Influences of A- and B-site cations on the physicochemical properties of perovskite-structured A(In1/3Nb1/3B1/3)O3 (A = Sr, Ba; B = Sn, Pb) photocatalysts

doi:10.1016/j.jphotochem.2006.03.014

Journal of Photochemistry and Photobiology A: Chemistry

Volume 183, Issues 1–2, 30 September 2006, Pages 176-181

https://doi.org/10.1016/j.jphotochem.2006.03.014 Get rights and content

Abstract

We have investigated the effects of A- and B-site cation substitution on the physicochemical properties of perovskite-structured A(In_1/3Nb_1/3B_1/3)O₃ (A = Sr, Ba; B = Sn, Pb) photocatalysts. X-ray diffraction, X-ray absorption and diffuse UV–vis spectroscopic analyses reveal that tetravalent Pb^IV or Sn^IV ions can be successfully incorporated into the octahedral B-site of the perovskite lattice, leading to a narrowing of bandgap energy (E_g). The substitution of such electronegative cations gives rise to the enhancement of photocatalytic activity to effectively decompose organic molecules. Interestingly, alkaline earth metal ions in the dodecahedral A-site with ionic bonding environment also affect significantly the band structure and photoefficiency of the perovskite compounds; a larger cation is beneficial for creating visible light driven photocatalytic activity through a decrease of E_g. This observation could be understood in terms of the weakening of transition metal–oxygen bond upon the expansion of unit cell. The present relationship between the chemical bonding nature of substituent cation and the band structure provides an efficient tool for designing and developing new efficient visible light active photocatalysts.

Introduction

Over the past decades, semiconductive inorganic solids have attracted intense research interest due to their photocatalytic activity for the degradation of organic pollutants and/or the decomposition of water molecules [1], [2]. Such a photodissociation driven by a semiconductor originates from a redox reaction with transient electrons or holes generated by an electronic transition from valence band (VB) to conduction band (CB). The efficiency of photocatalysts depends strongly on their band structures such as bandgap energy (E_g), the positions of VB and CB, etc. In this regard, there have been many attempts to optimize the band structure of the semiconductive photocatalyst such as dye sensitization, a coupling of different semiconductors, and so on [3], [4], [5]. Recently attempts have been made to modify the bandgap energy simply by substituting metal or oxygen ions in part in oxide lattice [6], [7]. In particular, a perovskite-structured ABO₃ metal oxide can be a promising matrix for the chemical substitution, since it is stable enough to form a solid solution with diverse metal ions. In fact, there have been some reports about the substitution of transition metal ions into UV-active photocatalysts, leading to the decrease of E_g [8], [9]. More recently we have shown that visible light driven photocatalytic activity can be induced by the incorporation of electronegative cations into an octahedral B-site of perovskite lattice [10]. On the other hand, there have been limited number of literatures about the influence of cation in a dodecahedral A-site on the bonding nature of photocatalysts [8], [11], since a covalent bond between B-site cation and oxygen is believed to be a main factor in determining the electronic structure of metal oxide photocatalyst near Fermi energy, rather than an ionic bond between A-site cation and oxygen.

In this study, we have investigated the influence of A-site cations as well as the B-site ones on the physicochemical properties of perovskite-structured A(In_1/3Nb_1/3B_1/3)O₃ (A = Sr, Ba; B = Sn, Pb) photocatalysts and their chemical bonding nature and band structure have been systematically studied using X-ray diffraction (XRD), X-ray absorption (XAS) and diffuse UV–vis spectroscopy. In addition, the evolution of photocatalytic activities upon cation substitution has been studied by examining the decomposition of organic pollutants under UV–vis or visible light irradiation. On the basis of chemical bonding character, the effects of cation substitution on the A- and B-sites on the photoefficiency have been explained in terms of the evolution of metal–oxygen bonds.

Section snippets

Sample preparation and characterization

Polycrystalline A(In_1/2Nb_1/2)O₃ and A(In_1/3Nb_1/3B_1/3)O₃ (A = Sr, Ba; B = Sn, Pb) samples were prepared by conventional solid-state reaction with the stoichiometric mixture of SrCO₃, BaCO₃, PbO₂, SnO₂, In₂O₃ and Nb₂O₃ at 950–1050 °C for several days in oxygen atmosphere. The crystal and band structures of the obtained perovskite-type compounds have been determined by XRD and diffuse UV–vis spectroscopy, respectively. Diffuse reflectance UV–vis spectra were obtained on a Perkin-Elmer Lambda 35

Powder XRD and FE-SEM analysis

Fig. 1 represents the powder XRD patterns of the quaternary metal oxides A(In_1/3Nb_1/3B_1/3)O₃ (A = Sr, Ba; B = Sn, Pb), together with those of the ternary A(In_1/2Nb_1/2)O₃ compounds. All the diffraction peaks could be well indexed on the basis of perovskite structure with the cubic symmetry [8]. As listed in Table 1, the Ba-based compounds show a larger lattice parameter a than the corresponding Sr-based ones, since the Ba^II ion is bigger than the Sr^II ion [14]. Taking into account the fact that the

Conclusions

In this study, we have shown that the cation substitution into the A- as well as B-sites of perovskite-structured semiconductor affects significantly its band structure and photocatalytic activity. While the substitution into the octahedral B-site leads to a direct change of CB position with metal nd/(n + 1)s character, the A-site substitution alters the bandgap energy through the variation of metal–oxygen bond strength induced by the change of unit cell volume. Based on the present results, we

Acknowledgments

This work was supported by grant no. R08-2003-000-10409-0 from the Basic Research Program of the Korea Science & Engineering Foundation and partly by the SRC/ERC program of MOST/KOSEF (grant: R11-2005-008-03002-0). The experiments at Pohang Light Source (PLS) were supported in part by MOST and POSTECH.

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Influences of A- and B-site cations on the physicochemical properties of perovskite-structured A(In1/3Nb1/3B1/3)O3 (A = Sr, Ba; B = Sn, Pb) photocatalysts

Abstract

Introduction

Section snippets

Sample preparation and characterization

Powder XRD and FE-SEM analysis

Conclusions

Acknowledgments

J. Am. Chem. Soc.

J. Phys. Chem. B

Nature

Chem. Rev.

J. Photochem. Photobiol. A

J. Photochem. Photobiol. A

J. Chem. Soc., Faraday Trans.

J. Mater. Chem.

J. Phys. Chem.

Nature

Science

J. Phys. Chem. B

Chem. Mater.

J. Phys. Chem. B

Influences of A- and B-site cations on the physicochemical properties of perovskite-structured A(In_1/3Nb_1/3B_1/3)O₃ (A = Sr, Ba; B = Sn, Pb) photocatalysts