7254 DOI: 10.1021/la904377f Langmuir 2010, 26(10), 7254–7261 Published on Web 01/04/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Charge Separation in a Niobate Nanosheet Photocatalyst Studied with Photochemical Labeling Erwin M. Sabio, † Miaofang Chi, ‡ Nigel D. Browning, §,^ and Frank E. Osterloh* ,† † Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, ‡ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, § Department of Chemical Engineering and Materials Science, University of California, Davis, One Shields Avenue, Davis, California 95616, and ^ Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550 Received November 19, 2009. Revised Manuscript Received December 15, 2009 Photolabeling was employed to probe charge separation and the distribution of redox-active sites on the surface of nanosheets derived from the layered photocatalysts KCa 2 Nb 3 O 10 . Electron microscopy reveals 1-50 nm particles of silver, gold, iridium oxide, and manganese dioxide particles and small atomically sized clusters of platinum and IrO x on the nanosheet surfaces and along the edges. The sizes, shapes, and particle densities vary with the deposition conditions, i.e., the precursor concentration and the presence of sacrificial agents. Overall, the study shows that photogenerated electrons and holes are accessible throughout the nanosheets, without evidence for spatial charge separation across the sheet. Introduction Oxides of early transition metals (Ti, Nb, Ta) have evolved as effective photocatalysts for water splitting under UV irradia- tion;a process of potential importance for the conversion of abundant sunlight into renewable fuel. 1-3 The efficiency of these catalysts is determined by several factors, including the visible light absorption characteristics of the material, the electrochemi- cal overpotentials for the coupled water redox reactions, and the degree of photochemical charge separation. Measuring and understanding these separate processes are critical for raising the efficiency of photocatalysts. One way to obtain information about charge separation in photocatalysts is by using photochemical labeling. During photo- chemical labeling, a catalyst powder is irradiated in the presence of a metal compound that deposits on the catalysts surface after a redox step. The locations of the deposited particles then pinpoint the redox-active sites. For TiO 2 anatase and rutile crystals, it was shown by Ohno et al. that photochemical deposition of PbO 2 (from Pb 2þ ) selectively occurs onto the (011) face, whereas platinum particles deposit reductively onto the (110) face. 4 This indicates that the PbO 2 - and Pt-labeled crystals facets are the preferred locations for water oxidation and reduction, respec- tively. Similarly, photochemical labeling on La-doped NaTaO 3 showed that Pb 2þ oxidatively deposits as PbO 2 in grooves on the catalyst surface, which were thus identified as sites for water oxidation. 5 For microcrystals of the layered BaLa 4 Ti 4 O 15 , deposi- tion of PbO 2 identified the basal plane as water oxidation sites and the edge sites as water reduction sites (Au deposition). 6 Finally, photochemical deposition has also been employed on single crystalline titanate nanosheets. Here, Cu 2 O, Au, and Cu were found to grow reductively on edge sites and MnO 2 to grow oxidatively on face sites of TiO 2 nanosheet. 7 This was interpreted as evidence for electron hole separation occurring in the nano- sheets, driving electrons to the edge sites and hole to the facets. In this study, we apply photolabeling to evaluate charge separation and active site distribution in nanosheets derived from the Dion-Jacobsen phase KCa 2 Nb 3 O 10 . 8,9 KCa 2 Nb 3 O 10 is a well- known photocatalyst for H 2 evolution from water and from solutions of sacrificial electron donors. 10-19 It has a layered structure that is composed of individual Ca 2 Nb 3 O 10 - sheets (Figure 1), with each sheet made of layers of μ 2 -O bridged NbO 6 octahedra and with Ca 2þ ions filling the voids. The layered structure type is believed to enhance catalytic activity because the reduced symmetry is thought to aid the separation of photochemically generated electrons and holes. For example, in the layered K 2 La 2 Ti 3 O 7 it has been speculated *Corresponding author: Fax (þ1) 530 752 8995; Tel (þ1) 530 752 6242; e-mail fosterloh@ucdavis.edu. (1) Osterloh, F. E. Chem. Mater. 2008, 20(1), 35–54. (2) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38(1), 253–278. (3) Kamat, P. V. J. Phys. Chem. C 2007, 111(7), 2834–2860. (4) Ohno, T.; Sarukawa, K.; Matsumura, M. New J. Chem. 2002, 26(9), 1167– 1170. (5) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125(10), 3082– 3089. (6) Miseki, Y.; Kato, H.; Kudo, A. Energy Environ. Sci. 2009, 2(3), 306–314. (7) Matsumoto, Y.; Ida, S.; Inoue, T. J. Phys. Chem. C 2008, 112(31), 11614– 11616. (8) Dion, M.; Ganne, M.; Tournoux, M. Mater. Res. Bull. 1981, 16(11), 1429– 1435. (9) Fukuoka, H.; Isami, T.; Yamanaka, S. J. Solid State Chem. 2000, 151(1), 40– 45. (10) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A.; Onishi, T. Catal. Lett. 1990, 4(4-6), 339–343. (11) Ebina, Y.; Tanaka, A.; Kondo, J. N.; Domen, K. Chem. Mater. 1996, 8(10), 2534–2538. (12) Takata, T.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Catal. Today 1998, 44(1-4), 17–26. (13) Ebina, Y.; Sasaki, T.; Harada, M.; Watanabe, M. Chem. Mater. 2002, 14 (10), 4390–4395. (14) Kim, J.; Hwang, D. W.; Kim, H. G.; Bae, S. W.; Lee, J. S.; Li, W.; Oh, S. H. Top. Catal. 2005, 35(3-4), 295–303. (15) Yamashita, Y.; Hyuga, K.; Petrykin, V.; Kakihana, M.; Yoshimura, M.; Domen, K.; Kudo, A. J. Ceram. Soc. Jpn. 2007, 115(1344), 511–513. (16) Hata, H.; Kobayashi, Y.; Bojan, V.; Youngblood, W. J.; Mallouk, T. E. Nano Lett. 2008, 8(3), 794–799. (17) Maeda, K.; Mallouk, T. E. J. Mater. Chem. 2009, 19(27), 4813–4818. (18) Maeda, K.; Eguchi, M.; Youngblood, W. J.; Mallouk, T. E. Chem. Mater. 2009, 21(15), 3611–3617. (19) Ebina, Y.; Sakai, N.; Sasaki, T. J. Phys. Chem. B 2005, 109(36), 17212– 17216.