Facile Catalyst-Free One-Pot Synthesis and Optical Properties of MgO Nanocrystals with Different Morphologies in Atmospheric Air

Abstract

MgO nanocrystals with cube, wire and flower shapes were fabricated by thermal evaporation technique in air at atmospheric pressure. Mg powder mixed with graphite powder was used as the source material and no catalyst was used in the synthesis process. The morphology of the MgO nanocrystals was significantly changed from cube shape to wire and then flower-like shape with increasing the mass ratio of graphite to Mg in the source material. This indicates that the mass ratio of Mg/graphite in the source material played a crucial role in the morphological change of MgO nanocrystals. All the MgO crystals had a cubic crystal structure. The cube-shaped MgO crystals exhibited a blue emission centered at 420 nm, whereas the wire and flower-shaped MgO crystals showed an ultraviolet emission at 380 nm.

MgO nanocrystals synthesized via thermal evaporation of Mg/graphite powders with different mass ratios of (a) 5:1 and (b) 1:1.

1. Introduction

Nanocrystalline materials have attracted considerable attention due to their novel electrical and optical properties that they do not exhibit in bulk materials. Their novel properties are affected by the shape as well as the particle size of materials. Thus, much effort has been devoted to control the morphology of nanocrystalline materials. Nowadays, metal oxide nanocrystalline materials have received extensive interest for their widespread applications in electronics and optics.^1–5) Among the metal oxides, magnesium oxide (MgO) is one of the interesting functional oxides with various fascinating properties.

(MgO) is an insulating oxide with a wide band gap of 7.8 eV, low heat capacity, high melting point and high dielectric constant. Its interesting properties make it ideal for many applications in the field of catalyst,⁶⁾ high Tc superconductors,⁷⁾ heat resistance,⁸⁾ solar cells,⁹⁾ and metal oxide semiconductor transistors.¹⁰⁾ In particular, MgO has an exciton binding energy of 80 meV, which is advantageous for UV light emitter and detector applications.¹¹⁾ MgO nanocrystals also have potential applications in optoelectronic devices.

MgO nanocrystals have been obtained by diverse methods including thermal decomposition,¹²⁾ thermal evaporation,¹³⁾ hydrothermal,¹⁴⁾ sol-gel,¹⁵⁾ combustion¹⁶⁾ and chemical precipitation.¹⁷⁾ Among these methods, thermal evaporation method has the excellent advantages of low cost and simplicity. However, the thermal evaporation process has a disadvantage to be usually performed in vacuum, which makes the process a little bit complicated. In order to make the thermal evaporation process simpler, it is worthwhile to develop an effective synthesis process of MgO nanocrystals in air at atmospheric pressure. Furthermore, thus far MgO nanocrystals have been mainly synthesized using metal catalysts.^18–21) Catalysts can create contamination in the obtained nanocrystals. Thus it is important to explore a synthesis route of MgO nanocrystals without using catalyst.

On the other hand, in thermal evaporation process, the concentration of oxygen during the growth of oxide nanocrystals is very important because oxygen affects the volatility of the source materials and the stoichiometry of the vapor phase, which in turn influences the morphology of products. In this work, the reducing agent of graphite was mixed with Mg source powder so as to control the oxygen concentration in the atmosphere. The control of oxygen concentration led to the change in the morphology of MgO nanocrystals.

In this study, morphology of MgO nanocrystals synthesized by thermal evaporation process in air at atmospheric pressure was investigated. In particular, the morphology of the MgO nanocrystal could be changed by using Mg powder mixed with graphite powder as the source material. The effect of graphite on the morphology and luminescence property of the as-synthesized MgO nanocrystals was also studied.

