## 1 Introduction  
Photoelectrochemical (PEC) water splitting is a promising technology for the direct conversion from solar energy to hydrogen energy. Since [PERSON] and [PERSON] proposed electrochemical photolysis of water at a TiO\({}_{2}\) electrode in 1972 [1], extensive studies have been carried out in this field. Photoelectrodes, which are the key component of a PEC system, should meet the following requirements: (1) appropriate energy levels for conduction band and valence band; (2) effective separation of photogenerated electron-hole pairs; (3) resistance to chemical- and photo-corrosion. Various semiconductor materials such as TiO\({}_{2}\)[2, 3], \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)[4, 5, 6], ZnO [7, 8], WO\({}_{3}\)[9, 10, 11], SrTiO\({}_{3}\)[12, 13], BiVO\({}_{4}\)[14, 15], etc. have been examined for their potential as photoelectrodes. Among them, \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) shows great potential due to its favorable band gap (2.1-2.2 eV), abundant natural reserves, non-toxicity and high chemical stability. Theoretically, its maximum photocurrent density is 12 mA/cm\({}^{2}\) and the maximum energy efficiency is 15% at 1.23 V vs. RHE (reference hydrogen electrode) under one solar irradiation (AM1.5,100 mW/cm\({}^{2}\)) [16]. However, the actual PEC activity of \(\alpha\)-Fe2O3 is far below its theoretical value, which was likely caused by a short hole diffusion length (2-3 nm) and a poor electrical conductivity (10\({}^{-2}\) cm\({}^{2}\)/V-s) [4; 5; 17]. Nanostructure design and doping are two common strategies to improve PEC performances. It has been reported that oriented one-dimensional (1D) \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanostructures such as nanorods and nanotubes exhibit a higher PEC activity than nanoporous and nanogranular \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)[16; 18; 19]. This is because 1D nanostructures have the following merits. On the one hand, 1D nanostructures provide a short transport pathway for photoinduced holes to reach the semiconductor-electrolyte interface, which reduces the possibility of electron-hole recombination [17; 20; 21]. On the other hand, the migration of photoinduced electrons towards a substrate would suffer less resistance if \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) preferentially grows along the (110) direction [17; 22]. This could be explained by the anisotropic conductivity of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). The conductivity of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) within the (001) basal plane (i.e., in the (110) direction) is up to 4 orders of magnitude higher than that vertical to the [001] plane [17]. Since oriented 1D \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanostructures have these advantages, various preparation methods have been developed. They include anodization [18], aqueous chemical growth [17; 20], hydrothermal synthesis [2], chemical vapor deposition (CVD) [22], electrochemical deposition [23], spin-coating deposition [24], and chemical co-precipitation [25], etc. Among these methods, aqueous chemical growth attracts attention for its mild synthesis condition and easy combination with doping. Doping is another effective strategy for the improvement of PEC activities [26]. In the past decade, numerous metal and non-metal elements such as Cr [16; 27], Ti [28; 29], Yb [30], Zn [31], Sn [6], Se [32], Nb [33], Ta [33], S [5; 34], P [35], etc. have been doped into \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). The effects of doping include: (1) to increase the concentration of charge carriers and thus enhance the electrical conductivity [5]; (2) to extend the range of light absorption [36]; (3) to facilitate the separation of electron-hole pairs by acting as electron traps [16]. The combination of these two strategies is expected to enhance the PEC performance further [5; 6; 16]. Moreover, it is desirable that the introduction of foreign elements and the formation of 1D nanostructures occur simultaneously. However, the in situ doping may cause changes in the morphology and crystallinity of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\).