Composite of TiO₂ nanowires and Nafion as humidity sensor material

Abstract

Homogeneous TiO₂ nanowires were fabricated by hydrothermal method. SEM pictures proved the yield of nanowires to be more than 90%. Composite humidity sensing films were made by using TiO₂ nanowires, TEOS and Nafion. FTIR absorption spectroscopy was used as a semi-quantitative method to get information about the protonation. The sensing films were prepared by a dip-coating method. The composite films coated on a pair of gold electrodes were tested for humidity sensors of resistance type. The measurement was carried out at five fixed humidity points in the range of 12–97% relative humidity, which were controlled by employing five different salt solutions. Resistance changes were about three orders of magnitude. The nanowires-based humidity sensors showed moderate sensitivity, short response and recovery time (<2 min) at relative humidity less than 76%, and good long-term stability.

Introduction

Needs for humidity sensors are growing in industrial and agricultural applications for monitoring and controlling the surroundings is growing. Different measuring techniques, like impedance [1], [2], capacity [3], [4], [5], field effect transistors (FET) [6], surface-acoustic wave (SAW) [7], quartz crystal microbalance (QCM) [8], [9], [10], [11], fiber optic [12], [13], [14], [15] and microwave sensors [16], have been explored for humidity detection. In recent years, nanorod and nanowire films were fabricated and their humidity sensitive characteristics have been investigated [17], [18], [19], and these nanomaterial films were found to be efficient humidity sensors. In consideration of quality and cost, impedance type humidity sensors are becoming more prevalent. Humidity-sensitive materials used in various fields are classified into three groups: electrolytes, organic polymers and porous ceramics [20].

Ceramic humidity sensors usually show better chemical resistance and mechanical strength than polymer sensors. TiO₂ sensing materials are commonly used in research for the reason of easy fabrication. Gusmano and co-workers [21], [22] used the TiO₂ modified with 1–10% K⁺ and Li⁺ through a sol–gel method as a sensing material. The electrical resistance of the material showed a variation of seven orders of magnitude with the change in relative humidity (RH) from 4 to 90%. A humidity sensing material ZrO₂–TiO₂ increased the sensitivity by doping with Li⁺ in the research of Jain et al. [23]. Nitsch et al. [24] used an active thick film layer based on ZnO–TiO₂–Cr₂O₃ as a sensing material. Traversa and co-workers [25] used the technology of electrochemical impedance spectroscopy to investigate the humidity-sensing electrical conduction mechanism of the films of TiO₂ doped with 1–10% K⁺ and Li⁺ in the RH range of 4–87% RH.

TiO₂ nanowires are a kind of nano-scale material and have been successfully synthesized by some research groups through hydrothermal treatment, chemical vapor deposition or other methods [26], [27], [28]. The TiO₂ nanowires are very intriguing as a humidity-sensing material. In the present study, therefore, composite films of TiO₂ nanowires and Nafion were made, since such composites of fine ceramic particles and polymers are often used as humidity sensors [1], [2], [31].