Influence of fuel-to-oxidizer ratio, potential of hydrogen and annealing temperature on the structural and optical properties of nanocrystalline MgO powders synthesized by the hydrothermal method

Figures & data

Figure 1. Flowchart for hydrothermal synthesis of MgO nanocrystalline powders.

Table 1. Experimental conditions for synthesizing MgO nanoparticles.

Samples	Fuel-to-oxidizer ratios	pH	Annealing temp. (°C)
S1	0.25	12	500
S2	0.5	12	500
S3	0.75	12	500
S4	1	12	500
S5	0.75	12	750
S6	0.75	12	1000
S7	0.75	8	500
S8	0.75	10	500

Figure 2. X-ray diffraction patterns of nanocrystalline MgO powders synthesized at (a) different F/O ratios (b) different pH and (c) different annealing temperatures.

Table 2. Lattice parameter (a), crystalline size, X-ray density (${\rho }_x$), percentage of porosity, dislocation density (δ) and lattice strain (ε) of MgO nanocrystal for different ratio of fuel-to-oxidizer, deferent pH and different annealing temperature.

		Sample	a (A^o)	D (nm)	ρx (g/cm^3)	P %	S (m^2/g)	δ (nm^-2)	ε
Different F/O	0.25	S1	̶	̶	̶	̶	̶	̶	̶
	0.50	S2	4.217	18.55	7.14	50.81	36.40	0.0019	0.29
	0.75	S3	4.209	22.82	7.18	51.10	20.59	0.0006	0.24
	1.00	S4	4.212	19.85	7.16	50.98	33.38	0.0016	0.27
Different pH	8	S7	4.209	31.62	7.18	51.11	17.63	0.0004	0.17
	10	S8	4.207	29.44	7.19	51.17	19.55	0.0005	0.18
	12	S3	4.209	22.82	7.18	51.10	20.59	0.0006	0.24
Different annealing temp.	500	S3	4.209	22.82	7.18	51.10	20.59	0.0006	0.24
	750	S5	4.209	33.48	7.19	51.11	16.15	0.0004	0.16
	1000	S6	4.2096	49.06	7.18	51.10	7.89	0.0001	0.11

Figure 3. Crystallite size and dislocation density values of the nanocrystalline MgO powders at (a) different F/O (b) different pH and (c) different annealing temperatures.

Figure 4. The Rietveld refinements of nanocrystalline MgO powders synthesized at F/O (0.75): (a–c) pH 12, annealed at 500 °C, 750 and 1000, respectively (d) pH 10, annealed at 500 °C (e) pH 8, annealed at 500 °C.

Table 3. Profile factor (R_p), weighted residual factor (R_wp), expected residual factor (R_exp) and goodness of fit (χ²) of MgO samples from Rietveld refinement.

Sample	Rexp	Rp	Rwp	χ^2
S3	4.12	4.50	5.98	1.45
S5	3.98	4.59	6.10	1.53
S6	4.53	5.47	7.59	1.67
S7	4.97	5.55	7.14	1.43
S8	4.55	4.78	6.25	1.37

Table 4. EDS data for MgO at various synthesis conditions.

		Sample	EDS wt%
			Mg	O
Different ratio	0.25	S1	60.3	39.7
	0.50	S2	62.6	37.4
	0.75	S3	68.5	31.5
	1.00	S4	67.2	32.8
Different pH	8	S7	71.0	29.0
	10	S8	61.0	39.0
	12	S3	68.5	31.5
Different annealing temp.	500	S3	68.5	31.5
	750	S5	68.1	31.9
	1000	S6	72.5	27.5

Figure 5. FESEM images and EDS spectra of the MgO samples annealed at temperatures of (a) 500 °C, (b) 750 °C and (d) 1000 °C.

Figure 6. FTIR spectrum of nanocrystalline MgO powders synthesized att (a) different F/O (b) different pH and (c) different annealing temperatures.

Figure 7. Diffuse reflectance spectra of the nanocrystalline MgO powders synthesized at (a) different F/O ratios (b) different pH and (c) different annealing temperatures.

Figure 8. The Kubelka–Munk elaboration of the nanocrystalline MgO powders synthesized at (a) different F/O ratios (b) different pH and (c) different annealing temperatures.

Table 5. Direct and indirect band gap of nanocrystalline MgO powders at various synthesis conditions.

		Sample	Type of transition
			Eg (n = ½)	Eg (n = 2)
Different ratio	0.25	S1	̶	̶
	0.50	S2	5.875	5.88
	0.75	S3	5.790	5.870
	1.00	S4	5.895	5.902
Different pH	8	S7	5.762	5.855
	10	S8	5.778	5.864
	12	S3	5.790	5.870
Different annealing temp.	500	S3	5.790	5.870
	750	S5	5.765	5.836
	1000	S6	5.726	5.830

Table 6. Comparison of bandgap widths and average particle size of MgO nanoparticles.

Bandgap width (eV)	Average particle size (nm)	Reference
2.91	27	[1]
4.2	9.2	[46]
5.89 5.95	10 − 30 20 − 50	[47]
5.96 4.42	16 − 19 78 − 90	[10]
6.38	43 − 75	[48]
3.17 − 3.45	8 − 15	[49]
4.8	7	[50]
2.54	13 − 25	[51]
5.25	7 − 38	[52]
5.35	10 − 30	[53]
5.7 − 5.8	18 − 49	Results obtained in the present study

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Data availability

All data that support the findings of this study have been included in the article.