Fabrication of Hollow Silica Particles Using a Self-Assembled Polyethylene Granule as a Template

We developed a novel method preparing nonspherical hollow silica particles (HSP) using a micron-sized granule self-assembled from partially oxidized PE wax. The morphology of the granule was closely investigated in terms of concentration and acid value of PE wax and cooling rate. Due to the oxidized unit in PE wax, magnetic nanoparticle was incorporated into the granule during the self-assembly, and silica was coated on the granule surface via the self-assembly. Silica-coating condition was studied by varying water content and reaction time. After the PE wax was removed by calcination, nonspherical HSP or magnetic HSP was obtained. This cost-effective HSP is expected to be useful for practical applications.

Spherical HP has been fabricated via oil-in-water emulsion [15][16][17][18], hydrothermal synthesis [19], and self-assembly techniques [20] using a template soft enough having spherical morphology to minimize interfacial energy.By comparison, nonspherical HP was fabricated using an anisotropically grown or clustered structure as a template.For example, hematite (α-Fe 2 O 3 ) synthesized in various morphologies such as spindle, cube, rod, ellipsoidal, peanut-like, and polyhedron was frequently used as an inorganic template [21][22][23][24], and polystyrene colloid clusters having polyhedral structures were used as an organic template [25].Nonspherical shape is favorable for enhanced physicochemical properties mainly due to a higher specific surface area compared to a spherical one.Nonspherical titanium oxide [14] and NiS HP [26] showed outstanding properties such as a high specific capacitance and good cycling stability.
Considering practical applications of HP, there are some requirements in the template formation as for cost-effective HP fabrication.Surface of either inorganic or organic template should be suitable for inorganic coating usually based on sol-gel reaction.Otherwise, additional surface treatments need to be done in advance.For example, carbon nanotube template for preparing hematite hollow nanohorns needs strong acid treatments for creating surface-active groups [27].For surface activation, PS colloid was prepared from polymerization of PS monomer mixing with a silane coupling agent [28] or PS colloid was treated by ammonia [29].
Another consideration is the efficiency of template removal.In most cases, template removal is required for creating vacancy inside, which can be done by processes such as calcination and solvent and acid washing.Usually, organic templates are easily removed by calcination in air or solvent washing while inorganic templates such as hematite require strong acid washing.It seems that calcination process is more cost-effective.
Self-assembly process of polyethylene (PE) crystallization has been known since the 1960s [30].Spherulite structure with few tens micron in size can be obtained from its melt state by a cooling process which originated from stacking of lamellae with interlamellar thickness of ~12 nm [31].Single lamella was obtained from cooling of hot and dilute PE in various solvents [32].Recently, we found that PE wax could form micronsized ellipsoidal granule composed of multilamellae from a concentrated solution by controlling a cooling rate [33].
In this work, we have proposed a novel approach to synthesize HSP using ellipsoidal PE granule as a template.We fabricated uniform micron-sized and nonspherical PE granule via self-assembly of partially oxidized PE wax in solution, which was successively coated by silica based on a sol-gel reaction without additional surface treatments.By calcination, nonspherical HSP was obtained.It is believed that this is the most cost-effective way to prepare nonspherical HSP.Furthermore, magnetite nanoparticle was incorporated in the granule during the self-assembly so that the resulting HSP is capable of being attracted to the magnet.As a potential application, efficient removal of toxic chemical from the waste water was demonstrated.
2.2.Ellipsoidal HSP Formation.PE wax was mixed with toluene and heated at about 100 °C until the solution become clear.Then, the solution was cooled down to room temperature with a controlled cooling rate using a water bath.PE granule was collected by centrifugation at 6000 rpm for 10 min and washed 3 times using ethanol.Silica coating was done via a sol-gel method known as a Stöber method [34].The PE granule was dispersed in ethanol, and then TEOS, deionized water, and ammonia solution were successively added while mild stirring at room temperature.After the reaction, the silica-coated PE granule was washed 3 times with ethanol and dried.To obtain HSP, PE granule was calcinated at 500 °C for 2 hr.
2.3.Characterization.Wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements were conducted at PLS-II 9A U-SAXS beamline of Pohang Accelerator Laboratory (PAL) in Korea.Diffraction angles were calibrated by a precalibrated sucrose [35].And the sample-to-detector distance was about 225 mm and 2.5 m for WAXD and SAXS, respectively.The X-ray wavelength was 0.6218 Å. WAXD and SAXS patterns were recorded with a 2D CCD detector (Rayonix SX165), and X-ray irradiation time was few seconds dependent on the saturation level of detector.

