Hou Bingya, Shen Lang, Shi Haotian, Kapadia Rehan, Cronin Stephen B
Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, USA.
Department of Material Science, University of Southern California, Los Angeles, California 90089, USA.
Phys Chem Chem Phys. 2017 Jan 25;19(4):2877-2881. doi: 10.1039/c6cp07542h.
We report measurements of photocatalytic water splitting using Au films with and without TiO coatings. In these structures, a thin (3-10 nm) film of TiO is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (∼μA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1 μA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film. The samples with TiO produce a larger AC photocurrent indicating that hot electrons are being injected from the metal into the TiO semiconductor where they then reduce hydrogen ions in solution forming H (i.e., 2H + 2e → H). The AC photocurrent exhibits a narrow peak when plotted as a function of reference potential, which is a signature of hot electrons. Here, we photoexcite a monoenergetic source of hot electrons, which produces a peak in the photocurrent, as the electrode potential is swept through the resonance with the redox potential of the desired half-reaction. This stands in contrast to conventional bulk semiconductor photocatalysts, whose AC photocurrent saturates beyond a certain potential (i.e., light limited photocurrent). The photocurrents produced at the metal-liquid interface are smaller than those of the metal-semiconductor system, mainly because, in the metal-semiconductor system, there is a continuum of energy and momentum states that each hot electron can be injected into, while for an ion in solution, the number of energy and momentum states are very small.
我们报告了使用有和没有TiO涂层的金膜进行光催化水分解的测量结果。在这些结构中，通过原子层沉积（ALD）在100nm厚的金膜顶部沉积一层薄的（3 - 10nm）TiO膜。我们采用交流锁相技术，这使我们能够检测由金属中光激发产生的寿命较短的热电子所产生的相对较小的光电流（~μA）。在光照下，由于金膜中光激发产生的热电子和热空穴，裸金膜在析氢反应（HER）和析氧反应（OER）中均产生较小的交流光电流（<1μA）。带有TiO的样品产生更大的交流光电流，这表明热电子从金属注入到TiO半导体中，在那里它们随后还原溶液中的氢离子形成H（即，2H + 2e → H）。当将交流光电流作为参考电位的函数绘制时，会出现一个窄峰，这是热电子的特征。在这里，我们光激发单能热电子源，当电极电位扫过与所需半反应的氧化还原电位的共振时，会在光电流中产生一个峰值。这与传统的体相半导体光催化剂形成对比，传统体相半导体光催化剂的交流光电流在超过一定电位后会饱和（即光限制光电流）。在金属 - 液体界面产生的光电流小于金属 - 半导体系统的光电流，主要是因为在金属 - 半导体系统中，每个热电子可以注入到连续的能量和动量状态中，而对于溶液中的离子，能量和动量状态的数量非常少。
