Nano letters Si TiO2 H/H v ZHaO nmic contact Figure 1. Schematics of the asymmetric nanoscale tree-like heterostructures used for solar-driven water splitting, a, Structural schematics of the notree heterostructure. The small diameter TiO2 nanowires(blue)were grown on the upper half of a Si nanowire (gray), and the two miconductors absorb different regions of the solar spectrum. The two insets display the photoexcited electron-hole pairs that are separated at the miconductor-electrolyte interface to carry out water splitting with the help of cocatalysts (yellow and gray dots on the surface). b, Energy band diagram of the nanotree heterostructure for solar-driven water splitting. The photogenerated electrons in Si and holes in TiO2 move to the surface to perform water splitting, while the holes in Si and electrons in TiO2 recombine at the ohmic contact between the two semiconductors materials was applied here because of its large surface area and the shorter distances that carriers must travel to reach the semiconductor-electrolyte surface. Upon illumination photo- excited electron-hole pairs are generated in Si and TiO, which absorb different regions of the solar spectrum. Because of the band-bending at the semiconductor-electrolyte interfaces Time(min) migrate to the surface and reduce protons to genelde k ( Figure 1b), the photogenerated electrons in the Si nanowire eanwhile the photogenerated holes in the TiO2 nanowires oxidize water to evolve O2. The holes from Si and electrons from TiO2 recombine at the ohmic contact, completing the imilar to that of natural photosynthesis. Owing to the difference of the two materials in catalytic and electrical transport properties, a nanoscale tree- like light-harvesting architecture is employed to maximize the Figure 2. Optimized Si and nanowire photoelectrodes. a, SEM haracterization of Si(top)and TiO2(bottom)nanowire electrodes performance( Figure 1a). As compared to Si nanowires, TiO2 Nanowires of different length scales for the two semiconductors are nanowires with smaller diameters and higher surface area are shown. When the two semiconductor nanowire electrodes were preferred because of TiO2's shorter carrier diffusion length and electrically connected, immersed in water, and illuminated, a nonzero slower O2 evolution kinetics. An ohmic contact was created hotocurrent flowed from TiO 2 to Si to carry out the water-splitting between the Si/TiO, interface where majority carriers can reaction.b, Photocurrent density(mA/cm2)of Si and TiO2 nanowires recombine. Cocatalysts for both reactions were loaded to the applied voltage(v)referenced to a reduce the reactions overpotential. The overall system reversible hydrogen electrode(RHE) Simulated one-sun illumination resembles a nanoscale tree in which the Si nanowire serves as here. c, Current density(mA/cm)of externally short-circuited Si and the trunk and the TiO2 nanowires as the branches( Figure la) ns as b bu Such a nanotree system possesses large surface area that using chopped light exposure. The measurement scheme is shown in a favorable for catalytic reactions. At the same time the spatially d, Photocurrent measurement to test the stability of the short-circuited separated photoelectrodes with a local ohmic contact help to nanowire photoelectrodes as shown in a, under simulated three-sun illumination. e, Comparison of the evolved H2 and O2 gases and the nanoscale tree-like design is in principle applicable for other charge through the circuit in d; the ratio close to 4:2: 1 for charge, Hy Z-scheme"materials in solar-to-fuel conversion(Supporting and Oz proves water splitting with few losses. The scale bars in a are Information), and the combination of Si and TiO,is demonstrated here as a proof of concept. To realize this nanoscale tree-like architecture for artificial O2 evolution, but they usually are tested under aqueous photosynthesis, we started by optimizing the individual electrolytes of different pH for material stability and optimal components of the integrated nanosystem. Substrates with performance. In addition the cocatalyst used in one half- highly ordered arrays of Si nanowires 800 nm in diameter were react reaction may either cross-contaminate the other cocatalyst or tested as Hx- generating photocathodes. For O2 evolution, the induce the back-reaction of the other half reaction 19,29 In this photoanodes consisted of single-crystalline rutile TiO2 nano- work Si nanowire photocathodes loaded with platinum wires with diameters about 80-100 nm made using hyd nanoparticles and TiO2 nanowire photoanodes loaded with thermal synthesis(Figure 2a). Both Si-0 and TiO2. iridium oxide nanoparticles were tested to ensure they could nanowires have been well-studied as model systems for H2 and function in acidic electrolyte together( Supporting Informa 2990 dxdoloran0.1021/l401615 tI Nano Lett.2013,13.2989-2992materials was applied here because of its large surface area and the shorter distances that carriers must travel to reach the semiconductor−electrolyte