1. Introduction

In the last decades, composites are playing a vital role in synthesizing new materials. Composites provides high quality mechanical and physical properties because it combines the most desirable properties of its constituents while suppressing their least desirable properties [1]. The attractive properties shown by the composites are high thermal or electrical conductivity [2a], a low coefficient of thermal expansion [2b], adsorbent [2c, d, e], surfactant [2f], catalytic [2g], photo catalytic [2h] and elastic properties [2h]. The properties shown by the composites could not be achieved by either of the constituent materials alone.

In recent years, Silica coated Iron oxide composites have attracted a great deal of attention in the field of chemistry and applied sciences. As a result extensive study have been carried in the last few years globally on it [3, 4]. In the Silica coated Iron oxide composite, usually SiO₂ matrix structure plays the role of host for Iron oxide fine particles, which nucleates in the Silica pores, conveniently. High performance technological and scientific and operational applications such as magnetic technology [5-7], catalysis [8], sensors [9-11], optics [12], adsorbent [13] properties has widen the research on the Silica coated Iron oxide composite. Hence Silica-coated composite nanoparticles (Fe₃O₄@SiO₂/ core-shell) have been synthesized and studied by many groups [14-17]. Recently, Silica coated magnetite composite have been successfully applied to extract some heavy metals, like Cd²⁺, Cu²⁺, Hg²⁺, and Pb²⁺ from water in a wide pH range [18].

Different physical methods, chemical methods and electro-chemical methods have been using to synthesize the Silica-Coated composite materials [19]. In the present investigation, we focused our attention on the preparation of Silica coated Iron-oxide composites by Sol-Gel method because it offers wide range of advantages over other techniques [20-21]. Despite of all these, scientists all over the world are still trying to develop a technologically simple, cost effective and the most versatile sol-gel synthetic route for preparation of specific type of composite materials with desired chemical composition.

In this work, the Silica coated Iron-oxide composite particles of different ratios were synthesized through two steps. At first, the hematite particles were prepared by electrochemical method [22] and then the composite material were synthesized through the modified sol-gel method using the prepared hematite and tetraethylorthosilicate (TEOS). TEOS was used as the precursor of Silicon (IV) oxide. The precursors and composite materials were characterized with Fourier-transform infrared spectroscope, Simultaneous Thermal Analyzer (TGA/DTA) and X-Ray Diffractiometer (XRD). The compatibility and morphology of the materials were investigated by using the scanning electron microscope. Composite materials prepared by different method can be used for the removal of different heavy metals from wastewater. Thus we can use composite materials for environmental applications.

2. Experimental

2.1. Materials Used

All the chemical reagents used were of analytical grades. Double Distilled Water (DDW), pure ethanol (Merck, Germany) and analytical grade acetone (Merck, Germany) were used as solvent. Other reagents were Tetraethylorthosilicate (TEOS) (Aldrich, Germany), Hydrochloric acid, HCl (Merck, Germany), Sulphuric acid, H₂SO₄ (Merck, Germany), Phosphoric acid, H₃PO₄ (Merck, Germany), Ammonium Hydroxide (Merck, India), Sodium Chloride (Merck, India) and Sodium Hydroxide (Merck, India).

2.2. Electrochemical Preparation of Iron Oxide

The electrochemical experiment was carried out in a two-electrode electrochemical cell containing iron as sacrificial electrodes. The experimental set up included the electrode assembly, a DC power supply unit, a voltage stabilizer, a resistance box to regulate the current and a multimeter to read the current values. The electrode assembly consisted of pairs of iron plates (7.0 cm × 7.0 cm × 0.15 cm) placed in parallel arrangement. A photograph of the experimental set up is shown in Figure 1 which is based on the diagram reported by Mollah et al [22]:

Figure 1(a). Schematic diagram of the cell. 1. Anode, 2.Cathode , 3. Pyrex beaker, 4. Variable resistance, 5. Ammeter, 6. D.C. source

During the process, the electrodes were immersed in 0.025M aqueous NaCl (99.5%) solution. Electrolysis was then carried out using 1A current for about one hour and suspension of oxyhydroxides was generated into the electrolytic solution. At the end of each run, the suspension was kept in open air for 24 hours to settle completely and then filtered through a 2.5 μm pore size membrane filter and the recovered solid was washed with double distilled water until free from chloride. The solid mass was dried in an oven at a temperature of 120℃ for 24 hours.

