Structure of the Photo-catalytically Active Surface of SrTiO₃

Abstract

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Since the initial reports of the ultraviolet (UV) activity of TiO 2 and SrTiO 3 in the 1970's, researchers have pursued a fundamental understanding of the mechanistic and molecular-level phenomena involved in photo-catalysis. [1][2][3][4][5][6][7] Although it requires UV light, after four decades SrTiO 3 is still the gold standard for splitting water. It is chemically stable and catalyzes both the hydrogen and the oxygen reactions without applied bias. While ultrahigh vacuum (UHV) surface science techniques have provided useful insights, 8 we still know relatively little about the structure of electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a SrTiO 3 electrode during water splitting, before and after training with a positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy (SECM) at open circuit correlates this training with a tripling of the activity toward photoinduced water splitting. A novel first-principles joint density-functional theory (JDFT) simulation constrained to the X-ray data via a generalized penalty function identifies an anatase-like structure for the more active, trained surface.Wide bandgap, n-doped, metal oxide semiconductors such as SrTiO 3 , TiO 2 and WO 3 absorb UV light to form photo-generated (electrons and holes) charge carriers, capable of driving redox reactions at the interface with an electrolyte. 9-11 SrTiO 3 is a prototypic perovskite structure metal oxide ( Figure 1a). The perovskites exhibit a vast range of attractive physico-chemical properties including promising energy conversion activity. 12 In particular, SrTiO 3 has very attractive photocatalytic properties. It is highly stable in base, displays high quantum efficiency for the electro-oxidation of water under UV illumination, and performs the light-driven water splitting reaction (i.e., photo-generation of both O 2 and H 2 from H 2 O) under an applied bias, 2,13 at open circuit aided by an auxiliary metal electrode, 2,13 and on free-standing crystals. 4,5,[13][14][15][16] The fundamental mechanisms underlying surface photochemical reactions, however, remain unclear. While the bulk d-band structure can correlate with activity, 12 surface defects and surface structure are critically important and it is typically difficult to decouple the bulk and surface contributions to observed changes in reactivity. [17][18][19] Here, we illustrate the critical role that surface structure plays by demonstrating under operando conditions that the electrochemical activation (training) of n-doped SrTiO 3 (001) in basic media induces an irreversible surface reordering that enhances (by 260%) its activity for photocatalytic water splitting.The operando structural characterization of SrTiO 3 before and after training wa...

Dynamic Stabilization of Metal Oxide–Water Interfaces

“…Surface pKa prediction based on bond valence analysis suggests that water exchange will influence the proton transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide-water interfaces.The interfaces between metal oxides and water are among the most important in nature and in emerging energy applications, with wide ranging impacts from photocatalytic water splitting [1][2][3][4] to the geochemical cycling of elements 5,6 . Key chemical processes such as adsorption, electron transfer, growth, and dissolution all depend principally on the atomic structure adopted at these interfaces.…”

“…For example, dissolution and solute adsorption are regulated by the structure of interfacial water 7,8 . Surface acid/base chemistry 9,10 arises from the types and arrangement of terminal metal-coordinating aquo/hydroxyl groups 3,[11][12][13] .…”

Influence of surface restructuring on the activity of SrTiO3 photoelectrodes for photocatalytic hydrogen reduction

“…Through surface-sensitive characterization and electronic-structure calculation, it has been shown that SrTiO 3 can undergo a TiO 2 -rich surface reconstruction [7][8][9][10][11][12]. This result has been further confirmed by the detailed comparison of computationally predicted structures with accurate x-ray reflectivity data [9,13].…”

“…Previously reported terminations include the single-TiO 2 layer [9,12,35], and the stoichiometric double-TiO 2 terminated interface in the (1×1) and (2×1) surface unit cells (2 ML (1×1) and 2 ML (2×1)) [7,9,10,12]. In addition, a unique type of SrTiO 3 surface structure induced under electrochemical conditions has been recently reported by Plaza and coworkers [13]. Specifically, the SrTiO 3 interface has been shown to undergo a substantial reconstruction upon "training" at positive bias, forming a non-stoichiometric, triple-TiO 2 terminated interface, which exhibits significantly improved activities in alkaline solutions.…”

Surface Reconstructions of SrTiO3(110) Calibrated with STM and LEED

“…Strontium titanate (SrTiO 3 ) as a functional metal oxide material has attracted extensive research interests due to novel structures and properties exhibited at its surfaces . It is a paradigmatic substrate for epitaxial growth of photocatalyst, high‐ T c superconductors, ferroelectrics . Moreover, the existence of two‐dimensional (2D) electron gas and liquid on the surface and interface of SrTiO 3 makes it an appealing substitute for conventional semiconductors .…”