1. Modular assembly of photochemical water splitting catalysts from KCa 2 Nb 3 O 10 nanosheets, IrO 2, and Pt nanoparticles Owen C. Compton, Elizabeth C. Carroll, Cory Mullett, Shirley Chiang, Delmar S. Larsen, Frank E. Osterloh  Department of Chemistry, University of California, Davis, CA 95616 *fosterloh@ucdavis.edu Introduction References Acknowledgment This work was funded by the Energy Innovations Small Grant (EISG) program of the California Energy Commission. Conclusion Photochemical H 2 evolution Excited state dynamics Photoreduction of IrO 2 (cit) Predicted increases in global energy consumption and climate change driven by fossil fuel emissions have sparked a renewed interest in carbon- neutral energy sources. A recent analysis has shown that the conversion of solar energy into hydrogen is one of the most promising renewable energy technologies. 1 Exfoliation of the layered perovskite KCa 2 Nb 3 O 10 produces 1.6 nm thin nanosheets (bandgap 3.53 eV) composed of triple stacks of NbO 6 octahedra. Nanosheet edge lengths range from 0.2 nm to 1.8 nm with a mean value of 1.2 nm. We have previously observed that dispersions of exfoliated TBA[Ca 2 Nb 3 O 10 ] nanosheets are photocatalytically active for the conversion of water into H 2 and unidentified water oxidation products. 2 Viewing these nanosheets as templates, we have developed a facile method for the synthesis of multi-component photocatalysts. Here we demonstrate that the activity of the nanosheets can be altered by addition of IrO 2 (cit) and Pt(cit) nanoparticles serving as co-catalysts. The nanoparticles are either linked through γ -aminoalkylsilyl groups, or, in the case of Pt, photochemically grown on the nanosheet surface. The resulting nanostructures were characterized by TEM, HRTEM, UV/vis, fluorescence, XPS, and IR spectroscopy. The performance of the catalysts was tested under UV irradiation with a low pressure Hg lamp. Transient absorption spectroscopy was used to determine the excited state lifetimes of the nanosheets. In conclusion we have demonstrated a modular assembly approach for the generation of two-component nanostructures for photocatalytic hydrogen evolution from water. The structures are supported by direct chemical bonds between the nanomaterials or held together by γ -aminoalkylsilyl linkers. Under UV irradiation, all structures are catalytically active for photochemical H 2 evolution from water with decreasing activity in the series Pt-[HCa 2 Nb 3 O 10 ] (3) > Pt(cit)-APS-[Ca 2 Nb 3 O 10 ] (5) > Pt(cit)-AEAUS-[Ca 2 Nb 3 O 10 ] (4) > IrO2(cit)-APS-[Ca 2 Nb 3 O 10 ] (7) > TBA[Ca 2 Nb 3 O 10 ] (2). The trend can be rationalized by assuming that citrate molecules reduce the catalytically active area on Pt and organic linkers impede electron transport from the nanosheets to the Pt cocatalyst. The same trend is observed when the electrochemical potentials for water reduction are compared. In the absence of Pt, little H 2 is evolved due to the lack of water reduction sites on the catalysts. Hydrogen but no O 2 evolution from water is achieved with IrO2(cit)-APS-[Ca2Nb3O10] (7), which undergoes photoreduction to Ir-APS-[Ca2Nb3O10]. Ongoing work 5 in this laboratory is devoted to elucidating the excited state dynamics in these systems, determining the fate of O 2, and to extending the synthetic concept