Annealing temperature–dependent electronic properties in hydrothermal TiO2 nanorod arrays

Abstract

Single-crystalline semiconductor nanostructures of metal oxides as-synthesized by solution methods generally demand high-temperature treatment, so as to achieve enhanced performance in energy and (opto)electronic devices. However, the mechanism remains unclear. In this work, we have monitored charge dynamic properties of hydrothermal TiO2 nanorod arrays (NRAs) with their annealing temperatures in model dye-sensitized solar cells. Results indicate that electron collection efficiencies are in a trend of 400 > 530 > 80 > 250 °C due to a synergistic effect of electron transport and recombination, which are consistent with the device performance variations. We have further built up the relations between surface properties and charge dynamics of TiO2 NRAs with annealing temperatures for the first time. Results show that at low temperatures (≤ 250 °C), residual modifiers (i.e., Cl− and nano-sized carbon) anchored on TiO2 surface serve as recombination centers, which inhibit the electron collection; high-temperature annealing renders a clean TiO2 surface, enabling a substantially enhanced electron collection efficiency as high as ~ 95%. This mechanistic study would promote more applications of this class of cheap nanomaterials in a variety of fields such as solar cells, photocatalysis, supercapacitors, and batteries.