Nano letters observed from dispersions of Si nanowires, TiO2 nanowires, or of the U.S. Department of Energy under Contract No. DE- their mixture( Supporting Information), the solar-driven water ACO2-05CH11231 splitting does result specifically from the integrated design. The nanotree heterostructure displayed much higher photoactivity REFERENCES than the configuration where a TiO, thin-film is partially (1)Hall, D. Rao, K. Photosynthesis, 6th ed. Cambridge University features for our nanoscale-tree architecture are essential due to (2)Barber, J. Chem. Soc. Rev. 2009, 38, 185-196. 3)Blankenship, R. E; Tiede, D. M; Barber, ]; Brudvig, G. materials used in this study( Supporting Information ) Fleming, G; Ghirardi, M; Gunner, M R; Junge, W. Kramer, DA the vastly different optical and electrical properties of two Additionally, system-level optimization anotree Melis, A; et al. Science 2011, 332, 805-809. heterostructure is carried out. Because of the current-matching 4)Bolton, J. R; Strickler, S. J; Connolly, I. S. Nature 1985, 316, 495-500. requirement in a"Z-scheme"system, the overall rate of water (s)Nocera, D G. The artificial leaf. Acc. Chem. Res 2012, 45, 767 splitting is limited by the photoelectrode that produces the smaller photocurrent output. By varying the percentage of the 776 (6)Fujishima, A; Honda, K Nature 1972, 238, 37-38 nanotree that is covered in nanowires,an optimized 7) gratzel,M.Nate2001,414,38-34 water-splitting photoactivity can be found. Figure 4b compares Walter, M. G. Warren, E. L; McKone, J. R Boettcher, S. W; Mi, the experimental results of nanotree water splitting activity at Q Santori, E. A; Lewis, N S. Chem. Rev. 2011, 110,6446-6473 different TiO2 percentages(Supporting Information), with an (9)Tachibana, Y; Vayssieres, L; Durrant, J.R. Nat Photonics 2011 estimation based on the j-v data of the separate Si and TiO2 6,511-518 nanowire photoelectrodes in Figure 2. As expected, both data (10)Nozik, A J. AppL. Phys. Lett. 1976, 29, 150-153 sets show the best performance from a geometry in which TiO (11) Gray, H. B. Nat. Chem. 2009, 1, 7. is 50-80% of the heterostructure's total length. The optimized (12)Turner, J. Nat. Mater. 2008,7,770-771 nanotree geometry allows these heterostuctures to reach an 3755-3755 (13)Liu, C. Hwang, Y J; Jeong, H E Yang, P Nano Lett. 2011, 11, nergy conversion efficiency of 0. 12% under simulated sunlight, (14)Reece, S. Y Hamel, J. A; Sung, K- Jarvi, T. D Esswein, A. J. which is comparable to that of photosynthesis in plants. The Pijpers, J.J. H. Nocera, D. G. Science 2011, 334, 645-648 weight-normalized photoactivity of these nanotree heter (15)Kainthla, R. C Khan, S. U. M. Bockris, J. O. M Int. J. Hydrogen structures(about 875 umol/h H2 for 1 g of material)is highe Energy1987,12,381-392 than that for both electrode and powder approaches (16) Haslev,O. Science1998,280,425-427. ( Supporting Information), demonstrating the importance of (17)Miller, E. L. Paluselli, D. Marsen, B. Rocheleau, R. E. Sol overall nanosystem design and interface optimization. Future Energy Mater. Sol. Cells 2005, 88, 131-144 improvements in the material quality and the synthetic (18)Kudo, A. MRS Bull..2011,36,32-38 methods, along with using earth-abundant cocatalysts, (19)Maeda, K; Domen, K. Phys. Chem Lett. 2010, 1, 2655-2661 would make this nanotree architecture more efficient and (20) Yang, P. Tarascon, I.-M. Nat Mater. 