# Removal of Metal Ions in Spin-on Hardmask Using Functionalized Porous Silica Adsorbents
## 3 Result and Discussion
### Metal Ion Adsorption  
To evaluate the adsorption performance of the functionalized mesoporous silica materials, metal ion removal experiments were initially conducted under solvent-only conditions to isolate the intrinsic adsorption capacity of each surface functional group without interference from other organic components. This approach allows for a direct comparison of the affinity of each functional group (-OH, -CH3, -SH, and -NH2) toward the target metal ions (Al3+, Cr3+, Cu2+, Fe3+, Ni2+, and Ti4+), providing a clear understanding of how surface chemistry influences ion removal efficiency. The removal efficiency (%) was calculated based on the difference between the initial and equilibrium metal ion concentrations using ICP-OES analysis, as described in Section 2.5. A consistent trend in ion removal efficiency was observed across different adsorbents, primarily influenced by the type of functional group introduced on the surface rather than  
Figure 4: Zeta potential values of functionalized (**a**) MCM-41 and (**b**) SBA-15 samples at pH 7.  
by pore size or surface area. This highlights the dominant role of surface chemistry in adsorption behavior. As illustrated in Figure 5a, unmodified MCM-41-OH and SBA-15-OH both demonstrated a common feature of selectively removing Al\({}^{3+}\) and Ti\({}^{4+}\) ions. Notably, MCM-41-OH also demonstrated moderate adsorption of Ni\({}^{2+}\), which was not observed for SBA-15-OH. Figure 5b shows that methyl-functionalized samples (MCM-41-CH\({}_{3}\), SBA-15-CH\({}_{3}\)) exhibited the lowest overall adsorption performance. Only limited removal of Al\({}^{3+}\) and Ti\({}^{4+}\) was observed, indicating the hydrophobic nature of the -CH\({}_{3}\) group may hinder effective interaction with solvated metal ions. In Figure 5c, MCM-41-SH, and SBA-15-SH modified with thiol groups (-SH) on the surface, selectively removed Al\({}^{3+}\), Cu\({}^{2+}\), and Ti\({}^{4+}\) ions. In particular, the selective adsorption of Cu\({}^{2+}\) ions, which is not achieved by other functionalized adsorbents, is especially noteworthy. As observed in Figure 5d, MCM-41-NH\({}_{2}\) and SBA-15-NH\({}_{2}\) modified with amine groups (-NH\({}_{2}\)) on the surface, demonstrated exceptional ion removal efficiency compared to other functionalized materials. They exhibited consistently superior ion removal efficiency for all tested ions, especially with MCM-41-NH\({}_{2}\) achieving over 99% removal efficiency across the board. This outstanding performance is attributed to the strong  
Figure 5: Removal efficiency of various metal ions by functional groups: (**a**) –OH, (**b**) –CH\({}_{3}\), (**c**) –SH, and (**d**) –NH\({}_{2}\), applied to MCM-41 and SBA-15 under solvent-only conditions. chelation ability and high affinity of amine groups for metal cations. Chelation occurs as the lone pair of electrons on nitrogen atoms in the amine groups coordinate with metal cations to form stable complexes. In addition, amine groups enable multiple interaction mechanisms, including electrostatic attraction, hydrogen bonding, and covalent coordination, all of which contribute to enhanced adsorption efficiency [25]. These results suggest that amine-functionalized mesoporous silica is a promising adsorbent for the removal of trace metal contaminants in semiconductor applications, offering high efficiency with low environmental and economic impact. When comparing the overall adsorption performance across different functional groups, the trend -NH\({}_{2}\) > -SH > -OH > -CH\({}_{3}\) was observed, showcasing superior adsorption performance in that order. Based on these results, MCM-41-NH\({}_{2}\) and SBA-15-NH\({}_{2}\) were selected for further experiments using metal ion solutions containing monomers and polymers. For the adsorption isotherm experiments, only MCM-41-NH\({}_{2}\) was chosen due to its outstanding performance. Moreover, the adsorption affinity of functional groups was closely related to metal ion properties such as valence, size, and softness.