Enhanced ethanol sensing performance of Fe: TiO₂ nanowires and their mechanism of sensing at room temperature

Abstract

TiO₂ and Fe-doped TiO₂ nanowires were synthesized by spray pyrolysis technique to study their gas sensing properties towards ethanol. Charge transfer from metal dopant to TiO₂, and modification of TiO₂ with Fe doping was investigated for their ability to enhance gas sensing activity. The X-ray diffraction results indicate that the Fe dopant was substitutionally incorporated by replacing Ti⁴⁺ cations. Fourier transform infrared spectral analysis confirmed the presence of brookite TiO₂. The UV–visible spectra showed the increase in absorption with Fe doping when compared with undoped TiO₂ film, and optical band gap decreased slightly with Fe doping. SEM images revealed the presence of one dimensional structure of straight nanowires for undoped TiO₂ and curved nanowires for Fe doped TiO₂ films. To understand the enhancement of sensing performance of TiO₂ film with Fe doping, the gas sensing mechanism of the film towards Ethanol at room temperature was studied and discussed.

Introduction

Ethanol vapor sensing finds large application in the field of food processing, biomedical, chemical industry, and breath analysis. For these applications, it is essential to provide high sensitivity, high selectivity, high stability, low working temperature, and short response and recovery times. Therefore, a great deal of research has been focused on the development of functional materials for high-performance of ethanol vapor sensing. In the past decades, semiconductors were widely used for gas sensing application, in that metal oxide semiconductors were extensively used because of their significant change in resistance upon exposure of gases to trace concentration of particular gas. Metal oxide semiconductors like ZnO, WO₃, SnO₂, TiO₂ and V₂O₅ were used as gas sensors. Among them, TiO₂ has been investigated extensively for gas sensing due to its higher surface reactivity to gases [1]. In order to enhance the gas sensitivity of TiO₂, nanostructures such as nanoparticles (0D) [2], nanowires (1D) [3], nanotubes (1D) [4], nanosheets (2D) [5] and hierarchical nanostructures (3D) [6], with high surface area were synthesized [7]. TiO₂ nanowires were fabricated to improve gas sensing characteristics on a large scale, as nanowires being one dimensional, nanostructure with uniform morphology and a large surface area with controllable less agglomeration have potential applications. However, there are many disadvantages of TiO₂ being used as gas sensors, due to high working temperatures, longer response, recovery time, and lower sensitivity. Recently, many methods were investigated with the focus of improving the gas sensing performance of TiO₂ nanowires. Doping with components such as Au, Pt, Pb, and Ag is known to be effective, because active sites can be produced for particular gas species by doping. However, these sensitive materials can be poisoned easily in some gas atmospheres, which can lead to reduction in sensitivity and stability. Iron has been considered an appropriate candidate for doping TiO₂, as the radius of Fe³⁺ being similar to that of Ti⁴⁺. Therefore, Fe³⁺ ions might easily be incorporated into TiO₂ lattice [8]. As the band gap of iron is 2.6 eV, it will reduce the band gap of TiO₂, thereby increasing the performance of the sensor at lower temperature with large response and lower recovery time.

TiO₂ and Fe doped TiO₂ thin films were deposited by different methods, such as sol–gel process [9], chemical spray pyrolysis [10], [11], sputtering [12], hydrothermal technique [13], reactive pulsed laser deposition [14] and electron beam physical vapour deposition [15]. Among which spray pyrolysis offers a number of advantages over other deposition processes, such as scalability of the process, cost-effectiveness, easiness of doping, operation at moderate temperatures and large uniform surface area. Fe-functionalized Brookite TiO₂ nanowires were assessed to detect a range of gases, but their sensing properties toward ethanol gas were not reported as far as we know. Hence, in the present work a novel ethanol sensor based on TiO₂ films were fabricated by spray pyrolysis technique. The effect of Fe doping on the structural, optical, and morphological properties of TiO₂ were investigated for enhancing sensing performance of TiO₂ towards ethanol at room temperature.