| Methods
| Particle size (TEM) (nm)
| Crystal size (XRD) (nm) | References |
| (i) Air oxidation | Not mentioned | 26 | [23] | (ii) Ball milling | 50 | Not mentioned | [24] | (iii) Combustion reaction | 20–30 | 18–61 | [25] | (iv) Combustion route | 10–20 and 25–80 | | [26] | (v) Solid state reaction | 10 | 15–30 | [27] | (vi) Autocombustion assisted | 7–8 | 7.4 | [28] | (vii) Sol-gel method | Not mentioned | 35 | [29] | (viii) Combustion reaction | 28–46 | 28–46 | [30] | (ix) Microwave hydrothermal method | 2 | 3 | [31] | (x) Solution combustion synthesis | Figure 6 | 51.9, 50.9, 34.4, 27.9, 24.4, 16.4 | Present works |
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