Sensors and Actuators A 187 (2012) 37–42 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Morphology-dependent humidity adsorption kinetics of ZnO nanostructures N. Asar a , A. Erol a,∗ , S. Okur b , M.C. Arikan a a Physics Department, Istanbul University, Faculty of Science, Vezneciler, 34134 Istanbul, Turkey b Department of Metallurgy, Faculty of Engineering, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey article info Article history: Received 20 May 2012 Received in revised form 6 August 2012 Accepted 9 August 2012 Available online 19 August 2012 Keywords: ZnO Nanowire Nanoparticle Sensor Humidity sensor Langmuir adsorption model QCM abstract The humidity-sensing characteristics of ZnO nanostructures are investigated using a quartz crystal microbalance (QCM) measurement. ZnO nanostructures are synthesized via sol–gel route in nanoparti- cle (ZnO-NP) and nanowire (ZnO-NW) morphologies with diameter about 20–30 nm. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods are used to determine the morphology and crys- tal structure of ZnO nanostructures. Humidity sensing capabilities are discussed in terms of the different morphologies. The results show that ZnO-NP is more sensitive to humidity changes than ZnO-NW. QCM results are analyzed using Langmuir adsorption model to determine adsorption rates, Gibbs free energy of adsorption (G), and adsorbed mass amount by the synthesized ZnO nanostructures. Negative value of G for humidity adsorption on ZnO nanostructures indicates that the process is spontaneous and adsorption capacity increases with size reduction. Gibbs free energy of the ZnO-NP is found to be more negative, indicating that the ZnO-NP has more favorable adsorption sites compared to the ZnO-NW. Experimental and theoretical results exhibit that humidity-sensing properties of ZnO nanostructures are morphology-dependent. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The use of nanostructures as sensor materials has attracted sig- nificant attention due to their enhanced sensitivity and selectivity, miniaturized size, fast response and low cost compared to their bulk and thin film counterparts [1–3]. One of the reasons why nano- structures show superior chemical and physical properties is that they have high surface to volume ratio which is directly related to particle size and morphology. When the size of nanostructure is reduced below 5 nm, surface to volume ratio becomes larger dras- tically. For a 5 nm particle, 50% of atoms are on its surface and surface energy increases with the amount of low coordination sur- face atoms. Therefore, nanostructures are more chemically reactive than bulk materials [4]. Humidity monitoring is very important in wide range of fields such as meteorology, agriculture, automotive industry, food pro- cessing, textile, medicine and device manufacturing. It is possible to measure relative humidity based on changing resistance or capac- itance of sensitive material. On the other hand, using QCM for sensing analysis has advantages of high sensitivity, simple and room temperature operation compared to the conventional analy- sis methods. QCM is a mass sensitive tool that utilizes piezoelectric property of quartz crystal to measure frequency shift due to mass ∗ Corresponding author. Tel.: +90 2124555700; fax: +90 2124400069. E-mail addresses: ayseerol@gmail.com, ayseerol@istanbul.edu.tr (A. Erol). loading. Quartz resonators are capable of measuring mass changes as small as a fraction of a monolayer of atoms related to frequency change by Sauerbrey relation [5]: m =-A √  2f 2 0 f (1) where m and f represent mass and frequency changes, f 0 ,  and  denote fundamental frequency, shear modulus and density of quartz crystal, respectively. A net change of 1 Hz corresponds to 1.34 ng of materials adsorbed onto the crystal surface area (A) of 0.196 cm 2 . ZnO is one of the most promising metal oxide semiconductors for gas/vapor/humidity-sensing applications and has pronounced sensitivity to gases such as NH 3 , NO 2 , CO, H 2 , and ethanol [6–9]. It has been observed that ZnO nanostructures are more sensitive due to their high surface to volume ratio and have more chemi- cally active centers [10]. In the literature, there are several papers focused on QCM-based ZnO nanostructure humidity sensors. Zhang et al. [11] studied frequency responses of ZnO nanowire and nanorod coated QCM humidity sensors with diameters of 30–40 nm and 300 nm, respectively. They observed larger frequency response from ZnO nanowires at 97% RH, due to rough and larger surface area. Wang et al. [12] investigated humidity sensitivity of ZnO nanotetrapods depending on film thickness in RH changing from 30 to 80%. They found that sensitivity increased up to a certain thickness of 91 nm and then saturated. Zhou et al. [13] devel- oped a wireless humidity sensor prototype using combination of 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.08.019