Wet-Chemical Synthesis Of Silicon Dioxide
Silicon dioxide, also known as silica. It is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. Si atom is bonded to 4 oxygen atoms and each oxygen atom to two Si atoms. It has a very high melting and boiling point . It does not conduct electricity.
In principle we can classify the wet chemical synthesis of nanomaterials into two broad groups:
- The top down method: where single crystals are etched in an aqueous solution for producing nanomaterials, For example, the synthesis of porous silicon by electrochemical etching.
- The bottom up method: consisting of sol-gel method, precipitation etc. where materials containing the desired precursors are mixed in a controlled fashion to form a colloidal solution.
Wet-Chemical Synthesis (Sol-gel process)
The sol-gel process, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The precursors for synthesizing these colloids consist usually of a metal or metalloid element surrounded by various reactive ligands. The starting material is processed to form a dispersible oxide and forms a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the gel produces the oxide. MOR + H2O → MOH + ROH (hydrolysis) MOH + ROM → M-O-M + ROH (condensation).
Recently the superhydrophobic surfaces with a water contact angle higher than 150° have received much interest, due to their convenience in many fields, Such as self-cleaning, snow inhibition and contamination inhibition. These widespread applications have motivated great efforts to improve fabrication techniques of superhydrophobic surface. In recent years, it has been found that, if a surface with a rough or micro textured structure has a low interfacial free energy, the CA can reach almost 180°, and the surface will remain dry as a water droplet easily slides across it. The typical example is the self-cleaning of nature’s lotus leaf, on which the CA is about (161) ° and the sliding angle (SA) only 2°. So far, many methods have been used for fabricating the superhydrophobic surface. All of these different methods for fabricating the superhydrophobic surfaces are almost based on the two main methods, which are to create the rough structure on the hydrophobic surface and to modify a rough surface by materials with low surface free energy. Therefore, the roughness is crucial for the preparation of the superhydrophobic surface.
It is well known that the wettability of solid surface could be characterized by contact angel (CA) of water droplets. If the CA of a surface is lower than 90°, the surface is described as a hydrophilic surface. In contrast, if that of a surface is higher than 90°, the surface is described as hydrophobic surface for smooth surfaces; CA can be represented by Young’s equation. For rough surfaces, the wetting state can be described by the Wenzel and cassie-baxter models. The Wenzel model assumes that the liquid is in intimate contact everywhere with the rough surface, and completely fill any surface structures. The roughness factor in the Wenzel equation can enhance the natural state of material, and gives hydrophilic surfaces a lower contact angle and hydrophobic surfaces a larger contact angle. The cassie-Baxter model introduces another new wetting state, in which the liquid allows very low friction during droplet movement, which introduces a low sliding angle. Both the Cassie- Baxter and Wenzel states can cause high static contact angles, but only the cassie-Baxter state can lead to very low sliding angle. Spraying is thought to be one effective technique to create artificial superhydrophobic surfaces due to its simplicity, low-cost and low-temperature method.
Many researches aim to fabricate superhydrophobic coating with little attention in improving the layer stability and suffer from low durability. we fabricated superhydophobic surface coating with the help of adhesive on the glass, paper and metal which is much durable than the normal coating processes.
