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Structure and photocatalytic property of perovskite and perovskite-related compounds
Materials Chemistry and Physics 96 (2006) 234–239
Structure and photocatalytic property of perovskite and perovskite-related compounds Yang Yang a,b , Yanbin Sun a,b , Yinshan Jiang a,b,∗ a
b
Key Laboratory of Automobile Materials, Jilin University, Ministry of Education, Changchun 130026, China Department of Materials Science and Engineering, Jilin University, Changchun 130026, China Received 9 March 2005; received in revised form 2 July 2005; accepted 7 July 2005
Abstract Perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds were prepared by citrate–nitrate combustion method. Crystal structure was verified by X-ray diffraction. Absorbency was determined by UV–vis spectrophotometer. FTIR was also used to determine whether the decoloration of methyl orange solution was caused by photocatalysis. SEM was used to compare the morphology of the samples prior to and after photocatalysis. Calcination temperature of the samples was maintained at 450 ◦ C for 2 h and 800 ◦ C for 4 h. It was supposed that the presence of different photocatalytic properties among synthesized perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds was derived from structural differences among those materials; moreover, transition-metal ions (Fe3+ and Co2+ ) in oxygen coordinated octahedra were the main source of photocatalytic abilities and alkaline-earth metal ions (Ca2+ , Sr2+ and Ba2+ ) play important roles in stabilizing perovskite structure. And photocatalysis has no influence on the morphology of the sample. © 2005 Elsevier B.V. All rights reserved. Keywords: Perovskite; Brownmillerite; Photocatalysis; Transition-metal; Alkaline-earth metal
1. Introduction In recent years, the treatment of water contaminated with traces of toxic organic compounds has attracted scientists’ attention from various fields. In particular, photocatalytic degradation in the presence of a semiconductor catalyst has been shown to be a promising method for the destruction of toxic chemicals [1]. Among those semiconductor catalysts, TiO2 has been regarded as a desirable photocatalyst for the destruction of polluting materials [2,3]. Its action mechanism has been discussed widely; in short, semiconductor catalyst in the contaminated water absorbs a photon ultraviolet band gap energy resulting in the formation of electron donor (reductive) sites and electron acceptor (oxidizing) sites. Thus, the carbon containing pollutants are oxidized into car∗ Corresponding author. Present address: College of Material Science and Engineering, Jilin University, 6, West-Minzhu Avenue, Changchun 130026, China. E-mail address: [email protected] (Y. Jiang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.007
bon dioxide, water and other anions such as nitrate, sulphate or chloride [4]. Recently, scientists have given their attention to looking for new types of semiconductor catalysts and improving TiO2 for destruction of polluting materials [5,6]. But there is little attention paid on perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds for the same purpose. The perovskite oxide with general formula of ABO3 is frequently encountered structure in inorganic chemistry, and this structure can accommodate most of the metallic ions in the periodic table with a significant number of different anions. An ideal perovskite structure has an ABO3 stoichiometry and a cubic crystal structure, which is composed of a threedimensional framework of corner-sharing BO6 octahedron. In this structure, BO6 octahedron is considered to be the basic cell, those octahedra are connected via their vertices, and A-site cation is often alkaline-earth metal elements or rare earth elements while B-site cation is often transition-metal elements. B-site cation locates in octahedral vacancy and Asite cation fills the 12 coordinate cavities formed by BO6
Y. Yang et al. / Materials Chemistry and Physics 96 (2006) 234–239
network [7,13]. Perovskite-related compound is a kind of distorted perovskite (brownmillerite, A2 B2 O5 ) structure which is often caused by the excessive absence of O2− ion, because the excessive absence of O2− ion can transform BO6 octahedron into BO4 tetrahedron and change the connection mode of the basic cell [14,15]. Perovskite and perovskite-related structures are important crystal structures due to their diverse physical/chemistry properties [7,8] over a wide temperature range. For example, perovskite ceramics with ferroelectric or piezoelectric properties, such as BaTiO3 and Pb(ZrTi)O3 , play a dominant role in the electroceramics industry, etc. [19,20]. Scientists have investigated the structures and properties of perovskite and perovskite-related materials widely [9,10]; however, little work has been reported on photocatalysis of perovskite and perovskite-related materials [16–18]. In this paper, we discussed the relation between structural characteristics and photocatalytic property of perovskite and perovskite-related materials.
