field and then stabilized at a magnetic field of 800 Oe. The Fe3O4 exhibited a saturation magnetization of 47.98 emu/g which is lower than that observed for magnetite with a size of 37 nm (92 emu/g) [35,46]. The difference in magnetization can be correlated with the size of the magnetite particle. In fact, it has been reported that a reduction in Fe3O4 particle size leads to a decrease in the saturation magnetization [35]. This phenomenon, which has already been observed in many other magnetic nanoparticles, can be explained by the presence of a magnetically inert layer on the
Fig. 2. TGA and DTG curves of Fe3O4 (a) and Fe3O4@H-SiO2/KCC-1 (b).
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Fig. 3. Magnetic hysteresis loops for Fe3O4 (a) and Fe3O4@H-SiO2/KCC-1 (b).
surface of the nanoparticles [47]. Saturation magnetization is controlled by the surface effect directed by an effective volume, which is a fraction of the nominal volume. As the particle size decreases, the magnetic effective volume accounts for a smaller proportion of the total volume, thus reducing the specific saturation magnetization. The saturation magnetization decreased dramatically for the Fe3O4@H-SiO2/KCC-1 sample to 17.89 emu/g. This drop in the saturation magnetization value after coating of the grafted silica onto the Fe3O4 surface can be attributed to the presence of the silica shell around the core [48]. Another factor that might be responsible for lowering magnetization level is that part of the silica is bonded onto the surface of the magnetite particles through the FeAOASi chemical bonding [49]. The magnetic moment of the iron ions bonded with the silicon on the surface of the magnetite NPs disappeared through the formation of the FeAOASi bonds, thus leading to a decrease in magnetism following the silica coating [50]. This finding can be considered evidence of successful silica coating on the magnetite NPs. The literature includes reports of similar findings that indicate a diminished magnetization value after magnetic particles have been coated with silica [51–53]. On the other hand, the coercivity (Hc) of the hydrophobic iron oxide/silica core-shell NPs was 81.94 Oe, which is lower than the coercivity of the as-synthesized Fe3O4 NPs (84.68 Oe). This effect
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could be caused to the formation of an interfacial structure, and similar magnetic behavior has been observed for SiO2/Fe2O3 nano-architectures [54,55]. The microstructure of the as-prepared magnetite was also analyzed using TEM. The particles have polygonal shapes. Fitting the particle size histogram to a smooth curve, a mean particle diameter of 22 nm is obtained as depicted in Fig. 4. SEM was used for studying the morphology of the surface of the Fe3O4@H-SiO2/KCC-1 nanoparticles as illustrated in Fig. 5(a). The SEM data acquired showed that the material consisted of uniform and well-defined spherical particles with a particle size ranging from 50 nm to approximately 200 nm. Furthermore, the micrographs obtained from TEM analysis indicate that the material consisted of dendrimeric fibers spreading out in radial manner and distributed flower‐like in all directions as illustrated in the Fig. 5 (b). Further analyses involving the application of EDS were conducted in order to prove the presence of the different components of our samples. The EDS analysis verified the co-existence of both the silica and magnetite nanoparticles (Fig. S1, supporting information). Importantly, the SEM of the prepared superhydrophobic surface revealed that silica coated magnetite nanoparticles were with different sizes and shapes randomly stacked over the substrate and made densely packed protrusions after curing. Additionnaly, Fe3O4@H-SiO2/KCC-1 particles became larger because they were agglomerated (Fig. S2, supporting information). Contact angles were also measured as a means of evaluating the wettability of the produced samples. Because of the presence of hydroxyl functional groups, the contact angle of the control sample coated only with cured epoxy was 65°. The measured contact angles of the prepared superhydrophobic Fe3O4@H-SiO2/KCC-1 core-shell surface prepared were around 175°. Obtaining, the wettability measurements of this magnetic superhydrophobic surface, in fact, was difficult because water droplets rolled off the surface toward the sides very easily and rarely stabilized. The measurements were collected by carefully placing the droplets of water in a perfectly flat region of the sample. The wetting properties of the superhydrophobic magnetic surface were then compared to those of the hydrophobic silica nanoparticles without a magnetic core prepared by the Ströber and hydrothermal processes and grafted with trimethoxy(octyl)silane using same procedure described in this work. Interestingly, the contact angles measured were found to be 135° and 154°, respectively, as illustrated in Scheme 3. The Wenzel and Cassie–Baxter models are two classical models generally used for describing the wetting properties of
Fig. 4. TEM of as-prepared Fe3O4 nanoparticles and their particle size distribution.
