Shape Control of Silica Powder Formation

The purpose of the present research is the different morphologies production of crystalline and amorphous-silica powder. It’s a basic material for many pharmaceutical and environmental applications as well. And, it’s produced using the combination of the alkali chemical etching process and the ultra-sonication technique. The critical preparation conditions are KOH concentration (weight %) and the sonication time (hour). The paper presents the chemical mechanism of the silica particle formation as well as the different morphology. The results show the formation of crystalline and amorphous-porous-silica particles in the micrometer range with the porous order network that has pore sizes range in micrometer too. This synthetic uses commercial silicon, which could be useful for large-scale production. Also, the nano-sphere and nano-cubic shapes of silica powder are formed starting by commercial silicon powder.


Introduction
The silica (SiO 2 ) powder is noted as competent materials because of their unique characteristics (low density, high surface area and high specific strength) [1] [2] [3].SiO 2 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-SiO 2 preparation techniques in previous studies [8]; the synthesis of porous-SiO 2 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 SiO 2 particles [1] [8].In this work, it demonstrated a combination between wet alkali chemical etching (ACE) process and ultra-sonication technique that controls the SiO 2 shapes.

Materials & Methods
The combination of ACE process and ultra-sonication technique is attractive because of its simplicity.The preparation of porous-SiO 2 different morphologies is started by 7 g commercial polycrystalline Si-powder (99%, Sigma-Aldrich).It's added to the NPA (30 Vol %), KOH conc (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-SiO 2 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-SiO 2 morphology.In addition, the formation of nano-porous-SiO 2 chemical bonds is determined by Fourier Transform Infrared spectroscopy (Shimadzu FTIR-8400s, Japan).

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, (SiO 2 /Si), crystalline-SiO 2 and amorphous-SiO 2 using a combination between
Finally, Figure 1(b) and Figure 1(c) (t = 5 hours), the SiO 2 -peak high intensity appeared at 2θ = 20˚ that corresponds to the complete transformation to amorphous-SiO 2 [13].Then, the preparation conditions during the crystalline-SiO 2 formation are [4.5 and 6 weight % KOH at t = 4 hours].But, the amorphous-SiO 2 is formed at t = 5 hours.From the reported data, (t) has a great effect in the controlling process during the SiO 2 type preparation.
Figure 2 shows the chemical bonding formation of SiO 2 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 SiO 2 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   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-SiO 2 chemical formula is close to SiO x yH 2 O form [16].In addition, the shoulder at 2341 cm −1 is assigned to the stretching-vibrations of Si-OH groups in the amorphous-SiO 2 structure [15], which also confirm the XRD data as shown in Figure 1.The pores interconnection is the most important structure characteristics of porous-SiO 2 prepared in this work, by which the type of application is determined.Then, at the increasing of (t) values, the verification of SiO 2 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 SiO 2 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-SiO 2 .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 SiO 2 powder synthesis are presented in the following mechanism: At high pH range (alkaline medium) [1]: The SiO 2 morphology [nano-porous-sphere and nano-porous-cubic] is visualized by TEM-images. Figure 4  Then, it has high surface area that forms anion surfactants, which is used in many industrial applications [19].Also, the crystalline-SiO 2 [1] is recorded in Figure 4 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.

Conclusion
The pore size of the interconnected porous-SiO 2 network in the micron range is successfully formed using commercial Si-powder as a cheap source of SiO 2 (different morphological shapes).The particle size of the crystalline-porous-SiO 2 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 SiO 2 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 (a) and Figure 2(b), respectively.