Neutral buffered electrolytes guarantee ideal band-edge pinning for semiconductor photoanodes

The photoelectrochemical (PEC) processes occurring in semiconductor photoelectrodes have been regarded as similar to the physical processes occurring at the semiconductor/metal interfaces. In contrast, the physicochemical processes occurring in the electrolyte have been considered to be unaffected by the (photo)electrode materials that are employed. We found that these "ideal" situations are not always guaranteed during performing the actual PEC reaction. That is, the present study based on impedance and hydrodynamic voltammetry analyses proposes that the band diagram at the interface between a semiconductor photoanode and an electrolyte can be affected by transient physicochemical phenomena in the electrolyte during the PEC oxygen evolution reaction. Specifically, in the case that a neutral unbuffered electrolyte was employed, a local pH gradient was formed during the reaction and produced a positive shift in the flat-band potential. This means the breakdown of the ideal band bending at the Schottky-like junction. Meanwhile, a neutral buffered phosphate-based electrolyte suppressed the formation of this pH gradient and thus guaranteed ideal band-edge pinning at the photoanode/electrolyte interface. This study provides insights demonstrating that PEC water splitting occurring at the semiconductor/electrolyte interface are distinct from simple analogy to the conventional semiconductor physics and to the physicochemical processes in the electrolyte.