Mass-sensitive TiO2-based acoustic sensor element

Abstract

The sorption properties with respect to gaseous NH3 of e-beam evaporated TiO2 layers were investigated by a quartz crystal microbalance (QCM) allowing the detection of nanograms of sorbed mass. The TiO2 (249 nm thick) layers were deposited on 16 MHz resonators. The as-deposited TiO2 layers were characterized by ellipsometry, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The measurements of the QCM-TiO2 system's sorption properties were carried out on a laboratory setup allowing operation in dynamic and static modes under strict control of the process parameters which were recorded and processed automatically by an electronic measuring system. The sorption properties of TiO2 were determined using the frequency shift (ΔF) of the QCM in NH3 with concentrations ranging from 10 ppm to 1000 ppm. The time of sorption and de-sorption were defined; the maximal QCM frequency shifts for each NH3 concentrations were measured and the sorbed masses were calculated. It was determined that ΔF increases from 8 Hz at 10 ppm to 216 Hz at 1000 ppm, which corresponds to sorbed masses of 2.78 ng and 75.19 ng, respectively. The results show that the QCM-TiO2 system could very successfully be used as a sensor element for NH3 detection in the range 10 – 1000 ppm.