# Removal of Metal Ions in Spin-on Hardmask Using Functionalized Porous Silica Adsorbents
## 3 Result and Discussion
### Metal Ion Adsorption
Thiol-functionalized mesoporous silica (-SH) showed strong affinity for soft divalent cations like Cu\({}^{2+}\), Pb\({}^{2+}\), and Cd\({}^{2+}\), due to covalent bonding based on [PERSON]'s hard-soft acid-base (HSAB) theory. Its selectivity over hard trivalent ions (e.g., Cr\({}^{3+}\) and Al\({}^{3+}\)) is attributed to the high charge density and strong hydration shells of the latter [26]. Methyl-functionalized mesoporous silica (-CH\({}_{3}\)) exhibited negligible uptake due to its hydrophobicity and lack of donor atoms. However, co-functionalization with thiol groups can improve steric accessibility and prevent oxidation [26]. Unmodified mesoporous silica with hydroxyl groups (-OH) selectively adsorbed hard trivalent cations such as Al\({}^{3+}\), Ti\({}^{4+}\), and Fe\({}^{3+}\) through inner-sphere complexation with deprotonated silanols at a favorable pH, although its capacity was modest [27; 28]. In the subsequent experiments using monomer-containing metal ion solutions, the ion removal efficiencies of the adsorbents were evaluated, as presented in Figure 6. Compared to the results obtained in the solvent-only system (Figure 5), both MCM-41-NH\({}_{2}\) and SBA-15-NH\({}_{2}\) exhibited significantly reduced adsorption performance. Moreover, the previously observed trend of uniform ion adsorption across different metal ions was no longer maintained. The effective molecular size of phenol is approximately 0.75 nm, and that of formaldehyde is approximately 0.25 nm [29; 30]. According to Table 1, the pore sizes of MCM-41-NH\({}_{2}\) and SBA-15-NH\({}_{2}\) are 3.1381 nm and 7.1297 nm, respectively. Therefore, it can be interpreted that the dissolved monomers partially infiltrate the mesopores of the adsorbents, thereby hindering access of metal ions to the active adsorption sites on the internal surfaces. In addition to this steric hindrance, potential chemical interactions between phenol and metal ions may further contribute to the observed reduction in adsorption efficiency. Recent studies have demonstrated that phenol can undergo oxidation via proton-coupled electron transfer (PCET) or stepwise proton transfer/electron transfer (PTET) mechanisms in the presence of Cu\({}^{2+}\) complexes, leading to the generation of phenoxy radicals and the reduction of Cu\({}^{2+}\) to Cu\({}^{+}\)[31]. Such reactivity suggests that phenol may chemically bind or react with certain metal ions in solution, altering their speciation and reducing their availability for adsorption. Furthermore, phenol is also known to form hydrogen bonds with amine groups on the adsorbent surface, which could competitively occupy active sites or modify the surface polarity. While these effects were not the primary focus of this study, they may partially explain the reduced and non-uniform adsorption performance in monomer-containing systems. adsorption experiments were also carried out in polymer-containing metal ion solutions, and the resulting ion removal efficiencies are presented in Figure 7. Compared to the solvent-only condition (Figure 5), a decrease in adsorption performance was observed, although the values were notably higher than those obtained in the monomer-containing solution (Figure 6). This trend can be interpreted as follows: due to the relatively larger molecular size of the polymer compared to monomers, it cannot easily diffuse into the mesopores of the adsorbents, thereby limiting its interference with metal ion adsorption. In the case of MCM-41-NH\({}_{2}\), adsorption efficiency in the polymer-containing solution was improved compared to that in the monomer-containing condition. However, it did not exhibit uniform adsorption behavior, showing high removal efficiency only for certain metal ions. In contrast, SBA-15-NH\({}_{2}\) demonstrated a significant enhancement in adsorption performance relative to its behavior in the monomer-containing solution. Although its efficiency remained lower than that observed under solvent-only conditions, it effectively removed all target ions with relatively uniform performance.