2012, 11, 560-563 (21)Yang, P Yan, R; Fardy, M. Nano Lett. 2010, 10, 1529-1536 cost-effective. Also an ion-conductive membrane could be (22)Foley, J. M; Price, M J; Feldblyum, J. I; Maldonado, S.Energy potentially incorporated between the silicon nanowires and the Environ.Sci2012,S,5203-5220. TiO, nanowires to realize macroscopic separation of H2 and (23)Liu, B. Aydil, E. S. Am. Chem. Soc. 2009, 131, 3985-3990 O. 11,12 30 These nanoscale tree-like heterostructures illustrate (24)Boettcher, S. W Warren, E. L; Putnam, M. C. Santori, E.A the feasibility of integrating individual nanoscale components Turner-Evans, D. Kelzenberg, M. D Walter, M. G; McKone, J. R into a functional system that mimics the nanoscopic integration Brunschwig, B S; Atwater, H. A; Lewis, N S.J. Am. Chem. Soc. 2011 design of this 133, 1216-1219 overall nanosystem also provides a pathway toward better solar- (25)Yuan, G; Aruda, K; Zhou, S; Levine, A Xie, J; Wang, D to-fuel conversion efficiency as it allows newly discovered Angew. Chem, Int. Ed 2011, 50, 2334-2338 individual components to be readily plugged in (26)Hou, Y. Abrams, B. L; Vesborg, P. C. K; Bjorketun, M. E; Herbst, K; Bech, L; Setti, A. M. Damsgaard, C. D. Pedersen, T i Hansen, O. Nat. Mater. 2011, 10, 434-438 ■ ASSOCIATED CONTENT (27) Liu, G. Wang, L; Yang, H. G. Cheng, H-M Lu, G.Q. I S Supporting Mater..chem.2010,20,831. Detailed method for nanostructure synthesis and character- ization of solar-to-fuel conversion This material is available free 5060-5069 S Y. J; Hahn, C; Liu, B Yang, P. ACS Nano 2012, 6, ofchargeviatheInternetathttp://pubs.acs.org (29)Sayama, K; Arakawa, H J. Chem. Soc, Faraday Trans. 1997, 93, ■ AUTHOR INFORMATION (30)Spurgeon, J M; Walter, M. G; Zhou, J; Kohl, P A; Lewis,N S. Energy Environ. Sci. 2011, 4, 1772. Corresponding Author FE-mail: P yang@berkeley. edu. NOTE ADDED AFTER ASAP PUBLICATION Author Contributions This paper was published ASAP on May 6, 2013 with an error C L and J T. contributed equally to this work. in the two equations on page 9 of the Supporting Information Notes ile. The corrected version was re-posted on May 13, 2013. The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank S. Brittman for helpful discussion. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, 2992 dxdoloran0.1021/l401615 tI Nano Lett.2013,13.2989-2992observed from dispersions of Si nanowires, TiO2 nanowires, or their mixture (Supporting Information), the solar-driven water splitting does result specifically from the integrated design. The nanotree heterostructure displayed much higher photoactivity than the configuration where a TiO2 thin-film is partially deposited onto Si nanowire, suggesting that the structural features for our nanoscale-tree architectrure are essential due to the vastly different optical and electrical properties of two materials used in this study (Supporting Information). Additionally, system-level optimization20 or the nanotree heterostructure is carried out. Because of the current-matching requirement in a “Z-scheme” system, the overall rate of water splitting is limited by the photoelectrode that produces the smaller photocurrent output.8 By varying the percentage of the nanotree that is covered in TiO2 nanowires, an optimized water-splitting photoactivity can be found. Figure 4b compares the experimental results of nanotree water splitting activity at different TiO2 percentages (Supporting Information), with an estimation based on the J−V data of the separate Si and TiO2 nanowire photoelectrodes in Figure 2. As expected, both data sets show the best performance from a geometry in which TiO2 is 50−80% of the heterostructure’s total length. The optimized nanotree geometry allows these heterostuctures to reach an energy conversion efficiency of 0.12% under simulated sunlight, which is comparable to that of photosynthesis in plants.2,3 The weight-normalized photoactivity of these nanotree hetero￾structures (about 875 μmol/h H2 for 1 g of material) is higher than that for both electrode and powder approaches (Supporting Information), demonstrating the importance of overall nanosystem design and interface optimization. Future improvements in the material quality and the synthetic methods, along with using earth-abundant cocatalysts,5,26 would make this nanotree architecture more efficient and cost-effective. Also an ion-conductive membrane could be potentially incorporated between the silicon nanowires and the TiO2 nanowires to realize macroscopic separation of H2 and O2. 