A novel ethanol gas sensor based on TiO2/Ag0.35V2O5 branched nanoheterostructures

Abstract

Much greater surface-to-volume ratio of hierarchical nanostructures renders them attract considerable interest as prototypical gas sensors. In this work, a novel resistive gas sensor based on TiO2/Ag0.35V2O5 branched nanoheterostructures is fabricated by a facile one-step synthetic process and the ethanol sensing performance of this device is characterized systematically, which shows faster response/recovery behavior, better selectivity, and higher sensitivity of about 9 times as compared to the pure TiO2 nanofibers. The enhanced sensitivity of the TiO2/Ag0.35V2O5 branched nanoheterostructures should be attributed to the extraordinary branched hierarchical structures and TiO2/Ag0.35V2O5 heterojunctions, which can eventually result in an obvious change of resistance upon ethanol exposure. This study not only indicates the gas sensing mechanism for performance enhancement of branched nanoheterostructures, but also proposes a rational approach to design nanostructure based chemical sensors with desirable performance.

Figure 1. Fabrication of TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures and characters of the heterostructures.

(a) Schematic illustration of the fabrication process for TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures, which are first prepared by electrospinning and then annealed at 450 °C in ambient air. (b–e) SEM images at three different magnifications of TiO₂ nanofibers (b,c) and TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures (d,e), where a great many small branches extend out of the fiber backbones. (f) N₂ adsorption–desorption isotherms and (g) XRD patterns of TiO₂ nanofibers and TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures.

Figure 2. Morphology and structure of TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures.

(a) TEM image of the nanoheterostructures, illustrating the formation of branched fiber-like nanostructures. (b) High-magnification TEM (HRTEM) image of the TiO₂/Ag_0.35V₂O₅ branched nanoheterostructure. (c,d) Magnified parts of the typical backbone and branch taken from the boxed areas in (b). (e–j) STEM image of the TiO₂/Ag_0.35V₂O₅ branched nanoheterostructure (e) and corresponding EDX elemental maps of O (f) Ti (g) V (h) and Ag (i,j), respectively, where only O, V, and Ag elements present in the branches.

Figure 3. XPS spectra of TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures.

(a) survey spectrum, (b–d) high resolution XPS spectra of Ti 2p, V 2P, and O 1s core-level binding energy, respectively.

Figure 4. Gas sensing performance of the sensors.

(a) Gas sensing properties versus different operating temperatures of the pure TiO₂ nanofibers and TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures based sensors exposed to 100 ppm ethanol. The inset shows the corresponding responses. (b) Reproducibility of the two sensors exposed to 100 ppm successive ethanol vapors (10 cycles) at 350 °C. The inset is the typical response and recovery curves of the two different types of sensors. (c) Dynamic response-recovery curves of the two sensors to ethanol vapors at 350 °C in the concentration sequence of 20, 50, 100, 500 and 1000 ppm. The inset is the corresponding responses. (d) Selective tests of TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures compared with TiO₂ nanofibers based sensors exposed to 100 ppm ethanol (Eth), acetone (Ace), ammonia (Amm), methanol (Met), and toluene (Tol) at 350 °C.

Figure 5. The scheme of the proposed gas sensing mechanism of the TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures based sensor.

(a) band structure model in air and in ethanol (E_V: valence band; E_C: conduction band; E_F: Fermi level; qΦ: effective energy barrier of the heterojunction); (b) model of the TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures based sensor exposed in air (step 1, step2, and step 3) and ethanol vapor (step 4 and step 5), respectively.