Received 26 February 2015; accepted 17 April 2015; published 21 April 2015

ABSTRACT

A new type of photocatalytic material, double-perovskite oxides, LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) was synthesized via a conventional solid-state reaction process using Ln₂O₃, BaCO₃ and Co₂O₃ as raw materials. X-ray diffraction results show that the crystalline structures are a pure orthorhombic lattice and are consistent with LnBaCo₂O_5+δ microparticles. The photocatalytic activity of the LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders was further demonstrated in the degradation of Congo red (CR) under ultraviolet light irradiation with the dye solution concentration of 25 or 50 mg∙L⁻¹. The double-perovskite oxides LnBaCo₂O_5+δ show a certain photocatalytic activity during the degradation of CR under ultraviolet light, which means that they are one kind of the promising photocatalytic materials for the degradation of the azo dyes.

Keywords:

Double-Perovskite, LnBaCo₂O_5+δ, Photocatalysis, Congo Red

1. Introduction

With the society and economic development, the problem of environmental pollution is outstanding day by day. Especially, the contamination of water due to color effluents coming from different industries, such as textiles, dyestuff, and paper, which are toxic and low-biodegradable, discharges into the aquatic systems and leads to the serious destroy of the survival environment as well as serious damage to human health [1] -[5] . Among these pollution, azo dyes, characterized by the presence of the N=N linkage, are the mostly and frequently used for colorization in textile industries [6] . Faced with the global environmental pollution, the process as efficient as possible in the azo dyes degradation has widely been pursued [7] . Several studies of physical, chemical and biological methods have been carried out. However, since their very stable and complex aromatic structures, the traditional treatment means used for azo dyes effluents are sometimes ineffective, non-destructive and merely transfer pollutants from water to another phase, resulting in secondary pollution, and even generating greater toxicity aromatic amine materials [8] - [10] . In recent years, the photocatalytic process on semiconducting materials has greatly been concerned because it provides a green way to directly degrade waste water pollutants [11] . As a result, numerous semiconductor photocatalytic materials, such as TiO₂ [12] , SnO₂ [13] , ZnO [14] , and BiVO₄ [15] , have been developed and applied in photocatalytic degradation of organic pollutants due to their high efficiency, commercial availability and high chemical stability.

Perovskite oxides with the general formula of ABO₃ in which A-site (with ionic radius of larger than 1.0 Å) is a rare-earth or an alkali-earth element and B-site (having ionic radius in the order of 0.6 - 0.8 Å) is typically a 3D transition metal become the hotspot of material science because of their stable structure and unique physical and chemical properties [16] . The ideal perovskite ABO₃ has a cubic crystal structure, which is composed of a three-dimensional framework of eight corner-sharing BO₆ octahedrons. Among the structure, BO₆ octahedron is often considered as the basic cell and B-site cation locates in octahedral vacancy while A-site cation fills the twelve coordinate cavities formed by BO₆ network [17] [18] . Due to their unique properties, such as various types of oxygen vacancy order, intrinsic oxygen reduction reaction activity, high conductivity and magnetic properties, perovskites have been used in solid oxide fuel cells, oxygen separation, membrane reactor for partial oxidation of methane to syngas and also used as catalysts for controlled partial hydrocarbon oxidation and photocatalysis [19] - [21] . Especially, their high performances in photocatalytic reactions, such as overall water splitting and photo degradation of organic pollutants, attracted much attention because of their narrower band gap (often less than 3.0 eV) which could be easily excited under visible light or ultraviolet light irradiation [22] . As a photocatalyst, perovskites, such as SrSnO₃ [23] , SrTiO₃ [24] and LaTiO₃ [25] , have been extensively studied in the field of photo degradation of organic pollutants.

Double-perovskite oxides which named after perovskites ABO₃ are a large family and have wide application fields. Among them, the oxygen deficient ordered double perovskite cobaltates with the general formula LnBaCo₂O_5+δ have drawn significant interest as potential IT-SOFC (Intermediate Temperature-Solid Oxide Full Cell) cathodes, chemical gas censors, magnetic and oxygen permeation materials because of their high electronic con- ductivity, oxygen transport properties, oxygen surface exchange coefficient and superior oxide ionic diffusivity [26] - [29] . The crystal structures of these oxides are formed by stacking sequence CoO₂|LnO_δ|CoO₂|BaO|CoO₂ along the c direction, and the oxygen vacancies are mainly localized into LnO_δ layer. Transformation of a simple cubic perovskite with randomly occupied A-sites into a layered crystal with alternating lanthanide and alkali- earth planes reduces the strength of oxygen binding and provides disorder-free channels for ionic motion [30] . Since perovskite oxides possess high photocatalytic performances, as with close links to the perovskite compounds, double-perovskite oxides can be conjectured that they are a kind of potential photocatalytic material. Nevertheless, little work has been carried out to investigate their photocatalytic performances.

