NANO TTER pubs. acs org/AnolE A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting Chong Liu, t Jinyao Tang, Hao Ming Chen, Bin Liu, and Peidong Yang* ., 3 'Department of Chemistry and'Department of Materials Science and Engineering, University of California, Berkeley,California 94720, United States Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Informatic ABSTRACT: Artificial photosynthesis, the biomimetic ap proach to converting sunlight's energy directly into chemical aims to imitate nature by using an integrated system of nanostructures, each of which plays a specific role in the unlight-to-fuel conversion process. Here we describe a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. simulated sunlight, a 12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural p synthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance KEYWORDS: Artificial photosynthesis, water splitting, nanowire-based heterostructure n natural photosynthesis the energy of absorbed sunlight conductors loaded with cocatalysts. Upon exposure to light, the produces energized carriers that execute chemical reactions minority carriers in the two semiconductors will be used to in separate regions of the chloroplast. The electrons used to carry out the individual half reactions, while the majority produce NADPH are excited via the"Z-scheme ombine at an ohmic contact between the absorbing photosystems I and II. 2 The energy of the materials. Such an ohmic contact is the inorganic analogue photoexcited charge carriers is then used to overcome the of the electron transport chain in a chloroplast. Previous studies thermodynamic barrier and to provide any kinetic overpotential of solar water splitting without any applied bias employed needed to drive the photosynthetic reactions. Compared to the excitation of a single light absorber, excitation of the two light of two semiconductor powders. Here we demonstrate a absorbers, or a"Zscheme"system, allows capture of lower fully integrated functional nanosystem for direct solar water energy photons and thus a larger part of the solar spectrum, splitting, in which all of the individual components, for which can potentially lead to a higher efficiency. Moreover, example, two nanowire photoelectrodes with large surface photosystems I and II are arranged side by side on the area, an ohmic contact, and two cocatalysts, are carefully them for efficient charge transfer. In addition, the spatial efficiency. Such a modular approach demonstrates syster separation of the reduction and oxidation catalytic centers level materials design and integration at the nanoscale for minimizes the undesirable back-reaction of the photosynthetic efficient and cost-effective solar-to-fuel conversion. products. This careful arrangement of photosynthetic con model"Z-scheme system with two light-absorbin stituents results in a fully integrated system that facilitate materials is chosen here to demonstrate the capability of an onversion of solar energy into chemical fuels. The average integrated nanostructure to use sunlight to split water. Earth rate of energy captured by this photosynthetic proces abundant and stable semiconductors, silicon(Si)and titanium approaches 130 terawatts, about six times larger than the dioxide(TiO2), were chosen as the hydrogen-generating current worldwide power consumption. photocathode and oxygen-generating photoanode, respectively ame concept of an integrated system of nanostructures (Figure 1). Moreover the nanowire morphology of both can be applied to artificial photosynthesis. A general path for mimicking natural photosynthesis is to generate O2 and H2 Received: May 3, 2013 separately via the "Z-scheme" using two inorganic semi- Published: May 6, 2013 ACS Publications o2013 American Chemical Society 2989 dxdoloro/o.102/n401615t| Nano Lert.2013,13.2989-299A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting Chong Liu,†,§ Jinyao Tang,† Hao Ming Chen,† Bin Liu,† and Peidong Yang*,†,‡,§ † Department of Chemistry and ‡ Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States *S Supporting Information ABSTRACT: Artificial photosynthesis, the biomimetic ap￾proach to converting sunlight’s energy directly into chemical fuels, aims to imitate nature by using an integrated system of nanostructures, each of which plays a specific role in the sunlight-to-fuel conversion process. Here we describe a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance. KEYWORDS: Artificial photosynthesis, water splitting, nanowire-based heterostructure I n natural photosynthesis the energy of absorbed sunlight produces energized carriers that execute chemical reactions in separate regions of the chloroplast. The electrons used to produce NADPH are excited via the “Z-scheme” of light￾absorbing photosystems I and II.1,2 The energy of the photoexcited charge carriers is then used to overcome the thermodynamic barrier and to provide any kinetic overpotential needed to drive the photosynthetic reactions. Compared to the excitation of a single light absorber, excitation of the two light absorbers, or a “Z-scheme” system, allows capture of lower energy photons and thus a larger part of the solar spectrum,3 which can potentially lead to a higher efficiency.4 Moreover, photosystems I and II are arranged side by side on the thylakoid membrane with the electron transport chain between them for efficient charge transfer. In addition, the spatial separation of the reduction and oxidation catalytic centers minimizes the undesirable back-reaction of the photosynthetic products. This careful arrangement of photosynthetic con￾stituents results in a fully integrated system that facilitates conversion of solar energy into chemical fuels.5 The average rate of energy captured by this photosynthetic process approaches 130 terawatts, about six times larger than the current worldwide power consumption.1,2 The same concept of an integrated system of nanostructures can be applied to artificial photosynthesis.6−9 A general path for mimicking natural photosynthesis is to generate O2 and H2 separately via the “Z-scheme” 10 using two inorganic semi￾conductors loaded with cocatalysts. Upon exposure to light, the minority carriers in the two semiconductors will be used to carry out the individual half reactions, while the majority carriers recombine at an ohmic contact between the materials.11−13 Such an ohmic contact is the inorganic analogue of the electron transport chain in a chloroplast. Previous studies of solar water splitting without any applied bias employed either electrode-based macroscopic devices10,14−18 or a mixture of two semiconductor powders.18,19 Here we demonstrate a fully integrated functional nanosystem for direct solar water splitting, in which all of the individual components, for example, two nanowire photoelectrodes with large surface area, an ohmic contact, and two cocatalysts, are carefully positioned in order to maximize the energy conversion efficiency. Such a modular approach demonstrates system￾level materials design and integration20 at the nanoscale for efficient and cost-effective solar-to-fuel conversion. A model “Z-scheme” system with two light-absorbing materials is chosen here to demonstrate the capability of an integrated nanostructure to use sunlight to split water.13 Earth￾abundant and stable semiconductors, silicon (Si) and titanium dioxide (TiO2), were chosen as the hydrogen-generating photocathode and oxygen-generating photoanode, respectively (Figure 1). Moreover the nanowire morphology of both Received: May 3, 2013 Published: May 6, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 2989 dx.doi.org/10.1021/nl401615t | Nano Lett. 2013, 13, 2989−2992