Sensors and Actuators B 231 (2016) 166–174 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Low level NO 2 detection under humid background and associated sensing mechanism for mesoporous SnO 2 Adelina Stanoiu a , Simona Somacescu b , Jose Maria Calderon-Moreno b , Valentin Serban Teodorescu a , Ovidiu Gabriel Florea a , André Sackmann c , Cristian Eugen Simion a,∗ a National Institute of Materials Physics, Atomistilor 105bis, P.O. Box MG-7, 077125 Bucharest-M ˘ agurele, Romania b “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania c AG Weimar, Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 15, D-72076 Tübingen, Germany a r t i c l e i n f o Article history: Received 13 October 2015 Received in revised form 18 February 2016 Accepted 27 February 2016 Available online 3 March 2016 Keywords: Mesoporous SnO2 Surface hydroxylation Electrical resistance and work function changes Sensing mechanism a b s t r a c t Mesoporous SnO 2 prepared by a hydrothermal synthesis route assisted by the ionic surfactant Cetyltrimethylammonium bromide, has rutile-type tetragonal symmetry, small homogeneous nanocrys- tallite size of ∼4 nm and good thermal stability. Porosity analysis revealed high surface area ∼127 m 2 /g and a narrow pore size distribution, with an average pore diameter ∼4 nm. The mesoporous structure is likewise advantageous towards enhancing the surface reactivity and subsequent gas sensing perfor- mances. The role played by the surface hydroxylation on the NO 2 sensing mechanism was discussed with respect to the associated photoelectron spectral components. Under humid air, associated with the in-field conditions, the highest sensitivity was attained at 150 ◦ C, were the sensor signal towards NO 2 is 4 times higher than the one recorded in dry air. This feature has been experimentally demonstrated by simultaneous electrical resistance and work function changes measurements conducted in the range of 400–5000 ppb NO 2 . © 2016 Elsevier B.V. All rights reserved. 1. Introduction Diesel powered cars are the main NO 2 pollutants in crowded urban areas as a result of the combustion process. Due to its high oxidizing ability, NO 2 induces harmful effects on the respiratory system. The Scientific Expert Group on Occupational Exposure Lim- its (SCOEL) has issued very restrictive limits towards NO 2 exposure in order to ensure safety ambient working conditions [1]. Thus, the recommended values are: 0.5 ppm for 8-h TWA (time weighed average) and 1 ppm for STEL −15 min (short term exposure limit). Gas sensing devices based on semiconducting metal oxide (SMOX) might be a suitable alternative (low cost and low power consumption) to the optical analyzers used for traffic pollution monitoring. Since SnO 2 based (SMOX) sensors are already on the market for detecting reducing agents, research efforts have been undertaken to spread their sensing ability towards oxidizing gasses. For instance, Jaswinder et al. have reported the NO 2 sensitivity and selectivity of undoped and WO 3 -doped SnO 2 . Temperature of ∗ Corresponding author. Fax: +40 213690177. E-mail address: simion@infim.ro (C.E. Simion). 150 ◦ C was optimum for 500 ppm NO 2 detection [2]. Zhang et al. investigated the potential of SnO 2 hollow spheres mediated by car- bon microspheres to sense NO 2 , exhibiting a signal of 2471 for 50 ppm of NO 2 and no response to the same concentrations of C 2 H 5 OH, gasoline, methanol, CCl 4 , acetone and NH 3 when oper- ated at 160 ◦ C [3]. Sharma et al. have studied SnO 2 thin film with surface integrated nanoclusters of WO 3 [4,5] and also with differ- ent metal oxide catalysts (WO 3 , TeO 2 , Al 2 O 3 , NiO, CuO, In 2 O 3 , ZnO, TiO 2 , Ag 2 O and PdO) for NO 2 detection. The maximum sensor signal towards 10 ppm of NO 2 was obtained at low operating tempera- ture (100 ◦ C). In the report of Choi et al. Pd/Pt functionalized SnO 2 nanowires have been studied for their high selective response of around 880 when only 100 ppb of NO 2 was dynamically dosed over the sensors operated at 300 ◦ C [6]. Zhang et al. highlighted that SnO 2 -reduced graphene oxide (SnO 2 -rGO) nanocomposites syn- thesized via hydrothermal route shows a response of 3.5 when exposed to 5 ppm NO 2 in comparison with less than 1.5 for the same concentrations of Cl 2 , NO, CO and 25% RH at 50 ◦ C [7]. Maeng et al. have identified the NO 2 sensing mechanism with SnO 2 nanoslab showing a response of 75 for 5 ppm NO 2 at 300 ◦ C using N 2 as carrier gas [8]. SnO 2 hollow nanofiber based sensors made via elec- trospinning showed a sensor signal of 81.4 at 2 ppm NO 2 , when http://dx.doi.org/10.1016/j.snb.2016.02.137 0925-4005/© 2016 Elsevier B.V. All rights reserved.