surface.21 Upon illumination photo￾excited electron−hole pairs are generated in Si and TiO2, which absorb different regions of the solar spectrum. Because of the band-bending at the semiconductor−electrolyte interfaces7 (Figure 1b), the photogenerated electrons in the Si nanowires migrate to the surface and reduce protons to generate H2; meanwhile the photogenerated holes in the TiO2 nanowires oxidize water to evolve O2. The holes from Si and electrons from TiO2 recombine at the ohmic contact, completing the relay of the “Z-scheme”, 10,13 similar to that of natural photosynthesis. Owing to the difference of the two materials in catalytic and electrical transport properties, a nanoscale tree￾like light-harvesting architecture is employed to maximize the performance (Figure 1a). As compared to Si nanowires, TiO2 nanowires with smaller diameters and higher surface area are preferred because of TiO2’s shorter carrier diffusion length and slower O2 evolution kinetics.22 An ohmic contact was created between the Si/TiO2 interface where majority carriers can recombine. Cocatalysts for both reactions were loaded to reduce the reactions’ overpotential. The overall system resembles a nanoscale tree in which the Si nanowire serves as the trunk and the TiO2 nanowires as the branches (Figure 1a). Such a nanotree system possesses large surface area that is favorable for catalytic reactions. At the same time the spatially separated photoelectrodes with a local ohmic contact help to segregate the products to mitigate back-reactions.11 This nanoscale tree-like design is in principle applicable for other “Z-scheme” materials in solar-to-fuel conversion (Supporting Information), and the combination of Si and TiO2 is demonstrated here as a proof of concept. To realize this nanoscale tree-like architecture for artificial photosynthesis, we started by optimizing the individual components of the integrated nanosystem. Substrates with highly ordered arrays of Si nanowires 800 nm in diameter were tested as H2-generating photocathodes. For O2 evolution, the photoanodes consisted of single-crystalline rutile TiO2 nano￾wires with diameters about 80−100 nm made using hydro￾thermal synthesis23 (Figure 2a). Both Si24−26 and TiO2 6,27,28 nanowires have been well-studied as model systems for H2 and O2 evolution, but they usually are tested under aqueous electrolytes of different pH for material stability and optimal performance. In addition the cocatalyst used in one half￾reaction may either cross-contaminate the other cocatalyst or induce the back-reaction of the other half reaction.19,29 In this work Si nanowire photocathodes loaded with platinum nanoparticles and TiO2 nanowire photoanodes loaded with iridium oxide nanoparticles were tested to ensure they could function in acidic electrolyte together (Supporting Informa￾Figure 1. Schematics of the asymmetric nanoscale tree-like heterostructures used for solar-driven water splitting. a, Structural schematics of the nanotree heterostructure. The small diameter TiO2 nanowires (blue) were grown on the upper half of a Si nanowire (gray), and the two semiconductors absorb different regions of the solar spectrum. The two insets display the photoexcited electron−hole pairs that are separated at the semiconductor-electrolyte interface to carry out water splitting with the help of cocatalysts (yellow and gray dots on the surface). b, Energy band diagram of the nanotree heterostructure for solar-driven water splitting. The photogenerated electrons in Si and holes in TiO2 move to the surface to perform water splitting, while the holes in Si and electrons in TiO2 recombine at the ohmic contact between the two semiconductors. Figure 2. Optimized Si and TiO2 nanowire photoelectrodes. a, SEM characterization of Si (top) and TiO2 (bottom) nanowire electrodes. Nanowires of different length scales for the two semiconductors are shown. When the two semiconductor nanowire electrodes were electrically connected, immersed in water, and illuminated, a nonzero photocurrent flowed from TiO2 to Si to carry out the water-splitting reaction. b, Photocurrent density (mA/cm2 ) of Si and TiO2 nanowires in 0.5 M H2SO4 solution versus the applied voltage (V) referenced to a reversible hydrogen electrode (RHE). Simulated one-sun illumination was used, and the absolute value of the photocurrent density is shown here. c, Current density (mA/cm2 ) of externally short-circuited Si and TiO2 nanowire photoelectrodes under the same conditions as b but using chopped light exposure. The measurement scheme is shown in a. d, Photocurrent measurement to test the stability of the short-circuited nanowire photoelectrodes as shown in a, under simulated three-sun illumination. e, Comparison of the evolved H2 and O2 gases and the charge through the circuit in d; the ratio close to 4:2:1 for charge, H2, and O2 proves water splitting with few losses. The scale bars in a are 10 μm (top) and 1 μm (bottom). Nano Letters Letter 2990 dx.doi.org/10.1021/nl401615t | Nano Lett. 2013, 13, 2989−2992