电位对α-Fe2O3和掺钛α-Fe2O3光氧化水电荷转移速率常数的影响

外加一定的阳极电位可提高未掺杂的α-Fe₂O₃和Ti掺杂的α-Fe₂O₃(Ti-Fe₂O₃)电极的光电流或光电化学氧化水的速率, 但文献中通常假定电位全部降落在半导体固体一侧(带边钉扎), 其对界面电荷转移速率常数的影响鲜见报道. 本文应用电化学阻抗谱研究了外加电位对这两种电极光电化学氧化水时界面电荷转移速率常数的影响.结果表明: 随着外加阳极电位增大,两种电极的界面电荷转移速率常数均增大,但速率常数增幅比理论预期的要小, 表明电位并不是全部降落在电极的亥姆霍兹层, 而是同时降落在空间电荷层和亥姆霍兹层(费米能级钉扎). 表面态电容测量结果表明光生电荷可在表面态中积累, 导致了电位在电极界面重新分布并提高了界面电荷转移速率常数.相同电位下, 光强越强, 光生空穴在表面态积累越多, 降落在亥姆霍兹层中的电位增加,电荷转移速率常数也更大. 与α-Fe₂O₃相比,外加阳极电位对Ti-Fe₂O₃的界面转移速率常数提高更为明显.

Influence of the Potential on the Charge-Transfer Rate Constant of Photooxidation of Water over α-Fe2O3 and Ti-Doped α-Fe2O3

It has been reported that applying a certain external anodic potential over un-doped (α-Fe₂O₃) and Ti-doped α-Fe₂O₃ (Ti-Fe₂O₃) electrodes can improve the photocurrent or the photoelectrochemical oxidation rate of water. However, it is assumed in the literature that the potential drops completely across the side of the solid semiconductor (band edge pinning), and the influence of the potential on the interfacial charge-transfer rate constant is rarely reported. In this article, the impact of the applied potential on the interfacial charge-transfer rate constant during photoelectrochemical oxidation of water over the two electrodes was investigated by electrochemical impedance spectroscopy. The results showed that by increasing the applied anodic potential, the interfacial charge-transfer rate constants for both electrodes were increased. The smaller increase in the magnitude of the rate constant than determined by theory indicates that not all of the applied potential drops across the Helmholtz layer, but takes place in both the space charge and Helmholtz layers simultaneously (Fermi level pinning). The results of the surface-state capacitance measurements suggested that the photo-generated charge can be accumulated in the surface states, resulting in the re-distribution of the potential at the interface and an improvement in the rate constant. Under the same applied potential, the higher the light intensity is, the more the photogenerated holes accumulated in the surface states. This causes an increase in the potential drop across the Helmholtz layer and consequently increases the charge-transfer rate constant. Compared with the α-Fe₂O₃, the improvement of the charge-transfer rate constant by the anodic potential is more obvious.