Morphology and structure of nano MgO prepared by a novel nitrogen-protective pressurization method

Abstract

A new micro stereoscopic structure of nano MgO was prepared by a novel nitrogen-protective pressurization method and characterized by SEM, EDS, and XRD. The nano MgO was formed by spherical units, and under the action of adsorption, the spherical units formed nanosheets by connecting to one another. Moreover, the units and sheets further gathered into laminations of nano MgO. The grain sizes of the nanosheet and laminate nano MgO were 11.8 nm and 367.6 nm, respectively, which were prepared by a novel method. The influence of nano MgO on morphology and structure were revealed by changing the amount of PVA, calcination time, and pressure.

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Introduction

In recent decades, new materials have been developed,^1–5 and nano MgO is a new type of highly functional fine inorganic material^6,7 that is widely used in many fields.^8–10 It has different special functions,^11,12 and is a material that can be used in relation to heat, light, electricity, mechanics, and chemistry. Nano MgO can be prepared by many methods, although the solid,¹³ liquid^14,15 and gas phase^16,17 methods are usually adopted. The solid phase method easily introduces impurities into the product. The gas phase method is accompanied by high cost, complicated preparation process, poor efficiency, and high energy consumption. The liquid phase method is the best method for the preparation of nano MgO and includes the sol–gel method,^18,19 microemulsion method,^20,21 and precipitation method.^22,23 The simplex liquid phase method also has some disadvantages, such as products that easily coacervate, making it hard to control the morphology and industrialize. Therefore, new methods need to be explored.

Under the above circumstances, we developed a novel nitrogen-protective pressurization method based on a liquid phase precipitation method. Using this novel method, the morphologies of nano MgO products are presented by changing the calcination time with different amounts of PVA. In addition, the micro stereoscopic structure of nanosheets is proposed.

Experimental

The reaction device for the N₂-protective pressurization method is shown in Fig. 1. This novel method increases the pressure of the reaction system so that the boiling point of water increases and water evaporation is reduced. This allows the reaction to be more homogeneous in the liquid phase, which is beneficial for the formation of products with high dispersity and uniformity. In addition, air was isolated through the entire reaction process, which prevented impurities from being introduced.

Fig. 1 Reaction device for the nitrogen-protective pressurization method.

To prepare the nano MgO, 10 g MgCl₂·6H₂O (analytically pure, Sinopharm Chemical Reagent) was dissolved in 50 ml of deionized water, and then 6.7 g (10 wt%) or 15 g (20 wt%) PVA-1500 (analytically pure, Sinopharm Chemical Reagent) was added to the solution. The mixture was stirred for 1 h at room temperature until the solid material fully dissolved, and then the solution was poured into the burette. 250 ml of 2.5 wt% NH₃·H₂O (Sinopharm Chemical Reagent) was added to a 500 ml reactor. The solution in the burette was added dropwise into the reactor, and the liquid in the reactor was kept at 40 °C with continuous stirring. The drop rate was 0.3 ml min⁻¹ until the burette was empty. N₂ was added throughout the reaction process in order to keep the pressure at 0.1 MPa (gauge pressure) during the entire reaction system. White precipitates were constantly produced during the process.

The pH value of the system was adjusted to 9.8 after the reaction process. The mixture was aged for 24 h until there was adequate precipitation of a white solid. The solid was washed with deionized water until no Cl⁻ ion could be found and was then dried in vacuum for 12 h. The precursor of Mg(OH)₂ (ref. 24) was obtained. Finally, the precursor was calcined at 700 °C for 2 h, 3 h, and 4 h in a muffle furnace to obtain nano MgO.

The precursor of Mg(OH)₂ was also prepared under normal atmospheric pressure using the same reaction conditions described above except no additional N₂ was added. Nano MgO was acquired by calcining the precursor at 700 °C for 3 h.

Results and discussion

SEM analysis revealed the morphology change for nano MgO. The images were obtained by scanning electron microscopy (SEM, JSM-6390LV). As shown in Fig. 2a, when the addition of polyvinyl alcohol (PVA) is 10 wt% and calcination time is 2 h, aggregation of MgO nanosheets occurs. These nanosheets cannot be clearly distinguished from each other. When the calcination treatment was prolonged to 3 h, the morphology became more regular, and the nanosheets displayed a proper morphology (Fig. 2b), with the product being dispersive. When the calcination time was extended to 4 h, it can be seen from Fig. 2c that the nanosheets are overburnt with an irregular form. Aggregation of spherical particles occurs along with the formation of sheets; however, the nanosheets are difficult to discern. This may have occurred because the long calcination time caused a collapse of the morphology.

Fig. 2 SEM images of nano MgO prepared with 10 wt% PVA with different calcination times by the novel method. (a) 2 h, (b) 3 h, and (c) 4 h (scale bar = 100 nm).

Fig. 3a shows that when the addition of PVA is 20 wt% and the calcination time is 2 h, a cotton-like nano MgO emerged, which may have formed because insufficient PVA was removed. When the calcination treatment is prolonged to 3 h, the morphology becomes regular. The product with the structure of laminations is produced in Fig. 3b, although it is uncertain whether this is the optimal morphology of nano MgO. The calcination time was extended to 4 h. Fig. 3c shows that the MgO is overburnt based on the laminations of nano MgO. Aggregation of spherical particles in the lamella is faintly visible, and the lamination pattern was mostly destroyed. The reason is in accordance with the above.

Fig. 3 SEM images of nano MgO prepared with 20 wt% PVA at different calcination times by the novel method. (a) 2 h, (b) 3 h, and (c) 4 h (scale bar = 100 nm).

