An Investigation of the Ammonia-Sensing Mechanism of α-MoO3-Based Chemi-Resistive Sensors
Authors: Ebaugh Thomas Allen , Bonville Leonard J. , Maric Radenka 
Published: December 2020

Chemi-resistive ammonia sensors based on alpha-phase molybdenum trioxide (α-MoO3) are a simple and selective technology for measuring the concentration of gaseous ammonia for applications such as human breath sensing and monitoring selective catalytic reduction (SCR) systems [1-7]. The basic operating principle of these sensors is the correlation between the electrical resistance of the sensor and the concentration of ammonia to which the sensor is exposed. Typically, the sensor consists of an interdigitated electrode substrate covered with the α-MoO3 film [1-7]. The electrical resistance of the α-MoO3 film changes as a function of ammonia concentration. This change in resistance of the α-MoO3 film is commonly attributed to the alteration of the charge depletion layer (Λ) of the α-MoO3 particles through the reaction of ammonia with adsorbed oxygen species [5-9]. For α-MoO3, the charge depletion layer model predicts that the resistance of the film should decrease with increasing ammonia concentration. In addition to the charge depletion layer model, other sensing mechanisms have been suggested, such as the partial reduction by ammonia of the surface of the α-MoO3 film [1,2] and the formation of oxygen vacancies in the α-MoO3 lattice [3].
The α-MoO3 film can be fabricated by a variety of methods, including sol-gel synthesis, ion beam deposition, hydrothermal synthesis, and flame spray pyrolysis [1-6]. In this work, the α-MoO3 film is fabricated using Reactive Spray Deposition Technology (RSDT) followed by annealing to develop crystallinity. The RSDT is a flame-based process that can be used for the fabrication of α-MoO3 films at a commercial scale. The initial development of ammonia sensors with RSDT-fabricated α-MoO3 films of varying morphology was presented at the 236th ECS Meeting as an oral presentation, “Ammonia-Sensing Properties of α-MoO3 Fabricated by Reactive Spray Deposition Technology (RSDT).” From these initial sensors, the sensor with the strongest response to ammonia was selected for further testing at concentrations between 0.1 ppm and 5 ppm ammonia in dry air at 400°C. These sensing results and the materials characterization of the sensor by SEM and XRD before and after 80 hours of continuous testing are reported in Ebaugh et al. [7] and are shown in Figures 1-3. In Figure 1, the sensing results among the three test cycles comprising the 80 hours of testing are quite repeatable; however, the results consistently depart from the prediction of the charge depletion layer model between 3 ppm and 5 ppm.
To better explain this unexpected sensing behavior, additional electrochemical testing and materials characterization is needed. Specifically, normal pulse voltammetry (NPV) can be used to provide data pertaining to the electronic path through the α-MoO3 film and the reaction and transport of chemical species adsorbed on the α-MoO3 film. TEM analysis of the α-MoO3 can show the nanoscale morphology of the sensing material. It is well established that the nanoscale morphology of the metal-oxide film impacts its sensing behavior [8-10]. XPS can also be used to investigate the possible reduction of the surface of the α-MoO3 particles and the formation of oxygen vacancies in the α-MoO3 lattice. Data from the materials characterization of the sensing film (XRD, SEM, TEM, and XPS) will be used in conjunction with sensing results and NPV data to develop an explanation for the sensor behavior observed in [7]. An understanding of the ammonia-sensing mechanism will help to guide the optimization of the RSDT-fabricated α-MoO3 film to improve the performance of the sensor for a wide range of applications.
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Figure 1