Proposal of a Sensing Mechanism of α-MoO3 Chemi-Resistive Sensors for Gaseous Ammonia in Air
Authors: Ebaugh Thomas Allen , Bonville Leonard J. , Maric Radenka 
Published: May 2020

Chemi-resistive ammonia sensors based on α-MoO3 are a simple and selective technology for measuring the concentration of gaseous ammonia [1-4]. Typically, the sensor consists of an interdigitated electrode substrate covered with the α-MoO3 film [2-4]. The measured electrical resistance of the α-MoO3 film on the substrate changes as a function of gaseous ammonia concentration to which the film is exposed. α-MoO3 is a n-type semiconductor [4], meaning that the electrical conductivity of α-MoO3 is dominated by free electrons as opposed to holes. The mechanism of ammonia sensing by n-type semiconducting metal oxides is commonly attributed to the alteration of the charge depletion layer (Λ) of the metal oxide particles through the reaction of ammonia with adsorbed oxygen species [4-10]. For n-type semiconducting metal oxides, the charge depletion layer model predicts that the resistance of the sensing film (α-MoO3) decreases with increasing concentration of reducing gas, such as ammonia, to which the sensor is exposed. 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 MoO3 film and the formation of oxygen vacancies in the MoO3 lattice [1, 2, 11].
It is generally understood that the sensitivity of the sensor can be tailored based on the particle size and morphology of the metal oxide film [5-10]. Therefore, as initial work, sensors with α-MoO3 films of varying morphology on gold-interdigitated electrodes with alumina substrate (DropSens P/N: IDEAU200) were fabricated by Reactive Spray Deposition Technology (RSDT) and tested for sensitivity to ammonia in synthetic air at 400ºC. This work 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 sensors, the sensor with the strongest response to ammonia was selected for further testing in ammonia and synthetic air at 400ºC with constant 1V DC bias across the sensor. This additional testing consisted of three 12-hour cycles, in which six ammonia concentrations were tested: 0.1, 0.3, 0.5, 1, 3, and 4.9ppm. Between each concentration, the sensor was flushed with synthetic air. Between each 12-hour cycle, the sensor was held at 400ºC in synthetic air for approximately 12 hours. For each ammonia concentration in each cycle, the sensing response (S) was calculated as S=Rgas/Rair where Rgas is the resistance of the sensor for a particular concentration of ammonia and Rair is the resistance of the sensor for the air flush immediately before the particular concentration of ammonia. Sensing results are shown in Figure 1. The results from cycle to cycle are quite repeatable; however, the results consistently depart from the prediction of the charge depletion layer model between 3ppm and 4.9ppm. In an attempt to assess whether these unexpected results could be attributed to morphological or crystallographic changes in the MoO3 film, SEM images and XRD plots were compared before and after the three 12-hour test cycles. SEM images were taken on both the gold and alumina portions of the electrode. Figures 2 and 3 show the SEM images and XRD plots, respectively. Neither SEM nor XRD results suggest that the unexpected sensing behavior may be attributed to morphological and/or crystallographic changes of the MoO3 film.
To better explain the sensing behavior, additional testing, including electrochemical impedance spectroscopy (EIS) and X-Ray photoelectron spectroscopy (XPS), will be performed on the sensor. XPS has been reported to provide information regarding the partial reduction of the surface of the MoO3 film [1]. EIS can be used to develop equivalent circuits that are representative of the sensor. It has been shown that different properties of the sensing film, such as grain size and particle contact, correspond to different equivalent circuits [12]. Therefore, EIS data at varying ammonia concentrations can help to develop an understanding of the film properties involved in the sensing mechanism. Results from EIS, XPS, SEM, and XRD, along with an understanding of the charge depletion layer model, will be used to propose a thorough ammonia sensing mechanism for α-MoO3.
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Figure 1