Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Photocatalytic activity of Nb heterostructure (NaNbO 3 /Na 2 Nb 4 O 11 ) and Nb/ clay materials in the degradation of organic compounds Yvan J.O. Asencios a, ⁎ , Marcella V. Quijo a , Francielle C.F. Marcos b,c , André E. Nogueira d , René R. Rocca a , Elisabete M. Assaf b, ⁎ a Institute of Marine Science, Federal University of São Paulo (IMar/UNIFESP), 11030-100 Santos, SP, Brazil b São Carlos Institute of Chemistry, University of São Paulo, 13566-590 São Carlos, SP, Brazil c University of São Paulo, Escola Politécnica of the University of São Paulo, 05508-010 São Paulo, SP, Brazil d Federal University of Ouro Preto, 35400-000 Ouro Preto, MG, Brazil ARTICLE INFO Keywords: Niobium heterostructure Clays Materials Photocatalysts Rhodamine B ABSTRACT In this article, Nb heterostructure-based (NaNbO 3 /Na 2 Nb 4 O 11 ) and Nb/Clay-based materials were prepared, characterized and evaluated for the Rhodamine B (C 28 H 31 ClN 2 O 3 ) photodegradation under UV radiation (253.7 nm). As far as this research understands, this is the first time that the photocatalytic activity of the Nb heterostructure (NaNbO 3 /Na 2 Nb 4 O 11 ) is reported. Nb heterostructure is composed of sodium niobate and dis- odium tetraniobium hendecaoxide (NaNbO 3 /Na 2 Nb 4 O 11 ). The material was synthesized by a simple and eco- nomic method via basic precipitation of ammoniacal niobium oxalate (NH 4 [NbO (C 2 O 4 ) 2 ·H 2 O] XH 2 O) followed by thermal treatment at 500 °C. Nb/Clay-based materials were synthesized from the natural clay suspension (Montmorillonite) precursor. The addition of Nb to the natural clay by the methodology employed in this work modified the band-gap energy and increased the specific surface area of the resultant materials (to almost twice the value of the original specific surface area). Furthermore, this addition also decreased the luminescence and trap centres and potentiated the photocatalytic activity of the Nb/Clay materials. The Nb heterostructure showed higher photocatalytic activity in Rhodamine B photodegradation (recorded up to 95% removal at natural con- ditions). Similar values were reported to Nb/Clay materials. Despite the natural clay having more luminescence and trap centres than Nb/Clay materials, it recorded very low photocatalytic activity. Nb addition to natural clay increased the photocatalytic activity of the material, which would give other use to clay along with being used as an adsorbent. 1. Introduction The synthetic dyes are organic substances used in the manufacture of various products; they are mostly found in dumped effluents from textile industries. Their presence in water is dangerous since they are toxic and can cause mutations in aquatic organisms; such substances require specific treatment for complete removal (Kumar et al., 2014; Genuino et al., 2013; Chowdhury and Bhattacharyya 2016; Lacerda et al., 2015). The Rhodamine B (RhB, C 28 H 31 ClN 2 O 3 ) is a synthetic dye that has red colour and belongs to the family of xanthenes, is water soluble, fluorescent and widely used in the textile industry (Ramuthai et al., 2009; Xu et al., 2012). Both chemical and physical processes such as adsorption, pre- cipitation, ultra-filtration, reverse osmosis, and flocculation are applied for the removal of the toxic substances (including synthetic dyes) contained in industrial effluents, however these processes can result in the formation of large amounts of sludge which can reduce the lifetime of operation and increase operating cost (for equipment maintenance). The heterogeneous photocatalysis allows the removal of several types of organic pollutants (including textile dyes) in a simple way by using a solid-semiconductor (E.g. TiO 2 , ZnO, Fe 2 O 3 ) and light (with energy larger than the band-gap energy of the semiconductor) to gen- erate of % OH radicals (a strong oxidant) which lead to the mineraliza- tion of a variety of organic pollutants (Klabunde, 2013; Xu et al., 2011; Fan et al., 2015). It has been demonstrated that heterogeneous photo- catalysis using semiconductors are more efficient than conventional methods because the photocatalytic processes gradually break down the contaminant molecules, with no remaining residues of the original or- ganic matter and therefore not requiring disposal of sludge to landfill (Ibhadon and Fitzpatrick, 2013). Moreover, heterogeneous https://doi.org/10.1016/j.solener.2019.10.005 Received 29 April 2019; Received in revised form 18 September 2019; Accepted 2 October 2019 ⁎ Corresponding authors. E-mail addresses: yvan.jesus@unifesp.br (Y.J.O. Asencios), eassaf@iqsc.usp.br (E.M. Assaf). Solar Energy 194 (2019) 37–46 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T