Selective Ammonia Detection at Ppb-Level with Si-Doped MoO3 Sensors for Food and Environmental Applications
Authors: Weber Ines C. , Güntner Andreas T. , Pratsinis Sotiris E. 
Published: May 2020

Introduction

Ammonia (NH3) is an important marker for food spoilage[1] as well as a toxic environmental pollutant emitted from concrete walls, fertilizers or refrigeration systems[2]. Therefore, low-cost compact sensors for the detection of hazardous NH3 are urgently needed. Metal-oxide (chemo-resistive) gas sensors are attractive for NH3 detection as they offer simple operation, low power consumption and can be integrated into portable devices.[3] In specific, the α-phase of MoO3 is promising for detection of NH3 down to 50 parts per billion (ppb) at dry conditions. However, due to the high operating temperatures of MoO3 gas sensors (250 to 500 °C), the material must be stabilized by doping or the addition of foreign oxides. Here, we show how tailoring material morphology, phase composition and particle size enables high NH3 sensing performance[4].

Method

Pure and Si-doped MoO3 (0 – 20 wt%) nanoparticles were made by flame spray pyrolysis (FSP) and directly deposited onto Al2O3 sensor substrates with interdigitated Pt electrodes[4]. Subsequently, the sensor films were annealed at 450 °C for 5 h in an oven for thermal stabilization. The nanoparticles were analyzed using X-ray diffraction (XRD) patterns, nitrogen adsorption (Brunauer-Emmett-Teller, BET) as well as transmission electron microscopy (TEM) and electron diffraction (ED). The sensing performance was characterized as a function of SiO2 content with synthetic gas mixtures at a sensor temperature of 400 °C on a dynamic mixing setup. Furthermore, the selectivity towards possible interfering molecules of different chemical classes (e.g. acetone, CO and NO) was evaluated.

Results and Conclusions

The as-prepared, pure MoO3 powder shows fine crystallinity confirmed by transmission electron microscopy (TEM) (Figure 1a) and ED patterns (Figure 1b). By annealing the sensor substrates at 450 °C for 5 h, pure MoO
x
particles grow to ribbon-like structures (Figure 1d) with large crystallites, as indicated by XRD (triangles) and BET (open circles) and confirmed by bright spots in ED patterns (Figure 1c). Doping with Si reduces particle and crystal growth significantly, decreasing the crystal size from 147 to 65 nm and the particle size from 272 to 83 nm resulting in superior thermal stability. Excess Si (above 3 wt%) is not incorporated into the MoO3 lattice, but forms amorphous SiO2 domains, inhibiting further crystal growth, as subjected by the constant dXRD (Figure 1) and visible by TEM. Due to those structural changes, the sensor response and selectivity depend on the SiO2 doping content.
Figure 2 shows the response of pure and Si-doped MoO3 sensors to 1000 ppb NH3 (triangles), acetone (circles), NO (squares) and CO (diamonds) at 400 °C. Increasing the SiO2 content from 0 to 3 wt% almost triples the NH3 response from 0.22 to 0.53 with only minor changes for the other analytes. This results from the reduced sinter neck size of MoO3 due to segregated SiO2 domains which locally narrow the conduction channel leading to an increased electron depletion and change the morphology resulting in a higher surface-to-volume ratio[4]. Therefore, charge carrier mobility and thus film resistance are dominated by surface phenomena increasing the sensor’s sensitivity. Moreover, Si-doping (3 wt%) improved the NH3 selectivity toward other interfering gases achieving a response ratio to acetone (S
NH3
/ SA
) of 3.6 and to NO of 7.9 and no sensitivity for CO (Figure 2). Above 3 wt% SiO2 content, the film resistance continuously increases (inset in Figure 2) due to the formation of large and inert SiO2 domains. In addition to the enhanced response and selectivity towards NH3, the sensor could clearly detect NH3 down to 400 ppb even in the presence realistic relative humidity (up to 90%)[4]. As a result, this sensor is promising for integration into a portable device for food spoilage detection and environmental monitoring.

References

[1] S. Matindoust, A. Farzi, M. Baghaei Nejad, M. H. Shahrokh Abadi, Z. Zou, and L. R. Zheng, Ammonia gas sensor based on flexible polyaniline films for rapid detection of spoilage in protein-rich foods, J. Mater. Sci. Mater. Electron., 28, (2017) 7760–7768.
[2] J. Wang, X. Wei, and P. Wangyang, Gas-Sensing Devices Based on Zn-Doped NiO Two-Dimensional Grainy Films with Fast Response and Recovery for Ammonia Molecule Detection, Nanoscale Res. Lett., 10, (2015) 1–9.
[3] A. T. Güntner, S. Abegg, K. Königstein, P. A. Gerber, A. Schmidt-Trucksäss, and S. E. Pratsinis, Breath Sensors for Health Monitoring, ACS Sens., 4, (2019) 268–280.
[4] A. T. Güntner, M. Righettoni, and S. E. Pratsinis, Selective sensing of NH3 by Si-doped α-MoO3 for breath analysis, Sensors Actuators, B Chem., 223, (2016) 266–273.

Figure 1