Sensors for inorganic vapor detection based on carbon nanotubes and poly(o-anisidine) nanocomposite material
Introduction
The high surface area, size, hollow geometry and chemical inertness remarkable properties of CNTs make them attractive for demanding applications in the field of gas sensing. To date studies on possible applications of CNTs have been focused either on individual single-walled carbon nanotubes as sensitive materials towards O2, NO2 and NH3[1], [2], [3], [4] or on multi-walled carbon nanotubes (CNTs) mats as NH3, CO, CO2 humidity and O2 gas sensors [5], [6], [7]. More recently we have reported on the preparation of CNTs thin films by radio frequency plasma enhanced chemical vapor deposition on Si3N4/Si substrates, provided with interdigital Pt electrodes, for NO2 monitoring at low concentrations (10–100 ppb in air) [8].
Nanotube sensors offer significant advantages over conventional metal-oxide-based electrical sensor materials in terms of sensitivity and small sizes needed for miniaturization and construction of massive sensor arrays. Nevertheless, several outstanding issues remain. Firstly, for sensing purposes, it is desirable to reliably obtain devices consisting of semiconductor CNTs operating at room temperature [1], [4], [9]. Secondly, molecular sensing requires strong interactions between a sensor material and target molecules. This is also the case for nanotubes. We have found that nanotubes are not sensitive to many types of molecules (i.e. CO, H2, HCl), indicating an apparent lack of specific interactions between nanotube and these molecules. We believe that nanotube sensors with molecular specificity can be obtained through rational chemical and/or physical modification of nanotubes. Chemical modification may include sidewall functionalization by desired molecular groups [10] while physical modification may involve simple deposition of suitable species on the nanotubes. Here, we present our recent results in tackling the issues above.
Polyaniline (PANI) and its derivatives have been deeply studied for their good electric properties, easy methods of synthesis and high environmental stability [11], [12], [13], [14]. The chemistry of polyanilines is generally more complex respect to other CP. This fact is due to their dependence on both the pH value and the oxidation states, described by three different forms known as leucoemeraldine base (LB) (fully reduced form), emeraldine base (EB) (50% oxidised form), and pernigraniline base (PB) (fully oxidised form). The most important is the EB form and its protonation by means of H+ ions generated from protic acids gives the emeraldine salt form (ES), responsible of the strong increment of conducting properties [15] (see Fig. 1). This process is reversible and it is possible for the presence of imine groups basic sites located along the conducting polymer backbone [16], [17]. The remarkable fact that the chemical–physical properties of PANI and its derivatives are pH sensitive has led to the study of these materials as sensors [18], [19].
The present work focuses on CNTs thin films prepared by radio frequency pulsed plasma enhanced chemical vapor deposition glow discharge system for the detection of HCl. In this work we show that excellent molecular HCl sensors can be enabled by adding on CNTs poly(o-anisidine) (POAS). The POAS modified CNTs sample exhibit significant electrical conductance modulation upon exposure to small concentrations of HCl in air. Importantly, these advanced sensing characteristics are obtained at room temperature. Furthermore, we demonstrate that CNTs and poly(o-anisidine) nanocomposites [20] can be easily scaled up with ensemble of self-assembling of CNTs made by a simple plasma deposition growth approach.
Section snippets
Experimental details
The CNTs thin film was grown using a radio frequency pulsed plasma enhanced chemical vapor deposition (RF PECVD) system. Prior to the nanotube growth, a Si3N4/Si substrate was patterned with platinum film (60 nm thick) by vacuum deposition through shadow masks, containing rectangular stripes 30 μm wide and a back deposited thin film platinum heater commonly used in gas sensor applications [3]. A thin film (3 nm) of Ni catalyst was deposited onto the Si3N4/Si substrates using thermal evaporation
Results and discussion
High-resolution field emission SEM images of CNTs deposited with pulsed plasma are plotted in Fig. 2b. In Fig. 2b the edge zone between the Pt electrode and the sensitive CNTs film is shown. The reason why CNTs did not grow on Pt, as shown in Fig. 2b, can be attributed to the lack of fragmentation of the Ni film on the platinum electrode. This fact can be due to a diffusion of Ni atoms through Pt during the annealing process at 650 °C, eventually leading to the formation of a binary Ni–Pt
Conclusions
In conclusion, CNTs thin films prepared by pulsed RF PECVD demonstrated their potentiality as a new class of materials for HCl detection for environmental applications. Moreover, polymer functionalization enhances the sensitivity to these devices. The advances made here shall pave the way for future work in developing CNTs sensor arrays for highly sensitive and specific molecular detection and recognition in gases and in solutions.
Acknowledgements
This work was supported by MIUR through the project FIRB. One of the authors (I.A.) gratefully acknowledge the financial support from the National Institute of Materials Science and Technology. We are grateful to Dr. Jenny Alongi (Dipartimento di Chimica e Chimica Industriale – University of Genova) for access to transmission electron microscopy as well as technical support. The authors would like to thank Professor S. Santucci for helpful discussions and the free supply of the sensor layout.
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