TiO2/Ti1−xSnxO2 heterojunction nanowires: characterization, formation, and gas sensing performance

Abstract

Tetragonal rutile TiO₂/Ti_1−xSn_xO₂ heterojunction nanowires were successfully fabricated through carbothermal reaction with hydrogen gas at 1050 °C. These heterojunction nanowires partially formed Ti_1−xSn_xO₂ superlattice structures and SnO₂ nanocrystals epitaxially grew on the TiO₂ nanowire surfaces. Based on the evaluations of Gibbs free energy, all reaction processes were governed by spontaneous reactions with vapor–solid–liquid (VLS). The nanowires exhibited many defect states, such as dislocations, stacking faults, and vacancies, revealed by high-resolution transmission microscopy (HRTEM) and the cathodoluminescence spectrum. The Raman spectrum revealed obvious redshift and asymmetric broadening effects, especially in the position of A_1g (604 cm⁻¹) and E_g (427 cm⁻¹) modes when compared with a previous study (J. M. Wu et al., J. Cryst. Growth, 2005, 281, 384.). The size-induced phonon confinement effect and the presence of dislocations with internal strain in the vicinity of the nanowire surfaces led to a downshift and broadening of the first order Raman line. The results are consistent with our observations from the HRTEM images. Operating temperatures of ∼160 °C with a gas sensitivity of ∼350% were achieved when detecting an ethanol concentration of ∼300 ppm. The material characterizations and formation mechanism are investigated in detail.