2. Experimental Procedure

Mg powder mixed with graphite powder was used as the source material. Mg powder was mixed with graphite powder at different mass ratios of 5:1, 2:1, 1:1 and 1:2 to examine the effect of graphite on the formation of MgO nanocrystals. The powder mixtures were mixed in a ball mill for 5 h for homogeneous mixing. The source materials were placed in alumina crucibles and then the alumina crucibles were inserted into the center of a muffle furnace. Next, the furnace was heated to 1000°C and the furnace temperature was maintained for 1 h for evaporation and oxidation process. In addition, the effects of heating temperature and time on the morphology of MgO products were investigated. For the investigation, Mg/graphite powder mixtures with a mass ratio of 1:1 were heated for 1 h at 600–800°C, and for 20–40 min at 1000°C. After the process, the as-synthesized products were collected for characterization.

X-ray diffractometry (XRD) with Cu K_α radiation was used to identify the crystalline phases of the products. Field emission scanning electron microscope (FESEM) and energy dispersive X-ray (EDX) spectroscope were employed to study the morphology and components of the products. The cathodoluminescence measurement was performed at room temperature.

3. Results and Discussion

Figure 1 shows the XRD patterns of the products prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of 5:1, 2:1, 1:1 and 1:2. The XRD patterns of the products show the same diffraction peaks. The diffraction peaks are well coincident with the cubic crystal structure of bulk MgO which has the lattice constants of a = b = c = 0.421 nm. The positions of the diffraction peaks at 36.9°, 42.9°, 62.3°, 74.7° and 78.6° correspond to (111), (002), (022), (113) and (222) planes of MgO with cubic crystal structure. No other impurity phases are detected, indicating that the products are single-phase MgO.

Fig. 1

XRD patterns of the products prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of (a) 5:1, (b) 2:1, (c) 1:1 and (d) 1:2.

Figure 2 shows the EDX spectra of the products prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of 5:1, 2:1, 1:1 and 1:2. For all the products, only the elements of Mg and O are identified in the EDX spectra. No other elements except Mg and O are detected, which confirms that pure MgO phase was formed. The EDX result is in good agreement with the XRD result.

Fig. 2

EDX spectra of the products prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of (a) 5:1, (b) 2:1, (c) 1:1 and (d) 1:2.

The morphology of the MgO products was observed with SEM. Figure 3 shows the SEM images of the MgO products prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of 5:1, 2:1, 1:1 and 1:2. When the Mg/graphite powder mixture with a mass ratio of 5:1 was used as the source material, three-dimensional cube-shaped MgO nanocrystals are observed in the product. The side lengths of the MgO cubes are in the range of 200∼500 nm. The edges and corners are very sharp, and the side surfaces are very smooth and well-defined. In the case of the Mg/graphite source material with a mass ratio of 2:1, cube-shaped MgO crystals are also observed, but most of the MgO crystals have an irregular cube shape with rounded edges. As the mass ratio of graphite to Mg in source material increased to 1:1, nanowires with uniform diameter distribution and high aspect ratio were formed in large-quantity. The nanowires have an average diameter of 250 nm and lengths of several tens of micrometers. On the other hand, the Mg/graphite source material with a mass ratio of 1:2 produced novel flower-like MgO crystals as shown in Fig. 3(d). The petals of the flowers show a cone shape with a sharp tip. The flowers have the sizes ranging from 1.7 µm to 10 µm. The SEM results reveal that the mass ratio of graphite to Mg powder in source material have a strong effect on the morphology of MgO nanocrystals.

Fig. 3

SEM images of the MgO products prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of (a) 5:1, (b) 2:1, (c) 1:1 and (d) 1:2.

When the source material was a Mg/graphite powder mixture with a mass ratio of 5:1, Mg was evaporated and the Mg vapor reacted with oxygen in air at the process temperature to form MgO nuclei. MgO has a face-centered cubic crystal structure. Thus MgO nuclei with the cubic structure grow into cube-shaped crystals. With increasing the mass ratio of graphite to Mg to 2:1, graphite might prevent the oxidation of Mg vapor. Hence, MgO nuclei would not have well-defined cubic structure, resulting in the growth of irregular cube-shaped crystals with rounded edges. As the mass ratio of graphite to Mg further increased to 1:1, Mg was much more difficult to oxidize to MgO, which would lead to the low supersaturation level of MgO vapor. It is known that the low supersaturation level is favorable to the growth of one-dimensional crystals.²²⁾ Under the low supersaturation of growth species, growth species adsorbed on the surface of nuclei have the mean free path enough to reach energetically favorable sites. Then the growth species are incorporated into these sites, resulting in an anisotropic growth of crystal. This process leads to the growth of one-dimensional structures such as wires. When the mass ratio of Mg and graphite in the source material was 1:2, Mg powder was evaporated at the process temperature of 1000°C. However, the Mg vapor was not oxidized due to the reducibility of graphite, resulting in the formation of Mg and(or) MgOx (X < 1) crystals. The Mg and(or) MgOx (X < 1) crystals would be aggregated because of their low melting temperatures, leading to the formation of the micrometer-sized crystals with flower shape.²³⁾

