| Summary: | Surface electron traps on semiconductor photoanodes mediate surface recombination and deteriorate the photoelectrochemical (PEC) water oxidation performance of the photoanode. Developing convenient methods to reduce surface electron traps is therefore essential for high efficiency PEC water oxidation on semiconductor photoanodes, particularly for nanostructured photoanodes with large surface area. Herein, we employ a O<sub>2</sub> plasma treatment to boost the PEC water oxidation performance of Ti-doped Fe<sub>2</sub>O<sub>3</sub> (Ti-Fe<sub>2</sub>O<sub>3</sub>) nanowires photoanodes, aiming to reduce surface oxygen vacancies, the dominant electron traps on Ti-Fe<sub>2</sub>O<sub>3</sub> surface. X-ray diffraction (XRD), scanning electron microscopy and spectroscopic analyses show that the oxygen plasma treatment changes the structural, morphological and optical properties negligibly, but it does reduce the content of surface oxygen vacancies, as estimated from O1s X-ray photoelectron spectroscopy spectra. An optimal O<sub>2</sub> plasma treatment (200 W, 70 s) increases the photocurrent density of the Ti-Fe<sub>2</sub>O<sub>3</sub> nanowire photoanode to 2.14 mA·cm<sup>−2</sup> (1.23 V vs. RHE) under air mass 1.5G simulated solar light, which is 1.95 times higher than the pristine Ti-Fe<sub>2</sub>O<sub>3</sub> nanowire photoanode. The surface hole transfer efficiency is also improved by 1.66 times due to the reduced surface recombination. The work suggests that O<sub>2</sub> plasma treatment is a convenient but effective method to boost the PEC water oxidation performance of Ti-Fe<sub>2</sub>O<sub>3</sub> photoanode and might be applicable to other semiconducting oxide photoanodes for high efficiency PEC water splitting.
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