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.

Engineering using liquids confined in channels 10-1000 nm in dimension, or "extended-nanofluidics," is the next target of microfluidic science. Liquid properties at this scale were unrevealed until recently because of the lack of fundamental technologies for investigating these ultrasmall spaces. In this article, the fundamental technologies are reviewed, and the emerging science and technology in the extended-nanospace are discussed.

Well-aligned polycrystalline Ta3N5-NRs provide enhanced light harvesting and efficient generation and extraction of charge carriers, leading to completely saturated photocurrent.

Heterostructure-based photoanodes have been investigated to enhance light absorption and promote the generation and extraction of charge carriers for efficient solarto-hydrogen energy conversion. Oxy(nitride) semiconducting materials are promising candidates to harvest the visible solar spectrum; however, the realization of stable and efficient oxy(nitride) heterostructure-based photoanodes remains a challenge. Here, we demonstrate a core−shell heterojunction photoanode of Ta 3 N 5 -nanorods/BaTaO 2 N that is obtained by combining glancing angle deposition and dip coating techniques. The heterojunction photoanode homogeneously covered by a FeNiO x cocatalyst (Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x ) generates a stable photocurrent of ∼4.5 mA cm −2 at 1.23 V RHE under simulated AM 1.5G sunlight. The stoichiometric evolution of O 2 and H 2 from water occurs steadily over an hour when the covered heterojunction photoanode is connected to a Pt counter electrode with faradaic efficiencies of 90%−95%. This work may open a new path to fabricating efficient and stable oxy(nitride) photoactive materials for solar energy conversion.

A semitransparent Ta3N5 photoanode is designed for efficient and durable solar water splitting. The Ta3N5-CuInSe2 tandem device exhibits an initial and stabilized solar-to-hydrogen efficiency of ∼9% (highest for metal oxides/nitrides) and 4%, respectively.

Detailed numerical simulations are performed to probe performance loss mechanisms and limiting parameters of Ta3N5-NRs based photoanodes. Device modelling enables the development of design strategies to realize efficient solar water oxidation.

Nanofluidics have recently attracted significant attention with regard to the development of new functionalities and applications, and producing new functional devices utilizing nanofluidics will require the fabrication of nanochannels. Fused silica nanofluidic devices fabricated by top-down methods are a promising approach to realizing this goal. Our group previously demonstrated the analysis of a living single cell using such a device, incorporating nanochannels having different sizes (102–103 nm) and with branched and confluent structures and surface patterning. However, fabrication of geometrically-controlled nanochannels on the 101 nm size scale by top-down methods on a fused silica substrate, and the fabrication of micro-nano interfaces on a single substrate, remain challenging. In the present study, the smallest-ever square nanochannels (with a size of 50 nm) were fabricated on fused silica substrates by optimizing the electron beam exposure time, and the absence of channel breaks was confirmed by streaming current measurements. In addition, micro-nano interfaces between 103 nm nanochannels and 101 μm microchannels were fabricated on a single substrate by controlling the hydrophobicity of the nanochannel surfaces. A micro-nano interface for a single cell analysis device, in which a nanochannel was connected to a 101 μm single cell chamber, was also fabricated. These new fabrication procedures are expected to advance the basic technologies employed in the field of nanofluidics.