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so after calcination we had to grind the final products into powder. 2.3. Photocatalytic properties experiment The photocatalytic decoloration experiment was conducted in homemade photochemistry reactor equipped with 250 W high pressure mercury lamp. Firstly, we prepared 20 mg l−1 methyl orange solution beforehand. After that, we weigh up 0.01 g samples accurately in beakers and added 20 ml methyl orange solution into those beakers. After samples were dispersed adequately in methyl orange solution, put the beakers in homemade reactor for 2 h; during this process, we took out the beakers and extracted the clear liquid to measure absorbency of methyl orange solution every 10 min, those values we named Ai . Besides, we also measured original methyl orange solution, this value we named A0 . Absorbencies were measured via UV-754 UV–VIS spectrophotometer at 459 nm.The photocatalytic decoloration rate of methyl orange were computed via the formula: d = (A0 − Ai )/A0 .
2. Experiment 2.4. Measurement methods 2.1. Materials For the preparation of perovskite and perovskite-related compounds, ferric nitrate (A.R. 98.5%), cobalt nitrate (A.R. 99.0%), calcium nitrate (A.R. 99.0%), strontium nitrate (A.R. 99.5%), barium nitrate (A.R. 99.5%), citric acid (A.R. 99.8%), ammonia solution (A.R. 25.0–28.0%), methyl orange (A.R.) and deionized water were used. 2.2. Sample preparation In order to prepare perovskite and perovskite-related compounds, we selected some elements to satisfy the structural formula of ABO3 (Ca, Sr and Ba in A-site and Fe and Co in B-site). Samples were synthesized by citrate–nitrate combustion method, in which stoichiometric amounts of metalnitrates were mixed and dissolved in deionized water to form 0.1 mol l−1 solution; citrate acid was dissolved in deionized water to form 0.5 mol l−1 solution (the molar ratio of citrate/metal ions was a little more than 1). Then, put the metal-nitrates mixed solution under constant stirring, followed by adding citrate acid solution drop by drop 30 min later. In order to combine citric acid with metal ions adequately, ammonia solution was added to the mixed solution to keep the pH at 9 and heat the mixed solution at 70 ◦ C. Constant stirring was used during the whole process. The mixed solution was then polymerized under infrared irradiation for more than 10 h till gel-like product was formed. After drying, this gel-like product was calcined at 450 ◦ C for 2 h to let the product self-ignite and burn off the organic compound in the material, then at 800 ◦ C for 4 h to form perovskite or perovskite-related compounds. Because of the self-igniting and expanding of the precursors during calcinations, the final products were several times larger than their precursors,
X-ray diffraction measurements were used to determine the structures of products. X-ray diffraction measurements were performed on a Shimadzu XD-3 diffractometer (Cu ˚ 40 kV, 30 mA, 2θ from 20◦ to 60◦ ). K␣ = 1.54 A, Besides, FTIR was also used to detect whether the decoloration was caused by photocatalysis. The FTIR of the specimens were measured by a Nexus-670 FTIR spectrometer. Before conducting photocatalytic experiments, we tested the spectra of original products which we named the former spectrum. After photocatalytic experiments, we filtrated out the same products and tested the spectrum of them which we named the latter spectrum. Through comparing the former and the latter spectrums, we can confirm whether the decoloration was caused by photocatalysis, because if there were curves of methyl orange in the latter spectrum, it indicated that the decoloration may be derived from absorption and vice versa. In order to detect the photocatalytic decoloration’s influences on the morphology of samples, we also used SEM in this experiment. We observed the SEM micrographs of the samples prior to and after photocatalysis. SEM micrographs were recorded with the help of a JEM-2000FX (JEOL Ltd.) scanning electron microscope (operated at 20 keV).
3. Results and discussion 3.1. Crystal structure of final products Black powder of BaFeO3 , Sr2 Fe2 O5 , BaCoO2.93 , Sr2 Co2 O5 and orange powder of Ca2 Fe2 O5 were synthesized by citrate–nitrate combustion method; the experiment results are summarized in Table 1.
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Fig. 1. The powder XRD pattern of: (a) BaFeO3 (() BaFeO3 and () BaCO3 ); (b) () Sr2 Fe2 O5 ; (c) () Ca2 Fe2 O5 .
