TiO2 /Ferroelectric Heterostructures as Dynamic Polarization-Promoted Catalysts for Photochemical and Electrochemical Oxidation of Water

Abstract

Using first-principles density functional theory calculations, we explore the chemical activity of epitaxial heterostructures of TiO 2 anatase on strained polar SrTiO 3 films focusing on the oxygen evolution reaction (OER), the bottleneck of water splitting. Our results show that the reactivity of the TiO 2 surface is tuned by electric dipoles dynamically induced by the adsorbed species during the intermediate steps of the reaction while the initial and final steps remain unaffected. Compared to the OER on unsupported TiO 2 , the combined effects of the dynamically induced dipoles and epitaxial strain strongly reduce rate-limiting thermodynamic barriers and significantly improve the efficiency of the reaction. DOI: 10.1103/PhysRevLett.112.196102 PACS numbers: 82.45.Jn, 82.45.Un, 82.50.−m, 77.55.fp Titanium dioxide (TiO 2 ) is widely used in photocatalysis and solar energy conversion due to its many favorable properties, like stability against photocorrosion and proper band alignment relative to the water redox potentials [1][2][3]. However, the efficiency of TiO 2 is notoriously low, and efforts at improving it through modifications such as doping [4] or synthesis of (nano)crystals exposing highly reactive facets [5] have encountered only limited success. Among the possible alternatives to TiO 2 , polar materials have recently attracted significant interest because their surface dipoles can react easily with charged species and supply built-in electric potentials to increase the carrier mobility [6,7]. However, control of the surface reactivity of these materials is difficult [8], e.g., because exposed surface dipoles often cause undesired atomic or electronic reconstructions and unexpected adsorption of charged species which can interfere with the reaction of interest [9][10][11]. Less well known than TiO 2 and polar semiconductors are heterostructures composed of stable TiO 2 thin films supported by a ferroelectric substrate [12,13]. Experimental studies have shown that the surface reactivity of TiO 2 is directly influenced and improved by the underlying ferroelectric domain structure, an effect that has been attributed to the combined efficient absorption and charge separation in the ferroelectric substrate and transport across the TiO 2 =substrate interface [12,13]. Despite some encouraging results [13,14], however, the interest in such TiO 2 =ferroeletric heterostructures has remained limited. In particular, their physical properties are largely unexplored and an atomic-scale understanding of their reactivity is missing.We present here a first-principles density functional theory (DFT) study of TiO 2 =ferroelectric heterostructures, which provides evidence that these composite materials can have a substantially enhanced reactivity relative to TiO 2 . We focus on model heterostructures composed of a TiO 2 (001) film of anatase (the TiO 2 polymorph most efficient for photocatalysis [15]) supported by a nearly ferroelectric SrTiO 3 (STO) substrate that can easily acquire a macroscopic polarization in the p...

Structure and water oxidation activity of 3d metal oxides

“…More recently, Lee and Selloni investigated OER PES on anatase (0 0 1) on polar substrate SrTiO 3. They show that the polarization of the substrate can significantly increase the OER activity by modifying the thermodynamics.…”

Surface charged species and electrochemistry of ferroelectric thin films

“…11 However, few studies and ab initio modeling have considered the capacity of water molecules to dissociate and participate in redox chemical reactions at ferroelectric surfaces, [12][13][14][15][16] which would be expected to provide a large additional screening contribution, 17,18 in particular on highly reactive perovskite oxides. 19,20 Moreover, surface chemistry is not only important for the stabilization of ferroelectric surfaces, 21 but in fact polarization switching kinetics are intrinsically coupled to surface electrochemical phenomena. [22][23][24][25][26][27] Finally, not only the surface but also the bulk chemical composition is susceptible to the polarization direction and switching.…”

Localized states induced by an oxygen vacancy in rutile TiO2

“…Near the surface, vacancies serve as active sites to dissociate molecules and are essential for photocatalysis. [1][2][3][4][5][6][7] In the bulk, vacancies can lead to ferromagnetism 8-10 enabling a possible use of TiO 2 as a magnetic semiconductor in spintronics. 11 Several experimental techniques have been used to characterize oxygen vacancies.…”