(441g) PANI/MoS2 based Room Temperature Gas Sensor for Sub-Ppm NH3 detection: Experimental and DFT Insights

The high levels of hazardous gas emissions have an adverse effect on both human health and the environment. Therefore, it is imperative to monitor harmful gases to protect both humans and the environment. This creates a substantial need for low-cost, dependable environmental monitoring and safety gas sensors.

Molybdenum disulfide (MoS₂), a form of 2D transition-metal dichalcogenide, has been extensively studied for chemical vapor detection because of its long interlayer distance (1.24 A°), high absorption coefficient, and large surface-to-volume ratio. MoS₂-based gas sensors, on the other hand, have a lower room-temperature sensing response and lack reversibility, which is most likely related to their low electric conductivity, ease of 2D nanosheet restacking, and significant oxygen adsorption in air. As a result, it encourages the fabrication of MoS₂-based sensors with superior sensing reversibility, chemical stability, and sensing responsiveness for use in practical detection applications. Meanwhile, conducting polymers provide room-temperature sensing and better selectivity, especially polyaniline (PANI). However, the environmental stability and sensing response of pristine PANI-based gas sensors is low, thus having a negative influence on their use as sensors. To improve sensing capabilities, several fabrication techniques have been put forth in the literature. These include creating p-n heterojunctions of PANI with metal oxides, carbon nanotubes, rGO, and noble metals, which would alter the band structure and best utilize the characteristics of each component in the composite. Few studies, nonetheless, have discussed the use of PANI in conjunction with two-dimensional (2D) materials.

In our work, the PANI/MoS₂ composite, screen-printed on PET substrate, is utilized as a flexible room-temperature-based sensor to investigate its behaviour with ammonia (NH₃) and various other gases with biological and environmental implications. The sensing capabilities are analysed experimentally and through theoretical studies, i.e., DFT studies. Further, two different synthesis routes have been explored to synthesize the composite to study the effect of the morphology of PANI/MoS₂ on sensing.

Methodology:

Different routes for the synthesis of PANI-MoS₂ composite.

1) Route 1: PANI-MoS₂ composite was synthesized using interfacial polymerization of aniline in the presence of bulk MoS₂. Further, to evaluate the influence of MoS₂ in sensing and find a suitable ratio that provides the best sensing performance, polymerization was carried out in the presence of varying concentrations of MoS₂.

2) Route 2: Multi-layered MoS₂ nanosheets were developed using hydrothermal treatment via precursors (ammonium molybdate and thiourea with a specific ratio reported elsewhere). Few layers MoS₂ nanosheets synthesized by hydrothermal methods were added to the polymer reaction system from the initiation stage. After the pre-optimized duration of 12 hours, the polymerization was stopped, and through a rigorous purification process, the composite material was collected and stored under vacuum conditions until further used.

Once the composite formation was confirmed, conducting inks were formulated using α-terpinol. The chemiresistive-based sensors were fabricated by screen printing on PET sheets with specifically designed silver electrodes. The screen-printed devices were dried at 80 °C for four hours and stored in a vacuum desiccator until further use.

Sensing:

This study was carried out using an in-house gas sensing system where compressed dry air (CDA) is used as a carrier gas. A regulated amount of CDA is sent to the sensing chamber to remove any impurities in the chamber or previously adsorbed species. The analyte gas was transported to the mixing chamber in a particular ratio with CDA to achieve the necessary gas concentration. The mixture was fed to the sensing chamber and exposed to the sensor film. After a successful sensing investigation, an inert gas purge was performed to remove any remaining vapors from the sensing chamber. The relative change in the resistance of the sensor brought on by exposure to the target gas was used to measure the sensor response. All the measurements were conducted at a relative humidity (RH) of 10% and a temperature range between 25-30 °C, i.e., room temperature.

Results and Implications:

The sensor response of the composite markedly improved from that of the pristine PANI, showing an improvement of 11% for 10 ppm of NH₃. The reason for the superior sensing response of the composites over the pristine PANI is the synergistic effect of conducting polymer and 2D material; the EIS studies indicated that the p–n junction depletion layer formed at the interface of PANI and MoS₂ enhanced the sensing response. Additionally, the gas-sensing properties of the PANI/MoS₂ composites were reinforced by their abundance of reactive sites.

Using the PANI/MoS₂ device, the detection limit for NH₃ was found to be 122 ppb with a 2.9% response. There was no need for external heating because the sensing device could sense and recover at ambient temperature. One essential factor that impacts sensor responsiveness is humidity. The sensing response of the device was evaluated at different relative humidity levels, and it was discovered that the response was steady up to 70% relative humidity, but above that point, an increased response to NH₃ was seen. Such behavior might be because absorbed moisture or the production of conductive H₃O⁺ further protonate PANI. Over nine months, the response values of the sensor fluctuate. However, this fluctuation is minimal, with a divergence of 2% demonstrating the consistency of the sensor over time. The composite synthesized via two routes exhibited structural and morphological diversity, demonstrated by all the characterization techniques. Morphology, however, had no appreciable effect in enhancing the gas-sensing response. This might be explained by the low content of MoS₂ in the composites, which lessened the impact of morphology on sensing. However, during sensing studies, it was observed that composites synthesized via route 2 showed better noise resistance and durability against environmental effects than the ones synthesized via route 1. Furthermore, the detection limit of NH₃ was in the sub-ppm range (122 ppb), but the limit of detection for other gases, including hydrogen sulfide, acetone, ethanol, carbon dioxide, and methanethiol, was in the ppm range (20 ppm-70 ppm) which is high compared to NH₃, signifying the sensitivity of our device towards NH₃. Added to that, to confirm sensing experiments and determine the rationale for sensing, a DFT analysis (Bader charge, charge density difference, density of states) was conducted, and one of the significant findings was that adding MoS₂ to PANI increased NH₃ adsorption while reducing the adsorption of other gases making this device more sensitive towards NH₃, which supported our sensing results. Furthermore, hydrogen sulfide, carbon dioxide, and dimethyl disulfide were shown to have physisorption, whereas NH₃, NO₂, acetone, ethanol, and methanethiol showed quasi-molecular interaction.

Further, the ability of our device to respond selectively to low quantities (sub-ppm range) of NH₃ suggests that the material has a significant potential for the non-invasive diagnosis of kidney diseases in humans as NH₃ in human breath exhaled by healthy individuals ranges between 20–688 ppb, but those with renal disorders have levels around 2000 ppb.