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Fig. 5. SEM (a) and TEM (b) images of Fe3O4@H-SiO2/KCC-1 nanoparticles.
Scheme 3. The contact angles of superhydrophobic silica (a) and (b) prepared by the Stöber and hydrothermal processes, respectively, and those of magnetic superhydrophobic silica (c).
superhydrophobic surfaces [56,57]. Both models portray surface structure and chemical composition as together being the determinant factors in surface wetting. In our study, the ultra-water repellency of this magnetic superhydrophobic surface was most probably caused by the low surface energy of the hydrocarbon chains and by the formation of the SiAOASi network, as well as by an increased surface roughness due to the dendritic fibrous surface morphology present in silica particles as revealed by TEM analysis [58]. The long-term stability of superhydrophobic coatings is a crucial parameter for the determination of their performance and for the feasibility of their industrial application. In this study, the chemical stability of the prepared superhydrophobic coating was therefore evaluated based on immersion in a 3.5 wt% NaCl aqueous solution. The evaluation entailed an examination of the relationship between the water contact angle and the immersion time as indicated in Fig. 6. It is clear that the contact angle measured on the superhydrophobic surface decreased slightly over time, but a contact angle of 165.9 ± 1.7° was still maintained after 40 days of immersion. Additionally, the sliding angles values always remain below 5°, which indicates the low adhesion of the as-prepared superhydrophobic surface. From a wettability perspective, these results demonstrate the good durability and hydrophobicity of the as-prepared surface in sodium chloride environments, as 40 days of immersion had no influence on the surface states. The chemical stability of the prepared superhydrophobic surface was also evaluated under two different pH conditions namely
pH = 1, and pH = 14 as can be seen from Fig. 7. Hydrochloric acid and sodium hydroxide were used for adjusting the pH of the solutions. Similar stability to the salt aqueous solution were obtained
Fig. 6. Change in the contact angle as a function of the immersion time of the asprepared superhydrophobic magnetic surface in a 3.5 wt% NaCl aqueous solution.
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Fig. 7. Variation in the contact angle of the as-prepared superhydrophobic magnetic surface for different pH values.
where a high contact angle in the range of 166.5 ± 1.4° were maintained for 24 h which indicates that the prepared superhydrophobic surface also possesses also excellent chemical stability with respect to both acidic and alkaline environments. 5. Conclusion In summary, a superhydrophobic magnetic material based on fibrous mesoporous Fe3O4@H-SiO2/KCC-1 nanoparticles with a core-shell structure was successfully prepared using a simple approach. The structural characterizations of the produced particles revealed a dendrimeric fibers morphology spreading out in radial manner as flower‐like. To increase roughness and reduce surface energy, the Fe3O4@HSiO2/KCC-1 core-shell nanoparticles were uniformly dispersed on the surface of an epoxy resin coating. The resulted superhydrophobic magnetic surface exhibits outstanding superhydrophobic properties and good stability as well as good durability under different corrosive conditions. This could be explained by the co-existence of low surface energy and the high surface roughness caused by the grafted hydrophobic alkyl chains due to the dendritic fibrous surface morphology and the flexible fibrous structure the fibrous silica. This could was be explained by the co-existence of low surface energy and high surface roughness caused by the excellent accessibility of grafted hydrophobic alkyl chains and by the dendritic fibrous surface morphology present in silica. This facile and low-cost approach developed in this work may also be expanded for the preparation of other magnetic superhydrophobic organicinorganic hybrid materials that can have several applications such oil-water separation, novel easy-clean coatings and in electronic. The use of these ferromagnetic material as fillers to enhance the adhesion and the corrosion protection of polymeric coatings are underway in our laboratory. Acknowledgments We are very thankful to Juhani Haitham, Hawraa Bin saad and Nada Qari for their day to day aid in laboratory life. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.11.042.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Self-cleaning superhydrophobic epoxy coating based on fibrous silica-coated iron oxide magnetic nanoparticles Haleema Alamri, Abdullah Al-Shahrani, Enrico Bovero, Turki Khaldi, Gasan Alabedi, Waleed Obaid, Ihsan Al-Taie, Aziz Fihri ⇑ Oil and Gas Network Integrity Division, Research & Development Center, Saudi Aramco, Dhahran 31311, Saudi Arabia
