Simultaneous control of morphology and PEC water splitting activity of hematite nanostructures by silicon doping

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 intrinsic properties such as low conductivity and short holes diffusion length (only few nanometers). Among the methods used for improving its photocurrent, Si-doping has driven intense research during recent years delivering Si-doped α-Fe2O3 photoanodes with record efficiency around 3-4 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 electrochemical properties remains elusive. [4,5] In this contribution, we prepared α-Fe2O3 nanostructures through the solvothermal reaction of Fe (II) acetate in ethanol at 150°C for 15 h. Si-doped α-Fe2O3 samples were synthesized by adding a proper amount (1-5-10-15-20 mol%) of tetramethoxysilane as Si source. The resulting materials were all goethite (α-FeOOH) 100% in phase composition and after a thermal treatment at 550°C for 1h they turned into pure α-Fe2O3. SEM and TEM analysis revealed that pure α-Fe2O3 crystallized in hollow spheres morphology. Interestingly, the introduction of Si induced the morphology transition to Si-α-Fe2O3 with acicular shape. Raman, synchrotron radiation powder diffraction, XPS and EDX/STEM measurements were employed to detect the structural changes due to the Si inclusion in the α-Fe2O3 lattice. As the amount of Si in the α-Fe2O3 nanostructures increased, the atomic % of oxygen and Fe2+ augmented. This pointed out to a doping mechanism where the additional charge introduced by the substitution of Fe3+ with Si4+ was compensated both by iron valence reduction and interstitial oxygen. The variation of the atomic composition of α-Fe2O3 structure was reflected by increased structural disorder as detected by Raman spectroscopy and trend of lattice atomic distances as resulting from synchrotron radiation powder diffraction. Finally, α-Fe2O3 powders were deposited on FTO conductive glass through electrophorethic deposition and tested in a traditional three-electrodes PEC cell under AM1.5G illumination. Voltammetric analysis were performed 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.