Results and Discussion
3.1.Morphology of Micron-Sized PE Granule 3.1.1.Acid Value of PE Wax.Even though the morphology of single lamella has been extensively studied from the dilute solution and the melt [31,32], self-assembly of PE wax having broad molecular weight distribution from a nondilute solution has been rarely studied, in particular, not for partially oxidized PE wax.The oxidation degree of PE wax can be defined by acid value where PE wax possesses oxygencontaining functional groups such as carboxyl, hydroxyl, and carbonyl, expecting reduction in crystallinity of selfassembled structures.
As shown in Figure 1(a), the hexagonal-shaped granule was formed from RC1 solution where the concentration and cooling rate were 1.0 wt% and 10 °C/min, respectively.The dimension of the granule was found to be few hundred nm in thickness and 2-3 μm in size, indicating that hundreds of single lamella with ~12 nm in thickness were stacked to form multilamellar structure due to strong van der Waals interaction between lamellae [33].It is noted that this crystallization and stacking process takes only few minutes.As the acid value increases, the granule shape became more rounded and ellipsoidal as shown in Figures 1(b) and 1(c).
WAXD spectrum of each granule is shown in Figure 1(d).There are two characteristic peaks of PE granule corresponding to 110 and 200 which moved toward a lower angle as the acid value increases, meaning larger interspacing.From the peak area, the crystallinity of each PE granule also was calculated, which is 79.2% for RC1, 74.6% for RC2, and 68.2% for RC3, respectively.As expected, the crystallinity decreases as the acid value increases.
The SAXS spectra and 2D scattering images of each PE template are shown in Figure 1(e).Interlamellar distance of the PE granule was calculated to 11.3 nm (RC1), 10.8 nm (RC2), and 10.3 nm (RC3) using Bragg's law, which became thinner as the acid value increased.
Silica coating has been done on each granule using a solgel method.As shown in the inset of Figures 1(a)-1(c), there was no silica formed on RC1 granule surface, and silica particles were sparsely attached on RC2 while on RC3 granule surface silica, silica particles are densely covered.As expected, RC1 having 0 acid value was not able to be a template for silica coating because the surface has a very low 2 Journal of Nanomaterials surface tension, that is, no polarity.As a template for HSP, the surface should be adequate for silica coating based on a sol-gel process.The granule prepared from PE wax with certain acid value can be a template for silica coating on which oxygen-containing groups exist to be served as active surface that can be wetted by TEOS.From now on, we focused on RC3 for fabricating HSP.In the figure, we clearly see that the granule is composed of many plies, indicating that a single lamella stacks face-toface to form the granule.From the interlamellar spacing obtained from SAXS, the granule is composed of ~60 lamellae in case of 1.0 wt%.It is noted that lamellar stacking is faster on the edge of a granule at high concentrations, resulting in a recessed structure in the center.

Cooling Rate.
Cooling rate is also an important factor to control granule morphology.Figures 2(e)-2(h) show the SEM images of the RC3 granule with a cooling rate of ~150, ~15, ~7, and ~3°C /min, respectively.In case of a fast cooling rate of 150 °C/min, thin and flexible granule was formed.As the 3 Journal of Nanomaterials cooling rate decreased, the granule size became bigger.From the WAXD spectrum, crystallinity of a granule was measured to 71.5% and 64.2% for ~3 and ~15 °C/min of cooling rate, respectively, indicating that a slower cooling rate induced not only bigger crystal but also higher crystallinity.
The slower cooling rate effect on the granule morphology was similar to the higher concentration.However, it seems that controlling the cooling rate provided more regularity and reproducibility in regulating granule morphology than concentration.Therefore, the experimental condition for  granule formation using RC3 was chosen to be a concentration of 1.0 wt% with a cooling rate of 10 °C/min, which will be applied for HSP fabrication.