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 4 October 2017 Revised 13 November 2017 Accepted 14 November 2017 Available online 15 November 2017 Keywords: Magnetic nanoparticles Fiber core-shell Superhydrophobicity Wettability
a b s t r a c t Inspired by the self-cleaning lotus leaf, a facile method of fabricating superhydrophobic silica coated magnetite nanoparticles using a cost-effective process is presented in this work. The structural characterizations and magnetic properties of the obtained core-shell magnetic nanoparticles were characterized by means of X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). TEM analysis revealed that the particles present flower‐like dendrimeric fibers morphology. The particles were uniformly dispersed on the surface of an epoxy resin coating with the purpose to increase the roughness and reduce the surface energy of the surface. The resulting superhydrophobic surface provides robust water-repellent surface under harsh conditions, thanks to its self-cleaning characteristic. The superhydrophobicity of this surface was confirmed based on the measurements of a water contact angle around 175°, which surpasses the theoretical limit of the superhydrophobicity. The simplicity and the costeffectiveness of the process developed in this study appears to be a promising route for the preparation of other magnetic superhydrophobic organic-inorganic hybrid materials that would be beneficial in a wide variety of applications. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (A. Fihri). https://doi.org/10.1016/j.jcis.2017.11.042 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
Ideal prototypes for superhydrophobic surfaces can be found in nature, in entities such as lotus leaves and the unique body
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structures of a number of insects, all of which are characterized by low surface energy and have contact angles greater than 150° [1,2]. Such natural structures have inspired the design of artificial superhydrophobic surfaces, an area that has attracted extensive attention over the last two decades in both academic and industrial fields due to their potential benefits in a variety of applications, such as anticorrosion [3,4], self-cleaning [5,6], anti-icing [7], oilwater separation [8]. Today, the mechanism behind these superhydrophobic natural structures is no longer a mystery, and several studies have been conducted involving the use of Scanning electron microscopy analysis for unveiling the full mechanism underlying this phenomenon [9]. These studies provide evidence of the coexistence of a low surface energy material with a high degree of roughness in hierarchical micro/nanostructure. Both of these characteristics are, in fact, necessary for the preparation of artificial superhydrophobic surfaces [10]. These findings have inspired scientists to explore a range of pathways for the fabrication of a variety of superhydrophobic structures that have varied functional systems, with the goal of developing the next generation of hybrid/functional materials [11,12]. Combining the synthesis of a superhydrophobic structure with materials that are responsive to different external stimuli such as mechanical force, temperature, or magnetic and electric fields, is currently of major interest in the materials science research community [13,14]. In the context of responsive materials, magnetic nanoparticles (MNPs) have become a topic of tremendously expanded study owing to their outstanding magnetic properties, which makes them highly useful in many diverse areas, including magnetic separation, water purification, and biomedical applications such as the hyperthermic treatment of cancerous tumors [15–17]. Of these magnetic materials, magnetite (Fe3O4) nanoparticles have been increasingly under investigation due to their unique physical and chemical properties [18]. It has been reported that adding an inert layer on the surfaces of the magnetite particles helps preventing their aggregation in a dispersant, thus enhancing their chemical reactivity and stability [19]. The recent synthesis of magnetic core-shell materials has attracted considerable scrutiny and the synthesis of various kinds of Fe3O4 core-shell composite materials has been reported, including Fe3O4/polymers [20], Fe3O4/carbon [21], and Fe3O4/MOF [22]. However, the majority of studies have been focused on the synthesis of a silica-coated magnetite particle core-shell. This emphasis is attributable to the high level of stability of magnetic silicates and the possibility of modifying their surface using a variety of functional groups, based on the presence of reactive silanol groups that enable control of the surface properties and porous structure of the materials [23–26]. An additional factor is that Fe3O4 particles possess a large number of magnetic dipoles that could absorb a significant amount of incident electromagnetic radiation. When these later propagate throughout