A promising photoelectrode material for solar-driven water splitting, hematite (a-Fe 2 O 3 ) is non-toxic, abundant, chemically stable, low-cost, and has a bandgap of approximately 2.1 eV, [1][2][3][4][5] which accounts for a maximum theoretical solar-tohydrogen (STH) efficiency of 15 %. [4] This last property compares favorably with the most studied metal oxide materials for photoeletrochemical (PEC) water splitting, including TiO 2 , [6][7][8][9][10] ZnO, [11] and WO 3 . [12][13][14][15] However, the reported STH efficiencies of hematite photoelectrodes are substantially lower than the theoretical value, owing to several limiting factors such as poor conductivity, short excited-state lifetime (< 10 ps), [16] poor oxygen evolution reaction kinetics, [17] low absorption coefficient, [18] short diffusion length for holes (2-4 nm), [19] and lower flat-band potential in energy for water splitting. [4,20] Enormous efforts have been made to overcome these limitations of hematite, including the incorporation of oxygen evolving catalysts to reduce the kinetic barrier for water oxidation on the hematite surface, [21][22][23] the development of nanostructures to increase the effective surface area and to reduce diffusion length for carriers, [4,5,24] as well as the development of element-doped hematite for improving electrical conductivity and/or light absorption. [2] Recently, we demonstrated that TiO 2 nanowires thermally treated in hydrogen showed increased donor density and PEC performance as a result of the formation of oxygen vacancies. [6] We anticipated that creating oxygen vacancy (V O ), and thereby Fe 2+ , sites in hematite could significantly increase the conductivity of the material through a polaron hopping mechanism. [25,26] Although V O can be created by sintering hematite in a reductive atmosphere such as hydrogen, it may introduce hydrogen as a dopant into the structure. Addition-ally, hematite can be easily reduced in hydrogen to produce magnetite (Fe 3 O 4 ), which is photo-inactive. [27] Herein, we report an alternative method for the preparation of highly conductive and photoactive hematite through thermal decomposition of b-FeOOH in an oxygen-deficient atmosphere (N 2 + air). The resulting hematite sample showed substantially enhanced photoactivity compared to the pristine hematite prepared in air. The oxygen content during thermal activation significantly affects the formation of V O and thereby the photoactivity of hematite nanowires for water oxidation. This is the first demonstration of highly photoactive hematite nanowire arrays at a relatively low activation temperature without a dopant element.Akaganeite nanowires were prepared through the hydrolysis of FeCl 3 (0.15 m) in an environment with a high ionic strength (1m NaNO 3 ) and low pH value (pH 1.5, adjusted by HCl) at 95 8C for 4 h. [28] The resulting yellow film on a fluorine-doped tin oxide (FTO) substrate was covered with nanowire arrays with an average diameter and length of 70 nm and 700 nm, respectively (Figure 1 a). X-ray diffractio...
