Silica (silicon dioxide) is an oxide complex i.e. combination of silicon and oxygen atoms joined consecutively. Within the silica molecules, every silicon atom connects with four oxygen atoms while every oxygen atom connects with two silicon atoms by Si-O bonds. The silicon and oxygen atoms have a bond angle of 109.5o, and this bond has a length that varies from 1.54 to 1.69 Å. [1].The siloxane bonds, which are silicon-oxygen-silicon bonds have angle that can vary from 120ο to 180 ο based on changes in bond energy. These unique properties of silica are attributed to the various shapes and compositions due to high bond energy (621.7 kJ/mol) of Si-O bonds [1, 2]. These silica materials have high level of chemical inertness with excellent level of thermal resistance and a good mechanical properties as compared to other oxides like ZnO, TiO2 [3]. The relevance and interest in silica thin films has grown recently, not only in the scientific but also in industrial fields [4, 5], such as catalysis, chemical mechanical polishing [6],pigments [7] and stabilizers [8].
For synthesis of silica thin films, various methods were proposed, such as chemical vapor deposition (CVD), plasma manufacturing, microemulsion method, electron-beam evaporation, sol–gel synthesis, and magnetron sputtering [9–13]. All these techniques have many advantages such as fast deposition speed, require of low temperature, multiple layers of coating, and minimal damage to the film, but still these techniques have a few drawbacks. However, the film deposition rate of the CVD process is quite slow, and it often involves flammable, very toxic, or explosive reactants such as SiH4 and SiH2Cl2. Guo et al. [14] fabricated an ultrathin SiO2 layer by using SiCl4 at room temperature in CVD method. Although magnetron sputtering is an advantageous method for thin film deposition, it has some major drawbacks such as strong magnetic materials cannot achieve high speed sputtering at low temperatures due to plasma instability and limitations in magnetic flux [15], which make this method complex. In addition, oxygen deficiency is another inadequacy of silica films prepared by magnetron sputtering [16]. Overall, the vacuum-based techniques are not budget friendly and requires high-cost equipment such as expensive crucible, electron-gun and its power supply are required in e-beam evaporation method. Sol-gel method is regarded as the most significant and frequently used technique due to various advantages including its ability to synthesise at low temperatures with ideal PH to yield high purity and its versatility in adjusting reaction mixture compositions to control the rate of reaction. Again, the sol-gel synthesis process can be classified as precipitated synthesis, Stöber method, and biomimetic sol-gel synthesis process [17, 18]. It has been illustrated that the Stöber process is the most convenient and efficient method for fabricating spherical silica nanoparticles because the reaction state is easy to control and carried out, and the reactants are normal [19]. This approach involves hydrolysis and condensation of tetraethyl orthosilicate (TEOS) as silicon alkoxide which hydrolysed with ethanol and water in presence of ammonia as basic catalyst that described by the following three equations:
Hydrolysis:
≡ Si–OR + H2O ↔≡ Si–OH + ROH (1)
Alcohol condensation:
≡ Si–OR + ≡ Si–OH ↔≡ Si–O–Si ≡ + ROH (2)
Water condensation:
≡ Si–OH + ≡ Si–OH ↔≡ Si–O–Si ≡ + H2O (3)
It is widely reported that the diameter of silica nanoparticles can be controlled by the concentration of TEOS, water, and ammonia [20–22]. Wang et al. [23] verified that by altering TEOS concentration, the size of silica nanoparticles increased while the molar concentrations of water and ammonia remained constant. Whenever the concentration of TEOS enhanced, more primary particles caused to raise the hydrolytic rate. Due to production of additional short chains of TEOS, the primary particles are spontaneously aggregated to form stable secondary particles that significantly increase particle size distribution in polydisperse pattern [24, 19]. Additionally, when water concentration increased, the hydrolysis rate as well as the interaction of silica molecules during the condensation process both decreased, resulting in smaller particles with an uneven distribution [24]. Ammonia is the most significant experimental parameter that influences the morphology of silica particles more than the concentrations of TEOS and water [25]. There are several methods reported for deposition process such as dip coating, spray pyrolysis, spin coating and drop casting method. But the spin coating approach has been reported as the most practical and useful way to deposit various materials as solutions, especially polymers, biomaterials, and nanoparticles [26, 27]. Thus, it can be concluded that Stober process with spin coating method is one of the suitable method for the fabrication of silica thin films due to minimal cost.
In this work, silica solution was synthesized by Stober method with variation in one of the growth parameters i.e. ammonia (catalyst) and coating process was followed by spin coating technique using glass substrates. The purpose of this work is to understand how the concentration of ammonia affects various characteristics of silica thin films like structure, particle size and its distribution, bandgap, and wetting behaviours. The present work included a brief discussion of the impact of ammonia concentrations on the optical, structural, morphological, and wetting characteristics of silica thin films.