Figure 1(b). Electro-coagulator used in the process

2.3. Preparation of Silicon (IV) Oxide

Silicon (IV) oxide was prepared by hydrolysis of tetraethylorthosilicate (TEOS) in ethanol. The ionic equation for the reaction between tetraethylorthosilicate (TEOS) and water in ethanol is:

A simple one step protocol was used for the preparation of silica, which involves with the hydrolysis and condensation of tetraethylorthosilicate (TEOS) in ethanol water mixture under acidic condition at room temperature. TEOS, water and ethanol were mixed at 1.00: 10.20: 3.85 molar ratio followed by 9.00 mL water containing 0.20 mL molar HCl [23]. It was then stirred constantly using a thermostated magnetic stirrer at ambient temperature for about one hour. The stirred mixture was then allowed to stand for two days for ageing so that gel formation was completed.

The gel of silicon (IV) oxide was collected as a residue by filtration. Distilled water was poured on the silicon (IV) oxide gel several times to ensure removal of chloride or any other undesirable ions from silicon (IV) oxide because the presence of impurity will cover up the surface and hence the actual surface area will decrease. The gel was dried in an oven at 110℃ for 4 hours. Silicon (IV) oxide thus prepared was preserved in a desiccator for characterization. Schematic representation of the process is shown below:

2.4. Preparation of Silica Coated Iron-oxide Composite by Sol-Gel Method

In a beaker, small amount of water was taken and reddish brown precipitate of iron oxide (prepared according to the process described in Section 2.2) was instantly added into it. Then the mixture was sonicated for about 30 minutes. After that the ethanol solution containing TEOS was added into that iron hydroxide. Dilute HCl solution was added to maintain the pH value of 5.0 and the mixture was sonicated for about 30 minutes. After sonication, the mixture was left for 24 hours at ambient temperature to form SiO₂-Fe(OH)₃ composite. The composite settled at the bottom of the beaker. Double distilled water was added to the composite and stirred for half an hour and then again left for 2 hours to settle the composite. This process was done to ensure complete removal of chloride and other undesirable ions from the composite. Water from the composite was evaporated by gently heating with a low flame and then the composite was heated in the woven at 300℃ for 6 hours to convert it into composite.

The silica coated iron-oxide composite thus prepared was preserved for characterization and subsequent uses.

All three samples of the composite of different ratios were prepared in the same process (only the molar ratio were maintained as 3:7, 1:1 and 7:3).

2.5. Characterization

Silicon (IV) oxide, Iron oxide and the composites prepared in this research were characterized by FT-IR, XRD, TGA-DTA and SEM –EDS techniques.

The molecular characterization were carried out by FT-IR spectrophotometer (Model: FT IR -8400/8900) using potassium bromide pellet. The phase composition of prepared silicon (IV) oxide, iron oxide and composites were analysed by an X-ray Diffractiometer. The XRD analysis of the samples were carried out by an X-ray Diffractiometer (XRD) (Model: Philips X'pert) with CuK_α radiation (λ = 1.54178 A⁰ ) source with a nickel filter. Thermo gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) of the samples were carried out using simultaneous Thermal Analyzer (DTA/TGA) (Model-SII EXSTAR6000 TG-DTA6300 Seiko Instrument Inc., Japan). Liquid nitrogen gas was used as a stripping gas. The samples of silicon (IV) oxide, iron oxide and silica coated composites were analysed by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Analyses were carried out on a JOEL JSM6490LA, (Japan) equipped with germanium detector and diamond window.

3. Results and Discussion

3.1. FT-IR Spectroscopic Analysis

Close examination of the Figure 3 (a) shows that major absorption bands of the Silicon (IV) oxide are in good agreement with the reported values of its authentic sample. Therefore, the FT-IR analysis supports that the sample prepared was Silicon (IV) oxide.

