Effect of Orientation on Bulk and Surface Properties of Sn-doped Hematite (α-Fe2O3) Heteroepitaxial Thin Film Photoanodes

The orientation dependence on the photoelectrochemical properties of Sn-doped hematite photoanodes was studied by means of heteroepitaxial film growth. Nb-doped SnO2 (NTO) was first grown heteroepitaxially on c, a, r, and m plane single crystal sapphire substrates in three different orientations. Hematite was then grown in the (001), (Ho), and (100) orientations on the NTO films. The structural, morphological, optical, and photoelectrochemical properties of the photoelectrodes were studied. The hematite photoanodes possessed high crystallinity and smooth surfaces. Hole scavenger measurements made in H2O2-containing electrolyte revealed that the flux of photogenerated holes arriving at the surface was not significantly affected by orientation. Cathodic shifts in the onset potential for water photo-oxidation of up to 170 mV were observed for (110) and (100) oriented hematite photoanodes as compared to (001) oriented films. These results suggest that varying the orientation of heteroepitaxial thin film Sn-doped hematite photoelectrodes primarily affects charge transfer into the electrolyte arising from the surface properties of the different crystal faces rather than affecting hole transport through the bulk under illumination. Electrochemical techniques were then used to probe the existence of surface states which were found to vary with both exposed crystal face as well as foreign dopant inclusion. Kelvin probe force microscopy (KPFM) measurements revealed correlation between the work function of the hematite films (measured in air) and the flat-band and onset potentials for water photo-oxidation (in alkaline aqueous solution).