The comparison of various methods for the synthesis of cobalt oxide nanoparticles.

Method	Material	Solvent	Treatment time	Temperature (°C)	Sintering/calcination		Size (nm)	Cobalt oxide phase	Ref.
					Temperature (°C)	Time (h)
Sol gel	Cobalt nitrate hexahydrate	Ethylene glycol	>2 h	1st step at room temperature, 2nd step at 120	700	1, 2, & 3	60.099, 52.611, & 84.168	Co3O4	15
	Cobalt nitrate hexahydrate	Distilled water	>2 h	1st step at 30 and, 2nd step at 80	400	4	13–15	Co3O4	16
Hydrothermal	Cobalt nitrate hexahydrate	Ethylene glycol	6 h	100	In N2 gas at 450 and 550	3	25, 38, and 51	Co3O4 and Co3O4/CoO	4
	Cobalt nitrate hexahydrate	Distilled water and thioglycolic acid (TGA)	5–7 h	100–160	—	—	17.5	Co3O4	10
	Cobalt nitrate tetrahydrate	Deionized water	6 h	180	450	5 h (ramp of 10 °C min−1)	51, 38, & 25	Co3O4	17
	Cobalt nitrate hexahydrate	Deionized water	30 h	150			100	Co3O4	51
Solvothermal	Cobalt acetate tetrahydrate	Ethanol	1st step at 160 °C for 2 h, supernatant from 1st step standing for 72 h		In air gas at 200, 350, 300, & 350	10 (ramp of 1 °C min−1)	5.5, 7.5, 8.6, 10.5	Co3O4	6
	Cobalt nitrate hexahydrate and cobalt acetate tetrahydrate	Ethanol	5 h	250	250	3, 6, and 16 h	70	Co3O4	52
	Cobalt(ii) and (iii) acetylacetonate	Mixed solution between ethanol and distilled water (1 : 15)	8 h	120, 140, 160, 180, 200	500	5 h	40	Co3O4	53
Spray pyrolysis	Cobalt nitrate	Ethylene glycol	—	—	500 to 800 in an air atmosphere	—	12, 27, and 43	Co3O4	18
Microemulsion	Cobalt acetate tetrahydrate	Water and ammonium oxalate	—	—	—	—	25 and 35	Co and Co3O4	19
	Cobalt 2-ethylhexanoate and cobalt nitrate hexahydrate	Water	—	—	—	—	3–10	Co3O4	20
Microplasma	Cobalt nitrate hexahydrate	Distilled water	25 min	—	450	4	10–17	Co3O4	This work