Table 3.1. FT-IR absorption bands of Silicon (IV) oxide with band assignments and reported bands of its pristine sample

Figure 2. Illustration of the silica coated iron-oxide composite preparation based on the diagram reported by Meera [24]

Figure 3(a). FT-IR spectrum of SiO2

Figure 3(b). FT-IR spectra of Iron-oxide

Study of the Figure 3 (b) reveals that the bands at 457 cm^-1 and 580 cm^-1 were assigned to the stretching and torsional vibration modes of the hematite Fe-O bonds in the tetrahedral and octahedral sites respectively, which are in good agreement with the results reported by Mollah M.Y.A [22]. The absorption bands at 3420 cm^-1 and 1630 cm^-1 are due to stretching and bending vibrations of O-H bonds in surface water molecules. Thus the absorption bands shown by the synthesized iron oxide are in good agreement with the reported absorption bands of hematite type of iron oxide. Therefore, the FT-IR analysis provides the evidence that hematite type of Iron oxide was electrochemically generated in this research.

Close examination of Figure 3 (c) shows that major absorption bands at 462 cm^-1, 795 cm^-1, 955 cm^-1, 1080 cm^-1, 1636 cm^-1, 2360 cm^-1 and 3420 cm^-1 shown by the Silica coated Iron-oxide composite were in good agreement with those shown by its constituents Silicon (IV) oxide and Iron oxide. These FT-IR results indicate the presence of both silicon (IV) oxide and Iron oxide in the composite.

FT-IR results of the composite (3:7) and (7:3) are shown in the Figure 3 (d) and Figure 3 (e) respectively which also indicate the presence of both silicon (IV) oxide and Iron oxide in the composite.

Figure 3(c). FT-IR of the Silica coated Iron oxide (1:1)

Figure 3(d). FT-IR of Silica coated Iron-oxide composite (3:7)

Figure 3(e). FT-IR of Silica coated Iron-oxide composite (7:3)

3.2. XRD Analysis

Figure 4 (a) shows the XRD analysis of the prepared Silica which indicates characteristics diffuse peak at about 20º. The result reveals that the silica particles are amorphous. This demonstrates that a high percentage of these particles are amorphous, but a few of them are crystalline. The result is in good agreement with the result reported by Meera basa [24] and Tabatabaei [26].

Figure 4(a). XRD of Silicon (IV) oxide

Figure 4(b). XRD of Iron oxide

Iron-oxide prepared by electrochemical method were characterized by XRD technique. Figure 4 (b) represents the X-ray diffraction spectra of iron oxides. The 2θ value of iron oxide sample is found to match with JCPDS-33-0664, confirming the formation of iron oxide.

Figure 4 (c) shows the XRD patterns of the thermally treated Silica coated Iron oxide composite. In the sample, besides the broad band characteristic for the presence of the amorphous silica gel, the amorphous iron-oxide was evidenced. This could be assigned to the bonding of the iron into Si-O-Fe-O-Si polymeric bonds that do not break at low temperatures of the thermal treatment. The results are in good agreement with the result reported by Raileanu et al [27].

Figure 4(c). XRD Pattern of Silica coated Iron oxide composite

3.3. TGA-DTA Analysis

TGA of SiO₂ (Fig.5a) shows gradual weight loss from 40° to 160℃ (11.0%), almost a steady state from 160℃ onward due to removal of free water (absorbed from atmosphere) and presence of any structural water.

Thermogram of Differential Thermal Analysis (DTA) (Figure 5a) shows an endothermic effect between 86-150℃ temperature ranges due to removal of structural water. The result is in good agreement with the result reported by Qingyin Wu [28].

Thermograph of iron oxide (Figure 5b) shows gradual weight loss from 26° to 176℃ (10.4%) and 176° to 378℃ (8.2%), almost a steady state from 378℃ onward. Weight loss may be due to removal of free water absorbed from atmosphere and presence of any structural water. Gradual weight loss from 250° to 400℃ (8.2%) was due to dehydroxylation for structural OH.

