Remarkably enhanced H2 response and detection range in Nb doped rutile/anatase heterophase junction TiO2 thin film hydrogen sensors

Abstract

Hydrogen sensors combining wide detection range and high response, that can operate at room temperature, are urgently demanded for monitoring different concentrations of hydrogen on a single device. Here, a novel one-step hydrothermal method is reported to construct Nb doped rutile/anatase TiO2 heterophase junctions for a highly sensitive H2 sensor. The Nb concentration in the solvent plays an important role for the heterophase juncton self-assemble, and the reaction time determines the surface morphology giving either a rutile TiO2 nanorod decorated anatase form, or a Nb doped rutile/anatase TiO2 bi-layer structure. An inverse dependence of the detectivity on H2 concentration is obtained as theoretically predicted but previously not experimentally verified for the high concentration region at low temperatures. The H2 concentration detection range is remarkably expanded, extending from 1 ppm to 12000 ppm for nanorod decorated film. The H2 response at 1 ppm is significantly enhanced to 22.5%, and reaches 98.9% in 8000 ppm H2 for the bilayer structure. The remarkablely extended detection range is the result of the massive accumulation of reactive pre-absorbed O2− on the surface due to the Nb doping, and the significantly enhanced response at room temperature results from the joint contributions of pre-absorbed O2−, film compactness and the heterophase junction. Moreover, Nb doped rutile/anatase TiO2 heterophase juncton films show high stability and humidity resistance for hydrogen sensing.