Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, USA.
J Am Chem Soc. 2012 Mar 7;134(9):4294-302. doi: 10.1021/ja210755h. Epub 2012 Feb 23.
Hematite (α-Fe(2)O(3)) constitutes one of the most promising semiconductor materials for the conversion of sunlight into chemical fuels by water splitting. Its inherent drawbacks related to the long penetration depth of light and poor charge carrier conductivity are being progressively overcome by employing nanostructuring strategies and improved catalysts. However, the physical-chemical mechanisms responsible for the photoelectrochemical performance of this material (J(V) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by atomic layer deposition to study the photoelectrochemical properties of this material under water-splitting conditions. We employed impedance spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity, and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat-band potential. These findings demonstrate the utility of impedance spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement.
赤铁矿（α-Fe(2)O(3)）是一种很有前途的半导体材料，可通过水分解将阳光转化为化学燃料。其固有缺点是光的穿透深度长和载流子电导率差，通过采用纳米结构策略和改进的催化剂来逐步克服。然而，这种材料的光电化学性能（J(V)响应）的物理化学机制仍未得到很好的理解。在本研究中，我们通过原子层沉积制备了薄膜赤铁矿电极，以研究该材料在水分解条件下的光电化学性质。我们采用阻抗谱来确定在不同电压、光强和电解质 pH 条件下产生光电流的主要步骤。提出了一个通用的物理模型，其中包括在半导体/液体界面处存在表面态，空穴在该表面态中积累。该表面态的充电与电荷转移电阻和光电流起始之间的强烈相关性为在半导体/电解质界面处的表面态中积累空穴提供了新的证据，这些空穴是水氧化的原因。在光照下，这个表面态的充电也与测量的平带电位的偏移有关。这些发现证明了阻抗谱在赤铁矿电极研究中的实用性，它可以提供光电极的关键参数，而测量相对简单。
