Synthesis , characterization and optical band gap of Pirochromite ( MgCr 2 O 4 ) Nanoparticles by Stearic Acid Sol-Gel Method

Article history: Received January 21, 2016 Received in revised form July 10, 2016 Accepted 11 July 2016 Available online 11 July 2016 Pirochromite (MgCr2O4) nanoparticles were successfully prepared in this study. During synthesis of the pirochromite nanoparticles, a sol-gel was prepared by using magnesium acetate and potassium dichromate as magnesium and chromium sources and by using stearic acid as the network. Infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDX) were used for the elemental analysis, and diffuse reflectance spectroscopy (DRS) and vibrating sample magnetometer (VSM) were used in order to identify, provide a fuzzy diagnosis, and determine the size and morphology of the particles, as well as to analyze the optical and magnetic properties of the particles. The particle size of MgCr2O4 nanoparticles was observed to fall within a range of 39 nm–71 nm. © 2016 Growing Science Ltd. All rights reserved.


Introduction
Nanoparticles have unique properties, such as high resistance, toughness, and rigidity, due to their high surface area to volume ratio.Magnesium oxide is a powder nanocompound; its spinel structure consists of a very large family of compounds with a wide range of physical properties, which has attracted great interest in the scientific community 1 .MgCr2O4 is traditionally created by a solid state reaction that requires a high temperature, which leads to a lower specific surface area and lower catalytic activity for the synthesized material, Synthesis and Characterization of Pirochromite (MgCr2O4) Nanoparticles by Stearic Acid Sol-Gel Method was successfully prepared in this study.Magnesium chromate spinel, MgCr2O4, is a well-known refractory material due to its high-melting point (2350°C), high chemical inertness against both acidic and basic slags, and high strength at elevated temperatures.MgCr2O4 is used in a wide range of industrial furnaces such as in secondary refining, non-ferrous metal metallurgy, cement rotary kilns, thermocouple protection tube, coal gasifier lining, glass furnace, etc.For its many applications, the nanosized spinel particles are greatly desired because nanocrystalline MgCr2O4 spinel particles certainly offer higher sintering reactivity, consequently decrease the sintering temperature of the product, and improve the properties of the products.The spinel structure includes a large family of compounds that exhibit a wide variety of physicochemical properties with important practical applications 2 .Thus, spinel oxides (AB2O4), pirochromite (MgCr2O4), and other related binary oxides have important technological applications, including their use as magnetic materials, high-temperature ceramics 3 , strengthening agents, sensor elements, interconnecting materials for solid oxide fuel cells, combustion catalysts, or catalytic supports [4][5][6][7][8] .For many of these applications, achievement of high surface area materials is greatly desired.The spinel MgCr2O4 displays a cubic structure belonging to the space group Fd3m, in which Mg and Cr ions occupy the tetrahedral and octahedral sites, respectively.MgCr2O4 has been taken as a model transition metal oxide combustion catalyst for chromite-related materials.The catalytic activity of this material in the oxidation of propane and propene has been also investigated, confirming that MgCr2O4 is an efficient complete combustion catalyst [9][10][11][12] .Magnesium oxide is an insulating material that has a high heat transfer capacity, and its binary spinel (MgCr2O4) and the other relevant binary oxide (Fe, Co, Ni) have important applications in different technologies, such as magnetic materials, high-temperature ceramics, and reinforcing agents, and can be used as sensors or elements that are intelligent and combustion catalysts 13 .The spinel structure of MgCr2O4 represents a cubic structure belonging to the Fd3m space group, in which magnesium and chromium ions are occupied in hexagonal and octagonal molecular shapes 14 .Although the metallurgical use of magnesium oxide is to provide resistance to decay and to create a brilliant shine as a component in alloys, such as stainless steel and chrome plating, and to serve as a catalyst in applications, MgCr2O4 is also used as a catalyst for the oxidation of propene and propane 15 .To overcome this limitation, in this study, nano-sized MgCr2O4was prepared by using a sol-gel synthesis process, as shown in Scheme 1.

Experiment
The chemical compounds that were employed in this work were obtained from Merck and were used with no purification.Transmission electron microscope (TEM) Scanning electron microscopy (SEM) analysis was performed on a Philips XL-30 field-emission scanning electron microscope operated at 16 kV.X-ray diffraction, XRD, and EDX were performed on the EM 3200model, manufactured by KYKY.The TEM pictures were recorded with Philips model EM 208 instrument at the accelerating voltage of 100 kV.The fine powders were dispersed in amyl acetate on a carbon-coated TEM copper grid.SEM was equipped with a LEO1455 UP, Oxford, UK.The UV-vis diffused reflectance spectra (DRS) were obtained from UV-vis Scinco 4100 spectrometer with an integrating sphere reflectance accessory.BaSO4 was used as reference material; UV-vis absorption spectra were recorded using a Shimadzu 1600 PC in the spectral range of 190-900 nm.

Synthesis of the MgCr2O4Nanoparticles
To synthesize the MgCr2O4 nanoparticles, stearic acid (0.4 mol) was melted in a beaker at 73°C.Magnesium acetate (0.1 mol) was then added to it.At this stage, a homogeneous solution was obtained.A potassium dichromate solution (0.2 mol) was added afterward and stirred until a homogeneous solution was obtained.This solution was cooled to room temperature and put in the oven for 6 hours to obtain a dry gel.Finally, the dry gel was calcined at 750°C; after it was cooled to room temperature for a few hours, MgCr2O4 nanoparticles were obtained 16 .

