Chemistry Department, Imperial College London, South Kensington Campus, SW7 2AZ, UK.
J Am Chem Soc. 2011 Jul 6;133(26):10134-40. doi: 10.1021/ja200800t. Epub 2011 Jun 15.
Competition between charge recombination and the forward reactions required for water splitting limits the efficiency of metal-oxide photocatalysts. A key requirement for the photochemical oxidation of water on both nanostructured α-Fe(2)O(3) and TiO(2) is the generation of photoholes with lifetimes on the order of milliseconds to seconds. Here we use transient absorption spectroscopy to directly probe the long-lived holes on both nc-TiO(2) and α-Fe(2)O(3) in complete PEC cells, and we investigate the factors controlling this slow hole decay, which can be described as the rate-limiting step in water oxidation. In both cases this rate-limiting step is tentatively assigned to the hole transfer from the metal oxide to a surface-bound water species. We demonstrate that one reason for the slow hole transfer on α-Fe(2)O(3) is the presence of a significant thermal barrier, the magnitude of which is found to be independent of the applied bias at the potentials examined. This is in contrast to nanocrystalline nc-TiO(2), where no distinct thermal barrier to hole transfer is observed.
在限制金属氧化物光催化剂效率的电荷复合与水分解所需的正向反应之间存在竞争。在纳米结构 α-Fe(2)O(3) 和 TiO(2) 上进行光化学水氧化的一个关键要求是产生具有毫秒到秒量级寿命的光空穴。在这里，我们使用瞬态吸收光谱直接探测完整 PEC 电池中 nc-TiO(2) 和 α-Fe(2)O(3) 上的长寿命空穴，并研究控制这种缓慢空穴衰减的因素，这可以被描述为水氧化的速率限制步骤。在这两种情况下，这个速率限制步骤都被暂时分配到从金属氧化物到表面结合的水分子的空穴转移。我们证明了 α-Fe(2)O(3) 上缓慢空穴转移的一个原因是存在显著的热势垒，其大小发现与所研究的电势下的外加偏压无关。这与纳米晶 nc-TiO(2) 形成对比，在 nc-TiO(2) 中没有观察到明显的空穴转移热势垒。
