A Salt-Assisted Combustion Method to Prepare Well-Dispersed Octahedral MnCr 2 O 4 Spinel Nanocrystals

Well-dispersed nanocrystalline MnCr 2 O 4 was prepared by a salt-assisted combustion process using low-toxic glycine as fuel and Mn(NO 3 ) 2 and Cr(NO 3 ) 3 ⋅9H 2 O as raw materials. The obtained products were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared (FT-IR) spectroscopy, Raman spectroscopy, Transmission ElectronMicroscopy (TEM), and Scanning Electron Microscopy (SEM).The fabrication process was monitored by thermogravimetric and differential thermal analysis (TG-DTA).The phase formation process was detected by XRD, and MnCr 2 O 4 single phase with high crystallinity was formed at 700C. TEM and SEM images revealed that the products were composed of well-dispersed octahedral nanocrystals with an average size of 80 nm. Inert salt-LiCl played an important role in breaking the network structure of agglomerated nanocrystallites.


Introduction
In recent years, nanostructured materials have found extensive applications such as catalytic, electrochemical sensors for biological and pharmaceutical analysis, and high capacity anode materials due to exceptional properties of nanostructured materials where at least one dimension of the structure is less than 100 nm [1][2][3][4][5].
Mixed metal oxides represented by the general formula AB 2 O 4 are spinel structure oxides with a variety of interesting electrical, magnetic, and optical properties [6,7].Due to the exceptional properties, the complexes are widely applied in wastewater treatment and photocatalytic field [8,9].Generally, A is bivalent cation, and B is trivalent cation.A cation is in fourfold coordination and B cation retains the sixfold coordination.MnCr 2 O 4 is ferromagnetic spinel, in which the Mn 2+ cations occupy the tetrahedral (A) sites and the Cr 3+ cations occupy the octahedral (B) sites.Due to their remarkable magnetic and electric properties, they have received broad interests in theoretical and experimental investigations for application purpose [10,11].Many reports found that MnCr 2 O 4 spinel structure was usually presented on the top of chromic scale as coatings in carburization attack in many petrochemical industrial processes [12,13].Moreover, MnCr 2 O 4 exhibits much better resistance to carbonaceous attack than Cr 2 O 3 [14].It is reported that MnCr 2 O 4 nanocomposite has a vital effect on the NO 2 sensing property for YSZ-based potentiometric sensor [15].Therefore there has been a growing interest focused on the investigation of synthesis and properties of nanostructured MnCr 2 O 4 materials.
Traditionally, MnCr 2 O 4 was prepared by solid-state reaction using a stoichiometric mixture of MnO 2 and Cr 2 O 3 powders with an atomic ratio of 1 : 1 at 1000 ∘ C sintering for 10 h [16,17].Although it was simple, this process had several serious drawbacks, including the high reaction temperature and the limited degree of chemical homogeneity.Precursor method was one of the typical strategies to synthesize well-dispersible nanometal oxides [18].In our previous study, the complex oxides nanocrystalline were easily obtained by a salt-assisted combustion method [19,20].In this paper, we present the preparation and characterization of well-dispersed MnCr 2 O 4 nanocrystals by the salt-assisted combustion method.The fabrication procedure of MnCr 2 O 4 can be referred to in the literature [20].Mn(NO 3 ) 2 and Cr(NO 3 ) 3 ⋅9H 2 O were used as precursors of Mn and Cr, respectively.The molar ratio of Mn : Cr was 1 : 2. Glycine was used as fuel.Firstly, an appropriate amount of glycine was dissolved in deionized water.Then proper amounts of Mn(NO 3 ) 2 , Cr(NO 3 ) 3 ⋅9H 2 O, and LiCl were added to the glycine aqueous solution in turn.The mixed solution was vigorously stirred for 2 h at 60 ∘ C and evaporated at 120 ∘ C. At this stage, the viscous liquids were swelled with the evolution of gases, and self-propagating solution combustion slowly occurred to yield the loose powders.The obtained powders were calcined at different temperatures ranging from 400 to 700 ∘ C for 3 h in air.In order to remove salt, the as-calcined powders were filtered and washed with hot deionized water and ethanol until the Cl − was eliminated.Finally, the product was dried in an oven at 80 ∘ C. To study the influence of the addition of inert salt during the reaction on the product particles, MnCr 2 O 4 nanocrystals were also prepared under the condition without adding LiCl in the process of reaction.

