CO 2 Preactivation in Photoinduced Reduction via Surface Functionalization of TiO 2 Nanoparticles Daniel Finkelstein-Shapiro, †,‡ Sarah Hurst Petrosko, §,▽ Nada M. Dimitrijevic, §,#,‡ David Gosztola, § Kimberly A. Gray,* ,⊥,‡ Tijana Rajh,* ,§ Pilarisetty Tarakeshwar, ∥ and Vladimiro Mujica* ,∥,†,§ † Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Institute for Catalysis in Energy Processes, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ∥ Department of Chemistry and Biochemistry, Arizona State University, Physical Sciences Building, Room D-102, P.O. Box 871604, Tempe, Arizona 85287, United States ⊥ Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States # Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States * S Supporting Information ABSTRACT: Salicylate and salicylic acid derivatives act as electron donors via charge- transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO 2 using a combination of ultraviolet−visible spectroscopy (UV−vis), surface enhanced Raman scattering (SERS), 13 C NMR, and electron paramagnetic resonance (EPR). We find CO 2 binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO 2 , suggesting successful charge transfer from the sensitizer to the neighboring site of CO 2 . This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO 2 on semiconducting surfaces and proposes a set of tools and experiments to do so. SECTION: Energy Conversion and Storage; Energy and Charge Transport T iO 2 photocatalyzes the conversion of CO 2 to methane but suffers from a limited response to the solar spectrum because its band gap lies in the ultraviolet. 1 Dyes can be used to extend its absorption range into the visible and add functionality by providing the possibility for tailored binding sites. 2,3 Two mechanisms exist for charge injection from an adsorbed molecule into the semiconductor conduction band (CB): (i) excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the molecule with subsequent injection into the CB, and (ii) direct excitation from the HOMO to the CB, that is, excitation of a charge transfer (CT) state. The first mechanism has been extensively studied in dye-sensitized solar cells while the second is just beginning to receive attention for its use in sensing and photocatalysis. 4−7 Metal oxide nanoparticles functionalized with salicylic acid and salicylic acid derivatives represent examples of bioinorganic complexes with CT states. In these systems, the hydroxyl oxygens coordinate in a bidentate form to an undercoordinated Ti atom at the surface. Molecules that form a CT state with semiconductors also show surface enhanced Raman scattering (SERS) through the chemical effect. 8−12 This opens the possibility of probing surface molecular species spectroscopi- cally to follow the details of a reaction or to detect a molecule of interest. Amine groups can bind CO 2 to form stable carbamates 13 and are thus a means for increasing the affinity of CO 2 toward a surface. 14−19 In nature, the lysine group of the enzyme RuBisCO is activated by intake of a CO 2 molecule and formation of a carbamate. 20 Amines grafted onto photocatalytic oxide substrates can efficiently adsorb CO 2 , but are degraded by the irradiated photocatalytic substrate and cannot convert CO 2 to higher energy products. 19 Recently, a pyridine p-GaP photoelectrochemical cell was shown to convert CO 2 to methanol with high efficiencies. 18 If we are to use amines tethered to metal oxide semiconductors, increased stability of Received: December 7, 2012 Accepted: January 17, 2013 Published: January 17, 2013 Letter pubs.acs.org/JPCL © 2013 American Chemical Society 475 dx.doi.org/10.1021/jz3020327 | J. Phys. Chem. Lett. 2013, 4, 475−479