Fig. 3: X-ray photoelectron spectroscopy of the photocathode before and after stability measurements.

a, b XPS O 1 s spectra of the n⁺-GaN nanowires/Si photocathode as-grown (a) and tested (b) under conditions described in the caption of Fig. 2, taken with the incident angle \(\theta=60^\circ\). The definition of the angle \(\theta\) is schematically shown in (d). The as-grown photoelectrode (a) showed deconvoluted peaks at 531.3 eV and 532.6 eV for O-Ga (red curve) and O-H (blue curve), respectively. After 10 hours of chronoamperometry (CA) (b), an additional deconvoluted peak at 531.6 eV corresponding to gallium oxynitride species (violet curve) emerged. The grey curves in (a) and (b) are the fitted curves of their respective raw data. c Valence band maximum measurements of the photocathode before (red dots) and after 10 hours of CA (blue dots), where the quantity E_FS – E_VS increased by \(\triangle {E}_{{{\mbox{S}}}}\approx 0.5\,{{\mbox{eV}}}\). The intersection between the background intensity flatline (at binding energy less than 2 eV) and the linear fit of the onset of the photoelectron signal intensity is the position of the surface Fermi level (E_FS) relative to the surface valence band maximum (E_VS).