Overcoming Charge Collection Limitation at Solid/Liquid Interface by a Controllable Crystal Deficient Overlayer

Bulk and surface charge recombination of photoelectrode are two key processes that significantly hinder solar-to-fuel conversion of photoelectrochemical cell (PEC). In this study, the function of a “crystal-deficient” overlayer is unveiled, which outperforms a traditionally used amorphous or crystalline overlayer in PEC water splitting by exhibiting a high conductivity and large electron diffusion length to enable unlimited electron collection. The optimized ≈2.5 nm thickness of the “crystal-deficient” shell results in a depletion layer with a width of 3 nm, which overcomes the flat band limitation of the photovoltage and increases the light absorptivity in the wavelength range from 300 to 420 nm. In addition, a 50-fold increase in the conductivity yields a one-order-of-magnitude increase in the diffusion length of an electron (L_n)(≈20 μm), allowing for unlimited electron collection in the 1.9 μm TiO₂ nanowire array with the “crystal-deficient” shell. The controllable “crystal-deficient” overlayer in rutile TiO₂ nanowires photoanode achieves a photocurrent density greater than 2.0 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE), a 1.18% applied bias photon-to-current efficiency at 0.49 V versus RHE, a faradaic efficiency greater than 93.5% at 0.6 V versus Pt under air mass 1.5G simulated solar light illumination (100 mW cm⁻²).