Effect Of Morphology Control And Copper Doping Of Tungsten Trioxide Based Gas-sensing Materials On Acetone Responses

Acetone is an important organic synthetic chemical and an excellent solvent that can solve a lot of organic compounds. However, acetone has high volatility even at room temperature. When people inhale high concentrations of acetone, the central nervous system will suffer inhibition and anesthesia and the liver, kidney and pancreas and other organs of the human body may be damaged. Besides, acetone is a product of the metabolism of the human body. The level of acetone in exhaled breath can reflect the health status of the human body. Therefore, the detection of acetone concentration has great significance whether in industrial process or in medical field.Metal-oxide-based semiconductor gas sensors are widely used in the detection of various target gases due to their characteristics such as high sensitivity, fast response, small size,low energy consumption and cost, simple operation, etc. WO₃ is one of the most investigated gas-sensing materials for the detection of acetone. The gas-sensing mechanism is based on the redox reaction of acetone molecules with oxygen species adsorbed on the surface of WO₃. Morphology and microstructure control as well as doping of WO₃ may influence the adsorption and chemical reaction of gas molecules on the surface of the material, and ultimately, exact great influence on the gas-sensing properties of the material. In this paper, we synthesized three different morphologies of WO₃, that is, hollow spherical, urchin-like and plate-like tungsten trioxides by solvothermal methods. Moreover, hollow spherical tungsten trioxide copper-doped also was obtained. The prepared WO₃ were characterized systemically by X-ray powder diffraction, Scanning electron microscope, Transmission electron microscope, High resolution Transmission electron microscope, N2 adsorption/desorption isotherms, TG analysis and X-ray photoelection spectra. The effect of morphology, microstructure and doping of the prepared WO₃ materials on the sensitivity, selectivity to acetone as well as the stability of the materials were investigated thoroughly. The main results are as follows:1) Hollow spherical, urchin-like and plate-like tungsten trioxides were synthesized via solvothermal methods. The surface area of hollow spherical tungsten trioxide with ca.520 nm in diameter was 110 m2/g, and the surface area of urchin-like tungsten trioxide with ca. 460 nm in diameter was 69 m2/g. The effects of calcination temperature and operating temperature on the gas-sensing performance of materials with different morphologies were studied. The results showed that hollow spherical tungsten trioxide calcined at 550 oC exhibited the optimal response towards acetone under operating temperature of 340 oC. The selectivity of hollow spherical tungsten trioxide was also studied. Under the same conditions, there were almost no responses for the gas molecules,including ethanol, methanol, benzene, formic acid, formaldehyde and ammonia, even if the vapor concentrations reached up to 5 ppm. Thus it could be seen that the materials possessed high selectivity to acetone. Finally, the effect of the relative humidity on the acetone responses was further investigated. When the relative humidity was 15%, the sensitivity was 3.32, 4.24 and 5.20 for 0.5 ppm, 1 ppm and 2 ppm acetone concentration,respectively. When the relative humidity was above 95%, the sensitivity was 1.65, 1.81 and 2.23 for 0.5 ppm, 1 ppm and 2 ppm acetone concentration, respectively.2) The effect of copper doping on the gas-sensing responses for the hollow spherical with the best gas-sensing activity was studied. The results of XPS analyses showed that the peak of Cu2p3/2 appeared, and the shift of W4 f towards higher binding energy increased with the increase of the amount of copper dopping, indicating that copper atoms had doped into the lattice of tungsten oxide. The effects of the amount of copper doping, calcination temperature and operating temperature on the gas-sensing performance of doped hollow spherical tungsten trioxide were studied. The results showed that the optimal amount of copper doping was 3%, the optimal calcination temperature was at about 550 oC and the optimal operating temperature was at around340 oC. The selectivity of hollow spherical tungsten trioxide with 3% copper doping was also studied. The results indicated that under the similar conditions, there were hardly responses to some gase molecules, such as ethanol, methanol, benzene, formic acid,formaldehyde and ammonia, even though the vapor concentrations reached up to 5 ppm.Finally, the effect of the relative humidity on the acetone responses was further investigated. When the relative humidity was 15%, the sensitivity was 4.01, 4.96 and7.61 for 0.5 ppm, 1 ppm and 2 ppm acetone concentration, respectively. When the relative humidity was above 95%, the sensitivity was 2.02, 2.25 and 2.72 for 0.5 ppm, 1ppm and 2 ppm acetone concentration, respectively. In comparison to the undoped WO₃,copper dopping in hollow spherical tungsten trioxide obviously improved the gas-sensing activity to acetone.