XRD Pattern of the MgCr2O4 Composition
To determine the composition of the MgCr2O4 crystalline structure, the XRD pattern was recorded.As shown in Fig. 1, the XRD pattern of the composition is in agreement with the reference pattern.The sharp peaks observed at 18° (2θ) , 37° (2θ), and 43° (2θ) indicated the presence of Mg, Cr, and O. Additionally, the observed peaks at 56 ° (2θ) and 63° (2θ) are related to the formation of Cr2O3 and CrO, respectively.As expected, the highest degree of crystallite was achieved when the calcined temperature occurred in the synthesis process 17 .

Morphology of samples
XRD has been performed on powder and the particle size L has been estimated with Scherrer formula (Eq.( 1)) 18 : (1) where K is the form factor (equal to 0.9), λ = 0.15418 nm, 2θ the peak position and Δ (2θ) the full width at half maximum of the diffraction peak in terms of radians.In this way, we find that the crystallite size of the powders Calcined at 600°C, 700°C and 750°C were about 39 nm. Figure 2

EDX Elemental Analysis
This type of spectrum is obtained as a result of bombarding samples with X-rays, electrons, or protons to determine the elements .Fig. 3 shows the EDX pattern of the MgCr2O4 nanoparticles.The EDX analysis of the calcined nano-powder created in the 900°C and 750°C environment illustrated the presence of Mg, Cr, and O, which confirms the production of MgCr2O4.

DRS Analysis
The absorption coefficient and optical band gap of a material are two important parameters by which the optical characteristics and its practical applications in various fields are judged.Figure 4 shows the DRS of pure MgCr2O4.In Fig. 4, a sharp absorption peak is observed around 250 nm, indicating the optical band gap attributed to the Cr3+ to Cr2+ charge-transfer interaction XP spectra reveal that a fraction of chromium appears in the form of Cr6+ species.Taking into account that its reduction is relatively easy when it is present in unsupported chromium oxides, the first reduction step (below 750°C) can be assigned to the reduction of Cr6+ to Cr3+.Besides, the reduction of Cr3+ to Cr2+ can apparently take place in the region above 750°C.The system operates based on casting light on the material surface and measuring the amount of reflected light in comparison with a standard (nonabsorption of the dispersion, usually BaSO4).Measuring dispersive-reflection is particularly useful for evaluating the properties of powdered materials.The DRS chart of MgCr2O4 nanoparticles is shown in Figure 4, in which the maximum absorbance was apparent at 250 nm 19 .Fig. 4 .The DRS chart of MgCr2O4 nanoparticles

Band-GapAnalysis
For direct band gap determination, a plot of versus is presented in Figure 5. Band gap value was obtained by extrapolating the straight portion of the graph on the h axis at =0, as indicated by the solid line in Fig. 5. Usually, the band-gap of the nanomaterials is larger than that of nano-crystallines or of non-nano materials.With an increase in the band-gap, the particle size of the semiconductor material decreases.The energy gap is an important feature of semiconductor materials, which determines their application abilities in optoelectronics 20 .Band-gap measurements are obtained through the absorption curve, in which α is a linear absorption coefficient of the matter.A semiconductor with a large band-gap cannot absorb a lot of light in solar cells.Thus, the use of synthesized salts on solar panels was introduced.Solar cells have material with a large band-gap.Synthesized salts are organic molecules that can absorb light on the surface of porous electrodes and that start the conversion of light to electrical current [21][22] .However, the use of a material with a large band-gap is not efficient enough for solar cells; two large band-gap materials are needed, as this current has to come to a nanowire.Fig. 5 shows the band-gap diagram of MgCr2O4 nanoparticles at a rate of approximately 1.8 eV, while the value in the MgCr2O4 bulk is 1.3 eV.This increase in the value of the band-gap indicates that the material has gained semiconductor properties 23 .

Conclusions
This paper investigated the synthesis and characterization of MgCr2O4 nanoparticles.The results obtained from the use of FT-IR, DRS, SEM, EDX, XRD, and VSM, and the subsequent comparison with reference patterns indicated that the preparation process of the nanoparticles in this study is appropriate.The particle size of the synthesized MgCr2O4 was in the range of 39 nm to 71 nm.The band-gap of the MgCr2O4 nanoparticles was also obtained and showed the rate of approximately 1.8 eV.Additionally, the VSM results indicated that the MgCr2O4 nanoparticles are paramagnetic.Their magnet value was approximately 0.2 emu g -1 .Although the metallurgical use of magnesium oxide is to provide resistance to decay and to create a brilliant shine as a component in alloys, such as stainless steel and chrome plating, and to serve as a catalyst in applications, MgCr2O4 is also used as a catalyst for the oxidation of propene and propane.

Fig. 1 .
Fig. 1.The XRD Diffraction Patterns of the MgCr2O4 Composition Calcined at (a) 600°C; (b) 700°C and (c) 750°C (a) shows a typical bright-field TEM image of the MgCr2O4 nanoparticles from the solution.All MgCr2O4 nanoparticles have a narrow size distribution.The particles are in the size 39 nm to 71 nm, and the average size of particles obtained from two methods is close approximately.Figure 2(b) shows the morphologies of MgCr2O4 products by SEM.It indicated that the size of the particles was homogeneous.