**3. Materials and Methods**

## *3.1. Material*

Lanthanum (III) nitrate hexahydrate, citrate acid, cobalt (II) nitrate hexahydrate, potassium nitrate, and glycol were bought from Shanghai Aladdin. Gallium (III) nitrate nonahydrate was purchased from Beijing HWRK Chem. All of above materials were used without further purification.
