Photocatalytic behaviour of Bi 2 MO 6 polymetalates for rhodamine B degradation C. Belver a, *, C. Ada ´n b , M. Ferna ´ ndez-Garcı ´a b a Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Cantoblanco, C/ Sor Juana Ines de la Cruz 3, E-28049 Madrid, Spain b Instituto de Cata ´lisis y Petroleoquı´mica, CSIC, Campus Cantoblanco, E-28049 Madrid, Spain 1. Introduction In the past decades, the photocatalytic technology has attracted much attention and been widely studied with the final aim of efficiently eliminating the undesired chemical substances for environmental conservation [1,2]. The photocatalytic materials have mainly focused on TiO 2 semiconductor because of their high reactivity, low cost and environmentally friendly features [3,4]. Nevertheless, the TiO 2 needs UV light irradiation to produce the electronic transition responsible to its photoactivity because of its large band gap of 3.2 eV. The ultraviolet radiation constitutes less that 4% of whole energy of incoming solar spectrum, while the visible light is more than 50%. Hence, effective utilization of the solar energy appears nowadays as the main aim of photochemical researchers [5,6]. In this sense, many efforts have been dedicated to modify the TiO 2 with the goal to shift the spectral response of TiO 2 into the visible region (380 nm < l < 750 nm) and improve its photocatalytic performances. Some of the most feasible modifications are doping (cationic or anionic), metal deposition and surface sensitization [4,7,8]. On the other hand, in recent years many efforts have been paid to develop new type of photocatalytic materials active under sunlight mainly based on oxide semiconductors [5,9,10]. The strategies described to enhance the visible light absorption of the oxide semiconductors are focused in the modification of their band gap [11]. In general, these oxides are based on metal cations with d 0 or d 10 configurations. The conduction bands (CB) are usually formed by the empty orbitals of the metal cations, whereas the valence band (VB) is based on O 2p orbitals. A way to modify the band gap consists of creating an electron donor level between the VB and CB, as occurred during the metal doping of TiO 2 [6] or the continuous modification of the conduction band for high levels of doping cations [12]. Another way to the band gap modification could be the formation of new VB employing elements with orbitals other than O 2p. In this sense different metalates, described as A x B y O z , have been studied as optimal materials for visible light absorption [13,14]. The development of these materials is influenced by their photocatalytic applications, being photodegradation reactions and water splitting the most common [15,16]. The presence of cations such as Bi 3+ , In 3+ , Sn 2+ (s 2 configuration) or Ag + (d 10 configuration) in an oxide system manages to elevate the valence band by means of the combination of their respective orbitals with the O 2p orbital from the oxygen, narrowing the band gap of the semiconductor. In this way, oxides such as BiVO 4 , AgVO x , CaBi 2 O 4 , InMO 4 (M = V, Nb, Ta), Bi 2 WO 6 or MIn 2 O 4 (M = Ca, Sr, Ba) have been described as effective photocatalysts for contaminant degradation [17–22]. The conducting properties of these materials mainly depend on their electronic properties, as an example it has been reported that the introduction of s orbital components increases the charge mobility, and, at the same time, tunes the relative positions of CB and VB for InMO 4 (M = V, Nb, Ta) and BiVO 4 systems [23,24]. By other side, the structural properties of these materials are also relevant. The crystallization on complex Catalysis Today 143 (2009) 274–281 ARTICLE INFO Article history: Available online 30 October 2008 Keywords: Polymetalates Aurivillius structure Photocatalysis RhB degradation ABSTRACT Bi 2 MO 6 (M = W or Mo) have been successfully synthesized using the citrate complex method. The samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and UV–vis spectroscopy. The solids crystallized in the orthorhombic Aurivillius-type structure and display optimum light absorption properties to be used as photocatalysts. The photoactivity power of the samples was investigated systematically using the rhodamine B degradation under different irradiation wavelengths. The behaviour of the solids and the photodegradation mechanism was studied as a function of the irradiation light (UV or visible) employed. Direct solar light was also employed as irradiation source showing that this type of structures could drive to a plausible strategy for developing finest photocatalyst to degrade wastewaters by using solar light. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 91 334 9000; fax: +34 91 372 0623. E-mail address: cbelver@icmm.csic.es (C. Belver). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2008.09.011