IMPEDANCE AND INTENSITY MODULATED PHOTOCURRENT SPECTROSCOPY AS COMPLEMENTARY DIFFERENTIAL METHODS IN PHOTOELECTROCHEMISTRY

Theoretical relations were derived for the impedance and the intensity modulated photocurrent response of illuminated p-InP photoelectrodes (bare and platinum coated). The relations were verified experimentally for the hydrogen evolution reaction. Coupling of these differential methods reduces or even eliminates known problems of impedance spectroscopy (i.e. data transformability) and modulated photocurrent transfer functions (i.e. data scaling). The results confirm the model of a Schottky barrier with subsequent charge transfer across the semiconductor(catalyst)-electrolyte interface. Values of the Helmholtz capacity both at bare p-InP under low current densities and at electrodes coated with monolayer amounts of platinum were found to be smaller (approximately 1 muF cm-2) than typical values for metallic surfaces. The consequences of this finding, such as the problem of experimental distinction of the surface state capacity from the Helmholtz capacity, are discussed and literature data are commented upon.