As shown in Fig. 4a, when the addition of PVA is 10 wt%, the products of nano MgO appear to be grainy, adherent, and with irregular morphology. The addition of PVA was 20 wt% in Fig. 4b. The morphology of nano MgO in Fig. 4b is similar to that shown in Fig. 4a but with some differences. The grain size is bigger in Fig. 4b than that in Fig. 4a. The aggregation of nano MgO particles is regular in Fig. 4b, and they tend to form a sheet. This may be due to the influence of additional PVA. Compared with Fig. 2b and 3b, the products of nano MgO in Fig. 4 are aggregative by many particles that have gathered, and therefore, there is increased dispersity of nano MgO that was prepared by the novel method.

Fig. 4 SEM images of nano MgO prepared with a calcination time for 3 h and different amounts of PVA under ordinary pressure. (a) 10 wt% PVA and (b) 20 wt% PVA (scale bar = 100 nm).

Qualitative and quantitative analysis of nano MgO was performed by energy dispersive spectrometry (EDS, Falcon). Fig. 5a shows the spectrum of the nanosheet (Fig. 2b), illustrating that the mass fraction ratio of O to Mg is 26.73% : 73.27%, and the atomic ratio of O to Mg is 35.36% : 64.64%. In Fig. 5b, the ratio of O to Mg is 26.10 wt% : 73.90 wt%, and the atomic ratio of O to Mg is 34.61% : 65.39%. The mass fraction ratios and atomic ratios are close to each other, and the atomic ratio of O to Mg is approximately 1 : 2. Based on the cubic structure of MgO and the networked rectangular MgO nanostructures²⁵ along with the SEM and EDS images of MgO nanosheets, a hypothetical simple micro stereoscopic structure is proposed in Fig. 5c. It is composed of many spherical units that gathered and were connected to each other by adsorptive force.²⁶ This would enable MgO nanosheets of different lengths and widths to be prepared. The nanosheets may be curly and gather again when the strengths of the adsorption force are inconsonant among the units. Thus, laminations of nano MgO are formed. It is proposed that the morphology of nano MgO is related to the amount of additive²⁷ in the reaction. The less the amount of PVA added, the weaker the adsorption force among the MgO spherical units, and thus, it is easier to shatter the spherical units into the morphology of small size in the calcination process. This hypothesis is consistent with the SEM images.

Fig. 5 The EDS spectra of nano MgO for a (a) nanosheet and (b) lamination product and (c) hypothetical simple micro stereoscopic structure of nano MgO prepared by the nitrogen-protective pressurization method.

The phase purity of the products was examined by X-ray powder diffraction (D8 Advance). As shown in Fig. 6, all the diffraction peaks were clearly indexed to the pure MgO crystal. The characteristic peaks²⁸ of MgO remain at the positions of (111), (200), (220), (311), and (222). No other phase can be detected in the XRD pattern, indicating high purity of the products. The XRD pattern of the nanosheets (Fig. 6a) shows low peak intensity and large FWHM values compared to the XRD pattern of the lamination products (Fig. 6b). This suggests that the average size of the nanosheets is smaller than that of the lamination products. These data are in accordance with the previous SEM images and EDS spectra.

Fig. 6 XRD patterns of (a) nanosheet and (b) lamination products.

The pore structure of the products was analyzed by a surface area analyser (ASAP2020), and the grain size was measured with a nano laser granulometer (Zetasizer Nano). The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were utilized to calculate the BET specific surface area and the pore size distributions, respectively. As shown in Table 1, the multipoint BET specific surface area (30.9 m² g⁻¹), total pore volume of a single point (0.573074 cc g⁻¹), adsorption micropore volume of a single point (0.012632 cc g⁻¹), and the BJH pore size (37.1 nm) of the nanosheets are more than that (26.2 m² g⁻¹, 0.308601 cc g⁻¹, 0.010612 cc g⁻¹ and 23.5 nm) of the lamination product. However, the difference in the specific surface area between the two products is not large. In contrast, the grain size (11.8 nm) of the nanosheet is less than that (367.6 nm) of the lamination product, which is in agreement with the comparison of SEM images, and EDX and XRD patterns. A comparison of the properties of nano MgO products that were prepared under ordinary pressure shows that they are similar to that of the products prepared by the novel method, and are also in accordance with the SEM images in Fig. 4.

Table 1 Comparison of the properties of nano MgO products prepared by (A) the novel method and (B) under ordinary pressure

Properties of products	Nanosheets (A)	Laminations (A)	Product with 10 wt% PVA (B)	Product with 20 wt% PVA (B)
Multipoint BET specific surface area (m^2 g^−1)	30.9	26.2	32.0	27.0
Total pore volume of a single point (cc g^−1)	0.573074	0.308601	0.687784	0.327847
Adsorption micropore volume of a single point (cc g^−1)	0.012632	0.010612	0.014067	0.009850
BJH pore size (nm)	37.1	23.5	60.9	24.3
Grain size (nm)	11.8	367.6	26.6	39.8

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Conclusions

In summary, sheets and laminations of nano MgO were prepared by the nitrogen-protective pressurization method for the first time. Changes in morphology and structure were revealed by altering the amount of PVA and calcination time. Moreover, a simple micro stereoscopic structure of the MgO spherical units was proposed according to EDS spectra. Adsorptive force influenced the base units so that they assembled into sheets and laminations of nano MgO. Future research will concentrate on discovering how this novel method influences the morphology and structure of nano MgO. In addition, the exact structures of the nano MgO will be elucidated in future works.

Acknowledgements

This work is financially supported by the National Sci-Tech Support Plan of China (no. 2007BAE25B01) and the National Natural Science Foundation of China (NSFC) under Grant no. 20776086 and 21076126.

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