Flexible humidity sensor based on TiO₂ nanoparticles-polypyrrole-poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride composite materials

Abstract

A novel flexible resistive-type humidity sensor was fabricated by the in situ photopolymerization of TiO₂ nanoparticles/polypyrrole (TiO₂ NPs/PPy) and TiO₂ nanoparticles/polypyrrole/poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride (TiO₂ NPs/PPy/PMAPTAC) composite thin films on a polyester (PET) substrate. The effect of the TiO₂ NPs concentration on the electrical and humidity sensing properties of the TiO₂ NPs/PPy composite thin films on a PET substrate was investigated. PMAPTAC was incorporated into the TiO₂ NPs/PPy composite thin films to increase the flexibility and sensitivity of the composite films for practical use. Characterizations of the thin films were performed by Fourier transform infrared spectroscopy (FTIR). The flexible humidity sensors based on the TiO₂ NPs/PPy/PMAPTAC composite thin films had the best flexibility, highest sensitivity, least hysteresis and greatest linearity. Other sensing properties such as the effects of applied frequency and ambient temperature, response and recovery times, and long-term stability were also studied.

Introduction

Humidity sensors are extensively adopted in many fields, for example, to improve quality of life and enhance industrial processes. Many materials have been studied for their use in fabricating humidity sensors, including polymers [1], [2], [3], [4], [5], [6], [7], [8], ceramics [9], [10], [11], [12] and composites [13], [14], [15], [16]. Most of these sensor systems are based on the use of rigid substrates such as ceramics or SiO₂/Si. The ceramic substrates have good thermal and chemical stabilities and are suitable as substrate materials for humidity sensors. However, integrating many sensors and control systems on a ceramic substrate is very difficult. Silicon substrates have favorable electrical and mechanical properties and are good materials for integrated circuits. However, their fabrication is complicated and high-cost.

Today, organic electronic transistors, such as light emitting diodes, photo-diodes, solar cells and lasers, can be realized entirely from organic materials. They can be produced on a low-cost flexible carrier, using a high-volume production process, such as reel-to-reel manufacturing. Another feature of organic materials is the application of flexible chemical sensors based on ions or electrons as charge carriers [17], [18]. Nilsson et al. [18] reported a humidity sensor made of an organic electrochemical transistor (OECT) combined with Nafion as the sensing material. Manohar and co-workers [19] reported a flexible vapor sensor based on films of single-walled carbon nanotube bundles deposited on a poly(ethyleneterephthalate) substrate. In our previous study [20], a flexible humidity sensor made of a methyl methacrylate/[3-(methacrylamino)propyl] trimethyl ammonium chloride (MMA/MAPTAC) copolymer material was successfully prepared by in situ co-polymerization on a polyester (PET) substrate. Importantly, flexible sensor devices must be manufactured at low temperature.

Conducting polymers such as polythiophene, polypyrrole (PPy) and polyaniline have been intensively studied because of their remarkable mechanical and electrical properties, which can be exploited in actuators, sensors and electrochromic devices [21], [22], [23]. Among conducting polymers, PPy has attracted much interest because it is easily synthesized; it has relatively good environmental stability, and its surface charge characteristics can easily be modified by changing the dopant species in the material during synthesis.

An ultraviolet (UV)-irradiation technique uses radiation as the driving force to induce electron transfer from the monomer species in a cast solution film to the electron acceptor. The main advantage of the photopolymerization process over electrochemical and/or chemical polymerization is that it allows conducting polymer films to be easily designed and optimized by incorporating molecular species into the polymer structure on a non-conducting substrate surface [24]. Additionally, a simple single-pot room temperature reaction that can be scaled up to meet industrial requirements has been used to synthesize PPy/manganese zinc ferrite nanocomposites [25]. In our earlier study [26], a humidity sensor was fabricated by the in situ photopolymerization of TiO₂ nanoparticles/PPy (TiO₂ NPs/PPy) composite thin films on an alumina substrate. However, no attempts have been made to construct flexible resistive-type humidity sensors based on TiO₂ NPs/PPy composite thin films by photopolymerization. In this work, in the first step, TiO₂ NPs/PPy composite thin films were prepared on a PET substrate as flexible resistive-type humidity sensors using in situ UV-irradiation technique. The humidity sensing and electrical properties of the TiO₂ NPs/PPy composite thin films were studied. In the second step, PMAPTAC, a polymer electrolyte, was incorporated into the TiO₂ NPs/PPy composite thin films to improve their flexibility. The films were characterized by Fourier transform infrared spectroscopy (FTIR). The humidity sensing properties of the TiO₂ NPs/PPy/PMAPTAC composite thin films, including sensitivity, hysteresis, influence of ambient temperature, response and recovery times and stability were also studied.