Photoelectrochemical reactions, important types

Description of the Most Important Types of Photoelectrochemical Reactions... 

Photoelectrochemical (PEC) reduction of CO2 with a p-type semiconductor electrode can be regarded as one of the solar energy conversion technologies and is important from a view-point of the global environmental problems. The reaction proceeds by essentially the same mechanism as photosynthesis and is of much interest as an artificial model for it. A number of studies have been made [1], but the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. We reported [2-4] that a p-Si electrode modified with small metal (Cu, Au and Ag) particles worked as an ideal-type electrode for the PEC reduction of CO2 in aqueous solutions. In the present paper we will report that the electrode of this type is also effective for the PEC reduction of CO2 in non-aqueous solutions which have high CO2 solubility. 

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

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