the materials, the Fe3O4 magnetic dipoles and the electromagnetic radiation collide, thus inducing an energy reduction [27]. Fe3O4 can therefore play a role as an electromagnetic interference shield against radiation penetration. However, the use of Fe3O4 as an electromagnetic interference shielding material is limited because of its susceptibility to contamination, especially under harsh environmental conditions such as rain. This challenge can be overcome by making use of the selfcleaning and anti-contamination properties of a superhydrophobic shell around the Fe3O4. The use of silica as a shell for preventing Fe3O4 corrosion seems an appealing solution, and a large number of superhydrophobic surfaces have recently been prepared by processing a variety of functional silica nanoparticles [28–30]. The importance of magnetic silica nanoparticles in this context is also related to their remarkable magnetic properties, which open up new avenues for future functional materials research. More recently, the development of magnetic superhydrophobic coatings on wood composites [31] and on a wide range of substrates such as
glass [11] copper, and aluminum [32] has gained prominence due to their wide-ranging potential applications [14]. However, little research has been devoted to the design of magnetic superhydrophobic coatings with improved stability and durability, and only a few published reports have described the functionalization of MNPs using fluoride moieties to lower the surface energy of coated MNPs [33,34]. The reason of this scarceness is that these fluorinated moieties are typically expensive, and a more costeffective approach is usually preferable. In this work, we report the development of an alternative, simpler approach that employs inexpensive reagents for the synthesis of a superhydrophobic silica shell grafted around core MNP-based materials (Fe3O4@H-SiO2/ KCC-1). The fabricated magnetic-core/silica-shell-grafted nanoparticles exhibited superior and durable superhydrophobic performance under a variety of corrosive conditions. These superhydrophobic MNPs are easily separable by an external magnetic field, which makes them good candidates for a variety of crucial applications such as anti-icing, oil-water separation, and anticorrosion. Moreover, when these superhydrophobic nanoparticles were deposited on a crosslinked epoxy matrix, they also exhibited an excellent self-cleaning ability with good acid-base resistance. 2. Experimental 2.1. Materials High purity iron (II) chloride (FeCl24H2O, 99%), ammonia (28%), tetraethoxysilane (TEOS, 98%), trimethoxy(octyl)silane (OTS, 97.5%), cetyltrimethylammonium bromide (CTAB, 99%), urea (99%), toluene (99.7%) and ethanol (99.8%) were purchased from the Sigma-Aldrich chemical company and were used without further purification. 2.2. Synthesis of the Fe3O4 magnetic nanoparticles Magnetite nanoparticles were prepared according to the procedure reported in the literature but with a minor modification [35]. Starting with a typical synthesis, 2 g of FeCl24H2O were dissolved in 100 mL of water and stirred at room temperature for 30 min. Then 15 mL of ammonium hydroxide were added, and the mixture was stirred continuously for 60 min, thus permitting the iron to be oxidized. The resulting solution was stirred for 60 min at room temperature and then transferred into a Teflon hydrothermal reactor to be heated at 120 °C for 5 h in an oven. The black powder obtained was collected and purified by repeated centrifugation, washed several times with deionized water, and then dried at 120 °C for 5 h. 2.3. Preparation of the Fe3O4@SiO2/KCC-1 The Fe3O4 nanoparticles were covered with silica particles to permit further growth of fibrous silica and were prepared as described in an earlier work with minor modifications [36,37]. Firstly, 2 g of Fe3O4 NPs were dispersed in a mixture composed of 200 mL of ethanol, 50 mL of deionized water, and 10 mL of ammonia solution and the mixture was stirred for 60 min following which, 4 mL of TEOS were added dropwise. The mixture was then stirred at room temperature for 6 h, following which, the solid obtained was collected by filtration, washed thoroughly with deionized water and ethanol, and then dried. To make the Fe3O4@SiO2/KCC-1, 2 g of the Fe3O4@SiO2 powder obtained above were dispersed in 120 mL of an aqueous solution containing 2.4 g of urea and 4 g of CTAB. The suspension was then added to a mixture composed of 6 mL of 1-pentanol, 120 mL of cyclohexane, and 10 g of TEOS. The resultant mixture was stirred at room temperature for
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1 h and then transferred into a sealed stainless steel autoclave. Heat was applied at 120 °C for 5 h in an oven, after which the autoclave was cooled to room temperature and the Fe3O4@SiO2/KCC-1 obtained was isolated by filtration, washed several times with deionized water and ethanol, dried overnight in a vacuum drying oven at 150 °C.
