A selective room temperature formaldehyde gas sensor using TiO₂ nanotube arrays

Abstract

A new gas sensor using TiO₂ nanotube arrays was fabricated and explored for formaldehyde detection at room temperature. Highly ordered vertically grown TiO₂ nanotube arrays were synthesized by using the conventional electrochemical anodization process. The sensor using the fabricated nanotube arrays as the sensing elements demonstrated a good response to different concentrations of formaldehyde from 10 to 50 ppm and a very good selectivity over other reducing gas species such as ethanol and ammonia at room temperature. While the exact sensing mechanism is unclear, some possibilities are briefly discussed.

Introduction

Formaldehyde (HCHO) is a colorless, pungent-smelling gas and can cause several symptoms such as watery eyes, burning sensation in the eyes and throat, nausea, and difficulty in breathing in some humans exposed at elevated levels (above 0.1 parts per million). High concentrations may trigger heart attack in people with asthma [1], [2], [3], [4], [5], [6]. HCHO is also a possible carcinogen and may cause central nervous system damage, immune system disorders, as well as blindness and respiratory diseases [1], [2], [3]. Such highly health-threatening gas species can exist in substantial concentrations both indoors and outdoors. Sources of formaldehyde in the home include building materials and household products [1], [2], [3], [4], [5], [6]. Therefore, there is a high demand on deployment of portable, cheaper gas detectors at home and work place for real-time monitoring the levels of formaldehyde so that precautions can be taken in the presence of this toxic gas.

Metal oxide semiconductor gas sensors to detect toxic gas species are attractive due to their simple working principle, high sensitivity, portability and low cost [7], [8], [9], [10], [11], [12], [13], [14], [15]. For formaldehyde detection, various metal oxides have been investigated including SnO₂ or doped SnO₂ [4], [5], [6], [8], [9], WO₃ [9], LaFe_1−xZn_xO₃ [10], NiO thin film [11], [12], CdO-mixed In₂O₃ [13] and doped ZnO [14], [15]. These sensors indicate good sensitivity to ppm or sub-ppm levels of formaldehyde and have potential to be employed in real situation. However, one of the obvious disadvantages is that they operate at elevated temperatures usually around 200–400 °C [7], [8], [9], [10], [11], [12], [13], [14], [15] and require a heater integrated with the sensor. This would raise the operating cost and also add requirements on packaging materials, further complicating the sensor design. Therefore, recent research has focused on modifying the conventional materials including utilization of nanosized particles or nanostructural architectures, sometimes aided with the UV–visible illumination during the sensor operation [16], [17], [18], [19], [20], [21], [22].

Since the highly ordered vertically grown TiO₂ nanotubes have been successfully synthesized by Grimes et al. [23], they have been explored for various applications such as in solar cells, photocatalysis and chemical sensing [24], [25], [26], [27], [28], [29], [30], [31], [32]. The flexibility in adjustment of the microstructure in terms of tube length, pore diameter and chemical composition has offered an excellent platform that can be tailored to meet different requirements [26]. Particularly, in gas sensing, it has been studied for the detection of ppm-level oxygen, H₂, acetone, and humidity at relatively low temperatures [24], [25], [26], [29], [30], [31], [32]. The basic working principle involves adsorption and desorption of the dissociated oxygen due to the large specific surface area of the nanotubes for oxygen, humidity and acetone detection. In the case for H₂ detection in pure nitrogen background, the splitting of H₂ and direct interaction between hydrogen and the nanotube surface is involved [26], [33].

In this work, TiO₂ nanotube arrays have been studied for the detection of formaldehyde at room temperature for the first time to the best of our knowledge.