Structural characterization of LaCoO 3 perovskite nanoparticles synthesized by sol–gel autocombustion method

Nanostructure perovskites such as LaMO 3 (where M = transition metal such as Mn, Co, Ni, and Fe) have captured attention in materials science fields due to their promising catalytic properties. In this study, the LaCoO 3 perovskite nanoparticles were synthesized by a two-step route via the sol–gel autocombustion method. In this method, lanthanum nitrate and cobalt nitrate were used as metals sources, after dissolving in distilled water. PVP was used as a surfactant, while urea and glycine were applied as fuel. The sol was formed at the stirring stage at 60 ◦ C, and then continued to gelation through water evaporation at 90 ◦ C, to end up in the autocombustion state. The product of combustion was washed, centrifuged three times, and heat-treated at 600 ◦ C for 2 h. Synthesized nanoparticles were characterized by scanning electron microscopy, X-ray powder diffraction (XRD), and particle size analyzer. Characterization results show that nanoparticles were synthesized in a narrow size range, below 100 nm, with perovskite structure using sol–gel autocombustion method; these particles were spherical in shape and without visible porosity on the surface. The purity and crystalline size of nanoparticles were studied through XRD analysis indicating that variation in these parameters depends on the fuel and fuel-to-oxidizer ratio, as impurities decreased by increasing the fuel ratio, for both glycine and urea. In addition, using glycine is demonstrated to result in better purity as compared with urea as fuel.

nanoparticles is carried out through diverse chemical methods, including sol-gel, reduction-oxidation, hydrothermal hydroxide oxidation, and metal-salts decomposition. In order to find the most attractive method of synthesis, parameters such as production cost, ability to operate continuously, and production rate are considered. The solution combustion synthesis (SCS) or sol-gel autocombustion method appears to be a promising method due to its low production cost since it requires low-cost precursor materials and equipment. This method allows synthesis of a wider range of metal oxides nanoparticles, and also shows to be more efficient compared with other methods. [1][2][3][4][5] The perovskite oxides are mixed metal oxides with the general formula AMO 3 , where A is a larger cation (usually an alkaline-earth or rare-earth metal cation) and M is the smaller cation (usually a transition metal cation: e.g., Mn, Co, Fe, Ni) surrounded by six oxygen atoms in an octahedral coordination. 6 However, perovskite oxides of the form LaMO 3 , where M is a transition metal, containing Mn, Co, Fe, and Ni have been synthesized in different structures such as nanostructured films, hollow spheres, and mesoporous nanowires, nanoparticles, porous, and fibrous structures, [7][8][9][10][11][12] and have been used in variety of applications, including gas sensors (CO, propane, and ethanol sensing), photocatalytic reduction of CO 2 with visible light, electronic devices, electrochemical systems, solid oxide fuel cell, solar cells, and catalysts. 11,[13][14][15][16][17][18][19][20][21] Similarly, to synthesize and develop the perovskite phase of LaCoO 3 as nanoparticles and nanowires, different methods have been used. These methods include thermal decomposition, hydrothermal decomposition, microwave-assisted coprecipitation, coprecipitation, spray-flame, sol-gel, and combustion methods. With the exception of sol-gel and combustion, these methods need different facilities and precursors that make the processes to be time-consuming and costly. 10,18,[21][22][23][24][25][26][27][28][29] LaCoO 3 perovskite nanoparticle as a catalyst for NO x gas reduction is a promising substitute of three-way catalysts (TWCs) based on noble metals. Narrow size destitution of catalysts particles are vital characteristics.
This study aims to synthesize spherical solid nanoparticles of perovskite phase of LaCoO 3 in a two-step by combining the sol-gel and autocombustion methods in narrow size distribution. The influence of fuel ratio and different fuels on nanoparticles' characteristics is also addressed in this research work since such particles can be used as a catalyst in the reduction of nitrogen oxide gases (NO x ).

EXPERIMENTAL
The following materials were used as precursors for the synthesis of perovskite particles: Lanthanum Nitrate (La(NO 3 ) 3 -6H 2 O) and Cobalt Nitrate (Co(NO 3 ) 2 -6H 2 O) as a supplier of metal elements, and glycine (C 2 H 5 NO 2 ) and urea (CH 4 N 2 O) as fuel, PVP as a surfactant. All of the precursor materials were obtained from Merck-Germany and used in the experiments without further purification.
Step 1: A 0.1M solution of lanthanum nitrate and cobalt nitrate was prepared in 100 ml of distilled water, using a 250 ml beaker. The solution was stirred on a magnetic stirrer for 15 min until the material was completely dissolved, forming a uniform pink solution.
Step 2: Four different samples were prepared, as shown in Table 1. An appropriate amount of glycine and urea was slowly added to the solutions. The glycine/total metals and urea/total metals ratios were kept as 1 and 2 in this experiment (F/O = 1 and 2). The temperature of the solutions was increased to 60 • C. Five weight percent PVP of the available metals was added. PVP played the role of surfactant in the experiment to obtain a uniform shape and narrow size distribution. Solutions were vigorously mixed on a magnetic stirrer for 30 min.
Step 3: The temperature of the solutions was increased to 90 • C to gradually evaporate water. The solutions were stirred using a magnetic stirrer until gelation and combustion occurred. The powders were rinsed using distilled water and centrifuged at 2000 RPM for 15 min, three times. The collected nanoparticles were heat treatment in a furnace at 600 • C for 2 h.  Equation (1) provides the overall combustion reaction for the preparation of lanthanum cobalt oxide during combustion stage, using different fuels. 30 During the experiment, all parameters that influence the properties of nanoparticles such as heating rate, exposure to air, and stirring speed were kept constant. The variable parameters were fuel and fuel-to-oxidizer (F/O) ratio.
The following figure represent the schematic process of preparation method ( Figure 1).

