Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it’s optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72 mA cm−2 under 1 sun illumination at 1.23 VRHE that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56 mA cm−2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light.

We fabricated an NMR cell equipped with 10-100 nm scale spaces on a glass substrate (called extended nanospaces), and investigated molecular structure and dynamics of water confined in the extended nanospaces by (1)H NMR chemical shift (delta(H)) and (1)H and (2)H NMR spin-lattice relaxation rate ((1)H- and (2)H-1/T(1)), (1)H NMR spin-spin relaxation rate ((1)H-1/T(2)), and (1)H NMR rotating-frame spin-lattice relaxation rate ((1)H-1/T(1rho)) measurements of H(2)O and (2)H(2)O. The delta(H) and (1)H- and (2)H-1/T(1) results showed that size-confinement produces slower translational motions and higher proton mobility of water, but does not affect the hydrogen-bonding structure and rotational motions. Such unique phenomena appeared in the space size of 40 to 800 nm. However, the (1)H-1/T(1) value at 40 nm was still different from that in 4 nm porous nanomaterial, because translational and rotational motions were inhibited for H(2)O molecules in the nanomaterial. By examining temperature- and deuterium-dependence of the (1)H-1/T(1) values, the molecular translational motions of the confined water were found to be controlled by protonic diffusion invoking a proton hopping pathway between adjacent water rather than hydrodynamic translational diffusion. Furthermore, we clarified that proton exchange between adjacent water molecules in extended nanospaces could be enhanced by the chemical exchange of protons between water and SiOH groups on glass surfaces, ( identical with SiO(-)...H(+)...H(2)O) + H(2)O --> triple bond SiO(-) + (H(3)O(+) + H(2)O) --> triple bond SiO(-) + (H(2)O + H(3)O(+)), based on (1)H-1/T(2) measurements. An enhancement of proton exchange rate of water due to the reduction of space sizes was verified from the results of (1)H-1/T(1rho) values, and the rate of water in the 100 nm sized spaces is larger by a factor of more than ten from that of bulk water. Such size-confinement effects were distinctly observed for hydrogen-bond solvents with strong proton-donating ability, while they did not appear for aprotic and nonpolar solvent cases. Based on these NMR results, we suggested that an intermediate phase, in which protons migrate through a hydrogen-bonding network and the water molecules are loosely coupled within 50 nm from the surface, exists mainly in extended nanospaces. This model could be supported by a three-phase theory based on the weight average of three phases invoking the bulk, adsorbed, and intermediate phases.

Understanding fluid and interfacial properties in extended nanospace (10-1000 nm) is important for recent advances of nanofluidics. We studied properties of water confined in fused-silica nanochannels of 50-1500 nm sizes with two types of cross-section: (1) square channel of nanoscale width and depth, and (2) plate channel of microscale width and nanoscale depth. Viscosity and wetting property were simultaneously measured from capillary filling controlled by megapascal external pressure. The viscosity increased in extended nanospace, while the wetting property was almost constant. Especially, water in the square nanochannels had much higher viscosity than the plate channel, which can be explained considering loosely coupled water molecules by hydrogen bond on the surface within 24 nm. This study suggests specificity of fluids two-dimensionally confined in extended nanoscale, in which the liquid is highly viscous by the specific water phase, while the wetting dynamics is governed by the well-known adsorbed water layer of several-molecules thickness.