Nano letters Figure 3. Structural characterization of the nanotree heterostructures. a, False-colored, large-scale SEM image of a Si/TiO2 nanotree array. b, Comparison of the optical images of a TiO2 nanowire substrate, a Si nanowire substrate, and a Si/TiO2 nanotree substrate. c, SEM image of the details of a nanotree heterostructure d, Magnified SEM image showing the large surface area of the TiO2 segment used for water oxidation. The scale bars are 10 um(a)and 1 um(c, d) tion). The optimized J-V photocurrent data of both Si and high surface area of the TiO, nanowires(Figure 3d)provide TiO2 nanowire photoelectrodes under simulated sunlight are abundant reactive sites for the sluggish O2 evolution. The Si plotted in Figure 2b. A current density intersection of 0.35 mA/ nanowire embedded underneath the TiO2 nanowire cm" suggests a non-zero current flow under illumination when anode provides an ohmic contact for recombination of majority the Si nanowire photocathode and TiO 2 nanowire photoanode carriers and serves as a charge collector( Supporting are directly linked together. This result is confirmed by the Information), which takes advantage of the high carrier systems transient current response under chopped illumination mobility of single-crystalline Si nanowires. As a fully integrated emally linked for si and TiO2 nanowires when they of the essential features in natural photosynthese tures many (Figure 2 c), implying that solar-driven water splitting without nanosystem, the entire nanotree heterostructure as Solar-driven water splitting without any applied bias is Prolonged testing of the two illuminated nanowire photo achieved under simulated sunlight using the nanotree electrodes under short-circuit conditions was also performed. heterostructures. Figure 4a displays the evolution of H, and Figure 2d shows that the photocurrent first decreased and then stabilized at 70% of its original performance, render 0.15 occurrent of about 0.7 mA 1.0 three-sun illumination. Under illumination, gas bubbles were evolved from the surface of both electrodes; gas chromatog aphy measured a stoichiometric 2: 1 hydrogen-to-oxygen ratio as is expected for water splitting(Figure 2e). Moreover, comparison of the quantities of gases produced and the amount of charge that passed through the circuit shows that these nanowire electrodes exhibit a 91% charge-to-chemical Faradic efficiency. TiO segment percentage (%) To realize overall water splitting within an integrated Figure 4. Solar-driven water splitting using nanotree heterostructures. anosystem, we synthesized the nanotree heterostructure tha ombines the optimized photocathode and photoanode a, The evolution of H, and O2 gases measured by gas chromatography under simulated sunlight of 150 mW/cm"(1.5 suns).Nanotree Figure dispersed in 0.5 M H SO, solution. After microscope(SEM) image of a large area of the substrate that 90 min of measurement, the gas in the reactor was purged and refile contains many nanotree heterostructures with Si nanowire with helium. The 2:1 ratio of H, versus O2 confirmed the wate trunks and TiO2 nanowire branches, that is, an artificial forest. splitting reaction. b, Measured energy conversion efficiency of We prepared Si nanowire arrays by reactive ion etching(RIE suspensions of nanotree heterostructures(left axis)with different since it is readily available for wafer-scale fabrication, but other ercentages of TiO2 covering the Si nanowires. The estimation for the synthetic methods, for example, chemical vapor deposition 4 or ail -ayed for comparison (yellow curve) normalized relative phe of the nanotrees(right axis) electroless etching, can also be used. As shown in Figure 3b, uniform and large-scale arrays of nanotree heterostructures are synthesized. The color of the nanotree array combines the O2 gases from the Si/TiO2 nanotree heterostructures dispersed white scattering of TiO2 nanowires and the visible-light in electrolyte. The 2: 1 stoichiometry between H, and O, absorption of the silicon nanowire array. a closer examination confirms the water-splitting photoactivity, and the linear of an individual nanotree(Figure 3c)shows a core-shell increase of gas concentrations(2. 1 and 1. 