Abstract
In this paper, we present the relative humidity (RH) sensing response of a chemiresistive sensor, employing sensing layers based on a ternary nanohybrids comprised of holey carbon nanohorns (CNHox), titanium (IV) oxide, and polyvinylpyrrolidone (PVP) at 1/1/1/(T1), 2/1/1/(T2), and with 3/1/1 (T3) mass ratios. The sensing device is comprised of a silicon-based substrate, a SiO2 layer, and interdigitated transducer (IDT) electrodes. The sensitive layer was deposited via the drop-casting method on the sensing structure, followed by a two-step annealing process. The structure and composition of the sensing films were investigated through scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD). The resistance of the ternary nanohybrid-based sensing layer increases when H increases between 0% and 80%. A different behavior of the sensitive layers is registered when the humidity increases from 80% to 100%. Thus, the resistance of the T1 sensor slightly decreases with increasing humidity, while the resistance of sensors T2 and T3 register an increase in resistance with increasing humidity. The T2 and T3 sensors demonstrate a good linearity for the entire (0–100%) RH range, while for T1, the linear behavior is limited to the 0–80% range. Their overall room temperature response is comparable to a commercial humidity sensor, characterized by a good sensitivity, a rapid response, and fast recovery times. The functional role for each of the components of the ternary CNHox/TiO2/PVP nanohybrid is explained by considering issues such as their electronic properties, affinity for water molecules, and internal pore accessibility. The decreasing number of holes in the carbonaceous component at the interaction with water molecules, with the protonic conduction (Grotthus mechanism), and with swelling were analyzed to evaluate the sensing mechanism. The hard–soft acid-base (HSAB) theory also has proven to be a valuable tool for understanding the complex interaction of the ternary nanohybrid with moisture.
Highlights
Humidity is a critical parameter for human life and other species, as its variation influences their vital activity
The surface topography of the sensing films based on TiO2 /CNHox/PVP ternary nanohybrid were investigated by scanning electron microscopy (SEM, Thermo Scientific, Waltham, MA, USA)
This paper reports the relative humidity (RH) sensing response of a chemiresistive sensor employing sensitive films based on a ternary nanohybrid comprised of CNHox, titanium (IV) oxide, and PVP at 1/1/1/(T1), 2/1/1/(T2), and 3/1/1 (T3) mass ratios
Summary
Humidity is a critical parameter for human life and other species, as its variation influences their vital activity. The interest for sustainable, reliable, low-cost, and highly sensitive humidity sensors increased in the last years due to their usefulness in a wide variety of industrial, commercial, and residential applications such as in HVAC (heating, ventilation, and air conditioning), food/beverage processing, the medical field (incubators, respirators, and sterilizers), meteorology, structural health monitoring, agriculture, robotics, and so forth [1,2]. Devices used for humidity control and monitoring may include sensors with either a capacitive response [3], a gravimetric [4] or thermal conductivity change [5], and an optical [6] or a resistive signal [7]. Performances of the resistive humidity sensors, such as related to sensitivity, hysteresis, response time and recovery time, and reproducibility, are closely related to the properties of sensing materials [8]. Various sensing layers and their physical–chemical response to humidity change have been widely explored in the past
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