Magnetite Nanoparticle Trapping.
During the multilamellar self-assembly, quantum dot can be trapped in between lamellae [33].Here, magnetite nanoparticle (Fe 3 O 4 , diame-ter~20 nm) was added when PE wax granule was selfassembled. Figure 3(a) shows that the particles (30 wt% based on PE amount) are trapped inside the ellipsoidal granule and evenly distributed.The cross-sectional TEM image (thickness of the slice for the TEM sample was about 100 nm) on the right provided that the particles exist only in between the lamellae, implying that the nanoparticle was entrapped after the single lamella formation depicted as Figure 3(b).After the self-assembly, the solution was centrifuged to get granules, and the supernatant was found to be clear, confirming that all the particles were trapped in the granule during the self-assembly.We found that PE wax with no acid value (RC1) could not trap any particle inside, which indicates that entrapping nanoparticles is mainly due to polar interaction between oxygen-containing functional groups on the lamella and the nanoparticle.Figure 3(c) shows 10, 30, and 50 wt% magnetite nanoparticle trapped granules in toluene solution, and all the granules were attracted to a magnet.

Hollow Silica Particle (HSP) Formation
3.2.1.Water Concentration.The morphology of silica particles based on the sol-gel method was affected by various factors such as the precursor, catalyst, reaction time, and reaction temperature.Water content is important for initiating hydrolysis determining the initial reaction rate and condensation kinetics, which is related to silica colloid size in the reaction [36].A granule made from RC3 dispersed in ethanol was used for silica coating.The reaction condition was that the concentration of TEOS, ammonia, and reaction time was fixed to 0.3 M, 1.0 M, and 2 hr, respectively, while the concentration of water changed from 0.82 to 7.2 M. By observation, the reaction solution became opaque within a few minutes for water content of greater than 2.2 M while for the lowest water concentrations, turbid solution started to appear after an hour.Therefore, a reaction time of 2 hr was chosen to consume TEOS as much as possible for finding water effect.From the SEM image of silica-coated granule in Figures 4(a)-4(d), silica particle was uniformly coated and grew on the surface of the granule.Silica particle size on the granule was measured from the SEM images and displayed 3.2.2.Effect of Reaction Time.The effect of the sol-gel reaction time on the morphology of silica was also monitored.The reaction time was controlled from 0.5 hr to 4 hr while the concentration of TEOS, ammonia, and deionized water was fixed at 0.13 M, 1.0 M, and 3.1 M, respectively.Figures 5(a)-5(c) showed the SEM pictures of silica-coated granule and, after calcination, broken pieces of hollow silica particle (HSP) which was deliberately broken to check silica-coating thickness, d. Figure 5(d) summarized d as a function of the reaction time.At 0.5 hr, ~50 nm silica particle was coated on the surface and d of HSP was found to be ~190 nm.For 2 and 4 hr, d was ~320 nm and ~410 nm, and the particle size on the surface was getting bigger while smaller particles are still found underneath.In summary, silica coating on the granule underwent a process in which small particles grew on the surface and bigger particles grew on the next layer as the reaction time, in which behavior is similar to colloidal silica growth mechanism as previously reported [38].
3.2.3.Calcination.Figure 5(e) showed the TGA thermograms of the RC3 granule with and without silica coating where silica-coating condition was fixed to be at a concentration of TEOS, ammonia, and deionized water and reaction time of 0.13 M, 1.0 M, 2.0 M, and 2 hr, respectively.The granule started to lose weight at 400 °C and was totally removed at 500 °C while the silica-coated granule left its weight in half, indicating that coated silica amount is similar to the granule weight.Therefore, calcination condition to prepare HSP was fixed to 500 °C for 2 hr in the air environment, which was applied to the samples in Figures 5(a)-5(c).TEM image of the HSP is shown in Figure 5(f) where it is confirmed that a hollow space exists inside and 40-70 nm silica particles are densely packed on the surface.
3.2.4.Magnetic HSP (mHSP).Figure 6(a) shows the SEM and TEM images of silica-coated magnetite nanoparticle trapped RC3 granule after calcination.From the TEM image, it is found that the nanoparticle was thoroughly adsorbed in the silica shell instead of cluster formation.The calcination process could induce to transform Fe 3 O 4 (magnetite) to α-Fe 2 O 3 (hematite) or γ-Fe 2 O 3 (maghemite) [39].We checked the Fe 3 O 4 particle crystal structure using XRD after annealing as shown in Figure 6(b).Here, 50 nm Fe 3 O 4 particle was used.After annealing at 300 °C for 2 hr, all the peaks were shifted to 7 Journal of Nanomaterials 0.2 °to the right indicating that most of the magnetite phases seemed to be transformed into maghemite because both crystals have the same spinel ferrite structure with slightly different spacing.After annealing at 500 °C for 2 hr (calcination condition for removing PE granule), XRD peaks indicated that there was almost no magnetite left in the sample, transforming to hematite and maghemite mixtures.This HSP incorporated with hematite and maghemite nanoparticles is called magnetic HSP (or mHSP).
As one of the HSP applications, removal of toxic chemical such as iodine contaminated in water has been suggested [40].We duplicated the iodine removal test according to the reference using mHSP.Figure 6(c) shows that iodine in the water phase was transferred to butyl acetate phase filled with mHSP, and then the mHSP was easily separated by attaching a magnet.Therefore, this magnetically attractable HSP could be useful for toxic chemical removals.

