Sensors and Actuators B 216 (2015) 41–48 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo ur nal home page: www.elsevier.com/locate/snb Mesoporous ceramic oxides as humidity sensors: A case study for gadolinium-doped ceria L. Almar a , A. Tarancón a , T. Andreu a , M. Torrell a , Y. Hu b , G. Dezanneau b , A. Morata a,∗ a Advanced Materials for Energy, Catalonia, Institute for Energy Research (IREC), Jardí de les dones de negre, 1, Planta 2, 08930 Sant Adrià del Besòs (Barcelona), Spain b Laboratoire Structures, propriétés et Modélisation des Solides, CentraleSupelec, CNRS, Grande voie des vignes, 92295 Chatenay-Malabry Cedex, France a r t i c l e i n f o Article history: Received 11 February 2015 Received in revised form 1 April 2015 Accepted 8 April 2015 Available online 17 April 2015 Keywords: Mesoporous Humidity sensors Ceramic Ceria Thermal stability a b s t r a c t Mesoporous materials have been studied as high performance sensing materials due to their singular microstructure and extremely high surface-to-volume ratio. However, the lack of stability of these nano- structures is assumed as one of the major drawbacks toward their application in real devices. In this work, this limitation is overcome by the synthesis of thermally stable mesoporous gadolinium doped ceria. Humidity sensors were fabricated and tested under different (i.e. humidity and temperature) conditions. The mesoporous layers were attached to the substrate at 900 ◦ C preserving mesoporous structure intact. This process at high temperature provides the layer with a mechanical strength and allows self-cleaning cycles at high temperatures if required. The humidity sensing mechanism is presented and discussed in detail by means of impedance spectroscopy. An ionic type of conduction mechanism is corroborated. Fast response and recovery, as well as very low hysteresis and no drift are observed. It was also shown that the response of the devices can be straightforwardly tuned by changing layer thickness or pore size, allowing to fulfill sensing needs of different applications. All the mentioned prop- erties joined to the simplicity of the fabrication and the flexibility of the used fabrication route for synthesizing any other metal oxide make this kind of devices a potential group for developing high performance and fast gas sensors. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The humidity control is of utmost importance as it affects both living organisms and materials. Therefore, humidity sensors are currently used in many fields such as manufacturing processes, environmental and health monitoring or domestic applications [1]. Porous ceramics form a common group of materials to sense humidity [2,3]. The adsorption mechanism of the water molecules on the surface of the ceramic oxides is well known (Fig. 1). First, a chemisorbed layer is formed on the available sites of the oxide surface, causing dissociation of water molecules to surface hydro- xyls. After the first adsorbed layer is formed, subsequent layers are physisorbed. The molecules in the first physisorbed layer are bounded through double hydrogen bonds and cannot move. At higher humidity levels, water molecules are singly bounded and form a network with a liquid-like behavior. Thus, the water molecules from the second layer can move or rotate freely (like ∗ Corresponding author. Tel.: +34 606522639. E-mail address: amorata@irec.cat (A. Morata). in bulk liquid water) and the Grotthuss mechanism i.e. the proton transport becomes dominant [2]. From the surface adsorption mechanism, it is clear that large pore volumes are desirable in order to get highly sensitive humidity sensors. Moreover, the response and recovery times of the ceramic sensors can be controlled by the pore size. The detection limit can be set very low by decreasing the pore size [3]. In general homogeneity and controllability of porous structures are important properties in sensor applications. Mesoporous mate- rials provide high area structures with a very precise shape, which make them perfect candidates for sensing applications [4–7]. In this respect, many studies have reported high sensitivity in silica based mesoporous humidity sensors [8–11]. However, during the fabri- cation of the sensor, the mesoporous powder is usually attached to the electrodes at very low temperatures in order to preserve the nanostructure. This limitation makes these devices mechan- ically unstable and affects reproducibility, limiting their viability for commercial applications. Moreover a heat treatment at temperatures over 400 ◦ C is nor- mally used in commercial ceramic humidity sensors to recover the initial state [1,3]. However, if this heat treatment process is http://dx.doi.org/10.1016/j.snb.2015.04.018 0925-4005/© 2015 Elsevier B.V. All rights reserved.