Fig. 1(a) shows the powder XRD pattern of BaFeO3 . From the pattern, we can see that the relatively pure perovskite compound was synthesized with little BaCO3 which were inevitable in the process of synthesizing perovskite compound [11,12]. Fig. 1(b) shows the powder XRD pattern of Sr2 Fe2 O5 and Fig. 1(c) shows the powder XRD pattern Table 1 Experiment results A-site element
B-site element
Final product
Crystal structure
Ba Sr
Fe Fe
BaFeO3 Sr2 Fe2 O5
Ca
Fe
Ca2 Fe2 O5
Ba Sr
Co Co
BaCoO2.93 Sr2 Co2 O5
Perovskite Perovskite-related (brownmillerite) Perovskite-related (brownmillerite) Perovskite Perovskite-related (brownmillerite)
of Ca2 Fe2 O5 . As has been described above, we synthesized materials with fixed stoichiometric amount of original stuff, namely the molar ratio of A-site element:B-site element:O = 1:1:3. But during the process of calcination, SrFeO3 and CaFeO3 were more likely to lose excessive oxygen in oxygen-absent atmosphere. Based on the XRD patterns of final products, we consider that it is more likely to form perovskite structure with large ion in A-site when fixed B-site element. According to the basic theory of crystallography, in the ABO3 -type compounds, A-site ion and O ion stack cubically, B-site ion and O ion form BO6 octahedron which connects corner to corner with each other to form threedimensional structure with A-site ion lying in the vacancy of BO6 octahedron network [7]. Accordingly, if A-site element is not large enough, A-site ion is not likely to stack with O ion compactly. In our experiment, when we select Ba as A-site element, it is easy to form perovskite structure.
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Fig. 2. The powder XRD pattern of: (a) BaCoO2.93 (() BaCoO2.93 and () BaCO3 )) and (b) () Sr2 Co2 O5 .
But when we select Sr and Ca as A-site element, it is more likely to form an unstable perovskite structure which is due to the excessive loss of oxygen during calcination. It makes the BO6 octahedron transform into BO4 tetrahedron which makes the crystal structure less stable. This kind of less stable perovskite structure is called to be perovskite-related structure (brownmillerite, A2 B2 O5 ) [15]. In order to confirm our assumption, we synthesized similar materials with Co in B-site instead of Fe. Fig. 2(a) shows the powder XRD pattern of BaCoO2.93 and Fig. 2(b) shows the powder XRD pattern of Sr2 Co2 O5 . This result further confirms that in forming perovskite structure, A-site element plays important role in stabilizing crystal structure. With the reducing radius of A-site element, the stability of synthesized perovskite structure compounds reduces correspondingly.
3.2. Photocatalysis After photocatalysis experiments, we find that perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds have photocatalytic properties. But the photocatalytic properties of perovskite structure materials are better than brownmillerite structure materials obviously. Fig. 3(a) shows the comparison of photocatalytic properties among BaFeO3 , Sr2 Fe2 O5 and Ca2 Fe2 O5 . It is easy to see that the photocatalytic property of BaFeO3 is better than Sr2 Fe2 O5 and Ca2 Fe2 O5 . It is due to the different structures between perovskite and brownmillerite, because we suppose the photacatalytic properties of perovskite and brownmillerite are derived from the BO6 octahedron. According to the theory of crystal field, when B-site ions (Fe and Co) lie in the octahedral crystal field, 3d-orbit will split into eg and t2g . 3d-
Fig. 3. (a) The comparison of photocatalytic properties among BaFeO3 , Sr2 Fe2 O5 and Ca2 Fe2 O5 . The photocatalytic decoloration rate of BaFeO3 can achieve above 90%. (b) The comparison of photocatalytic properties between BaCoO2.93 and Sr2 Co2 O5 . The photocatalytic decoloration rate of BaCoO2.93 can achieve above 80%.
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Fig. 4. The comparison of FTIR spectra before (a) and after (b) photocatalysis.
Orbit electrons in those splited orbitals have activities. This kind of activity is supposed to be the source of photocatalysis of perovskite and brownmillerite. Under the radiation of ultraviolet ray, the active 3d-orbit electron can be activated and produce electron–hole pair which can induce oxidation of organic molecule. As we discussed above, the basic cell of the structure of brownmillerite is not BO6 octahedron exclusively, there are lots of BO6 octahedron transformed into BO4 tetrahedron. This reduces the potential ability to produce electron–hole pair. Contrarily, the basic cell of perovskite structure is BO6 octahedron, there are abundant sources of electron–hole pair. This can explain the phenomenon of better photacatalytic property of BaFeO3 than Sr2 Fe2 O5 and Ca2 Fe2 O5 .
