Influence of Soluble Salt Matrix on Mechanochemical Synthesis of Lanthanum Cobaltate Nanoparticles

Introduction

LaCoO₃ has many practical applications owing to its excellent physical and chemical properties. The LaCoO₃-based materials have been known to show high catalytic properties for oxidation of carbon monoxide, methane, hexane, and toluene^1-4. Thus, it can be used as the catalyst for combustion, automobile exhaust and waste gas purification^5-7. Moreover, it could be used as electrode materials for solid-electrode fuel cells and gas sensors^8,9. The syntheses of LaCoO₃ have been achieved by many methods including conventional ceramic powder technology which leads to low the specific surface area, poorly active materials, requires high temperatures and long calcination periods. To overcome these limitations, several techniques were developed including sol–gel^4,6, amorphous heteronuclear complex decomposition synthesis¹⁰, the polymerizable complex method¹¹, combustion synthesis¹², the molten chloride flux method¹³, the alkaline coprecipitation method¹⁴ and electrochemical oxidation¹⁵. These methods can provide products of fine and homogeneous particles with high the specific surface area, but the processes are generally complicated and the reagents used are very expensive.

Recently, mechanochemical processing has been used for the synthesis of ultrafine powders. In this process chemical precursors undergo reaction, either during milling or during subsequent low temperature heat treatment, to form a composite powder consisting of fully dispersed ultrafine particles embedded within a soluble salt by-product. The ultrafine powder is then recovered by removing the salt by-product with a simple washing procedure. This technique has successfully been used to synthesize a wide variety of materials including transition metals¹⁶, magnetic intermetallics¹⁷, sulfide semiconductors¹⁸, and oxide ceramics^19-21. The mechanochemical processing technique allows significant control to be exercised over the characteristics of the final washed powder. For example, the average particle size can readily be controlled through the use of inert diluents and post-milling heat treatments. Furthermore, the simultaneous formation of ultrafine particles with an intervening salt matrix suggests that agglomerate formation can more readily be avoided than is possible with other synthesis techniques since the salt matrix inherently separates the particles from each other during processing. In addition, Achimovičová et al.²² have been reported that the effect of soluble salt matrix on the nanoparticles of lead sulphide (PbS) prepared by mechanochemical processing. In this study revealed that the addition of soluble salt matrix can be improved the specific surface area, the average particle size and homogeneity of PbS particles were synthesized.

The purpose of this research was to study of the effect of soluble salt matrix addition on the average particle size and the specific surface area of LaCoO₃ nanoparticles which synthesized by mechanochemical method.

Experimental

Materials and synthesis

Starting materials used in the experiment were LaCl₃ (Fluka, 99.0%), CoCl₂ (Sigma-Aldrich, 97%), Na₂CO₃ (Fluka, 99.0%) and NaCl (Merck, 99.9%). All the starting materials were dried at 80^oC for 2 h. The NaCl was used as soluble salt matrix and added to the starting powders at different concentrations. The mixture of starting powders was milled by planetary ball mill (Retsch, PM 100) at speed 300 rpm under atmospheric. The mechanochemical processing was used to synthesis of LaCoO₃ nanoparticles under condition with no addition of soluble salt matrix and performed according to the equation (1). On the other hand, the addition of soluble salt matrix was performed according to equations (2) and (3). The weight and the molar ratio between the reactants were selected empirically in the initial powder mixture.

Twenty grams mixture was put in a stainless steel pot of 250 cm³ inner volume with fourteen stainless steel balls of 20 mm diameter and milled for 2 h. The ground sample was calcined at 600^oC with a heating rate of 10^oC/min for 90 min, followed by washing with distilled water to remove NaCl phase from by-product. Then, the sample was dried in an oven at 110^oC for 2 h.

