Sensing performance of palladium-functionalized WO3 nanowires by a drop-casting method
Graphical abstract
Highlights
► Tungsten oxide nanowires were grown by close-spaced vapor transport technique. ► The nanowires were functionalized with palladium nanoparticles deposited by a drop-casting method. ► A comparative study of the sensing performance to reducing gases was conducted. ► It is shown that this simple method improves the sensing performance of the tungsten oxide nanowires.
Introduction
There is an increasing need of modern societies to monitor toxic gases in the environment for assuring people's health and security. The environment is getting more polluted by the enormous industrial development, automotive pollution, and biological hazards. The main gases to be monitored in the industry and the environment are NOx, CO, SO2, H2S, NH3, and volatile organic compounds (VCO's) [1]. However, the high cost of the monitoring systems limits the control and monitoring of the air quality. Alternatively, it has been shown that sensors based on semiconducting metal oxides can provide good sensitivity, selectivity and stability for obtaining portable and cheap monitoring systems [2].
In recent years, one-dimensional metal oxide nanostructureshave attracted great interest because they have proved to be superior to both the thin and thick film gas sensors. Yamazoe and co-workers showed that reduction of the crystal size in policrystalline films increases the sensor's sensitivity [3]. However, it has been observed that in these traditional policrystalline films, the moderate operating temperatures required for the surface reactions to take place induce a grain coalescence which affects the long-term stability of the sensor [4].
Quasi one-dimensional metal oxide nanostructures have shown a great potential for their use as chemical sensors because they have a high surface to volume ratio and superior stability due to their high crystallinity and relatively simple preparation methods. In addition, the size of these nanostructures approaching the material Debye length make them prospective for high sensitive gas sensors [5]. Several materials like SnO2, TiO2 and ZnO and others have been studied for such applications [6]. Among them, WO3 has been shown to be a good candidate for the detection of both oxidizing and reducing gases such as NOx [7], NO2 [8], O3 [9] and O2 [10], NH3 [11], H2S [12], CO [13], CH4 [14], Hydrocarbon [15], and VCO's [16].
Therefore, many efforts have been made to improve the morphology and size of the WO3 nanostructures since these technological parameters are directly related to the sensor performance. In the literature several chemical and physical methods have been developed successfully to synthesize nanostructurated tungsten oxides with peculiar morphologies such as nanofibers [17], nanotubes [18], nanowires [19], nanospheres [20], and nanorods [21]. However, the control of the synthesis of WO3 nanostructures with small crystalline size and superior surface properties to fulfill the requirements of gas sensor applications is still a fundamental challenge for scientists worldwide [22]. Furthermore, many authors have shown that to further improve the sensor performance it is necessary to functionalize the nanostructures by adding noble metal nanoparticles (Pt, Pd, etc.) or metal oxides as additives [23], [24], [25]. Thus, it is required to combine the nanostructure characteristics with those of the additives for obtaining advanced hybrid materials that allow the development of high sensitive and selective gas sensors.
Tungsten oxide nano-materials are synthesized by several techniques mainly using high temperature evaporation [26], precipitation [27], electrochemical anodizing [28], sputtering [29], topochemical [30] and solvothermal synthesis approaches [31]. However, the above mentioned techniques have some drawbacks in the fabrication of the sensors because they include several intermediate processes to have the final device; i.e., nanowires growth, nanowires collection and dispersion in a solution, deposition and alignment of the nanowires. These techniques require expensive equipments such as electron beam lithography, focus ion beam and sputtering systems. In addition, these techniques also present a series of non-controllable processes such as sonification and the dispersion of the nanowires onto prefabricated inter-digitated electrodes.
This work reports the gas sensing characteristics of a sensor based palladium-sensitized tungsten oxide nanowires by a a drop-casting method. The tungsten-oxide nanowires were synthesized by the close-spaced vapor transport technique, this technique is a variant of the chemical vapor deposition technique (CVD) that allows deposition of the nanowires on a final substrate without intermediate steps for the sensor fabrication (collection, dispersion of nanowires on a solution and nanowires alignment on patterned electrodes). The sensor performance was obtained for both the bare and the functionalized tungsten oxide nanowires when exposed to hydrogen and volatile organic compounds for a wide range of operating temperatures and concentrations. The results reveal that using this simple functionalization technique, the sensor performance is significantly enhanced with regard to the response time, recovery time, and sensitivity. The best sensing performance results were obtained at an operating temperature of 300 °C showing that the sensor based on the palladium-functionalized WO3-NWs becomes more selective to hydrogen and xylene. The improvement on the gas sensing characteristics is attributed to the effect of the palladium particles anchored on the surface of the nanowires.
Section snippets
Experimental
The experimental procedure for the synthesis of the WO3-NWs by the CSVT technique is described as follows: First, the nanostructures sourcewas obtained from the wet thermal oxidation of tungsten foils, which previously were cut into squares of approximately 1 cm2 and cleaned using conventional solvents (xylene, acetone, and methanol). In the oxidation process, the operating temperature, nitrogen flow (carrier gas), chamber pressure, and oxidation time were fixed to 750 °C, 100 sccm, atmospheric
Results and discussion
Fig. 2 shows a SEM image of the WO3-NWs film after thermal annealing of the as-grown nanowires. In this figure it is clear that the WO3-NWs are distributed randomly forming a network in the film and it is also clear that the WO3-NWsdiameters are in the range 50–200 nm, and their lengths are above 5 μm. Finally, the transverse section of the networked film (not shown) revealed that the nanostructures growth is almost perpendicular to the substrate, and most of them are in contact. Then, thin films
Conclusions
WO3-NWs were synthesized by close-spaced vapor transport technique, showing that this is a versatile and simple technique that allows deposition of nanowires on a substrate without intermediate steps for the sensor fabrication. Morphological and structural characterizations showed that after thermal annealing the dimensions of the as-grown nanowires are not modified and they correspond to WO3 in the orthorhombic phase. It has been shown that it is possible to functionalize the surface of WO3
Acknowledgements
We thank Nicolás Morales López, Adolfo Tavira-Fuentes, Miguel GalvánArellano, Gaspar CasadosCruz and Josue Esau Romero Ibarra from the “Laboratorio Avanzado de Nanoscopia Electronica” at CINVESTAV-IPN for their technical assistance in the morphological and structural characterizations. This project was supported by PROMEP-SEP, No. 103.5/11/6673, México.
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