Upward Shift in Conduction Band of Ta 2 O 5 Due to Surface Dipoles Induced by N‑Doping Ryosuke Jinnouchi,* ,† Alexey V. Akimov, ‡ Soichi Shirai, † Ryoji Asahi, † and Oleg V. Prezhdo* ,‡ † Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States * S Supporting Information ABSTRACT: Density functional theory calculations were executed to clarify the mechanism of the experimentally observed upward shift in conduction band minimum (CBM) and valence band maximum (VBM) of N-doped Ta 2 O 5 , which is used as a photosensitizer in CO 2 reduction. Calculations reproduce well the experimental energy levels (with respect to vacuum) of nondoped Ta 2 O 5 and N-doped Ta 2 O 5 . Detailed analyses indicate that N-doping induces formations of defects of oxygenated species, such as oxygen atom and surface hydroxyl group, in the Ta 2 O 5 , and the defect formations induce charge redistributions to generate excess negative charges near the doped nitrogen atoms and excess positive charges near the defect sites. When the concentration of the doped nitrogen atoms at the surface is not high enough to compensate positive charges induced at the surface defects, the remaining positive charges are compensated by the nitrogen atoms in inner layers. Dipole moments normal to the surface generated in this situation raise the CBM and VBM of Ta 2 O 5 , allowing photogenerated electrons to transfer from N-doped Ta 2 O 5 to the catalytic active sites for CO 2 reduction as realized with Ru complex on the surface in experiment. 1. INTRODUCTION Artificial photosynthesis under visible light to produce organic species is an important energy conversion method to resolve the fossil fuel shortage and global warming problems. 1-9 One of the promising methods to realize artificial photosynthesis is Z-scheme, 2,8 where two semiconductor electrodes are used to activate two half-cell redox reactions. In the photosynthesis device proposed by Sato et al., 2 a semiconductor modified with metal-complex electrocatalyst (SC/MCE) used as a photo- cathode activates the following CO 2 reduction, + + → + − 2CO 4H 4e 2HCOOH 2 (R1) while a Pt loaded TiO 2 semiconductor used as a photoanode activates the following oxygen evolution reaction, → + + + − 2HO O 4H 4e 2 2 (R2) In the photocathode semiconductor, such as InP, GaP, and N- doped Ta 2 O 5 , excited electrons are injected from the conduction band of the semiconductor to the LUMO of MCE, such as Ru complex, and the injected electrons participate in the CO 2 reduction reaction R1. In the photoanode TiO 2 , photogenerated holes oxidize water molecules to evolve oxygen molecules through reaction R2. By combining the two semiconductor electrodes, the following net photosynthesis reaction is realized: + → + 2CO 2H O 2HCOOH O 2 2 2 (R3) The photocathode catalyst is a key material in the photosynthesis device. To achieve efficient and selective conversion of CO 2 to the desired product, formic acid in the above reaction R3, the photocatalyst requires efficient electron injections 10-12 and selective catalytic conversions. 13-19 The former is driven by the suitable energy alignment between semiconductor and MCE; the LUMO level must be higher than the redox level of the CO 2 reduction, i.e., -4.4 eV in vacuum scale for the case of reaction R1, 20 and the energy level of the conduction band minimum (CBM) must be further higher than the LUMO level to make the electron injections possible. 10,18 It should be also noted that the band gap of the semiconductors must be narrow enough to make the solar light available for the electron excitations. 18,21,22 To meet these requirements, a wide variety of semiconductors has been developed, 2,18,21-23 but little is known on the mechanisms dominating the energy alignments. A typical example is shown by N-doped Ta 2 O 5 (N-Ta 2 O 5 ) modified with Ru complexes, which is the first photocathode utilized for selectively reducing CO 2 under visible light. 18 As shown in Figure 1, redox levels of Ru complexes, [Ru- (bpy) 2 (CO) 2 ] 2+ (bpy: 2,2′-bipyridine), [Ru(dcbpy)(bpy)- (CO) 2 ] 2+ (dcbpy: 4,4′ -dicarboxy-2,2′ bipyridine), [Ru- (dcpby) 2 (CO) 2 ] 2+ , and [Ru(dpbpy)(Cl) 2 CO) 2 ] (dpbpy: 4,4′- Received: July 17, 2015 Revised: November 6, 2015 Published: November 9, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 26925 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925-26936 Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 8, 2019 at 00:12:43 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.