Ammonia has extremely high annual production in fields such as industry, agriculture, and refrigeration, but its strong corrosiveness and acute toxicity pose serious threats to the environment and human health, making the development of efficient gas sensors for real-time monitoring crucial. Gas sensors based on metal oxide semiconductors (MOS), although widely used due to high sensitivity, low cost, and ease of integration, generally face an inherent performance trade-off in practical deployment, namely difficulty in simultaneously achieving high sensitivity, good selectivity, and stable room-temperature operation, and are often limited by relatively high operating temperatures and limited stability under environmental fluctuations.

Existing studies mostly rely on constructing heterojunctions, noble metal modification, element doping, and surface functionalization, but such methods mostly focus on macroscopic interface regulation or surface chemical reactions. Noble metal modification also suffers from high cost, easy agglomeration and deactivation, and humidity sensitivity. Heterojunction design often introduces complex interface states that affect long-term device stability. At present, in the field of gas sensing, strategies that use quantum confinement effects to precisely regulate carrier localization behavior, energy-level density of states, and trap-state charge storage at the quantum scale are still rarely explored in depth, causing the comprehensive performance of sensors in complex real environments to remain difficult to meet stringent requirements.

To address the above issues, a joint team composed of Sichuan Agricultural University and Southwest Jiaotong University used the ZEPTOOLS ZEM series desktop scanning electron microscope for in-depth research, the team constructed planar quantum wells by in situ growing CdS quantum dots on a cicada-pupa-like Fe₂O₃ substrate, synergistically inducing energy-level discretization and interfacial defect trap-state charge storage, and combined photoexcitation enhancement with size exclusion effects to successfully overcome the trade-off among sensitivity, selectivity, and room-temperature operation of traditional MOS sensors, achieving high-performance NH₃ detection.

Title:Light-Enhanced CdS Quantum Dot and Cicada-Like Fe2O3 Collaboratively Induce Band-Level Discretization and Trap-State Charge Storage for Advanced Ammonia Detection 
Journal:Inorganic Chemistry 
URL:http://dx.doi.org/10.1021/acs.inorgchem.6c03409

To address the bottleneck that traditional metal oxide semiconductor gas sensors find it difficult to simultaneously achieve room-temperature operation, high sensitivity, and high selectivity, this study first focuses on the construction mechanism and precise microstructural regulation of nano-heterocomposite sensing materials. The author team first synthesized a Fe₂O₃ substrate with a cicada-pupa-like micromorphology by a hydrothermal method, used the directional etching effect of SO₄²⁻ to endow it with a high specific surface area and hierarchical pore structure, and then in situ grew CdS quantum dots on the Fe₂O₃ surface. During this synthesis process, the team clarified that butylamine (BTA) acts as a core auxiliary agent to inhibit bulk CdS precipitation, promote controlled decomposition of thiourea, and protect quantum dot formation. They also systematically elucidated the decisive influence of Fe₂O₃ precursor addition amounts from 100 mg, 200 mg, to 300 mg on the nucleation kinetics and dispersion density of quantum dots, and finally determined that a 200 mg loading amount can form a uniformly anchored, uniformly sized, highly dispersed Fe-CdQDs composite structure.

To precisely confirm the micromorphological evolution and structural uniformity of the materials, the experimental team carried out systematic microstructural characterization of samples synthesized by different processes. During morphological analysis, the ZEPTOOLS ZEM series desktop scanning electron microscope was used to accurately characterize the surface micromorphology and particle distribution features of the samples, providing clear imaging evidence for intuitive observation of the micromorphological differences between Fe-Cd and Fe-CdQDs. The observation results confirmed that the Fe-Cd sample without butylamine formed coarse bulk CdS precipitates on the Fe₂O₃ surface, whereas Fe-CdQDs with the auxiliary agent exhibited fine particles tightly anchored at the substrate edges. Combined with transmission electron microscopy analysis of lattice fringes and interfacial mismatch defects, the interfacial contact state between quantum dots and substrate was fully clarified.

Subsequently, the research focused on the sensitive response behavior and multi-field coupling sensitization characteristics of this composite system under room-temperature ammonia conditions. Gas sensing tests showed that the Fe-CdQDs 200 sensor exhibited a response value as high as 68% to 100 ppm NH₃ at 20°C room temperature, with response and recovery times of 17 s and 43 s, respectively, and a detection limit of 6 ppm. It also maintained excellent operational stability and outstanding humidity resistance over multiple cycles and continuous 30-day long-term testing. More importantly, CdS quantum dots possess significant photoelectric excitation properties. Under 254 nm light excitation, the response value of the composite further jumped to 84.73%, while responses to interfering gases such as trimethylamine, ethanol, acetone, and hydrogen sulfide were effectively suppressed below 15%, fully verifying the key promoting role of external light field excitation in multiplying room-temperature response sensitivity and molecular recognition accuracy.

Finally, through energy band theory analysis, electrochemical impedance spectroscopy, and first-principles density functional theory calculations, the study systematically revealed the intrinsic physicochemical mechanism of synergistic sensitization by quantum confinement effects and defect-state charge regulation. Theoretical and spectral analyses confirmed that the strong quantum confinement effect of CdS quantum dots broadens their band gap to 1.27 eV and renders energy levels quantized and discrete, prompting the interfacial built-in electric field to drive electrons backflow from Fe₂O₃ and accumulate on the CdS side, where they are deeply captured by abundant mismatched lattice defect states and trap centers at the interface, thereby constructing a high-resistance "charge reservoir" in air. Once exposed to reducing NH₃, the trapped electrons are rapidly and massively released back to the conduction band, causing a significant conductivity mutation. At the same time, closely packed CdS quantum dots act as a size-selective molecular sieve physical barrier, effectively blocking macromolecular interferents and promoting highly selective NH₃ adsorption, fundamentally providing solid and reliable microscopic mechanistic support for the quantum band engineering design of high-performance room-temperature gas sensors.

The ZEPTOOLS ZEM series scanning electron microscope is a highly integrated, portable, and cost-effective scientific research instrument. It features rapid vacuum pumping, high imaging speed, and a variety of signal detector options, making it suitable for morphology observation and compositional analysis, and it can also adapt to various in situ experimental requirements. The instrument has low installation environment requirements, is not floor-sensitive, and is simple to operate; non-specialists can quickly get started. It can be widely applied in many fields such as new material R&D, life sciences, failure analysis, and industrial quality inspection, providing scientific research institutes and enterprise users with a powerful microcharacterization solution that combines high performance and high cost-effectiveness.