drophobic material to the epoxy, the dispersal was then pressurized on the top of the surface. The epoxy coating was cured at 40 °C for 2 days, following which, the excess of Fe3O4@H-SiO2/ KCC-1 that was not attached to the surface was removed using flowing deionized water and ethanol, and the surface was then dried at 50 °C for 4 h.
2.4. Preparation of the Fe3O4@H-SiO2/KCC-1
3. Characterization and measurement
Prior to silanization, the Fe3O4@SiO2/KCC-1 NPs were dried for 2 h at 80 °C. According to the usual practice, 20 g of Fe3O4@SiO2/ KCC-1 were dispersed in 100 mL of dry toluene and stirred for 1 h at room temperature. The next step was to add 40 mL of trimethoxy(octyl)silane and reflux the resulting mixture for 2 days as depicted in Scheme 1. The solution was then left to be cooled at room temperature, and the powder obtained was isolated by centrifugation, thoroughly washed with toluene and ethanol, and then dried in an oven at 120 °C for 6 h so that it could be systematically characterized.
Thermogravimetric analysis- Derivative Thermogravimetry (TGA-DTG) was performed with a TA Instruments SDT Q600 apparatus under air at a heating rate of 10 °C/min. X-ray diffraction (XRD) of the samples was performed at room temperature using a Rigaku Ultima IV X-ray diffractometer equipped with Cu Ka radiation in Bragg-Brentano geometry (h–2h). To determine the surface wettability, the contact angles of water droplets (4 lL) on the surface coating were measured using an optical tensiometer (Attension Theta, Biolin Scientific, Germany) at room temperature and analysis was performed by fitting drops with a Young-Laplace formula using the Theta Software. Magnetic properties were measured at room temperature using a vibrating sample magnetometer (VSM) (Lake Shore Company). Scanning electron microscopy (SEM) analyses were recorded on a FEI Helios NanoLab microscope. SEM aluminum stub holders with conductive carbon tape were used for fixing the powdered materials. The SEM was operated in low vacuum mode with varied water vapor pressure in order to reduce charging on imaging quality during analysis. Transmission electron microscopy (TEM) analyses were recorded on a Tecnai Twin TEM microscope operated at a 120 kV accelerating voltage. The chemical composition was analyzed using an energy disper-
2.5. Preparation of the superhydrophobic magnetic epoxy coating A schematic illustration of the fabrication process is shown in Scheme 2. Briefly, 10 g of acetone were mixed with 4 g of diglycidyl ether of bisphenol A epoxy resin. Next, 2 g of isophoronediamine were added, and the resulting solution was thoroughly mixed. The epoxy pre-coating was applied on a pressure-cleaned carbon steel substrate by hand brushing, and Fe3O4@H-SiO2/KCC-1 powder was then dispersed onto the epoxy surface coating using a 450 lm sieve. To enhance the adhesion of the magnetic superhy-
Scheme 1. Schematic representation of the Fe3O4@H-SiO2/KCC-1 preparation process.
Scheme 2. Schematic illustration of the preparation of the magnetic superhydrophobicity epoxy surface.
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sive X-ray spectrometer (EDS) embedded on the TEM microscope. The diameter of particles was statically analyzed using ImageJ free software and the distribution histogram is obtained by measuring at least 160 nanoparticles.
