Which phase of iron oxyhydroxides (FeOOH) is more competent in overall water splitting as a photocatalyst, goethite, akaganeite or lepidocrocite? A DFT-based investigation

Iron oxyhydroxides (FeOOH) are the general form of iron element existing on the earth that has many applications, including organic pollutant degradation, water treatment, solar cell, photocatalysis and so on. Recent research found that FeOOH exhibits remarkable oxygen evolution reaction (OER) activities in photocatalysis, which indicate that FeOOH could be a suitable candidate for overall water splitting reaction. However, the theoretical understanding of different phases iron oxyhydroxides are still insufficient. In this work, we apply the ab initio calculations based on density functional theory (DFT + U and HSE06) to goethite (alpha-FeOOH), akaganeite (beta-FeOOH) and lepidocrocite (gamma-FeOOH) with an aim to select the promising photocatalyst for overall water splitting reactions. The results of surface energy manifest that akaganeite is more accessible for crystal engineering. Moreover, the band edge calculation shows the valence band maximum (VBM) and conduction band minimum (CBM) of akaganeite can both satisfy the requirements of hydrogen evolution reaction (HER) and oxygen evolution OER at the same time.