Fabrication and Characterization of Silica Nanoparticles from Beach Sand

Silica nanoparticles (SNPs) have many important applications including anti-reflection coating, self-cleaning surface and drug carriers. They are usually synthesized from the commercial precursor such as tetraethyl orthosilicate (TEOS). On the other hand, the natural silica can be found abundantly in organic materials such as rice husk and palm shell or in inorganic material such as beach sand and clay. Therefore, obtaining SNPs from the natural resources is very strategic for economic and technology considerations. This motivates the current study performing the synthesis and characterization of SNPs derived from the beach sand as one of natural resources available. Fort this purpose, the silica sands were mixed with sodium hydroxide (NaOH) for 2 hours at 90⁰ C, prior to filtering process for gaining the sodium silicate solution (SSS) which was further refluxed with hydrochloric acid (HCl) of 3 and 5 M until its pH reached the value of 7, and finally dried. The characterizations carried out on the resulting SNPs included UV-Vis and FTIR spectroscopies, XRD and SEM. The XRD study confirmed that the resulting samples are amorphous silica phase with the average crystallite size of 4.8 and 2.9 nm for the SNPs synthesized with 3 and 5 M chloric acid, respectively. The image analysis of SEM results revealed that the obtained SNPs have the average diameter of 11.6-12.00 nm. The formation of SNPs was further confirmed with the UV-Vis and FTIR spectroscopies. On the basis of investigation results, it was shown that the desired silica nanoparticles can be successfully derived from the beach sand.


Introduction
Silica nanoparticles (SNPs) have vast applications in science, engineering, and health.It was used for anti-reflection coating and self-cleaning windows due to the ease of surface modification so it can be hydrophilic, hydrophobic, and superhydrophobic [1].It also can be used as a carrier for drugs and vaccines because it can be modified as biocompatible material [2].Since the SNPs can be applied in various applications, it has attracted researcher to elaborate more on basic research, laboratory tests, and prototype.
The synthesis method of nanomaterials can be classified as bottom-up or chemical method (chemical vapor deposition, hydrothermal, and sol-gel) and top-down or physical method (thermal evaporation, laser ablation, and milling) [3].The sol-gel method is well known due to its simple process and controllable parameters.One of its methods is the Stober method conducted by mixing tetraethyl orthosilicate (TEOS) as precursor and ethanol with NH4OH as catalyst [4,5].However, commercial TEOS which is frequently used is expensive [1].
Silica is one of the most abundant materials on earth that can be stored in many natural resources, such as rice husk, palm shells, clay, and silica sand.However, silica sand has the highest amount of silica percentage, which can reach 99% [6][7][8][9][10].Indonesia has a large stock of silica sand due to the very long coastline which encourages many researchers to optimize this potential.So in this paper, the fabrication SNPs was synthesized using silica sand from Belitung Island with sodium silicate solution (SSS) route then followed by neutralization using hydrochloric acid (HCl).

Experimental Details
The experiment is conducted in two steps, they are formation of sodium silicate solution (SSS) from silica sand and synthesis of silica nanoparticles (SNPs) from SSS. Sodium silicate solution were synthesized by mixing milled silica sand and sodium hydroxide 8 M using magnetic stirrer on 90°C temperature for 2 hours in 400 rpm speed.Then, the mixture was cooled in room temperature and filtered by whatman paper to get SSS.hydrochloric acid was dropped to the solution gradually with different concentration of 3 M (SSS-3M) and 5 M (SSS-5M).So, the results from this synthesis were SSS control (SSS-0), SSS-3M and SSS-5M.To confirm the results, Ultraviolet-Visible (UV-Vis) and Fourier Transform Infrared (FTIR) characterization was conducted.
The SSS was settled down for 24 hours until it become gel then followed by washing process using centrifuge method in 10.000 rpm until it reach pH 7. Furthermore, the sample was dried in room temperature to gain SNPs that noted as SNP3M and SNP5M.It was characterized and analyzed by X-Ray Diffraction (XRD) to gain phase information and Scanning Electron Microscope (SEM) to gain its microstructure.

Results and Discussion
The sodium silicate solution was formed by reaction of silica sand and sodium hydroxide as follows.
The UV-Vis characterization is showed in Figure 1 for three types of sodium silicate solution.The figure shows the absorbance of the solution raise due to increasing of acid concentration [11].The peak of the spectra is about 260-310 nm that reflect that concentration of dissolved solid particle.It informs that there is increase of solid particle due to addition of HCl to the SSS and significant change is shown after HCl addition to SSS.The absorbance of solution in visible range (about 500 nm) is very high.

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Engineering Chemistry Vol. 6 FTIR result confirms UV-Vis one where the addition of acids increase the peak of spectra as shown in Figure 2. The two peak intensity between 3000-3500 cm -1 and between 1500-200 cm -1 was attributed to Si-OH where the result of neutralization of SSS.The peak about 800 cm -1 and about 400 cm -1 were attributed to Si-O-Si bending vibration [8,12].It can be stated that formation of silica nanoparticles was on progress.It will be fully formed after washing and drying.
Silicate contain two double bonds between silicone and oxygen that change become silica hydroxide ion with charge -1 where silicone bond with hydroxide ion from sodium hydroxide cause a double bond between silicon and oxygen become single bond.Another double bond and hydrogen from the ion release become silicone trioxide ion that has -2 charges [8].Sodium ions that positively charge attract the silicon trioxide ion to form sodium silicate.The presence of bonding of Si-OH, Si-O-Si and other bonding that appear in FTIR spectra indicate the reaction and some incomplete proses of change.It is attributed to solution may experience reverse reaction.

Fig. 2. FTIR Spectra of SSS
The dried silica nanoparticles were identified using XRD to find out if it was formed.The XRD spectrum shows for different acids concentration of 3 M and 5 M are shown in Figure 3.Both are amorphous as only gave a peak around 23⁰ and SNP5M is more amorphous rather than another one.It means the crystal orientation of SNPs is random.The XRD spectrum is confirmed as the work by other researcher [6].Quantitative analysis was applied to get crystallite size using Scherrer equation [13].The computations result 4.8 nm for SNP3M and 2.9 nm for SNP5M where the higher hydrochloric acid concentration cause smaller crystallite size.The width of the XRD spectra shows the trend which wider peak represents the smaller crystal.
The formation of SNPs as following equation ( 2) where sodium silicate react with hydrochloric acid and produce silica, sodium chloride and water.The washing process using centrifuges remove the sodium chloride where the drying process removes the water.
It can be confirmed from SEM image that show small SNPs were agglomerated as shown in Figure 4. Agglomeration is a characteristic of nanoparticles due to the natural state of it is unstable.It is then confirmed that silica nanoparticles was formed and the size in nanometer.The agglomeration can be avoided by change the functional group on SNPs surface.It result the similar charge that exert repulsion among the particles.