Single-atom catalysts where active metal atom is dispersed on the surface of the support have received great attention due to their high specific activity and unique selectivity. It is well known that single-atom is anchored on the defect site at the surface. However, there has been little effort to correlate the number and type of defects and the resultant catalytic activity.
Hematite is one of the most prominent photoelectrochemical catalysts for water oxidation reaction, in virtue of its good optical property and cost-effectiveness. However, its widespread adaptation to commercial usage has been limited due to its modest charge transport property and poor surface reaction kinetics. To overcome these, in this work, the concept of single atom catalyst is applied and the contribution of defect to the catalytic activity is investigated. Hematite is defect chemically modified by atmosphere change and doping, varying type (Fe-vacancy or O-vacancy) and number of defects. Subsequently, iridium, which is known to be the best water oxidation catalyst yet very expensive, is deposited at the trace amount, as a form of the single-atom anchored at the surface defect sites. Through this, both the carrier conduction in the bulk and the reaction rate at the hematite-electrolyte interface are optimized. As a result, photocurrent is enhanced up to 3 mA/cm3 at 1.23 VRHE, and 5 mA/cm3 at 1.4 VRHE by donor doping, while undoped or acceptor doped system show 3-fold or much less photocurrent. Materials properties related with these are analyzed and the defect chemical origin underlying these is elucidated.