RESULTS
X-ray powder diffraction (XRD) patterns were recorded by a D8 Advance (Bruker) with Cu Kα (wavelength 1.54 Å) after nanoparticles were heat treated at 600 • C for 2 h. Scanning electron microscopy (SEM; FEI Company, USA-Quanta 200) was used to investigate the morphology, size, and uniformity of the products. A particle size analyzer (PSA) was used to determine the size distribution. During the PSA test, the ethanol was used as a dispersant, and the temperature was set at 25 • C. The duration of the test was 50 s. The powder XRD data shown in Figure 2 Figure 3(A,B) shows that using urea as fuel may cause changes in purity and crystalline properties of the final product. Production of LaCoO 3 nanoparticles, with Co 3 O 4 , La 2 O 3 , and La 2 O 2 (CO 3 ) as impurities, was obtained by using urea in both F/O = 1 and F/O = 2 ratios. It can be concluded that a higher ratio of glycine causes complete oxidation, while low and high ratios of urea cause partial oxidation and lower degrees of incorporation of La and Co oxide into the perovskite structure. Because combustion enthalpy of fuel molecule in glycine is higher than the urea and it may cause complete oxidation of metals. 30 Finally, crystallite size and the average crystallite size was estimated according to the Debye and Scherrer formula from the (110), (024), and (214) reflection of the LaCoO 3 phase at 2θ ≈ 32.9 • , 47.5 • , and 58.9 • , respectively, and is presented in Table 2.
where: D is the crystallite size is the X-ray wavelength (1.5418 Å) is the full width of the half maximum of the diffraction peak is the Bragg diffraction angle Table 2 shows that LaCoO 3 synthesized with glycine exhibited the lowest crystallite size (∼14.9 and 28.1 nm) compared with the synthesis with urea (∼41.6 and 25.7 nm). This result reveals that glycine and urea act in the opposite manner; while increasing fuel ratio in glycine increase the crystallite size and purity, increasing urea decreases crystallite size and improves purity.   Results from particle size analyses show the narrow size distribution as illustrated by Figures 4 and 5, and the SEM micrograph indicates that.
Particle size analysis confirms the SEM result, which increasing the glycine ratio causes increasing particle size, while urea acts oppositely.
The morphologies of LaCoO 3 synthesized with the sol-gel autocombustion method using glycine and urea as fuel are presented in Figures 6(A,B) and 7(A,B), respectively. The SEM images show the non-porous nature of the synthesized perovskite particles with spherical shape in all samples. The porous nature can be due to the nitrate precursors because burning of the nitrates at a higher temperature may lead to porous structures.
A comparison of these micrographs reveals that the glycine in the ratio of 2 resulted in a lower particle size and greater homogeneity, while the powder prepared by the urea showed a relatively bigger particle size. Some agglomeration and inhomogeneity in shape and size are also observed. The average particle size of LaCoO 3 synthesized by glycine is below 70 nm with a spherical shape; whereas the powder obtained using urea has a broader particle size distribution and inhomogeneity in shape. However, SEM results of particle size are different from the results of crystallite size estimation (see Table 2). Particle size is different from crystallite size suggesting that individual particles have several crystallites. It would seem a particle is made up of several different crystallites.

CONCLUSION
LaCoO 3 perovskite nanoparticles can be synthesized using sol-gel autocombustion, two-step method. This method is low-cost, efficient, and does not require advanced equipment for synthesis. Investigation of the lattice structure, morphology, and particle size by XRD, SEM, and PSA analysis methods indicate that particles are uniform in shape and size. These characteristics will play an important role in the use of perovskite nanoparticles as catalysts and lead to uniformity in the properties of the catalyst. The uniform size nanoparticles produced through this method is a result of the use of surfactants in the synthesis process. The results confirm the successful synthesis of LaCoO 3 nanoparticles of spherical shape and narrow size distribution, below 100 nm in size. LaCoO 3 perovskite nanoparticle as an excellent catalyst for CH 4 oxidation, CO oxidation, and NO reduction, is a promising substitute for TWCs based on noble metals, as they are limited by restricted resources, high price, and particle growth at high temperatures.