  Indium phosphide and derived compound semiconductors are materials often
involved in high-efficiency solar water splitting due to their versatile
opto-electronic properties. Surface corrosion, however, typically deteriorates
the performance of photoelectrochemical solar cells based on this material
class. It has been reported that (photo)electrochemical surface
functionalisation protects the surface by combining etching and controlled
corrosion. Nevertheless, the overall involved process is not fully understood.
Therefore, access to the electrochemical interface structure under operando
conditions is crucial for a more detailed understanding. One approach for
gaining structural insight is the use of operando reflection anisotropy
spectroscopy. This technique allows the time-resolved investigation of the
interfacial structure while applying potentials in the electrolyte. In this
study, p-doped InP(100) surfaces are cycled between anodic and cathodic
potentials in two different electrolytes, hydrochloric acid and sulphuric acid.
For low, 10 mM electrolyte concentrations, we observe a reversible processes
related to the reduction of a surface oxide phase in the cathodic potential
range which is reformed near open-circuit potentials. Higher concentrations of
0.5 N, however, already lead to initial surface corrosion.
