LetterA selective room temperature formaldehyde gas sensor using TiO2 nanotube arrays
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], LaFe1−xZnxO3 [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.
Section snippets
Experimental
Titanium sheets (purity: 99.4%) were successively sonicated in acetone, ethanol and deionized water for 10 min at each step to remove any grease on the surface before the electrochemical anodization. The anodization was performed using a two-electrode cell. The as-treated Ti plates were used as the working electrodes and a stainless steel served as the counter electrode. The samples were anodized in a solution containing 0.27 M NH4F consisting of mixtures of DI water and glycerol
Results and discussion
Fig. 2 shows XRD patterns of the as-fabricated and calcined TiO2 nanotubes at 400 °C and 500 °C. The as-fabricated sample was amorphous. It has been reported that this amorphous structure would persist until it is calcined above 300 °C [30], [34]. To obtain the crystalline phase, thermal treatment was carried out above 300 °C. The clear sharp peaks shown in the XRD patterns of the two samples calcined at 400 °C and 500 °C indicate the presence of a crystalline phase. As calcination temperature
Conclusions
In summary, a new room-temperature formaldehyde gas sensor using the TiO2 nanotube array is reported in this work. The sensor using TiO2 nanotube array annealed at 400 °C shows the best response among the samples investigated to different concentrations of formaldehyde from 10 to 50 ppm and good selectivity toward formaldehyde over 50 ppm ethanol and 1000 ppm ammonia in humidified air at room temperature.
Acknowledgements
Dr. Shiwei Lin appreciates the financial support from the Program for New Century Excellent Talents in University (NCET-09-0110) and National International Cooperation Program (2009DFA92551). Dr. Xiaogan Li would like to thank the foundations from the Fundamental Research Funds for the Central Universities. The authors thank Prof. Jing Wang for providing the sensor testing facility. Help rendered by Mr. Pengjun Yao and Mr. Yangong Zheng on the experiments is also appreciated.
Dr. Shiwei Lin currently is a professor at Hainan University, PR China. He received his Ph.D. degree from the University of Manchester in Electrical and Electronic Engineering in the U.K. in 2006. His research interests include self-organized nanotubes, metal-oxide semiconductors and their photoelectric properties.
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Dr. Shiwei Lin currently is a professor at Hainan University, PR China. He received his Ph.D. degree from the University of Manchester in Electrical and Electronic Engineering in the U.K. in 2006. His research interests include self-organized nanotubes, metal-oxide semiconductors and their photoelectric properties.
Ms. Dongrong Li is a final-year undergraduate student in Hainan University, Haikou, PR China.
Mr. Jian Wu is a final-year undergraduate student in the School of Electronic Science and Technology in Dalian University of Technology, Dalian, Liaoning, PR China.
Dr. Xiaogan Li is an associate professor in the School of Electronic Science and Technology in Dalian University of Technology, Dalian, Liaoning, PR China. He received his Ph.D. in Materials Science and Engineering from University of Leeds, U.K. in 2007. Then he conducted a two-year postdoctoral research in chemical gas sensors at The Ohio State University in USA from 2007 to 2009. His current research interests are in chemical sensors, inorganic materials chemistry and physics, and biosensors.
Prof. Sheikh Akbar received his Ph.D. in materials and engineering from Purdue University. He is currently a professor of materials science and engineering at The Ohio State University. Prof. Akbar also founded the NSF Center for Industrial Sensors and Measurements (CISM) at Ohio State, with primary focus on the R&D of sensors for applications in hostile industrial environments.