Metal-modified and vertically aligned carbon nanotube sensors array for landfill gas monitoring applications

Abstract

Vertically aligned carbon nanotube (CNT) layers were synthesized on Fe-coated low-cost alumina substrates using radio-frequency plasma enhanced chemical vapour deposition (RF-PECVD) technology. A miniaturized CNT-based gas sensor array was developed for monitoring landfill gas (LFG) at a temperature of 150 degrees C. The sensor array was composed of 4 sensing elements with unmodified CNT, and CNT loaded with 5 nm nominally thick sputtered nanoclusters of platinum (Pt), ruthenium (Ru) and silver (Ag). Chemical analysis of multicomponent gas mixtures constituted of CO(2), CH(4), H(2), NH(3), CO and NO(2) has been performed by the array sensor responses and pattern recognition based on principal component analysis (PCA). The PCA results demonstrate that the metal-decorated and vertically aligned CNT sensor array is able to discriminate the NO(2) presence in the multicomponent mixture LFG. The NO(2) gas detection in the mixture LFG was proved to be very sensitive, e.g.: the CNT:Ru sensor shows a relative change in the resistance of 1.50% and 0.55% for NO(2) concentrations of 3.3 ppm and 330 ppb dispersed in the LFG, respectively, with a wide NO(2) gas concentration range measured from 0.33 to 3.3 ppm, at the sensor temperature of 150 degrees C. The morphology and structure of the CNT networks have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. A forest-like nanostructure of vertically aligned CNT bundles in the multi-walled form appeared with a height of about 10 microm and a single-tube diameter varying in the range of 5-35 nm. The intensity ratio of the Raman spectroscopy D-peak and G-peak indicates the presence of disorder and defects in the CNT networks. The size of the metal (Pt, Ru, Ag) nanoclusters decorating the CNT top surface varies in the range of 5-50 nm. Functional characterization based on electrical charge transfer sensing mechanisms in the metal-modified CNT-chemoresistor array demonstrates high sensitivity by providing minimal sub-ppm level detection, e.g., download up to 100 ppb NO(2), at the sensor temperature of 150 degrees C. The gas sensitivity of the CNT sensor array depends on operating temperature, showing a lower optimal temperature of maximum sensitivity for the metal-decorated CNT sensors compared to unmodified CNT sensors. Results indicate that the recovery mechanisms in the CNT chemiresistors can be altered by a rapid heating pulse from room temperature to about 110 degrees C. A comparison of the NO(2) gas sensitivity for the chemiresistors based on disorderly networked CNTs and vertically aligned CNTs is also reported. Cross-sensitivity towards relative humidity of the CNT sensors array is investigated. Finally, the sensing properties of the metal-decorated and vertically aligned CNT sensor arrays are promising to monitor gas events in the LFG for practical applications with low power consumption and moderate sensor temperature.

Electrophoretic deposition of Au NPs on CNT networks for sensitive NO<sub>2</sub> detection

“…Past Raman spectroscopy on pristine CNTs revealed the presence of some amount of amorphous carbon in the CNTs network (Penza et al, 2008(Penza et al, , 2010; specifically, the high intensity ratio (I (D)/I (G)) of D peak, associated with the disordered graphitic layer, to G peak, due to the tangential modes of the graphitic planes, revealed qualitatively the presence of a large amount of disorder and defects. On the surface-functionalized CNTs, Au NPs effectively decorate MWCNT sidewalls, forming isolated nanoclusters, with a density increase upon increasing deposited Au content.…”

“…The experimental set-up used for gas sensing measurements is reported elsewhere (Penza et al, 2010). The CNT-based gas sensors were placed in a sealed stainless test cell (500 mL Pristine CNTs 95.0 ± 0.5 -5.0 ± 0.5 Au NPs/CNTs t: 90 s 94.4 ± 0.5 0.3 ± 0.2 5.3 ± 0.5 Au NPs/CNTs t: 600 s 91.2 ± 0.5 1.1 ± 0.2 7.8 ± 0.5 volume) for gas exposure measurements.…”

Supported plasma-made 1D heterostructures: perspectives and applications

“…Among the advantages of plasma processes, it has been quoted that they are relatively low cost procedures, environmentally friendly (i.e., solventless method) and scalable at wafer scale for direct deposition onto electronic and photonic devices while providing a precise control over the composition of the deposited materials. Due to these advantages, during the last years plasma technology has being systematically used for the fabrication of 1D nanofibres and other heterostructures [25,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. As already mentioned, most works in this field deal with the fabrication of heterostructures formed by CNTs and carbon nanofibers where the metal particle acts as a catalyst of the growth process.…”

“…Just as examples of this type of heterostructures let us mention the case of Co nanoparticles encapsulated in CNTs that were synthesised by a one-step PECVD method at high pressures and substrate temperatures [32], that of Ti-coated CNTs fabricated by inductively-coupled plasma enhanced chemical vapour deposition (ICP-CVD) and Ti sputtering for field emission applications [33][34][35] or Cu-C hybrid nanosystems produced by means of plasma deposition of carbon on copper metal seeds [28]. The results of Penza et al about the decoration of CNTs with metal (Ag, Au, Pt and Ru) nanoparticles are also very promising in the field of nanosensor applications [36].…”

Room temperature carbon nanotube based sensor for carbon monoxide detection

“…However, SWCNTs have been modified, especially with metal loading, to provide a response in the form of a change in resistance upon exposure to CO. Rh-loaded SWCNTs were shown to respond to 2500 ppm of CO at room temperature (Star et al, 2006). Vertically aligned CNTs decorated with Pt, Ru and Ag clusters responded to 0.1 % CO at 150 • C when exposed to a mixture of CO, CO 2 , NH 3 , CH 4 and NO 2 , with the mixture representing a landfill gas (Penza et al, 2010). Adding CNTs to an otherwise nonresponsive polyaniline allows CO sensing in the range of 100-1000 ppm at room temperature (Wanna et al, 2006).…”