Frequency selective alcohol sensing performance of Pd/TiO2 nanotube array/Ti (MIM) devices in capacitive mode

Optimal gas/vapor sensing performance by adopting either manipulation of the sensing layers or fabricating different device structures like resistive, metal insulator semiconductor (MIS), metal insulator metal (MIM) and thin... [ view full abstract ]

Optimal gas/vapor sensing performance by adopting either manipulation of the sensing layers or fabricating different device structures like resistive, metal insulator semiconductor (MIS), metal insulator metal (MIM) and thin film field effect transistor (TFET) sensors receives paramount importance since last 3-4 decade. These sensors were operated in resistive mode (current/voltage) and the fractional resistance change resulting from effective change in carrier concentrations under exposure of gases/vapor was used as indexed term to identify the sensor response. As far as price and durability is concerned, the resistive gas sensor is reasonable to commercialize. But, poor selectivity and limited sensor response are the two main bottlenecks which force to sensor researcher to find alternative method for superior gas detection. Earlier studies revealed that, doping or surface functionalization by use of noble metals were mostly used for better selectivity, whereas grain size control found to play a pivotal role for improved sensor response. But, improved results were still far from satisfactory and needs further improvements. In this context, capacitive measurement under exposure of gases/vapor is relatively new and far accurate than the resistive one. The capacitive measurement is predominantly governed by the change in relative dielectric permittivity (ɛr) of the medium in presence of gases/vapor and is less sensitive to the charge effects.
Pd/TiO2 nanotube/Ti (MIM) devices were investigated at the temperature range of 27-200°C and over a frequency range of 0.01-200KHz towards 2-propanol, ethanol and methanol targeting concentrations range of 1-100ppm. From the impedance measurement, the device resonance was found at 2.2 KHz in air ambient and shifted towards lower frequency regime of 0.40 KHz, 0.25 KHz and 0.16 KHz upon exposure to 100 ppm of 2-propanol, ethanol and methanol, respectively (shown in Fig 2 (a)). Selectivity window attributed frequency selective alcohol sensitivity of the TiO2 nanotubes. Adsorption induced shifts in resonance frequency may be attributed to the (i) variation in dielectric permittivity (ɛr) of the nanotube void region and (ii) change in density of the adsorbed molecules. Further, the sensor showed maximum capacitive response magnitude of 406%, 556% and 669%, towards 100 ppm of 2-propanol, ethanol and methanol, respectively.