Effects of F doping on TiO2 acidic sites and their application in QCM based gas sensors

Titanium dioxide (TiO2) and fluorine doped titanium dioxide (F-TiO2) particles were prepared by a one-pot hydrothermal synthesis, and applied on quartz crystal microbalance (QCM) resonators to detect trace levels of nerve agent stimulant, dimethyl methylphosphonate (DMMP). DMMP sensing measurements exhibited that TiO2 functionalized QCM resonators had fast response and recovery as well as satisfactory sensitivity. F-TiO2 functionalized QCM resonators showed even faster response and higher sensitivity but rather slow recovery. The sensing mechanism and the effect of fluorine doping were examined by diffuse reflectance infrared Fourier transform spectroscopy, revealing that the adsorption took place at Brønsted acid sites at the initial stage and at Lewis acid sites later. Fluorine doping on TiO2 greatly enhanced the acidity of Lewis acid sites, resulting in faster response but slower recovery than pure TiO2. X-ray diffraction analyses demonstrated that the fluorine doping could enhance the crystalinity of anatase.