Instrumentation.
The thermal decomposition process of the sol was investigated by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) using Beijing WCT-2A thermal analyzer from 50 ∘ C to 750 ∘ C, with a heating rate of 20 ∘ C/min and Al 2 O 3 as reference.The crystalline phase structure was determined by Bruker D8 Advance Xray diffractometer (XRD) using Cu K radiation.FT-IR spectra of KBr powder-pressed pellets were recorded on a Bruker Vector 22 spectrometer.Raman spectra were run on a Renishaw in Raman microscope.Transmission electron microscopy (TEM) image was recorded on a JEOL JEM-2100 transmission electron microscope operating at 200 kV.Scanning electron microscopy (SEM) image was recorded on a JSM-7500F scanning electron microscope.Energy dispersive spectrum (EDS) analysis was taken with EDAX electron microscope.

TG-DTA Analysis.
In order to study the thermal behavior of the precursor, the corresponding TG and DTA curves are shown in Figure 1.The TG curve shows that the weight loss started at 50 ∘ C; the first endothermic peak can be attributed to the removal of the solvent water in the precursor.The exothermic peaks at 228 ∘ C and 352 ∘ C with the weight loss (12.45%) are due to the burning of the glycine.The third exothermic peak is related to the formation of the oxide as no other exothermic process was observed after 457 ∘ C.These are identical to the XRD patterns (Figure 2).The measured overall weight loss 34.01%was slightly less than the theoretical weight loss 37.14%, which may be due to the incomplete burning of the glycine in the self-propagation process.In order to further investigate the dispersibility of obtained particles, SEM images under different magnifications of MnCr 2 O 4 particles are shown in Figure 5.It is clear that the average particle size is 80 nm.The nanoparticles are of tetrahedral shape.The SEM micrographs also reveal that the samples have good dispersibility.It can be seen that there are a few abnormal large grains.However, most of the grains are uniform and well-dispersed.

Crystal Growth Process.
As is well known, when solvent evaporates to exceed the saturated solubility of solute in the heating process, solute will precipitate, especially in seed precipitation.Since the self-propagating combustion reaction released a large amount of heat in an instant, the particles could be formed under this condition.The salt precipitation in situ was completed in an instant to form a thin layer of salt crust on the surface of the newly formed nanoparticles.After the rapid cooling, the salt-coated particles were trapped in the salt matrix, which prevented the reagglomeration of the particles.Therefore, the introduction of LiCl in the process of traditional solution combustion reaction could effectively prevent nanocrystallites from forming the inseparable threedimensional network during the calcination.Instead, welldispersed nanoparticles were formed.in the formation of well-dispersed nanocrystals.The developed procedure is simple and well controlled.

2. 1 .
Preparation of MnCr 2 O 4 Nanocrystals.All reagents were of analytical grade and used without further purification.

Figures 3 (
Figures 3(a) and 3(b), respectively.The bands at 555 cm −1 and 652 cm −1 are the characteristic vibration peaks of spinel MnCr 2 O 4 nanocrystals.Figure 3(b) shows the IR spectra of MnCr 2 O 4 nanocrystals.There are two absorption peaks at 516 cm −1 and 621 cm −1 , which are attributed to the Mn-O vibration frequency of the metal at tetrahedral clearance and octahedral clearance, respectively [21].3.4.Morphology Analysis.The size, shape, and agglomeration state of the MnCr 2 O 4 particles obtained by the salt-assisted combustion method at 700 ∘ C are shown in Figure 4. TEM image of MnCr 2 O 4 particles obtained in the reaction process without inert salt is shown in Figure 4(a).It reveals that the MnCr 2 O 4 particles are composed of cube-like agglomerated structures.As shown in Figure 4(b), MnCr 2 O 4 particles obtained by the salt-assisted combustion method are uniform in both morphology and crystallite size and are cubic-like with good dispensability.The average size calculated from the TEM image is 80 nm, which is consistent with the result from

Figure 5 :
Figure 5: Representative SEM images under different magnification of the nanocrystals MnCr 2 O 4 obtained by precursor calcined at 700 ∘ C for 3 h.

3. 6 .
EDS Analysis.EDS was used to further confirm the composition of the obtained samples.The EDS analysis of the obtained products indicates that MnCr 2 O 4 nanocrystals are composed of manganese, chromium, and oxygen with an approximate molar ratio Mn/Cr/O ≈ 1/2/4, giving a stoichiometric formula of MnCr 2 O 4 with no chemical segregation phenomenon (Figure6).
MnCr 2 O 4 nanocrystals were successfully made by the salt-assisted combustion method at a relatively low temperature.The calcination temperature had an important effect on the crystal sizes and lattice distortion.TEM results indicated that the introduction of inert salt-LiCl into the solution combustion synthesis process broke up the network structure of agglomerated nanocrystallites and resulted

Figure 6 :
Figure 6: EDS analysis of MnCr 2 O 4 nanocrystals obtained by precursor calcined at 700 ∘ C for 3 h via the salt-assisted method.