Figure 4 shows the CL spectra of the MgO crystals prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of 5:1, 2:1, 1:1 and 1:2. The CL spectra show two emission bands. One is an UV emission band centered at approximately 380 nm and the other is a blue emission band with a maximum intensity at 420 nm. The blue emission band is observed in the CL spectra of the MgO crystals prepared using the Mg/graphite source materials with the mass ratios of 5:1 and 2:1. The UV emission band is observed from the MgO crystals prepared using the Mg/graphite source materials with the mass ratios of 1:1 and 1:2. It has been reported that the blue emission originates from oxygen-vacancy related defects (F-type centers) in MgO²⁴⁾ and the UV emission at 380 nm is attributed to low-coordinate oxide ions present at edge sites.²⁵⁾ The SEM images show that the cube-shaped MgO nanocrystals had higher surface area as compared to the wire- and flower-shaped crystals. Accordingly, the MgO nanocubes had the larger concentration of surface oxygen vacancies, which might be responsible for the blue emission.

Fig. 4

CL spectra of the MgO crystals prepared via thermal evaporation of Mg/graphite source materials with different mass ratios of (a) 5:1, (b) 2:1, (c) 1:1 and (d) 1:2.

Figure 5 shows the SEM images of the MgO products prepared via thermal evaporation of Mg/graphite source materials with a mass ratio of 1:1 for 1 h at 600°C and 800°C. The MgO products are composed of nanocrystals with irregular shapes. Well-defined shapes associated with cubic were not observed. Specially, MgO nanowires were grown in large quantity at 1000°C, whereas wire-shaped crystals were not formed at the temperatures lower than 800°C.

Fig. 5

SEM images of the MgO products prepared via thermal evaporation of Mg/graphite source materials with a mass ratio of 1:1 at (a) 600°C and (b) 800°C.

Figure 6 shows the SEM images of the MgO products prepared via thermal evaporation of Mg/graphite source materials with a mass ratio of 1:1 for 20 min and 40 min at 1000°C. When the process was performed for 20 min, MgO nanowires started to grow. As the growth time increased to 40 min, the density and size of MgO nanowires increased significantly.

Fig. 6

SEM images of the MgO products prepared via thermal evaporation of Mg/graphite source materials with a mass ratio of 1:1 for (a) 20 min and (b) 40 min at 1000°C.

4. Conclusion

By controlling the mass ratio of Mg/graphite in the source material, MgO nanocrystals with various morphologies were synthesized by thermal evaporation method. In addition, the synthesis process was carried out in air at atmospheric pressure, which made it simpler and less costly. The increase in the mass ratio of graphite to Mg in the source material transformed the morphology of the MgO crystals from cube shape to wire and flower shape. This result reveals that the mass ratio of graphite to Mg in the source material has an important effect on the morphology of MgO crystals. Two emission signals were detected in the CL spectra of the MgO crystals. A blue emission centered at 420 nm was observed from the MgO crystals with cube shape, while an UV emission at 380 nm was observed in the MgO crystals with wire and flower-like shapes. As a result, it is concluded that luminescence properties can be controlled by altering the morphology of MgO crystals. The MgO nanocrystals with different luminescence properties have potential applications in optoelectronic devices.