Differential Thermal Analysis (DTA) (Figure 5b) shows two endothermic peak at 94℃ and 246℃ temperature. The endothermic effect at 94℃ may be due to removal of physically bound water and the endothermic effect at 246℃ is due to removal of structurally bound water. Exotherm between 250° to 300℃ is due to recrystallization process involving transformation of smaller crystals into larger one. The DTA/TG result shown above for iron oxide is in good agreement with the data reported by Balek et al [29].

Thermograph of the silica coated iron-oxide (1:1) (Figure 5c) shows gradual weight loss from 26℃ to 290℃ (11.0%), almost a steady state from 290℃ onward. Weight loss may be due to removal of free water absorbed from atmosphere and presence of any structural water.

Figure 5(a). TGA/DTA of Silicon (IV) oxide

Figure 5(b). TGA/DTA of Iron Oxide

Figure 5(c). TGA/DTA of Silica coated composite

Differential Thermal Analysis (DTA) (Figure 5c) shows two endothermic peak at 80.4℃ and 250℃ and three exothermic peak at 160℃, 300℃ and 360℃. The endothermic effect at 80.4℃ and 250℃ may be due to removal of structural water. Endothermic peak at 341.9℃ corresponds to decomposition of goethite phase. Exothermic peak at 300℃ can be considered to be due to the growth of goethite (α-FeOOH), an intermediate step of hematite. Other exothermic peaks are due to recrystallization process involving transformation of smaller crystals into larger one. The DTA/TGA result shown above are in good agreement with the result shown by Ennas et al [30] and Palomares [31].

3.4. SEM and EDS

Figure 6(a). SEM & EDS image of Silicon (IV) oxide

Figure 6(b). SEM & EDS image of Iron oxide

Figure 6(c). SEM & EDS image of Silica Coated iron oxide composite

The SEM result of Silicon (IV) oxide shows that the particles are in aggregated form. EDS result suggests the presence of Si and oxygen elements, confirming the formation of SiO₂ particles. Theoretically Si-O ratio is 0.875 and experimentally the value is 0.72, which is very close to the expected value. EDS result indicate the presence of trace amount of Cl as impurities.

The above Figure 6 (b) represents the SEM and EDS images of iron oxide which suggests the presence of aggregated particles and small grains at the surface. However the elemental analysis suggests the presence of Fe and O atomic percentage that indicates the formation of the iron oxide. Theoretically Fe-O ratio is 2.33 and experimentally the value is 2.04, which is very close to the expected value. EDS result indicate the presence of small amount of Cl as impurities.

The SEM and EDS results of the prepared Silica coated Iron-oxide composite, shown in the above Figures 6 (c) indicate the formation of the composite. The EDS result indicates the formation of the composite of specific composition. Theoretically Fe-Si to O ratio is 1.75 and experimentally it is found as 1.99, which is close to the expected value, confirming the formation of Silica coated Iron oxide composite. The EDS result also indicate the presence of small amount of Cl as impurities.

4. Conclusions

Silica coated iron oxide composite of different ratios (3:7, 1:1 and 7:3) were prepared by Sol-Gel method. This chemical method provides a simple, general and cost effective process for the synthesis of Silica coated Iron-oxide. XRD and FT-IR results confirm that Hematite type of iron oxide and Tridymite (SiO₂) type of Silicon (IV) oxide were prepared in this research. Silica coated Iron oxide composites of varied ratios prepared by different method have been characterized by FT-IR, XRD, TGA/DTA, SEM and EDS techniques. Analytical results obtained by the above techniques lead us to conclude that Silica coated Iron-oxide composites were formed due to interaction between Silicon (IV) oxide and Iron oxide.

The composites synthesized in this work will have important applications not only in advanced materials but also in environmental fields such as adsorbent to remove toxic heavy metals from wastewater.

ACKNOWLEDGEMENTS

We express gratitude to Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh for providing the research facilities.