1. Introduction
The silica (SiO2) powder is noted as competent materials because of their unique characteristics (low density, high surface area and high specific strength) [1] [2] [3]. SiO2 is a very interesting material for using in many fields [4] [5]. And, it’s proving a promising material due to its good thermal and mechanical stability. In addition, it has many excellent physical and chemical properties [6] [7].
There’re various porous-SiO2 preparation techniques in previous studies [8] ; the synthesis of porous-SiO2 thin films is described with respect to pore structures [9]. It’s derived from the presence of active (OH-groups) on the particle surface. So, the (Si-O-Si) linkages between neighboring particles are produced as a result of the hydrogen-bond formation. In case of the (OH-groups) deactivation, the nanoparticles aggregation is stopped [7]. There are several chemical techniques used for controlling the particle shapes, and size of fine SiO2 particles [1] [8]. In this work, it demonstrated a combination between wet alkali chemical etching (ACE) process and ultra-sonication technique that controls the SiO2 shapes.
2. Materials & Methods
The combination of ACE process and ultra-sonication technique is attractive because of its simplicity. The preparation of porous-SiO2 different morphologies is started by 7 g commercial polycrystalline Si-powder (99%, Sigma-Aldrich). It’s added to the NPA (30 Vol %), KOHconc (3, 4.5, and 6 weight %) and at different time values (2, 3, 4, and 5 hours). The product is filtered, washed, and then slowly dried at room temperature. The structure of the nano-porous-SiO2 is characterized using X-ray diffraction (XRD; Cu Kα radiation of 1.5405 Å, Shimadzu 7000 diffractometer). The scanning electron microscope (SEM; JEOL-JSM-5300) and transmission electron microscope (TEM; JEOL, Japan), present the nano-porous-SiO2 morphology. In addition, the formation of nano-porous-SiO2 chemical bonds is determined by Fourier Transform Infrared spectroscopy (Shimadzu FTIR-8400s, Japan).
3. Results & Discussion
XRD is considered as a reference technique for the product powder characterization [10]. Figure 1 shows the diffraction features of the polycrystalline-Si, (SiO2/Si), crystalline-SiO2 and amorphous-SiO2 using a combination between ACE and ultra-sonication process [at different values of KOHconc and (t)]. Figures 1(a)-(c) show the diffraction patterns at 28.22˚, 47.19˚, 56.03˚, 68.97˚, 76.25˚, 87.93˚, and 94.8370˚ (JCPDS card nos. 01-079-0613 and 00-027-1402) [11], which correspond to planes (111), (220), (211), (400), (331), (422), and (511), respectively. As shown in Figure 1(b) (t = 3 hours), amorphous-SiO2 appears as a broadened peak at 2θ = 21.5˚ for producing (SiO2/Si). And, Figure 1(b) and Figure 1(c) (t = 4 hours) show the enhancement broadening peak that corresponds to the crystalline-SiO2 [12].
Finally, Figure 1(b) and Figure 1(c) (t = 5 hours), the SiO2-peak high intensity appeared at 2θ = 20˚ that corresponds to the complete transformation to amorphous-SiO2 [13]. Then, the preparation conditions during the crystalline-SiO2 formation are [4.5 and 6 weight % KOH at t = 4 hours]. But, the amorphous-SiO2 is formed at t = 5 hours. From the reported data, (t) has a great effect in the controlling process during the SiO2 type preparation.
Figure 2 shows the chemical bonding formation of SiO2 types; the broadening bands (820 and 1300 cm−1) assign to Si-O-Si asymmetric-stretching-vibrations [14], and the narrow bands (1089 and 1093 cm−1) correspond to the SiO2 presence [15]. The bands (801 and 803 cm−1) are due to symmetric-stretching-vibrations of siloxane-groups (Si-O-Si), as shown in Figure 2(a) and Figure 2(b), respectively.
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Figure 1. XRD of Si-polycrystalline, SiO2/Si crystalline SiO2 and amorphous-SiO2 powders obtained with different KOHconc and t; (a) 3 wt% KOH, (b) 4.5 wt% KOH, and (c) 6 wt% KOH.
