Acetone sensing properties of CuO nanowalls synthesized via oxidation of Cu foil in aqueous NH 4 OH Vu Xuan Hien a, * , Nguyen Hoang Minh a , Dang Tuan Son a , Nguyen Thanh Nghi a , Luong Huu Phuoc a , Cao Tien Khoa b , Dang Duc Vuong a , Nguyen Duc Chien a , Young-Woo Heo c a School of Engineering Physics, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hanoi, Vietnam b Department of Physics, Thai Nguyen University of Education, 20 Luong Ngoc Quyen Street, Thainguyen, Vietnam c School of Materials Science and Engineering, Kyungpook National University (KNU), Daegu, 702-701, Republic of Korea article info Article history: Received 4 January 2018 Received in revised form 12 January 2018 Accepted 15 January 2018 Available online 31 January 2018 Keywords: Nanowalls CuO Oxidation Solution Gas sensor abstract High-density and uniform CuO nanostructures are synthesized via the facile oxidation of Cu foil in aqueous NH 4 OH below 80  C. The CH 3 COCH 3 sensing properties of the synthesized materials are investigated at the operating temperature of 200e360  C. A good sample is further tested with C 2 H 5 OH and NH 3 for demonstrating the gas selectivity at the optimal operating temperature. The selectivity between acetone and ethanol of the CuO nanowalls synthesized using this method is considered and compared with the results obtained by other studies. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Copper is one of the most abundant metals on earth. Its oxide (CuO) is an intrinsically p-type metal-oxide semiconductor with a bandgap (E g ) of 1.2e2.0 eV [1]. With the development of nano- science and nanotechnology, copper oxide nanomaterials are considered as promising materials for a wide range of applications e.g., lithium ion batteries, dye-sensitized solar cells, photocatalytic hydrogen production, photodetectors, and biogas sensors [2e7]. Various micro- and nano-structures of CuO such as microflowers, nanoparticles, nanorods, nanowires, and nanoplates/nanowalls/ nanosheets have been synthesized using various physical and chemical approaches [8e14]. The first such approach was proposed by D. W. Shoesmith et al. [15] wherein Cu metal was oxidized, forming CuO nanorods in an alkaline solution of Li(OH) 2 . Recently, high-density and uniform CuO nanostructures have been grown on Cu foils in alkaline solutions [3,16e25]. In 2011, T. Soejima et al. proposed a facile process for the oxidation synthesis of CuO nanobelt arrays using NH 3 eH 2 O 2 aqueous solution [26]. In this method, humid NH 3 vapor was believed to be the main reactant toward Cu foil, thus forming Cu(OH) 2 and subsequently CuO nanostructures. Nevertheless, this synthesis process and a later study by T. Soejima et al. ignored the sinking of the Cu foil area in the aqueous NH 3 solution [27]. In 2017, G. Kaur et al. synthesized CuO nanostructures by treating Cu foils in NH 4 OH at room tem- perature for different treatment times [28]. The surface treatment of Cu in alkaline aqueous solutions is a potential method for the mass fabrication of CuO nanostructures with high uniformity and density. It is interesting to compare the gas sensing properties among CuO nanomaterials synthesized by this approach and by others. Nevertheless, none of above studies investigated the gas sensing properties of as-synthesized CuO nanomaterials. In this study, CuO nanowalls versus nanoparticles were syn- thesized via the oxidation process of Cu foil in NH 4 OH solution at 50e70  C. The gas sensing properties of the as-prepared CuO nanoplates were examined with C 2 H 5 OH, CH 3 COCH 3 , and NH 3 at 200e360  C. The sensing data were compared with the gas sensing properties of CuO nanostructures synthesized using other methods. * Corresponding author. E-mail address: hien.vuxuan@hust.edu.vn (V.X. Hien). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum https://doi.org/10.1016/j.vacuum.2018.01.030 0042-207X/© 2018 Elsevier Ltd. All rights reserved. Vacuum 150 (2018) 129e135