Fig. 3(b) shows the comparison of photocatalytic properties between BaCoO2.93 and Sr2 Co2 O5 . Because of the same reason as illustrated above, the differences between photocatalytic properties of BaCoO2.93 and Sr2 Co2 O5 are easy to understand. Because in this experiment FTIR spectra are used to detect methyl orange, so we only take parts of those spectra for example. Fig. 4 shows the comparison of FTIR spectra before and after photocatalysis, (a) is the FTIR spectrum of BaFeO3 before photocatalysis and (b) is the FTIR spectrum of BaFeO3 after photocatalysis. Through comparing (a) and (b), we find that there is no sign of methyl orange. This indicates that the decoloration of methyl orange solution is derived from photocatalysis rather than absorption. But we also find the curves at 769 cm−1 , 627 cm−1 and 559 cm−1 in spectrum (a) disappear in spectrum (b); instead of them, curve at 464 cm−1 appears. This may be due to the influence of photocatalysis; we will study it further in our future research. 3.3. SEM micrographs of samples Because of the same synthesizing method, the morphologies of different samples have nothing different obviously, so we select the SEM micrograph of BaFeO3 to discuss the photocatalysis’ influences on the sample’s morphology. Fig. 5(a) shows the morphology of sample before photocatalysis; in this micrograph product particles congregate irregularly. Fig. 5(b) shows the morphology of sample after photocatalysis; the figure and conglomeration state of particles are the same as the morphology of sample before photocatalysis. So, we believe that the photocatalysis has no influence on the morphology of the sample.
Fig. 5. The SEM micrograph of samples: (a) before photocatalysis and (b) after photocatalysis.
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4. Conclusions We can see that if the A-site ions are large enough, the perovskite structure will be formed rather easily. From the viewpoint of crystallography, when fixed B-site ions, larger A-site ion can stack with O ion much more compactly and form more stable structure. So under the same condition, BaFeO3 and BaCoO2.93 can form perovskite structure, whereas Sr2 Fe2 O5 , Ca2 Fe2 O5 and Sr2 Co2 O5 form less stable perovskite-related structure (namely brownmillerite). We suppose that the source of photocatalysis of perovskite and brownmillerite structure compounds is derived from the BO6 octahedron in both structures, because we believe that transition-metal ions in octahedral crystal field can activate the 3d-orbit electrons which under the radiation of ultraviolet ray can produce electron–hole pair. As has been reported before [21–25], electron–hole pair can induce oxidation of organic molecules, which has lots of applications in disposal of sewage. Photocatalysis has no influence on the morphology of the sample.
Acknowledgements This experiment is supported by Project 985-Automotive Engineering of Jilin University and the authors are grateful to Ms. Yujie Wang for X-ray analysis.
References [1] D.F. Ollis, E. Pelizzetti, N. Serpone, Photocatalysis: Fundamentals and Applications, Wiley, New York, 1989.
