Article pubs.acs.org/JPCC
Density Functional Characterization of the Electronic Structures and Band Bending of Rutile RuO2/TiO2(110) Heterostructures Wei Wei,†,‡ Ying Dai,*,† Baibiao Huang,† Xiaoke Li,‡ Florian Nag̈ ele,‡ Herbert Over,§ Myung-Hwan Whangbo,∥ and Timo Jacob*,‡ †
School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Shanda South Road 27, 250100 Jinan, People’s Republic of China ‡ Institute of Electrochemistry, Ulm University, Albert-Einstein-Allee 47, D-89081 Ulm, Germany § Physical Chemistry Department, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany ∥ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States ABSTRACT: The enhanced catalytic and photocatalytic activities of a RuO2 layer deposited on the TiO2(110) surface were examined by constructing model RuO2/TiO2(110) heterostructures with and without oxygen vacancies and performing density functional calculations. The formation of the heterojunction only weakly affects the atomic structure of the interface due to a pseudomorphic deposition but causes a strong electron density accumulation in the interface as well as a bending of the valence and conduction bands of TiO2. The electron accumulation in the interface creates a strong internal electric field, which helps to effectively separate photogenerated electron−hole pairs during a photocatalytic process. Finally, we report on the catalytic role of oxygen vacancies at the surface.
I. INTRODUCTION Oxide heterostructures possess a broad range of novel physical properties, paving the way for a variety of new functional devices. Because of the subtle interplay between charge, lattice, spin, and orbital degrees of freedom, new quantum states have been uncovered at the interface in oxide heterostructures.1−3 For instance, in one of the most studied heterostructures, LaAlO3/SrTiO3, the occurrence of a two-dimensional electron gas (2DEG) has been demonstrated, opening up new possibilities for manipulating the electronic properties.3−9 In addition, much attention has been drawn to fascinating interface-related structural, electronic, and magnetic properties, which include enhanced ionic conductivity, formation of new layered structures, interionic exchange interactions, and noncollinear spins, as found in LaGaO3/MgAl2O4, Fe2O3/MgO, ZnO/Zn(Mg)O, SrTiO3/SrZrO3, and CrO2/RuO2.10−14 Since the report on its capability for splitting water into hydrogen and oxygen, TiO2 has engendered lots of interest as a potential photocatalyst material.15 However, its wide band gap and high recombination probability of photogenerated electrons and holes seriously limit the photocatalytic efficiency of TiO2. As a consequence, many efforts have been devoted to improve the photocatalytic activity of TiO2, primarily focused on doping foreign elements or depositing noble metals on the surface. Recently, TiO2 coated with a RuO2 layer, hereafter referred to as RuO2/TiO2 heterostructure, has been shown to be an efficient photocatalyst toward water splitting, decomposition of organics, water-gas shift reaction, and oxidation of © XXXX American Chemical Society
molecules, with evidently improved catalytic activity with respect to the TiO2 counterpart.16−20 This significantly improved photocatalytic activity has been suggested to arise from an efficient separation of photogenerated electron−hole pairs, as verified by XPS and UPS measurements in RuO2/TiO2 heterojunction.16 In addition, band bending and efficient charge separation have also been illustrated in RuO2/ZnO heterojunction,21 which shows enhanced light absorption in the visible region due to surface plasmon resonance. However, there is no concrete evidence from theory for this suggestion, and the structural and electronic properties of RuO2/TiO2 heterostructures are still not well understood at the atomic level. The rutile RuO2/TiO2 heterostructure is a perfect model system for a direct comparison between theory and experiment because thin RuO2 layers deposit pseudomorphically on the TiO2 substrate. Thus, in the present work, we study the fundamental structural and electronic properties of model RuO2/TiO2 heterostructures on the basis of first-principles density functional electronic structure calculations. We show that interface effects on the atomic and electronic properties are strongly localized in the interface region, the improved photocatalytic activity of the RuO2/TiO2 heterostructure originates from a strong charge accumulation in the interface, Received: February 25, 2015 Revised: May 11, 2015
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DOI: 10.1021/acs.jpcc.5b01884 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C and surface oxygen vacancies play an important role in the catalytic and photocatalytic activities.
II. CALCULATION METHODS Our first-principles density functional theory (DFT) calculations employed the projector augmented wave (PAW) method as implemented in the vienna ab initio simulation package (VASP).22,23 The generalized gradient approximation (GGA)24 of Perdew−Burke−Ernzerhof (PBE)25 was used for the exchange-correlation functional with the plane-wave cutoff energy of 450 eV. The Monkhorst and Pack scheme of k-point sampling was employed for the integration over the first Brillouin zone:26 a 9 × 9 × 15 grid was used to prerelax the rutile bulk structures and a 12 × 12 × 18 grid to obtain the total energies; in the case of surface and interface structures, 8 × 4 × 1 and 10 × 6 × 1 grids were used for the geometry optimization and total energy calculations, respectively. Gaussian smearing was used to determine how the partial occupancies are set for each wave function. The conjugate-gradient algorithm is adopted to fully relax the lattice constants and atomic positions (up to
