Ammonia sensor based on polypyrrole–graphene nanocomposite decorated with titania nanoparticles

Abstract

A composite of polypyrrole (PPy) and graphene nanoplatelets (GNs) decorated with titanium dioxide (TiO₂) nanoparticles (TiO₂@PPy–GN) was synthesized by a sol–gel process combined with in situ chemical polymerization. The microstructure, morphology, and crystallinity of the TiO₂@PPy–GN nanocomposite were characterized by Fourier-transform infrared spectrometry, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results indicate that the TiO₂ nanoparticles with a diameter of 10–30 nm were well dispersed on the PPy–GN composite. The TiO₂@PPy–GN nanocomposite exhibited good electrical-resistance response to NH₃ at room temperature. Compared to the PPy–GN thin film, the TiO₂@PPy–GN thin film showed enhanced NH₃-sensing properties in the form of higher sensitivity and much faster response under the same conditions. The selectivity and stability of the NH₃ sensor based on theTiO₂@PPy–GN nanocomposite were also investigated.

Introduction

Nanostructured materials have been widely explored for the fabrication of gas sensors because of their high surface area, flexibility, and excellent electrical properties [1], [2], [3]. Among these materials, conducting polymers have attracted particular interest. Conducting polymers such as polypyrrole (PPy) and polyaniline can be simply prepared in solution and have unique electrical conductivity, redox property, and good stability at room temperature [4], [5], [6]. The morphology and surface area of these polymers can also be tuned by changing the dopant species during synthesis [7]. These features are expected to be utilized in gas sensors to lower the detection limit, decrease the response time, and improve the sensitivity [8], [9]. Doping conducting polymers with graphene has been considered an effective way to improve their sensing performance because of the unique properties of graphene and the combined effect of the two materials [10], [11], [12], [13].

Graphene, a two-dimensional carbon nanomateiral, has a large surface-to-volume ratio, high specific area, superior electronic conductivity, and excellent chemical stability, which make it desirable for use as a supporting framework for other nanoparticles to carry out sensing processes [14]. Furthermore, its two-dimensional honeycomb structure allows its carbon atoms to be directly exposed to the environment, thus maximizing the sensor׳s surface area per unit volume and increasing its sensitivity to gas molecules. Moreover, it has intrinsically low electrical noise owing to its crystal lattice, which tends to screen charge fluctuations more effectively than one-dimensional carbon nanomaterials such as carbon nanotubes [15], [16]. Therefore, a number of graphene-based gas sensors have been fabricated and excellent sensing performance has been reported [17], [18], [19]. To further improve the sensitivity and selectivity of the gas sensors, many other materials, such as conducting polymers, metal oxides, and metal nanoparticles, have been added to modify the surface of the graphene sheets [11], [17], [20].

Titanium dioxide (TiO₂) is another widely employed material for H₂, NH₃, CO, and NO₂ detection because of its inert surface and high sensing abilities. TiO₂ is an n-type semiconductor material with an energy band gap of about 3.33 eV, which cannot be found in other nanomaterials, thus rendering it suitable for providing useful sensing ranges [8]. However, most of the TiO₂-based gas sensors only work at high temperature, which may restrict its application. Therefore, it is desirable to develop a gas sensor that operates at room temperature while retaining the sensing properties of TiO₂ nanoparticles at the same time [20].

Although the sensing performance of TiO₂ nanoparticles has been studied extensively, to the best of our knowledge, no attempt has been made to examine the performance of a conducting-polymer–graphene composite decorated with TiO₂ nanoparticles. In the present work, a composite of PPy and graphene nanoplatelets (GNs), PPy–GN, was decorated with well-dispersed TiO₂ nanoparticles by the sol–gel method. An ultra-sensitive and highly selective room-temperature NH₃ sensor with fast response was successfully fabricated by drop-coating the nanocomposite on integrated indium–tin oxide (ITO) electrodes. The reproducibility and stability of the sensor were also examined.