11,12,30 These nanoscale tree-like heterostructures illustrate the feasibility of integrating individual nanoscale components into a functional system that mimics the nanoscopic integration in chloroplasts. More generally, the modular design of this overall nanosystem also provides a pathway toward better solar￾to-fuel conversion efficiency as it allows newly discovered individual components to be readily plugged in. ■ ASSOCIATED CONTENT *S Supporting Information Detailed method for nanostructure synthesis and character￾ization of solar-to-fuel conversion. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: p_yang@berkeley.edu. Author Contributions C.L. and J.T. contributed equally to this work. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS We thank S. Brittman for helpful discussion. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE￾AC02-05CH11231. ■ REFERENCES (1) Hall, D.; Rao, K. Photosynthesis, 6th ed.; Cambridge University Press: New York, 1999. (2) Barber, J. Chem. Soc. Rev. 2009, 38, 185−196. (3) Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; et al. Science 2011, 332, 805−809. (4) Bolton, J. R.; Strickler, S. J.; Connolly, J. S. Nature 1985, 316, 495−500. (5) Nocera, D. G. The artificial leaf. Acc. Chem. Res. 2012, 45, 767− 776. (6) Fujishima, A.; Honda, K. Nature 1972, 238, 37−38. (7) Gratzel, M. Nature 2001, 414, 338−344. (8) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2011, 110, 6446−6473. (9) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Nat. Photonics 2011, 6, 511−518. (10) Nozik, A. J. Appl. Phys. Lett. 1976, 29, 150−153. (11) Gray, H. B. Nat. Chem. 2009, 1, 7. (12) Turner, J. Nat. Mater. 2008, 7, 770−771. (13) Liu, C.; Hwang, Y. J.; Jeong, H. E.; Yang, P. Nano Lett. 2011, 11, 3755−3758. (14) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Science 2011, 334, 645−648. (15) Kainthla, R. C.; Khan, S. U. M.; Bockris, J. O. M. Int. J. Hydrogen Energy 1987, 12, 381−392. (16) Khaselev, O. Science 1998, 280, 425−427. (17) Miller, E. L.; Paluselli, D.; Marsen, B.; Rocheleau, R. E. Sol. Energy Mater. Sol. Cells 2005, 88, 131−144. (18) Kudo, A. MRS Bull. 2011, 36, 32−38. (19) Maeda, K.; Domen, K. J. Phys. Chem. Lett. 2010, 1, 2655−2661. (20) Yang, P.; Tarascon, J.-M. Nat. Mater. 2012, 11, 560−563. (21) Yang, P.; Yan, R.; Fardy, M. Nano Lett. 2010, 10, 1529−1536. (22) Foley, J. M.; Price, M. J.; Feldblyum, J. I.; Maldonado, S. Energy Environ. Sci. 2012, 5, 5203−5220. (23) Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, 3985−3990. (24) Boettcher, S. W.; Warren, E. L.; Putnam, M. C.; Santori, E. A.; Turner-Evans, D.; Kelzenberg, M. D.; Walter, M. G.; McKone, J. R.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. J. Am. Chem. Soc. 2011, 133, 1216−1219. (25) Yuan, G.; Aruda, K.; Zhou, S.; Levine, A.; Xie, J.; Wang, D. Angew. Chem., Int. Ed. 2011, 50, 2334−2338. (26) Hou, Y.; Abrams, B. L.; Vesborg, P. C. K.; Bjö rketun, M. E.; Herbst, K.; Bech, L.; Setti, A. M.; Damsgaard, C. D.; Pedersen, T.; Hansen, O. Nat. Mater. 2011, 10, 434−438. (27) Liu, G.; Wang, L.; Yang, H. G.; Cheng, H.-M.; Lu, G. Q. J. Mater. Chem. 2010, 20, 831. (28) Hwang, Y. J.; Hahn, C.; Liu, B.; Yang, P. ACS Nano 2012, 6, 5060−5069. (29) Sayama, K.; Arakawa, H. J. Chem. Soc., Faraday Trans. 1997, 93, 1647−1654. (30) Spurgeon, J. M.; Walter, M. G.; Zhou, J.; Kohl, P. A.; Lewis, N. S. Energy Environ. Sci. 2011, 4, 1772. ■ NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on May 6, 2013 with an error in the two equations on page 9 of the Supporting Information file. The corrected version was re-posted on May 13, 2013. Nano Letters Letter 2992 dx.doi.org/10.1021/nl401615t | Nano Lett. 2013, 13, 2989−2992