In this paper, the mainly research is focused on the preparation process and removal properties of organic pollutants CR from the wastewater using the double-perovskite oxides LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm).

2. Experimental

2.1. Materials Synthesis

All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification and treatment.

LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders were prepared by a conventional solid-state reaction method. Ln₂O₃, BaCO₃ and Co₂O₃ served as the starting materials for the necessary metal ions. The process flow chart is shown in Figure 1. Typically, Ln₂O₃, BaCO₃ and Co₂O₃ powders were mixed in stoichiometric ratios and thoroughly ground in an agate mortar using ethanol as a solvent for 1 h. After the evaporation of ethanol, the mixed powders were calcined at 1000˚C for 6 h under the air atmosphere and then slowly cooled down to room temperature in the furnace.

2.2. Materials Characterization

The crystal structures of the synthesized materials were characterized by X-ray powder diffraction (XRD) using

a Rigaku D/Max-3B instrument with copper target and Kα radiation (λ = 1.54056 Å). The accelerating voltage and applied current were 40 kV and 200 mA. The samples were scanned from 10˚ to 90˚ (2θ) in steps of 0.02˚.

2.3. Measurement of Photocatalytic Activity

CR (C₃₂H₂₂N₆Na₂O₆S₂, molecular weight: 696.67 g∙mol⁻¹, from Guangzhou Reagent Corporation, China) was used as a model dye to evaluate the photocatalytic activity of the LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) samples. The degradation tests of CR were conducted in an aqueous solution at room temperature under ultraviolet light irradiation with the dye solution concentration of 25 or 50 mg∙L^{− 1 . In a typical degradation process, under ultraviolet light irradiation and stirring conditions, 20 mg of the synthesized samples were added into 50} mL CR aqueous solutions with an initial concentration of 25 or 50 mg∙L⁻¹ in a quartz beaker, respectively. At a given time, 4 mL of the dispersion was continually extracted and subsequently centrifuged to separate LnBaCo₂O_5+δ powders and dye solutions at 4000 rev∙min^-1 for 30 min. The reaction mixtures were irradiated by a Xe-lamp (30 W) with the wavelength range of 320 - 400 nm in which a peak wavelength of 365 nm for UV light and 400 - 800 nm for visible light, and magnetically stirred throughout the photocatalytic experiment under air. The initial concentration (C₀ ) and the instant concentration (C ) of the aqueous solution of CR were determined with a UV-2401PC spectrophotometer at λ_max = 498 nm . The degradation rate of CR dye was calculated as C/C₀.

3. Results and Discussion

3.1. The Crystal Structures of the As-Synthesized Samples

Figure 2 shows the X-ray diffraction patterns of the oxides which prepared from the appropriate chemical precursors via calcination at 1000˚C for 6 h under an air atmosphere. The XRD patterns of samples with the nominal compositions of LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) are represented at a single-phase double-perovskite without any impurity phase. All the diffraction peaks of LnBCO samples can be indexed well with orthorhombic structure EuBaCo₂O_5.52 (JCPDS No. 53-0136, a = 3.915 Å, b = 3.882 Å, c = 7.549 Å), GdBaCo₂O_5.42 (JCPDS No. 53-0135, a = 3.918 Å, b = 3.879 Å, c = 7.545 Å), SmBaCo₂O_5.54 (JCPDS No. 53-0133, a = 3.913 Å, b = 3.889 Å, c = 7.576 Å), space group Pmmm, and these findings are in agreement with previously published reports [31] - [33] .

3.2. The Photocatalytic Activity of the LnBCO Powders

The as-synthesized LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders with well crystal were used for photocatalytic tests under ultraviolet light irradiation. CR was used as a test contaminant since it has been extensively used as

