Direct Bonding of Amino-Modified Silica and Metal Oxides: Condensation Reactivity Explained by Acidity/Basicity of Metal-Hydroxyl Groups

A low-temperature bonding technique is utilized for bonding different kinds of materials together. However, it is still challenging to achieve sufficient bonding strength at lower temperatures. To overcome the limitation of low-temperature bonding, we must clarify the bonding mechanism at the functional group level. Nevertheless, no comprehensive explanation for the material dependence of the bonding strength has been demonstrated. In the present study, the bonding strength between various metal oxides and silica (SiO2) modified with hydroxyl (Si–OH) or amino groups (Si–NH2) is clarified. Modification with Si–NH2 to SiO2 surfaces improved the bonding strength to SiO2, TiO2, SnO2, MnO2, WO3, and CrO3, but it rather deteriorated the strength to NiO, Al2O3, and MgO. The material dependence of the bonding strength originated from the number of covalent bonds formed between SiO2 and metal oxides as well as their bond energy. The reactivity of the bond formation between the metal oxides and Si–NH2-modified SiO2 cannot be explained by the areal density of the hydroxyl groups on metal oxides (M–OH), which is the most general expectation. In contrast, we found that the reactivity of Si–NH2 was well explained by the acidity/basicity of M–OH, which has rarely been considered in the field of low-temperature bonding.