4. Results and discussion The entire process was conducted under mild conditions, and no complicated procedures were needed. For this proposed procedure, we took advantage of the nature of the Fe3O4 nanoparticles which have a high density of hydroxyl groups estimated to be in the range of 5.1–9.8 OH groups per nm2 [38]. The active hydroxyl groups on the surface of the Fe3O4 nanoparticles, were reacted with the hydrolyzed acidic moieties formed by the TEOS during the hydrolysis stage. At this stage the presence of ammonia increases the pH value, thus allowing the polymerization of the silica, which occurs through the condensation of the silanol groups (Si-OH) and begins the formation of the first coating layer of silica, based on the creation of covalent siloxane bonds (SiAOASi) [39]. After being washed and dried, the Fe3O4@SiO2 along with CTAB as a directing agent, was used to be covered with a mesoporous silica shell. Alkyl chains were then grafted onto the mesoporous shell to render it superhydrophobic, with superior properties and durability under a variety of corrosive conditions. Fig. 1(a) shows that the several diffraction peaks at 2h values of 18.278°, 30.066°, 35.413°, 37.044°, 43.038°, 53.391°, 56.914°, and 62.497° of the corresponding XRD pattern of the Fe3O4 NPs match the 111, 220, 311, 222, 400, 422, 511, and 440 planes, respectively, of the Fe3O4 cubic inverse spinel structure. The XRD pattern is in good agreement with the data for pure cubic Fe3O4 as reported in JCPDS number card: 01-076-1849 and are consistent with those reported in the literature [40,41]. The sharp diffraction peaks confirm the formation of a pure phase of Fe3O4 with excellent crystallinity and no characteristic impurity peaks were observed under the experimental conditions in place. The other XRD diffraction pattern, Fig. 1(b) indicates that the Fe3O4@SiO2/KCC-1 has diffraction peaks similar to those of the parent Fe3O4 NPs with an additional broad peak at 21.364° corresponding to the amorphous silica. These findings indicate that the silica was successfully coated onto the surface of the Fe3O4 nanoparticles and that the coating later retained its phase structures despite being calcined at 550 °C. Thermogravimetric analysis was employed in order to examine the thermal stability of Fe3O4 and Fe3O4@H-SiO2/KCC-1 samples for temperatures ranging from room temperature to 800 °C; the results are displayed in Fig. 2. For the uncoated Fe3O4, the TGA curve showed three weights losses of about 4% from room temper-
Fig. 1. XRD patterns of Fe3O4 (a) and Fe3O4@H-SiO2/KCC-1 core-shell nanoparticles (b).
atures up to 455 °C. The first weight loss at 25–200 °C can be attributed to the evaporation of surface bonded water. The second weight loss. The second weight loss occurred between 210 and 340 °C can be attributed to the conversion of the magnetite into hematite. It important to stress that it was reported that the magnetite converted into maghemite at around 270 °C which will be then transformed into hematite at around 320 °C [42]. This oxidation process is expected to increase the sample weight [43]. In addition, the small weight loss observed at 450 °C can be attributed to release of the pore bound water [44]. In contrast, two different weight loss regions can be identified for the Fe3O4@H-SiO2/ KCC-1. The first was characterized by rather slow weight loss up to 230 °C due to the evaporation of physisorbed water and pore bound water, whereas in the second region, the major weight loss of 5.2% observed between 230 °C and 600 °C can be attributed to the conversion of magnetite to hematite and to the loss of the covalently bonded octylsilane group [45]. This weight loss does not occur at a specific temperature since the organic silane groups decompose slowly from the silica surface. The magnetic hysteresis loops of the Fe3O4 and Fe3O4@H-SiO2/ KCC-1 are shown in Fig. 3. It is clearly evident that, for both samples, magnetization increased quickly with an increase in the magnetic field and then stabilized at a magnetic field of 800 Oe. The Fe3O4 exhibited a saturation magnetization of 47.98 emu/g which is lower than that observed for magnetite with a size of 37 nm (92 emu/g) [35,46]. The difference in magnetization can be correlated with the size of the magnetite particle. In fact, it has been reported that a reduction in Fe3O4 particle size leads to a decrease in the saturation magnetization [35]. This phenomenon, which has already been observed in many other magnetic nanoparticles, can be explained by the presence of a magnetically inert layer on the
Fig. 2. TGA and DTG curves of Fe3O4 (a) and Fe3O4@H-SiO2/KCC-1 (b).
