Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Humidity impedimetric sensor based on vanadium pentoxide xerogel modified screen−printed graphite electrochemical cell Maria G. Trachioti, Mamas I. Prodromidis ∗ Department of Chemistry, University of Ioannina, 45 110, Ioannina, Greece ARTICLEINFO Keywords: Relative humidity sensor Impedimetric sensor screen−printed electrodes Vanadium pentoxide xerogel ABSTRACT The development of a humidity sensor utilizing vanadium pentoxide xerogel (V 2 O 5 ·nH 2 O, VPX) is described. Thin films of VPX were drop−cast onto a low−cost, screen−printed graphite three−electrode electrochemical cell (SPC) and the resulting transducing surface was assessed as a relative humidity (RH%) sensor. The mor- phology of VPX, its interaction with water vapors as well as the electrochemical properties of VPX/SPC were characterized by scanning electron microscopy, ATR-infrared spectroscopy and electrochemical impedance spectroscopy (EIS), respectively. The sensor possesses high sensitivity (190–500 Ohm/RH%) over a wide range of RH (10–93%), sensor response of 93%, low hysteresis, sufficient storage stability, and a fast response and recovery time, of 52 and 21 s, respectively. EIS data obtained at different RH% values were sufficiently modeled to a single equivalent electric circuit, which describes the conduction mechanism within the VPX film and the electrochemical properties at the electrode/film interfaces. Results demonstrate that the designed sensor is suitable for on−site and real−time monitoring of relative humidity at ambient conditions. 1. Introduction Humidity plays an important role in environmetal control, in- dustrial processing as well as in many aspects of human life and, therefore, there is increased interest for sensor development in order to monitor humidity changes in real world conditions. Low cost, simple fabrication, wide detection range, high sensitivity and fast response are the major features of an ideal humidity sensor [1]. Regarding the transduction technique, humidity sensors can be sorted as resistive, capacitive, quartz crystal microbalance- (QCM) [2], surface acoustic wave- (SAW) [3,4], and fiber optic−based [5], while the plethora of the materials that have been utilized as sensing surfaces, including ceramics [1,6], conducting polymers [7,8], graphene and its derivatives [8–10], low dimensional transition metal dichalcogenides [11,12], organic semiconductors [13] etc. have been comprehensively reviewed [6,9,14]. Among them, metal oxide semiconductors (SnO 2 , TiO 2 , CuO, MoO 3 , WO 3 , MnO 2 , etc.) have received increasing attention, because they are non−toxic and they can be easily synthesized [1,6]. The majority of metal oxide−based humidity sensors employ the resistive type humi- dity−sensing mechanism. Water is adsorbed at the oxide−air interface creating firstly a monolayer of chemically bonded water molecules and subsequently multilayers of physically bonded water molecules. Sensor electrical properties differ by altering the humidity and they can be restored due to the reversibility of water adsorption [2,11,15,16]. Orthorombic V 2 O 5 is a layered material exhibiting a high surface-to- volume ratio [17], which due to its interaction with water vapors has been widely used for the development of humidity sensors in the last few years. Pawar et al. [18] developed a humidity sensor based on ultra−thin V 2 O 5 nanosheets synthesized by a hydrothermal method, Yin et al. [19] proposed a humidity sensor by utilizing a single V 2 O 5 micro/nano−tube synthesized via a modified chemical vapor deposi- tion process, while a humidity sensor based on ultrathin epitaxial films of V 2 O 5 formed by plasma−enhanced atomic layer deposition has also been proposed [20]. Orthorombic V 2 O 5 has also been widely used as a dopant of various single [21,22] or mixed [15,23] semiconducting metal oxides in order to improve the response characteristics of the resulting humidity sen- sors. Remarkably, orthorombic V 2 O 5 doping was found to regulate the p− to n−type conduction transition and thus to prevent the emergence of non−monotonous signal variations (with respect to the water vapor pressure) which are known to hamper the use of the resulting sensors for quantitative purposes [15,23]. V 2 O 5 layered structure acts as an excellent host lattice for the in- tercalation of ionic species [17,24,25]. When water is the only inter- calant, the material is referred to as vanadium pentoxide hydrate https://doi.org/10.1016/j.talanta.2020.121003 Received 13 February 2020; Received in revised form 30 March 2020; Accepted 3 April 2020 ∗ Corresponding author. E-mail address: mprodrom@uoi.gr (M.I. Prodromidis). Talanta 216 (2020) 121003 Available online 08 April 2020 0039-9140/ © 2020 Elsevier B.V. All rights reserved. T