Silicon-driven morphology, structure, and water splitting activity in hematite nanostructures

Hematite (α-Fe2O3) is a promising photoanode in solar photoelectrochemical (PEC) water splitting with a theoretical solar-to-fuel conversion of 14-17%, which corresponds to a photocurrent of 11–14 mA cm−2. [1-3] However, α-Fe2O3 performances are limited by its low conductivity and short holes diffusion length (only few nm). Among the methods used to improve its photocurrent, Si-doping has driven intense research delivering α-Fe2O3 photoanodes with record efficiency of 4 mA/cm2. [1] Besides the increased conductivity, the understanding of the effects produced by the introduction of tetravalent dopants such as Si4+ (or Ti4+) on structural and PEC properties remains elusive. [4,5] In this contribution, we analyzed the morphological, structural, and PEC activity changes due to the systematic insertion of Si (1, 5, 10, 15, 20%) in α-Fe2O3 nanostructures prepared through a solvothermal route. SEM and TEM analysis revealed that pure α-Fe2O3 crystallized as hollow nanorods. The introduction of Si induced a transition to Si-α-Fe2O3 with aspherical shape. Raman, synchrotron radiation powder diffraction (SRPD), XPS and EDX/STEM measurements were employed to detect the structural changes due to Si inclusion in α-Fe2O3 lattice. As the amount of Si in the α-Fe2O3 increased, the atomic % of O and Fe2+ augmented. This pointed out to a doping mechanism where the additional charge due to the substitution of Fe3+ with Si4+ was compensated both by Fe valence reduction and interstitial O. The variation of the atomic composition of α-Fe2O3 structure was reflected by increased structural disorder and trend of atomic distances. Finally, α-Fe2O3 powders were tested in a traditional three-electrodes PEC cell under AM1.5G illumination observing an optimum of 1% Si-doping. Through impedance measurements, the charge transfer resistance and donor density were extracted and correlated to Si-content, structure and morphology of α-Fe2O3 nanostructures.