PHOTOCURRENT DOUBLING DURING THE OXIDATION OF FORMIC-ACID AT N-CDS - AN INVESTIGATION BY INTENSITY MODULATED PHOTOCURRENT SPECTROSCOPY

The oxidation of formic acid at illuminated single crystal n-CdS electrodes has been investigated using intensity modulated photocurrent spectroscopy (IMPS). The quantum yield for the process is 1.8, indicating that hole capture is followed by electron injection ("current doubling"). The IMPS response in the saturation photocurrent region provides evidence that electron injection involves a short-lived intermediate with a first-order lifetime of 2 x 10(-5) s. It is suggested that electron injection occurs indirectly via a surface state located about 0.5 eV below the conduction band of CdS. It has been shown that Fe(III) scavenges the intermediate, leading to a progressive reduction of the quantum yield as the concentration is increased. The rate constant for the scavenging reaction was found to be 10(7) dm3 mol-1 s-1.