Materials Science in Semiconductor Processing 117 (2020) 105157 Available online 16 May 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved. The hierarchical nanostructured Co-doped WO 3 /carbon and their improved acetone sensing perfomance Valentine Saasa a, b , Thomas Malwela a , Yolandy Lemmer c , Mervyn Beukes d , Bonex Mwakikunga a, e, * a DSI/CSIR-Centre for Nanostructured and Advanced Materials, PO Box 3951, South Africa b Department of Biochemistry, Genetics and Microbiology, University of Pretoria, Pretoria, 0001, South Africa c CSIR-Next Generation Health, Pretoria, 0001, South Africa d Department of Biochemistry, Stellenbosch University, Western Cape, South Africa e Department of Physics, Tshwane University of Technology, PO Box 680, Pretoria 0001, South Africa A R T I C L E INFO Keywords: Acetone Cobalt doped tungsten oxide Sensor Metal oxides Semiconductor Sensitivity ABSTRACT Hierarchical nanostructured Co-doped WO 3 with carbon as template has been successfully synthesised through facile sol-gel method. The synthesised Co-doped WO 3 was characterized by X-ray diffraction, Scanning electron microscopy, Transmission electron microscopy, Energy dispersive X-ray spectrometry, and Brunauer-Emmett- Teller and X-ray photoelectron spectroscopy. The gas sensing properties of WO 3 doped with Co from 0 to 0.8 wt % were also investigated on various VOCs. The fabricated sensor based on 0.6 wt% Co-doped WO 3 with carbon as a template showed good sensitivity, selectivity, fast response and recovery time towards 1.5 ppm of acetone at 50 � C under 90% relative humidity. The excellent gas sensing properties could be attributed to high surface area, small crystallite size, defect of WO 3 and Co catalysis effect which promotes gas adsorption and most importantly the stabilized monoclinic phase of WO 3 , which accounts for the good selectivity. 1. Introduction Semiconducting metal-oxides gas sensors have attracted consider- able scientifc interest in the feld of gas sensing due to their fascinating properties such as low-cost, ease of use, and portability [1,2]. To name the few, gas sensors such as ZnO, SnO 2 , In 2 O 3 , and WO 3 show a signif- icant resistance change upon exposure to a trace concentration of reducing or oxidizing gases [3,4]. Furthermore, they are best suited for detecting volatile organic gases at low concentration levels in view of sensitivity, stability, robustness and so on [5]. Thus makes them an ideal candidate for monitoring trace amount of gases such as acetone in human breath, toxic gases in the environment and indoors [6]. More importantly, nanoporous semiconducting metal oxides with large specifc surface area are ideal materials for improving gas sensing performances by enhancing sensing sites and total exposures to target gases [7,8]. Although many oxides have been investigated for acetone sensor development, there are still a few problems regarding selectivity towards a single gas. This is followed by a selective sensor that could operate at low or room temperature condition. The addition of noble metals onto the metal oxides surface has been mostly applied as a way to solve selectivity problems of the gas sensors [9]. This approach has shown improvement towards the selectivity of the sensor, particularly for acetone detection. Qi et al., 2008, reported a good selectivity of acetone over other gases, although the acetone concentration was very high (500 ppm) including very high operating temperature (350 C). In a study conducted by Koo et al., 2017 [10], the authors reported 1 ppm acetone selectivity using Pd functionalised CO 3 O 4 at 350 C. A rapid acetone selectivity of a WO 3 nanotube sensor to 100 ppm was reported by Chi et al., 2015 [11], however the operating temperature was very high for gas sensor devices. Many other reported acetone selectivity includes [12–15], 100 ppm at 360 C, 50 ppm at 350, 50 ppm at 200 C, 2 ppm at 300 C and 0.9 ppm at 450 C, respectively. Based on the reported literature, there is a great need to develop a gas sensor material which can detect low ppm volatile organic com- pounds, more specifcally acetone gas [16]. There is a high demand of breath acetone sensor application in the market. Hence the current study seeks to develop a gas sensor based on Co-doped WO 3 with carbon nanospheres as a template to selectively detect low ppm of acetone at lower operating temperatures. As we might know, some diseases are associated with VOCs such as an increase in breath acetone (0.8 ppm and * Corresponding author. DSI/CSIR-Centre for Nanostructured and Advanced Materials, PO Box 3951, South Africa. E-mail address: bmwakikunga@csir.co.za (B. Mwakikunga). Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp https://doi.org/10.1016/j.mssp.2020.105157 Received 29 November 2019; Received in revised form 18 April 2020; Accepted 20 April 2020