Low-Cost Ammonia Sensor Using Carbon-Based Nanocomposite

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Apr 17 2024

Researchers recently published an article in Scientific Reports proposing a low-cost, flexible ammonia gas sensor that operates at room temperature. The sensor uses a nitrogen-doped carbon nano-onions/polypyrrole (NCNO-PPy) composite fabricated through hydrothermal and in situ chemical polymerization methods and mounted on an inexpensive membrane substrate.

Background

Ammonia (NH₃) is a pungent colorless gas originating from several industrial, agricultural, and commercial sources and is dangerous for human health and the environment. The safe limit for human exposure is 25 ppm NH₃ for 8 hours, beyond which it can cause severe respiratory and cardiac problems. Thus, highly sensitive and low-cost ammonia detection devices are required for early intervention during ammonia leaks.

Numerous ammonia sensors have recently been fabricated using various gas-sensing materials, such as polymers, metal oxides, transition metal dichalcogenides, carbon nanomaterials, and metal hybrids. However, their practical application is limited by high working temperatures, low selectivity, and tedious preparation.

Alternatively, organic conducting polymers like polypyrrole (PPy) are favorable for ammonia detection due to their low operating temperature, good sensitivity, and compatibility with flexible substrates. Yet, organic conducting polymers still face limitations in terms of low response, long response time, and poor damp-heat stability, which can be overcome by incorporating other efficient sensing materials into their matrix.

For instance, novel carbon-based nanomaterials like carbon dots and carbon nano-onions (CNOs) exhibit high surface area and good electrical properties, making them ideal for gas sensing applications.

Sensor Fabrication

The present study employed in situ chemical oxidative polymerization to synthesize an NCNO-PPy composite for room-temperature ammonia sensing. Flame pyrolysis was used to prepare CNOs from waste oil, followed by hydrothermal nitrogen doping using urea as the precursor. Subsequently, the synthesized materials were characterized by field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) surface analysis, and X-Ray diffraction (XRD).

After thorough characterization, the NCNO-PPy nanocomposite was coated on a polyvinylidene fluoride (PVDF) substrate with aluminum electrodes to prepare a flexible ammonia sensor. The researchers evaluated the gas sensing performance of the NCNO-PPy sensor at room temperature for various ammonia concentrations in an acrylic chamber.

This gas sensing performance was then compared to a pristine PPy sensor. The researchers subsequently performed a computational study on the optimized NCNO-PPy nanocomposite both with and without NH₃ interaction using density functional theory (DFT) via the Materials Studio program.

Results

The structural and morphological characterization of the NCNO-PPy nanocomposite revealed its elemental composition and surface features. For example, FE-SEM images and XRD patterns showed a homogeneous dispersion of CNOs within the PPy matrix, leading to the formation of inter-junction surfaces. Furthermore, BET analysis indicated a large specific surface area and a mesoporous structure in the 5 wt.% NCNO-PPy composite. These characteristics thus enhance the sensor's high sensitivity to ammonia.

The flexible NCNO-PPy sensor proposed in this study exhibited exceptional performance for ammonia detection at room temperature. A response of 17.32 % was observed for 100 ppm ammonia concentration with a low response time of 26 seconds. In contrast, the PPy sensor’s response was only 3.62 % for 200 ppm ammonia concentration, which is too low for practical applications.

Additionally, the NCNO-PPy ammonia sensor has a lower detection limit of 1 ppm, high selectivity (against hydrogen, carbon dioxide, carbon monoxide, ethanol, and nitrogen dioxide), good repeatability, and long-term durability. This flexible sensor can bend up to 90° and remain intact for 500 bending cycles, exhibiting mechanical robustness under extreme bending conditions.

The researchers attributed the enhanced performance of the NCNO-PPy ammonia sensor to the NCNO-PPy bonding and synergistic interaction, defects formed from nitrogen doping, and high reactive sites present on the surface of NCNO-PPy composites. These features enable a strong interaction of NCNO-PPy with NH₃ molecules at an enhanced adsorption rate.

Additionally, the high surface area and porous structure of CNOs and their high electron mobility with PPy lead to an effective transduction and diffusion process for ammonia. All these factors lead to enhanced gas sensing performance of NCNO-PPy compared to PPy.

Conclusion

Overall, the researchers demonstrated an efficient and flexible room-temperature ammonia sensor using a carbon-based nanocomposite. Thanks to its wide functional range (1 to 200 ppm ammonia), stability, and selectivity, this sensor could be used in various fields like food quality assessment, public safety, and health monitoring. In addition, the robust bending performance of the NCNO-PPy ammonia sensor makes it suitable for implementation in wearable electronics.

The DFT analysis of the optimized NCNO-PPy nanocomposite reveals its electronic properties—band gap, electron affinity, and ionization potential—enhancing our understanding of its high-efficiency sensing capabilities. This study appears to be the first to use CNOs in affordable, flexible gas sensors, potentially advancing the real-time, room-temperature detection of ammonia.

Journal Reference

Shiv Dutta Lawaniya, Kumar, S., Yu, Y., & Awasthi, K. (2024). Nitrogen-doped carbon nano-onions/polypyrrole nanocomposite based low-cost flexible sensor for room temperature ammonia detection. Scientific Reports, 14(1). https://doi.org/10.1038/s41598-024-57153-4, https://www.nature.com/articles/s41598-024-57153-4 

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