| 초록 |
Previous studies on photoelectrode for photoelectrochemical water splitting cells have been mainly focused on synthesizing oxide semiconductors with wide band gaps. Unfortunately, these pristine oxide photoanodes without any catalysts have relatively low photocurrent densities because of the inherent limitation of insufficient visible light absorption due to the wide band gap. Unlike typical oxide photoanode materials with wide band gaps and moderate band bending, phpotoanode materials with relatively narrow band gaps and lower work functions can provide advantageous situations by effectively absorbing visible and infrared light below 2 eV and forming large surface potentials at the same time. In this regard, chalcogenide materials are highly likely to allow high-efficient PEC operation because of their relatively narrow band gap, In essence, it is accepted that the photo-driven electrochemical water splitting reaction process from typical semiconductor photoelectrodes is carried out via the following three steps: 1) light absorption, 2) subsequent separation and migration of photo-generated charge carriers in the depletion region of the electrode/electrolyte junction, and 3) water oxidation/reduction reactions. Here, the charge separation efficiency was strongly affected by the energy band bending of photoelectrodes at the junction, and the formed energy band bending was dictated by the position of the work function of the photoelectrodes. n-type chalcogenides as photoanodes for plain demonstration of large band bending are inferred to be very attractive, despite hte difficulty of obtaining high quality n-type conductivity. In this study, we propose a fabrication desigh for the synthesis of n-type binary chalcogenide by control of the formation of defects. From electrocheical analyses, the energy band diagram of the n-type compound was established and the mechanisms of such a large improvement in the photocurrent, which was remarkably high (approximately 5 mA/cm2 at 1.23 V vs. RHE), were assumed to be bande on the strong inversion model of typical semiconductors. This model makes it possible to understand the abrupt generation in the photocurrent, resembling the current profiles of an electrolysis reaction from metal catalysts rather than that of typical semiconductor photoanodes. |
