The photocurrent density measurements reveal an improved photoactivity for NTs grown in high water content electrolytes (1.1 mA/cm2 in 12 wt.% H2O vs 0.75 mA/cm2 in 2 wt.% H2O at 1 V vs RHE). Similarly the Incident Photon to Current Efficiency (IPCE) of NTs obtained in 12 wt.% H2O is consistently higher than for 2 wt.% H2O over the range 350-300 nm (with values > 40% at 320-300 nm). Electrochemical Impedance Spectroscopy (EIS) analysis shows similar electron transport and charge transfer properties for NTs grown in low and high water contents. EIS is mainly sensitive to the bottom of the NTs, whereas the NTs wall is not active at potentials more positive than the flat band potential. EIS also reveals that the poorer photoelectrochemical properties of NTs grown in 2 wt.% H2O are due to a larger density of deep intra band-gap energy states in comparison with 12 wt.% H2O.
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