Gas sensing behaviour of mat-like networked tungsten oxide nanowire thin films

Abstract

Here, we present the results of the resistive response of tungsten trioxide nanowire (mat-like, nanowire networks) and nanoparticle thin films subjected to N 2 O gas in the temperature range of 373-773 K. The nanowire mats exhibited an order of magnitude higher response in the resistivity change compared to that of nanoparticle films at temperatures above 523 K. Nanowire mats also exhibited relatively faster adsorption and desorption times. Impedance spectroscopy studies showed that the gas sensing mechanism for nanowire mats involves changes in both the nanowire and grain boundary resistances, whereas for nanoparticle films only the grain boundary resistance governs the sensor properties upon exposure to gases.

Chemical Vapour Deposition of Gas Sensitive Metal Oxides

“…In addition, by manipulating CVD conditions to favour formation of networked nanowire mats rather than NPs the sensitivity of tungsten oxide to N 2 O was improved by an order of magnitude compared to NP films ( Figure 3). This was related to the higher surface-to-volume ratio of NS compared to NPs, although this study also highlights the need to use highly networked nanowires as opposed to (quasi) aligned nanowires for optimum gas sensing performance, as these behave similarly to single nanowire or parallel nanowire arrays [17]. Similar observations were also reported for gas microsensors based on tungsten oxide nanoparticles and nanowires grown via AACVD (Figure 3) [32].…”

“…These materials are typically monoclinic or tetragonal phases with a variety of morphologies reported including films, particles and low dimensional structures, with the formation of nanostructures (NS) demonstrated below 600˝C for AACVD [15] and at 800˝C for hot filament CVD. The starting materials reported in the production of gas sensitive tungsten oxide include metallic W [16,17], WO 3 (powder, pellet) [18,19], WCl 6 [20], W(OCl 4 ) [21], W(CO) 6 [22][23][24] [20,21,26,27], silicon- [16,22,23,25] or polymer-based [28] gas sensing devices. The localized CVD of tungsten oxide nanostructures on Si-based microhotplates ( Figure 1) via heating provided from the sensor platform itself, rather than from the reactor chamber, has also been demonstrated as a viable method for the fabrication of gas sensors based on tungsten oxide [23], which provides interesting new possibilities for sensor processing.…”

Important considerations for effective gas sensors based on metal oxide nanoneedles films

“…The high orientation of the crystals in the NN bulk favours the electron transport, and therefore the transducer function of individual NN. However, and contrary to single NN sensors, the electron transport in NN films also depends on the NN/NN boundary as previously reported [24]. Although, in our experiments we have chosen mat-like films to enhance the NN/NN boundary in the film, it is apparent that the electrode configuration (with wide gaps typically used for polycrystalline films) does not improve the interconnected networked NN mats, indicating the need of special electrodes configuration -with electrode gaps similar to the NN length as suggested in reference [32] -that effectively interconnect the NN in the film, improving the sensor response, and possibly the response and recovery time of the NN based gas sensors.…”

“…The influence of solvent on the morphology of AACVD tungsten oxide films was recently considered in detail in reference [8]. Similar non-aligned and mat-like tungsten oxide NN, growth by self-catalysed direct vapour-solid mechanism, have been reported previously [11,24], as well as other nanostructures such as tube-like [25], rod-like [26], or flake-like [27]. According to reference [24], NN mat-like morphologies are effective structural arrangements for gas sensing applications, which suggests that NN films deposited at 500 ºC from an acetone/toluene mixture (Figure 2c) are attractive for gas sensor fabrication.…”

Improved acetone sensing properties of flat sensors based on Co-SnO2 composite nanofibers

“…These results indicate Co-SnO 2 composite nanofibers are good candidates for fabrication of high performance acetone sensors for practical application. Establishing effective methods for monitoring and detecting toxic and flammable gases such as carbon monoxide, acetone, benzene and toluene is important because of the regulations that exist in many countries [1][2][3][4][5]. Although many modern monitoring methods, such as gas chromatography and infrared spectroscopy, have high sensitivity, they are expensive and cannot be used for real-time measurements.…”