to optimized three component nanostructures with separate sites for water oxidation and reduction. TEM images of photocatalysts Optical properties Transient absorption measurements of APS-[Ca 2 Nb 3 O 10 ] (2) sheets in water. Transient spectra at 1 ps, 10 ps, 100 ps, 1 ns, and 6.8 ns show that transient decay was faster for APS functionalized nanosheets than non-fuctionalized. Normalized kinetics at 400 nm, 500 nm, 600 nm, and 695 nm reveal recombination of shallowly trapped carriers occurs relatively quickly compared to deeply trapped carriers at longer wavelengths. Disperse reflectance spectra of photocatalysts show a consistent band gap absorption at 350 nm (3.53 eV). IrO 2 (cit) and Pt(cit) nanoparticles have absorption bands in the vislble. TEM images reveal variation in the attachment of co-catalysts. In absence of APS, photoreduced Pt nanoparticles grow in clusters. When APS is present isolated Pt nanoparticles are formed instead. IrO 2 (cit) nanoparticles are found as globular clusters ~30 nm in diameter that are supported by interactions among the citrate surfactants. 4 1.Lewis, N. S.; Nocera, D. G., PNAS, 2006, 103, 15729-15735. 2.Compton, O. C.; et al., J. Phys. Chem. C, 2007, 111, 14586-14592. 3.Hara, M.; et al., Electrochim. Acta., 1983, 28, 1073-1081. 4.Hoertz, P. G.; et al., J. Phys. Chem. B, 2007, 111, 6845-6856. 5.Carroll, E. C.; et al., J. Phys. Chem. C, 2008, 112, 2394-2403. Assembly of photocatalysts [Ca 2 Nb 3 O 10 ] - IrO 2 (cit) Linker (L) Si H 3 CO R Pt (cit) [HCa 2 Nb 3 O 10 ] (1) Pt-[HCa 2 Nb 3 O 10 ] (3) H 2 PtCl 6 /h ν MeOH APS or AEAUS DMSO L-[Ca 2 Nb 3 O 10 ] (2) Pt-APS- [Ca 2 Nb 3 O 10 ] (4) Pt (cit)-APS- [Ca 2 Nb 3 O 10 ] (5) IrO 2 (cit)-APS- [Ca 2 Nb 3 O 10 ] (7) H 2 PtCl 6 /h ν MeOH Pt (cit) H2OH2O IrO 2 (cit) H2OH2O Platinization of the nanosheets significantly increases photocatalytic activity. Activity of platinized catalysts decreases due to the presence of chemical linkers or citrate surfactant. No O 2 was evolved. Results are for 100 mg of catalyst suspended in 50 mL of water irradiated by 750 W Hg lamp. 200 nm 1 2 3 4 100 nm 5 6 D 1 3 2 4 5 7 Infrared spectra Strong absorption bands for ν (Si-O) in APS and AEAUS functionalized species confirm attachment of the γ -aminoalkylsilyl linkers. Irradiation of IrO 2 (cit)-APS-[Ca 2 Nb 3 O 10 ) reduces IrO 2 nanoparticles from an initial mixture of Ir 4+ /Ir 3+ to a final mixture of Ir 4+ /Ir(0), with 10-20% of Ir(0). The latter is observed in the XPS spectrum as a high saddle between the peaks and the shoulder on the 4f 7/2 peak. 3 TEM data reveals that globular IrO 2 (cit) clusters transform to islands of individual particles after irradiation. 200 nm IrO 2 (cit) Pt (cit) 50 nm 64.7 61.8 64.5 61.9 (7) after irradiation 1 3 2 4 5 6 R = (APS) (AEAUS) NH NH 2 Pt (cit)-AEAUS- [Ca 2 Nb 3 O 10 ] (6) 7 pre-irr. post-irr. 1 3 2 4 5 6 7 Cyclic voltammetry measurements at pH=14 show that the overpotential for water reduction is reduced when Pt is present as a cocatalyst. Increasing the linker length from C 3 to C 12 in AEAUS increases the water reduction overpotential. Attachment of IrO 2 (cit) reduces the water oxidation overpotential by 0.6V. However, the catalytic effect of IrO 2 is reduced when the potential is cycled repeatedly to -1.47V. All potentials are given vs. NHE. Electrochemical measurements