## 5 Discussion: Mechanism of Surface Hydroxylation of Fused Silica  
The surface energy is saturated due to the intermolecular force symmetry of the atoms inside the solid. However, the atoms on the solid surface are at a higher energy level due to the unbalanced intermolecular force, which makes the solid surface have a higher reactivity. Molecular dynamics simulations were used in Ref. [20] to show that the band gap on the silica surface is significantly smaller than that of the bulk structure (Figure 4), indicating that surface atoms are more likely to have chemical reactions with external atoms than the bulk atoms. On the other hand, the waviness of microscopic fluctuations on the solid surface is mainly manifested in the form of lattice defects, vacancies, and dislocations at the atomic scale. The chemical state of surface atoms is also different due to the different structural forms of atoms at different surface defect locations. The silicate glass surface composed of [SiO4] mainly contains unsaturated structures such as non-bonded oxygen, tri-bonded silicon, and cyclic silicon, as shown in Figure 5.  
During the sputtering process, many atoms leave their lattice position under the bombardment of incident ions, resulting in the fracture of chemical bonds or the formation of new chemical bonds on surface atoms, which will cause chemical structure defects such as lattice defects, vacancies, and dislocations, making the chemical bonding state of surface atoms significantly different from that of the internal bulk atoms. The asymmetry of the chemical bond energy of the surrounding atoms and the existence of multiple unbound electrons in the outer layer of the atoms lead to the enhancement of the physical properties  
Figure 4: The band gap between the body structure and the surface of fused silica. and chemical activity of surface atoms. The chemical properties of fused silica are very stable and water-soluble. However, there are many unsaturated structures on the surface after IBS treatment, which has a strong adsorption capacity for polar molecules. [PERSON] et al. [22] found that the broken or unbroken Si-O-Si bonds can undergo electrophilic or nucleophilic reactions with water molecules to weaken the Si-O bond strength so that the silica structure is continuously destroyed to form a surface adsorption layer of OH- groups. Water plays an extremely important role in the SiO\({}_{2}\) medium, and the water molecules can penetrate into the SiO\({}_{2}\) molecule under hydrostatic force at room temperature to break the Si-O bond, making it change into a Si-OH bond with lower bond energy. The diffusion of water molecules causes surface hydroxylation of SiO\({}_{2}\) molecules, thus forming a soft layer of Si(OH)\({}_{4}\), which can be expressed as:  
\[(SiO_{2})_{x}+H_{2}O\leftrightarrow(SiO_{2})_{x-1}+Si(OH)_{4} \tag{2}\]  
[PERSON] [23] believed that the hydroxylation generated by the chemical reaction between siloxane (Si-O-Si) and water is mainly determined by the chemical properties of the processed surface. When the IBS surface of fused silica is immersed in water, the polar water molecules easily adsorb with surface atoms and hydroxylate these surface atoms, thus forming a layer of Si(OH)\({}_{n}\) structure on the silica surface so that the Si atoms on the surface form a relatively stable structure. Due to the presence of H+ and OH- ions in aqueous solutions, the two ions chemically interact with different types of unsaturated structures on the surface, respectively. [PERSON] et al. [24] found that the surface hydroxylation process is that H+ reacts with non-bonded oxygen in an aqueous solution, while OH- reacts with unsaturated silicon to form OH- groups.