1 umol/h for H2 heterostructure with the photoanode of TiO2 nanowires on the and O2 from 2.4 mg of nanotree heterostructures )reveals stable top and the photocathode of Si nanowires on the bottom. The ytic perform Since no direct water slitting dxdoloran0.1021/l401615 tI Nano Lett.2013,13.2989-2992tion). The optimized J−V photocurrent data of both Si and TiO2 nanowire photoelectrodes under simulated sunlight are plotted in Figure 2b. A current density intersection of 0.35 mA/ cm2 suggests a non-zero current flow under illumination when the Si nanowire photocathode and TiO2 nanowire photoanode are directly linked together. This result is confirmed by the system’s transient current response under chopped illumination (Figure 2c), implying that solar-driven water splitting without applied bias is possible for Si and TiO2 nanowires when they are externally linked. Prolonged testing of the two illuminated nanowire photo￾electrodes under short-circuit conditions was also performed. Figure 2d shows that the photocurrent first decreased and then stabilized at 70% of its original performance, rendering a stabilized photocurrent of about 0.7 mA/cm2 under simulated three-sun illumination. Under illumination, gas bubbles were evolved from the surface of both electrodes; gas chromatog￾raphy measured a stoichiometric 2:1 hydrogen-to-oxygen ratio, as is expected for water splitting (Figure 2e). Moreover, comparison of the quantities of gases produced and the amount of charge that passed through the circuit shows that these nanowire electrodes exhibit a 91% charge-to-chemical Faradic efficiency. To realize overall water splitting within an integrated nanosystem, we synthesized the nanotree heterostructure that combines the optimized photocathode and photoanode (Supporting Information). Figure 3a shows a scanning electron microscope (SEM) image of a large area of the substrate that contains many nanotree heterostructures with Si nanowire trunks and TiO2 nanowire branches, that is, an artificial forest. We prepared Si nanowire arrays by reactive ion etching (RIE)26 since it is readily available for wafer-scale fabrication, but other synthetic methods, for example, chemical vapor deposition24 or electroless etching,25 can also be used. As shown in Figure 3b, uniform and large-scale arrays of nanotree heterostructures are synthesized. The color of the nanotree array combines the white scattering of TiO2 nanowires and the visible-light absorption of the silicon nanowire array. A closer examination of an individual nanotree (Figure 3c) shows a core−shell heterostructure with the photoanode of TiO2 nanowires on the top and the photocathode of Si nanowires on the bottom. The high surface area of the TiO2 nanowires (Figure 3d) provides abundant reactive sites for the sluggish O2 evolution. The Si nanowire embedded underneath the TiO2 nanowire photo￾anode provides an ohmic contact for recombination of majority carriers and serves as a charge collector (Supporting Information), which takes advantage of the high carrier mobility of single-crystalline Si nanowires. As a fully integrated nanosystem, the entire nanotree heterostructure captures many of the essential features in natural photosynthesis. Solar-driven water splitting without any applied bias is achieved under simulated sunlight using the nanotree heterostructures. Figure 4a displays the evolution of H2 and O2 gases from the Si/TiO2 nanotree heterostructures dispersed in electrolyte. The 2:1 stoichiometry between H2 and O2 confirms the water-splitting photoactivity, and the linear increase of gas concentrations (2.1 and 1.1 μmol/h for H2 and O2 from 2.4 mg of nanotree heterostructures) reveals stable catalytic performance. Since no direct water splitting is Figure 3. Structural characterization of the nanotree heterostructures. a, False-colored, large-scale SEM image of a Si/TiO2 nanotree array. b, Comparison of the optical images of a TiO2 nanowire substrate, a Si nanowire substrate, and a Si/TiO2 nanotree substrate. c, SEM image of the details of a nanotree heterostructure. d, Magnified SEM image showing the large surface area of the TiO2 segment used for water oxidation. The scale bars are 10 μm (a) and 1 μm (c, d). Figure 4. Solar-driven water splitting using nanotree heterostructures. a, The evolution of H2 and O2 gases measured by gas chromatography under simulated sunlight of 150 mW/cm2 (1.5 suns). Nanotree heterostructures were dispersed in 0.5 M H2SO4 solution. After every 90 min of measurement, the gas in the reactor was purged and refilled with helium. The 2:1 ratio of H2 versus O2 confirmed the water￾splitting reaction. b, Measured energy conversion efficiency of suspensions of nanotree heterostructures (left axis) with different percentages of TiO2 covering the Si nanowires. The estimation for the normalized relative photoactivity of the nanotrees (right axis) is displayed for comparison (yellow curve). Nano Letters Letter 2991 dx.doi.org/10.1021/nl401615t | Nano Lett. 2013, 13, 2989−2992