LaCl₃ + CoCl₂ + 2.5Na₂CO₃ + 0.25O₂              LaCoO₃ + 5NaCl + 2.5CO₂             (1)

LaCl₃ + CoCl₂ + 2.5Na₂CO₃ + 0.25O₂ + 2NaCl         LaCoO₃ + 7NaCl + 2.5CO₂    (2)

LaCl₃ + CoCl₂ + 2.5Na₂CO₃ + 0.25O₂ + 4NaCl         LaCoO₃ + 9NaCl + 2.5CO₂    (3)

Characterizations

The milled, calcined and washed samples were characterized by X-ray diffraction analysis (Philips X’Pert), using Cu-Ka radiation in an angular range from 2q = 5^o to 80^o, to identify the phases existing in the product. The average particle size was determined using a laser diffraction method fitted with a wet sampling system (Mastersizer S, Malvern), this particle diameter reported was calculated using volume distribution (D[4,3]). The specific surface area (SSA) of the sample was measured by a nitrogen gas adsorption instrument (Quantachrome, Version 1.11) based on the BET method. Morphology and microstructure of the prepared sample was observed by scanning electron microscope (SEM) and analyzed the chemical composition by energy dispersive spectrometer (EDS) technique (JSM-6301F, JEOL).

Results and Discussion

Fig. 1 shows XRD patterns of samples before and after milled at speed 300 rpm for 2 h by grinding mixture of LaCl₃, CoCl₂ and Na₂CO₃. Fig. 1(a) shows peaks which associated with LaCl₃, CoCl₂ and Na₂CO₃ mixture before milling and their peaks intensity decrease gradually and vanished after milled for 2 h. However, Fig. 1(b) shows XRD pattern of sample with no addition of soluble salt matrix accorded to the reaction equation (1). It clearly shows the new peaks of NaCl and these peaks correspond to the standard powder diffraction patterns (JCPDS) of NaCl file No. 05-0628. The appearance of the NaCl phase indicated that displacement reactions between LaCl₃, CoCl₂ and Na₂CO₃ occurred during milling and other phases such as various types of carbonate and possibly unreacted starting compounds, are present in the form of amorphous or minor phases in the ground product²³. This reaction occurred indicates that the mechanochemical (MC) solid-state reaction can be proceeded by grinding mixture of starting material powders using a high-energy ball mill at room temperature^19,24-26. In addition, the NaCl phase was formed in the ground product and it plays a big role to disperse or enable well separate nanoparticles of LaCoO₃²⁷. Moreover, XRD pattern of sample with addition of soluble salt matrix at the different concentrations which accorded to the reactions equations (2) and (3) also shows high intensity of NaCl diffraction peaks as shown in Figs. 1(c) and 1(d). Increasing of the intensity peaks were increased the concentration of soluble salt matrix addition, this resulted due to increase in the crystalline volume fraction of NaCl which added into the initial starting mixture during the grinding process²⁸.

Figure 1: XRD pattern of reactant mixture and milling samples; (a) starting mixture before milling, (b) no addition of soluble salt and milled for 2 h, (c) addition of 2 mol soluble salt and milled for 2 h and (d) addition of 4 mol soluble salt and milled for 2 h Click here to View figure

 

Fig. 2 shows XRD pattern of samples after milled and calcined at 600^oC for 90 min. The XRD pattern of all samples clearly revealed the new peaks of LaCoO₃ which were observed after heat treatment and associated with the NaCl phase. These results indicate that the reaction between the starting materials occurred during calcinations which can be given by equations (1)-(3). The presence of LaCoO₃ phase after heat treatment as shown in Figs. 2(a), 2(b) and 2(c), indicates that the milling of starting powders by mechanochemical processing leads to form of desired phase at low temperature (600^oC)^27-30.

Figure 2: XRD pattern of samples after milled for 2 h and heated at 600oC; (a) no addition of soluble salt matrix, (b) addition of 2 mol NaCl and (c) addition of 4 mol NaCl   Click here to View figure

 

Fig. 3 shows XRD pattern of samples after washed with distilled water. It was found that the peaks of LaCoO₃ are present in all conditions and do not have any impurities. Therefore, it can be concluded that the NaCl phase in the by-product was removed out by the washing operation^27,28. Moreover, the intensity of LaCoO₃ peaks show slightly broadening peak and decrease gradually with an increasing in concentrations of soluble salt matrix addition as shown in Figs. 3(a), 3(b) and 3(c). Decreasing of the intensity attributed that the NaCl phase within the starting powders can be prevent reaction occurring, due to the soluble salt matrix separate the reactants, reducing the reaction volume between the reactant particles, leading to a decrease in reaction rate and the rate of heat generation in the calcination process^25,28. Furthermore, the XRD patterns of LaCoO₃ peaks correspond to the standard powder diffraction patterns (JCPDS) file No. 25-1060 with perovskite-type structure.

