Au-Doped Tungsten Trioxide Thin Films for NO2 Sensing

The information provided is given as reference. The gas molecules of Nitrogen dioxide (NO2) are not benefit to the human, plant, and animals. The gas can cause various problems, such as negative effect to lungs, smog rain, and is mainly fashioned from the burning of fossil fuels inside the combustion engines, alongside the other forms of NOx, such as NO and N2O.

Tungsten troxide (WO3) is an n type semiconductor that with a band-gap of 2.6–2.8 eV, has been widely studied as a promising sensing material for NO2 detection for its excellent sensitivity and selectivity.

Recently, gold (AU) is reported to be doped on tungsten trioxide thin films by decorating Au nanoparticles on the synthesized pristine WO3 thin film using the electron beam evaporator. To see the effects of Au nanoparticles, we fabricated the gas sensors based on pristine WO3 thin films and gold-incorporated (Au-incorporated) WO3 thin films with different Au contents. Thus, sensors based on the Au-doped tungsten trioxide thin films has been successfully prepared by solution process, the product exhibit high sensitivity and good selectivity for NO2 sensing. The preparation process of WO3 thin films is as below:

The distance between platinum (Pt, 150 nm thick) and titanium (Ti, 30 nm thick) interdigital electrodes (IDE) is 4 μm, which is patterned on the SiO2/Si substrate using photolithography and an electron beam evaporator. After ultrasonic cleaning of the IDE patterned substrate, the mask is attached to the substrate to deposit the WO3 film on the precise sensing area. Prepare a light yellow tungsten solution by dissolving 200 mg (NH4)2WS4 in 1 ml N,N-dimethylformamide (DMF). Add AuCl3 to the prepared solution (with different concentrations of 5, 10 and 20 mM to determine the optimal amount of Au content) and use an ultrasonic bath to uniformly disperse to form a uniform solution of Au doped into WO3 film. It will be prepared After the solution was dropped on the masked Pt IDE, a uniform film was prepared by a spin coater at a speed of 4000 rpms for 60 s, and then the mask was removed. After drying in air, the coated substrate was annealed in an oven at 500°C for 6 h. During the annealing process, the AuCl3/(NH4)2WS4 solution is transformed into a gold-doped WO3 film. Then, a gas sensor based on gold-doped WO3 film was obtained.

In short, sensors based on the Au-doped tungsten trioxide thin films has been prepared by solution process, the product exhibit high sensitivity and good selectivity for NO2 sensing. The morphologies of the pristine WO3 and Au-incorporated WO3 thin films were observed by FESEM and AFM, showing that the Au-incorporated WO3 thin film had a higher porosity than the pristine WO3 thin film. In addition, the HRTEM images confirmed that the Au nanoparticles are uniformly distributed throughout the film. 

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