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[2] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [3] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [4] A.K. Ray, A.A.C.M. Beenackers, Catal. Today 40 (1998) 73. [5] K. Vidya, V.S. Kamble, P. Selvam, N.M. Gupta, Appl. Catal. B: Environ. 54 (2004) 145. [6] C. Sahoo, A.K. Gupta, A. Pal, Dyes Pigments 66 (2005) 189. [7] Z.L. Wang, Z.C. Kang, Functional and Smart Materials, Plenum Press, New York, 1998. [8] M.W. Lufaso, P.M. Woodward, Acta Cryst. B57 (2001) 726. [9] V.V. Vashook, M.V. Zinkevich, Yu.G. Zonov, Solid State Ionics 116 (1999) 129–138. [10] S. Stemmer, A. Sane, N.D. Browning, T.J. Mazanec, Solid State Ionics 130 (2000) 71–80. [11] O.A. Harizanov, Mater. Lett. 34 (1998) 232–236. [12] I. Maclaren, C.B. Ponton, J. Eur. Ceram. Soc 20 (2000) 1267– 1275. [13] O. Muller, R. Roy, The Major Ternary Structural Families, Springer Verlag, New York, 1974. [14] E.F. Bertaut, P. Blum, A. Sagnieres, Acta Cryst. 149 (1959) 12. [15] L.M. Liu, T.H. Lee, Y.L. Qiu, Y.L. Yang, A.J. Jacobson, Mater. Res. Bull. 31 (1996) 29–35. [16] K. Domen, A. Kudo, T. Onishi, J. Catal. 102 (1986) 92. [17] Q.S. Li, K. Domen, S. Naito, T. Onishi, K. Tamaru, Chem. Lett. 3 (1983) 321. [18] S. Ahuja, T.R.N. Kutty, J. Photochem. Photobiol. A: Chem. 97 (1996) 99. [19] R. Konta, T. Ishii, H. Kato, A. Kudo, J. Phys. Chem. B 108 (2004) 8992. [20] J. Wang, S. Yin, Q. Zhang, F. Sato, T. Sato, J. Ceram. Soc. Jpn. 112 (2004) S1408. [21] A.A. Abdel-Wahab, A.M. Gaber, J. Photochem. Photobiol. A 114 (1998) 213. [22] J.J. Liang, T.J. Liu, J. Chin. Chem. Soc. 32 (1986) 133. [23] T. Ibusuki, K. Takeuchi, Atmos. Environ. 20 (9) (1986) 133. [24] M.A. Fox, C.C. Chen, J.N.N. Yonnathan, J. Org. Chem. 40 (1984) 1969. [25] M.A. Fox, Top. Curr. Chem. 142 (1987) 721.
Structure and photocatalytic property of perovskite and perovskite-related compounds Yang Yang a,b , Yanbin Sun a,b , Yinshan Jiang a,b,∗ a
b
Key Laboratory of Automobile Materials, Jilin University, Ministry of Education, Changchun 130026, China Department of Materials Science and Engineering, Jilin University, Changchun 130026, China Received 9 March 2005; received in revised form 2 July 2005; accepted 7 July 2005
Abstract Perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds were prepared by citrate–nitrate combustion method. Crystal structure was verified by X-ray diffraction. Absorbency was determined by UV–vis spectrophotometer. FTIR was also used to determine whether the decoloration of methyl orange solution was caused by photocatalysis. SEM was used to compare the morphology of the samples prior to and after photocatalysis. Calcination temperature of the samples was maintained at 450 ◦ C for 2 h and 800 ◦ C for 4 h. It was supposed that the presence of different photocatalytic properties among synthesized perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds was derived from structural differences among those materials; moreover, transition-metal ions (Fe3+ and Co2+ ) in oxygen coordinated octahedra were the main source of photocatalytic abilities and alkaline-earth metal ions (Ca2+ , Sr2+ and Ba2+ ) play important roles in stabilizing perovskite structure. And photocatalysis has no influence on the morphology of the sample. © 2005 Elsevier B.V. All rights reserved. Keywords: Perovskite; Brownmillerite; Photocatalysis; Transition-metal; Alkaline-earth metal
1. Introduction In recent years, the treatment of water contaminated with traces of toxic organic compounds has attracted scientists’ attention from various fields. In particular, photocatalytic degradation in the presence of a semiconductor catalyst has been shown to be a promising method for the destruction of toxic chemicals [1]. Among those semiconductor catalysts, TiO2 has been regarded as a desirable photocatalyst for the destruction of polluting materials [2,3]. Its action mechanism has been discussed widely; in short, semiconductor catalyst in the contaminated water absorbs a photon ultraviolet band gap energy resulting in the formation of electron donor (reductive) sites and electron acceptor (oxidizing) sites. Thus, the carbon containing pollutants are oxidized into car∗ Corresponding author. Present address: College of Material Science and Engineering, Jilin University, 6, West-Minzhu Avenue, Changchun 130026, China. E-mail address: [email protected] (Y. Jiang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.007
bon dioxide, water and other anions such as nitrate, sulphate or chloride [4]. Recently, scientists have given their attention to looking for new types of semiconductor catalysts and improving TiO2 for destruction of polluting materials [5,6]. But there is little attention paid on perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds for the same purpose. The perovskite oxide with general formula of ABO3 is frequently encountered structure in inorganic chemistry, and this structure can accommodate most of the metallic ions in the periodic table with a significant number of different anions. An ideal perovskite structure has an ABO3 stoichiometry and a cubic crystal structure, which is composed of a threedimensional framework of corner-sharing BO6 octahedron. In this structure, BO6 octahedron is considered to be the basic cell, those octahedra are connected via their vertices, and A-site cation is often alkaline-earth metal elements or rare earth elements while B-site cation is often transition-metal elements. B-site cation locates in octahedral vacancy and Asite cation fills the 12 coordinate cavities formed by BO6
Y. Yang et al. / Materials Chemistry and Physics 96 (2006) 234–239
network [7,13]. Perovskite-related compound is a kind of distorted perovskite (brownmillerite, A2 B2 O5 ) structure which is often caused by the excessive absence of O2− ion, because the excessive absence of O2− ion can transform BO6 octahedron into BO4 tetrahedron and change the connection mode of the basic cell [14,15]. Perovskite and perovskite-related structures are important crystal structures due to their diverse physical/chemistry properties [7,8] over a wide temperature range. For example, perovskite ceramics with ferroelectric or piezoelectric properties, such as BaTiO3 and Pb(ZrTi)O3 , play a dominant role in the electroceramics industry, etc. [19,20]. Scientists have investigated the structures and properties of perovskite and perovskite-related materials widely [9,10]; however, little work has been reported on photocatalysis of perovskite and perovskite-related materials [16–18]. In this paper, we discussed the relation between structural characteristics and photocatalytic property of perovskite and perovskite-related materials.
