Photovoltage and Photocurrent at Tetraphenylporphyrin and Zinc Tetraphenylporphyrin Electrodes in Acidic Solutions

Photoelectrochemical properties of tetraphenylporphyrin and zinc tetraphenylporphyrin spread on a platinum plate were investigated in acidic solutions containing a variety of electroactive species. It was found that the photovoltage measured in 0.5 M sulfuric acid solutions depended strongly on the redox potential of the electroactive species; species having a redox potential of around 1.0 V vs NHE (such as oxygen and dichromate ions) generated the largest photovoltage. A similar dependency was also observed in the photocurrent, although a little ambiguous. These phenomena are discussed from a point of semiconductor electrochemistry. The magnitude of the photocurrent was found to be influenced by solution pH, suggesting that protonation of the porphyrin film surface plays an important role in the charge transfer process.