4.2. Preparation of Copper Aluminate

The CuAl₂O₄ nanoparticle was synthesized by a modified citrate combustion method according to the previous reports.³³ Briefly, the precursors with an atomic ratio of copper and aluminum of 1:2 were dissolved in 100 mL of ultrapure water. Then, the specific amount of citric acid (0.03 M) was dissolved in 100 mL of ultrapure water and was drop-wise added to the solution containing metal precursors under magnetic stirring for 2 h to form a mixed solution. The obtained homogeneous solution was constantly stirred at 90 °C until the formation of a blue sticky gel, and then the gel was calcined at 400 °C for 4 h. Finally, the resultant brown particles were ground, washed with ultrapure water until the pH reached neutral, and dried at 80 °C for 24 h.

Pure CuO and Al₂O₃ nanoparticles were used as a comparison in this work and were prepared according to the aforementioned methods used for CuAl₂O₄, but without aluminum nitrate and copper nitrate, respectively.