Published: June 29, 2011 r2011 American Chemical Society 929 dx.doi.org/10.1021/cs2001434 | ACS Catal. 2011, 1, 929–936 RESEARCH ARTICLE pubs.acs.org/acscatalysis Photocatalytic Conversion of CO 2 to Hydrocarbon Fuels via Plasmon-Enhanced Absorption and Metallic Interband Transitions Wenbo Hou, † Wei Hsuan Hung, § Prathamesh Pavaskar, ‡ Alain Goeppert, † Mehmet Aykol, ‡ and Stephen B. Cronin* ,†,‡ † Department of Chemistry, ‡ Department of Electrical Engineering, and § Department of Materials Science, University of Southern California, Los Angeles, California 90089, United States b S Supporting Information ’ INTRODUCTION Photocatalytic conversion of carbon dioxide into hydrocar- bons is of great interest for its potential to convert an abundant greenhouse gas to useful hydrocarbon fuels. In 1979, Inoue et al. first demonstrated the photoelectrocatalytic reduction of aqu- eous carbon dioxide to produce formic acid, formaldehyde, methyl alcohol, and methane using semiconducting photocata- lytic powders, including TiO 2 , ZnO, CdS, GaP, SiC, and WO 3 . 1 In addition, Halmann reported formic acid production from aqueous CO 2 at the p-type GaP photocathode in an electro- chemical photocell 2 and oxide semiconductors in a photoche- mical solar collector. 3 Hemminger and co-workers demonstrated photosynthetic reduction of carbon dioxide in water vapor to form methane on SrTiO 3 crystalline surfaces without any externally applied potential and in the absence of a liquid electrolyte. 4 TiO 2 is one of the most promising photocatalysts for carbon dioxide reduction; however, it does not absorb light in the visible region of the electromagnetic spectrum. Because of TiO 2 ’s short wavelength cutoff, only a small fraction of solar photons (∼4%) can be used to drive this photocatalyst. The resulting low photocatalytic yield of TiO 2 is perhaps its main disadvantage for the photocatalytic conversion of carbon dioxide into hydro- carbons. Several attempts have been made to increase its yield. 5 For example, copper-, 5À7 copper oxide-, 8,9 silver-, 7,10 and ruthe- nium dioxide-doped 11 TiO 2 have resulted in increased yields. Previously, our group and several others have reported plasmon resonant enhancement of dye photodegradation, 12 oxidation of CO 13 and organic compounds, 14 and photoelectrochemical reactions. 15,16 However, the mechanism for this increased photo- catalytic activity is controversial. Tatsuma’s group and others treat the plasmon excitation in the metals as having an energy separation between the electrons and holes, which enables electron transfer from the Au nanoparticles to the adjacent TiO 2 . 14À16 However, surface plasmons consist of the collective oscillation of charge bound to the Au surface, and therefore have no highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) or valence band-con- duction band energy separation associated with them. It is well- known that a Schottky barrier is formed at metal-semiconductor junctions. In this paper, we provide a rigorous analysis of this charge transfer process by calculating the electron transfer from the plasmon excitation in the Au nanoparticle to the TiO 2 semiconductor using the electric potentials calculated from numerical electromagnetic simulations together with the ideal diode equation for a Au/TiO 2 Schottky junction. Noble metal nanoparticles combined with semiconductors have been widely studied for improved charge separation of photogenerated electronÀhole pairs, thus enhancing the overall photocatalysis of semiconductors under UV illumination. 17À22 Received: March 14, 2011 Revised: June 27, 2011 ABSTRACT: A systematic study of the mechanisms of Au nanoparticle/ TiO 2 -catalyzed photoreduction of CO 2 and water vapor is carried out over a wide range of wavelengths. When the photon energy matches the plasmon resonance of the Au nanoparticles (free carrier absorption), which is in the visible range (532 nm), we observe a 24-fold enhancement in the photocatalytic activity because of the intense local electromagnetic fields created by the surface plasmons of the Au nanoparticles. These intense electromagnetic fields enhance sub-bandgap absorption in the TiO 2 , thereby enhancing the photocatalytic activity in the visible range. When the photon energy is high enough to excite d band electronic transitions in the Au, in the UV range (254 nm), a different mechanism occurs resulting in the production of additional reaction products, including C 2 H 6 , CH 3 OH, and HCHO. This occurs because the energy of the d band excited electrons lies above the redox potentials of the additional reaction products CO 2 /C 2 H 6 , CO 2 /CH 3 OH, and CO 2 /HCHO. We model the plasmon excitation at the Au nanoparticle-TiO 2 interface using finite difference time domain (FDTD) simulations, which provides a rigorous analysis of the electric fields and charge at the Au nanoparticle-TiO 2 interface. KEYWORDS: photocatalytic, photocatalysis, plasmon, plasmonic, interband transition, CO 2 , hydrocarbon fuels