Ammonia-Sensing Properties of α-MoO3 Fabricated By Reactive Spray Deposition Technology (RSDT)
Authors: Ebaugh Thomas Allen , Poozhikunnath Abhinav , Bonville Leonard J. , Maric Radenka 
Published: February 2020

A vast body of literature exists focusing on the measurement of gaseous ammonia up to 1000ppm for various applications, ranging from healthcare to industry. Among the various sensing methods used for these measurements, the implementation of chemi-resistive sensors based on α-MoO3 is promising due to these sensors’ simplicity and selectivity toward ammonia [1-4]. Chemi-resistive sensors based on metal oxides typically involve a film of the metal oxide particles between two electrodes. The measured resistance of this metal oxide film changes as a function of gaseous ammonia concentration to which the film is exposed. The sensitivity of the sensor can be tailored based on the particle size and morphology of the metal oxide film [5-10].

α-MoO3 is a n-type semiconductor [4], meaning that the electronic conductivity of α-MoO3 is dominated by the 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]. Rout et.al. [5] describe the formation of the charge depletion layer by the transfer of electrons from the surface of the metal oxide particles to chemisorbed oxygen species, as shown in Equation (1).

O2+2e-→2O-
ads (Equation 1)

Because the charge depletion lower has a lower concentration of free electrons as a result of oxygen adsorption, the surface of the metal oxide particles has a higher electronic resistance than the bulk. Upon the introduction of gaseous ammonia, the ammonia reacts with oxygen species adsorbed on the surface of the metal oxide particles as shown in Equation (2), thereby releasing electrons back into the surface of the metal oxide particles [5]. This release of electrons decreases the electronic resistance of the surface of the metal oxide particles, which ultimately decreases the resistance of the sensor.

2NH3+3O-
ads →N2+3H2O+3e- (Equation 2)

Despite the wide acceptance of the charge depletion layer model as the method for ammonia sensitivity by metal oxides, the sensing literature does not appear to address the specific reaction pathway(s) involving both the adsorption of ammonia onto the MoO3 surface and the reaction of adsorbed ammonia species with adsorbed oxygen species. However, reaction pathways for the heterogeneous catalytic oxidation of ammonia on MoO3 are proposed in the literature.

In this work, chemi-resistive α-MoO3 ammonia sensors with two distinct morphologies, see Figure 1, are fabricated by RSDT. These sensors are characterized with XRD, SEM, and TEM. XRD results confirm the presence of α-MoO3 in both sensor morphologies. The differences in sensor morphologies are consistent with the differences in processing parameters. For example, the larger structures in Sensor A are consistent with the higher processing and annealing temperatures as compared with those used for Sensor B. The response of the sensors to gaseous ammonia in dry synthetic air up to 4.9ppm is measured using an in-house-built testing apparatus. Initial results suggest that there is a strong influence of α-MoO3 morphology on sensing response (Figure 2). Through the correlation of sensor morphology with sensing response, along with the application of the charge depletion layer model and catalytic oxidation mechanisms, a comprehensive reaction pathway for the adsorption and reaction of gaseous ammonia on α-MoO3 sensors will be proposed.

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Figure 1