Effects of 1D/2D Heterostructure Formation on the Charge Carrier Recombination Dynamics of TiO₂ Nanotube Photoanodes for Solar Photoelectrochemical Water Splitting

Heterostructures of low-dimensional semiconductors are employed as photoelectrodes in photoelectrochemical (PEC) water splitting cells to synergize the functions of component materials and enhance the performance by improving primarily the light absorption, charge transfer at the semiconductor-electrolyte interface and/or photo-generated carrier separation in the semiconductor. In recent years, the heterostructures formed by several one and two dimensional (1D/2D) materials were reported to have unique functionalities capable of enhancing the efficiency of PEC water splitting. For example, we recently demonstrated that hot carrier transfer would occur from hematene (2D hematite) to titania nanotubes in their heterostructure photoanodes. Nonetheless, a clear understanding of the light induced processes taking place in such structures is necessary to make further advancements leading to the realization of commercially viable devices. We developed an intensity modulated photovoltage/photocurrent spectroscopy (IMVS/IMPS) apparatus for this purpose. The technique uses a small time-varying perturbation in the monochromatic light incident on the device to cause a frequency dependent response. It is a powerful tool for revealing the behavior of photo-generated charge carriers, especially the minority carrier lifetime, diffusion length and interfacial charge transfer rates. We developed photoanodes consisting of heterostructures of titanium dioxide nanotubes with various 2D materials and studied the PEC characteristics. In this presentation, we discuss the correlation of these characteristics with the charge carrier recombination dynamics studied using IMVS/IMPS.