Synthesis of Ca- and Ce-Doped Mesoporous Silica Nanoparticles

Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, St. Louis, MO, USA) was used as the silica source, and cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich, St. Louis, MO, USA) as the structure-directing agent. Cerium(III) nitrate hexahydrate (Alfa Aesar, Haverhill, MA, USA) was used as the cerium ions’ source, while calcium nitrate tetrahydrate (ITW Reagents, Barcelona, Spain) was used as the calcium ions’ source.
The Ca-doped MSNs consisted of 60%mol SiO₂ and 40%mol CaO, and the Ca/Ce co-doped MSNs consisted of 60%mol SiO₂ 37.5%mol CaO and 2.5%CeO.
Calcium- and cerium-doped mesoporous silica nanoparticles (MCM-41 type) were synthesized in basic (pH 12–12.5) aqueous solution via the surfactant-assisted cooperative self-assembly process. A typical synthesis was conducted using two solutions. Solution 1 consisted of 2 g CTAB dissolved in 800 mL of aqueous NaOH 1.25% w/v at 80 °C. Solution 2 was prepared by dissolving 10 g of TEOS in 200 mL of deionized H₂O at room temperature, followed by the addition of 8 g calcium nitrate tetrahydrate or 7.8 g calcium nitrate tetrahydrate and 0.28 g cerium nitrate hexahydrate to produce Ca-doped or Ca/Ce co-doped nanoparticles, respectively. Solution 2 was added to solution 1 dropwise, and the resulting mixture was left under stirring for 2 h at 80 °C. The mixture was then thermally aged for 24 h at 100 °C. The separation of the synthesized material from the aqueous medium was performed via filtration. It was then washed once with ethanol, three times with deionized water, and was left to dry for 3 days. The removal of the organic phase (CTAB) was achieved by calcination in an oxidative atmosphere for 6 h at 550 °C (heating rate of 1 °C·min⁻¹). The molar ratios of the reactants are presented in Table 1.