an indicator for the photocatalytic activity. After photocatalysis experiments, we found that double-perovskite oxides LnBaCo₂O_5+δ could remove the CR effectively, which mean they have photocatalytic properties. Figure 3 shows the photodegradation rate of CR solutions with different initial concentration in the presence of samples. From Figure 3(a), it is easy to see that the degradation rate of CR solutions (C₀ = 25 mg∙L⁻¹) by EuBaCo₂O_5.52 is rapid at the first 16 h of the reaction time. Following the rapid degradation, the rate decreased and the decolorization efficiency was 92.6% for 24 h. The other two samples, GdBaCo₂O_5.42 and SmBaCo₂O_5.54, display similar photocatalytic degradation rates of CR comparing with EuBaCo₂O_5.52. In general, the catalytic activity is tightly related to the structure of the catalyst [34] . Double-perovskite oxides LnBaCo₂O_5+δ is a kind of distorted perovskite structure which is often caused by the excessive absence of O²⁻ ion. The same structure can explain the phenomenon of similar photocatalytic property of EuBaCo₂O_5.52, GdBaCo₂O_5.42 and SmBaCo₂O_5.54. And likewise, as shown in Figure 3(b), because of the same reason, the three samples have similar photocatalytic degradation rates of a higher concentration of CR solutions (C₀ = 50 mg∙L⁻¹) except the time of photodegradation extended to 60 h.

To further clarify the photocatalytic performance and mechanism of CR, the continuous UV-Vis spectra of the centrifuged solution after catalytic reactions at the different intervals were used to record and in contrast to initial CR solution. Figure 4 displays the UV-Vis absorption spectra of CR solutions (C₀ = 25 or 50 mg∙L⁻¹) before and after treatment with as-synthesized LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders. From the UV-vis absorbance spectrum of CR initial solution, we can see three absorbance peaks at 241, 342 and 498 nm, respectively. The peaks at 241 nm and 342 nm are attributed to benzene ring and naphthalene ring structures, while the peak at 498 nm is attributed to the azo bonds of CR molecule [35] . The characteristic absorption peaks of CR at 342 nm and 498 nm were chosen as the parameter that was monitored. As shown in Figure 4, in the presence of LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders, all absorption peaks went down dramatically along with increasing degradation time and correspondingly the solution became decolorized from red to near colorless. The amplitude of the characteristic peaks continuously decreased with respect to the reaction time and finally to disappear in Figure 4 suggests that the azo bonds and the naphthyl rings were destroyed to form phenyl derivatives. UV light illumination of the aqueous CR solution in the presence of LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders causes the absorption bands of the CR dye in the visible region to disappear basically, which indicates that the dye is degraded completely. When LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders used to degrade CR aqueous solutions, the color of solution was bleached and the absorption of solution at 498 nm was close to zero. In accordance with changes of the absorbance intensity at 498 nm, the decolorization efficiency of 25 mg∙L⁻¹ CR solution in the presence of LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders after 3 h degradation reaction was

achieved about 47%. Complete decolorization of CR solution (C₀ = 25 mg∙L⁻¹) was achieved within 26 h. These results are in agreement with the analysis shown in inset. It is clearly seen that the cardinal red color of starting solution gradually disappears along with increasing the degradation time, which also depicts the degradation process of CR.

The possible catalytic mechanism of LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders is proposed like that of the perovskite oxides. We suppose the photocatalytic property of double-perovskite is derived from the BO₆ octahedron. In LnBaCo₂O_5+δ, 2p-orbit of O^2? constitutes the valence band, while the 3d-orbit of Co³⁺ constitutes the conduction band. Under the ultraviolet light irradiation, the valence bands electrons of LnBaCo₂O_5+δ were excited and transferred into conduction bands, generating highly reactivity electron-hole pairs which can induce oxidation of organic molecule, by leaving holes (h⁺) in the valence bands. The hole (h⁺) is a strong oxidant on the surface of LnBaCo₂O_5+δ and could oxidize the adsorbed water molecules or hydroxyl ions into ・OH radicals (H₂O or OH^? + h⁺ → ・OH ) which is highly radicals to oxidize or degrade the adjacent CR dye molecules. At the same time, the adsorbed oxygen reacted with electrons to form superoxide ions (O₂ + e^? →) which could decompose the organic molecules effectively [23] [36] .

4. Conclusion

Double-perovskite oxides, LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) microparticles were successfully prepared by a conventional solid-state reaction method. The samples were characterized by X-ray diffraction (XRD), showing that the resulting particles were highly crystalline LnBCO particles. The photocatalytic activity of the LnBaCo₂O_5+δ (Ln = Eu, Gd, and Sm) powders was investigated by the degradation of CR, and the results revealed that LnBCO had a certain photocatalytic activity, which indicated that they could be a promising photocatalyst for the degradation of organic molecules.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No.51262029), the Key Project of the Department of Education of Yunnan Province (ZD2013006), Program for Excellent Young Talents, Yunnan University (XT412003), Yunnan University Graduate Program for Research and Innovation, and the Department of Science and Technology of Yunnan Province via the Key Project for the Science and Technology (Grant No.2011FA001).

References

NOTES

^*Corresponding author.