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so after calcination we had to grind the final products into powder. 2.3. Photocatalytic properties experiment The photocatalytic decoloration experiment was conducted in homemade photochemistry reactor equipped with 250 W high pressure mercury lamp. Firstly, we prepared 20 mg l−1 methyl orange solution beforehand. After that, we weigh up 0.01 g samples accurately in beakers and added 20 ml methyl orange solution into those beakers. After samples were dispersed adequately in methyl orange solution, put the beakers in homemade reactor for 2 h; during this process, we took out the beakers and extracted the clear liquid to measure absorbency of methyl orange solution every 10 min, those values we named Ai . Besides, we also measured original methyl orange solution, this value we named A0 . Absorbencies were measured via UV-754 UV–VIS spectrophotometer at 459 nm.The photocatalytic decoloration rate of methyl orange were computed via the formula: d = (A0 − Ai )/A0 .
2. Experiment 2.4. Measurement methods 2.1. Materials For the preparation of perovskite and perovskite-related compounds, ferric nitrate (A.R. 98.5%), cobalt nitrate (A.R. 99.0%), calcium nitrate (A.R. 99.0%), strontium nitrate (A.R. 99.5%), barium nitrate (A.R. 99.5%), citric acid (A.R. 99.8%), ammonia solution (A.R. 25.0–28.0%), methyl orange (A.R.) and deionized water were used. 2.2. Sample preparation In order to prepare perovskite and perovskite-related compounds, we selected some elements to satisfy the structural formula of ABO3 (Ca, Sr and Ba in A-site and Fe and Co in B-site). Samples were synthesized by citrate–nitrate combustion method, in which stoichiometric amounts of metalnitrates were mixed and dissolved in deionized water to form 0.1 mol l−1 solution; citrate acid was dissolved in deionized water to form 0.5 mol l−1 solution (the molar ratio of citrate/metal ions was a little more than 1). Then, put the metal-nitrates mixed solution under constant stirring, followed by adding citrate acid solution drop by drop 30 min later. In order to combine citric acid with metal ions adequately, ammonia solution was added to the mixed solution to keep the pH at 9 and heat the mixed solution at 70 ◦ C. Constant stirring was used during the whole process. The mixed solution was then polymerized under infrared irradiation for more than 10 h till gel-like product was formed. After drying, this gel-like product was calcined at 450 ◦ C for 2 h to let the product self-ignite and burn off the organic compound in the material, then at 800 ◦ C for 4 h to form perovskite or perovskite-related compounds. Because of the self-igniting and expanding of the precursors during calcinations, the final products were several times larger than their precursors,
X-ray diffraction measurements were used to determine the structures of products. X-ray diffraction measurements were performed on a Shimadzu XD-3 diffractometer (Cu ˚ 40 kV, 30 mA, 2θ from 20◦ to 60◦ ). K␣ = 1.54 A, Besides, FTIR was also used to detect whether the decoloration was caused by photocatalysis. The FTIR of the specimens were measured by a Nexus-670 FTIR spectrometer. Before conducting photocatalytic experiments, we tested the spectra of original products which we named the former spectrum. After photocatalytic experiments, we filtrated out the same products and tested the spectrum of them which we named the latter spectrum. Through comparing the former and the latter spectrums, we can confirm whether the decoloration was caused by photocatalysis, because if there were curves of methyl orange in the latter spectrum, it indicated that the decoloration may be derived from absorption and vice versa. In order to detect the photocatalytic decoloration’s influences on the morphology of samples, we also used SEM in this experiment. We observed the SEM micrographs of the samples prior to and after photocatalysis. SEM micrographs were recorded with the help of a JEM-2000FX (JEOL Ltd.) scanning electron microscope (operated at 20 keV).
