Uncovering Energetic Positions of Surface Trap States in α-FeO Treated with Cobalt Phosphate (Co-Pi) Using Charge Carrier-Selective Heterodyne Transient Grating Technique.

Near-field heterodyne transient grating (NF-HD-TG) responses of hematite (α-FeO) treated with cobalt phosphate (Co-Pi) were measured with the burn lasers inducing the depletion of the response by the removal of the trapped charge carriers in the target state, which is called charge carrier-selective heterodyne transient grating (CS-HD-TG) spectroscopic technique. We found that two distinct trap states co-existed in Co-Pi loaded on the surface of α-FeO. One of them named r-SS2, of which potential was similar to that of r-SS1 in the surface of α-FeO, acted as a recombination centre but could increase the lifetime of the trapped holes by the charge separation. We also revealed that the energetic position of the other (i-SS2), which has been regarded as the intermediate state for oxygen evolution reaction with low overpotential, was higher than that of i-SS1 in α-FeO but lower than those of r-SS1 and r-SS2.