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

Abstract

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), (110), 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 photo-generated holes arriving at the surface was not significantly affected by orientation. Cathodic shifts in the onset potential for water photooxidation 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 Sndoped 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).

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

“…Materials Characterization : The crystalline quality of the hematite films was previously examined via X‐ray diffraction (Rigaku Smartlab). The films have (0001) orientation, high degree of crystallinity, and epitaxial alignment to the sapphire substrate, which is described elsewhere in more detail . X‐ray reflectivity measurements were performed in order to extract the thicknesses of the hematite films and are shown in Figure S10 (Supporting Information).…”

Two-site H2O2 photo-oxidation on haematite photoanodes

“…The samples consisted of a heteroepitaxial (110)-oriented Sn-doped (1%) haematite thin film photoanode (thickness ~30 nm) on Nb-doped SnO 2 (NTO) transparent electrode (thickness ~350 nm) deposited on an a -plane sapphire substrate. Details of the fabrication process and the microstructural characteristics of the photoanode can be found elsewhere 65 .…”

Understanding charge transfer, defects and surface states at hematite photoanodes

“…For hematite, conductivity along crystal orientation (110) has been reported to be four orders of magnitude higher than along (001). 84 However, these two orientations offer the same hole ux, 85 presumably because there is no difference in free electron density, giving a similar chance of bulk recombination.…”