3. Results and discussion 3.1. Crystal structure of final products Black powder of BaFeO3 , Sr2 Fe2 O5 , BaCoO2.93 , Sr2 Co2 O5 and orange powder of Ca2 Fe2 O5 were synthesized by citrate–nitrate combustion method; the experiment results are summarized in Table 1.
Y. Yang et al. / Materials Chemistry and Physics 96 (2006) 234–239
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Fig. 1. The powder XRD pattern of: (a) BaFeO3 (() BaFeO3 and () BaCO3 ); (b) () Sr2 Fe2 O5 ; (c) () Ca2 Fe2 O5 .
Fig. 1(a) shows the powder XRD pattern of BaFeO3 . From the pattern, we can see that the relatively pure perovskite compound was synthesized with little BaCO3 which were inevitable in the process of synthesizing perovskite compound [11,12]. Fig. 1(b) shows the powder XRD pattern of Sr2 Fe2 O5 and Fig. 1(c) shows the powder XRD pattern Table 1 Experiment results A-site element
B-site element
Final product
Crystal structure
Ba Sr
Fe Fe
BaFeO3 Sr2 Fe2 O5
Ca
Fe
Ca2 Fe2 O5
Ba Sr
Co Co
BaCoO2.93 Sr2 Co2 O5
Perovskite Perovskite-related (brownmillerite) Perovskite-related (brownmillerite) Perovskite Perovskite-related (brownmillerite)
of Ca2 Fe2 O5 . As has been described above, we synthesized materials with fixed stoichiometric amount of original stuff, namely the molar ratio of A-site element:B-site element:O = 1:1:3. But during the process of calcination, SrFeO3 and CaFeO3 were more likely to lose excessive oxygen in oxygen-absent atmosphere. Based on the XRD patterns of final products, we consider that it is more likely to form perovskite structure with large ion in A-site when fixed B-site element. According to the basic theory of crystallography, in the ABO3 -type compounds, A-site ion and O ion stack cubically, B-site ion and O ion form BO6 octahedron which connects corner to corner with each other to form threedimensional structure with A-site ion lying in the vacancy of BO6 octahedron network [7]. Accordingly, if A-site element is not large enough, A-site ion is not likely to stack with O ion compactly. In our experiment, when we select Ba as A-site element, it is easy to form perovskite structure.
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Fig. 2. The powder XRD pattern of: (a) BaCoO2.93 (() BaCoO2.93 and () BaCO3 )) and (b) () Sr2 Co2 O5 .
But when we select Sr and Ca as A-site element, it is more likely to form an unstable perovskite structure which is due to the excessive loss of oxygen during calcination. It makes the BO6 octahedron transform into BO4 tetrahedron which makes the crystal structure less stable. This kind of less stable perovskite structure is called to be perovskite-related structure (brownmillerite, A2 B2 O5 ) [15]. In order to confirm our assumption, we synthesized similar materials with Co in B-site instead of Fe. Fig. 2(a) shows the powder XRD pattern of BaCoO2.93 and Fig. 2(b) shows the powder XRD pattern of Sr2 Co2 O5 . This result further confirms that in forming perovskite structure, A-site element plays important role in stabilizing crystal structure. With the reducing radius of A-site element, the stability of synthesized perovskite structure compounds reduces correspondingly.
