Highly Efficient Hydrogen Formation using Photoexcited Semiconductro Surface

太阳能是一种非常有用的能源，它被用作光热的来源，或者转化为化学能、电能等。对于光能的能量转换，光合作用是有代表性的。在自然界中，植物可以通过光合作用将太阳能储存在自身体内。我们对与光有关的化学现象很感兴趣。利用光能诱导化学反应是我们主要关心的问题。利用人工光合作用系统生产氢这一清洁能源正在积极探索光电化学方法。作为人工系统，我们提出了一种半导体电极系统。重点是在光反应半导体固体层与水之间存在一个界面，并在界面处发生光致反应。当半导体电极与电解质溶液接触时，在薄半导体表面区域内形成空间电荷层。然后，该电极接收能量大于其带隙E_g的光子，在空间电荷层内产生电子-空穴对。所生成的载流子与电极表面的析氢或析氧有关，这取决于半导体的类型。我们首次用n型二氧化钛（TiO_2）电极与铂黑电极连接，前者暴露在近紫外光下的系统证明了太阳能光解的可能性。本课题利用p型半导体金刚石电极在紫外光下实现了高效制氢。

Solar energy is a very useful energy, and it is used as a source of light and heat, or it is transformed into chemical energy, electric energy and so on. For an energy conversion of light energy, photosynthesis is representive. In natrure, plants can store solar energy in themselves by way of photosynthesis reaction. We have been interested in light-related chemical phenomena. Utilization of light energy to induce chemical reactions is our main concern. The photoelectrochemical approaches are actively pursued for the production of hydrogen, a clean energy source by an artificial photosynthesis system.As the artificial system, we proposed a semiconductor electrode system. The point is that there exists an interface between solid layr of photo-reactive semiconductor and water and photo-induced reaction occurs at the interface. When a semiconductor electrode is in contact with an electrolyte solution, a space charge layr within a thin semiconductor surface region is formed. Then, this electrode receives photons with energy greater than its band gap E_g, electron-hole pairs are generated within the space charge layr.The generated carriers are concerned in the hydrogen or oxygen evolution on the electrode surface depending on types of semiconductor.The possibility of solar photoelectrolysis was demonstrated by us for the first time with a system in which an n-type titanium dioxide (TiO_2) electrode was connected to a platinum black electrode, the former exposed to near-uv light. This research project has achieved to form hydrogen efficiently by using P-type semiconducting diamond electrode under uv light.