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Figure 2. FTIR of the precipitated SiO2-nanoparticle obtained with different KOHconc prepared by ACE; (a) t = 4 hr, (b) t = 5 hr.
The O-H bending-vibration bands of the adsorbed water are located at 1649 and 1644 cm−1, and the O-Hstretching-vibration bands at 3441 and 3445 cm−1 (at t = 4 and 5 hours). The presence of water molecules proves that the amorphous-SiO2 chemical formula is close to SiOx yH2O form [16]. In addition, the shoulder at 2341 cm−1 is assigned to the stretching-vibrations of Si-OH groups in the amorphous-SiO2 structure [15], which also confirm the XRD data as shown in Figure 1.
Figure 3 presents SEM-micrographs of SiO2 powder morphology; Figure 3(e) shows the inversely proportional relation between (t) and SiO2 construction. At 6 weight % KOH; Figures 3(a)-(d) correspond to t = 4 and 5 hours, respectively. The pores interconnection is the most important structure characteristics of porous-SiO2 prepared in this work, by which the type of application is determined. Then, at the increasing of (t) values, the verification of SiO2 became more obvious. This relation proves the porosity percent enhancement at increasing (t). Figure 3(c) shows the particle size value (0.59 - 1.16 µm) and the pore size that’s nearly in the same range. But, at t = 5 hours (Figure 3(b)), the SiO2 particle size range decrease to (0.08 - 0.34 µm). So, SEM-images are in line with XRD data, as a result of complete conversion to the amorphous-SiO2. So, the product powder is suitable for using as heavy metal removal and dye removal as shown in last literatures [17].
The basic chemical reactions of SiO2 powder synthesis are presented in the following mechanism:
[18] (1)
At high pH range (alkaline medium) [1] :
(2)
(3)
The SiO2 morphology [nano-porous-sphere and nano-porous-cubic] is visualized by TEM-images. Figure 4(a) and Figure 4(c) show the nano-porous-sphere SiO2 (90 nm), with different geometric projections, that’s synthesized at the
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Figure 3. SEM images of porous-SiO2 obtained with (a) and (c) 6 wt% KOH and t = 4 hr, (b) and (d) 6 wt% KOH and t = 5 hr. (e) The relation between t (hr) and the pore size (µm).
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Figure 4. TEM-micrographs of SiO2-nanoparticles obtained with (a) and (c) 4.5 wt% KOH and t = 3 hr, (b) and (d) 4.5 wt% KOH and t = 4 hr.
preparation conditions 4.5 weight % KOH and t = 3 hours. But, at t = 4 hours, the nano-porous-cubic particles (45 nm) are appearing that agree the XRD-data in Figure 1(b). Then, it has high surface area that forms anion surfactants, which is used in many industrial applications [19]. Also, the crystalline-SiO2 [1] is recorded in Figure 4(b) and Figure 4(d). Then, the critical preparation conditions of porous-crystalline and amorphous-SiO2 are 6 weight % KOH at (t = 4 and 5 hours), as shown in Figure 1(c), Figures 3(a)-(d). Either in Figure 1(b) and Figures 4(a)-(d), at the different preparation conditions [4.5 weight % KOH at t = 3 and 4 hours], the nano-sphere and nano-cubic-SiO2 shapes, respectively, are described.
Thus, it’s found that the sonication time is controlled in the form and size of powder produced. Due to the ultra-sonication process is a growth inhibition technique especially in presence of alkali chemical etching process. So, the enhancement volume process is intermittently synchronized with the base etching process, causing the final shape of the product to change.
4. Conclusion
The pore size of the interconnected porous-SiO2 network in the micron range is successfully formed using commercial Si-powder as a cheap source of SiO2 (different morphological shapes). The particle size of the crystalline-porous-SiO2 powder is in range (0.59 - 1.16 µm), but in case of amorphous one is in range (0.08 - 0.34 μm). The successful production of SiO2 powder is through a combination ACE and ultra-sonication technique and subsequent drying at room temperature. It provided optimal conditions for the generation of micro-porous-crystalline and amorphous-SiO2 network. So, it’s a novel method for controlling the synthesis of several SiO2-nano-shapes {spherical (90 nm) and cubic (45 nm)} production.