Zinc Oxide (ZnO)
Similar to TiO,, ZnO also has a broad bandgap of ~3.2 eV, but with a higher carrier mobility. Moreover, ZnO is an environmentally friendly and inexpensive semiconductor, making it suitable for use as a photoanode for PEC water splitting [84]. Nevertheless, the effective applications of ZnO as a photoanode still require the improved design and modification techniques to overcome several limitations of ZnO, including the large bandgap that prevents visible light absorption, the chemical instability in acidic and alkaline electrolytes, and photo corrosion under both anodic and cathodic bias [84].
Tungsten Oxide (WO3)
Compared to other metal oxides, WO, has many important merits, such as the long minority carrier diffusion length (150 nm) and carrier lifetime, and excellent chemical stability in acidic conditions. Additionally, it has a bandgap of 2.5-2.8 eV that can harvest ~12% of the solar spectrum [85]. As a result, WO, has been widely studied as photoanode material for PEC water splitting over the past years [85]. However, the sluggish OER kinetics and slow charge transfer rates retard the practical application of WO,-based PEC systems [85]. Moreover, WO, photoanode could produce peroxo species besides 02 during the water oxidation process, resulting in a gradual loss of the PEC activity [86]. To improve the performance, different modification methods, such as the electrodeposition of Co-Pi cocatalyst, have been developed for enhancing the long-term stability and promoting the charge transfer [87].
6.3.2.4 Hematite (a-Fe203)
As one of the most stable forms of iron oxides, a-Fe,0, has attracted tremendous attention recently owing to its high abundance, low cost, non-toxicity, superior chemical stability, and favorable bandgap (2.0-2.2 eV) that allows it to absorb all the UV and a significant portion of visible light [82,88-100]. Theoretically, the maximum STH conversion efficiency of hematite photoanode can reach 16.8% [88], which is higher than that of the benchmark STH efficiency required for practical applications (>10%). However, to date, the highest reported STH efficiency of hematite is only around 3.4% [89], which is far below both the benchmark and theoretical values. The low STH efficiency is mainly caused by several inherent shortcomings of the hematite photoanodes, such as poor conductivity (on the order of 10~2 cm2 V-1 s_l), short lifetime of the photogenerated charge carriers (<10 ps), a very short hole diffusion length (2-4 nm), and a slow OER kinetics [82,88-100].
To overcome these obstacles, considerable efforts have been made in the past decades. For example, nanostructuring of hematite to shorten the charge diffusion pathway [90], elemental doping (e.g., Ti [91 ], Sn [92], P [93], and oxygen vacancies [94]) to improve the bulk electronic conductivity, heterojunction construction (such as Fe20,/Ti02 [95], Fe20,/MgFe204 [96], Fe20,/ZnFe204 [97], and Fe20,/Fe2Ti05 [98,99]) to promote the charge separation, and surface treatment to facilitate the charge transfer from hematite into electrolyte and accelerate the OER kinetics [99,100]. Although great progresses have been achieved, the PEC behavior of hematite and the exact role of different modifications are still not completely understood because the PEC water splitting process is complicated, involving multiple steps. The development of more efficient hematite photoanodes to meet the requirements of practical application is still a formidable challenge.
