Trap density measurement in three-dimensional nanostructured photoelectrode of titanium dioxide with various morphologies

Abstract

Three-dimensional TiO2 nanostructures take advantages of large surface area and superior light-harvesting efficiency, as well as appropriate charge carrier transport and separation features. Here in this research, we report for the first time on measurement of density of localized trap states in three-dimensional TiO2 nanostructures with various morphologies. Bare 1-dimensional TiO2 nanofibers have been synthesized via two different methods: electrospinning and hydrothermal processes. The surface of TiO2 nanofibrous has been modified with nanoparticles and nanobranches thorough second hydrothermal process. Various morphologies of nanostructured TiO2 have been fabricated including nanoparticulated nanofiber (NP-NF), nanostar (NS), nanofiber decorated with nanoparticle (NF-NP) and nanobranched nanofiber (NB-NF). The density and depth of localized trap states as well as the position of quasi-Fermi level have been measured using a double-switch method. The trap depths in various structures have been measured in the range of 250–370 meV and have been compared with typical nanoparticular electrodes. These results imply that in spite of morphological order in surface-modified one-dimensional nanostructures, high density of deep trap states might impose direct effect on electron transport features. Based on the results, the electronic structure of nanostructured photoelectrodes should be considered in all novel nanostructured optoelectronic devices.