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Fig. 3. Magnetic hysteresis loops for Fe3O4 (a) and Fe3O4@H-SiO2/KCC-1 (b).
surface of the nanoparticles [47]. Saturation magnetization is controlled by the surface effect directed by an effective volume, which is a fraction of the nominal volume. As the particle size decreases, the magnetic effective volume accounts for a smaller proportion of the total volume, thus reducing the specific saturation magnetization. The saturation magnetization decreased dramatically for the Fe3O4@H-SiO2/KCC-1 sample to 17.89 emu/g. This drop in the saturation magnetization value after coating of the grafted silica onto the Fe3O4 surface can be attributed to the presence of the silica shell around the core [48]. Another factor that might be responsible for lowering magnetization level is that part of the silica is bonded onto the surface of the magnetite particles through the FeAOASi chemical bonding [49]. The magnetic moment of the iron ions bonded with the silicon on the surface of the magnetite NPs disappeared through the formation of the FeAOASi bonds, thus leading to a decrease in magnetism following the silica coating [50]. This finding can be considered evidence of successful silica coating on the magnetite NPs. The literature includes reports of similar findings that indicate a diminished magnetization value after magnetic particles have been coated with silica [51–53]. On the other hand, the coercivity (Hc) of the hydrophobic iron oxide/silica core-shell NPs was 81.94 Oe, which is lower than the coercivity of the as-synthesized Fe3O4 NPs (84.68 Oe). This effect
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could be caused to the formation of an interfacial structure, and similar magnetic behavior has been observed for SiO2/Fe2O3 nano-architectures [54,55]. The microstructure of the as-prepared magnetite was also analyzed using TEM. The particles have polygonal shapes. Fitting the particle size histogram to a smooth curve, a mean particle diameter of 22 nm is obtained as depicted in Fig. 4. SEM was used for studying the morphology of the surface of the Fe3O4@H-SiO2/KCC-1 nanoparticles as illustrated in Fig. 5(a). The SEM data acquired showed that the material consisted of uniform and well-defined spherical particles with a particle size ranging from 50 nm to approximately 200 nm. Furthermore, the micrographs obtained from TEM analysis indicate that the material consisted of dendrimeric fibers spreading out in radial manner and distributed flower‐like in all directions as illustrated in the Fig. 5 (b). Further analyses involving the application of EDS were conducted in order to prove the presence of the different components of our samples. The EDS analysis verified the co-existence of both the silica and magnetite nanoparticles (Fig. S1, supporting information). Importantly, the SEM of the prepared superhydrophobic surface revealed that silica coated magnetite nanoparticles were with different sizes and shapes randomly stacked over the substrate and made densely packed protrusions after curing. Additionnaly, Fe3O4@H-SiO2/KCC-1 particles became larger because they were agglomerated (Fig. S2, supporting information). Contact angles were also measured as a means of evaluating the wettability of the produced samples. Because of the presence of hydroxyl functional groups, the contact angle of the control sample coated only with cured epoxy was 65°. The measured contact angles of the prepared superhydrophobic Fe3O4@H-SiO2/KCC-1 core-shell surface prepared were around 175°. Obtaining, the wettability measurements of this magnetic superhydrophobic surface, in fact, was difficult because water droplets rolled off the surface toward the sides very easily and rarely stabilized. The measurements were collected by carefully placing the droplets of water in a perfectly flat region of the sample. The wetting properties of the superhydrophobic magnetic surface were then compared to those of the hydrophobic silica nanoparticles without a magnetic core prepared by the Ströber and hydrothermal processes and grafted with trimethoxy(octyl)silane using same procedure described in this work. Interestingly, the contact angles measured were found to be 135° and 154°, respectively, as illustrated in Scheme 3. The Wenzel and Cassie–Baxter models are two classical models generally used for describing the wetting properties of
Fig. 4. TEM of as-prepared Fe3O4 nanoparticles and their particle size distribution.
