With growing demand for precise environmental monitoring comes the need for compact, low-power gas sensors. Ammonia, widely used in industrial processes such as selective catalytic reduction (SCR) systems for NOx removal and as a hydrogen carrier in clean energy applications, poses a dual risk.
Overdosing in SCR systems can lead to atmospheric release of the gas, while their are concerns around ammonia's storage and transport due to its high volatility. Ammonia also reacts with sulfur dioxide and nitrogen oxides to form fine particulate matter (PM2.5), further contributing to air pollution. At concentrations above 50 ppm, ammonia becomes a serious health hazard, capable of causing irreversible respiratory damage.
As a result, detecting ammonia leaks rapidly and reliably, especially at ambient conditions, is critical.
Existing Technologies
Chemiresistive sensors, with their simple design, low cost, and easy integration into miniaturized platforms, are one answer. However, most ammonia sensors in this category suffer from two main issues: poor selectivity and the need for elevated operating temperatures.
Metal-organic frameworks (MOFs), known for their tunable porosity and high surface area, have potential in sensing applications, but their poor electrical conductivity has constrained their practical use. This low conductivity results from insulating organic linkers and limited electron pathways.
Pyrolyzing MOFs into metal oxide semiconductors is a way around this, improving conductivity but retaining the desirable porous structure and enhancing surface defect sites that contribute to gas adsorption and signal response.
While several MOF-derived materials have shown promise in gas sensing, few studies have successfully demonstrated cerium-based MOF (Ce-MOF) as a viable room-temperature ammonia sensor.
The MOF-to-Oxide Design
The study published in Chemosensors, presents a method for fabricating nanostructured cerium oxide-carbon composites (CeO2-C) by thermally treating Ce-BDC (cerium-terephthalic acid) MOF precursors.
The team fine-tuned the crystallinity and particle size of these Ce-BDC precursors by adjusting the concentration of cerium ions and organic linkers. Through controlled solvothermal synthesis followed by pyrolysis at 500?°C, they produced a series of CeO2-C-X materials (X = 0.5, 1, 1.5, 2), each with distinct structural features.
Importantly for their use in sensors, the thermal conversion retained residual carbon from the organic ligands, which enhanced electrical conductivity and reduced sensitivity to humidity.
Structural and Performance Optimization
Detailed characterization using SEM, TEM, XRD, XPS, Raman spectroscopy, and nitrogen adsorption measurements revealed that increasing the concentration of raw materials resulted in more uniform nanoparticle dispersion.
Among the series, the Ce-BDC-1 precursor and its derivative CeO2-C-1 were unique. It exhibited the smallest change in particle size before and after pyrolysis (from 8.67 to 7.96 nm), indicating minimal structural collapse and greater retention of desirable physical properties.
Higher precursor concentrations (X?≥?1.5) caused structural collapse and excessive oxygen-defect clustering, lowering Ce4+ content and weakening redox performance, degrading sensing capability. This made CeO2-C-1 the best performing sample, balancing porosity, crystallinity, and surface chemistry.
Strong Room-Temperature Sensing Performance
When tested under ambient conditions (25?°C, 40 % relative humidity), CeO2-C-1 demonstrated a high response of 82 % to 100 ppm ammonia, with fast response and recovery times of 22 and 66 seconds, respectively. It also exhibited a low detection limit of 0.31 ppm and strong long-term stability, maintaining ~80 % of its response over 30 days.
Selectivity tests showed that CeO2-C-1 responded significantly less to interfering gases like H2S (27 %) and NO2 (11 %), highlighting its potential where cross-sensitivity is a challenge. These performance advantages were attributed to its optimized mesoporous structure, small pore size, and moderate concentration of oxygen vacancies, all key features that support efficient gas adsorption and rapid charge transport.
Download your PDF now!
The researchers also evaluated the sensor’s performance under oxygen-free environments and across varying humidity levels. While the response time slightly increased in these settings, CeO2-C-1 retained a strong signal, reinforcing its robustness in diverse operating conditions.
Mechanistically, the sensing behavior is governed by depletion-layer modulation. Surface-adsorbed oxygen species (O2-, O-) capture electrons, forming a depletion region in the semiconductor.
When ammonia is introduced, it reacts with these oxygen species, releasing electrons back into the conduction band, reducing resistance and generating the sensor signal. The residual carbon content in CeO2-C also aids in maintaining stable conductivity, even in humid conditions.
Conclusion
This study demonstrated that by carefully tuning the precursor concentration and preserving structural integrity during synthesis, researchers can develop high-performance ammonia sensors that work reliably at room temperature.
CeO2-C-1, derived from a precisely engineered Ce-MOF precursor, is a compelling material with fast response times, high selectivity, and operational stability, that does not require either elevated temperatures or complex circuitry.
Journal Reference
Wang, L. et al. (2025). Nanostructured CeO2-C Derived from Ce-BDC Precursors for Room-Temperature Ammonia Sensing. Chemosensors, 13(10), 362. DOI: 10.3390/chemosensors13100362, https://www.mdpi.com/2227-9040/13/10/362
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.