Gas-sensing technology has been employed extensively for environmental, industrial, and medical applications. The development of sensitive and reliable materials that detect small amounts of harmful contaminants in the air has been and remains a major challenge [1
]. Various metal oxide semiconductors (MOS), used as conventional chemical gas-sensing materials in resistive type gas sensors, have been increasingly investigated because of their operational simplicity, lightweight, high sensitivity, and low cost [3
]. Among these oxides, tungsten oxide thin films happen to be one of the promising candidates for sensing hazardous gases. Hence, a variety of tungsten oxide semiconductors synthesis techniques have been reported, including electron beam evaporation [6
], thermal or anodic oxidation [8
], electrochemical deposition [10
], chemical vapor deposition (CVD) [11
], sputtering [13
], pulsed laser deposition [16
], sol-gel [18
] as well as screen printing of thick films based on ultra-fine powder [19
To date, with different synthesis techniques, tungsten oxides of various crystalline structures and morphologies ranging from zero-dimensional (nanoparticles and quantum dots), to 1D (nanorods, nanowires, nanofiber, nanobricks and nanotubes), 2D (nanosheets, nanoplates and nanolamella), and 3D (nanoflowers and nanospheres) [20
] have been fabricated. The tungsten oxides-based gas sensors for the measurement of H2
], NO [22
], CO [30
] and organic gasses [32
] such as acetone, methanol, ethanol and formaldehyde. Compared with bare tungsten trioxide (WO3
) materials, the sensing properties have been significantly enhanced by the doping of other elements, functionalization of noble metal nanoparticles, or heterojunction formed with other semiconductors.
Since it was first reported as an ammonia
sensor in 2000 [34
], a pure tungsten trioxide-based gas sensor has demonstrated success in NH3
leak detection in both indoor and outdoor air quality monitoring [3
]. However, these sensors normally operated at high temperatures ranging from 150 to 500 °C, which complicated the design requirements for temperature monitoring and control, as well as environmental safety. For example, Leng et al. reported that the optimal sensitivity occurred at 500 °C when exposed to 100 ppm NH3
]. Ji et al. reported recently that the sensitivity peaked at 350 °C when exposed to 100 ppm NH3
]. More recently, a great effort has been made to decrease the operating temperature, especially for wearable applications. As a result, ammonia detection around 150 °C was developed [37
], as well as room temperature ammonia sensors by compositing carbon nanotubes [38
], or hybridizing polyaniline [39
], with tungsten oxide nanocrystals. Nevertheless, either the response time or recovery time was long, up to a few minutes. In order to have the sensing material return to initial conductance quickly after each cycle, a short pulse of annealing treatment at 250 °C was introduced.
Great advances have been achieved in designing and fabricating different dimensional WO3 materials. However, it is still challenging to achieve high-performance WO3 gas sensors. As an n-type semiconductor, its conductance increases when the chemisorbed oxygen species on the sensing material surface reacts with the adsorbed target gas molecules. Therefore, the material morphology and crystalline structure play a fundamental role in determining the sensing performances because the interaction occurs primarily on the surface and boundary. A quick, low-cost and environmental green device prototyping method is highly needed to optimize process parameters with a controllable approach and to explore the relationship between the featured morphologies and sensing performances.
The device performances reported to date have been evaluated for WO3 gas sensors fabricated based on a combination of different processing and design parameters, such as synthesis method, material composition, device dimension and electrode design. Hence, it is difficult to compare and optimize their performances. In this paper, we report on the simple post-annealing-free hot-filament CVD method to prepare tungsten oxide samples where tungsten oxides are grown by direct thermal evaporation of the tungsten filament under medium vacuum. Then, the evolution of the material compositions, crystalline structures and surface morphologies at different substrate temperatures is studied using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), Raman spectroscopy, and X-ray diffraction (XRD) techniques. The gas-sensing properties based on the materials synthesized are examined systematically and compared with pure counterparts. Finally, a low-cost, high-performance gas sensor operating at room temperature is demonstrated for monitoring air pollutants such as NH3, CO and CH4.
The simple post-annealing-free hot filament CVD method has been used to effectively prepare crystalline tungsten oxides. Compared with the other synthesizing methods, this simple approach offers some advantages such as its easy implementation, cost effectiveness, and the avoidance of harmful agents. The controllable compositions and morphologies, including NPs, NRs and NFs, have been obtained at a relatively high deposition rate by varying the substrate temperature. Based on these as-grown tungsten oxide membranes, simple, low-cost gas-sensing prototypes have been designed, fabricated, and tested. The NF-based gas sensor gives the best performance, followed by the NR-based device and lastly the NP device. When operated at room temperature, besides high sensitivity and fast response time, the NF-based gas sensor offers the additional advantage of good repeatability and stable baseline. Several ways have been suggested to further enhance the gas-sensing sensitivity. Besides the functionalization of the active layer surface with noble metal nanoparticles, it is found that in general, hybrid nanocomposite or multistructure-based sensors have a higher sensitivity than a conventional device constructed solely from one material when tested under identical experimental conditions, suggesting a synergistic effect between the two components. Overall, such a simple fabrication method can be further use to optimize the sensor performance by fine tuning processing parameters such as oxygen content inside the vacuum chamber, substrate temperatures for other morphologies like nanotubes, and thermal annealing processes. In addition, this method can also be modified for doping with other elements, or functionalization with noble metal particles, etc., to further optimize the material properties for sensing different target gases.