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Fig. 5. SEM (a) and TEM (b) images of Fe3O4@H-SiO2/KCC-1 nanoparticles.
Scheme 3. The contact angles of superhydrophobic silica (a) and (b) prepared by the Stöber and hydrothermal processes, respectively, and those of magnetic superhydrophobic silica (c).
superhydrophobic surfaces [56,57]. Both models portray surface structure and chemical composition as together being the determinant factors in surface wetting. In our study, the ultra-water repellency of this magnetic superhydrophobic surface was most probably caused by the low surface energy of the hydrocarbon chains and by the formation of the SiAOASi network, as well as by an increased surface roughness due to the dendritic fibrous surface morphology present in silica particles as revealed by TEM analysis [58]. The long-term stability of superhydrophobic coatings is a crucial parameter for the determination of their performance and for the feasibility of their industrial application. In this study, the chemical stability of the prepared superhydrophobic coating was therefore evaluated based on immersion in a 3.5 wt% NaCl aqueous solution. The evaluation entailed an examination of the relationship between the water contact angle and the immersion time as indicated in Fig. 6. It is clear that the contact angle measured on the superhydrophobic surface decreased slightly over time, but a contact angle of 165.9 ± 1.7° was still maintained after 40 days of immersion. Additionally, the sliding angles values always remain below 5°, which indicates the low adhesion of the as-prepared superhydrophobic surface. From a wettability perspective, these results demonstrate the good durability and hydrophobicity of the as-prepared surface in sodium chloride environments, as 40 days of immersion had no influence on the surface states. The chemical stability of the prepared superhydrophobic surface was also evaluated under two different pH conditions namely
pH = 1, and pH = 14 as can be seen from Fig. 7. Hydrochloric acid and sodium hydroxide were used for adjusting the pH of the solutions. Similar stability to the salt aqueous solution were obtained
Fig. 6. Change in the contact angle as a function of the immersion time of the asprepared superhydrophobic magnetic surface in a 3.5 wt% NaCl aqueous solution.
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Fig. 7. Variation in the contact angle of the as-prepared superhydrophobic magnetic surface for different pH values.
where a high contact angle in the range of 166.5 ± 1.4° were maintained for 24 h which indicates that the prepared superhydrophobic surface also possesses also excellent chemical stability with respect to both acidic and alkaline environments. 5. Conclusion In summary, a superhydrophobic magnetic material based on fibrous mesoporous Fe3O4@H-SiO2/KCC-1 nanoparticles with a core-shell structure was successfully prepared using a simple approach. The structural characterizations of the produced particles revealed a dendrimeric fibers morphology spreading out in radial manner as flower‐like. To increase roughness and reduce surface energy, the Fe3O4@HSiO2/KCC-1 core-shell nanoparticles were uniformly dispersed on the surface of an epoxy resin coating. The resulted superhydrophobic magnetic surface exhibits outstanding superhydrophobic properties and good stability as well as good durability under different corrosive conditions. This could be explained by the co-existence of low surface energy and the high surface roughness caused by the grafted hydrophobic alkyl chains due to the dendritic fibrous surface morphology and the flexible fibrous structure the fibrous silica. This could was be explained by the co-existence of low surface energy and high surface roughness caused by the excellent accessibility of grafted hydrophobic alkyl chains and by the dendritic fibrous surface morphology present in silica. This facile and low-cost approach developed in this work may also be expanded for the preparation of other magnetic superhydrophobic organicinorganic hybrid materials that can have several applications such oil-water separation, novel easy-clean coatings and in electronic. The use of these ferromagnetic material as fillers to enhance the adhesion and the corrosion protection of polymeric coatings are underway in our laboratory. Acknowledgments We are very thankful to Juhani Haitham, Hawraa Bin saad and Nada Qari for their day to day aid in laboratory life. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.11.042.
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