Characterization and low-cost, green synthesis of Zn²⁺ doped MgO nanoparticles

1 Introduction

Magnesium oxide (MgO) nanoparticles with large surface area have been investigated comprehensively in various applications, such as in catalysts, catalyst support, hazardous waste treatment, antimicrobial materials, refractory materials, destructive adsorbent for a large number of pollutants, and superconductor materials [1], [2], [3].

Furthermore, doping is an important parameter used for nanoparticles due to the reformation of physicochemical properties of metal oxide, which has broad applications, namely in optoelectronics, dilute magnetic semiconductors, and photo detectors [4]. Recently, various elements like Fe, Cd, Mn, Ca, and Zn have doped MgO nanoparticles, making significant versatile properties. As a vivid illustration, with Mn doped MgO nanoparticles, the magnetic property changed from diamagnetic to ferromagnetic [5], [6], [7], [8].

In addition, the wide range of applications can be affected by the particle size and crystal morphology of particles determined by scientists to concentrate on the synthesis of MgO nanoparticles. Different synthesis methods have been expressed, including thermal evaporation [9], flame spray pyrolysis [10], the sol-gel technique [11], chemical vapor deposition [12], hydrothermal [13], the auto combustion method [8], and the microwave-induced combustion process [3]. Nevertheless, some of these preparation methods are expensive and complicated processes. Moreover, the use of environmentally cancerous chemicals and organic solvents, which are poisonous, leads to large quantities of wastes inserted into water sources and environments [8], [9], [10], [11], [12], [13]. To solve these problems, a simple and viable alternative to chemical and physical methods, namely a green method, has drawn attention, employing fungi, actinomycetes, juices, and plant extracts, which are low-cost, non-toxic, and eco-friendly precursors for the synthesis of nanostructures.

Recently, the biosynthesis of MgO nanoparticles has successfully attracted more attention and interest and some green methods have been developed, including the use of ethanolic fruit extracts of Aloe vera gel [6], Emblica officinalis [14], Acacia gum [15], and concentrated solar energy [16].

In this paper, an attempt was made to synthesize Zn²⁺-doped MgO nanoparticles, mainly due to ionic radius – the ionic radius of Mg²⁺ (0.72 A) is similar to that of Zn²⁺ (0.74 A) [17].

This substitution occurs as the result of a green co-precipitation method in the presence of an extract of Aloe Vera latex, and then it was investigated with respect to both structural characteristics and morphology. Analyses of the Zn²⁺-doped MgO nanoparticles were carried out using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Fourier transform infrared (FT-IR).

2 Materials and methods

3 Results and discussion

The Zn²⁺ doped MgO nanoparticles were synthesized via a green co-precipitation method with extract of Aloe vera latex as a stabilizer and capping agent to control the crystal growth of nanoparticles. The green co-precipitation method is an economical, non-toxic method which generally leads to the formation of uniform nanostructures.

The correct crystallinity corresponding to that of a single-phase cubic structure (space group Fm3̅m), with lattice parameters 𝑎 = 4.2160Å (JCPDS 087-0651), which is represented in Figure 1, was observed in the XRD pattern of powders. Overall, the diffraction pattern can show a peak broadening, owing to the effects of particle size and strain factor by using the slope of the Williamson-Hall plot [19]:

Figure 1:

The X-ray diffraction (XRD) patterns of Zn²⁺ doped MgO nanoparticles calcinated at: (A) 600°C, (B) 700°C, and (C) 800°C.

Based on the powder diffraction peak broadening, the strain values (ε) are less than 0.01, due to chemical synthesis and crystalline size of MgO nanoparticles presented in Table 1, confirming that the greater the calcination temperature is, the higher the crystalline size and the better the crystals growth will be. The sharp peaks demonstrate high crystalline size with no indication of any detectable secondary phases. In other words, Zn has been doped in the structure of MgO, albeit it cannot be seen in the XRD pattern. Putting the crystalline size (according to the Debye-Scherrer formula), lattice constant a and width of a diffraction peak at half maximum intensity β into the following equation [20]:

the dislocation density (δ) in the samples was calculated to be around 8.9×10¹⁴ in Table 1. It is agreed that a larger dislocation density implies a greater hardness in the nanoparticles, because of increasing grain boundary in the sample caused by a decrease in the grain size, preventing the movement of a dislocation by the others at the grain boundaries [21].

EDS analysis was selected to examine the chemical doping of Zn in MgO nanostructures after calcination at 800°C (Figure 2). Characteristic peaks for Zn, Mg, and O in the sample and any trace of the other additives or pollutions are observed. It is shown that the atomic ratio of Mg to Zn in the synthesized samples is about Mg:Zn=0.907:0.093. Mg_0.907Zn_0.093O nanoparticles confirmed the amount of Zn in comparison with XRD results.

Figure 2:

Energy dispersive X-ray spectrums after calcination at 800°C.

