Visible-Light Photochemical Activity of Heterostructured Core-Shell Materials Composed of Selected Ternary Titanates and Ferrites Coated by TiO 2 Li Li, † Xuan Liu, † Yiling Zhang, † Noel T. Nuhfer, † Katayun Barmak, ‡ Paul A. Salvador, † and Gregory S. Rohrer* ,† † Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ‡ Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States ABSTRACT: Heterostructured photocatalysts comprised of micro- crystalline (mc-) cores and nanostructured (ns-) shells were prepared by the sol-gel method. The ability of titania-coated ATiO 3 (A = Fe, Pb) and AFeO 3 (A = Bi, La, Y) catalysts to degrade methylene blue in visible light (λ > 420 nm) was compared. The catalysts with the titanate cores had enhanced photocatalytic activities for methylene blue degradation compared to their components alone, whereas the catalysts with ferrite cores did not. The temperature at which the ns-titania shell is crystallized influences the photocatalytic dye degradation. mc-FeTiO 3 / ns-TiO 2 annealed at 500 °C shows the highest reaction rate. Fe-doped TiO 2 , which absorbs visible light, did not show enhanced photocatalytic activity for methylene blue degradation. This result indicates that iron contamination is not a decisive factor in the reduced reactivity of the titania coated ferrite catalysts. The higher reactivity of materials with the titanate cores suggests that photogenerated charge carriers are more easily transported across the titanate-titanate interface than the ferrite-titanate interface and this provides guidance for materials selection in composite catalyst design. KEYWORDS: core-shell, photocatalyst, TiO 2 , dye degradation, ferrite, titanate 1. INTRODUCTION TiO 2 is one of the most widely studied photocatalysts because of its appropriate electronic structure, photostability, chemical inertness, and commercial availability. 1-7 Titania’s photo- chemical efficiency in visible light is limited by several factors, such as the recombination of photogenerated charge carriers, the back reaction of intermediates, insufficient active sites for the redox reactions, and especially its wide band gap. Various methods have been used to modify TiO 2 to make it more active under visible light, such as metal and nonmetal doping, 3,8-12 dye sensitization, 13,14 the formation of junctions with other semiconductors, 15,16 and coupling to narrow band gap semiconductors. 17-24 For example, CdS, 25 Cu 2 O, 26 BiOI, 22 ZnFe 2 O 4 , 27 and CuInS 2 , 28 were combined with TiO 2 for visible-light sensitization and the heterostructured materials showed enhanced reactivity for organic pollutant degradation and hydrogen production. The combination of titania with different semiconductors is potentially promising because heterostructured photocatalysts can combine multiple functionalities within a single struc- ture. 24,29 Among different configurations for heterostructured photocatalysts, the core/shell structure has been widely investigated as a means to enhance light absorption, charge transfer, and surface area. 30,31 Several heterostructured catalysts comprised of micocrystalline, visible light absorbing cores (FeTiO 3 and PbTiO 3 ), coated by nanostructured titania, have recently been shown to have enhanced visible light photo- chemical reactivity. 32,33 These materials combine the high surface area that is necessary to provide enough active sites for high reactivity with the good crystallinity required to transport photogenerated carriers without recombination. 34 Finally, the addition of an internal field within the light absorbing core separates photogenerated charge carriers and thus decreases recombination. This internal field can arise from different sources, such as ferroelectric spontaneous polarization, 35-37 or polar surface terminations. 38,39 For example, a BaTiO 3 /TiO 2 core/shell heterstructure shows enhanced photocatalytic hydro- gen production compared to its components alone. 40 On the basis of the success of FeTiO 3 /TiO 2 , PbTiO 3 /TiO 2 , and BaTiO 3 /TiO 2 heterostructured catalysts with microcrystal- line cores and nanocrystalline shells, it is reasonable to hypothesize that cores with narrower gaps will absorb more visible light and be more reactive. The purpose of this paper is to test this hypothesis. In general, ferrites have narrower bandgaps than titanates, so we will compare the photochemical activities of heterostructured catalysts with Fe containing cores Received: March 10, 2013 Accepted: May 6, 2013 Published: May 6, 2013 Research Article www.acsami.org © 2013 American Chemical Society 5064 dx.doi.org/10.1021/am4008837 | ACS Appl. Mater. Interfaces 2013, 5, 5064-5071