Role of a pore network for band energy configuration in mesostructured materials

Abstract

This work is focused on the charge transfer process of mesoporous amorphous titania to build a band energy diagram by spectro- and photoelectrochemical characterization. The surface topology of mesoporous titania is completely different from a nanocrystalline film, as transmission electron microscopy confirmed. Mesoporous titania consists of an amorphous framework of titania walls where cylindrical pores are ordered in a hexagonal arrangement. Two features have been attributed to the surface topology of mesoporous titania during electrochemical characterization: (i) dominance of capacitive surface-confined electrochemical processes due to the huge surface area of amorphous titania walls showing a metallic behavior; (ii) a band energy denominated “mesoscopic” band which intermediated charge transfer from the substrate into the surface states and defect sites (Ti4+∕Ti3+) resulting in a cathodic current when mesoporous titania acted as photovoltaic solar cells. The spectroelectrochemical characterization confirmed that mesostructured titania has a different band energy diagram determined by analysis of the filling of empty electronic states during a lithium intercalation process. A surface model for mesostructured materials is introduced in this work where quantum sized particles are surrounded by hollow titania particles, modifying their optical and electrical properties. These hollow particles contain surface states and defect sites (Ti4+∕Ti3+) ordered in a hexagonal arrangement due to a porous network of mesoporous titania and, consequently, a mesoscopic band appears. This conception of band energy can give a different insight to build functional devices like solar cells, electrochromical windows and batteries where mesostructured materials can act as a cathode transporting holes through their pore network.