REFERENCES

• 1)   J.  Ma,  L.  Wang,  Y.  Wu,  X.  Dong,  Q.  Ma,  C.  Qiao,  Q.  Zhang and  J.  Zhang: Mater. Trans. 55 (2014) 1900–1902.
• 2)   L.G.  Teoh and  K.  Li: Mater. Trans. 53 (2012) 2135–2140.
• 3)   P.  Wang,  D.  Liu and  D.  Li: Mater. Trans. 53 (2012) 1892–1895.
• 4)   W.  Li,  W.  Banerjee,  S.  Maikap and  J.  Yang: Mater. Trans. 52 (2011) 331–335.
• 5)   H.  Sasaki,  G.  Saito,  N.  Sakaguchi,  R.  Ota and  H.  Takahashi: Mater. Trans. 60 (2019) 688–692.
• 6)   A.  Hellman,  S.  Klacar and  H.  Grönbek: J. Am. Chem. Soc. 131 (2009) 16636–16637.
• 7)   B.H.  Moeckly,  S.E.  Russek,  D.K.  Lathrop,  R.A.  Buhrman,  J.  Li and  J.W.  Mayer: Appl. Phys. Lett. 57 (1990) 1687–1689.
• 8)   H.  Wang,  B.  Glaser and  D.  Sichen: Metall. Mater. Trans. B 46 (2015) 749–757.
• 9)   K.  Tennakone,  J.  Bandara,  P.K.M.  Bandaranayake,  G.R.A.  Kumara and  A.  Konno: Jpn. J. Appl. Phys. 40 (2001) L732.
• 10)   G.  Jiang,  A.  Liu,  G.  Liu,  C.  Zhu,  Y.  Meng,  B.  Shin,  E.  Fortunato,  R.  Martins and  F.  Shan: Appl. Phys. Lett. 109 (2016) 183508–183510.
• 11)   D.M.  Roessler and  W.C.  Walker: Phys. Rev. 159 (1967) 733.
• 12)   R.  Sathyamoorthy,  K.  Mageshwari,  S.S.  Mali,  S.  Priyadharshini and  P.S.  Patil: Ceram. Int. 39 (2013) 323–330.
• 13)   Y.  Su,  Z.  He,  Y.  Chen and  D.  Cai: Rare Met. 25 (2006) 74–78.
• 14)   A.H.  Chowdhury,  I.H.  Chowdhury and  M.K.  Naskar: Mater. Lett. 158 (2015) 190–193.
• 15)   A.  Kumar and  J.  Kumar: J. Phys. Chem. Solids 69 (2008) 2764–2772.
• 16)   R.  Halder and  S.  Bandyopadhyay: J. Alloys Compd. 693 (2017) 534–542.
• 17)   F.  Meshkani and  M.  Rezaei: Powder Technol. 196 (2009) 85–88.
• 18)   J.  Zhan,  Y.  Bando,  J.  Hu and  D.  Golberg: Inorg. Chem. 43 (2004) 2462–2464.
• 19)   K.P.  Kalyanikutty,  F.L.  Deepak,  C.  Edem,  A.  Govindaraj and  C.N.R.  Rao: Mater. Res. Bull. 40 (2005) 831–839.
• 20)   C.  Tang,  Y.  Bando and  T.  Sato: J. Phys. Chem. B 106 (2002) 7449–7452.
• 21)   Y.  Su,  Z.  He,  Y.  Chen and  D.  Cai: Rare Met. 25 (2006) 74–78.
• 22)   X.  Wang,  J.  Song and  Z.L.  Wang: Chem. Phys. Lett. 424 (2006) 86–90.
• 23)   X.S.  Fang,  C.H.  Ye,  L.D.  Zhang,  J.X.  Zhang,  J.W.  Zhao and  P.  Yan: Small 1 (2005) 422–428.
• 24)   H.  Li,  M.  Li,  X.  Wang,  X.  Wu,  F.  Liu and  B.  Yang: Mater. Lett. 102–103 (2013) 80–82.
• 25)   M.  Sterrer,  T.  Berger,  O.  Diwald and  E.  Knözinger: J. Am. Chem. Soc. 125 (2003) 195–199.