The initial pHs of the various metal solutions were increased to
around 12 by the addition of NaOH. (The corresponding final pH
was 11.5.) For the Ni(NH3)6Cl2 solution, only ammonia hydroxide
can be applied to adjust pH; otherwise, precipitates occur. Then
0.5 g of silica was weighed out and added into the metal ammine
complex solutions. After 1 h of shaking, the solid was filtered,
washed, and then dried over night in flowing air. The metal elemental
analysis determined by ICP.