Additional study of Zn²⁺ doped MgO nanostructures, after calcination at 800°C (Figure 3), shows that the thermal treatment results in the essential disappearance of the IR absorption. Peaks related to the hydroxyl groups of adsorbed water on the surface of MgO nanoparticles were observed at 3430 cm⁻¹ and 1429 cm⁻¹. They are respectively related to the stretching and bending vibrations of the absorbed water on the surface of the samples during the handling to record the spectra, because nanoparticles have mixed with KBr [22], [23], [24]. The peak at 1056 cm⁻¹ indicates the formation of ZnO. A peak at 51 cm⁻¹ is attributed to Zn-O and Mg-O bending and stretching vibration of the Mg_0.907Zn_0.093O sample [25], [26], [27].

Figure 3:

Fourier transform infrared (FT-IR) spectra of Zn²⁺-doped in MgO nanoparticles after calcination at 800°C.

In this study, extractive Aloe vera latex was used as an efficient controlling agent of size and morphology. Four major C-glycosyl constituents in the Aloe vera latex are aloin A, aloin B, aloesin, and aloeresin A, shown in Figure 4. The presence of relatively permanent benzene rings in all of these compounds was controlling and most likely responsible for uniform shapes and sizes of the ensuing discrete nanoparticles [28], [29], [30]. A schematic diagram illustrating the effect of glycosyl from extractive Aloe vera latex is shown in Figure 5.

Figure 4:

Chemicals present in latex prepared from Aloe vera [23].

Figure 5:

Schematic of effect of C-glycosyl constituents in the Aloe vera latex in synthesis of Zn²⁺ doped in MgO nanoparticles.

The morphology and particle size distribution of the as-prepared samples were investigated using the FE-SEM technique, as shown in Figures 6 and 7 . FE-SEM images of the as-prepared nanoparticles using 100 ml of Aloe vera extract latex, being calcined at various temperatures of 600°C, 700°C, and 800°C are shown in Figure 6. Nanoparticles were distributed in a homogenous and uniform spherical-shape manner. In other words, they grew radially from the nucleation points on the surface of the aggregated nanoparticles. Segregation of nanoparticles via growth of regular crystalline grains during synthesis is observed, which may lead to greater availability of active sites. Based on Figure 7, with an increase of calcination temperature, the particle size distributions increased in the range of 30–65 nm, thanks to green synthesis. In fact, the C-glycosyl compound in Aloe vera extract latex in synthesis advances the growth of particles and prevents agglomeration or micro size structure [6], [31].

Figure 6:

Field emission scanning electron microscope (FESEM) images of of Zn²⁺ doped MgO nanoparticles with two magnitude that calcinated at: (A) and (B) 600°C; (C) and (D) 700°C; (E) and (F) 800°C.

Figure 7:

Particle size distribution of Zn²⁺ doped MgO nanoparticles that calcinated at: (A) 600°C, (B) 700°C, and (C) 800°C.

TEM images, high-resolution TEM (HRTEM) images, and selected area of electron diffraction of the Zn²⁺ doped MgO nanoparticles after calcination at 700°C are shown in Figure 8. Clearly, in Figure 8A and B it could be seen that the particles have an elliptical shape with diameters of 45–68 nm, and particle distribution is uniform, which is nearly equal to particle size in FESEM images and XRD data. Based on Figure 8C, the lattice fringes between the two adjacent planes is 0.24 nm, which corresponds to the MgO (111) plane, obviously affirming XRD values. The monocrystalline nature of cubic phase MgO nanoparticles is confirmed in Figure 8D. In HRTEM, there was no evidence of the presence of a ZnO crystalline phase, which confirmed Zn²⁺ doped MgO lattice, which was clearly observed in the EDS spectrum of the nanoparticles.

Figure 8:

Transmission electron microscopy (TEM) image of of Zn²⁺ doped MgO nanoparticles: (A) and (B) with different magnifications, (C) high-resolution transmission electron microscopy (HRTEM) image, (D) selected area of electron diffraction (SAED) pattern.

The crystallinity was estimated by comparison of crystallite size as ascertained by TEM/SEM particle size determination. Crystallinity index of nanoparticles was estimated using Eq. (3) [32]:

where I_cry is the crystallinity index, D_P is the average of particle size obtained by SEM or TEM images and D_cry is particle size calculated from the XRD data. Table 1 depicts the crystallinity index for all samples that scored near one, confirming a monocrystalline unit that was subsequently confirmed by HRTEM images.

4 Conclusion

In summary, for the first time, Zn²⁺ doped MgO nanoparticles with spherical morphology were synthesized in extract Aloe vera latex media, which is a facile, eco-friendly and inexpensive route. Then, their properties were investigated by XRD, EDX, FT-IR, FESEM, and HRTEM techniques. The XRD patterns indicate the structure of MgO without any impurity trace. The EDX analysis confirmed Zn²⁺ has doped in MgO nanoparticles because of its proportion. FT-IR spectra confirmed the formation of MgO nanoparticles. In addition, FESEM analysis showed uniform, homogenous particles with a diameter of around 45–65 nm, due to the green method, which has some advantages such being a non-toxic and low-cost method to obtain nanoparticle uniformly. HRTEM results confirmed the formation of monocrystalline MgO nanoparticles by interplanar spaces. This is in adjustment with XRD results.