Synthesis of LaCoO3 nanoparticles by microwave process and their photocatalytic activity under visible light irradiation

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Abstract

We have investigated the photocatalytic activity for the decomposition of methyl orange on the LaCoO3 perovskite-type oxides prepared at different conditions using microwave process. In the case of LaCoO3 catalysts calcined above 500 °C, the formation of the perovskite crystalline phase was confirmed. From the results of UV–Vis DRS, all the catalysts have the similar absorption spectrum up to visible region. The chemisorbed oxygen plays an important role on the photocatalytic decomposition of methyl orange and the higher the contents of chemisorbed oxygen, the better the performance of photocatalyst.

Introduction

Titanium dioxide has been widely used as a photocatalyst because of its low cost, non-toxicity and photochemical stability [1]. However, TiO2 can only absorb UV light (λ < 387 nm) which is only 3–5% of solar light due to its wide band gap of 3.2 eV [2]. Various methods have been employed to improve the visible light photocatalytic efficiency of TiO2 in the visible region, such as metal or non-metal ion doping, noble metal deposition, conjugated polymer modification and composite semiconductor formation [3]. To maximize the use of sunlight, a new type of photocatalyst with a perovskite structure has been exploited as a highly effective photocatalysts in the visible region in recent years [4].

LaCoO3 with typical ABO3-type perovskite structure has attracted considerable attention due to its high catalytic activity, low cost, and environmental friendliness. However, there have been a very limited number of literature reports about its photocatalytic property [5]. In addition, a variety of methods, such as solid state reaction, sol–gel, hydrothermal technology, the polymerized complex technique and so on, have been applied to synthesize perovskite-type oxides [6], [7]. Conventional processes of synthesis involve many defects, for instance, high calcination temperature for a long time and no control of particle size. In order to solve these problems, the microwave-assisted method can be applied for the preparation of perovskite-type oxides. Microwave heating has lately popular in the crystallization of materials such as zeolites and other inorganic materials, due to its higher yields, cleaner synthetic method and shorter crystallization time compared with the conventional method. The synthesis of metal oxides with microwave heating has many advantages such as fast crystallization [8], increased phase purity and selectivity, narrow size distribution and facile morphology control.

In this paper, we prepared nanosized LaCoO3 perovskite-type oxides at different conditions using microwave process. The LaCoO3 perovskite oxides were characterized using X-ray diffraction (XRD), DRS, and XPS in order to determine the nature of the metal species. In addition, their activity as photocatalysts for the decomposition of methyl orange was investigated.

Section snippets

Experimental

Analytical-grade lanthanum nitrate (La(NO3)3·6H2O), cobalt nitrate (Co(NO3)2·6H2O) and malic acid were used as raw materials. In preparation of the perovskite-type oxides, 1.5 mol of malic acid was added into a mixed aqueous solution of metal nitrates. The solution was then evaporated to dryness with stirring, and further dried at 160 °C. Microwave assisted synthesis was performed using modified 300 W, 2.45 GHz home microwave digestion system (LG Electronic Co.). When the microwave process is used,

Results and discussion

After the drying process of the mixed solution of metal nitrates and malic acid at 160 °C, precursor is obtained by following the sol–gel state. If this precursor is calcined at 650 °C directly, a lot of impurities are able to be mixed in the catalyst. Thus we have to calcine the precursor stepwise to prevent the formation of impurities.

Fig. 1 shows the thermogravimetric analysis of powder precursor in air. TGA peak shows the decrease of weight three times at the range 200–280 °C, 300–450 °C, and

Conclusions

In this study, we have investigated the photocatalytic activity for the decomposition of methyl orange on the LaCoO3 perovskite-type oxides prepared at different conditions using microwave process. From XRD and TEM results, the formation of the perovskite crystalline phase, in the case of LaCoO3 catalysts calcined above 500 °C, was confirmed. From the results of UV–Vis DRS, all the catalysts have the similar absorption spectrum up to visible region. The chemisorbed oxygen plays an important role

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0006722) and Brain Busan 21 Project in 2010.

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