Broadband Dielectric Spectroscopy Detection of Volatile Organic Compounds with ZnO Nanorod Gas Sensors
Authors: Amoah Papa K. , Lin Pengtao , Baumgart Helmut - , Obeng Yaw 
Published: December 2020

Historically, metal-oxide semiconductor-based gas sensors, such as SnO2 and ZnO, have been used to detect gases based on chemiresistivity [1]. In this approach, the electrical changes in response to analytes are measured with electrical circuits, that necessarily involve making electrical contacts to the sensing materials. These contacts are imperfect and introduce errors into the measurements. In this paper, we will demonstrate the feasibility of BDS-based metrology in addressing this gap in metrology to avoid these tool-measurand interactions / parasitic errors. Specifically, we will discuss how radio frequency (RF) propagation characteristics can be applied to probe and study charge transfer chemical changes in these metal-oxide VOC sensing materials.
For this study, the solid-state gas sensor device under test consisted of hydrothermally synthesized ZnO nanorods grown on a fine grain seed layer of atomic layer deposition (ALD) ZnO on a silicon substrate. The sample was placed on a waveguide situated in a controlled environment to study the redox reactions of the ZnO sensor detecting ethanol target gas under thermal heating. In this configuration, the ZnO gas sensing element couples to the microwave through an evanescent mode. The temperature of the assembly was varied from room temperature to approximately 180 °C, in a variety of gaseous environments, while continuously monitoring the microwave signal going through the waveguide.
Analysis of the BDS data (at specific frequencies) gave us electrical information that can be correlated to charge-transfer reactions involving the defects in the ZnO material and the molecules in the gaseous environment. For example, according to the S21 data, the sample became more resistive as temperature increased. This can be attributed to the redox reactions occurring on the ZnO nanorod gas sensor surface. Illustratively, Figure 1 shows time-lapse evolution on microwave signal loss in the experimental setup attributable to the discrete events occurring in the ZnO in response to various environmental conditions.
The resistivity (as indicated by the S21) of the ZnO is stable below 120°C. At 120C, the system becomes increasing resistive with time, possibly as adsorbed water molecules on the ZnO rods desorb and to the healing of the ZnO surface defects, in a distinct two-step process [1]. Finally, the system became less lossy (i.e., electrical resistivity decreased) when ethanol target gas (technical grade C2H5OH, EtOH) was injected into the reactor. However, this decrease in resistivity might have been influenced by the decrease in temperature once the EtOH was introduced. From the S-parameters, we calculate a microwave attenuation constant (i.e. the real part of the microwave propagation constant) which quantified energy dissipation into the ZnO material. The attenuation constant data suggest the redox reaction induced changes in the semiconductor rods are reversible within the temperature range in this experiment.
To the best of our knowledge, such detailed description of the discrete events that occur in the metal-oxide sensing material cannot be obtained with the traditional electrical DC resistivity measurements. In the future, we may be able to use this technique to discern differences, and distinguish between ZnO surface- and bulk reactions, as well as identify / characterize point defects.

References

H-J. Kim, and J.-H. Lee, “Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview”, Sensors and Actuators B 192 (2014) 607– 627

Heine, F. Girgsdies, A. Trunschke, R. Schlögl, and M. Eichelbaum, “The model oxidation catalyst α-V2O5: insights from contactless in situ microwave permittivity and conductivity measurements”, Applied Physics A, vol. 112, pp. 289–296, 2013 DOI 10.1007/s00339-013-7800-6

Figure 1