Photoelectrochemical activity on Ga-polar and N-polar GaN surfaces for energy conversion

Hydrogen generation through direct photoelectrolysis of water
was studied using photoelectrochemical cells made of different facets of
free-standing polar GaN system. To build the fundamental understanding at
the differences of surface photochemistry afforded by the GaN {0001} and
{000 1} − polar surfaces, we correlated the relationship between the surface
structure and photoelectrochemical performance on the different polar
facets. The photoelectrochemical measurements clearly revealed that the
Ga-polar surface had a more negative onset potential relative to the N-polar
surface due to the much negative flat-band potential. At more positive
applied voltages, however, the N-polar surface yielded much higher
photocurrent with conversion efficiency of 0.61% compared to that of 0.55% by using the Ga-polar surface. The reason could be attributed to the variation in the band structure of the different polar facets via MottSchottky analyses. Based on this work, understanding the facet effect on
photoelectrochemical activity can provide a blueprint for the design of materials in solar hydrogen applications.