Conclusion
In this work, we have proposed a novel method to synthesize hollow silica particles (HSP) using a polyethylene granule as a template for silica shell.We fabricated micron-sized and nonspherical PE polyethylene granule via self-assembly of partially oxidized PE wax, which was successively coated by silica based on a sol-gel reaction without additional surface treatments.Furthermore, magnetite nanoparticles were entrapped in the granule during the self-assembly so that the resulting HSP was capable of being attracted to the magnet extending its potential applications such as toxic chemical removal from the waste water.It is believed that this process is the most cost-effective way to prepare HSP.

3. 1 . 2 .
Concentration.The morphology of the self-assembled granule should be dependent on PE wax concentration.Figures2(a) and 2(b) show the SEM images of granules self-assembled from RC3 with a concentration of 0.1, 1.0, 2.0, and 3.0 wt%.The cooling rate was fixed at 10 °C/min.The average size of the granule increased from 1.8 to 7.5 μm as the concentration of solution increased.And average thickness of the granule was measured to ~100 nm, ~0.7 μm, ~2.0 μm, and ~4.0 μm, respectively.

Figure 1 :
Figure 1: SEM images of the granule from (a) RC1, (b) RC2, and (c) RC3.(d) WAXD and (e) SAXS pattern and its corresponding 2D diffraction images of each crystal.The concentration and cooling rate for crystal formation were fixed at 1.0 wt% and 10 °C/min, respectively.Insets in (a)-(c) show the silica-coated granules.

Figure 3 :
Figure 3: TEM images of (a) 30 wt% Fe 3 O 4 nanoparticle entrapped PE wax granule and its cross-sectional view.(b) Schematic diagram for Fe 3 O 4 nanoparticle entrapped between lamellae.(c) 10, 30, and 50 wt% Fe 3 O 4 nanoparticle entrapped PE granule in toluene solution before and after the magnetic field was applied.

Figure 4 :Figure 5 :
Figure 4: SEM images of silica-coated RC3 template as a function of water concentration of (a) 0.82 M, (b) 3.1 M, (c) 5.9 M, and (d) 7.2 M. (e) Silica particle size is displayed according to water concentration.

Figure 6 :
Figure 6: (a) SEM and TEM images of silica-coated RC3 granule incorporated with Fe 3 O 4 nanoparticle after calcination at 500 °C for 2 hr.(b) XRD spectra of 50 nm Fe 3 O 4 nanoparticle before and after heat treatment in air.(c) mHSP separation by a magnet after iodine absorption from the iodine contaminated water.

Table 1 :
Physical properties of polyethylene waxes used.