Dear all, I am preparing nano Cu particle by reaction of CuCl2 with NaBH4 in PVP solution. CuCl2 and PVP were mixed ultrasonically in pure water. Before dropping NaBH4 solution, the water was bubbled by N2 to remove O2. After dropping NaBH4, the solution turns into gold color. This means that the nano Cu particle was formed. The problem is: Several weeks later, we carried out the experiments again but they did not repeat (I mean the reactions had not been like that before). After dropping NaBH4, the solution turns into gold color and then (after 3min) it turns into black. The particles seem to be agglomerated (See video: https://www.youtube.com/watch?v=so7UjVDDtR0&feature=youtu.be ). We do many ways to solve this problem but it is not success. Please give me your suggestions! Thank you very much!