To formulate the Cu2+ precursor solutions labeled as A, B and C, a solvent was created by combining 10 ml of DMF with 10 ml of methanol. Into this solvent, CuCl2•2H2O was introduced in amounts of 0.02 g for solution A, 0.06 g for solution B, and 0.1 g for solution C. The preparation of the Mn2+ precursor solution followed an analogous procedure. 换一种表述