Sensors and Actuators B 221 (2015) 760–768 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Highly flexible room temperature NO 2 sensor based on MWCNTs-WO 3 nanoparticles hybrid on a PET substrate Usman Yaqoob, Duy-Thach Phan, A.S.M. Iftekhar Uddin, Gwiy-Sang Chung ∗ School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 680-749, South Korea a r t i c l e i n f o Article history: Received 3 January 2015 Received in revised form 16 June 2015 Accepted 26 June 2015 Available online 3 July 2015 Keywords: Flexible NO2 sensor MWCNTs WO3 NPs Hybrid PET substrate a b s t r a c t Fabrication and evolution of a flexible nitrogen dioxide (NO 2 ) sensor made of a multi-walled carbon nanotubes tungsten trioxide nanoparticles (MWCNTs-WO 3 NPs) hybrid on a polyethylene terephthalate (PET) substrate has been studied. Hydrothermally synthesized WO 3 NPs and commercial MWCNTs were mixed with the assistance of -terpineol to obtain the viscous gel of MWCNTs-WO 3 NPs hybrid. The as- synthesized hybrid was gel casted on a PET substrate to fabricate the flexible sensor device. The flexible sensor showed enhanced NO 2 sensing performance at room temperature with excellent mechanical flexibility, even at a curvature angle of 90 ◦ and after bending/relaxing several times (10 3 , 10 4 , 10 6 , and 10 8 ). A maximum response of 14% (5 ppm) to NO 2 with a limit of detection (LOD) of 0.1 ppm and relatively better recovery time (27 min) were obtained. Along with the enhanced sensing properties, lightweight and mechanical robustness of the as-prepared MWCNTs-WO 3 NPs hybrid might makes it a promising building block for a flexible NO 2 sensor. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Nitrogen dioxide (NO 2 ) is a reddish-brown, nonflammable, poisonous, oxidizing gas with an irritating odor. This typical air pol- lutant belongs to the family of highly reactive gases called nitrogen oxides (NO x ). NO 2 is usually produced by the combustion of fos- sil fuels, which is very harmful to human beings and threatens the environment. It is responsible for acid rain, global warming and the production of ozone (O 3 ) [1]. As a consequence, to ensure human health and environmental safety, it is essential to develop a highly efficient NO 2 gas sensor that can reliably detect and monitor this pollutant, even at very low concentrations. With a view to fulfilling the increased demand for a highly efficient and practical NO 2 sen- sor, numerous research results have been reported in the literature [2,3]. In recent years, flexible gas sensors have attracted the attention of many researchers because of their low cost, mechanical stabil- ity, and biocompatibility, making them promising candidates for various applications, such as aeronautic transportation, portable electronics and small textiles, or radio frequency identification (RFID) [4]. The main concern is not only to develop a completely flexible device, but to also maintain the stability of the sensor in ∗ Corresponding author. E-mail address: gschung@ulsan.ac.kr (G.-S. Chung). URL: http://home2.ulsan.ac.kr/user/gschung (G.-S. Chung). harsh environments (such as high temperatures or mechanical and electrical fields). Specifically, demand for a highly flexible NO 2 sen- sor with a low limit of detection (LOD) and excellent stability has become increasingly important for the detection of NO 2 from auto- mobiles or other combustion processes, and for gas monitoring in environments and in human breath. A number of flexible NO 2 sensor-based research results have been reported in the literature. However, full recovery, stability in a highly flexible environment, lower limits of detection, selectivity, and synthesis-process com- plexity are still great challenges. Recently, Cho et al. [5] reported on an enhanced flexible NO 2 sensor made with chemical vapor deposition (CVD)-grown multi- layer graphene film decorated with aluminum (Al) nanoparticles at 150 ◦ C. Huang et al. [6] reported on sulfur-doped reduced graphene oxide (RGO)-decorated silver (Ag) nanoparticles (NPs) on a polyimide (PI) substrate with a very fast response/recovery time (0.16/0.33 min). Choi et al. [7] reported on CVD-grown mul- tilayer graphene, and Lee et al. [8] on 3D graphene foam via CVD at a very slow recovery. Su and Shieh [9] used in situ reduction of graphene oxide (GO) on a polyethylene terephthalate (PET) sub- strate. In addition, graphene [10], carbon nanotubes (CNTs)/RGO [11], and self-assembly of multiwalled carbon nanotubes (MWC- NTs) [12], etc., have been studied for NO 2 sensors on different flexible substrates. Among the various usable flexible substrates, PET is considered as an interesting material due to its excellent adhesive functionality with different materials, and good thermal and chemical stability. http://dx.doi.org/10.1016/j.snb.2015.06.137 0925-4005/© 2015 Elsevier B.V. All rights reserved.