Improved BTEX gas sensing characteristics of thermally treated TiO2 hierarchical spheres manifested by high-energy {001} crystal facets

Abstract

Aromatic hydrocarbons like benzene, toluene, ethylbenzene and xylene (BTEX) are lethal volatile organic compounds, which should be precisely monitored to maintain occupational safety and indoor air quality. Thus, herein, the superior sensitivity towards toluene and ethylbenzene, among other BTEX vapours was exhibited by thermally treated sea-urchin like TiO2 hierarchical spheres achieved via hydrothermal method. X-Ray diffraction patterns displayed an improvement in crystallinity with annealing temperature and a total phase transformation to rutile phase, due to thermal instability of anatase phase. The Brunauer-Emmett-Teller surface area analyzer showed that the pore diameter increased with annealing temperature, whereas a thermal quench after 500 °C was observed on the photoluminescence results. To demonstrate the uniqueness of hierarchical structure and the effect of annealing at different temperatures, the gas sensing performances towards BTEX vapours was evaluated at various conditions. The sensor annealed at 500 °C (T5) displayed temperature-dependent selectivity towards ethylbenzene at 75 °C, with a detection limit of ∼ 0.1 ppm (i.e. 100 ppb). The sensor annealed at 700 °C (T7) revealed the highest response of 13 towards toluene at 150 °C, due to exposed high surface energy {001} facets, containing plentiful active oxygen species, which are more active for adsorption of toluene. The T7 sensor demonstrated a clear stability towards toluene for about 90 days, displaying only minimal drift. The fundamental gas sensing mechanism associated to the detection of BTEX vapour was discussed in detail.