Figure 3: XRD pattern of samples after washed with distilled water; (a) no addition of soluble salt matrix, (b) addition of 2 mol NaCl and (c) addition of 4 mol NaCl Click here to View figure

 

Table 1 shows the effect of soluble salt matrix addition on the average particle size (D[4,3]) and the specific surface area of samples. The results show that the average particle size and specific surface area of samples depend on the concentration of soluble salt matrix which added in the initial reactants mixture. The average particle size was decreased with increased in the concentration of NaCl, while specific surface area was increased. The results are accordance with previously studies of Dodd and McCormick²⁸ and Hector et al.³¹ which found that the addition of soluble salt matrix to the reactant mixtures can be reduced the average particle size and crystallite size. The average particle size of LaCoO₃ with no addition of soluble salt matrix is 3.14 mm, while the samples with addition of 2 mol NaCl and 4 mol NaCl decrease down to 2.94 and 2.63 mm, respectively. On the other hand, the specific surface areas were increased from 25.67 up to 26.93 and 27.48 m²/g when addition of 2 mol NaCl and 4 mol NaCl, respectively. Decreasing of the average particle size and increasing the specific surface area due to the effect of soluble salt matrix in the initial reactant mixture. The salt matrix is not only separate the LaCoO₃ particles to prevent agglomeration, but also enable well dispersed of LaCoO₃ particles after calcinations^24,25,28.

Table 1: Average particle size and specific surface area of samples

Samples	Average particle size (D[4, 3])             Specific surface area (mm)                                                       (m2/g)
LaCoO3                                3.14 ± 0.02                                              25.67 ± 0.01 LaCoO3 + 2NaCl                 2.94 ± 0.02                                              26.93 ± 0.01 LaCoO3 + 4NaCl                 2.63 ± 0.02                                              27.48 ± 0.01

 

The morphology of samples after washing was investigated by scanning electron microscope (SEM). SEM images clearly illustrated that the significant influence of the soluble salt matrix addition can improve the dispersion of LaCoO₃ nanoparticles. These results indicated that the existence of soluble salt matrix phase in the initial reactant mixture will prevent the agglomeration of particles or separate the LaCoO₃ nanoparticles during calcination. Fig. 4(a) shows SEM micrograph of the LaCoO₃ nanoparticles which no addition of soluble salt matrix, is comprised of highly agglomerate particles. In the contrary, Figs. 4(b) and 4(c) show SEM micrographs of LaCoO₃ nanoparticles which addition of 2 mol NaCl and 4 mol NaCl show loosely agglomerated of particles and clearly separate of particles. Furthermore, EDS spectrums as shown in Fig. 5 illustrated evident peaks of La, Co and O which accord to chemical elements of LaCoO₃ nanoparticles.

Figure 4: SEM micrograph of samples after washing; (a) no addition of soluble salt matrix Click here to View figure

 

Figure 4(b): addition of 2 mol NaCl  Click here to View figure

 

Figure 4(c) addition of 4 mol NaCl Click here to View figure

 

Figure 5: EDS spectrums of LaCoO3 after washing Click here to View figure

 

Conclusions

LaCoO₃ nanoparticles can be successfully prepared by heat treatment of the milled powder obtained by mechanochemical reaction of LaCl₃, CoCl₂, and Na₂CO₃ with NaCl as a soluble salt matrix. The soluble salt matrix can separate and prevent the agglomeration of LaCoO₃ nanoparticles after calcinations at 600^oC. The addition of the soluble salt matrix in the reactant mixture can reduce the average particle size and increase the specific surface area. Moreover, SEM micrographs show loosely agglomerate nanoparticles.

Acknowledgments

I would like to thank the Commission on Higher Education, Thailand for supporting by grant fund under the program Strategic Scholarships for Frontier Research Network for the Ph.D. Program Thai Doctoral degree for this research. Also, I would like to thank the Faculty of Science and Graduate School of Chiang Mai University for partial financial support.

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