3.2. Photocatalysis After photocatalysis experiments, we find that perovskite (ABO3 ) and perovskite-related (brownmillerite, A2 B2 O5 ) compounds have photocatalytic properties. But the photocatalytic properties of perovskite structure materials are better than brownmillerite structure materials obviously. Fig. 3(a) shows the comparison of photocatalytic properties among BaFeO3 , Sr2 Fe2 O5 and Ca2 Fe2 O5 . It is easy to see that the photocatalytic property of BaFeO3 is better than Sr2 Fe2 O5 and Ca2 Fe2 O5 . It is due to the different structures between perovskite and brownmillerite, because we suppose the photacatalytic properties of perovskite and brownmillerite are derived from the BO6 octahedron. According to the theory of crystal field, when B-site ions (Fe and Co) lie in the octahedral crystal field, 3d-orbit will split into eg and t2g . 3d-
Fig. 3. (a) The comparison of photocatalytic properties among BaFeO3 , Sr2 Fe2 O5 and Ca2 Fe2 O5 . The photocatalytic decoloration rate of BaFeO3 can achieve above 90%. (b) The comparison of photocatalytic properties between BaCoO2.93 and Sr2 Co2 O5 . The photocatalytic decoloration rate of BaCoO2.93 can achieve above 80%.
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Fig. 4. The comparison of FTIR spectra before (a) and after (b) photocatalysis.
Orbit electrons in those splited orbitals have activities. This kind of activity is supposed to be the source of photocatalysis of perovskite and brownmillerite. Under the radiation of ultraviolet ray, the active 3d-orbit electron can be activated and produce electron–hole pair which can induce oxidation of organic molecule. As we discussed above, the basic cell of the structure of brownmillerite is not BO6 octahedron exclusively, there are lots of BO6 octahedron transformed into BO4 tetrahedron. This reduces the potential ability to produce electron–hole pair. Contrarily, the basic cell of perovskite structure is BO6 octahedron, there are abundant sources of electron–hole pair. This can explain the phenomenon of better photacatalytic property of BaFeO3 than Sr2 Fe2 O5 and Ca2 Fe2 O5 .
Fig. 3(b) shows the comparison of photocatalytic properties between BaCoO2.93 and Sr2 Co2 O5 . Because of the same reason as illustrated above, the differences between photocatalytic properties of BaCoO2.93 and Sr2 Co2 O5 are easy to understand. Because in this experiment FTIR spectra are used to detect methyl orange, so we only take parts of those spectra for example. Fig. 4 shows the comparison of FTIR spectra before and after photocatalysis, (a) is the FTIR spectrum of BaFeO3 before photocatalysis and (b) is the FTIR spectrum of BaFeO3 after photocatalysis. Through comparing (a) and (b), we find that there is no sign of methyl orange. This indicates that the decoloration of methyl orange solution is derived from photocatalysis rather than absorption. But we also find the curves at 769 cm−1 , 627 cm−1 and 559 cm−1 in spectrum (a) disappear in spectrum (b); instead of them, curve at 464 cm−1 appears. This may be due to the influence of photocatalysis; we will study it further in our future research. 3.3. SEM micrographs of samples Because of the same synthesizing method, the morphologies of different samples have nothing different obviously, so we select the SEM micrograph of BaFeO3 to discuss the photocatalysis’ influences on the sample’s morphology. Fig. 5(a) shows the morphology of sample before photocatalysis; in this micrograph product particles congregate irregularly. Fig. 5(b) shows the morphology of sample after photocatalysis; the figure and conglomeration state of particles are the same as the morphology of sample before photocatalysis. So, we believe that the photocatalysis has no influence on the morphology of the sample.
Fig. 5. The SEM micrograph of samples: (a) before photocatalysis and (b) after photocatalysis.
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4. Conclusions We can see that if the A-site ions are large enough, the perovskite structure will be formed rather easily. From the viewpoint of crystallography, when fixed B-site ions, larger A-site ion can stack with O ion much more compactly and form more stable structure. So under the same condition, BaFeO3 and BaCoO2.93 can form perovskite structure, whereas Sr2 Fe2 O5 , Ca2 Fe2 O5 and Sr2 Co2 O5 form less stable perovskite-related structure (namely brownmillerite). We suppose that the source of photocatalysis of perovskite and brownmillerite structure compounds is derived from the BO6 octahedron in both structures, because we believe that transition-metal ions in octahedral crystal field can activate the 3d-orbit electrons which under the radiation of ultraviolet ray can produce electron–hole pair. As has been reported before [21–25], electron–hole pair can induce oxidation of organic molecules, which has lots of applications in disposal of sewage. Photocatalysis has no influence on the morphology of the sample.
Acknowledgements This experiment is supported by Project 985-Automotive Engineering of Jilin University and the authors are grateful to Ms. Yujie Wang for X-ray analysis.
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